FR2931309A1 - POWER MONOMODE LASER DEVICE AND AMPLIFICATION SYSTEM COMPRISING THE LASER DEVICE - Google Patents

Abstract

The present invention relates to a power laser device, which can be used for lidar systems, and to an amplification system comprising the device. The device comprises an input fiber-reinforced section (1) for amplifying an optical beam. multi-mode incident (3), a fiber conversion section (4) for the conversion of the multimode incident optical beam into a single-mode power beam (8) and an intermediate fiber section (9) allowing the multimode incident optical beam (3) to propagate towards the fiber-conversion section (4) and allowing the reflected monomode optical beam (8) to exit the device. The invention also relates to a single-mode power amplification system comprising the single-mode laser power device of the invention. 'invention.

Description

The present invention relates to a single-mode laser power device and an amplification system comprising the laser device. In recent years, considerable progress has been made in the field of fiber optic lasers, mainly due to the surge in telecommunications applications. At the same time, high power fiber lasers (above 100 W) have been studied for industrial applications. The advantages of fiber lasers over conventional solid state lasers (Nd: YAG for example) are indeed numerous, among which we can mention: - High robustness, the optical bench in a fiber laser being the fiber itself there is no problem of misalignment as in a conventional laser; - High efficiency with optical-to-optical efficiencies up to twice as high as for a solid laser pumped diodes, the latter being obtained by guiding the pump and the spatial overlap between the pump and the amplified signal; - A better distribution of the heat in the active medium reducing the problems related to the management of the thermal one; Better mode guidance to obtain a beam of high spatial quality (single-mode beam); - The ability to achieve significant gains and achieve effective amplifiers; - The possibility of obtaining an eye-safe emission (1.5 μm) of very high efficiency (Erbium doped fibers); A potentially low cost. These advantages make optical fiber lasers extremely attractive for industrial (cutting, machining ...) and optronic (Lidar, rangefinders, illuminator) applications. Even if quite remarkable results have already been validated with monomode fibers, obtaining even greater power is made impossible by problems of resistance to the luminous flux of the single-mode fiber and the appearance of parasitic non-linear phenomena ( effects Raman, Brillauin,., ..). These parasitic phenomena become even more important for Lidar systems in which the laser source operates in pulse mode, single-mode space and mono-frequency spectral finesse. The development of compact optical fiber laser sources for Lidar applications is also of prime importance for aircraft-based systems for detecting vortices and turbulence, created especially near airport runways by aircraft during takeoffs and landings (which can disrupt the flight conditions of the planes that follow them and therefore require to space them strongly). So today there is a real wait to find innovative solutions to overcome these limitations. It has already been proposed to use very large diameter (> 100 μm) very high multimode fiber amplifying power lasers as the last amplifier stage. The large diameter of the fiber makes it possible to reduce the intensity in the fiber and to avoid the appearance of parasitic non-linear phenomena. In contrast, the output beam of the fiber is highly multimode. To limit the energy density in the core of the fiber, LMA (Large Mode Area) fibers having a core diameter of 20 μm to 50 μm larger than that of the core of a standard single mode fiber are generally used. In order to obtain a spatial single-mode beam, the core has a small numerical aperture (less than 0.1 - typically 0.06) which makes it possible to introduce losses on the high modes. Nevertheless, and as suggested above, the bigger the heart, the more difficult it is to maintain only the fundamental mode. Moreover, it is extremely difficult to produce a doped core having a constant index profile over the entire diameter of the core. The non-uniformity of the index profile favors the propagation of a distorted (donut-shaped) mode that is unusable for the intended applications. Obtaining a polarized beam from heart symmetry rupture techniques also becomes very difficult with the increase in the diameter of the heart. The use of LMA fibers makes it possible today to obtain pulse energies limited to 300 μJ, which is too little for Lidar airborne turbulence detection applications for which an energy of at least 1 mJ is required. An approach proposed in a previous patent application (process for producing a power laser beam and implementation device, Thales patent No. 05 09093) consists of using a highly multimode amplifying fiber with a very large core (greater than 50 pm, typically 75 μm) with which it is possible to extract large energies by pulse on a spatial and depolarized multimode beam. The multimode beam is then converted into a single-mode beam, polarized in another passive index gradient fiber. This mode conversion is achieved by beam cleanup using stimulated Brillouin scattering. In a first fiber stage, the maximum energy is extracted without worrying about the beam quality or the polarization. The desired beam quality and polarization are obtained in a second fiber stage. The multimode beam injected into the passive fiber and the monomode beam obtained propagate in the opposite direction along the same axis of propagation. This requires using a Faraday isolator to recover the single mode beam. The use of this isolator is restrictive since it forces a propagation in free space between the amplifying multimode fiber and the passive gradient index fiber (thus losing the interest of the fiber source). In addition to its voluminous nature, the insulator introduces high losses (typically more than 50%). Furthermore, it is also difficult to maintain the polarization of the monomode beam over time because of the thermal stresses experienced by the index gradient fiber. The present invention aims to solve the limitations of the solution mentioned above and allows in particular: the suppression of the Faraday isolator and the associated losses between the amplifying multimode fiber and the index gradient fiber; - the elimination of free-space beam propagation in order to increase the compactness of the source, its robustness, its insensitivity to vibrations and shocks; control of the polarization of the output beam by passive or active techniques.

For this purpose, the present invention provides a fully fiber-optic single-mode laser device for converting a multimode source into a single-mode power beam. More precisely, the subject of the invention is a single-mode power laser device comprising an input fiber-bundle section comprising one or more amplifying optical fibers intended to amplify a multimode incident optical beam, a fiber-conversion section comprising an optical fiber intended for conversion. the multimode incident optical beam into a single-mode power beam, an intermediate fiber-optic section comprising an optical fiber enabling the multimode incident optical beam to propagate towards the conversion fiber-bundle, characterized in that: the fiber-conversion section comprises an optical fiber index gradient passive signal, said passive optical fiber having a length such that by Brillouin effect the device generates by reflection of the multimode incident optical beam, a monomode optical power beam; the input fiber section and the intermediate fiber section respectively comprise at least a first fiber and a second fiber comprising fiber cores in contact so as to transmit the multimode incident optical beam in the fiber-conversion section; the intermediate fiber bundle having a free end constituting the output for a monomode power beam of the device; A variant of this device is characterized in that the intermediate fibered section is composed of a double core index jump fiber whose large core is adapted to the diameter and the numerical aperture of the core of the gradient fiber. index comprising the fiber-conversion section, and whose small core is adapted to the size of the single-mode reflected beam created by the Brillouin effect in the passive index gradient fiber. A variant of this device is characterized in that the fiber conversion section and the intermediate fiber section are composed of the same passive index gradient fiber. A variant of this device is characterized in that the fiber input section comprises several amplifying fibers intended to amplify single-mode optical beams and capable of constituting by combination the multimode incident optical beam.

A variant of this device is characterized in that the fiber input section comprises a single amplifying fiber for amplifying a multimode incident optical beam. A variant of this device is characterized in that the fiber input section comprises a plurality of amplifying fibers intended to amplify multimode optical beams and capable of constituting by combination the multimode incident optical beam. A variant of this device is characterized in that the core of the gradient index passive fiber is composed of silica (SiO2). A variant of this device is characterized in that the core of the graded index passive fiber is composed of chalcogenide (As2S3, for example). The invention also relates to a single-mode power amplification system comprising the single-mode power laser device and a fiber-mode single-mode reference oscillator generating a monomode beam of frequency vo, characterized in that: it comprises a monomode fiber coupler allowing dividing the monomode beam of frequency vo into first and second optical beams; the first optical beam generated by the fiber-based single-mode reference oscillator being amplified to obtain a multimode power beam and introduced to the input of the single-mode power laser device; the second optical beam generated by the fiber-based monomode reference oscillator being shifted at the frequency vo + SvE; to account for Brillouin SvB offset; the second optical beam thus shifted in frequency being transmitted to the monomode power laser device by its input. A variant of this single-mode power amplification system is characterized in that it comprises at least one fiber preamplifier, a fiber isolator, a multimode fiber amplifier in order to amplify the first of the two optical beams generated by the oscillator. monomode fiber reference. A variant of this single-mode power amplification system is characterized in that it comprises a frequency modulator for frequency shifting the second of the two optical beams generated by the monomode reference oscillator fiber at the frequency vo + SvB. A variant of this single-mode power amplification system is characterized in that it comprises a polarization controller connected to the input of the single-mode power laser device. A variant of this single-mode power amplification system is characterized in that a fiber is connected at the output of the frequency modulator, said fiber having a periodic curvature, the wavelength of said curvature being chosen in such a way that modes that are not affected by the curvature are eliminated by coupling. A variant of this single-mode power amplification system is characterized in that it comprises a first feedback circuit making it possible to adapt the output of the modulator to variations in the Brillouin shift occurring in the passive gradient-index fiber. of the single-mode laser power device. A variant of this single-mode power amplifier system according to the claim is characterized in that the first feedback circuit comprises a weakly reflecting plate and a photodiode. A variant of this single-mode power amplification system is characterized in that it comprises a second feedback circuit for improving the polarization of the optical beam at the output of the single-mode power laser device. A variant of this single-mode power amplifier system is characterized in that the second feedback circuit comprises a polarizer, a photodiode and a polarization controller.

The invention will be better understood and other advantages will become apparent on reading the following description given by way of non-limiting example and with reference to the appended figures in which: FIG. 1 illustrates a first variant of a laser device according to the invention; FIG. 2 illustrates a second variant of a laser device according to the invention; FIG. 3 illustrates a third variant of a laser device according to the invention; FIG. 4 illustrates a first variant of a single-mode power amplifier system; FIG. 5 illustrates a second variant of a single-mode power amplifier system; FIG. 6 illustrates the concept of modal filtering by periodic curvature; FIG. 7 illustrates a third variant of a single-mode power amplifier system.

A first variant of the invention is illustrated in FIG. 1, the device comprises an input fiber-bundle section 1 composed of a fiber adapted to the propagation of a multimode optical beam 2 comprising a sheath 10 and a core 11. variant also comprises a fiber-conversion section 4 composed of a graded index passive fiber 5 comprising a sheath 6 and a core 7 and performing the Brillouin mode mode conversion and an intermediate fiber-bundle section 9 composed of a fiber An optical multimode incident beam presented at the input 17 of the device then propagates in the entered fiber section 1, then in the intermediate fiber section 9 to reach the fiber conversion section 4. The Brillouin reflection of the incident wave generates a single-mode optical beam of power leaving the device through the output 13. It should be noted that an optical beam can also be introduced by the end 12 of the device. This point will be explained later in the description. The length of the conversion fiber bundle 4 composed of a graded index passive fiber 5 is chosen to be large enough for the threshold of the Brillouin effect to be crossed. This length can be, for example, several meters. A remarkable property of the Brillouin effect in a sufficiently long index gradient fiber is that the reflected wave (called Stokes wave) created in the index gradient fiber in the direction of propagation opposite to that of the wave. incident (the incident wave 3 is that which enters the fiber 2 by its end 17 to then propagate in the index gradient fiber 5), is a unique mode of the fiber, whereas the incident wave is multimode . The choice of a gradient index profile for the passive fiber 4 is therefore motivated by the fact that during the Brillouin reflection phenomenon, the fundamental mode is preferred. Indeed, it was pointed out in the article by L. Lombard, A. Brignon, J.-P. Huignard and E. Lallier entitled Beam cleanup in a self-aligned gradient-index Brillouin cavity for high-power multimode fiber amplifiers , Optics letters / Vol. 31, No. January 2/15, 2006, that when a multimode optical wave is introduced into a graded index passive fiber, the Brillouin reflectivity gain is maximized for the fundamental mode of the fiber. It has been previously described that the device at the heart of this invention makes it possible to convert a multimode source into a single-mode source of power. FIG. 1 illustrates an example in which a multimode optical beam 3 is introduced into the device in an input fiber bundle 1 composed of a multimode fiber 2 in order to constitute a multimode optical source. The sections 4 and 9 are made within a single fiber characterized by a graded index profile. By contacting the core of the fiber or fibers of the input section 1 with the core of the fiber constituting the intermediate section 9, the index profile of the fiber comprising the intermediate section 9 may be altered and so that the monomode transmission generated by the Brillouin effect is disturbed. In this case, and as shown in FIG. 2, it may be judicious to use a double-core index jump fiber 22 whose large core 20 is adapted to the diameter and the numerical aperture of the core 7 of the Index gradient fiber constituting the conversion section. The small core 21 is adapted to the propagation of the monomode beam created by the effect: Brillouin in the index gradient fiber 5. The index jump fiber 22 is simply soldered or connectorized to the index gradient fiber 5 The concept of adiabatic coupling between the two types of fibers can also be used by performing a stretching operation on one end of the index jump fiber or on the intermediate coupling zone between the index jump fiber structures. and index gradient. In order to minimize the transmission losses between the index jump fiber 22 (also referred to as the index step) and the index gradient fiber 5, it is possible to optimize the opto-geometric parameters of these two structures. . The number of modes that can be conveyed by a highly multimode optical fiber is expressed via the relation: N = a azk2n, A a + 2 where has characterized the distribution of the index gradient of the core of the fiber, a is the radius of the core, n1 is the optical refractive index of the core material, n2 is the refractive index of the optical cladding and 0 is the relative index difference (0 = n, n2) n, from this relation defining the number of modes that can be excited and considering the parameters a = o. (Characterizing an index jump fiber) and a = 2 (characterizing a gradient index fiber), it is observed that for identical opto-geometric parameters, the transmission losses are 3dB. A common alternative is to use a gradient fiber that satisfies the number of mode conservation rule, which is equivalent to employing a multimode index gradient fiber having a relative index difference twice as high as Multimode fiber with index jump. This remark should be tempered, however, if the modal filtering property of the combination of index jump fiber and index gradient fiber is used.

In order to generate the multimode optical source applied at the input of the device, a multimode optical source can be used as is the case for the examples of FIGS. 1 and 2.

An alternative is to use several single-mode sources. In this case, the single-mode power laser device is adapted accordingly, and this by selecting an appropriate configuration of the input fibular section 1. A possible configuration is illustrated by means of the example shown in FIG. 3. In this example, the source Multimode optics is generated by combining two single-mode 31 and 3: 2 optical beams. The input fiber bundle 1 is composed in this case of two monomode fibers 33 and 34. Different configurations of the input fiber bundle 1 may be chosen so as to adapt to the number of monomode sources required for generating the source. multimode desired. In order for the incident optical beam entering the device to propagate in the fiber-conversion section 4, two embodiments can be envisaged in order to put the heart or hearts of the input fiber bundle 1 into contact with the heart of the Intermediate bundle 9 In a first embodiment, the constituent fiber (s) of the input fiber bundle 1 are welded to the fiber of the intermediate bundle section 9. It will be noted that to simplify the representation of the device in FIGS. only this embodiment has been considered. The sections 1 and 9 being made of different fibers, no optical isolation is necessary. A second embodiment is to bring the core of the fiber (s) making up the input fiber bundle 1 into contact with the core of the fiber composing the intermediate fiber bundle 9 by using the fiber fusion technique, a well-known method. so that an evanescent wave appears and can be transmitted from the input fiber section 1 to the intermediate fiber section 9 and then to the fiber-conversion section 4. Furthermore, the Brillouin reflectivity R is directly related to the Brillouin gain g, the input intensity I and the length L of the graded index fiber 5 constituting the fiber-conversion section 4 by the formula R = gxlxL. Since the Brillouin gain g is specific to the constituent material of the core of the index gradient fiber 5, its reflectivity can be influenced by selecting the material constituting the fiber. Thus, one can choose to use an index gradient fiber with a silica core (SiO2) or a chalcogenide core (As2S3) to obtain a larger gain Brillouin. These fibers with a chalcogenide core have a higher refractive index than that associated with silica. Moreover, the non-linear refractive index (called Kerr index) is high, which can positively affect the generation of the Brillouin process but make the threshold of optical damage weaker or generate an extended spectrum by auto-modulation. phase.

The single-mode power laser device, several variants of which have been described with the support of FIGS. 1 to 3, can be used within a single-mode power amplifier system. This amplification system and its variants are illustrated in FIGS. 4, 5 and 7. In order to maintain the spectral width of the reference oscillator, it is possible to inject a small amount of power at the input 12 of the single-mode laser device. part of the monomode beam from the reference oscillator 41 and frequency shifted Brillouin shift (about 10 GHz). A first variant of said system is shown in FIG. 4. The single-mode laser power device 40 is soldered to the other fibers involved in the system architecture. The complete source is composed of a fibered single-mode reference oscillator 41. The output of the oscillator is divided into two beams 50 and 51 by means of a single-mode fiber coupler. A part is amplified by means of laser diode pumped preamplifiers 42 followed by a fiber isolator 43. The high energy is finally obtained by a last amplifier stage using a highly multimode fiber 44. The output of this fiber is soldered to the input 17 of the single-mode laser power device 40. The other part of the reference oscillator is shifted in frequency Brillouin shift SvB by means of a modulator 45. The output of the modulator 45 is connected to the input 12 of the device single-mode power laser 40 and the various elements of this branch are connected by monomode fibers 46. The single-mode beam obtained from the multimode beam is then recovered at the output 13 of the device at which a collimator 47 is positioned. As shown in FIG. 5, the amplification system described above can be improved by controlling the polarization of the beam by means of a fiber polarization controller 48 which can advantageously use PLZT ceramic (device for the dynamic control of the polarization of a beam). optical wave and method of manufacturing the device, Thales patent No. 02 15994). The resulting monomode output is connected to the input 12 of the single-mode laser power device 40. The single-mode beam obtained from the depolarized multimode beam is then recovered at the output 13 of the single-mode power laser device 40. note that by choosing the appropriate polarization at the polarization controller, it is possible to obtain a linearly polarized beam at the output of the system. In place of the polarization controller, it is also conceivable to apply a well-adapted periodic period stress on the index gradient fiber to promote linear polarization. Indeed, the application of a curvature to an optical fiber induces a coupling of the guided modes with the continuum of the radiating modes. In an optical fiber that is perfectly symmetrical axially and in a transverse plane, all the eigenvalues (modes) can be decomposed according to two eigenfunctions (polarization states). Under the effect of a micro-curvature, the index profile is modified in the plane of application of the curvature. As illustrated in FIG. 6, the implementation of a periodic curvature on a fiber 71 makes it possible to preferentially couple the modes in a state of polarization to the continuum provided that the orientation of the x-y axes is retained. Under the effect of curvature, this change in the index profile results in a change in the spacing between two adjacent modes in the phase space. By applying a periodic micro-curvature, the coupling between adjacent modes is favored. Thus, the application of a periodic disturbance with a correctly chosen wavelength 70 will favor the coupling elimination of modes that are not affected by the curvature. This modal filtering will be accompanied by a selection of a linear polarization state. A variation of this system is shown in Fig. 7 and includes active control of bias and frequency shift. Indeed, the thermal disturbances of the system can cause variations in the polarization of the output beam or energy variations caused by fluctuations in the Brillouin shift frequency. To avoid these fluctuations, a weakly reflecting (0.1%) plate 65 is placed at the exit of the system to take a small part of the beam. The photodiode 64 makes it possible to control the power of the beam. This power is maximized by adjusting by means of an electronic feedback loop 63 the frequency offset introduced by the modulator 45. In the same way, the photodiode 61 is placed on the polarization component orthogonal to that which one seeks to obtain through a polarizer 60. The polarization controller 48 is adjusted by a feedback loop 62 to minimize the signal on the photodiode 61.

Claims (16)

CLAIMS1- A single-mode power laser device comprising an input fiber-bundle section (1) comprising one or more amplifying optical fibers (2) intended to amplify a multimode incident optical beam (3), a fiber-conversion section (4) comprising a fiber optical system (5) for converting the multimode incident optical beam into a single-mode power beam (8), an intermediate fiber-bundle section (9) comprising an optical fiber allowing the multimode incident optical beam (3) to propagate towards the fiber-bundle section conversion device (4) and characterized in that: - the fiber conversion section (4) comprises a graded index passive optical fiber (5), said passive optical fiber having a length such that by Brillouin effect the device generates by reflection of the multimode incident optical beam (3), a single-mode optical power beam (8), - the input fiber section and the intermediate fiber section re respectively comprise at least a first fiber and a second fiber comprising fiber cores (11, 14) in contact so as to transmit the multimode incident optical beam (3) in the fiber-conversion section (4). - The intermediate fiber bundle having a free end (13) constituting the output for a monomode power beam of the device; 2- Device according to claim 1, characterized in that the intermediate fibred section (9) is composed of a double-core index jump fiber whose large core (20) is adapted to the diameter and numerical aperture of the core (7) of the index gradient fiber (5) constituting the conversion fiber-bundle section (4), and whose small core (21) is adapted to the size of the Brillouin shine-filtered monomode reflected beam (8) in the gradient index passive fiber (5). 3- Device according to claim 1, characterized in that the converted fiber section (4) and the intermediate fiber section (9) are composed of the same passive index gradient fiber. 4- Device according to one of claims 1 to 3, characterized in that the fiber input section comprises a plurality of amplifying fibers for amplifying single-mode optical beams and capable of constituting the multimode incident optical beam combination. 5. Device according to one of claims 1 to 3, characterized in that the fiber input section (1) comprises a single amplifying fiber (2) for amplifying a multimode incident optical beam (3). 6. Device according to one of claims 1 to 5, characterized in that the core (7) of the index gradient passive fiber (5) is composed of silica (SiO2). 7- Device according to one of claims 1 to 5, characterized in that the core (7) of the index gradient passive fiber (5) is composed of chalcogenide (As2S3). 8. A singlemode power amplification system comprising the single-mode power laser device (40) according to one of the preceding claims and a fiber-optic monomode reference oscillator (41) generating a monomode beam of frequency vo, characterized in that: comprises a monomode fiber coupler for dividing the monomode beam of frequency vo into first and second optical beams (50) and (51); the first optical beam (50) generated by the fibered single-mode reference oscillator (41) being amplified to obtain a multimode power beam and introduced to the input (17) of the single-mode power laser device (40); the second optical beam (51) generated by the fiber-optic monomode reference oscillator (41) being shifted at the frequency vo + SvB in order to take account of the Brillouin SvB shift; the second optical beam thus shifted in frequency being transmitted to the monomode power laser device (40) via its input (12). 9- power single-mode amplification system according to claim 8 characterized in that it comprises at least one fiber preamplifier (42), a fiber isolator (43), a multimode fiber amplifier (44) to amplify the first of the two optical beams (50) generated by the fibered single-mode reference oscillator (41). 10- power single mode amplification system according to one of claims 8 or 9 characterized in that it comprises a frequency modulator (45) for frequency shift the second of the two optical beams (51) generated by the oscillator fiber-mode single-mode reference (41) at the frequency vo + SvB. 11- power single-mode amplification system according to one of claims 8 to 10 characterized in that it comprises a bias controller (48) connected to the input (12) of the single-mode power laser device (40). 12- power single-mode amplification system according to one of claims 8 to 10 characterized in that a fiber is connected at the output of the frequency modulator, said fiber having a periodic curvature, the wavelength (70) of said curvature being chosen such that modes which are not affected by the curvature are removed by coupling; 13- power single-mode amplification system according to one of claims 8 to 12 characterized in that it comprises a first feedback circuit (63) for adapting the output of the modulator (45) to the variations of the shift of Brillouin involved in the index gradient passive fiber of the single mode laser power device (40). 14- power single-mode amplification system according to claim 12 characterized in that the feedback circuit (63) comprises a weakly reflecting plate (65) and a photodiode (64). 15- power single-mode amplification system according to one of claims 8 to 14 characterized in that it comprises a second feedback circuit (62) for improving the polarization of the optical beam at the output (13) of the single-mode power laser device (40). 16- power single-mode amplification system according to claim 15 characterized in that the second feedback circuit (62) comprises a polarizer (60), a photodiode (61) and a polarization controller (48).