P Levinsky1
email: levinsky@fzu.cz

C Candolfi1

A Dauscher1

B Lenoir1

J Hejtmánek2

Thermoelectric properties of magnesium-doped tetrahedrite Cu12-xMgxSb4S13

Keywords: Thermoelectric, Tetrahedrite, Magnesium substitution, Material synthesis

Tetrahedrites, naturally occurring sulfosalt minerals, have been shown to exhibit peak ZT values close to unity near 700 K due to the combination of semiconducting-like properties and extremely low lattice thermal conductivity. A wide range of elements can be substituted into tetrahedrites, each of them affecting the thermoelectric properties. Interestingly, all tetrahedrites reported to date contain exclusively d-and p-block elements of the periodic table. Here, we demonstrate that magnesium, an s-block element, can be successfully introduced in Cu12Sb4S13. We successfully prepared a series of polycrystalline samples Cu12-xMgxSb4S13 with nominal compositions x = 0.5, 1.0, 1.5. Powder X-ray diffraction and chemical mapping confirmed that approximately half of the Mg atoms were incorporated into the tetrahedrite unit cell, while the other half forms electrically insulating MgS precipitates.

Thermoelectric properties, measured between 5 and 673 K, show that the effect of Mg2+ is similar to that of other aliovalent elements substituting for either Cu or Sb. In particular, 2 increasing the Mg content drives the system closer to a semiconducting behavior, leading to a concomitant increase in the thermopower and electrical resistivity and a decrease in the electronic part of the thermal conductivity. Because these two trends counterbalance each other, the overall effect of Mg on the ZT of Cu12Sb4S13 is found to be marginal with a peak ZT of 0.55 at 673 K.

Introduction

Thermoelectric (TE) materials allow direct, solid-state conversion between heat and electricity. The efficiency with which heat is converted into electricity and vice-versa is directly related to the dimensionless thermoelectric figure of merit ZT = S2T/(ρκ), where T is the absolute temperature, S is the Seebeck coefficient or thermopower, ρ is the electrical resistivity and κ is the total the thermal conductivity. [START_REF] Goldsmid | Thermoelectric Refrigeration[END_REF][START_REF] Rowe | Thermoelectrics and Its Energy Harvesting[END_REF] Tetrahedrites, a family of compounds based on an eponymous sulfosalt mineral that can be mined worldwide, have recently been found to possess favorable thermoelectric properties. [START_REF] Suekuni | [END_REF] Tetrahedrites can be described by the general chemical composition Cu10(Zn,Fe,Cu,…)2(Sb,As)4S13 which is a multicomponent variant of the ternary tetrahedrite Cu12Sb4S13. [4] While naturally occurring tetrahedrite minerals are not directly suitable for TE applications because of their nearly intrinsic semiconducting electrical properties synonymous with high electrical resistivity values, [5] synthetic tetrahedrites with optimized chemical composition have been shown to reach peak ZT values close to unity near 700 K. [6,7,8] Such good thermoelectric performance is mainly due to their extremely low lattice thermal conductivity with values as low as 0.5 W m-1 K-1 at 700 K. [9] These low values along with the glass-like temperature dependence have been shown to originate from strongly anharmonic, low-energy optical modes associated with unconventional thermal vibrations of some of the Cu atoms. [10,11,12] Partial substitutions by transition metals or chalcogenides on the Cu, Sb or S sites help to drive the metallic properties of Cu12Sb4S13 towards a heavilydoped semiconducting state more favorable for thermoelectric performance. In this context, a wide range of elements have been considered with the aim of determining their influence on the thermoelectric properties of Cu12Sb4S13. These substitutions include Se [13,14] on the S site, As [15], Te [16,17] and Bi [18] on the Sb site, and a dozen of different elements on the Cu site such as Fe [6], Co [7], Ni [19,20], Zn [6], Cd [21] or Sn [22]. Several elements can be substituted simultaneously. [4,23,24,25,26,27,28] Nevertheless, all tetrahedrites reported to date contain exclusively d-and p-block elements of the periodic table. Here, we demonstrate for the first time that Mg, an s-block element, can be successfully substituted for Cu in Cu12Sb4S13. We present analyses performed by powder X-ray diffraction and scanning electron microscopy and transport property measurements carried out over a wide range of temperatures.

Experimental Details

Polycrystalline samples of Cu12-xMgxSb4S13 were prepared via powder metallurgy. Pure elements (Cu powder 99.95 %, Mg powder 99.99 %, Sb shots 99.999 %, S pieces 99.999 %) were mixed in stoichiometric quantities in an Ar-filled glove box and sealed in evacuated silica tubes under secondary vacuum. These tubes were placed in a rocking furnace and heated at 670 °C for 12 h with slow warming and cooling rates (0.3 K min-1 and 0.5 K min-1, respectively) because of the high vapor pressure of sulfur. Resulting ingots were crushed and cold pressed into cylindrical pellets which were sealed under secondary vacuum in silica tubes. Subsequent annealing at 450 °C for 5 days was completed by quenching the tubes in cold water. The pellets were ground into fine powders which were consolidated by spark plasma sintering (SPS) at 80 MPa for 4 min at 450 °C using the SPS Syntex Dr Sinter 515 apparatus. The consolidated pellets had a relative density ranging between 95 % and 98 % of the theoretical density, the sample with the lowest Mg content being the densest.

Crystal structure and phase purity were investigated by powder X-ray diffraction (PXRD) at 300 K using a Bruker D8 Advance diffractometer (Cu K1 radiation). Rietveld refinements were performed using the FullProf Software [29] to determine the lattice parameters of the samples and to estimate the fraction of secondary phases. An FEI Quanta 650 FEG scanning electron microscope (SEM) was used to study the sample morphology by back-scattered electron (BSE) imaging and chemical composition by energydispersive X-ray (EDX) spectroscopy.

The dense pellets were cut into bar-shaped and disc-shaped samples with a diamond wire saw for measurements of the thermoelectric properties. High-temperature electrical resistivity and thermopower were measured with an Ulvac-Riko ZEM-3 commercial apparatus in the temperature range 300 -700 K. In this temperature range, the thermal conductivity was calculated according to the relation κ=αcpD. The thermal diffusivity α was measured on disc-shaped samples by the laser flash technique (Netzsch LFA 427). The specific heat cp was estimated by the Dulong-Petit law, while the density D was measured at 300 K. The overall experimental uncertainty in ZT is estimated to be 17 %. [30] The electrical resistivity, thermopower and thermal conductivity were measured at low temperatures (5 -300 K) using a custom made apparatus for the Mg-substituted samples and a physical property measurement system (PPMS, Quantum Design) for the ternary tetrahedrite. Both systems employ the four-point electric and thermal transport method.

Results and Discussion

PXRD patterns of the samples sintered by SPS are shown in Fig. 1a. These results confirm that the samples are composed mainly of the targeted tetrahedrite phase together with a small amount of secondary phases. These were identified as Cu3SbS4 and CuSbS2 in all the samples and a copper-rich tetrahedrite phase in the x = 0.5 sample. Note that the Cu3SbS4 phase is usually observed in polycrystalline ternary Cu12Sb4S13, if it were to be prepared by ball-milling or by powder metallurgy. [31,32] The fraction of this impurity phase (4, 3 and 1 wt.% for the x = 0.5, 1.0, and 1.5 samples, respectively) is found to scale with the Mg content with an increase in the Mg content resulting in a lower amount of Cu3SbS4 present in the sample. The weight fraction of the other phases is around 2 % as determined from Rietveld refinements. The inset in Fig. 1a indicates that the reflections shift towards a lower angle as x increases, which confirms that Mg has been successfully introduced into the tetrahedrite unit cell. Rietveld refinements against the PXRD patterns show evidence that the lattice parameter a increases linearly with the nominal Mg content up to x = 1.5 (see Fig. 1b) according to the relation a = 10.324+0.051x Å. As we shall see below, the nominal value of x is approximately twice as high as the actual Mg content in the tetrahedrite phase. Thus, a increases approximately by 0.1 Å per Mg atom per formula unit. This value is larger than what could be expected based solely on Shannon's ionic radius of Mg [33] which is comparable to other d-block elements. The value estimated for Mg is closer to 0.1 Å per atom which is equivalent to substitutions by large atoms such as Hg [34] and Cd [21]. BSE images collected on the two end-member samples of this series, x = 0.5 and x = 1.5, are shown in Fig. 2. Both samples are composed mostly of the tetrahedrite phase in agreement with the PXRD analysis. According to EDX, this phase contains approximately one half of the nominal concentration of Mg atoms. The other half is found to form inclusions of the electrically insulating binary MgS also visible in the BSE images. The Cu3SbS4 and CuSbS2 phases observed in the PXRD patterns can also be identified. According to EDX, neither of these two phases contains Mg atoms.

The thermoelectric properties measured between 5 and 673 K are shown in Fig. 3. The data measured on the ternary Cu12Sb4S13 are consistent with those reported by Suekuni et al. [START_REF] Suekuni | [END_REF] In particular, the metal-to-insulator transition that sets in near 85 K, leaving signatures in all transport coefficients, can be clearly seen. In addition, these measurements show hysteretic behavior above 200 K when the measurements are performed upon cooling and warming whose exact origin remains so far elusive. As shown in Fig. 3a, both features are still observable in the x = 0.5 sample, although the steep rise in the electrical resistivity values below the metal-to-insulator transition is strongly lessened. The fact that this transition survives partial substitution is in line with recent results obtained on As-substituted samples, revealing that this transition is suppressed upon substitution smoothly rather than abruptly.

[15] Intriguingly, the hysteretic behavior observed between 200 and 300 K is more pronounced than in the ternary compound. This change might hint towards an impuritytriggered decomposition since this sample contains a higher amount of secondary phase (Cu3SbS4) compared to the ternary sample. Upon further increasing the Mg content to x = 1.0 and 1.5, these two features are fully suppressed, leaving a steep rise in  below 100 K characteristic of thermally-activated electrical properties. Above this temperature, the  values smoothly decrease with increasing temperatures up to 300 K, above which they remain nearly constant (see inset of Fig. 3a).

Above 300 K, the  values increase with increasing Mg content. The thermopower (Fig. 3b) follows a similar trend with x, with the highest measured value of 170 V K-1 achieved at 673 K in the x = 1.5 sample. This behavior is consistent with an increase in the semiconducting character as x increases, as observed for other partial substitutions on the Cu or Sb site. Substituting Mg for Cu therefore has a similar influence on the electronic band structure, a fact further supported by theoretical calculations [35]. Mg2+ progressively fills the two holes per formula unit present in the valence bands of Cu12Sb4S13, reducing the carrier concentration and thereby resulting in a concomitant increase in the electrical resistivity and thermopower values. The magnitude of this effect is comparable to other substituents [22,36] and further supports the fact that Mg is an effective doping species in the Cu12Sb4S13 tetrahedrite.

The metal-to-insulator transition in the x = 0.0 sample as well as the hysteretic behavior observed in the x = 0.0 and x = 0.5 samples also have a profound influence on the total thermal conductivity shown in Fig. 3c as a function of temperature. Indeed, upon cooling from 300 to 100 K, the x = 0.0 sample shows a dip near 200 K in addition to a continuous decrease. A second dip is observed near 85 K which closely corresponds to the metal-toinsulator transition accompanied by a distortion of the cubic unit cell. [37] The introduction of a small amount of Mg significantly enhances the first anomaly with a rather abrupt decrease between 300 and 200 K from 1.1 to 0.55 W m-1 K-1, respectively. This stronger decrease, which corresponds to the temperature range where the hysteretic behavior is observed in the electrical resistivity, is mainly due to the higher  values measured in this sample compared to the ternary compound. This leads to a significant decrease in the electronic contribution which drops well below 0.1 W m-1 K-1. The second anomaly near 85 K is no longer visible, which is in agreement with the general fact that a partial substitution on the Cu, Sb or S site tends to suppress the low-temperature metal-to-insulator transition. Upon further increasing the Mg content to x = 1.0 and eventually to x = 1.5, the conventional, glass-like temperature dependence widely observed in tetrahedrites is recovered. At high temperatures, the total thermal conductivity decreases with increasing Mg content. This effect is also due to the increase in the electrical resistivity values which leads to a decrease of the electronic contribution to the thermal conductivity. The minimum thermal conductivity value of 0.8 W m-1 K-1 has been measured at 673 K for the x = 1.5 sample, a value typically observed in substituted tetrahedrites.

Finally, Fig. 3d shows the temperature dependence of the dimensionless thermoelectric figure of merit ZT. The decrease in the total thermal conductivity is counterbalanced by the decrease in the power factor due to a significant rise in the electrical resistivity values upon substituting Mg for Cu. This substitution therefore leaves the ZT values unchanged with respect to the ternary sample. A peak ZT value of 0.55 at 673 K is equivalent for all the samples.

Conclusion

We successfully synthesized a series of Mg-substituted tetrahedrite samples Cu12-xMgxSb4S13 and confirmed the incorporation of Mg into the unit cell from EDX and PXRD analyses. To the best of our knowledge, an s-block element has not been considered before in the crystal structure of tetrahedrites. As all other elements of the d-and p-block studied so far, Mg was found to drive the metallic ternary Cu12Sb4S13 closer to a semiconducting state. This behavior has been experimentally confirmed by a concomitant increase in the electrical resistivity and thermopower with increasing x. In agreement with prior studies on tetrahedrites, the thermal conductivity of all samples is very low, reaching 0.8 W m-1 K-1 at 673 K in the x = 1.5 sample.

Nevertheless, the combination of these effects leaves the dimensionless thermoelectric figure of merit ZT mostly unaffected because of the high electrical resistivity values. As a result, a peak ZT value of 0.55 at 673 K was found to be equivalent in all three Mg-containing samples and in pristine Cu12Sb4S13. This study demonstrates that other s-block elements may be worth considering to determine whether they can be beneficial for optimizing the thermoelectric performance of tetrahedrites.

Fig. 2 :

 2 Fig. 2: BSE images of polished surfaces of the SPS sintered samples with (a) x = 0.5 and (b) x

Fig. 3 :

 3 Fig. 3: Temperature dependence of the thermoelectric properties of the studied samples: (a)

Acknowledgements

This work was performed with the financial support of the Czech Science Foundation (project No. 14-36566G and 18-12761S), the Operational Programme Research, Development and Education (Center of Advanced Applied Sciences project No. CZ.02.1.01/0.0/0.0/16_019/0000778) and the Grant Agency of the Czech Technical University in Prague (grant No. SGS16/245/OHK4/3T/14).

List of Figures


D Magliacano
email: dario.magliacano@univ-fcomte.fr

M Ouisse

S De Rosa

F Franco

A Khelif

DESIGN GUIDELINES FOR THE ACOUSTIC PERFORMANCE IMPROVEMENT OF A PERIODIC POROUS MATERIAL

In this paper, some guidelines are provided in order to predict at which frequency the 1 st performance peak (related to periodicity effects: half of the wavelength = periodicity dimension) appears, together with its amplitude, as functions of the unit cell dimensions. Conversely, also the link between the unit cell dimensions and the 1 st performance peak amplitude as functions of the design frequency is shown. Furthermore, some additional guidelines are provided in order to predict at which frequency the 1 st performance peak appears, together with its amplitude, as functions of the foam airflow resistivity.

INTRODUCTION

The design based on periodic elements is a powerful strategy for the achievement of lightweight sound packages and represents a convenient solution for manufacturing aspects [START_REF] Allard | Propagation of Sound in Porous Media: Modelling Sound Absorbing Materials[END_REF]. An interesting research target is the inclusion of vibroacoustic treatments at early stage of product development through the use of porous media with periodic inclusions, which exhibit proper dynamic filtering effects [START_REF] Magliacano | Computation of dispersion diagrams for periodic porous materials modeled as equivalent fluids[END_REF]; this address different applications in transportation (aerospace, automotive, railway), energy and civil engineering sectors, where both weight and space, as well as vibroacoustic comfort, still remain as critical issues. Here we propose some design guidelines in order to tune the performances of a JCA-modeled foam [START_REF] Johnson | Theory of dynamic permeability and tortuosity in fluid-saturated porous media[END_REF], [START_REF] Champoux | Dynamic tortuosity and bulk modulus in air-saturated porous media[END_REF], which make possible to obtain a device whose frequency efficiency outperforms existing designs.

DESIGN GUIDELINES

Some guidelines are provided in order to predict at which frequency the 1 st performance peak (related to periodicity effects: half of the wavelength = periodicity dimension) appears, together with its amplitude, as functions of the unit cell dimensions. Conversely, also the link between the unit cell dimensions and the 1 st performance peak amplitude as functions of the design frequency is shown. The test campaign is carried out in the 0 -18000 Hz frequency range, through the use of a repetition of five 3D unitary cells constituted by a 2 cm cube with a 0.5 cm radius perfectly rigid cylindrical inclusion, where the dimension of the inclusion changes accordingly to those of the unit cell (the ratio between the unit cell and the inclusion dimensions is kept constant). According to the results obtained in Figure 1 and Figure 2, and considering that a typical acoustic excitation in aeronautics lays in the range of 20 -2000 Hz [START_REF] Peiffer | Full frequency vibro-acoustic simulation in the aeronautics industry[END_REF], one should choose a unit cell dimension between 0.065 m and 0.1 m in order to obtain a transmission loss improvement of averagely 25% respect to the use of a simple foam layer of the same thickness. Considering an automotive application, instead, the typical acoustic excitation lays in the range of 20 -4000 Hz [START_REF] Mohanty | An overview of automobile noise and vibration control[END_REF], and therefore one should choose a unit cell dimension between 0.035 m and 0.1 m in order to obtain a transmission loss improvement of averagely 35% respect to the use of a simple foam layer of the same thickness. Furthermore, some guidelines are also provided in order to predict at which frequency the 1 st performance peak appears, together with its amplitude, as functions of the airflow resistivity value of the foam. The test campaign is carried out in the 0 -17000 Hz frequency range, by comparing a repetition of five melamine unit cells, where the airflow resistivity value is artificially changed. Looking at Figure 3, it is clear that the static airflow resistivity has no meaningful impact on the position of the periodicity peak in the frequency range. Instead, one may notice that 𝜎 has a nonnegligible effect on the variation of the non-homogeneous values, compared to the homogeneous ones, in correspondence of the periodicity peak: in particular, this variation reduces its amplitude at increasing airflow resistivity values, both for absorption coefficient and transmission loss performances. This is probably due to the fact that, as shown in Figure 4 and, in an even more evident manner, in Figure 5, for a homogeneous layer of foam, when 𝜎 increases absorption coefficient performances decrease, while transmission loss ones gets better. This is an expected phenomenon, since the airflow resistivity parameter may be considered as an "acoustical hardness" indicator of a foam, in the sense that, the higher it is, the less air permeability there is. It is evident, then, that the general effect of the presence of any external inclusion in the foam reduces at increasing 𝜎, and the non-homogeneous curves tends to assume the same behavior of the homogeneous one, still maintaining a bias difference in the average value (as it can be clearly seen from Figure 5). Indeed, already starting from 𝜎 = 50000 Pa * s m 2 , periodicity peaks are no more precisely identifiable.

Density [kg/m

CONCLUSIONS

In this paper, some guidelines are provided in order to predict at which frequency the 1 st performance peak appears, together with its amplitude, as functions of the unit cell dimensions and of the airflow resistivity; conversely, also the link between the unit cell dimensions and the 1 st performance peak amplitude as functions of the design frequency is shown. Results are very promising for aeronautic and automotive applications.

Figure 1 :

 1 Figure 1: Absorption coefficient design curves as a function of the unit cell dimension (on the left) and the frequency of the 1st peak (on the right).

Figure 2 :

 2 Figure 2: Transmission loss design curves as a function of the unit cell dimension (on the left) and the frequency of the 1st peak (on the right).

Figure 3 :

 3 Figure 3: Absorption coefficient (on the left) and transmission loss (on the right) design curves as functions of the foam airflow resistivity value.

Figure 4 :

 4 Figure 4: Absorption coefficient value as a function of frequency and foam airflow resistivity; homogeneous case (on the left) and case with a cylindrical perfectly rigid inclusion (on the right).

Figure 5 :

 5 Figure 5: Transmission loss value as a function of frequency and foam airflow resistivity; homogeneous case (on the left) and case with a cylindrical perfectly rigid inclusion (on the right).

3 ] Prandtl number

			Dynamic viscosity	Adiabatic bulk	Specific heat
			[kg/(m*s)]	modulus [kg/(m*s 2 )]	ratio
	1.205	0.713	1.983*10 -5	1.42*10 5	1.401

Table 1 :

 1 Air properties.

	Porosity Tortuosity Resistivity	Viscous characteristic	Thermal characteristic
			[Pa*s/m 2 ]	length [mm]	length [mm]
	0.99	1.02	8430	0.138	0.154

Table 2 :

 2 Acoustic parameters of the foam used for the numerical test campaign.

ACKNOWLEDGEMENTS

This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No. 675441 (ViPer project).


D Grojo

M Chambonneau

L Lavoute

D Gaponov

M Castaing

V Y Fedorov

A Hideur

S Février

S Tzortzakis

O Utéza

THREE-DIMENSIONAL WRITING INSIDE SILICON USING A 2-µM PICOSECOND FIBER LASER Paper # Micro 1404

Three-dimensional writing inside silicon using a 2-µm picosecond fiber laser

Introduction

First experiments using femtosecond lasers in the prospect of a 3D laser writing technology applicable to silicon (Si) have revealed strong nonlinear effects affecting energy delivery. These include highly efficient multiphoton absorption and important plasma effects far before reaching the focal region, which prevent from localizing enough energy density in the bulk of Si to induce any permanent material change [1][2][3]. Here, we introduce a solution based on the development of a µJclass Thulium-doped fiber laser operating in the picosecond regime. Supported by nonlinear propagation simulations and energy flow analyses, we reveal that nonlinear propagation effects persist in the investigated regime near critical power conditions. However, the conditions for reliable writing deep into Si wafers are accessible. Moreover, we anticipate a direct benefit of the Thulium-doped fiber laser technology used in these experiments with the possibility for bulk processing of even narrower band gap materials (e.g. Germanium). This represents a new challenge in this rapidly growing field of research after the recent demonstrations of the first in-chip functional microsystems fabricated inside Si by laser writing [4][5][6].

Results and discussions

To target a pulse energy level consistent with the application of bulk machining in silicon we rely on the development of a Thulium-doped fiber system delivering 2-ps pulses at 1970 nm. While table-top systems deliver outstanding performances on the order of hundreds of μJ [START_REF] Gaida | Thulium-doped fiber chirped-pulse amplification system with 2 GW of peak power[END_REF], monolithic all-fiber systems are usually limited to sub-μJ energy level [START_REF] Wan | High pulse energy 2 μm femtosecond fiber laser[END_REF] [START_REF] Tan | 1 μJ, sub-300 fs pulse generation from a compact thuliumdoped chirped pulse amplifier seeded by Raman shifted erbium-doped fiber laser[END_REF]. We have developed a 5 μJ-class ultrafast thulium-doped fiber system. The chirped pulse amplification approach was applied to the seed pulses generated in a dissipative soliton oscillator [START_REF] Gaponov | 2μm all-fiber dissipative soliton master oscillator power amplifier[END_REF]. This concept allows all-fiber integration, high seed average power, adjustable spectrum to fit dispersive transmission band components, and pre-stretched pulse. The power amplifier stage features 25-micron core diameter active fibers instead of rod-type photonic crystal fibers. In order to boost the pulse energy above the μJ level, the nonlinear threshold, imposed by the active fiber core size, was increased by stretching the seed pulse to 550 ps by means of a chirped fiber Bragg grating stretcher. The system also includes pigtailed acousto-optical modulator for repetition rate control. Finally, the output pulse was compressed using chirped volume Bragg grating. The linear amplification regime was observed up to an output average power of 500 mW at a repetition rate of 100 kHz.

To investigate the damage response of bulk silicon, we carried out measurements using a conventional machining experiment. Linearly polarized pulses are focused with NA=0.85 at different depths in an intrinsic Si wafer (<100>-orientation, double side polished) using an asphere mounted on a motorized stage (along the optical axis Z). XY positioning is performed by motorized motion of the Si wafer. The theoretical spot diameter is about 1.5 µm (FWHM) according to calculations. We employ dark-field microscopy for detecting the light scattered by any produced damage in the bulk. In practice, the sample is illuminated at grazing angle with white-light and a microscope unit equipped with an InGaAs camera is installed for in situ observation. Irradiations are systemically repeated 10 times for each tested condition so that we can make a statistical analysis of the damage initiation and discuss the results in terms of damage probability.

The figure 1 illustrates the unusual damage response resulting from irradiation of Si with 10000 pulses. It shows the measured damage probability as a function of the energy of the 2-ps pulses. The corresponding experiments have been repeated after placing the geometrical focus at two different depths: 100 and 150 µm. Each measurement results from irradiations separated by 50 µm to avoid any side effects from previous irradiations. For both depths, we first note a transition from 0% to 100% damage probability for a pulse energy of  250 nJ thus showing that the damage threshold can be exceeded in contrary to femtosecond laser experiments for similar focusing conditions. In practice, as soon as the transition region near threshold is exceeded, we observe highly repeatable modifications. However, a surprising feature occurs at higher energy (around 450 nJ and shown by an arrow in Fig. 1) where the damage probability drops down to zero indicating a reduction of the delivered fluence below the damage threshold. It is also interesting to note that by increasing the energy, it remains possible to overcome this reduction and retrieve the 100% damage probability with apparently less localized modifications as more important scattering by the damage sites is detected.

To understand this unusual damage response, we turned to nonlinear propagation simulations. All the details on the simulations method were reported in Ref. [START_REF] Fedorov | [END_REF]. The results in the picosecond regime and comparisons to experiment are available in Ref. [12]. In brief, our simulations account for Kerr refraction, nonlinear ionization and the associated plasmas effects. It was originally developed to quantify the intensity clamping process generally experienced by propagating intense pulses and being severely pronounced when ultrashort pulses are tightly focused in bulk Si [3]. In this previous work, the comparisons with measurements showed that the essential contributions to the intensity clamping are correctly accounted in the model [3]. We have identified the progressive depletion of the pulse by pre-focal nonlinear absorption, plasma screening and defocusing as contributing factors to the clamping. However, we also found the importance of Kerr refraction for a fully quantitative agreement. Interestingly, without introducing any additional physical contribution in the simulations, we found a small delocalization and relocalization of the laser energy flux above clamping conditions in the picosecond regime [12]. This can be directly associated to the non-monotonic damage probability which is found above clamping. Kerr focusing being the only nonlinear process localizing the laser energy, it is natural to expect that it is the one competing with the previously mentioned effects clamping the intensity to cause the modulation of the laser pulse energy density.

With the knowledge of this peculiar damage response shown in figure 1 we derived the conditions to achieve well-localized modifications in two distinct planes. The energy of the 2 ps pulses is first set slightly above the modification threshold but below the delocalization energy range (320 nJ per pulse focused with a numerical aperture NA=0.85) for reliable digital writing of binary data at a depth of 150 µm below the surface of a Si wafer. Data of a second information are subsequently inscribed on the surface using the same procedure at reduced pulse energy for pixels defined by clean ablation (confirmed by SEM observations). As shown in figure 2, the data in the two planes are clearly distinguished using the infrared dark-field microscope provided that the depth of field is short enough using a high magnification microscope objective. This demonstrates the possibility for multi-layer writing and reading information inside silicon wafers by picosecond lasers.

Summary and conclusion

Bulk damage test experiments in crystalline silicon have revealed that the energy flux of 2-µm pulses is strongly affected by nonlinear effects. However, in contrary to the femtosecond regime, the modification threshold can be exceeded when 0.85 NA focusing is used. This enabled us to successfully inscribe data inside a Si wafer and, more importantly this indicated that microfabrication applications can be addressed on the basis of the rapidly developing Thulium-doped fiber laser technologies.

Figure 1 .

 1 Figure 1. Damage probability measurements with 10000 pulses focused below the surface of Si. Two focusing depths are tested: 100 and 150 µm. Damage occurrence is established using dark field IR microscopy. Several transitions between 0% to 100% damage probability evidence a non-monotonic change of the delivered fluence due to the nonlinear effects. The absence of damage in an energy range above threshold is attributed to nonlinear delocalization of the radiation.

Figure 2 .

 2 Figure 2. Bulk-surface two-plane data inscription by laser processing. The sample is observed in IR dark field microscopy. With 20× magnification (a), the depth of field allows to observe both planes at the same time but the two layers are clearly distinguished by changing the focusing depth with a 50× magnification objective [(b) and (c)].

Acknowledgements

This research has been performed in the frame of the International Associated Laboratory "MINOS" and has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Grant Agreement No. 724480).


Abdul Basit
email: abdul.basit3@ptcl.net.pk1

Ullah Safi

Omair Khan

Sabir

Safi Ullah Khan
email: safiullah.khan@ptcl.net.pk2

Omair Sabir
email: omair.sabir@gmail.com

Application-based forwarding in Delay-Tolerant Networks

Keywords: Energy-Efficient, Delay-Tolerant Networks, Buffer Management, Computer Networks. I

come

INTRODUCTION

The growing use of powerful mobile devices, together with the current global, fast-paced development of technology and consumption habits, has raised the users' expectancies and criteria; people are craving for reliable connectivity while on the move. This has resulted in a networking situation where the mobile industry focuses on overcoming the power, CPU, and memory constraints by proposing more sophisticated and better, heterogeneous devices and wireless networks. Nonetheless, the intermittent connectivity even in urban scenarios is still a problem since the availability of wireless shadowing, the expensive internet services and closed access points would contribute to the potentially low throughput and the significant delay. With the improvement of the Internet, the mobile ad hoc networks (MANET) have begun to operate by themselves, and most people already maintain those continuously connected to the Internet, thus forming a dynamic and autonomous topology of a self-configuring network of mobile and portable devices. Because they are linked wirelessly, those devices rely on almost no infrastructure, and because of the capacity of MANETs to be forwarding unrelated traffic constantly, they also function as routers. The challenge is to equip each device in such a consistent manner as to permit it to continuously maintain the information required to route traffic properly.

In the scenario of Internet and MANETs, there is always end-to-end connectivity between the source and the destination nodes. The network operates through a global protocol due to the connectivity of all nodes in the network [START_REF] Mohamed | Simulation comparison and analysis of network throughput in DSDV and AODV and DSR mobile Adhoc Network Protocols[END_REF]. Delay-tolerant networks are challenged networks in which there is no end-toend connectivity between the source and the destination nodes, mostly due to the mobility of those nodes. In the scenario of DTN, the routing protocol is to forward data by using the Storeand-Carry approach. In this method, data loss prevention is ensured by storing the data in the buffer until it is transmitted to another node and reaches its destination [START_REF] Sobin | A survey of routing and data dissemination in delay tolerant networks[END_REF].

Content-oriented routing schemes consider the nature and the interest of data, instead of the host and the location of a node, which increases the performance of the network [START_REF] Libo | Routing in Mobile Opportunistic Networks[END_REF]. Social networks have played a vital role in improving the performance by leveraging the challenges of the huge amount of content existing in the network. Because an entry is created in the routing table for each content item, even if we are capable of compressing data, we must cope with the situation of fastmoving content which is produced, moved or deleted with a high speed and occurrence. The paper utilizes the social benefit of different nodes, and the processed information is forwarded to the node if their interest profile matches, or else, the data is forwarded to the static throw-box. While mobile throw-boxes implement the transportation system and increase the performance of the network, static throw-boxes are placed in different locations by the point of interest [START_REF] Faisal | Social interest-based routing in Delay tolerant networks[END_REF].

Buffer management policies improve the performance of the network, but little research has been done about implementing it in throw-boxes. In the FIFO ("first in, first out") policy, the message is deleted as soon as the buffer is full. The disadvantage is that the priority messages will also be deleted [START_REF] Momina | An adaptive and efficient buffer management scheme for resource-constrained delay tolerant networks[END_REF]. www.ijew.io

The MOPR buffer management technique used a metric Fp to delete the messages based on Fp=Fpold + p, where Fp represents the most favourably forwarded message and p is the delivery probability of that message. If the buffer becomes full, the data that has highest Fp value [START_REF] Momina | An adaptive and efficient buffer management scheme for resource-constrained delay tolerant networks[END_REF] will be deleted.

The MOFO buffer management technique deletes the data that has most copies or is propagated the most in the network since that data would have travelled in the network for a period of time and so it has had higher chances to reach its destination [START_REF] Kawakib | Routing Strategies and Buffer Management in Delay Tolerant Networks[END_REF].

The nodes have smartphones for forwarding and receiving data, though most research findings do not consider the energy aspect of smartphones. Our contribution in the paper is to provide options and demonstrate that an efficient hybrid buffer management policy might be implemented in throw-boxes to reduce the congestion and improve the energy efficiency when the throw-boxes get full and new data is arriving. For every user, interests are defined and data is relayed if the interest of the receiver node is the same as that of the sender. If any data arrives at the throw-box when the same is full, the data which is going to be deleted first is the one that has popular interest or increased diffusion level and high hop count as it would have already been propagated to many nodes and other throw-boxes. This will reduce the congestion and increase the energy efficiency, and new data can be spread through the network while the previous one will have the chances to be stored somewhere else. In this hybrid buffer scheme, data with lower interest will also reach the destination node.

II. RELATED WORK

Delay-tolerant is a sparse network. There is no end-to-end connectivity due to the mobility of the nodes and the partitioning of the network so that they depend on a store-and-forward mechanism. A message is stored in the buffer and is relayed to another node when it encounters the same. Infrastructure-based communication is very expensive, and people living in the rural areas are not wealthy enough to pay for increased communication expenses, so the DTN is a more appropriate solution we must work on to deploy and develop. Direct Delivery [START_REF] Kawakib | Routing Strategies and Buffer Management in Delay Tolerant Networks[END_REF] is a routing protocol in which the source will deliver the message directly to the destination, or otherwise, it will not relay the message to another node. In other words, this means that if the node of origin does not meet the target node, the message will never reach the goal. Epidemic routing [START_REF] Vahdat | Epidemic routing for partially connected ad hoc networks[END_REF] floods the message to every node it meets. It does not consider the buffer space. It has a maximum delivery ratio, but the resource consumption in this routing protocol is too big. In the Spray-and-Wait [START_REF] Spyropoulos | Efficient routing in intermittently connected mobile networks: the multiple-copy case[END_REF] routing protocol, the source node sprays the message to "T" nodes, where "T" is a value predefined by the user. As the node is left with only one message, the direct delivery routing protocol is used. These routing protocols forward the message to another node without having any knowledge or utility metrics. The Prophet routing protocol depends on the history of encounter nodes. If node A meets node B regularly, and node B meets node C also regularly, then node A can also send the message to node C. In this routing protocol, we also take the Aging factor into account [START_REF] Lindgren | Probabilistic routing in intermittently connected networks[END_REF].

Social networks have received a lot of interest from researchers. These are networks in which routing is done through the social ties and is characterized by strained relations and interactions between human beings. Social features rely on behavioural habits and as such are considered to be more stable and long-term [START_REF] Daly | Social network analysis for routing in disconnected delay-tolerant manets[END_REF]. A community is defined as people living in the same area and having more chances of meeting one another than the individuals living in different areas. LABEL [START_REF] Hui | How small labels create big improvements[END_REF] is a routing protocol which takes into account social characteristics. Residents in the same area, or people who have the same profession, are given a LABEL based on their affiliation. The message is forwarded to the node which has the same label, or otherwise, it is not forwarded. The drawback in this routing protocol is that if the source node does not meet the node having the same LABEL, the message will never be delivered to the destination. The SimBet [START_REF] Daly | Social network analysis for routing in disconnected delay-tolerant manets[END_REF] routing protocol provides the capability to forward a decision based on two social metrics, between "centrality" and "similarity."

The betweenness centrality acts as a bridge for forwarding the message to different communities. The node which has similar neighbours with the destination node is another metrics used in this routing protocol. The message is transmitted to the node having both a high betweenness centrality and a high similarity value. Bubble Rap [START_REF] Hui | Bubble rap: Social-based forwarding in delay-tolerant networks[END_REF] is a routing protocol which forwards the message to another node based on the metrics degree centrality. The central node can be considered the traditional node as the popular node will be connected to more people. In this routing protocol, there is a global centrality and a local (geographical) centrality. First, the message is bubbled up globally, and as soon as it reaches the destination community, it bubbles up on the local centrality. In [START_REF] Momina | An adaptive and efficient buffer management scheme for resource-constrained delay tolerant networks[END_REF], the social interests of users are exploited, and data is forwarded to the nodes with matching interests. Static throw-boxes are placed in Point-ofinterest locations, and mobile throw-boxes are used for data forwarding. While improving on delivery probability, we need to reduce the congestion and apply an efficient buffer management policy. In [START_REF] Costa | Socially-aware routing for publish-subscribe in delay-tolerant mobile ad hoc networks[END_REF] data is forwarded on the basis of the users' interests; if their interests are the same, data is forwarded, else their weight towards that interest is evaluated in that time interval-if the interest is higher, the threshold data is forwarded, otherwise, it is not. Efficient buffer management scheme is not implemented in the routing method that reduces the energy efficiency and performance of a network. Limited and insufficient research is done on implementing the buffer management policies in throw-boxes. Only two techniques are identified to use both the buffer of throw-boxes and the buffer management policies [START_REF] Gupta | DirMove: the direction of movement-based routing in DTN architecture for the post-disaster scenario[END_REF], [START_REF] Zhao | Capacity enhancement using throw-boxes in mobile delay tolerant networks[END_REF]. In both static and mobile throwboxes, the routing scheme implements the FIFO buffer management policy for discarding the old messages as the buffer of the throw-boxes becomes full. One of the disadvantages of this scheme is that the priority of data is not considered; there can be some messages that need to reach the destination but will simply be discarded if they are old [START_REF] Zhao | Capacity enhancement using throw-boxes in mobile delay tolerant networks[END_REF]. One buffer management technique that is used is MOFO [START_REF] Momina | An adaptive and efficient buffer management scheme for resource-constrained delay tolerant networks[END_REF] in which data that has a greater number of copies in the network needs to be deleted. This technique does not consider the data that has passed over a greater number of hops from the data it receives from the nodes. These algorithms are not implemented in throw-boxes for evaluating the efficiency of a network.

Another buffer technique that is used is SHLI [START_REF] Libo | Routing in Mobile Opportunistic Networks[END_REF] which deletes the data whose remaining TTL is much lower; this data cannot reach the destination so it is deleted to free up buffer space. This technique performs well until the throw-box becomes full; in that case, the data with high TTL cannot be deleted so this technique fails.

The LEPR buffer management scheme [START_REF] Yin | An Effective Buffer Management Policy for Opportunistic Networks[END_REF] deletes the data based on predictive delivery probability; the data will lower predictive delivery probability is deleted first which frees buffer space.

III. PROPOSED BUFFER MANAGEMENT MECHANISM

The limitation of existing routing protocols is that they do not consider any buffer management scheme in the nodes and throw-boxes. Epidemic [START_REF] Vahdat | Epidemic routing for partially connected ad hoc networks[END_REF] routing protocol floods the data to every node it encounters and considers buffer capacity unlimited. Prophet [START_REF] Lindgren | Probabilistic routing in intermittently connected networks[END_REF] routing doesn't flood the data to every node it encounters, instead, it forwards the data based on past node records, i.e., if node A frequently encounters node B, there is a higher probability that they will meet again, and data is forwarded based on the history of encountered nodes. This routing protocol also considers the buffer unlimited. The SCORP [START_REF] Moreira | Impact of human behavior on social opportunistic forwarding[END_REF] routing protocol forwards the data based on interests: if the interest of the encountered nodes is the same, data is relayed to the node, else the weight of that interest in that time interval is evaluated, and if it is greater than the threshold the data is relayed, otherwise it is not. This approach is good in terms of reducing the congestion of the network, but SCORP does not consider that the buffer of the nodes is limited. The limitation of the protocols explained above is that they consider the nodes to have unlimited space, and so data is forwarded to the nodes as it is received, which is not a practical approach. In practice, the smartphones have limited buffer space, and so a buffer management scheme needs to be implemented. Another limitation is that the above routing scheme does not consider that the battery of the nodes can be depleted and an energy efficient algorithm needs to be implemented. The proposed algorithm considers the above two limitations of routing protocols and offers a routing protocol that takes into account the buffer's limited space and the node's smartphone limited battery capacity. Until now, very little work has been done on implementing the buffer management technique in throw-boxes, and in this research, the buffer scheme is implemented in throw-boxes and nodes. The proposed algorithm is compared with SCORP [START_REF] Moreira | Impact of human behavior on social opportunistic forwarding[END_REF], Prophet [START_REF] Lindgren | Probabilistic routing in intermittently connected networks[END_REF] and Epidemic [START_REF] Vahdat | Epidemic routing for partially connected ad hoc networks[END_REF] routing schemes for evaluation. Above is the algorithm of the proposed routing protocol, and its flow chart is shown in Fig. 2. 1. The data in delay-tolerant networks is in the form of bundles that are completely relayed to another node they encounter and which is within range. Bluetooth and wi-fi can be used for sending data to another node, www.ijew.io but we only consider Bluetooth for data forwarding when nodes encounter each other, as these are challenged networks and wi-fi facilities are not available everywhere. The range of a Bluetooth is 10 meters. In Fig. 1, suppose that Node A is in range of Node B (within 10 meters) so that data in bundle format will be relayed to Node B, and similarly, Node B is in range of Node C (within 10 meters), so Node B will forward the bundle to Node C and data can reach the destination.

2. Nodes are defined as different interests. The data in the buffers of the throw-boxes is labeled with interests.

One of the techniques is deleting those messages that have less popularity or fewer subscribers, but in this method, the disadvantage is that data with less popularity will never reach the destination.

3. If the node encounters any other node or throw-box, the first step is to search for data with TTL<5 hours and delete it as these messages have very low chance to reach the destination node due to the lower TTL. This buffer space gets empty and new data can be relayed to the throw-boxes.

4. As the first step is completed, apply step two: Delete all those messages that have highest diffusion level and hop counts. The following equation is implemented for finding the highest value of Pa.

Pa= P +1 + Hp ( 1
)
Where Pa is the equation used for deleting the messages from the throw-boxes, P is the number of copies spread in the network, and Hp is the hop count factor of the data.

5. The algorithm searches for data that has highest Pa value a, has highest number of copies spread and has passed a large number of hops.

6. Perform step 4 in descending order as the buffer gets empty.

7. If the node encounters any other node or throw-box, the above procedure is applied.

V. SIMULATION RESULTS AND PERFORMANCE ANALYSIS

ONE simulator is used for the evaluation of the routing schemes for the delay-tolerant networks [START_REF] Ker¨anen | The one simulator for DTN protocol evaluation[END_REF]. The working day movement model is used for simulation. The nodes are distributed throughout the map. The home, offices and evening activity places have also been pre-defined in the map. There are 17 groups which deploy in the settings. Each group is defined with interest. In this simulation, there are ten [START_REF] Hui | How small labels create big improvements[END_REF] unique interests which are assigned randomly to specific groups of nodes that do not overlap with each other. The external event is used for generating the message in the simulation. The message is created according to the user's specifications (the user can specify the source and the destination node). The ten [START_REF] Hui | How small labels create big improvements[END_REF] different interests are: agriculture, tourism, sport, medicine, reading, games, business, news, movies, and weather. Each node in a group is randomly assigned with interest. The simulation was paralleled with a study without the implementation of the buffer scheme [START_REF] Momina | An adaptive and efficient buffer management scheme for resource-constrained delay tolerant networks[END_REF] and a study with the same parameters but with a buffer scheme in throw-boxes applied. For analysis of the energy efficiency of the routing protocols, the energy module [START_REF] Rodrigues-Silva | Energy Impact Analysis on DTN Routing Protocols[END_REF] is developed in One simulator used for determining the energy consumption of the nodes. For reference, Samsung S6 Mobile is used with Bluetooth for configuration settings. The parameters dependent on energy consumption of the devices are scan (-38.61 mAs), receive (-51.47 mAs) and transmit (-51.57 mAs). The simulation parameters are shown in Table 1. It can be defined as the number of messages created and the number of messages delivered to a destination. The higher the distribution probability, the better the performance of the network. The simulation results are shown in Fig. 3. The first scenario, in which buffer scheme EEHIMBS is implemented, shows 0.85 delivery probability with TTL of 1 day, compared to Epidemic, Prophet and SCORP routing scheme which shows 0.73, 0.74 and 0.80, respectively. The 3-day TTL of Epidemic, Prophet and SCORP routing protocol shows 0.76, 0.77 and 0.82 delivery probabilities, respectively, and with the buffer scheme being implemented-the delivery probability increases to 0.87. The 1-week TTL of Epidemic, Prophet and SCORP routing protocol is 0.78, 0.79 and 0.84, respectively, but with buffer scheme implemented, the delivery probability increases from 0.85 to 0.90. The 7-week TTL performed on Epidemic, Prophet and SCORP routing protocol shows 0.82, 0.85 and 0.87; with the buffer scheme implemented, it reaches 0.94. The increase in the delivery probability is due to the implemented interest-based buffer scheme; the scheme uses the hybrid approach in which even the data with less popularity reaches to the destination node. On the other hand, Epidemic routing protocol has low delivery probability due to its flooding approach; the Prophet routing protocol does not implement any buffer management protocol but neither does it flood data to all nodes, so it has higher delivery probability than the Epidemic routing protocol. SCORP routing protocol forwards data based on interests, so it has a higher delivery probability as data reaches the area where the node has high chances of receiving data. The proposed routing scheme achieves highest delivery probability due to buffer management scheme implemented; the data with less priority also gets delivered to the destination. It can be defined as the time it takes for the created message to reach the destination node. The lower the average latency, the better the performance of the network. The 1-day TTL using Epidemic, Prophet and SCORP routing protocol shows an average latency of 32, 28 and 27ms; with the buffer scheme being implemented the latency is 25ms. The 3-day TTL using Epidemic, Prophet and SCORP routing protocol shows 32, 30 and 28ms of average latency, and with EEHIMBS the average latency decreases from 25ms to 23ms. The 1-week TTL using the same protocol shows an average latency of 35, 31 and 28ms, and with EEHIMBS it has an average latency of 18ms. The 7week TTL using the same protocols shows the average latency of 39, 33 and 29ms respectively, and with EEHIMBS it has an average latency of 18ms. The average latency when using the buffer scheme has reduced the average latency in comparison to the one used without enacting the buffer scheme. The Epidemic routing scheme floods the data to throw-boxes and nodes and does not have buffer scheme implemented and so it has high average latency. Similarly, the Prophet and SCORP routing protocols do not flood the data so they have less latency compared to the Epidemic routing protocol. The proposed buffer scheme deletes data with high diffusion or high popularity level, and more space is created in the throw-boxes while the average latency is reduced. The results are shown in Fig. 4. The overhead ratio using Epidemic, Prophet and SCORP routing protocol with a 1-day TTL is 0.70, 0.64 and 0.63, and with EEHIMBS implemented, the overhead ratio is 0.60. The overhead ratio of the above same protocols with 3-day TTL shows 0.77, 0.69 and 0.64, and with EEHIMBS implemented, it drops from 0.64 to 0.59. The overhead ratio with 1-week TTL using the above routing protocols shows 0.80, 0.76 and 0.78, and when using EEHIMBS, it drops from 0.59 to 0.55. The overhead ratio using the same protocols with a 7-week TTL has the values 0.81, 0.76 and 0.69, and when using EEHIMBS, it decreases from 0.55 to 0.54. The Epidemic routing protocol has highest overhead ratio due to its flooding nature, compared to SCORP and Prophet routing protocol. SCORP routing protocol forwards data based on interests, so the overhead ratio is less compared to Epidemic and Prophet routing protocol. EEHIMBS has lowest overhead ratio due to deleting the data as the buffer gets full, and data will TTL less than 5 hours also gets deleted. The overhead ratio can be defined as the number of extra bytes sent in the network for data to reach the destination node. The lower the overhead ratio, the better the performance of the network. The results are shown in Fig. 5.

IX. AVERAGE REMAINING ENERGY

The energy efficiency of the proposed routing protocol is compared to the Epidemic, Prophet and SCORP routing protocol. The results are shown in Fig. 6. Initial energy of the nodes in all routing protocols is 2100 mAh when the number of nodes is 100. As the number of nodes is increased to 200, the Epidemic routing protocol has the lowest remaining energy of 1700 mAh, SCORP has a remaining energy of 1900 mAh, Prophet routing's remaining energy is 1800 mAh and EEHIMBS remaining energy is 2000 mAh. As the number of nodes is raised from 200 to 300, the Epidemic routing has the lowest remaining energy of 1400 mAh, Prophet routing has 1500 mAh remaining energy, SCORP's remaining energy is 1700 mAh and EEHIMBS has a remaining energy of 1800 mAh. When the number of nodes is increased from 300 to 400, Epidemic routing has lowest average remaining energy 1000 mAh, Prophet routing's remaining energy is 1300 mAh, SCORP's routing protocol remaining energy is 1400 mAh, and www.ijew.io EEHIMS has 1500 mAh remaining energy. Overall, EEHIMBS performed best due to the buffer management scheme, as the buffer is always free, the time is not wasted in scanning and energy efficiency is achieved. SCORP forwards data on interest metric, so it performs well compared to Prophet and Epidemic routing protocol. The epidemic has lowest remaining energy due to its flooding approach-it sends data to all nodes as it receives it so it wastes all energy in scanning and transmission of data.

CONCUSLION

An interest-based buffer management scheme is proposed in throw-boxes to increase the performance of the network. When the throw-box is full, we delete the data with high diffusion level instead of deleting the data with less popularity. The proposed scheme has good energy efficiency compared to other routing protocols. This hybrid approach increases the delivery probability of the network by also delivering the data with less popularity. The overall average latency and overhead ratio are also decreased with this buffer management policy, and we also double-checked and prevented some critical data discrepancies. In future research, this buffer management scheme can be compared with other buffer management policies.

Figure 1 .

 1 Figure 1. Encounter of Nodes.

IV

  with highest value of Pa: Pa=P+1+Hp Delete the Higest value of Pa and Perform Operation in descending order until space gets Empty }

Figure 2 .

 2 Figure 2. Proposed Buffer Management Flowchart

Figure 3 .

 3 Figure 3. Delivery Probability VII. AVERAGE LATENCY

Figure 4 .

 4 Figure 4. Delivery Probability

Figure 5 .

 5 Figure 5. Overhead Ratio

Figure 6 .

 6 Figure 6. Average remaining Energy

TABLE I .

 I

		SIMULATION PARAMETERS
	Parameters	Values
	Simulation area	4500m x 3400m
	Mobility Model	Working Day Movement Model
	Simulation time	5 days
	Interface	Bluetooth
	Interface range	10 m
	Groups	17
	Nodes	150
	Nodes speed	0.5, 1.5 meters
	Initial energy	2000 mAh
	Scan energy	38.61
	Transmit energy	51.47
	Receive energy	51.47
	VI. DELIVERY PROBABILITY


Volker Zorngiebel
email: zorngiebel@isl.tm.fr

S Scharnholz

E Spahn

P Brosselard

Jean-Pierre Chante

Dominique Planson

J.-P Chante

Fabrication and Characterisation of High Voltage SiC-Thyristors

Keywords: SiC, high power devices, thyristors, GTO, EGR, MESA, JTE

published or not. The documents may come   L'archive ouverte pluridisciplinaire

Introduction

The physical properties, above all its high electric field, good thermal conductivity and high temperature capability make SiC to be the top choice for high power semiconducting switches [START_REF] Chow | SiC and GaN bipolar power devices[END_REF]. These properties could be used to design higher voltage devices with higher current densities in order to reduce the number of devices used in a switching system. On the basis of numerical simulations published by N. Arssi et al [START_REF] Arssi | Study based on the Numerical Simulation of a 5 kV Asymmetrical 4H-SiC Thyristor for High Power Pulses Application[END_REF] we designed a thyristor in reference to the studies for future pulsed power applications as reported by S. Scharnholz et al. [START_REF] Scharnholz | High-Power MOSFETs and Fast-Switching Thyristors Utilized as Opening Switches for Inductive Storage Systems[END_REF] and E. Spahn et al. [START_REF] Spahn | Modular Pulsed Power Supply Units for active Protection Systems[END_REF]. In order to define our devices, we looked about what is the state of the art for high power silicon thyristors.

Fabrication

Device Layout. Starting the simulation with a fixed point for the breakover voltage the structure for the thyristor was defined as in Fig. 1 to sustain a VBO of 5 kV. The first simulations were done without including the periphery into the calculations. We only looked to the pn-junction which has to block the voltage. So with an 35 µm thick low doped epitaxial blocking layer, different kinds of periphery protection were calculated like MESA, EGR and JTE. The first device we realized had epitaxial guard rings (EGR's). In Fig. 2 shows a picture of the device displaying the layout of the structure with interdigitated gate and anode areas. The structures realized on the masks vary from 170 to 330 µm in length and 35 to 50 µm in width Moreover the number of fingers was varied and so the total anode area ranges from 0.7x10 -3 to 5.7x10 -3 cm 2 . The sizes of the gate structures are varying in the same range as the anode. The number of guard rings was set to five with a width of 12 µm and a distance of 2 µm. The guard rings are terminated by a trench with a width of 45 µm. These parameters were also defined by the simulation. For the MESA protected devices we used the same variations for the size of the structures. The distance from the edge of the MESA to the anode structure was set to 13 µm as a minimum. Process Flow. For the realization of the devices we used three different sets of masks, one for each kind of termination. In principle the process flow of all three terminations has the same sequence. The first step was to etch the trench for the gate contacts through the top epi-layer. The mask was a Ti mask or alternatively a Ti/Cr-mask for the ECR RIE etching. With this step the structure of the anode and also the alignment marks for the following steps were defined. The second step was to etch the termination area. We used the same etching parameters as before and the same mask metal. Fabricating the devices with EGRs we tried to stop etching accurately on the p drift layer. As contrast to this for the MESA and for the JTE devices we etched deep into the p layer. In the finished devices we obtained an over etching for the EGRs of 150 nm and a MESA of 1,3 µm into the p layer. The simulation shows that the breakover voltage is enhanced with increasing the MESA depth. Limited by the technical equipment we are not able to etch more than 1,3 µm deep in one step. Also as a result from the etching the sidewall angle of the structures vary from 93° to 98°. The third step was to define the metal contacts for anode and gate after deposition of the passivation oxide. The oxide was deposited in two different ways. For the first samples we deposited 800 nm oxide by PECVD and for the second series a 1600 nm thick oxide was deposited by CVD at 900°C. The oxide was annealed in two steps with a final dry reoxidation as described from Scharnholz et al. [START_REF] Scharnholz | Utilization of RPECVD oxide in U-MOS structures and different MOSFETs on SiC[END_REF]. The metal contacts for the anode consisted of a Al/Ti double layer. For the gate and the cathode we used a Ti/Ni double layer. The contacts where annealed at 1000°C for 180 s in an RTA furnace. For the JTE there was an additional step for the implantation which required one more mask layer to define the implantation area in a deposited protection oxide.

Up to now we realised thyristors with an EGR and a MESA termination. The devices terminated by a JTE and a MESA with larger etching depth are still in process.

Characterisation

Blocking-state. The results shown in Fig. 3 are measured on a sample of the first series with devices having a MESA protection. In this particular case the etched MESA was 1,3 µm in depth.

The maximum breakover voltage VBO measured on this sample was almost 1500 V. The measurement was done in oil to prevent a breakdown in air close to the surface of the devices. Because of the technological process flow, there was a small crack between the metal contacts and the passivation oxide. This was changed for the second series of samples. Manuscript for Proc. 4th European Conf. on SiC and Related Materials (ECSCRM), Linköping, Sweden, 1-5 Sep. 2002 N + = 7 10 18 cm -3 In a second case we measured the breakover voltage VBO on samples with EGRs. This time the measurement was done with the samples put into perfluorinated fluid Galden™ HT270. As to be seen in Fig. 4 the breakover voltage amounts up to about 1900 V for these devices. The measurement was stopped when the leakage current reached 1 µA to prevent from a premature destruction. Switching. Applying a gate current pulse with a length of 1 µs an amplitude of at least 27 mA was necessary to switch on the devices. In Fig. 5 we show the gate pulse and the corresponding signals of anode-cathode voltage and the cathode current. Using the same pulse we could switch on the devices with anode-cathode voltages ranging from 45 V to 300 V. In all these cases the cathode current was limited to 100mA with series resistances ranging from 0,4 to 3 kW. The switch-off of the devices was investigated by using a gate pulse of 1µs and a variable amplitude. With an anode cathode voltage of 100 V we switched off cathode currents of more than 60 mA. We also investigated the influence of the duration of the gate pulse on the switch-off behaviour. Therefore we vary the pulse length from 1µs up to 1 ms. To switch off a device with a anode cathode voltage of 100 V and a cathode current of 100 mA we need a gate pulse amplitude of 61,5 mA for the short pulse and 43,5 mA for the long pulse.

Conclusion

Based on simulation results thyristors on SiC with different terminations were fabricated. We characterised the devices according to the breakover voltage as well as the switch-on and -off behaviour. The breakover voltage attains values up to 1900 V for devices terminated by epitaxial guard rings. Investigations concerning the switching behaviour showed that the devices could be turned on and off by short gate pulses. Besides this the experimental results show a good correlation to the simulations. Based on this further simulations were performed showing that high power thyristors are feasible with the chosen design and process flow. Therefore future work will focus on the realisation of such high power devices on SiC.

Fig. 1 :Fig. 2 :

 12 Fig.1: Half cell of the thyristor with the doping Fig. 2: Device with EGR termination (5 rings Level and thickness given by Creeä of 12µm width and a distance of 2 µm)

Fig. 3 : 4 :

 34 Fig. 3: Simulation of the dependence of the breakover Fig. 4: Measurement of the breakover voltage with IG = 0mA voltage from MESA depth on different devices at room temperature

Fig. 5 :

 5 Fig. 5: Switching on by different anode cathode voltage and constant current


Roberta Nipoti

Francesco Moscatelli

Andrea Scorzoni

Antonella Poggi

Gian Carlo Cardinali

Mihai Lazar

Christophe Raynaud

Dominique Planson

Marie-Laure Locatelli

Jean-Pierre Chante

Contact resistivity of Al/Ti ohmic contacts on p-type ion implanted 4H-and 6H-SiC

Al-Ti alloys with 72 wt% Al were employed for the realisation of ohmic contacts on 4´10 19 cm -3 p-type ion implanted 4H-and 6H-SiC samples. Contact resistivity characterisations by TLM measurements were done at wafer level in the temperature range 28-290°C. Analysis of the TLM measurements took into account current crowding at the metal pads. More than half of the evaluated contact resistivity reached the minimum value detectable by the used TLM devices, that was slightly higher than 1´10 -6 Wcm 2 . Above this limit value, contact resistivity decreased for increasing temperature and was spread over a few decades. The maximum contact resistivity at 28°C was 2´10 -4 Wcm 2 , which changed to 5´10 -6 Wcm 2 at 290°C. The thermal behaviour of these TLM structures featured thermionic-field emission conduction.

INTRODUCTION

Al-Ti alloys have been subjects of several electrical, chemical and physical investigations aimed at making ohmic contacts on p-type SiC [START_REF] Crofton | Titanium and aluminum-titanium ohmic contacts to p-type SiC[END_REF]. The alloy with the 70 wt% Al was found to give the more reproducible specific contact resistivity rc after vacuum annealing at 1000°C for 2 min. These rc values ranged from 1.5´10 -4 Wcm 2 to 9.5´10 -6 Wcm 2 for doping concentration ranging from 7´10 18 cm -3 to 1.3´10 19 cm -3 , respectively, independently of the 4H-or 6H-SiC polytype [START_REF] Crofton | Finding the optimum Al-Ti alloy composition for use as an ohmic contact to p-type SiC[END_REF]. Moreover, the lowest rc values were found to be independent of the alloy ratio [START_REF] Crofton | Finding the optimum Al-Ti alloy composition for use as an ohmic contact to p-type SiC[END_REF][START_REF] Crofton | Contact resistance measurements on ptype 6H-SiC[END_REF]. These studies featured as grown SiC epilayer, mesa etching, Al-Ti sintered targets of the desired weight ratio and lift off technology to define the metal pattern for the rc evaluation.

The purpose of this work was to transfer the 70 wt% Al recipe [START_REF] Crofton | Finding the optimum Al-Ti alloy composition for use as an ohmic contact to p-type SiC[END_REF] to p-type ion implanted 4H-and 6H-SiC specimens. Due to technological constraints some steps were modified. In particular, single element targets were used and the layer thickness ratio was chosen so to obtain the desired Al/Ti weight ratio. The metal patterns were defined by chemical etching. Different gases in the annealing ambient were studied, namely high purity Ar and vacuum.

The Transmission Line Method (TLM) was used to evaluate the rc of these Al-Ti alloyed contacts. In particular, the resistance measurements on TLM structures were analysed accounting for two-dimensional (2D) and three-dimensional (3D) current flow.

EXPERIMENTAL

<0001> 8° off-axis 4H-SiC and <0001> on-axis 6H-SiC, n-type wafers supplied by CREE were used for this study. Almost box shaped doping profiles were made at the sample surface via multiple energy Al + implantation and high temperature thermal annealing. The substrate temperature during implantation was varied in the range 27-300°C from sample to sample. The implanted wafers were annealed one by one at 1700°C in an inductively heated J.I.P.ELEC TM furnace equipped with a glassy carbon crucible covered by a SiC disk. The annealing gas was high purity Ar at a partial pressure just above the atmospheric one. Secondary Ion Mass Spectrometry showed the as implanted and annealed doping profiles were the same, while Hall effect measurements at room temperature showed hole density values corresponding to a complete electrical activation of the implanted Al. The p-type implanted layers had doping concentration and thickness equal to 4´10 19 cm -3 and 1.35 µm for the 6H-SiC wafer, 4.5´10 19 cm -3 and 0.38 µm for the 4H-SiC one.

<0001> 3.5° off-axis 6H-SiC wafer by CREE with a 2 µm thick 1´10 19 cm -3 p-type epilayer on an n-type substrate was also used for comparison.

The TLM metal patterns were realised by the following recipe. The samples were cleaned consecutively in boiling trichlorethylene, acetone and isopropyl alcohol. This degreasing in organic solvent was followed by the next surface cleaning recipe H2SO4:H2O2 (1:1) for 5min, deionised water for 3min, HF:H2O=1:10 for 60s, deionised water for 3min. At the end of this cleaning process, the substrates were dried with N2 gas. Before loading in the vacuum chamber for metal deposition, the samples were dipped in HF: H2O = 1:1 for 30s and dried with N2 gas. First Ti (80 nm) and then Al (350 nm) were deposited by sputtering. The chosen thicknesses correspond to 72 wt% Al. Photolithography process and subsequent metal etching were employed to define the metal patterns. Post deposition annealing was done at 1000°C for 2 min and different annealing ambients were tested: high purity Ar gas and vacuum (10 -6 torr).

TLM structures featured two mask levels in the case of the implanted wafers and one mask level in the case of the epilayer specimen. In fact, in the former case doped strips, below the metal pads, were defined by mesa etching or selective ion implantation, while in the epilayer case the conductive paths were not confined. In the implanted specimens Van der Pauw (VdP) devices, for a direct extraction of the sheet resistance Rsh of the implanted layers, were defined too. Different sets of TLM devices were used. On the 6H-SiC wafer, 341 µm wide doped strips with 296 µm wide and 132 µm long metal pads with increasing distances between consecutive pads of 18-27-37-57-108 µm were realised (TLM1). On the 4H-SiC wafer, the TLM structures were 150 µm wide doped strips with 94 µm wide and 94 µm long metal contacts with increasing distances between consecutive pads of 9-18.5-28-37.5-47-57 µm (TLM2). While on the epilayer, TLM metal patterns equal to those used for the 4H-SiC specimen were produced. The given figures were measured after contact alloying and by using an optical microscope, with measurement error equal to ± 0.5 µm.

Current-voltage (I-V) measurements on the TLM and VdP devices were performed as a function of the temperature in the range 28-290°C using a computer controlled parametric characterisation system. This was equipped with a Micromanipulator MM6620 semiautomatic probe station, a Temptronic TP315B hot chuck, a Keithley K707 switching matrix (equipped with 7072 and 7174 semiconductor cards) and a Keithley K90 I-V measuring system, composed by a Keithley K2361 synchronisation controller and four Keithley K238 source measure units. An average of ten TLM and ten VdP devices homogeneously distributed over a 35 mm or 50 mm wafers were typically measured per sample type.

rc was evaluated by resistance measurements on TLM structures. These TLM structures were included in masks designed for the realisation of given SiC devices and it was found their geometry with respect to the sheet resistance of the implanted layers could not be treated by the usual one-dimensional (1D) approach. Neither the measurements of non-confined TLM structures could be treated by a 1D approach. Thus, lateral current crowding at the metal pads was taken into account under a 2D approximation. As shown in ref. [START_REF] Scorzoni | Metal /semiconductor contact resistivity and its determination from contact resistance measurements[END_REF][START_REF] Scorzoni | Ohmic contact resistance evaluation in silicon planar structures: application to the CoSi2/n + interface[END_REF][START_REF] Finetti | Lateral current crowding effects on contact resistance Measurements in four terminal resistor test patterns[END_REF] this can be done under the assumptions that the sheet resistance of the doped layer between and that below the pads are equal, and that the transfer length is at least three times, or more, the doped strip depth. Taking into account the transfer length is rc dependent the check of the latter condition was done once the 2D simulated rc was obtained. Should not this condition be fulfilled, a correction factor for rc would be computed following the indication given in ref. [START_REF] Schuldt | An exact derivation of contact resistance to planar devices[END_REF].

The sheet resistance of the strip layer was a variable parameter in the 2D code and at the same time it could be directly measured by means of VdP devices. The comparison between these computed and measured values was used as reliability check for the code outputs.

RESULTS

Fig. 1(a,c) shows the total resistance Rtotal data versus pad distance L and their simulation under 1D and 2D approximations. The search for optimal rc and Rsh values was done through a 2D best fit of the Rtotal versus L. The TLM measured resistances and simulations shown in this picture belong to the 6H-SiC ion implanted wafer, but their trend was representative of all those here studied. In particular, the 2D approximation always accounted better than the 1D approach for the curvature of the experimental data. The 2D simulations always compute Rtotal values at L = 0 lower than those computed by the 1D model. The differences between the intercepts at L = 0 were always approximately independent of rc but dependent on and almost constant for given TLM device geometry and doped layer Rsh. This is evidenced in Fig. 1(b,d) that is an enlarged view of Fig. 1(a,c) next to L = 0. Such differences amounted at (25 + 5) W and (196 + 5) W for the TLM1 and TLM2 pattern type, respectively. Fig. 2 shows the Rsh values extracted from the 2D model for the doped strips of the TLM structures (TLM Rsh ) versus the corresponding ones obtained from the VdP measurements (VdP Rsh). The VdP devices had square shape with a side parallel to the wafer flat, while the TLM doped strip were aligned both parallel and orthogonal to the wafer flat. The 4H-and 6H-SiC implanted samples at the minimum and maximum measurement temperatures were chosen as representative of all the samples here studied. The evident spread of the VdP Rsh values for each measurement temperature is a consequence of the temperature gradient across the wafer diameter during the ion implantation annealing process as discussed in ref. [START_REF] Lazar | A comparative study of high-temperature aluminium postimplantation annealing in 6H-and 4H-SiC, non uniform temperature effects[END_REF]. Simulated and measured values are linearly related through a factor equal to (1.06 + 0.01).

Taking into account the rc evaluated by the 2D model and the corresponding transfer length with respect to the thickness of the doped layer, the 2D rc values had to be transformed into the equivalent 3D ones by applying correction factors in the range 1.2 and 3 [START_REF] Schuldt | An exact derivation of contact resistance to planar devices[END_REF]. Generally, lower were the 2D rc values higher were the correction factors. Moreover, it was found that the used TLM geometry did not allow us to extract rc values lower than approximately 1´10 -6 Wcm 2 . The equivalent 3D rc values are plotted in Fig. 3 versus measurement temperature. Fig. 3(a,b) shows the rc for the not confined TLM devices, i.e. for the 1´10 19 cm -3 6H-SiC epilayer. Fig. 3(a) corresponds to high purity Ar annealing ambient, while Fig. 3(b) to vacuum ambient. The rc values for the ion implanted 4H-SiC and 6H-SiC specimens, i.e. 4´10 19 cm -3 doping density, are shown in Fig. 3(c) and Fig. 3(d), respectively. All the rc values below the minimum resolution of the method were set equal to 1´10 -7 Wcm 2 . These last rc values were not meaningful but indicated a very good ohmic performance. The error bars are shown only for one data set per picture and account for the minimisation process between simulated and experimental results.

Finally, the yield of the rc values shown in Fig. 3(c,d) is plotted in Fig. 4 for different measurement temperatures.

DISCUSSION

The linearity of the Fig. 2 plot is an indirect proof of the reliability of the results provided by the simulation code, i.e. of the estimated rc values. The fact the proportionality factor differs from 1 within the errors may be due to almost unavoidable non-ideal effects in the VdP geometry and/or a consequence of the approximations used for the 2D simulation of the resistance measurements of the TLM structures. In such a case the correction factors applied to the 2D rc might be different than the applied ones. Nevertheless it can be remarked that these correction factors were so low that the corrected rc remained of the same order of magnitude and had the same trend versus temperature as the 2D rc.

A typical problem of the Al/Ti alloyed contact annealed in Ar is the oxidation of the pad top surface [START_REF] Moscatelli | Al/Ti Ohmic Contacts to p-type Ion Implanted 6H-SiC: Mono-and Two-dimensional Analysis of the TLM Data[END_REF]. Such an oxide is hard to remove and makes difficult the wafer level characterisation of the devices. Therefore, post annealing in vacuum is generally preferred. The results plotted in Fig. 3(a) and in Fig. 3(b), show that, once the probing problem was solved, the rc performances for annealing in high purity Ar or in vacuum are equivalent. In the light of this result, the rc values of all the samples here presented were analysed independently of the annealing ambient, as done in Fig. 4.

The comparison between Fig. 3(a,b) and Fig. 3(c,d) evidences the spread of the rc values is negligible in the case of the epitaxial wafer, while it is as large as a few decades for the ion implanted specimens. Such a difference might be attributed to local differences in the electrical activation of the implanted doping species [START_REF] Lazar | A comparative study of high-temperature aluminium postimplantation annealing in 6H-and 4H-SiC, non uniform temperature effects[END_REF]. In fact, when the estimated rc values above the minimum resolution of the method are compared with the literature data [START_REF] Crofton | Titanium and aluminum-titanium ohmic contacts to p-type SiC[END_REF][START_REF] Crofton | Finding the optimum Al-Ti alloy composition for use as an ohmic contact to p-type SiC[END_REF][START_REF] Crofton | Contact resistance measurements on ptype 6H-SiC[END_REF] the plot of Fig. 5 is obtained. Looking at this figure, we can remark the data of the implanted specimens fall in a region where rc is expected to be high sensitive to small variation of the doping concentration.

A further remark can be done about Fig. 5. The rc evaluated at the highest temperature has a better agreement with the trend of the published values for contacts with an equivalent weight percent of Al. It looks like the barrier height for conduction was higher in the ion implanted samples than in the epitaxial ones. In general, the thermal behaviour of these rc featured thermionic-field emission conduction at the contact interface. Nevertheless, even at room temperature, many rc values were below the minimum detectable limit (see Fig. 4), i.e. they were better than the published values. In this case we might speculate about reasons to justify a lower barrier height for conduction in the implanted specimens with respect to the epitaxial substrates. We think such non-homogeneous behaviour of the contacts at wafer level might be understood looking for correlation among electrical performances of different devices at different temperatures. This work is in progress. Finally, the yields shown in Fig. 4 suggest the technology for the realisation of ohmic contacts to SiC is so mature that the Kelvin resistor method [START_REF] Scorzoni | Metal /semiconductor contact resistivity and its determination from contact resistance measurements[END_REF] could be introduced for the evaluation of rc on ion implanted SiC, both 4H and 6H.

CONCLUSION

This work demonstrates the technology of Al/Ti alloyed contacts on p-type ion implanted 4H-or 6H-SiC can give quite low values of rc. In fact, in spite of the large spread of the results, the rc values were always lower than 2´10 -4 Wcm 2 for a medium/high doping concentration of the implanted layers, i.e. 4´10 19 cm -3 . Moreover, a non-negligible yield of values below 1´10 -6 Wcm 2 was found. The results for 4H-and 6H-SiC were similar.

Moreover, this study demonstrates the analysis of non-ideal TLM test structures can be faced and solved by taking into account current crowding, thus reaching a better knowledge about the contact performance.

Figure 1 .

 1 Figure 1. Two meaningful cases from the 6H-SiC ion implanted wafer have been chosen to show the trend of the experimental (exp) and simulated (1D, 2D) data. [a] and [c] show the whole measurement range, [b] and [d] are enlarged views close to L = 0.

Figure 2 .

 2 Figure 2. Sheet resistances of the TLM structures extracted from the 2D model versus the corresponding values obtained from the VdP measurements.

Figure 3 .

 3 Figure 3. Contact resistivity versus temperature for the not confined TLM patterns alloyed in Ar [a] or vacuum ambient [b], and for the confined TLM devices of the ion implanted 4H-SiC [c] and 6H-SiC [d] wafers. All the evaluated rc data are shown, while the error bars corresponding to the minimisation procedure between simulated and measured TLM measurements values are shown for only one data set in each sub-figure.

Figure 5 .

 5 Figure 5. Contact resistivity versus p-type doping: comparison between literature (stars) and this work data at room temperature (open square and dots) and 290°C (full square and dots). The literature data referring to the 70 wt% Al are evidenced as full stars.

Figure 4 .

 4 Figure 4. Yield of the ohmic contact process for the implanted wafers with respect to the contact resistivity mapped at wafer level. 4H-and 6H-SiC results were joined.

  Work partially funded by the Italian Contracts CNR-MADESS-II and ASI/CNR(1/R/073/01). The clean room staff of CNR-IMM Sezione di Bologna is acknowledged for TLM metal pattern realisation.


Marie Guionie

Aurélien Thorette

Marco Romanelli

Emmanuel Pinsard

Laurent Lablonde

Benoît Cadier

Marc Vallet

Marc Brunel

Dual-polarization DFB fiber laser stabilized by frequency-shifted feedback

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

Dual-polarization DFB fiber laser stabilized by frequencyshifted feedback

Marie Guionie 1 , Aurélien Thorette Distributed-feedback (DFB) fiber lasers can sustain the oscillation of two orthogonal polarizations at different frequencies, due to the fiber Bragg grating UV-induced birefringence [START_REF] Loh | 1.55 µm phase-shifted distributed feedback fibre laser[END_REF]. Such dual-frequency fiber lasers (DFFL) are promising as heterodyne sources but, in the context of microwave photonics, the stabilization of the beat frequency becomes necessary. Locking the beat frequency of a dualfrequency laser to a local oscillator by optical frequency-shifted feedback (FSF) has been proved to be efficient [START_REF] Kervevan | Beat-note jitter suppression in a dual-frequency laser using optical feed-back[END_REF], and can be applied in principle to any dual-polarization laser. Here we investigate this FSF method to a Er3+-doped DFB fiber laser, resulting in an all-fibered 1.5 μm system emitting a stabilized beat note at 1 GHz.

The set-up is depicted in Fig. 1(a). The DFFL is pumped at 976 nm through the WDM and emits at 1547 nm two orthogonal polarizations (with associated frequencies νx and νy). The free-running beat note is around 1 GHz; its line-width is equal to 3 kHz and presents a drift of a few MHz over hours [START_REF] Guionie | Beat note stabilzation in dual-polarization DFB fiber lasers by an optical phase-locked loop[END_REF]. The polarization-maintaining FSF loop is based on a polarization beam splitter whose output ports are closed on one another after passing through the frequency shifter, here an intensity modulator EOM driven at fLO and an optical isolator. The polarization controller PC permits to select one of the polarization states, say νx (νx < νy), which is re-injected into the laser. The EOM thus generates two sidebands at frequencies νx ± fLO. Hence, for a small detuning Δν = fLO -|νy -νx|, the re-injected field contains an optical sideband resonant with νy. Intracavity coupling can then lead to frequency locking between νy and the νx+fLO component of the reinjected field. This results in the transfer of the spectral purity from fLO to the beat frequency |νy -νx|. The observed dynamics depend on both detuning Δν and injection strength Γ. First, we find a frequency locking between the beat note and the synthesizer, delimited by the experimental blue points (see Fig. 1(b)), over a 4 MHz range at maximum injection strength. At large excitation rates, the laser may display a complicated, possibly chaotic dynamics on Δν > 0 side (delimited by the red points). In the locking zones, the stabilized beat frequency exhibits a narrow linewidth, smaller than 1 Hz (see Fig. 1(c)). The associated phase noise is measured to be -100 dBc/Hz at 1 kHz from carrier. The laser remains phase-locked for days in laboratory conditions.

Fig. 1 (

 1 Fig. 1 (a) Experimental scheme. The optional EDFA permits here to control the re-injected power. (b) Normalized injection strength Γ with respect to the detuning Δν at two different excitation ratio η. Colored areas delimit the different regimes. White: unlocked, blue: locked, red: chaotic. (c) RF Spectrum of the locked DFFL beat frequency.

  1 , Marco Romanelli 1 , Emmanuel Pinsard 2 , Laurent Lablonde 2 , Benoît Cadier 2 , Marc Vallet 1 , Marc Brunel 1

1. Univ Rennes, CNRS, Institut FOTON -UMR 6082, F-35000 Rennes, France 2. iXBlue Photonics, Rue Paul Sabatier, 22300 Lannion, France

Such sources are advantageous in terms of carrier tuning, compactness and integration and could find application such as in radio-over-fiber. From the dynamics point of view, the linewidth enhancement factor of such lasers could be measured using this method [START_REF] Thorette | Linewidth enhancement factor measurement based on FM-modulated optical injection: application to rare-earth-doped active medium[END_REF]. Finally, similar studies on DFL fiber laser at 10 GHz are under investigation.