FR3143892A1 - Dispersive optical device and short pulse laser system comprising such a dispersive optical device - Google Patents

Abstract

A dispersive optical device (50, 51) adapted to modify a group velocity dispersion of a beam (10) of light pulses. According to the invention, the dispersive optical device comprises a polarization splitter cube (11), a first volumetric Bragg grating (13) with variable pitch and a first quarter-wave plate (12) with flat and parallel faces, one face of the first quarter-wave plate (12) being fixed to an input-output face (23) of the first Bragg grating (13), the first volumetric Bragg grating (13) with variable pitch being photoinscribed in a material along planes parallel to the input-output face (23) of the first Bragg grating (13), another face of the first quarter-wave plate (12) being made integral with one of the two lateral faces (19 , 20) of the polarization separator cube (11). Figure for abstract: Fig. 3

Description

Dispersive optical device and short pulse laser system comprising such a dispersive optical device

The present invention relates to the technical field of pulse power lasers.

More specifically, the present invention relates to a dispersive optical device capable of modifying the frequency drift of a beam of laser pulses to stretch or compress these laser pulses.

The advent of ultra-short laser pulses has enabled numerous applications in scientific, industrial and medical fields. However, the direct amplification of short pulses is limited by the appearance of non-linear optical effects during propagation in optical amplifiers. Several solutions have been found to overcome this difficulty, including frequency drift amplification. In all cases, the production of very intense and very short laser pulses involves the amplification of pulses of long duration, for example of several tens of picoseconds, and the compression of the amplified pulses towards a significantly shorter duration, for example example of 10 femtosecond to 1 picosecond. A compressor capable of providing pulse compression conventionally comprises a dispersive delay line. The dispersive delay line generally comprises one or more prism, diffraction grating, frequency drift mirror, dispersive material, Bragg grating and/or any other dispersive optical element. To obtain sufficient dispersion to modify the duration of the pulses by several tens of picoseconds, only compressors using diffraction gratings and/or Bragg gratings are suitable. For example, the Treacy grating compressor typically uses two reflective diffraction gratings and a set of mirrors or prisms. However, such a compressor poses size difficulties (typically more than 10cm x 10cm x 10 cm or even up to 100cm x 50cm x 20 cm). In addition, such a compressor is extremely sensitive to vibrations and mechanical movements which induce misalignments. Such a compressor is therefore not suitable for being mounted on moving parts and even less suitable for being held by hand. These dispersive optical systems all operate in reflection, that is to say that the beam of compressed laser pulses exits the compressor parallel to the incident beam of amplified laser pulses, but in the opposite direction. This configuration requires the use of another optical system, often composed of one or more mirrors, in order to send the beam of compressed laser pulses towards the target, making the whole thing even more sensitive to misalignments.

A very important advance in the production of ultra-short pulses by frequency drift amplification has been made using variable pitch volumetric Bragg gratings (RBVPV). A volumetric Bragg grating with variable pitch is a one-piece, foolproof and very compact optical component. A variable pitch Bragg grating can also be inscribed in an optical fiber to form a variable pitch fiber Bragg grating (or chirped FBG). However, a fiber Bragg grating with variable pitch has a limited surface area of the order of a few tens of square microns, which is generally too small for applications with intense laser pulses. A fiber Bragg grating with variable pitch can be used to stretch light pulses before amplification. Furthermore, the dispersion introduced by a volumetric or fiber Bragg grating with variable pitch is fixed and cannot be adjusted. Finally, a variable pitch Bragg grating also operates in a reflection configuration, returning the beam in the direction opposite to that of the incident beam on the grating. Analogously with a Treacy diffraction grating compressor, the laser beam coming from a compressor using a variable pitch Bragg grating can be returned towards the target, that is to say in a direction different from that of the incident beam but always using a set of mirrors or prisms in free space whose orientation is adjusted for this use. This set of mirrors and prisms introduces increased sensitivity to vibrations.

Patent document US 2014/0168755 A1 (Clowes et al.) discloses a pulse compression system based on a volumetric or fiber Bragg grating with variable pitch, comprising a polarization separator and a quarter-wave phase plate on the path of the beam in order to return the beam emerging from the compressor at a right angle or to two volumetric or fiber Bragg gratings with variable pitch which makes it possible to increase the dispersion introduced. However, such a system requires precise optical alignment of each optical component and does not make it possible to adjust the group velocity dispersion introduced by the Bragg grating(s).

Furthermore, frequency drift amplification requires perfect compensation of the frequency drift introduced by the stretcher and the amplification chain by that of the compressor. Different methods exist to achieve this perfect match. The simplest is to vary the distance between the optical components of a Treacy compressor. This traditional method requires placing at least one component on a translation table and makes the system even more sensitive to misalignment. It requires moving the components perfectly parallel to the axis of the beam incident on said component.

Another solution consists of using a fiber Bragg grating with variable pitch as a pulse stretcher and varying its frequency drift by adjusting its temperature. This method was described by Frankinas et al in “Efficient ultrafast fiber laser using chirped fiber Bragg grating and chirped volume Bragg grating stretcher/compressor configuration,” Proc. SPIE 9730, Components and Packaging for Laser Systems II, 973017 (22 April 2016). Although very precise, this method requires the use of a fiber pulse stretcher and only allows very small dispersion excursions of the order of 3fs/K. It would therefore be necessary to increase the temperature by 300°C for an adjustment of around 1 ps.

One of the aims of the invention is to propose a pulse compression device which is robust, compact, light and adjustable in group speed dispersion without changing the direction of the beam of compressed laser pulses.

For this purpose, the present disclosure relates to a dispersive optical device adapted to modify a group velocity dispersion of a beam of light pulses, the dispersive optical device comprising a polarization separator cube, a first volumetric Bragg grating with variable pitch and a first quarter-wave plate, the polarization splitter cube having an entrance face, an exit face parallel to the entrance face, two side faces perpendicular to the entrance face and an interface inclined by 45 degrees relative to the entry face and the two lateral faces, the entry face being able to receive the beam, the first quarter-wave plate having flat and parallel faces, one face of the first quarter-wave plate being fixed to an input-output face of the first Bragg grating, the first volumetric Bragg grating with variable pitch being photoinscribed in a material following planes parallel to the input-output face of the first Bragg grating, another face of the first quarter-wave plate being made integral with one of the two lateral faces of the polarization separator cube so that the normal to the plane of the fringes of the first volumetric Bragg grating with variable pitch makes an angle of 45 degrees with the normal at the interface.

Advantageously, the dispersive optical device is in one piece.

According to certain embodiments, the other face of the first quarter-wave plate is fixed to one of the two side faces of the polarization splitter cube.

According to another embodiment, the dispersive optical device comprises a right angle prism having a first face forming an angle of 90 degrees with a second face of the right angle prism, the first right angle prism face being fixed to one of the two side faces of the polarization splitter cube and the second face of the right-angle prism being fixed to the first quarter-wave plate.

According to a particular and advantageous aspect, the dispersive optical device comprises a second quarter-wave plate and a reflective optical component, the second quarter-wave plate having flat and parallel faces, the second quarter-wave plate being arranged between the reflector optical component and the other of the two lateral faces of the polarization splitter cube, one face of the second quarter-wave plate being fixed to the reflector optical component and another face of the second quarter-wave plate being fixed to the other of the two lateral faces of the polarization separator cube.

Advantageously, the reflective optical component comprises a mirror or a reflective treatment applied directly to said other face of the second quarter-wave plate.

Alternatively, the reflector optical component comprises a second volumetric Bragg grating with variable pitch, the second quarter-wave plate being fixed to an input-output face of the second Bragg grating, the second volumetric Bragg grating with pitch variable being photoinscribed in a material following planes parallel to the entry-exit face of the second Bragg grating.

According to another particular and advantageous aspect, the dispersive optical device comprises an optical isolator fixed to the output face of the polarization separator cube.

Optionally, the dispersive optical device comprises means for tilting the dispersive optical device by rotation around an axis parallel to the intersection between the input face and the interface of the polarization separator cube.

The present disclosure also relates to a laser system with pulses of duration between 10 femtoseconds and 1 nanosecond and power of between 1 W and 1 kW, the laser system comprising a source capable of generating linearly polarized source pulses, an optical amplifier system, a stretcher and/or a compressor comprising at least one dispersive optical device according to one of the embodiments described.

The present disclosure also relates to a laser system with pulses of adjustable duration between 10 femtoseconds and 10 picoseconds comprising a stretcher and/or a compressor comprising at least one dispersive optical device according to one of the embodiments described.

The dispersive optical device of the present disclosure makes it possible to introduce a temporal frequency drift on a laser pulse without changing the direction of the laser beam or by deflecting it by 90 degrees, depending on the embodiment.

The dispersive optical device makes it possible to compress or stretch a laser pulse, preferably without changing the direction of the incident beam.

Particularly advantageously, the temporal frequency drift introduced is adjustable by means of a rotation of the dispersive optical device around a single axis of rotation, while maintaining a direction of the laser beam parallel to the incident direction or, respectively, to 90 degrees from the incident direction.

Of course, the different characteristics, variants and embodiments of the invention can be associated with each other in various combinations as long as they are not incompatible or exclusive of each other.

In addition, various other characteristics of the invention emerge from the appended description made with reference to the drawings which illustrate non-limiting forms of embodiment of the invention and where:

is a schematic diagram of a dispersive optical device for temporally stretching a pulse,

is a schematic diagram of a dispersive optical device for temporally compressing a pulse,

is an example of a dispersive optical device according to a first embodiment,

is an example of a dispersive optical device according to a variant of the first embodiment,

is an example of a dispersive optical device according to another variant of the first embodiment,

is an example of a dispersive optical device according to a second embodiment,

is an example of a dispersive optical device according to a third embodiment,

illustrates a group speed dispersion adjustment method of a dispersive optical device according to the first, second or third embodiment,

is an example of a dispersive optical device according to a fourth embodiment,

illustrates a group speed dispersion adjustment method of a dispersive optical device according to the fourth embodiment,

is a diagram of a frequency drift pulse amplification chain using two dispersive optical devices according to the present disclosure, in which the stretcher is tunable;

is a diagram of a frequency drift pulse amplification chain using two dispersive optical devices according to the present disclosure, in which the compressor is tunable,

is an amplified and compressed laser pulse autocorrelation trace obtained experimentally;

is a diagram of a frequency drift pulse amplification chain using two dispersive optical devices according to the present disclosure, in which the stretcher and the compressor are tunable.

It should be noted that in these figures the structural and/or functional elements common to the different variants may have the same references.

detailed description

In this document, frequency drift is understood to mean the modification of a group speed of a laser pulse, which results in a spectral dispersion of the pulse.

The dispersive optical device is based on the use of one or more volumetric Bragg gratings with variable pitch having the particularity that the physical face of the grating is perfectly parallel to the plane of the fringes of the grating inscribed in the material. This volumetric Bragg grating with variable pitch is fixed by welding or optical bonding or adhesion to a set of optical components to form the dispersive optical device. Such a dispersive optical device makes it possible to introduce spectral dispersion onto the incident signal, preferably without modifying its direction. The dispersive optical device of the present disclosure finds particularly interesting applications for laser systems for producing ultra-short pulses in the industrial, scientific and/or medical fields.

There schematically represents a dispersive optical device 50 used to temporally stretch a laser pulse, for example before amplification. Source 1 produces laser pulses 2 of duration between 10 fs and 10 ns and having a negative, zero or positive frequency drift. This laser pulse 2 propagates along a propagation direction. A laser pulse 2 is incident on the dispersive optical device 50 which modifies its frequency drift without altering the direction of propagation of the pulse. In the case where the initial laser pulse 2 has a zero or very small frequency drift compared to that introduced by the dispersive optical device 50, or if the frequency drift of the laser pulse 2 is of the same sign as that introduced by the dispersive optical device 50, at the output of the dispersive optical device 50, the pulse 3 is stretched and its duration generally lengthened compared to the initial laser pulse 2.

There schematically represents a dispersive optical device 51 used to temporally compress a laser pulse. The source 1, possibly followed by a stretcher, for example a first dispersive optical device 50 as described in connection with the , produces laser pulses 2, or 3, of duration between 30 ps and 10 ns and having a non-zero, negative or positive frequency drift. The dispersive optical device 51 introduces a frequency drift of sign opposite to that of the laser pulse 2, or 3. Consequently, the pulse 4 is compressed at the output of the dispersive optical device 51. The final duration of the pulse 4 is a function of the sign of each of the frequency drifts, their absolute values and the initial duration of pulse 2, 3.

We will now describe the structure and operation of the dispersive optical device 50, 51 according to different embodiments, as well as the characteristics of the pulses propagating in such a dispersive optical device.

The dispersive optical device 50, 51 of the present disclosure is based on the manufacture of a massive or one-piece dispersive optical device by bonding, welding or optical adhesion of several optical components having very precise characteristics. In order to be able to compress laser pulses exhibiting a frequency drift, the device uses one or more volumetric Bragg gratings with variable pitch, denoted RBVPV.

There schematically represents a dispersive optical device 50, 51 according to a first embodiment. We have represented an orthonormal coordinate system XYZ, the plane XZ being in the plane of the . The dispersive optical device 50, 51 comprises a polarization splitter cube 11, a first quarter-wave plate 12, a first volumetric Bragg grating 13 with variable pitch, a second quarter-wave plate 14 and a reflector optical component 15 The optical reflector component 15 is for example a plane mirror with flat and parallel faces. Alternatively, the optical reflector component 15 consists of a reflective treatment applied directly to the external face of the second quarter-wave plate 14, the two faces of the second quarter-wave plate 14 being polished in parallel with a angle between the two faces less than or equal to 0.05 degree.

The polarization separator cube 11 comprises an entry face 17, an exit face 21 parallel to the entry face 17, two lateral faces 19, 20 perpendicular to the entry face 17 and an interface 18 inclined by 45 degrees relative to the entry face 17 and the two side faces 19, 20. The side faces 19, 20 are parallel to each other and perpendicular to the entry face 17. Advantageously, the polarization separator cube 11 thus forms a parallelepiped rectangle, cut along its diagonal by interface 18.

The first quarter-wave plate 12 and the second quarter-wave plate 14 are plates with flat and parallel faces. The first quarter-wave plate 12 is arranged between the lateral face 19 of the polarization separator cube 11 and the input-output face 23 of the first volumetric Bragg grating 13 with variable pitch. More precisely, the first quarter-wave plate 12 is fixed by gluing, welding or optical adhesion, on the one hand, to the lateral face 19 of the polarization separator cube 11 and, on the other hand, to the face of input-output 23 of the first volumetric Bragg grating 13 with variable pitch. The second quarter-wave plate 14 is arranged between the other side face 20 of the polarization separator cube 11 and the optical reflector component 15. The second quarter-wave plate 14 is fixed by gluing, welding or optical adhesion, on the one hand, to the lateral face 20 of the polarization separator cube 11 and, on the other hand, to the optical reflector component 15.

In this document, “fixed” means the fact that two optical elements are in contact, directly or via optical glue, and in an invariable position relative to each other, without possibility of relative movement or rotation between the two optical elements fixed to each other.

The first volumetric Bragg grating 13 with variable pitch has an input-output face 23. The first volumetric Bragg grating 13 with variable pitch is manufactured and chosen to be a volumetric Bragg grating photoinscribed in a material (for example a block of glass) along planes parallel to this entry-exit face 23. In this way, the normal to the plane of the fringes of the first volumetric Bragg grating 13 with variable pitch forms an angle of 45 degrees ± 0.1° with the normal to the interface 18 of the polarization separator cube 11.

Variable-pitch volumetric Bragg gratings are produced by various companies including, for example, Optigrate Corporation, 3267 Progress Drive, Orlando, Florida 32286, USA. A periodic variation in the refractive index of a photosensitive glass is recorded in a block of glass using a UV laser. This network behaves like a Bragg grating by reflecting the wavelengths corresponding to the period of the network. Varying the period of the grating along the axis of the glass block allows different wavelengths to be reflected at different positions in the glass block. We thus introduce a delay between the wavelengths, which results in a drift in the optical frequency of the pulse reflected by the volumetric Bragg grating with variable pitch. The direction of the reflected wave is determined by the angle of incidence on the fringe planes of the diffraction grating and not by the physical input-output face 23 of the Bragg grating 13.

The dispersive optical device 50, 51 is made up of optical components whose reflecting surfaces are perfectly parallel, to within 0.1 degree, to the physical surfaces except for the polarizing surface 18 which is exactly 45° ± 0.1° from the physical surfaces 19 and 20. In particular, the planes of the fringe network inscribed in the RBVPV are parallel to the entry-exit face 23 of the glass block in which the network is inscribed. This precision makes it possible to guarantee that an incident beam perpendicular to the entry-exit face 23 of the glass block comprising the network 13 is reflected exactly on itself by the network 13.

The elements of the dispersive optical device 50, 51 are brought into contact by their optical faces, that is to say the faces crossed by the laser beam. The assembly of these elements is carried out by bonding, using an optical glue transparent to the laser beam, by optical contact, or by welding. The elements 12, 13, 14, 15 are assembled so that their respective optical faces are parallel to the lateral faces 19, 20 of the polarization separator cube 11. The dispersive optical device 50, 51 thus forms a compact, one-piece optical component , perfectly aligned by construction and unadjustable. As a non-limiting example, the polarizer cube has dimensions of 8 mmx8 mmx8 mm and the optical faces of all elements have the same dimensions of 8 mmx8 mm. These dimensions are variable and all of the elements do not need to have faces of uniform dimensions.

The dispersive optical device 50, 51 is arranged to receive a beam 10 of light pulses incident on the input face 17 of the polarization separator cube 11. The beam 10 of light pulses is of linear polarization perpendicular to the plane of the . In other words, the beam 10 is polarized S, parallel to a line of intersection between the input face 17 and the interface 18, this line of intersection being parallel to the Y axis.

The interface 18 makes it possible to direct the S-polarized light beam 10 by reflection towards the first quarter-wave plate 12 and the first volumetric Bragg grating 13 with variable pitch . The axes of the first quarter-wave plate 12 are aligned at 45 degrees from the incident polarization so that the beam 10 of linear polarization S is transformed into a beam 120 of circular polarization as it passes through the first quarter-wave plate wave 12. The beam 120 of light pulses of circular polarization is incident on the first volumetric Bragg grating 13 with variable pitch. The different spectral components of the pulse are reflected on the first Bragg grating 13 at different positions along the direction of propagation of the beam. This difference in optical path for the different spectral components results in a frequency drift which is added to that of the incident pulse. Consequently, the beam reflected 130 by the first volumetric Bragg grating 13 with variable pitch has a modified dispersion or a modified frequency drift relative to the laser beam 10. The orientation of the planes of the diffraction fringes registered in the grating 13 determines the direction of the beam reflected 130 by the first Bragg grating 13. In the example of the , the direction of propagation of the beam 120 coming from the first quarter-wave plate 12 is perpendicular to the fringe plane, consequently the reflected beam 130 propagates in the opposite direction to the beam 120 coming from the first quarter-wave plate 12. The reflected beam 130 again passes through the first quarter-wave plate 12 and forms a beam 135 incident on the face 19 of the polarization separator cube 11. The reflected beam 130 of circular polarization is thus transformed into a beam 135 of linear polarization , exactly perpendicular to the polarization of the incident beam 10, therefore in a state P, in other words along the Z axis.

The interface 18 transmits the beam 135 of polarization P towards the second quarter-wave plate 14 whose axes are aligned at 45 degrees from the polarization P. The second quarter-wave plate 14 transforms the linear polarization P of the beam 135 in a circular polarization. The mirror 15 reflects the circular polarization beam and forms a beam 140 which passes through the second quarter-wave plate 14. The circular polarization beam 140 is transformed by the second quarter-wave plate 14 into a polarization beam 150 linear S. The interface 18 reflects the beam 150 of linear polarization S towards the output face 21 of the polarization separator cube 11 and forms an output beam 200 propagating in a direction which is fixed by the orientations of the different optical components and in particular of the mirror 15, of the first volumetric Bragg grating 13 with variable pitch and of the polarization separator cube 11.

As illustrated on the , when the beam 10 is incident on the entrance face 17 of the polarization separator cube 11 at a zero angle of incidence, in other words under normal incidence, the beam 200 at the output of the dispersive optical device 50, 51 propagates along the same direction and in the same direction as the incident beam 10. As indicated above, the fringes of the first volume Bragg grating 13 with variable pitch and the plane of the mirror 15 are parallel, to within 0.1 degrees. The beam 200 emerging from the dispersive optical device 50, 51 is then perfectly parallel to the incident beam 10 whatever the angle between the first volume Bragg grating 13 with variable pitch and the polarization separator cube 11. The beam 200 is thus perfectly aligned and propagates in the same direction as the beam 10 incident on the entrance face 17 under normal incidence and with a polarization identical to the polarization of the beam 10. The beam 200 has a total dispersion equal to the sum of the induced dispersion by the volumetric Bragg grating 13 with variable pitch and the initial dispersion of the incident laser beam 10, taking into account the sign of each of these dispersions.

The dispersive optical device 50, 51 of the allows you to compress or stretch a laser pulse while maintaining the direction of the laser beam. The dispersive optical device 50 of the can be used to make one-piece stretcher or one-piece compressor.

The dispersive optical device 50, 51 is thus advantageously inserted in the path of a beam of laser pulses 10 to modify the group speed dispersion of the pulses, without modifying the direction or the position of the laser beam 200 at the output. The dispersive optical device 50, 51 determines by construction the direction of the output beam 200 relative to the incident beam 10.

According to a variant of the first embodiment illustrated on the , the location of the first quarter-wave plate 12 fixed to the first volumetric Bragg grating 13 with variable pitch is swapped with that of the second quarter-wave plate 14 fixed to the optical reflector component 15. This permutation does not modify in any way the operation of the dispersive optical device 50, 51. In fact, in this case, the light beam is transmitted twice through the second quarter-wave plate 14 and reflected on the optical reflector component 15 before passing through the separator cube of polarization 11 to be reflected by the first volumetric Bragg grating 13 with variable pitch and transmitted twice through the first quarter-wave plate 12. In this variant, the beam 10 incident on the S-polarized input face 17 is reflected on the interface 18, then transmitted via the side face 19 through the second quarter-wave plate 14 to form a beam 140 of circular polarization which is reflected on the optical reflector component 15 to form a beam 150 of circular polarization. The beam 150 passes through the second quarter-wave plate 14 to form a beam 155 of linear polarization P. The beam 155 of linear polarization P is transmitted through the polarization separator cube 11 towards the other side face 20. beam 155 of linear polarization P is transmitted through the first quarter-wave plate 12 and forms a beam 120 of circular polarization. The circularly polarized beam 120 is reflected on the fringes of the first Bragg grating 13 to form a reflected beam 130 of circular polarization. The reflected beam 130 is transmitted through the first quarter-wave plate 12 and forms a beam 150 of linear polarization S in the direction of the polarization separator cube 11. The interface 18 reflects the beam 150 of linear polarization S and forms the beam 200 output. As described in connection with the , the output beam 200 has a total dispersion equal to the sum of the dispersion induced by the volumetric Bragg grating 13 with variable pitch and the initial dispersion of the incident laser beam 10, taking into account the sign of each of these dispersions.

In particular, as illustrated in , when the incident beam 10 is perpendicular to the entrance face 17, the beam reflected by the interface 18 is perfectly perpendicular to the reflecting surface of the optical reflector component 15. The beam 155 reflected by the optical reflector component 15 and transmitted two times through the second quarter-wave plate is perpendicular to the fringes of the first volumetric Bragg grating 13 with incident variable pitch. The beam 150 reflected by the first Bragg grating 13 and transmitted twice through the first quarter-wave plate is inclined exactly 45 degrees on the interface 18 and the output beam 200 is perfectly parallel to the incident beam 10. In the case where the incident light laser beam 10 is perpendicular to the entrance face 17 of the polarization separator cube 11, the output light beam 200 has the same direction, the same polarization and the same position as the light beam 10 of entrance. In other words, when the beam 10 is incident on the input face 17 of the polarization separator cube 11 under normal incidence, we also obtain an output beam 200 perfectly aligned and propagating in the same direction as the incident beam 10.

Numerous variants of the dispersive optical device 50, 51 are considered here.

There further illustrates a variant according to which the optical reflector component 15 consists of a reflective treatment applied to the external face of the second quarter-wave plate 14, at the wavelength of the incident laser beam 10. In this configuration the two faces of the second quarter-wave plate 14 are polished parallel with an angle between the two faces not exceeding 0.05 degrees.

According to a variant illustrated on the , the dispersive optical device 50, 51 further comprises an optical isolator arranged on the output light beam 200. The optical isolator comprises for example a polarizer cube 26 and a third quarter-wave plate 27. Advantageously, the polarizer cube 26 and the third quarter-wave plate 27 have input and output faces perfectly parallel to each other and are assembled for example by gluing on these faces, in order to maintain the orientation of the components. More precisely, the polarizer cube 26 is fixed to the output face 21 of the polarization separator cube 11 and the third quarter-wave plate 27 is fixed to the opposite face of the polarizer cube 26. The polarizer cube 26 is oriented at 90 degrees of the polarization separator cube 11. When the polarization separator cube 11 reflects the polarization S in a horizontal plane, the polarizer cube 26 reflects the polarization S in a vertical plane and vice versa. The third quarter-wave plate 27 at the wavelength of the laser beam 200 is oriented so that its axes are at 45 degrees from the axes of the polarizer cube 26. The dispersive optical device 50, 51 provided with this optical isolator remains in one piece. The optical isolator thus formed does not introduce any deviation or movement of the output laser beam 250.

The laser beam 200 at the output of the polarization separator cube 11 is polarized S. The polarizer cube 26 transmits the output laser beam 200 polarized S. By crossing the third quarter-wave plate 27, this polarization becomes circular and forms the beam of output 250. If a reflecting surface reflects all or part of the laser power towards the third quarter-wave plate 27, the circularly polarized beam passes through the third quarter-wave plate again and its polarization is transformed into a rectilinear polarization P. This polarization P is reflected by the polarizer cube 26 perpendicular to the axis of the output laser beam 250 and does not propagate in the dispersive optical device 50, 51 towards the laser source. An optical insulation effect is thus obtained.

According to a second embodiment, illustrated on the , the dispersive optical device 50, 51 further comprises a right-angle prism 28 disposed between the lateral face 19 of the polarization separator cube 11 and the first quarter-wave plate 12. More precisely, the right-angle prism 28 has a first face and a second face forming an angle of 90 degrees. The first face of the right-angled prism 28 is fixed to the side face 19 of the polarization separator cube 11. The second face of the right-angled prism 28 is fixed to one face of the first quarter-wave plate 12. The prism 28 at right angle makes it possible to deflect by 90 degrees the beam entering and leaving the first Bragg grating 13. This configuration makes it possible to reduce the bulk of the dispersive optical device 50, 51 linked to the length of the first Bragg grating 13 in particular when the network 13 has a length greater than the dimensions of its entrance face. This is generally the case for stretched pulses of long duration.

Optionally, the dispersive optical device 50, 51 according to the second embodiment comprises an optical isolator, as described above and illustrated in the , without increasing the bulk of the dispersive optical device 50, 51. The optical isolator can be fixed not only to the polarization separator cube 11 but also to the first Bragg grating 13, which makes it possible to further strengthen the robustness of the dispersive optical device 50 , 51. This configuration is particularly suitable for long networks.

According to a third embodiment, illustrated on the , the optical reflector component 15 is replaced by a second volumetric Bragg grating 25 with variable pitch. The joint use in the same dispersive optical device 50, 51 of two volumetric Bragg gratings with variable pitch 13 and 25 makes it possible to increase the dispersion of group speeds introduced by the dispersive optical device 50, 51. This configuration makes it possible to obtain a very high dispersion while using RBVPVs of reduced dimensions and relatively easy to manufacture. Optionally, the dispersive optical device 50, 51 according to the third embodiment comprises an optical isolator, as described in connection with the .

Indeed, in the third embodiment, illustrated in , the incident laser beam 10 is reflected on the interface 18 in the direction of the first quarter-wave plate 12. The laser beam is transmitted a first time through the first quarter-wave plate 12, reflected by the first network of Bragg 13, then transmitted a second time through the first quarter-wave plate 12 to form the once-scattered beam 130. The beam 130 propagates through the polarization splitter cube 11 towards the second quarter-wave plate 14. The beam 130 is transmitted a first time through the second quarter-wave plate 14, reflected by the second grating of Volume Bragg 25 with variable pitch, then transmitted a second time through the second quarter-wave plate 14 to form a beam 150 dispersed twice: a first dispersion by the first volume Bragg grating 13 with variable pitch and a second dispersion by the second volumetric Bragg grating 25 with variable pitch. The beam 150 is reflected on the interface 18 of the separator cube 11 and forms the output beam 200. Here too, when the beam 10 is incident on the input face 17 of the polarization separator cube 11 under normal incidence, we obtain also an output beam 200 perfectly aligned and propagating in the same direction as the incident beam 10. The beam 200 here has a total dispersion equal to the sum of the dispersion induced by the first volumetric Bragg grating 13 with variable pitch, of the dispersion induced by the second volume Bragg grating 25 with variable pitch and the initial dispersion of the incident laser beam 10, taking into account the sign of each of these dispersions.

According to a variant of the third embodiment, a right-angle prism is placed between the polarization separator cube 11 and each of the two volumetric Bragg gratings with variable pitch. This configuration makes it possible to reduce the bulk of the dispersive optical device 50, 51 using two volumetric Bragg gratings with variable pitch.

As illustrated in connection with the , the dispersive optical device 50, 51 of the present disclosure makes it possible to easily adjust the dispersion introduced by the device. Consider a dispersive optical device 50, 51 according to any one of the first, second or third embodiments described above. On the , an example of a dispersive optical device 50, 51 according to the first embodiment has been shown. Let us consider a rotation of the dispersive optical device 50, 51 by an angle alpha (denoted α) around an axis Y perpendicular to the plane of the . For this purpose, the dispersive optical device 50, 51 is placed on an opto-mechanical mount adjustable in angle. If the alpha angle is zero, the emerging beam is parallel and coincides with the incident beam. However, the rotation of a non-zero alpha angle does not affect the direction of the emerging beam 200 which remains perfectly parallel to the incident laser beam 10. The rotation of an angle alpha around an axis perpendicular to the plane of the , that is to say perpendicular to the plane containing the first Bragg grating 13 and the optical reflector component 15 only introduces a translation of the output beam 200 in the plane XZ by a quantity d which is proportional to the angle alpha and to the distance L between the mirror 15 and the middle of the zone of the first Bragg grating 13 in which the fringes of said grating are inscribed. The translation d of the beam is given approximately by the formula:

For an alpha angle less than 5 degrees, a distance L of the order of 10 mm, the distance d is of the order of 1.7 mm small compared to the diameter of the incident laser beam 10. This small lateral movement without angular deviation , allows easy positioning of the dispersive optical device 50, 51 on the incident laser beam 10. The positioning of the dispersive optical device 50, 51 does not require fine angular adjustment, but only within a few degrees. It is nevertheless necessary to ensure that the XZ plane is parallel to the incident beam to avoid introducing an angular deviation in the direction perpendicular to this plane.

This property of invariance by rotation in the XZ plane makes it possible to use the dispersive optical device 50, 51 as an adjustable stretcher or compressor. Indeed, the dispersion introduced onto the pulse by the network 13 depends on the angle of incidence of the beam on the volumetric Bragg grating with variable pitch 13. However, the variation of the angle of incidence of the beam on the volumetric Bragg grating 13 with variable pitch makes it possible to modify the dispersion introduced by the volumetric Bragg grating 13 with variable pitch. It is thus possible to finely adjust the spectral dispersion introduced by the dispersive optical device 50, 51 in order to exactly compensate for that of the incident pulse or simply in order to vary the residual dispersion of the emerging pulse from the dispersive optical device 50, 51.

According to the fourth embodiment, illustrated in Figures 9 and 10, the dispersive optical device 50, 51 does not include a second quarter-wave plate 14, nor a reflective optical component 15, the other components being identical to those described in link with the For example. The dispersive optical device 50, 51 according to the fourth embodiment makes it possible to generate an output beam 135 deviated by exactly 90 degrees relative to the incident beam while modifying the frequency drift of the pulses which pass through it. The dispersive optical device 50 according to the fourth embodiment of the allows compressing a laser pulse while rotating the laser beam 90 degrees relative to the incident direction. The dispersive optical device 50 of the can be used to make one-piece compressor or one-piece stretcher.

A rotation of an alpha angle of the dispersive optical device 50, 51 according to the fourth embodiment around an axis perpendicular to the axis of the incident beam also makes it possible to adjust the dispersion introduced by the volumetric Bragg grating with variable pitch without significant movement in the direction of the reflected wave. By orienting the device around this axis, it is thus possible to adjust the group speed dispersion introduced by the device without modifying the direction of the emerging beam.

It is thus possible to obtain a source of high-power femtosecond or picosecond laser pulses using one or more dispersive optical devices 50, 51 according to the present disclosure.

In particular, the presents a frequency drift pulse amplifier system comprising a source 1 of short pulses incident on a first dispersive optical device 50 configured to temporally stretch the pulses 3. The stretched pulses 3 are then amplified in an amplifying chain 6 comprising one or several solid crystalline amplifying media or made of doped glass or based on doped optical fibers. After amplification, a second dispersive optic 51 is configured to present a dispersion opposite to that of the first dispersive optical device 50, so as to temporally compress the pulses 4. The compensation tuning can be finely adjusted by adjusting the angle of incidence on one of the two dispersive devices 50,51.

There illustrates for example an adjustment of the dispersion by rotation of the first dispersive optical device 50 or stretcher. The dispersive optical device 50, 51 according to the present disclosure can be used to compensate for the dispersion introduced in an assembly including a source of short pulses and optical components which introduce a group speed dispersion on said pulses and therefore modify their duration. For example, a femtosecond laser source incident on a microscope using an optical fiber undergoes group velocity dispersion introduced by the fiber and consequently the duration of the pulses focused by the microscope on the sample under observation is greatly lengthened. The dispersive optical device 50, 51 allows adjustable compensation in a very compact environment and preferably without modifying the direction of the incident beam on the microscope.

There illustrates for example an adjustment of the dispersion by rotation of the second dispersive optic 51 or compressor. This rotational adjustment has no effect on the direction of the beam and therefore allows the pulse duration to be adjusted without any further adjustment in the laser system.

The dispersive optical device 50, 51 thus makes it possible to obtain a source of amplified pulses of high power and tunable duration without modifying the direction of the beam.

There schematically represents an example of a laser system comprising a source 1 producing pulses 2 of 300 fs which are stretched to a duration of 100 to 500 ps by a first dispersive optical device 50 according to one of the embodiments described. The first dispersive optical device 50 comprises at least one first volumetric Bragg grating with variable pitch configured to have a dispersion equal to 30 ps/nm. The amplifying chain 6 comprises a series of active fibers doped with ytterbium or crystal amplifiers doped with ytterbium. The amplifying chain 6 receives the stretched pulses 3 and generates amplified stretched pulses 4. The second dispersive optics 51 comprises another volume Bragg grating with variable pitch configured to present a dispersion of the order of -30 ps/nm. The second dispersive optic 51 receives the amplified stretched pulses 4 and generates compressed amplified pulses 5 having a duration of around 300 fs. In practice, the second dispersive optics 51 can be based on the same volumetric Bragg grating with variable pitch as the first dispersive optical device 50: it is sufficient to orient the RBVPV in an inverted manner, the rear face becoming the front face and vice versa. versa.

Another use of the dispersive optical device 50, 51 is illustrated in the . A source 1 of picosecond laser pulses, for example with a duration of 50 ps, is amplified in an amplifier chain 6 comprising a series of active fibers doped with ytterbium. Pulses 3, 4 undergo strong phase self-modulation during their propagation which has the effect of broadening the spectrum by creating a frequency drift. A dispersive optical device 51 makes it possible to compensate for this frequency drift and compress the pulses to a much shorter duration. In a particular example, the initial pulses 3, 4 have a duration of 50 ps, are amplified up to an energy of 15 µJ before being compressed by the dispersive optical device 51 as described in connection with the , comprising an RBVPV having a dispersion of between -10 and -20 ps/nm. There shows an example of an autocorrelation trace of a pulse 5 produced by such a laser system, which has a duration of approximately 980 fs.

The dispersive optical device 50, 51 described here can be used in all applications for which a so-called TREACY compressor is used, or in which a pulse stretcher is used.

In one application, the present disclosure proposes a dispersive optical device 51 making it possible to compress a laser pulse with a duration of between 20 ps and 100 ps to a final duration of between 0.1 ps and 3 ps, in an extremely compact volume and, preferably without changing the direction or position of the laser beam. The laser source is for example a picosecond source amplified in a fiber or solid state amplifier. It can also be a femtosecond source stretched in a grating stretcher or using the dispersion of the materials then amplified and finally recompressed by the dispersive optical device according to one of the embodiments described below.

Particularly advantageously, the dispersive optical device can be used to adjust the duration of the compressed pulses without modifying the direction of the laser beam. A simple rotation of the dispersive optical device on itself makes it possible to obtain a variable pulse duration of approximately 1 ps for a few degrees of rotation angle.

Particularly advantageously, the dispersive optical device is sufficiently compact to be attached to the end of a flexible fiber laser. Such a fiber laser equipped with the dispersive device allows the production of a beam of ultra-brief and very intense laser pulses. The soft end of the fiber laser can take any position and direction, depending on the application. The position and direction of the ultra-short laser pulse beam can be changed quickly and without being limited by the compressor, which remains stable.

In another application, the dispersive optical device 50 makes it possible to stretch the pulses before amplification and to adjust the duration of the amplified and possibly compressed pulses, preferably without realignment of the amplifying chain.

Of course, various other modifications may be made to the present disclosure within the scope of the appended claims.

Claims (11)

Dispersive optical device (50, 51) adapted to modify a group velocity dispersion of a beam (10) of light pulses, the dispersive optical device comprising: a polarization splitter cube (11), a first Bragg grating ( 13) volumetric with variable pitch and a first quarter-wave plate (12), the polarization separator cube (11) having an input face (17), an output face (21) parallel to the input face (17), two side faces (19, 20) perpendicular to the entry face (17) and an interface (18) inclined by 45 degrees relative to the entry face (17) and the two side faces (19 , 20), the input face (17) being able to receive the beam (10), the first quarter-wave plate (12) having flat and parallel faces, one face of the first quarter-wave plate ( 12) being fixed to an input-output face (23) of the first Bragg grating (13), the first volumetric Bragg grating (13) with variable pitch being photoinscribed in a material following planes parallel to the face of input-output (23) of the first Bragg grating (13), another face of the first quarter-wave plate (12) being made integral with one of the two lateral faces (19, 20) of the polarization separator cube ( 11) so that the normal to the plane of the fringes of the first volumetric Bragg grating with variable pitch makes an angle of 45 degrees with the normal to the interface (18). Dispersive optical device (50, 51) according to claim 1 in which the dispersive optical device is in one piece. Dispersive optical device (50, 51) according to claim 1 or 2 in which the other face of the first quarter-wave plate (12) is fixed to one of the two lateral faces (19, 20) of the cube separator of polarization (11). A dispersive optical device (50, 51) according to claim 1 or 2 comprising a right angle prism (28) having a first face forming an angle of 90 degrees with a second face of the right angle prism, the first face being a right angle prism being fixed to one of the two side faces (19, 20) of the polarization splitter cube (11) and the second face of the right angle prism being fixed to the first quarter-wave plate (12). Dispersive optical device (50, 51) according to claims 3 or 4, comprising a second quarter-wave plate (14) and a reflective optical component (15, 25), the second quarter-wave plate (14) being with faces plane and parallel, the second quarter-wave plate (14) being arranged between the optical reflector component (15, 25) and the other of the two lateral faces (19, 20) of the polarization separator cube (11), one face of the second quarter-wave plate (14) being fixed to the optical reflector component (15, 25) and another face of the second quarter-wave plate (14) being fixed to the other of the two lateral faces (19, 20) to the polarization separator cube (11). Dispersive optical device (50, 51) according to claim 5 in which the reflective optical component (15, 25) comprises a mirror (15) or a reflective treatment applied directly to said other face of the second quarter-wave plate (14). . Dispersive optical device (50, 51) according to claim 5 in which the reflective optical component (15, 25) comprises a second volumetric Bragg grating (25) with variable pitch, the second quarter-wave plate (14) being fixed to an entry-exit face of the second Bragg grating (25), the second volumetric Bragg grating (25) with variable pitch being photoinscribed in a material along planes parallel to the entry-exit face of the second Bragg grating (25). Dispersive optical device (50, 51) according to one of claims 1 to 7, comprising an optical isolator fixed to the output face (21) of the polarization separator cube (11). Dispersive optical device (50, 51) according to one of claims 1 to 8 comprising means for tilting the dispersive optical device (50, 51) by rotation around an axis parallel to the intersection between the entrance face (17) and the interface (18) of the polarization separator cube (11). Laser system with pulses of duration between 10 femtoseconds and 1 nanosecond and power of between 1 W and 1 kW, the laser system comprising a source capable of generating linearly polarized source pulses, an optical amplifier system, a stretcher and/or a compressor comprising at least one dispersive optical device (50, 51) according to one of claims 1 to 9. Laser system with pulses of adjustable duration between 10 femtosecond and 10 picosecond comprising a stretcher and/or a compressor comprising at least one dispersive optical device (50, 51) according to one of claims 1 to 9.