Full metadata record
| DC Field | Value | Language |
|---|---|---|
| dc.contributor | Department of Electrical and Electronic Engineering | en_US |
| dc.contributor.advisor | Jin, Wei (EEE) | en_US |
| dc.creator | Guo, Linhao | - |
| dc.identifier.uri | https://theses.lib.polyu.edu.hk/handle/200/13693 | - |
| dc.language | English | en_US |
| dc.publisher | Hong Kong Polytechnic University | en_US |
| dc.rights | All rights reserved | en_US |
| dc.title | Optical fiber photothermal spectroscopy for gas sensors and phase modulators | en_US |
| dcterms.abstract | Optical fiber photothermal spectroscopy (PTS) has been used for gas sensors over the last decade, with high selectivity and sensitivity. Involving a pump-probe configuration, PTS with hollow core fibers (HCFs) enables efficient photothermal (PT) phase modulation at frequencies up to tens of kHz, which ensures gas detection with high signal-to-noise ratio (SNR). In this thesis, we conduct further research to improve the sensor performance and reduce the cost, with an ultra-compact gas sensor using single mode fibers (SMFs) for real-time gas detection, and pump-probe-alternating and Fourier-transform-PTS techniques for multi-component gas sensing. Meanwhile we explore another application of the PTS for phase modulators (PMs). | en_US |
| dcterms.abstract | Real-time gas sensing is very important from daily life to the industrial and agricultural production. Thus, we propose a miniature optical fiber PT gas sensor with fast response and large dynamic range. The sensing region is an air gap formed between the cleaved ends of two SMFs. Theoretical formulations of photothermal phase modulation and interferometric phase detection are presented. Numerical simulation and experimental investigation are carried out to optimize the system parameters to maximize the photothermal signal. A gas sensor with an air gap of 130 μm demonstrates a noise equivalent concentration (NEC) of 45 ppb and dynamic range of 2×107 for acetylene (C2H2) detection, with a response time of 0.9 s. The sensor is simple to construct and may be used for real-time gas detection in a confined space. | en_US |
| dcterms.abstract | Moreover, in conventional fiber optical PTS (FO-PTS) systems, in addition to the pump lasers tuned to the specific absorption lines of the target gas species, an additional laser is used as the probe beam for PT phase detection. For this, we demonstrate an C2H2 /methane (CH4) gas sensor based on HCF-PTS with a pump-probe-alternating technique. This technique utilizes two distributed-feedback lasers as pump and probe beams alternatively for two gas components through time-division multiplexing. With a 2.5-cm-long HCF, NECs of 370 ppb and 130 ppb are demonstrated for methane and acetylene, respectively. The proposed technique eliminates the need for an additional laser in the traditional PTS setup, enabling the construction of a sensitive yet more cost-effective multi-component gas detection system. | en_US |
| dcterms.abstract | Furthermore, as the number of targeted gas species increases, FO-PTS systems normally require more pump lasers. To counteract this, we demonstrate a gas sensor by combining the techniques of HCF-enhanced FO-PTS with Fourier transform spectroscopy. A broadband light source (BLS) is used as the pump beam, with the light intensity modulated at different frequencies for different wavenumbers by passing through a scanning two-beam interferometer. The PT spectrum is obtained by Fourier transform of the measured interferogram. With a 10-cm-long HCF, NECs of ~465 ppb and ~457 ppm are demonstrated for C2H2 and carbon dioxide (CO2), respectively, with only one pump beam. Detection of the two-component gas of C2H2 and CO2 is demonstrated with the same system. The proposed method enables the construction of a PTS system with a compact size for multi-component gas detection. | en_US |
| dcterms.abstract | Meanwhile, we explore the PTS for the application of PMs with HCFs. We study the effect of varying gas concentration, buffer gas, length and type of fibers on the performance of optical fiber photothermal PMs based on C2H2-filled hollow-core fibers. For the same control power level, the PM with Ar as the buffer gas achieves largest phase modulation. For a fixed length of hollow-core fiber, there exists an optimal C2H2 concentration that achieves the largest phase modulation. With a 23-cm-long hollow-core anti-resonant fiber filled with 12.5% C2H2 balanced with Ar, phase modulation of π-rad at 100 kHz is achieved with a control power of 200 mW. The modulation bandwidth of the PM is 150 kHz. The modulation bandwidth is extended to ~1.1 MHz with a hollow-core photonic bandgap fiber (HC-PBGF) of the same length filled with the same gas mixture. The measured rise and fall time of HC-PBGF PM are 0.57 μs and 0.55 μs, respectively. | en_US |
| dcterms.extent | xxviii, 113 pages : color illustrations | en_US |
| dcterms.isPartOf | PolyU Electronic Theses | en_US |
| dcterms.issued | 2025 | en_US |
| dcterms.educationalLevel | Ph.D. | en_US |
| dcterms.educationalLevel | All Doctorate | en_US |
| dcterms.LCSH | Optical fiber detectors | en_US |
| dcterms.LCSH | Photothermal spectroscopy | en_US |
| dcterms.LCSH | Gas detectors | en_US |
| dcterms.LCSH | Phase modulation | en_US |
| dcterms.LCSH | Hong Kong Polytechnic University -- Dissertations | en_US |
| dcterms.accessRights | open access | en_US |
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