Laser system and method for generating laser pulses

Abstract

An embodiment relates to a laser system (20) for generating laser pulses having a determined carrier-envelope offset (CEO), comprising a Cr-doped II-VI based laser oscillator system (22) having a resonator cavity (112), the laser oscillator system (22) adapted to emit laser pulses (10) from the resonator cavity (112) having a peak power of at least 0.75 MW. The laser system further comprises a nonlinear optical element (26) for spectrally broadening at least a portion of the emitted laser pulse (10) incident on the nonlinear optical element to obtain a laser pulse (10) having octave-spanning spectral components, and a frequency doubling element (34) for generating a second harmonic frequency component of at least a portion of the octave-spanning spectral components of the spectrally broadened laser pulse (10) when the spectrally broadened laser pulse incidents on the frequency doubling element (34) such that the second harmonic frequency component spectrally overlaps with the remainder of the octave-spanning spectral components of the laser pulse (10). The laser system further comprises an f-2f interferometer device (40) for generating a beating signal of at least a portion of the overlapped spectral components that interfere with each other in the f-2f interferometer device and for determining and / or controlling the CEO of the emitted laser pulse (10) based on the beating signal. Another embodiment relates to a method for generating a laser pulse (10) having a determined carrier-envelope offset.

Description

Technical Field

[0001] Embodiments of the present invention relate to a laser system and method for generating laser pulses having a determined carrier-envelope offset. Accordingly, the present embodiments relate to laser technology.

Background Art

[0002] Carrier-envelope offset stabilized mode-locked lasers are often used for spectroscopy in the frequency domain or time domain. In particular, frequency combs operating in the mid-infrared (MIR) spectral region, which is a region containing most of the characteristic rotational-vibrational molecular transitions, are useful light sources for spectroscopic applications. Phase-stabilized MIR radiation can be generated by realizing a high-power femtosecond laser with an actively stabilized carrier (carrier wave) envelope phase and nonlinearly converting its output to the long wavelength region (see Non-Patent Document 1). Cr-doped II-VI lasers having an ultra-wideband emission spectrum centered around 2.3 μm are particularly suitable for generating frequency combs in MIR based on a fiber pump laser system (see Non-Patent Document 2). However, using a directly diode-pumped Cr-doped II-VI laser for frequency comb generation promises significantly better synchronization performance compared to a fiber pump laser system by reducing the relative intensity noise of the output (see Non-Patent Document 3).

[0003] Generally, the generation of frequency combs requires the stabilization of the carrier - envelope offset frequency (CEO) of the driving laser source, which is usually an important task and can be carried out with interferometer technology. An approach based on a Cr - doped II - VI laser for detecting the CEO is described, for example, in Patent Document 1 (US10886690B2), according to which the optical harmonics of the laser output generated in a non - linear laser medium are employed to generate beat notes. However, such a solution has limitations in that when generated in a laser amplifier crystal, it has been shown that at a resolution bandwidth of 100 kHz, the signal - to - noise (S / N) value of the beat notes significantly exceeds 30 dB (see Non - Patent Document 4). The threshold of 30 dB is a normal reference value, and generally, stable phase synchronization can be ensured when this threshold is exceeded.

[0004] According to another conventional approach, which is particularly used for lasers operating at wavelengths shorter than 2 μm, an interferometer - type configuration generally including an f - 2f interferometer is adopted (see Non - Patent Document 5). This approach is based on the beat between two spectral components, one of which is taken from the original spectrum and the other is generated through frequency doubling of the original spectrum in a non - linear medium. Therefore, in this method, a super - broadband spectrum spanning at least an optical octave is required such that the non - linearly generated frequency component spectrally overlaps with the original component to generate beat notes.

[0005] Since the output of a conventional oscillator has a lower peak power compared to an amplifier, when using an optical waveguide such as an optical fiber to spatially confine and propagate the waveguide, spectral broadening was achieved by non-linearly expanding the spectrum of the pulse (see Non-Patent Document 6). However, such waveguides usually have very small apertures on the order of several micrometers. Therefore, minute fluctuations in the pointing of the incident beam lead to differences in the amount of light coupled into the waveguide, ultimately affecting the spectral spread and its stability. Furthermore, the optical coupling efficiency into the waveguide is often limited to 70% or less, resulting in undesirable power losses. Additionally, when propagating through the interior of a non-linear fiber, strong phase distortion occurs, and there may be cases where the second harmonic and the fundamental wave do not overlap temporally. In such cases, additional delay compensation is required, leading to complexity and noise.

[0006] Pages 1 - 2 of Non-Patent Document 7 describe a laser system having a Cr:ZnS laser oscillator that emits laser pulses with a peak power of 0.5 MW and a Cr:ZnS amplifier that amplifies the laser pulses emitted from the oscillator. Furthermore, it is described that due to the non-linear characteristics of polycrystalline Cr:ZnS, amplification, spectral broadening, and non-linear pulse compression can be performed simultaneously, and the generation of optical harmonics is also possible. Therefore, all these steps are carried out with the exact same amplifier crystal.

[0007] Pages 101930U - 101930U of Non-Patent Document 8 describe a Cr:ZnS laser system that emits laser pulses with a peak power of 1 MW.

[0008] Page 2458 of Non-Patent Document 9 describes a Cr:ZnS oscillator having a resonator length of approximately 1.9 m and a repetition rate of approximately 79 MHz.

[0009] Patent Document 2 (JP 2002 171015 A) describes a multipass arrangement that forms part of a resonator for reflecting a laser beam multiple times back and forth to increase the beam path length while keeping the external dimensions compact.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0011]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Non-Patent Document 9

Summary of the Invention

Problems to be Solved by the Invention

[0012] Therefore, it is desirable to provide a solution for generating laser pulses in the MIR spectral region having a determined carrier-envelope offset with high stability and low noise at wavelengths around 2 μm or longer wavelengths. The solution should enable high repetition rates, as well as low technical complexity and low manufacturing costs.

Means for Solving the Problems

[0013] The solution is provided by embodiments having the features of the independent claims. Optional embodiments are the subject of the dependent claims and the specification.

[0014] One embodiment relates to a laser system for generating laser pulses having a determined carrier envelope offset (CEO). The laser system comprises a Cr-doped II-VI based laser oscillator system having a resonator cavity, and the laser oscillator system is adapted to emit from the resonator laser pulses having a peak power of at least 0.75 MW. The laser system further comprises a non-linear optical element for spectrally broadening at least a portion of the emitted laser pulses, the non-linear optical element imparting to the laser pulses irradiated thereon spectral components of an octave span. Further, the laser system comprises a frequency doubling element which, when irradiating the spectrally broadened laser pulses on the frequency doubling element, generates second harmonic spectral components of at least a portion of the spectral components of the octave span of the spectrally broadened laser pulses, such that a portion of the second harmonic spectral components spectrally overlaps with a portion of the remaining spectral components of the octave span of the laser pulses. Further, the laser system comprises an f-2f interferometer device for generating a beating signal of at least a portion of the overlapping spectral components emerging from the frequency doubling element and interfering with each other in the f-2f interferometer device, and for determining and / or controlling the CEO of the laser pulses emitted based on the beating signal.

[0015] Another embodiment relates to a method of generating laser pulses having a determined carrier - envelope offset (CEO). The method comprises supplying a laser pulse having a peak power of at least 0.75 MW and spectral components in the range of 1.8 μm to 2.4 μm radiated from a resonator cavity of a laser oscillator system. The method further comprises spectrally broadening the laser pulse to impart spectral components of an octave span to the laser pulse, and in a frequency - doubling element, generating second - harmonic spectral components of at least a portion of the spectral components of the octave span of the spectrally - broadened laser pulse such that a portion of the spectral components of the second - harmonic overlaps with a portion of the spectral components of the remaining octave span of the laser pulse. Additionally, the method comprises the steps of emitting from the frequency - doubling element and interfering with each other in an f - 2f interferometer device to generate a beating signal of at least a portion of the overlapping spectral components, and determining and / or controlling the CEO of the radiated laser pulse based on the beating signal.

[0016] The carrier - envelope offset can be determined and / or controlled in the time domain and / or the frequency domain. In the time domain, the carrier - envelope offset is represented by the carrier - envelope offset phase, the CEO phase, which is also referred to as the carrier - envelope phase, CEP. The CEP is the phase offset between the carrier light wave and the envelope, that is, the change in the amplitude of the light wave over time. The amplitude of the light wave may be the amplitude of the electric field or the amplitude of the power of the light wave, and these reach their maximum at the same time point. When the local maximum of the carrier light wave coincides temporally with the maximum amplitude of the electric field, the carrier - envelope offset is defined as zero. When the phase of the oscillation is shifted with respect to the maximum electric field, the CEP becomes non - zero. Since the CEP is a phase and thus related to an angle, its quantity is usually expressed in radians. In the frequency domain, the CEO is the carrier - envelope offset frequency, the CEO frequency, fCEO is determined by. The CEO frequency f CEO The amount of is usually expressed in Hertz. The CEO frequency f CEO can vary, for example, from 0 Hz to the repetition rate of the laser oscillator system. The CEO frequency f stabilized at 0 Hz CEO is such that the CEP is stabilized at a fixed value, i.e., the CEP does not change with respect to the laser pulse. Similarly, when the CEO frequency f CEO is stabilized at a frequency equal to the repetition rate f of the laser oscillator rep the CEP becomes the same for different laser pulses. However, an uncontrolled CEO frequency f CEO , or a CEO frequency f stabilized at a frequency different from 0 Hz and the repetition rate f rep results in the CEP changing from pulse to pulse. Thus, controlling the CEO frequency f CEO to a specific value (not equal to 0 and the repetition rate) will result in the CEP having a defined variation, and only stabilizing the CEO frequency f CEO to 0 or the repetition rate f CEO results in the CEP being constant. On the other hand, controlling the CEO to a specific CEP in the time domain necessarily results in the CEO frequency being stabilized. Depending on some applications, controlling the CEO may be necessary to stabilize the CEP to a predetermined value, while in other applications, controlling the CEO frequency f rep may be sufficient and / or advantageous. CEO By controlling.

[0017] Saying that the CEO is determined means that the CEO is known and may be considered when using the laser pulse. Thus, in some embodiments, the CEO may be controlled, for example, set to a predetermined value. In some other embodiments, the CEO may be measured and the variation of the CEO may be considered in the application of the laser pulse.

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

[0019] A Cr-doped II-VI-based laser oscillator system is a laser oscillator system having a Cr-doped II-VI gain medium. The Cr-doped II-VI gain medium includes a II-VI bulk medium doped with chromium atoms. The II-VI medium is composed of chemical elements of Group 2 and Group 6 of the periodic table. The II-VI medium may include a II-VI crystal doped with chromium. In some optional embodiments, the Cr-doped II-VI-based laser oscillator system includes or consists of a gain medium including Cr-doped ZnS and / or Cr-doped ZnSe. However, according to other embodiments, different II-VI materials may be used. The Cr-doped II-VI gain medium enables a wide spectral range for laser activity from about 1.8 μm to about 3.0 μm and is thus well-suited for the generation of laser pulses in the MIR spectral region, particularly femtosecond laser pulses in the MIR spectral region. However, as long as it is suitable for the generation of laser pulses in the MIR spectral region that spectrally assist the femtosecond pulse width, alternatively or additionally, one or more other gain media may be used.

[0020] When a laser pulse is emitted from the resonator cavity of a laser oscillator system having a peak power of 0.75 MW, it means that a laser pulse having the peak power can be supplied only by the laser oscillator system without requiring additional external amplification (such as an external amplification stage). For example, the laser oscillator system may have a repetition rate of 50 MHz or less to supply a laser pulse having a peak power of 0.75 MW or more.

[0021] When spectral components in the range of 1.8 μm to 2.4 μm are emitted from the resonator cavity, it means that the emitted light from the resonator cavity spectrally covers at least a part in the range of 1.8 μm to 2.4 μm. The spectral components may cover the entire range from 1.8 μm to 2.4 μm, or may cover a part of the range. The fact that the laser pulse has spectral components in the range of 1.8 μm to 2.4 μm does not further exclude the possibility of having spectral components outside the range of 1.8 μm to 2.4 μm, for example, spectral components having longer wavelengths and / or shorter wavelengths.

[0022] A nonlinear optical element is an optical element having a non-zero nonlinear refractive index. In particular, the nonlinear optical element may have a second-order refractive index n2 suitable for performing nonlinear optical applications based on a laser pulse. For example, the second-order refractive index n2 may be at least 10 -15 cm 2 / W for performing desired nonlinear optical applications such as spectral broadening by self-phase modulation and / or multi-wave mixing.

[0023] A spectrally broadened laser pulse having spectral components of an octave span means that the frequency spectrum of the spectrally broadened laser pulse extends from at least a first frequency to a second frequency that is twice the first frequency. Throughout this disclosure, the spectrum is considered to extend to a particular frequency at which the spectral intensity equals 0.001 times the maximum spectral intensity. In other words, the spectrum of the laser pulse is considered to extend to a wavelength or frequency at which the spectral intensity is attenuated by 30 dB, i.e., 1,000 times, compared to the maximum spectral intensity.

[0024] A frequency doubling element is a non-linear optical element that is suitable for and / or optimized for non-linearly optically generating the second harmonic spectral components of a laser pulse. The frequency doubling element can be optimized with respect to material and / or cleavage angle and / or thickness to achieve the desired conversion of the spectral components of the laser pulse propagating through the frequency doubling element to second harmonic spectral components having a frequency that is twice that of the fundamental spectral components of the laser pulse.

[0025] The fact that the second harmonic spectral components spectrally overlap with a portion of the remaining octave span spectral components of the laser pulse means that a particular wavelength range and frequency range are covered by the fundamental spectral components of the laser pulse as well as the generated second harmonic spectral components.

[0026] An f-2f interferometer device is a device that enables the generation and detection of a beating signal of at least a portion where the (fundamental) spectral component and the second harmonic spectral component of a laser pulse overlap spatially / temporally. The f-2f interferometer device does not require two separate interferometer arms. Instead, the f-2f interferometer device can be adapted such that both the fundamental spectral component and the second harmonic spectral component of the laser pulse propagate within one and the same interferometer arm. The f-2f interferometer may include a detector for detecting an overlapping spectral component that generates a beating signal as an electrical signal that can be evaluated by at least an electrical circuit and / or an electronic circuit. For example, the beating, in particular the frequency of the beating signal, may depend on the CEO of the laser pulse. In particular, the frequency of the beating signal may correspond to the CEO frequency f CEO and. The beating signal may have a frequency within the radio frequency range.

[0027] Embodiments provide the advantage that laser pulses having a determined carrier - envelope offset (CEO) can be generated in the mid - infrared (MIR) with low noise, i.e., no variation of the CEO or low - level variation of the CEO. In particular, a laser system according to embodiments can exhibit a CEO phase noise (integrated between 1 Hz and 1 MHz) of 200 mrad or less, optionally 100 mrad or less, optionally 50 mrad or less, and further optionally 30 mrad or less. This is particularly advantageous by using laser pulses emitted by a Cr - doped II - VI - based laser oscillator system having a peak power of at least 0.75 MW. Since the peak power of the laser pulses emitted from the laser oscillator system is high, the laser pulses can be used directly for spectral component generation and frequency doubling of the octave span without the need for an additional external laser amplifier outside the laser oscillator system. Thus, the present embodiments do not require an external laser amplifier stage, thereby avoiding additional sources of noise and CEO variations. Furthermore, since no additional laser amplifier stage is required, the complexity, spatial expansion, and manufacturing cost of the laser system for generating laser pulses having a determined CEO can be kept at a low level. In particular, due to the spectral wavelength region in the MIR, the laser system can open up new applications for CEO - stabilized laser pulses for spectroscopic applications in cost - sensitive environments and / or environments that require a small spatial expansion of the laser system.

[0028] In particular, when using a directly diode - pumped mode - locked Cr - doped II - VI laser oscillator system, it is possible to achieve both a low noise level and a compact size and low manufacturing cost.

[0029] Furthermore, embodiments provide the advantage that, because the laser pulses directly from the laser oscillator system have high peak power, there is no need to use an optical waveguide (usually having a length of 1 cm or more) to achieve the spectral broadening necessary to obtain the spectral components of the octave span. Instead, a bulk nonlinear optical element having a thickness of less than 1 cm may be used, which results in lower optical losses than an optical waveguide. This also provides the further advantage that embodiments do not need to compensate for differences in the optical path length of waveguides, which often results from the high optical dispersion of such waveguides. In spectral broadening using conventional waveguides, it was necessary to correct for different optical path lengths due to optical dispersion before splitting the fundamental spectral component and the second harmonic spectral component into two separate interferometer arms and recombining them to generate a beating signal. However, this drawback of conventional systems can also be avoided by embodiments because they do not require an optical waveguide to achieve the desired spectral broadening. Thus, in an embodiment, it is possible to provide only a single mutual interferometer arm of f-2f interferometry for the fundamental spectral component and the second harmonic spectral component, thereby maintaining the complexity at a low level and avoiding the generation of additional noise of the beating signal generated from separate interferometer arms. Therefore, embodiments enable achieving high stability in CEO stabilization and / or CEO determination. Some embodiments provide the advantage that the CEO can be stabilized such that the total variation of the CEO is as low as 200 mrad or less, i.e., the integrated phase noise of the stabilized CEO frequency integrated in the range of 1 Hz to 1 MHz is 200 mrad or less. Optionally, the integrated phase noise may be 100 mrad or less, optionally 50 mrad or less, optionally 30 mrad or less.

[0030] Furthermore, since the non-linear optical element and the frequency doubling element for spectral broadening are provided as separate elements, the present disclosure offers the advantage that the spectral broadening process and the frequency doubling process can be optimized independently of each other. In other words, the non-linear optical element for spectral broadening is optimized to obtain high-efficiency spectral broadening, and the frequency doubling element can be optimized to obtain high-efficiency frequency doubling of the spectrally broadened laser pulse. Thereby, a specific high peak power can be achieved in the laser oscillator system without adding an external amplifier that may adversely affect the stabilization of the CEO. Further, the non-linear optical element for spectral broadening and the frequency doubling element may be separated from the gain medium within the laser oscillator system, that is, no amplification of the laser pulse occurs in the non-linear optical element and the frequency doubling element for spectral broadening. This makes it possible to optimize the gain of laser amplification in the laser oscillator system independently of spectral broadening and frequency doubling, and vice versa.

[0031] In some optional embodiments, the overlapping second harmonic spectral components emitted from the frequency doubling element propagate collinearly and temporally overlap with the remaining fundamental spectral components along a common optical path. In other words, the fundamental spectral components and the second harmonic spectral components of the laser pulse are not split into two different interferometer arms. Thereby, noise, that is, fluctuations in the beating signal that may occur in some cases, and thus fluctuations in the CEO frequency can be suppressed to a very low level. Therefore, this further improves the stability of the laser pulse with respect to the CEO.

[0032] In some optional embodiments, the beating signal is generated only by the interference and overlapping spectrum of the second harmonic spectrum component of the laser pulse remaining after the frequency doubling element and the spectrum component of the octave span. In other words, in order to generate the second harmonic spectrum component and to obtain the basic beam for the f-2f interferometer device to generate the beating signal, there is no need to split the laser beam to obtain a separate basic laser pulse. Instead, the basic laser pulse remaining after the generation of the second harmonic spectrum component may be used for the generation of the beating signal. Thereby, the noise can be kept at a very low level. Therefore, further, the stability regarding the CEO of the laser pulse can be improved.

[0033] In some optional embodiments, the laser system further includes a spectral filter element for spectrally filtering the overlapping spectral components before generating the beating signal. In some embodiments, the spectral filter element may include a spectral bandpass filter or may be composed of a spectral bandpass filter. Thereby, the possibility of the influence of spectral components that do not contribute to the generation of the beating signal can be reduced and / or avoided. Therefore, further noise can be reduced.

[0034] In some optional embodiments, the step of controlling the CEO of the emitted laser pulse includes the step of adjusting the laser oscillator system to emit a laser pulse having a predetermined CEO. For example, in some optional embodiments, the step of adjusting the laser oscillator system may include the step of adjusting the intracavity dispersion of the resonator, and / or the step of adjusting the excitation power for exciting the gain medium of the laser oscillator system, and / or the step of adjusting the internal optical loss of the resonator. Thereby, a means for efficiently controlling the CEO of the laser pulse is provided. For example, the internal optical loss of the resonator can be controlled using an acousto-optic modulator in the resonator.

[0035] In some optional embodiments, the step of controlling the CEO includes the step of stabilizing the CEO to a desired CEO, i.e., a predetermined CEO frequency and / or a desired CEP. For example, the CEO may be controlled such that the CEO frequency becomes zero or near zero. This provides the advantage that the waveform of the laser pulse can be stabilized periodically or constantly, i.e., the pulse-to-pulse variation of the waveform can be reduced or minimized. Further, the CEP may be controlled such that the CEP becomes zero or near zero. This may provide the advantage that the maximum amplitude of the electric field is obtained at the maximum value of the temporal envelope. However, according to other optional embodiments, the CEO may be controlled to stabilize at a CEO frequency and / or a CEP different from zero. Depending on the application, it may be advantageous to have a constant CEO frequency and / or a constant CEP, while the specific values of the CEO frequency and / or the CEP are not so important. Stabilizing the CEO to a non-zero frequency / phase angle may reduce the technical effort for such applications. According to still other optional embodiments, the CEO may not be static and may be controlled to change in a predetermined manner. This is beneficial for applications involving different CEO frequencies.

[0036] In some optional embodiments, the nonlinear optical element for spectral broadening has a thickness of 1 mm or less. This can provide the advantage that, even after broadening the spectral width with the nonlinear optical element, a high quality of the beam profile of the laser pulse is maintained, i.e., a high beam quality factor M2 close to 1.2 is maintained. This can be particularly advantageous when only a part of the spectrally broadened laser pulse is used for CEO determination and / or control, and another part of the spectrally broadened laser pulse is used for intended applications such as experiments, spectroscopy, medical, and / or machining applications. Further, this can improve the coherence of the emitted light and the signal-to-noise ratio of the beating signal. However, according to other embodiments, a thicker nonlinear optical element may be selected for spectral broadening. For example, a thickness between 1 mm and 2 mm may be selected.

[0037] In some optional embodiments, the nonlinear optical element for spectral broadening comprises or consists of rutile-type TiO2. The nonlinear optical element comprising or consisting of TiO2 has a high nonlinear refractive index n2 of about 10 -14 cm 2 / W (in the case of the rutile crystal structure) and exhibits transparency suitable for laser pulses in the mid-infrared spectral region. In particular, the nonlinear optical element thus comprises or consists of rutile. This can provide advantageous properties for broadening the spectrum of the laser pulse emitted by a Cr-doped II-VI laser oscillator system having a central wavelength in the range of about 1.8 μm to 2.6 μm to a shorter wavelength, i.e., a spectral region below the fundamental wavelength spectrum of the laser pulse emitted by the laser oscillator system. For example, TiO2, and in particular a rutile-based nonlinear optical element, can be used to spectrally broaden the laser pulse to a wavelength of about 1.2 μm (30 dB attenuation with respect to the maximum value of the spectral power distribution). In addition, TiO2, particularly a rutile-based nonlinear optical element, can broaden the spectrum up to a longer wavelength MIR spectral region.

[0038] In some optional embodiments, a Cr-doped II-VI-based laser oscillator system may include an imaging unit that forms part of a resonator cavity, the imaging unit being adapted to decouple the spot size of the intracavity laser beam in the gain medium from the intracavity length of the resonator cavity, and the resonator cavity and the imaging unit being adapted such that the laser oscillator system emits laser pulses at a repetition rate of 50 MHz or less. The imaging unit is an optical configuration for extending the length of the resonator cavity (also referred to as the intracavity length) and reducing the repetition rate of the laser pulses emitted from the laser oscillator system. The imaging unit may be adapted to image the intracavity laser beam so as to at least somewhat maintain 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 plane mirrors and / or curved mirrors. The imaging unit may be incorporated into the resonator cavity. In some embodiments, the imaging unit can include at least one of the end mirrors of the resonator cavity. This facilitates the integration of the imaging unit into the resonator cavity and may facilitate the adjustment of the resonator length and thus the repetition rate of the laser oscillator system. The imaging unit can be adapted such that even if the length of the imaging unit is adjusted and accordingly the length of the resonator is adjusted, the position of the imaging surface of the end mirror in the resonator cavity included in the imaging system does not change. Thereby, the spatial characteristics of the laser emission mode in the resonator, particularly in the gain medium, are kept constant for different length adjustments by the imaging unit. Thus, by imaging, the repetition rate of the laser oscillator system can be reduced, as a result of which the energy per laser pulse is increased, and accordingly, a higher peak power can be obtained compared to a conventional laser oscillator system having a high repetition rate.As a result, by providing an imaging unit within the resonator, it is possible to realize laser pulses with peak powers of more than 0.75 MW emitted directly from the Cr-doped II-VI laser oscillator system without the need for further amplification.

[0039] The imaging unit may include one of the end mirrors of the resonator. Furthermore, the imaging unit may be adapted to image the end mirror of the resonator included in the imaging unit onto the imaging plane. This allows the distance of the imaging plane from the other end mirror of the resonator cavity to remain unchanged when adapting the length of the resonator cavity by the imaging unit. This therefore allows a complete decoupling of the mode size of the laser radiation in the resonator cavity, in particular the size of the laser beam or laser pulse at the gain medium for different resonator lengths adjusted by the imaging unit.

[0040] A laser system with a repetition rate of 50 MHz or less means that the laser system is operated in a pulsed mode, e.g., mode-locked operation, and the frequency of the laser pulses emitted from the laser system is 50 MHz or less, and therefore the time distance between two successively emitted laser pulses is about 20 ns or more.

[0041] In some optional embodiments, the laser oscillator system is adapted to emit laser pulses having a pulse width of 40 fs or less FWHM, which allows for peak powers of 0.75 MW or more to be achieved with moderate pulse energies. Furthermore, pulse widths of 40 fs or less allow for efficient driving of nonlinear optical processes such as spectral broadening and / or multiwave mixing applications.

[0042] In some optional embodiments, the laser system is adapted such that the beating signal has a signal-to-noise ratio of 40 dB or more measured with a high-frequency resolution bandwidth of 100 kHz. This can be achieved, in particular, by reducing and / or avoiding noise, as described in the embodiments above. In particular, in an f-2f interferometer, by avoiding an external laser amplification stage and / or by avoiding a separate interferometer arm, it is possible to reduce noise and achieve a signal-to-noise ratio of 40 dB or more measured with a high-frequency resolution bandwidth of 100 kHz. Thereby, the laser pulse is advantageous for various applications that are sensitive to slight changes in the CEO. This is particularly advantageous for achieving the stability of the CEO such that the integrated phase noise of the CEO frequency is 30 mrad or less.

[0043] It will be understood by those skilled in the art that the features described above and the features in the following description and figures are not only disclosed in the explicitly disclosed embodiments and their combinations, but also other technically feasible combinations and isolated features are constituted by the present disclosure. In the following, without limiting the present invention to the described embodiments, some optional embodiments and examples will be described with reference to the drawings.

Brief Description of the Drawings

[0044] Hereinafter, with reference to the drawings, further optional embodiments will be described.

Fig. 1A

Fig. 1B

Fig. 2

Fig. 3

DETAILED DESCRIPTION OF THE INVENTION

[0045] In the drawings, the same reference numerals are used for corresponding or similar features in different drawings.

[0046] Figures 1A and 1B schematically illustrate the physical principle of the carrier-envelope offset, also referred to as the carrier-envelope phase. Figure 1A shows an optical pulse train 10, i.e., a laser pulse 10 irradiated by a mode-locked femtosecond laser oscillator system according to an optional embodiment. The time distance T between two consecutive laser pulses 10 is given by the reciprocal (value) of the repetition rate f rep of.

[0047] The carrier (carrier) light wave 12 (solid line) oscillates at a frequency determined by the central wavelength of the laser pulse. The carrier light wave 12 is modulated by a periodic envelope 14 (dashed line) at the pulse repetition rate f rep . The oscillations of the carrier light wave 12 and the envelope 14 each have a periodic phase, and accordingly, the carrier light wave 12 and the envelope phase 14 have a relative phase relationship represented by the carrier-envelope offset CEO.

[0048] The three illustrated laser pulses 10 have different CEP values. The leftmost laser pulse 10 shows CEP = 0, and as a result, the phases of the carrier light wave 12 and the envelope 14 coincide, and both show local maxima at the same time. As can be seen, when CEP = 0, the maximum value of the envelope 14 coincides temporally with the maximum value of the carrier light wave 12. Therefore, the phase shift between the carrier light wave 12 and the envelope 14, and thus the CEP, is zero.

[0049] The laser pulse 10 shown in the middle indicates CEP = π / 2, which means that the phase of the carrier light wave 12 is shifted by π / 2 with respect to the envelope. Therefore, the maximum value of the envelope 14 coincides with the zero crossing of the carrier light wave 12.

[0050] The laser pulse 19 shown on the right indicates CEP = π, and as a result, the maximum amplitude of the carrier light wave 12 in the negative direction, i.e., the local minimum value, coincides temporally with the maximum value of the envelope 14.

[0051] Therefore, FIG. 1A shows that the temporal evolution of the laser pulse 10, especially the maximum electric field, can vary significantly depending on different values of CEP, even though the envelope 14 does not change.

[0052] FIG. 1B schematically shows the laser pulse 10 of FIG. 1A in the frequency domain, i.e., the Fourier transform of the signal shown in FIG. 1A. The vertical axis represents the spectral power density 16 with respect to the frequency f shown on the horizontal axis. The spectral power density 16 includes a comb of a plurality of spectral components 18 arranged at equal intervals, and the interval between the individual spectral components 18 is equal to the repetition rate f of the laser oscillator system. rep Furthermore, all the individual spectral components have a common offset given by the CEO frequency f. The CEO frequency f representing the initial offset frequency shown in FIG. 1B CEO indicates the frequency interval of the first frequency component from the zero point in the frequency scale. Since this is generally in the high-frequency spectral region, it may be measured using conventional electronic devices. This offset frequency f CEO may be determined, in particular, by measuring the frequency of the beating signal generated by the f - 2f interferometer device due to the overlap between the spectral components of the laser pulse (in the octave span) and the second harmonic spectral components generated by the laser pulse in the octave span. CEO ​

[0053] CEO frequency f CEO By stabilizing it, the waveform of the laser pulse can be stabilized periodically and / or constantly, which can be achieved by performing a controlled adjustment of the laser oscillator system based on the measured beating signal. Further, by stabilizing the CEO frequency to zero or f rep CEP can be stabilized.

[0054] FIG. 2 schematically shows a laser system 20 for generating a laser pulse having a determined CEO according to an optional embodiment. The laser system 20 includes a Cr-doped II-VI-based laser oscillator system 22 having a resonator cavity, and the laser oscillator system 22 is adapted to emit a laser pulse from a resonator cavity having a peak power of at least 0.75 MW. The Cr-doped II-VI laser oscillator system 22 may include a direct diode-pumped gain medium formed of a Cr-doped ZnS or ZnSe crystal. A more detailed description of the laser oscillator system 22 is described below with reference to FIG. 3. The optical path 24 of the emitted laser pulse is indicated by a solid line.

[0055] The laser system 20 further includes an arrangement for spectrally broadening the laser pulse, the arrangement including a non-linear optical element 26 and two optical elements 28 for focusing the laser pulse onto the non-linear optical element 26 and for collimating the laser pulse behind the non-linear optical element 26. Further, the arrangement includes two steering mirrors 30 for controlling the propagation path of the laser pulse passing through the focusing optical element 26 and the non-linear optical element 26. The spectrally broadened components of the laser pulse are symbolized by a dotted optical path.

[0056] The non-linear optical element 26 is optimized to broaden the spectrum of the laser pulse propagating through the non-linear optical element 26. The laser pulse emitted by the laser oscillator system 22 covers a spectral region of approximately 2.0 μm to approximately 2.6 μm. Therefore, the non-linear optical element 26 is formed of bulk rutile-type TiO2 having a thickness of approximately 0.5 mm, which provides properties suitable for achieving a significant spectral broadening of the laser pulse towards shorter wavelengths. The fact that the non-linear optical element 26 is formed of bulk TiO2 means that, in contrast to the use of conventional photonic fibers etc., the non-linear optical element 26 according to this embodiment does not apply spatial confinement of the laser beam propagating through the non-linear optical element 26.

[0057] In particular, by focusing the laser pulse on the non-linear optical element 26, a spectrally broadened laser pulse is produced that covers a spectral region spanning at least an optical octave, such as a spectral wavelength region span of at least 1.2 μm to at least 2.6 μm.

[0058] The beam splitter 32 is arranged behind the arrangement for expanding the spectrum, branches off the first part of the power of the laser pulse for further use in the intended application, and reflects the first part in the desired direction. The second part of the power of the laser pulse passes through the beam splitter 32 and is used to determine and / or control the CEO of the emitted laser pulse. Alternatively or additionally, a part of the power of the laser pulse may be branched off before the spectral broadening. This is advantageous when using the CEO-stabilized laser pulse emitted from the laser oscillator system 22 without further spectral broadening. For this purpose, the beam splitter 32 or an additional beam splitter (not shown) may be arranged between the laser oscillator system 22 and the arrangement for spectral broadening. The transmitted power of the laser pulse must be sufficient for spectral broadening of the laser pulse and subsequently generating second harmonic spectral components. The remaining power of the laser pulse can be used for applications such as spectroscopy, medical, and / or machining.

[0059] In the next step, the laser pulse enters the arrangement for frequency doubling. The arrangement includes a frequency doubling element 34, two optical elements 36 for focusing the laser pulse onto the frequency doubling element 34 and collimating the laser pulse behind the frequency doubling element 34, and two steering mirrors 38 for controlling the beam propagation path through the frequency doubling element 34. According to this embodiment, the frequency doubling element 34 is a non-linear optical element adapted to cause second harmonic generation from the laser pulse propagating through the frequency doubling element 34. The material and / or thickness and / or cut angle and / or periodic structure of the frequency doubling element may be optimized with respect to the central wavelength of the laser pulse and the desired conversion efficiency. For example, the frequency doubling element may be formed of a periodically poled lithium niobate (PPLN) non-linear optical crystal having a poling period of 36 μm and a thickness of 1 mm.

[0060] Behind the frequency multiplier element 34, the spectral components of the remaining octave span of the laser pulse (fundamental laser pulse) and the generated second harmonic spectral components propagate collinearly. This is indicated by the dashed line extending collinearly showing the optical path of the second harmonic spectral components.

[0061] Since the spectrally broadened laser pulse spectrally covers at least the entire optical octave, a part of the generated second harmonic spectral components and the spectral components of the octave span of the spectrally broadened laser pulse have a spectrally overlapping region. In other words, to some extent, the fundamental spectral components of the spectrally broadened laser pulse and the generated second harmonic spectral components cover exactly the same spectral wavelength range. This spectral overlap region is on the short wavelength side of the spectrally broadened laser pulse. Furthermore, the spectrally broadened laser pulse and the spectral components of the generated second harmonic overlap spatially and temporally with each other.

[0062] These collinearly propagating laser pulses, and the spectral components of the second harmonic that spectrally, spatially, and temporally overlap, are then used in an f-2f interferometer device 40 to generate a beating signal. Due to the collinear propagation direction, these spatial overlaps and these temporal overlaps are achieved by keeping the optical dispersion during frequency doubling at a low level, and no further measures are necessary to achieve the spatial, temporal, and spectral overlaps required for f-2f interferometric spectroscopy. The f-2f interferometer device 40 includes a photodiode 42 for detecting the incident overlapping spectral components having spatial, temporal, and spectral overlaps at the detection surface of the photodiode 42, and thus generates a detectable high-frequency region beating signal based on the output signal of the photodiode 42. The laser system may further include an optional spectral filter element 44, which can be an optical bandpass filter 44, for separating the spectrally overlapping spectral components from the other remaining spectral components of the laser pulse and the generated second harmonic spectral components, and these cannot be used for generating the beating signal.

[0063] Hereinafter, the frequency of the beating signal and the related spectral components will be described.

[0064] Due to the operation of the pulsed laser, the laser pulse has a spectrum of a frequency comb with a plurality of frequency spikes, as shown in FIG. 1B. Thus, the frequency spectrum of the laser pulse spectrally broadened over an octave span has the following frequencies: f = f CEO +n·f rep And, further, twice the frequency: 2f = f CEO +2n·f rep including frequency spikes at.

[0065] The parameter f CEOshows the frequency offset by the CEO, f rep shows the repetition rate or repetition frequency of the laser pulses emitted by the laser oscillator, and n is an integer ≥ 1 indicating that the frequency is the nth frequency spike, i.e., n times the repetition rate (not related to the frequency offset).

[0066] The second harmonic frequency 2f′ generated by frequency multiplication of the above fundamental frequency spike having the frequency f can be described as follows: 2f′ = 2·f = 2·f CEO + 2·n·f rep

[0067] Thus, the frequency 2f contained in the spectrally broadened laser pulse of the octave span which is the fundamental wave, and the frequency 2f′ generated by frequency multiplication, clearly differ only by the CEO frequency f CEO representing the beating signal: 2f′ − 2f = f CEO

[0068] As a result, the CEO frequency is generated by an f - 2f interferometer device and directly corresponds to the beating signal that can be utilized as an electrical signal by the f - 2f interferometer device. Therefore, by determining and / or controlling and / or stabilizing the beating signal, the CEO of the laser pulse can be determined and / or controlled and / or stabilized in the time domain.

[0069] The determination and / or control and / or stabilization of the CEO may be performed by tuning the laser oscillator system to stabilize and / or control the beating signal to a predetermined frequency, e.g. a frequency of zero. Tuning the laser oscillator system may include adjusting the internal dispersion of the resonator cavity and / or adjusting the pump power for pumping the gain medium of the laser oscillator system and / or adjusting the internal optical losses of the resonator cavity. This can ensure that the laser pulses emitted by the laser oscillator system exhibit the determined CEO.

[0070] This embodiment offers the advantage of minimal power loss and insensitivity to beam pointing fluctuations. Moreover, the output beam, which retains a good spatial profile and ultrashort pulse width, can be utilized for other applications besides beat-note detection. Furthermore, due to the short propagation length through the nonlinear optical element for spectral broadening, the possibility of walk-off between frequency components can be minimized, and high spatial and temporal overlap of the spectral components of the laser pulse and the generated second harmonic can be achieved without the need to separately tune the individual optical paths. This helps to achieve a high signal-to-noise ratio of the detected CEO beat-tone, and therefore gives the possibility of achieving low-noise operation in a stable state.

[0071] FIG. 3 illustrates generally an optional embodiment of a Cr-doped II-VI laser oscillator system 22 having a resonator cavity 112 and adapted to emit laser pulses having a peak power of at least 0.75 MW therefrom.

[0072] The laser oscillator system 22 includes a resonator cavity 112 for confining the intracavity laser beam 113. Cavity mirrors 112a and 112b are disposed at both ends of the resonator cavity 112, respectively. The cavity mirrors 112a and 112b may also be referred to as end mirrors. According to an optional embodiment, one of the cavity mirrors 112a and 112b may include the function of an outcoupler for extracting a portion of the intracavity laser beam 113 out of the resonator cavity 112. For example, the cavity mirror 112a forming the outcoupler may be partially transparent to transmit a slight portion of the intracavity laser beam.

[0073] Furthermore, the laser oscillator system 22 includes a Cr-doped II-VI gain medium 114 that functions as a laser active medium. According to this embodiment, the gain medium 114 may be a Cr:ZnSe or Cr:ZnS gain medium suitable for amplifying optical radiation in the spectral region of about 1.8 μm to 3.0 μm. The gain medium may be directly diode-pumped by a suitable laser diode (not shown). Two optical elements 116 are provided to appropriately focus and collimate the intracavity laser beam 113 in order to shape the intracavity laser beam 113 to show an appropriate spot size 1000, that is, an appropriate beam waist, in the gain medium 14 and within the gain medium 14. The optical element 116 may be provided as an optical lens.

[0074] According to this embodiment, the gain medium 114 not only functions as a laser active medium for amplifying the intracavity laser beam 113, but also functions as a Kerr medium for realizing the Kerr-lens mode locking of the laser oscillator system 22. In other words, the gain medium 114 combines the gain medium and the Kerr medium in one single element. According to other embodiments, the laser oscillator system 22 may include a Kerr medium (not shown) separate from the gain medium 114, which enables independent adjustment of the gain and mode locking.

[0075] In addition, the laser oscillator system 22 includes an imaging unit 118 for decoupling the spot size 1000 of the intracavity laser beam 113 from the intracavity length 1002 of the resonator cavity 112, as indicated by the dashed double arrow. According to this embodiment, the imaging unit 118 is formed by a 4f telescope 120 in the vicinity of the cavity mirror 112b (in this case, f represents the focal length of the telescope lens). The 4f telescope includes two optical lenses 122 each having a focal length, and the two optical lenses 122 are arranged at a distance twice the focal length from each other as indicated by the arrows. Further, one of the optical lenses 122 is arranged at a distance corresponding to the focal length from the cavity mirror 112b. In this way, the imaging unit 118 is configured to image the intracavity laser beam 113 from the imaging plane 1004 to the cavity mirror 112b arranged adjacent to the imaging unit 118. Therefore, due to the optical configuration of the resonator cavity 112 including the imaging unit, an image of the cavity mirror 112b is virtually obtained on the imaging plane 1004. Accordingly, the resonator mode of the intracavity optical beam 113 in the portion extending from the left cavity mirror 112a of the resonator cavity 112 to the imaging plane 1004 defines the same resonator mode as if the right cavity mirror 112b were arranged on the imaging plane 1004. Therefore, even if the intracavity length 1002 of the resonator cavity 112 is extended by the imaging unit 18, the resonator mode does not change, and in particular, it does not affect the spot size 1000 of the intracavity laser beam 113 in the gain medium 114. This is in contrast to the case where the intracavity length 1002 of the resonator cavity 112 is simply extended without the imaging unit 118. In this simply extended case, the beam waist changes with the increase in the intracavity length 1002 due to the reconstruction of the intracavity laser beam 113 by shifting the cavity mirrors 112a and 112b.

[0076] By using the imaging unit 118, the length of the resonator cavity 112 becomes longer. Therefore, the repetition rate of the laser oscillator system 22 decreases as compared with the case where the cavity mirror 112b is disposed on the imaging surface 1004. Thereby, a repetition rate of 50 MHz or less can be realized. In some embodiments, a repetition rate of 40 MHz or less, or 30 MHz or less can also be realized. Since the average laser output power (which does not essentially change) is concentrated in a smaller number of pulses, the decrease in the repetition rate makes it possible to achieve higher pulse energy and thus higher peak power of the emitted laser pulses. In particular, in the present embodiment, a repetition rate of 40 MHz corresponding to an intracavity length of 3.75 m, or a repetition rate of 25 MHz corresponding to an intracavity length of 6.0 m can be realized. Therefore, the laser oscillator system can supply femtosecond laser pulses having a peak power of 1 MW or more.

[0077] According to an optional embodiment, the laser oscillator system 22 has an adjustable intracavity length. For example, the positions of the cavity mirror 112b and optionally the imaging unit 118 may be moved to shorten and / or extend the intracavity length 1002 of the resonator cavity 112. For example, the length of the resonator cavity may be continuously adjustable and / or adjustable in steps. According to some optional embodiments, the intracavity length 1002 of the resonator cavity 112 can be changed to some extent without requiring a change in the optical element 122 of the imaging unit 118. According to some embodiments, changing the intracavity length 1002 of the resonator cavity 1112 may require replacing at least one of the optical elements 122 with a different optical element having a different focal length.

[0078] The laser oscillator system 22 according to this embodiment may be used to determine and / or stabilize the CEO in order to also provide a CEO-stabilized laser pulse. In particular, the laser oscillator system 22 may be adjusted to yield a CEP-stabilized pulse. In particular, the laser oscillator system performs the determination and / or control of the CEO and does not require an external laser amplification stage to achieve an appropriate pulse energy and peak power for using the CEO-stabilized pulse for a desired application.

Explanation of Signs

[0079] 10 Laser pulse 12 Carrier light wave 14 Envelope 16 Spectral power density 18 Spectral component / frequency spike 20 Laser system 22 Laser oscillator system 24 Optical path of laser pulse 26 Nonlinear optical element for spectral broadening 28 Optical element 30 Steering mirror 32 Beam splitter 34 Frequency doubling element 36 Optical element 38 Steering mirror 40 f-2f interferometer 42 Photodiode 44 Spectral bandpass filter 112 Resonator cavity 112a, 112b Cavity mirror / end mirror 113 Laser beam in cavity 114 Gain medium 116 Optical element 118 Imaging unit 120 4f-telescope 122 Optical element of imaging unit 1000 Spot size / beam waist in gain medium 1002 Cavity internal length of resonator cavity 1004 Imaging plane of 4f-telescope CEO Carrier - envelope - offset

Claims

1. A laser system (20) for generating a laser pulse having a determined carrier envelope offset, CEO, comprising: - A Cr-doped II-VI based laser oscillator system (22) having a resonator cavity (112), adapted to emit a laser pulse (10) having a peak power of at least 0.75 MW from said resonator cavity (112); said laser oscillator system (22); - A non-linear optical element (26) for spectrally broadening at least a portion of said emitted laser pulse (10), said non-linear optical element (26) imparting an octave-spanning spectral component to said laser pulse (10) irradiated thereon; said non-linear optical element (26); - A frequency doubling element (34) which, when irradiating said spectrally broadened laser pulse (10) onto said frequency doubling element (34), generates a second harmonic spectral component of at least a portion of the octave-spanning spectral component of said spectrally broadened laser pulse (10), such that a portion of said second harmonic spectral component spectrally overlaps with a portion of the remaining octave-spanning spectral component of said laser pulse (10); said frequency doubling element (34); - An f-2f interferometer device (40) for generating a beating signal of at least a portion of said overlapping spectral components which emerge from said frequency doubling element (34) and interfere with each other in said f-2f interferometer device, and for determining and / or controlling the CEO of said laser pulse (10) emitted based on said beating signal; said f-2f interferometer device (40); A laser system comprising the above components.

2. The laser system (20) according to claim 1, wherein said overlapping second harmonic spectral components emerging from said frequency doubling element propagate collinearly and temporally overlap with said remaining fundamental spectral components along a common optical path.

3. In the laser system (10) according to any one of claims 1 or 2, the beating signal is generated only by spectral components that interfere with and overlap the second harmonic spectral component of the laser pulse (10) remaining after the frequency doubling element (34) and the spectral components of the octave span. Laser system.

4. A laser system (20) according to any one of claims 1 or 2, further comprising a spectral filter element (44) for spectrally filtering the overlapping spectral components before generating the beating signal. Laser system.

5. In the laser system (20) according to any one of claims 1 or 2, the step of controlling the CEO of the emitted laser pulse (10) comprises adjusting the laser oscillator system (22) to emit a laser pulse having a predetermined CEO. Laser system.

6. In the laser system (20) according to claim 5, the step of adjusting the laser oscillator system (22) comprises adjusting the internal dispersion of the resonator cavity (112), and / or adjusting the excitation power for exciting the gain medium (14), and / or adjusting the internal optical loss of the resonator cavity (112). Laser system.

7. In the laser system (20) according to any one of claims 1 or 2, the step of controlling the CEO comprises stabilizing the CEO to a predetermined CEO. Laser system.

8. In the laser system (20) according to any one of claims 1 or 2, the nonlinear optical element (26) for spectral broadening has a thickness of 1 mm or less. Laser system.

9. In the laser system (20) according to any one of claims 1 or 2, the non-linear optical element (26) for spectral broadening comprises rutile-type TiO ２ or consists of rutile-type TiO ２ A laser system.

10. In the laser system (20) according to any one of claims 1 or 2, the Cr-doped II-VI group-based laser oscillator system (22) includes Cr-doped ZnS and / or Cr-doped ZnSe, or includes a gain medium composed of Cr-doped ZnS and / or Cr-doped ZnSe. Laser system.

11. In the laser system (20) according to any one of claims 1 or 2, the Cr-doped II-VI group-based laser oscillator system (22) includes an imaging unit (118) that forms a part of the resonator cavity (112), and the imaging unit (118) is adapted to decouple the spot size (1000) of the intracavity laser beam (113) in the gain medium (114) from the intracavity length (1002) of the resonator cavity (112). The resonator cavity (112) and the imaging unit (118) are adapted such that the laser oscillator system (22) emits laser pulses at a repetition rate of 50 MHz or less. Laser system.

12. In the laser system (20) according to claim 11, the imaging unit (118) includes end mirrors (112a, 112b) of the resonator cavity (112). Laser system.

13. In the laser system (20) according to claim 12, the imaging unit (118) is adapted to image the end mirrors (112a, 112b) of the resonator cavity (112) included in the imaging unit (118) onto an imaging surface. Also, when the imaging unit (118) adapts the length of the resonator cavity (112), the imaging unit (118) is adapted to keep the distance of the imaging surface from another end mirror in the resonator cavity (112) constant. Laser system.

14. In the laser system (20) according to any one of claims 1 or 2, the laser oscillator system (22) is adapted to emit the laser pulse (10) having a pulse width of FWHM 40 fs or less. Laser system.

15. In the laser system (20) according to any one of claims 1 or 2, the laser system (20) is adapted such that the beating signal measured with a high-frequency resolution bandwidth of 100 kHz has a signal-to-noise ratio of 40 dB or more. Laser system.

16. A laser system (20) according to any one of claims 1 or 2, further comprising a diode-based pump light source (22) for optically exciting the gain medium of the Cr-doped II-VI group-based laser oscillator system.

17. The laser system (20) according to claim 16, wherein the diode-based pump light source includes one or more light-emitting diodes and / or one or more laser diodes.

18. A method for generating a laser pulse (10) having a determined carrier envelope offset, CEO, comprising the following steps, namely: - Supplying a laser pulse (10) having a peak power of at least 0.75 MW and a spectral component in the range of 1.8 μm to 2.4 μm, emitted from a resonator cavity (112) of a Cr-doped II-VI group-based laser oscillator system (22); - Spectrally broadening the laser pulse (10) so as to impart an octave-span spectral component to the laser pulse (10); - Generating a second harmonic spectrum of at least a part of the octave-span spectral component of the spectrally broadened laser pulse (10) in a frequency doubling element (34), such that a part of the second harmonic spectral component spectrally overlaps with a part of the spectral component of the remaining octave-span of the laser pulse; - Emitting from the frequency doubling element (34) and interfering with each other in an f-2f interferometer device to generate a beating signal of at least a part of the overlapping spectral component; - Determining and / or controlling the CEO of the emitted laser pulse (10) based on the beating signal; A method comprising the above steps.

19. The method according to claim 18, wherein the step of controlling the CEO of the emitted laser pulse comprises adjusting the laser oscillator system (22) to emit a laser pulse (10) having a predetermined CEO.

Images