Waveguide based passively demodulated photothermal interferometer for light absorption measurements of trace substances

Abstract

In this contribution, we present the development of a passively demodulated interferometer based on 3 × 3 waveguide couplers to measure light absorption of trace gases and aerosol particles via the photothermal effect. In contrast to a “classical” interferometer with two outputs, active quadrature control is not required to ensure a high sensitivity of the system. An algorithm for the evaluation of the photothermal interferometry signal from the outputs of asymmetric 3 × 3 couplers is detailed. The performance of the algorithm is demonstrated with N O 2 calibration experiments using couplers with different working principles (i.e., fused-fiber and planar-waveguide based). The results of a laboratory measurement campaign using aerosolized nigrosin are discussed, and the measured aerosol absorption is compared to a reference instrument. A noise analysis shows interferometer phase noise to be the primary noise component. Improvements to the setup are recommended, which should improve the current instrumental detection limit in terms of absorption coefficient to below the current value of 100 M m − 1 ( 1 σ , 60 s). This corresponds to mass concentrations of about 10 µ g / m 3 for submicrometer-size black carbon particles.

A Concise Review of the Progress in Photonic Sensing Devices

“…Hence, the development of smaller and more cost-effective devices can authorize widespread surveillance of air quality, letting policymakers and individuals make informed decisions to improve air quality. The advancement of a passively demodulated interferometer established on 3 × 3 WG couplers for measuring light absorption of trace gases and aerosol particles through the photothermal effect is presented [106]. This system does not require active quadrature control and uses an algorithm for evaluating the photothermal interferometry signal.…”

Gas Sensing System Based on an All-Fiber Photothermal Microcell

“…Long-term stability, temperature invariability, and low susceptibility to vibrations [13] thus present a major concern and limitation. The excitation light is applied mostly in a direction perpendicular to the probe laser beam [13,14], thus decreasing the volume of the heated region significantly [8] (refractive index changes only in a limited volume of the probe beam, i.e., the probe beam is usually focused into a spot along the probe beam, while the usage of the excitation power is weak as the relevant absorption occurs over distances that can not exceed the probe beam diameter). These perpendicular configurations lead to an increase in engineering complexity due to the tight alignment requirements as the probe and excitation beams must intersect [13].…”

Comment on egusphere-2023-2641

“…Photo-thermal interferometry is a technique that combines laser-induced heating with interferometric measurements to quantify light-absorbing aerosols (Lee and Moosmüller, 2020;Visser et al, 2023;Li et al, 2016). By directing a laser beam onto the aerosol sample, the absorbed light generates local temperature gradients, causing changes in the refractive index and thus altering the phase of a probe laser beam passing through the sample.…”

“…The deployment of photo-thermal and photo-acoustic instruments in field settings has been limited due to operational challenges, which has hindered the widespread adoption of these techniques. However, there has been a recent resurgence of interest in these methods with the development of a new generation of photometers and interferometers (Drinovec et al, 2022;Visser et al, 2020Visser et al, , 2023.…”