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Title: Photothermal gas spectroscopy with hollow-core photonic bandgap fibre : from fundamental mechanism to distributed sensing application
Authors: Lin, Yuechuan
Advisors: Jin, Wei (EE)
Keywords: Gas detectors -- Materials
Optical fibers
Gases -- Analysis -- Equipment and supplies
Issue Date: 2018
Publisher: The Hong Kong Polytechnic University
Abstract: Photothermal interferometry (PTI) is a powerful spectroscopic technique for trace gas analysis with characteristics of ultra-sensitivity, unique selectivity and non-intrusiveness. It usually adapts a pump-probe configuration: the absorption of periodically modulated pump beam in gas molecules causes localized heating, resulting in changes in temperature, pressure and density, and hence modulating the refractive index of the probe beam, that propagates in the same gas sample and accumulates phase modulation. The photothermal phase modulation of the probe beam is detected via an optical interferometer with high precision. PTI has been well developed in free-space optics arrangements but its performance is limited by the beam divergence, beam spot size, bulky system size and complexity in optical alignment. It usually requires high pump power to achieve ultra-sensitivity. The hollow-core photonic bandgap fibres (HC-PBFs) can confine both the light and gas molecules within the central hollow core over a long length with low loss and provide a higher light intensity with a given optical power. Since the PTI signal is directly proportional to gas absorption length and pump light intensity, PTI in HC-PBFs can achieve much better photothermal (PT) efficiency while makes it possible for the development of all-fibre optical gas sensors with ultra-sensitivity, high selectivity and compact size. It is also capable of developing remote, multiplex points and distributed sensing. Our group has demonstrated a first PTI in HC-PBFs and obtained a limit of detection (LOD) as good as 2.3×10‾⁹cm‾¹, which is comparable with that in free-space optics arrangements but uses only tens of mW pump power. It enhances the detection sensitivity by nearly 3 orders of magnitude and achieves unprecedented dynamic range of nearly six orders of magnitude, compared with the previously reported gas sensors using HC-PBFs. However, the mechanism and dynamics of PT phase modulation in HC-PBFs are still not well understood, even though they are essential for the further development of PTI gas sensors. On the other hand, the system used in the preliminary work on PTI with HC-PBFs is still far behind the requirement for practical in-situ gas sensing applications. The objectives of this dissertation are to investigate the fundamental mechanism and dynamics of PT phase modulation, to optimize the performance of PTI sensors and to exploit the possibility of distributed gas sensing application with HC-PBFs. To investigate the mechanism and dynamics of PT phase modulation in HC-PBFs, we developed a theoretical model and designed a PTI sensor using a pulsed pump source. We proposed a numerical model solved by finite element method to analyze the thermal conduction process in HC-PBFs. Several parameters that affect the efficiency of PT phase modulation in HC-PBFs, i.e. the thermal conduction time, pulse durations of pump beam and the size of hollow-core, are studied. It has been found that the PT efficiency will increase as increasing the pump pulse duration until it reaches ~ 1.2μs. For pump pulse duration > 1.2μs, further increasing the pump pulse duration would not enhance the PT efficiency any further. Meanwhile, for pulsed pump source with pulse duration > 1.2μs, the normalized photothermal phase modulation coefficient is found to be ~ 1.5rad· cm · mW⁻¹· m⁻¹, normalized to 1cm-1 gas absorption coefficient, 1 mW peak pump power and 1m effective absorption length. The characteristic time constants of leading/trailing parts of PTI output pulse signals are determinedby the thermal conduction time of buffered gas in HC-PBFs and non-radiative relaxation time of absorptive gas molecules. The numerical simulation provides an effective way to comprehend and explain the PT phase modulation in HC-PBFs and forms a fundamental reference to optimize the design of PTI sensors. The results of our numerical model agree well with the experimental results. The LOD down to ppb (parts-per-billion in volume ratio) level of C₂H₂ gas detection using pulsed pump source can be expected.
Towards the real industrial applications of PTI, we studied optimized performance of all-fibre PTI HC-PBFs sensors. The frequency dependence of PT phase modulation in HC-PBFs using a continuous-wave (CW) intensity-modulated pump beam was investigated by use of a fibre-optic Mach-Zehnder interferometer (MZI). We found that there is no significant difference of PT phase modulation efficiency for pump modulation frequency fp < 330 kHz. However, for fp from 440 kHz to 2 MHz, the PT phase modulation efficiency rolls off quickly. The experimental results agree well with that of our numerical simulation and the normalized PT phase modulation coefficient for a CW sinusoidally intensity-modulated pump source is ~ 0.76rad· cm · mW⁻¹· m⁻¹. Such a frequency-dependent PT response can benefit to further optimize the performance of photothermal gas spectroscopy system. The PTI sensors with Mach-Zehnder interferometer (MZI) requires active stabilization with an electric servo-loop control and also requires a nearly perfect length match between the sensing and reference arms to reduce the laser phase noise, which is complex and inconvenient for the applications in harsh environment, remote and multiplexed sensing. We developed a modified all-fibre Sagnac interferometer with passively stabilization based on a 3× 3 loop coupler. The minimum detectable phase of Sagnac interferometer is measured as 4× 10⁻⁷rad/√ Hz. With PT Sagnac interferometer, we achieved LOD of 7.8× 10⁻⁸cm⁻¹ (or 67 ppb for C₂H₂ detection, 1-s integration time) with a 1.1-m-long HC-PBFs and 45.6 mW peak pump power working at optimized pump modulation frequency. The PT Sagnac interferometry demonstrated a satisfactory long-term stability with maximum signal fluctuations of 1% for over 4.5 hrs under laboratory environment. The PTI with a modified Sagnac interferometer is capable of developing remote and multiplex points sensors. We studied distributed gas sensing with fibre-optic PTI. With CW intensity-modulated pump source, we derived a general formula to estimate the magnitude of PT phase modulation distributed over a long length HC-PBFs in the presence of absorptive as molecules. We demonstrated a quasi-distributed photothermal gas spectroscopy with 2 sections of sensing HC-PBFs (each with 28 m length) using a dual-pulse heterodyne multiplexed phase demodulation system. The quasi-distributedgas sensor provides a LOD down to 10 ppb (1.2× 10⁻⁸cm⁻¹) for C₂H₂ detection with 55 mW peak pump power. Theoretically, it is possible to multi-plex ~ 20 HC-PBF sensors. By incorporating phase-sensitive optical time domain reflectometry (φ - OTDR) with dual-pulse heterodyne phase detection, we demonstrated a distributed photothermal gas sensor witha 200-m-long HC-PBFs by use of backscattering signals from the surface scattering of HC-PBFs, which results from random fluctuations of the hollow-core dimensions along fibre. Without signal averaging, we achieved LOD down to 5 ppm (5.8× 10⁻⁶cm⁻¹) for C₂H₂ detection with 62.5 mW peak pump power. The distributed sensing length as long as a few kilometers of HC-PBFs could be expected with high sensitivity. This work is the first demonstration of distributed gas sensor using PTI in HC-PBFs and is of full potential for developing an all-fibre and non-destructive distributed gas sensor using a single long-length HC-PBFs with high sensitivity.
Description: xxxiii, 181 pages : color illustrations
PolyU Library Call No.: [THS] LG51 .H577P EE 2018 Lin
Rights: All rights reserved.
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