Laser oscillator system and method for generating optical pulses

Abstract

Some embodiments relate to a laser oscillator system (10) comprising a resonator cavity (12) for confining an intracavity laser beam (13). The laser oscillator system further comprises a Cr-doped II-VI gain medium (14) disposed within the resonator cavity (12) and an imaging unit (18) forming part of the resonator cavity (12). The imaging unit (18) is adapted to decouple a spot size (100) of the intracavity laser beam (13) at the gain medium (14) from an intracavity length (102) of the resonator cavity (12). Furthermore, the resonator cavity (12) and the imaging unit (18) are adapted such that the laser oscillator system (10) emits laser pulses at a repetition rate of 50 MHz or less. Further embodiments relate to a laser system (30) and a method for generating optical pulses having a spectral content of at least 2 μm wavelength.

Description

[Technical Field]

[0001] Embodiments of the present invention relate to laser oscillator systems, laser systems, methods for generating optical pulses having a spectral component at a wavelength of at least 2 μm, the use of laser oscillator systems to generate laser pulses having a peak power of at least 0.75 MW at a repetition rate of 50 MHz or less, and the use of rutile TiO for nonlinear spectral broadening of laser pulses. Thus, the present embodiments relate to laser technology. [Background technology]

[0002] Femtosecond light sources in the mid-infrared (MIR) region with high brightness and high photon flux are desired for a variety of applications, including spectroscopic applications, both in the frequency and time domains. The MIR spectral region is of particular interest because many molecules, especially biomolecules, exhibit characteristic spectral absorption signatures in this spectral region. Furthermore, the study and utilization of nonlinear optical processes benefit from such MIR laser pulses with femtosecond pulse durations, while at the same time generating pulses with low amplitude and timing fluctuations (noise) to achieve a high level of measurement sensitivity.

[0003] For such applications, few-cycle pulses with peak powers of approximately 0.75 MW or more can extend the spectral range not only into the MIR but also into the ultraviolet spectral region through nonlinear frequency conversion (see Non-Patent Document 1). The availability of such optical pulses at repetition rates of several MHz will enable the exploration of many new applications in molecular science as well as nanoscience.

[0004] Direct diode-pumped mode-locked laser oscillator systems are commonly used to generate femtosecond laser pulses. Compared with mode-locked lasers pumped by other solid-state (e.g., fiber) lasers, direct diode-pumped ultrashort pulse laser oscillators are particularly suitable for efficiently generating few-cycle pulses that exhibit low intensity noise, as described in, for example, Patent Document 1 (US 8976821 B2, “Directly diode-pumped, Kerr-lens mode-locked, few-cycle Cr:ZnSe oscillator,” Opt. Express 27, 24445 (2019)) and Non-Patent Document 2.

[0005] However, such systems can only reach peak power levels above 100 kW, which is insufficient to exploit optical nonlinearities for effective frequency conversion or other applications. MW-level peak powers at repetition rates of several MHz can only be achieved by externally amplifying the entire pulse train emitted from such oscillators with an external laser amplifier. External amplification generally has the disadvantage of adding noise and reducing the achievable measurement sensitivity.

[0006] Non-Patent Document 3 describes the use of a 300 nm thin film of rutile using a titanium sapphire laser with a wavelength of 800 nm, and the nonlinear refractive index of this rutile is 2.7 × 10 -17 m 2 / W.

[0007] Non-Patent Document 4 describes the use of rutile TiO2 in the fabrication of waveguides pumped by a titanium sapphire laser with a wavelength of about 800 nm.

[0008] Non-Patent Document 5 describes the use of TiO2 in the fabrication of waveguides excited at a wavelength of 1.550 nm. [Prior art documents] [Patent documents]

[0009]

Patent Document 1

Non-licensed literature

[0010] [Non-licensed document 1] S. Vasilyev et al. Octave-spanning Cr:ZnS femtosecond laser with intrinsic nonlinear interferometry, Optica 6, 126-127 (2019 [Non-licensed document 2] N. Nagl, et al., “Directly diode-pumped few-optical-cycle Cr:ZnS laser at 800 mW of average power,” CLEO, paper SF3H.5 (2020) [Non-licensed document 3] Long et al.: "Third-order optical nonlinearities in anatase and rutile TIO2 thin films", THIN SOLID FILMS, ELSEVIER, AMSTERDAM, NL, vol. 517, no. 19, 3 August 2009 (2009-08-03), pages 5601-5604

Non-licensed Document 4

Non-licensed Document 5

[0011] It is therefore desirable to provide a solution for generating femtosecond pulses in the mid-infrared spectral range with a peak power of at least several hundred kW at high repetition rates and with low technical complexity and manufacturing costs. It is further desirable to provide a solution for achieving efficient spectral broadening for generating few-cycle optical pulses in the MIR range with low technical complexity and manufacturing costs. [Means for solving the problem]

[0012] The solution is provided by embodiments having the features of the independent claims. Optional embodiments are the subject matter of the dependent claims and the description.

[0013] One embodiment relates to a laser oscillator system including a resonator cavity that confines an intra-cavity laser beam and a Cr-doped II-VI gain medium disposed within the resonator cavity. The laser oscillator system further includes an imaging unit forming part of the resonator cavity, the imaging unit adapted to decouple a spot size of the intra-cavity laser beam on the gain medium from an intra-cavity length of the resonator cavity. The resonator cavity and the imaging unit are adapted such that the laser oscillator system emits laser pulses at a repetition rate of 50 MHz or less.

[0014] Another embodiment relates to a laser system comprising a laser oscillator system according to an embodiment, the laser oscillator system adapted to emit laser pulses having a peak power of at least 0.75 MW, the laser system further comprising a nonlinear optical element having a thickness of 1 mm or less, the laser system adapted to irradiate the laser pulses emitted by the laser oscillator system onto the nonlinear optical element to spectrally broaden the laser pulses such that the spectrally broadened laser pulses span at least a half optical octave.

[0015] Yet another embodiment relates to a method for generating optical pulses having spectral components at wavelengths of at least 2 μm. The method includes providing laser pulses emitted by a laser oscillator having a pulse width of 30 fs or less with a FWHM of at most 30 fs, a peak power of at least 0.75 MW, and a center wavelength of at least 1.8 μm. The method further includes providing a laser having a thickness of at most 1 mm and a nonlinear refractive index, n2, of at least 5·10 at a wavelength of 2 μm. -19 m 2 The method includes focusing the laser pulse onto a nonlinear optical element.

[0016] Yet another embodiment relates to the use of a laser oscillator system according to the embodiments to generate laser pulses having a peak power of at least 0.75 MW at a repetition rate of 50 MHz or less.

[0017] Yet another embodiment relates to the use of bulk rutile TiO for nonlinear spectral broadening of laser pulses having spectral components with wavelengths of at least 1 μm, optionally at least 2 μm, which may be in the wavelength range of 1 μm to 4 μm, and optionally in the wavelength range of 2 μm to 3 μm.

[0018] A laser oscillator system is a laser oscillator that provides laser activity within a gain medium in a resonator cavity. A laser oscillator system does not include external amplification of the laser pulses after extraction from the resonator cavity. A laser oscillator system may include an external excitation means, such as a pump laser, which may be part of the laser oscillator system or may be separate from the laser oscillator system. For example, a laser oscillator system may be directly diode-pumped by radiation provided by a light-emitting diode and / or a laser diode.

[0019] An intracavity laser beam is a laser beam that is confined within a resonator cavity and is maintained for multiple round trips within the laser cavity, where a small portion of the intracavity laser beam is coupled out by one of the resonator mirrors in the resonator cavity.

[0020] The Cr-doped II-VI gain medium includes a II-VI bulk medium doped with chromium atoms. The II-VI medium is composed of elements from the second and sixth main groups of the periodic table. The II-VI medium may include a chromium-doped II-VI crystal. In particular, the II-VI material may include ZnS and / or ZnSe. However, in other embodiments, different II-VI materials may be used. The Cr-doped II-VI gain medium provides properties suitable for generating femtosecond laser pulses in the MIR spectral region. However, one or more other gain media may alternatively or additionally be used, as long as they are suitable for generating laser pulses in the MIR spectral region that spectrally correspond to femtosecond pulse widths.

[0021] The imaging unit is an optical configuration for extending the length of the resonator cavity (also referred to as the intracavity length) and thereby reducing the repetition rate of the laser pulses emitted by the laser oscillator system. The imaging unit can be adapted to image the intracavity laser beam so as to at least partially preserve the transverse mode and / or beam profile of the intracavity laser beam. The imaging unit may include transmissive optical elements, such as one or more optical lenses, and / or reflective optical elements, such as flat and / or curved mirrors. The imaging unit may be integrated into the resonator cavity. In some embodiments, the imaging unit may include at least one end mirror of the resonator cavity.

[0022] A laser oscillator system with a repetition rate of 50 MHz or less means that the laser oscillator system is operated in a pulsed mode, e.g., mode-locked operation, and the frequency of the laser pulses emitted from the laser oscillator system is 50 MHz or less, and therefore the time distance between two consecutively emitted laser pulses is about 20 ns or more.

[0023] A laser system comprising a laser oscillator system may further include other means for modifying the emitted laser pulse. For example, the laser system may include other means for spectrally broadening the laser pulse. The laser system may further include a laser amplifier and / or an optical parametric amplifier for further amplifying the laser pulse emitted by the laser oscillator system.

[0024] The peak power of a laser pulse is the power achieved when the laser pulse has the maximum electric field intensity. In other words, the peak power is the maximum power of the laser pulse. A laser pulse having a peak power of 0.75 MW or more means that the laser pulse has an electric field intensity equivalent to a power of 0.75 MW at the maximum value of the electric field in the time domain.

[0025] A thickness of the nonlinear optical element of 1 mm or less means that the spatial extent of the nonlinear optical element in a direction parallel to the propagation direction of the incident laser pulse is 1 mm or less.

[0026] A spectrally broadened laser pulse spanning at least half an octave means that the laser pulse covers a range of frequencies extending from a specific first frequency to a second frequency having a frequency at least 1.5 times that of the first frequency. The threshold power at which the spectral range extends is the respective wavelength or frequency at which the spectral power distribution is attenuated by 30 dB compared to the wavelength or frequency with the maximum power.

[0027] The laser pulse width is indicated using the commonly used parameter FWHM, which stands for full width at half maximum.

[0028] The use of rutile TiO2 means that a bulk TiO2 medium having a rutile crystal structure is used. The use of TiO2 without a specifically designated crystal structure may include the use of TiO2 of any existing crystal structure, such as anatase, brookite, or rutile. However, in embodiments specifying rutile TiO2, only the rutile crystal structure of TiO2 is used for the respective purpose. The use of rutile TiO2 may include or consist of the use of bulk rutile TiO2. "Bulk" in this sense refers to the use of a bulk piece of rutile TiO2 as a nonlinear optical element, instead of using a waveguide structure that includes or consists of rutile TiO2.

[0029] Some embodiments provide the advantage of enabling high-peak-power, few-cycle laser pulses in the mid-infrared spectral region to be generated directly from a laser oscillator system. In other words, some embodiments provide the advantage of enabling high-peak-power, few-cycle laser pulses in the mid-infrared spectral region to be generated without the need for an additional external laser amplifier system in addition to the laser oscillator system. By generating such high-peak-power laser pulses directly from an oscillator system, a compact system can be realized, which may facilitate the integration of such laser pulse sources into various other systems, such as spectroscopy devices, security devices, medical devices, and / or machining tools.

[0030] Furthermore, some embodiments provide the advantage that the technical complexity of such laser oscillator systems can be kept significantly lower than that of conventional laser systems including laser amplifier systems. Furthermore, some embodiments provide the advantage that the laser pulse source can be provided at a lower manufacturing cost compared to conventional laser pulse sources including laser amplifier systems.

[0031] It is advantageous to increase the intracavity length of the resonator cavity, thereby reducing the repetition rate of the laser pulses emitted from the laser oscillator system, thereby achieving higher pulse energies and therefore higher peak powers of the individual pulses than laser oscillator systems with higher repetition rates.

[0032] While this advantage and realization of the embodiment have been recognized by the present inventors, it has been traditionally believed in the art that achieving high peak power in laser oscillator systems using Cr-doped II-VI gain media presents technical disadvantages or problems due to the high nonlinear refractive index. For example, the nonlinear refractive index n2 of ZnSe is approximately 1·10 at wavelengths of 2-3 μm. ―14 cm 2 / W. Due to the high nonlinear refractive index, spatial and temporal parasitic effects, and hence instabilities in Kerr-lens mode-locking, were expected to occur already at low power levels in Cr:ZnS and Cr:ZnSe laser gain media compared to Ti:sapphire laser gain media with comparable lasing parameters.

[0033] Thus, in the past, the following were considered to be obstacles to Cr-doped II-VI lasers: (i) the longer the intracavity length of the resonator, the smaller the spot size of the intracavity laser beam in the gain medium, resulting in increased intensity; and (ii) the longer the intracavity length, the higher the peak power due to a lower repetition rate, resulting in a further increase in intensity. These obstacles were previously thought to be due to the high nonlinear refractive index, preventing the generation of high-peak-power laser pulses from Cr-doped II-VI laser oscillators. Therefore, it is surprising that the inventors have discovered that by using an imaging unit to extend the intracavity length of a Cr-doped II-VI laser oscillator system, the intracavity length can be increased and the repetition rate correspondingly reduced, while avoiding the obstacles previously thought to be due to the high nonlinear refractive index. The combination of the Cr-doped II-VI gain medium and the imaging unit allows for decoupling of the spot size from the intracavity length, thus enabling the repetition rate to be reduced without significantly reducing the spot size of the intracavity laser beam in the gain medium. Therefore, even if the imaging unit increases the intracavity length and, as a result, the peak power of the laser pulse is increased, the nonlinear effects and related disorders in the gain medium can be avoided or reduced to an acceptable level, and a laser oscillator system based on a Cr-doped II-VI gain medium can be realized to generate laser pulses with peak powers of 0.75 MW or more.

[0034] The use of bulk rutile TiO2 allows for supercontinuum-like spectral broadening in bulk media, which always requires a balanced interplay between self-focusing, self-phase modulation, material dispersion, and plasma generation via multiphoton absorption. In particular, the use of bulk rutile TiO2 allows for spectral broadening of laser pulses in the 2 μm-3 μm, and optionally 1 μm-4 μm, wavelength range in bulk media, eliminating the need for a waveguide structure to confine the laser pulse to a small radius over long propagation distances. Instead, the bulk material itself determines the dispersion for spectral broadening. According to the present disclosure, spectral broadening in bulk rutile TiO2 is achieved with an interaction length of 1 mm or less while maintaining a high-quality beam profile that facilitates the use of the broadened laser pulse. Maintaining a good beam profile is advantageous for focusing the broadened laser pulse. Efficient expansion of laser pulses with wavelengths of 1 μm–4 μm, especially 2 μm–3 μm, and pulse energies of nJ in bulk materials strongly demands material properties such as optimal group delay and high-order dispersion, high damage threshold, and wide band gap.

[0035] As the inventors have discovered, rutile TiO2 not only provides strong spectral broadening, but also long-term stability that may be lacking in some other materials. Even among materials with similar dispersion properties, the inventors have found essential differences in their broadening behavior, with rutile TiO2 exhibiting highly beneficial spectral broadening performance in the 1 μm-4 μm, and especially 2 μm-3 μm, spectral region.

[0036] The unique combination of rutile's high nonlinear refractive index, large optical bandgap, and zero-crossing optical dispersion around 2-3 μm enables Cr-doped II-VI lasers to generate laser pulses with nJ-level pulse energies as a spectral supercontinuum without strong residual multiphoton absorption. Furthermore, the use of thin rutile plates with thicknesses of 1 mm or less allows for a near-Gaussian beam profile, demonstrating high spatial quality of the laser beam. However, the thin rutile plate should have a minimum thickness of at least 100 microns, which is sufficient for nonlinearity to build up and result in spectral broadening. Rutile layers as thin as a few hundred nanometers will not lead to substantial spectral broadening when combined with Cr-doped II-VI laser oscillators.

[0037] If the characteristics of the beam profile after spectral broadening are not essential to the intended application of the laser pulse, thicker bulk rutile TiO2 may be used, for example, between 1 mm and 5 mm thick, which provides greater nonlinear interactions and therefore higher spectral broadening. However, keeping the thickness of the bulk rutile TiO2 below 1 mm allows for adequate spectral broadening while maintaining the quality of the beam profile.

[0038] In some optional embodiments, the imaging unit is adapted to obtain a variable intracavity length. The intracavity length may be variable within a predetermined range. For example, the imaging unit may include one or more telescopes, and tuning the intracavity length may include moving at least one or some of the mirrors and / or lenses comprising the one or more telescopes. The imaging unit may enable continuous tuning of the cavity length and / or tuning of the resonator length in predetermined step sizes. Tuning the intracavity length allows the laser oscillator system to still perform mode-locked operation. In particular, tuning the intracavity length may be performed such that the spot size of the intracavity laser beam does not change with the change in the intracavity length due to tuning. According to other optional embodiments, the spot size of the intracavity laser beam in the gain medium may change slightly in response to tuning the intracavity length of the resonator cavity. For example, changing the intracavity length by about 10% results in a change in the spot size of the intracavity laser beam by about 10%. Tuning the intracavity length of the resonator cavity can be performed without the need to replace and / or change optical elements in one or both of the resonator cavity and / or the imaging unit. According to other embodiments, tuning the intracavity length may require replacing and / or changing optical elements in one or both of the imaging unit and / or the resonator. The tunability of the intracavity length may provide the advantage of tuning the repetition rate of the laser oscillator system over a corresponding range. In this way, tuning the intracavity length can provide the ability to adjust the repetition rate of the laser oscillator system for a desired application.

[0039] In some optional embodiments, the spot size of the intracavity laser beam in the gain medium is adjustable, thereby adjusting the intensity of the intracavity laser beam in the gain medium, and thus the gain, and / or the occurrence or avoidance of effects due to the nonlinear refractive index of the gain medium. For example, the resonator cavity may include additional focusing elements to focus the intracavity laser beam into the gain medium. Adjusting these additional focusing elements allows the spot size in the gain medium to be adjusted.

[0040] In some optional embodiments, the imaging unit includes one or more telescopes for imaging the intracavity laser beam, the one or more telescopes optionally including one or more 4f-telescopes. This allows for maintaining the transversal mode of the resonator cavity, particularly the beam diameter of the intracavity laser beam in the portion of the resonator cavity outside the imaging unit. This further allows for maintaining the spot size of the intracavity laser beam in the gain medium, facilitating decoupling of the intracavity length from the spot size in the gain medium. In some embodiments, an end mirror of the resonator cavity is positioned at one of the imaging planes of the one or more telescopes. This further facilitates maintaining the appropriate resonator mode of the intracavity laser beam.

[0041] In some optional embodiments, the resonator cavity and, optionally, the imaging unit comprise one or more multipass cells, which optionally include one or more Herriott-type cells. In other words, the imaging unit may include a multipass cell to increase the intracavity length of the resonator cavity. The multipass cell may be based on a reflective optical element, such as a plane mirror and / or a curved mirror. This has the advantage of keeping the dispersion within the resonator cavity low. It also has the advantage of achieving the damage threshold. Using an imaging unit including a multipass cell facilitates increasing the intracavity length. Alternatively or additionally, the resonator cavity may include a multiple-pass cell so that the intracavity laser beam passes through the gain medium multiple times during each half-circle within the resonator cavity. This can further reduce the repetition rate and increase the laser gain per round trip.

[0042] In some embodiments, the Cr-doped II-VI gain medium includes or consists of ZnS and / or ZnSe. The II-VI gain medium may be provided as polycrystalline ZnSe and / or ZnS. Cr-doped ZnS and Cr-doped ZnSe gain media are preferred because they are widely used as laser gain media and are abundantly available in suitable quality. In some embodiments, the gain medium is oriented at the Brewster angle at the central wavelength of the intracavity laser beam, or at the normal incidence angle of the intracavity laser beam if the gain medium is optionally coated with an anti-reflection coating. This can reduce losses due to unwanted reflections from the intracavity laser beam off the gain medium.

[0043] In some optional embodiments, the resonator cavity and imaging unit are adapted to cause the laser oscillator system to emit laser pulses at a repetition rate of 40 MHz or less, optionally 30 MHz or less, optionally 20 MHz or less, and optionally 10 MHz or less, which can further increase the pulse energy and therefore the achievable peak power of the emitted laser pulses.

[0044] In some embodiments, the laser oscillator system is adapted to emit laser pulses having a pulse width of 30 fs or less FWHM. The emitted laser pulses may have a peak power of at least 0.75 MW, optionally at least 1 MW. These laser pulses may be suitable for a wide variety of nonlinear optical applications, such as spectral broadening and / or time-resolved spectroscopy applications.

[0045] In some optional embodiments, the emitted laser pulses cover at least the spectral range from 2.0 μm to 2.8 μm. The wavelength at which the spectral power distribution is attenuated by 30 dB compared to the maximum of the spectral power distribution, i.e., the wavelength at which the spectral power is 1,000 times lower than the maximum, is considered the cutoff wavelength for the spectral broadening. A spectral broadening from 2.0 μm to 2.8 μm supports the generation of 30 fs pulses.

[0046] In some optional embodiments, the laser oscillator system is adapted as a Kerr-lens mode-locked laser oscillator system. This allows for efficient generation of femtosecond laser pulses from the laser oscillator system. In some optional embodiments, the gain medium is adapted to provide the functionality of a Kerr medium for Kerr-lens mode-locking. In other words, the gain medium can provide the functionality of a laser medium and also the functionality of a Kerr medium. Alternatively, the laser oscillator system can include a Kerr medium that is separate from the gain medium. This allows for the spot size of the intracavity laser beam in the gain medium to be adjusted independently from the spot size in the Kerr medium, thereby separating laser activity from Kerr-lens mode-locking.

[0047] In some optional embodiments, the Cr-doped II-VI gain medium is directly diode-pumped. The gain medium may be directly diode-pumped using optical radiation provided by a light-emitting diode and / or a diode laser. Direct diode pumping offers the advantage that lower amplitude noise can be achieved compared to other pumping techniques, and therefore may provide a more stable laser output, which may result in higher measurement sensitivity for laser pulse-based applications. Furthermore, direct diode-pumped laser oscillator systems can be realized in a more compact manner than laser oscillator systems pumped by fiber lasers. In addition, laser diodes for directly pumping laser oscillators often have lower manufacturing costs than fiber lasers, which may enable their use in cost-sensitive applications. Furthermore, direct diode-pumped laser oscillators may have higher wall-plug efficiency and potentially reduce power consumption.

[0048] In some optional embodiments, the laser system is adapted to focus the laser pulses onto the nonlinear optical element, thereby achieving high intensity within the nonlinear optical element and thus efficiently utilizing the nonlinear optical effects occurring within the nonlinear optical element. In particular, focusing the laser pulses onto the nonlinear optical element allows the thickness of the nonlinear optical element to be reduced while achieving the desired nonlinear optical effect, which may be beneficial for maintaining a high-quality beam profile, such as having a low beam quality factor M2, optionally close to 1.2.

[0049] In some optional embodiments, the laser system is adapted so that the spectrum of the laser pulse corresponds to a pulse width of 15 fs or less FWHM after propagation through the nonlinear optical element. In other words, the spectrum may be adapted after spectral broadening in the nonlinear optical element so that the Fourier transform of the spectral power distribution corresponds to the temporal power distribution of a laser pulse having a pulse width of 15 fs or less. To provide laser pulses having a pulse width of 15 fs or less, it is advantageous to control dispersion. For dispersion pre-compensation, pulse compression of the laser pulse can be applied after the nonlinear optical element and / or before the nonlinear optical element to obtain laser pulses having a pulse width of 15 fs or less.

[0050] In some optional embodiments, the nonlinear optical element comprises an anti-reflective coating on the surface facing the incident laser pulse, which may reduce optical loss due to unwanted reflections off the front surface of the nonlinear optical element.

[0051] In some optional embodiments, the nonlinear optical element is positioned at the Brewster angle relative to the direction of incidence of the laser pulse at the center wavelength of the laser pulse. The nonlinear optical element may be formed of a birefringent crystal cut at an angle such that the k-vector of the incident laser pulse is parallel to the optic axis of the birefringent crystal. Thus, the nonlinear optical element may include or consist of a crystal cut at a specific angle suitable to meet both requirements.

[0052] In some optional embodiments, the nonlinear optical element comprises or consists of TiO. The nonlinear optical element comprises or consists of TiO. The nonlinear optical element comprises or consists of TiO. 2 The high nonlinear refractive index n2 of TiO2 / W (in the rutile crystal structure) provides suitable transparency for laser pulses in the mid-infrared spectral region. In particular, nonlinear optical elements may therefore contain or consist of rutile. Additionally, rutile TiO2 features dispersion zero crossings in the mid-infrared region, which are beneficial for octave-wide spectral broadening, such as supercontinuum broadening, in the 2-3 μm and, optionally, 1-4 μm spectral wavelength regions. This may provide advantageous properties for broadening the spectrum of laser pulses emitted by Cr-doped II-VI laser oscillator systems with center wavelengths in the range of approximately 1.8 μm to 2.6 μm to shorter wavelengths, i.e., spectral regions below the fundamental wavelength spectrum of the laser pulses emitted by the laser oscillator system. For example, nonlinear optical elements based on TiO2, and particularly rutile, can be used to spectrally broaden laser pulses down to wavelengths of approximately 1.2 μm (30 dB attenuation relative to the maximum of the spectral power distribution). In addition, TiO2, and especially rutile-based nonlinear optical elements, can further broaden the spectrum towards longer wavelengths in the MIR spectral region.

[0053] In some optional embodiments, the laser system further includes a second nonlinear optical element for spectral broadening in the mid-infrared spectral region. The second nonlinear optical element optionally includes or consists of ZnGeP2 (also known as ZGP). The laser system may be adapted so that laser pulses propagating through the second nonlinear optical element undergo nonlinear frequency conversion. According to some optional embodiments, the nonlinear frequency conversion may include intrapulse difference frequency generation. Using the second nonlinear optical element for spectral broadening in the MIR spectral region allows for optimizing the spectral broadening toward shorter wavelengths separately from optimizing the spectral broadening toward longer wavelengths. This may provide additional flexibility for spectral broadening. The nonlinear optical element and the second nonlinear optical element may be arranged in a cascade configuration, such that the laser pulse propagates through the nonlinear optical element before propagating through the second nonlinear optical element. However, the order of the nonlinear optical elements may be reversed. In some optional embodiments, the laser pulse is compressed after propagating through the nonlinear optical element and before propagating through the second nonlinear optical element. Alternatively or additionally, the pulse may be compressed after propagating through the second nonlinear optical element. For example, the laser system may include one or more laser pulse compression elements, such as a diffraction grating, a prism, a grism, and / or a chirped mirror. In some embodiments, the nonlinear optical element may be formed from TiO2, particularly rutile, and the second nonlinear optical element may be formed from ZGP. Thus, in some embodiments, the method for generating optical pulses having a spectral component at least at a wavelength of 2 μm further includes focusing the laser pulse on a second nonlinear optical element including or consisting of ZnGeP2, and the laser pulse propagating through the second nonlinear optical element undergoes nonlinear frequency conversion. This method may be applied to a laser oscillator system according to one of the optional embodiments provided.

[0054] In some embodiments, the laser system can be used to generate supercontinuum optical pulses covering at least the 1.5 μm to 3.5 μm spectral range and having pulse widths of 15 fs FWHM or less.

[0055] One embodiment relates to the use of rutile TiO for nonlinear spectral broadening. In some embodiments, this involves irradiating the rutile with laser pulses having a peak power of at least 0.75 MW and a spectral component at least 2 μm in wavelength. This allows for the provision of few-cycle laser pulses in the MIR spectral region based on laser pulses emitted from a laser oscillator system without the need for external amplification.

[0056] In some optional embodiments, using rutile TiO for nonlinear optical applications includes or consists of multi-wave mixing applications. Further, in some optional embodiments, using rutile includes using rutile nonlinear optical elements in nonlinear optical applications.

[0057] In some optional embodiments, TiO2 is used for spectral broadening of laser pulses emitted by Cr-doped II-VI laser oscillator systems, particularly to shorter wavelengths. Traditionally, TiO2 has been used for spectral broadening and supercontinuum generation only in waveguides and spectral regions other than the MIR (see, e.g., "Spectral broadening in anatase titanium dioxide waveguides at telecommunication and near-visible wavelengths," Opt. Express 21, 18582-18591 (2013) and K. Hammani et al., "Octave Spanning Supercontinuum in Titanium Dioxide Waveguides," Applied Sciences 8, 543 (2018)).

[0058] To date, TiO2, especially rutile, has not been used for spectral broadening of pulses emitted from Cr-doped II-VI laser oscillators. One reason may be that to achieve strong spectral broadening of more than one octave in a thin nonlinear medium, especially for the 1 MW peak power of femtosecond Cr-doped II-VI oscillators, the medium must have a very large nonlinear refractive index (n2). However, a large n2 typically results in a small bandgap, which leads to strong multiphoton absorption (MPA) for laser beams with small spot sizes in nonlinear optical devices. MPA significantly reduces the broadening capability and, in extreme cases, can even lead to irreversible crystal degradation. In addition, the dispersion of rutile TiO2 is characterized by a zero crossing in the spectral region corresponding to that of Cr-doped II-VI oscillators. Therefore, self-compression and the resulting self-focusing may further improve spectral broadening. However, the present inventors have found that TiO2, and in particular rutile, has properties that make it suitable for use in thin nonlinear optical elements for broadening the spectrum of laser pulses in the spectral region around 2 μm wavelength. -14 cm2 This is a rather unusual material, possessing a large n value of 1 / W and a large band gap of approximately 3.2 eV, making it suitable for spectral broadening with minimal MPA in the spectral region around 2 μm wavelength.

[0059] Furthermore, the use of rutile TiO2 as a nonlinear optical element for nonlinear spectral broadening, especially with a thickness of 1 mm or less, allows substantial spectral broadening of femtosecond pulses in the MIR spectral range, with the added benefit of maintaining high spatial and / or temporal beam quality, which is beneficial for further utilization of spectrally broadened laser pulses and, in particular, their focusability.

[0060] The use of bulk rutile TiO2 as a nonlinear optical element offers the advantage of being insensitive to small variations in the pointing of the incident beam, resulting in no large variations in transmittance and spectral broadening stability, as is often observed in optical waveguides based on spatial confinement.

[0061] In another aspect, a laser system is provided, the laser system comprising a laser oscillator system adapted to emit laser pulses having a peak power of at least 0.75 MW, the emitted laser radiation having a center wavelength in the range of 1 μm to 4 μm, optionally in the range of 2 μm to 3 μm. The laser system further comprises a nonlinear optical element having a thickness of 1 mm or less. The laser system is adapted to irradiate the laser pulses emitted by the laser oscillator system onto the nonlinear optical element to spectrally broaden the laser pulses such that the spectrally broadened laser pulses span at least a half-optical octave. The nonlinear optical element may comprise or consist of TiO2, and in particular may comprise or consist of rutile TiO2. The laser oscillator system may be adapted as a thulium laser, i.e., a laser having a thulium-based gain medium.

[0062] The nonlinear optical element may have a thickness that is no greater than approximately 10 times the one-sided Rayleigh length of the laser pulse that is focused onto the nonlinear optical element. "Essentially" in this context means that the deviation between the thickness and ten multiples of the one-sided Rayleigh length is less than 10% of the thickness of the nonlinear optical element.

[0063] It is understood by those skilled in the art that the above-mentioned features and features in the following description and drawings are not only disclosed in the explicitly disclosed embodiments and combinations thereof, but also other technically feasible combinations and individual features are included in the present disclosure. Hereinafter, some optional embodiments and specific examples will be described with reference to the drawings for illustrative purposes, without limiting the present invention to the described embodiments. [Brief explanation of the drawings]

[0064] Further optional embodiments are described with reference to the following drawings. [Figure 1] 1 is a schematic diagram of a laser oscillator system according to a first optional embodiment. [Figure 2] FIG. 1 is a schematic diagram of a laser oscillator system according to a second optional embodiment. [Figure 3] FIG. 10 is a schematic diagram illustrating the use of a nonlinear optical element for spectral broadening of a laser, according to an optional embodiment. [Figure 4] 1 is a plot of normalized spectral intensity versus wavelength before and after spectral broadening. [Figure 5] FIG. 1 is a diagram of a laser system according to an optional embodiment. [Figure 6] FIG. 10 is a diagram of a laser system according to another optional embodiment. [Figure 7] 1 is a diagram showing an exemplary spectral power distribution of generated MIR radiation.

[0065] In the drawings, the same reference numbers are used for corresponding or similar features in different drawings.

[0066] FIG. 1 schematically illustrates a laser oscillator system 10 according to a first optional embodiment. The laser oscillator system 10 includes a resonator cavity 12 for confining an intracavity laser beam 13. Respective cavity mirrors 12a and 12b are disposed at both ends of the resonator cavity 12. The cavity mirrors 12a and 12b are sometimes referred to as end mirrors. According to an optional embodiment, one of the cavity mirrors 12a and 12b may include an out-coupler function for extracting a portion of the intracavity laser beam 13 outside the resonator cavity 12. For example, the cavity mirror 12a forming the out-coupler may be partially transparent to transmit a small portion of the intracavity laser beam 13.

[0067] The laser oscillator system 10 further includes a Cr-doped II-V I gain medium 14 that serves as the laser active medium. According to the illustrated embodiment, the gain medium 14 may be a Cr:ZnSe or Cr:ZnS gain medium suitable for amplifying optical radiation in the spectral region of approximately 1.8 μm to 3.0 μm. The gain medium may be directly diode-pumped by a suitable laser diode (not shown). To shape the intracavity laser beam 13 to exhibit a suitable spot size 100, i.e., a suitable beam waist, at and within the gain medium 14, two optical elements 16 are provided for appropriately focusing and collimating the intracavity laser beam 13. The optical elements may be optical lenses.

[0068] According to this embodiment, the gain medium 14 not only functions as a laser active medium for amplifying the intracavity laser beam 13, but also functions as a Kerr medium for realizing Kerr-lens mode locking in the laser oscillator system 10. In other words, the gain medium 14 functions as both a gain medium and a Kerr medium in one and the same element.

[0069] The laser oscillator system 10 additionally includes an imaging unit 18 for decoupling the spot size 100 of the intracavity laser beam 13 from the intracavity length 102 of the resonator cavity 12, indicated by the dashed double arrow in FIG. 1 . According to this embodiment, the imaging unit 18 is formed by a 4f-telescope 20 located near the cavity mirror 12b. The 4f-telescope includes two optical lenses 22, each having a focal length f, that are positioned at a distance of twice the focal length f, i.e., 2f, from each other. Furthermore, one of the optical lenses 22 is located at a distance corresponding to the focal length f from the cavity mirror 12b. Thus, the imaging unit 18 is configured to image the intracavity laser beam 13 from an image plane 104 to the cavity mirror 12b located adjacent to the imaging unit 18. The optical configuration of the resonator cavity 12, including the imaging unit, therefore virtually provides an image of the cavity mirror 12b at the image plane 104. The resonator modes of the intracavity light beam 13 in the portion of the resonator cavity 12 extending from the left cavity mirror 12a to the image plane 104 therefore define resonator modes that are identical to the resonator modes as if the right cavity mirror 12b were located at the image plane 104. Therefore, extending the intracavity length 102 of the resonator cavity 12 by the imaging unit 18 does not change the resonator modes and, in particular, does not affect the spot size 100 of the intracavity laser beam 13 in the gain medium. This is in contrast to simply extending the intracavity length 102 of the resonator cavity 12 without the imaging unit 18, in which case the beam waist 100 changes with increasing intracavity length 102 due to focusing of the intracavity laser beam 13 by the cavity mirrors 12a and 12b.

[0070] Due to the extended length of the resonator cavity 12 resulting from the use of the imaging unit 18, the repetition rate of the laser oscillator system 10 is reduced compared to placing the cavity mirror 12b at the image plane 104. This allows repetition rates of 50 MHz or less to be achieved. In some embodiments, repetition rates of 40 MHz or less, or even 30 MHz or less, may be achieved. Because the average laser output power (which remains essentially unchanged) is concentrated into a smaller number of pulses, the reduced repetition rate allows for higher pulse energies, and therefore higher peak powers, of the emitted laser pulses to be achieved. In particular, the presented embodiment can achieve a repetition rate of 25 MHz, corresponding to an intracavity length of 6.0 meters. Thus, the laser oscillator system can provide femtosecond laser pulses with peak powers of 1 MW or more.

[0071] According to optional embodiments, the laser oscillator system 10 has an adjustable intracavity length. For example, the positions of the cavity mirrors 12b and, optionally, the imaging unit 18 may be moved to shorten and / or lengthen the intracavity length 102 of the resonator cavity 12. For example, the intracavity length may be continuously adjustable and / or may be adjustable in steps. According to some optional embodiments, the intracavity length 102 of the resonator cavity 12 can be changed to some extent without requiring changes to the optical elements 22 of the imaging unit 18. According to some embodiments, changing the intracavity length 102 of the resonator cavity 12 may require replacing at least one of the optical elements 22 with a different optical element having a different focal length.

[0072] 2 is a schematic diagram of a second optional embodiment of the laser oscillator system 10, which corresponds in most respects to the first optional embodiment of the laser oscillator system 10. However, the second optional embodiment differs from the first optional embodiment in that a Kerr medium 24 for Kerr-lens mode-locking is provided separately from the gain medium 14. In addition, the second optional embodiment of the laser oscillator system 10 further includes two optical elements 26 for focusing and collimating the intracavity laser beam 13 onto the Kerr medium 24. These additional features enable the second optional embodiment of the laser oscillator system 10 to independently adjust the spot size of the intracavity laser beam 13 at the gain medium and the spot size at the Kerr medium 24. Thus, gain can be controlled by adjusting the spot size of the intracavity laser beam 13 in the gain medium and by selecting and adjusting the focal length and positioning of the optical elements 16 surrounding the gain medium 14, and Kerr-lens mode-locking can be independently controlled by tuning the Kerr effect by selecting and adjusting the focal length and positioning of the optical elements 26 surrounding the Kerr medium 24. This provides additional degrees of freedom for controlling the parameters of the laser oscillator system 10.

[0073] This decoupling further enables scaling of the peak power of the laser pulses emitted by the laser oscillator system 10, i.e., the spot size 100 in the gain medium 14, and optionally the overlap between the pump beam and the intracavity laser beam 13 for soft aperture mode-locking, can be optimized for maximum laser gain independently of optimizing the Kerr nonlinearity for optimal onset and maintenance of mode-locked operation, which can be optimized via the thickness and / or position and / or focusing spot size in the separate Kerr medium 24.

[0074] The resulting achievable peak power of the laser oscillator system 10, approaching or exceeding 1 MW, is large enough to efficiently drive nonlinear processes such as spectral broadening via self-phase modulation (SPM) in suitable nonlinear media. In addition to spectral broadening, the spectrally broadened pulses can also be compressed in the time domain to shorter durations.

[0075] 3 shows a schematic representation of the use of a nonlinear optical element 28 according to an optional embodiment for spectral broadening of the incident laser pulse as a laser beam 29, which has been focused and collimated by a respective optical element 27. The nonlinear optical element 28 is made of bulk TiO2 with a rutile crystal structure and is provided as a homogeneous material without any macroscopic structure that would impart waveguiding properties to the incident laser light 29. The nonlinear optical element has a thickness of 1 mm or less in the direction of propagation of the laser beam. This provides a high beam quality factor M that ensures high focusability after spectral broadening of the laser beam 29. 2 This allows maintaining the spectrally broadened laser pulses and therefore the good usability of the spectrally broadened laser pulses for applications requiring strong focusing. The limited thickness of the nonlinear optical element limits the possible degradation of the beam profile during spectral broadening, resulting in a high beam quality factor. To achieve significant spectral broadening with a thin nonlinear optical element, it is advantageous to position the nonlinear optical element 28 closer to the focus of the laser beam 29 compared to the typical position of a thicker nonlinear optical element having a thickness of several millimeters. The small spot size of the laser beam 29 within the thin nonlinear optical element 28 leads to a strong Kerr lens effect, which remixes the wavelength components of the laser beam 29, thus increasing the homogeneity of the spectral distribution on the beam profile. This homogenization of the spectral components reduces the degradation of the beam profile associated with spectral broadening.

[0076] Thus, by using a thin nonlinear optical element 28 having a thickness of 1 mm or less for spectral broadening, the transmitted laser beam retains high spatial and temporal quality that is advantageous for further use, such as the subsequent generation of mid-infrared radiation.

[0077] 4 shows a graph 400 of normalized spectral intensity (vertical axis, logarithmic scale) versus wavelength (in nanometers). Graph 402 represents the normalized spectral intensity of a laser pulse emitted by a Cr-doped II-VI laser oscillator system according to one embodiment, before any additional spectral broadening. As can be seen, the spectral intensity peaks near a wavelength of 2.2 μm and extends to shorter wavelengths down to about 2.05 μm before decreasing sharply. The cutoff wavelength is attenuated by about 30 dB compared to the maximum, i.e., 10 -3 , reaching a wavelength of approximately 1.95 μm. On the longer wavelength side, the spectrum broadens to approximately 2.45 μm. Thus, the spectrum of the laser pulse emitted by the laser oscillator system 10 according to the optional embodiment spans from approximately 1.95 μm to approximately 2.45 μm. After spectrally broadening the laser pulse with the apparatus detailed with reference to FIG. 3, the spectrum significantly acquires additional spectral components, as shown in graph 404. Spectral broadening of the laser pulse was achieved by focusing the laser pulse on a nonlinear optical element 28 formed of a 0.5 mm thick bulk rutile TiO plate. Graph 404 shows that significant spectral broadening occurs, especially on the shorter wavelength side, resulting in a spectral intensity distribution that extends to a wavelength of approximately 1.2 μm before disappearing in the noise. Similarly, on the longer wavelength side, the spectral intensity increased in the wavelength range from approximately 2.2 μm to approximately 2.4 μm. Thus, spectral broadening significantly increased the spectral content on both the short-wavelength and long-wavelength sides of the original spectrum of the laser pulse emitted from the laser oscillator system.

[0078] In some optional embodiments, laser pulses can be used to generate mid-infrared radiation that extends to longer wavelengths within the MIR spectral region, with or without spectral broadening in a nonlinear optical element 28, for example, as shown in Figure 3. The generation of the MIR radiation can be performed via nonlinear frequency conversion using laser pulses emitted by a laser oscillator system without additional spectral broadening, or using laser pulses produced by a laser system that includes spectral broadening, as shown in Figure 3. Both approaches are suitable for generating MIR radiation without the need for further amplification of the laser pulses in an amplifier stage outside the laser oscillator system.

[0079] In one embodiment of the laser system 30 shown in FIG. 5 , laser pulses emitted by a Cr-doped II-VI laser oscillator system 10 having a peak power of at least 0.75 MW are directly focused onto a (second) nonlinear optical element for nonlinear frequency conversion and MIR generation. To reduce optical dispersion and the resulting degradation of the pulse shape of the emitted laser pulses, the laser system 30 for nonlinear frequency conversion shown in FIG. 5 includes reflective optical elements, including two steering mirrors 32 and two off-axis parabolic mirrors 34, for focusing the laser pulses onto the nonlinear optical element for nonlinear frequency conversion and generation of MIR radiation 36 and collimating the laser pulses (also referred to as the second nonlinear optical element 36). The dotted line 38 indicates the optical path of the laser pulses. The dashed line 40 indicates the optical path of the MIR radiation generated by the laser pulses in the second nonlinear optical medium by nonlinear frequency conversion, specifically intrapulse difference frequency generation. As shown, the propagation directions of the laser pulses and the generated MIR radiation are the same.

[0080] Figure 6 shows a laser system 30 including nonlinear frequency conversion according to another embodiment, which in most respects corresponds to the embodiment of the apparatus 30 presented in Figure 5. However, the apparatus 30 according to this embodiment differs from the apparatus 30 shown in Figure 5 in the feature that the laser pulses used for generating MIR radiation are subjected to prior spectral broadening and pulse compression in a nonlinear optical element (as exemplarily shown in Figure 3). To this end, the laser pulses emitted by the Cr-doped II-VI laser oscillator system 10 are applied to corresponding apparatuses 42 and 44 for nonlinear spectral broadening and temporal pulse compression, respectively, before focusing the laser pulses into a second nonlinear optical element 36 for nonlinear frequency generation and generation of MIR radiation.

[0081] 7 shows an exemplary diagram 700 of the spectral power distribution of MIR radiation generated by the laser system 30 presented with respect to FIG. 5, where nonlinear frequency conversion and MIR radiation generation were driven by laser pulses emitted by a Cr:ZnS laser oscillator system with a peak power of 1 MW. No spectral broadening or pulse compression was applied prior to MIR radiation generation.

[0082] Diagram 700 shows spectral power (unit: mW / nm) on the vertical axis, wavelength (unit: micrometer) on the lower horizontal axis, and frequency (unit: THz) on the upper horizontal axis. The solid line in graph 702 shows the spectral power obtained by nonlinear frequency conversion driven by a laser pulse, which is 87 GW / cm in the nonlinear optical medium for nonlinear frequency conversion placed at the focus of the laser pulse. 2 Graph 702 provides an estimated peak intensity of 10 -1 mW / nm to about 10 -6 The spectral power distribution spans approximately 15 μm, with spectral power in the mW / nm range, exceeding the peak power achievable with conventional Cr-doped II-VI laser oscillator systems, which is only 13 GW / cm. 2The substantial amount of MIR radiation generated in this process becomes even more apparent when compared to the spectral power (graph 704) of the (essentially non-existent) MIR radiation generated at a focal peak intensity of 13 GW / cm. As graph 704 shows, graph 704 corresponds roughly to the detection noise, so 2 Thus, diagram 700 illustrates that while Cr-doped II-VI laser oscillator systems according to embodiments that provide laser pulses with peak powers of at least 0.75 MW, or even at least 1 MW, are well suited for generating MIR radiation without prior spectral broadening and pulse compression, conventional Cr-doped II-VI laser oscillator systems do not provide laser pulses with sufficient peak power to generate MIR radiation without further external amplification. [Explanation of symbols]

[0083] 10 Laser oscillator system 12 Resonator Cavity 12a, 12b Cavity mirror / end mirror 13 Intracavity laser beam 14 Gain medium 16 Optical Elements 18 Imaging unit 20 4f-telescope 22 Optical elements of the imaging unit 24 Car Media 26 Optical Elements 27 Optical Elements 28 Nonlinear optical elements (for spectral broadening) 29 Laser Beam 30 Laser System 32 Steering mirror 34 Parabolic Mirror 36 (Second) Nonlinear Optical Element (for nonlinear frequency conversion) 38 Laser pulse path 40 Optical path of generated MIR radiation 42 Spectral broadening devices 44 Apparatus for temporal pulse compression f focal length of optical element 22 100 spot size / beam waist in gain medium 102 Intra-cavity length of resonator cavity 104 4f-Telescope image plane 400 Plot showing spectral intensity before and after spectral broadening 402 Normalized intensity of emitted laser pulse 404 Normalized intensity after spectral broadening Diagram showing the spectral intensity after 700 MIR generation 702 87GW / cm 2 Spectral power of the MIR radiation generated by 704 13GW / cm 2 Spectral power of the MIR radiation generated by

Claims

1. A laser system (30), comprising: A laser oscillator system (10), comprising: a resonator cavity (12) for confining an intracavity laser beam (13); a Cr-doped II-VI gain medium (14) disposed within the resonator cavity (12); an imaging unit (18) forming part of the resonator cavity (12); Equipped with the imaging unit (18) is adapted to decouple a spot size (100) of the intracavity laser beam (13) in the gain medium (14) from an intracavity length (102) of the resonator cavity (12); the resonator cavity (12) and the imaging unit (18) are adapted so that the laser oscillator system (10) emits laser pulses at a repetition rate of 50 MHz or less; the laser oscillator system (10) adapted to emit laser pulses having a peak power of at least 0.75 MW and a center wavelength of 1.8 μm or greater; a nonlinear optical element (28) having a thickness of 1 mm or less and a nonlinear refractive index n 2 of at least 5×10 −19 m 2 / W at a wavelength of 2 μm; Equipped with The laser system is adapted to focus the laser pulses emitted by the laser oscillator system (10) onto the nonlinear optical element (28) to broaden the spectrum of the laser pulses such that the spectrally broadened laser pulses span at least a half optical octave.

2. A laser system (30) as described in claim 1, wherein the laser pulse emitted by the laser oscillator system (10) has a pulse width of FWHM 30 fs or less.

3. 2. The laser system (30) of claim 1, wherein the nonlinear optical element (28) is TiO. ２ or TiO ２ A laser system consisting of:

4. 4. The laser system (30) of claim 3, wherein the nonlinear optical element (28) is rutile TiO. ２ or rutile TiO ２ A laser system consisting of:

5. 5. The laser system (30) of claim 1, further comprising a second nonlinear optical element (36) for spectral broadening in the mid-infrared spectral region, the second nonlinear optical element (36) optionally comprising or consisting of ZnGeP2, and wherein the laser system is adapted such that the laser pulses propagating through the second nonlinear optical element (36) undergo nonlinear frequency conversion.

6. 5. The laser system (30) of claim 1, wherein the laser system is adapted to support a pulse width of 15 fs or less after the spectrum of the laser pulse has propagated through the nonlinear optical element (28).

7. The laser system (30) of any one of claims 1 to 4, wherein the nonlinear optical element (28) includes an anti-reflection coating on a surface facing the incident laser pulse.

8. 5. The laser system (30) according to claim 1, wherein the nonlinear optical element (28) is arranged at Brewster's angle with respect to the direction of incidence of the laser pulse at the center wavelength of the laser pulse, and the nonlinear optical element (28) is formed of a birefringent crystal cut at an angle such that the k-vector of the incident laser pulse is parallel to the optical axis of the birefringent crystal.

9. 1. A method for generating optical pulses having a spectral component at least at a wavelength of 2 μm, comprising the steps of: providing a laser pulse emitted from a laser oscillator having a pulse width of 30 fs or less FWHM, a peak power of at least 0.75 MW, and a center wavelength of 1.8 μm or greater; Thickness 1mm or less and at least 5.10-19m at wavelength 2μm ２ / W nonlinear refractive index n ２ focusing the laser pulses onto a nonlinear optical element having A method comprising:

10. 10. The method of claim 9, wherein the nonlinear optical element is ZnGeP. ２ or ZnGeP ２ A method for nonlinear frequency conversion comprising:

11. 10. The method of claim 9, wherein the nonlinear optical element (28) is TiO ２ or TiO ２ and optionally rutile TiO ２ or rutile TiO ２ A method comprising:

12. 12. The method of claim 11, wherein the nonlinear optical element (28) has a thickness of less than or equal to 10 times the Rayleigh length of the laser pulse focused onto the nonlinear optical element.

13. 13. The method of claim 11 or 12, further comprising: directing the laser pulses to a ZnGeP ２ or ZnGeP ２ wherein the laser pulses propagating through the second nonlinear optical element (36) undergo nonlinear frequency conversion.

14. The method according to any one of claims 9 to 12, wherein the laser oscillator (10) is a laser oscillator system (10), a resonator cavity (12) for confining an intracavity laser beam (13); a Cr-doped II-VI gain medium (14) disposed within the resonator cavity (12); an imaging unit (18) forming part of the resonator cavity (12); Equipped with the imaging unit (18) is adapted to decouple a spot size (100) of the intracavity laser beam (13) in the gain medium (14) from an intracavity length (102) of the resonator cavity (12); The resonator cavity (12) and the imaging unit (18) are a laser oscillator system (10), wherein the laser oscillator system (10) is adapted to emit laser pulses at a repetition rate of 50 MHz or less.

15. Use of a laser system (30) according to any one of claims 1 to 4 for generating supercontinuum light pulses covering at least the spectral range from 1.5 μm to 3.5 μm and having a pulse width of FWHM 30 fs or less.

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