Author: Ho, Hoi-lut
Title: Multi-point fiber optic gas sensor systems
Degree: Ph.D.
Year: 2002
Subject: Hong Kong Polytechnic University -- Dissertations
Optical fiber detectors
Chemical detectors
Department: Department of Electrical Engineering
Pages: iv, 136 leaves : ill. ; 30 cm
Language: English
Abstract: Multiplexing is a very important issue for fiber optic sensors. It enhances the system utilization and the competitiveness of fiber sensors against conventional technologies. Although, multiplexing techniques for optical fiber sensors have been proposed and theoretically studied over the last 20 years, implementations in chemical (gas) sensors context are rarely reported experimentally. It is believed that an increasing attention will focus in this area in the coming years. The main objective of this project is to develop and investigate multi-point gas detection systems for quantitative measurement. Two different multiplexing systems for fiber optic gas sensors, one named time-division-multiplexed (TDM) system, the other frequency modulated continuous wave (FMCW) system, are investigated theoretically and experimentally in this project. In our systems, quantitative measurement of gas concentration based on wavelength modulation spectroscopy (WMS) and second-harmonic detection are demonstrated by applying low-frequency wavelength modulation to an external-cavity tunable diode laser. The tunable laser operating at 1.53 μm region is used to detect acetylene (C2H2) based on direct absorption through the use of micro-optic cells. A time-division multiplexed three-sensor system with a forward-coupled ladder topology have been implemented and experimentally tested. The experimental system using micro-optic cells of length 25mm have demonstrated sensitivity of 81 ppm/√Hz.. The crosstalk between the sensors was found to be -30 dB. Power budget analysis shows that a sensor network consisting of 90 sensors could be realized with the same multiplexing topology. The performance of such a networked system has been examined in terms of shot noise limit, inter-channel crosstalk and detection sensitivity. Apart from the TDM techniques, we also applied a frequency modulated continuous wave (FMCW) technique for multiplexing optical fiber gas sensors. The sensor network is again of a ladder topology and is interrogated by the same tunable laser. For the FMCW system, the optical delays of each channel are carefully designed in order to satisfy the basic requirements that will be discussed in the following chapters. The system performance in terms of detection sensitivity and crosstalk between sensors was investigated and found limited by the coherent mixing between signals from different channels. The system performance can be improved significantly by use of appropriate wavelength modulation/scanning coupled with low pass filtering. Computer simulation shows that an array of 37 acetylene sensors with a detection accuracy of 2000 ppm for each sensor may be realized. A three-sensor network of ladder topology is experimentally demonstrated for the detection of acetylene gas. A minimum detectable concentration of 270 ppm/√Hz is obtained with 25mm gas cells under atmospheric pressure which corresponds to a minimum detectable absorbance of 1.48 x 10-4 /√Hz. The crosstalk between the sensors is below -20 dB. With the theoretical and experimental justifications, the relative advantages and disadvantages of the TDM and FMCW systems can be summarized as follows. TDM system works with a simple concept where simple circuitries are employed for either encoding or decoding. It shows excellent system tolerance compared with the FMCW system. For the TDM system, the performance is mainly affected by the extinction ratio of the optical switch that can be simply improved by using a double M-Z type modulator or switch. In contrast, the performance of the FMCW system is heavily influenced by a number of factors such as the biases of the fiber delays, the settings of the triangular chirped carrier and the linearity of the VCO. Deviations from the optimized values of the above parameters would cause a raise in inter-channel crosstalk and unwanted interferometric signals (noise). If a highly coherent light source is used and no noise suppression techniques are implemented, the sensitivity and the crosstalk of the FMCW system are definitely worse than that of the TDM system. However, these unwanted interferometric signals can be effectively suppressed by using a low coherent source and specified signal processing techniques, therefore the FMCW system would have superior performance because of its better optical power utilization where a continuous optical signal is fed into the network instead of the optical pulses used in the TDM system. Better signal-to-noise ratio would be obtained with the FMCW system since the detected signal would be greatly enhanced compared with the optical-power-related noise (shot noise). The FMCW system is shown to have a better sensitivity when the performance of the system is shot-noise ("quantum") limited.
Rights: All rights reserved
Access: open access

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