WO2007028783A1

WO2007028783A1 - Method for producing a power laser beam and device therefor

Info

Publication number: WO2007028783A1
Application number: PCT/EP2006/065979
Authority: WO
Inventors: Laurent Lombard; Arnaud Brignon; Jean-Pierre Huignard; Patrick Georges
Original assignee: Thales
Priority date: 2005-09-06
Filing date: 2006-09-04
Publication date: 2007-03-15
Also published as: FR2890496B1; FR2890496A1

Abstract

The invention concerns a device for producing a power laser beam, characterized in that it comprises a master oscillator followed by preamplifying stages (1), a multimode amplifying fiber (2) and a loop circuit (3) including a circulator (8 to 12) and a multimode optic fiber (4), a spatial filtering device (6) and an output coupling device (14).

Description

METHOD FOR PRODUCING A POWER LASER BEAM AND DEVICE FOR IMPLEMENTING THE SAME

The present invention relates to a method for producing a power laser beam, and to a device for implementing this method.

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 (> 100 W) have been studied for industrial applications. The advantages of fiber lasers on conventional solid state lasers (Nd: YAG for example) are indeed numerous: - High robustness - the optical bench in a fiber laser is the fiber itself -> no misalignment problem as in a classic laser,

High Efficiency - Through pump guidance and spatial overlap between the pump and the amplified signal, electrical-optical efficiencies up to twice as high as for a pumped solid diode laser can be achieved. Better distribution of heat in the active medium, which reduces the problems related to the management of thermal mode guidance, which allows to obtain a beam of high spatial quality (single mode beam),

Ability to achieve significant gains and realize effective amplifiers,

Possibility of obtaining an eye-safe emission (1, 5 μm) of very high efficiency (Er-doped fibers), - 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 will be made impossible by problems of resistance to the luminous flux of the single-mode fiber and the appearance of parasitic nonlinear phenomena (Raman effects, Brillouin, ...). 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 the use of power lasers fiber amplifying very large diameter (> 100 microns), very strongly multimode, 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 convert the multimode beam into a single-mode beam, it is possible to use nonlinear interactions such as two-wave mixing or phase conjugation. The use of the two-wave mixture in photorefractive materials and the stimulated Brillouin scattering phase conjugation in a fiber are known to achieve mode conversion. But these two techniques are not well adapted to the Lidar application. Indeed, the photorefractive materials are not yet well optimized at the wavelength of 1, 5 microns (ocular safety zone). Moreover, the phenomenon of Brillouin phase conjugation in the fibers starts on a noise distributed randomly all along the fiber (spontaneous Brillouin emission). This random distribution of the noise has the effect of returning a conjugate wave which is strongly temporally modulated, which broadens the laser emission spectrally and renders the laser pulse unusable for a Lidar.

The present invention relates to a method of producing a power laser beam for producing a single-mode beam of high power (greater than 100 W) which is not affected by parasitic phenomena, in particular non-linear, and having a very small spectral width (less than a few MHz).

The present invention also relates to a device for producing a power laser beam, making it possible to obtain a beam having very good spectral and temporal characteristics, which device is the lightest and the least bulky possible.

The method according to the invention is characterized in that the laser beam of a master laser oscillator is amplified with the aid of an amplifying optical fiber with a large core, which is converted into a multimode laser beam thus amplified. in a monomode beam by Brillouin-effect two-wave mixing in a multimode fiber. Advantageously, this mixing is carried out in a multimode gradient index fiber.

The power amplification device according to the invention is of the type comprising a master oscillator followed by pre-amplification stages, and it is characterized in that it comprises, at the output of the preamplification stages, a power amplifying fiber. multimode and a loop circuit comprising a circulator, a multimode optical fiber, a spatial filtering device and an output coupler device.

The present invention will be better understood on reading the detailed description of an embodiment, taken by way of nonlimiting example and illustrated by the appended drawing, in which:

FIG. 1 is a simplified block diagram of a power laser source according to the invention, and

FIGS. 2 to 4 are simplified block diagrams of variants according to the invention of the laser source of FIG. 1.

FIG. 1 shows a master oscillator 1 with its preamplifiers making it possible to reach a power (for example 1W) or an energy per pulse sufficient to be able to saturate the amplifier stage 2 which follows it. The set of preamplifiers of the oscillator 1 is advantageously entirely made of doped amplifying optical fibers and may include, in a well known manner, optical isolators and spectral filters. At the output of the set 1 master oscillator + preamplifiers, the beam is single-mode space. The amplifier 2 consists of a very highly multimode optical fiber with a large heart (for example having a diameter greater than 100 microns) to avoid all nonlinear parasitic phenomena such as stimulated Brillouin scattering, as well as the optical damage of the fiber. On the other hand, the amplified beam at the output of this last stage 2 is highly multimode spatial. At the output of this fiber 2, the optical energy is for example between 1 and 10 mJ. In order to find a monomode beam, necessary for most Lidar type applications, the invention proposes to use a Brillouin effect two-wave mixing interaction in a multimode optical fiber with fundamental mode conservation, which is, according to a preferred embodiment of the invention, an index gradient optical fiber (hereinafter referred to as "Gl fiber") for example. It may also be possible to use an optical index jump fiber (SI fiber) provided that the single-mode beam is able to propagate without deformation or depolarization along such a fiber (what is can obtain, for example, when this fiber is kept perfectly straight). To simplify the explanations of the operation of the devices of FIGS. 1 to 3, only a fiber G 1 will be considered below, it being understood that it will be possible to use an IS fiber in its place if the condition mentioned therein is respected. - above. FIG. 1 shows a first embodiment of an optical circuit 3 performing in particular the multimode-monomode conversion and incorporating a fiber Gl referenced 4. The circuit 3 is in the form of a loop essentially comprising, in addition to the fiber 4, a circulator 5 disposed at the output of the multimode amplifying fiber 2, a spatial filter 6 and an output coupler 7. In the present embodiment, the circulator 5 comprises a first collimation lens 8 whose object focus is confused with the output of the amplifying fiber 2, a first polarizer 9, a Faraday rotator 10, a second polarizer 11 and a second collimation lens 12 whose focal point is coincidental with one of the ends of the fiber 4, end referenced 4A . The polarizer 9 also serves as a splitter blade. Part of the beam that it receives from the lens 8 is sent to the rotator 10, and the other is sent to a half-wave plate 13 followed by a polarizer-separator 14 constituting the aforementioned coupler 7. Following the polarizer-separator 14, in the axis of the beam coming from the half wave plate 13, a mirror 15 is provided which returns the portion of the beam from the separator 14 in the direction of said axis to the spatial filtering device 6. The spatial filter 6 comprises, for example, a first lens 16 at the focus of which is disposed a filtering hole 17, followed by another collimating lens 18 whose object focus coincides with the image focus of the lens 16. The lens 18 is followed by a focusing lens 19 whose object focus coincides with the second end, referenced 4B, of the optical fiber 4.

In the device of FIG. 1, the multimode beam coming from the large-core amplifying fiber 2 is injected into a fiber Gl 4 by means of lenses (8 and 12) after having passed through the rotator-shaped Faraday isolator. Faraday (10) and two polarizers (9 and 11). The set of elements 8 to 12 constitutes a circulator. The characteristics of the fiber Gl (numerical aperture ONQ _I and core diameter DQ _I ) are chosen such that ON _G | ^χ D _G I> ONampX D _am p, where ON _am p is the numerical aperture of the amplifying fiber 2 and D _am p its diameter. The length of the fiber G 1 is chosen to be large enough for the threshold of the Brillouin effect to be crossed in this fiber. This length can be, for example, several meters.

A remarkable property of the Brillouin effect in a long enough fiber Gl is that the wave (called Stokes wave), which is created in the fiber G1 in the direction of propagation opposite to that of the incident wave (the incident wave is that which enters the fiber 4 by its end 4A), is a unique mode of the fiber, while the incident wave is multimode. On the other hand, the single mode which returns in the fiber 4 by its end 4B is not necessarily the Gaussian fundamental mode that one wishes but can be for example the LP11 mode, according to the conditions of coupling of the multimode beam incident in the fiber Gl. It is therefore a question of lowering the threshold of the mode that one chooses compared to the other modes. The solution of the invention is to browse the loop 3 by the fundamental mode only using a spatial filter. The Stokes wave is reflected by the polarizer 9 and, by means of the mirror 15 and the lens 19 is reinjected into the end 4B of the fiber G1. In order to inject only the fundamental mode and to favor only the latter, said spatial filter, constituted in this example by two lenses and a filtering hole, is inserted in the path of the Stokes wave. The monomode Stokes wave injected at the end 4B of the fiber will then be amplified preferentially by the multimode incident wave. via a two-wave mixing process. This process can be very efficient and the multimode incident wave can transfer substantially all of its energy to the monomode Stokes wave thus created with a typical conversion rate that can be greater than 90%. To extract the single-mode system wave, simply insert on the Stokes wave path a half-wave plate (13) and a polarizer (14). Adjusting the ratio transmitted part / reflected part of the half-wave plate makes it possible to adjust the fraction of the wave that is extracted out of the system (output beam 20) and the fraction that is fed back into the loop 3. A study theoretical shows that a re-injection of about 10% of the total power of the Stokes wave can be sufficient for optimal operation.

The advantage of the process described above is that it is completely self-adapted. Indeed, for a Brillouin two-wave mixing process, it is necessary that the wave that one wants to amplify is shifted in frequency by a quantity δv _B with respect to the frequency v ₀ of the incident wave. δv _B is called Brillouin Shift and is about 15 GHz in a silica fiber. Here, the Brillouin shift is automatically obtained because the wave reinjected at the end 4B of the fiber G1 is none other than the Stokes wave obtained by the stimulated Brillouin effect and thus affected by the Brillouin frequency shift. In the process described above, you only need to inject the fundamental mode into the loop. This means that the filter hole 17 must be very precisely placed and perfectly conjugated with the Fiber Gl. It is advantageous to replace the spatial filter formed by two lenses and a filtering hole of FIG. 1 by a single-mode fiber directly welded to the back of the fiber G 1 as shown in FIG.

In Figure 2, the same elements as those of Figure 1 have been assigned the same reference numbers. These same elements are those referenced 1 to 5 and 7 to 15 and the lens 19 which directly follows the mirror 15. At the image focus of the lens 19, there is an end of a monomode optical fiber 21 whose other end is welded (welded 22) to the end 4B of the fiber 4.

The interest of the single-mode fiber is that it performs the filtering on the higher modes and especially that it is aligned very precisely with the Fiber Gl by means of a standard weld. The input coupling of the single-mode fiber can be coarse: it reduces the efficiency but does not risk not to excite higher modes.

Another improvement is to recycle the depolarized portion of the multimode indicative beam as shown in FIG. 3. In the device of this FIG. 3, all the elements of the device of FIG. 2, which have the same reference numerals, have been taken up, and added to the polarizer 9 a mirror 23, and facing both the mirror 24 and the polarizer 11 a mirror 24, these two mirrors being arranged to reflect a component ("vertical" in this case) of the multimode beam of the way described below.

In this architecture of FIG. 3, the multimode and depolarized beam from the large-core amplifying fiber is separated into two beams having orthogonal polarizations by means of the polarizer 9. The horizontally polarized component (in the plane of the drawing for example) passes through the polarizer 9 and the Faraday RF rotator and is then injected into the fiber 4. The vertically polarized component (perpendicular to the plane of the drawing in the example) is reflected by the polarizer 9 and is returned by means of the two mirrors 23, 24 on the polarizer 11 to be then also injected into the fiber 4. It should be noted that the monomode Stokes wave is horizontally polarized throughout the fiber, and that we rely on the rapid depolarization modes of the Multimode incident wave so that both polarizations of the incident wave can interfere with the Stokes wave. The horizontally polarized Stokes wave passes through the polarizer 11 and the rotator 10 and is reflected by the polarizer 9 in the direction of the half-wave plate 14. If a fraction of the Stokes wave were to be vertically polarized, the insertion a Faraday isolator between the two mirrors 23 and 24 would be necessary to avoid any return in the amplifying fiber which could damage the system.

FIG. 4 shows another embodiment of the device of the invention. This device of FIG. 4 comprises a master oscillator 25 and preamplifiers 26 whose output is followed by a multimode amplifying optical fiber 27. The fiber 27 is connected to a circulator 28 to which is also connected an end of an optical fiber G 1 29. The other end of the fiber 29 is connected to the output of the oscillator 25 by a frequency shift device 30. At the output 31 of the circulator 28, an amplified single-mode laser beam is collected. Oscillator 25 and the preamplifiers 26 may be the same as those of all 1 of the previous embodiments. Similarly, the elements 27, 28 and 29 may be the same as the elements 2, 8 to 12, and 4, respectively.

According to the embodiment of FIG. 4, the monomode wave injected at the rear of the fiber G 1 is not the Stokes wave but a fraction of the monomode wave coming directly from the master oscillator 25. The monomode wave thus injected is then amplified by two-wave mixing by the Brillouin effect in the fiber Gl 29 by the intense and multimode beam obtained at the output of the amplifying fiber 27. In order for the two-wave mixture to function, an offset must be introduced. in frequency on the single-mode beam injected at the rear of the fiber Gl 29. This offset must be equal to the Brillouin shift of the fiber G 1 used. This frequency shift of about 10 to 15 GHz can be obtained by means of an electro-optical component such as a LJNbθ ₃ modulator for example. The accuracy of this shift is about 10 MHz at the wavelength of 1.5 μm.

This device of FIG. 4 is therefore not self-adaptive as was that of FIG. 3, but has the advantage of being able to very precisely control the energy or power of the single-mode beam that is injected as well as its temporal characteristics (pulse shape). The device of FIG. 4 can be entirely made of optical fibers using optical fiber components such as those developed for telecommunication applications. On the other hand, if these components do not have sufficient laser flux resistance, the device can also be made in free space. The optical fiber circulator 28 shown in FIG. 4 can for example be replaced by a Faraday isolator as in FIG. 3, and then formed of two polarizers (9, 11) and a Faraday rotator (10). The recycling of the depolarized part of the multimode incident beam can also advantageously be realized in this variant, in the same way as in the device of FIG. 3. It should be noted that the reference monomode beam taken after the master oscillator 25 Figure 4 can also be taken after the pre-amplification stages 26 if a higher energy is needed on this beam.

This architecture can be sensitive to any variation in temperature, which would modify the value of the frequency offset. To get rid of this difficulty, it is possible to add adaptive control of the frequency offset by driving the electro-optical component by means of a feedback loop controlled by the power or energy of the single-mode output beam. The important parameters of two exemplary embodiments of the device of the invention will be given below.

1. System operating at 1.5 μm using erbium doped fibers, ytterbium for embedded Lidar application (detection of vortices and turbulence):

- Characteristic of the beam at the output of the "master oscillator + fiber pre-amps" assembly: energy 100 μJ pulse duration: 400 ns single-mode spectral and polarized spectral beam

- Characteristic of the output beam of the optical fiber amplifier (double-core fiber, diameter of the 100 μm doped core, diameter of the pump core: 400 μm, length about 3 m): energy 2 mJ pulse duration: 300 ns spatial multimode and depolarized spectral monomode

- Characteristics of the fiber Gl: core diameter: 100 μm length: a few tens of meters

- Characteristics of the beam at the output of the system: energy 1 mJ (total efficiency of the conversion system approximately 50%) pulse duration: 300 ns spatial monomode and spectral polarized

2. Continuous system operating at 1 μm using ytterbium doped fibers for industrial applications: - Characteristics of the output beam of the "master oscillator + fiber optic pre-amps" assembly: 1W continuous power polarized single-mode and spectral power

- Characteristics of the output beam of the fiber amplifier (dual-core fiber, diameter of the 100 μm doped core, diameter of the pump core: 800 μm, length approximately 3 m): output power: 400 W (for approximately 1, 2 pump kW) multimode spatial and single-mode spectral depolarized

- Characteristics of the fiber Gl: core diameter: 100 μm length: 30 m

- Characteristics of the beam at the output of the system: power 200 W continuous (total efficiency of the conversion system approximately 50%) polarized spatial and spectral monomode

Claims

A method of producing a power laser beam, characterized in that the laser beam of a master laser oscillator (1) is amplified by means of a large-core amplifying optical fiber (2), that the multimode laser beam thus amplified is converted into a monomode beam by two-wave mixing by Brillouin effect in a multimode fiber (4).

2. Method according to claim 1, characterized in that the two-wave Brillouin effect mixture is performed in a multimode index gradient fiber.

3. Method according to claim 1, characterized in that the Brillouin effect two-wave mixing is performed in a multimode index jump fiber.

4. A device for producing a power laser beam, characterized in that it comprises a master oscillator followed by pre-amplification stages (1), a multimode power amplifying fiber (2) and a loop circuit (3). comprising a circulator (8 to 12, 28), a multimode optical fiber (4, 29), a spatial filtering device (6, 30) and an output coupler device (14, 28).

5. Device according to claim 4, characterized in that the multimode optical fiber of the loop circuit is of the index gradient type.

6. Device according to claim 4, characterized in that the multimode optical fiber of the loop circuit is of the index jump type.

7. Device according to one of claims 4 to 6, characterized in that the spatial filter comprises a filter hole (17) and collimation lenses (16, 18).

8. Device according to one of claims 4 to 6, characterized in that the spatial filter is constituted by a monomode optical fiber (21) welded (22) at one end of the multimode fiber of the loop circuit.

9. Device according to one of claims 4 to 8, characterized in that the circulator comprises a Faraday rotator (10) disposed between two polarisers (9, 11).

10. Device according to claim 9, characterized in that it comprises at the entrance of the loop a device (23, 24) for recycling the depolarized portion of the multimode beam from the amplifying fiber.

11. Device according to claim 10, characterized in that the recycling device comprises two mirrors each disposed facing one of the polarizers of the recycling device.

12. Device according to one of claims 4 to 6, characterized in that the loop comprises an output pump-coupler (28), a multimode optical fiber (29), and a spatial filtering device (30).

13. Device according to claim 12, characterized in that the spatial filtering device comprises a frequency shift device (30) connecting the master oscillator or the preamplifiers to the multimode optical fiber of the loop.