Short-pulse laser system and method for generating laser pulses

Abstract

The invention relates to an optical system comprising a laser source (1) generating pulsed laser radiation consisting of a temporal sequence of laser pulses and at least one pulse compression device (3) located in the beam path and having a nonlinear medium (7), wherein the laser pulses undergo a nonlinear spectral broadening during the propagation through the medium (7) and a chirp is applied to the laser pulses. The object of the invention is to provide an optical system which makes it possible to generate nonlinearly compressed laser pulses with an improved temporal pulse contrast or with an improved pulse quality. According to the invention, a group delay dispersion which varies along the beam path and at least partially compensates the chirp is applied to the laser pulses by the pulse compression device (3).

Description

Technical Field

[0001] The present invention relates to an optical system having a laser source that generates pulsed laser radiation consisting of a time sequence of laser pulses; and at least one pulse compression device located in the beam path and having a nonlinear medium, wherein the laser pulses undergo nonlinear spectral broadening during propagation through the medium, and a chirp is applied to the laser pulses.

[0002] The present invention also relates to a method for generating laser pulses, wherein pulsed laser radiation consisting of a time sequence of laser pulses is generated, and the generated laser pulses are spectrally broadened in a nonlinear manner by applying chirp. Background Technology

[0003] Laser systems for generating ultrashort laser pulses in the picosecond and femtosecond range have been a focus of attention for many years.

[0004] Various applications of this system require shorter pulse durations than those supported by the gain medium of the laser system. Furthermore, effects in the optical amplifier, such as saturation or gain narrowing, can reduce the spectral bandwidth of the laser radiation, leading to an undesirable increase in pulse duration at the laser system output.

[0005] One known method for shortening pulse duration is to coherently generate new spectral components using nonlinear effects. The corresponding nonlinear interactions can occur in the gain medium (nonlinear amplification) as pulse compression devices, or in a separate component downstream of the optical amplifier in the beam path. To increase spectral bandwidth, the most commonly utilized nonlinear interaction between laser radiation and the medium is self-phase modulation (SPM). Spectral broadening induced by SPM can be achieved in media of various geometries, such as optical waveguides in optical fibers.

[0006] SPM (Spectral Purification) provides additional frequency components to the laser pulse, thus increasing the bandwidth of the laser radiation. To shorten the pulse duration using these newly generated frequency components, the laser pulse must be as unaffected by chirp as possible, i.e., unaffected by the different time delays of the different frequency components of the laser radiation. Therefore, pulse compression devices typically include a dispersive element downstream of a nonlinear medium to compensate for the chirp generated by the SPM as much as possible, thereby compressing the laser pulse in time. The goal is to achieve a pulse duration as close as possible to the generated spectral bandwidth, i.e., a bandwidth-limited laser pulse with a minimum pulse duration. The compression factor achieved by the dispersive element can be limited by various influences such as ionization, achievable nonlinearity, loss, or the finite spectral bandwidth of the nonlinear medium.

[0007] A known problem is that the pulse quality of nonlinearly compressed laser pulses is imperfect, and a certain amount of pulse energy remains in the temporal background of the secondary pulse or laser radiation. This is due to the spectral broadening property of SPM, which is reflected in the significant modulation of spectral intensity (see Agrawal, GP, 2007, Nonlinear Fiber Optics, 4th edition, Amsterdam, Academic Press). Even with perfect chirp elimination, some pulse energy remains outside the timing master pulse. It is well known that for larger temporal compression factors, temporal pulse contrast, or pulse quality, generally decreases. A measure of pulse quality is the proportion of total pulse energy falling within a specific time window around the pulse's maximum intensity.

[0008] In this context, the object of the present invention is to provide an optical system capable of generating nonlinear compressed laser pulses with improved time pulse contrast or improved pulse quality. Summary of the Invention

[0009] This invention achieves this objective starting with an optical system of the type shown at the beginning, because a group delay dispersion (GDD) that varies along the beam path and at least partially compensates for the chirp is applied to the laser pulse by a pulse compression device.

[0010] The present invention also achieves this objective by a method for generating laser pulses, wherein pulsed laser radiation consisting of a time sequence of laser pulses is generated, and the generated laser pulses are spectrally broadened in a nonlinear manner by applying chirp, wherein a variable group delay dispersion along the beam path is applied to the laser pulses, the group delay dispersion affecting at least partial compensation of the chirp.

[0011] The basic idea of ​​this invention is a pulse compression device in which spectral broadening and chirp compensation are distributed over as many individual steps as possible (in the limiting case, infinitesimally small steps), corresponding to (quasi-)adiabatic pulse compression. Ideally, the compression factor (i.e., the time pulse shortening factor) of each step is kept as small as possible, which reduces spectral modulation in the SPM-broadened spectrum of the laser pulse. Preferably, the compression factor for each step should be less than four, preferably less than three, and more preferably less than two. This reduces the energy content in the secondary pulse and effectively increases the peak pulse power. After each step of spectral broadening caused by SPM, the laser pulse is compressed accordingly by applying group delay dispersion (GDD). In the simplest case, the intensity of the nonlinear interaction remains constant in subsequent steps. However, due to the continuous reduction in pulse duration and the resulting increase in pulse peak power, spectral broadening increases, and therefore the chirp applied in each step also changes. Therefore, the group delay dispersion must vary with each step, i.e., along the beam path, in order to compensate for the chirp applied in each step as much as possible.

[0012] In a possible embodiment of the invention, the group delay dispersion varies continuously or in stages along the beam path. In fact, nonlinear compression can be asymptotic, whereby the nonlinear medium is divided into two or more separate portions, through which laser radiation continuously passes. Each portion of the nonlinear medium is followed in the beam path by a dispersive optical element associated with that portion, wherein these optical elements are distinct from each other with respect to the group delay dispersion. In this design, portions of the nonlinear medium alternate with the dispersive elements assigned to them. Each portion of the nonlinear medium and its associated dispersive element are assigned to a step of nonlinear compression. The dispersive elements are designed for the group delay dispersion such that the chirp generated in the associated step is largely compensated, thereby reducing the overall spectral modulation during nonlinear compression.

[0013] SPM is an intensity-dependent effect, meaning that stronger spectral broadening occurs in regions of higher intensity interaction (between the nonlinear medium and laser radiation) compared to regions of lower intensity. Therefore, a laser beam with a typical Gaussian beam distribution experiences spatially inhomogeneous spectral broadening during propagation through a nonlinear medium (such as a glass plate). Spectral broadening is more pronounced near the beam axis than in peripheral regions further away. However, many applications require the spectral bandwidth of the laser pulse to be uniform across the beam distribution. A known method for spatially uniform spectral broadening of pulsed laser radiation (see Nenad Milosevic, Gabriel Tempea, and Thomas Brabec, “Optical pulse compression: bulk media versus hollow waveguides,” Opt. Lett. 25, 672-674, 2000) utilizes the fact that spectral broadening is spatially uniform in the medium located within an imaging mirror array, which is designed as a multipass cell of a stable resonator. Therefore, in the optical system according to the invention, the nonlinear medium can advantageously be located within the multipass cell through which the laser radiation passes multiple times. A multipass cell comprises an arrangement of two or more (partially focused) mirrors that redirect the laser beam coupled to the multipass cell at each reflection point, such that beam propagation is confined to a predetermined volume along a controlled propagation path within the multipass cell until the laser beam leaves the multipass cell after multiple reflections and thus passes through its volume. Known multipass cell designs are referred to, for example, White cells or Herriott cells. Spatially uniform spectral broadening using a multipass cell requires that the mirrors be shaped and arranged such that the multipass cell forms a stable optical resonator, characterized by the presence of a Gaussian beam as a transverse eigenfunction of the resonator, undergoing the desired spatial homogenization of spectral broadening in the same manner as transverse eigenfunctions in a nonlinear waveguide.

[0014] Dielectric materials (e.g., glass plates) or gases (e.g., rare gases) can be used as nonlinear media located in a multipass cell and thus through which laser radiation passes multiple times. It is also conceivable to arrange several nonlinear elements in the multipass cell; glass plates of different thicknesses can be used, or regions with different gas pressures can be provided within the multipass cell.

[0015] The destruction threshold of the mirrors used to implement the multi-pass cell limits the compressible pulse energy or peak pulse power that can be coupled into the cell. The destruction threshold depends on the intensity of the laser radiation. In principle, the intensity on the mirror surfaces can be reduced by increasing the distance between the mirrors. Furthermore, it is possible to operate with a near-concentric mirror configuration, resulting in the largest beam radius on the mirror surfaces in all symmetrical arrangements. However, this configuration results in a small focal point of the laser radiation, which in turn must take into account the nonlinear interactions in the medium during the design. On the one hand, destruction or over-ionization of the medium must also be avoided; on the other hand, the cumulative nonlinear phase per revolution must not exceed a certain limit in order to keep the nonlinear pulse compression at each step sufficiently low within the context of this invention.

[0016] The chirp compensation according to the invention can be achieved because the mirrors of the multipass cell are designed to be dispersive (e.g., as dielectric mirrors). Therefore, at least one of the mirrors can be appropriately divided, wherein the laser radiation is continuously reflected at different portions of the mirror as it passes through the multipass cell multiple times. At least two portions of the mirrors are different from each other with respect to the applied group delay dispersion, such that for each compression step, i.e., for each passage of the laser radiation through the nonlinear medium located in the multipass cell, an appropriate group delay dispersion is applied to the laser radiation during subsequent reflections at the corresponding mirror to largely compensate for the chirp generated in each case, thereby maintaining low spectral modulation.

[0017] The multipass cell can be designed together with associated (segmented) mirrors so that nonlinear compression is performed by subdividing it into a relatively large number of steps. Thus, the laser radiation passes through the multipass cell (i.e., the focal point of the multipass cell) at least three times, preferably at least five times, particularly preferably at least ten times or even more than twenty times.

[0018] Because the laser radiation passes through the nonlinear medium in the multipass cell multiple times, it experiences a substantially constant nonlinear polarization at each step along the entire beam path. However, it is also conceivable that multipass cell sequences of this type may have different polarizations along the beam path. Attached Figure Description

[0019] Exemplary embodiments of the invention are explained in more detail with reference to the accompanying drawings. In the drawings:

[0020] Figure 1 A schematic diagram of the optical system according to the present invention is shown in block diagram form;

[0021] Figure 2 A schematic diagram of a pulse compression device based on optical fiber according to the present invention is shown;

[0022] Figure 3A schematic diagram of a pulse compression device based on a multi-pass pool according to the present invention is shown;

[0023] Figures 4a to 4c A schematic diagram showing the pulse duration, spectrum, and time pulse order shortening of a nonlinear compressed laser pulse is presented. Detailed Implementation

[0024] The spatially separated (dispersion characteristics) are designed such that a group delay dispersion varying along the beam path (i.e., from reflection to reflection) is applied to the laser pulse to compensate for chirp. Advantageously, the mirrors can be segmented here (see...). Figure 6 ), in which the laser radiation, when passing through the multi-pass pool 6 multiple times, is reflected in different parts of the reflectors 8 and 8' ( ). Figure 6 The reflections are sequentially performed at points 1 to 20. The portions of mirrors 8 and 8' differ from each other during the reflection process with respect to the applied group delay dispersion, so as to largely compensate for the chirp for each compression step.

[0025] The following uses Figures 4a to 4c The schematic diagram illustrates the principle of this invention. The output point is a laser pulse (Gaussian pulse) generated (and amplified) by a laser source 1 with a pulse duration of 300 fs and a pulse energy of 1 mJ. The nonlinear parameter for each compression step of the nonlinear medium subjected to multiple irradiations is γ = 2.5 × 10⁻⁶. -7 (W·m) -1 This nonlinear parameter acts over a distance of 250 μm (e.g., the irradiation thickness of the medium). After each step of SPM spectral broadening, the laser pulse is compressed by applying group delay dispersion. The intensity of the nonlinear interaction remains constant in subsequent steps. However, the spectral broadening increases with each step due to the reduced pulse duration and the resulting increase in pulse peak power. For simplicity, higher-order losses and nonlinear effects are neglected, and pulse compression is not considered in the context of higher phase terms (e.g., third-order dispersion). Figures 4a to 4c The diagram illustrates the (simulated) results of pulse compression performed in stages in this manner. Figure 4a The schematic diagram illustrates the pulse duration achieved after each step and the group delay dispersion required for this in each step. Figure 4b and Figure 4cThe results of 27th-order pulse compression in the spectral and temporal ranges are shown (curves 9 and 10) compared to the conventional single-order pulse compression simulation (curves 11 and 12). The spectral width increases to 42.7 nm, while the 300 fs input pulse is compressed to 28.5 fs. The decisive factor is the pulse energy content within the + / -50 fs window near the pulse maximum, which accounts for 90% of the total pulse energy, thus significantly higher than the single-order nonlinear compression case also shown, where only 73% of the total pulse energy is contained within the same time window. Figures 4a to 4c The examples provided are for illustrative purposes only and do not represent an optimized solution. Pulse contrast can be further improved by adjusting the number of steps, the nonlinear intensity of each step, and the chirp compensation for each step.

[0026] Figure 5 Explanation Figure 6 The mirror design achieves the pulse duration after each step and the required group delay dispersion for each step. The output point is still a 300 fs laser pulse with a pulse energy of 1 mJ. The nonlinear parameter for each broadening step of the multiple-irradiation nonlinear medium 7 is γ = 2.5 × 10⁻⁶. -7 (W·m) -1 This nonlinear parameter acts on a length of 250 μm. Therefore, each laser pulse propagates 13 times through the nonlinear medium 7, undergoing spectral broadening during this process without any compensation for the chirp generated. This is followed by reflection from a mirror, which imposes a group delay of -7000 fs. 2 This shortens the laser pulse to approximately 120 fs. The following three steps include nonlinear spectral broadening (each for -1000 fs). 2 Group delay dispersion), followed by three steps of nonlinear spectral broadening (each for -500 fs). 2 Group delay dispersion), finally through nonlinear medium 7, and finally a single -200fs 2 Group delay dispersion. The original spectral bandwidth of the 5.234 nm laser pulse is thus increased to 45.8 nm (full width at half maximum, FWHM). The compressed pulse duration at the end is 28.6 fs, and the energy fraction in the + / -50 fs window around the pulse maximum is 89%, which is very close to... Figures 4a to 4c The values ​​of the finely graded nonlinear compression shown (which has 27 steps) are obtained through... Figure 6 The segmented reflectors 8 and 8' shown in the diagram significantly simplify the implementation.

[0027] Figure 6Each of the two reflectors 8 and 8' consists of ten parts, of which the first twelve parts (numbered 1-12) overlapped by the beam path have vanishing group delay dispersion, and part 13 on reflector 8' has -7000 fs. 2 The group delay dispersion, portions 14 and 16 on mirror 8 and portion 15 on mirror 8', has -1000 fs. 2 The group delay dispersion, portion 18 on mirror 8 and portions 17 and 19 on mirror 8', has -500 fs. 2 The group delay dispersion, and the portion 20 on the mirror 8 again has a group delay dispersion that becomes zero (in this case, the final compression step occurs outside the multipath pool).

[0028] It should also be mentioned that, Figure 6 The example shown is for illustrative purposes and not an optimized configuration. It should also be noted that the functionality according to the invention does not necessarily have to be implemented in a multi-pass pool. For example, twenty separate, at least partially curved, mirrors with corresponding dispersion characteristics, or dispersion applied not through mirrors but through additional elements, provide the same result. Similarly, the use of only a single nonlinear element is not mandatory.

[0029] In the method of the present invention for improving the pulse contrast or pulse quality of a nonlinear compressed laser pulse, it should be noted, particularly with respect to a specific exemplary embodiment having a multipass cell 6, that the pulse peak power, which increases with multiple passes through the nonlinear medium, should not cause undesirable effects in the multipass cell, such as damage to the mirrors, interference ionization in the focal point between the mirrors, or excessively strong nonlinear interactions with each focusing pass.

[0030] This will manifest as a deterioration in the spatial spectral uniformity of the laser beam in the output beam AL. The effects of these limitations must be considered when designing the multi-pass cell 6. One possible solution is a sequence of multi-pass cells of this type with different nonlinearities and / or different mirror configurations (e.g., regarding radius of curvature and spacing).

[0031] It should also be noted that even when using lossy spectral broadening methods (e.g., in a capillary or in a multipass cell with a metal mirror), the method of the present invention may still outperform conventional single-step spectral broadening due to the preservation of strong nonlinear interactions.

[0032] As previously mentioned, it is also conceivable to use adiabatic nonlinear pulse compression in a series arrangement of two or more multipass cells of the type according to the invention, each cell having a suitable mirror and nonlinear medium, in order to produce pulse durations in only a few optical cycles while having high temporal quality.

Claims

1. An optical system comprising a laser source (1) for generating pulsed laser radiation consisting of a time sequence of laser pulses and at least one pulse compression device (3) located in the beam path and including a nonlinear medium (7), wherein, The laser pulse undergoes nonlinear spectral broadening during propagation through the medium (7), and a chirp is applied to the laser pulse. The pulse compression device (3) applies a group delay dispersion that varies continuously along the beam path to the laser pulse, thereby at least partially compensating for the chirp.

2. The optical system according to claim 1, wherein, The nonlinear medium (7) is divided into two or more separate portions (4), through which the laser radiation passes continuously, wherein at least some or each of the portions (4) of the nonlinear medium is followed in the beam path by a dispersive optical element (5, 5', 5'') associated with that portion, wherein at least two of the optical elements (5, 5', 5'') are different from each other with respect to the group delay dispersion thus applied.

3. The optical system according to claim 2, wherein, The compression factor, i.e. the time pulse shortening factor of the laser pulse for each of the portions (4), is less than four, and each of the portions (4) has separately assigned dispersive optical elements (5, 5', 5'').

4. The optical system according to claim 3, wherein, The compression factor is less than three.

5. The optical system according to claim 3, wherein, The compression factor is less than two.

6. The optical system according to claim 1, wherein, The nonlinear medium is located in the multi-pass cell (6) through which the laser radiation passes multiple times.

7. The optical system according to claim 6, wherein, The multi-pass cell (6) has at least two mirrors (8, 8') and the laser radiation is reflected back and forth between the at least two mirrors.

8. The optical system according to claim 7, wherein, The shape and arrangement of the reflectors (8, 8') are chosen to make the multipass cell (6) form a stable optical resonator.

9. The optical system according to claim 7 or 8, wherein, The reflectors (8, 8') are spherical mirrors and are arranged concentrically, wherein the nonlinear medium (7) is located at the center of the arrangement.

10. The optical system according to claim 7 or 8, wherein, The reflectors (8, 8') are dispersive.

11. The optical system according to claim 10, wherein, The mirrors (8, 8') are different from each other with respect to the applied group delay dispersion.

12. The optical system according to any one of claims 7, 8, and 11, wherein, At least one of the reflectors (8, 8') is divided into portions, wherein the laser radiation is continuously reflected at different portions of the reflector (8, 8') as it passes through the multipass pool (6) multiple times.

13. The optical system according to claim 12, wherein, At least two portions of the mirrors (8, 8') are different from each other with respect to the applied group delay dispersion.

14. The optical system according to any one of claims 6 to 8, 11 and 13, wherein, The laser radiation passes through the multipass cell (6) at least three times.

15. The optical system according to any one of claims 6 to 8, 11 and 13, wherein, The laser radiation passes through the multipass cell (6) at least five times.

16. The optical system according to any one of claims 6 to 8, 11 and 13, wherein, The laser radiation passes through the multipass cell (6) at least ten times.

17. The optical system according to any one of claims 6 to 8, 11 and 13, wherein, Compression factor, which is the time pulse shortening factor for each laser pulse passing through, is less than four.

18. The optical system according to any one of claims 6 to 8, 11 and 13, wherein, Compression factor, which is the time pulse shortening factor for each laser pulse passing through, is less than three.

19. The optical system according to any one of claims 6 to 8, 11 and 13, wherein, Compression factor, which is the time pulse shortening factor for each laser pulse passing through, is less than two.

20. The optical system according to any one of claims 1 to 8, 11 and 13, wherein, The nonlinear polarizability of the medium (7) is constant along the beam path.

21. A method for generating laser pulses, wherein, Pulsed laser radiation consisting of a time sequence of laser pulses is generated, and the spectral broadening of the generated laser pulses is performed nonlinearly by applying chirp. Specifically, a group delay dispersion that varies continuously along the beam path is applied to the laser pulse to at least partially compensate for the chirp.

22. The method according to claim 21, wherein, The nonlinear spectral broadening and the corresponding compensation for the chirp are performed in two or more consecutive single steps, wherein the compression factor, i.e., the time pulse shortening factor of the laser pulse for each single step, is less than four.

23. The method according to claim 22, wherein, The compression factor is less than three.

24. The method according to claim 22, wherein, The compression factor is less than two.

Images