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

Abstract

The invention relates to a dispersive optical device (50, 51) suitable for modifying a group velocity dispersion of a beam (10) of light pulses. According to the invention, the dispersive optical device comprises a polarizing cube beamsplitter (11), a first variable-spacing volume Bragg grating (13) and a first quarter-wave plate (12) with parallel planar faces, one face of the first quarter-wave plate (12) being attached to an entrance-exit face (23) of the first Bragg grating (13), the first variable-spacing volume Bragg grating (13) being photo-inscribed in a material along planes that are parallel to the entrance-exit face (23) of the first Bragg grating (13), another face of the first quarter-wave plate (12) being integral with one of the two side faces (19, 20) of the polarizing cube beamsplitter (11).

Description

[0001] Dispersive optical device and short pulse laser system comprising such a dispersive optical device

[0002] Technical field

[0003] The present invention relates to the technical field of pulsed power lasers.

[0004] 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.

[0005] Prior art

[0006] The advent of ultra-short laser pulses has enabled a large number of applications in scientific, industrial and medical fields. However, 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 long-duration pulses, for example several tens of picoseconds, and the compression of the amplified pulses to a significantly shorter duration, for example from 10 femtoseconds to 1 picosecond. A compressor capable of providing pulse compression typically includes a dispersive delay line.The dispersive delay line generally comprises one or more prisms, diffraction gratings, frequency drift mirrors, dispersive materials, Bragg gratings and / or any other dispersive optical elements. To obtain sufficient dispersion to modify the pulse duration 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 space constraints (typically more than 10cm x 10cm x 10cm or even up to 100cm x 50cm x 20cm). In addition, such a compressor is extremely sensitive to vibrations and mechanical movements that induce misalignments. Such a compressor is therefore not suitable for being mounted on moving parts, let alone handheld.These dispersive optical systems all operate in reflection, meaning that the compressed laser pulse beam 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, to send the compressed laser pulse beam towards the target, making the assembly even more sensitive to misalignments.

[0007] A very important advance in the production of ultrashort pulses by frequency drift amplification has been made using variable-pitch volume Bragg gratings (VPGB). A VPGB is a single-piece, foolproof, and very compact optical component. A VPGB can also be inscribed in an optical fiber to form a chirped FBG (or variable-pitch fiber Bragg grating). However, a VPGB 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 VPGB can be used to stretch light pulses before amplification. Furthermore, the dispersion introduced by a VPGB or a chirped FBG is fixed and cannot be adjusted.Finally, a variable-pitch Bragg grating also operates in a reflective configuration, returning the beam in the opposite direction to that of the incident beam on the grating. Similarly to a Treacy grating compressor, the laser beam from a compressor using a variable-pitch Bragg grating can be returned to the target, i.e. in a different direction than the incident beam but still using a set of mirrors or prisms in free space whose orientation is adjusted for this purpose. This set of mirrors and prisms introduces increased sensitivity to vibrations.

[0008] US patent document 2014 / 0168755 A1 (Clowes et al.) discloses a pulse compression system based on a variable-pitch volume or fiber Bragg grating, comprising a polarization splitter and a quarter-wave phase plate in the beam path to return the beam emerging from the compressor at right angles or two variable-pitch volume or fiber Bragg gratings that increase the introduced dispersion. However, such a system requires precise optical alignment of each optical component and does not allow adjustment of the group velocity dispersion introduced by the Bragg grating(s).

[0009] 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 obtain this perfect agreement. The simplest consists of varying 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.

[0010] Another solution is to use a variable-pitch fiber Bragg grating as a pulse stretcher and to vary 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 accurate, this method requires the use of a fiber pulse stretcher and only allows very small dispersion excursions of the order of 3 fs / K. Therefore, the temperature would have to be increased by 300°C for an adjustment of the order of 1 ps.

[0011] One of the aims of the invention is to provide a pulse compression device which is robust, compact, lightweight and adjustable in group velocity dispersion without changing the direction of the compressed laser pulse beam.

[0012] Statement of the invention

[0013] 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 splitter cube, a first variable-pitch volume Bragg grating and a first quarter-wave plate, the polarization splitter cube having an input face, an output face parallel to the input face, two side faces perpendicular to the input face and an interface inclined at 45 degrees relative to the input face and the two side faces, the input face being adapted to receive the beam, the first quarter-wave plate having planar 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 variable-pitch volume Bragg grating being photoinscribed in a material along 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 splitter cube so that the normal to the plane of the fringes of the first variable-pitch volume Bragg grating makes an angle of 45 degrees with the normal to the interface.,

[0014] Advantageously, the dispersive optical device is a single unit.

[0015] According to some embodiments, the other face of the first quarter-wave plate is attached to one of the two side faces of the polarization splitter cube.

[0016] According to another embodiment, the dispersive optical device comprises a right angle prism having a first face forming a 90 degree angle with a second face of the right angle prism, the first face of the right angle prism being attached to one of the two side faces of the polarization splitter cube and the second face of the right angle prism being attached to the first quarter wave plate.

[0017] According to a particular and advantageous aspect, the dispersive optical device comprises a second quarter-wave plate and a reflector 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 splitter cube.

[0018] Advantageously, the reflective optical component comprises a mirror or a reflective treatment applied directly to said other face of the second quarter-wave plate.

[0019] Alternatively, the reflective optical component comprises a second variable-pitch volume Bragg grating, the second quarter-wave plate being fixed to an input-output face of the second Bragg grating, the second variable-pitch volume Bragg grating being photoinscribed in a material along planes parallel to the input-output face of the second Bragg grating.

[0020] According to another particular and advantageous aspect, the dispersive optical device comprises an optical isolator fixed to the output face of the polarization splitter 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 splitter cube.

[0021] The present disclosure also relates to a laser system with pulses of duration between 10 femtoseconds and 1 nanosecond and of power 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.

[0022] The present disclosure also relates to a 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 according to one of the embodiments described.

[0023] The dispersive optical device of the present disclosure allows for introducing 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.

[0024] The dispersive optical device allows a laser pulse to be compressed or stretched, preferably without changing the direction of the incident beam.

[0025] Particularly advantageously, the introduced time frequency drift is adjustable by means of a rotation of the dispersive optical device around a single rotation axis, while maintaining a direction of the laser beam parallel to the incident direction or, respectively, at 90 degrees to the incident direction.

[0026] Of course, the various features, variants and embodiments of the invention may be combined with each other in various combinations to the extent that they are not incompatible or mutually exclusive.

[0027] Brief description of the drawings

[0028] In addition, various other characteristics of the invention emerge from the appended description given with reference to the drawings which illustrate non-limiting embodiments of the invention and where:

[0029] Figure 1 is a block diagram of a dispersive optical device for time-stretching a pulse, Figure 2 is a block diagram of a dispersive optical device for time-compressing a pulse,

[0030] Figure 3 is an example of a dispersive optical device according to a first embodiment,

[0031] Figure 4 is an example of a dispersive optical device according to a variant of the first embodiment,

[0032] Figure 5 is an example of a dispersive optical device according to another variant of the first embodiment,

[0033] Figure 6 is an example of a dispersive optical device according to a second embodiment,

[0034] Figure 7 is an example of a dispersive optical device according to a third embodiment,

[0035] Figure 8 illustrates a method for adjusting the group velocity dispersion of a dispersive optical device according to the first, second or third embodiment,

[0036] Figure 9 is an example of a dispersive optical device according to a fourth embodiment,

[0037] Figure 10 illustrates a group velocity dispersion adjustment method of a dispersive optical device according to the fourth embodiment,

[0038] Figure 11 is a schematic of a frequency drift pulse amplification chain using two dispersive optical devices according to the present disclosure, in which the stretcher is tunable;

[0039] Figure 12 is a schematic of a frequency drift pulse amplification chain using two dispersive optical devices according to the present disclosure, in which the compressor is tunable,

[0040] Figure 13 is an experimentally obtained autocorrelation trace of an amplified and compressed laser pulse;

[0041] Figure 14 is a schematic 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.

[0042] It should be noted that in these figures the structural and / or functional elements common to the different variants may have the same references.

[0043] Detailed description In this document, frequency drift is understood to mean the modification of a group velocity of a laser pulse, which results in a spectral dispersion of the pulse.

[0044] The dispersive optical device is based on the use of one or more variable-pitch volume Bragg gratings 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 variable-pitch volume Bragg grating is fixed by welding or bonding or optical 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 on 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.

[0045] Figure 1 schematically represents a dispersive optical device 50 used to temporally stretch a laser pulse, for example before amplification. The 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 propagation direction 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.

[0046] Figure 2 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 Figure 1, 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 opposite sign 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 the pulse 2, 3.

[0047] 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.

[0048] The dispersive optical device 50, 51 of the present disclosure is based on the manufacture of a solid or single-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 variable-pitch volume Bragg gratings, denoted RBVPV.

[0049] Figure 3 schematically represents a dispersive optical device 50, 51 according to a first embodiment. An orthonormal XYZ reference frame is represented, the XZ plane being in the plane of Figure 3. The dispersive optical device 50, 51 comprises a polarization splitter cube 11, a first quarter-wave plate 12, a first variable-pitch volume Bragg grating 13, a second quarter-wave plate 14 and a reflective optical component 15. The reflective optical component 15 is for example a plane mirror with flat and parallel faces. Alternatively, the reflective optical 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 an angle between the two faces less than or equal to 0.05 degrees.

[0050] The polarization splitter cube 11 comprises an input face 17, an output face 21 parallel to the input face 17, two lateral faces 19, 20 perpendicular to the input face 17 and an interface 18 inclined at 45 degrees relative to the input face 17 and to the two lateral faces 19, 20. The lateral faces 19, 20 are parallel to each other and perpendicular to the input face 17. Advantageously, the polarization splitter cube 11 thus forms a rectangular parallelepiped, cut along its diagonal by the interface 18.

[0051] 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 splitter cube 11 and the input-output face 23 of the first variable-pitch volume Bragg grating 13. 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 splitter cube 11 and, on the other hand, to the input-output face 23 of the first variable-pitch volume Bragg grating 13. The second quarter-wave plate 14 is arranged between the other lateral face 20 of the polarization splitter 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 splitter cube 11 and, on the other hand, to the optical reflector component 15.

[0052] In this document, "fixed" means that two optical elements are in contact, either directly or by means of an optical glue, and in an invariable position relative to each other, without the possibility of relative movement or rotation between the two optical elements fixed to each other.

[0053] The first variable-pitch volume Bragg grating 13 has an input-output face 23. The first variable-pitch volume Bragg grating 13 is fabricated and chosen to be a photo-inscribed volume Bragg grating in a material (e.g., a glass block) along planes parallel to this input-output face 23. In this way, the normal to the fringe plane of the first variable-pitch volume Bragg grating 13 forms an angle of 45 degrees ± 0.1° with the normal to the interface 18 of the polarization splitter cube 11.

[0054] Variable-pitch volume Bragg gratings are produced by various companies, including Optigrate Corporation, 3267 Progress Drive, Orlando, Florida 32286, USA. A periodic variation in the refractive index of a photosensitive glass is recorded in a glass block using a UV laser. This grating behaves like a Bragg grating by reflecting wavelengths corresponding to the grating period. A variation in the grating period along the axis of the glass block allows different wavelengths to be reflected at different positions in the glass block. This introduces a delay between the wavelengths, which results in a drift in the optical frequency of the pulse reflected by the variable-pitch volume Bragg grating. 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 at 45° ± 0.1° from the physical surfaces 19 and 20. In particular, the planes of the fringe grating inscribed in the RBVPV are parallel to the input-output face 23 of the glass block in which the grating is inscribed. This precision makes it possible to guarantee that an incident beam perpendicular to the input-output face 23 of the glass block comprising the grating 13 is reflected exactly on itself by the grating 13.

[0055] 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 gluing, 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 splitter cube 11. The dispersive optical device 50, 51 thus forms a single-piece optical component, compact, perfectly aligned by construction and foolproof. As a non-limiting example, the polarizer cube has dimensions of 8 mmx8 mmx8 mm and the optical faces of all the 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.

[0056] 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 splitter cube 11. The beam 10 of light pulses is of linear polarization perpendicular to the plane of FIG. 3. In other words, the beam 10 is polarized S, parallel to a straight line of intersection between the input face 17 and the interface 18, this straight line of intersection being parallel to the Y axis.

[0057] 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 variable-pitch volume Bragg grating 13. The axes of the first quarter-wave plate 12 are aligned at 45 degrees to the incident polarization so that the linearly polarized beam 10 S is transformed into a circularly polarized beam 120 upon passing through the first quarter-wave plate 12. The beam 120 of circularly polarized light pulses is incident on the first variable-pitch volume Bragg grating 13. The different spectral components of the pulse are reflected on the first Bragg grating 13 at different positions along the beam propagation direction. 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 variable-pitch volume Bragg grating 13 has a modified dispersion or a modified frequency drift relative to the laser beam 10. The orientation of the planes of the diffraction fringes inscribed in the grating 13 determines the direction of the beam reflected 130 by the first Bragg grating 13. In the example of FIG. 3, the direction of propagation of the beam 120 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 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 splitter 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.

[0058] The interface 18 transmits the beam 135 of polarization P towards the second quarter-wave plate 14 whose axes are aligned at 45 degrees to the polarization P. The second quarter-wave plate 14 transforms the linear polarization P of the beam 135 into a circular polarization. The mirror 15 reflects the circularly polarized beam and forms a beam 140 which passes through the second quarter-wave plate 14. The circularly polarized beam 140 is transformed by the second quarter-wave plate 14 into a linearly polarized beam 150 S. The interface 18 reflects the linearly polarized beam 150 S towards the output face 21 of the polarization splitter 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 variable-pitch volume Bragg grating 13 and of the polarization splitter cube 11.

[0059] As illustrated in Figure 3, when the beam 10 is incident on the input face 17 of the polarization splitter cube 11 at a zero angle of incidence, in other words at normal incidence, the beam 200 at the output of the dispersive optical device 50, 51 propagates in the same direction and in the same sense as the incident beam 10. As indicated above, the fringes of the first variable-pitch volume Bragg grating 13 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 regardless of the angle between the first variable-pitch volume Bragg grating 13 and the polarization splitter 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 dispersion induced by the variable-pitch volume Bragg grating 13 and the initial dispersion of the incident laser beam 10, taking into account the sign of each of these dispersions.

[0060] The dispersive optical device 50, 51 of Figure 3 allows a laser pulse to be compressed or stretched while maintaining the direction of the laser beam. The dispersive optical device 50 of Figure 3 can be used to make a one-piece stretcher or a one-piece compressor.

[0061] The dispersive optical device 50, 51 is thus advantageously inserted in the path of a laser pulse beam 10 to modify the group velocity dispersion of the pulses, without modifying the direction or position of the output laser beam 200. The dispersive optical device 50, 51 determines by construction the direction of the output beam 200 relative to the incident beam 10.

[0062] According to a variant of the first embodiment illustrated in FIG. 4, the location of the first quarter-wave plate 12 fixed to the first variable-pitch volume Bragg grating 13 is swapped with that of the second quarter-wave plate 14 fixed to the reflector optical component 15. This swapping does not in any way modify the operation of the dispersive optical device 50, 51. Indeed, in this case, the light beam is transmitted twice through the second quarter-wave plate 14 and reflected on the reflector optical component 15 before passing through the polarization splitter cube 11 to be reflected by the first variable-pitch volume Bragg grating 13 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 lateral face 19 through the second quarter-wave plate 14 to form a circularly polarized beam 140 which is reflected on the reflecting optical component 15 to form a circularly polarized beam 150. The beam 150 passes through the second quarter-wave plate 14 to form a linearly polarized beam 155 P. The linearly polarized beam 155 P is transmitted through the polarization splitter cube 11 towards the other lateral face 20. The linearly polarized beam 155 P is transmitted through the first quarter-wave plate 12 and forms a circularly polarized beam 120. 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 splitter cube 11. The interface 18 reflects the beam 150 of linear polarization S and forms the output beam 200. As described in connection with FIG. 3, the output beam 200 has a total dispersion equal to the sum of the dispersion induced by the variable-pitch volume Bragg grating 13 and the initial dispersion of the incident laser beam 10, taking into account the sign of each of these dispersions.

[0063] In particular, as illustrated in Figure 4, when the incident beam 10 is perpendicular to the input 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 twice through the second quarter-wave plate is perpendicular to the fringes of the first incident variable-pitch volume Bragg grating 13. 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 laser beam 10 is perpendicular to the input face 17 of the polarization splitter cube 11, the output light beam 200 has the same direction, the same polarization and the same position as the input light beam 10. In other words, when the beam 10 is incident on the input face 17 of the polarization splitter cube 11 under normal incidence, a perfectly aligned output beam 200 propagating in the same direction as the incident beam 10 is also obtained. Numerous variants of the dispersive optical device 50, 51 are considered here.

[0064] Figure 4 further illustrates a variant according to which the reflective optical 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 in parallel with an angle between the two faces not exceeding 0.05 degrees.

[0065] According to a variant illustrated in Figure 5, 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 that are 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 splitter 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 to the polarization splitter cube 11. When the polarization splitter cube 1 1 reflects the S polarization in a horizontal plane, the polarizer cube 26 reflects the S polarization 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 to the axes of the polarizing cube 26. The dispersive optical device 50, 51 provided with this optical isolator remains monobloc. The optical isolator thus formed does not introduce any deviation or displacement of the output laser beam 250.

[0066] The laser beam 200 at the output of the polarization splitter cube 11 is S polarized. The polarizer cube 26 transmits the S polarized output laser beam 200. By passing through the third quarter-wave plate 27, this polarization becomes circular and forms the output beam 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 isolation effect is thus obtained.

[0067] According to a second embodiment, illustrated in FIG. 6, the dispersive optical device 50, 51 further comprises a right-angle prism 28 disposed between the lateral face 19 of the polarization splitter 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 lateral face 19 of the polarization splitter cube 11. The second face of the right-angled prism 28 is fixed to a face of the first quarter-wave plate 12. The right-angled prism 28 makes it possible to deflect the beam entering and leaving the first Bragg grating 13 by 90 degrees. This configuration makes it possible to reduce the size of the dispersive optical device 50, 51 linked to the length of the first Bragg grating 13, in particular when the grating 13 has a length greater than the dimensions of its entry face.This is generally the case for stretched pulses of long duration.

[0068] Optionally, the dispersive optical device 50, 51 according to the second embodiment comprises an optical isolator, as described above and illustrated in FIG. 6, without increasing the size of the dispersive optical device 50, 51. The optical isolator can be fixed not only to the polarization splitter cube 11 but also to the first Bragg grating 13, which makes it possible to further reinforce the robustness of the dispersive optical device 50, 51. This configuration is particularly suitable for long gratings.

[0069] According to a third embodiment, illustrated in FIG. 7, the reflector optical component 15 is replaced by a second variable-pitch volume Bragg grating 25. The joint use in the same dispersive optical device 50, 51 of two variable-pitch volume Bragg gratings 13 and 25 makes it possible to increase the group velocity dispersion introduced by the dispersive optical device 50, 51. This configuration makes it possible to obtain a very significant 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 FIG. 5.

[0070] Indeed, in the third embodiment, illustrated in figure 7, 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 Bragg grating 13, then transmitted a second time through the first quarter-wave plate 12 to form the beam 130 dispersed once. The beam 130 propagates through the polarization splitter cube 11 in the direction of 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 variable-pitch volume Bragg grating 25, 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 variable-pitch volume Bragg grating 13 and a second dispersion by the second variable-pitch volume Bragg grating 25. The beam 150 is reflected on the interface 18 of the splitter cube 11 and forms the output beam 200. Here too, when the beam 10 is incident on the input face 17 of the polarization splitter cube 11 under normal incidence, a perfectly aligned output beam 200 propagating in the same direction as the incident beam 10 is also obtained.The beam 200 here has a total dispersion equal to the sum of the dispersion induced by the first variable-pitch volume Bragg grating 13, the dispersion induced by the second variable-pitch volume Bragg grating 25 and the initial dispersion of the incident laser beam 10, taking into account the sign of each of these dispersions.

[0071] According to a variant of the third embodiment, a right-angle prism is arranged between the polarization splitter cube 11 and each of the two variable-pitch volume Bragg gratings. This configuration makes it possible to reduce the size of the dispersive optical device 50, 51 using two variable-pitch volume Bragg gratings.

[0072] As illustrated in connection with Figure 8, 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. Figure 8 shows an example of a dispersive optical device 50, 51 according to the first embodiment. Consider a rotation of the dispersive optical device 50, 51 by an angle alpha (denoted a) around an axis Y perpendicular to the plane of Figure 8. For this purpose, the dispersive optical device 50, 51 is placed on an angle-adjustable opto-mechanical mount. If the angle alpha is zero, the emerging beam is parallel and coincident with the incident beam. However, the rotation by a non-zero angle alpha 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 Figure 8, that is to say perpendicular to the plane containing the first Bragg grating 13 and the reflecting optical component 15 introduces only a translation of the output beam 200 in the XZ plane of 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: d = 2. L. sin(c ).

[0073] For an angle alpha 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 displacement 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 to within a few degrees. However, it is advisable 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.

[0074] This property of rotational invariance 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 on the pulse by the grating 13 depends on the angle of incidence of the beam on the variable-pitch volume Bragg grating 13. However, the variation of the angle of incidence of the beam on the variable-pitch volume Bragg grating 13 makes it possible to modify the dispersion introduced by the variable-pitch volume Bragg grating 13. 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 pulse emerging from the dispersive optical device 50, 51.

[0075] According to the fourth embodiment, illustrated in figures 9 and 10, the dispersive optical device 50, 51 does not comprise a second quarter-wave plate 14, nor a reflector optical component 15, the other components being identical to those described in connection with figure 3 for example. The dispersive optical device 50, 51 according to the fourth embodiment makes it possible to generate an output beam 135 deflected 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 figure 9 makes it possible to compress a laser pulse while rotating the laser beam by 90 degrees relative to the incident direction. The dispersive optical device 50 of figure 9 can be used to manufacture a single-piece compressor or a single-piece stretcher.

[0076] A rotation of an angle alpha 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 variable-pitch volume Bragg grating without significant displacement of the direction of the reflected wave. By orienting the device around this axis, it is thus possible to adjust the group velocity dispersion introduced by the device without modifying the direction of the emerging beam.

[0077] It is thus possible to obtain a source of high-power femtosecond or picosecond laser pulses by using one or more dispersive optical devices 50, 51 according to the present disclosure.

[0078] In particular, Figure 11 shows 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 amplifier chain 6 comprising one or more solid crystalline or doped glass or doped optical fiber-based amplifier media. After amplification, a second dispersive optic 51 is configured to have 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.

[0079] Figure 11 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 velocity dispersion on said pulses and therefore modify their duration. For example, a femtosecond laser source incident on a microscope by means of an optical fiber undergoes the 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 extended. The dispersive optical device 50, 51 allows adjustable compensation in a very compact environment and preferably without modifying the direction of the beam incident on the microscope.

[0080] Figure 12 illustrates for example an adjustment of the dispersion by rotation of the second dispersive optic 51 or compressor. This rotation adjustment has no effect on the beam direction and therefore allows the pulse duration to be adjusted without any other adjustment in the laser system.

[0081] 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.

[0082] Figure 14 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 variable-pitch volume Bragg grating configured to have a dispersion equal to 30 ps / nm. The amplifier chain 6 comprises a series of active fibers doped with ytterbium or amplifiers with ytterbium-doped crystals. The amplifier chain 6 receives the stretched pulses 3 and generates amplified stretched pulses 4. The second dispersive optics 51 comprises another variable-pitch volume Bragg grating configured to have 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 the order of 300 fs.In practice, the second dispersive optics 51 can be based on the same variable pitch volume Bragg grating as the first dispersive optical device 50: it is sufficient to orient the RBVPV in reverse, the rear face becoming the front face and vice versa.

[0083] Another use of the dispersive optical device 50, 51 is illustrated in Figure 12. 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. The pulses 3, 4 undergo during their propagation a strong self-phase modulation 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 to 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 to an energy of 15 pJ before being compressed by the dispersive optical device 51 as described in connection with FIG. 5, comprising an RBVPV having a dispersion of between -10 and -20 ps / nm.Figure 13 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.

[0084] The dispersive optical device 50, 51 described herein can be used in all applications for which a so-called TREACY compressor is used, or in which a pulse stretcher is used.

[0085] In one application, the present disclosure provides a dispersive optical device 51 for compressing 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 modifying 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 may also be a femtosecond source stretched in a grating stretcher or using the dispersion of materials then amplified and finally recompressed by the dispersive optical device according to one of the embodiments described below.

[0086] Particularly advantageously, the dispersive optical device can be used to adjust the duration of the compressed pulses without changing 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 about 1 ps for a few degrees of rotation angle.

[0087] Particularly advantageously, the dispersive optical device is compact enough to be attached to the end of a flexible fiber laser. Such a fiber laser equipped with the dispersive device enables the production of a beam of ultra-short and very intense laser pulses. The flexible end of the fiber laser can take any position and direction, depending on the application. The position and direction of the beam of ultra-short laser pulses can be changed quickly and without being limited by the compressor, which remains stable.

[0088] 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 amplifier chain.

[0089] Of course, various other modifications may be made to the present disclosure within the scope of the appended claims.

Claims

Claims 1. 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 variable-pitch volume Bragg grating (13) and a first quarter-wave plate (12), the polarization splitter 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 input face (17) and an interface (18) inclined at 45 degrees relative to the input face (17) and to the two side faces (19, 20), the input face (17) being adapted to receive the beam (10), the first quarter-wave plate (12) having planar 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 variable-pitch volume Bragg grating (13) 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 splitter cube (11) so that the normal to the plane of the fringes of the first variable-pitch volume Bragg grating makes an angle of 45 degrees with the normal to the interface (18)., 2. Dispersive optical device (50, 51) according to claim 1 wherein the dispersive optical device is a single-piece device.

3. Dispersive optical device (50, 51) according to claim 1 or 2 wherein the other face of the first quarter-wave plate (12) is fixed to one of the two lateral faces (19, 20) of the polarization splitter cube (11).

4. A dispersive optical device (50, 51) according to claim 1 or 2 comprising a right-angle prism (28) having a first face forming a 90-degree angle with a second face of the right-angle prism, the first right-angle prism face being attached 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 attached to the first quarter-wave plate (12).

5. Dispersive optical device (50, 51) according to claim 3 or 4, comprising a second quarter-wave plate (14) and a reflective optical component (15, 25), the second quarter-wave plate (14) having flat and parallel faces, the second quarter-wave plate (14) being arranged between the reflective optical component (15, 25) and the other of the two lateral faces (19, 20) of the polarization splitter cube (11), one face of the second plate quarter-wave plate (14) being fixed to the reflector optical 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 splitter cube (11).

6. Dispersive optical device (50, 51) according to claim 5 wherein 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).

7. Dispersive optical device (50, 51) according to claim 5 wherein the reflective optical component (15, 25) comprises a second variable-pitch volume Bragg grating (25), the second quarter-wave plate (14) being fixed to an input-output face of the second Bragg grating (25), the second variable-pitch volume Bragg grating (25) being photoinscribed in a material along planes parallel to the input-output face of the second Bragg grating (25).

8. 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 splitter cube (11).

9. 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 input face (17) and the interface (18) of the polarization separator cube (11).

10. Laser system with pulses of duration between 10 femtoseconds and 1 nanosecond and power 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. 1 1. 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.

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