DE102014118880A1 - Optical device for modulating the spectral phase of laser pulses - Google Patents

Abstract

An optical device (1) for modulating the spectral phase of laser pulses, in particular for temporal broadening or compressing the duration of a laser pulse in the chirped pulse amplification or the amplification of time-stretched laser pulses with subsequent compression comprises at least one in a predeterminable wavelength range An angle grating diffraction grating (2) and a folding optics (4, 7) for such a convolution of the beam path that multiple diffractions of the laser pulse at the at least one diffraction grating (2) can be generated. According to the invention, the folding optics (4, 7) are designed in such a way that at least eight diffractions can be generated on the at least one diffraction grating (2) or on an array comprising several diffraction gratings (2) in order to achieve the same chromatic dispersion in comparison to the conventional concept (cf. For example, BBEB Treacy "IEEE J. Quantum Electron., QE-5, 454 (1969)) allows for a more compact size or, for the same size, to produce multiple chromatic dispersion.

Description

The invention relates to an optical device for modulating the spectral phase of laser pulses; in particular for time broadening or for compressing the duration of a laser pulse in the case of chirped pulse amplification, z. B. for amplifying time-spread, so-called stretched pulses (English: Chirped Pulse Amplification, CPA), with at least one in a predetermined wavelength range angular dispersion having diffraction gratings and a folding optics for such folding of the beam path that multiple diffractions of the laser pulse to the at least one diffraction grating can be generated.

Laser systems are known from the prior art, in which an amplification of a laser pulse provided by a laser source takes place by means of the so-called CPA methodology. In laser systems designed in this way, the spectral phase of the laser pulses before and after amplification in an optically active amplifier medium is modulated by means of optical devices which correspond to chromatic dispersive filters in order to reduce the peak intensity occurring during amplification and thus a possible non-linear disturbance of the pulse shape or even to counteract damage to the amplifier medium. It is customary to reduce the peak intensity by a factor of 500 to 10,000 by extending / broadening the pulse duration by just this factor. The chromatically dispersive filter is commonly referred to as laser pulse stretcher. After amplification, a laser pulse compressor typically compensates for the chromatic dispersion previously imposed by the laser pulse expander and the transmitted optical media such that the pulse shape, duration, and contrast ideally equal those before amplification or a desired defined manner, e.g. B. shortened or extended, correspond.

For example, in the spectral range around 1030 nm, which finds increasing interest in the field of laser material processing, in particular due to the scalability of the average power and the pulse energy in Yb-doped media, common diffraction grating frequencies are 1600 lines per mm or 1740 lines per mm. With likewise customary spectral bandwidths of 2 nm to 6 nm (based on the half-width, Full Width at Half Maximum, FWHM) and time stretching factors of about 1000, the path between two diffractions at those diffraction gratings, which is a modulation of the spectral phase of the laser pulses by means of angular dispersion, in typical applications one to two meters.

Laser pulse compressors or laser pulse stretchers, which are based on angular dispersion, in which therefore the dispersive optical elements are formed for example as prisms and diffraction gratings, require increasing space with increasing amount of the required second order dispersion, since the dispersion caused proportional to the propagated path under the influence of the angular dispersion is. In any case, the angular dispersion caused by the dispersive optical element is limited: in prisms, the angular dispersion is limited due to the maximum refractive index difference between the prism material and its surroundings. For diffraction gratings, the limitation is caused by the minimum grating period. The latter must be greater than half the wavelength of the radiation to be modulated, since otherwise the desired effect in conventional configurations is absent. Furthermore, the grating period is limited by the fact that the adjustment sensitivity increases non-linearly with decreasing period length.

Recently, dielectric diffraction gratings have been presented, which are formed as reflection gratings and have a diffraction efficiency of more than 99% (cf. Optics Letters, Vol. 2, pages 323-326 ). These reflection grids are close to industrial availability. First laboratory tests with a laser pulse compressor comprising two reflection gratings, wherein four diffractions per pass are produced (so-called four-pass laser pulse compressor) were demonstrated in the above-mentioned publication.

Out "Aberration-free stretcher design for ultrashort-pulse amplification", G. Cheriaux, et al., Optics Letters, Vol. 21, Issue 6, pp. 414-416 (1996) , For example, a aberration-poor or -freies extensor configuration, known as opener Strecker known. The structure is based on an original configuration of A. normally closed (cf. US 3,748,015 ).

The open-type laser pulse expander is further characterized by Zhang et al. in "Compact and material-dispersion offner stretcher for chirped pulse amplifications", Optics Communications 206 (2002) 7-12, 15 May 2002 , treated.

Based on this prior art, it is an object of the present invention to provide an optical device which has a particularly compact design and causes a laser amplification sufficiently large chromatic dispersion.

According to the invention the object is achieved by an optical device of the type mentioned above with the characterizing features of claim 1.

Advantageous embodiments of the invention are the subject of the dependent claims.

In an optical device for modulating the spectral phase of laser pulses, in particular for time broadening or compressing the duration of a laser pulse in the chirped pulse amplification, at least one in a predetermined wavelength range angular dispersion exhibiting diffraction grating and a folding optics for folding the beam path is provided. The folding optical system is designed such that a plurality of diffractions of the laser pulse can be generated at the at least one dispersive diffraction grating. According to the invention, the folding optical system is designed such that at least eight diffractions can be generated on the at least one dispersive diffraction grating or on an arrangement comprising a plurality of dispersive diffraction gratings.

The invention is based on the observation that the space required to produce a sufficiently large dispersion for laser amplification can be reduced in that the laser pulse is guided several times through the optical device, which can be designed in particular as a laser pulse compressor or expander. The optical device has one or more dispersive diffraction gratings, wherein multiple diffractions can be generated at each diffraction grating. For this purpose, the folding optical system has one or more optical elements for folding the beam path, so that the guided laser pulse is diffracted several times at each diffraction grating. Thus, at least eight diffractions can be generated per pass on the diffraction grating (s), so that the stretching or compression factor is increased by a factor of at least two compared to known 4-pass laser pulse recompressors or 4-pass laser pulse compressors required space to increase significantly. Conversely, this means that the space required to produce a given second order dispersion can be reduced by folding the beam path by a factor of at least two. Accordingly, the present invention is characterized by a particularly compact arrangement, which is simultaneously designed to produce a comparatively high dispersion.

According to preferred embodiments, the beam path is folded such that a total of more than eight diffractions, in particular 8 to 40 diffractions, in each case in sets of four, can be generated. However, as the number of diffractions produced on the diffraction grating increases, the diffraction efficiency of the diffraction gratings used and the influence of the beam quality (roundness, astigmatism, M ² value, etc.) by the arrangement of accumulated wavefront errors caused by the optics used are Significance, since the losses occurring and the disturbance of the beam quality for an efficient use of the optical device in the laser pulse amplification must remain below a certain threshold. This threshold depends on the application and can vary accordingly. For example, if a total efficiency of 50% is tolerable, then 64 to 68 inflections would be acceptable for an exemplary single diffraction efficiency of 99%, provided that the beam quality still meets the respective requirements.

The at least one diffraction grating of the optical device is designed as a transmission grating or as a reflection grating. In embodiments that provide arrangements of multiple diffraction gratings, combinations of transmission and reflection gratings may also be used. In any case, it is advantageous to provide diffraction gratings with high diffraction efficiency. The diffraction efficiency of the diffraction gratings is preferably greater than 92%, particularly preferably greater than 98% or 99%.

Particularly preferably, the at least one diffraction grating of the optical device is designed as a reflection grating. Reflection gratings in particular offer the highest scaling reserves with regard to the average and peak power used. Preferred high-quality reflection gratings have a multiplicity of dielectric layers which are arranged one above the other on a substrate and are applied, for example, by means of a coating method, in particular by means of ion beam sputtering (IBS).

Alternatively, especially in the case of low average powers of, for example, less than 2 W at 1030 nm centroid wavelength of the laser, metallically coated diffraction gratings are preferred. As metals are particularly suitable for gold and silver.

Preferably, the multi-layer system consists of layers that are applied alternately and consist of materials that have different refractive indices, cf. dielectric multilayer mirror. Such an alternating layer sequence with finely tuned, varying thicknesses of the individual layers causes a particularly high degree of reflection of the reflection grating thus formed.

The materials of the layers particularly preferably comprise Ti ₂ O ₅ , Ta ₂ O ₅ , HfO ₂ , AlF ₃ , Al ₂ O ₃ , Nb ₂ O ₅ , Sc ₂ O ₃ , TiO ₂ , Y ₂ O ₃ , ZrO ₂ and / or SiO ₂ . Depending on the laser wavelength, the dielectric layers of the reflection grating, taking into account the angle of incidence, effectively have a thickness on the order of a quarter of the wavelength (see "quarter-wave stacks"), ie at 1 micron laser wavelength up to a few 100 nm. The thickness of the layers can vary, in particular the layers lying near the surface can have an increased or a reduced thickness, which reduces the diffraction efficiency sometimes strongly influenced by a predetermined angle and accordingly makes it possible to achieve high diffraction efficiencies for special laser parameters apart from the Littrow angle (cf. Optics Letters, Vol. 2, pages 323-326 ).

The folding optics preferably comprise an arrangement of one or more optical elements, which are designed as mirrors, lenses, retroreflector prisms and / or step mirrors. The individual optical elements of the folding optics are arranged such that the beam path is passed through the optical device several times and the beam acquires or retains a desired shape.

Preferably, the beam path is folded in such a way by means of the folding optics that an integer multiple of four diffractions can be generated at the intended diffraction gratings or at the provided diffraction grating. In each case less than four individual diffraction gratings are provided. In other words, it is intended to irradiate a diffraction grating multiple times.

The optical device with such a folded beam path is characterized by a particularly compact design.

Particularly preferably, the folding optics is arranged in particular in the case of the laser pulse compressor such that 8 to 40 diffractions can be set on the at least one diffraction grating or on the arrangement comprising a plurality of diffraction gratings. In the application as a laser pulse compressor, a limitation of the diffractions is considered to be a total transmission of more than 75%, whereby this definition is arbitrary and depends on the respective context. In another context, higher or lower total emissions could also be set.

In one embodiment of the invention, four diffraction gratings are provided so that the generation of 8 to 24 diffractions corresponds to a two to six fold convolution of the beam path within the optical device. It is advantageous from an economic point of view and with respect to the beam quality that diffraction gratings with high diffraction efficiency are used for this, because the higher the diffraction efficiency, the fewer folds are necessary and the lower are disturbances of the beam quality as a rule. In particular, it is intended to produce 8 to 12 diffractions if the diffraction efficiency of the grating (s) used is 97% to 98%. However, if the diffraction efficiency is 99%, then 24 diffractions can be adjusted to achieve a total efficiency of more than 75%.

In one possible embodiment, exactly one dispersive diffraction grating is provided. It is thus defined to generate the at least eight diffractions on the same diffraction grating. In an alternative embodiment, only two diffraction gratings are provided for realizing the arrangement. In any case, the optical device is characterized by a particularly compact design and reduced manufacturing costs.

The optical device is formed in a preferred embodiment as a laser pulse expander for temporally broadening the duration of the laser pulse. The time-stretched or "chirped" laser pulse thus has, after passing through the optical device, an extended duration due to the positive or negative dispersion of the laser pulse stretcher.

In another preferred embodiment, the optical device is designed as a laser pulse compressor for temporally compressing the duration of the laser pulse. In particular, it is provided to compensate for a dispersion of a chirped laser pulse by means of the laser compressor thus formed after it has passed an amplifier medium.

In a laser system comprising a laser source for providing a laser pulse, a laser pulse expander for temporally broadening the duration of the laser pulse, an optical amplifier medium for amplifying the time-stretched laser pulse, and a laser pulse compressor for temporally compressing the duration of the stretched and amplified laser pulse, the laser pulse expander and or the laser pulse compressor formed by an optical device described above. In particular, the dispersion caused by multiple passages within the optical device and thereby increased in size can also be used to generate laser pulses with increased laser pulse energy while maintaining the same laser pulse quality. The increased second-order dispersion allows the increase of the laser pulse energy such that the energy density or peak intensity in the amplifier medium remains below a critical threshold, so that the occurrence of disturbing nonlinear effects can be avoided. The critical threshold can be determined in particular by means of the known B integral.

The invention will be described below with regard to further features and advantages with reference to the Description of exemplary embodiments and with reference to the accompanying schematic drawings explained in more detail. Show

1 an optical device according to a first embodiment with four reflection gratings to produce a chromatic dispersion in a plan view;

2 the optical device according to the first embodiment in a side view;

3 an optical device according to a second embodiment with two dispersive reflection gratings in a plan view;

4 the optical device according to the second embodiment in a side view;

5 an optical device according to a third embodiment with a dispersive reflection grating in a plan view;

6 the optical device according to the third embodiment in a side view;

7 a designed as a laser straightener optical device with two reflection gratings according to a fourth embodiment in a plan view;

8th the optical device according to the fourth embodiment in a side view;

Corresponding parts are provided in all figures with the same reference numerals. In particular, the center of a four-pass single pass is marked with a small cross in all drawings.

1 and 2 show a first embodiment of the designed as a multi-pass dispersion filter optical device 1 in a top view and in a side view. In order to produce a sufficiently large dispersion, a total of four dispersive diffraction gratings are provided in the first exemplary embodiment 2 provided, which are designed as a reflection grating. Correspondingly, there are four diffractions per pass of the laser beam 3 at the four diffraction gratings 2 produced.

folding optics 4 for folding the beam path of the laser beam 3 are arranged at the edge. Every folding optics 4 includes several optical elements, namely mirrors 5 and lenses 6 , As in the side view of 2 is easy to see, by means of the edge arranged folding optics 4 causes a height offset so that the laser beam 3 several times at the diffraction gratings 2 is bent. Each through the folding optics 4 accordingly, induced convolution leads to another four diffractions at the four diffraction gratings 3 of the first embodiment.

It is intended to effect several passes to a sufficiently large dispersion in the laser beam 3 to create. In any case, so by means of the folding optics 4 an integer multiple of four diffractions at the proposed diffraction gratings 2 produced.

The diffraction gratings designed as reflection gratings 2 are highly efficient and comprise a plurality of dielectric layers superimposed on a substrate made of Ti ₂ O ₅ , Ta ₂ O ₅ , HfO ₂ , AlF ₃ , Al ₂ O ₃ , Nb ₂ O ₅ , Sc ₂ O ₃ , TiO ₂ , Y ₂ O ₃ , ZrO ₂ , SiO ₂ or the like. Layers of different materials are arranged alternately one above the other.

As an alternative to these dielectric diffraction gratings 2 are diffraction gratings 2 preferred with gold coating.

The lenses 6 the folding optics 4 are used to adjust the divergence / convergence of the laser beam 3 so that it can have, for example, the same diameter and the same divergence / convergence at the output of the chromatically dispersive filter as can be given a desired shape at the input or the beam.

For Gaussian beam cross sections, the lenses are 6 and optionally further, not shown, beam-forming elements arranged such that the beam waist of the laser beam 3 in the middle of a single passage or in the middle of all of the folding optics 4 caused passageways lies.

3 and 4 show the optical device 1 according to a second embodiment, the only two designed as a reflection grating diffraction grating 2 includes. To generate four diffractions per pass is an internal folding optics 7 provided, which is arranged at the edge and by means of further mirrors 8th causes a height offset of the beam path.

The number of folding optics provided 4 determines, as in the first embodiment, the number of passes, which is selected according to the size of the dispersion to be generated and the maximum size of the arrangement.

5 and 6 show a further, fourth embodiment of the optical device 1 with only one dispersive diffraction grating 2 , which - as in the previously shown embodiments - is designed as a reflection grating.

To generate four diffractions per pass cause the intended internal folding optics 7 compared to the one in the 3 and 4 shown embodiments, a further convolution. This is in the fourth embodiment by an additional level mirror 9 reached.

The in the 1 to 6 Embodiments shown include reflection gratings as dispersive elements. Embodiments in which the diffraction gratings 2 are formed as a transmission grating, are also implemented and provided. This applies to both in the 1 to 6 exemplified optical devices 1 , which are designed as laser pulse compressors, as well as designed as a laser pulse expander optical devices 1 ,

7 and 8th show such as a multi-pass dispersion filter formed optical device 1 in a side view and in a top view. In the beam path of the laser beam 3 are additional imaging optics 10 provided, which are embodied in the example shown as spherical lenses with positive refractive power and focal length f. One of the two intended imaging optics 10 is from one of the two diffraction gratings 2 arranged at a distance X. The other distances are fixed and correspond to the focal length f or 2f. Depending on which value is selected for the distance X with respect to the focal length f, a second order positive or negative dispersion can be generated. In that sense, in the 7 and 8th shown optical device 1 be easily adapted to generate dispersions with opposite signs and is thus suitable both for the formation of laser pulse stretching and for the formation of laser pulse compressors. The folding optics 4 and the internal folding optics 7 cause this - as in the embodiments of the 1 to 6 - A sufficient number of passes or diffractions at the diffraction gratings 2 ,

In addition to spherical lenses can also be aspherical or cylindrical lenses 6 . 10 Use, depending on the optimization objective and layout of the arrangement.

In another embodiment, not shown, the imaging optics 10 as spherical concave mirrors and the diffraction gratings 2 designed as a transmission grating. These optical elements can be arranged for example in the known opener-Strecker configuration for generating four individual diffractions, for which purpose also a reaction with only one diffraction grating 2 is conceivable. In any case, the external folding optics 4 provided to a sufficient number of passes and thus a sufficient dispersion at the diffraction gratings 2 to create.

The invention has been described above with reference to preferred embodiments. It is understood, however, that the invention is not limited to the specific embodiment of the exemplary embodiments shown, but rather the person skilled in the art can derive variations from the description without departing from the essential basic idea of the invention.

LIST OF REFERENCE NUMBERS

1

 optical device

2

 diffraction grating

3

 laser beam

4

 folding optics

5

 mirror

6

 lens

7

 folding optics

8th

 mirror

9

 stepped mirror

10

 imaging optics

X

 distance

f

 focal length

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 3748015 [0006]

Cited non-patent literature

• Optics Letters, Vol. 2, pages 323-326 [0005]
• "Aberration-free stretcher design for ultrashort-pulse amplification", G. Cheriaux, et al., Optics Letters, Vol. 21, Issue 6, pp. 414-416 (1996) [0006]
• Zhang et al. in "Compact and Material-Dispersion Offer stretcher for chirped pulse amplifications", Optics Communications 206 (2002) 7-12, 15 May 2002 [0007]
• Optics Letters, Vol. 2, pages 323-326 [0018]

Claims (14)

Optical device ( 1 ) for modulating the spectral phase of laser pulses, in particular for temporal broadening or temporally compressing the duration of a laser pulse in the case of chirped pulse amplification, with at least one diffraction grating having an angular dispersion in a predefinable wavelength range (US Pat. 2 ) and a folding optics ( 4 . 7 ) for such folding of the beam path, that a plurality of diffractions of the laser pulse at the at least one diffraction grating ( 2 ) are producible, characterized in that the folding optics ( 4 . 7 ) is designed such that at least eight diffractions on the at least one diffraction grating ( 2 ) or at a plurality of diffraction gratings ( 2 ) comprehensive arrangement can be generated. Optical device ( 1 ) according to claim 1, characterized in that the diffraction grating ( 2 ) is designed as a transmission grating or as a reflection grating. Optical device ( 1 ) according to claim 2, characterized in that the reflection grating comprises a plurality of dielectric layers superimposed on a substrate. Optical device ( 1 ) according to claim 3, characterized in that the layers consist of two different refractive indices having materials, wherein layers of respectively different materials are arranged alternately superimposed. Optical device (1) according to claim 3 or 4, characterized in that the materials of the layers Ti ₂ O ₅ , Ta ₂ O ₅ , HfO ₂ , AlF ₃ , Al ₂ O ₃ , Nb ₂ O ₅ , Sc ₂ O ₃ , TiO ₂ , Y ₂ O ₃ , ZrO ₂ and / or SiO ₂ . Optical device ( 1 ) according to claim 1 or 2, characterized in that the diffraction grating ( 2 ) has a metallic coating. Optical device ( 1 ) according to one of the preceding claims, characterized in that the folding optics ( 4 . 7 ) an arrangement of one or more mirrors ( 6 . 8th ), Lenses ( 6 . 10 ), Retroreflector prisms and / or levels ( 9 ). Optical device ( 1 ) according to one of the preceding claims, characterized in that the folding optics ( 4 . 7 ) is arranged such that an integer multiple of four diffractions at the at least one diffraction grating ( 2 ) or at the multiple diffraction gratings ( 2 ) comprehensive arrangement can be generated. Optical device ( 1 ) according to claim 8, characterized in that the folding optics ( 4 . 7 ) is arranged such that 8 to 40 diffractions at the at least one diffraction grating ( 2 ) or at the multiple diffraction gratings ( 2 ) comprehensive arrangement can be generated. Optical device ( 1 ) according to one of the preceding claims, characterized in that exactly one diffraction grating ( 2 ) is provided. Optical device ( 1 ) according to one of the preceding claims 1 to 9, characterized in that exactly two diffraction gratings ( 2 ) is provided. Optical device ( 1 ) according to one of the preceding claims, characterized in that the optical device ( 1 ) is designed as a laser pulse expander for temporally broadening the duration of a laser pulse. Optical device ( 1 ) according to one of the preceding claims, characterized in that the optical device ( 1 ) is designed as a laser pulse compressor for temporally compressing the duration of a laser pulse. A laser system comprising a laser source for providing a laser pulse, a laser pulse extender for temporally broadening the duration of a laser pulse, an optical amplifier medium for amplifying the time-stretched laser pulse and a laser pulse compressor for temporally compressing the duration of the stretched and amplified laser pulse, characterized in that the laser pulse expander and or the laser pulse compressor of an optical device ( 1 ) is formed according to one of claims 1 to 12.

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