Ballandras

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

Acoustic Wave Filter Based on Periodically Poled Lithium Niobate

Emilie Courjon, Florent Bassignot, Gwenn Ulliac, Sarah Benchabane, and Sylvain Ballandras

I. I

T  demand for highly coupled high-quality acoustic wave devices has generated strong innovative activity, yielding the investigation of new excitation principles [START_REF]Advances in Surface Acoustic Wave Technology, Systems And Applications[END_REF]. Recently, we have investigated periodically poled transducers (PPTs) on single-crystal lithium niobate (LiNbO 3 ) Z-cut plates. A PPT consists of two electrically conductive media embracing a periodically poled ferroelectrics layer [START_REF] Courjon | Lamb wave transducer built on periodically poled Z-cut LiNbO 3 wafers[END_REF]. Its fabrication is performed by locally forcing the LiNbO 3 ferroelectric polarization using a poling bench [START_REF] Myers | Quasi-phase-matched optical parametric oscillator[END_REF]. The poling bench mainly consists of a high-voltage amplifier used to submit the wafer to an electric field via liquid electrodes (a saturated lithium chloride solution) strong enough to override the material coercive field and, hence, to invert domains defined by an in situ photo-resist mask. PPTs then allow for the excitation of symmetrical Lamb modes [START_REF] Courjon | Lamb wave transducer built on periodically poled Z-cut LiNbO 3 wafers[END_REF] with operating frequencies twice as high (considering a given mechanical period) as the operating frequencies obtained using SAWs excited by standard interdigital transducers (IDTs). This principle has been successfully implemented and tested for the excitation of waves at frequencies ranging from a few megahertz [START_REF] Ballandras | A novel surface wave transducer based on periodically poled piezoelectric domain[END_REF] up to several gigahertz [START_REF] Sarin Kumar | A high frequency surface acoustic wave device based on thin film piezoelectric interdigital transducers[END_REF].

In this work, the fabrication of PPTs on single-crystal LiNbO 3 Z-cut substrates is proposed to develop acoustic filters. For that purpose, the possibility of transposing the well-known principle of transversal SAW filters (based on tapered IDTs [START_REF] Morgan | Wave Devices for Signal Processing[END_REF]) to PPTs has been considered. The interest of this approach relies on the possibility of controlling the impulse response of the transducer using plane electrodes which are easy to shape according to the intended spectral function. Because this technology is revealed to be more robust than IDT-based solutions (particularly against short circuit and lithography defects) [START_REF] Courjon | Lamb wave transducer built on periodically poled Z-cut LiNbO 3 wafers[END_REF], [START_REF] Ballandras | A novel surface wave transducer based on periodically poled piezoelectric domain[END_REF], [START_REF] Sarin Kumar | A high frequency surface acoustic wave device based on thin film piezoelectric interdigital transducers[END_REF], it should represent an attractive alternative for the development of RF filters. PPTs exhibiting a poling period of 40 µm have been successfully fabricated for the experimental assessment of the proposed approach. Several designs of electrodes have been tested to fabricate a band-pass filter operating near 100 MHz. Test devices have been measured using an RF tip-probing machine. Although these devices did exhibit multi-mode responses because of the thickness of the poled LiNbO 3 plate, typical filter responses have been observed and analyzed. Furthermore, acoustic-optical measurements have been performed on the corresponding devices, enabling observation of the actual distribution of the excited wave under the electrodes. These measurements enable the definition of the appropriate electrode design to fabricate an acoustic filter.

The first section of the paper is devoted to a brief review of PPTs' basic principles and specific features. Simulation fundamentals and basic fabrication methods devoted to PPTs are also summarized to introduce the filter implementation sections. Thus, the design of the filter is described first, followed by a report of the experimental characterization of the test devices, yielding discussions on the actual operation of PPT-based band-pass filters. As a conclusion, further developments of such devices are considered.

II. P P T

A. PPTs Versus IDTs

A PPT is based on a periodically poled piezoelectric substrate metalized on its both sides to obtain a capacitive dipole in which an elastic wave can be excited by phase construction [START_REF] Courjon | Lamb wave transducer built on periodically poled Z-cut LiNbO 3 wafers[END_REF]. Fig. 1 presents a comparison between classical IDTs and PPTs.

PPTs take advantage of two principal features compared with standard periodic interdigitated electrode transducers deposited atop piezoelectric materials. The first feature is the robustness of the excitation to defects or surface contamination (nondistributed electrodes with separated polarity on each side of the substrate secure the excitation and no passivation is required to avoid short-circuit as in the case of standard IDTs). The second feature is the possibility of exciting waves exhibiting a wavelength equal to the poling period, contrary to standard IDTs, for which the wavelength is twice the mechanical period of the grating (the Bragg condition). This characteristic, however, implies that the device operates in a second-harmonic regime, yielding the superimposition of progressive direct and backward modes. For this situation, one may expect some operational differences compared with standard IDTs. This point will be discussed in the sections devoted to filter design and manufacture.

B. PPT Simulation

A PPT can be defined by a period equal to the acoustic wavelength, a thickness, an acoustic aperture and an overall length. Because the acoustic aperture usually is much larger than the thickness of the plate and the period of the grating, a 2-D analysis can be advantageously implemented without any loss of generality. Because of the inhomogeneous nature of the material properties of the periodically poled substrate, numerical computation tools such as finite element analysis (FEA) are well adapted to predict the electrical response of PPTs. Furthermore, PPTs' structures are often periodic, with several tenths of periods along the transducer length. Therefore, meshing only one period of the transducer consisting of two rectangles representing the two opposite ferroelectric domains and considering periodic boundary conditions allows for reliable and effective computations of PPTs harmonic admittance [START_REF] Ballandras | Finite-element analysis of periodic piezoelectric transducers[END_REF]. The mesh used in our simulations is shown in Fig. 2.

As explained previously, all of this area must be meshed because of the heterogeneous piezoelectric prop-erties of one PPT period. According to the definition in the IEEE Std-176 standard [8], the Z+ polarized domain corresponds to the (YXl)/90° cut whereas the second domain (Z-polarized) is a (YXl)/-90° cut. The numbers on the mesh edges correspond to the references characterizing the boundary conditions applied to the mesh limits. In our case, the references labeled 1 and 2 correspond to the applied electric potential and the references labeled 3 and 4 are representative of the periodicity conditions. The simulations based on periodic FEA allow derivation of the harmonic admittance of the devices [START_REF] Ballandras | Finite-element analysis of periodic piezoelectric transducers[END_REF], which provides information about the PPT's characteristics. One can determine the nature and the synchronization frequency of the excited waves, but also the electromechanical coupling of the corresponding mode and dispersion properties, considering appropriate assumptions [START_REF] Courjon | Pure longitudinal plate mode excited by poled domains transducers on LiNbO 3[END_REF]. Fig. 3 shows the harmonic admittance of a 500-µm-thick LiNbO 3 PPT with a 50-µm period.

Two main contributions are indicated. The first mode is excited at 76 MHz [corresponding to a phase velocity (Vφ) equal to 3800 m•s -1 ] and the second mode at 131 MHz (Vφ = 6550 m•s -1 ). The polarization of these modes has been determined by our FEA simulation tool [START_REF] Ballandras | Finite-element analysis of periodic piezoelectric transducers[END_REF]. The first mode can be compared with the fundamental symmetrical Lamb waves (usually called S 0 ). Because the particle motions on each side of the plate exhibit an elliptical shape, this wave is called an elliptic wave hereafter. The second mode is similar to a longitudinal mode with an extension and compression motion of all of the material points of the structure along the propagation direction, and is therefore called a longitudinal wave [START_REF] Courjon | Pure longitudinal plate mode excited by poled domains transducers on LiNbO 3[END_REF]. In both cases, the resonance occurs at the end of the stopband, although the notion of stopband here is more difficult to comprehend than in the case of SAW operating at Bragg condition. In the later case, the stopband is usually representative of the wave scattering onto the periodic obstacles (electrodes). For PPTs, one can assume that the stopband provides information about dispersion properties of both progressive direct and backward waves forming the mode. Therefore, the width of the frequency stop-band gives an indication of the effectiveness of the mode confinement within the grating.

C. PPT Fabrication

A periodically poled transducer is fundamentally based on a periodically poled piezoelectric medium. Such a periodically poled structure can be advantageously achieved on ferroelectric materials such as lead zirconium titanate (PZT) [START_REF] Ballandras | A novel surface wave transducer based on periodically poled piezoelectric domain[END_REF], [START_REF] Sarin Kumar | A high frequency surface acoustic wave device based on thin film piezoelectric interdigital transducers[END_REF] because of the rather small value of its coercive electric field (the absolute value of the electric field above which the spontaneous polarization can be inverted, close to 5 kV/mm in the case of PZT), or lithium tantalate [START_REF] Kadota | Acoustic wave devices using periodical poled Z-cut LiTaO 3 plate[END_REF].

As previously presented, all of the test devices have been manufactured using a dedicated poling bench (inspired by [START_REF] Myers | Quasi-phase-matched optical parametric oscillator[END_REF]) which allows for controlling the poling of optical quality Z-cut LiNbO 3 plates. It mainly consists of a high-voltage amplifier used to submit the ferroelectric wafer to an electric field strong enough to invert the negative polarization of the material. The coercive field of lithium niobate is approximately 21 kV/mm [START_REF] Wengler | Poling dynamics of lithium niobate crystals[END_REF], [START_REF] Kim | Coercive fields in ferroelectrics : A case study in lithium niobate and lithium tantalate[END_REF]; consequently, the voltage needed to invert the domains is approximately 11 kV for 500-µm-thick wafers. The domains to be poled are defined using a photo-resist pattern on top of a plate surface, as proposed in [START_REF] Myers | Quasi-phase-matched optical parametric oscillator[END_REF]. A set of masks has been used for that purpose with poling periods (i.e., acoustic wavelengths) equal to 10, 20, 40, and 50 µm (corresponding to 5-10-, 20-, and 25-µm line widths, respectively). The plate is held in a poly(methyl methacrylate) (PMMA) mounting by means of two O-ring joints which create two cavities filled with a saturated lithium chloride solution used as a liquid electrode (as shown in the scheme of Fig. 4).

An appropriate poling sequence is applied to stabilize the final material state [START_REF] Courjon | Lamb wave transducer built on periodically poled Z-cut LiNbO 3 wafers[END_REF], [START_REF] Myers | Quasi-phase-matched optical parametric oscillator[END_REF], providing reproducible operating conditions and effective poling yields. The poling result can be easily controlled by a simple optical postobservation, because it generates a contrast at the edge of the poled domains.

D. Experimental Results and Comparison With the Theory

Each device was found to operate according to theoretical predictions and allowed for electrical measurements. Fig. 5 shows the comparison of experimental and theoretical admittances of a 50-µm-period PPT.

Although the response is somewhat difficult to interpret because of numerous parasitic bulk wave residuals, two strong contributions are observed at 76 and 130 MHz. The first mode corresponds to the elliptic mode and the second mode to the longitudinal mode. It is important to note that the latter cannot be excited with a classical IDT, because the corresponding boundary conditions are difficult to meet with such a transducer. Even if the two modes are slightly shifted considering theoretical predictions, this plot points out the agreement between the results. In a first-order approximation, it is possible to calculate the equivalent phase velocity of each contribution, yielding values of 3800 and 6500 m•s -1 respectively (in agreement with the predicted values reported in Section II-B). We can, however, observe a difference of magnitude between the theoretical and experimental results, which is mainly the result of acoustic losses defined in the simulation tool.

III. D   PPT-B F

Considering the predictive and manufacturing capabilities, the use of PPTs for the fabrication of band-pass filters in place of IDTs has been investigated. For that purpose, the idea was to attempt to transpose the usual transversal SAW filter techniques [START_REF] Morgan | Wave Devices for Signal Processing[END_REF] to PPTs, taking advantage of the ease of patterning shaped electrodes according to the desired impulse response.

A. Analogy Between IDT-Based Filters and PPT-Based Filters

Fig. 6(a) presents a classical band-pass IDT-based filter. It consists of two IDTs deposited on a piezoelectric substrate. The transducers are made of interleaved metal electrodes which are used to launch and receive the waves, so that an electrical signal is linearly converted to an acoustic wave and then back to an electrical signal using fundamental piezoelectric properties. A basic advantage is that acoustic waves travel very slowly (typically 3000 m•s -1 ), so large delays can be obtained with smaller device dimensions compared with electromagnetic delay lines. In designing a SAW filter, the overall frequency response characteristics are determined by deriving two impulse responses for the two transducers, whose transforms are added together in decibels [START_REF] Royer | Ondes Élastiques Dans les Solides, Tome 2 : Génération, Interactions Acousto-Optiques, Applications[END_REF].

In this configuration, a rectangular frequency response should be obtained. Therefore, this concept has been used to design our PPT-based filters [see Fig. 6(b)] assuming that the waves emitted by PPTs actually are propagating like IDT-excited waves.

In our PPT-based filters, the electrode shapes are designed on previously poled substrates according to the desired frequency operation. To validate the principle, it was first decided to design basic electrode shapes using rectangles and diamonds. In theory, with those transducers, the frequency responses should be obtained as:

• (sin X/X) 2 for a filter based on 2 rectangular electrodes, • (sin X/X) 3 for a filter based on one rectangular electrode and one diamond-shaped electrode, • (sin X/X) 4 for a filter based on 2 diamond-shaped electrodes.

The frequency response for those 3 different filters is presented in Fig. 7. Although usually geometric tapering of both IDTs is not preferred using standard SAWs (one is geometrically tapered and the second one is functionalized using source suppression), all of these configurations were tested to exemplify the actual operation and interaction of PPTs.

B. Fabrication of the PPT-Based Filters

The first step of the fabrication consists of poling the piezoelectric substrate; in our case, a 3-in (76.2-mm) LiN-bO 3 wafer. A 40-µm poling period was used, corresponding to the excitation of the elliptically polarized wave near 100 MHz. Several electrodes of various shapes and lengths were then patterned on the different poled areas to test many configurations. Several different gaps between the input and output electrodes have also been studied. The chosen design is shown in Fig. 8. Finally, the wafer is diced so that each poled area has access to enough electrodes on each device (typically 2 to 4) for testing purposes.

IV. E R

A. Acoustic Measurements

Transmission measurements have been performed by means of a tip-probing bench linked to a ZVC network analyzer (Rohde & Schwarz, Munich, Germany). Although all the transducers exhibited reflection responses in agreement with theoretical predictions, only one device was found to present a significant transfer function according to the expected filtering function. This device was composed of two rectangular electrodes whose characteristics are:

• First rectangle electrode length: 10 λ • Gap between the electrodes: 15 λ • Second rectangle electrode length: 20 λ As mentioned in the preceding sections, this device exhibited a multi-mode response because of the LiNbO 3 thickness. However, two contributions are easily observed at around 88 and 95 MHz. When zooming-in near the first one, the typical signature of a band-pass filter can be observed (see Fig. 9).

The characteristics of this filter are: a center frequency of 88 MHz, a passband width equal to 350 kHz and insertion losses of about 1.6 dB. Furthermore, two peaks can be clearly observed on the top of the passband width. It seems that two modes contribute to the response, which suggests that the filter is based on a coupling mode mechanism.

To validate this hypothesis, the reflection coefficient, S11, of one of the transducers was measured; see Fig. 10.

This plot actually shows two principal reflection contributions coinciding with the previously mentioned peaks of the transfer function. Because the distance between the two transducers is relatively small (only 15λ), the possibility of coupling energy from another transducer via evanescent wave behavior is realistic. This reinforces the interpretation of the stopband of the transducer dispersion curve even in the second-harmonic regime (stationary wave conditions). Furthermore, it would explain the operation of the filter reported in [START_REF] Kadota | Acoustic wave devices using periodical poled Z-cut LiTaO 3 plate[END_REF] on periodically poled lithium tantalite, which exhibits a behavior very close to the one described here. To better understand the transducer operation, optical measurements have been performed to observe the acoustic vibration distribution under and near the PPT. One can also observe a resonance with extremely low insertion losses (about 0.3 dB) at 95 MHz (see Fig. 11). Although not well explained, this result is very encouraging because such a resonator could be advantageously used to fabricate an oscillator because the unloaded quality factor was revealed to be close to 50 000 at about 100 MHz.

To better understand the transducer operation and the way the filter works, acoustic-optical measurements were performed to observe the acoustic vibration distribution under and near the PPT.

B. Acoustic-Optical Measurements

As previously explained, acoustic-optical measurements were implemented here to analyze the mechanical wave distribution during the electrical excitation of an electrode. The experimental setup used for the measurements of the elastic wave displacement is a scanning optical heterodyne interferometer similar to the one described in [START_REF] Vairac | New structures for heterodyne interferometric probes using double-pass[END_REF], but here only allowing amplitude detection. The probe basically consists of a Michelson interferometer folded into a single arm, in which the output coupler of the laser is used as one of the mirrors constituting the interferometer. A schematic of this heterodyne laser vibrometer is displayed in Fig. 12.

The 500-µm-thick 40-µm-period transducer was then characterized using this experimental protocol. The upper electrode corresponds to a few tens of wavelengths metal layer deposited on a several-millimeter-long periodically poled area.

Only modes having a significant deformation along the axis normal to the plate (here the Z-axis) are detectable by the laser vibrometer. Therefore, the distribution of the elliptic mode propagation at 95 MHz was particularly observable during this experiment. Fig. 13 shows the amplitude variation of the material vibrations around the excited electrode.

A maximum of vibration amplitude of about 0.2 nm can be observed with a waviness actually corresponding to the grating periodicity. Moreover, one can observe that the vibrations are mainly localized under the electrode, although some vibration traces are recorded beside the excited surface.

For an in-depth analysis of this phenomenon, a graph representing a cross-section of the previous figure along the y-axis has been plotted (see Fig. 14). This graph confirms that the maximum of vibration amplitude is located under the electrode. These vibrations exponentially decrease over a distance of 1 mm on both sides of the electrode. This measurement confirms that the elastic waves excited by a PPT are quasi-standing waves which extend only a few wavelengths outside the electrode regions. Finally, these observations are in agreement with the operation principle reported in Section I-B. The excitation of the grating is achieved for a second-harmonic operation condition of the grating, yielding the superimposition of direct and backward waves, which does not allow for launching waves in a manner similar to IDTs. This result explains why the only operating PPTbased filter is the one whose electrodes are the closest, allowing for a mode-coupling operation of the resulting quadripole.

The same experiment was performed with an electrode with a different shape (i.e., a diamond). The same behavior as the one reported here was observed, i.e., the localization of the principal surface motion right under the electrodes and an evanescent regime outside the metalized area. These results indicate that the propagation of elastic waves in a PPT does not depend on the electrode shape. This result is crucial because it demonstrates that the initially proposed approach is not well adapted for the fabrication of transversal filters by simply transposing classical IDT concepts to PPT. However, it also provides essential information about PPTs operation and the possibility for coupling modes from one transducer another. The very small losses observed on the corresponding transfer function also suggest that such transducers should be particularly well adapted for such filter operations.

V. C

In this paper, a new concept of acoustic band-pass filters has been presented. This device is based on the use of PPTs in lithium niobate instead of classical IDTs. The possibility of fabricating PPTs in lithium niobate has been demonstrated for different periods. This kind of transducer enables excitation of an elliptical mode with a resonance frequency twice as high as a wave excited by a classical IDT with the same mechanical period. This device is thus very interesting for the fabrication of RF acoustic devices such as band-pass filters.

For that purpose, PPT-based filters were fabricated, copying the principle of IDT-based transversal filters. Although most of the tested structures did not exhibit the expected transfer functions, one device did present a transfer function with marked rectangular spectral windows close to the expected response. The analysis of the electrical parameters of the device has oriented the investigation toward a coupled-mode operation regime. Acoustic-optical measurements have indirectly confirmed this hypothesis. It appears that the elastic waves excited by a PPT are quasi-standing waves, with electrically stimulated vibration clearly localized under the metalized area and an evanescent behavior in the adjacent grating. As a consequence, even if simple transversal filter operations are still beyond reach because of the inner transducer principle, the possibility of fabricating coupled-mode filters is clearly emphasized and exemplified here, with remarkable loss features and compactness characteristics related to the PPTs' fundamental properties. Although more work is clearly needed to set definitive design rules for such transducer, the path is open for an effective alternative to SAW for RF filter manufacturing exploiting periodically poled structures. R

Abstract-

  Solutions for the development of compact RF passive transducers as an alternative to standard surface or bulk acoustic wave devices are receiving increasing interest. This article presents results on the development of an acoustic band-pass filter based on periodically poled ferroelectric domains in lithium niobate. The fabrication of periodically poled transducers (PPTs) operating in the range of 20 to 650 MHz has been achieved on 3-in (76.2-mm) 500-µm-thick wafers. This kind of transducer is able to excite elliptical as well as longitudinal modes, yielding phase velocities of about 3800 and 6500 m•s -1 , respectively. A new type of acoustic band-pass filter is proposed, based on the use of PPTs instead of the SAWs excited by classical interdigital transducers. The design and the fabrication of such a filter are presented, as well as experimental measurements of its electrical response and transfer function. The feasibility of such a PPT-based filter is thereby demonstrated and the limitations of this method are discussed.

Fig. 1 .

 1 Fig. 1. (a) Classical interdigital transducer and (b) periodically poled transducer.

Fig. 2 .Fig. 3 .

 23 Fig. 2. Two-dimensional mesh of a periodically poled transducer.

Fig. 4 .

 4 Fig. 4. Schematic diagram of the poling bench.

Fig. 5 .

 5 Fig. 5. Comparison between experimental results and theoretical predictions for a 500-µm-thick LiNbO 3 transducer (λ = 50 µm).

Fig. 6 .

 6 Fig. 6. Comparison between interdigital transducer-based and periodically poled transducer-based band-pass filters.

Fig. 7 .Fig. 8 .

 78 Fig. 7. Frequency responses which should be obtained with three different periodically poled transducer-based filters.

Fig. 9 .

 9 Fig. 9. Transfer function obtained with a filter composed of two rectangular electrodes (10λ and 20λ, respectively) separated by 15λ: typical signature of a band-pass filter.

Fig. 10 .

 10 Fig. 10. Reflection coefficient S11 observed for one of the two transducers of the operating filter.

Fig. 11 .

 11 Fig. 11. Resonance observed at 95 MHz on the same device.

Fig. 12 .

 12 Fig. 12. Schematic diagram of the laser vibrometer bench.

The authors are with the Franche Comte Electronique Mecanique Thermique et Optique-Sciences et Technologies (FEMTO-ST) Institute, Besançon, France (e-mail: emilie.courjon@femto-st.fr).


Min Won Lee

Thibaut Sylvestre

Michaël Delqué

Alexandre Kudlinski

Arnaud Mussot

J.-F F Gleyze

Alain Jolly

Hervé Maillotte

Demonstration of an All-Fiber Broadband Optical Parametric Amplifier at 1 µm

Keywords:

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

I. INTRODUCTION

F IBER optical parametric amplifiers (FOPAs) have evinced tremendous interest over the years in view of their potential application to all-optical signal processing technologies [START_REF] Marhic | Fiber Optical Parametric Amplifiers, Oscillators and Related Devices[END_REF]- [START_REF] Mckinstrie | All-optical signal processing by fiber-based parametric devices[END_REF]. In addition to providing large gain and wide bandwidth, they offer access to arbitrary wavelength ranges, simultaneous wavelength conversion and low noise figures. Much effort has been spent on FOPAs for telecommunication applications in the 1.5 m wavelength band and some outstanding progress have been made such as 70 dB net gain and bandwidth of a few hundred nanometers in standard and highly-nonlinear dispersion shifted optical fibers [START_REF] Marhic | Fiber Optical Parametric Amplifiers, Oscillators and Related Devices[END_REF]. The use of microstructured optical fibers (MOF) in parametric devices such as amplifiers, generators and oscillators has been the subject of much recent work [START_REF] Fan | Parametric amplification in a microstructure fiber[END_REF]- [START_REF] Sharping | Microstructure fiber based optical parametric oscillators[END_REF]. MOFs indeed offer the promise of extending the range of attainable wavelengths and efficiency of parametric devices. FOPA in the 1 m band is also of great interest because of the need of all-optical broadband signal processing techniques for Ytterbium-doped fiber laser and amplifier systems. Recently, an FOPA has been reported in the 1m band by using an MOF that provides anomalous dispersion in the band [START_REF] Sylvestre | Parametric amplification and wavelength conversion in the 1040-1090 nm band by use of a photonic crystal fiber[END_REF]. This FOPA was demonstrated in free space as a proof of principle and cannot be implemented in practical fiber-based systems. In this work, optical parametric amplification in the 1m band is experimentally achieved, for the first time to our knowledge, in a fully integrated all-fiber and polarization-maintaining (PM) optical system, paving the way for practical applications to all-optical pulse processing in the 1m window. The FOPA demonstrated in this work is implemented with a pump pulsed by an electro-optic intensity modulator specially designed to operate at 1 m with a minimum insertion loss, enhanced extinction ratio and a high optical damage threshold. Efficient parametric gain and wavelength conversion are demonstrated by pumping at 1065 nm near the zero-dispersion wavelength of the MOF and by using a signal at a centre-wavelength of 1053 nm. The choice of the signal wavelength, which is located in the anti-Stokes band of the FOPA, matches the specific needs of the search of Inertial Confinement Fusion (ICF) in large laser systems [START_REF] Besnard | The megajoule laser program -ignition at hand[END_REF], [START_REF] Haynam | National ignition facility laser performance status[END_REF]. Our experimental results show a net parametric gain of greater than 30 dB at 1053 nm with a good quality of optical amplification and higher idler wavelength conversion efficiency due to the additional contribution of the Raman gain. This all-fiber pulsed-pump parametric device provides efficient amplification and conversion for applications such as optical parametric chirped-pulse amplification (OPCPA) [START_REF] Hanna | Fiber optical parametric pulse amplification in the femtosecond regime[END_REF], [START_REF] Caucheteur | Experimental demonstration of optical parametric chirped pulse amplification in optical fiber[END_REF], pulse optical replication or regeneration, ultrafast optical sampling and next generation optical oscilloscopes [START_REF] Jolly | Demonstration of a true single-shot 100 GHz-bandwidth optical oscilloscope at 1053-1064 nm[END_REF], [START_REF] Andrekson | Ultrahigh bandwidth optical sampling oscilloscope[END_REF]. These applications may take a large benefit from FOPA to provide a fairly uniform spectral gain over broad bandwidth.

II. EXPERIMENTAL SETUP

Setting an all-fiber PM OPA in the 1-m band is a big challenge because of the need for low insertion loss fiber optic components, electro-optical modulators and a suitable optical fiber as a nonlinear medium. At such a wavelength, anomalous dispersion can be provide only by the use of a MOF whose photonic crystal cladding ensures a high single-mode field confinement together with the dispersion characteristics [START_REF] Russell | Photonic crystal fibers[END_REF], [START_REF] Wadsworth | Supercontinuum and four-wave mixing with Q-switched pulses in endlessly single-mode photonic crystal fibres[END_REF]. MOF has been widely used for fiber optical parametric oscillators. In our experimental setup shown schematically in Fig. 1, the pump is provided by a 6 mW tunable DFB laser source at 1060 nmwith a tuning range from 1040 nm to 1080 nm. The linearly-polarised CW emission of the laser is converted to pulse emission by an LiNbO electro-optic Mach-Zhender intensity modulator manufactured by Photline Technologies (NIR-MX series). The modulator is specially designed for operation at 1060 nm with low insertion loss ( dB), high extinction ratio and high optical damage threshold. A pulse generator is used to generate rectangular pulses and an RF amplifier amplifies pulses so as to drive the modulator over of modulation. In this experiment, a short pulse ( ns) is used to avoid any Brillouin backscattering that limits the OPA gain [START_REF] Marhic | Fiber Optical Parametric Amplifiers, Oscillators and Related Devices[END_REF]. The pulsewidth and duty cycle of pulse train are then set to 3.4 ns and 1/16, respectively. A DC offset is added into the modulator to suppress residual CW emission in the pulse emission. A high-power polarization-maintaining (PM) Ytterbium-doped fiber amplifier (YDFA) amplifies the pump pulse. A bandpass filter with a 2.5 nm of 3 dB-linewidth is adjusted to 1065.8 nm of center-wavelength to ensure suppression of amplified spontaneous emission (ASE) of the YDFA. Another distributed feedback (DFB) PM fiber laser emitting CW at 1053 nm is used as a signal source to be amplified by the fiber OPA. A 99/1 PM fiber optical coupler at 1060 nm is used to couple both the pump and the signal at the MOF's input. Only 1% of the signal power is injected into the fiber amplifier whilst 99% of the pump is injected into the amplifier to keep the pump power at a maximum level. As parametric amplification is polarization dependent, the states of polarization (SOP) of the pump and signal are maintained in the whole system as well as in the MOF by use of PM elements. We additionally use a polarization controller (PC) before the MOF to align both the pump and signal polarisations parallel to one of the birefringent axes of the MOF. The signal power is set to be 40 dB lower than the maximum average pump power. The input signal power is then measured as dBm. The fiber output is split into two outputs by another 90/10 PM fiber optical coupler. The 10% port is sent to an optical spectrum analyzer (OSA) and the 90% port is injected into another bandpass filter centered at the signal wavelength of 1053 nm. The amplified signal is then detected by a photodetector (PD) and recorded by a real-time digital oscilloscope.

The MOF used in this work was specifically designed and manufactured to provide both anomalous dispersion required for phase-matching and a high nonlinear coefficient for efficient the duty cycle and taking into account the optical loss ( dB) due to the fiber splicing at the input of FOPA. A peak pump power of as much as 4 W is coupled into the MOF.

III. RESULTS

First, we have performed OPA experiments in the absence of an input signal by tuning the center-wavelength of the pump so as to find the best pump wavelength that provides the maximum parametric gain for the signal at 1053 nm. Fig. 3 depicts the experimental output spectra as a color plot when scanning the pump wavelength from 1064 nm to 1069 nm. Here the power of the YDFA is set to yield approximatively 3.6 W of output pump power. The ASE spectra reveal a number of new frequency components created by four-wave mixing (FWM) process at both the sides of the pump wavelength in a range of 1030-1100 nm [START_REF] Wadsworth | Supercontinuum and four-wave mixing with Q-switched pulses in endlessly single-mode photonic crystal fibres[END_REF]. We can see in particular a large power asymmetry in the output spectra from 1060 nm pump wavelength. This asymmetry is mainly due to the additional contribution of Raman scattering that leads to higher-power (lower-power) Stokes (anti-Stokes) components [START_REF] Chestnut | Raman-assisted fiber optical parametric amplifier and wavelength converter in highly nonlinear fiber[END_REF]- [START_REF] Hsieh | Combined effect of raman and parametric gain on single-pump parametric amplifiers[END_REF]. This combined contribution of both the parametric and Raman gains is significant because the maximum Raman gain is located at 1100 nm. Nevertheless, we are able to identify the maximum ASE power at 1053 nm for a pump wavelength located at 1065.8 nm. This pump wavelength is kept in the following work.

The peak pump power is now set to W to undertake parametric amplification process in presence of an input signal. Fig. 4 shows input and output spectra of FOPA when pump is switched on and off, respectively. For a fair comparison, the spectra are recorded with the 90/10 optical coupler at the input of OSA. In Fig. 4(a) the pump at nm and signal at nm are present at the input of FOPA. The signal power at pump-on is measured as dBm. The signal power is 40 dB lower than that of the pump. On the other hand, Fig. 4(b) shows dBm of the signal power at the output of FOPA when the pump is on. Comparing these two powers, it is seen that the signal is amplified by 16.4 dB in FOPA. This gain is obtained with the average power of the amplified signal. As the signal is converted from CW to rectangular pulses, the duty cycle of pump pulses needs to be taken into account. The duty cycle is 1/16 and it gives 11.1 dB of extra gain. Hence, this results in 27.5 dB of net gain. The output signal filtered at 1053 nm is also depicted in Fig. 4(a) that shows a good quality of amplification. A gross gain can also be obtained simply by comparing the output signal powers at pump-on and -off from optical spectra in Fig. 4(b). The signal power at pump-off is as low as dBm at the output. Therefore, the gross gain of FOPA is obtained as much as 35.6 dB. This difference of 8.1 dB originates from the optical loss in the MOF. In Fig. 4(b), the idler appears at 1079 nm with the similar power to the signal. Cascaded four-wave mixing (FWM) signals also exist at 1040.7 nm and 1092.4 nm, respectively [START_REF] Ng | Cascaded four-wave mixing in fiber optical parametric amplifiers: Application to residual dispersion monitoring[END_REF]. Second-order cascaded FWM signals can also be observed with the peak power over 3.4 W. Therefore, the FOPA exhibits high wavelength conversion efficiency. Despite of the filter at 1065.8 nm, residual ASE noise from the YDFA still remains at the FOPA input in Fig. 4(a). Due to this input ASE noise, parametric amplified spontaneous emission (ASE) bands are very significant in Fig. 4(b). As the input ASE noise is quite flat, the parametric gain bandwidth of FOPA would be estimated from the parametric ASE noise. This will be discussed later.

Time traces of the amplified signal at the output of FOPA are also measured. The output of FOPA is filtered at 1053 nm to suppress the residual pump and parametric ASE noise and measured by the PD and oscilloscope. Fig. 5 shows time traces of the amplified signal and pulsed pump at W. For the sake of clarity, the time trace of the pump is displaced; the upper trace (blue trace) is the signal trace amplified by the pulsed pump at the output of FOPA and the lower trace is the pulsed pump trace at the input of FOPA. The pulsewidth and duty cycle of the amplified signal are measured as 3.4 ns and 1/16, respectively. These values correspond to those of the pump pulses and the signal is transformed to pulses by the pulsed pump. As such, it is clearly shown from the figure that a good quality of parametric amplification is achieved.

The parametric gain efficiency is also investigated with respect to the peak pump power . Fig. 6(a) displays parametric gains for peak powers from 0.4 W to 3.8 W. These gains are net parametric gains. The circle dots are parametric gains calculated with optical powers of the signal between input and output spectra of FOPA measured from OSA. On the other hand, the square dots are parametric gains obtained by comparing with RF outputs of the signal from the oscilloscope, i.e., time traces at the output of FOPA. Because of the low sensitivity of the PD, the RF gains below W could not be measured. The negative gains below W are in fact losses due to 8.1 dB of the fiber loss. The gains after W overcome the fiber loss. As the power increases thereafter, the gains for the 1053 nm signal increases exponentially. However, the gain seems to saturate for the power over 2.8 W. This is because of the pump depletion and particularly parametric ASE noise. In fact, the parametric ASE noise increases significantly above W. It is seen from the figure that the maximum net parametric gain is obtained as 35 dB for the optical gain at W and 32.7 dB for the RF gain at W. By taking into account 8.1 dB of the fiber loss, the maximum gross optical gain can be estimated as 43.1 dB.

Wavelength conversion efficiency of the FOPA is further studied with the idler at 1079 nm. Fig. 6(b) shows optical output powers of the signal and idler measured from optical spectra. The idler power is lower than that of signal below W. However, the idler power overtakes the signal one at W and the idler power is greater than the signal one thereafter. It is speculated that Raman gain involves in the idler power. Although the pump power is estimated below the Raman threshold, a small amount of Raman gain is expected to be present in the idler which is located at 3.2 THz of frequency offset from the pump. This leads to Raman-assisted parametric frequency conversion, as previously demonstrated [START_REF] Sylvestre | Raman-assisted parametric frequency conversion in a normally dispersive singlemode fiber[END_REF].

IV. CURVE FITS

In order to compare the experimental measurements with the theory, the gain bandwidth and gain over power are fitted. The theoretical parametric gain is given by [START_REF] Marhic | Fiber Optical Parametric Amplifiers, Oscillators and Related Devices[END_REF] (1) where is the parametric gain factor by unit length, and , the phase mismatch.

. is the group velocity dispersion coefficient at the pump wavelength.

is the fourth-order dispersion as s m obtained from numerical simulations. The fiber length is 400 m, but the effective length is obtained as m by taking into account the fiber loss [START_REF]Nonlinear Fiber Optics[END_REF].

Fig. 6(b) clearly shows output power asymmetry between the signal and idler because of the involvement of Raman gain. The Raman gain is given by [START_REF]Nonlinear Fiber Optics[END_REF] (2) where is the Raman gain coefficient for fused silica fiber. As the signal is located on the anti-Stokes side of the pump, we must consider the whole Raman gain including both the anti-Stokes and Stokes gain band [START_REF] Hsieh | Combined effect of raman and parametric gain on single-pump parametric amplifiers[END_REF]. Therefore, the total gain can be approximated by [START_REF] Mckinstrie | All-optical signal processing by fiber-based parametric devices[END_REF] As the laser used for signal generation is not tunable, the experimental gain bandwidth at W can be extracted readily from Fig. 4(b) for the case of gain bandwidth fitting. The extracted gain bandwidth is shown in Fig. 7(a). The signal, pump, idler and FWM sidebands are all suppressed and the gain values are adjusted so that the gain at 1053 nm matches to 35.6 dB of gross gain obtained in Fig. 4(b). Fig. 7(a) also shows in red the fitting curve obtained from (3). In this curve fit, and are set to be the fitting parameters and obtained as 1065 nm, 9.1 W km and m/W respectively. These values are close to those estimated by a finite element method. By taking into account the whole Raman gain curve including both the anti-Stokes and Stokes bands, the fitted curve in Fig. 7(a) shows a power asymmetry of the gain bandwidth and matches closely to the experimental curve. The idler power is greater than the signal power by around 2 dB. The power asymmetry is similar to the result reported in [START_REF] Hsieh | Combined effect of raman and parametric gain on single-pump parametric amplifiers[END_REF]. From the figure, the gain bandwidth of FOPA is estimated as a range of 1049-1083 nm for the given pump power. This is in good agreement with our previous measurement done in free space [START_REF] Sylvestre | Parametric amplification and wavelength conversion in the 1040-1090 nm band by use of a photonic crystal fiber[END_REF].

A gain-power curve is obtained from Fig. 6(a) for the case of gain-power fitting. As the gain seems to be saturated beyond W, those values are not considered in the curve fit. The gains in Fig. 7(b) are set to gross gains. The same parameters as in the case of gain bandwidth fitting are used in the fitting. A fitting of gain-power curve gives nm, W km and m/W. The ZDW is slightly different to that obtained in the case of gain bandwidth fitting whilst remains the same in both the cases.

V. C ONCLUSION

In conclusion, we have demonstrated a microstructured fiberbased optical parametric amplifier in a wavelength range near the 1 m band in a fully all-fiber polarization-maintaining optical configuration. This has been achieved using a pump wave pulsed by an LiNbO electro-optic intensity modulator specially designed for operating at 1060 nm. A CW input signal at 1053 nm is converted to a pulsed signal and amplified by the pulsed pump at 1065.8 nm. Maximum net gains of 35 dB and 32.7 dB have been measured both optically and electrically, respectively. From the amplified signal time traces, it is shown that a good quality of parametric amplification has been achieved in terms of optical contrast and temporal stability. By investigating the idler conversion efficiency, it is observed that the idler power is greater than the signal power because of Raman-assisted parametric wavelength conversion. The gain bandwidth of FOPA has also been estimated by means of parametric ASE spectra in a range of 1049-1083 nm. A fitting with the experimental results has given the values of and ZDW which are in a good agreement with those estimated from numerical simulations.

The FOPA demonstrated here may be used for optical sampling, optical parametric chirped-pulse amplification (OPCPA) and short pulse processing in the 1m Ytterbium band. It is important to stress here that parametric amplification has been undertaken in the continuous-wave pumping regime by use of a phase modulator specially designed at 1060 nm but only 3.5 dB net gain has been obtained. This can easily be improved by reducing the whole optical loss of the fiber optic components and the microstructured fiber, and using the Stokes band instead of the anti-Stokes one of the FOPA. For completeness, we expect in a future work to measure the whole gain bandwidth and the noise figure of the FOPA with a tunable signal source.

  M. W. Lee, T. Sylvestre, M. Delqué and H. Maillotte are with the Département d'optique P. M. Duffieux, Institut FEMTO-ST, Université de Franche-Comté, CNRS UMR 6174, F-25030 Besançon cedex, France (email: thibaut. sylvestre@univ-fcomte.fr). A. Kudlinski and A. Mussot are with the Laboratoire de Physique des Lasers Atomes et Molécules, IRCICA, Université de Lille 1, CNRS UMR 8523, 59655 Villeneuve d'Ascq Cedex, France. J.-F. Gleyze and A. Jolly are with the Commissariat à l'Energie Atomique (CEA), Centre d'Etudes Scientifiques et Techniques Aquitaine, Chemin des sablières, 33144 Le Barp, BP2, France.

Fig. 1 .

 1 Fig. 1. Schematic diagram of the experimental setup. RF Amp: RF amplifier, YDFA: Ytterbium-doped fiber amplifier, OC: optical coupler, PC: polarisation controller, PD: photodetector, OSA: optical spectrum analyzer. The polarisation is maintained in the whole setup.

Fig. 2 .

 2 Fig. 2. Dispersion curve for the fundamental mode exhibiting a zero-dispersion wavelength at 1065 nm and nonlinear coefficient as a function of the wavelength. The inner plot shows a SEM image of the microstructured fiber cross-section.

Fig. 3 .

 3 Fig. 3. Color plot of output spectra of the microstructured optical fiber as a function of the pump wavelength and without input signal. The pump power is set to P =2:4 W. The vertical line indicates the signal wavelength used for parametric amplification.

Fig. 4 .

 4 Fig. 4. Spectra of input and output of the microstructured fiber-based OPA for the 1053 nm signal at P =2:4 W. In (a) the blue curve is a spectrum of the FOPA input and the red curve is a spectrum of the FOPA output filtered at 1053 nm. In (b) the blue curve is a spectrum of the FOPA output at pump-on whilst the red curve at pump-off.

Fig. 5 .

 5 Fig. 5. Time traces recorded by the oscilloscope. The upper trace is a time trace of the signal amplified at P =3:6 W at the output of FOPA. The lower trace is a time trace of the pulsed pump at the input of FOPA. The pump power is attenuated for PD.

Fig. 6 .

 6 Fig. 6. (a) Net parametric gains with respect to the pump power. Circle dot (blue): parametric gains calculated with powers of the signal from optical spectra. Square dot (red): parametric gains calculated with RF signal outputs from the PD. (b) Optical output power of the signal (blue curve) and the idler (red curve).

Fig. 7 .

 7 Fig. 7. (a) Gross parametric gain bandwidth. Circle dots (blue): Experimental gain bandwidth extracted from Fig. 4(b). Solid line (red): fitted curve using (3). (b) Gross parametric gain with respect to the peak pump power. Dotted line (blue): parametric gains retrieved from Fig. 6(a). Solid line (red): fitted curve.

Authorized licensed use limited to: McGill University. Downloaded on April 19,2021 at 14:10:40 UTC from IEEE Xplore. Restrictions apply.

ACKNOWLEDGMENT

The authors acknowledge the European INTERREG-IVA programme and the Fond Européen de Développement Régional (FEDER). They thank Photline Technologies for manufacturing the electro-optic modulator.