FR3018144A1 - HYBRID OPTICAL AMPLIFICATION DEVICE - Google Patents

Abstract

The invention relates to a device (10) for hybrid optical amplification adapted to amplify an input signal (S) pulse and having a signal wavelength (AO) of between 1 and 2.5 microns comprising: -a fiber coupler (FC) configured to inject into the same fiber the input signal (S) and a pump signal (P) having a pump wavelength (λp), an amplifying optical fiber (FA) doped with an earth ion -rare, configured to amplify the input optical signal (S) so as to output a first amplified signal (SA1) and a residual pump signal (Pr), -a boost crystal (AC) doped with said ground ion -Rare and configured to amplify the first amplified signal (SA1) to generate a second amplified signal (SA2), -An optical adapter (OA) disposed between an output end of the amplifying fiber (FA) and the amplifying crystal ( CA), configured to adapt the first amplified signal (SA1) and the residual pump signal (Pr) to the boost crystal (CA).

Description

FIELD OF THE INVENTION The invention relates to the field of near-infrared light sources, typically wavelengths between 1 μm and 2.1 μm, produced from an amplified laser oscillator. using optical components, fibers or crystals, doped with earth-rare ions, typically Erbium, Ytterbium, Thulium, Holmium ions.

More particularly, the invention relates to laser sources for LiDAR applications for "Light Detection and Ranging" in English terminology, or industrial (machining, marking), based on nanosecond pulses, of duration typically between 0.1 ns and 1000 ns. , and having a high peak power, typically between 100 kW and 10 MW. STATE OF THE ART For LIDAR applications, it is desired to have nano-second pulsed laser sources emitting in the near infrared with a high peak power. The nanosecond domain maximizes the peak power while remaining compatible with existing detection systems (eg avalanche photodiodes). The spectral range towards 1.5 pm, referred to as ocular security, has as such a particular interest. It is also desired to be able to control the profile of the pulses in order to adapt it to different applications and situations. For example, the generation of nanosecond or even sub-nanosecond pulses allows centimeter distance resolution that opens the way to 3D imaging and allows partially hidden objects to be revealed. Conversely, pulses of 100-1000 ns are required when it is desired to implement a coherent detection for Doppler velocity measurement. It has also been shown that the ability to modulate in intensity, in phase, or in frequency a nanosecond pulse could allow a Radar type processing on reception and improve the temporal resolution for a given pulse duration, and the extraction of information such as radial velocity even in incoherent detection. The level of peak power required depends on the range sought and therefore varies according to the application, but it can exceed 1 MW (106 Watts) for a range greater than 10 km. Thus, it is sought to have a laser source architecture capable of generating nanosecond pulses of high peak power in the near infrared and allowing control of the waveform at the output of the laser source. For this we use laser sources and amplifiers doped with different rare-earth ions allow access to different spectral bands of the near infrared. Ytterbium ion Yb3 + allows emission to 1-1.1 pm, Erbium ion Er3 + emission to 1.5-1.6 pm, Thulium ion Tm3 + emission to 1.9-2.0 pm, and Holmium ion Ho3 + emission to 2.05-2.15 pm. The emission of the Er3 + ion towards 1.5-1.6 μm is of particular interest for the production of sources with ocular safety and is compatible with the use of uncooled sensitive detectors based on InGaAs. The fiber laser technology, shown schematically in FIG. 1, partly meets the need mentioned. Starting from a low power OL oscillator, for example a fiber laser diode emitting 10 mW peak, emitting a Sinit signal whose time profile is simply controlled. Then an amplification chain SA composed of a series of optical fiber amplifiers FA1, FA2, FA3 (such as for example an EDFA for "Erbium Doped Fiber Amplifier" in English terminology) which generates amplified signals is used. respectively Sinit / A1, Sinit / A2 Sinit / A3. Each amplifier is pumped by a pump laser, respectively P1, P2> P1, P3> P2, whose intensity increases with the power density of the amplified signal. For example, the use of Erbium doped fiber amplifiers makes it possible to design an amplification chain with a very large gain (-70 dB). Thus, it leads to a peak power of up to more than 100 kW using for example three amplifiers in cascade. In such a chain, illustrated in FIG. 2, the different amplification stages 35 and the inter-stage components, pumping coupler CF1, CF2, CF3, optical isolator 11, 12, 13, bandpass filters, are interconnected. by fiber, which gives this architecture a great robustness. Each stage has its own pump, the residual pump signal from the lower stage being generally stopped with filters or absorbents. Indeed, certain components of the chain, for example optical isolators, do not transmit at the pump wavelength and could therefore be damaged. For Lidar applications, it is important that a single-mode signal propagates and is amplified to the end of the chain to obtain monomodal emission for the LIDAR with a minimum of divergence (beam limited by diffraction). The pump can propagate singly or multimode. In practice, for a standard EDFA type amplifier with a monomode core fiber, the pump signal is monomode if the pump is at 1480 nm but slightly multimode if it is at 980 nm, as is often the case in commercial amplifiers. In order to maximize the extractable energy, it is sought to successively increase the size of the mode of the signal to be amplified propagating in the amplifiers by increasing the diameter of the core Dc of the fiber while decreasing the difference in index Ln between the core and the sheath. , to ensure monomode propagation. Indeed, we define a parameter V which is a normalized propagation constant: V = -rr / À .Dc. ON rze Tr / À .Dc. V2. nl. An At wavelength of the optical signal propagating in the fiber Dc core diameter of the fiber ON numerical aperture of the fiber n1 index of the core A difference in index between the sheath and the core of the fiber. It is shown that V <2.4 for a single-mode propagation.

The standard fiber fabrication technology (MCVD) makes it impossible to control the index difference below about 10-3. This limitation makes it difficult in practice to increase the core diameter of the Erbium doped fibers beyond about 20 pm while guaranteeing monomode propagation for near-infrared wavelengths, for example 1.5 pm. , and a numerical aperture of 0.06. As a result, the extractable energy is of the order of a few hundred micro joules and the peak power limited to a hundred kW for pulses of 1 ns (reference 1). These values limit the range to a few kilometers for LIDAR applications. Other fiber manufacturing technologies can reduce the index difference. This is the case, for example, of micro-structured fibers for which the sheath is made of silica with air holes of sub-wavelength dimension conferring on the structure an effective index slightly lower than that of the doped silica. of the heart. Nevertheless, if such structures effectively increase the diameter of the mode (for example up to 40 μm), the optical guidance is so small that it becomes impossible to bend the fiber without loss, which poses a problem. congestion for many applications. To overcome the limitations associated with fiber laser technology, it is possible to switch to solid-state laser technology using doped crystals such as free propagation Erbium. In this case, because of the large transverse dimensions of the crystal, one can freely choose the diameter of the mode and it is possible to extract much more energy. For example, an Er: YAG oscillator / amplifier architecture emitting at 1645 nm and pumped using 1470 nm multimode laser diodes made it possible to obtain 100 mJ with pulses of about 100 ns. In this case the oscillator was a Q-switched triggered Er: YAG laser generating pulses of about 50 mJ. In this type of laser, a modulator is inserted inside the cavity, which prevents the laser laser until the material is pumped to the maximum. At this time the modulator allows the laser effect and all the energy stored in the cavity is emitted in a short time. The gain of the amplifiers is only about 3 dB, due to the limited length of the crystals, a few centimeters compared to a few meters for the fibers, and in this particular case the low pumping density resulting from the use of laser diodes multimode.

By pumping around 1533 nm using a single-mode erbium-doped fiber laser, a gain of more than 10 dB was demonstrated. However, this remains insufficient to design an amplification chain based solely on this technology from a low power oscillator. In fact, the known studies on the subject use a Q-switch type oscillator delivering energy pulses (-100 pJ) that can then be amplified beyond one milli joule. Although solid laser technology makes it possible to eliminate the limitation in energy per pulse and in peak power, it presents a number of disadvantages. In particular, the use of a Q-switch oscillator followed by an amplifier makes the complex architecture because using many optical or electro-optical components whose alignment is critical. On the other hand, a disadvantage of these Q switch lasers is that the pulse is poorly controlled, and can not be precisely phase and / or amplitude modulated, unlike fiber amplifier structures, in which the weak signal starting from eg a laser diode can be easily modulated. Thus, this type of oscillator does not make it possible to generate pulses with an arbitrary temporal profile, that is to say to amplify complex waveforms. The object of the invention is to overcome the aforementioned drawbacks, and more particularly to achieve a hybrid amplifier having the advantages of fiber amplifiers and to deliver a much higher peak power than that accessible using only fiber amplifiers, the same an order of magnitude than that accessible with solid-state laser technology, while retaining the ability to process complex waveforms.

DESCRIPTION OF THE INVENTION The present invention relates to a hybrid optical amplification device adapted to amplify a pulse input signal and having a signal wavelength of between 1 and 2.5 dam comprising: a configured fiber coupler for injecting into the same fiber the input signal and a pump signal having a pump wavelength, an optical fiber amplifying amplifier doped with a rare-earth ion, connected to the fiber coupler, and configured to amplify the optical signal inputting by 5 energy transfer from the pump signal so as to output a first amplified signal and a residual pump signal, -an amplifying crystal doped with said rare-earth ion and configured to amplify the first signal amplified by energy transfer from the residual pump signal, so as to generate a second amplified signal, an optical adapter disposed between an end of nettle of the amplifying fiber and the amplifier crystal, configured to adapt the first amplified signal and the residual pump signal to the amplifier crystal. Advantageously, the amplifying fiber is monomode for signal and pump wavelengths. According to one embodiment, the fiber coupler is configured to inject the input signal and the pump signal into a single monomode fiber. Alternatively, the fiber coupler comprises a transport fiber directly connected to the amplifying fiber. Advantageously, the rare earth ion is the Erbium ion and wherein the signal wavelength is between 1532 nm and 1617 nm. Advantageously, the crystal is a YAG crystal. According to one embodiment, the output end of the amplifying fiber is soldered to a non-core bit and wherein the optical adapter comprises a refractive lens. According to another embodiment, the optical adapter comprises a graded index fiber segment directly welded to the output end of the amplifying fiber. Alternatively, the device according to the invention further comprises a polarizer disposed at the input of the amplifier crystal, and a quarter wave plate then a mirror on the opposite side of said amplifier crystal, so that the amplification of the first signal amplified by double pass. Alternatively, the device according to the invention further comprises an optical isolator disposed at the input of the amplifier crystal, and a quarter-wave plate and then a mirror on the opposite side of said amplifier crystal, so that the amplification of the first Amplified signal is made by quadruple pass.

According to another aspect, the invention relates to a hybrid amplification system comprising the hybrid amplification device according to the invention, as well as a laser oscillator configured to generate an initial signal and a preamplifier comprising at least one configured amplifying fiber. for amplifying the initial signal and generating the input signal of said device. Advantageously, the laser oscillator is a fiber diode laser directly modulated current, or modulated by an external electro-optical modulator. According to one embodiment, the laser oscillator is configured to generate an initial signal comprising pulses with a duration of between 0.1 and 1000 ns and having a variable repetition rate. In a variant, the laser oscillator is configured to generate an initial signal comprising frequency and / or amplitude modulated pulses. Advantageously, the system according to the invention further comprises a pump laser 15 configured to generate a pump signal which has, at the input of the amplifying fiber, a power greater than the power necessary to saturate the absorption of the amplifying fiber with the pump wavelength. Advantageously, the pump laser and the laser oscillator are configured so that the pump and signal wavelengths respectively correspond to the excitation and de-excitation of the same energy level of the earth-ion. rare in the crystal matrix. Advantageously, the pump laser comprises an Erbium or Erbium-Ytterbium doped fiber laser emitting around 1533 nm. According to one embodiment, the pump laser is further configured to generate an additional pump signal coupled to the output of the amplifying fiber using an additional fiber coupler and wherein the amplifier crystal is configured to amplifying the first amplified signal from and the residual pump signal and the additional pump signal. Other features, objects and advantages of the present invention will appear on reading the detailed description which follows and with reference to the appended drawings given as non-limiting examples and in which: FIG. 1 already cited illustrates the amplification principle by optical fiber doped from a laser oscillator. FIG. 2 already mentioned illustrates the architecture and the various optical components used in an optical fiber amplifier. FIG. 3 schematizes the hybrid amplification device according to the invention. FIG. 4 illustrates an example of hybrid amplification device according to the invention. FIG. 5 illustrates another example of hybrid amplification device according to the invention, in which the fiber coupler comprises a transport fiber. FIG. 6 illustrates another example of hybrid amplification device 10 according to the invention, in which the offset of the signal and of the pump is made by two different fibers upstream of the fiber coupler. FIG. 7 illustrates an optical adapter variant of an amplification device according to the invention. FIG. 8 illustrates another variant of the optical adapter of an amplification device according to the invention. FIG. 9 illustrates an energy level of a crystal according to the invention. FIG. 10 shows the variation as a function of wavelength at absorption and emission effective cross sections of erbium in silica. FIG. 11 shows the variation as a function of wavelength at erbium absorption and emission cross sections in a YAG crystal. FIG. 12 shows the peak power obtained at the output of the amplifying fiber as a function of the length of this fiber. FIG. 13 shows the evolution of the pumping power as a function of the length of the amplifying fiber. FIG. 14 shows the peak power obtained at the output of the crystal, the Er: YAG amplifier as a function of the length of the crystal. FIG. 15 shows the evolution of the power of the pump Ppr as a function of the length of the crystal. FIG. 16 illustrates an embodiment of the device according to the invention in which the signal makes a double pass through the amplifier crystal. FIG. 17 illustrates an embodiment of a system according to the invention. DETAILED DESCRIPTION OF THE INVENTION FIG. 3 depicts a hybrid optical amplification device 10 according to the invention. The device 10 is able to amplify an input signal S 5 pulse signal wavelength λ0 between 1 and 2.5 pm is the near infrared. The device 10 comprises an FC fiber coupler configured to inject into the same fiber the input signal S and a pump wavelength pump signal λp. Preferentially, the input signal S is derived from an oscillator OL pre-amplified by a PreA amplifier comprising at least one conventional amplifying fiber, and the pump signal comes from a pump laser PL, detailed below. An example of a conventional fiber coupler, illustrated in FIG. 4, is of the directional type. This type of coupler is made by fusion / stretching from two single-mode fibers and allows a coupling by evanescent waves in the silica without interfaces with very low insertions losses, thus allowing to withstand a high power. The coupler can also be made from a dichroic mirror mounted in a fiber bundle. The device 10 also comprises an earth-rare-ion doped FA amplifying optical fiber, connected to the FC fiber coupler, and configured to amplify the input optical signal S by energy transfer from the pump signal P of to output a first amplified signal Sm and a residual pump signal Pr. The amplifying fiber FA is of conventional technology. The fiber coupler FC is a conventional fiber coupler which injects the input signals S and pump P directly into the same fiber which is the amplifying fiber FA, as shown in FIGS. 3 and 4. These couplers are for example standard components used for optical telecoms, possibly "hardened" to support high power (> 10 W). According to a variant illustrated in FIG. 5, the FC fiber coupler comprises a monomode FT transport fiber directly connected to the amplifying fiber FA. The signals P and S are in this case injected into the transport fiber FT by means of a conventional fiber coupler made by drawing fusion or using dielectric mirrors. The fiber offset of the input signal and the pump is advantageous in many embedded applications. The single-mode aspect of the transport fiber guarantees a good overlap of the P and S beams at the input of the amplifying fiber FA, as well as a small divergence of the signal beam at the output of the transport fiber, as explained above.

According to another variant illustrated in FIG. 6, the offset is made separately by two different fibers, a transport fiber FT for the offset of the input signal S, and another fiber for the pump signal P, which is part of the pump laser. or connected to it.

The device 10 further comprises an amplifying crystal CA doped with the same rare-earth ion as the amplifying fiber, which is configured to amplify the first amplified signal SA1 by transfer of energy from the residual pump signal Pr, so as to generating a second amplified signal SA2. Thus the power of the pump signal P must be sufficient at the input of the amplifying fiber FA, so that the latter does not absorb all of its energy, and and the residual pump power Pr at the output of the amplifying fiber FA is sufficient to serve as a pump signal input to the amplifier crystal to perform an amplification of SAI in the AC amplifier crystal.

Finally, the device 10 comprises an optical adapter OA disposed between the output end of the amplifying fiber FA on the side of the crystal CA and the amplifying crystal CA, configured to adapt the first amplified signal SAI and the residual pump signal Pr to the amplifier crystal. IT. This adaptation consists in collimating and widening the respective diameters of the first amplified signal SAI and the residual pump signal Pr in order to adapt the dimensions of the signal and pump beams to the amplifying crystal CA. This last amplification stage is therefore carried out in free propagation. Alternatively, an optical isolator is positioned between the optical adapter and the CA crystal.

The signals at the output of the fiber FA are divergent, and it is appropriate to optimize the amplification in the crystal to illuminate it with beams having a diameter adapted to the amplification in the AC crystal, that is to say say bundles configured to minimize the volume of the pumped crystal, given the diffraction of the pump beams and the signal, and the length of the crystal. When the pumped volume is minimized, the pumping density and gain are maximized. The optical adapter fulfills this role by achieving optimal focusing of the pump and signal beams. An approximate value of the optimum diameter D (typically at the center of the crystal) is obtained by considering a Gaussian beam focused at the center of the crystal and calculating the diameter of the beam D at the focusing point (waist) minimizing the volume of the beam in the crystal. This diameter D is: L D n.rc. With, L: length of the crystal n: optical index of the crystal: signal wavelength / pump Thus the hybrid amplification device 10 comprises an optical fiber amplifier FA, following any other stages of amplifications to fiber, and whose last amplification stage comprises a CA doped crystal in free propagation. This architecture makes it possible to push back the energy / peak power limitations related to the sole use of amplifying fibers, while retaining most of their advantages. In fact, compared to an all-fiber system, all that is required is to add a doped crystal rod at the end of the chain (a conventional power fiber laser system is generally terminated by a collimator, that is to say a lens of collimation possibly followed by an isolator). This free-propagation last stage is relatively simple to align, and is capable of delivering pulses having energy and peak power equivalent to that available with solid-state laser technology.

In addition, the architecture of the device is such that the pump signals Pr and signal to be amplified Sm originate from the same fiber FA: there is necessarily a good spatial overlap between them, which minimizes the number of optical components of the unguided portion. .

According to a variant illustrated in FIG. 7, the output end of the amplifier crystal-side amplifying fiber is provided with a core-free tip E (commonly referred to as "end-cap"), which is conventionally used to prevent optical damage to the amplifier. interface with the air. The optical adapter OA comprises in this case a simple refractive lens Lr, low cost, available and allowing great flexibility for adapting the size of the mode. According to another variant illustrated in FIG. 8, the optical adapter OA comprises an index gradient lens segment Lgi directly welded to the output end of the amplifying fiber. The length of the lens and the amplitude of the index gradient are chosen to allow collimation of the Pr and SA1 beams with the desired diameter. This eliminates a component in free propagation (the refractive lens). According to a preferred variant, the amplifying fiber FA is compatible with a monomode amplification for the signal wavelength λ 0, to obtain a first amplified signal of low divergence. According to a preferred variant, the amplifying fiber FA is also monomode for the pump wavelength λp. This property is obtained by the choice of the numerical aperture and the core diameter of the FA fiber. The single-mode character of the propagation in the FA fiber of the amplified signal and the pump allows a good recovery of these beams in the crystal. To obtain this property, the wavelengths λ0 and λp must be close enough. This is verified when, advantageously, the pump wavelengths Ap and the signal O0 respectively correspond to the excitation and the deexcitation of the same energy level 90 of the rare-earth ion in the crystal matrix, respectively from and to the fundamental level 91, as shown in Figure 9.

At the output of the amplifying fiber FA, the signals Pr and SA1 are automatically aligned, which allows to obtain automatically, without alignment, a very good recovery in the crystal using a single lens as an optical adapter.

In order for the energy transfers to operate correctly, the wavelength λ 0 must lie in the gain band of the doped fiber FA and coincide with one of the emission lines of the doped crystal CA, and the length of the lep wave must be in the absorption band of the doped fiber FA and coincide with one of the absorption lines of the CA doped crystal.

Advantageously, the rare earth ion is the Erbium ion and the crystal is a Y3A15O12 or YAG matrix. Erbium doping allows emission in the 1.5 μm ocular safety band and the use of InGaAs avalanche detectors. FIG. 10 shows the wavelength-dependent variation at the o-aeff and creeff emission cross sections of erbium in silica, and FIG. 11 shows the pump line absorption lines. And "laser lines" emission of erbium in a YAG crystal. Thus, for an erbium doping, the signal wavelength λ 0 is preferably between 1532 nm and 1617 nm, and is advantageously chosen such that it coincides with an erbium-doped YAG laser line, for example λ 0 = 1617 nm. This wavelength is preferably pumped with a pump signal λp = 1533 nm, which corresponds to the absorption maximum in the erbium-doped silica and to an absorption line in the erbium-doped YAG. Other Erbium doped crystalline matrices are characterized by emission lines at shorter wavelengths than 1617 nm, which reduces the length of the amplifying fiber FA because the gain is greater. Reducing this fiber length minimizes the nonlinear effects that can distort the pulse and broaden its optical spectrum and thus maximize the peak power at the output of the FA fiber. The wavelength of 1617 nm is in fact at the limit of the gain band of the erbium-doped silica fiber amplifiers (FIG. 10). By way of example, the YLF matrix allows emission towards 1601 nm, and the YVO4 matrix allows emission towards 1580 nm. In general, there are various possible matrices in the form of single crystals or ceramics allowing emission in the range 1532-1617 nm, which coincides with the gain band of the fiber amplifiers. In addition, depending on the rare earth ion used, appropriate signal and pump wavelengths should be chosen, with the constraint that they correspond to commercial sources (laser oscillator and pump laser). According to one example, the rare earth ion is the Ytterbium ion, the signal wavelength λ0 is between 1 μm and 1.1 μm, and the pump laser PL is a neodymium doped fiber laser of pump wavelength λp = 910 nm or a Ytterbium doped fiber laser of pump wavelength λp = 980 nm. According to another example, the rare earth ion is the Thulium ion, the signal wavelength λ0 is between 1.9 μm and 2.05 μm and the pump laser is an Erbium doped fiber laser with wavelength λp between 1530 nm and 1600 nm. According to another example, the rare earth ion is the Holmium ion, the signal wavelength λ0 is between 2.05 μm and 2.15 μm, and the pump laser is a Thulium doped fiber laser with pump wavelength λ. between 1900 nm and 2000 nm. We will now describe a preferred embodiment of a hybrid amplification device according to the invention, specifying the sources, signal and pump, adapted to its implementation in the example. Its architecture is that of FIG. 5. A pulse laser oscillator OL consists of a modulated fiber-optic diode emitting at λ = 1617 nm which generates an initial signal Sinit. As previously described, this wavelength corresponds to an amplification line of the Er: YAG crystal and is also compatible with the gain band of the erbium doped fiber amplifiers. According to one embodiment, the laser oscillator is directly modulated by the injection current. According to another embodiment, the laser oscillator operates continuously and the modulation is performed using an external fiber optic electro-optical modulator.

Advantageously, the laser oscillator OL is configured to generate an initial Sinit signal comprising pulses with a duration of between 0.1 and 1000 ns and having a variable repetition rate. Advantageously, the laser oscillator OL is configured to generate an initial Sinit signal comprising frequency-modulated and / or phase-modulated pulses, that is to say configured to generate a complex waveform to be amplified. The initial modulated signal of this Sinit diode is then pre-amplified using a PreA preamplifier comprising a plurality of conventional Erbium Doped Fiber Amplifiers (EDFAs) up to a peak power level. compatible with FT fiber transport over several meters without harmful non-linear effects. The signal resulting from this pre-amplification and then transported in the fiber FT forms the signal S injected at the input of the hybrid amplification device. The optical fiber offset of this first part of the source is not mandatory but may be advantageous in a large number of embedded applications. The pre-amplified laser diode emits gaussian envelope pulses with a half-height width of 1 ns with a peak power of 100 W at a rate of 10 kHz. This power is propagated without distortion in the transport fiber. More than one fiber pumping laser PL emitting at the 1533 nm wavelength compatible with both the pumping of the erbium doped fiber amplifiers and the pumping of the erbium doped crystal amplifiers, as described previously, is available. This type of laser is commercially available. This is for example a laser emitting 50 W continuously, which is an erbium doped fiber laser or a co-doped ytterbium-Erbium fiber laser. The pump laser PL generates a signal P which is coupled with the signal S at 1617 nm in the transport fiber FT by means of a conventional fiber coupler made by melting fibers or using mirrors dielectrics. At the end of the FT transport fiber, an erbium-doped fiber section FA is soldered with a compatible core diameter and numerical aperture of monomode amplification and making it possible to extract a maximum of energy (typically one hundred micro joules). with a core diameter of 20 μm and a numerical aperture of 0.09). In addition, the pumping power at the input of this amplifying fiber is sufficient to saturate the absorption in the fiber FA so that the major part of the pumping power remains available at the output of this fiber FA (corresponding to Pr ) for the next amplification stage in the CA crystal. In other words, the pump laser PL is configured to generate a pump signal which has, at the input of the amplifying fiber FA, a power greater than the power necessary to saturate the absorption of the amplifying fiber FA with the pump wavelength λp throughout the fiber. The saturation power Pp, sat defined as the power necessary to excite half of the ions of the ground state to the excited state for the absorption of the pump, is given by the following formula: hv A Pp, sat - e where ae, p and oe, p are the emission and absorption cross sections at the pump wavelength, z the lifetime of the excited level, and average area of the pump beam in the fiber. In this example, the power of 50 W corresponds to about 1000 times the saturation power at the pump wavelength for the fiber in question. Moreover, it is also necessary that the energy of the signal to be amplified at the input of the fiber is very low compared to the saturation energy of the emission, and becomes comparable to it only at the end of the fiber. amplifying so as not to saturate the absorption. The saturation energy Es, sat corresponds to the incident energy necessary for the deexcitation of half of the excited ions by stimulated emission, and is given by the following formula: h. v s. At Cre, its, $) where, 0, s, and cra, s are the emission and absorption cross sections at the wavelength of the signal, and at the average area of the signal beam. In the example taken, the incident energy of the signal at the input of the fiber FA is 0.1 pJ, the amplified energy at the output of fiber FA is about 200 pJ for 7.5 m of fiber, and the saturation energy Es, sat is about 500 pJ.

Note that the power dissipated in this fiber amplifier FA is low enough to be able to overcome a cooling system and it is therefore possible to simply have the amplifying fiber FA in the optical cable at the end of the transport fiber. At the output of the amplifier, the amplified signal Sm and the residual pump Pr are automatically aligned and with a very good spatial overlap since the two wavelengths λ0 and λp are very close. This configuration allows using a single lens to couple the two beams Sm and Pr with the required diameter in the Er: YAG crystal for a final amplification step.

The following figures were obtained by numerical simulation, using the known spectroscopic parameters of the erbium-doped silica fibers and the erbium-doped YAG matrix. FIG. 12 shows the peak power Pc1 (signal SAI) obtained at the output of the fiber amplifier FA as a function of the length of this fiber. The selected fiber corresponds to an erbium doped fiber commercially available with a core of 20 μm, a numerical aperture of 0.09, and a doping level corresponding to an absorption of 120 dB / m at 1532 nm. A peak power of 200 kW (200 pJ) is obtained for a fiber length of approximately 7.5 m. FIG. 13 shows the evolution of the pumping power Pp (signal P) as a function of the length of the fiber FA. After 7.5 m, the residual pumping power is Pr = 45 W. The thermal power dissipated in the amplifying fiber is less than 3 W, which does not require special thermal management. This value of 45 W is large enough that Pr can again serve as a pump in the CA crystal. FIG. 14 shows the peak power Pc2 obtained at the output of the CA Er: YAG amplifier (signal SA2) as a function of the length of the crystal 35 with an incident pump power Pr of 45 W and an incident peak power of the signal of 200. kW. It was assumed that the beams had a diameter of 200 μm in the crystal. About 1 MW of peak power Pc2 is obtained for an interaction length of 10 cm and 1.75 MW for 15 cm. Figure 15 shows the evolution of the pump Ppr under the same conditions. There is a very significant amplification of the signal by the CA crystal. Alternatively, if it is necessary to cool, the amplifying fiber it is possible to arrange the fiber FA in the emitting head, for example for a higher rate of fire. In order to reduce the crystal length necessary to obtain a given peak power, according to one embodiment, a double pass of the signal or of the signal and of the pump is made using a Pol polarizer, a quarter blade. LQO wave, and a mirror M as illustrated in Figure 16. This assumes to have a rectilinear polarized beam for at least the signal and therefore the upstream use of polarization-maintaining fibers (PM). Likewise, four passages are made in the crystal by replacing the polarizer with an optical isolator and adding a mirror. These modifications reduce the length of the crystal. According to another aspect, the invention also relates to a hybrid amplification system 100 comprising the hybrid amplification device 10 that an OL laser oscillator configured to generate an initial Sinit signal, and a PreA preamplifier comprising at least one amplifying fiber. FA1, FA2, FA3 ... configured to amplify the initial signal Sinit and to generate the input signal S of the device 10, as illustrated in FIGS. 5 and 6. As previously described, advantageously the laser oscillator OL is a Fiber laser diode is directly modulated in current, or continuous and modulated by an external electro-optical modulator. Advantageously, the laser oscillator OL is configured to generate an initial signal comprising pulses with a duration of between 0.1 and 1000 ns and having a variable repetition rate. Advantageously, the laser oscillator OL is configured to generate an initial signal comprising frequency and / or amplitude modulated pulses.

Advantageously, the system 100 further comprises a pump laser PL configured to generate a pump signal having, at the input of the amplifying fiber FA, a power greater than the power required to saturate the absorption of the amplifying fiber FA with the pump wavelength λp. This condition ensures that a significant portion of the pump is not absorbed by the fiber and remains available for pumping the AC crystal. Advantageously, as explained above, the pump laser PL and the laser oscillator OL are configured so that the pump wavelength λp and the signal λ0 correspond to the excitation (for the pump) and the de-excitation (for the emission of the amplified signal) of the same energy level of the rare-earth ion in the CA crystal matrix. This property ensures wavelengths λp and λ0 sufficiently close so that the amplifying fiber FA is single-mode for the two wavelengths, ensuring at output of fiber FA a self-alignment of the two beams, allowing these beams to adapt to diameter required in the CA crystal using a simple lens, refractive or gradient index. The pump laser can operate continuously or in pulse mode.

Advantageously, the pump laser PL comprises an Erbium doped fiber laser or co-doped Ytterbium-Erbium emitting around 1533 nm. These sources are commercially available with the required conditions: power of 50 W, singlemode, and having a good performance and a small footprint.

According to another embodiment, the pump laser PL is a Raman fiber laser laser emitting at about 1480 nm which also corresponds to a pumping wavelength common to the Er-doped silica fibers and the Er-doped crystals (in FIG. especially the YAG). This type of laser, which is obtained by Raman cascade from a Yb fiber laser, is commercially available. This source makes it possible to have more gain per unit of length, thus to minimize the length of the fiber and thus to allow more peak power with the appearance of harmful effects. According to an embodiment illustrated in FIG. 17, the pump laser PL of the system 100 is further configured to generate an additional pump signal Padd coupled with the output of the amplifying fiber by means of an additional fiber coupler Cadd. In this case, the amplifying crystal CA is configured to amplify the first amplified signal Sm from and the residual pump signal Pr and the additional pump signal Padd. This arrangement makes it possible to lift the condition on saturation of the absorption throughout the fiber and on the input level of the signal in the fiber FA, which introduces more flexibility for the design of the amplification device.

Claims (9)

CLAIMS1 Hybrid optical amplification device (10) capable of amplifying an impulse input signal (S) and having a signal wavelength (λ 0) of between 1 and 2.5 pm comprising: a fiber coupler (FC) configured to inject into the same fiber the input signal (S) and a pump signal (P) having a pump wavelength (λp), an optical fiber an amplifier (FA) doped with a rare-earth ion, connected to the fiber coupler (FC), and configured to amplify the input optical signal (S) by energy transfer from the pump signal (P) so as to outputting a first amplified signal (SA1) and a residual pump signal (Pr); -a boost crystal (AC) doped with said rare-earth ion and configured to amplify the first amplified signal (SA1) by transfer of energy from the residual pump signal (Pr), so as to generate a second amplified signal (SA2), -an optical adapter (OA) disposed between an output end of the amplifying fiber (FA) and the amplifying crystal ( CA) configured to adapt the first amplified signal (SA1) and the pump signal r esiduel (Pr) to the amplifier crystal (CA). 2. Device according to claim 1 wherein the amplifying fiber (FA) is monomode for signal (A0) and pump (Ap) wavelengths. 25 3. Device according to one of the preceding claims wherein the fiber coupler is configured to inject into the same single-mode fiber the input signal (S) and the pump signal (P). 4. Device according to one of the preceding claims wherein the fiber coupler (FC) comprises a transport fiber (FT) directly connected to the amplifying fiber (FA). 5. Device according to one of the preceding claims wherein the rare earth ion is the Erbium ion and wherein the signal wavelength is between 1532 nm and 1617 nm. 6. Device according to one of the preceding claims wherein the crystal is a YAG crystal. 7. Device according to one of the preceding claims wherein the output end of the amplifying fiber is welded to a nozzle (E) 10 devoid of heart and wherein the optical adapter comprises a refractive lens (Lr). 8. Device according to one of claims 1 to 6 wherein the optical adapter (OA) comprises a graded index fiber segment (Lgi) 15 directly welded to the output end of the amplifying fiber. 9. Device according to one of the preceding claims further comprising a polarizer (Pol) disposed at the input of the amplifier crystal, and a quarter wave plate (LQO) and a mirror (M) on the opposite side of said crystal amplifier , so that amplification of the first amplified signal is by double pass. Apparatus according to one of claims 1 to 8, further comprising an optical isolator disposed at the input of the amplifier crystal, and a quarter-wave plate (LQO) and a mirror (M) on the opposite side of said amplifier crystal, so that the amplification of the first amplified signal takes place by quadruple pass. A hybrid amplification system (100) comprising the hybrid amplification device (10) according to one of the preceding claims as well as a laser oscillator (OL) configured to generate an initial signal (Sinit) and a preamplifier ( PreA) comprising at least one amplifying fiber (FA1, FA2, FA3) configured to amplify the initial signal (Sinit) and generate the input signal (S) of said device (10). The system of claim 11 wherein the laser oscillator (OL) is a fiber laser diode directly modulated in current, or modulated by an external electro-optical modulator. 13. System according to one of claims 11 or 12 wherein the laser oscillator (OL) is configured to generate an initial signal comprising pulses with a duration between 0.1 and 1000 ns and having a variable repetition rate. 14. System according to one of claims 11 to 13 wherein the laser oscillator (OL) is configured to generate an initial signal comprising pulse modulated frequency and / or amplitude. 15. System (100) according to one of claims 11 to 14 further comprising a pump laser (PL) configured to generate a pump signal which has, at the input of the amplifying fiber (FA), a power greater than the power required to saturate the absorption of the amplifying fiber (FA) at the pump wavelength (λp), 16. The system of claim 15 wherein the pump laser (PL) and the laser oscillator (OL) are configured so that the pump (λp) and signal (λ0) wavelengths respectively correspond to the excitation and de-excitation of the same energy level of the rare-earth ion in the crystal matrix. . 17. System according to one of claims 15 or 16 wherein the pump laser (PL) comprises an Erbium-doped or Erbium-Ytterbium co-doped fiber laser emitting at 1533 nm. The system of one of claims 15 to 17 wherein the pump laser (PL) is further configured to generate an additional pump signal (Padd) coupled to the output of the amplifying fiber (FA) using an additional fiber coupler (Cadd) and wherein the amplifier crystal (CA) is configured to amplify the first amplified signal (Sm) from and the residual pump signal (Pr) and the additional pump signal (Padd) .