A compact compressor and a stretcher for spectrally-divided broadband chirped pulse amplification

Abstract

Chirped pulse amplification with an optical pulse stretcher (13) for dividing an optical input pulse (14) in temporally and spectrally separated collinear pulses and a compressor for applying a negative dispersion to an optical signal (2), the compressor comprising: - a first dispersive element (3); - a second dispersive element (5), wherein the second dispersive element (5) is arranged parallel to the first dispersive element (3) and downstream of the first dispersive element (3); - a reflective element (7) downstream of the second dispersive element (5) configured for reflecting the optical signal (2) onto the second dispersive element (5); and -a first mirror (8) and a second mirror (9) arranged downstream the first dispersive element (3) and upstream of the second dispersive element (4), wherein the first mirror (8) and the second mirror (9) are parallel to each other and are configured for increasing an optical path length of the optical signal (2).

Description

[0001] A compact compressor and a stretcher for spectrally-divided broadband chirped pulse amplification

[0002] The present invention relates to the manipulation of optical pulses , in particular to applying a negative or a positive dispersion to an ultrafast optical pulse .

[0003] In particular, the invention relates to a device for applying a negative dispersion to an optical signal , comprising :

[0004] - a first dispersive element , in particular a first grating , configured for receiving an optical signal ;

[0005] - a second dispersive element , in particular a second grating , wherein the second dispersive element is arranged parallel to the first dispersive element and downstream of the first dispersive element ;

[0006] - a reflective element downstream of the second dispersive element configured for receiving the optical signal from the second dispersive element and configured for reflect ing the optical signal onto the second dispersive element ; wherein the first dispersive element , the second dispersive element and the reflective element are preferably configured in a Treacy compressor configuration .

[0007] The invention also relates to a method for applying a positive dispersion to an optical signal .

[0008] Furthermore , the invention concerns a method and an optical pulse stretcher for dividing an optical input pulse in temporally and spectrally separated collinear pulses with distinct positive dispersions .

[0009] In addition, the invention concerns a method for chirped pulse amplification as well as a chirped pulse amplification device for amplifying an optical input pulse with an optical pulse stretcher , an amplification unit , and a device for applying a negative dispersion .

[0010] Ultrafast optical pulses refer to electromagnetic pulses with durations in the order of picoseconds ( 10-12seconds ) , femtoseconds ( 10-15seconds ) or even less . To achieve such short durations in the time-domain, these pulses must have a broad optical spectrum. Individual spectral components within this spectrum must maintain a fixed phase relationship , which needs to be precisely aligned to result in an ultrafast pulse . Methods and devices for generating such ultrafast optical pulses are well known in the prior art and have been made commercially available . Applications of ultrafast pulses find widespread use in applications such as time-resolved spectroscopy, microscopy and imaging , quantum control , and metrology, to name a few . Ultrafast optical pulses play a crucial role in scientific research as well as industrial applications .

[0011] To tailor pulses to specific applications , it is often necessary to manipulate ( i . e . , to adj ust ) these pulses accordingly . For example , adj usting the pulse duration is a common requirement . This adj ustment can be achieved through dispersion control . Dispersion causes the spectral components of a pulse to shift in time relative to each other, allowing for pulse stretching or compression as needed .

[0012] Many applications require relatively high intensities , for example to achieve non-linear effects . Therefore , pulse amplification is often necessary . To prevent damages of amplifying media and components , chirped pulse amplification ( CPA) is typically employed . In CPA, an initial pulse is stretched ( in the time domain ) by applying positive dispersion, then the stretched pulse is amplified, and finally the amplified pulse is compressed by applying a negative dispersion, which counteracts the positive dispersion applied earlier during stretching . The positive dispersion applied during stretching and the negative dispersion applied during compression need to be precisely matched .

[0013] This technique is well known in the art and is also used in commercially available laser systems .

[0014] The resulting intensity of the amplified beam is often limited by damage threshold of the amplifier . Therefore , the more the pulse is stretched before amplifying, the higher the possible intensity of the compressed pulse .

[0015] Practical applications are , however , not limited by pulse stretching but by pulse compression . Any optical element employed after the amplifier will cause a loss of power and consequently intensity due to absorption and / or scattering. Therefore, it is crucial to employ as little optical components as possible after the amplifier. Losses before the amplifier, i.e. , in the stretcher, are less crucial since these losses can be compensated by the amplifier. Consequently, the maximum amplification in CPA is often limited by the amount of dispersion that can be introduced by the compressor without resulting in significant losses in power and intensity.

[0016] A compressor is, for example, known from Treacy, E. (1969) . Optical pulse compression with diffraction gratings. IEEE Journal of quantum Electronics, 5 (9) , 454-458.

[0017] The so-called "Treacy compressor" (inspired by E. Treacy cited above) is, for example, also disclosed in Ivanov, V. (2022) . Compact optical grating compressor. Optics Express, 30(20) , 35338-35347.

[0018] Typically, a pair of parallel gratings is used to introduce negative dispersion. A mirror is arranged downstream of the grating pair, which reflects the pulses such that each pulse interacts with each grating twice. Overall, spectral components with shorter wavelengths have a shorter optical path though the compressor than spectral components with higher wavelengths. This leads to a negative dispersion. The opposite case, i.e., when shorter wavelength components are delayed with respect to longer wavelength components, is referred to as positive dispersion.

[0019] A compressor configuration that is widely used nowadays was proposed and shown in Martinez, O. "3000 times grating compressor with positive group velocity dispersion: Application to fiber compensation in 1.3-1.6 pm region." IEEE Journal of Quantum Electronics 23.1 (1987) : 59-64.

[0020] An approach to enhance the maximum dispersion of a compressor is to enhance the dispersive power (spectral resolution) of the employed gratings. However, the spectral resolution of gratings is proportional to the density of lines and / or grooves. In addition, the maximum dispersion is limited by the diffraction angle of the gratings, which is also proportional to the line / groove density. High diffraction angles pose practical difficulties in the technical implementation .

[0021] Another approach is to extend the overall optical path length in order to achieve a higher difference path length between spectral components and therefore a higher dispersion. This approach, however, typically leads to relatively bulky devices, which is often an issue in laboratories and facilities .

[0022] In addition, the overall dispersion achievable is limited by the size (i.e. , the aperture) of the gratings used. As the path length increases, the spatial separation of spectral components in a direction perpendicular to the main direction of the pulse increases, due to the angular dispersion of the pulse. Therefore, large gratings would be necessary to achieve high dispersions. However, in particular with respect to high-precision gratings designed for high spectral resolution, it is difficult to increase the grating size without introducing defects or compromising uniformity. The size of the grating is ultimately constrained by manufacturing challenges . In addition, the costs of gratings scale exponentially with the size. Tiling individual (smaller) gratings (see for example Flamand, J. , de Villele, G., Cotel, A. , Touzet, B. , & Kane, S. (2006, June) . New MLD gratings adapted for tiling in petawatt-class lasers. In Journal de Physique IV (Proceedings) (Vol. 133, pp. 601-605) . EDP sciences. ) is possible but also implies increased cost of multiple gratings and additional fine kinematic mechanical components required to precisely position the gratings .

[0023] Another device for applying an optical dispersion is, for example, known from Wu, Jianglai, et al. "Active, large-scale tuning of optical dispersion by free-space angular-chirp-enhanced delay (FACED) ." CLEO: Science and Innovations. Optica Publishing Group, 2016. The device mainly consists of a pair of tilted mirrors. An input broadband pulsed laser beam is first angularly dispersed by a diffraction grating, and is then coupled into the mirror-pair entrance by a lens system. The beam is then decomposed into a set of spatially-chirped zig-zag paths in the device. The dispersed light is back-reflected and can be restored to its original input beam profile after the grating. The zig-zag paths lead to an enhanced path length and an increased dispersion . In addition, due to the different zig-zag paths of different spectral components , the output signal comprises discrete spectral modes . Since no technique for counteracting the effect of compressor is provided or known in the prior art , the disclosed device is not suited as a compressor for CPA, since the compressor and a suitable stretcher need to precisely counteract one another .

[0024] A related method for producing spatially shapable GHz burst pulses is known from Shimada, Keitaro , et al . "Spectrum shuttle for producing spatially shapable GHz burst pulses . " Advanced Photoni cs Nexus 3 . 1 ( 2024 ) : 016002 -016002 . An ultrashort pulse is dispersed by a grating pair and is then incident on a pair of parallel mirrors . The incident light travels back and forth between these parallel mirrors , wherein daughter pulses with specific wavelength components are guided to a third mirror ( or an SLM) at every lap , whereby the daughter pulses are vertically aligned . Subsequently, the reflected pulses return along their initial incoming paths and are picked off with a beam splitter as a spatially nondispersive and spectrally separated pulse train . The parallel mirrors introduce a time delay between separate spectral components of the incident pulse and lead to the formation of the pulse burst . The overall dispersion of each daughter pulse is limited by the size of the grating . The disclosure does not provide a method for counteracting the effect of the spectrum shuttle . Since no suitable counteracting stretcher is known, the "spectrum shuttle" cannot be used as a compressor in CPA .

[0025] In summary, the maximum dispersion that can be introduced in a broadband pulse is limited by the aperture of the dispersive elements used . Presently known methods with enhanced dispersion comprise discrete spectral modes and offer dispersion of only one sign, which hinder their employment as a compressor in chirped pulse amplification .

[0026] It is an obj ect of the invention to at least alleviate or eliminate these drawbacks of the prior art . In particular , it is an obj ect of the invention to provide a device and a method for applying a negative dispersion to an optical signal ( i . e . , a compressor ) with an enhanced dispersion, as well as a pulse stretcher and a method for dividing an optical input pulse in temporally and spectrally separated collinear pulses with distinct positive dispersions , which is suited to counteract the effect of the compressor . Furthermore , it is an obj ect of the invention to provide a method and a device for chirped pulse amplification, which provide an enhanced amplification and an enhanced output intensity of the amplified output pulse .

[0027] This obj ect is solved with a device for applying a negative dispersion to an optical signal , comprising :

[0028] - a first dispersive element , in particular a first grating , configured for receiving an optical signal ;

[0029] - a second dispersive element , in particular a second grating , wherein the second dispersive element i s arranged parallel to the first dispersive element and downstream of the first dispersive element ;

[0030] - a reflective element downstream of the second dispersive element configured for receiving the optical signal from the second dispersive element and configured for reflecting the optical signal onto the second dispersive element ; wherein the first dispersive element , the second dispersive element and the reflective element are preferably configured in a Treacy compressor configuration; wherein

[0031] - a first mirror and a second mirror are arranged downstream of the first dispersive element and upstream of the second dispersive element , wherein the first mirror and the second mirror are parallel to each other and are configured for increasing an optical path length of the optical signal .

[0032] Furthermore , this obj ect is solved with a method for applying a negative dispersion to an optical signal , preferably by means of a device for applying a negative dispersion to an optical signal according to the invention, with the steps :

[0033] - Receiving an optical signal having an optical spectrum;

[0034] - Applying an angular chirp to the optical signal by means of a first dispersive element , such that each spectral component of the optical signal is directed in a distinct direction;

[0035] - Increasing an optical path length of the optical signal having the angular chirp by means of a first mirror and a second mirror , wherein the first mirror is arranged parallel to the second mirror ; - Guiding the optical signal onto a second dispersive element downstream of the first mirror and the second mirror, wherein the second dispersive element is parallel to the first dispersive element ;

[0036] - Reflecting the optical signal by means of a reflective element positioned downstream of the second dispersive element onto the second dispersive element , such that the optical signal is propagated back to the first dispersive element via the first and the second mirror;

[0037] - Eliminating the angular chirp by means of the first dispersive element ;

[0038] - Outputting the optical signal having a negative dispersion .

[0039] The obj ect is also solved with a method for dividing an optical input pulse in temporally and spectrally separated collinear pulses with distinct positive dispersions with the steps :

[0040] - Receiving an optical input pulse with an input spectrum and a main direction of propagation;

[0041] - Applying a spatial chirp to the optical pulse , such that different wavelengths of the input spectrum are separated in a direction transverse to the main direction of propagation;

[0042] - Separating the optical input pulse having the spatial chirp in at least a first portion with a first spectrum, a second portion with a second spectrum and a third portion with a third spectrum, wherein the first portion, the second portion and the third portion are spatially non-overlapping, wherein the first spectrum, the second spectrum and the third spectrum are spectrally nonoverlapping;

[0043] - Applying a first positive dispersion to the first portion, a second positive dispersion to the second portion and a third positive dispersion to the third portion;

[0044] - Adj usting a first time delay between the first portion and the second portion, and a second time delay between the second portion and the third portion;

[0045] - Aligning the first portion, the second portion and the third portion collinearly along a main output axis ;

[0046] - Outputting a train of temporally and spectrally separated pulses , wherein the first portion with the first spectrum and the first positive dispersion forms a first pulse of the train, wherein the second portion with the second spectrum and the second positive dispersion forms a second pulse , wherein the third portion with the third spectrum and the third positive dispersion forms a third pulse

[0047] Furthermore , the obj ect is solved with an optical pulse stretcher for dividing an optical input pulse in temporally and spectrally separated collinear pulses , comprising :

[0048] - a spatial chirp application unit for applying a spatial chirp to an optical input pulse ;

[0049] - a separation unit for separating the optical input pulse comprising the spatial chirp in at least a first portion with a first spectrum, a second portion with a second spectrum and a third portion with a third spectrum, wherein the first spectrum, the second spectrum and the third spectrum are spectrally nonoverlapping;

[0050] - a positive dispersion unit for applying a positive dispersion to the first portion, the second portion and the third portion;

[0051] - a delay stage for generating a first time delay between the first portion and the second portion , and for generating a second time delay between the second portion and the third portion;

[0052] - an alignment unit for aligning the first portion, the second portion and the third portion collinearly along a main output axis .

[0053] The obj ect is also solved with a method for chirped pulse amplification with the steps :

[0054] - Providing an optical input pulse ;

[0055] - Stretching the optical input pulse by means of method for dividing an optical input pulse in temporally and spectrally separated collinear pulses with distinct positive dispersions according to the invention, in particular with an optical pulse stretcher according to the invention ;

[0056] - Amplifying the first pulse , the second pulse and the third pulse ; and

[0057] - Compressing the amplified first pulse , second pulse and third pulse by means of the method for applying a negative dispersion according the invention, preferably by means of a device for applying a negative dispersion according to the invention .

[0058] Additionally, the obj ect is also solved with a chirped pulse amplification device for amplifying an optical input pulse with an optical pulse stretcher according to the invention, an amplification unit for amplifying optical pulses , and a device for applying a negative dispersion according to the invention .

[0059] The optical signal is an electromagnetic signal . The optical signal may be an optical pulse with an optical spectrum. The optical pulse may be referred to as broadband pulse and / or ultrafast pulse . The optical pulse may comprise a pulse duration in the order of picoseconds or femtoseconds . The optical spectrum may, for example , have a center wavelength between 180 nm and 25 pm . The optical pulse may comprise a bandwidth between 0 . 1 nm and 300 nm for example .

[0060] The optical signal may comprise one or more optical pulses , i . e . , the optical signal may comprise a train of pulses .

[0061] In ultrafast optics , the term temporal chirp refers to a temporal variation in the frequency of an optical pulse over its duration . Specifically, temporal chirp describes how the frequency of the light within the optical pulse changes with time . By introducing a temporal chirp in an optical pulse , a duration of the optical pulse can be manipulated . A temporal chirp can be introduced by applying a dispersion to the pulse , such that the path lengths of individual spectral components of the optical pulse differ , which results in a phase delay between these components .

[0062] In addition, a spatial chirp may be introduced . The term spatial chirp relates to a variation or separation of spectral components in a cross-section of the pulse orthogonal to the direction of propagation of the pulse ( i . e . , a main direction of the pulse ) .

[0063] Furthermore , an angular chirp may be introduced . The term angular chirp refers to a variation in the propagation direction or angle of different spectral components of an optical pulse . In other words , it relates to different wavelengths within an optical signal , in particular within a single pulse , being spread out at different angles , typically caused by elements like diffraction gratings or prisms .

[0064] The manipulation of (pulsed) optical signals relies on the control of these different kinds of chirp, for example , by means of dispersion and dispersive elements .

[0065] Compressor The optical signal may initially ( i . e . , before any interaction with the device for applying a negative dispersion ) be essentially free from an angular chirp .

[0066] The device for applying a negative dispersion may also be referred to as compressor , since it may be employed as a compressor in chirped pulse amplification (CPA) . In the following the terms "device for applying a negative dispersion" and "compressor" are used synonymously . Although it is preferred to use the device for applying a negative dispersion as a compressor in CPA, the device may in principle also be utilized in applications other than CPA.

[0067] The device for applying a negative dispersion comprises a first dispersive element , in particular a first grating , configured for receiving the optical signal . The first dispersive element diffracts different spectral components in different angles and therefore applies an angular chirp to the optical signal . A second dispersive element and a reflective element are arranged downstream of the first dispersive element .

[0068] The term "downstream" and the term "upstream" refer to an order of interaction of the optical signal with different components . For, example , an optical system may comprise elements A, B and a mirror redirecting the optical signal back to an input . The optical signal may comprise a main direction of propagation and may be guided such that it interacts first with element A, then with element B , followed by the mirror , then B and finally again A. The order of interaction is in this example A, B, mirror , B, A . The terms "downstream" and "upstream" may refer to the first interaction of the optical signal with each element , such that in this simplified example element B is arranged downstream of element A . Consequently, element B is arranged upstream of the mirror , in this simplified example .

[0069] The second dispersive element , in particular a second grating , is arranged parallel to the first dispersive element and downstream of the first dispersive element . The reflective element is arranged downstream of the second dispersive element and is configured for receiving the optical signal from the second dispersive element and configured for reflecting the optical signal (back) onto the second dispersive element . The first dispersive element , the second dispersive element and the reflective element are preferably configured in a Treacy compressor configuration (cf., for example, fig. 1 (a) and in Ivanov, V. (2022) . Compact optical grating compressor. Optics Express, 30 (20) , 35338-35347) .

[0070] The reflective element may be a mirror, preferably a roof mirror. The roof mirror may comprise two mirror surfaces arranged perpendicular to another, such that reflected light is parallel to incoming light, wherein the reflected light is spaced apart from the incoming light due to a parallel offset introduced by the roof mirror .

[0071] In addition, a first mirror and a second mirror are arranged downstream of the first dispersive element and upstream of the second dispersive element (i.e., in between the first dispersive element and the second dispersive element) , wherein the first mirror and the second mirror are parallel to each other and are configured for increasing an optical path length of the optical signal.

[0072] The first mirror and the second mirror are preferably arranged such that the optical signal is reflected by the first mirror multiple times and by the second mirror multiple times, in particular at least two, preferably at least four times. The optical signal may propagate in a zig-zag path in between the parallel first and second mirror. Due to the angular chirp introduced by the first diffractive element, an incident angle on the first mirror depends on the wavelength of each spectral component. Therefore, different spectral components have different optical paths and are reflected a different number of times. Due to the different (integer) number of reflections, the path lengths between adjacent spectral components differ significantly. If, for example, the optical signal is a single broadband pulse, the optical signal is split in a train of sub-pulses, wherein the sub-pulses do not overlap spectrally and are separated by time delays resulting from the different number of reflections in between the two parallel mirrors .

[0073] The parallel first and second mirror form a channel with an inlet and an outlet, wherein the first dispersive element is arranged in front of (or at) the inlet and the second dispersive element is arranged after (or at) the outlet. The spectral components of the optical signal spatially overlap at the outlet and at the second dispersive element. In other words, different spectral components of the optical signal may be incident on the second dispersive element at the same point . Therefore , the effective size of the second optical grating is enhanced . In comparison, in commonly used compressor designs , different spectral components are incident on gratings at distinct positions . Due to the overlapping of spectral components , which ultimately results from the differences in the number of reflections , the second dispersive element can be used more efficiently . The effective size of the second dispersive element is a multiple of the actual size due to a spreading and folding of the optical signal ( comprising the angular chirp ) by means of the parallel first and second mirror .

[0074] The mirror is arranged such that it reflects the optical signal backwards , such that the optical signal again propagates to the first dispersive element via the first and the second mirror . The angular chirp is then eliminated by the first dispersive element .

[0075] The optical signal interacts twice with the first dispersive element and twice with the second dispersive element . The optical signal interacts once with the reflective element . The optical signal is reflected the same integer number of times by the first and the second mirror . The integer number may be wavelength-dependent .

[0076] Overall , the device for applying a negative dispers ion applies a negative dispersion to the optical signal . In case the optical signal is a broadband pulse , the output signal comprises discrete spectral bands . The output signal then comprises distinct subpulses , which are separated spectrally and temporal ly . Each of the sub-pulses comprises a distinct negative dispersion, which depends ( inter alia ) on the number of reflections in between the parallel mirrors . The overall achievable negative dispersion is enhanced manifold compared to devices known form the prior art and dispersive elements of the same size .

[0077] For example , the optical signal may comprise a spectral component with a wavelength of 800 nm ( red ) and another spectral component with a wavelength of 400 nm (blue ) . The first dispersive element may diffract red at a greater diffraction angle than blue . The first mirror may be arranged such that an incident angle of red is steeper than an incident angle of blue . Therefore , the red component will be reflected in between the first and the second mirror more often than the blue component . A length of the first mirror and a length of the second mirror (perpendicular to a normal distance between the first and the second mirror ) may be equal to or , for example , greater than the normal distance between the first and the second mirror .

[0078] Optionally, the first mirror and the second mirror are arranged transverse to the first dispersive element .

[0079] Optionally, a length of the second dispersive element is greater than a normal distance between the two parallel mirrors .

[0080] Preferably, the first mirror and the second mirror each comprise a mirror end facing the second dispersive element , wherein the second dispersive element extends from the mirror end of the first mirror to the mirror end of the second mirror .

[0081] Optionally, the first dispersive element and the second dispersive element comprise the same or essentially the same dispersive power . For example , the first dispersive element and the second dispersive element may be gratings , wherein both gratings comprise the same or essentially the same line density .

[0082] The first dispersive element and / or the second dispersive element may be a transmission grating . The first dispersive element and / or the second dispersive element may be a reflection grating . The first dispersive element and / or the second dispersive element may be a prism.

[0083] The method for applying a negative dispersion to an optical signal , preferably by means of the device according to the invention, comprises the steps of :

[0084] - Receiving an optical signal having an optical spectrum;

[0085] - Applying an angular chirp to the optical signal by means of a first dispersive element , such that each spectral component of the optical signal is directed in a distinct direction;

[0086] - Increasing an optical path length of the optical signal having the angular chirp by means of a first mirror and a second mirror , wherein the first mirror is arranged parallel to the second mirror ;

[0087] - Guiding the optical signal onto a second dispersive element downstream of the first mirror and the second mirror, wherein the second dispersive element is parallel to the first dispersive element ; - Reflecting the optical signal by means of a reflective element positioned downstream of the second dispersive element onto the second dispersive element , such that the optical signal is propagated back to the first dispersive element via the first and the second mirror;

[0088] - Eliminating the angular chirp by means of the first dispersive element ;

[0089] - Outputting the optical signal having a negative dispersion .

[0090] Optionally, increasing the optical path length comprises folding the optical signal . By folding , different spectral components overlap at the second dispersive element , such that the effective size of the second dispersive element is increased . Folding may be understood as overlapping different spectral components at least at the second dispersive element .

[0091] The method and the device for applying a negative dispersion according to the invention is especially simple , comprises a relatively low number of components , i s simple to adj ust and can be implemented in a particularly small setup .

[0092] These advantages are especially relevant for CPA. However , in order to employ the device for applying a negative dispersion according to the invention for CPA, it is necessary to provide a stretcher that is capable of providing an input signal for the compressor, which the compressor is able to re-combine to a single pulse . Since the compressor introduces discrete spectral modes and a wavelengthdependent group delay, this may lead to a relatively high complexity of the stretcher . However, a high complexity of the stretcher is favorable compared to a high complexity of the compressor . The advantages of the simple and effective compressor design j ustify a high degree of complexity of the stretcher . The compressor and the stretcher are therefore interrelated devices that complement each other in the context of CPA.

[0093] Stretcher

[0094] In the following , an optical pulse stretcher as well as a ( corresponding ) method for dividing an optical input pulse in temporally and spectrally separated collinear pulses with distinct positive dispersions are described . This method and the stretcher ( i . e . , the optical pulse stretcher ) are a suitable counterpart of the compressor and the method discussed above , such that the combination of the compressor according to the invention and the stretcher according to the invention is suitable for CPA.

[0095] The method for dividing an optical input pulse into temporally and spectrally separated collinear pulses with distinct positive dispersions comprises the following steps :

[0096] - Receiving an optical input pulse with an input spectrum and a main direction of propagation;

[0097] - Applying a spatial chirp to the optical input pulse , such that different wavelengths of the input spectrum are separated in a direction transverse to the main direction of propagation;

[0098] - Separating the optical input pulse having the spatial chirp into at least a first portion with a first spectrum, a second portion with a second spectrum and a third portion with a third spectrum, wherein the first portion, the second portion and the third portion are spatially non-overlapping , wherein the first spectrum, the second spectrum and the third spectrum are spectrally non- over lapping;

[0099] - Applying a first positive dispersion to the first portion, a second positive dispersion to the second portion and a third positive dispersion to the third portion;

[0100] - Adj usting a first time delay between the first portion and the second portion, and a second time delay between the second portion and the third portion;

[0101] - Aligning the first portion, the second portion and the third portion collinearly along a main output axis ;

[0102] - Outputting a train of temporally and spectrally separated pulses , wherein the first portion with the first spectrum and the first positive dispersion forms a first pulse of the train, wherein the second portion with the second spectrum and the second positive dispersion forms a second pulse , wherein the third portion with the third spectrum and the third positive dispersion forms a third pulse .

[0103] The optical input pulse may, for example , be provided by a mode - locked laser . The optical input pulse may be referred to as broadband pulse and / or ultrafast pulse . The optical input pulse may, similar to the optical pulse discussed above , comprise a pulse duration on the order of picoseconds or femtoseconds . The optical spectrum may, for example , have a center wavelength between 180 nm and 25 m. The optical pulse may comprise a bandwidth between 0.1 nm and 300 nm for example.

[0104] Optionally, the optical input pulse may be separated in more than three portions, for example, in exactly or at least four portions, at least five or at least ten portions, wherein each portion has a separate spectrum.

[0105] Each of the separated portions (i.e. , at least the first, the second and the third portion) is subjected to a separate positive dispersion (i.e. , the first positive dispersion, the second positive dispersion and the third positive dispersion) . These positive dispersions are preferably different from each other.

[0106] Adjusting the first time delay between the first portion and the second portion, and the second time delay between the second portion and the third portion may, for example, be effected by means of different optical path lengths of the portions with respect to one another .

[0107] Outputting a train of temporally and spectrally separated pulses, wherein the first portion with the first spectrum and the first positive dispersion (i.e., a first positive chirp) forms a first pulse of the train, wherein the second portion with the second spectrum and the second positive dispersion (i.e., a second positive chirp) forms a second pulse, wherein the third portion with the third spectrum and the third positive dispersion (i.e., the third positive chirp) forms a third pulse.

[0108] The first time delay and the second time delay can preferably be adjusted without changing the respective dispersion.

[0109] The optical pulse stretcher for dividing an optical input pulse in temporally and spectrally separated collinear pulses, comprises a spatial chirp application unit for applying the spatial chirp to the optical input pulse.

[0110] The spatial chirp application unit preferably comprises a first dispersive element and a second dispersive element, wherein the first dispersive element and the second dispersive element are arranged parallel to one another. The first and / or the second dispersive element preferably comprise a grating. Preferably, the first and the second dispersive element comprise the same or essentially the same dispersive power. Optionally, the spatial chirp application unit comprises a lens , in particular a first cylindric lens upstream of the first dispersive element , and another lens , preferably a second cylindric lens , downstream of the second dispersive element . Preferably, the spatial chirp application unit comprises at least one stack of mirrors ( a mirror stack ) , in particular a first mirror stack and a second mirror stack . Each mirror stack may comprise at least three ( adj acent ) mirror surfaces . The first cylindric lens may be configured to form a Fourier plane on the first mirror stack in order to separate spectral components . The second cylindric lens may be configured to recollimate the beam with the same size as the input . The mirror stack may separate the spatially chirped beam into portions and direct these portions at the third grating at different angles of incidence . The spatial chirp may be set by a distance between the first dispersive element and the second dispersive element in the spatial chirp application unit . Preferably, the spatial chirp is not influenced by the mirror stack ( s ) .

[0111] The first mirror stack may be configured to redirect the portions of the spectrum in mutually different directions . The second mirror stack may be configured to direct these portions on the third dispersive element of the positive dispersion unit at different angles such that these portions form a straight line on the third dispersive element ( i . e . , after interacting with the third dispersive element ) .

[0112] Furthermore , the optical pulse stretcher comprises a separation unit for separating the optical input pulse comprising the spatial chirp in at least a first portion with a first spectrum, a second portion with a second spectrum and a third portion with a third spectrum, wherein the first spectrum, the second spectrum and the third spectrum are spectrally non-overlapping .

[0113] Optionally, the separation unit comprises a set of mirrors with at least a first separation mirror configured for receiving the first portion, a second separation mirror for receiving the second portion and a third separation mirror for receiving the third portion . The first separation mirror may be arranged to intercept the first portion, the second mirror may be arranged to intercept the second portion and the third mirror may be arranged to intercept the third portion . This interception is feasible due to the spatial chirp that has been applied to the optical input pulse earlier and, in particular , due to the focusing with the first cylindric lens .

[0114] Optionally, the first , the second and the third mirror are parallel to one another , wherein a center of the first mirror, a center of the second mirror and a center of the third mirror lie on a common straight line . The first separation mirror, the second separation mirror and the third separation mirror may comprise different lateral dimensions with respect to one another .

[0115] The positive dispersion unit for applying a positive dispersion to the first portion, the second portion and the third portion preferably comprises :

[0116] - a third dispersive element configured for applying an angular chirp to the optical input pulse ;

[0117] - a focusing lens , wherein the focusing lens is configured for receiving the first portion, preferably reflected off the first separation mirror, the second portion, preferably reflected off the second separation mirror , and the third portion, preferably reflected off the third separation mirror , wherein the first portion, the second portion and the third portion comprise the angular chirp; and preferably

[0118] - a roof mirror downstream of the focusing lens , wherein the roof mirror is configured to receive the first portion, the second portion and the third portion, and configured to redirect the first portion, the second portion and the third portion towards the focusing lens , wherein a distance between the roof mirror and the focusing lens is equal to the focal length of the focusing lens .

[0119] This leads to a particularly compact stretcher, since the same ( third ) dispersive element , focusing lens and the same roof mirror are used for applying the first positive dispersion, the second positive dispersion and the third positive dispersion . The dispersion can be controlled by adj usting the respective path lengths in between the third dispersive element and the focusing lens , which is particularly simple . The roof mirror leads to an even more compact layout , since only one focusing lens is necessary . As an alternative to the roof mirror, an additional focusing lens and a fourth dispersive element may be used , for example .

[0120] Without the focusing lens , a negative dispersion would be introduced . The focusing lens switches the sign of the applied dispersion such that a positive dispersion is applied. The amount of positive dispersion can be adjusted by adjusting an optical path length of the respective portion between the third dispersive element and the focusing lens. The smaller the path length between the third dispersive element and the focusing lens, the greater the positive dispersion (i.e., the greater the group delay) . The optical path length in between the third dispersive element and the focusing lens of the first, second and third portion can be adjusted independently. Therefore, each of the portions is subjected to a distinct positive dispersion (i.e. , positive chirp) . Preferably, the separation unit is arranged downstream of the third dispersive element and upstream of the focusing lens (i.e. , in between the third dispersive element and the focusing lens) . By adjusting a position of the first separation mirror, the second separation mirror and the third separation mirror, the respective path length may be adjusted in order to adjust the respective positive dispersion of the respective (separated) portion.

[0121] The delay stage for generating a first time delay between the first portion and the second portion, and for generating a second time delay between the second portion and the third portion preferably comprises at least a first delay mirror for receiving the first portion, a second delay mirror for receiving the second portion and a third delay mirror for receiving the third portion, wherein a distance between the first delay mirror and the second delay mirror is adjustable to adjust the first time delay, wherein a distance between the second delay mirror and the third delay mirror is adjustable to adjust the second time delay. The delay stage is preferably arranged outside of the positive dispersion unit, such that the first time delay and the second time delay can be adjusted by means of the delay stage without influencing the first, second and / or third positive dispersion. Preferably, the first portion, the second portion and the third portion do not comprise an angular chirp when interacting with the delay stage. The delay stage may be configured to receive the first portion, the second portion and the third portion from the positive dispersion unit.

[0122] The delay stage may be configured to re-direct (i.e., reflect) the optical signal into the positive dispersion unit. For example, the first, second and third portions may interact with the third dispersive element and the focusing lens four times in this case. The alignment unit for aligning the first portion, the second portion and the third portion collinearly along a main output axis may be a separate unit or may be effected by the spatial chirp application unit . In other words , the spatial chirp application unit may also be the alignment unit . By inserting the first portion, the second portion and the second portion into the spatial chirp application unit in a reverse direction compared to the optical input pulse , these portions are collinearly aligned along a main output axis , which coincides with the main direction of propagation of the optical input pulse before the spatial chirp application unit .

[0123] Chirped pulse amplification (CPA)

[0124] In CPA an optical input pulse is stretched in time , the stretched pulse is amplified, and finally the amplified pulse is compressed again such that essentially the original pulse duration of the input pulse is recovered .

[0125] Stretching the optical input pulse is effected by means of the method for dividing an optical input pulse in temporally and spectrally separated collinear pulses with distinct positive dispersions , in particular with an optical pulse stretcher according to the invention .

[0126] Amplifying the first pulse , the second pulse and the third pulse may, for example , be effected by an amplification unit . The amplification unit may, for example , comprise a regenerative amplifier and / or a multi-pass and / or a single-pass amplifier .

[0127] Compressing the amplified first pulse , the amplified second pulse and the amplified third pulse is effected by means of the method for applying a negative dispersion according to the invention, and preferably by means of a device for applying a negative dispersion according to the invention .

[0128] The chirped pulse amplification device for amplifying an optical input pulse comprises an optical pulse stretcher according to the invention, an amplification unit for amplifying optical pulses , and a device for applying a negative dispersion according to the invention .

[0129] Preferably, the first positive dispersion, the second positive dispersion the third positive dispersion, as well as the first time delay and the second time delay are adjusted such during stretching that the amplified first pulse, second pulse and third pulse are combined to a single optical pulse upon compressing. The stretcher is more complex and has more degrees of freedom than the compressor. The compressor introduces (separated) spectral bands that cannot be adjusted individually but depend on the wavelength of the pulse (s) , the dispersive power of the first and second dispersive element, an angle of incidence at the first mirror, the distance between the first and the second mirror and a length of the first and second mirror. By adjusting the compressor, all spatial modes are affected. In contrast, the stretcher, can be adjusted to introduce spectral bands, which the compressor can compensate. For example, in the stretcher, the separation can be adjusted such that the spectrum of the input pulse is separated at the wavelengths that correspond to an incremental increase in the number of reflections between the first and the second mirror in the compressor. The first, the second and the third positive dispersion can be adjusted individually to compensate the respective negative dispersion introduced by the compressor. Independently of the dispersion control, the first time delay and the second time delay (and if necessary further time delays between further portions) can be adjusted individually to match the compressor. In other words, the (complex) stretcher can be adjusted to have precisely the opposite effect on the optical signal than the (simple, robust, efficient, and cost-effective) compressor. Without these degrees of freedom of the stretcher and the ability to match the compressor, the compressor could not be used in CPA and its advantages could not be utilized in CPA.

[0130] By way of example, the disclosure is further explained with respect to some selected embodiments shown in the figures. However, these embodiments shall not be considered limiting for the disclosure.

[0131] Figures 1, 2 and 3 schematically show perspective views of an embodiment of a device for applying a negative dispersion 1 to an optical signal according to the invention (a compressor) .

[0132] Fig. 4 schematically shows an overview of an embodiment of an optical pulse stretcher for dividing an optical input pulse in temporally and spectrally separated collinear pulses .

[0133] Fig. 5 schematically shows the spatial chirp application unit of fig. 4 in more detail. Fig . 6 schematically shows the separation unit , the positive dispersion unit , the delay stage in more detail .

[0134] Fig . 7 schematically shows the separation unit of figures 4 and 6 in more detail .

[0135] Fig . 8 schematically shows a similar embodiment as figures 4 to 7 with added annotations as commonly used in the field of ultrafast optics .

[0136] Fig . 9 schematically shows a comparison of group delay introduced by a spectrally divided compressor scheme and a traditional Treacy pair .

[0137] Fig . 10 schematically shows a chirped pulse amplification device with an optical pulse stretcher, an amplification unit and compressor .

[0138] Figure 1 schematically shows a device for applying a negative dispersion 1 to an optical signal 2 , comprising :

[0139] - a first dispersive element 3 , in this example a first grating 4 , configured for receiving the optical signal 2 ;

[0140] - a second dispersive element 5 , in this example a second grating 6 , wherein the second dispersive element 5 is arranged parallel to the first dispersive element 3 and downstream of the first dispersive element 3 ;

[0141] - a reflective element 7 downstream of the second dispersive element 5 configured for receiving the optical signal 2 from the second dispersive element 5 and configured for reflecting the optical signal 2 onto the second dispersive element 5 ; and

[0142] - a first mirror 8 and a second mirror 9 arranged downstream of the first dispersive element 3 and upstream of the second dispersive element 5 , wherein the first mirror 8 and the second mirror 9 are parallel to each other and are configured for increasing an optical path length of the optical signal 2 .

[0143] The dispersive effect of the first dispersive element 3 and the second dispersive element 5 is schematically indicated . Longer wavelength spectral components of an optical spectrum of the optical signal 2 are diffracted by the first dispersive element at a greater diffraction angle than shorter wavelength spectral components . The first mirror 8 is arranged such that an incident angle of longer wavelength spectral components is steeper than an incident angle of shorter wavelength spectral components . Therefore , a longer wavelength spectral component will be reflected in between the first mirror 8 and the second mirror 9 more often than a shorter wavelength spectral component . Due to discrete steps in the path length at distinct wavelengths , the input signal is split in spectrally distinct sections . These sections are folded by the parallel first mirror 8 and the second mirror 9 such that they spatially overlap at the second dispersive element 5 .

[0144] The first mirror 8 and the second mirror 9 are arranged transverse to the first dispersive element 3 .

[0145] A length 10 of the second dispersive element 5 is greater than a normal distance 11 between the first mirror 8 and the second mirror 9 .

[0146] The first mirror 8 and the second mirror 9 each comprise a mirror end 8a and 9a facing the second dispersive element 5 , wherein the second dispersive element 5 extends from the mirror end 8a of the first mirror 8 to the mirror end 9a of the second mirror 9 .

[0147] A method for applying a negative dispersion to an optical signal 2 , by means of the device 1 comprises the steps :

[0148] - Receiving the optical signal 2 having an optical spectrum;

[0149] - Applying an angular chirp to the optical signal 2 by means of the first dispersive element 3 , such that each spectral component of the optical signal 2 is directed in a distinct direction;

[0150] - Increasing an optical path length of the optical signal 2 having the angular chirp by means of the first mirror 8 and the second mirror 9 , wherein the first mirror 8 is arranged parallel to the second mirror 9 ;

[0151] - Guiding the optical signal 2 onto the second dispersive element 5 downstream of the first mirror 8 and the second mirror 9 , wherein the second dispersive element 5 is parallel to the first dispersive element 3 ;

[0152] - Reflecting the optical signal 2 by means of a reflective element 7 positioned downstream of the second dispersive element 5 onto the second dispersive element 5 , such that the optical signal 2 is propagated back to the first dispersive element 3 via the first mirror 8 and the second mirror 9 ;

[0153] - Eliminating the angular chirp by means of the first dispersive element 3 ;

[0154] - Outputting the optical signal 2 having a negative dispersion .

[0155] Figure 2 shows a schematic view of the device for applying a negative dispersion 1 . The reflective element 7 is in this exemplary embodiment a roof mirror 12 with two mirror surfaces 12 a and 12b arranged perpendicular to another, such that reflected light is parallel to incoming light , wherein the reflected light is spaced apart from the incoming light due to a parallel offset introduced by the roof mirror 12 . Therefore , the optical signal 2 received by the device 1 at the input is parallel and spaced apart from the optical signal 2 at the output .

[0156] Figure 3 schematically shows another view of the device for applying a negative dispersion 1 shown in figures 1 and 2 . In particular , the effect of the roof mirror 12 can be seen .

[0157] Due to the arrangement of the first mirror 8 and the second mirror 9 , the effective path lengths of the spectral components are extended, thereby increasing the dispersion of the device . Different spectral components overlap at the second dispersive element 5 .

[0158] In this embodiment , the first dispersive element 3 and the second dispersive element 5 both are transmission gratings . Alternatively, the first dispersive element 3 and / or the second dispersive element 5 may be or may comprise a reflection grating or a prism, for example .

[0159] Figure 4 schematically shows an overview of an embodiment of an optical pulse stretcher 13 for dividing an optical input pulse 14 in temporally and spectrally separated collinear pulses , comprising :

[0160] - a spatial chirp application unit 15 ( details see fig . 5 ) for applying a spatial chirp to an optical input pulse 2 ;

[0161] - a separation unit 16 ( details see fig . 6 ) for separating the optical input pulse 142 comprising the spatial chirp in at least a first portion 17 ( see fig . 6 ) with a first spectrum, a second portion 18 ( see fig . 6 ) with a second spectrum and a third portion 19 ( see fig . 6 ) with a third spectrum, wherein the first spectrum, the second spectrum and the third spectrum are spectrally nonoverlapping;

[0162] - a positive dispersion unit 20 ( details see fig . 6 ) for applying a positive dispersion to the first portion 17 , the second portion 18 and the third portion 19 ;

[0163] - a delay stage 21 ( detail see fig . 6 ) for generating a first time delay between the first portion and the second portion, and for generating a second time delay between the second portion and the third portion;

[0164] - an alignment unit 22 ( detail see fig . 6 ) for aligning the first portion 17 , the second portion 18 and the third portion 19 collinearly along a main output axis .

[0165] Fig . 5 schematically shows the spatial chirp application unit 15 of fig . 4 in more detail . In this exemplary embodiment , the spatial chirp application unit 15 comprises a f irst dispersive element 23 and a second dispersive element 24 , wherein the first dispersive element 23 and the second dispersive element 24 are arranged parallel . Furthermore , the spatial chirp application unit 15 comprises a pair of cylindric lenses with a first cylindric lens 25a and a second cylindric lens 25b for controlling a cross -section of the optical input pulse 14 and for focusing the optical input pulse along a line . Downstream of the first dispersive element 23 and the second dispersive element 24 is a first mirror stack 26 and a second mirror stack 27 , which both comprise four essentially planar mirror surfaces , that are arranged with a predefined angle to one another .

[0166] The first cylindric lens 25a is configured to form a Fourier plane on the first mirror stack 26 in order to separate spectral components . The second cylindric lens 27 is configured to recollimate the beam with the same size as the input . The first mirror stack 26 separates the spatially chirped beam into four portions and directs these portions at a third dispersive element 28 at different angles of incidence . The spatial chirp is set by a distance between the first dispersive element 23 and the second dispersive element 24 .

[0167] The first mirror stack 26 is configured to redirect the portions of the spectrum in mutually different directions . The second mirror stack 27 is configured to direct these portions on the third dispersive element 28 of the positive dispersion unit 20 at different angles such that these portions form a straight line on the third dispersive element 28 ( i . e . , after interacting with the third dispersive element 28 ) . Note that the output of the spatial chirp application unit 15 comprises a rectangular or elliptic cross section , wherein the wavelength of the output increases from a first end to a second end of the cross section . Note that the optical input pulse 14 is inj ected from the left side as depicted in figure 5 .

[0168] Downstream of the spatial chirp application unit 15 ( i . e . , after the second cylindric lens 25b ) is a third dispersive element 28 , which is associated with the positive dispersion unit 20 , as will be discussed in more detail in connection with figure 6 .

[0169] Fig . 6 schematically shows the separation unit 16 , the positive dispersion unit 20 , and the delay stage 21 in more detail . The optical input pulse 14 is inj ected in the optical pulse stretcher 13 from the left side , in particular in the spatial chirp application unit 15 .

[0170] The separation unit 16 comprises a set of mirrors 29 with at least a first separation mirror 30 configured for receiving the first portion 17 , a second separation mirror 31 for receiving the second portion 18 and a third separation mirror 32 for receiving the third portion 19 . In this embodiment , the optical input pulse 14 is separated in four portions , therefore , the separation unit comprises a fourth separation mirror 34 configured for receiving a fourth portion 35 . The first separation mirror 30 , the second separation mirror 31 , the third separation mirror 32 and the fourth separation mirror 34 are parallel roof mirrors in this exemplary embodiment . The separation mirrors ( i . e . , the first separation mirror 30 , the second separation mirror 31 , the third separation mirror 32 and the fourth separation mirror 34 ) are staggered and displaced in a direction normal to the figure plane of fig . 6 , such that each separation mirror intersects the corresponding portion .

[0171] The positive dispersion unit 20 comprises :

[0172] - the third dispersive element 28 configured for applying an angular chirp to the optical input pulse 14 ;

[0173] - a focusing lens 36 , wherein the focusing lens 36 is configured for receiving the first portion 17 reflected off the first separation mirror 30 , the second portion 18 reflected off the second separation mirror 31 , and the third portion 19 reflected off the third separation mirror 32 , wherein the first portion 17 , the second portion 18 and the third portion 19 comprise the angular chirp . In addition, the focusing lens 36 is configured for receiving the fourth portion 35 reflected off the fourth separation mirror 34 . The number of portions in which the optical input pulse 14 is separated can be adj usted by providing the respective number of separation mirrors in a respective arrangement and by adj usting the number of mirror surfaces of the first mirror stack 26 and the second mirror stack 27 accordingly .

[0174] Furthermore , the positive dispersion unit 20 comprises a roof mirror 37 downstream of the focusing lens 36 , wherein the roof mirror 37 is configured to receive the first portion 17 , the second portion 18 and the third portion 19 as well as the fourth portion 35 , and configured to redirect the first portion 17 , the second portion 18 and the third portion 19 as well as the fourth portion 35 towards the focusing lens 36 , wherein a distance between the roof mirror 37 and the focusing lens 36 is equal to the focal length of the focusing lens 36 .

[0175] The dispersion of each of the portions 17 , 18 , 19 and 35 can be controlled by adj usting the respective path lengths in between the third dispersive element 28 and the focusing lens 36 , in this case by means of adj usting a position of the first 30 , second 31 , third 32 and fourth separation mirror 34 . The smaller the path length between the third dispersive element 28 and the focusing lens , the greater the positive dispersion ( i . e . , the greater the group delay) . The optical path length of each portion 17 , 18 , 19 and 35 in between the third dispersive element 28 and the focusing lens 36 can be adj usted independently . Therefore , each of the portions is subj ected to a distinct positive dispersion ( i . e . , positive chirp ) .

[0176] The separation unit 16 is arranged downstream of the third dispersive element 28 and upstream of the focusing lens 36 ( i . e . , in between the third dispersive element and the focusing lens ) . This leads to a particularly compact setup .

[0177] Downstream of the positive dispersion unit 20 and the separation unit 16 is the delay stage 21 . The delay stage 21 comprises at least a first delay mirror 38 for receiving the first portion 17 , a second delay mirror 39 for receiving the second portion 18 and a third delay mirror 40 for receiving the third portion 19 , wherein a distance between the first delay mirror 38 and the second delay mirror 39 is adj ustable to adj ust the first time delay, wherein a distance between the second delay mirror 39 and the third delay mirror 40 is adj ustable to adj ust the second time delay .

[0178] In this exemplary embodiment , the delay stage 21 comprises a fourth delay mirror 41 for receiving the fourth portion 35 . By adj usting a distance between the fourth delay mirror 41 and the third delay mirror 40 , a third time delay between the third portion 19 and the fourth portion 35 can be adj usted .

[0179] In this exemplary embodiment , the delay stage 21 directs the portions 17 , 18 , 19 and 35 back to the third dispersive element 28 , such that the portions 17 , 18 , 19 and 35 have another round-trip in the positive dispersion unit 20 and the separation unit 16 in reversed direction . This way, the portions 17 , 18 , 19 and 35 pick up further positive dispersion (positive chirp ) and are guided back towards the spatial chirp application unit 15 . When inj ected with a signal with a spatial chirp at the output , the spatial chirp application unit 15 eliminates the spatial chirp . In other words , the spatial chirp application unit 15 is in this particularly compact embodiment also utilized as an alignment unit 22 ( details of the spatial chirp application unit 15 see fig . 5 ) for aligning the first portion 17 , the second portion 18 and the third portion 19 ( as well as the fourth portion 35 ) collinearly along a main output axis .

[0180] Overall , the optical input pulse 14 takes a round-trip in the optical pulse stretcher 13 and interacts twice with the spatial chirp application unit 15 ( once to apply the spatial chirp and once to eliminate it again) . The first portion 35 interacts four times with the third dispersive element 28 , four times with the first separation mirror 30 , four times with the focusing lens 36 , two times with the roof mirror 37 and once with the first delay mirror 38 . The same applies mutatis mutandi s to the second 18 , the third 19 and the fourth portion 35 .

[0181] The method for dividing an optical input pulse 14 in temporally and spectrally separated collinear pulses with distinct positive dispersions comprises the steps :

[0182] - Receiving the optical input pulse 14 with an input spectrum and a main direction of propagation;

[0183] - Applying a spatial chirp to the optical input pulse 14 , such that different wavelengths of the input spectrum are separated in a direction transverse to the main direction of propagation; - Separating the optical input pulse 14 having the spatial chirp in at least the first portion 17 with the first spectrum, the second portion 18 with the second spectrum and the third portion 19 with a third spectrum, wherein the first portion 17 , the second portion 18 and the third portion 19 are spatially non-overlapping , wherein the first spectrum, the second spectrum and the third spectrum are spectrally non-overlapping ;

[0184] - In this exemplary embodiment : Using the first mirror stack 26 and the second mirror stack 27 for directing portions 17 , 18 and 19 at the third dispersive element 28 at different angles such that the main propagation axes of the portions 17 , 18 and 19 are collinear upon ( i . e . , after ) diffraction on the third dispersive element 28 ;

[0185] - Applying a first positive dispersion to the first portion 17 , a second positive dispersion to the second portion 18 and a third positive dispersion to the third portion 19 ;

[0186] - Adj usting a first time delay between the first portion 17 and the second portion 18 , and a second time delay between the second portion 18 and the third portion 19 ;

[0187] - Aligning the first portion 17 , the second portion 18 and the third portion 19 collinearly along a main output axis ;

[0188] - Outputting a train of temporally and spectrally separated pulses , wherein the first portion 17 with the first spectrum and the first positive dispersion forms a first pulse of the train, wherein the second portion 18 with the second spectrum and the second positive dispersion forms a second pulse , wherein the third portion 19 with the third spectrum and the third positive dispersion forms a third pulse .

[0189] In this exemplary embodiment the optical input pulse 14 is separated in four portions , consequently the train of temporally and spectrally separated pulses comprises a fourth pulse , wherein the fourth portion 35 with a fourth spectrum and a fourth dispersion forms the fourth pulse . The optical input pulse 14 can be separated in any number of portions as needed in the specific application .

[0190] Note that this exemplary embodiment utilizes the same components for all separated portions , wherever possible and / or applicable ( e . g . , there is only one focusing lens ) . This leads to a compact and efficient setup . However , after separating the portions , each portion could in principle be processed individually with individual components ( e . g . , with an individual focusing lens per portion etc . ) .

[0191] Fig . 7 schematically shows the separation unit of figures 4 and 6 in more detail in a perspective view . The first separation mirror 30 comprises a first width in z-direction 43 , the second separation mirror 31 comprises a second width in z -direction 43 , the third separation mirror 32 comprises a third width and the fourth separation mirror comprises a fourth width . The first width is smaller than the second width, the second width is smaller than the third width, and the third width is smaller than the fourth width . The optical input pulse 14 is inj ected in the separation unit 16 at an upper level with respect to the z -direction 43 . The roof mirror 37 ( see fig . 6 ) is configured to effect an offset in z -direction 43 , such that the separated first 17 , second 18 , third 19 and fourth 35 portions then interact with the separation unit 16 on a lower level ( lower in z-direction 43 and relative to the upper level ) . The delay stage 21 essentially reverts the direction of the portions , such that the portions are ( after interacting with the delay stage ) propagated through the lower level and are reverted by the roof mirror 27 to the upper level of the separation unit 16 , such that the portions are finally re-inj ected in the spatial chirp application unit 15 ( then acting as alignment unit 22 ) to eliminate the spatial chirp and to align the portions collinearly along a main output axis . The main output axis coincides with a main input axis .

[0192] Fig . 8 schematically shows an embodiment similar to the embodiment of figures 4 to 7 and with added annotations as commonly used in the field of ultrafast optics .

[0193] Fig . 9 schematically shows a comparison of group delay introduced by a spectrally divided compressor scheme and a traditional Treacy pair .

[0194] High saturation fluence of Yb3H — doped crystals provides robust average power scaling in cw-pumped solid-state amplifiers but complicates energy extraction because of high intensity in the laser crystal . Typically, for higher output energy under cw pumping , the beam spot size in the crystal is expanded and the average output power and slope efficiency are sacrificed . Recently, a cw-pumped 130-fs Yb : CALGO regenerative amplifier was developed that delivers flat output power of >115 W in a wide range of repetition rates from 10 kHz to 100 kHz . At the energy of ~11 mJ at 10 kHz , crystal damage occurs for pulses chirped to FWHM of ~0 . 5 ns . A possible dispersion management solution is presented , summarized in Fig . 9 , which answers the challenge of further energy scaling without changing the working aperture of the stretcher and compressor gratings , ensures mechanical robustness and compactness and does not introduce additional diffraction losses in the compressor grating pair .

[0195] Fig . 9 shows a comparison of group delays introduced by spectrally divided compressor scheme ( solid curve ) and a traditional Treacy compressor ( dashed curves ) . a ) Superposed group delay of the corresponding compressors , b ) Assumed pulse spectrum centered at 1035 nm. c ) Temporal profile of the pulse inside the regenerative amplifier . The spectral division scheme generates a chirped pulse train where each spectral segment is stretched to a similar duration as in the case of the standard scheme represented by dashed curves . In both cases , identical dispersive power and working apertures of compressor and stretcher diffraction gratings are used . ( Diffraction groove period 571 nm, Littrow angle 65 ° at 1035 nm, corresponding to industry' s standard for broadband Yb lasers . ) The effective illuminated aperture of the large grating is 10 cm, corresponding in the case of the classical Treacy pair to 40 cm separation between the gratings and to 155 cm in the case of the zigzag mirror fold . Note different group delay slopes (GDD ) due to the variation of the effective inter-grating separation for each spectral segment .

[0196] Traditionally, the practical stretching and compression ratio in fs CPA is limited by the width of the diffraction gratings and associated with the prohibitive cost , substrate translation range in ion / electron beam machines , etc .

[0197] Doubling dispersive power by double-passing the compressor is not advisable as it adds another four bounces on the diffraction gratings and squares the diffraction losses .

[0198] As seen, for example , in Figures 1 , 2 and 3 , a very compact compressor can be obtained by a zigzag fold on a pair of parallel mirrors ( the first mirror 8 and the second mirror 9 ) . By running a different integer number of reflection bounces on the mirror fold, different spectral segments overlap spatially on the same aperture of the larger grating . This scheme corresponds to a tiled grating geometry, except that all spectral segments are intercepted by the same grating instead of physically tiling. The footprint is 25x13.5 cm. Equivalent geometries with reflection gratings and a variable number of resultant spectral segments are also easily obtained. Note that reflection losses on the mirror fold (first mirror 8 and second mirror 9) are negligible compared to diffraction losses.

[0199] Parallelism of the mirror pair is critical to eliminate spatial chirp as unequal amounts of bounces on nonparallel mirrors 8 and 9 would change incidence angles on the second grating 6.

[0200] Fig. 8 shows ray tracing of a matching positive dispersion stretcher (i.e., the optical pulse stretcher 13) . (a) Top view, (b) isometric projection showing beam folding, (c) closeup on the spectral divider (the spatial chirp application unit) implemented in the Fourier plane of an auxiliary perpendicular small diffraction grating pair (DG1,2) placed between a pair cylinder-lenses CL1,2. Spectral segments are picked off in the Fourier plane F on a segmented mirror stack MSI and recombined on the main grating DG3 (the third dispersive element) with a mirror stack MS2. The only elements that are not in common path are roof mirrors RM1-4 for GDD control and delay mirrors DM1-4. The Fourier plane of the main cylinder lens CL3 with f=F3 is the symmetry plane of RM. Note that RM1-4 have different heights to intercept beams at the upper and lower levels. For presentation clarity, CL3— RM leg is shown unfolded. Additional polarization control elements (Faraday rotator, in- and outcoupling TFP and a X / 2 waveplate before DG3 ) are omitted in the schematic.

[0201] According to the damage scaling on the stretched pulse duration in the ^’"'“regime combined with pulse division into a train, the damagelimited energy of the amplifier can be increased by at least a factor of 3 after adopting the proposed spectral division scheme.

[0202] Figure 10 schematically shows a chirped pulse amplification device 44 for amplifying an optical input pulse 14 with an optical pulse stretcher 13 (see figures 4-7) , an amplification unit 45 for amplifying optical pulses, and a device 1 for applying a negative dispersion (see figures 1-3) , which acts as a compressor.

[0203] The optical input pulse 14 is injected into chirped pulse amplification device 44, in particular into the optical pulse stretcher 13 from the left side. Accordingly, an amplified optical pulse 46 exits the chirped pulse amplification device 44 at the right side of the schematic .

[0204] A method for chirped pulse amplification comprises the steps :

[0205] - Providing an optical input pulse 14 ;

[0206] - Stretching the optical input pulse 14 by means of method for dividing an optical input pulse in temporally and spectrally separated collinear pulses with distinct positive dispersions , in this embodiment with an optical pulse stretcher ;

[0207] - Amplifying the first pulse , the second pulse and the third pulse ; and

[0208] - Compressing the amplified first pulse , second pulse and third pulse by means of the method for applying a negative dispersion, in this exemplary embodiment by means of the device 1 for applying a negative dispersion .

[0209] The first positive dispersion, the second positive dispersion the third positive dispersion ( as well as the fourth positive dispersion ) , as well as the first time delay and the second time delay ( and additionally the third time delay) are adj usted such during stretching that the compressed amplified first pulse , second pulse and third pulse are combined to a single optical pulse upon compressing . The optical pulse stretcher 13 and the compressor are adj usted to precisely counteract each other .

Claims

Claims :

1. Device (1) for applying a negative dispersion to an optical signal (2) , comprising:- a first dispersive element (3) , in particular a first grating (4) , configured for receiving an optical signal (2) ;- a second dispersive element (5) , in particular a second grating (6) , wherein the second dispersive element (5) is arranged parallel to the first dispersive element (3) and downstream of the first dispersive element (3) ;- a reflective element (7) downstream of the second dispersive element (5) configured for receiving the optical signal (2) from the second dispersive element (5) and configured for reflecting the optical signal (2) onto the second dispersive element (5) ; wherein the first dispersive element (3) , the second dispersive element (5) and the reflective element (7) are preferably configured in a Treacy compressor configuration; characterized by- a first mirror (8) and a second mirror (9) arranged downstream the first dispersive element (3) and upstream of the second dispersive element (5) , wherein the first mirror (8) and the second mirror (9) are parallel to each other and are configured for increasing an optical path length of the optical signal (2) .

2. Device (1) according to claim 1, characterized in that the first mirror (8) and the second mirror (9) are arranged transverse to the first dispersive element (3) .

3. Device (1) according to claim 1 or claim 2, characterized in that a length of the second dispersive element (5) is greater than a normal distance between the first mirror (8) and the second mirror (9) .

4. Device (1) according to claim 3, characterized in that the first mirror (8) and the second mirror (9) each comprise a mirror end facing the second dispersive element (5) , wherein the second dispersive element (5) extends from the mirror end (8a) of the first mirror (8) to the mirror end (9a) of the second mirror (9) .

5. Method for applying a negative dispersion to an optical signal (2) , preferably by means of a device (1) according to anyone of claims 1 to 4, with the steps:- Receiving an optical signal (2) having an optical spectrum;- Applying an angular chirp to the optical signal (2) by means of a first dispersive element (3) , such that each spectral component of the optical signal (2) is directed in a distinct direction;- Increasing an optical path length of the optical signal (2) having the angular chirp by means of a first mirror (8) and a second mirror (9) , wherein the first mirror (8) is arranged parallel to the second mirror (9) ;- Guiding the optical signal (2) onto a second dispersive element (5) downstream of the first mirror (8) and the second mirror (9) , wherein the second dispersive element (5) is parallel to the first dispersive element (3) ;- Reflecting the optical signal (2) by means of a reflective element (7) positioned downstream of the second dispersive element (5) onto the second dispersive element (5) , such that the optical signal (2) is propagated back to the first dispersive element (3) via the first mirror (8) and the second mirror (9) ;- Eliminating the angular chirp by means of the first dispersive element (3) ;- Outputting the optical signal (2) having a negative dispersion .

6. Method for dividing an optical input pulse (14) in temporally and spectrally separated collinear pulses with distinct positive dispersions with the steps :- Receiving an optical input pulse (14) with an input spectrum and a main direction of propagation;- Applying a spatial chirp to the optical input pulse (14) , such that different wavelengths of the input spectrum are separated in a direction transverse to the main direction of propagation;- Separating the optical input pulse (14) having the spatial chirp in at least a first portion (17) with a first spectrum, a second portion (18) with a second spectrum and a third portion (19) with a third spectrum, wherein the first portion (17) , the second portion (18) and the third portion (19) are spatially nonoverlapping, wherein the first spectrum, the second spectrum and the third spectrum are spectrally non-overlapping;- Applying a first positive dispersion to the first portion (17) , a second positive dispersion to the second portion (18) and a third positive dispersion to the third portion (19) ;- Adjusting a first time delay between the first portion (17) and the second portion (18) , and a second time delay between the second portion (18) and the third portion (19) ;- Aligning the first portion (17) , the second portion (18) and the third portion (19) collinearly along a main output axis;- Outputting a train of temporally and spectrally separated pulses, wherein the first portion (17) with the first spectrum and the first positive dispersion forms a first pulse of the train, wherein the second portion (18) with the second spectrum and the second positive dispersion forms a second pulse, wherein the third portion (19) with the third spectrum and the third positive dispersion forms a third pulse.

7. An optical pulse stretcher (13) for dividing an optical input pulse (14) in temporally and spectrally separated collinear pulses, comprising :- a spatial chirp application unit (15) for applying a spatial chirp to an optical input pulse (14) ;- a separation unit (16) for separating the optical input pulse (14) comprising the spatial chirp in at least a first portion (17) with a first spectrum, a second portion (18) with a second spectrum and a third portion (19) with a third spectrum, wherein the first spectrum, the second spectrum and the third spectrum are spectrally non- over lapping;- a positive dispersion unit (20) for applying a positive dispersion to the first portion (17) , the second portion (18) and the third portion (19) ;- a delay stage (21) for generating a first time delay between the first portion (17) and the second portion (18) , and for generating a second time delay between the second portion (18) and the third portion (19) ;- an alignment unit (22) for aligning the first portion (17) , the second portion (18) and the third portion (19) collinearly along a main output axis .

8. Optical pulse stretcher (13) according to claim 7, characterized in that the spatial chirp application unit (15) comprises a first dispersive element (23) and a second dispersive element (24) , wherein the first dispersive element (23) and the second dispersive element (24) are arranged parallel.

9. Optical pulse stretcher (13) according to claim 7 or claim 8, characterized in that the separation unit (16) comprises a set of mirrors with at least a first separation mirror (30) configured for receiving the first portion (17) , a second separation mirror (31) for receiving the second portion (18) and a third separation mirror (32) for receiving the third portion (19) , wherein the first separation mirror (30) , the second separation mirror (31) and the third separation mirror (32) are preferably roof mirrors, in particular parallel roof mirrors .

10. Optical pulse stretcher (13) according to anyone of claims 7 to 9, characterized in that the positive dispersion unit (20) comprises :- a third dispersive element (28) configured for applying an angular chirp to the optical input pulse (14) ;- a focusing lens (36) , wherein the focusing lens (36) is configured for receiving the first portion (17) , preferably reflected off the first separation mirror (30) , the second portion (18) , preferably reflected off the second separation mirror (31) , and the third portion (18) , preferably reflected off the third separation (32) mirror, wherein the first portion (17) , the second portion (18) and the third portion (19) comprise the angular chirp; and preferably- a roof mirror (37) downstream of the focusing lens (36) , wherein the roof mirror (37) is configured to receive the first portion (17) , the second portion (18) and the third portion (19) , and configured to redirect the first portion (17) , the second portion (18) and the third portion (19) towards the focusing lens (36) , wherein a distance between the roof mirror (37) and the focusing lens (36) is equal to the focal length of the focusing lens (36) .

11. Optical pulse stretcher (13) according to anyone of claims 7 to 10, characterized in that the delay stage (21) comprises at least a first delay mirror (38) for receiving the first portion (17) , a second delay mirror (39) for receiving the second portion (18) and a third delay mirror (40) for receiving the third portion (19) , wherein a distance between the first delay mirror (38) and the second delay mirror (39) are adjustable to adjust the first time delay, wherein a distance between the second delay mirror (39) andthe third delay mirror (40) is adjustable to adjust the second time delay.

12. Method for chirped pulse amplification with the steps:- Providing an optical input pulse (14) ;- Stretching the optical input pulse (14) by means of method for dividing an optical input pulse in temporally and spectrally separated collinear pulses with distinct positive dispersions according to claim 6, in particular with an optical pulse stretcher (13) according to anyone of claims 7 to 11;- Amplifying the first pulse, the second pulse and the third pulse; and- Compressing the amplified first pulse, second pulse and third pulse by means of the method for applying a negative dispersion according to claim 5, preferably by means of a device (1) for applying a negative dispersion according to anyone of claims 1 to 4.

13. Method according to claim 12, characterized in that the first positive dispersion, the second positive dispersion, the third positive dispersion, as well as the first time delay and the second time delay are adjusted such during stretching that the compressed amplified first pulse, second pulse and third pulse are combined to a single optical pulse upon compressing.

14. Chirped pulse amplification device (44) for amplifying an optical input pulse (14) with an optical pulse stretcher (13) according to claim 7, an amplification unit (45) for amplifying optical pulses, and a device (1) for applying a negative dispersion according to anyone of claims 1 to 4.

Images