| Author: | Hong, Yingzhen |
| Title: | Performance enhancement of fiber optic photothermal interferometry for gas sensing |
| Advisors: | Jin, Wei (EEE) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Gas detectors Laser spectroscopy Photothermal spectroscopy Optical fibers Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Electrical and Electronic Engineering |
| Pages: | xxx, 144 pages : color illustrations |
| Language: | English |
| Abstract: | Trace gas sensing holds importance in numerous domains, encompassing air pollution surveillance, medical examination, and industrial process regulation. Laser absorption spectroscopy (LAS) has been widely used in gas detection, utilizing the unique "fingerprint" absorption characteristics of gas molecules. Photothermal interferometry (PTI) is a derivative of LAS that offers high sensitivity for trace gas detection, involving two lasers for pumping and probing. A modulated pump laser generates heat in the gas medium, modulates its local refractive index (RI) and subsequently the phase of a probe beam propagating within the medium. This modulation can be detected via optical interferometry. Diverse gas species have been detected in the laboratory environment with noise-equivalent- concentrations (NEC) down to parts-per-billion (ppb) level, utilizing different interferometric configurations. However, the ultra-low detection limit does not directly applicable to practical applications that demand high sensitivity and long-term stability. Thus, the objective of this thesis is to improve the performance of hollow-core fiber (HCF)-based PTI gas sensors to make them more applicable to real-world situations. Several interferometric schemes have been scrutinized to detect gases exhibiting strong absorption in the near-infrared (NIR) spectrum. However, gases like oxygen do not have strong absorption in the NIR. Given the inherent simplicity of the Fabry-Perot interferometer (FPI), which enables the independent utilization of the pump and probe transmission optics, we propose a visible-pump and NIR-probe FPI scheme. This approach allows for efficient access to the strong absorption lines within the visible spectrum, while also providing a cost-effective solution for using a NIR probe interferometer. We further propose a high-finesse Fabry-Perot (FP) cavity with a short HCF, yielding superior performance compared to the low-finesse FPI. With a 1-cm-long HCF gas cell, we demonstrate an oxygen detection down to 6 parts-per-million (ppm) with a 1000-s averaging time. In PTI systems, the probe source commonly employed is a narrow-linewidth laser. However, the use of such a laser induces unwanted intensity fluctuations due to parasitic interference. To counteract this, we propose a fiber optic low-coherence (LC) PTI system, which employs a broadband probe source with a short coherence length. With a 10-cm-long HCF, we achieve acetylene detection with measurement precision of 0.025% and instability of ±0.038% over 3 hours. Theoretical analysis and experimental investigation of parasitic interference reveal that LC-PTI offers improved measurement precision and long-term stability compared to traditional PTI. To further enhance the gas detection sensitivity, we demonstrate the optical phase-modulation amplification (OPMA) by using an HCF FP cavity and a dual-mode interferometer (DMI), respectively. By locking the probe wavelength to the resonance of an FP cavity, OPMA of over two orders of magnitude is achieved, enabling ultra-sensitive gas detection with large dynamic range. However, it requires accurate wavelength locking, which is complex for practical applications. Hence, we further demonstrate the OPMA by operating a DMI at destructive interference and use it to achieve carbon dioxide detection with NEC of 1 ppb with a 50-cm-long HCF. The OPMA can be employed to improve the sensitivity of phase modulation-based sensors. |
| Rights: | All rights reserved |
| Access: | open access |
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