L Zhilin

P Basyrova

V Loiko

Popkov

K Eremeev

O Dymshits

A Khubetsov

I Alekseeva

S Zapalova

M Tsenter

A Zhilin

L Basyrova

P Loiko

V Popkov

Influence of Fe 2+ doping concentration on the structure and spectroscopic properties of transparent glass-ceramics based on Fe 2+ :ZnAl2O4 nanocrystals

may come

Influence of Fe 2+ doping concentration on the structure and spectroscopic properties of transparent glass-ceramics based on Fe2+:ZnAl2O4 nanocrystals K Eremeev, O Dymshits, A Khubetsov, I Alekseeva, S Zapalova, M Tsenter,

Introduction

Coherent light sources emitting in the mid-infrared spectral range (the so-called molecular fingerprint region containing the absorption lines of various bio-and atmospheric species) are of practical use for such applications as molecular spectroscopy, gas sensing and medicine. There is a demand for gain media and saturable absorbers of mid-infrared lasers. Materials doped with tetrahedrally (Td) coordinated ferrous (Fe 2+ ) ions and demonstrating intense broadband absorption in the spectral range of 2-3 μm are the suitable candidates. The crystals of ZnS and ZnSe doped with Fe 2+ ions are clear examples of such materials [1]. Nevertheless, it is of great interest to search for other materials for Fe 2+ doping. One of the groups of such materials are spinels doped by Fe 2+ ions, e.g., Fe:MgAl2O4, [2]. The structural formula of spinel, AB2X4, can be expressed as IV [A1-B] VI [B2-A]X4, where IV [] and VI [] represent the tetrahedral (Td) and octahedral (Oh) sites, respectively;  is the so-called inversion parameter reflecting the degree of disorder. There are two ordered configurations of cations in spinels: the normal spinel with  = 0 and the inverse spinel with  = 1. Gahnite, ZnAl2O4, also belongs to the spinel family. It demonstrates a certain degree of disorder with increasing the annealing temperature [3]. Recently, transparent ceramics of Fe:ZnAl2O4 were fabricated and their spectroscopic parameters were evaluated [START_REF] Fedorov | Proc. SPIE[END_REF]. The Fe 2+ content in Td and Oh sites in gahnite was estimated as nearly equal. As compared to optical ceramics, zinc aluminosilicate (ZAS) transparent glass-ceramics (GCs) containing nanosized gahnite crystals possess an advantage of the flexible and convenient fabrication method based on the glass technology, and, moreover, they allow for alteration of the nanocrystals compositions, structures and sizes. Previously, we developed cobalt-doped transparent GCs in the ZAS system and revealed that the CoO facilitated amorphous phase separation of glasses that resulted in the formation of Co-doped amorphous zinc aluminotitanate (ZAT) phase and gahnite nanocrystals [START_REF] Alekseeva | [END_REF]6]. In this work, we developed and studied novel transparent GCs based on Fe 2+ :ZnAl2O4 nanocrystals with the aim to elucidate the role of iron ions in their formation.

Experimental

We studied the glasses of the ZnO-Al2O3-SiO2 system nucleated by TiO2, both undoped and doped with 0.6 and 1.0 mol% FeO (400 g in weight). They were melted at 1580 °C with stirring, cast onto a metal plate and annealed at 680 °C. The initial glasses were transparent. To promote the growth of gahnite nanocrystals, the glasses were subjected to two-stage heat-treatments at temperatures from 750 to 1200 °C with a holding time of 6 hours (h) at each stage. The first (nucleation) heat-treatment was always at 720 °C. The undoped glass was pale-grey while glasses doped with iron oxide were browngrey coloured. The colour of the samples steadily changed to black after the heat-treatment at the temperatures ranging from 720 to 1000 °C. The obtained GCs were transparent. The samples subjected to heat-treatments at higher temperatures became opaque, Figure 1.

The X-ray powder diffraction (XRD) patterns of the initial glasses and GCs were recorded using a Shimadzu XRD-6000 diffractometer with Cu Kα radiation and a Ni filter. The parameters of the unit cell of gahnite nanocrystals were determined from the Rietveld refinement using the MAUD software. The mean crystal sizes were estimated from the broadening of the X-ray diffraction peak at 2θ ≈ 65.0° according to the Scherrer equation [START_REF] Alekseeva | [END_REF]. The error of the crystal size estimation was ~5%. The Raman spectra were measured with a Renishaw inVia confocal Raman microscope using a Leica ×50 microscope objective and an edge filter. The excitation wavelength was 514 nm (Ar + ion laser line). The absorption spectra were measured with a Shimadzu 3600 spectrophotometer in the spectral range of 250 -3300 nm with a resolution of 1 nm. Plane-parallel polished plates with a thickness of 1 mm were used. The absorption coefficient αabs was calculated taking into account the Fresnel losses.

Results and discussion

Initial glasses

The undoped glass is X-ray amorphous while the traces of gahnite nanocrystals are visible in the XRD patterns of glasses doped with FeO, Figure 2(a). Their Raman spectra are formed by two broad bands, one with two local maxima at ~300 and ~445 cm -1 and another one with two maxima at about 800 and 920-930 cm -1 , Figure 2(b). The bands at ~445 and 800 cm -1 are attributed to vibrations of tetrahedra of the aluminosilicate network in the glass structure. The more intense band at higher frequencies is usually connected with vibrations of TiO4 tetrahedra entering the aluminosilicate network [START_REF] Alekseeva | [END_REF]6]. The shift of positions of the Raman bands centered at ~928 and 797 cm -1 for the undoped glass to lower frequencies with FeO doping manifests the beginning of liquid phase separation [START_REF] Alekseeva | [END_REF]6] during glass casting and annealing. The absorption spectra of the glasses doped with FeO are drastically different from that of the undoped glass, Figure 2(c). They are formed by the UV absorption edge probably caused by the overlapping O-Ti 4+ , O-Fe 2+ and O-Fe 3+ oxygen-to-metal charge transfer (OMCT) bands, intense, wide and unstructured absorption band in the visible which is assigned to spin-forbidden d-d transitions of ferrous and ferric ions, and the bands with the maxima at ~1080 nm (the 5 T2 → 5 E ( 5 D) transition) and ~1860 nm (the 5 E → 5 T2 ( 5 D) one) ascribed to absorption of Fe 2+ ions with Oh and Td coordination, respectively. A broad asymmetric band at the wavelengths of 2.7 -3.1 μm is caused by O-H vibrations of hydroxyl groups in the glass. The almost linear dependence of αabs on the concentration of Fe 2+ ions, Figure 2(d), indicates the implementation of Beer's law, i.e., the distribution of Fe 2+ ions between the Oh and Td sites in the initial glasses is almost constant.

Heat-treatment at the first stage

After the heat-treatment of the undoped glass at 720 °C, it contained only traces of gahnite crystals and the gahnite crystallinity fraction increased with the FeO doping, Figure 3(a). The addition of FeO leads to a pronounced change in the Raman spectra, Figure 3

(b).

There is an intensity redistribution between the bands at 800 -900 cm -1 in favour of the band at ~800 cm -1 . These changes are associated with the development of liquid phase separation and formation of the amorphous ZAT phase wherein titanium ions form TiO5 and TiO6 polyhedrons giving rise to a characteristic band at about 800 cm -1 superimposed on the band due to vibrations of tetrahedra of the aluminosilicate network. In the absorption spectra of FeO-doped samples, Figure 3(c), the broad band in the visible is connected with Fe 2+ + Ti 4+ → Fe 3+ + Ti 3+ intervalence charge transfer (IVCT) transitions and spin-forbidden transitions of Fe 2+ and Fe 3+ ions in the ligand field. The bands related to the transitions of Fe 2+ ions in the Oh and Td sites in the gahnite nanocrystals are now observed at ~1030 and ~1820 nm, respectively. The Beer's law is implemented for both bands, Figure 3(d). The structuring of the absorption bands due to the O-H vibrations is connected with OH -entering the spinel structure in FeO-doped GCs.

Heat-treatment at the crystallization stage

Gahnite crystallizes within the whole studied temperature range up to 1200 °C, Figure 4(a,b). Simultaneously with gahnite crystallization, the first diffraction maximum in the XRD patterns (an amorphous halo) decreases and shifts to smaller angles (to the position of the diffraction maximum of a quartz glass), because in the course of liquid phase separation and gahnite crystallization the residual glass composition becomes gradually depleted in Zn, Al, Ti and Co oxides [START_REF] Alekseeva | [END_REF]6]. According to the XRD data, the GCs prepared in the temperature range of 720 to 950 °C were of single phase nature containing only gahnite nanocrystals. Rutile, the stable modification of TiO2, crystallized starting from the temperature of 1000 °C and its amount increased with the FeO addition, Figure 4(a). Cristobalite appeared during the heat-treatment at 1200 °C only in FeO-doped GCs, Figure 4(b). The appearance of the broad band with two maxima at 725 and 765 cm -1 , Figure 4(c), and distinct bands at 705 and 737 cm -1 , Figure 4(d), in the Raman spectra of FeO-doped GCs reveals disordering of gahnite structure resulting from Fe 2+ and probably Ti 4+ entering the gahnite crystals. Anatase and rutile peaks are visible in the Raman spectra of GCs prepared by the heat-treatments with the second stage at 1000 °C, Figure 4(c), rutile peaks are enhanced in the Raman spectra of all FeO-doped GCs, Figure 4(c,d).

In the absorption spectrum of the undoped GC, a broad band spanning from the visible until near IR spectral range is probably connected with the 2 T2g  Eg transition of Ti 3+ ions in octahedral sites, as well as with the Ti 3+ /Ti 4+ IVCT band, Figure 5 (a). The absorption edge experiences a notable red-shift hindering the absorption band at 1000 nm and the intensity of the absorption band around 2000 nm greatly increases. The position of its maximum is now shifted to ~2035 nm. Moreover, the absorption band of OH --groups is structured and its intensity raises. A sharp increase in absorption in the visible may be due to the Fe 2+ -Fe 3+ IVCT transitions, Fe 2+ and Fe 3+ d  d transitions in the ZAT amorphous phase, in gahnite and a residual glass. The absorption band with a maximum at ~2035 nm is associated with the incorporation of Fe 2+ ions into the gahnite nanocrystals in Td positions (the 5 E  5 T2( 5 D) transition). The Beer's law is implemented for this band. The structuring of the IR absorption band caused by OH -groups is connected with gahnite crystallization as they enter into the gahnite structure.

Conclusions

We report on the synthesis of transparent glass-ceramics based on Fe 2+ :ZnAl2O4 nanocrystals and their structure, phase transformation and spectroscopic properties, for the first time, to the best of our knowledge. FeO doping and increasing the FeO concentration in the initial glasses accelerates liquid phase separation and gahnite crystallization during pouring and annealing of the as-cast glasses and during the first (nucleation) stage of the heat-treatment resulting in the formation of zinc aluminotitanate amorphous regions and gahnite nanocrystals. It implies that iron ions enter both phases. The developed transparent glass-ceramics exhibit a broadband (1.5-2.5 μm) absorption due to the 5 E → 5 T2 ( 5 D) transition of Fe 2+ ions in tetrahedral sites in gahnite nanocrystals. In addition, the absorption band of Fe 2+ ions in the octahedral sites was observed with a maximum at ~1 μm. The glass-ceramics are promising as saturable absorbers of lasers emitting at 2-3 μm.

Figure 1 .

 1 Figure 1. Photographs of undoped and 0.6 mol% FeO-doped initial glass and GCs obtained by heattreatments at temperatures of 720 -1200 °C indicated in photographs. The holding time is 6 h.

Figure 2

 2 Figure 2(a-d). Characterization of initial glasses, undoped and doped with 0.6 and 1.0 mol% FeO: (a) XRD patterns, (b) Raman spectra, (c) absorption spectra and (d) the dependences of αabs in the absorption bands due to the 5 T2( 5 D)  5 E and the 5 E  5 T2( 5 D) transitions of Fe 2+ ions (at 1080 and 1860 nm, respectively) on the FeO concentration.

Figure 3 (

 3 Figure3(a-d). Characterization of undoped and 0.6 and 1.0 mol% FeO doped glasses after the heat-treatment at 720 °C for 6 h: (a) XRD patterns, (b) Raman spectra, (c) absorption spectra and (d) the dependences of αabs in the absorption bands due to the 5 T2( 5 D)  5 E and the 5 E  5 T2( 5 D) transitions of Fe 2+ ions (at 1030 and 1820 nm, respectively) on the FeO concentration.

Figure 4 (Figure 5

 45 Figure4(a-d). XRD patterns and Raman spectra of glass-ceramics undoped and doped with 0.6 and 1.0 mol% FeO prepared by the heat-treatments: (a,c) at 720 °C and at 1000 °C, (b,d) at 720 °C and at 1200 °C. The holding time at each stage is 6 h.

Figure 6 (

 6 Figure 6(a,b). Dependences of the unit cell parameter (a) and the mean size of gahnite nanocrystals (b) in the glass-ceramics on the FeO concentration.

Acknowledgements

This work was partly supported by the RFBR (Grant 19-03-00855).


V V Vitkin

A V Kovalev

K M Grigorenko

V V Kurikova

E E Popov

P A Loiko

Resolving the 12 CH 4 and 13 CH 4 lines of Raman spectrum

We demonstrate a method to determine the volume fraction of gaseous isotopologues of methane 12 CH4 and 13 CH4 in a mixture from a Raman spectrum under spectral overlap and limited resolution.

Introduction

Resolving densely spaced spectral lines is a common spectroscopic problem. For example, methane isotopologues 12 CH4 and 13 CH4 Raman scattering lines are at a distance of less than 4 cm -1 from each other [START_REF] Vitkin | Raman Laser Spectrometer: Application to 12C/ 13C Isotope Identification in CH4 and CO2 Greenhouse Gases[END_REF], making it a problem to distinguish between them in a gas mixture. There is also a demand for a high-dynamic range, versatile, and precise tool for isotopic analysis, e.g., to confirm the origin of biomethane, and Raman spectroscopy is a right candidate for that [START_REF] Petrov | Pressure dependence of the Raman signal intensity in high-pressure gases[END_REF][START_REF] Vitkin | Raman gas analyzer for detecting carbon isotopologues[END_REF]. This paper presents a method to define the isotopologue volume fraction using a nonlinear regression model complemented by a two-peak Lorentz approximation of the scattering spectrum.

Theory

At the preparation step, Raman spectra of pure gases mixed with nitrogen are measured under the varied setup parameters, i.e., laser power, camera exposure time, the pressure of the gas in the cuvette. Then they are fitted with a single-peak Lorentz function:

( ) ( , , , ) S S S S k L k k k k S f A      =+ , (1)
where kindex (1 for 12 CH4, 2 for 13 CH4); ν -Raman frequency in wavenumbers; A S kamplitude of the Raman scattering signal peak; μ S kcentral wavenumber of the Raman scattering signal peak; σ S kwidth of the Raman scattering signal peak measured at the half of the amplitude; β S kbaseline parameter; fLthe Lorenz function:

22 ( , , , ) / [1 4( ) / ] L f A A       = + - . (2)
The Lorenz profile is used due to the good characterization of the Raman spectrum of gases [START_REF] Rothschild | Dynamics of molecular liquids[END_REF]. The approximation by eq. ( 1) involves evaluation of A S k, μ S k, σ S k, β S k parameters employing the least-squares fitting method. After the approximation, the nonlinear regression model is build based on extracted values A S k, and the experimental parameters. The following equation describes the model:

.% 1, .%,

S k vol k vol k k A P W X p PW X p  =+ , 2, ( , , , )
where Ppressure in the cuvette (in atmospheres); Wlaser power; τexposure time of camera; Xvol.%,kvolume fraction of the component; p1k, p2kparameters determined by the regression. The parameter p1k describes fixed unspecified experimental parameters. Parameter p2k describes the camera noise. The volume fraction can be determined from (3) using an equation:

.%, 2, 1, ( , , , ) ( ) / ( )

S vol k k k k X P W A A p p PW  =- , (4)
In order to determine each component's portion in the gas mixture, its Raman scattered signal is approximated by the sum of two Lorentz functions. The two-peak model of the signal can be written as the equation:

L k k k k M f A      = =+  , ( 5
)
where values with upper dashes are fixed and defined from a single-peak approximation by eq. ( 1). Extracting the peak intensities of the corresponding gas component and putting it and the experimental parameters into eq. ( 4). we can calculate its volume fraction.

Experimental results

To verify our method, we conducted a series of experiments measuring the volume fraction of 12 CH4 and 13 CH4 in gaseous mixtures. The experimental setup is described in our previous work [START_REF] Vitkin | Raman gas analyzer for detecting carbon isotopologues[END_REF]. Figure 1 demonstrates both measured and fitted Raman scattering signals (green lines are for measured spectra; red lines are for 12 CH4 Lorenz profile; blue lines are for 13 CH4 Lorenz profile; black lines are their sum as given by eq. ( 5)). (b) 13 CH4 -92,8%, N2res.; (в) 12 CH4 -0,01028%, 13 CH4 -0,01057%, N2res.;

(г) 12 CH4 -4,84%, 13 CH4 -4,98%, N2res.

The results, presenting measured volume fraction values and relative errors for a gaseous mixture containing 4.84% of 12 CH4 and 4.98% of 13 CH4, are shown in table 1. The error value depends on the volume fraction and accuracy of measuring the experimental parameters and does not exceed 11% (for the 100 ppm gaseous mixtures).

Conclusion

We demonstrate a method that can determine the volume fraction of gaseous components in densely spaced lines. The relative error depends on volume fraction and accuracy of measuring the experimental parameters but does not exceed 11%.

Acknowledgement

This research was funded by Ministry of Science and Higher Education of the Russian Federation, grant number RFMEFI57518X0180.

Fig. 1 .

 1 Fig. 1. Approximation of different gaseous mixtures: (а) 12 CH4 -50,8%, N2res.;(b) 13 CH4 -92,8%, N2res.; (в) 12 CH4 -0,01028%, 13 CH4 -0,01057%, N2res.;(г) 12 CH4 -4,84%, 13 CH4 -4,98%, N2res.

Table 1 .

 1 Experimental results

	Component	Measured value, %	Real value, % Relative error, %
	12 CH4	4.88	4.84	0.8
	13 CH4	4.99	4.98	0.2


A Alles

Z Pan

J M Serres

P Loiko

K Tang

S Yingming

Y Wang

Y Zhao

E Dunina

A Kornienko

P Camy

W Chen

L Wang

U Griebner

V Petrov

R M Solé

M Aguiló

F Díaz

X Mateos

Growth, Spectroscopy and Laser Operation of Tm 3+ , Li + -Codoped Ca 3 Ta 1.5 Ga 3.5 O 12 -Type Disordered Garnet Crystal

Calcium niobium gallium garnet (CNGG) crystals doped with thulium (Tm 3+ ) ions possess disordered structure leading to inhomogeneously broadened emission bands which makes them attractive for generation of ultrashort pulses at ~2 μm [START_REF] Wang | 78 fs SWCNT-SA mode-locked Tm:CLNGG disordered garnet crystal laser at 2017 nm[END_REF]. The actual composition of CNGG deviates from the stoichiometry and cationic vacancies are present to ensure charge compensation. They can be eliminated by codoping with univalent alkali cations (Li + , Na + ). The related calcium tantalum gallium garnet (CTGG) shows better thermal properties than CNGG [START_REF] Ma | Spectroscopic, thermal, and laser properties of disordered garnet Nd:CLTGG crystal[END_REF]. Here, we report on the growth, spectroscopy and first laser action in a Tm 3+ ,Li + -codoped CTGG (Tm:CLTGG) crystal.

The Tm:CLTGG single crystal was grown by the Czochralski method using an [111]-oriented seed and argon atmosphere in an iridium crucible. The crystal was transparent with slight green coloration, inset in Fig. 1(a). The Tm 3+ concentration was determined to be 3.17 at.%. Tm:CLTGG belongs to the cubic class (sp. gr. O 10 h , a = 12.5158(0) Å). Its structure was refined by the Rietveld method, Fig. 1(a). The Raman spectroscopy revealed a maximum phonon energy of 842 cm -1 . The absorption cross-section, σ abs , for the 3 H 6 → 3 H 4 Tm 3+ transition is 3.1×10 -21 cm 2 at 795.0 nm, Fig. 1(b). The maximum SE cross-section for the 3 F 4 → 3 H 6 Tm 3+ transition σ SE is 3.7×10 -21 cm 2 at 1865 nm with smooth and broad spectrum extending well above 2 μm, Fig. 1(c). The luminescence lifetime of the 3 F 4 state τ lum is 5.26 ms, Fig. 1(d). The transition probabilities for the Tm 3+ ion were calculated using the modified Judd-Ofelt (mJ-O) theory yielding a radiative lifetime τ rad ( 3 F 4 ) = 5.33 ms. CW laser operation was achieved with a compact plane-parallel cavity and a fiber-coupled 793 nm AlGaAs laser diode. The uncoated crystal was 8.19 mm-thick. The Tm:CLTGG laser (unpolarized) generated a maximum output power of 1.08 W at 1995 and 2003 nm with a slope efficiency of 23.8% and a laser threshold of 0.91 W (for T OC = 5%), Fig. 2. Tm:CLTGG is promising for broadly tunable and mode-locked lasers emitting above 2 μm.

Fig. 1

 1 Fig. 1 Tm:CLTGG crystal: (a) X-ray powder diffraction (XRD) pattern showing the Rietveld refinement, insetphotograph of the as-grown crystal; (b) σ abs spectrum for the 3 H 6 → 3 H 4 transition; (c) σ abs and σ SE spectra for the 3 F 4 ↔ 3 H 6 transition (RM: reciprocity method, F-L: Fuchtbauer-Ladenburg equation); (d) luminescence decay curve, λ exc = 808 nm, λ lum = 1866 nm.

Fig. 2

 2 Fig. 2 Diode-pumped Tm:CLTGG laser: (a) input-output dependences, T OC : output coupler transmission, η -slope efficiency; (b) typical spectra of laser emission; (c) typical spatial profile of the laser mode in the far-field.


Anna Volokitina

Pavel Loiko

Anatoly Pavlyuk

Sami Slimi

Rosa Maria Sole

Magdalena Aguilo

Francesc Diaz

Xavier Mateos

Growth and Polarized Spectroscopy of Red-Emitting Monoclinic Eu:CsGd(MoO 4 ) 2 Crystal with a Layered Structure

Growth and Polarized Spectroscopy of Red-Emitting Monoclinic

Eu:CsGd(MoO 4 ) 2 Crystal with a Layered Structure Anna Volokitina Trivalent europium ions (Eu 3+ ) are known as luminescent centers leading to red emission of high color purity; the property utilized in red phosphors. Eu 3+ ions are also recognized as structural probes (their luminescence is sensitive to the local site symmetry and its distortion). Recently, CW laser action was also demonstrated in certain Eu 3+ -doped crystals such as double tungstates KRE(WO 4 ) 2 (RE = Gd or Y) [START_REF] Dashkevich | Red Eu,Yb:KY(WO 4 ) 2 laser at ~702 nm[END_REF]. The laser operation was relying on the 5 D 0 → 7 F 4 transition corresponding to the deep-red spectral range (~700 nm). We report on the growth, structural study and polarized spectroscopy of another promising crystal, CsGd(MoO 4 ) 2 , suitable for Eu 3+ doping in high concentrations.

Single-crystals of cesium gadolinium double molybdate, 17 at.% Eu:CsGd(MoO 4 ) 2 (Eu:CsGdMo) were grown from the flux under low thermal gradients using cesium trimolybdate (Cs 2 Mo 3 O 10 ) as a solvent by A. Pavlyuk [START_REF] Loiko | Growth, spectroscopy and laser operation of monoclinic Nd:CsGd(MoO 4 ) 2 crystal with a layered structure[END_REF]. The as-grown crystals were transparent and had rose coloration, Fig. 1 Under excitation by an UV lamp, Eu:CsGdMo provides intense red emission, see Fig. 2(a). Its color coordinates (CIE 1931) are determined. Eu 3+ ions in CsGdMo exhibit extremely strong polarization-anisotropy of emission properties, Fig. 2(b), which is due to the low symmetry of the rare-earth site (C 2 ) and the layered crystal structure. The observed emissions originate from the 5 D 0 → 7 F J (J = 1...6) Eu 3+ transitions. No spectroscopic evidence of Eu 2+ species is found. The most intense emission at ~614 nm is due to the ED allowed transition to the 7 F 2 multiplet. The stimulated-emission (SE) cross-sections, σ SE , for the potential laser transition, 5 D 0 → 7 F 4 , are determined using the experimental branching ratio β JJ' of 15.9%, see Fig. 2(с). The maximum σ SE is 1.81×10 -20 cm 2 at 703.3 nm for light polarization E || b. The luminescence decay from the metastable 5 D 0 state is single exponential with a luminescence lifetime τ lum = 693 µs. Highly-doped Eu:CsGdMo crystals are promising for solid-state lighting and laser applications. The reported study was funded by RFBR, project number 19-32-90199.

  (a). Their structure (monoclinic, sp. gr. P2/c) was determined by powder XRD (lattice parameters: a = 9.5180(8) Å, b = 5.0775(3) Å, c = 8.0791(4) Å, β = 90.8998(0)°, obtained by the Le Bail method), Fig. 1(b). Eu:CsGdMo possesses a layered structure: the stacking of alternating layers -[Gd|EuO 8 ] -[MoO 4 ] -Cs -...occurs along the a-axis, Fig. 1(c). This determines the perfect natural cleavage of this material along the (100) plane, Fig. 1(d).

Fig. 1

 1 Fig. 1 17 at.% Eu:CsGdMo crystal: (a) Photograph of the as-grown crystal, the growth direction is along the [100] axis; (b) X-ray powder diffraction (XRD) pattern showing the Le Bail refinement, (hkl) -Miller's indices; (c) fragment of structure: a supercell (2a × 2b × 2c) in projection to the a-c plane; (d) ESEM image of a cleaved crystal plate (side view).

Fig. 2

 2 Fig. 2 Emission properties of Eu 3+ ions in monoclinic CsGdMo: (a) photograph of red luminescence under excitation by an UV lamp; (b) polarized luminescence spectra, λ exc = 488 nm; (c) SE cross-sections, σ SE , for the 5 D 0 → 7 F 4 transition; (d) luminescence decay curve, λ exc = 532 nm, λ lum = 616 nm.

1,2,* , Pavel Loiko 3 , Anatoly Pavlyuk 4 , Sami Slimi 1 , Rosa Maria Solé 1 , Magdalena Aguiló 1 , Francesc Díaz 1 , and Xavier Mateos 1

	1. Universitat Rovira i Virgili (URV), Marcel•lí Domingo 1, 43007 Tarragona, Spain
	2. ITMO University, 49 Kronverkskiy Pr., 197101 St. Petersburg, Russia. *E-mail: anna.itmo@gmail.com
	3. Centre de Recherche sur les Ions, les Matériaux et la Photonique (CIMAP), UMR 6252 CEA-CNRS-ENSICAEN, Université de Caen
	Normandie, 6 Boulevard du Maréchal Juin, 14050 Caen Cedex 4, France
	4. A.V. Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Sciences, 3 Lavrentyev Ave., 630090 Novosibirsk, Russia


Esrom Kifle

Florent Starecki

Pavel Loiko

Solenn Cozic

Franck Joulain

Thibaud Berthelot

Thierry Georges

Dragan Stojcevski

Damien Deubel

Patrice Camy
email: patrice.camy@ensicaen.fr

Watt-level visible laser in double-clad Pr 3+ -doped fluoride fiber pumped by a GaN diode

We report on a red Praseodymium fiber laser delivering 1.07 W at 634.5 nm with a slope efficiency of 20.7% (vs. the incident pump), a laser threshold of 0.55 W and a single-mode output (M 2 x,y <1.1) in the quasi-continuouswave regime. It is based on a 0.6 mol% Pr 3+ -doped ZBLAN double-clad fiber with a 5.5 μm core, a double D-shaped (diameters: 115/125 μm) inner cladding and a length of 5.0 m. The fiber is pumped by a multimode 443-nm GaN diode. The laser design is optimized using a numerical model. The proposed concept is suitable for the development of diode-pumped high brightness watt-level visible Praseodymium fiber lasers.

Trivalent praseodymium (Pr 3+ ) is the most widespread rare-earth ion for direct generation of visible laser light [1]. It exhibits high absorption cross-sections in the blue, relatively long lifetime of the upper laser level and abundant emission lines falling into the blue, green, orange and red spectral ranges. Pr 3+ ions also enable wavelength tunable laser operation [2,3]. Visible Pr lasers are of practical use for solid-state lighting, data storage, fiber optics communications, biomedical applications and quantum optics.

Among the host matrices for Pr 3+ doping, fluoride materials were found to be particularly suitable as they provide low phonon energies leading to long luminescence lifetimes of the emitting state (thermally coupled 3 PJ/ 1 I6 levels) and weak crystal fields leading to relatively high position of energy levels of the 4f n-1 5d 1 configuration responsible for strong excited-state absorption in Pr 3+ -doped materials [1]. So far, various fluoride single-crystals (e.g., LiYF4, KY3F10, BaY2F8, etc.) doped with Pr 3+ ions were employed in visible bulk lasers [4][5][6][7]. Metz et al. designed a Pr:LiYF4 laser delivering 2.9 W at 523 nm with a record-high slope efficiency of 72% using an optically pumped semiconductor laser (2ω-OPSL) as a pump source [5].

Pr 3+ -doped fluoride glasses (e.g., those in the ZrF4 system) in the form of fibers were also employed in visible fiber lasers [8][9][10][11]. The development of fiber sources is particularly attractive because of their inherent advantages such as excellent beam quality, stability, small footprint and flexible configuration. Note that one of the key factors for the design of Pr lasers is the cross-relaxation (CR) among the adjacent active ions depopulating the upper laser manifold and shortening its lifetime [12]. The CR is enhanced with the doping level which limits the Pr 3+ concentration in gain media. This is however not critical for fiber lasers where high absorption efficiencies can be reached.

Fujimoto et al. used a 0.3 mol% Pr 3+ -doped fluoro-aluminate single-clad multimode fiber (4 cm-long) pumped by two GaN laser diodes to demonstrate watt-level laser emission in the green and red [10]. In the latter case, a quasi-continuous-wave (quasi-CW) laser generated a peak power of 1.24 W at 638 nm with a slope efficiency of 46.6% (vs. the absorbed pump power). Kajikawa et al. proposed to use the double-clad fiber design to allow for further power scaling [13]. Using the same fluoro-aluminate glass, the authors achieved 0.25 W at 639 nm with a slope efficiency of 43.4% corresponding to a single-mode output. Further power scaling reaching 2.0 W at 638 nm was demonstrated in [14]. Note that in [13,14], the potential of the double-clad design was not fully exploited because of the small inner cladding diameter (14 and 24 μm in [13,14], respectively) and most likely predominantly core pumping (evidenced by the short fiber length of 10 cm). Broadband wavelength tuning of Pr fluoride fiber lasers based on ZBLAN glasses was realized in [2]. Double-clad active fibers containing a single-mode core where the laser emission propagates and a multimode inner cladding for the pump are well suited for power scaling. The inner cladding being much larger than the core and having large numerical aperture (N.A.) enables easy coupling of pump light from laser diodes. Among the pump sources for Pr lasers, particularly GaN diodes offer the optimum solution in terms of compactness, costeffectiveness and power scalability [6,10,15] (as compared to 2ω-OPSLs). In the present work, we aimed to demonstrate a watt-level red Pr fiber laser fully exploiting the double-clad geometry making it very suitable for low-brightness GaN diode pumping.

First, we briefly characterize the key spectroscopic properties of the Pr 3+ -doped ZBLAN glass (measured in the present work). The absorption cross-section, σabs, spectrum in the blue spectral range is shown in Fig. 1(a). For the 3 H4 → 3 P2 transition, the maximum σabs is 1.08×10 -20 cm 2 at 442 nm and the absorption bandwidth (FWHM) is 10 nm. The emission spectrum of Pr 3+ ions is shown in Fig. 1(b). The stimulated-emission (SE) cross-section, σSE, for the 3 P0 → 3 F2 transition, Fig. 1(c), falling into the red spectral range was calculated using the Füchtbauer-Ladenburg (F-L) equation to be 3.82×10 -20 cm 2 at 635 nm. Here, we considered a radiative lifetime τrad( 3 P0) of 37 μs and a luminescence branching ratio B(JJ') of 12.1% [16]. The typical luminescence decay curve for red emission (the decay from the 3 P0 state) is shown in Fig. 1(d) for the doping level of 0.6 mol% Pr. It is well fitted using the Inokuti-Hirayama model considering a dipole-dipole transfer mechanism [17]. The mean luminescence lifetime <τlum> shows a relatively weak dependence on the Pr doping level, Fig. 1(e), and it equals 39.7 μs for 0.6 mol% Pr doping. As a gain material, we used a Pr 3+ -doped ZBLAN fluoride fiber (Le Verre Fluoré) in the double-clad geometry. The doping level was 0.6 mol% PrF3 (NPr = 1.16×10 20 cm -3 ). The core diameter was 5.5 μm and the inner cladding made of a lower-index fluoride glass had a truncated circular (double D-shaped) profile with diameters of 115/125 μm used to enhance the pump absorption. The outer cladding made of a resin polymer had a diameter of 180 μm. The fiber geometry is shown in Fig. 2(a). A confocal microscope image (in the reflection mode) of the polished fiber end-facet is shown in Fig. 2(b) (the core is not visualized due to the small refractive index contrast). The length of the fiber was 5.0 m. The refractive indices of the core and inner cladding were 1.5096 and 1.5052 at the laser wavelength (635 nm), respectively. The numerical aperture (N.A.) of the fiber for the pump was 0.40 and for the laser emission, it was 0.12. Both fiber ends were mounted into SMA connectors. The end-facet of the fiber was polished and highly-reflective (HR) coated at 635 nm serving as a HR end mirror (M2). The coating was a dielectric multilayer structure deposited using a standard sputtering machine for laser mirror elaboration. The input facet was also polished for normal incidence and left uncoated. It was used as an input mirror (M1) both for pump coupling and as the output coupler having a reflectivity R of 4.1% at the laser wavelength (determined only by the Fresnel loss). Both fiber ends were passively cooled. The fiber core supported up to 2 modes at the laser wavelength (LP01 and LP11, confinement factors: 92.0% and 72.6%, respectively, V-number: 3.12) and the inner cladding was highly multimode. The scheme of the laser is shown in Fig. 3. As a pump source, we used an open-space GaN laser diode (NUBM44, Nichia) emitting up to ~6 W at 443 nm (emission bandwidth: 1.5 nm, temperature drift of the emission wavelength: less than 2.5 nm), Fig. 1(a). The pumping was in quasi-CW or true CW regimes. In the former case, it was electronically modulated with duty cycles 1:10, 1:5, 1:3 or 1:2 (the duration of the pump pulse was ~0.5 ms). The pump was collimated using an antireflection (AR) coated cylindrical short focal length (f = 1.19 mm) lens (LIMO) and then focused into the fiber using an AR-coated achromatic lens (f = 50 mm, Thorlabs). The mode diameter (at the 1/e 2 level) at the fiber end-facet was 80±5 μm (pumping into the inner cladding). The propagation loss for the pump radiation at 443 nm αloss, clad = 1.54±0.2 dB/m was determined from pump-transmission experiments using different fiber lengths. This high value of αloss, clad is related to parasitic absorption of blue light by the UV-curable resin polymer. The pump coupling efficiency was simply estimated from the Fresnel loss at the uncoated input facet of the fiber, ηcoupl = 95.8%. A dichroic mirror coated for high transmission (HT, T >98%) at 430-480 nm and for HR (R >99.7%) at 620-650 nm was inserted before the focusing lens at an angle of 45°. The laser spectrum was measured with an optical spectrum analyzer (Ando AQ-6315E).

The pump absorption in the fiber was determined from pumptransmission (cut-back) measurements for several lengths of the fiber L, Fig. 4(a). The pump was almost completely absorbed for L = 5.0 m (ηabs = 93.4±1%). This value includes the propagation losses in the cladding at the pump wavelength. The points in Fig. 4(a) were fitted as Ptrans/Pcoupl = exp(-(αabs×Гp+αloss, clad)L), where αabs = 0.54×10 3 dB/m is the absorption coefficient at the pump wavelength, cf. Fig. 1(a), and Гp = 1.94×10 -3 is the power filling factor approximated as the ratio of the core / clad areas.

The spectral dependence of total losses for cladding-guided light (absorption by Pr 3+ ions in the core and propagation losses in the inner cladding) was evaluated in a pump-transmission experiment using a halogen lamp, as shown by the grey curve in Fig. 4(b). To precise the absolute values, we performed similar measurements with lasers at 535 and 664 nm yielding αloss, clad of 1.10±0.2 dB/m and 0.62±0.1 dB/m, respectively. Thus, we were able to estimate the spectral dependence of αloss, clad, see the green curve in Fig. 4(b). It rapidly increases for shorter wavelengths which is due to both light scattering in the fiber and absorption by the resin polymer as explained above. We also measured the total loss for light guided in the core using a He-Ne laser (633 nm) yielding the value of 0.43±0.1 dB/m. By subtracting the Pr 3+ reabsorption at this wavelength (a phonon sideband of the 3 H4 → 1 D2 band), we obtain αloss, core = 0.19±0.1 dB/m (due to light scattering). The input-output dependences of the Pr fiber laser operating in the quasi-CW regime with various duty cycles ranging from 1:10 to 1:2 are shown in Fig. 5(a). These data correspond to the bending diameter of the fiber of 20 cm showing almost no bending loss (<0.5% reduction in the output power). For the 1:10 duty cycle, the laser generated a maximum peak output power of 1.07 W at 634.5 nm with a slope efficiency of 21.6% (with respect to the coupled pump power Pcoupl). The laser threshold was at 0.55 W and the optical-to-optical efficiency was 18.1% for the maximum available peak pump power of 5.9 W. The output dependence was linear well above the laser threshold. The laser linewidth was ~0.6 nm, Fig. 5(b). The laser operated for >1 hour without any degradation of its output power. For smaller pump duty cycles, the maximum peak output power gradually decreased, and it was 0.85 W for the 1:2 duty cycle. A thermal roll-over was observed for Pcoupl > 3.5 W and assigned to parasitic heating of the outer cladding due to its absorption at the pump wavelength. To avoid the risk of thermal damage of the fiber, for the true CW experiment, we limited Pcoupl to 3.7 W, so that the output power amounted to 0.46 W. For further power scaling in the CW regime, additional attention should be paid for thermal management of the input fiber end. The typical far-field mode profile for the Pr fiber laser (bending diameter: 20 cm) is shown in Fig. 6(a). The mode is nearly circular and the 1D intensity profiles are well fitted using the Gaussian distribution, Fig. 6(a). The measured beam quality factors M 2 x,y are 1.03 and 1.04, respectively, Fig. 6(b). We also studied more compact laser design employing a bending diameter of 8 cm. This design did not lead to any noticeable alteration of the laser mode but a small power reduction was observed corresponding to the bending loss αbend of 0.02±0.01 dB/m. The performance of the red Pr fiber laser was simulated using the model of a strongly pumped quasi-four-level laser developed by Kelson and Hardy [19]. The pump is coupled into the inner cladding and propagates along the negative direction (backward pumping). The fraction of pump coupled in the core is determined by the Гp factor. The laser light propagating both in positive and negative directions is mainly concentrated in the core; the LP01 mode is considered. The rate equation model includes the groundstate absorption, stimulated emission and spontaneous emission into the laser mode. We consider the propagation losses for both the laser and the pump. The steady-state rate equations and the propagation equation for the pump and laser are solved numerically using fourth-order Runge-Kutta algorithm [20,21]. The spectroscopic data reported in Fig. 1 were used.

The results of the simulations are shown in Fig. 7. For a fixed coupled pump power, the output power of the laser monotonously increases with reducing the reflectivity of the input mirror R, see Fig. 7(a). This justifies the use of an uncoated input fiber end in the present work. For a laser operating at 635 nm, and for the given parameters, no significant improvement of the output power is expected when reducing R below the value set by Fresnel losses (e.g., when applying an AR coating), Fig. 7(b). However, in this case, the laser threshold will notably increase. For the designed laser, there exist an optimum fiber length (~5 m) which is determined by a counterplay between increasing pump absorption and raising cavity roundtrip losses in longer fibers, as shown in Fig. 7(c). For lower propagation losses, it is logically expected that the optimum L value will increase, Fig. 7(d).

The model well explains the experimental output dependence for the Pr fiber laser, as shown by the blue curve in Fig. 5(a), and it predicts the potential of such double-clad fiber laser pumped by a single GaN laser diode. To conclude, we report on a concept of watt-level, high brightness visible lasers based on Pr 3+ -doped double-clad fluoride fibers with large inner cladding diameter and nearly single-mode core allowing for their efficient pumping with high-power and lowbrightness GaN blue laser diodes emitting at 443 nm. The proof-ofthe-concept is verified using a 0.6 mol% Pr 3+ -doped ZBLAN fiber with the core / inner cladding diameters of 5.5 μm and 115/125 μm, respectively, which delivers >1 W of red emission (in the quasi-CW regime) at 635 nm with high slope efficiency of 19.4% corresponding to a single-mode output. The performance of this laser is simulated revealing the necessity to consider an optimum fiber length / doping level. We believe that further power scaling and improvement of the laser efficiency are feasible via (i) reducing the propagation losses for the pump in the inner cladding, (ii) better thermal management of the fiber preventing its thermal damage, and (iii) coupling of several pump laser diodes into this high N.A. double-clad fiber gain medium.

Funding. European Union within the European Regional Development Fund (ERDF) (FEDER FSE-2014-2020). This Project was also supported in a joint framework between the French State, under the Future Investments Program, and the Bretagne and Normandy regions (FIZIC project).

Disclosures. The authors declare no conflicts of interest.

Fig. 1 .

 1 Fig. 1. Spectroscopy of Pr 3+ ions in ZBLAN glass: (a) absorption crosssection, σabs, spectrum in the blue spectral range, blue curve -the spectrum of the GaN diode; (b) luminescence spectrum, λexc = 443 nm, the arrow indicates the laser line; (c) the scheme of energy levels of Pr 3+ ions in fluoride glass; (d) luminescence decay curve for 0.6 mol.% Pr doping, λexc = 465 nm, λlum = 635 nm, the curve represents the fit using the Inokuti-Hirayama model for s = 6 [17]; (e) luminescence lifetime of the 3 P0 state vs. the Pr doping level, the literature data are shown for comparison [16,18].

Fig. 2 .

 2 Fig. 2. (a) Scheme of the transverse profile of the double-clad fluoride fiber; (b) confocal laser microscope image of the polished end-facet of the fiber (the jacket is removed), reflection mode, λ = 405 nm.

Fig. 3 .

 3 Fig. 3. Scheme of the red Pr fiber laser: LD -GaN laser diode, CLcollimating cylindrical lens, FL -achromatic focusing lens, M1 and M2cavity mirrors, HR -highly-reflective coating, DM -dichroic mirror.

Fig. 4 .

 4 Fig. 4. Characterization of the double-clad Pr 3+ -doped fluoride fiber: transmission of the pump from the GaN diode vs. the fiber length: symbols -experimental data, curve -their fit; (b) propagation losses in the inner cladding and the core (symbols), the absorption losses due to Pr 3+ ions are subtracted. The attenuation spectra (the grey curves) for light propagating in the core and cladding are shown for comparison.

Fig. 5 .

 5 Fig. 5. Diode-pumped red Pr fiber laser: (a) input-output dependence, η -slope efficiency, black curve -simulation; (b) typical laser emission spectrum measured at Pcoupl = 2.0 W.

Fig. 6 .

 6 Fig. 6. (a) Typical far-field mode profile of the output beam from the Pr fiber laser; (b) 1D intensity profiles, symbols -experimental data, curves -their Gaussian fits; (c) evaluation of the M 2 x,y parameters. The fiber bending diameter is 20 cm.

Fig. 7 .

 7 Fig. 7. Simulation of the performance of the diode-pumped red Pr fiber laser: (a) output power vs. the input mirror reflectivity R, Pcoupl = 6.0 W, L = 5.0 m; (b) input-output dependences for different R values, L = 5.0 m; (c,d) output power vs. the fiber length L for different Pr doping levels: (c) αloss = 1.54 dB/m (inner cladding), 0.43 dB/m (core), (d) lossless fiber, αloss ≡ 0, Pcoupl = 6.0 W, R = 4%.


Esrom Kifle

Pavel Loiko

Carolina Romero

Javier Rodríguez Vázquez De Aldana

Viktor Zakharov

Yulia Gurova

Andrey Veniaminov

Valentin Petrov

Uwe Griebner

Romain Thouroude

Javier Rodríguez

Aldana

Mathieu Laroche

Magdalena Aguiló

Francesc Díaz

Patrice Camy

Xavier Mateos
email: xavier.mateos@urv.cat

Tm 3+ and Ho 3+ colasing in in-band pumped waveguides fabricated by femtosecond laser writing

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

The Holmium ion (Ho 3+ ) is attractive for solid-state laser emission at wavelengths slightly above 2 μm ( 5 I7 → 5 I8 transition), falling in the eye-safe spectral range. Holmium lasers are of practical interest for remote sensing, wind mapping, medicine and further frequency conversion into the mid-IR. A common way to excite the Ho 3+ ions is codoping of the host matrix with thulium (Tm 3+ ) ions acting as sensitizers [1,2]. Tm 3+ ions can be pumped at ~0.8 μm (to the 3 H4 state) transfering a part of the energy of the electronic excitation to the Ho 3+ ones via non-radiative energy-transfer (ET), Tm 3+ ( 3 F4) → Ho 3+ ( 5 I7) [3] and such codoped bulk laser can be very efficient [1]. A diode-pumped Tm,Ho:KLu(WO4)2 (Tm,Ho:KLuW) laser generated an output power of 451 mW at 2081 nm with a slope efficiency of 31% [2]. Tm 3+ ,Ho 3+ codoped materials are also attractive for mode-locked lasers at >2 μm [4,5]. Their emission wavelength is redshifted (with respect to singly Tm 3+ doping) avoiding the structured water vapor absorption and the combined gain bandwidth is broader favoring sub-100 fs pulses [4,5]. Optically "passive" surface waveguides (WGs) supporting light guiding at ~2 μm could find applications in bio-and environmental sensing based on evanescent-field interaction with material on the surface. The probing of "passive" WGs can be provided by another integrated ("active") device. "Active" surface WGs are also suitable for pulsed sources based on interaction with a surface-deposited saturable absorber [6]. Efficient surface Tm 3+ ,Ho 3+ WG lasers with a broadband emission around 2 μm are excellent candidates for these aims. Unfortunately, so far, only few studies have been devoted to this topic [7][8][9]. Planar Tm 3+ ,Ho 3+ codoped WGs based on KY(WO4)2 and LiYF4 layers grown by Liquid Phase Epitaxy (LPE) are known [7,8]. A Tm,Ho:LiYF4 planar WG laser generated 81 mW at 2051 nm with a slope efficiency of 24% [8]. In [9], buried channel WGs were fabricated in bulk Tm,Ho:ZBLAN glass by femtosecond Direct Laser Writing (fs-DLW) yielding 76 mW at 2052 nm with a similar slope. Note that in all of these publications, the conventional pump scheme (excitation to the 3 H4 level of Tm 3+ ) was used.

The difficulty in developing Tm 3+ ,Ho 3+ lasers in general, and such WGs in particular, is the strong thermal effect [2] due to energy losses occurring in the following processes: relaxation of Tm 3+ ions to the 3 F4 state, possible Ho 3+ → Tm 3+ back ET and energy-transfer upconversion (ETU) for both ions. In part, can be alleviated by direct (in-band) pumping of Tm 3+ to the 3 F4 state, Fig. 1(a). Bulk inband pumped Tm 3+ ,Ho 3+ lasers are known [10]. Recently, we reported the first singly Tm-doped in-band pumped channel WG laser delivering up to 2.05 W at 1881 nm with a record-high slope efficiency of 78.3% [11]. In the present work, we demonstrate the first in-band pumped Tm 3+ ,Ho 3+ codoped WG laser.

As a gain material, we employed monoclinic (sp. gr. C2/c) 5 at.% Tm, 0.5 at.% Ho:KLuW grown by the top seeded solution growth method using K2W2O7 as a solvent. The actual Tm 3+ and Ho 3+ doping levels were determined by electron probe microanalysis to be NTm = 2.30×10 20 cm -3 and NHo = 0.53×10 20 cm -3 . A rectangular sample was cut in the frame of the optical indicatrix of KLuW which is an optically biaxial crystal. The uncoated sample was 3.0-mm thick along the Ng-axis. Its 5.8(Nm)×1.5(Np) mm 2 input and output faces (pump and laser propagation) and the top surface were polished. Depressed-index guides with a half-ring cladding (type III) were fabricated by fs-DLW. To produce individual damage tracks, the output of a Ti:Sapphire regenerative amplifier (Spitfire, Spectra Physics) delivering 120 fs (795 nm central wavelength) at 1 kHz repetition rate was focused into the bulk volume of the material through the top surface (Nm×Ng). Only a small fraction of the pulse energy was employed (65 nJ). The writing optics comprised a 40× microscope objective (numerical aperture: N.A. = 0.65). The crystal was scanned at a speed of 500 µm/s along its Ng-axis resulting in the formation of the damage tracks. The polarization of the fs laser was perpendicular to the writing direction (E || Nm). No repolishing after the inscription of the damage tracks was applied.

The geometry of the guides was inspected using a confocal laser microscope (LSM 710, Carl Zeiss). The resolution was 0.24 μm. The end-facet view with polarized light (P || Np) in bright field, Fig. 2 To study the effect of the fs laser writing irradiation on the emission properties of rare-earth ions (Tm 3+ and Ho 3+ ), the intensity of the green Ho 3+ emission was monitored across the end-facet area, Fig. 3 Using a confocal Raman microscope, (InVia, Renishaw) equipped with a 50× objective, we measured the micro-luminescence spectra from the core region for E || Np and E || Nm, Fig. 3(c). This confirmed well-preserved strong anisotropy of the spectroscopic properties of the material in the core region. By focusing of the 488 nm radiation on three areas (focus diameter: 2 μm) in the surrounding bulk material, in the WG core and on the WG cladding, we compared the emission spectra of Ho 3+ ( 5 S2+ 5 F4 → 5 I7 transition) and Tm 3+ ( 3 H4 → 3 H6 transition) ions. As shown in Fig. 3(d), there is no reduction in intensity nor alteration of the spectral shape for the core region. The emission intensity is depressed in the cladding, in agreement with Fig. 3(a), while only slight changes in the spectral shape (e.g., peak shifts and broadening) are detected in our study. It suggests a slight and strongly spatially localized alteration of the crystal field around the rare-earth ions.

The laser experiments were performed in a linear plano-plano laser cavity containing a pump mirror (PM) coated for high transmission at the pump wavelength (HT, T = 93% at 1.68 μm) and for high reflection (HR) at 1.87-2.30 μm, and a set of output couplers (OCs) with an actual transmission at the laser wavelength TOC in the range of 1.5%...20%. OCs with higher transmissions (TOC = 30%-50%) resulted only in Tm 3+ lasing with deteriorated performance and, thus, they were not studied. Both cavity mirrors were gently pressed the crystal end-facets (without any indexmatching liquid) leading to a geometrical cavity length of 3.0 mm. The sample was fixed on a Cu-holder using a silver paint.

The pump source was a home-made Raman fiber laser (RFL) [11]. It comprised of a 1560 nm Erbium Master-Oscillator Power Amplifier (MOPA) and a polarization-maintaining GeO2-doped silica fiber with a Raman-active mode at ∼440 cm -1 . The RFL delivered 4.0 W at 1679 nm (emission bandwidth: 1 nm), linearly polarized and a spatially single-mode output (M 2 <1.1). The pump wavelength corresponded to the 3 H6 → 3 F4 Tm 3+ transition (in-band pumping), Fig. 1(b). The pump was focused through the PM using a spherical uncoated CaF2 lens (f = 40 mm, T = 94%). The measured pump spot diameter at the input WG facet 2wP was 30±5 μm. The pump polarization corresponded to E || Nm. More details about the laser set-up can be found elsewhere [11].

The pump coupling efficiency ηcoupl = Pcoupl/Pinc ≈ 89% was simply estimated from Fresnel losses (refractive index nm = 2.002), because the pump spot size is smaller than the WG dimensions and the condition (N.A.)pump < (N.A)WG ≈ 0.05 was satisfied. The absorption under non-lasing conditions, ηabs, NL = Pabs/Pcoupl, was determined from measurements of the transmitted pump. A strong absorption bleaching was detected: ηabs, NL gradually decreased below its smallsignal value, ηabs, 0 = 1 -exp(-σabs P NTmt) = 50.9%, where σabs P = 1.03×10 -20 cm 2 and t = 3.0 mm, Fig. 1(b). The small-signal singlepass pump absorption length was 4.2 mm. The absorption under lasing conditions, ηabs, L, was calculated from the ηabs, NL value at the threshold pump power for each OC accounting for the double-pass of the pump because the OCs were highly-reflective at ~1.68 μm. The ηabs, L value obtained, in the range 42 -49±1% (depending on the OC) ensured almost uniform pumping (inversion) in the WG [8].

The output beam was collimated with an uncoated CaF2 lens (f = 15 mm) and a cut-off filter (T < 0.001% at ~1.68 μm) was used to filter out the residual pump. An optical spectrum analyzer (OSA, AQ6375B, Yokogawa) equipped with a multimode low-OH optical fiber served for recording the emission spectra. The laser beam profile was captured using a FIND-R-SCOPE near-IR camera; the scale was calibrated by illuminating WGs with a known size.

The output dependences of the Tm,Ho WG laser are shown in Fig. 4(a). For all the studied output couplers, colasing of Tm 3+ ( 3 F4 → 3 H6 transition) and Ho 3+ ( 5 I7 → 5 I8 transition) ions was observed. Thus, we first measured the total (Σ = Tm+Ho) power. The maximum power was 448 mW at ~1942 nm (Tm 3+ ) and 2059 nm (Ho 3+ ) with a slope efficiency ηΣ of 40.6% (vs. Pabs) (TOC = 9%). The laser threshold occurred at Pabs = 334 mW and the optical-to-optical efficiency ηopt, Σ was 28.2% (vs. Pinc). The laser threshold increased with TOC, from Pabs = 120 mW (TOC = 1.5%) to 366 mW (TOC = 20%). The laser emission for the studied OCs was linearly polarized (E || Nm, horizontal) and the polarization state was naturally selected by the gain anisotropy for both Tm 3+ and Ho 3+ transitions. Typical spectra are shown in Fig. 4(b). The Ho 3+ laser wavelength (~2059 nm) was almost independent on TOC; the Tm 3+ emission occurred between 1915 -1963 nm, showing a tendency for a blue-shift with increasing the TOC due to the quasi-three-level Tm laser scheme with reabsorption. The assignment of the emissions is according to the gain spectra of Tm 3+ and Ho 3+ ions for E || Nm.

The wavelength difference for Tm 3+ and Ho 3+ allowed separating the corresponding power fractions using a dichroic mirror, e.g., that for Ho 3+ part: Ho/(Ho+Tm), Fig. 4(c). Both the Ho 3+ power fraction, Ho/(Ho+Tm), and the output power, Pout, Ho, showed a complex dependence on the output coupling. The maxima for both these parameters ocurred at the same intermediate TOC = 5% (for which the fraction Ho/(Ho+Tm) = 84±4%). For higher TOC, both parameters gradually decrease to zero (for TOC = 20%, Ho/(Ho+Tm) < 1%). Thus, the maximum Ho 3+ output power is observed for TOC = 5% amounting to 144 mW. Note that for this output coupler, the total (Tm+Ho) characteristics are: 174 mW at 1916 nm (Tm 3+ ) and 2059 nm (Ho 3+ ) with ηΣ = 15.1% and a laser threshold of 206 mW. The minimized Tm power fraction for this OC corresponding to a wavelength of 1916 nm can explain this blue-shift of the Tm 3+ emission.

The example power dependence of the Tm 3+ and Ho 3+ WG laser is illustrated in Fig. 5 for TOC = 3%. Note the slightly nonlinear dependence explained by a power redistribution between Tm 3+ and Ho 3+ lasing. The threshold is much lower for the Ho 3+ laser (161 mW) than for Tm 3+ (320 mW). The Ho 3+ power first increases linearly with a slope efficiency ηHo of 15.9%. As soon as the threshold for Tm 3+ lasing is reached, its power increases nonlinearly at the expense of the Ho 3+ one. The corresponding slope efficiency ηTm is 15.3%, obtained by fitting the output dependence well above the Tm 3+ laser threshold. The laser spectra, Fig. 5(b), broaden with increasing the pump power very pronounced for the Tm emission. The Tm 3+ and Ho 3+ emission channels both produced a spatially multimode output. The beam profiles were almost identical. The typical near-field profile of the Ho-laser captured using the near-IR camera and a dichroic mirror to eliminate Tm emission is shown in Fig. 6(a). More than 75% of the power was contained in the main lobe. It had an asymmetric shape extended along the horizontal direction (|| Nm) and it was localized not symmetrically with respect to the WG cladding, adjacent to its bottom. The lobe had nearly Gaussian intensity profiles in the horizontal and vertical directions,

Here, βTm = N2( 3 F4)/NTm and βHo = N7( 5 I7)/NHo are the inversion levels for Tm 3+ and Ho 3+ ions, respectively, σSE and σabs are the stimulatedemission and absorption cross-sections, respectively, for both the 3 F4 ↔ 3 H6 Tm 3+ and 5 I7 ↔ 5 I8 Ho 3+ transitions, and NΣ = NTm + NHo. The relation between βTm and βHo was determined accounting for the parameters of the bidirectional Tm 3+ ↔Ho 3+ ET [3] in Tm,Ho:KLuW, P28 = 1.69×10 -22 cm 3 μs -1 (direct ET) and P71 = 0.15×10 -22 cm 3 μs -1 (back ET) [12]. For low βTm <0.20, typical for bulk Tm 3+ ,Ho 3+ lasers, the local peaks at 2075 and 2059 nm are assigned to Ho 3+ emission. These peaks are most pronounced in the gain spectra. For higher inversion levels (βTm and, accordingly, βHo), the peaks at 1915, 1946 and 1960 nm (Tm 3+ ) and at 2034 and 2059 nm (Ho 3+ ) dominate and correspond to close gain leading to Tm 3+ ,Ho 3+ colasing.

To conclude, the combination of fs-DLW to fabricate surface channel WGs in anisotropic crystals and in-band pumping by a Raman fiber laser to alleviate thermal issues is a novel approach for the development of integrated light sources emitting at ~2 μm. We report on a record-high Ho 3+ output from a Tm 3+ ,Ho 3+ codoped WG laser. Single Ho 3+ emission will be possible at lower propagation losses (lower β) and lower Ho/Tm ratios.

Fig. 1 .

 1 Fig. 1. In-band pumping of Tm,Ho:KLuW: (a) energy-level scheme of Tm 3+ and Ho 3+ : red (green) arrow -pump (laser) transition, ET -energytransfer, ETU -energy-transfer upconversion; (b) absorption crosssections, σabs, for the 3 H6 → 3 F4 Tm 3+ and 5 I8 → 5 I7 Ho 3+ transitions and light polarized along the Np and Nm optical indicatrix axes.

Fig. 2 .

 2 Fig. 2. Confocal microscopy study of the fs-DLW Tm 3+ ,Ho 3+ codoped surface channel WGs in Tm,Ho:KLuW: (a,b) end-facet view -(a) bright and (b) dark field; (c,d) top surface view in bright field: (c) area adjacent to the input facet, (d) individual damage tracks. Transmission mode, polarized light P: (a,b) P || Np, (c,d) P || Ng. λ = 405 nm.

  (a), reveals a half-ring cladding formed by 31 vertically elongated (direction of the laser propagation) damage tracks. The diameter of the cladding is ~50 μm and the upper / lower tracks are located at 12 / 49 μm beneath the crystal surface. The cross-section size of each track is 1(horizontal, Nm) × 6(vertical, Np) μm 2 and their separation is 2 μm (horizontal). No macroscopic cracks in the core or in the surrounding bulk area are observed. The inspection of the same area in dark field, Fig. 2(b), reveals bright scattering centers coinciding with the written tracks. The top view with polarized light (P || Ng), Fig. 2(c), indicates that the damage tracks are continuous, and they propagate through the entire length of the sample reaching the end-facets at both sides. The cladding resembles a canvas-like structure. Individual tracks are visualized in Fig. 2(d) showing dark borders and a brighter inner part probably due to local expansion and partial amorphization of the material subjected to the fs temporal structure of the pulses.

  (a). It revealed a clear drop in the emission intensity within the cladding and almost unchanged luminescence response of the core. A close look on the individual damage tracks from the top surface, Fig. 3(b), indicated a strong localization of the depressed emission intensity within the dark "sidewalls" of the tracks.

Fig. 3 .

 3 Fig. 3. μ-luminescence study of the fs-DLW Tm 3+ ,Ho 3+ codoped surface channel WGs: (a,b) μ -luminescence mapping: (a) end-facet view, (b) top view, a closer look on the damage tracks. 5 S2+ 5 F4 → 5 I8 Ho 3+ emission, unpolarized light; (c) polarized emission spectra from the core region for light polarizations E || Np and E || Nm; (d) comparison of the emission spectra from Ho 3+ and Tm 3+ ions from the bulk, core and cladding regions of the WG, E || Np. The excitation wavelength λexc is 488 nm.

Fig. 4 .

 4 Fig. 4. CW in-band pumped Tm 3+ ,Ho 3+ codoped surface channel WG laser: (a) input-output dependences, η -slope efficiency; (b) emission spectra, Pabs = 0.9 W; (c) relative power fraction of Ho 3+ emission, Ho/(Tm+Ho), and the Ho 3+ output power vs. output coupling, Pabs = 0.9 W; (d) Effective gain cross-sections, σg, ,eff, for different Tm 3+ inversion ratios βTm = N2( 3 F4)/NTm, arrows indicate the observed laser wavelengths. The laser polarization is E || Nm.

Fig. 5 .

 5 Fig. 5. Power and spectral properties of Tm 3+ and Ho 3+ emissions from the in-band pumped Tm 3+ ,Ho 3+ codoped surface channel WG laser: (a) input-output dependences (total and individual Tm and Ho lasing), ηTm(Ho) -slope efficiency for Tm 3+ -and Ho 3+ -lasing, respectively; (b) laser emission spectra versus Pabs. The laser polarization is E || Nm.

Fig. 6 .

 6 Fig. 6. Near-field intensity profile of the Ho lasing mode of the in-band pumped Tm 3+ , Ho 3+ codoped surface channel WG laser: (a) 2D mode profile, dashed lines indicate the crystal / air interface and the WG cladding; (b) 1D intensity plots along the horizontal (|| Nm) and vertical (|| Np) directions, symbols -experimental data, curves -Gaussian fits. The laser polarization, E || Nm, is horizontal. TOC = 5%, Pabs = 1.0 W.

Fig. 6 (

 6 b), corresponding to the mode diameters (at the 1/e 2 level) 2wL of 24.5 μm (|| Nm) and 21.1 μm (|| Np). The mode was well localized within the WG cladding.To explain the observed colasing, we calculated the effective gain cross-sections, σg, eff, for the Tm 3+ ,Ho 3+ -codoped KLuW crystal:

Funding. Spanish Government, MINECO (MAT2016-75716-C2-1-R (AEI/FEDER,UE), FIS2017-87970-R); Generalitat de Catalunya (2017SGR755); Junta de Castilla y León (SA287P18); Ministry of Science and Higher Education of Russian Federation (Goszadanie 2019-1080); Agence Nationale de la Recherche (LABEX-EMC3); European Union (European Regional Development Fund, ERDF); Regional Council of Normandie (NovaMat).

Disclosures. The authors declare no conflicts of interest.


Vladimir Vitkin

Anton Polishchuk

Ian Chubchenko

Evgeniy Popov

Konstantin Grigorenko

Artem Kharitonov

Arsen Davtian

Anton Kovalev

Valeria Kurikova

Patrice Camy

Pavel Loiko

Magdalena Aguiló

Francesc Díaz

Xavier Mateos
email: xavier.mateos@urv.cat

Raman Laser Spectrometer: Application to 12C/13C Isotope Identification in CH4 and CO2 Greenhouse Gases

Keywords: Raman laser spectrometer, carbon isotopes, greenhouse gases, carbon dioxide, methane, human exhalation

teaching and research institutions in France or abroad, or from public or private research centers.

Introduction

Nowadays, the concentration of greenhouse gases (such as CO2, CH4, N2O, etc.) in the atmosphere is increasing due to the anthropogenic emissions. Thus, there is a need to determine the anthropogenic contribution against the natural background and to recognize emissions from various industries [START_REF] Rieker | Frequency-comb-based remote sensing of greenhouse gases over kilometer air paths[END_REF][START_REF] Ghosh | Stable isotope ratio mass spectrometry in global climate change research[END_REF][START_REF] Schneider | Detecting climatic change signals: are there any" fingerprints"?[END_REF]. One possibility for doing this is via the carbon isotope ( 12 C and 13 C) ratio measurements of carbon dioxide (CO2) and methane (CH4) in the atmospheric air [START_REF] Stevens | The carbon isotopic composition of atmospheric methane[END_REF][START_REF] Monteil | Interpreting methane variations in the past two decades using measurements of CH4 mixing ratio and isotopic composition[END_REF].

So far, the isotope ratio Raman spectroscopy of CO2 was addressed in several research works. In [START_REF] Li | Carbon isotopic compositions in carbon dioxide measured by micro-laser Raman spectroscopy[END_REF], the authors have found a way for determining the isotope-delta values (the relative difference of isotope ratios with respect to the reference material) δ 13 C using micro-Raman spectroscopy. A technique for monitoring 13 CO2 by cavity-enhanced Raman spectroscopy was developed in [START_REF] Keiner | Enhanced Raman multigas sensing-a novel tool for control and analysis of 13 CO2 labeling experiments in environmental research[END_REF].

Quantitative analysis of carbon isotopic composition in CO2 with the estimation of the measurement uncertainty was performed in [START_REF] Li | Quantitative measurement of carbon isotopic composition in CO2 gas reservoir by micro-laser Raman spectroscopy[END_REF]. A detailed experimental and theoretical study on isotopic surfaceenhanced Raman Spectroscopy was done in [START_REF] Zhang | Experimental and theoretical study on isotopic surface-enhanced Raman spectroscopy for the surface catalytic coupling reaction on silver electrodes[END_REF].

Defining the isotopic composition of greenhouse gases helps to constrain global budgets and to study sink and source processes [START_REF] Prinzhofer | Gas isotopes tracing: an important tool for hydrocarbons exploration[END_REF]. CH4 is an important anthropogenic and natural greenhouse gas and, moreover, it participates in atmospheric chemistry through its reaction with the hydroxyl radical [START_REF] Naik | Preindustrial to present-day changes in tropospheric hydroxyl radical and methane lifetime from the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP)[END_REF]. Since individual CH4 sources have characteristic isotope signatures, carbon and hydrogen isotope ratios of CH4 (e.g., δ 13 C -CH4) have been useful to constrain the global methane budget [START_REF] Bergamaschi | High-precision direct measurements of 13 CH4/ 12 CH4 and 12 CH3D/ 12 CH4 ratios in atmospheric methane sources by means of a long-path tunable diode laser absorption spectrometer[END_REF].

By precisely measuring the ratio of 13 C to 12 C, one is able to determine the source of methane [START_REF] Bréas | The global methane cycle: isotopes and mixing ratios, sources and sinks[END_REF]. The individual values depend on the mechanisms of the CH4 formation and consumption prior to its release to the atmosphere. It was shown [START_REF] Petrov | Raman gas analyzer (RGA): Natural gas measurements[END_REF] that it is possible to accurately measure the methane content in natural gases using Raman spectroscopy. The results of this work show the possibility of isotope-ratio analysis of methane.

Nowadays, isotope ratio measurement systems based on optical spectrometers are used because of several advantages. The first one is the fundamental possibility to distinguish the isotopologues (molecules that differ only in their isotope composition) with the same molecular weight but different isotopic composition like 16 O 13 C 16 O and 16 O 12 C 17 O (both representing carbon dioxide) [START_REF] Prokhorov | A novel method of carbon dioxide clumped isotope analysis with tunable infra-red laser direct absorption spectroscopy[END_REF]. Second, it is possible to perform calibration-free absolute measurement of isotopologues based on ab initio calculations of line intensities [START_REF] Polyansky | High-accuracy CO2 line intensities determined from theory and experiment[END_REF]. This is a relevant possibility in the metrology of isotope ratios because the International Committee for Weights and Measures (CIPM) encourages the development of absolute isotope ratio measurement values for reference materials [START_REF]International Committee for Weights and Measures[END_REF]. Third, such systems are relatively easy in use, field deployable and low cost. They require no complicated sample preparation, provide real-time data, and allow for in situ monitoring with a spectroscopic selectivity.

The disadvantage of the isotope ratio optical spectrometers compared to mass spectrometers is still low accuracy [START_REF] Van Geldern | Field-based stable isotope analysis of carbon dioxide by mid-infrared laser spectroscopy for carbon capture and storage monitoring[END_REF].

Raman spectroscopy has advantages over traditional methods in the analysis of pure isotopologues. Due to different selection rules, Raman spectroscopy can detect even diatomic homonuclear molecules such as O2 or N2 [START_REF] Siberio-Pérez | Raman spectroscopic investigation of CH4 and N2 adsorption in metal-organic frameworks[END_REF][START_REF] Keutel | Evidence for negative cross correlations in vibrational dephasing in liquids: Isotropic Raman-line shift and width phenomena in isotopic mixtures of N2 and O2[END_REF][START_REF] Cabaço | Transient dimer formation in supercritical carbon dioxide as seen from Raman scattering[END_REF][START_REF] Musso | Critical Raman line shape behavior of fluid nitrogen[END_REF]. The development of Raman-based gas analyzers was presented in several papers [START_REF] Yan | Analysis of fiber-enhanced Raman gas sensing based on Raman chemical imaging[END_REF][START_REF] Yu | Optimization of parabolic cell for gas Raman analysis[END_REF]. A system for analysis of mixtures of CO and H2 (synthesis gas) was proposed in [START_REF] Eichmann | Characterization of a fast gas analyzer based on Raman scattering for the analysis of synthesis gas[END_REF]. The Purcell enhanced Raman scattering (PERS) device was used for isotopic gas analysis in [START_REF] Petrak | Isotopic gas analysis through Purcell cavity enhanced Raman scattering[END_REF]. A Raman analyzer for sensitive natural gas composition analysis was described in [START_REF] Sharma | Raman analyzer for sensitive natural gas composition analysis[END_REF]. The main limitation is the low intensity of Raman scattering from gases. So far, application of Raman spectroscopy for routine trace gas analysis have not found widespread use due to the inherent weakness of Raman transitions and it was mainly employed in condensed phases [START_REF] John | X-ray Raman spectroscopic study of water in the condensed phases[END_REF].

The above mentioned limitation can be overcome by using intense excitation beams and highpressure gas cavities. For the reliable 13 C/ 12 C ratio measurements, it is critical to calibrate the instruments. The preparation of calibration gas mixtures requires the analysis of the parent gases, such as pure 12 CO2 and 13 CO2 [START_REF] Chubchenko | Development of a new type of reference standard for carbon isotopic composition[END_REF].

In the present work, we report on the method of quantitative analysis of 12 CO2, 13 CO2, 12 CH4 and 13 CH4 greenhouse gases based on Raman laser gas spectroscopy.

Materials and Methods

This study is dedicated to the development of a compact Raman laser gas spectrometer suitable for the analysis of gases under high pressures. The details of the spectrometer are described in Section 3.

The isotopically enriched carbon dioxide ( 12 CO2, 13 CO2) and methane ( 12 CH4 and 13 CH4) gases were studied. The CO2 gases were provided by PA EXP, Ltd. (Russia) and the CH4 ones -by the Cambridge Isotope Laboratories (USA). The chemical purity (CP) and carbon isotopic enrichment (δC) of the studied gases are specified in Table 1 (provided by the suppliers). The chemical purity shows the molar content of the main gas component and equals to 100% minus the total content of impurities (e.g., N2, O2, Ar, CO, etc.). It is measured by gas chromatography-mass spectrometry (GC-MS). The carbon isotopic enrichment shows the content of the target isotopologue in the main gas component and it is measured by isotope-ratio mass spectrometry (IRMS). The gas temperature was >99.5 >99.99 13 CH4 >99 >99.9 This work also includes the study of human exhalation. All subjects gave their informed consent for inclusion before they participated in the study. The study was conducted in accordance with the Declaration of Helsinki, and the protocol was approved by the Ethics Committee of ITMO University (project RFMEFI57518X0180).

Raman laser spectrometer

Excitation source

The scheme of the spectrometer is shown in Fig. 1. As the excitation source, we employed a continuous-wave diode-pumped solid-state laser (MSL-R-532, CNI-Lasers) emitting ~5 W of linearly polarized output at a wavelength of 532.123 nm with a full-angle divergence of 1.5 mrad, the beam quality parameter M 2 x,y < 1.1 (TEM00 mode) and a laser linewidth of <1 pm (~0.03 cm -1 ). The excitation beam was expanded using a ×7 Vega laser beam expander (λ = 532 nm, Edmund Optics) and focused into the gas cell using a specially designed doublet lens with a focal length f = 55.9 mm with both sides antireflection (AR) coated at 532 nm. The measured spot size in the focus was less than 10 μm (diameter, as measured by the optical knife method). The peak on-axis laser intensity was then about ~10 MW/cm 2 . More details about the focusing system can be found elsewhere [START_REF] Vitkin | Raman gas analyzer for detecting carbon isotopologues[END_REF].

Prior to the beam expanding system, we installed a system based on a pair of crossed polarizers (P1 and P2) and a λ/2 waveplate and a Faraday rotator placed between them. It prevented backscattered light from damaging the excitation laser. In addition, a pair of flat highly-reflective (HR) mirrors was used to align precisely the focal spot position in the gas cell, Fig. 1(b).

High-pressure gas cell

A special gas cell made of titanium was developed, Fig. 2. It was designed to work with gases in a limited volume with high pressure. The internal volume was only 0.57 cm 3 . To prevent reflections and light scattering, the internal surfaces of the gas cell were coated with an anti-glare enamel. The two optical windows for the excitation and scattered beams were made of K8 optical glass. The detection was at 90° to the optical axis of the laser beam. The gas cell can withstand pressures up to ~100 atm. Note that CO2 passes from the gaseous to liquid phase at a pressure of ~60 atm. The thickness of the optical windows was determined based on the calculations of the stress and displacement fields in the windows using the SolidWorks software, Fig. 3(a)-(d). The ultimate stress for both windows was assumed to be below the elastic limit of the K8 glass, ~40 MPa, Fig. 3(e). Under these conditions, the minimum thickness of the windows is 8 mm with a deviation from flatness of 2 μm when exposed to a pressure of 100 atm. We have selected the thickness of 8 mm for both windows. The clear aperture (diameter) of the windows was ~10 mm (for the excitation beam) and 15 mm (for the scattered light). The input window was AR coated for 532 nm and the output onefor 420-680 nm. The calculated fraction of the collected Raman signal determined by the geometry of the gas cell is 11%. The gas cell was equipped with a gas inlet and an outlet. First, a backing vacuum pump was connected to the outlet valve and the cell was evacuated to the pressure of 10 -3 mm Hg. Then, the inlet valve was opened and the cell was filled in with the studied or reference gas. After the measurement, we released the gas into the evacuation system through the outlet valve.

Spectrometer

The scattered light was collected using an achromatic doublet lens (f = 55.9 mm) AR-coated for 420-680 nm. The Rayleigh scattered light was filtered out using a pair of notch filters (attenuation at 532 nm: ×10 -12 ). Another achromatic doublet lens (f = 80.6 mm) was used to reimage the scattered light at the input slit of the spectrometer. It had the same AR coating as for the collecting lens.

The spectrometer was based on a Czerny-Turner scheme. A diffraction grating with 1800 grooves per mm with a blazing wavelength of 500 nm was used. Its spectral efficiency at 330-860 nm exceeded 40%. The linear dispersion of the system was 1.52 nm/mm and the spectral resolution (the instrumental linewidth, full width at half maximum (FWHM)) was 0.05 nm. The reproducibility of the wavelength setting was 0.03 nm. To find a compromise between the spectral resolution and the signal-to-noise (SNR) ratio, the width of the entrance slit was ~20 μm during the experiments. For detecting the spectra, we used a CMOS camera (Hamamatsu) with a 2048×122 pixel 2 grid (pixel size: 12×12 μm). It was cooled by a Peltier element to -40 °C. The exposure time varied from 10 μs up to 10 min. The projected spectral range was as long as 35 nm (0.017 nm per pixel). It was possible to shift this range within the interval 530-635 nm. More details can be found in [START_REF] Popov | Raman detector of carbon isotopes[END_REF].

The wavelength grading was performed using an Hg-He arc-discharge spectral lamp (DRGS-12). The calibration of the Raman spectrometer was performed using a certified reference material of toluene with a known Raman spectrum [START_REF]Standard Guide for Raman Shift Standards for Spectrometer Calibration[END_REF]. The determined relative standard deviation (∆rel) between the measured and standard peak Raman shifts of toluene was <0.01% for the range of Raman shifts of 1000-3300 cm -1 (for λexc = 532.123 nm). The spectral resolution of the spectrometer was ~1 cm - 1 (the instrumental linewidth, FWHM). The measurements are performed at room temperature (293 K).

Spectroscopy of 12 C/ 13 C isotopologues

The study of greenhouse gas mixtures containing the gases of interest (i.e., carbon dioxide and methane) requires a preliminary study of the reference gases with high chemical and isotopic purity.

The Raman spectra of the carbon isotopologues of carbon dioxide and methane are known. Here, we briefly describe and interpret the measured spectra. The CO2 molecule is linear and centrosymmetric. The Raman spectrum of CO2, see Fig. 4(a), contains two intense and sharp Q-branches which form the diad ν1 (1388 / 1370 cm -1 ) and 2ν2 (1285 / 1266 cm -1 ) for the 12 C / 13 C containing molecules, respectively [START_REF] Howard-Lock | Raman intensity measurements of the Fermi diad ν1, 2ν2 in 12 CO2 and 13 CO2[END_REF]. Adjacent to these lines, there are much weaker ones assigned as ν1 + ν2 -ν2 (1410 cm -1 for 12 C, for 13 C, it overlaps with the ν1 line) and 3ν2 -ν2 (1266 / 1248 cm -1 for 12 C / 13 C, respectively). The easiest attribution of the carbon isotopologues of CO2 is according to the ν1 and 2ν2 diad: the wavenumber shift between the corresponding lines for the 12 CO2 and 13 CO2 molecules is ~18-19 cm -1 . The Raman lines of isotopologues of carbon dioxide are easily detectable even at low gas pressure (1 atm.) using isotopically-enriched gases.

The Raman spectra of the 12 CH4 isotopologue have been systematically studied. Much less information is present about the second most abundant natural isotopologues, 13 CH4 [START_REF] Niederer | Analysis of the rovibrational spectrum of 13 CH4 in the Octad range[END_REF]. Only recently, several studies focused on filling in this gap [START_REF] Niederer | Global analysis of the infrared spectrum of 13 CH4: lines in the region 0 to 3200 cm -1[END_REF]. CH4 has tetrahedral symmetry (point group Td) with four normal modes of vibration. They are labeled by irreducible representations of the Td point group. The fundamental frequencies exhibit a simple relation, ν1(A1) ≈ ν3(F2) ≈ 2ν2(E) ≈ 2ν4(F2).

It leads to the so-called polyad structure in the Raman and IR spectra [START_REF] Boudon | The vibrational levels of methane obtained from analyses of high-resolution spectra[END_REF]. A polyad Pn gathers all vibrational states (ν1, ν2, ν3 and ν4) satisfying the condition n = 2(υ1 + υ3) + υ2 + υ4, where υi are the vibrational quantum numbers for i-th (i = 1, 2, 3, 4) mode. Each set of {υi} defines a vibrational level.

The pentad P2 corresponds to 5 vibrational levels: 2ν4, ν2 + ν4, 2ν2, ν1 and ν3. The lines in the Raman spectra corresponding to the pentad of methane are observed at ~3000 cm -1 and they can be used for detection of isotopologues of CH4.

The Raman spectra of isotopologues of methane around ~3000 cm -1 have a rich structure due to the complex vibration-rotation polyad interaction. The most intense line is observed at ~2919 cm -1 ( 12 CH4) and 2915 cm -1 ( 13 CH4) and is assigned as ν1. The isotope wavenumber shift is only ~4 cm -1 . This line is well detected even at low gas pressure and short exposure, Fig. 4(b). Other prominent lines are those at ~3024 cm -1 ( 12 CH4) and 3011 cm -1 ( 13 CH4) showing higher isotope wavenumber shift whilst much lower intensity.

The spectra from Fig. 4 were analyzed to determine the limit of detection (LOD) for each of the gases, Table 2. First, the Raman intensity at the characteristic Raman shift (ν) was determined both for the signal (Isignal) and noise (Inoise) yielding the useful signal, ΔIsignal = Isignal -Inoise. The room mean square value of the noise, (r.m.s.)noise, was also determined. The noise analysis was performed using the spectra of isotopically enriched gases in the wavenumber range free of Raman lines covering about ~1000 cm -1 . The LOD is:

noise vol% vol% signal 3( . . .) LOD . r m s X I =  (1)
Here, Xvol% is the volume fraction of the studied gas which is close to 100% for chemically pure isotopically enriched gases, cf. Table 1. The results on LOD are shown in Table 2 being expressed both in vol% and parts per million (ppm), LODppm = 10 4 ×LODvol%.

The upper limit for the error in the LOD estimation was determined by analyzing 10 repetitive measurements for the 12 CH4 gas. The maximum deviation from the average LOD value was ~14%. a The Raman spectra are shown in Fig. 4, gas pressure: 1 atm., exposure time: 60 s.

The estimated LOD values for the pressure of 50 atm. are according to Fig. 5. The error in the specified LOD values is 14%. The estimated values are given in italics.

The LOD can be decreased in two ways. One is the increase of the gas pressure. The limit of detection is proportional to inverse of the gas pressure p, LOD ~ 1/p, see Eq. ( 1). Indeed, the useful Raman signal of a gas in proportional to the gas pressure [START_REF] Gooijer | Chemical Analysis: A Series of Monographs on Analytical Chemistry and Its Application[END_REF], while the noise level is mostly determined by the detector and thus is independent on the pressure. To confirm this, we have measured the Raman spectra of chemically pure isotopically enriched 12 CO2 gas at various gas pressures in the range of 1 -9 atm. keeping the same exposure time of 60 s. The resulting LOD values (according to the 1388 cm -1 Raman line) are shown in Fig. 5. The data agree well with the LOD ~ 1/p law, yielding an estimation of the limit of detection at the maximum available gas pressure (50 atm.) of 227 ppm (for 60 s exposure). Note that particularly for CO2 at high pressures (>50 atm.), it was pointed out in [START_REF] Prinzhofer | Gas isotopes tracing: an important tool for hydrocarbons exploration[END_REF] that the exponential law better describes the dependence of the Raman signal on the gas pressure. Further studies are thus required at such elevated pressures.

Using the data measured at 1 atm., we interpolated the dependences for other gases, as shown in Fig. 5 by dashed lines. The corresponding LOD(50 atm.) values are listed in Table 2. In particular, for methane, they are as low as 49 ppm ( 12 CH4) and 26 ppm ( 13 CH4). The dependence of LOD on the exposure time Δt, plotted in a double-logarithmic scale, is shown in Fig. 6(b). From statistics, it is known that the r.m.s. of an average value is reduced by a factor of N -0.5 with respect to that for a single measurement, where N is the number of measurements which are averaged. Thus, if Δt0 = 1 s measurement yields a limit of detection of LOD0, any longer measurement with Δt = s×Δt can be represented as a set of s elementary measurements with LOD(Δt) = LOD0×s -0.5 = LOD0×(Δt/Δt0) -0.5 . By plotting Fig. 6(b) in double-logarithmic scale and fitting the experimental points by a linear function, we obtain the slope n of the dependence:

0 0 LOD( ) LOD . n t t t -    =      (2)
In our case, n = -0.54±0.01 which agrees well with the theoretical considerations.

By considering both the effects of the increased gas pressure (p = 50 atm.) and the exposure time (Δt = 300 s), we obtained lower-limit estimations of LOD for all four studied gases, cf. Table 2, i.e., <100 ppm for the isotopologues of carbon dioxide and <25 ppm for those of methane.

Towards applications: Human exhalation

Raman spectroscopy is well suited for the simultaneous detection of various gases in the analysis of human respiration [START_REF] Okita | A Raman cell based on hollow optical fibers for breath analysis[END_REF]. Natural carbon isotopes ( 12 C and 13 C) present in the human breath can be used to identify various diseases. Carbon isotopes in exhaled CO2 can be a valuable real time biomarker of cachexia [START_REF] Butz | Changes in the natural abundance of 13 CO2/ 12 CO2 in breath due to lipopolysacchride-induced acute phase response[END_REF] (depletion of the body), associated with the acute phase of the reaction caused by endotoxemia (accumulation of toxic substances in the body). The acute phase reaction causes shifts in stable carbon isotopes in exhaled CO2, which can be used to monitor nutrient metabolism. Isotope mass spectrometry (IRMS) exists to determine CO2 carbon isotopes in human respiration [START_REF] Braden | Clinically feasible stable isotope technique at a reasonable price: analysis of 13 CO2/ 12 CO2-abundance in breath samples with a new isotope selective-nondispersive infrared spectrometer[END_REF]. This method has high accuracy, sensitivity and stability, but it is rather complex, expensive and requires large expenditures on equipment and staff training. There is a less expensive way to detect CO2 isotopes using an isotope-selective non-dispersive infrared spectrometer (NDIRS) [START_REF] Barth | Determination of 13 CO2/ 12 CO2 ratio by IRMS and NDIRS[END_REF], but it is only suitable for simple breath tests where a small number of samples are required, for example, to detect diseases of the gastrointestinal tract and detect Helicobacter bacteria pylori [START_REF] Koletzko | Isotope-selective nondispersive infrared spectrometry for detection of Helicobacter pylori infection with 13 C-urea breath test[END_REF].

This is due to a decrease in the correlation of repeated measurements with longer series of measurements. An overview Raman spectrum of a human exhalation measured at ambient pressure (1 atm.) is shown in Fig. 7(a). Its shows several intense Raman peaks at 1285 and 1388 cm -1 (both -12 CO2), 1556 cm -1 (O2), 2328 cm -1 (N2) and 2919 cm -1 ( 12 CH4). By focusing on the spectral range around ~1300 cm -1 and applying longer exposure (600 s), we were able to resolve clearly the Raman lines assigned to the isotopologues of carbon dioxide ( 12 CO2 and 13 CO2). The spectral position of the lines and the isotopoc wavenumber shifts well agree with the data for isotopically enriched gases, cf. Fig. 4.

The Raman spectra of isotopologues of carbon dioxide were analyzed from the poit of view of LOD in order to confirm the values obtained with the isotopically enriched gases, see Fig. 4. Note that LOD can be calculated for any gas in a gas mixture if its volume fraction Xvol% is known, see Eq. ( 1). The volume fractions of 12 CO2 and 13 CO2 gases in the human exhalation depends on different factors, e.g., on the time between breathing in and out [START_REF] Normand | C appearance in plasma glucose and breath CO2 during feeding with naturally 13 C-enriched starchy food in normal humans[END_REF]44]. We will use the mean values of 4 vol% Fig. 7(b), we have determined LOD to be 225 ppm (12 CO2) and 75 ppm (13 CO2) at a gas pressure of 1 atm. and an exposure time of 600 s. These values agree well with the analysis performed in Table 2 considering the difference in the exposure time.

Conclusions

To conclude, we have developed a sensitive and compact Raman laser spectrometer suitable for analyzing gas mixtures containing such relevant greenhouse gases as carbon dioxide and methane.

In particular, it is capable of identifying the contributions of carbon isotopologues, such as 12 CO2 / 13 CO2 and 12 CH4 / 13 CH4. The feature of the developed spectrometer is a specially designed gas cell with the following advantages: (i) it can withstand high gas pressures (up to 100 atm.), (ii) it has a relatively small internal volume of ~0.6 cm 3 relaxing the requirements for sample preparation, and

(iii) together with the optical scheme of the spectrometer, it ensures good collection efficiency for the scattered light and, thus, low limits of detection (LODs) for the studied gases. By studying isotopically-enriched 12 CO2 / 13 CO2 and 12 CH4 / 13 CH4 gases, we estimated easily accessible LODs of less than 100 ppm for the isotopologues of carbon dioxide and less than 25 ppm for those of methane.

We also show the proof-of-the-concept of the suitability of the developed spectrometer for studying the human exhalation.

Further work will focus on developing a gas analyzer using calibrated gas mixtures. Such Raman gas spectrometer and gas analyzer are promising for studies of sources of pollution in the atmosphere, combustion processes and human exhalation.

Potentially, the developed Raman gas spectrometer can be used for analyzing virus structures in human exhalation.

Figure 1 .

 1 Figure 1. (a) Scheme of the Raman gas spectrometer: 1 -excitation laser, 2 -lens expander, 3 -focusing lens, 4 -gas cell, 5 -laser radiation absorber, 6 -collecting lens, 7 -notch filter, 8 -focusing lens, 9 -spectrometer, 10 -computer; (b) System for attenuation and optical alignment of the laser beam: P1 and P2 -polarizers, WP -half-wave plate, M1, M2 -HR mirrors.

Figure 2 .

 2 Figure 2. The scheme of the high-pressure gas cell: 1 -nut rings, 2 -PTFE gaskets, 3 -input window for passing the excitation light, 4 -useful volume for the studied gas, 5 -output window for collection of the scattered light.

Figure 3 .

 3 Figure 3. (a)-(d) Simulation of (a),(b) the stress field, σ, and (c),(d) the field of total displacements, u, in (a),(c) the input window of the gas cell and (b),(d) the output window used for collection of the scattered light. The gas pressure is 100 atm; (e) Calculated maximum stress in the window vs. its thickness, the dashed line represents the elastic limit of the K8 glass (the window material). The grain structure in (a)-(d) represents the mesh used in the simulations.

Figure 4 .

 4 Figure 4. Raman spectra of isotopically enriched (a) carbon dioxide ( 12 CO2 and 13 CO2) and (b) methane ( 12 CH4 and 13 CH4) gases. The excitation wavelength λexc = 532.123 nm; the resolution is ~1 cm -1 . Gas pressure: 1 atm. Exposure time: 60 s.

Figure 5 .

 5 Figure 5. Limit of detection (LOD) of chemically pure isotopically enriched carbon dioxide and methane gases vs. inverse of the gas pressure: symbols: experimental data for 12 CO2 using the 1388 cm -1 Raman line, solid line -their linear fit, dashed lines -interpolation for other gases according to the measurement at 1 atm. Exposure time: 60 s.

Figure 6 .

 6 Figure 6. (a) Raman spectrum of high-purity (>99%) isotopically-enriched (>99.9% 13 C) 13 CH4 gas. The excitation wavelength λexc = 532.123 nm; the resolution is ~1 cm -1 . Gas pressure: 1 atm. Exposure time Δt: 1 -30 s; (b) Double-logarithmic plot of LOD vs. the exposure time: symbols: experimental data, line: linear fit according to Eq. (2). Gas pressure: 1 atm.

Figure 7 .

 7 Figure 7. Raman spectrum of the human breath (exhalation): (a) an overview spectrum; (b) a close view on the 12 CO2 / 13 CO2 lines. The excitation wavelength λexc = 532.123 nm; the resolution is ~1 cm -1 . Gas pressure: 1 atm. Exposure time: (a) 30 s, (b) 600 s.

Table 1 .

 1 Chemical purity (CP) and Carbon isotopic enrichment (δC) of the studied carbon dioxide and methane gases.

	Gas	CP, %	δC, %
	12 CO2	>99.987	99.992
	13 CO2	>99.994	99.57
	12 CH4

Table 2 .

 2 Analysis a of the Sensitivity of the Raman Gas Spectrometer According to the Study of Chemically Pure Isotopically Enriched Carbon Dioxide and Methane Gases

	Parameter	12 CO2	13 CO2	12 CH4	13 CH4
	ν, cm -1	1388	1370	2919	2915
	Isignal, counts	7630	13754	37487	64654
	Inoise, counts	382	229	523	311
	(r.m.s.)noise, counts	28	27	30	28
	ΔIsignal, counts	7248	13525	36964	64343
	LOD(1 atm.), %	1.1731	0.5952 0.2428	0.1303
	LOD(50 atm.), %	0.0227	0.0119 0.0049	0.0026
	LOD(50 atm.), ppm	227	119	49	26
	LOD(50 atm., 300 s), ppm 95	50	20	11

CO2 and 0.04 vol%

CO2. Based on these volume fractions and the measured Raman spectra,

Funding: This research was funded by Ministry of Education and Science of Russian Federation, grant number RFMEFI57518X0180.

Author Contributions:

Conceptualization and methodology, E.P., I.C., A.Kh., P.C. and P.L.; validation, A.P. and A.D.; formal analysis, V.K.; investigation, V.V., I.C., E.P., A.P., A.D. and V.K.; data curation, A.K.; writing-original draft preparation, I.C., K.G.; writing-review and editing, P.L., M.A. and F.D.; visualization, K.G. and V.K.; supervision, V.V.; project administration, V.V. and X.M.; funding acquisition, V.V. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest:

The authors declare no conflict of interest.