Mid-infrared dual-comb spectroscopy for rapid temperature distribution characterization.

Due to the influence of chemical reactions, phase change, and other phenomena, the combustion system is a complicated high-temperature environment. Therefore, the spatio-temporally resolved monitoring of the temperature field is crucial for gaining a comprehensive understanding of the intricate combustion environment. In this study, we proposed a fast and high-precision temperature measurement technique based on mid-infrared (MIR) dual-comb spectroscopy with a high spectral resolution and fast refresh rate. Based on this technique, the spatio-temporally resolved measurement of a non-uniform temperature field was achieved along the laser path. To verify the capability of DCS for temperature measurement, the bandhead ro-vibrational lines of the CO2 molecule were acquired, and the 1-σ uncertainty of the retrieved temperature was 3.2°C at 800°C within 100 ms. The results demonstrate the potential of our fast and high-precision laser diagnostic technique which can be further applied to combustion kinetics.

Frequency comb with a spectral range of 0.4-5.2†µm based on a compact all-fiber laser and LiNbO3 waveguide.

This Letter presents a 0.4-5.2-µm frequency comb from a compact laser. We designed an integrated fiber device for a figure-9 laser and constructed an all-fiber laser system. The spectrum of the fiber laser was scaled to the broadband region using a chirped periodically poled lithium niobate waveguide. To use this system for gas sensing, a mid-infrared comb with a spectral range of 2.5-5.2 µm and average power of 2.1 mW was divided using an optical filter. The optical part was packaged in a 305 mm × 225 mm × 62 mm box. The comb was stabilized by locking the repetition rate and carrier-envelope offset frequency of the seed source. The system provided an ultrabroadband spectral range from 0.4 to 5.2 µm, which could be applied to spectroscopy, frequency metrology, and optical synthesizers.

Temperature-dependent determination of NO2 dimerization reaction based on dual-comb spectroscopy.

Dimerization reactions play a critical role in various fields of research, including cell biology, biomedicine, and chemistry. In particular, the dimerization reaction of 2NO2⇌N2O4 has been extensively applied in pollution control and raw material preparation. Spectroscopy, as a powerful tool for investigating molecular structures and reaction kinetics, has been increasingly employed to study dimerization reactions in recent years. In this study, we successfully demonstrated the application of dual-comb spectroscopy (DCS) to analyze NO2 dimerization reactions, making the first report on the application of this technique in this context. Parallel measurements of NO2 and N2O4 fingerprints spectra with high resolution at 3000 cm-1 was performed, benefiting from the unprecedented broadband and high-precision capability of DCS. The absorption cross-sections of N2O4 from 296 to 343 K was obtained from the measured spectra, which contributes to further research on the molecular spectrum of N2O4. These results demonstrate the potential of DCS for studying the dimerization reaction mechanism.

Dual-comb optical activity spectroscopy for the analysis of vibrational optical activity induced by external magnetic field.

Optical activity (OA) spectroscopy is a powerful tool to characterize molecular chirality, explore the stereo-specific structure and study the solution-state conformation of biomolecules, which is widely utilized in the fields of molecular chirality, pharmaceutics and analytical chemistry. Due to the considerably weak effect, OA spectral analysis has high demands on measurement speed and sensitivity, especially for organic biomolecules. Moreover, gas-phase OA measurements require higher resolution to resolve Doppler-limited profiles. Here, we show the unmatched potential of dual-comb spectroscopy (DCS) in magnetic optical activity spectroscopy (MOAS) of gas-phase molecules with the resolution of hundred-MHz level and the high-speed measurement of sub-millisecond level. As a demonstration, we achieved the rapid, high-precision and high-resolution MOAS measurement of the nitrogen dioxide [Formula: see text]+[Formula: see text] band and the nitric oxide overtone band, which can be used to analyze fine structure of molecules. Besides, the preliminary demonstration of liquid-phase chiroptical activity (as weak as 10-5) has been achieved with several seconds of sampling time, which could become a routine approach enabling ultrafast dynamics analysis of chiral structural conformations.

208-µs single-shot multi-molecular sensing with spectrum-encoded dual-comb spectroscopy.

Dual-comb spectroscopy (DCS) is a powerful spectroscopic technique, which is developing for the detection of transient species in reaction kinetics on a short time scale. Conventionally, the simultaneous determination of multiple species is limited to the requirement of broadband spectral measurement at the cost of the measurement speed and spectral resolution owing to the inherent trade-off among these characteristics in DCS. In this study, a high-speed multi-molecular sensing is demonstrated and achieved through using a programmable spectrum-encoded DCS technique, where multiple narrow encoding spectral bands are reserved selectively and other comb lines are filtered out. As a dual-comb spectrometer with a repetition rate of 108 MHz is encoded spectrally over a spectral coverage range of 1520 to 1580 nm, the measurement speed is increased 6.15 times and single-shot absorption spectra of multiple molecules (C2H2, HCN, CO, CO2) at a time scale of 208 µs are obtained. Compared to conventional single-shot dual-comb spectra, encoded dual-comb spectra have improved short-term signal-to-noise ratios (SNRs) by factors of 3.65 with four encoding bands and 5.68 with two encoding bands. Furthermore, a fiber-Bragg-grating-based encoded DCS is demonstrated, which reaches 17.1 times higher average SNR than that of the unencoded DCS. This spectrum-encoded technique can largely improve the DCS measurement speed, and thus is promising for use in studies on multi-species reaction kinetics.

Octave mid-infrared optical frequency comb from Er:fiber-laser-pumped aperiodically poled Mg: LiNbO3.

In this Letter, we report an octave-spanning mid-infrared (MIR) comb generation with a difference frequency generation (DFG) approach optimized for aperiodically poled Mg:LiNbO3 and nonlinear spectral broadening. An Er:fiber comb is delivered to two branches and amplified in an Yb:fiber and an Er:fiber amplifier, respectively. We demonstrate that the two-branch DFG can yield the spectrum tuned over an octave in a fan-out periodically poled lithium niobate. Thus, we obtain an optimized poling period profile and design the aperiodically poled Mg:LiNbO3. The results demonstrate that broadband combs can be generated in the MIR atmospheric window.

High-repetition-rate mid-IR femtosecond pulse synthesis from two mid-IR CW QCL-seeded OPAs.

Coherent pulse synthesis in the mid-infrared (mid-IR) domain is of great interest to achieve broadband sources from parent pulses, motivated by the advantages of optical frequency properties for molecular spectroscopy and quantum dynamics. We demonstrate a simple mid-IR coherent synthesizer based on two high-repetition-rate optical parametric amplifiers (OPAs) at nJ-level pump energy. The relative carrier envelope phase between the two OPAs was passively stable for a shared continuous wave (CW) quantum cascade laser (QCL) seed. Lastly, we synthesized mid-IR pulses with a duration of 105 fs ranging from 3.4 to 4.0 µm. The scheme demonstrated the potential to obtain broader mid-IR sources by coherent synthesis from multiple CW QCL-seeded OPAs.

130 W, 180 fs ultrafast Yb-doped fiber frequency comb based on chirped-pulse fiber amplification.

We report on a high-power fiber optical frequency comb consisting of a 250-MHz mode-locked fiber laser and a three-stage cascaded fiber chirped-pulse amplification system. After power scaling, the group velocity dispersion and third-order dispersion, generated in fiber stretcher and amplifiers, are compensated by a grism compressor, outputting a 132-W, 180-fs pulse train. The repetition rate and carrier-envelope offset frequency are locked to a Rb clock with the standard deviations of 1.07 and 0.87 mHz, corresponding to the fractional instability of 8.3×10-13 and 1.35×10-19, respectively. Moreover, we investigate the noise characteristics at high average powers, presenting a low-noise property of this high-power fiber OFC.

High-repetition-rate femtosecond mid-infrared pulses generated by nonlinear optical modulation of continuous-wave QCLs and ICLs.

We demonstrate an effective method to obtain high-repetition-rate femtosecond mid-infrared (mid-IR) pulses by nonlinear optical modulation of mid-IR continuous-wave (CW) quantum and interband cascade lasers (ICLs and QCLs). In the experiment, a high-repetition-rate femtosecond ytterbium-doped fiber laser with nanojoule-level pulse energy was used as the pump source of optical parametric amplifiers to modulate and amplify the mid-IR CW laser. Near transform-limited 84 fs duration (7.3 cycles) mid-IR pulses were generated with above 200 mW average power and a repetition rate of 160 MHz. Moreover, the spectral tunability was demonstrated using CW QCL at different wavelengths. The scheme offered a simple method to produce high-repetition-rate ultrashort pulses and that can be flexibly adopted in other mid-IR regions.