Keywords: Doped Fiber Amplifiers, Infrared Fiber Optics, Optical Fiber Devices, Thulium, 2 microns

We report experimental and simulated performance of a tandem (dual-stage) Tm-doped silica fiber amplifier with a high signal output power of 2.6 W in the 2 µm band. Combined high dynamic range, high gain, low noise figure, and high OSNR are achieved with our design.

I. INTRODUCTION

The recent progress in transmission experiments at signal wavelengths in the 2 µm band [START_REF] Liu | High-capacity Directly-Modulated Optical Transmitter for 2-µm Spectral Region[END_REF] shows the need for Thulium-doped fiber amplifiers (TDFAs) with a combination of high gain, low noise figure, and large dynamic range. Previous work has demonstrated single stage amplifiers operating from 1900-2050 nm and 1650-1850 nm [START_REF] Li | Thulium-doped fiber amplifier for optical communications at 2 µm[END_REF][START_REF] Jung | Silica-Based Thulium Doped Fiber Amplifiers for Wavelengths beyond the L-band[END_REF]. In this paper we report the experimental and simulated performance of a tandem single clad TDFA employing inband pumping around 1560 nm and designed for the 1900-2050 nm signal band. A combination of high gain (> 50 dB), output power of 2.6 W, > 30 dB dynamic range, and < 4 dB small signal noise figure are demonstrated with our design. Performance as a function of input power and signal wavelength is presented. The experimental data are in good agreement with steady-state simulations of our single clad tandem TDFA performance.

II. EXPERIMENTAL SETUP

Figure 1 shows the setup for measurements of the tandem TDFA which consists of a preamplifier (Stage 1) and a power booster (Stage 2). Signal light from a single frequency discrete mode laser (DML) source (Eblana Photonics) is coupled into the first TDF F1 through attenuator A. Signal input power is set by varying the attenuator. In stage 1, fiber 1 is co-and counter-pumped using wavelength division multiplexers (WDMs) with 1550 nm grating stabilized DFBs (P1 and P2) which deliver more than 200 mW each into F1.

Manuscript submitted on December 13, 2017. Robert E. Tench and Jean-Marc Delavaux are with Cybel LLC, 1195 Pennsylvania Avenue, Bethlehem, PA 18018 USA (e-mail: robert.tench@cybel-llc.com) (e-mail: jm@cybel-llc.com) Clement Romano is with Cybel LLC, 1195 Pennsylvania Avenue, Bethlehem, PA, 18018 USA, and Institut Telecom/Paris Telecom Tech, 46 Rue Barrault, 75634, Paris, France (e-mail: clement.romano@cybel-llc.com). The signal output of F1 is then coupled into the second TDF fiber F2, in Stage 2, which is counter-pumped either with a multi-watt 1560 nm fiber laser or a multi-watt 1567 nm fiber laser. Optical isolators I1 and I2 suppress parasitic lasing and ensure unidirectional operation. In our experiments, F1 is a 7 m length of OFS TDF designated TmDF200. Two types of fiber F2 are investigated, the first 5 m of OFS TmDF200 and the second 4.4 m of IXBlue TDF designated IXF-TDF-4-125.

III. EXPERIMENTAL RESULTS AND SIMULATIONS

Figure 2 shows the measured gain (G) and noise figure (NF) for the two amplifier configurations, first the OFS/OFS combination and then the OFS/IXBlue combination. In all of our data, which are displayed as points in the figures, input powers are referenced to the input of F1, and output powers are referenced to the output of F2. Maximum values of G of 54.6 dB and 55.8 dB, for OFS/OFS and OFS/IXBlue, respectively, were measured at a signal wavelength λs of 1952 nm for fibre laser pump powers Pp at 1560 nm of 1.95 W. The corresponding NF was measured to be in the range Pin between -30 dBm and +2 dBm.

The data demonstrate a large dynamic range of over 32 dB for an NF of 5.1 dB or less. For lower fiber laser pump powers (Pp=0.2 W to 0.8 W at 1560 nm), NF values as low as 3.2 dB were measured for the OFS/OFS configuration.

Simulations of these data were performed using fiber parameters measured in our laboratory [START_REF] Romano | Characterization of the 3F4-3H6 Transition in Thulium-doped Silica Fibres and Simulation of a 2µm Single Clad Amplifier[END_REF]. The simulation is based on a three level model of the Thulium ion in silica using the 3 H6, 3 F4, and 3 H4 levels including ion-ion interactions [START_REF] Romano | Simulation and design of a multistage 10 W thulium-doped double clad silica fiber amplifier at 2050 nm[END_REF]. The parameters of gain coefficient, absorption coefficient, and 3 F4 level lifetime were determined for the OFS and IXBlue fibers under test. Figure 3 plots the measured gain and absorption coefficients for the OFS fiber, which has a maximum absorption of 92 dB/m at 1630 nm. Figure 4 shows the gain and absorption coefficients for the IXBlue fiber, which has a maximum absorption of 140 dB/m at 1630 nm. The measured lifetimes are 650 µS for the OFS fiber and 750 µS for the IXBlue fiber. Other relevant parameters were taken from the literature. We note that our measurements of peak gain are lower than the peak absorption. This feature is consistent with some published data but not others [START_REF] Sincore | High Average Power Thulium-Doped Silica Fiber Lasers: Review of Systems and Concepts[END_REF][START_REF] Pisarik | Thulium-doped fibre broadband source for spectral region near 2 micrometers[END_REF][START_REF] Agger | Emission and absorption cross section of thulium doped silica fibers[END_REF][START_REF] Smith | Mode instability thresholds for Tm-doped fiber amplifiers pumped at 790 nm[END_REF].

The set of three level differential population equations [START_REF] Jackson | Theoretical modeling of Tmdoped silica fiber lasers[END_REF] was solved using a stiff solver, while the propagation set of differential equations was solved with a 4 th order Runge-Kutta The simulation accounts numerically for the amplified spontaneous emission (ASE) generated in the setup. Two stage simulation was carried out by sequentially applying the results of the single stage calculations.

As illustrated by the solid lines in Figure 2, the simulations agree well with the experimental data. Simulations of G are within 1.5 dB of the data for Pin > -25 dBm. Simulations of NF agree with the data to within 2 dB. These results validate the accuracy of our simulations for both high gain and highly saturated operating regimes.

Data illustrating the variation in output power Pout as a function of 1567 nm fiber laser pump power Pp are shown in Figure 5. For these data, Pin was set to between +1.3 and +2.2 dBm to saturate the amplifier and Pp was varied from 0.3 W to 3.2 W. For the OFS/OFS configuration, a maximum slope efficiency of 82% was observed at λs = 2004 and 1952 nm, corresponding to maximum output powers at these wavelengths of 2.60 W. The slope efficiency is defined as ΔPout / ΔPp. A reduced output power of 0.4 W was achieved at 2051 nm because of lower slope efficiency and the onset of lasing. Figure 6 illustrates the long term stability Abs.

Gain

Abs.

of Pout at 1952 nm and Pp = 2.43 W over a period of 6 hours. The variation in Pout over this time period was less than 4%.

No fiber nonlinear behavior such as Raman or Brillouin scattering was observed in our experiments.

Comparison of the data and simulations shows agreement to better than 0.5 dB for all experimental signal wavelengths as illustrated by the solid lines in Figure 5. These results validate the performance of our simulator as a function of signal wavelength.

Slope efficiency data as a function of λs for the OFS/OFS and OFS/IXBlue setups are shown in Figure 7. Simulated slope efficiencies, given by the solid lines in Figure 7, agree well with the experimental data for all the measured signal wavelengths. The simulations indicate that high slope efficiencies of >70% can be expected from 1900 nm to 2020 nm. The simulations also show that the single clad fiber can significant power 2051 nm with reduced efficiency. We attribute this behavior to the presence of lower wavelength ASE and to reabsorption at lower wavelengths. In 8 we contrast experimental output spectra obtained for the two TDFA amplifiers, for saturated input signals of +2.1 dBm at 1952 nm and fiber laser pump power at 1567 nm of 3.2 W. These data are taken under the same conditions and yield optical signal to noise ratios (OSNR) of 57 dB/0.1 nm for both configurations. The spectra observed for both setups exhibit small differences in the wavelength region below 1950 nm. We believe this is caused by the different doping of the two fibers. Nevertheless, the operating wavelength regions and bandwidths for the OFS and IXBlue fibers are largely equivalent. We attribute this similarity to the low concentration of Tm in the two fibers where the scattering and ion-ion interactions can be neglected. Figure 9 compares the experimental output spectrum for the OFS/IXBlue configuration with the results of our steadystate simulations. We find that the simulations predict the experimental data relatively well. At low wavelengths <1900 nm, we believe the differences between data and simulation are caused by the wavelength dependence in the passive components and the non-monochromatic spectrum of the single frequency laser source.

IV. DISCUSSION

The high measured internal gain of >55 dB represents a significant improvement over results previously reported [START_REF] Li | Thulium-doped fiber amplifier for optical communications at 2 µm[END_REF][START_REF] Jung | Silica-Based Thulium Doped Fiber Amplifiers for Wavelengths beyond the L-band[END_REF] for single stage TDFAs. Such a high small signal gain is promising for preamplifier, repeater, and low noise applications. Expt.

The high observed slope efficiency of 82% and output power of 2.6 W also show significant improvement over previously reported performance [START_REF] Li | Thulium-doped fiber amplifier for optical communications at 2 µm[END_REF][START_REF] Jung | Silica-Based Thulium Doped Fiber Amplifiers for Wavelengths beyond the L-band[END_REF]. The experimental SNR of 57 dB/0.1 nm for a saturated amplifier output is important for applications such as booster amplifiers.

The usable operating optical bandwidths of the tandem TDFAs, with the criterion of 10 dB down from the spontaneous emission peak (Figure 8), are estimated to be 122 nm for the OFS/OFS configuration and 130 nm for the OFS/IXBlue configuration. These values agree with previous work [START_REF] Li | Thulium-doped fiber amplifier for optical communications at 2 µm[END_REF][START_REF] Jung | Silica-Based Thulium Doped Fiber Amplifiers for Wavelengths beyond the L-band[END_REF] and are fully consistent with the simulated slope efficiencies in Figure 7.

Our steady state simulations of tandem TDFA performance agree well with the experimental over a range of Pin from -30 dBm to +2 dBm, for measurements of G, NF, and Pout. This agreement covers the measured wavelength range of 1910 -2051 nm. Future work will extend the studied wavelength range toward lower wavelengths. The good agreement between experiment and theory confirms that our simulator is a useful tool for the design of tandem high gain, high power TDFAs.

Finally, we note that both the OFS and IXBlue configurations of the tandem TDFA exhibit similar performance, both experimentally and in simulation, confirming [START_REF] Liu | High-capacity Directly-Modulated Optical Transmitter for 2-µm Spectral Region[END_REF][START_REF] Li | Thulium-doped fiber amplifier for optical communications at 2 µm[END_REF][START_REF] Jung | Silica-Based Thulium Doped Fiber Amplifiers for Wavelengths beyond the L-band[END_REF] that we can employ multiple commercial sources of Tm-doped fiber in our simulation and design of high performance tandem optical amplifiers.

V. SUMMARY

We have reported the design and experimental performance of a tandem single clad TDFA, in-band pumped around 1560 nm and operating in the 1900 -2050 nm signal band. Small signal gains >55 dB, output powers as high as 2.6 W, and small signal noise figures as low as 3.2 dB were experimentally measured. Slope efficiencies as high as 82% were also observed, and an SNR of 57 dB/0.1 nm was demonstrated with output powers >2 W. Comparison of our data with steady state simulations yielded good agreement, thereby validating our model for high gain and high saturated output powers from the tandem two-stage TDFA over a wavelength range of 1952-2051 nm. Our design is appropriate for high transmit power, preamplifier, and repeater applications in the 2 µm region.

Figure 1 .

 1 Figure 1. Tandem TDFA configuration.

Figure 2 .

 2 Figure 2. G and NF as a function of Pin for the two tandem amplifiers at 1952 nm.

of 4 .Broadband 2 W

 42 Figure, dB

Figure 3 .

 3 Figure 3. Measured gain and absorption coefficients for the OFS Tm-doped fiber.

Figure 4 .

 4 Figure 4. Measured gain and absorption coefficients for the IXBlue Tm-doped fiber.

Figure 5 .

 5 Figure 5. Saturated Pout vs. Pp for the OFS/OFS tandem amplifier.

Figure 6 .

 6 Figure 6. Long term stability of the TDFA output for Pp = 2.43 W.

Figure 7 .

 7 Figure 7. Slope efficiency as a function of λs for the two tandem configurations.

Figure 8 .

 8 Figure 8. Saturated output spectra for the two tandem configurations.

Figure 9 .

 9 Figure 9. Comparison of experimental and simulated output spectra for the OFS/IXBlue configuration.

VI. ACKNOWLEDGEMENTS

We are grateful to OFS and IXBlue for the Thulium-doped silica fibers, and to Eblana Photonics for the single frequency source lasers in the 2000 nm band.


Thierry Portafaix

Sophie Godin-Beekmann

Guillaume Payen

Martine De Mazière

Bavo Langerock

Susana Fernandez

Françoise Posny

Jean-Pierre Cammas

Jean-Marc Metzger

Hassan Bencherif

Ozone profiles obtained by DIAL technique at Maïdo Observatory in La Reunion Island: comparisons with ECC ozone-sondes, ground-based FTIR spectrometer and microwave radiometer measurements

Ozone profiles obtained by DIAL technique at Maïdo Observatory in La Reunion

Island: comparisons with ECC ozone-sondes, ground-based FTIR spectrometer and microwave radiometer measurements.

T. Portafaix (1)*, S. Godin-Beekmann (3), G. Payen (2), M. de Mazière (4), B. Langerock (4), S. Fernandez [START_REF] Neefs | BARCOS, an automation and remote control system for atmospheric observations with a Bruker interferometer[END_REF], F. Posny (1), J.P. Cammas (2), J. M. Metzger [START_REF] Fernandez | a novel ground based microwave radiometer for ozone measurement campaigns[END_REF], H. Bencherif [START_REF] Baray | Maïdo observatory: a new high-altitude station facility at Reunion Island (21° S, 55° E) for long-term atmospheric remote sensing and in situ measurements[END_REF] In addition, a microwave radiometer of University of Bern, has operated between late 2013 and early 2015.

STRATOSPHERIC DIAL SYSTEM AT REUNION ISLAND

This LIDAR was installed at Reunion Island in 2000 and moved to Maïdo facility in 2013 after instrumental updates.

Like any DIAL system, it requires the use of a pair of emitted wavelengths.

Laser sources are a tripled Nd:Yag laser (Spectra-Physics Lab 150) and a XeCl excimer laser (Lumonics PM 844). The Nd:Yag provides the non-absorbed beam at 355 nm with a pulse rate of 30 Hz and a power of 5W, and the excimer provides the absorbed beam at 308 nm with a pulse rate of 40 Hz and a power larger than 9W. An afocal optical system is used to reduce the divergence of the beam to 0.5 mrad.

The receiving telescope is composed of 4 parabolic mirrors (diameter: 500 mm). The backscattered signal is collected by 4 optical fibers located at the focal point of each mirror. The spectrometer used for the separation of the wavelengths is a Jobin Yvon holographic grating (3600 linesmm-1, resolution 3 Å.mm-1, efficiency >25 %).

The two Rayleigh beams at 308 and 355 nm are separated initially by the holographic grating and separated again at the output of the spectrometer by a lens system in the proportion 8% and 92 %, respectively, in order to adapt the signal to the non-saturation range of the photon-counting system. The optical signals are detected by 6 Hamamatsu non-cooled photomultipliers (PM). A mechanical chopper is used to cadence the laser shots and cut the signal in the lower altitude range where PM are saturated. This chopper consists of a steel blade rotating at 24 000 rpm in primary vacuum.

6 acquisition channels are recorded simultaneously: 2 channels at 355 nm corresponding to the lower and upper parts of the profile, 2 channels at 308 nm (lower and upper parts) and 2 Nitrogen Raman channels at 332 and 387 nm. In addition to the mechanical gating, both upper Rayleigh channels at 355 nm and 308 nm, are equipped with an electronic gating in order to cut the signals for the altitudes below 16 km and prevent signal-induced noise.

The system was moved to Maïdo Observatory by the end of 2012, after the update of the electronic system (now LICEL TR and PR transient recorders) and of the XeCl excimer laser. This new configuration allows us to obtain ozone profiles in the 15-45 km altitude range.

The lidar signals are recorded in a 3 min time file but averaged over the whole night acquisition (2 to 3h time integration per night) to increase the signal-to-noise ratio.

It is necessary to apply different corrections to the signal. The background signal is estimated and removed using an average or a linear regression in the high altitude range where the useful lidar signal is negligible (over 80 km). Another correction of the photomultiplier saturation for low layers is also required and applied.

OTHER STRATOSPHERIC OZONE INSTRUMENTS AT MAIDO FACILITY.

A ground-based microwave radiometer (GROMOS-C) designed to measure middle atmospheric ozone profiles has been installed at Maïdo Observatory in 2014 and removed in early 2015. It has been specifically designed for campaigns and is remotely controlled and operated continuously under all weather conditions. It measures the pressure broadened line at 110.836 GHz and can also measure the CO line at 115.271 GHz. The vertical profiles are retrieved by optimal estimation method [START_REF] Fernandez | a novel ground based microwave radiometer for ozone measurement campaigns[END_REF]. FTIR solar absorption measurements at high spectral resolution (from 0.0110 to 0.0035 cm -1 for ozone spectra) are performed by a Bruker 125HR spectrometer installed in 2013. This instrument is dedicated to NDACC measurements in the mid-infrared, covering the spectral range 600 to 6500 cm -1 (1.5 to 16 µm),and particularly the ozone retrievals are performed using the 1000-1005 cm -1 window in the 600-1400 cm -1 spectra (MCT detector, KBr beam-splitter). From the measured absorption spectrum, an inverse method (optimal estimation method) is used to trace back the vertical abundance profiles of gases present in the atmosphere. For ozone, information on about four independent layers in the atmosphere can be retrieved, roughly one in the troposphere and three in the stratosphere, up to about 45 km [START_REF] Vigouroux | Evaluation of tropospheric and stratospheric ozone trends over Western Europe from ground-based FTIR network observations[END_REF]. This instrument is operated remotely and automatically with an updated version of the BARCOS system [START_REF] Neefs | BARCOS, an automation and remote control system for atmospheric observations with a Bruker interferometer[END_REF]. In addition to the continuous monitoring of the atmospheric chemical composition and transport processes, the intention is also to participate to dedicated observations campaigns.

In addition, ECC ozone soundings are performed weekly at Reunion Island since 1998. The ozonesonde currently used is of ECC Z Ensci type with a 0.5% KI buffered solution from Droplet Measurement Technology [DMT]. It is coupled to a meteorological radiosonde M10 from MeteoModem. The effective vertical resolution of the ozone data is between 50 and 100 m [Thompson et al., 2003a[Thompson et al., ,b, 2007]]. The ozone measurement accuracy is around ±4% in the stratosphere below 10 mbar pressure level and the precision in total ozone column measured by the ECC sonde is around 5%. These ozone measurements are part of the SHADOZ (Thompson et al., 2003a(Thompson et al., , 2003b) and NDACC networks.

INTER-COMPARISONS

The first comparisons with ECC simultaneous sounding are very encouraging [START_REF] Baray | Maïdo observatory: a new high-altitude station facility at Reunion Island (21° S, 55° E) for long-term atmospheric remote sensing and in situ measurements[END_REF] with differences less than 10% throughout the profile. Figure 1 presents an example for June 23, 2014. Other comparison between DIAL and GROMOS -C ozone profiles after applying the averaging kernel show a very good agreement in the layer between 5 and 20 hPa, with differences less than 5 %. But differences are more important on the lower or upper layers. It can reach more than 15 % in the 20-100 hPa layer.

These comparisons with the microwave radiometer were made using the DIAL "Rapid Delivery" profiles for the NDACC network, using average parameters for photomultiplier desaturation and background signal removing.

These medium parameters can introduce some additional error in the lower or upper part of the resulting profiles. It will be important for the final version of this paper to make these comparisons from consolidated lidar profiles using refined parameters.

The stratospheric ozone LIDAR is already NDACC qualified. It should be noted however that an inter-comparison campaign of all the NDACC lidar systems (water vapor, temperature, ozone) installed the Maïdo Observatory with the mobile system of NASA-GSFC [START_REF] Mcgee | Improved stratospheric ozone lidar[END_REF] is planned for May 2015.

Comparisons with FTIR will be performed for the three layers between 15 and 45 km. The FTIR measurements in the ozone spectral range will be intensified during this 2015 intercomparison campaign.

  The ozone number density is retrieved from the slope of signals after derivation[START_REF] Godin-Beekmann | Systematic DIAL ozone measurements at Observatoire de Haute-Provence[END_REF].The lidar signals are corrected the Rayleigh extinction using a composite pressure temperature profile computed from nearby meteorological soundings performed daily at Reunion Airport and the Arletty model (based on meteorological data from the European Centre) It is also necessary in the DIAL technique to use a low-pass filter. The logarithm of each signal is fitted to a 2nd order polynomial and the ozone number density is computed from the difference of the derivative of the fitted polynomial. Varying the number of points on which the signals are fitted completes the filtering.

Fig 1 :

 1 Fig 1: Ozone profiles on 24 June 2013 by stratospheric DIAL (black line) and ECC-ozonesonde at Maïdo Observatory (blue).

ACKNOWLEDGEMENT

The present work is supported by LACy, OSU-Réunion and the FP7 European NORS project. The authors acknowledge the European Community, the Région Réunion, the CNRS, and the University of La Réunion for their support and contribution in the construction phase of the research infrastructure OPAR (Observatoire de Physique de l'Atmosphère à La Réunion). OPAR and LACy are presently funded by CNRS (INSU) and Université de La Réunion, and managed by OSU-R (Observatoire des Sciences de l'Univers à la Réunion, UMS 3365). We acknowledge Anne M. Thompson (NASA/GSFC, USA) the SHADOZ network principal Investigator, and E. Golubic, P. Hernandez and L. Mottet who are deeply involved in the routine lidar measurements at Maïdo facility.