Simple approach to broadband mid-infrared pulse generation with a mode-locked Yb-doped fiber laser.

Broadband mid-infrared (MIR) molecular spectroscopy demands a bright and broadband light source in the molecular fingerprint region. To this end, intra-pulse difference frequency generation (IDFG) has shown excellent properties among various techniques. Although IDFG systems pumped with 1.5- or 2-µm ultrashort pulsed lasers have been extensively developed, few systems have been demonstrated with 1-µm lasers, which use bulky 100-W-class high-power Yb thin-disk lasers. In this work, we demonstrate a simple and robust approach of 1-µm-pumped broadband IDFG with a conventional mode-locked Yb-doped fiber laser. We first generate 3.3-W, 12.1-fs ultrashort pulses at 50 MHz by a simple combination of spectral broadening with a short single-mode fiber and pulse compression with chirped mirrors. Then, we use them for pumping a thin orientation-patterned gallium phosphide crystal, generating 1.2-mW broadband MIR pulses with the -20-dB bandwidth of 480 cm-1 in the fingerprint region (760-1240 cm-1, 8.1-13.1 µm). The 1-µm-based IDFG system allows for additional generations of ultrashort pulses in the ultraviolet and visible regions, enabling, for example, 50-MHz-level high-repetition-rate vibrational sum-frequency generation spectroscopy or pump-probe spectroscopy.

Broadband complementary vibrational spectroscopy with cascaded intra-pulse difference frequency generation.

One of the essential goals of molecular spectroscopy is to measure all fundamental molecular vibrations simultaneously. To this end, one needs to measure broadband infrared (IR) absorption and Raman scattering spectra, which provide complementary vibrational information. A recently demonstrated technique called complementary vibrational spectroscopy (CVS) enables simultaneous measurements of IR and Raman spectra with a single device based on a single laser source. However, the spectral coverage was limited to ∼1000cm-1, which partially covers the spectral regions of the fundamental vibrations. In this work, we demonstrate a simple method to expand the spectral bandwidth of the CVS with a cascaded intra-pulse difference-frequency generation (IDFG). Using the system, we measure broadband CVS spectra of organic liquids spanning over 2000cm-1, more than double the previous study.

Microfluidic single-particle chemical analyzer with dual-comb coherent Raman spectroscopy.

Label-free particle analysis is a powerful tool in chemical, pharmaceutical, and cosmetic industries as well as in basic sciences, but its throughput is significantly lower than that of fluorescence-based counterparts. Here we present a label-free single-particle analyzer based on broadband dual-comb coherent Raman scattering spectroscopy operating at a spectroscopic scan rate of 10 kHz. As a proof-of-concept demonstration, we perform broadband coherent anti-Stokes Raman scattering measurements of polystyrene microparticles flowing on an acoustofluidic chip at a high throughput of >1000 particles per second. This high-throughput label-free particle analyzer has the potential for high-precision statistical analysis of a large number of microparticles including biological cells.

Fourier transform and Vernier spectroscopy using an optical frequency comb at 3-5.4 µm.

We present a versatile mid-infrared frequency comb spectroscopy system based on a doubly resonant optical parametric oscillator tunable in the 3-5.4 µm range and two detection methods: a Fourier transform spectrometer (FTS) and a continuous-filtering Vernier spectrometer (CF-VS). Using the FTS with a multipass cell, we measure high precision broadband absorption spectra of CH4 at 3.3 µm and NO at 5.25 µm, the latter for the first time with comb spectroscopy, and we detect atmospheric species (CH4, CO, CO2, and H2O) in air in the signal and idler ranges. Multiline fitting yields minimum detectable concentrations of 10-20 ppb Hz-1/2 for CH4, NO, and CO. For the first time in the mid-infrared, we perform CF-VS using an enhancement cavity, a grating, and a single detector, and we measure the absorption spectrum of CH4 and H2O in ambient air at ∼3.3 µm, reaching a 40 ppb concentration detection limit for CH4 in 2 ms.