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Coherent dual-comb spectroscopy (DCS) enables high-resolution measurements at high speeds without the trade-off between resolution and update rate inherent to mechanical delay scanning approaches. However, high system complexity and limited measurement sensitivity remain major challenges for DCS. Here, we address these challenges via a wavelength-tunable dual-comb optical parametric oscillator (OPO) combined with an up-conversion detection method. The OPO is tunable in the short-wave infrared (1300-1670 nm range) and mid-infrared (2700- 5000 nm range) where many molecules have strong absorption bands. Both OPO pump beams are generated in a single spatially-multiplexed laser cavity, while both signal and idler beams are generated in a single spatially-multiplexed OPO cavity. The near-common path of the combs in this new configuration enables comb-line-resolved and aliasing-free measurements in free-running operation. By limiting the instantaneous idler bandwidth to below 1 THz, we obtain a high power per comb line in the mid-infrared of up to 160 $\mu$W. With a novel intra-cavity nonlinear up-conversion scheme based on cross-comb spectroscopy, we leverage these power levels while overcoming the sensitivity limitations of direct mid-infrared detection, leading to a high signal-to-noise ratio (50.2 dB Hz$^{1/2}$) and record-level dual-comb figure of merit (3.5\times 10^8 Hz$^{1/2}$). As a proof of concept, we demonstrate the detection of methane with 2-ppm concentration over 3-m path length. Our results demonstrate a new paradigm for DCS compatible with high-sensitivity and high-resolution measurements over a wide spectral range., Comment: Supplementary Information included

Dual-comb generation from a single laser cavity provides a simple and high-performance solution to time sampling applications. We demonstrate a compact single-cavity dual-comb laser operating at gigahertz repetition rates and high repetition rate differences up to more than 100 kHz with sub-100 fs pulse duration. The single cavity approach leads to passive common noise suppression resulting in ultra-low relative timing jitter and fully resolvable comb lines in free-running operation. We showcase the laser performance with two application demonstrations: (a) time-domain spectroscopy of acetylene in the near-infrared via computational comb line tracking and (b) free-space THz time domain spectroscopy and thickness-measurements via adaptive sampling. For (b) we use efficient state-of the art iron-doped InGaAs photoconductive antennas to generate and detect the THz light. Here we operate these devices with an efficient Yb-based gigahertz repetition rate laser for the first time. One optical comb generates the THz light, while the other probes it via equivalent time sampling. We obtain signal strengths comparable to reference measurements with MHz repetition rate Er-based laser systems while achieving close to 1 GHz spectral resolution (defined by the comb line spacing) and generating THz frequencies up to 3 THz. By carrying out a careful investigation of the noise properties of the laser we confirm that the free-running gigahertz dual-comb oscillator provides a rapid yet highly precise optical delay sweep from a simple setup. Therefore, our approach will be beneficial for high-update rate time sampling and time-domain spectroscopy applications.

Femtosecond dual-comb lasers have revolutionized linear Fourier-domain spectroscopy by offering a rapid motion-free, precise and accurate measurement mode with easy registration of the combs beat note in the RF domain. Extensions of this technique found already application for nonlinear time-resolved spectroscopy within the energy limit available from sources operating at the full oscillator repetition rate. Here, we present a technique based on time filtering of femtosecond frequency combs by pulse gating in a laser amplifier. This gives the required boost to the pulse energy and provides the flexibility to engineer pairs of arbitrarily delayed wavelength-tunable pulses for pump-probe techniques. Using a dual-channel millijoule amplifier, we demonstrate programmable generation of both extremely short, fs, and extremely long (>ns) interpulse delays. A predetermined arbitrarily chosen interpulse delay can be directly realized in each successive amplifier shot, eliminating the massive waiting time required to alter the delay setting by means of an optomechanical line or an asynchronous scan of two free-running oscillators. We confirm the versatility of this delay generation method by measuring chi^(2) cross-correlation and chi^(3) multicomponent population recovery kinetics.

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 $\mathrm{Yb:CaF_2}$ 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 $8.0 \times 10^{-7} \, \mathrm{fs}^2/\mathrm{Hz} \; ( \sim 896 \: \mathrm{zs}/\sqrt{\mathrm{Hz}} )$, 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., Comment: 19 pages, 6 figures, 78 references; typos corrected, layout adjusted

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.

We present a near-infrared optical parametric chirped-pulse amplifier (OPCPA) and soft X-ray (SXR) high-harmonic generation system. The OPCPA produces few-cycle pulses at a center wavelength of 800 nm and operates at a high repetition rate of 100 kHz. It is seeded by fully programmable amplitude and phase controlled ultra-broadband pulses from a Ti:sapphire oscillator. The output from the OPCPA system was compressed to near-transform-limited 9.3-fs pulses. High-power operation up to an average power of 35 W was achieved, and a fully characterized pulse compression was recorded for a power level of 22.5 W, demonstrating pulses with a peak power greater than 21 GW. We demonstrate that at such high repetition rates, spatiotemporally flattened pump pulses can be achieved through a cascaded second-harmonic generation approach with an efficiency of more than 70%, providing a compelling OPCPA architecture for power-scaling ultra-broadband systems in the near-infrared. The output of this 800-nm OPCPA system was used to generate SXR radiation reaching 190 eV photon energy through high-harmonic generation in helium.

Coherent soft x-ray (SXR) sources enable fundamental studies in the important water window spectral region. Until now, such sources have been limited to repetition rates of 1 kHz or less, which limits count rates and signal-to-noise ratio for a variety of experiments. SXR generation at high repetition rate has remained challenging because of the missing high-power mid-infrared (mid-IR) laser sources to drive the high-harmonic generation (HHG) process. Here we present a mid-IR optical parametric chirped pulse amplifier (OPCPA) centered at a wavelength of 2.2 {\mu}m and generating 16.5-fs pulses (2.2 oscillation cycles of the carrier wave) with 25 W of average power and a peak power exceeding 14 GW at 100-kHz pulse repetition rate. This corresponds to the highest reported peak power for high-repetition-rate mid-IR laser systems. The output of this 2.2-{\mu}m OPCPA system was used to generate a SXR continuum extending beyond 0.6 keV through HHG in a high-pressure gas cell.