Method and apparatus for generating pulsed laser radiation with determination of temporal changes in the carrier-envelope phase

Abstract

Method for generating pulsed laser radiation, comprising the following process steps: - Generating first pulsed laser radiation, wherein the spectrum of the first pulsed laser radiation is an optical frequency comb with a number of equidistant spectral lines, - Generating a second pulsed laser radiation by frequency doubling of the first pulsed laser radiation, wherein the second pulsed laser radiation is polarized orthogonally to the first pulsed laser radiation, - Superposition of the individual pulses (3, 5) of the first and the second pulsed laser radiation to form a third pulsed laser radiation, - Detection of the polarization of the third pulsed laser radiation, - Deriving the temporal change of the carrier envelope phase of the individual pulses of the first pulsed laser radiation from the temporal evolution of the detected polarization.

Description

[0001] The invention relates to a method and a device for generating pulsed laser radiation.

[0002] A mode-locked laser can be used to generate a sequence of short laser pulses. A number of natural oscillations with different frequencies are excited in the laser's resonator. These natural oscillations are also referred to as modes. Each mode represents a spectral component in the spectrum of the generated electromagnetic radiation. Using suitable mechanisms, a fixed phase relationship can be established between the modes. This is also referred to as mode synchronization or mode locking. Due to mode locking, the laser pulses are emitted at a time interval corresponding to the round-trip time of a laser pulse in the laser's resonator. The temporal equidistance of the light pulses directly results in the spectrum of the electromagnetic radiation exhibiting equidistant spectral lines. Such a spectrum is also referred to as an optical frequency comb.The carrier envelope phase (CEP for short) describes the time difference between the envelope of the individual laser pulse and the fundamental oscillation of the laser pulse's electric field. The physical cause of the CEP and its temporal change is that the phase velocity and group velocity in the medium of the laser resonator are different and are also subject to fluctuations due to environmental influences. The CEP is of particular practical relevance for applications with very short laser pulses that consist of only a few cycles of the fundamental oscillation. The determination of the CEP can be used as part of a control system for temporal stabilization, so that, for example, all individual pulses in the generated pulse train exhibit a specific CEP.

[0003] State-of-the-art methods for determining CEPs are based on the so-called f-2f method (see Kakehata, M. et al., "Singleshot measurement of carrier-envelope phase changes by spectral interferometry," Optics Letters 26, 1436-1438, 2001). This method requires that the laser radiation has a spectral bandwidth of more than one octave. If this is not the case, spectral broadening must first be performed. The f-2f method involves frequency doubling (SHG for short, from English "Second Harmonic Generation") of the low-frequency range of the laser radiation spectrum, which also doubles the CEP. Overlaying this with the corresponding higher-frequency spectral range of the laser radiation results in the resulting beat signal being dependent on the CEP.However, the beat signal does not contain enough information for a complete, unambiguous determination and stabilization of the CEP at a specific value to generate a so-called CEP-stable frequency comb. Therefore, the current state of the art is based on temporal modulation of the CEP, in which the CEP increases or decreases by a specific value from individual pulse to individual pulse, resulting in a periodic variation of the CEP over the pulse sequence. The temporal variation of the CEP is determined by the so-called carrier envelope frequency (F for short). CEO(carrier envelope offset frequency). The f-2f beat signal oscillates at this frequency, which can be used to stabilize the frequency comb at a specific carrier envelope frequency. By stabilizing the repetition rate of the pulse sequence at this frequency (or a multiple of it), an optical frequency comb can be created that is CEP-stable, i.e., in which each individual pulse has the same CEP. This method can be extended by detecting the f-2f beat signal with spectral resolution. A time difference between the pulse sequence of the fundamental laser radiation and that of the frequency-doubled laser radiation results in a spectral modulation of the beat signal at a specific frequency. The period of the modulation depends on the time offset between the two pulse sequences, while the phase of the modulation depends on the CEP.The disadvantage of this approach is that suitable detectors for the spectrally resolved detection of the beat signal are comparatively slow, so that it is only possible to determine the CEP for each individual pulse with great effort.

[0004] DE 20 2007 017 492 U1 describes a device for stabilizing the CEP of a laser pulse generated by a mode-locked pulsed laser. The device comprises a laser oscillator, an interferometer, a detector, and a double feedback circuit. The interferometer generates laser pulses with a first and a second frequency component from the laser pulses generated by the mode-locked pulsed laser to generate first and a second interference signal. The detector receives the first and second interference signals and outputs third and fourth interference signals by inverting the phase of the second interference signal. The double feedback circuit controls the laser oscillator using the third and fourth interference signals to stabilize the CEP of the laser pulse generated by the mode-locked pulsed laser.

[0005] Against this background, it is the object of the invention to demonstrate an approach that is as simple, robust and practical as possible to generate pulsed laser radiation, namely with determination of the CEP or at least with determination of the temporal change of the CEP, and thus to enable the generation of a CEP-stable frequency comb.

[0006] The invention solves this problem by a method for generating pulsed laser radiation, comprising the following method steps: - generating first pulsed laser radiation, wherein the spectrum of the first pulsed laser radiation is an optical frequency comb with a number of equidistant spectral lines, - generating a second pulsed laser radiation by frequency doubling the first pulsed laser radiation, wherein the second pulsed laser radiation is polarized orthogonally to the first pulsed laser radiation, - superimposing the individual pulses of the first and second pulsed laser radiation to form third pulsed laser radiation, - Detection of the polarization of the third pulsed laser radiation, - Deriving the temporal change of the carrier-envelope phase of the individual pulses of the first pulsed laser radiation from the temporal course of the detected polarization.

[0007] The problem is also solved by a device for generating pulsed laser radiation, with - a mode-locked laser designed to generate first pulsed laser radiation whose spectrum is an optical frequency comb with a number of equidistant spectral lines, - a frequency doubler designed to generate second pulsed laser radiation by frequency doubling the first pulsed laser radiation in such a way that the second pulsed laser radiation is polarized orthogonally to the first pulsed laser radiation, and - a polarization detector designed to detect the polarization of a third pulsed laser radiation in which the individual pulses of the first pulsed laser radiation and the second pulsed laser radiation are superimposed.

[0008] According to the invention, the frequency-doubled (second) pulsed laser radiation has an orthogonal polarization compared to the fundamental (first) pulsed laser radiation. The individual pulses of the differently polarized laser radiation are superimposed (where the superimposed laser radiation is referred to as the third laser radiation within the meaning of the invention). Thus, unlike in the prior art, a beat signal is not derived from the superimposed laser radiation to determine and stabilize the CEP; instead, the polarization of the laser radiation resulting from the superimposed individual pulses is detected. This utilizes the fact that the phase difference between two superimposed individual pulses (except for a phase offset value dependent on the specific optical setup) corresponds to the CEP of the input laser pulse, i.e., the laser pulse of the first pulsed laser radiation.This phase difference is reflected in the polarization of the corresponding superimposed laser pulse of the third pulsed laser radiation. Detecting the temporal variation of the polarization (from individual pulse to individual pulse) thus directly yields the temporal change in the CEP of the input pulse. The invention thus enables direct access to the CEP information for each individual pulse, even at high repetition rates of the pulsed laser radiation up to the GHz range. No modulation of the CEP is necessary. This enables simple and robust measurement and also the stabilization of the CEP. Furthermore, polarization detection permits the use of readily available, inexpensive, and fast detectors, such as photodiodes.

[0009] In a preferred embodiment of the method according to the invention, spectral filtering is performed prior to the detection of the polarization, so that a spectral range is selected in the third pulsed laser radiation in which the spectra of both the first and the second pulsed laser radiation exhibit intensity. Those spectral ranges in which only the first or only the second pulsed laser radiation exhibit intensity are suppressed by the filtering in the third pulsed laser radiation. The spectral filtering ensures that, corresponding to the f-2f method, the polarization is only detected in that laser radiation in which the frequency-doubled low-frequency range of the frequency comb is superimposed on the corresponding higher-frequency range of the frequency comb. This results in the described dependence of the polarization on the CEP.For the purpose of spectral filtering, a correspondingly designed filter element can be provided in the beam path in front of the polarization detector and after the frequency doubler in the device according to the invention.

[0010] If necessary, the first pulsed laser radiation is appropriately spectrally broadened. For this purpose, a spectral broadening element can be provided in the beam path upstream of the polarization detector, e.g., in the form of a nonlinear optical fiber in which the spectral broadening is achieved by self-phase modulation.

[0011] In a further preferred embodiment, the superimposition of the individual pulses comprises a reduction of the time offset between the pulse sequence of the first pulsed laser radiation and the pulse sequence of the second pulsed laser radiation, such that the remaining time offset is smaller than the coherence time of the individual pulses. Any time offset that occurs between the pulse sequence of the fundamental laser radiation and the pulse sequence of the frequency-doubled laser radiation (e.g., due to dispersion) should be reduced as much as possible. In order for the inventive principle for detecting the phase difference based on the polarization of the superimposed laser pulses to function, the remaining time offset must be smaller than the coherence time of the laser pulses. This depends, among other things, on the bandwidth of the spectral filtering (see above).

[0012] To reduce the time offset, the device according to the invention can provide a delay element that reduces the time offset through polarization-dependent delay. The function of the delay element can be based, for example, on birefringence.

[0013] The invention advantageously enables the control, i.e., the stabilization, of the CEP of the first pulsed laser radiation, wherein the change in the carrier-envelope phase of the first pulsed laser radiation derived from the temporal profile of the detected polarization is the controlled variable. To generate a CEP-stable frequency comb, the control is expediently designed such that a temporally constant polarization of the laser pulses of the third pulsed laser radiation results, i.e., the CEP does not change over time.

[0014] For CEP stabilization, the device according to the invention can accordingly comprise a controller designed to control the mode-locked laser with a control signal for setting the CEP of the first pulsed laser radiation, and to derive the control signal from the detected polarization of the third pulsed laser radiation. The CEP can be set, for example, by adjusting the dispersion in the resonator of the mode-locked laser, as is known from the prior art.

[0015] A Hänsch-Couillaud detector, for example, is suitable as a polarization detector. It comprises the following components: - a λ / 4 wave plate or a λ / 2 wave plate, - a polarization beam splitter following the λ / 4 wave plate or the λ / 2 wave plate in the beam path, designed to split the laser radiation into two separate beam paths depending on the polarization, - two photodetectors, with each beam path assigned to a photodetector, and - a subtractor designed to generate a differential signal from the output signals of the two photodetectors. The function of the Hänsch-Couillaud detector with a λ / 4 waveplate or λ / 2 waveplate for detecting the phase difference between the orthogonally polarized laser pulses of the first and second laser beams is explained below using the exemplary embodiments.

[0016] Embodiments of the invention are explained in more detail below with reference to the drawings. They show: Fig. 1: Arrangement for generating frequency-doubled and orthogonally polarized laser pulses as a block diagram; Fig. 2: Polarization detector of the device according to the invention as a block diagram; Fig. 3: Device for generating pulsed laser radiation with detection of CEP changes as a block diagram; Fig. 4: Device for generating pulsed laser radiation with stabilization of the CEP as a block diagram.

[0017] The Fig. The partial arrangement shown in Fig. 1 (designated as a whole by 1) of the device according to the invention comprises an (optional) spectral broadening element 2, which is designed to polarize incoming pulsed laser radiation, indicated by the schematically drawn laser pulse 3 p→ (first pulsed laser radiation in the sense of the invention) spectrally broadened. A frequency doubler 4 is provided in the beam path, e.g. in the form of a non-linear crystal, which is designed to generate second pulsed laser radiation, indicated by the schematic laser pulse 5, in such a way that the second pulsed laser radiation is polarized orthogonally to the first pulsed laser radiation. This is shown in Fig. 1 by the polarization vector p→⊥ indicated. The frequency doubler 4 is followed by a filter element 6, which is designed to select a spectral range in which the spectra of both the first and the second pulsed laser radiation have intensity. Furthermore, a delay element 7 is provided, which is designed to reduce the time offset of the pulse sequence of the first pulsed laser radiation and the pulse sequence of the second pulsed laser radiation, ie the time interval between the laser pulses 3 and 5, by polarization-dependent delay, e.g. by means of a suitable birefringent medium. As in Fig. 1, the laser pulses 3, 5 should coincide after passing through the delay element 7; in any case, the remaining time offset should be smaller than the coherence time of the individual pulses. The phase difference Δφ between the two laser pulses 3, 5 corresponds (except for an offset value dependent on the specific optical arrangement) to the CEP of the input laser pulse 3. The phase difference Δφ is accordingly referred to below as the relative CEP. The detection of the temporal change in the relative CEP thus makes it possible to infer an identical change in the CEP of the input laser pulse 3. The mutually corresponding laser pulses 3, 5 are superimposed, i.e. the laser pulse 3 of the first pulsed laser radiation is superimposed as temporally coincident as possible with the laser pulse 5 of the second pulsed laser radiation obtained from this laser pulse 3 by frequency doubling.

[0018] According to the invention, the relative CEP Δφ is derived from the polarization of the laser pulse resulting from the superposition of the individual pulses 3, 5. A Hänsch-Couillaud detector can be used as the polarization detector, as described in Fig. 2. The polarization detector is designated therein as a whole by 8 and 8', respectively. The two superimposed individual pulses 3, 5 first pass through a wave plate 9 and then through a polarization-dependent beam splitter 19. The wave plate 9 is arranged at an angle of 45° to the axes of the beam splitter 9. At both outputs of the beam splitter, suitable photodetectors 11, 12 each provide a signal that depends on the energy of the incident laser pulse. A subtractor 13 forms a difference signal from the output signals of the two photodetectors 11, 12, which corresponds to a signal S HC= c · sin(Δφ) if the wave plate 9 (in the polarization detector 8) is a quarter-wave plate, and a signal S int = c · cos(Δφ), if the wave plate 9 (in the polarization detector 8') is a half-wave plate. Due to the collinear propagation of the fundamental (first laser radiation consisting of the laser pulses 3) and the frequency-doubled (second laser radiation consisting of the laser pulses 5) throughout the entire structure and the very short time interval (in the picosecond range or less), only minor phase fluctuations due to environmental influences are to be expected. The phase difference Δφ can be calculated accordingly by applying the inverse sine or cosine function to the output signal S HC or S intof the polarization detector 8 or 8'. The proportionality factor c can be easily determined in advance by suitable calibration. A signal S is generated for each individual pulse 3 of the pulse sequence of the first pulsed laser radiation. HC or S int , which is recorded in each case. The temporal sequence of the individual, correspondingly time-discretely recorded signals S HC or S int According to the invention, this results in the temporal change of the CEP of the individual pulses 3 of the first pulsed laser radiation, ie the change of the CEP from laser pulse to laser pulse.

[0019] In the device according to Fig. 3, a mode-locked laser 0 generates first pulsed laser radiation, which consists of a temporally equidistant sequence of laser pulses 3 ( Fig. 1 and Fig. 2), wherein the spectrum of the generated laser radiation is an optical frequency comb with a number of equidistant spectral lines. The first pulsed laser radiation passes through the arrangement 1 according to Fig. 1, whereby second pulsed laser radiation is generated from laser pulses 5 by frequency doubling of the first pulsed laser radiation, with polarization of the laser pulses 5 orthogonal to the polarization of the laser pulses 3, and whereby the laser pulses 3, 5 are superimposed to form third pulsed laser radiation, as described above. The third pulsed laser radiation is split into two branches (e.g., by means of a non-polarization-dependent beam splitter) and fed to the two polarization detectors 8 and 8', respectively, which are designed as described above. Since both output signals S HC or S int, which depend on the sine or cosine of Δφ, the relative CEP can be determined by inverting the sine or cosine over the full angular range, ie from -π to π.

[0020] In the device according to Fig. 4 is, insofar as it deviates from Fig. 3, only the phase detector 8 is provided, which has at its output the signal S HC This is fed to a controller 14 as a controlled variable. The controller 14 generates a control signal S at its output. s to control the mode-locked laser 0 to set the CEP of the first pulsed laser radiation. The control is carried out in such a way that a temporally constant signal S HC The control can also be based on a specific value of S specified as a reference variable. HC to generate a CEP-stable frequency comb in which the laser pulses have a specific, correspondingly predeterminable CEP.

Claims

[1] Method for generating pulsed laser radiation, comprising the following process steps: - generating first pulsed laser radiation, wherein the spectrum of the first pulsed laser radiation is an optical frequency comb with a number of equidistant spectral lines, - generating a second pulsed laser radiation by frequency doubling the first pulsed laser radiation, wherein the second pulsed laser radiation is polarized orthogonally to the first pulsed laser radiation, - superimposing the individual pulses (3, 5) of the first and second pulsed laser radiation to form third pulsed laser radiation, - Detection of the polarization of the third pulsed laser radiation, - Deriving the temporal change of the carrier-envelope phase of the individual pulses of the first pulsed laser radiation from the temporal course of the detected polarization. [2] Method according to claim 1, wherein spectral filtering is carried out before the detection of the polarization, so that in the third pulsed laser radiation a spectral range is selected in which the spectra of both the first and the second pulsed laser radiation have intensity. [3] Method according to claim 1 or 2, wherein the superimposing of the individual pulses (3, 5) comprises a reduction of the time offset between the pulse sequence of the first pulsed laser radiation and the pulse sequence of the second pulsed laser radiation, so that the remaining time offset is smaller than the coherence time of the individual pulses (3, 5). [4] Method according to one of claims 1 to 3, characterized by that a control of the carrier-envelope phase of the first pulsed laser radiation takes place, wherein the change in the carrier-envelope phase of the first pulsed laser radiation derived from the temporal course of the detected polarization is the controlled variable. [5] Method according to one of claims 1 to 4, wherein the first pulsed laser radiation undergoes spectral broadening. [6] Device for generating pulsed laser radiation, with - a mode-locked laser (0) designed to generate first pulsed laser radiation whose spectrum is an optical frequency comb with a number of equidistant spectral lines, - a frequency doubler (4) designed to generate second pulsed laser radiation by frequency doubling the first pulsed laser radiation in such a way that the second pulsed laser radiation is polarized orthogonally to the first pulsed laser radiation, and - a polarization detector (8, 8') designed to detect the polarization of a third pulsed laser radiation in which the individual pulses (3, 5) of the first pulsed laser radiation and the second pulsed laser radiation are superimposed. [7] Device according to claim 6, with a filter element (6) arranged in the beam path in front of the polarization detector (8, 8') and after the frequency doubler (4), designed to select a spectral range in which the spectra of both the first and the second pulsed laser radiation have intensity. [8] Device according to claim 6 or 7, comprising a delay element (7) designed to bring about a reduction in the time offset of the pulse sequence of the first pulsed laser radiation and the pulse sequence of the second pulsed laser radiation by polarization-dependent delay. [9] Device according to one of claims 6 to 8, comprising a controller (14) designed to control the mode-locked laser (0) with a control signal (S s ) to control the carrier envelope phase of the first pulsed laser radiation and thereby the control signal (S s) from the detected polarization of the third pulsed laser radiation. [10] Device according to one of claims 6 to 9, with a spectral broadening element (2) arranged in the beam path in front of the polarization detector (8, 8'), designed to spectrally broaden the first pulsed laser radiation. [11] Device according to one of claims 6 to 10, wherein the polarization detector (8, 8') is a Hänsch-Couillaud detector. [12] Device according to one of claims 6 to 11, wherein the polarization detector (8, 8') comprises the following components: - a λ / 4 or a λ / 2 wave plate (9), - a polarisation beam splitter (10) following the λ / 4 or λ / 2 wave plate (9) in the beam path, designed to split the laser radiation into two separate beam paths depending on the polarisation, - two photodetectors (11, 12), each beam path being assigned a photodetector (11, 12), and - a subtractor (13) designed to generate a difference signal (S HC , S int ) from the output signals of the two photodetectors (11, 12).

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