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

Dynamic gain driven mode-locking in GHz fiber laser

Ultrafast lasers have become powerful tools in various fields, and increasing their fundamental repetition rates to the gigahertz (GHz) level holds great potential for frontier scientific and industrial applications. Among various schemes, passive mode-locking in ultrashort-cavity fiber laser is promising for generating GHz ultrashort pulses (typically solitons), for its simplicity and robustness. However, its pulse energy is far lower than the critical value of the existing theory, leading to open questions on the mode-locking mechanism of GHz fiber lasers. Here, we study the passive mode-locking in GHz fiber lasers by exploring dynamic gain depletion and recovery (GDR) effect, and establish a theoretical model for comprehensively understanding its low-threshold mode-locking mechanism with multi-GHz fundamental repetition rates. Specifically, the GDR effect yields an effective interaction force and thereby binds multi-GHz solitons to form a counterpart of soliton crystals. It is found that the resulting collective behavior of the solitons effectively reduces the saturation energy of the gain fiber and permits orders of magnitude lower pulse energy for continuous-wave mode-locking (CWML). A new concept of quasi-single soliton defined in a strongly correlated length is also proposed to gain insight into the dynamics of soliton assembling, which enables the crossover from the present mode-locking theory to the existing one. Specifically, two distinguishing dynamics of Q-switched mode-locking that respectively exhibit rectangular- and Gaussian-shape envelopes are theoretically indicated and experimentally verified in the mode-locked GHz fiber laser through the measurements using both the standard real-time oscilloscope and emerging time-lens magnification. Based on the proposed criterion of CWML, we finally implement a GDR-mediated mode-locked fiber laser with an unprecedentedly high fundamental repetition rate of up to 21 GHz and a signal-to-noise ratio of 85.9 dB.

A simple and rapid simultaneous measurement strategy for optical rotatory dispersion and circular dichroism

A simple cavity-based technology capable of simultaneously measuring optical rotary dispersion and circular dichroism within milliseconds offers ultra-high sensitivity and unprecedented spectral resolution. This advancement holds significant potential for various biochemical applications, including drug development, clinical diagnosis, and food science and safety.

Sensitive Discrete Frequency Mid-Infrared Absorption Spectroscopy Using Digitally Referenced Detection

Broadly tunable mid-infrared (IR) lasers, including quantum cascade lasers (QCL), are an asset for vibrational spectroscopy wherein high-intensity, coherent illumination can target specific spectral bands for rapid, direct chemical detection with microscopic localization. These emerging spectrometers are capable of high measurement throughputs with large detector signals from the high-intensity lasers and fast detection speeds as short as a single laser pulse, challenging the decades old benchmarks of Fourier transform infrared spectroscopy. However, noise in QCL emissions limits the feasible acquisition time for high signal-to-noise ratio (SNR) data. Here, we present an implementation that is broadly compatible with many laser-based spectrometer and microscope designs to address these limitations by leveraging high-speed digitizers and dual detectors to digitally reference each pulse individually. Digitally referenced detection (DRD) is shown to improve measurement sensitivity, with broad spectral indifference, regardless of imbalance due to dissimilarities among system designs or component manufacturers. We incorporated DRD into existing instruments and demonstrated its generalizability: a spectrometer with a 10-fold reduction in spectral noise, a microscope with reduced pixel dwell times to as low as 1 pulse while maintaining SNR normally achieved when operating 8-fold slower, and finally, a spectrometer to measure vibrational circular dichroism (VCD) with a ∼ 4-fold reduction in scan times. The approach not only proves versatile and effective but can also be tailored for specific applications with minimal hardware changes, positioning it as a simple and promising module for spectrometer designs using lasers.

Non-uniform line-of-sight measurements of nitric oxide using mid-infrared Faraday rotation spectroscopy

Traditional absorption spectroscopy relies on detecting intensity variations along the line-of-sight to gauge average concentration and temperature. While methods like profile fitting and temperature binning offer insights into the non-uniformity of the path, they fall short of accurately capturing the precise spatial distribution with a single line-of-sight measurement. We propose a novel measurement scheme for non-uniformly distributed concentration of nitric oxide (NO) along the line-of-sight utilizing a single laser and path, by incorporating Faraday rotation spectroscopy with magnetic fields changing over time and space. We validate the proposed scheme by measuring a path of two regions in series with different NO concentrations, and comparing the measurement results with direct absorption spectroscopy of each respective region. In this work, the tuning range of the interband cascade laser used is from 1899.42 to 1900.97 cm-1, encompassing two sets of spectral lines corresponding to the 2Π1/2 and 2Π3/2 transitions of NO's R(6.5). The average relative uncertainty in the concentration measurement for each region is estimated to be within 1.5%, with the concentration for individual absorption cells ranging from 0.2% to 0.8%.

Reference-free dual-comb spectroscopy with inbuilt coherence

We demonstrate a simple system for dual-comb spectroscopy based on two inherently coherent optical frequency combs generated via seeded parametric downconversion. The inbuilt coherence is established by making the two combs share a common comb line. We show that the inbuilt coherence makes it possible to use a simple numerical post-processing procedure to compensate for small drifts of the dual-comb interferogram arrival time and phase. This enables long-time coherent averaging of the interferograms.

Near-Infrared Off-Axis Cavity-Enhanced Optical Frequency Comb Spectroscopy for CO2/CO Dual-Gas Detection Assisted by Machine Learning

Cavity-enhanced direct frequency comb spectroscopy (CE-DFCS) is widely used as a highly sensitive gas sensing technology in various gas detection fields. For the on-axis coupling incidence scheme, the detection accuracy and stability are seriously affected by the cavity-mode noise, and therefore, stable operation inevitably requires external electronic mode-locking and sweeping devices, substantially increasing system complexity. To address this issue, we propose off-axis cavity-enhanced optical frequency comb spectroscopy from both theoretical and experimental aspects, which is applied to the detection of single- and dual-gas of carbon monoxide (CO) and carbon dioxide (CO2) in the near-infrared. An erbium-doped fiber frequency comb with a repetition frequency of ∼41.709 MHz is coupled into a resonant cavity with a length of ∼360 mm in an off-axis manner, exciting numerous high-order modes to effectively suppress cavity-mode noise. The performance of multiple machine learning models is compared for the inversion of a single/dual gas concentration. A few absorbance spectra are collected to build a sample data set, which is then utilized for model training and learning. The results demonstrate that the Particle Swarm Optimization Support Vector Machine (PSO-SVM) model achieves the highest predictive accuracy for gas concentration and is ultimately applied to the detection system. Based on Allan deviation, the detection limit for CO in single-gas detection can reach 8.247 parts per million by volume (ppmv) by averaging 87 spectra. Meanwhile, for simultaneous CO2/CO measurement with highly overlapping absorbance spectra, the LoD can be reduced to 13.196 and 4.658 ppmv, respectively. The proposed optical gas sensing technique indicates the potential for the development of a field-deployable and intelligent sensor system capable of simultaneous detection of multiple gases.

High-SNR mid-infrared dual-comb spectroscopy using active phase control cooperating with CWs-dependent phase correction

Mid-infrared (MIR) dual-comb spectroscopy (DCS) is a highly effective method for molecular metrology of rovibrational transition spectra in a quick accurate manner. However, due to limited comb frequency instability, manipulating coherence between two frequency combs to accomplish high-quality spectral analysis in the MIR region is a huge challenge. Here, we developed a comb-teeth resolved MIR DCS based on active phase control cooperating with a CWs-dependent (CWD) interferogram timing correction. Firstly, four meticulously engineered actuators were individually integrated into two near-infrared (NIR) seed combs to facilitate active coherence maintenance. Subsequently, two PPLN waveguides were adopted to achieve parallel difference frequency generations (DFG), directly achieving a coherent MIR dual-comb spectrometer. To improve coherence and signal-to-noise ratio (SNR), a CWD resampled interferogram timing correction was used to optimize the merit of DCS from 7.5 × 105 to 2.5 × 106. Meanwhile, we carried out the measurement of MIR DCS on the methane hot-band absorption spectra (v3 band), which exhibited a good agreement with HITRAN by a standard deviation on recording residual of 0.76%. These experimental results confirm that this MIR DCS with CWD interferogram timing correction has significant potential to characterize the rovibrational transitions of MIR molecules.

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.

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.