SHORT PULSE LASER SYSTEM

Abstract

The disclosure relates to an optical system comprising a laser source (1) which generates pulsed laser radiation consisting of a temporal sequence of laser pulses in an input laser beam (EL), a splitting element (2) which follows the laser source (1) in the course of the beam and splits each of the laser pulses into laser pulse replicas separated spatially and/or temporally from one another, a combination element (4) which follows the splitting element (2) in the course of the beam and superimposes the laser pulse replicas in a respective laser pulse in an output laser beam. It is the task of the disclosure to provide an improved optical system compared to the prior art. It should be possible to generate particularly short and thus spectrally broadband laser pulses of high power with the optical system. The disclosure proposes that at least one multipass cell (3) is arranged in the beam path between the splitting element (2) and the combination element (4), through which the laser pulse replicas propagate, wherein the multipass cell (3) contains a medium in which the laser pulse replicas undergo nonlinear spectral broadening.

Claims

1. Optical system comprising: a laser source (1) which generates pulsed laser radiation consisting of a temporal sequence of laser pulses in an input laser beam (EL), a splitting element (2) following the laser source (1) in the beam path, which splits each of the laser pulses into laser pulse replicas separated spatially and/or temporally from each another, a combination element (4) which follows the dividing element (2) in the beam path and which superimposes the laser pulse replicas in a respective laser pulse in an output laser beam, wherein at least one multipass cell (3) is arranged in the beam path between the splitting element (2) and the combination element (4), through which multipass cell (3) the laser pulse replicas propagate, the multipass cell (3) containing a medium in which the laser pulse replicas undergo a nonlinear spectral broadening.

2. Optical system according to claim 1, comprising a reflector arranged in the beam path behind the multipass cell (3), which reflector reflects the laser pulse replicas after propagation through the multipass cell (3), after which the laser pulse replicas propagate in the reverse direction through the multipass cell (3), wherein the combination element (4) is formed by the splitting element (2), in that the splitting element (2) superimposes the laser pulse replicas after propagation through the multipass cell (3) in the backward direction in a respective laser pulse.

3. Optical system according to claim 1, wherein the dividing element (2) and/or the combination element (4) comprises a reflective element having zones of different reflectivity.

4. Optical system according to claim 3, wherein the splitting element (2) and/or the combination element (4) each comprise two or more reflective elements (A, B, C, D) at which the laser radiation is successively reflected one or more times.

5. Optical system according to claim 3, wherein the dividing element (2) and the combination element (4) have an identical construction.

6. Optical system according to claim 1, wherein the splitting element (2) and the combination element (4) each comprise at least one beam splitter and at least one optical delay path.

7. Optical system according to claim 1, wherein the laser pulse replicas propagate through the multipass cell (3) in spatially separated partial beams.

8. Optical system according to claim 7, wherein the partial beams form a two-dimensional array in a plane perpendicular to the beam path.

9. Optical system according to claim 1, comprising an error signal detector (7) which derives an error signal from the laser radiation, and a controller which derives at least one actuating signal from the error signal for driving at least one optical modulator (8) arranged in the beam path.

10. Optical system according to claim 8, wherein the optical modulator (8) comprises an array of phase modulators corresponding to the array of partial beams, each of the partial beams having a phase modulator associated therewith.

11. Optical system according to claim 1, wherein an arrangement of power actuators (6) is located in the beam path, wherein a power actuator (6) is associated with each partial beam which influences the power of the laser pulse replicas in that partial beam.

12. Optical system according to claim 1, wherein the multipass cell (3) comprises at least two mirrors whose shape and arrangement are selected such that the multipass cell (3) forms a stable optical resonator.

13. Optical system according to claim 12, wherein the multipass cell (3) comprises spherical mirrors in concentric arrangement.

14. Optical system according to claim 12, wherein the multipass cell (3) comprises metallic mirrors.

15. Optical system according to claim 12, wherein the multipass cell (3) comprises dielectric mirrors, wherein the medium and dielectric mirrors have anomalous total dispersion.

Description

[0032] FIG. 1 a schematic representation of an optical system according to the invention as a block diagram;

[0033] FIG. 2 splitting or combination element based on multiple reflection;

[0034] FIG. 3 schematic representation of an optical system according to the invention in a second embodiment as a block diagram;

[0035] FIG. 4 schematic representation of an optical system according to the invention in a third embodiment as a block diagram.

[0036] In the embodiment example of FIG. 1, an input laser beam of pulsed laser radiation coming from a laser source 1 (e.g. comprising a mode-locked oscillator with downstream amplifier) is split into a number of spatially separated (and preferably parallel) partial beams by means of a splitting element 2. The function of the splitting element 2 is expediently based on an arrangement of partially reflective mirrors or polarizing beam splitters in a cascaded arrangement, diffractive elements or an arrangement of mirrors with zones of different reflectivity (see below). The spatially separated partial beams are coupled into a multipass cell 3. This has at least two mirrors whose spacing and shape are selected according to a stable resonator configuration. The multipass cell contains a nonlinear medium (e.g. a transparent solid or a gas) which imprints a phase (predominantly) by SPM to the laser pulse replicas propagating in the partial beams and consequently causes a spectral broadening. Likewise, other nonlinear processes can also generate new spectral components. In this case, an approximately identical nonlinear phase is imprinted on all laser pulse replicas. The spatially separated partial beams do not exhibit optical path differences greater than the coherence length of the spectrally broadened laser pulse replicas. The spectrally broadened laser pulse replicas are coupled out of the multipass cell 3 (e.g. through a hole in one of the mirrors), and subsequently superimposed and spatially coherently combined by a combination element 4. This can be followed by a pulse compression stage 5, e.g. using suitable chirped mirrors. Likewise, the mirrors of the multipass cell 3 as well as the nonlinear medium contained therein can have an anomalous total dispersion in sum and thus cause a largely identical soliton self-compression of all partial beams or laser pulse replicas in the multipass cell 3.

[0037] FIG. 2 shows a splitting or combination element based on multiple reflection, as can be used in the system according to the invention. The element consists of four sub-elements A, B, C, D. The first sub-element A is a mirror with a reflectivity as high as possible. The second sub-element B comprises (in the example shown) exemplarily four zones with different reflectivity. The laser beams take the path shown in FIG. 2. The reflectivities of the zones of the sub-element B can be selected so that the incident input laser beam EL is divided into partial beams in a certain ratio. An example is a splitting in equal parts on all partial beams. This is achieved by choosing the reflectivities of the four zones to be 75%, 66%, 50% and 0%. The outgoing four partial beams then fall on plane-parallel surfaces of the two sub-elements C and D, which are tilted towards the sub-elements A, B. Sub-element C is again highly reflective. Sub-element D again has four zones of varying reflectivity (as before). As a result, as shown, a two-dimensional array of 16 partial beams is generated in a plane perpendicular to the beam path. The number of zones of different reflectivity at the sub-elements B and D can be arbitrary in each case, according to the desired number of partial beams, i.e. according to the splitting ratio. It should be noted that the number of zones does not necessarily have to be equal to the number of partial beams. A zone can also reflect the beam several times. The splitting element 2 and the combination element 4 can be identical and arranged in such a way that the resulting path lengths differences of the 16 partial beams almost cancel each other out (ideally within the coherence length).

[0038] It should be noted that the foci of the parallel partial beams in the multipass cell 3 may overlap. This can lead to undesirable non-linear interactions of the partial beams. The special feature of the splitting/combination element shown in FIG. 2 is that the laser pulse replicas of the parallel partial beams are temporally offset from each other, so that interactions between the laser pulse replicas are avoided. The time offset is determined by the distances of the highly reflective to the segmented mirrors and can be chosen according to the laser pulse duration in the input laser beam. If necessary, imprinted angles between the spatially separated partial beams can reduce or avoid an overlap of the foci.

[0039] The division into partial beams of as identical power as possible is important because all partial beams in the multipass cell 3 should undergo an almost identical nonlinear interaction, resulting in an almost identical pulse duration shortening and, moreover, this is the basis for a high combination efficiency in the downstream coherent superposition in the combination element 4 to generate the output laser beam. For this purpose, as shown in FIG. 3, an array of power actuators 6 adapted to the partial beam array can be provided. In the simplest case, this can be realized, for example, by an array of adjustable attenuators.

[0040] A detection of path length differences in the sub-wavelength range is performed in the embodiment example of FIG. 3 by means of an error signal detector 7. For this purpose, known arrangements can be used (see, e.g., Arno Klenke, Michael Müller, Henning Stark, Andreas Tünnermann, and Jens Limpert, “Sequential phase locking scheme for a filled aperture intensity coherent combination of beam arrays,” Opt. Express 26, 12072-12080, 2018). The correction or stabilization of the interferometric superposition in the combination element 4 can be performed by an array of phase modulators 8 (e.g., mirror array with piezo actuators), which in turn is adapted in its geometry to the partial beam array. The electronic control loop used for this purpose is not shown in FIG. 3. As an alternative to the active stabilization described above, passive approaches can also be used.

[0041] In the example shown in FIG. 4, the splitting element 2 splits the input laser pulses into at least two temporally separated laser pulse replicas with ideally identical pulse energy, in this example finely adjusted by a pulse-selective power control 9. In the multipass cell 3, the corresponding temporally separated spectral broadening of the laser pulse replicas is performed with subsequent coherent combination at 4 to generate the output laser beam. Also with the temporal splitting of the laser pulses the relative phase position of the laser pulse replicas and their stability is essential for a stable emission where most of the pulse energy is contained in the output laser beam. Known approaches for detection and active stabilization can also be used here. In the embodiment example, elements for detection of the relative phase position 10 and for corresponding active control 11 are included in the setup for this purpose. The electronic control components are again not shown in FIG. 4. Especially for the temporally separated spectral broadening of the laser pulse replicas, passive approaches (i.e. approaches that do not require control electronics) can be used to set the correct relative phase position of the laser pulse replicas in the combination.

[0042] To split the individual laser pulses of the input laser beam into a temporal sequence of laser pulse replicas at 13, partially reflecting mirrors, polarizing elements (e.g. thin film polarizer or polarization beam splitter) can be used, for example, or crystals with different transit times at different polarizations (birefringent crystals) can be used. A corresponding inverted arrangement allows the coherent combination at 4.

[0043] Alternatively (not shown), beam reversal can occur at the output of the system, i.e. after passing through the multipass cell 3, e.g. by means of a Faraday rotator in combination with a highly reflective mirror. After reflection, the laser pulse replicas propagate in the reverse direction through the multipass cell 3, using the splitting element 2 in the reverse direction for combination.

[0044] It is important that especially at the output of the system the optical components used support the spectral bandwidth of the nonlinear broadened laser pulses.

[0045] It should be noted that a multi-dimensional division, i.e. into laser pulse replicas separated from each other both spatially and temporally, is possible. This corresponds, for example, to a combination of the embodiments of FIGS. 3 and 4.

[0046] To overcome problems that may arise from the dispersion of the mirrors of the multipass cell 3 and the associated limitations in the generation of extremely broadband (few-cycle) laser pulses, metallic mirrors may advantageously be used in the multipass cell 3, possibly consisting of a metallic layer on a substrate characterized by good thermal conductivity (e.g. copper or sapphire).