Intra-cavity gas detection using Er-doped fiber lasers

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Intra-cavity gas detection using Er-doped fiber lasers

 

Author: Zhang, Min
Title: Intra-cavity gas detection using Er-doped fiber lasers
Degree: Ph.D.
Year: 2003
Subject: Hong Kong Polytechnic University -- Dissertations
Optical fiber detectors
Gas-detectors
Lasers
Optical fibers -- Measurement
Department: Dept. of Electrical Engineering
Pages: x, 139 leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1732938
URI: http://theses.lib.polyu.edu.hk/handle/200/3596
Abstract: Optical fiber sensors are expected to play a key role in real-time remote gas detection for industrial and environmental monitoring applications. Over the last 20 years, investigations on the mechanism of the single pass fiber gas sensors and the associated multiplexing technologies have been conducted theoretically and experimentally, and the potential and limitations of single pass sensors have been well understood. The main objective of this project is to investigate the use of fiber intra-cavity absorption laser sensor for quantitative gas measurement by inserting a gas cell into the cavity of the Er-doped fiber laser. Two configurations of the fiber intra-cavity absorption sensors based on the linear cavity fiber laser and the ring fiber laser are investigated theoretically and experimentally. Two possible mechanisms, one of which is based on the measurement of steady state output, the other on the oscillation build-up time are investigated in order to perform quantitative gas measurement. The laser dynamic, corresponding to the case when an intra-cavity absorber is placed in the cavity, is modeled using the rate equations based on the three level model of the Er-doped fiber laser. For an Er-doped fiber laser, the key parameters include cavity loss, pump power and laser cavity parameters. The relationships between such key parameters and the output parameters such as steady state output intensity, sensitivity enhancement factor, minimum detectable gas concentration, and the oscillation build-up time are estimated analytically or numerically. For steady state measurement, sensitivity enhancement factors of 55 and 33 are achieved by using a linear cavity and a ring cavity, respectively. A wavelength modulation/second harmonic detection technique is proposed and used to improve the sensor performance in terms of minimum detectable gas concentration. Experiments with the intra-cavity absorption gas sensor based on a ring cavity laser demonstrate a minimum detectable gas concentration of 0.017mol/m3 (422ppm) acetylene with a micro-optic gas cell of length 25mm. We have also explored the possibility of intra-cavity loss detection by measuring the laser oscillation build-up time. Theory shows that high measurement resolution (10-4dB) could be achieved if the measurement error in the oscillation build-up time could be reduced to be less than 9.5us. Preliminary experiments are conducted but the performance is not what we expected from the theoretical analysis, because of various limiting factors in the measurement of oscillation build-up time. Wavelength division multiplexing technology has been employed for multiplexing the intra-cavity absorption gas sensors in our work. The system performance in terms of sensor number, sensitivity enhancement factor, crosstalk between the sensors and the minimum detectable gas concentration is investigated respectively. Wavelength scanning technology combining with the wavelength modulation/second harmonic detection is also used to improve the performance of the sensor network. Preliminary experiments corresponding to an intra-cavity absorption gas sensor consisting of a 1 x 4 WDM and several fiber loop mirrors demonstrate that a signal to noise ratio of 187 can be obtained when a 25mm gas cell is filled with the 8.31mol/m3 (20%) acetylene gas, corresponding to a minimum detectable gas concentration of l000ppm. The crosstalk between the sensors is below the noise floor and is therefore negligible.

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