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The Research of TDM Based on Optical Sensers

Author: LinChengYuan
Tutor: CaoJiaNian
School: Harbin Engineering University
Course: Optical Engineering
Keywords: fiber gas sensor TDM weak signal detection quadrature lock-amplification
CLC: TP212
Type: Master's thesis
Year: 2011
Downloads: 17
Quote: 0
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Abstract


Tunable diode laser absorption spectroscopy (TDLAS) harmonics detection technology is a new high sensitivity detection scheme developed from the combination of laser and modulation technology. It has been widely applied in gas detection. The multi-point gas monitoring technique with tunable diode laser absorption spectroscopy (TDLAS) requires expensive lasers. The multiplexing technology sharing one set of laser and light detector can reduce the cost of a single measurement point.Based on the analysis of the traditional time-division multiplexing optical fiber gas sensor system, this paper presents a type of advanced topology in which the signal reaches the detector through layout fibers in different length and fibers used for delaying signal in the same lengths. It solves the problem caused by the layout fiber delaying the light signals in the traditional topology. On the analysis of the lock-amplification technology in TDLAS, the digital quadrature lock-amplification detection technology is used in harmonic signal extraction which can eliminate the impact of phase in amplitude detection automatically. It solves the trouble that the traditional lock-extraction method requires manual phase-shifted adjustment. The sampling method of the first harmonic improved that the sampling period is set to be same with the period of a modulation triangle wave,so that the starting sampling point can be any point. This new approach solves the trouble of designing synchronous square wave signal as the starting signal and manual phase-shifted adjustment in traditional way. A new calculation approach is presented that regarding the mean of maximum and minimum values as the mean of teseted signal in the sampling period. It saves system resources and makes parallel processing of multi-channel data possible. A digital time division multiplexing technology program based on TDLAS detection of methane gas detection is givenand a 4-way simulation system of it is builded in MATLB which has verifyed the theoretical feasibility of the system.A set of digital quadrature lock-in amplifier based FPGA established, and the dataof sinewave amplitude information extraction experiments shows the error of indicating data is under 2%. Finally, The capacity of digital quadrature lock-in amplifier is improved according to the features of time division multiplexing TDLAS. By the way of time-division multiplexing the key signal processing module, the required resources of the demodulation system is reduced greatly which solves the shortage of resource when the FPGA is processing multichannel datas.

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