Broadband Dual-comb Spectroscopy Research Articles

Sub-100 fs watt-level Kerr-lens mode-locked Yb:CaYAlO4 laser with a gigahertz repetition rate

We report a 1.04 GHz high-power Kerr-lens mode-locked Yb:CaYAlO4 laser pumped by a single-mode fiber laser at 976 nm. Based on a bow-tie cavity, stable unidirectional mode-locked operation is obtained with an output coupler of 1.6%. The oscillator delivers pulses with an average power of 1.46 W and with the pulse duration of 99 fs, which, to the best of our knowledge, is the first gigahertz-level Kerr-lens mode-locked laser based on the Yb:CaYAlO4 gain medium. We believe that the watt-level solid-state femtosecond laser at GHz would be an excellent source for developing time-resolved broadband dual-comb spectroscopy.

11-μs time-resolved, continuous dual-comb spectroscopy with spectrally filtered mode-locked frequency combs

Broadband dual-comb spectroscopy (DCS) based on portable mode-locked fiber frequency combs is a powerful tool for in situ, calibration-free, multi-species spectroscopy. While the acquisition of a single spectrum with mode-locked DCS typically takes microseconds to milliseconds, the applications of these spectrometers have generally been limited to systems and processes with time changes on the order of seconds or minutes due to the need to average many spectra to reach a high signal-to-noise ratio (SNR). Here, we demonstrate high-speed, continuous, fiber mode-locked laser DCS with down to 11 μs time resolution. We achieve this by filtering the comb spectra using portable Fabry–Perot cavities to generate filtered combs with 1 GHz tooth spacing. The 1 GHz spacing increases the DCS acquisition speed and SNR for a given optical bandwidth while retaining a sufficient spacing to resolve absorption features over a wide range of conditions. We measure spectra of methane inside a rapid compression machine throughout the 16 ms compression cycle with 133 cm−1 bandwidth (4000 comb teeth) and 1.4 ms time resolution by spectrally filtering one of the combs. By filtering both combs, we measured a single-shot, 25 cm−1 (750 comb teeth) spectrum of CO around 6330 cm−1 in 11 μs. The technique enables simultaneously high-speed and high-resolution DCS measurements, and can be applied anywhere within the octave-spanning spectrum of robust and portable fiber mode-locked frequency combs.

A fully stabilized low-phase-noise Kerr-lens mode-locked Yb:CYA laser frequency comb with an average power of 1.5 W

We report a fully stabilized self-referenced all-solid-state-laser frequency comb based on a home-built Kerr-lens mode-locked Yb:CaYAlO4 oscillator, which delivers ultrafast pulses with a pulse duration of 54 fs and an average power of 1.5 W. Free running carrier-envelope phase offset frequency (fceo) is observed with a signal to noise ratio of 40 dB at 100 kHz resolution bandwidth, which is phase-locked to a microwave frequency synthesizer with a residual phase jitter of 370 mrad [1 Hz–1 MHz] and with a standard frequency deviation of 0.8 mHz at 1-s gate time within 3-h, corresponding to the relative frequency instability of 2.9 × 10−18 at 1-s average time. It is the lowest phase noise and the highest frequency stability of fceo obtained in a watt-level all-solid-state bulk laser frequency comb to the best of our knowledge. A 13th repetition rate is simultaneously phase-locked to the same microwave standard with a frequency instability level of 10−12 at 1 s average time. Such fully stabilized power scaling Yb: CaYAlO4 laser frequency combs have great potential for applications in broadband dual comb spectroscopy as well as high loss cases such as long-distance time and frequency transfer.

Broadband coherent cavity-enhanced dual-comb spectroscopy

Cavity-enhanced spectroscopic instruments impact a broad range of scientific fields because they are sensitive, self-contained, and can be packaged for use by nonspecialists. Infrared-laser-based cavity-enhanced instruments are typically limited to measuring a few small molecules because they rely on narrowly tunable lasers that each measure a molecular transition. Broadband dual-comb spectroscopy (DCS) is a spectroscopic technique capable of simultaneously probing molecular absorption on thousands of narrowly spaced wavelengths using a single high-speed photodetector. With coherent averaging, DCS can measure multiple species, including large molecules, with a high signal-to-noise ratio (SNR). Here, we lock a high-finesse optical cavity to a broadband mode-locked frequency comb with an approach that enables a high SNR, broadband cavity-enhanced DCS with coherent averaging. With a 7.5 cm optical cavity, we attain a >12,000× path-length-enhancement factor, 60 nm bandwidth near 1660 nm (3.5% fractional bandwidth), and >27 dB SNR in 160 s. We measure the dense rovibrational spectrum of ethane in a background of overlapping H2O, CH4, and CO2 absorption. By combining the broadband, high-resolution, and single-detector nature of DCS with a compact optical cavity, we enable future self-contained instruments that are able to simultaneously detect a variety of molecules in realistic ambient conditions.

Kerr-lens mode-locked bidirectional dual-comb ring laser for broadband dual-comb spectroscopy

Fourier-transform spectroscopy is an indispensable tool for analyzing chemical samples in scientific research as well as the chemical and pharmaceutical industries. Recently, its measurement speed, sensitivity, and precision have been shown to be significantly enhanced by using dual-frequency combs. Moreover, recent demonstrations of inducing nonlinear effects with ultrashort pulses have enriched the utility of dual-comb spectroscopy. However, wide acceptance of this technique is hindered by its requirement for two frequency combs and active stabilization of the combs. Here, we overcome this predicament with a Kerr-lens mode-locked bidirectional ring femtosecond-pulse laser that generates two broadband frequency combs with slightly different pulse repetition rates and a tunable yet highly stable rate difference. Since these combs are produced by one and the same laser cavity, their relative coherence stays passively stable without the need for active stabilization. To show its utility, we demonstrate broadband dual-comb spectroscopy with the single laser.

Static FBG strain sensor with high resolution and large dynamic range by dual-comb spectroscopy

We demonstrate a fiber Bragg grating (FBG) strain sensor with optical frequency combs. To precisely characterize the optical response of the FBG when strain is applied, dual-comb spectroscopy is used. Highly sensitive dual-comb spectroscopy of the FBG enabled strain measurements with a resolution of 34 nε. The optical spectral bandwidth of the measurement exceeds 1 THz. Compared with conventional FBG strain sensor using a continuous-wave laser that requires rather slow frequency scanning with a limited range, the dynamic range and multiplexing capability are significantly improved by using broadband dual-comb spectroscopy.