High-power dual-comb thin-disk laser oscillator for fast high-resolution spectroscopy

Sub-100-fs Kerr-lens mode-locked Yb:YAG ring-cavity thin-disk oscillator

Ultrafast ring-cavity thin-disk oscillators combine high output power with the flexibility of generating output either unidirectionally or bidirectionally. Here, we report a Kerr-lens mode-locked ring-cavity Yb:YAG thin-disk oscillator delivering unidirectional 89-fs pulses by inducing additional spectral broadening with nonlinear plates. This is the shortest pulse duration for a ring-cavity mode-locked thin-disk oscillator. Bidirectional mode-locking was also realized. These results lay the foundation for the more efficient generation of high-order harmonics at MHz repetition rates and high-power dual frequency combs.

Coherently averaged dual-comb spectroscopy with a low-noise and high-power free-running gigahertz dual-comb laser

We present a new type of dual optical frequency comb source capable of scaling applications to high measurement speeds while combining high average power, ultra-low noise operation, and a compact setup. Our approach is based on a diode-pumped solid-state laser cavity which includes an intracavity biprism operated at Brewster angle to generate two spatially-separated modes with highly correlated properties. The 15-cm-long cavity uses an Yb:CALGO crystal and a semiconductor saturable absorber mirror as an end mirror to generate more than 3 W average power per comb, below 80 fs pulse duration, a repetition rate of 1.03 GHz, and a continuously tunable repetition rate difference up to 27 kHz. We carefully investigate the coherence properties of the dual-comb by a series of heterodyne measurements, revealing several important features: (1) ultra-low jitter on the uncorrelated part of the timing noise; (2) the radio frequency comb lines of the interferograms are fully resolved in free-running operation; (3) we validate that through a simple measurement of the interferograms we can determine the fluctuations of the phase of all the radio frequency comb lines; (4) this phase information is used in a post-processing routine to perform coherently averaged dual-comb spectroscopy of acetylene (C₂H₂) over long timescales. Our results represent a powerful and general approach to dual-comb applications by combining low noise and high power operation directly from a highly compact laser oscillator.

Control of the bipolarization emission of an Yb:YAG laser by the orientation of the pump polarization

We show that the polarized pumping can be used to control the relative powers of two linear, orthogonally polarized, eigenstates in a Yb:YAG laser. The experimental observations are in full agreement with a two-mode rate-equation model, highlighting the roles of both the gain anisotropy and the cross-saturation parameters, whose values are found to be ε=0.08 and β = 0.64, respectively, in a longitudinally pumped continuous-wave microchip laser. The application to dual-polarization frequency combs is discussed.

Efficient pump-probe sampling with a single-cavity dual-comb laser: Application in ultrafast photoacoustics

Ultrafast pump-probe measurements are used to characterize various samples, such as biological cells, bulk, and thin-film structures. However, typical implementations of the pump-probe apparatus are either slow or complex and costly hindering wide deployment. Here we combine a single-cavity dual-comb laser with a simple experimental setup to obtain pump-probe measurements with ultra-high sensitivity, fast acquisition, and high timing precision over long optical delay scan ranges of 12.5 ns that would correspond to a mechanical delay of about 3.75 m. We employ digital signal balancing to obtain shot-noise-limited detection compatible with pump-probe microscopy deployment. Here we demonstrate ultrafast photoacoustics for thin-film sample characterization. We measured a tungsten layer thickness of (700 ± 4) Å with shot-noise-limited detection. Such single-cavity dual-comb lasers can be used for any pump-probe measurements and are especially well-suited for ultrafast photoacoustic studies such as involving ultrasonic echoes, Brillouin oscillations, surface acoustic waves and thermal dynamics.

High-resolution dual comb spectroscopy using a free-running, bidirectional ring titanium sapphire laser

We report the first measurement of resolved molecular absorption lines with dual-comb spectroscopy using a Kerr-lens mode-locked bidirectional Ti:sapphire ring laser cavity. A 3 nm broad spectrum has been recorded in 5.3 ms with a spectral resolution of ≈ 1 GHz (0.05 cm-1) corresponding to a relative spectral resolution of 2.5 × 10^-6. The measurement of spectrally resolved molecular absorption lines have been demonstrated on the oxygen A-band at 394 THz (760 nm, 13 000 cm^-1) and was obtained with two free-running 100 fs Ti:sapphire trains of pulses without the need for active phase stabilization protocol nor real-time or post-processing correction. This work demonstrates that the bidirectional laser configuration enables a sufficient level of absolute and mutual coherence for dual-comb spectroscopy of resolved molecular absorption lines. Considering the high versatility of Ti:sapphire emission spectral range (from 600 to 1100 nm) with high-peak powers, the here reported results pave the way for Dual-Comb spectroscopy in the UV range at mW average output power using a standalone set-up, in the aim to extend its applicability for atmospheric remote-sensing.

Dual-comb optical parametric oscillator in the mid-infrared based on a single free-running cavity

We demonstrate a free-running single-cavity dual-comb optical parametric oscillator (OPO) pumped by a single-cavity dual-comb solid-state laser. The OPO ring cavity contains a single periodically-poled MgO-doped LiNbO₃ (PPLN) crystal. Each idler beam has more than 245-mW average power at 3550 nm and 3579 nm center wavelengths (bandwidth 130 nm). The signal beams are simultaneously outcoupled with more than 220 mW per beam at 1499 nm and 1496 nm center wavelength. The nominal repetition rate is 80 MHz, while the repetition rate difference is tunable and set to 34 Hz. To evaluate the feasibility of using this type of source for dual-comb applications, we characterize the noise and coherence properties of the OPO signal beams. We find ultra-low relative intensity noise (RIN) below -158 dBc/Hz at offset frequencies above 1 MHz. A heterodyne beat note measurement with a continuous wave (cw) laser is performed to determine the linewidth of a radio-frequency (RF) comb line. We find a full-width half-maximum (FWHM) linewidth of around 400 Hz. Moreover, the interferometric measurement between the two signal beams reveals a surprising property: the center of the corresponding RF spectrum is always near zero frequency, even when tuning the pump repetition rate difference or the OPO cavity length. We explain this effect theoretically and discuss its implications for generating stable low-noise idler combs suitable for high-sensitivity mid-infrared dual-comb spectroscopy (DCS).

Dual-comb thin-disk oscillator

Dual-comb spectroscopy (DCS) normally operates with two independent, relatively low power and actively synchronized laser sources. This hinders the wide adoption for practical implementations and frequency conversion into deep UV and VUV spectral ranges. Here, we report a fully passive, high power dual-comb laser based on thin-disk technology and its application to direct frequency comb spectroscopy. The peak power (1.2 MW) and the average power (15 W) of our Yb:YAG thin-disk dual-comb system are more than one-order-of-magnitude higher than in any previous systems. The scheme allows easy adjustment of the repetition frequency difference during operation. Both combs share all cavity components which leads to an excellent mutual stability. A time-domain signal recorded over 10 ms without any active stabilization was sufficient to resolve individual comb lines after Fourier transformation.

Achieving high output powers in dual-comb sources is important for possible applications like deep UV high resolution spectroscopy. Here the authors demonstrate a fully passive scheme of generating a set of high-power dual-combs from a thin-disc gain medium.

Dual-dispersion-regime dual-comb mode-locked laser

We report on the first, to the best of our knowledge, solid-state dual-comb mode-locked laser that simultaneously operates in different dispersion regimes. Due to the intrinsic polarization multiplexing in a birefringent Yb:Ca₃NbGa₃Si₂O₁₄ (Yb:CNGS) gain medium, the laser emits two cross-polarized pulse trains with a repetition rate offset of ∼ 4.8 kHz from a single cavity. We obtain dual pulse generation with a 20-fold difference in duration by setting the net cavity group delay dispersion to cross zero across the emission band of the employed gain medium. While the duration of the soliton-like pulses experiencing anomalous dispersion amounts to 117 fs, the second laser output, which is spectrally located in the normal dispersion region, is strongly chirped with a pulse duration of 2360 fs.

Timing jitter characterization of free-running dual-comb laser with sub-attosecond resolution using optical heterodyne detection

Pulse trains emitted from dual-comb systems are designed to have low relative timing jitter, making them useful for many optical measurement techniques such as optical ranging and spectroscopy. However, the characterization of low-jitter dual-comb systems is challenging because it requires measurement techniques with high sensitivity. Motivated by this challenge, we developed a technique based on an optical heterodyne detection approach for measuring the relative timing jitter of two pulse trains. The method is suitable for dual-comb systems with essentially any repetition rate difference. Furthermore, the proposed approach allows for continuous and precise tracking of the sampling rate. To demonstrate the technique, we perform a detailed characterization of a single-mode-diode pumped Yb:CaF₂ dual-comb laser from a free-running polarization-multiplexed cavity. This new laser produces 115-fs pulses at 160 MHz repetition rate, with 130 mW of average power in each comb. The detection noise floor for the relative timing jitter between the two pulse trains reaches 8.0 × 10^-7 fs²/Hz (∼ 896 zs/Hz), and the relative root mean square (rms) timing jitter is 13 fs when integrating from 100 Hz to 1 MHz. This performance indicates that the demonstrated laser is highly compatible with practical dual-comb spectroscopy, ranging, and sampling applications. Furthermore, our results show that the relative timing noise measurement technique can characterize dual-comb systems operating in free-running mode or with finite repetition rate differences while providing a sub-attosecond resolution, which was not feasible with any other approach before.

High-power modelocked thin-disk oscillators as potential technology for high-rate material processing

High average power femtosecond lasers have made spectacular progress in the last decades - moving from laboratory-based systems with maximum average powers of tens of watts to kilowatt-class mature industrial systems in a short time. The availability of such systems opens new possibilities in many fields; one of the most prominent ones that have driven many of these technological advances is precise high-speed material processing, where ultrashort pulses have long been recognized to provide highest precision processing of virtually any material, and high average power extends these capabilities to highest processing rates. Here, we focus our attention on one high-average power technology with large unexplored potential for this specific application: directly modelocked multi-MHz repetition frequency high-power thin-disk oscillators. We review their latest state-of-the-art and discuss future directions and challenges, specifically with this application field in mind.

Picosecond ultrasonics with a free-running dual-comb laser

We present a free-running 80-MHz dual-comb polarization-multiplexed solid-state laser which delivers 1.8 W of average power with 110-fs pulse duration per comb. With a high-sensitivity pump-probe setup, we apply this free-running dual-comb laser to picosecond ultrasonic measurements. The ultrasonic signatures in a semiconductor multi-quantum-well structure originating from the quantum wells and superlattice regions are revealed and discussed. We further demonstrate ultrasonic measurements on a thin-film metalized sample and compare these measurements to ones obtained with a pair of locked femtosecond lasers. Our data show that a free-running dual-comb laser is well-suited for picosecond ultrasonic measurements and thus it offers a significant reduction in complexity and cost for this widely adopted non-destructive testing technique.