2025. április 4. Publication number: MPI1107P12WO

This invention relates to a laser system with controlled carrier-envelope offset (CEO), based on a laser oscillator with Cr-doped II-VI gain material emitting laser pulses of ≥ 0.75 MW peak power. The output of the laser oscillator is broadened in a nonlinear optical element, providing laser pulses with octave-spanning spectral coverage. At least part of the broadened spectrum is frequency-doubled, such that the new second-harmonic components overlap spectrally with the fundamental. Interference of second harmonic and fundamental components (f-2f interferometer) generates a beating signal that allows to determine and/or control the CEO in a feedback loop.

Background

Many time- and frequency-domain spectroscopic experiments with ultrashort pulses depend on the phase of the electric-field oscillations with respect to the pulse envelope. Frequency combs operating in the mid-infrared spectral region can excite characteristic rotational and vibrational molecular transitions. Phase-stable mid-infrared frequency combs have applications, for example, in high-resolution [1] and field-resolved [2] molecular spectroscopies. Chromium-doped II-VI lasers provide broadband emission spectra centered around 2.3 μm that can be efficiently down-converted to longer wavelengths [3]. In addition, Cr:II-VI laser oscillators can reach exceptionally low noise performance, for example, by using direct diode pumping [4], making these lasers the ideal basis for mid-infrared spectroscopic applications. 

The phase offset of the electric field of a pulse with respect to the envelope maximum is defined as the carrier envelope phase (CEP, Fig. 1). The carrier-envelope offset (CEO) phase describes the shift in CEP in subsequent pulses, corresponding in frequency domain to an offset of the frequency comb of the laser output spectrum from the frequency origin, carrier envelope offset frequency (f_CEO). Stabilization of f_CEO to a non-zero value leads to pulse trains with a constant CEP slip, whereas setting the f_CEO to zero produces a train of identical pulses with a defined CEP (full CEP control). 

Carrier-envelope offset measurement and control typically relies on detecting a beating of frequency-doubled components of the laser output with the fundamental of the same laser pulses (f-2f interferometry). Due to the limited bandwidth of laser gain media, f-2f interferometry requires spectral broadening of the laser pulses to span at least one optical octave [5]. With the low-peak-power output of laser oscillators, such spectral broadening is conventionally achieved by propagating the pulses through a waveguide, for example, a nonlinear optical fiber [6], with the drawback of being prone to phase distortions and sensitive to beam pointing fluctuations. Another approach previously used spectral broadening during amplification of oscillator pulses from a Cr:ZnS laser oscillator, in combination with higher-order interferometry [7-9], at the expense of an additionally required external amplification stage and intrinsic coupling of the beat-signal generation to the amplification process.

The present invention directly broadens the output of a Cr:II-VI laser oscillator in a nonlinear medium to octave-spanning bandwidth, avoiding the drawbacks of waveguide-based broadening. Beat signals for CEO measurement and stabilization are generated with high signal/noise in a separate and individually tunable second-harmonic generation stage. Due to the low phase distortions in spectral broadening, f-2f interferometry may be implemented in a single beam, without the need of splitting and temporally overlaying fundamental and second-harmonic components, in this way further simplifying the scheme and increasing CEO measurement precision and active stabilization performance.   

Figure 1: The phase properties of the carrier wave of ultrashort pulses under the pulse envelope are described by the carrier-envelope offset (CEO) and carrier-envelope phase (CEP). The concepts are illustrated for a pulse train in time domain and the corresponding frequency comb in frequency domain. Abbreviations: frep -pulse-repetition frequency; T -pulse repetition period; f_CEO -carrier-offset frequency. 

Technology

The core of the laser system (Fig. 2) is a mode-locked Cr:II-VI oscillator. Using, for example, Cr:ZnS or Cr:ZnSe as the gain medium, supports emission in the 1.8-3 μm spectral range. Direct spectral broadening of the oscillator output is facilitated by peak powers of the oscillator-generated pulses of 0.75 MW or more.  For typical Watt-scale average power levels, such peak intensities can be achieved from a laser oscillator by operating at output pulse durations ≤ 40 fs, for example supported by Kerr-lens mode locking, and by reducing the pulse-repetition frequency to ≤ 50 MHz. The reduction in repetition frequency is achieved without changes in the oscillator mode by extending the laser cavity length with imaging units (not shown). 

At least octave-spanning spectral broadening is achieved by focusing the output of the laser oscillator (focusing/collimation are exemplary shown by parabolic mirrors) into a nonlinear element. A suitable nonlinear element that supports spectral broadening down to ca. 1.2 μm at the -30-dB level, is a rutile TiO₂ crystal. The material combines a high nonlinear refractive index n₂ of about 10-¹⁴ cm²/W and suitable transparency and dispersion characteristics that allow to achieve efficient spectral broadening in plates of as low as ≤ 1 mm thickness. Part of the recollimated broadened output may be optionally split off with a beam splitter to be used in experiments. 

For CEO measurement, the residual broadened fundamental pulses are focused into a frequency-doubling (SHG) element, for example a periodically poled lithium niobate (PPLN) crystal. In this configuration, the frequency-doubling stage can be optimized independently from the spectral broadening and other processes like amplification. The generated second-harmonic components spectrally overlap with the fundamental pulses. Interfering them (f-2f interferometer) produces a CEO-dependent intensity beating that is detected with a photodiode. Conveniently, the fundamental and second-harmonic components propagate collinearly. In addition, the low dispersion introduced in the bulk broadening process, compared to, for example, fiber-based broadening, supports intrinsic temporal overlap of fundamental and second-harmonic components, without the need for additional delay arms. Therefore, efficient and intrinsically stable f-2f interferometry can be achieved without any objects splitting directly in the collinear output beam of the second-harmonic generation stage. An optional optical filter may be used to isolate the interfering frequency components from the residual fundamental and improve the signal/noise level.

The measured beating signal gives direct access to the carrier-envelope offset frequency (f_CEO) and may be used in a feedback loop for active stabilization of the oscillator CEO and/or carrier envelope phase (CEP). Suitable control of the oscillator output phase can be achieved, for example, by modulating the power of the oscillator pump, the intra-cavity power and/or the intra-cavity dispersion.

Figure 2: Laser system for the generation of phase-stable infrared pulses, including a Cr:II-VI laser oscillator,  a spectral broadening stage in a nonlinear medium, frequency doubling of at least part of the fundamental pulse, and f-2f interferometry after an optional optical filter to produce a beating signal measured by a photodiode. The beating signal gives direct access to the carrier envelope offset frequency and can serve for measuring and/or controlling the CEO/CEP properties of the infrared pulse train.

Advantages of this invention

• Simplicity and compactness: Spectral broadening directly from the oscillator, with subsequent second-harmonic generation and f-2f interferometer in a single beam.
• Ultra-low noise performance drawing on the stability of Cr:II-VI laser oscillators, high beat-note signal/noise and intrinsically stable common-path f-2f interferometry.
• Phase-stable mid-infrared pulses directly from the oscillator.  Octave-spanning, phase-stable coherent pulses around 2.3 μm that can be converted efficiently to longer wavelengths, to cover, for example, the 3-12-μm range.

Applications

• Ultra-sensitive field-resolved infrared spectroscopy.
• Dual-comb spectroscopy.
• Seeding of mid-infrared amplifiers.

References
[1] G. Ycas et al., “High-coherence mid-infrared dual-comb spectroscopy spanning 2.6 to 5.2 μm”, Nature Photon. 12, 202-208 (2018).
[2] Pupeza, Ioachim, et al., “Field-resolved infrared spectroscopy of biological systems”, Nature 577.7788, 52-59 (2020).
[3] Wang, Qing, et al., “Broadband mid-infrared coverage (2–17 μm) with few-cycle pulses via cascaded parametric processes”, Optics letters 44.10 (2019): 2566-2569.
[4] N. Nagl et al., “Directly diode-pumped, Kerr-lens mode-locked, few-cycle Cr:ZnSe oscillator”, Opt. Expr. 27, 24445 (2019).
[5] H. R. Telle et al., “Carrier-envelope offset phase control: A novel concept for absolute optical frequency measurement and ultrashort pulse generation”, Appl. Phys. B 69, 327-332 (1999).
[6] N. Nagl et al., "Efficient femtosecond mid-infrared generation based on a Cr:ZnS oscillator and step-index fluoride fibers," Opt. Lett. 44, 2390-2393 (2019).
[7] S. Vasilyev et al., "Progress in Cr and Fe doped ZnS/Se mid-IR CW and femtosecond lasers", Proc. SPIE 10193, 101930U-101930U, (2017).
[8] S. Vasilyev et al., "2-cycle Cr:ZnS Laser with Intrinsic Nonlinear Interferometry", Laser Congress (ASSL, LAC, LS&C), pp. 1-2 (2019).
[9] S. Vasilyev et al., "Kerr-Iens mode-locked Cr:ZnS oscillator reaches the spectral span of an optical octave", Opt. Expr. 29, 2458 (2021).