Meddy Vanotti
email: meddy.vanotti@femto-st.fr

Virginie Blondeau-Patissier

David Rabus

Jean-Yves Rauch

Sylvain Ballandras

Selective Detection of Hydrogen with Surface Acoustic Wave Devices Using Palladium Layer Properties

Keywords: Gas sensors, Hydrogen, Palladium layer, SAW device, Rayleigh waves

For an increasing number of applications, hydrogen represents a solution of the future as it is the most common element in the Earth. However, due to its unstable properties in gas phase, a particular care must be dedicated to control possible gaseous leaks close to tanks and facilities using this resource. In this paper, surface acoustic wave sensors are proposed for detecting gaseous hydrogen in standard environmental conditions (atmospheric pressure and room temperature). The proposed Surface Acoustic Wave sensors consists in two Rayleigh-wave delay lines built on Quartz, one equipped with a palladium overlay and the other exhibiting a free path between the two interdigited transducers. A specific gas test cell has been developed to test various sensor configurations submitted to hydrogen-composed atmospheres. A particular care was paid to avoid hydrogen leakage in the working environment and to perform the regeneration of the gas absorbing layer. The developed device allows for identifying different concentrations of hydrogen (in the 1-4% range) diluted in nitrogen and is also able to detect hydrogen in current atmosphere. Surface Acoustic Wave devices exploiting hydrogen absorption capabilities of palladium thin films have been here used to make the detection and the identification of hydrogen concentrations in the 1-4% range and the influence of outer parameters such as temperature and relative humidity variations on the sensor operation is also reported.

Introduction

The raising shortages of fossil energy resources added to the increasing concern towards environmental issues have led to consider hydrogen as one of the most promising energy resource. This odorless and colorless gas being highly explosive over 4% concentration in air, the availability of a fast and accurate detection system close to storing facilities and equipping hydrogen-operated machines is mandatory for obvious security reasons. Such a system must exhibit a significant selectivity as it must detect the presence of gaseous hydrogen in air with concentrations smaller than the above-mentioned critical limit at standard conditions (room temperature and atmospheric pressure) as well as in harsher environment (very low or significantly high temperature). Although, some solutions have been proposed [START_REF] Katti | Passivated thick lm catalytic type H2 sensor operating at low temperature[END_REF] [2] [START_REF] Simon | Thermal and gas-sensing properties of a micromachined thermal conductivity sensor for the detection of hydrogen in automotive applications[END_REF] [4] [START_REF]Mems-based hydrogen gas sensors[END_REF] [6] [START_REF] Zhao | Alloptical hydrogen sensor based on a high alloy content palladium thin lm[END_REF], the current availability of such a detection system meeting modern specifications of hydrogen use and storage is still questionable. The main improvements for such sensors are their sensitivity, their selectivity and their reliability together with sensor size, cost reduction, energetic needs and response time [START_REF] Boon-Brett | Identifying performance gaps in hydrogen safety sensor technology for automotive and stationary applications[END_REF]. Many methods of detection of hydrogen and a comprehensive review can be found in the literature [START_REF] Hubert | Hydrogen sensors a review[END_REF], providing a substantial material base to try and address the above challenges. Among the possibilities, SAW (Surface Acoustic Wave) sensors have been widely studied in the last decades because of their attractive capabilities [START_REF] Anisimkin | Elastic properties of thin-film palladium for surface acoustic wave (SAW) sensors[END_REF] [26] [START_REF] Hines | Passive SAW-Based Hydrogen Sensor And System[END_REF]. Indeed, SAW devices exhibit high sensitivity to surface perturbation since most of the energy propagates in a region whose thickness is a few times the wavelength of the propagating acoustic wave. It is also a mature technology, SAW devices do exhibit limited size (less than 1 cm²) and they allow for wireless use [START_REF] Friedt | A wireless interrogation system exploiting narrowband acoustic resonator for remote physical quantity measurement[END_REF]. Initial works were made by D'Amico et al. [START_REF] D´amico | Palladium surface acoustic wave interaction for hydrogen detection[END_REF] using the properties of palladium layer to trap the targeted gas. Since this pioneer work, innovations concerning the selectivity and stability of sensitive layers versus external parameter have been proposed to improve hydrogen detection using SAW devices [START_REF] Jakubik | Bilayer structure for hydrogen detection in a surface acoustic wave sensor system[END_REF] [13] [14] [START_REF] Ippolito | Layered SAW hydrogen sensor with modified tungsten trioxide selective layer[END_REF] [16] [START_REF] Phan | Surface acoustic wave hydrogen sensors based on ZnO nanoparticles incorporated with a pt catalyst[END_REF] [18] [START_REF] Fardindoost | Pd doped wo3 films prepared by solgel process for hydrogen sensing[END_REF]. In this paper, a SAW sensor is proposed for detecting gaseous hydrogen in standard environmental conditions (atmospheric pressure and room temperature). The proposed SAW sensor consists in two Rayleigh-wave delay lines built on quartz, one equipped with a palladium (Pd) overlay and the other exhibiting a free path between the two interdigitated transducers (IDTs) used to excite and detect the acoustic wave. These IDTs are built using aluminum electrodes, as this metal is of low density and acts as a limited acoustic reflector. An innovative aspect of the proposed sensing system consists in the open-loop strategy for phase changes monitoring [START_REF] Rabus | A high sensitivity open loop electronics for gravimetric acousticwave-based sensors[END_REF]. Moreover, delay lines are monitored in parallel using a synchronous detection approach that provides high frequency measurement resolution and that permits a systematic characterization of the device before starting measurement. Along this approach, the impact of changes of intrinsic properties of the devices such as working frequency drift with aging can be minimized. These sensors have been tested in a specific gas cell developed to experiment with various sensor configurations submitted to hydrogen composed atmospheres. A particular care has been dedicated to avoid hydrogen leakage in the working environment and to promote the gas absorbing layer regeneration. The developed device allows for identifying different concentrations of hydrogen diluted in N 2 and is also able to detect H 2 in current atmosphere. The first section of the paper presents the SAW sensor as well as the exploited monitoring system. Experimental validation of H 2 detection then is reported, with a description of the chemical test bench and detection results for various H 2 concentrations. An analysis of the influence of H 2 adsorption on SAW propagation is proposed to provide routes for the proposed device optimization. In the same purpose, the last section of the article is devoted to characterizing the influence of external parameters such as temperature and relative humidity variations on the sensor operation, yielding conclusive discussions.

SAW sensor and associated monitoring system

Selective detection of hydrogen at room temperature and pressure have been achieved using SAW delay lines exploiting Rayleigh waves on AT-cut quartz, as the corresponding first order temperature coefficient of frequency (TCF) is close to zero, yielding frequencytemperature compensation for the above-mentioned operating conditions. The sensor structure correspond to a differential set-up in, which a sensitive track is achieved by depositing a Pd layer in between two IDTs whereas the reference track surface is left free to detect nonspecific gas/surface interaction. Along this approach, one can significantly increase the sensitivity of the device and its robustness to correlated perturbations (temperature, vibration, non-specific adsorption). The configuration of both generation and detection IDTs used for the sensor consist in 50 fingers pairs with a grating period of 10µm and path length of 5mm (the reactive surface). The wavelength is 40µm, yielding a frequency operation in the vicinity of 78MHz as the wave velocity approaches 3100m.s -1 . The Pd film was deposited by thermal evaporation on a single run and shaped by a lift-off technique. Its length along the propagation path was 3mm and its thickness equal to 300nm. The device configuration is shown on Fig. 1. Using a network analyzer, the transfer function of the device can be easily determined and hence the phase shift induced by gas absorption has been first monitored that way. However, the use of a dedicated electronics has been implemented and delivers similar information. That is represented on Fig. 2. This system includes two separate phase measurement schemes: a rough characterization stage using the AD8302 phase and magnitude detector, and a high sensitivity phase measurement using dedicated phase detectors and variable gain, low noise operational amplifiers as presented in another work [START_REF] Rabus | A high sensitivity open loop electronics for gravimetric acousticwave-based sensors[END_REF]. By increasing the gain on the operational amplifier, one can gradually improve the phase measurement resolution while remaining at a constant operating point. However, this approach also reduces the phase range due to the limited voltage range of the analog to digital converter (ADC). While the AD8302 is tuned to generate a voltage within the ADC range as the phase rotates from +90° to -90°, increasing the gain of the high sensitivity stage yields measurement ranges reduced to a fraction of the angle range. Thus, a tracking scheme is implemented in which the phase to frequency (locally linear) relationship is identified, and whenever the phase reaches a condition close to the boundary of the operating range of the ADC, the RF source frequency is tuned so that the phase moves back to the middle of the ADC range. This implementation is similar to a digital oscillator, since the frequency tracks a phase condition, with the added advantage than a precise phase measurement is provided as long as the voltage remains within the operating range of the ADC. The sensitivity of the set-up allows for some tens millidegrees resolution and is easily transportable. The response of the bare device and the functionalized one are respectively measured. This configuration has been used so as to make a systematic characterization of each new device used for H 2 detection. Fig. 3. illustrates the way the phase shift measurement is achieved.

Detection of hydrogen at room temperature and atmospheric pressure

Hydrogen detection in the percent order has been achieved at room conditions. Figs. 4 and5 present experimental results when using either nitrogen or air as carrier gas. As it can be observed in Fig. 5., the detection of about 95%vol of hydrogen in air at 35% RH and 20°C can be achieved with a response delay of about 20 seconds considering that the determination of the H 2 concentration in the melting gas is derived from the phase shift velocity during the exposure and not from the steady state. It is notable that this value depends on a large part on the performance of the hydrogen generation setup. Therefore, the intrinsic response delay of the device is expected to be shorter. Fig. 4. shows that the device allows for identifying different concentrations of hydrogen diluted in a nitrogen flow of 100sccm.

Analysis of hydrogen absorption on Pd films and its influence on SAW propagation

Absorption of hydrogen into Pd layer induces modifications on the SAW propagation conditions. The effective physical characteristics of the wave then vary with the amount of absorbed H 2 (especially its phase velocity). It has been shown [START_REF] Anisimkin | Elastic properties of thin-film palladium for surface acoustic wave (SAW) sensors[END_REF] that the two main parameters that influence the phase velocity change of elastic waves on quartz are mass-and elastic-loading as it can be seen on the equation bellow [START_REF] Anisimkin | Elastic properties of thin-film palladium for surface acoustic wave (SAW) sensors[END_REF]. Since only devices using quartz substrates have been used here, changes in electrodes conductivity are not considered as a possible origin of the observed phase velocity drift and therefore were not taken in account in the proposed analysis. Electromechanical coupling of Rayleigh waves on quartz substrates is actually small enough to consider conductivity changes negligible. The following equation has been derived for materials whose crystal structure is "cfc" and does not directly apply for the experiment presented in this paper but it provides a clear illustration of the two main principles governing the observed variations and an (although simplified) analytical formula for estimating the importance of each contribution (elastic and mass loadings).

(1)

In case of H 2 adsorption on Pd layer of thickness h through which a SAW of wavelength λ propagates, both the mass density ρ and the elastic constants C11 decrease, whereas the C44 constant increases. The values of normalized mechanical displacement A i for palladium are reported in [START_REF] Auld | Acoustic fields and waves in solids[END_REF] and [START_REF] Slobodnik | Microwave acoustic handbook[END_REF]. Numerical calculations of hydrogen absorption in Pd layers [START_REF] Anisimkin | Elastic properties of thin-film palladium for surface acoustic wave (SAW) sensors[END_REF] predict an increase of C44 elastic constant, yielding an increase of the phase velocity of the Rayleigh wave propagating under such an overlay. These calculations are in agreement with the experimental observations presented here.

Theoretical study and simulation results

Accurate calculations have been achieved, based on a set of programs dedicated to the analysis of stratified structures used as elastic wave guides. The basics of the corresponding developments have been presented in [START_REF] Reinhardt | Scattering matrix method for acoustic waves in piezoelectric, fluid, and metallic multilayers[END_REF] [START_REF] Pastureaud | Stable scattering-matrix method for acoustic waves in piezoelectric multilayers[END_REF]. The calculation principle is based on the derivation of a surface or interface effective permittivity yielding numerous information about the wave propagation characteristics such as the wave velocity, the mode coupling and possible propagation losses. The model accounts for all the known linear properties of the materials constitutive of the layered structure which directly condition the accuracy of the results as no assumption is made on the device operation (provided flat interfaces). Here are presented simulation results about mass loading effect. One can see on Fig. 6. the influence of a growing water film at the surface of the sensor. A phase decrease is observed. It is noticeable that the phase shift observed on Fig. 7. that is relative to the exposure of the sensor to humidity is consistent with the theoretical result. Indeed, that phase decrease corresponds to a reduction of the velocity of 133.10 -3 m.s -1 which would correspond to the presence, at the top, of the device of a water film that thickness is 4nm which is realistic. On another hand, calculations of theoretical velocity variation of the wave consecutive to changes of elastic properties of the palladium layer have been made. Simulation results presented on Fig. 8. were obtained on the basis of [START_REF] Anisimkin | Elastic properties of thin-film palladium for surface acoustic wave (SAW) sensors[END_REF] results. We can see here the influence of each elastic parameter on the velocity of the acoustic wave. According to [START_REF] Anisimkin | Elastic properties of thin-film palladium for surface acoustic wave (SAW) sensors[END_REF], we considered a decrease of 0,1% and 10% of the density (red solide curve) and the C11 (green triangles) coefficient of the palladium layer respectively. A increase of 40% of the C44 (blue circles) and the induced modification of non diagonals parameters (black squares) were also simulated. The global modification of the acoustic wave velocity is shown on the dashed black curve. One can notice that the velocity increases, upon a complete hydrogenation Fig. 6. Simulated condensation of water on the sensitive zone. (corresponding to X=0.66 (PdH X )), by 1.7m.s -1 . This value takes in account the modification of both Cij and density modifications of the palladium layer. It is also necessary to take into account the gravimetric effect due to H 2 adsorption. This phenomenon, shown on Fig. 9., results in a decrease of the wave velocity of 0.5m.s -1 when considering a complete hydrogenation. As a consequence, the predicted global velocity change is an increase of about 1.2m.s -1 . This result is in accordance with the observation made on Fig. 4. Indeed, the phase shift observed on this figure corresponds to an increase by 2.6m.s -1 of the acoustic wave velocity. The orders of magnitudes are consistent and the difference between these to values (2.6m.s -1 for the experiments and 1.2m.s -1 from the simulation) can be explained by the fact that the palladium layer used here has quite different geometrical properties than the one that was used in [START_REF] Anisimkin | Elastic properties of thin-film palladium for surface acoustic wave (SAW) sensors[END_REF] for the determination of the elastic properties modifications. Moreover, the sensitive layer has been here deposited using sputtering method contrary to [START_REF] Anisimkin | Elastic properties of thin-film palladium for surface acoustic wave (SAW) sensors[END_REF] where thermal evaporation was used.

The influence of humidity and temperature on hydrogen detection

As SAW devices are known to suffer from interference due to humidity and temperature, the influence of these parameters on the sensor operation have been experimentally investigated and the corresponding results are exposed here. The observations reported in this section are expected to provide information allowing for the improvement of the differential acquisition setup. One can see in Fig. 7. that an increase of the relative humidity (RH) of the injected gas causes a massloading effect that results in a decrease of the measured phase of the delay line. Indeed, the condensation of water onto the surface of the device naturally leads to a raise of the mass at the surface of the device (mass loading) yielding the decrease of the experimentally observed phase velocity. As said in the previous section, that behavior is sustained by theoretical observation. Furthermore, a damping phenomenon can clearly by seen on the transfer function of the device when exposed to humidity. Fig. 10. exhibits the impact of temperature variations on the capability of the SAW sensor to detect hydrogen at atmospheric pressure. Temperature changes lead to a decrease of the delay line synchronicity frequency characterized by a sensitivity of -219.10 -3 °.°C -1 . That phase decrease totally compensates the phase shift toward the high frequency observed when detecting hydrogen in absence of any temperature changes. It appears that the delay line used as a reference does not undergo any phase shift when exposed to heated gas since the device is temperature compensated. However, the TCF of the Pd detection channel notably changes as the Pd overlay tends to lower the 1 st order TCF. This behavior currently represents an obstacle to accurate hydrogen detection but can be avoided by the use of an appropriate reference (covered by a layer inert to hydrogen which induces the same TCF change as experienced here for the detection channel). As one can see on the second part of the graph, the detection of H 2 at 2%vol in N 2 is still clearly possible considering a stable operation temperature at room conditions.

Conclusion

In this paper, a differential SAW sensor based on Rayleigh wave on quartz has been developed and successfully tested. The sensor consist of two adjacent delay lines, one being used as a reference and the second one being functionalized with a Pd overlay, a metal known to be affected by gaseous hydrogen. The observation of specific phase shifts on the sensitive track compared to reference in presence of gaseous hydrogen using different gas carrier did assess the operation of the sensor. The use of the Pd overlay actually allows segregating the nature of the gas inserted in the reaction cell and provides quantitative information about its composition. The impact of temperature and humidity on the sensor operation also were analyzed. Although the differential nature of the sensor is expected to reject such interferences, it appears necessary to control this parameters at very minimum. Concerning the analysis of the sensor operation itself, some work still has to be carried out even if strong convictions arise from the experiment that the main change is related to the elastic properties of the Pd film due to hydrogen absorption. The crystalline structure of the film having an effective influence on the way hydrogen interact with the metallic film, it still has to be determined in order to improve simulation accuracy of the leverage of hydrogen absorption on the SAW device response. Toward this aim, further work will consist in the elaboration and characterization of palladium layers with different crystalline structures. Simulation of SAW device exhibiting different sensing layers will also be deepened in order to provide a comprehensive analysis of the experimental observations and to predict the structure of the delay-line to use in order to improve the differential acquisition setup. These investigations are expected to enhance the performance of the proposed sensor in terms of sensitivity and selectivity toward outer parameters such as temperature and humidity.

Fig. 1 .

 1 Fig. 1. Scheme of a SAW delay line using Rayleigh surface acoustic wave at 78MHz.

Fig. 2 .

 2 Fig. 2. Scheme and photograph of a SAW devices probing card.

Fig. 3 .

 3 Fig. 3. Phase shift measurement principle using a dedicated instrumentation.

Fig. 4 .

 4 Fig. 4. Detection of hydrogen at high concentration in air.

Fig. 5 .

 5 Fig. 5. Detection of hydrogen in the range 4% to 1%vol in N 2 .

Fig. 8 .

 8 Fig. 8. Wave velocity shift induced by changes in C11 (green triangles), C44 (blue circles), C12 (black squares), density (red solide line), global shift (black dashed line).

Fig. 9 .

 9 Fig. 9. Phase velocity variation due to hydrogen massloading effect.

Fig. 7 .

 7 Fig. 7. Influence of humidity on the detection of hydrogen at constant room temperature (19.3+/-0.1°C).

Acknowledgements

The authors want to address a special thank to J.M FRIEDT and G. MARTIN for their contributions to the development of the probing electronics. Fig. 10. Influence of temperature on the detection of hydrogen (RH in the 3-8% range).


Marc Loschonsky
email: marc.loschonsky@imtek.de

David Eisele

Jeremy Masson

Matthias Wieneke

Sebastien Alzuaga

Armin Dadgar

Sylvain Ballandras

Alois Krost

Leonhard Reindl

High-Overtone Bulk Acoustic Wave Resonator on Galliumnitride

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

INTRODUCTION

High-overtone bulk acoustic wave resonators (HBARs) devices are based on the effect of an almost total wave reflection at the substrates bottom interface to air at the backside of the substrate. The sputtered piezoelectric resonator layer, such as AlN or ZnO, is in between two metal layers, which are functioning as electrodes. Now, instead of using a sputter deposition process for the piezoelectric resonator layer on top of a thin film metal layer, a MOVPE process was used to guarantee highly and homogenous oriented a-plane GaN. Therefore, MOVPE growth directly on top of the r-plane sapphire substrate is required. Crystal growth of group-III-nitride layers is a currently well established process for light emitting devices and high-power transistors and recently high-frequency application as surface or bulk acoustic wave devices [START_REF] Yoder | Gallium nitride past, present, and future[END_REF], [START_REF] Misra | GaN-Based RF Power Devices and Amplif[END_REF], [START_REF] Müller | GaN micromachined FBAR structures for microwave applications[END_REF]. These works are mainly focused on c-plane oriented material like c-plane AlN or GaN most on silicone or some on c-plane sapphire substrates. Typical growth methods are molecular beam epitaxy (MBE) and MOVPE, the latter widely used in mass production. The layers are usually c-axis oriented when grown on the surfaces of sapphire ( 00 ), silicon carbide ( 01 1120 ) or silicon (111 ). By using r-plane sapphire or a-plane SiC, it is possible to switch the growth orientation and obtain a-plane GaN or AlN layers [START_REF] Lei | Heteroepitaxy, polymorphism, and faulting in GaN thin films on sil substrates[END_REF]. This is presently an active research topic with the focus on improving the efficiency of green LEDs. In LEDs grown with c-axis orientation, the piezoelectric polarization fields along the growth axis lead to a reduction in quantum efficiency, especially for longer wavelength LEDs.

The benefit of epitaxial growth as MOVPE compared to most other deposition techniques are a high crystalline quality, a low defect density, and usually a very good morphology. This also leads to improved acoustic properties, especially since the layer is a close to a single crystalline film, having only few boundaries, which originate in the initial island type growth. MOVPE grown GaN, therefore, seems to be an interesting candidate. Also, the MOVPE process was modified to enabling the growth of a conductive crystal layer [START_REF] Nishimura | a-plane GaN Shear Wave Thin Film Resonator[END_REF] serving as integrated buried electrode.

II. MOVPE GROWTH OF GALLIUM NITRIDE FOR THE DIFFERENT FUNCTIONAL LAYERS OF AN HBAR

To accomplish a-plane oriented GaN layers MOVPE growth was performed on epi-polished r-plane (10 12 ) sapphire substrates of 330 µm thickness. The MOVPE system is an AIXTRON AIX 200-4 RF/S with optical ports for in-situ measurement of curvature and layer thickness. To enable epitaxial growth, at first a ~20 nm GaN seeding layer was grown at ~700°C, followed by a 100 nm GaN:Si layer with a Si-concentration of more than 2E18 acting as shallow donors. This conductive crystalline layer served as a buried electrode. In the following, a pure 2 µm GaN layer was grown at 200 mbar and 1045°C. Both process steps were using standard high purity precursors as trimethyl-gallium and ammonia with high purity carrier gases H 2 and N 2 . Finally, a second GaN:Si layer was grown with the same concentration of Si as the first conductive layer, serving as a top electrode for the HBAR (Figure 1). The influence of the added Si-atoms for achieving a conductive layer degraded the surface morphology of the resulting grown crystal surface. The low thickness of 100 nm of the GaN:Si top layer did not allow any chemical or mechanical assisted polishing process after growth, therefore the MOVPE growth process itself had to be further improved to be competitive with already achieved previous results, refer to Figure 3, for providing a comparable surface quality as shown in Figure 2 III. DESIGN AND PROCESSING OF A-PLANE GAN HBARS A rectangular design was chosen for the HBARs where the resonator and electrode size was varied from 200 µm up to 400 µm in steps of 25 µm on each side. After the initial MOVPE crystal growth processes, a lithography step followed. To etch the resonators and the electrodes a chemical assisted ion beam etching process (CAIBE) was used. As contact pads to the GaN:Si electrode layers a 100 nm thick gold layer with a chromium adhesion layer was deposited by physical vapour deposition (PVD) process, followed by a classical wet etching of the contact pads.

Two different process designs where realised to demonstrate the feasibility of conductive GaN:Si layers serving as electrodes, as shown in

IV. SIMULATION AND MEASUREMENT

A. Simulation of the HBAR devices

The HBAR structure was simulated by a 2D-simulation software based on a scattering matrix method for multilayered media [8], [START_REF]IEEE/Cornell Conference on Advanced Concepts in[END_REF], which itself based on the transfer matrix method from Fahmy and Adler [START_REF] Fahmy | Propagation of acoustic surface waves in multilayers: A matrix description[END_REF]. Both a perfect reflecting and a non-reflecting backside of sapphire were simulated in a frequency range of 0 GHz up to 4 GHz and the static capacitance C 0 =3.8 fF was subtracted (Figure 5). Only the perfect reflecting backside condition enables the high overtones. By applying a FFT to the admittances, the respective impulse responses are obtained (Figure 6). The internal resonance in the piezoelectric layer splits into two shear and one longitudinal component wave at the bottom electrode-sapphire interface. The first three reflections in Figure 6 a) are the received first order reflections of these components, which are missing in Figure 6 b), because of the non-reflective backside. The corresponding bulk wave velocities in the sapphire are about 10707 m/s (fast shear), 6871 m/s (slow shear) and 5742 m/s (longitudinal).

B. Measurements of the HBAR devices

RF measurements and analysis were performed of the fabricated HBAR devices (Figure 7). The expected high overtones were missing and only a conventional resonance with a low quality factor Q of about 5 was found. The analysis of the impulse response show that the reflections almost totally disappeared. Examination of the internal resonance of the piezoelectric layer (Figure 8) indicates a slightly higher reflection between bottom electrode and sapphire than indicated by simulation. Additional simulations of the slowness curves of r-plane sapphire were made (Figure 9) and the directions of the power flow of the three bulk waves were investigated:

Fast shear wave:

x-tilt of -13°, y-tilt of -5° Slow shear wave: x-tilt of -6°, y-tilt of +7° Longitudinal wave: x-tilt of -4°, y-tilt of -17°O ne reasonable explanation for the missing of the high overtones and the conventional resonance with a Q of about 5 might be the crystal orientation of the r-plane sapphire itself. The r-plane orientation might deflect the acoustical wave in this way that reflections are strongly prevented. In addition, scattering effects at the interface of the GaN:Si and GaN crystal seed layer on the sapphire are possible and will be further investigated.

C. Temperature coefficient of resonance frequency

The temperature coefficient of frequency for different HBARs were measured with a Digit Concept Thermal Chuck on a Cascade Summit probe station in a temperature range from room temperature up to 200°C. For frequency analysis, the first resonance of each HBAR was measured with a ZVA-24 vector network analyzer from Rhode&Schwarz.

As shown in the different graphs of Figure 10 all resonators exhibit a negative temperature coefficient of frequency in a range of -13 to -17 ppm/K. The temperature coefficient of frequency seems to vary periodical with rising temperature. The elasticity of GaN with built in Si atoms may influence its elastically characteristics. V. CONCLUSION High-overtone bulk acoustic resonators acoustic wave devices based on MOVPE grown crystalline gallium nitride thin films with a-plane orientation gallium nitride were successfully fabricated and characterized, but there behavior totally deferred from what would have been expected. For a first understanding, additional simulations were and will be further achieved, considering the influence of r-plane sapphire's crystal orientation. In addition, the influence of the built-in Si atoms within the GaN crystal lattice to enable GaN becoming a conductive material will be analyzed in further research. This conductive crystalline layer could also affect the temperature coefficient of frequency with rising temperature due to its completely changed elastic behavior. Further work will be focused in extracting and measuring material parameters of gallium nitride thin films for matching simulation with measurements and an expansion of the focus of our work on other MOVPE grown a-plane oriented piezoelectric III-V compound semiconductors lik aluminum nitride. alkani for fruitful discussions along time for this work.

Figure 1

 1 Figure 1 Schematic view of layer stack of an HBAR with the corresponding thicknesses and the material composition. Not shown is the thin GaN crystal seed layer which is grown at first atop the r-plane sapphire.

Figure 2 Figure 3

 23 Figure 2 Two 3D surface profilometry scans of a GaN:Si surface whichshows both the surface quality of a 1.5x14.7 mm 2 scan field. The perspective of the first one is indicated by the perspective light, the second is an obvious perspective and shows from brown to white coloured minima and maxima. The roughneess value were found to be pretty good at R a = 42.62 nm, R q .= 72.92 nm and R t = 4.54 µm.

Figure 4

 4 Figure 4 Two different process designs for demonstration. left side: two resonators in series connected with an integrated GaN:Si conductive layer and two Cr/Au contact pads, right side: one single resonator connected with a contact pad atop the resonator and atop the integrated GaN:Si conductive layer, serving as a bottom electrode.

Figure 5

 5 Figure 5 2D-Simulated HBAR admittances, a) perfect reflecting b) nonreflecting backside of sapphire. The imaginary part of the static capacitance C 0 =3.8 fF was subtracted.

Figure 6

 6 Figure 6 2D-Simulated HBAR impulse responses normalized, a) perfect reflecting b) non-reflecting backside of sapphire.

Figure 7

 7 Figure 7 Measurement of fabricated HBAR, a) admittance b) normalized impulse response. The imaginary part of the static capacitance C 0 =0.143 fF was subtracted.

Figure 8 Figure 9

 89 Figure 8 Comparison of normalized impulse responses of a) simulated b) measured HBAR. Only the first nanoseconds with the internal resonance of the piezoelectric layer is shown.

Figure 10

 10 Figure 10 Temperature coefficient of resonance frequency at 25°C of different HBAR prototypes.

  iers," Proceedings of the IEEE, vol. 96, erlattices and icon and sapphire IDTs," Electronics Letters , vol.42, no.1, 5 Jan. 2006, pp. 62-63.

e [ 6

 6

ACKNOWLEDGMENT R. B. G. thanks Michael Wandt, Armin Baur, Michael Reichel, Nico Lehmann, Philip Katus, Christopher W. Hodges and Maya M the [8] A. Reinhardt, T. Pastureaud, S. Ballandras, and V. Laude, "Scattering matrix method for modelling acoustic waves in piezoelectric, fluid, and metallic multilayers," J. of Applied Physics, vol. 94, no. 10, 2003, pp. 2544-2546.


Jérôme Vasseur

Marc Hanna

John Dudley

Member, IEEE Jean-Pierre Goedgebuer

Alternate Multiwavelength Modelocked Fiber Laser

Keywords: Laser tuning, mode locked lasers, optical communication, optical fiber lasers, pulsed lasers

We demonstrate alternate mode-locking of three 30-ps pulse trains at three different wavelengths, each at a repetition rate of 3.8 GHz. The successive multiwavelength pulsed operation is reached in an actively mode-locked erbium-doped fiber laser in which an unbalanced Mach-Zehnder interferometer has been introduced. This component plays both the role of intensity modulator and tunable filter.

I. INTRODUCTION

T HE GENERATION of multicolor optical pulse trains has been intensively studied recently to obtain pulses at several wavelengths simultaneously or alternatively in the 1550-nm window. The main motivation for such sources is that the costs of current systems may be considerably reduced by using a single laser emitting simultaneously at different wavelengths. Such multiwavelength lasers are cost-effective sources for wavelength-division-multiplexing systems but also time-resolved spectroscopy and optical component characterization. In addition to being all-fiber systems, erbium-doped fiber amplifiers (EDFA) are adequate gain media for multiwavelength operation due to their wide gain curve. Several methods have been proposed to obtain multiwavelength operation in fiber lasers: the use of birefringent cavities [START_REF] Schlager | Dual wavelength pulse generation using mode-locked erbium-doped fiber ring laser[END_REF] or of second-order nonlinearity with comb filters [START_REF] Sun | Multiwavelength generation by utilizing second-order nonlinearity of LiNbO waveguides in fiber lasers[END_REF], the dispersion tuning method [START_REF] Lee | Alternate and simultaneous generation of 1-GHz dual-wavelength pulses from an electrically tunable harmonically modelocked fiber laser[END_REF], the introduction of fiber Bragg gratings [START_REF] Yao | Active mode locking of tunable multi-wavelength fiber ring laser[END_REF], [START_REF] Town | Dual wavelength modelocked fiber laser[END_REF] in the cavity, or more recently, the use of a frequency shifter combined with a Fabry-Pérot periodic filter [START_REF] Town | Applications of chirped grating filters in broadband optical fiber systems[END_REF], [START_REF] Maran | Erbium-doped fiber laser simultaneously mode locked on more than 24 wavelengths at room temperature[END_REF]. In this letter, we report the design and implementation of an alternate multiwavelength picosecond pulsed fiber laser that generates three pulse trains at three different wavelengths alternatively. This laser is actively mode-locked at a repetition rate of 3.8 GHz using an unbalanced Mach-Zehnder interferometer (UMZI). The UMZI plays both the role of optical filter and intensity modulator (IM). Results obtained in a laser with both the UMZI and a conventional IM are also presented. These sources might allow the implementation of two-dimensional (2-D) wavelength-time optical code-division multiple-access (OCDMA).

II. PRINCIPLE

The experimental configuration of the laser is shown in Fig. 1. It consists of a gain-flattened EDFA (Keopsys model 1 060 234), J. Vasseur and M. Hanna are with GTL-CNRS Telecom, UMR CNRS 6174, 57070 Metz, France (e-mail: jvasseur@georgiatech-metz.fr; mhanna@georgiatech-metz.fr).

J. Dudley and J.-P. Goedgebuer are with Laboratoire FEMTO-ST, UMR CNRS 6174, 25030 Besançcon Cedex, France (e-mail: john.dudley@univfcomte.fr; jean-pierre.goedgebuer@univ-fcomte.fr). a polarization controller, an unbalanced MZI (Photline Technologies custom order), and a 50/50 output coupler.

The cavity dispersion was measured to be less than 1 ps/nm/km, allowing mode-locking on a wide range of wavelengths. The UMZI is an integrated annealed proton exchanged LiNbO 10-Gb/s modulator that includes a fixed optical path difference of 40 m between its two arms. When no radio-frequency signal is applied onto the UMZI, it behaves as a spectral filter, with a sinusoidal frequency transfer function and a free spectral range (FSR) of 60 nm. Upon application of a time-varying electrical signal, this transfer function shifts accordingly. Equivalently, each wavelength sees a different temporal transfer function that depends on the bias of the MZI and the frequency and modulation depth used. This results in a wavelength-dependent mode-locking condition and in an alternate generation of pulses at different wavelengths. Considering the input field amplitude , with in the neighboorhood of a reference angular frequency , the transfer function of the UMZI is proportional to [START_REF] Schlager | Dual wavelength pulse generation using mode-locked erbium-doped fiber ring laser[END_REF] where is the delay introduced by the MZI, assumed to be of the form . We choose the reference angular frequency, , to be a maximum of transmission for the filter at :

, with an integer. Since , the transfer function is maximum for the angular frequency that satisfies . Using a Taylor expansion, can be evaluated as (2) Fig. 2 shows the theoretical evolution of the maximum of the transfer function of the UMZI as a function of time (with s, rad/s, rad/s, and s to fit the experimental parameters). Note that a similar principle was first used by Olsson and Tang [START_REF] Olsson | Time-wavelength multiplexing of modelocked external cavity semiconductor lasers[END_REF] in a bulk laser with separated tunable filter and mode-locker elements.

III. EXPERIMENTAL RESULTS

Fig. 3(a) and (b) display the temporal trace and spectrum of the signal at laser output, respectively. For a repetition rate of 3.8 GHz, three successive trains at three different wavelengths ( nm, nm, and nm) are observed using an optical spectrum analyzer with 0.07-nm resolution and a sampling oscilloscope with a 30-GHz photodiode. The wavelength of each pulse is identified by adding a 1.2-nm bandpass tunable optical filter at the laser output and the pulsewidths were recorded to be 24, 25, and 38 ps, yielding a time-frequency product of 0.81, and 1.85, respectively. Since the center wavelength of the filter varies with time, the generation of chirped pulses is expected. The three wavelengths' operation is stable for some hours at a constant room temperature. Nevertheless, since no stabilization system has been implemented, fluctuations of the peak power are observed. The time locations of these emitted wavelengths is plotted on Fig. 2. Experimental behaviors of and are in good agreement with the expected theoretical behavior. , however, is not accounted for by the filter transfer function. This might indicate that, for this filter position, the overall frequency selection is dominated by the EDFA transfer function. As shown in Fig. 4, the three pulse trains-three colors operation of the laser has been observed at different harmonics of the frequency axial mode separation for a range from some megahertz up to 3.8 GHz. As expected in a mode-locked configuration, the pulsewidth decreases with an increase of the modulation frequency [START_REF] Yariv | Optical Electronics in Modern Communications[END_REF].

Typically, the large homogeneous broadening occuring in the EDFA causes mode competition in a 10-nm window around each laser oscillation and prevents emission of closely separated wavelengths [START_REF] Chow | Multiwavelength generation in an erbium-doped fiber laser using in-fiber comb filters[END_REF], [START_REF] Sun | Multiwavelength erbium-doped fiber lasers exploiting polarization hole burning[END_REF]. Nevertheless, as mentioned in [START_REF] Town | Dual wavelength modelocked fiber laser[END_REF], temporal-spectral multiplexing helps overcome homogeneous broadening in multiwavelength pulsed lasers. In this laser, the wide separation of the emitted wavelengths (about 6 and 12 nm) seems to be inherent to the specific mode-locking technique used. The large FSR of the UMZI and the necessary adequation between the location of the pulse in the time domain and its spectral content, in the absence of any other constraint, are imposing such widely separated wavelengths. The exact emitted wavelengths are determined by local extrema of the gain curve of the EDFA.

IV. DISCUSSION

In order to have a better control on the timing of the emitted pulses, a conventional IM, integrated on LiNbO , is added in the ring cavity. The IM is synchronously driven at an integer multiple of the UMZI modulation frequency. It locks the emission of pulses on a temporal grid, while the central wavelength is determined by the UMZI. Fig . 5 shows the experimental results when the modulation frequency of the IM GHz is five times the modulation frequency of the UMZI MHZ). Two series of pulses at two different wavelengths ( nm and nm) are obtained. This confirms the possible laser emission at closely spaced wavelengths. The temporal pulses at and are separated by . The other locations on the temporal grid are left unoccupied, probably because the two emitted wavelengths capture all the available gain. In this case, the pulsewidths are recorded to be 50 and 45 ps, yielding a time-bandwidth product of 0.56 and 0.48, respectively. The birefringence of the IM combined with the polarizing behavior of the UMZI acts as a filter that imposes a wavelength spacing of 0.7 nm. This could be a means of anchoring the emitted wavelengths on the ITU grid.

Conversely, one could impose a constraint in the frequency domain by inserting a periodic filter in the cavity. The use of an intracavity Fabry-Pérot filter in conjunction with an acoustooptic frequency shifter [START_REF] Maran | Erbium-doped fiber laser simultaneously mode locked on more than 24 wavelengths at room temperature[END_REF] might enable us to force laser oscillations at closely spaced wavelengths, still in an alternate mode.

V. C ONCLUSION

We have demonstrated alternate pulsed operation at three wavelengths by use of a UMZI. The proposed method requires a limited number of optical components. By emitting successive pulses at different wavelengths, codes associating one time slot with one wavelength can be implemented. This might enable the implementation of dynamic 2-D OCDMA systems [START_REF] Salehi | Code-division multiple-access techniques in optical fiber networks-part I: Fundamental principles[END_REF], [START_REF] Tancevski | Wavelength hopping/time spreading code division multiple access[END_REF]. Further work will include the stabilization of the obtained pulse trains, the control of the emitted colors/pulse temporal locations, and the increase of the number of trains and wavelengths that can be emitted at higher frequencies.

Fig. 1 .

 1 Fig. 1. Schematic diagram of the alternate multiwavelength laser.

Fig. 2 .

 2 Fig. 2. Theoretical evolution of the maximum of the transfer function of the UMZI as a function of time (solid line) and experimental behaviors (points).

Fig. 3 .

 3 Fig. 3. (a) Time trace and (b) spectrum at the laser output.

Fig. 4 .

 4 Fig. 4. Full-width at half-maximum pulsewidth as a function of mode-locking frequency for the three emitted wavelengths ( =1 5 4 3 :2 nm, = 1549:6 nm, and = 1562:5 nm).

Fig. 5 .

 5 Fig. 5. (a) Time trace and (b) spectrum at the laser output in the presence of a conventional IM.