High-sensitivity dual-comb and cross-comb spectroscopy across the infrared using a widely-tunable and free-running optical parametric oscillator

Coherent dual-comb spectroscopy (DCS) is a form of Fourier transform spectroscopy that enables high-resolution measurements at high speeds without the trade-off between resolution and update rate inherent to mechanical delay scanning. However, high complexity of the optical system and limited sensitivity of the measurements remain major challenges for deploying broadband DCS in the short-wave infrared (SWIR, 1.4-3 um) and mid-infrared (mid-IR, >3 um) regions where many molecules have strong absorption bands. We address these challenges via a wavelength-tunable dual-comb optical parametric oscillator (OPO) combined with a new detection method. 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 beams in each cavity ensures that even in free-running operation the noise sources of the two combs are highly correlated, facilitating comb-line-resolved and aliasing-free measurements with 250-MHz spectral resolution. At an instantaneous bandwidth below 1 THz, high power per comb line of up to 70 uW (signal) and 150 uW (idler) is achieved. The accessible spectrum spans 1290 nm to 1670 nm (signal) and 2700 nm to 5160 nm (idler). In a SWIR DCS measurement, we achieve a signal-to-noise ratio (SNR) of 33 dB Hz^1/2. For mid-IR measurements, we introduce a novel cross-comb spectroscopy implementation to overcome limitations posed by traditional mid-IR detection, obtaining a record-high SNR of 41 dB Hz^1/2. Our results are a promising route towards dual-comb spectroscopy with high sensitivity and high resolution over a wide spectral range

Rapid-scan nonlinear time-resolved spectroscopy over arbitrary delay intervals

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

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 techniqu

Programmable pulse shaping for time-gated amplifiers

ISSN:1094-408

Absolute SESAM characterization via polarization-resolved non-collinear equivalent time sampling

Semiconductor saturable absorber mirrors (SESAMs) have enabled a wide variety of modelocked laser systems, which makes measuring their nonlinear properties an important step in laser design. Here, we demonstrate complete characterization of SESAMs using an equivalent time sampling apparatus. The light source is a free-running dual-comb laser, which produces a pair of sub-150-fs modelocked laser outputs at 1051 nm from a single cavity. The average pulse repetition rate is 80.1 MHz, and the full time window is scanned at 240 Hz. Cross-correlation between the beams is used to calibrate the time axis of the measurements, and we use a non-collinear pump-probe geometry on the sample. The measurements enable fast and robust determination of all the nonlinear reflectivity and recovery time parameters of the devices from a single setup, and show good agreement with conventional nonlinear reflectivity measurements. We compare measurements to a rate equation model, showing good agreement up to high pulse fluence values and revealing that the samples tested exhibit a slightly slower recovery at higher fluence values. Lastly, we examine the polarization dependence of the reflectivity, revealing a reduced rollover if cross-polarized beams are used or if the sample is oriented optimally around the beam axis.ISSN:0946-2171ISSN:1432-0649ISSN:0721-7269ISSN:0340-379

High-power OPCPA generating 1.7 cycle pulses at 2.5 µm

ISSN:1094-408

Decoupling phase-matching bandwidth and interaction geometry using non-collinear quasi-phase-matching gratings

ISSN:1094-408

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).ISSN:1094-408