EP1878096A1

EP1878096A1 - Laser device triggered by a photonic fibre

Info

Publication number: EP1878096A1
Application number: EP06743856A
Authority: EP
Inventors: Philippe Metivier; Sébastien ERMENEUX; Philippe Yvernault
Original assignee: Eolite Systems SAS
Current assignee: Eolite Systems SAS
Priority date: 2005-04-28
Filing date: 2006-04-27
Publication date: 2008-01-16
Also published as: WO2006114557A1; FR2885265B1; US7558298B2; KR20080002896A; US20080187010A1; JP2008539574A; JP5467629B2; FR2885265A1

Abstract

The invention relates to a device generating laser pulses with a duration of less than 30 ns, and comprising, along the internal optical axis thereof, a laser resonator having an optical length of less than 2m and comprising two reflective ends (6', 8) and a laser medium (6) which is continuously pumped by at least one laser diode pump wave (13), the laser medium being a medium wherein the laser wave has a gain with a very weak signal higher than 10 per passage. Said resonator also comprises an optical modulator (7). According to the invention, the optical modulator can deflect the internal laser axis by means of an electrical control in two stable directions, the first direction corresponding to an axis along which the laser radiation suffers sufficient loss to prevent the laser effect, and the second direction corresponding to an axis on which the laser beam is reflected at least partially by an optical redirection means (8); the other end of the resonator is closed by an at least partially reflective means (6'); and the modulator has a switching time which is longer than the time taken by the light to pass through the cavity, and has a much higher loss factor than the gain with a very weak signal of the resonator, the amplifying medium preserving the state of polarisation.

Description

Photonic fiber triggered laser device

The present invention relates to a laser-triggered photonic fiber device. The principle of producing high intensity laser pulses and short duration by triggering is known for a long time. It consists in preventing the regenerative amplification of a wave in a cavity containing a laser medium by introducing losses greater than the gain of the laser medium. After a pumping period that stores a significant energy in the gain medium, the optical transmission of the trigger is sharply increased to allow the creation of an intra-cavity wave that amplifies very rapidly and gives rise to the emission of a light pulse. The duration of the pulses produced is inversely proportional to the gain of the laser medium, and proportional to the length of the laser cavity. Conventionally, the gain medium is a laser bar and the trigger can be an acousto-optic or electro-optical modulator. Moreover, the possibility of producing light beams of very good quality and high average power is known by using double-sheath optical fibers whose core is doped with an ion having a laser transition. These systems have a relatively small active area (typically less than 10 microns in diameter) and therefore work mainly continuously to avoid damage to the faces of the fiber by high energy laser pulses (damage threshold about 20 to 50 J / cm ² for pulses of 10 ns). Photonic layer or MPF ("Multiclad Photonic Fiber") fiber lasers are also known which have been presented in the article by J. Limpert, N. Deguil-Robin, I. Manek-Hennninger, F. Salin, F. Roeser, A. Liem, T. Schreiber, S. Nolte, H. Zellmer, A. Tunnermann, J. Broeng, A. Petersson, and C. Jakobsen, "High-power rod- photonic crystal fiber laser type, "Opt.Express 13, 1055-1058 (2005) .MPF lasers comprise glass fiber optical amplifiers formed of a doped core and at least one peripheral sheath which provides guiding wave produced The core is doped by a rare earth ion, neodymium or Ytterbium in general.The guidance is provided by the implementation of a photonic structure obtained by a geometric set of channels or capillaries aqueques (holes). artificially lowers the index encountered by the wave produced and allows single-mode propagation for fiber core diameters of the order of 50 μm This large core diameter allows the energy of the wave produced to be spread over a larger diameter. the fundamental limitations of fiber amplifiers such as flux resistance and non-linear effects.With such technology it is conceivable to produce relatively short laser pulses. e 1 ns at 30ns with energies of the order of 1 mJ to 1 OmJ.

These MPF lasers can present extremely important gains thanks to the very strong confinement of the gain zone. This confinement usually imposes a long absorption length and a strong limitation in the energy produced by triggering. However, it is very difficult to maintain losses greater than the gain during the pumping period. It has, however, been found that particular photonic fibers can be used to both decrease the absorption length and increase the size of the active area. Limpert et al (Advanced SoMd State Photonics Conference, Vienna, February 2005) used one of these fibers to produce nanosecond pulses of high energy. Nevertheless, they used a very fast trigger (<5ns) per Pockels cell in order to block the cavity during the pumping phase. The laser is then limited to rates of the order of 100 kHz and the trigger system is particularly expensive. In addition, The system is sensitive to the polarization of the wave propagating in the resonator and its efficiency can be diminished by the depolarization during propagation in the fiber. The present invention makes it possible to produce very short pulses (<30 ns), while maintaining a beam quality close to the diffraction limit and very high average powers (> 50 W, see several hundred W). For this purpose, it is preferable to use a photonic layer laser fiber, referred to as MPF ("Multiclad Photonic Fiber"), associated with an acousto-optical modulator operating in a particular configuration.

Thus, the invention relates to a device for cyclic production of short laser pulses, the device comprising a laser resonator of optical length less than 2 m comprising two reflecting ends and incorporating a laser medium pumped continuously by at least one pump wave. derived from semiconductor laser diodes of powers focused by optical means of collimation in one end of the laser medium, the laser medium being a medium where an internal laser beam is guided and having a very low signal gain of greater than 10 per pass through the gain medium, the laser resonator also incorporating an optical modulator. The short laser pulses are approximately less than 30 ns.

According to the invention, the optical modulator can deflect the axis of the internal laser beam, said optical modulator being actuated by an electrical control, in two stable directions, the first direction corresponding to an axis in which the internal laser beam suffers sufficient losses. to prevent the laser effect from being triggered, the device then being in an open cavity mode, and the second direction corresponding to an axis in which the internal laser beam is reflected on itself at least partially by an optical return means closing the resonator at a first end, the device then being in a closed cavity mode, the second end of the laser resonator being closed by at least partially reflecting means located on the other side of the amplifying medium , the modulator having a switching time greater than the time taken by the light to traverse the cavity and it is used in a configuration where the loss factor it introduces in the state corresponding to the open cavity mode is greater than the gain at very weak resonator signal.

In various embodiments of the invention, the following means can be used alone or in any technically possible combination, are employed:

the laser medium is a photonic fiber MPF, the modulator is an acousto-optic modulator,

the modulator directly ensures the closing of the resonator in activated mode,

the first direction corresponds to an axis in which the laser radiation suffers losses sufficient to prevent the laser effect from being triggered (blocked mode), the device then being in an open cavity mode, and the modulator then not being activated ,

the second direction corresponds to an axis on which the laser beam is reflected on itself at least partially by an optical return means serving to close the resonator at a first of its two ends, the device then being in a closed cavity mode , and the modulator being activated,

the optical return means serving to close the resonator is positioned so that the angle formed by its normal and the laser beam at the output of the non-activated modulator is equal to the angle between the order 0 and the order 1 or - 1 of the acousto-optic modulator, the optical return means for closing the resonator is a reflective treatment for the laser wave on the output face of the optical modulator,

the modulator hardly consumes any electrical control energy during the blocked mode of the laser,

(also called pumping mode or non triggered mode)

the modulator is mechanical and acts by mechanical displacement of an optical means,

the modulator comprises means using the reflectivity variation of an initially transparent element,

the means using the variation of reflectivity allow the increase of reflectivity by the excitation of a longitudinal acoustic wave whose axis of propagation is collinear with the axis of the resonator, the device is monolithic, the elements of the cavity optic being made of silica and / or glass and being joined together,

in the monolithic optical cavity, the first end of the fiber is perpendicular to the normal to the longitudinal axis of the fiber,

in the monolithic optical cavity, a collimating lens is disposed between the first end of the fiber and the optical modulator,

the pump wave is produced by at least one laser diode and is focused by optical collimation means in the second end of the fiber,

the laser wave is extracted at the output of the device by a dichroic mirror disposed between the optical means of collimation,

the device comprises means for the pulsed laser radiation produced to be linearly polarized,

the amplifying medium retains the state of polarization,

the modulator introduces a different loss on two polarization states,

the assembly comprising the modulator and the fiber retains the polarization, the device is pumped by a fiber diode device of continuous fiber or non-fiber power,

the coupling of the pump with the resonator is done by means of an optical collimation device or by coupling a fiber,

- the average power is at least 10 W,

the average power is preferably at least 50 W,

the device comprises means making it possible to produce laser pulses of duration between 1 ns and 30 ns at rates greater than 50 kHz with a guided amplifying medium guaranteeing a laser output beam quality better than 1.5 times the diffraction,

- the peak power of the device is at least 33 kW, (average power> 50 W, rate greater than 50 kHz, pulse duration less than or equal to 30 ns),

the peak power of the device is typically greater than 50 kW (the peak power is the power measurable directly on the wave at the output of the laser cavity),

the device comprises means for producing harmonic radiation with nonlinear crystals,

- the device comprises means for producing new temporal frequencies using non-linear effects of the ^3rd order, in particular a photonic fiber,

the device comprises means for producing spectra covering a band much greater than 10 nm and capable of reaching several hundred nanometers,

a collimation lens is disposed between the first end of the fiber and the optical modulator,

the first end of the fiber comprises an anti-reflective means for the laser wave,

the first end of the fiber is inclined relative to the normal to the longitudinal axis of the fiber,

the angle of inclination of the first end of the fiber with respect to the normal to the longitudinal axis of the fiber is greater than the numerical aperture of the fiber core and is between 1 ° and 60 °,

the angle of inclination of the first end of the fiber with respect to the normal to the longitudinal axis of the fiber is approximately 8 °,

the anti-reflective means is a tip attached to the first end of the fiber,

the anti-reflective means is an antireflection treatment of the first end of the fiber; the laser comprises a ring resonator in which the laser wave does not travel exactly the same way in and out.

The laser of the invention is a high gain and guided mode (MPF fiber) triggered pulsed laser which has a short pulse duration and uses a simple optoelectronic triggering means whose triggering time is not critical. for the pulse duration and may be greater than the laser pulse duration.

The present invention will now be exemplified without being limited thereto with the description which follows in relation to the following figures: FIG. 1 which diagrammatically represents an MPF fiber-triggered laser device according to the invention, FIG. 2 which represents an example embodiment of the triggered laser device of the invention, Figure 3 which shows an application of the device of the invention.

In Fig. 1, a laser resonator is formed between a reflecting mirror 8 at the laser emission wavelength and a planar end 6 'of a photon fiber 6 MPF. The second end of the MPF fiber is cut or polished to form an angle of typically 8 ° (between 1 ° and 60 °) with the longitudinal axis of the fiber. The light beam emerging from the inclined end-slope fiber is collimated with a lens 14 on an acousto-optic modulator 7. This light beam is then incident on the acousto-optical modulator 7 which can output at least two different angular paths to the beam. The modulator 7 is activated at the desired firing rate by an electronic module 15, which controls a high frequency acoustic wave created in the modulator during activation. The mirror 8 is positioned so that its normal makes an angle Θ 10 corresponding to the angle between the beam incident on the modulator (and which corresponds to the unactivated modulator output beam) and the beam diffracted according to the order 1 or -1 in the activated modulator.

The MPF fiber is pumped continuously and longitudinally by a pump wave coming from one or more power laser diodes 13, the pump diode being preferably fibered. The pump wave is focused in the MPF fiber side non-inclined end, thanks to optical collimation means 1 1, in particular by corrected lenses of spherical aberrations (doublets, triplets or aspherical lenses ...). During the entire pumping phase, the supply of the electronic module 15 is kept at rest and it does not produce any control signal and the modulator therefore behaves like a block of isotropic material, transparent. The light beam from the MPF fiber passes through it without being deflected and strikes the mirror 8 at an angle Θ with the mirror normal and is therefore not returned to itself and can not return to the mirror. fiber. To trigger a laser emission activates the electronic module 15 which causes the creation of an acoustic wave in the modulator 7. This acoustic wave causes a deviation of the incident beam from the MPF fiber at an angle Θ which makes it arrive perpendicularly mirror 8 and the beam is then returned to itself and can be amplified in the MPF fiber. The face 6 'of the fiber, opposite the inclined face of the fiber, acts as a second partially reflecting mirror and causes a resonance effect that gives rise to the laser effect and thus to the production of a light pulse. A laser beam comprising pulses at a rate determined by the activation of the electronic module 15 is thus emitted through the face of the fiber MPF and is separated from the pump beam by a dichroic mirror 12.

The spatial quality of the pulse beam thus produced is fixed by the properties of the MPF fiber and can therefore be very close to the diffraction limit. One can for example use an MPF fiber with a core diameter of about 50 microns. For example, it is possible to use a "double clad" fiber structure comprising a waveguiding sheath having a cross section whose area is between 10,000 and 250,000 square microns, the amplifying medium having a radio length with a high section diameter: the area of the section being between 500 and 10,000 square microns and the length of the amplifying medium between 10 cm and 1.5 m. The amplifying medium of the fiber preferably has a very low signal gain typically greater than 10 per passage in the gain medium. The structure of the fiber allows the guidance of the laser wave in the amplifying medium.

Since the gain in the fiber is very large (greater than 10 and typically greater than 100 per pass), the duration of each pulse is fixed by the duration of a round trip in the laser cavity and not by the rise time of the laser. modulator. Thanks to the proposed configuration with a short MPF fiber (length less than 2 m and preferably less than 1 m), it is possible to produce pulses with a duration of less than 10 ns with a modulator whose response time is greater than 100 ns. To do this it is necessary to ensure that losses remain very high during the pumping period (otherwise the laser effect would occur independently of the trip). Triggering the laser on the first diffraction order of the acoustic module optical and pumping mainly when the acousto-optics is not active ensures that the cavity will remain open (no loopback possible) in the absence of a control signal on the modulator. In active mode, the very large gain of the fiber largely compensates for the losses caused by the fact that the modulator will work with a diffraction efficiency of less than 100%, whereas in passive mode the acousto-optic behaves like a passive optical component. and it is possible to guarantee a loss factor that is much greater than 100 (with, for example, a parasite return rate of the order of 2 per thousand).

It can thus be seen that the combination of an acousto-optic modulator operating on order 1 or -1 and a wide-mode photonic fiber makes it possible to guarantee the production of short, energetic pulses and with an excellent beam quality. , while working at high average power.

For optimum operation, a number of conditions must be met. In particular, the lens 14 at the inclined end of the MPF fiber must be chosen and positioned to ensure that the divergence of the beam passing through it coming from the fiber is less than the deflection angle Θ introduced by the acousto modulator. -optical. Likewise, it is preferable that the end of the fiber facing the modulator, i.e. the sloped end, is prepared to prevent laser oscillation between the two ends of the fiber. For this, it can be given an inclination with an angle relative to the normal to the fiber which is much larger than the numerical aperture of the core of the MPF fiber. In an alternative or complementary manner, it is possible to assemble on the end of the fiber, on the modulator side, a tip whose outer face (of exit) is not perpendicular to the axis of the fiber or which has received a treatment anti-reflective. Similarly, to obtain short pulses, the fiber must have a shorter length at 1 m to ensure pulses of less than 10 ns.

More generally, the modulator can use any other method of rapid modulation of the transmission or reflection of an optical system. By way of example, a micro-mechanical optical system makes it possible to obtain the angular optical switching effect necessary to trigger the pulse. Preferably, in the latter case, the reflecting mirror 8 is carried by the micromechanical optical system and can switch to move from a return position of the light beam on itself to another position.

It should be noted that the switching time of the modulator must be fast (typically less than a few hundred ns) without however having to be faster than the duration of a round trip in the laser cavity since the duration of the pulses is fixed by the gain of the amplifying medium and not by the switching speed of the switch.

Similarly, it is important to realize that the laser medium having in the invention a gain per pass very high (greater than 10), the simple activation of most optical modulators does not allow to introduce into the cavity sufficient losses to prevent the laser effect from occurring. We can no longer frustrate the laser emission and force it to appear as a pulse. It is therefore important in the invention to use a non-activated modulator during the energy storage phase in the laser medium, taking care that the beam passing through the modulator without being modified since the latter is not activated. , can not be sent back to the amplifier medium. This mode of operation is the opposite of the cavity triggering operations used in all the laser systems described before this invention, systems which traditionally uses the modification provided by an activated modulator to block the laser cavity during the storage phase. , and that triggers the emission of a pulse by returning the modulator to its non-activated state for a short time before the complete cycle (configuration which then minimizes the losses in the case of a low-gain laser). In an alternative embodiment (or complementary to the previous one for pumping by the two ends of the MPF fiber), the pump signal is incident on the MPF fiber by its end directly related to the modulator. In this case the modulator may be traversed by the pump wave or simply separated from the latter by a dichroic mirror disposed between the end (inclined, anti-reflection treated or having a tip) of the fiber 6 or the lens 14 and the modulator 7, allowing the longitudinal injection of the pump wave in the MPF fiber.

It is possible to realize the device in a substantially monolithic manner by grouping the main elements of the optical cavity, in particular the collimation optics 14, the modulator 7 and the mirrors 8, 6 ', in a material made of silica or in a glass which allows such an assembly. Figure 2 gives an example of such a monolithic assembly. In the case where the modulator is an acousto-optic modulator, it is possible to cut the external face of the modulator so that it makes an angle Θ with the axis of the fiber and to treat this face so as to make it reflective to the length of the fiber. laser wave to obtain an equivalent of the mirror 8 on this face.

Furthermore, the device of the invention can be followed very advantageously by one or more non-linear crystals 16 in order to produce harmonic radiations of the fundamental wave (especially for doubling, tripling, quadrupling, frequency quintupling). ). The combination of short pulses and a beam limited by diffraction makes it possible to maximize the frequency conversion efficiency and thus to produce visible or UV radiation of very high average power, which is difficult to obtain by the usual means. An example of implantation of one or more nonlinear crystals followed by dichroic mirrors for the separation of harmonics is shown in FIG. 3. The generation of harmonics is also possible with a monolithic device as represented in FIG. associating nonlinear crystals.

It is understood that other arrangements of the elements of the triggered laser device are possible without departing from the scope of the invention presented here.

Claims

1. Device for cyclic production of laser pulses of short duration, typically less than 30ns, the device comprising a laser resonator of optical length less than 2m comprising two reflective ends and incorporating a photonic fiber MPF pumped continuously by at least one pump wave power semiconductor laser diodes focused by fiber collimating optical means, the MPF having a medium where an internal laser beam is guided and having a very low signal gain of greater than 10 by passing through the gain medium; laser resonator also incorporating an optical modulator, the optical modulator being able to deflect the axis of the internal laser beam, said optical modulator being actuated by an electrical command, in two stable directions, the first direction corresponding to an axis according to which the internal laser beam undergoes losses sufficient to prevent the effe The laser is not triggered, the device then being in an open cavity mode, and the second direction corresponding to an axis according to which the internal laser beam is reflected on itself at least partially by an optical return means closing the resonator at a minimum. first of its ends, the device then being in a closed cavity mode, the second end of the laser resonator being closed by at least partially reflecting means located on the other side of the amplifying medium, characterized in that the device generates a peak power at least 33 KW and that the modulator has a switching time greater than the time taken by the light to traverse the cavity and that it is used in a configuration where the loss factor that it introduces in the state corresponding to the open cavity mode is greater than the very low signal gain of the resonator and that the amplifying medium maintains the state of polarisat ion.

2. Device according to claim 1, characterized in that the modulator directly ensures the closure of the resonator in activated mode.

3. Device according to claim 1, characterized in that the optical modulator is an acousto-optic modulator.

4. Device according to claim 3, characterized in that the optical return means for closing the resonator is positioned so that the angle formed by its normal and the laser beam output of the unactivated modulator is equal to the angle between the order 0 and the order 1 or -1 of the acousto-optic modulator.

5. Device according to claim 3 or 4, characterized in that the acousto-optic modulator consumes virtually no electrical control energy during the locked mode of the laser.

6. Device according to any one of the preceding claims, characterized in that it is monolithic, the elements of the optical cavity being silica and / or glass and being contiguous to each other.

7. Device according to claim 1 or 2, characterized in that the optical modulator is mechanical and acts by mechanical displacement of an optical means.

8. Device according to any one of the preceding claims, characterized in that it comprises means for producing laser pulses of less than 30 ns duration at rates greater than 50 kHz with a beam quality better than 1, 5 times diffraction.

9. Device according to any one of the preceding claims, characterized in that one end of the MPF fiber is inclined relative to the normal to the longitudinal axis of the fiber.

10. Device according to any one of the preceding claims, characterized in that the MPF fiber is pumped at both ends.

1 1. Device for cyclic production of laser pulses of short duration, typically less than 30ns, the device comprising a laser resonator of optical length less than 2m comprising two reflective ends and incorporating a photonic fiber MPF pumped continuously by at least one pump wave power semiconductor laser diodes focused by fiber collimating optical means, the MPF having a medium where an internal laser beam is guided and having a very low signal gain of greater than 10 by passing through the gain medium; laser resonator also incorporating an optical modulator, the optical modulator being able to deflect the axis of the internal laser beam, said optical modulator being actuated by an electrical command, in two stable directions, the first direction corresponding to an axis according to which the internal laser beam undergoes losses sufficient to prevent the effe The laser is not triggered, the device then being in an open cavity mode, and the second direction corresponding to an axis according to which the internal laser beam is reflected on itself at least partially by an optical return means closing the resonator at a minimum. first of its ends, the device then being in a closed cavity mode, the second end of the laser resonator being closed by at least partially reflecting means situated on the other side of the amplifying medium, characterized in that the modulator has a switching greater than the time taken by the light to traverse the cavity and that it is used in a configuration where the loss factor that it introduces in the state corresponding to the open cavity mode is greater than the very low signal gain of the resonator, in that the amplifying medium maintains the state of polarization and in that it comprises means for producing radiation ha with nonlinear crystals.