Dispersing optical device and short-pulse laser system comprising same

Abstract

The invention relates to a dispersive optical arrangement (50, 51) adapted to vary the group velocity dispersion of a pulsed light beam (10). According to the invention, the dispersive optical device comprises a polarizing cube beam splitter (11), a first variable periodic volume Bragg grating (13) and a first quarter-wave plate (12) having flat and parallel faces, one face of the first quarter-wave plate (12) being attached to the input / output face (23) of the first Bragg grating (13), and the other face of the first quarter-wave plate (12) being attached to the input / output face (23) of the first Bragg grating (13). The first variable periodic volume Bragg grating (13) is photoetched in the material along a plane parallel to the input / output face (23) of the first Bragg grating (13). The other surface of the first quarter-wave plate (12) is integrated with one of the two side surfaces (19, 20) of the polarizing cubic beam splitter (11).

Description

Technical Field

[0001] The present invention relates to the technical field of pulse power lasers.

[0002] More particularly, the present invention relates to a dispersive optical device capable of modifying the chirp of a laser pulse beam to either stretch or compress these laser pulses. Background Art

[0003] The advent of ultrashort laser pulses has enabled a large number of applications in science, industry and medicine. However, the direct amplification of short pulses is limited by the occurrence of nonlinear optical effects in optical amplifiers during propagation. Various solutions have been found to overcome this problem, including chirped pulse amplification. In all cases, the generation of extremely intense and extremely short laser pulses involves the amplification of long-duration pulses (e.g., tens of picoseconds) and the compression of the amplified pulses to a significantly shorter duration, e.g., from 10 femtoseconds to 1 picosecond. Compressors capable of providing pulse compression typically include dispersive delay lines. Dispersive delay lines typically include one or more prisms, diffraction gratings, dispersive mirrors, dispersive materials, Bragg gratings and / or any other dispersive optical elements. In order to obtain sufficient dispersion to modify the pulse duration by tens of picoseconds, only compressors using diffraction gratings and / or Bragg gratings are suitable. For example, Treacy grating compressors typically use two reflective diffraction gratings and a set of mirrors or prisms. However, such compressors present spatial challenges (often exceeding 10cm x 10cm x 10cm, and even as large as 100cm x 50cm x 20cm). Furthermore, such compressors are extremely sensitive to vibrations and mechanical movements that can cause misalignment. Consequently, such compressors are not suitable for mounting on moving parts or even for handheld use. These dispersive optical systems all operate in a reflective manner, meaning that the compressed laser pulse beam emerges from the compressor parallel to but in the opposite direction of the incoming beam of amplified laser pulses. This configuration requires the use of another optical system (typically including one or more mirrors) to direct the compressed laser pulse beam to the target, making the component even more sensitive to misalignment.

[0004] Significant progress has been made in generating ultrashort pulses through chirped pulse amplification using variable-period volume Bragg gratings (VPGBs). VPGBs are all-in-one, simple-to-use, and very compact optical components. VPGBs can also be inscribed in optical fibers to form chirped FBGs (or variable-period fiber Bragg gratings). However, the surface area of ​​VPGBs is limited, on the order of tens of square micrometers, which is generally too small for intense laser pulse applications. VPGBs can be used to stretch optical pulses before amplification. Furthermore, the dispersion introduced by VPGBs or chirped FBGs is fixed and cannot be adjusted. Finally, variable-period Bragg gratings also operate in a reflective configuration, returning the beam in a direction opposite to that of the incident beam on the grating. Similar to the Treacy grating compressor, the laser beam from a compressor using a variable-period Bragg grating can be returned to the target, i.e., in a direction different from that of 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.

[0005] U.S. Patent Document 2014 / 0168755A1 (Clowes et al.) discloses a pulse compression system based on a variable periodic Bragg grating or a fiber Bragg grating. This system includes a polarization beam splitter and a quarter-wavelength phase plate in the beam path to return the beam from the compressor or two variable periodic Bragg gratings or fiber Bragg gratings at right angles, which allows for increased introduced dispersion. However, this system requires precise optical alignment of each optical component and does not allow for adjustment of the group velocity dispersion introduced by the Bragg grating.

[0006] Furthermore, chirped-pulse amplification requires perfect compensation of the chirp introduced by the compressor with that introduced by the stretcher and amplifier chain. There are various approaches to achieving this perfect alignment. The simplest is to vary the distance between the optical components of the Treacy compressor. This traditional approach requires placing at least one component on a translation stage and makes the system more sensitive to misalignment. It also requires the component to be moved exactly parallel to the axis of the beam incident on it.

[0007] Another solution is to use a variable-period fiber Bragg grating as a pulse stretcher and change its chirp by adjusting its temperature. Frankinas et al. describe this approach 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 (April 22, 2016). While this approach is very precise, it requires the use of a fiber pulse stretcher and only allows for very small dispersion shifts of the order of 3 fs / K. Therefore, to make adjustments of the order of 1 ps, the temperature would have to be increased by 300°C.

[0008] One of the objects of the present invention is to provide a pulse compression device which is robust, compact, lightweight and in which the group velocity dispersion can be adjusted without changing the direction of the compressed laser pulse beam. Summary of the Invention

[0009] To this end, the present invention relates to a dispersive optical device adapted to change the group velocity dispersion of an optical pulse beam, the dispersive optical device comprising a polarization beam splitter cube, a first variable periodic Bragg grating, and a first quarter-wave plate, the polarization beam 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 45 degrees relative to the input face and the two side faces, the input face being adapted to receive the optical beam, the first quarter-wave plate having flat and parallel faces, one face of the first quarter-wave plate being fixed to the input and output faces of the first Bragg grating, the first variable periodic Bragg grating being photoetched in a material along a plane parallel to the input and output faces of the first Bragg grating, and the other face of the first quarter-wave plate being integrally formed with one of the two side faces of the polarization beam splitter cube, such that the normal to the fringe plane of the first variable periodic Bragg grating forms an angle of 45 degrees with the normal to the interface.

[0010] Advantageously, the dispersive optics are integral.

[0011] According to some embodiments, the other side of the first quarter wave plate is attached to one of the two side faces of the polarizing beam splitter cube.

[0012] According to another embodiment, the dispersive optical device includes a right-angle prism, a first face of the right-angle prism 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 sides of a polarizing beam splitter cube, and the second face of the right-angle prism being attached to a first quarter-wave plate.

[0013] According to a specific and advantageous aspect, the dispersive optical device includes 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 reflective optical component and the other of the two side faces of the polarizing beam splitter cube, one face of the second quarter-wave plate being fixed to the reflective optical component, and the other face of the second quarter-wave plate being fixed to the other of the two side faces of the polarizing beam splitter cube.

[0014] Advantageously, the reflective optics comprises a mirror or a reflective treatment applied directly to said other face of the second quarter wave plate.

[0015] Alternatively, the reflective optical component includes a second variable periodic Bragg grating, the second quarter wave plate is fixed to the input and output faces of the second Bragg grating, and the second variable periodic Bragg grating is photoetched in the material along a plane parallel to the input and output faces of the second Bragg grating.

[0016] According to another particular and advantageous aspect, the dispersive optical device comprises an optical isolator fixed to the output face of the polarizing beam splitter cube.

[0017] Optionally, the dispersive optical device comprises means for tilting the dispersive optical device by rotation about an axis parallel to the intersection line between the input face of the polarizing beam splitter cube and the interface.

[0018] The present disclosure also relates to a laser system having a pulse duration between 10 femtoseconds and 1 nanosecond and a 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 described embodiments.

[0019] The present disclosure also relates to a laser system having pulses of adjustable duration between 10 femtoseconds and 10 picoseconds, said laser system comprising a stretcher and / or compressor comprising at least one dispersive optical device according to one of the described embodiments.

[0020] The dispersive optics of the present disclosure allow for the introduction of temporal chirp onto laser pulses without changing the direction of the laser beam or deflecting it by 90 degrees, depending on the embodiment.

[0021] Dispersive optics allow laser pulses to be compressed or stretched, preferably without changing the direction of the incident beam.

[0022] Particularly advantageously, the introduced temporal chirp can be adjusted by rotating the dispersive optics about a single rotation axis while keeping the direction of the laser beam parallel to the incident direction or at 90 degrees to the incident direction, respectively.

[0023] Of course, the various features, variants and embodiments of the present invention can be combined with one another in various combinations, as long as they are not contradictory or mutually exclusive. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Furthermore, various other features of the invention emerge from the accompanying description given with reference to the accompanying drawings, which show a non-limiting form of embodiment of the invention, in which:

[0025] Figure 1 This is a schematic diagram of a dispersive optical device used to temporally stretch pulses.

[0026] Figure 2 This is a schematic diagram of a dispersive optical device used to time compress pulses.

[0027] Figure 3 is an example of a dispersive optical device according to the first embodiment,

[0028] Figure 4 is an example of a dispersive optical device according to a modification of the first embodiment,

[0029] Figure 5 is an example of a dispersive optical device according to another modification of the first embodiment,

[0030] Figure 6 is an example of a dispersive optical device according to a second embodiment,

[0031] Figure 7 is an example of a dispersive optical device according to a third embodiment,

[0032] Figure 8 A method for adjusting the group velocity dispersion of the dispersive optical device according to the first, second or third embodiment is shown.

[0033] Figure 9 is an example of a dispersive optical device according to a fourth embodiment,

[0034] Figure 10 A method for adjusting the group velocity dispersion of a dispersive optical device according to a fourth embodiment is shown.

[0035] Figure 11 is a schematic diagram of a chirped pulse amplification chain using two dispersive optical devices according to the present disclosure, wherein the stretcher is tunable;

[0036] Figure 12 is a schematic diagram of a chirped pulse amplification chain using two dispersive optical devices according to the present disclosure, where the compressor is tunable.

[0037] Figure 13 is the experimentally obtained autocorrelation trajectory of the amplified and compressed laser pulse;

[0038] Figure 14 is a schematic diagram of a chirped pulse amplification chain using two dispersive optical devices according to the present disclosure, where the stretcher and compressor are tunable.

[0039] It should be noted that in these figures, structural and / or functional elements common to different variants may have the same reference numbers. DETAILED DESCRIPTION

[0040] In this document, chirp is understood to mean a modification of the group velocity of a laser pulse, which leads to spectral dispersion of the pulse.

[0041] Dispersive optical devices are based on the use of one or more variable-periodic volume Bragg gratings (VPBGs), characterized by the physical planes of the gratings being perfectly parallel to the planes of the grating stripes etched into the material. The VPBGs are attached to a set of optical components by welding, gluing, or optical bonding to form the dispersive optical device. Such dispersive optical devices can introduce spectral dispersion into an incident signal, preferably without changing its direction. The disclosed dispersive optical devices find particular application in laser systems used to generate ultrashort pulses in industrial, scientific, and / or medical applications.

[0042] Figure 1 A dispersive optical device 50 for temporally stretching laser pulses (e.g., before amplification) is schematically shown. A light source 1 generates a laser pulse 2 with a duration between 10 fs and 10 ns and negative, zero, or positive chirp. The laser pulse 2 propagates along a propagation direction. The laser pulse 2 is incident on the dispersive optical device 50, which modifies its chirp without changing the propagation direction of the pulse. If the chirp of the initial laser pulse 2 is zero or very small compared to the chirp introduced by the dispersive optical device 50, or if the chirp of the laser pulse 2 has the same sign as the chirp introduced by the dispersive optical device 50, then at the output of the dispersive optical device 50, the pulse 3 is stretched and its duration is generally extended compared to the initial laser pulse 2.

[0043] Figure 2 A dispersive optical device 51 for temporally compressing laser pulses is schematically shown. The light source 1 may be followed by a stretcher, for example in combination with Figure 1The first dispersive optical device 50 described generates laser pulses 2 or 3 with a duration between 30 ps and 10 ns and a nonzero, negative, or positive chirp. Dispersive optical device 51 introduces a chirp of opposite sign to the chirp of laser pulses 2 or 3. As a result, pulse 4 is compressed at the output of dispersive optical device 51. The final duration of pulse 4 depends on the sign of each chirp, its absolute value, and the initial duration of pulses 2 and 3.

[0044] We will now describe the structure and operation of dispersive optical devices 50, 51 according to various embodiments, as well as the characteristics of pulses propagating in such dispersive optical devices.

[0045] The dispersive optical devices 50, 51 of the present disclosure are based on the fabrication of a solid or monolithic dispersive optical device by gluing, welding or optically bonding multiple optical components with high-precision properties. In order to be able to compress laser pulses exhibiting chirp, the device uses one or more variable period volume Bragg gratings, denoted as RBVPVs.

[0046] Figure 3 The diagram schematically shows a dispersive optical device 50, 51 according to a first embodiment. The diagram shows an orthogonal XYZ reference system, with the XZ plane located at Figure 3 The dispersive optical devices 50 and 51 include a polarization beam splitter cube 11, a first quarter-wave plate 12, a first variable period 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 having flat, parallel faces. Alternatively, the reflective optical component 15 includes a reflective treatment applied directly to the outer surface of the second quarter-wave plate 14, with the two faces of the second quarter-wave plate 14 being polished in parallel, and the angle between the two faces being less than or equal to 0.05 degrees.

[0047] Polarizing beam splitter cube 11 includes an input face 17, an output face 21 parallel to input face 17, two side faces 19 and 20 perpendicular to input face 17, and an interface 18 inclined 45 degrees relative to input face 17 and the two side faces 19 and 20. Side faces 19 and 20 are parallel to each other and perpendicular to input face 17. Advantageously, polarizing beam splitter cube 11 is thus formed as a rectangular parallelepiped, and is cut along its diagonal by interface 18.

[0048] The first quarter-wave plate 12 and the second quarter-wave plate 14 are both wave plates having flat and parallel surfaces. The first quarter-wave plate 12 is disposed between the side surface 19 of the polarization beam splitter cube 11 and the input / output surface 23 of the first variable periodic volume Bragg grating 13. More specifically, the first quarter-wave plate 12 is fixed to the side surface 19 of the polarization beam splitter cube 11 on the one hand, and to the input / output surface 23 of the first variable periodic volume Bragg grating 13 on the other hand by gluing, welding, or optical bonding. The second quarter-wave plate 14 is disposed between the other side surface 20 of the polarization beam splitter cube 11 and the reflective optical component 15. The second quarter-wave plate 14 is fixed to the side surface 20 of the polarization beam splitter cube 11 on the one hand, and to the reflective optical component 15 on the other hand by gluing, welding, or optical bonding.

[0049] In this document, "fixed" means that two optical elements are in contact directly or through optical adhesive and are in an unchanged position relative to each other, and there is no possibility of relative movement or rotation between the two optical elements fixed to each other.

[0050] The first variable-period volume Bragg grating 13 has an input-output face 23. The first variable-period volume Bragg grating 13 is fabricated and selected to be a volume Bragg grating photolithographically formed in a material (e.g., a glass block) along a plane parallel to the input-output face 23. Thus, the normal to the fringe plane of the first variable-period volume Bragg grating 13 forms an angle of 45 degrees ± 0.1 degrees with the normal to the interface 18 of the polarization beam splitter cube 11.

[0051] Variable-period volume Bragg gratings are manufactured by several companies, including Optigrate Corporation, 3267 Progress Drive, Orlando, Florida 32286, USA. Periodic variations in the refractive index of photosensitive glass are recorded in a glass block using an ultraviolet laser. The grating functions similarly to a Bragg grating by reflecting wavelengths corresponding to the grating period. Variations in the grating period along the axis of the glass block cause different wavelengths to be reflected at different locations in the block. This introduces a delay between the wavelengths, causing the optical frequency of the pulses reflected by the variable-period volume Bragg grating to shift. The direction of the reflected wave is determined by the angle of incidence on the fringe plane of the diffraction grating, rather than by the physical input and output surfaces 23 of the Bragg grating 13.

[0052] Dispersive optical devices 50 and 51 include optical components whose reflective surfaces are perfectly parallel to the physical surfaces within 0.1 degrees, with the exception of polarization surface 18, which is angled at exactly 45° ± 0.1° with physical surfaces 19 and 20. Specifically, the planes of the gratings inscribed in the RBVPV are parallel to the input and output faces 23 of the glass block in which the gratings are inscribed. This accuracy ensures that an incident light beam perpendicular to the input and output faces 23 of the glass block containing the grating 13 is precisely reflected by the grating 13 itself.

[0053] The elements of the dispersive optical devices 50, 51 are in contact with each other via their optical faces (i.e., the faces through which the laser beam passes). The assembly of these elements is achieved by gluing (using optical glue transparent to the laser beam), optical contact, or welding. Elements 12, 13, 14, 15 are assembled so that their respective optical faces are parallel to the sides 19, 20 of the polarization beam splitter cube 11. Thus, the dispersive optical devices 50, 51 form a monolithic optical component that is compact, perfectly aligned, and very reliable. As a non-limiting example, the dimensions of the polarizer cube are 8 mm x 8 mm x 8 mm, and the optical faces of all elements have the same dimensions, 8 mm x 8 mm. These dimensions are variable, and it is not necessary for all elements to have faces of uniform size.

[0054] The dispersive optical devices 50 and 51 are arranged to receive the light pulse beam 10 incident on the input face 17 of the polarization beam splitter cube 11. The light pulse beam 10 has a polarization perpendicular to the polarization beam splitter cube 11. Figure 3 In other words, the light beam 10 is S-polarized and parallel to the intersection line of the input face 17 and the interface 18, which is parallel to the Y axis.

[0055] The interface 18 can direct the S-polarized light beam 10 to the first quarter wave plate 12 and the first variable periodic Bragg grating 13 by reflection. The axis of the first quarter wave plate 12 is at a 45-degree angle to the incident polarization, so that the linearly polarized light beam 10S is converted into a circularly polarized light beam 120 when passing through the first quarter wave plate 12. The circularly polarized light pulse beam 120 is incident on the first variable periodic Bragg grating 13. Different spectral components of the pulse are reflected on the first Bragg grating 13 at different positions along the propagation direction of the beam. The difference in the optical paths of the different spectral components causes chirp, which is superimposed on the chirp of the incident pulse. Therefore, the light beam 130 reflected by the first variable periodic Bragg grating 13 has a modified dispersion or modified chirp relative to the laser beam 10. The orientation of the diffraction fringe plane engraved in the grating 13 determines the direction of the light beam 130 reflected by the first Bragg grating 13. In Figure 3In the example shown, the propagation direction of the light beam 120 from the first quarter wave plate 12 is perpendicular to the fringe plane, so the reflected light beam 130 propagates in the opposite direction to the light beam 120 from the first quarter wave plate 12. The reflected light beam 130 passes through the first quarter wave plate 12 again and forms a light beam 135 that is incident on the face 19 of the polarization beam splitter cube 11. Therefore, the circularly polarized reflected light beam 130 is converted into a linearly polarized light beam 135, which is completely perpendicular to the polarization of the incident light beam 10 and is therefore in the P state, in other words, along the Z axis.

[0056] Interface 18 transmits polarized P beam 135 toward second quarter wave plate 14, the axis of which is at a 45-degree angle to polarization P. Second quarter wave plate 14 converts the linear polarization P of beam 135 into circular polarization. Mirror 15 reflects the circularly polarized beam and forms beam 140, which passes through second quarter wave plate 14. Circularly polarized beam 140 is converted by second quarter wave plate 14 into linearly polarized beam 150S. Interface 18 reflects linearly polarized beam 150S toward output face 21 of polarization beam splitter cube 11 and forms output beam 200, which propagates in a direction fixed by the orientation of the various optical components, specifically, mirror 15, first variable periodic volume Bragg grating 13, and polarization beam splitter cube 11.

[0057] like Figure 3 As shown, when light beam 10 is incident on input facet 17 of polarization beam splitter cube 11 at zero angle of incidence (in other words, normal incidence), light beam 200 at the output of dispersive optical devices 50, 51 propagates in the same direction and with the same sense as incident light beam 10. As described above, the fringes of first variable periodic Bragg grating 13 are parallel to the plane of reflector 15 within 0.1 degrees. Regardless of the angle between first variable periodic Bragg grating 13 and polarization beam splitter cube 11, light beam 200 emerging from dispersive optical devices 50, 51 is perfectly parallel to incident light beam 10. Therefore, light beam 200 is perfectly aligned with incident light beam 10 at normal incidence on input facet 17 and propagates in the same direction as incident light beam 10, with the same polarization as light beam 10. The total dispersion of light beam 200 is equal to the sum of the dispersion caused by variable periodic Bragg grating 13 and the initial dispersion of incident laser beam 10, taking into account the sign of each of these dispersions.

[0058] Figure 3 The dispersive optical devices 50, 51 allow the laser pulses to be compressed or stretched while maintaining the direction of the laser beam. Figure 3 The dispersive optical device 50 can be used to make an integrated stretcher or an integrated compressor.

[0059] Therefore, the dispersive optical devices 50, 51 are advantageously inserted into the path of the 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 devices 50, 51 determine the direction of the output beam 200 relative to the incident beam 10 through their structure.

[0060] according to Figure 4 A variation of the first embodiment is shown, in which the position of the first quarter-wave plate 12, fixed to the first variable periodic Bragg grating 13, is interchanged with the position of the second quarter-wave plate 14, fixed to the reflective optical component 15. This interchange does not in any way alter the operation of the dispersive optical devices 50 and 51. In fact, in this case, the light beam is transmitted twice through the second quarter-wave plate 14 and reflected at the reflective optical component 15, then passes through the polarization beam splitter cube 11, is reflected by the first variable periodic Bragg grating 13, and is transmitted twice through the first quarter-wave plate 12. In this variation, the light beam 10 incident on the S-polarization input face 17 is reflected at the interface 18, then transmitted through the second quarter-wave plate 14 via the side 19 to form a circularly polarized light beam 140. This circularly polarized light beam is reflected at the reflective optical component 15 to form a circularly polarized light beam 150. Light beam 150 passes through the second quarter-wave plate 14 to form a linearly polarized P light beam 155. Linearly polarized P light beam 155 is transmitted through the polarization beam splitter cube 11 to the other side 20. The linearly polarized P beam 155 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 circularly polarized reflected beam 130. The reflected beam 130 is transmitted through the first quarter wave plate 12 and forms a linearly polarized S beam 150 in the direction of the polarization beam splitter cube 11. The interface 18 reflects the linearly polarized S beam 150 and forms an output beam 200. As shown in FIG. Figure 3 As described, the total dispersion of the output beam 200 is equal to the sum of the dispersion caused by the variable period volume Bragg grating 13 and the initial dispersion of the incident laser beam 10, taking into account the sign of each of these dispersions.

[0061] In particular, Figure 4As shown, when the incident beam 10 is perpendicular to the input face 17, the beam reflected by the interface 18 is perfectly perpendicular to the reflective surface of the reflective optical component 15. Light beam 155, reflected by the reflective optical component 15 and transmitted twice through the second quarter-wave plate, is perpendicular to the fringes of the first incident variable periodic volume Bragg grating 13. Light beam 150, reflected by the first Bragg grating 13 and transmitted twice through the first quarter-wave plate, is tilted exactly 45 degrees at the interface 18, and the output beam 200 is perfectly parallel to the incident beam 10. When the incident laser beam 10 is perpendicular to the input face 17 of the polarization beam splitter cube 11, the output beam 200 has the same direction, polarization, and position as the input beam 10. In other words, when the beam 10 is incident on the input face 17 of the polarization beam splitter cube 11 at normal incidence, we also obtain an output beam 200 that is perfectly aligned with the incident beam 10 and propagates in the same direction.

[0062] Various variations of the dispersive optical devices 50, 51 are contemplated herein.

[0063] Figure 4 A variant is further shown according to which the reflective optical component 15 comprises a reflective treatment applied to the outer surface 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 the angle between the two faces not exceeding 0.05 degrees.

[0064] according to Figure 5 In the illustrated variation, the dispersive optical devices 50 and 51 further include an optical isolator positioned above the output beam 200. This optical isolator, for example, comprises a polarization cube 26 and a third quarter-wave plate 27. Advantageously, the input and output faces of the polarization cube 26 and the third quarter-wave plate 27 are completely parallel to each other and are assembled together, for example, by gluing to these surfaces to maintain the orientation of the components. More specifically, the polarization cube 26 is attached to the output face 21 of the polarization beam splitter cube 11, and the third quarter-wave plate 27 is attached to the other face of the polarization cube 26. The polarization cube 26 is oriented at a 90-degree angle to the polarization beam splitter cube 11. When the polarization beam splitter cube 11 reflects S polarization in the horizontal plane, the polarization cube 26 reflects S polarization in the vertical plane, and vice versa. At the wavelength of the laser beam 200, the third quarter-wave plate 27 is oriented such that its axis forms a 45-degree angle with the axis of the polarization cube 26. The dispersive optical devices 50 and 51 provided with this optical isolator maintain a one-piece structure. The optical isolator thus formed does not introduce any deviation or displacement to the output laser beam 250 .

[0065] Laser beam 200 at the output of polarizing beam splitter cube 11 is S-polarized. Polarization cube 26 transmits S-polarized output laser beam 200. After passing through third quarter-wave plate 27, this polarization is converted to circular polarization, forming output beam 250. If a reflective surface reflects all or part of the laser power toward third quarter-wave plate 27, the circularly polarized beam passes through the third quarter-wave plate again, and its polarization is converted to linear polarization P. This polarization P is reflected perpendicularly to the axis of output laser beam 250 by polarization cube 26 and does not propagate toward the laser source in dispersive optical devices 50 and 51. This achieves an optical isolation effect.

[0066] according to Figure 6 In the second embodiment shown, the dispersive optical devices 50, 51 further include a right-angle prism 28 disposed between the side 19 of the polarization beam splitter cube 11 and the first quarter-wave plate 12. More specifically, the right-angle prism 28 has a first face and a second face that form a 90-degree angle. The first face of the right-angle prism 28 is affixed to the side 19 of the polarization beam splitter cube 11. The second face of the right-angle prism 28 is affixed to the face of the first quarter-wave plate 12. The right-angle prism 28 can deflect the light beams entering and exiting the first Bragg grating 13 by 90 degrees. This configuration can reduce the size of the dispersive optical devices 50, 51 relative to the length of the first Bragg grating 13, particularly when the length of the grating 13 is greater than the size of its input face. This is typically the case for long-duration stretched pulses.

[0067] Optionally, the dispersive optical device 50, 51 according to the second embodiment includes the above-mentioned Figure 6 The optical isolator shown does not increase the size of the dispersive optical devices 50, 51. The optical isolator can be fixed not only to the polarization beam splitter cube 11, but also to the first Bragg grating 13, which can further enhance the robustness of the dispersive optical devices 50, 51. This configuration is particularly suitable for long gratings.

[0068] according to Figure 7 In the third embodiment shown, the reflective optical component 15 is replaced by a second variable periodic volume Bragg grating 25. The combined use of two variable periodic volume Bragg gratings 13 and 25 in the same dispersive optical device 50, 51 can improve the group velocity dispersion introduced by the dispersive optical device 50, 51. This configuration can achieve very significant dispersion while using a RBVPV of relatively small size and relatively simple manufacturing. Optionally, the dispersive optical device 50, 51 according to the third embodiment includes a combination of Figure 5 Optical isolator described.

[0069] In fact, in the third embodiment, as Figure 7As shown, incident laser beam 10 is reflected at interface 18 in the direction of first quarter-wave plate 12. The laser beam transmits through first quarter-wave plate 12 for the first time, is reflected by first Bragg grating 13, and then transmits through first quarter-wave plate 12 a second time to form a first-dispersed beam 130. Beam 130 propagates through polarization beam splitter cube 11 toward second quarter-wave plate 14. Beam 130 transmits through second quarter-wave plate 14 for the first time, is reflected by second variable periodic Bragg grating 25, and then transmits through second quarter-wave plate 14 a second time to form a second-dispersed beam 150: the first dispersion produced by first variable periodic Bragg grating 13, and the second dispersion produced by second variable periodic Bragg grating 25. Beam 150 is reflected at interface 18 of beam splitter cube 11 and forms output beam 200. Similarly, when beam 10 strikes input facet 17 of polarization beam splitter cube 11 at normal incidence, a fully aligned output beam 200 is obtained, propagating in the same direction as incident beam 10. Here, the total dispersion of the light beam 200 is equal to the sum of the dispersion caused by the first variable periodic volume Bragg grating 13, the dispersion caused by the second variable periodic volume Bragg grating 25, and the initial dispersion of the incident laser beam 10, taking into account the sign of each of these dispersions.

[0070] According to a variation of the third embodiment, a right-angle prism is provided between the polarization beam splitter cube 11 and each of the two variable periodic Bragg gratings. This configuration can reduce the size of the dispersive optical devices 50, 51 using two variable periodic Bragg gratings.

[0071] If combined Figure 8 As shown, the dispersive optical devices 50, 51 of the present disclosure can easily adjust the dispersion introduced by the device.Consider a dispersive optical device 50, 51 according to any of the first, second or third embodiments described above. Figure 8 An example of a dispersive optical device 50, 51 according to a first embodiment is shown. Considering that the dispersive optical device 50, 51 is wound perpendicular to Figure 8 The Y-axis of the plane of the laser beam is rotated by an angle alpha (denoted as α). To this end, the dispersive optical devices 50 and 51 are placed on an angle-adjustable optomechanical support. If the angle alpha is zero, the outgoing beam is parallel to and coincides with the incoming beam. However, rotating the non-zero angle alpha does not affect the direction of the outgoing beam 200, which remains completely parallel to the incoming laser beam 10. Figure 8Rotating the axis by an angle alpha about the plane of the first Bragg grating 13 (i.e., perpendicular to the plane containing the first Bragg grating 13 and the reflective optical component 15) results in a translation of the output beam 200 only in the XZ plane, with the amount of translation d being proportional to the angle alpha and the distance L between the reflector 15 and the center of the region in which the grating stripes are etched in the first Bragg grating 13. The amount of translation d of the beam is approximately given by the following formula:

[0072] d=2.L.sin(α)

[0073] For angles alpha less than 5 degrees, distance L is on the order of 10 mm, and distance d is on the order of 1.7 mm, which is relatively small compared to incident laser beam 10. This small lateral displacement without angular deviation allows for easy positioning of dispersive optical devices 50, 51 relative to incident laser beam 10. Positioning of dispersive optical devices 50, 51 does not require fine angular adjustment; accuracy within a few degrees is sufficient. However, it is recommended to ensure that the XZ plane is parallel to the incident beam to avoid introducing angular deviations perpendicular to this plane.

[0074] This rotational invariance in the XZ plane allows the dispersive optical devices 50, 51 to function as adjustable stretchers or compressors. In fact, the dispersion introduced by the grating 13 to the pulse depends on the angle of incidence of the light beam on the variable periodic Bragg grating 13. However, changes in the angle of incidence of the light beam on the variable periodic Bragg grating 13 can modify the dispersion introduced by the variable periodic Bragg grating 13. Therefore, the spectral dispersion introduced by the dispersive optical devices 50, 51 can be finely adjusted to precisely compensate for the dispersion of the incident pulse, or simply to change the residual dispersion of the pulse emerging from the dispersive optical devices 50, 51.

[0075] according to Figure 9 and Figure 10 In the fourth embodiment shown, the dispersive optical devices 50, 51 do not include the second quarter wave plate 14 and the reflective optical component 15, and the other components are combined with, for example Figure 3 The components described are the same. The dispersive optical device 50, 51 according to the fourth embodiment can produce an output beam 135 that is precisely deflected 90 degrees relative to the incident beam, while changing the chirp of the pulse passing through the device. Figure 9 The dispersive optical device 50 of the fourth embodiment can compress laser pulses while rotating the laser beam 90 degrees relative to the incident direction. Figure 9 The dispersive optical device 50 in the embodiment can be used to manufacture an integrated compressor or an integrated stretcher.

[0076] According to the fourth embodiment, the dispersive optical devices 50 and 51 are rotated by an angle α about an axis perpendicular to the axis of the incident beam, thereby adjusting the dispersion introduced by the variable-period volume Bragg grating without significantly shifting the direction of the reflected wave. By adjusting the orientation of the device about this axis, the group velocity dispersion introduced by the device can be adjusted without changing the direction of the outgoing beam.

[0077] Thus, a high power femtosecond or picosecond laser pulse source may be obtained by using one or more dispersive optical devices 50, 51 according to the present disclosure.

[0078] in particular, Figure 11 A chirped pulse amplifier system is shown, comprising a short pulse source 1 incident on a first dispersive optical device 50, which is configured to temporally stretch the pulse 3. The stretched pulse 3 is then amplified in an amplifier chain 6, which includes one or more solid-state crystals or doped glass or doped fiber amplifier media. After amplification, a second dispersive optical device 51 is configured to have a dispersion opposite to that of the first dispersive optical device 50, thereby temporally compressing the pulse 4. Compensation tuning can be fine-tuned by adjusting the angle of incidence on either of the two dispersive devices 50, 51.

[0079] Figure 11 An example of adjusting dispersion by rotating the first dispersive optical device 50 or stretcher is shown. Dispersive optical devices 50 and 51 according to the present disclosure can be used to compensate for dispersion introduced in an assembly comprising a short pulse source and optical components that introduce group velocity dispersion into the pulses, thereby altering their duration. For example, a femtosecond laser source incident on a microscope via an optical fiber experiences fiber-induced group velocity dispersion, significantly extending the duration of the pulses focused by the microscope onto the sample being observed. Dispersive optical devices 50 and 51 allow for adjustable compensation in a very compact environment, preferably without changing the direction of the incident beam on the microscope.

[0080] Figure 12 An example of adjusting the dispersion by rotating the second dispersive optics 51 or the compressor is shown. This adjustment by rotation does not affect the beam direction, so the pulse duration can be adjusted without any other adjustments to the laser system.

[0081] Therefore, the dispersive optical devices 50, 51 can obtain an amplified pulse source of high power and adjustable duration without changing the direction of the light beam.

[0082] Figure 14An example laser system is schematically shown. The system includes a source 1 that generates 300 fs pulses 2, which are stretched to a duration of 100 to 500 ps by a first dispersive optical device 50 according to one of the described embodiments. The first dispersive optical device 50 comprises at least one first variable periodic volume Bragg grating (RBVPV) configured to have a dispersion of 30 ps / nm. An amplifier chain 6 comprises a series of ytterbium-doped active fibers or amplifiers with ytterbium-doped crystals. The amplifier chain 6 receives the stretched pulses 3 and generates amplified stretched pulses 4. A second dispersive optical device 51 comprises another variable periodic volume Bragg grating (VVGB) configured to have a dispersion of the order of -30 ps / nm. The second dispersive optical device 51 receives the amplified stretched pulses 4 and generates compressed amplified pulses 5 with a duration of the order of 300 fs. In practice, the second dispersive optical device 51 can be based on the same variable periodic volume Bragg grating as the first dispersive optical device 50: it is sufficient to orient the RBVPV in an inverted manner, i.e., backside becomes frontside, and vice versa.

[0083] Figure 12 Another use of dispersive optical devices 50, 51 is shown in FIG. A source of picosecond laser pulses 1 (e.g., with a duration of 50 ps) is amplified in an amplifier chain 6 comprising a series of ytterbium-doped active fibers. The pulses 3, 4 undergo strong self-phase modulation during their propagation, which has the effect of broadening the spectrum by generating chirp. The dispersive optical device 51 can compensate for this chirp and compress the pulses to a shorter duration. In a specific example, the initial pulses 3, 4 have a duration of 50 ps and are then combined with the optical fiber 6 to generate a pulse of 50 ps. Figure 5 The described dispersive optical device 51, comprising a RBVPV with a dispersion between -10 and -20 ps / nm, is amplified to an energy of 15 μJ before compression. Figure 13 shows an example of an autocorrelation trace of a pulse 5 generated by such a laser system, which has a duration of about 980 fs.

[0084] The dispersive optical devices 50, 51 described herein can be used in all applications where so-called TREACY compressors are used or where pulse stretchers are used.

[0085] In one application, the present disclosure provides a dispersive optical device 51 for compressing laser pulses with a duration between 20 ps and 100 ps to a final duration between 0.1 ps and 3 ps in an extremely compact form factor, preferably without changing the direction or position of the laser beam. The laser source can be, for example, a picosecond source amplified in an optical fiber or solid-state amplifier. It can also be a femtosecond source, stretched in a grating stretcher or using dispersion of a material, and then amplified and ultimately recompressed by a dispersive optical device according to one of the embodiments described below.

[0086] Particularly advantageously, the dispersive optics 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 optics itself can yield a variable pulse duration of approximately 1 ps for a rotation angle of a few degrees.

[0087] Particularly advantageously, the dispersive optical device is compact enough to attach to the end of a flexible fiber laser. Such a fiber laser equipped with this dispersive optical device is capable of generating ultrashort and extremely intense laser pulses. The flexible end of the fiber laser can be adjusted to any position and orientation depending on the application. The position and direction of the ultrashort laser pulses can be rapidly changed without being constrained by a compressor that maintains stability.

[0088] In another application, the dispersive optical device 50 can stretch the pulse before amplification and adjust the duration of the amplified and possibly compressed pulse, preferably without the need to realign the amplifier chain.

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

Claims

1. A dispersive optical device (50, 51) adapted to change the group velocity dispersion of an optical pulse beam (10), the dispersive optical device comprising: A polarization beam splitter cube (11), a first variable periodic volume Bragg grating (13), and a first quarter wave plate (12), wherein the polarization beam splitter cube (11) has an input surface (17), an output surface (21) parallel to the input surface (17), two side surfaces (19, 20) perpendicular to the input surface (17), and an interface (18) inclined at 45 degrees relative to the input surface (17) and the two side surfaces (19, 20), wherein the input surface (17) is adapted to receive a light beam (10), and the first quarter wave plate (12) has flat and parallel surfaces. , one surface of the first quarter wave plate (12) is fixed to the input and output surfaces (23) of the first Bragg grating (13), the first variable periodic Bragg grating (13) is photoetched in the material along a plane parallel to the input and output surfaces (23) of the first Bragg grating (13), and the other surface of the first quarter wave plate (12) is made into one piece with one of the two side surfaces (19, 20) of the polarization beam splitter cube (11), so that the normal of the stripe plane of the first variable periodic Bragg grating is at a 45 degree angle with the normal of the interface (18).

2. The dispersive optical device (50, 51) according to claim 1, wherein The dispersive optical device is integrated.

3. The dispersive optical device (50, 51) according to claim 1 or 2, wherein: The other side of the first quarter wave plate (12) is fixed to one of the two side faces (19, 20) of the polarization beam splitter cube (11).

4. The dispersive optical device (50, 51) according to claim 1 or 2, comprising a right-angle prism (28), a first face of the right-angle prism 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 (19, 20) of the polarization beam splitter cube (11), and the second face of the right-angle prism being attached to the first quarter-wave plate (12).

5. The 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), wherein the second quarter wave plate (14) has flat and parallel faces, and the second quarter wave plate (14) is arranged between the reflective optical component (15, 25) and the other side of the two side surfaces (19, 20) of the polarization beam splitter cube (11), one face of the second quarter wave plate (14) is fixed to the reflective optical component (15, 25), and the other face of the second quarter wave plate (14) is fixed to the other side of the two side surfaces (19, 20) of the polarization beam splitter cube (11).

6. The dispersive optical device (50, 51) according to claim 5, wherein The reflective optics (15, 25) include a mirror (15) or a reflective treatment applied directly to the other face of the second quarter wave plate (14).

7. The dispersive optical device (50, 51) according to claim 5, wherein: The reflective optical component (15, 25) comprises a second variable periodic Bragg grating (25), the second quarter wave plate (14) is fixed to the input and output surfaces of the second Bragg grating (25), and the second variable periodic Bragg grating (25) is photoetched in the material along a plane parallel to the input and output surfaces of the second Bragg grating (25).

8. The dispersive optical device (50, 51) according to any one of claims 1 to 7, comprising an optical isolator fixed to the output face (21) of the polarization beam splitter cube (11).

9. The dispersive optical device (50, 51) according to any one of claims 1 to 8, comprising means for tilting the dispersive optical device (50, 51) by rotation about an axis parallel to the intersection line between the input face (17) and the interface (18) of the polarization beam splitter cube (11).

10. A laser system having a pulse duration between 10 femtoseconds and 1 nanosecond and a power between 1 W and 1 kW, comprising a source capable of generating linearly polarized source pulses, an optical amplifier system, a stretcher and / or a compressor, the stretcher and / or compressor comprising at least one dispersive optical device (50, 51) according to any one of claims 1 to 9.

11. A laser system having pulses of adjustable duration between 10 femtoseconds and 10 picoseconds, comprising a stretcher and / or compressor comprising at least one dispersive optical device (50, 51) according to any one of claims 1 to 9.

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