WO2015056049A1

WO2015056049A1 - Laser pulse stretcher and compressor

Info

Publication number: WO2015056049A1
Application number: PCT/IB2013/059352
Authority: WO
Inventors: Romualdas DANIELIUS; Linas Giniunas
Original assignee: Uab Mgf Sviesos Konversija
Priority date: 2013-10-15
Filing date: 2013-10-15
Publication date: 2015-04-23

Abstract

A combined optical pulse stretcher-compressor system uses a common grating (1), preferably a holographic transmission grating. The grating is attached to a rotatable holder (3), to adjust the incidende angle of the beams to be stretched (4) and to be compressed (10), thereby adjusting third-order dispersion induced by the stretcher and the compressor assemblies. The grating may be formed from two grating regions having different substrate thickness. The grating region (2), having a thinner substrate serves for diffracting the output beam of the compressor assembly, thus minimizing distortion of beam profile or temporal pulse shape. In another embodiment, the grating is made of two dielectric layers (15, 16), arranged on both sides of the grating pattern, whereas one of the layers (15) is thinner and it is arranged for transmission of compressed pulses, after the beam strikes the grating for the last time (24) in the compressor assembly.

Description

LASER PULSE STRETCHER AND COMPRESSOR

FIELD OF INVENTION

This invention relates to the field of laser technology. More specifically it relates to optical pulse stretching or compression in chirped pulse amplification systems. BACKGROUND OF INVENTION

Modern laser technologies allow the generation of ultrashort pulses of electromagnetic radiation. Such pulses are generally generated by the use of ultrafast laser oscillators and can be shorter than a picosecond. Ultrashort laser pulses are an invaluable tool in novel industrial and scientific research applications, but usually, in order to achieve the desired results such pulses have to be amplified. The amount of times such pulse can be amplified is limited by the energy density (fluence) of light at the surface and in the volume of all the optical elements by the onset of nonlinear effects and laser damage due to the high peak power. Therefore the efficiency of such a laser amplifier is greatly limited if no additional means to combat the negative effects of ultrashort pulse propagation and amplification in a dispersive medium are employed. For reference and further reading see Frank Trager, "Springer Handbook of Lasers and Optics" 2nd ed., 2012.

In order to avoid such limitations, a technique, called chirped pulse amplification (CPA) has been developed. First principles of CPA have been described in scientific publication: D. Strickland and G. Mourou, "Compression of amplified chirped optical pulses", Opt. Commun. 56, 219 (1985). It allows to expand an ultra-short pulse in time, thus reducing the peak power of the pulse and making further amplification possible. After the pulse has been amplified, while circulating inside a laser cavity, such as a regenerative amplifier, the pulse is compressed, as close to its original duration as possible.

Usually, for pulse compression or stretching, one or more dispersive elements are used, which spatially separate different wavelength components of the pulse itself. Such dispersive element can be a diffraction grating, a pair of transparent prisms, a grade index plate or the like. Dispersion might occur during pulse propagation in dielectric media, featuring chromatic dispersion, as well. After spatially separating the wavelength components, and thus changing the length of the optical path, positive or negative group delay dispersion is achieved, otherwise called a chirp. After a chirp is induced, the spatially dispersed beam is collimated back into a single optical path by the use of a spatially dispersive element, thus creating an optical pulse with modified pulse duration.

The mentioned fundamental principles are well established and understood and have not undergone any fundamental changes during the past few decades. Therefore the technological problem is one of the optimization of the stretcher-compressor assemblies and the following inventions are discussed as characteristic examples of the prior art.

A US patent No. 5349591 describes the so-called 'back-to-back' stretcher- compressor assembly comprising two reflection gratings, arranged such, that the reflective surfaces of both grating face opposite directions and are mounted on a rotary stage, and the input beams of both stretcher and compressor are preferably parallel. In this solution, simultaneous rotation of the two gratings allows adjustment of the stretcher and compressor to operate at a specific wavelength in wavelength-tunable ultrafast laser systems, such as Tksapphire lasers.

Another US patent application US5329398 published on 12-07-1994 discloses a single-grating method and apparatus for a laser pulse stretcher and compressor. The method and apparatus exploits a two-layer vertical structure. One layer for the stretcher, and the other, for the compressor. Such a stretcher-compressor is particularly suitable for using in a chirped-pulse or regenerative laser amplifier where laser wavelength tuning is desirable. When a change in the wavelength is applied, only one rotational adjustment is required to resume the alignment of the whole stretcher and compressor. This apparatus shows significant simplification in structure and alignment of stretcher and compressor systems.

Another US patent application US6081543 published on 27-06-2000 discloses a system for a stretcher-compressor assembly having a single transmission grating. The invention provides a compact, combined stretcher-compressor assembly which utilizes a single grating element. The single grating element is preferably a holographic transmission grating element. The grating element has first and second surfaces which are opposed major surfaces with a thickness between the surfaces. The grating pattern is formed in a volume or on a surface of a grating element. The grating element is arranged in the assembly to receive a beam for stretching laser pulses in a stretcher beam path, and in the same arrangement, to receive a beam for compressing laser beam pulses in a compressor beam path. The stretcher and compressor beam paths pass through the single grating element. Respective reflecting means are arranged to provide a desired number of passes through the grating element by the stretcher beam path, and through the grating element by the compressor beam path.

Although, both solutions are described as utilizing a single grating for both compression and stretching of optical pulses, it is apparent that use of the same diffraction grating raises certain problems to be solved.

First is that an assembly with fixed single diffraction grating is difficult to adjust for compensation of third-order dispersion (TOD) as other optical components of the compressor-stretcher assembly have to be displaced and adjusted multiple times during tuning of such a laser. Such tuning is inconvenient and usually takes a long time as well as requires more bulky laser enclosures with lots of space for component displacement. Earlier patents describing stretcher-compressor designs, where the grating element has a rotational capability, including the solution described in the above-mentioned patent No. US6081543, do not provide a solution for third-order dispersion compensation, i.e. the compressor-stretcher assembly is arranged to be tuned without displacement of optical components, however the TOD problem is neither addressed nor solved. Third order dispersion is described more in detail in a book of R. Paschotta, 'Encyclopedia of Laser Physics and Technology' (2008) ISBN 978-3-527-40828-3.

The second problem stems from the use of reflective gratings, which usually are produced with a patterned reflective coating. This in turn presents an additional problem, since large gratings with large beam diameters on the grating surface must be employed in order to avoid reaching the optical damage thresholds. In addition to that, special measures must be taken to protect the metal coatings from degradation due to humidity in the operating environment. Furthermore, adjustment of reflection gratings is complicated due to the fact, that reflection angle changes twice as fast, as the grating is rotated, therefore use of a single reflection grating for stretcher and compressor is complicated in a sense, that reflection elements, such as roof-mirrors or prisms have to be of a large clear aperture, otherwise these reflective components have to be displaced.

Finally, most transmission grating assemblies feature another disadvantage.

Large aperture gratings are usually produced on a thick substrate of dielectric material, i.e. glass, quartz, etc. Sufficient thickness is necessary for reliability and easier mounting of the grating. However, the thicker the substrate is, the more non-linear optical effects occur when ultra-short pulses pass the substrate. Prior art solutions do not address this problem as well. SUMMARY

In order to eliminate the drawbacks indicated above and especially for compensation of third-order dispersion, this invention provides a compact stretcher- compressor assembly, which utilizes essentially a single diffraction grating element. The grating element is preferably a holographic transmission grating. In the most preferred embodiment, the grating comprises a first and a second surface with a dielectric material substrate between them and comprises a grating pattern on one of the surfaces or, preferably, in the volume of the grating element. The grating element is arranged and positioned with respect to optical assemblies of both - stretcher and compressor, whereas the grating is a common part of both assemblies. Input beams of both - stretcher and compressor - strike the diffraction grating element either from the same side or from opposite sides, with respect to the longitudinal axis (25) of the grating.

In case both input beams strike the grating element from the same side with respect to the longitudinal axis (25) of the grating, they must strike the grating at essentially similar angle of incidence, just from opposite directions, with respect to transversal axis (26) of the grating, as disclosed in Fig. 1. In such arrangement, when the grating (1 ) is rotated, angles of incidence for input beams of both stretcher and compressor change contrarily. This is essential for having possibility to adjust the third order dispersion (TOD) of the whole CPA system. In another case, the input beams (4, 10) must strike the grating from opposite sides, with respect to the longitudinal axis (25) of the grating, at essentially similar angles of incidence and from the same side with respect to the transversal axis (26) of the grating, as disclosed in Fig. 2.

In the most preferred embodiment, third-order dispersion is compensated by rotating the grating element and thus changing the angle of incidence (AOI) for both stretcher and compressor input beams such that during rotation, the AOI increases for one input beam, for example, the stretcher input beam (4), and decreases for the other input beam, for example, the compressor input beam (10) or vice versa. The most suitable angle of incidence for all incident beams is preferably close to a Littrow angle.

Fig.3 depicts calculated dependence of the TOD of the stretcher/compressor system versus the deviation of the angle of incidence (AOI) of the input beams of stretcher and compressor from the optimal Littrow angle. The second order dispersion is set to zero for each point, by optimization of proper length of the compressor. In another embodiment, the grating element (1 ) is made of an intra-volume grating pattern and two layers of dielectric material on both sides of the grating element, whereas the second layer (16) is thinner than the first layer (15). The grating element is positioned within the stretcher-grating assembly such that the second layer (16) transmits amplified and compressed pulses to the output of the compressor assembly.

In another embodiment, the grating is composed of two portions, having different substrate thickness. These two parts are rigidly attached to each other or formed from a monolithic dielectric slab. The thicker portion (1 ) features a larger aperture and is used for transmitting spatially expanded beams inside either stretcher or compressor, whereas the thin portion (2) is used for transmitting high intensity, spatially and temporally compressed pulses, which are passed towards the output of the compressor assembly (14). It is not essential to pass the stretcher beams through the thin portion of the grating, since the intensity of laser pulses in the stretcher assembly feature low energy, as compared to the compressor assembly. Ideally, both grating portions are rigidly attached together in order to avoid misalignments, due to change of ambient working conditions.

In the stretcher part of the assembly the grating element (1 ) together with the lens (8) and reflecting means (9) are placed within the optical path of the beam of optical pulses to be stretched. The optical path goes through the first surface of the grating element, passes the grating pattern, where the beam is diffracted, and then passes through the second surface. First reflecting means (9) face the second surface and redirect the diffracted beam back through the grating element on a parallel and offset optical path. Second reflecting means (7) face the first surface of the grating element and provide one additional pass through the grating element respectively. More reflective means can be arranged as to provide a desired number of passes.

In the compressor part of the assembly the beam path is arranged analogously to the stretcher, however the output and, optionally, the input beam goes preferably through the thinner part or a thinner layer of the grating (1 ). A possibility of choosing which side of the second grating will be considered as the first or second is available. DESCRIPTION OF DRAWINGS

In order to understand the invention better, and appreciate its practical applications, the following pictures are provided and referenced hereafter. Figures are given as examples only and in no way shall limit the scope of the invention. Fig. 1 illustrates a preferred embodiment of the stretcher-compressor system assembly, wherein input beams of both stretcher and compressor strike the diffraction grating from the same side, with respect to the longitudinal axis (25) of the grating, at almost similar angles of incidence.

Fig. 2 illustrates another possible configuration of the stretcher-compressor assembly, wherein input beams of both stretcher and compressor strike the diffraction grating from different sides, with respect to the longitudinal axis (25) of the grating, at almost similar angles of incidence.

Fig. 3 provides a graphic in which an interdependence of third-order dispersion (TOD) against deflection of grating orientation from the Littrow angle (opposite signs for stretcher and for compressor), is provided. The second order dispersion is optimized by the optimal compressor length for each calculated point. The calculation is done for 1700mm^"1 groove frequency of a diffraction grating and 100 mm length of the compressor.

Fig. 4 illustrates a schematic diagram of a side view of the diffraction grating (a) and a profile thereof (b), in which two layers of dielectric material are visible on both sides of the grating pattern, whereas the thickness differs between the two layers (1 5, 16).

Fig. 5 illustrates a schematic diagram of a side view of a diffraction grating, in two configurations: thinner portion is of entire height of the grating (a) or thinner portion forms just a part of the height (b).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

This invention provides a combined optical pulse stretcher-compressor system using a single adjustable grating assembly, preferably comprising a holographic transmission grating (1 , 2). It should be apparent to a person skilled in the art that the grating element or elements is/are adjusted inside the optical path as to operate essentially at the Littrow angle of incidence, however in our invention, the grating is arranged to operate at slightly different angles than the Littrow angle for third order dispersion compensation (for an in depth discussion see the references listed in chapter: Background of the invention).

In the most preferred embodiment, the stretcher-compressor assembly comprises a single diffraction grating element (1 ), which is mounted onto an adjusting means (3), such as a holder, having rotational capability, in order to provide a swift alignment capability. Therefore the grating (1 ) can be adjusted both for the stretcher and the compressor assemblies, at the same time. Input beams of both stretcher and compressor are arranged to strike the grating such, that rotational movement of the grating changes the AOI by contraries, i.e. AOI of the stretcher input beam increases and AOI for the compressor input beam decreases at the same amount, or vice versa. Both input beams (4, 10) strike the grating at essentially the same angle of incidence - typically close to the Littrow angle. There are two configurations of the stretcher- compressor assembly possible: either the input beams (4, 10) strike the grating element (1 , 2) from the same side with respect to the longitudinal axis (25) of the grating, as illustrated in Fig. 1 ; or the two input beams (4, 10) strike the grating element (1 , 2) from opposite sides with respect to the longitudinal axis (25) of the grating, as illustrated in Fig. 2. In either case, a rotational movement of the grating element (1 , 2) increases or decreases the angle of incidence for both input beams (4, 10) by contraries, but by the same amount. Such adjustment allows for convenient compensation of third-order dispersion. When such method is used for compensation of third-order dispersion, second order dispersion is immediately evident, after the grating (1 , 2) is rotated. However, it is pretty easy to compensate for the second-order dispersion by simply changing the length of the compressor, i.e. by translating the roof mirror (13) further or closer to the grating.

Yet in another embodiment, a single reflection grating is used (not illustrated in the drawings), whereas input beams of the stretcher and the compressor strike the grating from the reflective side of the grating, howeverfrom opposite sides with respect to the transversal axis (26) of such grating. One of the major differences from the optical design, based on a transmission grating, in this embodiment, all or majority of optical components used larger clear apertures - large enough to allow alignment of stretcher- compressor assembly and compensation of TOD by angular tuning of optical elements, rather than displacement. In this embodiment, TOD is compensated also by rotational movement of the reflection grating.

Herein we provide an exemplary explanation of a complete optical layout of the stretcher-compressor assembly. For the purpose of an example, we will use the basis of an optical layout illustrated in Fig. 1 , where preferably a holographic transmission grating is used. For those skilled in the art, it should be apparent, how beam tracing could be embodied in other optical layouts, including but not limited to Fig. 2.

Example For the stretcher assembly part, the incident beam (4) enters the assembly below a mirror (5), designed to separate the input beam (4) from the output beam (6). The beam propagates further below a second mirror (7), which can be a roof mirror or a prism, designed to shift the beam in the stretcher assembly to different height. Then the stretcher input beam (4) strikes the grating (1 ) at spot (17), at a preset angle. The beam is diffracted and leaves the grating (1 ), red shifted frequencies are diffracted more than blue ones, thus there is a strong angular dispersion evident after the grating. The diffracted beam passes through a lower portion of a lens (8), or a curved deflection mirror (this option is not shown in the drawings). The focused beam is then reflected by a flat mirror (9), which is placed in the focal plane of the lens (8). The beam then propagates in the same optical path, just in reverse and at different height. After passing the grating (1 ) for the second time at spot (18), pulses are already temporally chirped. However, in order to induce even higher temporal chirp, and to compensate a spatial chirp, the pulses are reflected with a prism or a roof mirror (7) and the beam is directed towards the grating (1 ) and shifted to a different height again. The beam strikes the grating (1 ) at spot (19), makes another round through the lens (8) and the mirror (9), strikes the grating (1 ) for the last time at spot (20) and is being redirected out of the stretcher assembly part by the tilted mirror (5). Thus a stretcher output beam (6) is formed.

For the compressor assembly part, the input beam (10) enters the compressor below a tilted mirror (1 1 ), is diffracted by the diffraction grating (1 or 2) at spot (21 ), and reflects from a roof mirror (13). The reflected beam strikes the grating (1 ) at spot (22) and is diffracted. Further the beam is shifted to a different height and reflected back by another roof-mirror (12). The beam follows the same initial path back, just at the different height, strikes the grating (1 ) at spot (23), then spot (24) and is deflected to the output (14) of the compressor, by the tilted mirror (1 1 ).

In such arrangement, third-order dispersion is compensated by rotating the transmission grating (1 ) and constantly adjusting the length of the compressor for resetting the second-order dispersion to zero. Graphically the desired result of such tuning is depicted in Fig. 3, where a change in angle of incidence (Δα) results in change of TOD of the stretcher/compressor system. When Δα=0, the orientation of the grating (1 ) is perfect for TOD compensation. The calculation is done for 1700mm^"1 groove frequency of a diffraction grating and 100mm length of the compressor.

For those skilled in the art it is obvious that different spots (17 to 24) on the grating element (1 or 2) may overlap with each other, without causing any significant negative effects to the operation of the stretcher-compressor assembly. Overlapping of the spots brings another significant advantage - the grating can be of smaller aperture, thus more robust.

Additional reflecting means can be used in the stretcher assembly in order to increase the temporal chirp, by making more passes through the grating (1 ). In addition to that, higher negative temporal chirp is induced by increasing the length of the compressor.

In one of the embodiments, the stretcher-compressor assembly comprises a grating element (1 , 2), which is formed by rigidly attaching two separate gratings (1 , 2) one to the other, whereas the second part of the grating (2) is thinner than the first part (1 ) and has preferably a smaller clear aperture. As a result, the grating element is designed to be of different thicknesses in at least two separate regions (1 , 2). In particular, it is preferable that high intensity (compressed) ultra-short pulses, which can induce strong nonlinear effects in the volume of a dielectric material, particularly in case of thick grating substrates, have an optical path inside the grating substrate, which is as short as possible. Therefore the grating part (2) is made thin and is arranged for transmitting and diffracting the output beam of the compressor. Since this input beam is collimated and of a relatively small diameter, as it strikes the grating, it creates high fluence on the surface and in the volume of the grating (2), thus inducing the appearance of self-phase modulation and self-focusing. Consequently, that distorts the spatial profile of the beam and, the temporal pulse shape. The thinner the grating (2) is, the less undesirable non-linear effects are induced. On the other hand, the grating region (2) has to maintain fairly good mechanical properties, i.e. to maintain its flatness at all times.

Yet in another embodiment, the grating element is formed from two layers of dielectric material (15, 16), as illustrated in Fig. 3. Herein the grating pattern is located between a thicker layer (15) and a thinner layer (16). Seeking to reduce appearance of negative effects, the grating (15, 16) is mounted in the stretcher-compressor assembly such, that the thinner layer (16) faces the output of the compressor, i.e. after passing the grating pattern for the last time, compressed pulses propagate only through the thin layer (16) of the dielectric material.

While this invention is described in terms of certain embodiments thereof, it is not intended to be limited to the description above, rather only to the extent set forth in the claims.

The stretcher-compressor assembly, as indicated above is suitable for use in chirped pulse amplification systems for making a high pulse energy ultrafast laser.

Claims

Optical pulse stretcher-compressor assembly, comprising a single grating element, arranged to work both for pulse stretching and compression, c h a r a c t e r i z e d in that said stretcher-compressor assembly is arranged for compensation of third-order dispersion by means of rotation of the diffraction grating, in order to change the angle of incidence for input beams of both - stretcher and compressor, whereas the grating element is mounted to a holder, having rotational capability.

The stretcher-compressor assembly according to claim 1, c h a r a c t e r i z e d in that said transmission grating element is arranged such, that during rotational movement, angle of incidence increases for the stretcher input beam and decreases for the compressor input beam or vice versa.

The stretcher-compressor assembly according to one of claims 1 or 2, c h a r a c t e r i z e d in that said grating is a holographic transmission grating.

The stretcher-compressor assembly according to one of claims 1 or 2, c h a r a c t e r i z e d in that said grating is a reflection grating.

The stretcher-compressor assembly according to one of claims 1 or 4, c h a r a c t e r i z e d in that it is arranged for compensation of second-order dispersion by means of optimization of the compressor length, whereas compensation of second-order dispersion is necessary after the grating is rotated for compensation of third-order dispersion.

The stretcher-compressor assembly according to one of claims 1 to 3, c h a r a c t e r i z e d in that said grating comprises two or more diffraction grating regions, having different substrate thickness in each of the regions, whereas the thinner region is arranged such that the last strike to the grating in the compressor assembly has a spot within said thinner region.

7. The stretcher-compressor assembly according to one of claims 1 to 3, c h a r a c t e r i z e d in that said grating comprises two layers of dielectric material on both sides of the grating pattern, wherein said two layers are of different thickness and the thinner layer is arranged to transmit the compressed pulses of a laser, after the beam strikes the grating for the last time in the compressor assembly.

8. An ultra-short pulse laser source, comprising stretcher and compressor, having a common transmission diffraction grating, c h a r a c t e r i z e d in that the grating is arranged as described in one of the claims 1 to 7 and orientation of stretcher and compressor input beams with respect to the grating is as described in claim 2.