Heterodyne photothermal spectroscopy of methane near 1651 nm inside hollow-core fiber using a bismuth-doped fiber amplifier

Highly sensitive and reliable optical fiber TDLAS gas detection system for methane in situ monitoring in near space

A highly sensitive and reliable tunable diode laser absorption spectroscopy gas detection system with a temperature-pressure compensation algorithm is demonstrated for detecting C H 4 concentrations in near space. Near space generally refers to the airspace 20-100 km away from the ground, where temperature and pressure changes are complex. Since the gas absorption spectrum is easily affected by temperature and pressure, a temperature-pressure compensation algorithm is proposed and used in the C H 4 sensor to improve the detection accuracy of the sensor. First, we measured the basic characteristics of the sensor in the laboratory, such as linearity and long-term stability. Experimental results showed that the linear correlation coefficient R-square can reach 0.999, and the concentration fluctuation of C H 4 is less than 0.17 ppm within 3.5 h. Then the sensor was applied to a research activity in Qinghai Province, China, in September, and the results show that the sensor can effectively monitor the C H 4 concentration in near space.

Amplified Photothermal Phase Modulation for Carbon Dioxide Detection by Operating a Dual-Mode Interferometer at Destructive Interference

Photothermal interferometry is a highly sensitive spectroscopic technique for trace gas detection. However, the performance of the state-of-the-art laser spectroscopic sensors is still insufficient for some high-precision applications. Here, we demonstrate optical phase-modulation amplification by operating a dual-mode optical fiber interferometer at destructive interference for ultrasensitive carbon dioxide detection. With a 50 cm long dual-mode hollow-core fiber, amplification of photothermal phase modulation by a factor of nearly 20 is achieved, which enables carbon dioxide detection down to 1 parts-per-billion with a dynamic range of over 7 orders of magnitude. This technique could be readily used to improve the sensitivity of phase modulation-based sensors with a compact and simple configuration.

Transmission of an optical vortex beam in antiresonant fibers generated in an all-fiber system

We report an experimental study on transmission of orbital angular momentum mode in antiresonant fibers generated with a dedicated all-fiber optical vortex phase mask. The vortex generator can convert Gaussian beam into vortex beams with topological charge l = 1. Generated vortex beam is directly butt-coupled into the antiresonant fiber and propagates over distance of 150 cm. The stability and sensitivity of the transmitted vortex beam on the external perturbations including bending, axial stress, and twisting is investigated. We demonstrate distortion-free vortex propagation for the axial stress force below 0.677 N, a bend radius greater than 10 cm.

Flow-Enhanced Photothermal Spectroscopy

Photothermal spectroscopy (PTS) is a promising sensing technique for the measurement of gases and aerosols. PTS systems using a Fabry-Pérot interferometer (FPI) are considered particularly promising owing to their robustness and potential for miniaturization. However, limited information is available on viable procedures for signal improvement through parameter tuning. In our work, we use an FPI-based PTS configuration, in which the excitation laser irradiates the target collinearly to the flowing gas. We demonstrate that the generated thermal wave, and thus the signal intensity, is significantly affected by the ratio between excitation modulation frequency and gas flow velocity towards another. We provide an analytical model that predicts the signal intensity with particular considerations of these two parameter settings and validate the findings experimentally. The results reveal the existence of an optimal working regime, depending on the modulation frequency and flow velocity.

Laser applications to chemical, security, and environmental analysis: introduction to the feature issue

This Applied Optics feature issue on laser applications to chemical, security, and environmental analysis (LACSEA) highlights papers presented at the LACSEA 2020 Seventeenth Topical Meeting sponsored by The Optical Society (OSA).