Architectures of Acousto-Optic Modulator, optical device and double-pass fiber optic amplifier.

Abstract

"Acousto-optic modulator, optical device and double-pass fiber optic amplifier architectures" The invention relates to an acousto-optic modulator comprising - a crystal, - a piezoelectric oscillator in contact with an injection face of the crystal, - an acoustic absorber arranged on an absorption face of the crystal; the acousto-optic modulator being characterized in that: - two opposite faces of the crystal through which one or more optical beams are intended to enter and / or exit the crystal, form a non-zero angle between them, and / or - at least one of the two opposite faces forms a non-zero angle with a direction of propagation of the acoustic wave in the crystal. Figure for the abstract: Fig.6

Description

Description Title of the invention: Architectures of an Acousto-Optic Modulator, an optical device, and a dual-fiber optical amplifier passage. technical field

[0001] = The present invention relates to the modulation and generation of signals Impulse optics, The present invention also relates to amplification of optical beams. The present invention aims, in particular, to generate signals high-rate, high-power, and narrow-spectrum pulsed signals.

[0002] — The present invention relates to an acousto-optic modulator, an optical device and a double-pass fiber optic amplifier. Prior art

[0003] The use of Acousto-Optical Modulators (AOMs) of the state of the previous technique for modulating an optical signal and for generating pulses of signal. Acousto-optical modulators are used to, among other things, modulate a signal in frequency and / or to modulate the intensity in order, among other things, to generate im- signal pulses. One inherent drawback of state-of-the-art DAWs lies in their losses by reflection corresponding to the parasitic optical signal reflected by the propagating MAOs in the opposite direction to the incident light. These losses by reflection reduce the per- forms of optical architectures in which DAWs are integrated and im- require adapting the operation and configuration of the architectures to counter the effect of these losses through reflection.

[0004] — A type of prior art architecture in which MAOSs are integrated Amplifiers, whose performance is limited by reflection losses, are the ones whose performance is limited by these losses. dual-pass optical fiber (AODP) including a DAW and an amplifier Doped fiber optics (EDFA type). Reflection losses from MAOs cause a distortion of the temporal shape of the signals emitted by the AOPD by generating over- unstable modulations and require, in order to prevent them, making this type more complex. of architecture by adding components, for example one or more additional DAWs- comments, and to limit the performance of certain components, for example decrease the gain from the EDFAs.

[0005] One object of the invention is, in particular: - to attenuate overmodulations in the time signal generated by losses by reflection in the AODPs, and / or - to mitigate losses due to reflection from the MAOs, and / or - to improve the performance of AODPs including at least one MAO. Presentation of the invention To this end, an acousto-optical modulator is proposed, comprising: - a crystal, - a piezoelectric oscillator in contact with one face of the crystal, called the injection face, capable of generating an acoustic wave propagating through the crystal, - an acoustic absorber arranged on one face of the crystal, called the absorption face, capable of absorbing, at least in part, the acoustic wave after it has propagated through the crystal from the injection face to the absorption face; The acousto-optic modulator is characterized in that: - two opposite faces of the crystal, called the entrance / exit faces, through which one or more optical beams are intended to enter and / or exit the crystal, form a non-zero angle between them, and / or - at least one of the two opposite faces forms a non-zero angle with a direction of propagation of the acoustic wave in the crystal, called the direction of propagation. According to the invention, the non-zero angle formed between the two entrance / exit faces and / or the non-zero angle formed by at least one of the two opposite faces with the direction of propagation are such that a beam intended to exit the crystal which is reflected in the crystal by one of the two entrance / exit faces presents, after exiting the crystal by the other of the two entrance / exit faces, a direction different from a direction: - of the optical beam(s) intended to enter the crystal from the other of the two entrance / exit faces, and / or - of the optical beam(s) intended to exit the crystal, after having exited the crystal, through the other of the two entrance / exit faces According to the invention, the direction of propagation of the acoustic wave in the crystal defines the direction in which the acoustic wave propagates inside the crystal. A beam intended to enter the MAO propagates from outside the crystal towards one of the inlet / outlet faces. A beam intended to exit the MAO propagates within the MAO towards one of the inlet / outlet faces. According to the invention, the crystal can be a parallelepiped. According to a first embodiment of the invention, the propagation direction can form any non-zero angle with a straight line, called the line of intersection, formed by the intersection of the two entrance / exit faces of the crystal. The line of intersection of the entrance / exit faces of the crystal corresponds to the line formed by the intersection of two planes including the entrance and exit faces of the crystal, respectively. The two inlet / outlet faces of the crystal can be arranged so that the angle non-zero formed between the propagation line and the line of intersection is such that a beam intended to exit the crystal which is reflected in the crystal by one of the two entrance / exit faces presents, after exiting the crystal by the other of the two entrance / exit faces, a direction different from a direction: - of the optical beam(s) intended to enter the crystal from the other of the two entrance / exit faces, and / or - of the optical beam(s) intended to exit the crystal, after exiting the crystal, through the other of the two input / output faces. According to the first variant of the invention, the line of intersection can be perpendicular to the direction of propagation. According to the first variant of the invention, the entrance / exit faces of the crystal can be arranged so that a zero angle is formed between: - a direction of the optical beam(s) intended to enter the crystal and / or exit the crystal, after exiting the crystal, through one of the two entry / exit faces, and - a direction of the optical beam(s) intended to enter the crystal and / or to exit the crystal, after exiting the crystal, through the other of the two entry / exit faces. According to a second variant of the invention, the direction of propagation can be parallel to the line of intersection. Preferably, according to the invention, the direction of propagation and the line of intersection can be included in a plane extending from the injection face to the absorption face. The plane formed between the direction of propagation and the line of intersection can be perpendicular to: - the direction of the optical beam(s) intended to enter and / or exit, after exiting, the crystal through one of the two entrance / exit faces, and / or - the direction of the optical beam(s) intended to enter and / or exit, after exiting, the crystal through the other of the two entry / exit faces. Preferably, according to the invention, an angle formed between one of the two inlet / outlet faces of the crystal and a direction of the optical beam(s) intended to enter the crystal and / or to exit the crystal, after exiting the crystal, through said one of the two inlet / outlet faces can be identical to an angle formed between the other of the two inlet / outlet faces of the crystal and a direction of the optical beam(s) intended to enter the crystal eV / or to exit the crystal, after exiting the crystal, through said other of the two inlet / outlet faces. According to a third variant of the invention, the inlet / outlet faces of the crystal can be parallel to each other and can form a non-zero angle with the direction of propagation. According to the third variant of the invention, the crystal can be an oblique prism. According to the third variant of the invention, an angle formed between one of the two inlet / outlet faces of the crystal and a direction of the optical beam(s) intended to enter the crystal and / or exit the crystal, after exiting the crystal, through said one of the two inlet / outlet faces may be different from an angle formed between the other of the two inlet / outlet faces of the crystal and a direction of the optical beam(s) intended to enter the crystal and / or exit the crystal, after exiting the crystal, through said other of the two inlet / outlet faces. Preferably, according to the invention, an angle formed between one of the two inlet / outlet faces of the crystal and the propagation direction can be identical to an angle formed between the other of the two inlet / outlet faces of the crystal and the propagation direction. Preferably, according to the invention, except for the third variant, the crystal can be a right prism. In a preferred manner according to the invention, except for the third variant, the crystal can be a right prism with a trapezoidal base. According to the invention, an optical device comprising is also proposed: - an Acousto-Optical Modulator (AOM), - a laser capable of generating an optical beam, - an optical fiber, called the upstream fiber, extending between the laser and the MAO, in which one or more optical beams are intended to propagate to and / or from the MAO, - an optical fiber, called the downstream fiber, located downstream of the MAO with respect to a direction connecting the upstream fiber to the MAO, in which the optical beam(s) destined for and / or coming from the MAO are intended to propagate, - a reflection means connected to the downstream fiber of the MAO and arranged to reflect a beam from the downstream fiber back into the downstream fiber. According to the invention: - the upstream fiber is a polarization-maintaining optical fiber capable of maintaining a linear polarization state of an optical beam, and / or - the downstream fiber is arranged such that the travel time of the optical beam in said downstream fiber from the MAO to the reflecting means is non-zero and less than or equal to half the opening time of the MAO, and / or -the MAO is a MAO according to the invention as defined above. Any one or more of the features of the MAO according to the invention can be introduced into the MAO of the device according to the invention. The downstream fiber can be arranged so that the travel time of the optical beam in said downstream fiber from the MAO to the reflecting means is non-zero and less than or equal to one-quarter of the opening time of the MAO. The downstream fiber can be arranged so that the travel time of the optical beam in said downstream fiber from the MAO to the reflecting means is equal to one-sixth of the opening time of the MAO. The beam travel time in the downstream fiber is modified by varying the length of the downstream fiber between the MAO and the reflection medium. The optical beam(s) intended to propagate in the upstream fiber to the MAO can be linearly polarized. Advantageously, the optical beam generated by the laser is linearly polarized. The upstream fiber can be a polarization-maintaining optical fiber, and the optical beam(s) propagating through the upstream fiber to the MAO can be linearly polarized to improve the reduction of temporal fluctuations by the MAO. The reduction of temporal fluctuations by the MAO is improved through the cancellation and / or reduction of interference in the downstream fiber between: - a parasitic beam propagating through the crystal towards the downstream fiber, and - a linearly polarized beam, originating from the upstream fiber, propagating through the crystal towards the downstream fiber. The parasitic beam can thus be linearly polarized normal to the polarization of the beam from the upstream fiber in order to suppress and / or reduce interference in the downstream fiber resulting from a mixture of parallel-polarized waves. The stray beam propagating within the crystal may be a portion reflected, within the crystal by one of the two input / output faces of the crystal located on the upstream fiber side, of a beam intended to exit the crystal of the MAO on the upstream fiber side. The upstream and downstream fibers may be connected, respectively, to one of the two input / output faces of the MAO crystal located on the upstream fiber side and to the other of the two input / output faces of the MAO located on the downstream fiber side. The term "connected" in the context of the invention does not mean linked or joined, but rather means that unilateral or bilateral transmission of beams between two "connected" components is permitted. The entry / exit angle formed by the arrangement of the upstream and downstream fibers, and / or the non-zero angle formed between the entry / exit faces, and / or the non-zero angle formed between the propagation direction and the parallel entry / exit faces of the crystal, ensures that a portion of a beam, reflected within the crystal by one of the two entry / exit faces and intended to exit the crystal to be coupled with one of the upstream and downstream fibers, has, after exiting the crystal through the other of the two entry / exit faces, a direction different from the direction of the optical beam(s) intended to propagate from the MAO to the other among the upstream and downstream fibers. In other words, the entry / exit angle formed by the arrangement of the upstream and downstream fibers, and / or the non-zero angle formed between the input / output faces of the crystal and / or the non-zero angle formed between the direction of propagation and the parallel input / output faces of the crystal, allows that a part reflected, in the crystal by one of the two input / output faces located on the upstream fiber side, or respectively on the downstream fiber side, of a beam intended to exit the MAO on the upstream fiber side, or respectively in the direction of the downstream fiber, presents, after exiting the crystal by the other of the input / output faces, a direction different from a direction of the beam intended to exit the MAO on the downstream fiber side, or respectively on the upstream fiber side.Put another way, the entry / exit angle formed by the arrangement of the upstream and downstream fibers, and / or the non-zero angle formed between the two entry / exit faces of the crystal and / or the non-zero angle formed between the direction of propagation and the parallel entry / exit faces of the crystal, allows that a part reflected, in the crystal by one of the two entry / exit faces, of a beam destined to exit the crystal is not coupled with one of the upstream and downstream fibers located on the side of the other of the entry / exit faces. According to the invention, the optical device may further comprise a separator arranged to connect unilaterally: - an input of the device to a portion of the upstream fiber connecting the separator to the MAO such that an optical beam emitted by the laser, propagating in a portion of the upstream fiber connecting the laser to the input of the optical device, is injected by the separator into the portion of the upstream fiber connecting the separator to the MAO in the direction of the MAO, and - the upstream part of the fiber connecting the MAO to the separator to an output of the optical device so that a return beam propagating in the upstream fiber from the MAO to the separator is injected by the separator towards the output of the optical device. According to the invention, the upstream fiber can be defined as comprising two optical fibers: - an optical fiber connecting the laser to the input of the optical device, and - an optical fiber connecting the MAO to the separator. The means of reflecting an optical beam may be capable of modifying the polarization of a polarized optical beam. According to the invention: - the reflecting means may be capable of modifying the polarization of a linearly polarized optical beam by a value of 90°. - the reflecting means may include a Faraday rotator mirror and / or a reflective Bragg grating. According to the invention, the downstream fiber can be a polarization-maintaining optical fiber. The upstream and downstream fibers can be polarization-maintaining so as not to require alignment to maintain the polarization state of the Optical beam during coupling in the downstream fiber. This reduces the alignment time of fiber collimators on each side of the MAO during its manufacture. According to the invention, the downstream fiber can be an optically pumped fiber amplifier. According to the invention, the device can be a double-pass fiber optic amplifier. According to the invention, the device may include a processing unit arranged and / or configured and / or programmed to control the laser and / or the acousto-optic modulator and / or pumping means associated with the optically pumped fiber such that the return beam at the output of the optical device is a pulsed optical beam exhibiting: - a frequency greater than 100 Hz and / or less than 500 kHz, and / or - a power greater than 10 mW and / or less than 500 W, and / or - a time width greater than | nanosecond (ns) and / or less than 1 ms. Description of figures Other advantages and features of the invention will become apparent upon reading the detailed description of implementations and embodiments, which are by no means limiting, and the following attached drawings: [fig.1] Figure 1 illustrates a schematic representation of a side view of a state-of-the-art MAO, [fig.2] Figure 2 illustrates a schematic representation of the reflection losses of a state-of-the-art DAW in side view, [fig.3] Figure 3 illustrates a schematic representation of a side view of a MAO according to a first embodiment of the first variant of the invention, [fig.4] Figure 4 illustrates a schematic representation of a bottom view of a MAO according to a second embodiment of the second variant of the invention, [fig.5] Figure 5 illustrates a schematic representation of a side view of a MAO according to a third embodiment of the third variant of the invention, [fig.6] Figure 6 illustrates a schematic representation of a state-of-the-art AODP, [fig.7] Figure 7 illustrates a simulation of a pulse, including common overmodulations, obtained by a state-of-the-art AODP, [fig.8] Figure 8 illustrates a simulation of an optical pulse obtained by a state-of-the-art AODP including the first improvement according to the invention, [fig.9] Figure 9 illustrates a simulation of an optical pulse obtained by a state-of-the-art AODP comprising the third improvement according to the invention, [fig.10] Figure 10 illustrates a simulation of an optical pulse obtained by a state-of-the-art AODP comprising the third improvement according to the invention, [fig.11] Figure 11 illustrates a simulation of an optical pulse obtained by a state-of-the-art AODP including the first and third improvements according to the invention, [fig.12] Figure 12 illustrates a simulation of an optical pulse obtained by a state-of-the-art AODP including the first and fourth improvements according to the invention. Description of the implementation methods The embodiments described below are not exhaustive; variants of the invention may include, in particular, a selection of the described features, isolated from the other described features (even if this selection is isolated within a sentence containing these other features), provided that this selection of features is sufficient to confer a technical advantage or to differentiate the invention from the prior art. This selection includes at least one feature, preferably a functional feature without structural details, or with only a portion of the structural details if this portion alone is sufficient to confer a technical advantage or to differentiate the invention from the prior art. Figure 1 illustrates a state-of-the-art Acousto-Optical Modulator (AOM) in a so-called classical arrangement. An AOM 1 comprises a crystal 2 made of a crystalline or amorphous material, for example, glass or quartz. It also includes a piezoelectric oscillator 3 in contact with one face of the crystal 4, called the injection face 4. An alternating signal is emitted at a given frequency by a transmitter 5. This alternating signal drives the piezoelectric oscillator 3, which vibrates at a frequency that is a function of the given frequency of the alternating signal. The vibrations of the piezoelectric oscillator generate an acoustic wave propagating in the crystal 2. The MAO 1 also includes an acoustic absorber 6 arranged on a face 7 of the crystal, called the absorption face 7. The absorber 6 absorbs the acoustic wave after it has propagated in the crystal 2 from the injection face 4 to the absorption face 7.The function of the absorber is to reduce the power of the acoustic wave by reflecting the acoustic wave back into the crystal, thus creating an additional parasitic intensity modulation of an optical beam propagating through the crystal. MAO 1s comprise two opposite faces 9, 91 and 9, 92, called input / output faces 9, 91, 92, parallel to each other. The inlet / outlet faces 9, 91, 92 are traversed by beams intended to enter 101, 102 into the crystal 2, called inlet beams, and beams intended to exit 103, 104 from the crystal 2. The inlet / outlet faces 9, 91, 92 are arranged so that the inlet beams 101, 102, after entering the crystal 2, propagate in the crystal 2 and become the beams intended to exit 103, 104 from the . Crystal 2. The inlet / outlet faces 9, 91, 92 are arranged so that the beams destined to exit 103, 104 of crystal 2, after passing through the inlet / outlet faces 9, 91, 92, become outgoing beams 105, 106 propagating in the same direction as the beams destined to enter 101, 102 into crystal 2. The propagation plane of the incident and reflected beams by the acoustic wave inside the crystal is called the propagation plane. Inside the crystal, the beam entering through the inlet face and the beam reflected by the acoustic wave and traveling towards the outlet face form an angle with each other and have a common point at the acoustic wave. These two beams therefore form a plane. State-of-the-art AOCs 1 cause the appearance of parasitic beams 11, also called reflection losses, when they are in operation. The inventors observed that when the input / output faces 9, 91, 92 are not parallel to each other, a considerable reduction in the appearance of such parasitic beams 11 is observed. Therefore, according to the invention, in order to limit, or even eliminate, the appearance of these parasitic beams 11, an acousto-optic modulator 1 is proposed, with reference to Figures 3 and 4, in which the two opposite input / output faces 9, 91, 92 of the crystal 2 form a non-zero angle with each other. With reference to Figure 2, the inventors have hypothesized that these parasitic beams 11 arise from the reflection, in the crystal 2, of a part of the beams intended to exit 103, 104 of the crystal 2 on the entrance / exit faces 9, 91, 92. These parasitic beams 11, that is to say the reflected part 11 of the beams intended to exit 103, 104 of the crystal 2, are not modulated by the MAO 1 and exit the crystal 2 in a direction parallel to the direction of the exit beams 105, 106 and the beams intended to enter 101, 102 into the crystal 2. With reference to Figures 3 and 5, the MAOs | according to the invention are arranged so that the parasitic beams 11 are not coupled with the outgoing beams 105, 106. The crystal 2 is arranged so that the parasitic beams, after exiting the crystal 2 through one of the input / output faces 9, 91, 92, have a different direction from the direction of the outgoing beams 105, 106. With reference to Figures 3 and 4, the angle formed between the two opposite inlet / outlet faces 9, 91, 92, or the angle formed by the faces with the propagation direction, results in the parasitic beams 11 propagating in crystal 2 along a different direction than the beams intended to exit 103, 104 from crystal 2, and exiting crystal 2 along a different direction than the outgoing beams 105, 106 and the beams intended to enter 101, 102 into crystal 2. The propagation direction is perpendicular to the piezoelectric oscillator 3. A line of intersection (not shown) of the opposite inlet / outlet faces 9, 91, 92 and the propagation direction 81 of the acoustic wave in crystal 2 lie in a plane 12 extending between the injection face 4 and the absorption face 7. The angle formed between the inlet face 9, 91 and the propagation direction is identical to the angle formed between the outlet face 9, 92 and the propagation direction. Furthermore, the angle formed between the inlet face 9, 91 and plane 12 is identical to the angle formed between the outlet face 9, 92 and plane 12. With reference to Figures 3 to 5, a 2 AMTIR crystal is used, excited with a 40MHz acoustic wave and crossed by beams 101, 102, 103, 104, 105, 106 of wavelength 1.54um in vacuum. Referring to Figures 3 and 4, the angle formed between the inlet face 9, 91 and the outlet face 9, 92 is between 0.5° and 5°, the angle is typically on the order of 1°. This allows for parallel output beams 105, 106 and input beams 101, 102. Referring to Figure 3, in a first embodiment, the inlet / outlet faces 9, 91, 92 of the MAO 1 form an angle with each other such that the inlet beams 101, 102 and the outlet beams 105, 106 are parallel to each other and perpendicular to the propagation direction 81 of the acoustic wave. The direction 82 along which the line of intersection extends is perpendicular to the propagation direction 81 of the acoustic wave. The propagation direction 81 and the line of intersection lie in the same plane. Referring to Figure 4, according to a second embodiment, the direction 82 along which the line of intersection extends is parallel to the direction of propagation 81 of the acoustic wave. The direction of propagation and the line of intersection are parallel. With reference to Figure 5, according to a third embodiment, the input / output faces 9, 91, 92 of the MAO 1 are parallel to each other and form a non-zero angle with the propagation direction 12. The propagation direction 81 and the line of intersection do not here belong to the same plane but belong to two distinct planes. According to the third embodiment, the angle [3 formed between the exit face 9, 92 and the beams 102, 106 passing through it is different from the angle formed between the input face 9, 91 and the beams 101, 105 passing through it. With reference to Figure 6, a state-of-the-art double-pass fiber optical amplifier (DPAO) 13 is shown. The DPAO 13 comprises a polarization-separating coupler (PBS) 14, an optical fiber 15, referred to as the upstream fiber 15, connecting the PBS 14 to a state-of-the-art MAO 1 as described above, a doped fiber optical amplifier (DFA) 16, whose component optical fiber is referred to as the downstream fiber 16, connecting the MAO 16 to a Faraday mirror 17, and a laser 18 emitting an optical beam 20 towards an input 21 of the PBS 14. The PBS 14 unilaterally connects the laser 18 to The upstream fiber 15 is connected so that the optical beam 20 emitted by the laser 18 propagates in the upstream fiber 15 towards the MAO 1. The PBS 14 also unilaterally connects the upstream fiber 15 to an output 19 of the AODP 13 so that a return optical beam 24, amplified twice and shaped, propagating in the upstream fiber 15 from the MAO 1 to the PBS 14, is injected by the PBS 14 towards the output 19 of the AODP 13. The optical beam 20, after exiting the MAO 1, is amplified once in the EDFA 16 to form an amplified forward beam 22, then is reflected by the Faraday mirror 17 and amplified a second time in the EDFA 16 before passing through the MAO 1 again. According to the invention, the AODP 13 comprises a processing unit arranged and / or configured and / or programmed to control the laser 18, the MAO 1, pumping means (not shown) associated with the AOFD 16 so that the return beam 24 at the output 19 of the AODP 13 is a pulsed optical beam 24 having a frequency between 100 Hz and 500 kHz, a power between 10 mW and 500 W, and a time width between 1 nanosecond (ns) and 1,000,000 ns. Figures 6 to 11 are simulations of optical pulses 24 obtained at the output 19 of optical architectures based on the AODP 13 described above. The simulations are performed using the Jones formalism and take into account the evolution of the polarization state and the transmitted power at every point of the AODP 13. Referring to Figure 6, when a state-of-the-art AODP 13 as defined above (without implementation of the MAO 1 to reduce reflection losses and without polarization-maintaining fibers or additional fiber length) is used to modify the optical beam 20 emitted by the laser 18 into amplified optical pulses 24, the return optical beam 24 at the output 19 of the AODP 13 exhibits overmodulation 23. It should be noted that the overmodulation profile 23 is unstable and unpredictable. This behavior is particularly detrimental to the practical use of this type of architecture. According to a first improvement of the AODP 13 of the state of the art described above, the upstream fiber 15 is a polarization-maintaining optical fiber 15 capable of maintaining the linear polarization state of the optical beam 20 emitted by the laser 18. This improvement allows the part reflected 11 by the entrance face 9, 91 of the crystal 2 of the MAO | The amplified return beam 25, corresponding to the incoming beam 102, propagating in the MAO 1 from the AOFD 16 towards the upstream fiber 15, exhibits a linear polarization state normal to that of the optical beam 20 emitted by the laser 18, corresponding to the incoming beam 101, propagating in the MAO 1 from the upstream fiber 15 towards the AOFD 16. It should be noted that even though the intensity of the reflected portion 11 is negligible compared to the intensity of the optical beam 20, the fact that it has already been amplified twice in the AOFD 16 makes its intensity non-negligible when coupled to the optical beam 20 at the input of the AODP 13. With reference to Figure 7, the effect of the first enhancement is illustrated, which partially cancels the interference between the spurious beams 11 and the optical beam 20 emitted by the laser 18 propagating in the MAO 1. It is observed that the overmodulations 23 equal to the modulation frequency of the MAO 1, called fuao, have been canceled. In general, all overmodulations 23 equal to n.fmao, where N is an odd number, are canceled. This corresponds to the case where the portion reflected 11 by the input face 9, 91 of the crystal 2 of the MAO 1 originates from an amplified return beam 25 that has made an odd number of round trips in the AODF 16. It is also observed that only the overmodulation 23 equal to twice the modulation frequency fuao of the MAO 1 is present. This corresponds to the case where the part reflected 11 by the entrance face 9, 91 of the crystal 2 of the MAO 1 comes from a return beam 25 return having made two round trips in the AOFD 16.The amplified return beams 25 that have made an even number of round trips in the AODF 16 greater than 2 are cut off by the closure of the MAO 1, the travel time in the downstream fiber 16, corresponding to the round-trip time, being greater than the opening time of the MAO 1. It is worth noting that, in addition to observing a single overmodulation 23, this overmodulation 23 exhibits high stability. Such stability can be used when a time profile exhibiting overmodulations 23 is of interest for controlling nonlinearities in the amplification stages. In particular, a reduction of the Brillouin effect can be achieved with this technique by raising its threshold of occurrence through the judicious choice of the overmodulation amplitude 23. According to a second improvement to the AODP 13 of the state-of-the-art described above, the downstream fiber 16 is also a polarization-maintaining optical fiber along all or part of its length. This second improvement prevents a change in the polarization of the optical beams 20, 22, 25, 106, 102 propagating in the downstream fiber 16. This second improvement also allows for the standardization of fibers 15 and 16 around the MAO, thus simplifying its manufacture. In this case, the fiber collimators used to inject and collect the light passing through the MAO 1 can be aligned without regard to maintaining polarization between the upstream fibers 15 and downstream fibers 16. With reference to Figures 8 and 9, according to a third improvement of the AODP 13 of the state of the art described above, it is proposed to increase the travel time of the optical beams 20, 22, 25, 106, 102, 11 propagating in the downstream fiber 16 from the MAO 1 to the Faraday mirror 17. In practice, increasing this delay consists of increasing the length of the downstream fiber 16. As illustrated in Figure 8, when the increase in the length of the downstream fiber 16 is such that the increase in the travel time of the optical beams 20, 22, 106, 102, 11 in the downstream fiber 16 is equal to half the opening time of the AOD 1, most of the overmodulations 23 are canceled because the spurious wave 11 reflected by the AOD during the first pass of the return wave 25 arrives at the AOD 1 when it is closed after a round trip. However, the duration of the amplified optical pulse 24 at the output 19 of the AODP 13 is also halved. As illustrated in Figure 9, when the increase in the length of the downstream fiber 16 is such that the increase in the travel time of the optical beams 20, 22, 106, 102, 11 in the downstream fiber 16 is equal to one-third of the opening time of the AOD 1, a significant portion of the overmodulations 23 are canceled. In this case, the duration of the amplified optical pulse 24 at the output 19 of the AODP 13 is reduced by only one-third. With reference to Figure 10, the simulation of overmodulations 23 generated by an AODP 13 is illustrated, including the first or second improvement and the third improvement for which the increase in the travel time of the optical beams 20, 22, 106, 102, 11 in the downstream fiber 16 is equal to one-third of the opening time of the MAO 1. The combination of these two improvements makes it possible to cancel most of the overmodulations 23, According to a fourth embodiment of the prior art AODP 13 described above, the prior art AODP 13 comprises the MAO 1 according to the invention. In this case, the spurious reflections 11 generated by the reflection on the entrance face 9, 91 of the crystal 2 of the MAO 1 of the return beam 22 from the AODF 16 are reduced by 40 to 45 dB or by 50 dB or by 60 dB or more. With reference to Figure 11, the simulation of overmodulations 23 generated by an AODP 13 incorporating the first and fourth improvements is illustrated. The combination of these two improvements makes it possible to cancel almost all of the overmodulations 23. According to the invention, the improvements made to the prior art AODP 13 are combinable. The effect of the improvements is cumulative. Therefore, according to the invention, the prior art AODP 13 comprises: - one of the improvements from the first, second, third or fourth improvement, or - any combination of the first and / or second and / or third and / or fourth improvements, or - all the improvements. Of course, the invention is not limited to the examples just described and many modifications can be made to these examples without departing from the scope of the invention. Thus, in combinable variants of the previously described embodiments: - the propagation direction 81 forms any angle greater than 0° and / or less than 90° with the direction 82 along which the line of intersection extends, and / or - the laser 18 is capable of emitting a continuous laser beam 20, and / or - the laser 18 is capable of emitting a pulsed laser beam 20, and / or - when the laser beam 20 emitted by the laser 18 is a pulsed beam, the processing unit is arranged and / or configured and / or programmed to control the laser 18 and the MAO 1 so that the opening of the MAO 1 coincides with the emission of the laser pulse 20 emitted by the laser 18. Furthermore, the different features, forms, variants and embodiments of the invention can be associated with each other in various combinations insofar as they are not incompatible or mutually exclusive.

Claims

Demands

1. Acousto-optical modulator comprising: - a crystal, - a piezoelectric oscillator in contact with one face of the crystal, called injection face, capable of generating an acoustic wave propagating in the crystal, - a sound absorber arranged on one face of the crystal, called the absorption, capable of absorbing, at least in part, the acoustic wave after it has propagated through the crystal from the injection face up to the absorption face; The acousto-optic modulator is characterized in that: - two opposite faces of the crystal, called entrance / exit faces, through into which one or more optical beams are intended to enter and / or exit of the crystal, form a non-zero angle between them, and / or - at least one of the two opposite faces forms a non-zero angle with a direction of propagation of the acoustic wave in the crystal, called direction of propagation.

2. Acousto-optic modulator according to claim 1, wherein the direction of propagation forms any non-zero angle with a a line, called the line of intersection, formed by an intersection between the two inlet / outlet faces of the crystal.

3. Acousto-optic modulator according to claim 2, wherein the The line of intersection is perpendicular to the direction of propagation.

4. Acousto-optic modulator according to the preceding claim, in in which the crystal's input / output faces are arranged such that a a zero angle is formed between: - a direction of the optical beam(s) intended to enter the crystal and / or to exit the crystal, after exiting the crystal, by one of the two input / output faces, and - a direction of the optical beam(s) intended to enter the crystal and / or to exit the crystal, after exiting the crystal, by the other of two input / output faces.

5. Acousto-optic modulator according to claim 1, wherein the the direction of propagation is parallel to a straight line, called a straight line intersection, formed by an intersection between the two faces crystal input / output.

6. Acousto-optic modulator according to any one of the claims 2 to 6, in which the direction of propagation and the line of intersection are included in a plane extending from the injection face towards the absorption face.

7. Acousto-optic modulator according to claim 1, wherein the The entrance / exit faces of the crystal are parallel to each other and form a non-zero angle with the direction of propagation.

8. Acousto-optic modulator according to the preceding claim, in which an angle formed between one of the two inlet / outlet faces of the crystal and a direction of the optical beam(s) intended to enter in the crystal and / or to exit the crystal, after exiting the crystal, by said one of the two inlet / outlet faces is different by an angle formed between the other of the two entrance / exit faces of the crystal and a direction of the or optical beams intended to enter and / or exit the crystal of the crystal, after having come out of the crystal, by the said other of the two faces input / output.

9. Acousto-optic modulator according to any one of the claims 1 to 8, in which an angle is formed between one of the two faces the crystal's input / output direction and the propagation direction are identical to a angle formed between the other of the two entrance / exit faces of the crystal and the direction of propagation.

10. Optical device comprising: - an Acousto-Optical Modulator (AOM), - a laser capable of generating an optical beam, - an optical fiber, called the upstream fiber, extending between the laser and the MAO, in which one or more optical beams are intended to propagate destination and / or origin of the MAO, - an optical fiber, called a downstream fiber, located downstream of the MAO relative to a direction connecting the upstream fiber to the MAO, in which is intended to the optical beam(s) propagate to and / or from originating from the MAO, - a means of reflection connected to the downstream fiber of the MAO and arranged for reflect a beam from the downstream fiber into the downstream fiber; The optical device is characterized in that: - the upstream fiber is a polarization-maintaining optical fiber suitable for maintain a linear polarization state of an optical beam, and / or - the downstream fiber is arranged so that a beam travel time optical in said downstream fiber from the MAO to the means of reflection is non-zero and less than or equal to half a duration opening the DAW, and / or -the MAO is an MAO according to any one of the claims | to 9.

11. | Optical device according to the preceding claim, comprising a se- comparator arranged to connect unilaterally: - an input of the device to a part of the upstream fiber linking the se- parator at the MAO so that an optical beam emitted by the laser, propagating in a portion of the upstream fiber connecting the laser to the input of the optical device, either injected by the separator into the part of the fiber upstream connecting the separator to the MAO in the direction of the MAO, and - the upstream portion of the fiber connecting the MAO to the separator at an output of the optical device such that a return beam propagating in the upstream fiber from the MAO to the separator is injected by the se- parser towards the output of the optical device.

12. Optical device according to claim 10 or 11, wherein the means reflection of an optical beam is capable of modifying the polarization of a polarized optical beam.

13. Optical device according to any one of claims 10 to 12, wherein the downstream fiber is a polarization-maintaining optical fiber.

14. An optical device according to any one of 10 to 13, wherein the downstream fiber is an optically pumped fiber amplifier.

15. Optical device according to claim 14, comprising a unit of processing arranged and / or configured and / or programmed to control the laser and / or the acousto-optical modulator and / or means of pumping associated with the optically pumped fiber so that the beam return at the output of the optical device, i.e., an optical beam im- impulsive presenting: - a frequency greater than 100 Hz and / or less than 500 kHz, and / or - a power greater than 10 mW and / or less than 500 W, and / or - a time width greater than 1 nanosecond (ns) and / or less alms.