EP4371197A1

EP4371197A1 - Pulse modification apparatus comprising a passive conversion device for compensating for environmental influences

Info

Publication number: EP4371197A1
Application number: EP22734533.7A
Authority: EP
Inventors: Andreas Enzmann
Original assignee: Trumpf Laser GmbH
Current assignee: Trumpf Laser GmbH
Priority date: 2021-07-12
Filing date: 2022-06-03
Publication date: 2024-05-22
Also published as: KR20240019844A; US20240154377A1; DE102021207332A1; CN117693872A; WO2023285028A1

Abstract

The invention relates to a pulse modification apparatus (1) in the form of a pulse stretching apparatus for the dispersive stretching of laser pulses (2) or a pulse compression apparatus for the dispersive compression of laser pulses (2), comprising at least one dispersive optical element (3, 3') for angle separation and the combination of spectral components (7) of the laser pulses (2), and a tuning device (4') for tuning the dispersion of the pulse modification apparatus (1) by influencing the spectral components (7). The pulse modification apparatus (1) further comprises at least one passive sensor element (5) having an output variable (A) that depends on at least one environmental parameter (U), and a passive conversion device (6) for converting a change (dA) in the output variable provided by the at least one passive sensor element (5) into a change (dS) in the tuning variable for the tuning device (4') in order to compensate for a change in the dispersion of the pulse modification apparatus (1) and/or at least one additional pulse stretching apparatus (25) and/or at least one additional pulse compression apparatus (26), said change in dispersion resulting from a change in the at least one environmental parameter (U).

Description

The invention relates to a pulse modification device in the form of a pulse stretching device for dispersive stretching of laser pulses or a pulse compression device for dispersive compression of laser pulses. The invention also relates to a chirped pulse amplification system. Laser pulses, in particular ultra-short laser pulses, ie laser pulses with pulse durations in the picosecond range and below, are used in numerous areas of technology, for example in material processing including laser welding and laser cutting. An important variable in the description of laser pulses is the phase of the electrical field of the laser pulses in the frequency domain so-called spectral phase. A distinction can thus be made between unchirped laser pulses, which have a spectral phase that is constant or linearly dependent on the frequency, and chirped laser pulses, whose spectral phase has a more complex frequency dependency. To put it simply, in a chirped laser pulse certain spectral components, for example lower-frequency ones, precede other spectral components, for example higher-frequency ones. Unchirped laser pulses are characterized by a minimal pulse duration for a given spectral width.

The properties of laser pulses can be influenced in a targeted manner by means of devices that have different effects on the individual spectral components of the laser pulses, which are therefore dispersive, in particular by means of devices that change the spectral phase. High pulse qualities, short pulse durations and high pulse intensities are generally desirable. Of particular importance are pulse stretching devices for dispersive stretching of laser pulses and pulse compression devices for dispersive compression of laser pulses. Unchirped and chirped laser pulses can be time-stretched using pulse stretching devices. The resulting time-stretched laser pulses are then chirped or more chirped. Chirped laser pulses can be temporally compressed using pulse compression devices. The resulting temporally compressed laser pulses are then chirped or unchirped to a lesser extent.

Pulse stretching and pulse compression devices are often used in combination. For example, at least one pulse stretching device, at least one pulse compression device and a pulse amplification device are integrated in a so-called chirped pulse amplification system. In combination with a seed laser, a chirped pulse amplification system is used to generate ultra-short laser pulses with the highest pulse intensities. Laser pulses of the seed laser, for example a fiber laser, are first stretched in time by means of the at least one pulse stretching device. The stretched laser pulses are then amplified in the amplification device, for example in a fiber amplifier. After the amplification, the amplified laser pulses are time-compressed again by means of the at least one pulse compression device. Without the time extension before amplification would result in the high intensity and the associated non-linear effects damage or destroy the medium of the amplification device and the pulse properties are impaired. Further details on the structure of ultrashort pulse lasers with a seed laser and a chirped pulse amplification system can be found, for example, in the article "Pulsed Lasers for Industrial Applications" by F. Jansen et al., Laser Technik Journal 2, 46 (2018) .

Pulse stretching and pulse compression devices are often designed as free-radiation devices, which means that the laser pulses or their spectral components propagate at least partially through air or another gas atmosphere. This is particularly advantageous when high intensities occur and/or high stretching factors are achieved must. This design includes grating and prism stretchers or grating and prism compressors, in which at least one grating or prism is used to separate and combine the individual spectral components. The separated spectral components have different propagation times in the stretchers or compressors before they are brought together again, which leads to the desired stretching or compression over time.

The dispersion of a pulse stretching or pulse compression device can be described mathematically via the accumulated spectral phase, f(w), of the laser pulses during propagation through the pulse stretching or pulse compression device. The pulse stretching or pulse compression device is typically characterized via the coefficients β of a Taylor expansion in the angular frequency, w, around the center frequency, w ₀ , of the laser pulses. Of particular importance is the group delay dispersion, ß ₂ , which describes the temporal divergence and convergence of the laser pulses in the lowest order. In the case of highly dispersive pulse stretching and pulse compression devices, however, the higher orders, in particular third-order dispersion, β ₃ , also play a role.

In the prior art, there are also devices for adjusting the dispersion of a pulse stretching and/or pulse compression device or a device described with a pulse stretching and / or a pulse compression device. Accurate adjustment of the dispersion is important in order to achieve the highest possible pulse quality, in particular the shortest possible pulse duration. In particular, this can also be used to compensate for changes in the environment that affect the pulse stretching and/or pulse compression device. The adjustment of the dispersion is based on a measurement of at least one pulse characteristic of the laser pulses.

For example, EP 3578287 A1 describes a laser system with a laser pulse source and a dispersion adjustment unit for pulse stretching or pulse compression of laser pulses with an arrangement with at least one dispersive element for generating angular dispersion and an optical unit arranged in the angular dispersion range. The optical unit comprises a plane-parallel optical plate that transmits the laser pulses and causes a parallel offset of the individual spectral components of the laser pulses that depends on the angle of incidence. A rotation of the optical disk affects the dispersion properties of the dispersion adjustment unit. In particular, the pulse duration of the laser pulses in an output beam of the laser system can be adjusted by rotating the optical disk. In an example, the laser system further includes a pulse duration measuring device and a control unit. The pulse duration measuring device is used to output a pulse duration-dependent measurement signal to the control unit. The control unit is used to control an angle setting device for setting an angular position of the optical disk as a function of the pulse duration measurement.

In US Pat. No. 7,822,347, a chirped pulse amplification system with a pulse generator, a pulse stretcher, a pulse amplifier and a pulse compressor, as well as an adjustment element and a pulse measuring device is also described. The setting element is suitable for setting the group velocity dispersion of the chirped pulse amplification system and for regulating the pulse duration of the compressed pulses. The adjustment element can either be used to adjust the dispersion of one of the existing elements of the chirped pulse amplification system, for example the pulse stretcher or the pulse compressor, or it is an additional dispersive element in the chirped pulse amplification system. The pulse measuring device is suitable for at least one pulse characteristic of the to measure compressed pulses, for example via multiphoton detection, an autocorrela tor or via a FROG (Frequency-Resolved Optical Gating) system. The setting element is also designed to react to an output signal from the pulse measuring device. In one example, the adjustment element serves to compensate for changes in dispersion that are due to changes in the environment or the optical path length of free-space elements. In further examples, the adjustment element is designed to apply a temperature gradient or an elongation gradient to a fiber Bragg grating.

However, known heart rate measuring devices, angle adjustment devices and control units, including those mentioned above, usually have a relatively complex structure and are relatively expensive.

In contrast, the object of the invention is to provide a pulse modification device in the form of a pulse stretching device or a pulse compression device, in which the exiting laser pulses have constant pulse properties independently of at least one environmental parameter and which is also characterized by a simple and inexpensive design.

According to a first aspect, this object is achieved by a pulse modification device in the form of a pulse stretching device for dispersive stretching of laser pulses or a pulse compression device for dispersive compression of laser pulses with at least one dispersive optical element for angle separation and combination of spectral components of the laser pulses and a control device for setting the Dispersion of the pulse modification device by influencing the spectral components and at least one passive sensor element with an output variable that depends on at least one environmental parameter, and a passive conversion device for converting an output variable change of the at least one passive sensor element into a manipulated variable change of the actuating device in order to change the dispersion of the pulse modification device and/or at least one further pulse stretching device and/or at least one further pulse compensating compression device resulting from a change in the at least one environmental parameter. As a result of this compensation, the emerging laser pulses have pulse properties that remain the same, regardless of the at least one environmental parameter.

If, in addition to the pulse modification device, the laser pulses also propagate through one or more other pulse stretching devices and/or one or more other pulse compression devices, instead of or in addition to changing the dispersion of the pulse modification device, the dispersion of the or at least one of the several other pulse stretching devices and/or the at least or at least one of the several further pulse compression devices are compensated. An exemplary application is chirped pulse amplification systems. The pulse modification device is preferably designed to compensate for the cumulative dispersion. The additional pulse stretching and/or pulse compression devices can be arranged in the beam path of the laser pulses before and/or after the pulse modification device.

The pulse modification device is designed as a free jet device. In the case of free-jet pulse stretching devices and free-jet pulse compression devices, a change in the at least one environmental parameter leads in particular to a change in the refractive index of the air or the gas atmosphere, which in turn leads to a change in the dispersion.

The at least one dispersive optical element is, for example, at least one diffraction grating or at least one prism. The separated spectral components of the laser pulses have different propagation times before they are recombined, which leads to the desired temporal compression or temporal stretching of the laser pulses.

In the case of a single passive sensor element, this can show a change in output size when only one environmental parameter changes or when several environmental parameters change. In the case of several passive sensor elements, this applies correspondingly to each of the passive sensor elements. The passive conversion device converts the change in the output variable into a change in the manipulated variable, with the change in the manipulated variable brought about leading to a compensation for the change in the dispersion that results from the change in the at least one environmental parameter.

A passive conversion device is understood here in particular as a conversion device that does not require any active elements, for example no motor, no additional energy sources and no electronics. This applies accordingly to the passive sensor element. As a result, the pulse modification device is relatively simple and inexpensive to maintain and easy to maintain or even maintenance-free.

In one embodiment, the adjusting device comprises a plane-parallel transmissive optical element, which is arranged in such a way that the angle-separated spectral components of the laser pulses pass through it and experience a parallel offset dependent on the angle of incidence, with the dispersion of the pulse modification device being controlled by a rotation of the plane-parallel transmissive optical element is adjustable and the manipulated variable change corresponds to a rotation angle of the plane-parallel transmissive optical element. The plane-parallel transmissive optical element can be a small glass plate, for example. The rotation of the plane-parallel transmissive optical element around the angle of rotation changes the beam path of the spectral components, which affects the dispersion of the pulse modification device. Details on setting the dispersion by means of a rotation of the plane-parallel transmissive optical element can be found in EP 3578287 A1, the content of which is hereby fully incorporated into the present application. In particular, the exact connection between the rotation angle and the group delay dispersion is set out there.

Alternatively, the adjusting device can also include a multi-wedge arrangement, in particular a double-wedge arrangement, which is arranged in such a way that the angularly separated spectral components pass through it, with the dispersion of the pulse modification device being adjustable by displacing at least one wedge of the multi-wedge arrangement and/or rotating the multi-wedge arrangement is. In egg ner double wedge arrangement or multi-wedge arrangement is an optical arrangement of two or more than two wedges. Wedges are understood here to mean wedge-shaped transmissive optical elements. In this case, the change in manipulated variable is typically a displacement path of the at least one wedge and/or an angle of rotation of the multi-wedge arrangement. In particular, a thickness of the double wedge arrangement can be varied by shifting a wedge of a double wedge arrangement. This leads to an adjustability of the dispersion of the pulse modification device, since the spectral components experience a beam displacement dependent on the angle of incidence when passing through the double wedge arrangement, which is dependent on the thickness of the double wedge arrangement. Further details on setting the dispersion of a pulse stretching or pulse compression device by means of such multi-wedge arrangements can also be found in EP 3578287 A1.

In another embodiment, the output of the passive sensing element or at least one of the passive sensing elements is a length. The length is understood here as the dimension of the passive sensor element or a part of the passive sensor element along any spatial direction. Alternatively or additionally, the output variable can also be an orientation of the passive sensor element or part of the passive sensor element. In particular, the output size change can result in a translation or a rotation or a superposition of a translation and a rotation.

In a further embodiment, at least two of the passive sensor elements are connected to one another in series, as a result of which their output variable changes, in particular their length changes, add up. For example, passive sensor elements can be connected in series, which show a change in output when the same environmental parameter changes. By adding the changes in the output variables, in particular the changes in length, a given change in the environmental parameter results in a stronger signal, in particular in a larger mechanical stroke. The passive conversion device manages with smaller levers. The signal-to-noise ratio improves. Overall, the compensation becomes more accurate and reliable. In a further embodiment, the passive conversion device is a mechanical transmission, preferably a linkage. A mechanical transmission is a device for the transmission of movements and forces through rigid components such as gears, racks, chains or belts. In particular, the passive conversion device can also include a so-called transmission chain. A linkage is a device consisting of two or more rods connected to one another via joints or rods, which is used to transmit forces and movements.

In a further development of this embodiment, at least one joint of the mechanical transmission or linkage is a flexure joint. Solid joints have the advantage of being friction-free or extremely low-friction. The output variable change can thus be converted more reliably and more precisely into the corresponding manipulated variable change, in particular into the rotation of the plane-parallel transmissive optical element by the corresponding angle of rotation. The typically limited range of motion of flexure joints does not represent a relevant limitation for the present application.

In a further embodiment, the passive conversion device is designed according to an experimentally determined calibration curve, which sets the change in the at least one environmental parameter in relation to the change in the dispersion of the pulse modification device and/or the at least one additional pulse stretching device and/or the at least one additional pulse compression device . From such a calibration curve and the known connection between the change in the manipulated variable, in particular the angle of rotation, and the resulting adaptation of the dispersion, there is a relationship between the change in the at least one environmental parameter and the value of the change in the manipulated variable, which is used to compensate for the change in the dispersion of the Pulse modification device leads.

In a further embodiment, the passive conversion means is according to a mathematical relationship of the form dß ₂ = F (ß ₃ , ß ₄ ß _m , w ₀ , dn) between a change, dn, of a refractive index within the pulse modification device and/or within the at least one additional pulse stretching device and/or within the at least one additional pulse compression device, which results from the change in the at least one environmental parameter, and the change caused thereby, dß ₂ , the group delay dispersion and the higher-order dispersion (ß ₃ , ß ₄ ß _m ) and the central frequency (w ₀ ) of the laser pulses (2) of the pulse modification device. The inventors have found that in the case of pulse modification devices in the form of free-radiation devices, the change in the group delay dispersion can be approximately described using mathematical relationships of the form specified. A relationship between the change in the at least one environmental parameter and the change in the group delay dispersion now results from a known relationship between the at least one environmental parameter and the refractive index, in the case of air, for example, from the Edlen formula. Since the connection between the change in the manipulated variable, in particular the angle of rotation of the plane-parallel transmissive optical element, and the change in the group delay dispersion is known, there is an overall relationship between the change in the at least one environmental parameter and the change in the manipulated variable leading to compensation. A corresponding design of the passive conversion device allows extensive or even complete compensation for the change in the dispersion, without the pulse duration or another pulse property having to be measured.

In a development of this embodiment, the mathematical relationship is the following: dß ₂ = b ₃ w ₀ άh. The mathematical relationship given describes, to at least a good approximation, the change in group delay dispersion for a given change in refractive index in many cases. Of the higher orders of dispersion, only the third-order dispersion, ß ₃ , is included in the mathematical relationship.

In a further embodiment, the change in manipulated variable, in particular the angle of rotation of the plane-parallel transmissive optical element, is at least approximately approximately proportional to the change in output variable, in particular to the change in length, of the at least one passive sensor element. The change in the refractive index is typically proportional, at least to a good approximation, to the change in the at least one environmental parameter, for example in the case of the Edlen formula. The same applies to typical changes in the output variable, in particular changes in the length of passive sensor elements as a function of the change in the at least one environmental parameter and the change in the group delay dispersion as a function of the change in the manipulated variable, in particular the angle of rotation. Consequently, the passive conversion device should preferably be designed in such a way that it causes a change in the manipulated variable that is proportional to the change in the output variable. In particular, the passive conversion device is preferably designed such that it causes the plane-parallel transmissive optical element to rotate by an angle of rotation that is proportional to the change in length or the changes in length of the passive sensor element or passive sensor elements. The proportionality factor, which leads to compensation for the change in dispersion, also results from the relationships mentioned.

In a further embodiment, the or one of the ambient parameters is an ambient pressure. The ambient pressure is particularly dependent on the weather, which means that it is subject to constant fluctuations. The ambient pressure has a relatively strong influence on the refractive index, which means that it has a relatively strong effect on the dispersion of free-radiation devices. Compensating for changes in dispersion resulting from a change in ambient pressure is therefore particularly important.

In a further development of this embodiment, the passive sensor element or at least one of the passive sensor elements comprises a pressure load cell, preferably an absolute pressure load cell. Pressure cells typically include two interconnected membranes that deform as a function of the pressure difference.

In the case of absolute pressure cells, the space between the membranes is evacuated. This has the advantage that they are insensitive to fluctuating ambient temperatures. In a further embodiment, the or one of the environmental parameters is a temperature. A change in temperature results in a change in the refractive index. However, thermal expansion of components of the pulse modification device, for example one or more base plates, can also occur. Temperature can also affect the grating period of diffraction gratings.

In a development of this embodiment, the passive sensor element or at least one of the passive sensor elements comprises a bimetallic element or a component, in particular a rod, which has a greater coefficient of expansion than the components of the passive conversion device, in particular than the rods of the linkage. In order to compensate for changes in dispersion caused by thermal fluctuations, in particular changes in pulse duration, materials with different thermal expansion coefficients can be used in a targeted manner. For example, at least one rod of the passive conversion device can be made of invar or titanium, which are characterized by a very small coefficient of thermal expansion. The passive sensor element, on the other hand, can comprise a rod made of aluminum, for example, which exhibits a significantly larger change in length when the temperature changes.

The passive sensor element may also comprise, for example, a plate or other component which is connected to a base plate of the pulse modification device and which expands (or contracts) with a change in the ambient temperature. Such a change in the output variable can, for example, shift a pivot point of the linkage, resulting in an adjustment of the manipulated variable.

Bimetallic elements consist of two interconnected metal layers that differ in their coefficients of thermal expansion. A temperature change therefore causes the bimetallic element to bend, which causes the output variable of the passive sensor element to change.

By means of at least one passive sensor element in the form of at least one partially evacuated pressure cell, both changes in the ambient pressure and Changes in ambient temperature are compensated. The pressure inside the partially evacuated pressure cell is to be selected appropriately. In order to achieve the same mechanical stroke for a given change in ambient pressure, a larger number of pressure cells is necessary when using partially evacuated pressure cells than when using absolute pressure cells.

The object mentioned at the outset is achieved according to a further aspect of the invention by a chirped pulse amplification system for amplifying laser pulses, comprising one or more pulse stretching devices for dispersive stretching of the laser pulses, a pulse amplification device for amplifying the stretched laser pulses and one or more Pulse compression devices for the dispersive compression of the amplified laser pulses, wherein the or at least one of the pulse stretching devices and/or the or at least one of the pulse compression devices is a pulse modification device as described above. Unlike in conventional chirped pulse amplification systems, the pulse parameters of the exiting laser pulses remain constant in such a chirped pulse amplification system, independently of the at least one environmental parameter. In particular, the pulse duration of the exiting laser pulses can be kept constant even in the case of pressure fluctuations.

In principle, in order to stabilize the pulse properties, in particular the pulse duration, of the laser pulse of a chirped pulse amplification system, the distance between the dispersive optical elements can also be tracked or readjusted. However, this is more complex and expensive than the compensation via the rotation of the plan-parallel transmissive optical element or via the displacement of at least one wedge of the double wedge arrangement.

Further features and advantages of the invention result from the following description of exemplary embodiments of the invention, with reference to the figures of the drawing, which show details essential to the invention, and from the claims. The individual features can each be implemented individually or together in any combination in a variant of the invention. Exemplary embodiments are shown in the schematic drawing and are explained below. In the figures, the same reference symbols denote the same or corresponding features. Show it:

Figs. 1a, 1b schematic representations of a pulse modification device in the form of a pulse compression device with two dispersive optical elements, a control device, comprising a plane-parallel transmissive optical element, a sensor element and a passive conversion device,

Figs. 2a-2c schematic representations of variants in Figs. 1a, 1b shown passive conversion device,

flight 3 shows a schematic representation of an adjusting device with a double wedge arrangement and

4 shows a simplified schematic representation of a chirped pulse amplification system for amplifying laser pulses with a pulse compression device, which is a pulse modification device according to the invention.

The Figs. 1a and b show an example of a pulse modification device 1 in the form of a pulse compression device for the dispersive compression of laser pulses 2 with two dispersive optical elements 3, 3', an actuating device 4', a passive sensor element 5 and a passive conversion device 6.

The pulse modification device 1 is a free beam device. The two dispersive optical elements 3,3' are used for angular separation and combination of the spectral components 7 of the laser pulses 2. Alternatively, the pulse modification device 1 can also have only one or more than two dispersive optical elements 3,3'. The dispersive optical elements 3, 3' shown are diffraction gratings, but they can also be other dispersive optical elements, for example prisms.

The incoming laser pulses 2, which are chirped, first impinge on the first dispersive optical element 3, as a result of which they are separated into their spectral components 7 and propagate in different directions. Through the second dispersive optical element 3 ', the spectral components 7 are parallelized and subsequently reflected back by a reflective optical element 8 ßend. The reflective optical element 8 can be a deflection prism, for example. In this case, the spectral components 7 that are reflected back are offset in height, so that the exiting laser pulses 2′ can be easily separated from the entering laser pulses 2. The individual spectral components 7 have different propagation times in the pulse modification device 1, which leads to the desired temporal compression of the laser pulses 2.

The actuating device 4' shown comprises, for example, a plane-parallel transmissive optical element 4, which is arranged in such a way that the angle-separated spectral components 7 of the laser pulses 2 pass through it and experience a parallel offset dependent on the angle of incidence. The dispersion of the pulse modification device 1 can be adjusted via a rotation 9 of the plane-parallel transmissive optical element 4 . However, the actuating device 4' can also be designed differently.

An output variable A of the passive sensor element 5 depends on an environmental parameter U from the environment. In the example shown, but not necessarily, the output variable A is the length L of the passive sensor element 5. When the environmental parameter U changes, the sensor element 5 consequently shows a change in output variable dA in the form of a change in length dL

The passive conversion device 6 serves to convert the change in output variable dA of the passive sensor element 5 into a change in the manipulated variable dS of the actuating device 4'. The effected change in the manipulated variable dS leads to a compensation for the change in the dispersion of the pulse modification device 1, which results from the change in the environmental parameter U. In the example shown, the change in manipulated variable dS corresponds to the angle of rotation da of the plane-parallel transmissive optical element 4.

In contrast to what is shown here, the pulse modification device 1 can also have a number of passive sensor elements 5 whose output variables A depend on the environment depend on the exercise parameters U. For example, at least two of the passive sensor elements 5 can also be connected in series, whereby their output variable changes dA, in particular their length changes dL, add up. The output variables A of the passive sensor elements 5 can also depend on several environmental parameters U. The output variable A of an individual sensor element 5 can also depend on a number of ambient parameters U, for example the ambient pressure and the ambient temperature.

In the example shown, the ambient parameter U is the ambient pressure. The sensor element 5 includes a pressure cell 10 in the form of an absolute pressure cell. If the ambient pressure changes, the membranes (not shown here) of the pressure cell 10 are deformed, which causes the change in length dL of the sensor element 5 . However, the sensor element 5 does not necessarily have to include a pressure cell 10 .

The ambient pressure affects in particular the refractive index n of the air or a gas atmosphere of the pulse modification device 1, as a result of which there is an indirect change in the dispersion of the pulse modification device 1.

The compensation for this change in dispersion is explained below by way of example: FIG. 1a shows the pulse modification device 1 at nominal pressure. The plane-parallel transmissive optical element 4 is on average perpendicular to the incident spectral components 7.

1b shows the pulse modification device 1 at a lower ambient pressure. The dispersion of the pulse modification device 1 is increased and the propagation time has to be shortened. As a result of the lower ambient pressure, the pressure cell 10 expands. The sensor element 5 shows a change in length dL; its length L is greater than in Fig. 1a. The sensor element 5 pushes on the passive conversion device 6 and rotates the plane-parallel transmissive optical element 4 by an angle of rotation there, as a result of which the transit time path length is shortened via the parallel offset in such a way that the increase in the dispersion is compensated for. In the case of increased ambient pressure, the compensation takes place in the same way. In this case, the dispersion in the pulse modification device 1 decreases and the propagation time has to be increased. The sensor element 5 pulls on the passive conversion device 6 and rotates the plane-parallel transmissive optical element 4 in such a way that the running time is lengthened due to the parallel offset. Unlike what is shown here, several pressure cells 10 can also be connected in series in order to further increase the mechanical stroke.

Deviating from the example shown, the environmental parameter U or one of the environmental parameters U can also be a temperature. A change in temperature can result not only in a change in the refractive index n but also in thermal expansion of components of the pulse modification device 1 . In this case, the change in length dL of the sensor element 5 or of the sensor elements 5 can be based on a thermal expansion of a material, for example in the form of a rod. For this purpose, the material preferably has a large coefficient of thermal expansion. For example, the material can be aluminum. Alternatively or additionally, the sensor element 5 can also comprise a bimetallic element. Changes in the dispersion of the pulse modification device 1, which are caused by changes in other environmental parameters U, can also be compensated for.

In the example shown, the passive conversion device 6 is a mechanical transmission, more precisely a linkage, i.e. it has rods 11 which are connected to one another or to a base plate via joints 1212, here including a fixed bearing 12'. The plane-parallel transmissive optical element 4 is also rotatably mounted by means of a joint 12''. The plane-parallel transmissive optical element 4 is arranged on the axis of rotation. In the example shown, the passive conversion device 6 can be understood as a mechanical lever system.

In the figs. 1a and 1b, the joints 12, 12' are flexure joints, but other joints can also be involved. Solid joints have the advantage of being friction-free or extremely low-friction. In contrast to what is shown here, the passive conversion device 6 does not have to be a mechanical transmission, in particular not a linkage. For example, the passive conversion device 6 can comprise a transmission chain. In this case, the passive conversion device 6 is a mechanical transmission, but not a linkage.

The passive conversion device 6 is designed, for example, such that the manipulated variable change dS in the form of the rotation angle da of the plane-parallel transmissive optical element 4 is at least approximately proportional to the output variable change dA in the form of the length change dL of the sensor element 5 . This leads to compensation for the change in dispersion, since in the example shown the change in length dL and the change in the refractive index n are proportional to the change in the environmental parameter U and the change in dispersion is proportional to the change in the refractive index n and the angle of rotation da and also the proportionality factor of the passive conversion device 6 between the change in length dL and the angle of rotation is chosen accordingly.

In the example shown, the change in length dL proportional to the change in the ambient parameter U is due to the fact that the pressure cell 10 deforms in proportion to the change in the ambient pressure. Pressure fluctuations are thus converted into a proportional mechanical stroke. The length L of the sensor element 5 is therefore linearly dependent on the ambient pressure U. The plane-parallel transmissive optical element 4 is rotated in proportion to the change in the ambient pressure and keeps the pulse properties, in particular the pulse duration, of the exiting laser pulses 2' constant. Proportional changes in the output variable dA can also be achieved by means of other sensor elements 5 and for other ambient parameters U.

However, the change in manipulated variable dS is not necessarily proportional to the change in output variable dA of sensor element 5. In general, the passive conversion device can also be converted according to a mathematical relationship of the form dß ₂ = F (ß ₃ , ß ₄ ß _m , w ₀ , dn) between a change, dn, in the refractive index n within the pulse modification device 1, which results from the change in the at least one ambient parameter U, and the resulting change, dß _2, in the group delay dispersion, ß _2, of the at least one pulse modification device 1 and the Central frequency, w ₀ , the laser pulses 2 and the higher-order dispersion voltage, ß _3, ß ₄ ß _m, the pulse modification device 1 be designed. In particular, the mathematical relationship can be: dß ₂ = b ₃ w ₀ άh. In many cases, however, such mathematical relationships result in a manipulated variable change dS that is proportional to the change in output variable dA, in particular in a rotation angle da that is proportional to the change in length dL. Alternatively, the passive conversion device 6 can be designed according to an experimentally determined calibration curve, which sets the change in the at least one environmental parameter U in relation to the change in the dispersion of the pulse modification device 1 .

Unlike in Figs. 1a and 1b, the pulse modification device 1 can also be a pulse stretching device for dispersive stretching of the laser pulses 2. In this case, the pulse modification device 1 can have, for example, two lenses in addition to the two dispersive optical elements 3, 3' in order to achieve a positive dispersion.

The pulse modification device 1 can also be designed to compensate for the change in the dispersion of at least one other pulse stretching and/or pulse compression device, not shown here, which results from the change in the at least one environmental parameter U.

In the figs. 2a, 2b and 2c are variants of the one shown in Figs. 1a and 1b shown passive conversion device 6 shown schematically. Also shown are the sensor element 5 comprising the pressure cell 10 and the plane-parallel transmissive optical element 4.

The bars 13 and joints 14 shown in dashed lines each correspond to two modified arrangements of the bars 11 and joints 12,12'. With such modifications ments of the arrangement, the angle of rotation achieved can be set for a given change in length dL of the sensor element 5 . More generally, the achieved manipulated variable change dS can be adjusted for a given output variable change dA by such modifications.

In FIG. 2c, the passive conversion device 6 is also of simpler design and has only two rods 11. FIG.

In FIG. 3, an adjusting device 4' is shown schematically, which comprises a double wedge arrangement 15 with two wedges 16, 16''. The double wedge arrangement 15 is arranged in such a way that the angularly separated spectral components 7 of the laser pulses 2 pass through it. About a displacement 17 of a wedge 16 of the double wedge arrangement 15, a thickness of the double wedge arrangement 15 and thus the Disper sion of the pulse modification device 1, not shown here, is adjustable. Instead of the double wedge arrangement 15, the adjusting device 4' can also have a multi-wedge arrangement.

4 shows an example of a chirped pulse amplification system 18 for amplifying laser pulses 2 with a pulse stretching device 19, a pulse amplifying device 20 and a first pulse compression device 21 and a second pulse compression device 22 in a simplified, schematic manner.

The incoming laser pulses 2 are stretched in time by means of the pulse stretching device 19 . The stretched laser pulses 23 are then amplified in the pulse amplification device 20 . The amplified laser pulses 24 are temporally compressed by means of the first pulse compression device 21 and the second pulse compression device 22 and the compressed laser pulses 2' exit the chirped pulse amplification system 15.

The second pulse compression device 22 is a pulse modification device 1 which is designed as described above. In particular, this is a free-radiation device with an actuating device 4', at least one passive sensor element 5 and a passive conversion device 6 for compensating for changes in the dispersion of the pulse modification device 1, which result from changes in the at least one environmental parameter U. The pulse stretching device 19 and the first pulse compression device 21 are, by way of example but not necessarily, a further pulse stretching device 25 or a further pulse compression device 26, the dispersion changes of which are also reflected in the pulse modification device 1 are compensated.

Deviating from the example shown here, the chirped pulse amplification system 15 can also have more than one pulse stretching device and/or only one or more than two pulse compression devices. The pulse stretching device 19 can also be a pulse modification device 1, which is designed as described above.

Claims

patent claims

1. Pulse modification device (1) in the form of a pulse stretching device for dispersive stretching of laser pulses (2) or a pulse compression device for dispersive compression of laser pulses (2).

- at least one dispersive optical element (3.3') for angular separation and combination of spectral components (7) of the laser pulses (2) and

- an adjusting device (4') for adjusting the dispersion of the pulse modification device (1) by influencing the spectral components (7), characterized by

- At least one passive sensor element (5) with an output variable (A) which depends on at least one environmental parameter (U), and

- a passive conversion device (6) for converting an output variable change (dA) of the at least one passive sensor element (5) into a manipulated variable change (dS) of the actuating device (4') in order to change the dispersion of the pulse modification device (1) and/or compensating for at least one further pulse stretching device (25) and/or at least one further pulse compression device (26) resulting from a change in the at least one environmental parameter (U).

2. Pulse modification device according to claim 1, characterized in that the adjusting device (4') comprises a plane-parallel transmissive optical element (4) which is arranged in such a way that the angle-separated spectral components (7) of the laser pulses (2) pass through it and experience an angle-dependent parallel offset, the dispersion of the pulse mo dification device (1) being adjustable via a rotation (9) of the plane-parallel transmissive optical element (4) and the manipulated variable change (dS) a rotation angle (da) of the plane-parallel transmissive optical element

(4) corresponds.

3. Pulse modification device according to claim 1 or 2, characterized in that it is the output variable (A) of the passive sensor element

(5) or at least one of the passive sensor elements (5) is a length (L).

4. Pulse modification device according to one of the preceding claims, characterized in that at least two of the passive sensor elements (5) are connected to one another in series, as a result of which their output variable changes (dA), in particular their length changes (dL), add up.

5. Pulse modification device according to one of the preceding claims, characterized in that the passive conversion device (6) is a mechanical transmission, preferably a linkage.

6. pulse modification device according to claim 5, characterized in that at least one joint of the mechanical transmission or the linkage is a solid joint.

7. Pulse modification device according to one of the preceding claims, characterized in that the passive conversion device (6) is designed according to an experimentally determined calibration curve which shows the change in the at least one environmental parameter (U) in relation to the change in the dispersion of the pulse modification device (1) and/or or the at least one further pulse stretching device (25) and/or the at least one further pulse compression device (26).

8. Pulse modification device according to one of claims 1 to 6, characterized in that the passive conversion device (6) according to a mathematical relationship of the form dß ₂ = F (ß ₃ , ß ₄ ß _m , w ₀ , dn) between a change (dn) of a refractive index (n) within the pulse modification device (1) and/or within the at least one additional pulse stretching device (25) and/or within the at least one additional pulse compression device (26), which results from the change in the at least one Ambient parameter (U) results, and the resulting change (dß ₂ ) in the group delay dispersion (ß ₂ ) and the higher-order dispersion (ß ₃ , ß ₄ ß _m ) and the central frequency (w ₀ ) of the laser pulses (2) is designed .

9. Pulse modification device according to claim 8, characterized in that the mathematical relationship is the following: dß ₂ = b ₃ w ₀ άh.

10. Pulse modification device according to one of the preceding claims, characterized in that the change in manipulated variable (dS), in particular the angle of rotation (da) of the plane-parallel transmissive optical element (4), is at least approximately proportional to the change in output variable (dA), in particular to the change in length (dL ), The at least one passive sensor element (5) is.

11. Pulse modification device according to one of the preceding claims, characterized in that the or one of the ambient parameters (U) is an ambient pressure.

12. Pulse modification device according to claim 8, characterized in that the passive sensor element (5) or at least one of the passive sensor elements (5) comprises a pressure cell (10), preferably an absolute pressure cell.

13. Pulse modification device according to one of the preceding claims, characterized in that the or one of the environmental parameters (U) is a temperature.

14. Pulse modification device according to Claim 10, characterized in that the passive sensor element (5) or at least one of the passive sensor elements (5) comprises a bimetallic element or a component, in particular a rod, which has a larger coefficient of expansion than the components of the passive conversion device (6 ), especially as the rods of the linkage.

15. Chirped pulse amplification system (18) for amplifying laser pulses (2), comprising: - one or more pulse stretching devices (19) for dispersive stretching of the laser pulses (2),

- A pulse amplification device (20) for amplifying the stretched laser pulses (23) and

- One or more pulse compression devices (21, 22) for the dispersive compression of the amplified laser pulses (24), the or at least one of the pulse stretching devices (19) and/or the or at least one of the pulse compression devices (21, 22) being a pulse modification device (1) acts according to any one of the preceding claims.