||Fiber-Optic Based Trace Methane Sensor using Mid-Infrared Light
||Department of Photonics
Trace gas detection
甲烷在現代生活中是一種被廣泛使用的溫室氣體，故如何有效地監測甲烷濃度一直以來都受到重視。本研究提出並討論三種使用中紅外線(λ= 3.3 μm)發光二極體的甲烷氣體感測系統的光學方案。
作為同類型研究的首例，本研究採用直接吸收法，其系統基本架構包含一個光源、一個中紅外線發光二極體、一個紅外線光感測器及一個具備適當氣體混合器系統的20 mm氣體槽。其輸出訊號可藉由訊號放大器或鎖相技術放大。使用訊號放大器時，感測極限估計為49 ppm，使用鎖相技術時，偵測極限估計為2.3 ppm。然因中紅外線光非人類肉眼能見，需透過熱顯像儀始能看到，致使光學對準是非常費時，是故在第二種感測系統的光學方案中，本研究改以微結構紅外線多模光纖代替氣體槽，並使用高功率Q開關雷射加工光纖表面，使光與甲烷分子此微加工光纖表面上產生交互作用，偵測極限估計為6.1 ppm，動態範圍估計為26 dB，比直接吸收還精確許多。
綜上研究經驗，本研究最終提出在中空光纖中使用中紅外線發光二極體的微量甲烷氣體感測系統的第三種光學方案。當來自中紅外線發光二極體的光聚焦在中空光纖上，將光與甲烷氣體之間的交互作用限制在直徑0.5 mm的光纖纖核內，並以內置之1m中空光纖增強光程，實現最佳的光與氣體的交互作用。透過採用直接吸收光譜技術，甲烷氣體濃度的感測極限擴展到低至17 ppb，動態範圍估計為42 dB。中紅外線發光二極體的400 nm頻寬可以涵蓋3.3 μm附近的所有強吸收線。此方案為應用於遠程和惡劣環境之低成本手持式甲烷檢測器設計，提供了一個可實現的工程願景。
Methane is a major greenhouse gas being widely used in modern daily life. The need to effectively monitor the methane concentration has always been emphasized.
In this study, we present and discuss three methane gas detection systems using a mid-infrared (MIR) (λ=3.3 µm) light emitting diode (LED). As a first of its kind, the direct absorption method was employed. It included a light source, an MIR LED, an infrared photodetector, and a 20-mm gas cell with a proper gas mixer system. The output signal could be amplified using a signal amplifier or a lock-in technique. The detection limit was estimated as 49 ppm using a signal amplifier and 2.3 ppm using a lock-in technique. MIR light is invisible to the naked eye but can be seen with a thermal camera. The optical alignment is time-consuming. A microstructured IR multimode fiber was adopted instead of a gas cell. The fiber surface was machined by a high-power Q-switched laser, and the interaction of light and methane molecules occurred on this micromachined fiber surface. The detection limit was estimated as 6.1 ppm. The dynamic range was estimated as 26 dB.
In addition, the study presents here an optical scheme for a trace methane gas detection system using mid-infrared (MIR) light emitting diode (LED) (λ=3.3 µm) in a hollow-core fiber (HCF). When light from the MIR LED was focused on the HCF, the latter confines the interaction of light and methane gas within a fiber core of 0.5 mm diameter. The 1-m-long HCF provided enhanced optical path, and optimum light-gas interaction. By employing direct absorption spectroscopy, the detection limit of methane gas concentration was extended to as low as 17 ppb, and the dynamic range was estimated as 42 dB. The 400-nm bandwidth of a MIR LED can cover all strong absorption lines in the vicinity of 3.3 µm. This scheme provides an engineering perspective to realize a low-cost, hand-held methane detector for remote and harsh environments.
Table of Contents IV
List of Tables VI
List of Figures VII
Chapter 1 Introductions 1
1.1 Research Backgrounds 1
1.2 Infrared spectroscopy 3
1.3 Research Motivations 7
1.4 Overview of this Thesis 8
Chapter 2 Methane Detection using Direct Absorption Method 10
2.1 Introductions 10
2.2 The Beer-Lambert law 13
2.3 The Database for molecular spectroscopy 14
2.3-1 HITRAN molecular spectroscopic database 14
2.3-2 PNNL molecular spectroscopic database 16
2.4 Experimental Scheme for Direct Absorption 17
2.5 Mid-Infrared (MIR) Light-Emitting Diode 19
2.5-1 HgCdTe (MCT) Photo detector 22
2.6 Methane Detection using Direct Absorption Method 25
2.7 The summary of Chapter 2 32
Chapter 3 Methane Detection Using Fiber-Optic Absorption Method 34
3.1 Introductions 34
3.2 The Design of Micro-Structured Fiber 35
3.3 The Experimental Scheme for Fiber-Optic Absorption Sensor 36
3.4 The summary of Chapter 3 39
Chapter 4 Trace Methane Sensor using Mid-Infrared Light Emitting Diode in Hollow-Core Fiber 41
4.1 Introductions 41
4.2 The Experimental Scheme for Trace Methane Sensor in Hollow-Core Fiber 44
4.2-1 the Hollow-Core Fiber in mid-infrared region 46
4.3 Experimental Data and Analysis 49
4.4 The Summary of Chapter 4 54
Chapter 5 Visual-Assisted Laser Microwelding of Carbon Microfiber on Metal Plates 56
5.1 Introductions 56
5.2 Infrared Thermal Imaging and Laser Melting 58
5.3 The Optical Setup and Design 61
5.3-1 The Optical Setup and Design 63
5.3-2 A Feedback Control of Laser Power During Laser Melting Process 65
5.4 Experiment Results and Discussions 67
5.5 The Summary of Chapter 5 70
Chapter 6 Conclusions and Prospects 72
6.1 Conclusions 72
6.2 Future works 73
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