FR2793612A1 - DEVICE FOR CORRECTING ABERRATIONS OF A LASER CHAIN WITH ULTRA-SHORT PULSES - Google Patents

Abstract

The device of the invention comprises an optical valve (1) divided into two regions (15, 16), one of which is linear (16), receives the incident beam (19) spread in wavelengths by a dispersive grating ( 20) and corrects spectral aberrations. The beam thus spectrally corrected is returned (23 to 27) to the other region (15) of the optical valve, which occupies the major part of its useful surface, and which corrects the spatial aberrations of the beam which is sent to it, thanks to to an optical addressing of this other region by an active matrix (35). The active matrix (35) is projected onto both regions of the valve.

Description

I

  DEVICE FOR CORRECTING ABERRATIONS OF A LASER CHAIN

ULTRA-SHORT PULSES

  The present invention relates to a correction device

  aberrations of an ultra-short pulse laser chain.

  Ultrashort pulse laser sources (of the order of a femtosecond: 10-15 s) are used in industrial applications, for example micromachining. These sources must have a relatively high peak power. They generally consist of a laser oscillator followed by a processing chain called "CPA chain" ("Chirp Pulse Amplifier") comprising devices for stretching, amplification and compression of ultrashort pulses. Crossing this chain

  lo causes aberrations (chromatic and geometric) of the impulses.

  These aberrations limit the power of the pulses.

  Chromatic aberration correction devices are known, as well as geometric aberration correction devices different from the first. To be able to correct all the aberrations, these two types of devices must be cascaded

  correction, which makes the whole bulky.

  The subject of the present invention is a device for correcting the two kinds of aberrations mentioned above which can affect ultrashort pulses, a device which is compact, less expensive than the above-mentioned cascade association, and whose performance is at least as good.

than those of this association.

  The correction device according to the invention comprises an optical valve, the major part of the useful surface of which is optically addressed by a spatial light modulator, and the rest of the useful surface of which, in the form of an oblong shape, is placed in the middle of a non-dispersive line 4F receiving the incident laser beam to be corrected, the beam from line 4F being returned in diverging form to said major part of the useful surface of the optical valve, at the outlet of which the

corrected beam.

  The present invention will be better understood on reading the

  detailed description of an embodiment, taken by way of example not

  limiting and illustrated by the appended drawing, in which FIG. 1 is a sectional view of an optical valve as used by the present invention; Figure 2 is a partial view, in simplified section of the valve of Figure 1 when illuminated by an addressing light beam; Figure 3 is a simplified diagram of an optical addressing device of the valve of Figure 1; Figure 4 is a plan view of the valve of Figure 1, showing its two correction zones; FIG. 5 is a simplified diagram of a correction device according to the invention; * Figure 6 is a simplified diagram of the non-dispersive line 4F of Figure 5, and, In Figure 7 is a block diagram of a laser source with its

  correction device according to the invention.

  The invention is described below with reference to a laser source called "ultrashort pulse" that is to say producing pulses of a duration of the order of a femtosecond, but it is understood that it is not limited to this single application, and that it can be implemented for longer pulses (for example of a duration up to the picosecond). Although in theory the duration of the pulses can be arbitrary, in practice the invention is only justified for pulses of

  duration less than one picosecond.

  Furthermore, the device of the invention is not limited to the only correction of aberrations, and can be used to produce pulses of determined shape or to produce several pulses with

  from one, preferably for ultra-short pulses.

  The optical valve 1 represented in FIG. 1 comprises a glass substrate 2 on which are successively deposited: a first electrode 3 transparent to the wavelengths used, a liquid crystal cell 4, a layer 5 of photoconductive material and a second electrode 6 The electrodes are generally made of ITO (tin and indium oxide), and the photoconductive material is for example BSO (oxide of

  bismuth and silicon) or amorphous silicon.

  As shown in FIG. 2, when the valve 1 is locally illuminated by a light beam 7 (coherent or not), having a wavelength to which the material of layer 5 is sensitive, the liquid crystal molecules being on the path of this beam undergo a rotation (because the addressing beam creates in the photoconductive layer a surface S of non-planar iso-potential), while the others remain in their initial state (parallel to the surface of layer). Thus, on the path of the beam 7, the liquid crystal has an optical index different from that which it has in the absence of an illumination beam. An optical lo phase is thus created which modulates the wave surface (p (which may or may not be plane) of a laser beam, surface (p which becomes the surface T after crossing

valve.

  There is shown in Figure 3 the simplified diagram of an optical addressing device of the valve 1, which is placed on the path of a beam 8 to be corrected. This addressing device comprises a light source 9 illuminating via an optical device 10 condenser and collimator a spatial light modulator 11 followed by an optical imaging device 12 and a dichroic plate 13 placed at 45 in the path of the beam 8, upstream of valve 1. The spatial light modulator is of the type with an electrically addressed liquid crystal matrix. The dichroic plate reflects (towards valve 1) the light at the wavelength of the source 9 (for example light

  visible), but is transparent at the wavelength of the laser beam 8.

  The light beam 14 downstream of the valve 1 has a wave surface modulated in phase by the valve, this modulation being able to be used to correct phase dispersions of the laser beam 8 (so that it is "flat wave" ") only to give a good waveform

determined to this beam.

  FIG. 4 shows the distribution of the useful zones of the valve 1. This valve has a useful surface of square shape, the side of which is between approximately 20 and 30 mm. This surface is divided into two regions, 15 and 16. Region 15 occupies most of the surface and is substantially square, while region 16 occupies a very narrow area (for example having a width of approximately 100 to 200 μm ) along one of the sides of the useful surface of the valve 1, and extends over the entire length of this side. The region 16 is separated from the region 15 by a narrow band 17, the width of which has substantially the same value as that of the region 16. The widths of the bands 16 and 17 depend on the resolution of the liquid crystal layer 4 of valve 1; these widths must be at least equal to the thickness of the liquid crystals of layer 4. Region 15 is intended for the correction of the spatial phase of the laser beam, while region 16

  is intended for the correction of the spectral phase.

  The region 15 is addressed optically by projection of the image from the matrix 11. This image generates a spatial distribution of voltage (see surface S of FIG. 2) which locally induces an orientation

  layer 4 liquid crystal molecules.

  It follows that the laser beam 7 (infrared or at a wavelength not absorbed by the photoconductive layer 5 of the valve 1) undergoes, after crossing the region 15 of the valve 1, a spatial phase modulation. The phase modulation law can be programmed to produce any waveform using the image displayed on the active matrix 11. This programming can be fixed, or even evolve in the

  time (as explained below with reference to Figure 7).

  Region 16, when placed in the (almost one-dimensional) area in which the spectrum of an ultrashort pulse is spread, enables active phase control of each spectral component of the pulse to be achieved. Thus, one can compensate for the dispersion effects undergone by the pulse during its propagation in a laser chain (comprising, for example, stretching, amplification and

pulse compression).

  FIG. 5 shows the simplified diagram of a device 18 according to the invention. The incident beam 19 (from a laser oscillator) is sent to a dispersive network 20 operating in transmission (or possibly in reflection). On the path of the beam from the network 20, there is a lens 21 (of focal length F) in the focal plane of which is disposed the region 16 of the valve 1. The spectrum of the pulses of the incident beam 19 is spread in this plane focal, in region 16. After crossing the valve 1, the laser beam passes through a second lens 22 identical to the lens 21 and a dispersive network 23, identical to the network 20. The lens 22 is symmetrical with the lens 21 with respect to the layer 4 of liquid crystals of the valve 1. The set of elements 20 to 23 is equivalent to a non-dispersive line 4F. The cylindrical laser beam from the array 23 is reflected by a set of three mirrors comprising two mirrors 24, 25 orthogonal to each other, and a mirror 26 placed at 45 relative to the mirror 25. These mirrors are arranged so as to return towards the region 15 the beam from network 23, parallel to the passing beam

through lens 22.

  At the outlet of the mirror 26, there is a lens 27 which forms, from the cylindrical beam reflected by the mirror 26, a beam converging at the focal point 28 of the lens 27 and which then diverges so as to illuminate the 1o almost all of the region 15 of the valve 1. A converging lens 29 is disposed adjacent to the region 15, for example upstream of the latter, as shown in FIG. 5. At the exit of the lens 29, the beam converges at its focus 30 , then diverges. A collimating lens 31 is arranged on this divergent path, shortly after the focal point, at the outlet of which a cylindrical laser beam 32 is collected, which is the beam treated by the method of the invention (with corrected aberrations and / or whose shape of the pulses has been modified). On the path of the beam from region 15, between valve 1 and focus 30, there is a dichroic blade 33, oriented at 45 relative to the axis of this beam. On another axis, perpendicular to the beam axis and intersecting it on the face of the blade 33 opposite the valve 1, there is, in order, a light source 34, an active matrix liquid crystal

  and an optic 36, the latter being the closest to the blade 33.

  The optics 36 produces the image of the matrix 35 on the part of the blade 33

  illuminated by the laser beam exiting the valve 1.

  Thanks to this device 18 of FIG. 5, the output beam 32 has the following properties: The shape of the pulses which it conveys can be modified compared to that of the pulses of the incident beam 19, and this, by action on the phase of each spectral component of the pulse (20, 23); At the spatial phase of the beam wave surface can be programmable through the corresponding control of the

active matrix 35.

  FIG. 6 shows the diagram, seen in a "horizontal" plane (that is to say perpendicular to the plane of FIG. 5 and including region 16), of the line "4F" ensuring phase control spectral of the incident beam. The same elements as those in Figure 5 are assigned the same reference numerals. At the exit of the dispersive network, the laser beam F1 (which was cylindrical before its incidence on the network) has a section of almost linear shape, parallel to the region 16, and is divergent. Its components, the number of which depends on the resolution of the network 20, have wavelengths 1 to X.n, and are spread from one end to the other of its section. The lens 21 collimates this divergent beam F1 into a beam F2 with parallel rays, each of these rays corresponding to one of the wavelengths% 1 to kn included in the incident beam 19. This beam F2 is directed towards the zone 16 of the valve 1, so that its section substantially coincides with this region. The width of this section is small (for example less than 100 μm), and its length can be practically equal to that of region 16 (that is to say approximately 20 to 30 mm, depending on the valve model used) . If a resolution of 100 μm is accepted for the liquid crystal layer of valve 1, it is possible to spread over region 16 from 200 to 300 spectral components of the incident laser beam, which is much higher than usual requirements.

  (less than 100 components in general).

  A device 37 for addressing the region 16, imaged on this region, reflecting on the plate 33, makes it possible to modulate the spectral phase of the various components% 1 to Xn in a similar manner to that used to modulate the spatial phase via the region 15 of the valve 1. This device 37 is similar to the device represented in FIG. 3 (elements 9 to 11) or to that represented in FIG. 5 (elements 34 to 36), but it is one-dimensional: it it is enough to optically modulate the line of liquid crystals of region 16. It is the same LCD screen which addresses the

  2D part of the spatial modulation and the 1 D part of the spectral modulation.

  FIG. 7 shows the block diagram of a laser source 38 with ultrashort pulses corrected according to the invention. This laser source 38 includes a laser oscillator 39 followed by a control module 40, a pulse stretcher 41, amplifiers 42 and a pulse compressor 43. The output beam 44 of the compressor 43 constitutes the output beam of the source 38. The control module 40 is produced in accordance with FIGS. 5 and 6, and the electrical control signals of its optical addressing devices are obtained, as described below, by means of a blade 44A disposed at 45 on the path of the output beam 44. The pulse stretcher 41 is produced in a manner known per se from a dispersive medium, from networks, etc. The compressor

  of pulses 43 may be of the network type.

  On the path of the beam 45 taken by the blade 44A, there is a cascade of a blade at 45 referenced 46 and a mirror 47. The blade 46 deflects part of the beam 45 towards a device 48 for measuring spectral phase, while the mirror 47 returns the remaining part of the beam 45 to a device 49 for measuring spatial phase. The measuring devices 48 and 49, of the interferometer type, are well known per se and will not be described here. The output signals from the devices 48 and 49 are sent to a processing device 50 also known per se, which is connected to the devices

  optical addressing (35 and 37) of the valve 1 of the control module 40.

  Thanks to the device of the invention, the ultra-short pulses produced by the laser source 39 have a pulse width close to the theoretical limits taking into account the width of the beam and of the band.

  spectral of the gain medium of the oscillator.

  According to an exemplary embodiment, the optical valve (1) is of the liquid crystal and photoconductive type in BSO or of the liquid crystal and photoconductive type in amorphous silicon, the liquid crystal cell operating in birefringence mode. The dimensions of the useful surface of this valve are 20 x 20 mm, - the active matrix (11, 35) is of the television screen type operating in transmissive (or reflective) mode, with a resolution typically between 600 x 500 pixels and 800 x 600 pixels. It is addressed in video mode - the laser oscillator (39) is of the Sapphire-Titanium type at nominal wavelength% 0 = 800 nm with a spectral band of AI = 10 to 50 nm. The duration of the pulses produced can be between 20 and 100 fs, depending on the spectral band AI used in the dispersive gratings 20 and 23, which operate in transmission or in reflection have a spatial period A -1 of approximately 1200 to 2000 mm. , for example, the diffracted beam has an average direction of 45 relative to the incident beam, the spatial frequency required for each of -1 these gratings is 2000 mm. These networks can be of the engraved type or of the holographic type inscribed on a photopolymer. The dispersion of a network is around 101 rd. The focal length of the lens 22, for a region 16 having a useful length of 20 mm, is then F = 20/10 = 200 mm; - the phase excursion that valve 1 can produce goes from 0 to 2 (in birefringent mode). The thickness of the liquid crystal cell must be% O / An = 4 pm for Xo = 800 nm (as

  specified above) and a variation in optical index An = 0.2.

  Advantageously, the image of the “pixelated” structure of the active matrix 35 is defocused on the photoconductor 5 of the valve 1, so as not to introduce a parasitic diffraction structure on the laser beam to be checked.

9 2793612

Claims (5)

  1. Device for correcting aberrations of a laser chain with ultrashort pulses, characterized in that it comprises an optical valve (1), the major part of the useful surface (15) of which is optically addressed by a spatial modulator of light (35), and of which the rest of the useful surface (16), of oblong shape, is disposed in the middle of a non-dispersive line 4F, receiving the incident laser beam to be corrected (19), the beam coming from the line 4F being returned, in divergent form (24 to 27) to said major part of the useful surface of the valve, at the outlet of which is recovered (29, 31) the corrected beam (32), the device controlling the phase   lo spatial wave and spectral phase of the pulse ..   2. Device according to claim 1, characterized in that the line 4F comprises, from upstream to downstream, a first dispersive network (20), a first lens (21), a second lens (22) and a second dispersive network (23), the second grating and lens being respectively symmetrical of the first compared to the valve.   3. Device according to claim 1 or 2, characterized in that the optical valve is of the liquid crystal type - BSO or of the crystal type liquid - amorphous silicon.   4. Device according to one of the preceding claims,   characterized in that the optical addressing device for said major part of the surface of the optical valve comprises a light source (34) illuminating an active matrix (35) and a dichroic blade (33) placed in front of the valve and returning the modulated beam from the active matrix to the valve.   5. Device according to one of the preceding claims,   characterized in that the image of the spatial light modulator (35) is defocused on the optical valve.