The phase-generated carrier (PGC) is the best-known signal-processing method for an optical fiber interferometer, and has been widely adopted for experimental systems such as hydrophones and optical fiber current sensors. It has many advantages, such as high resolution and passive sensor structure, and has been used for more than 20 years. In this paper, the signal dependence of the PGC is reported for the first time. Our results imply, in view of the highpass filter that is needed to suppress the temperature fluctuation of the interferometer, that the application of the PGC method is inappropriate for the measurement of low-frequency but sharp-rising-edge signals, such as seismic signals, whose frequency extends down to ~1 Hz and whose rising edge is very sharp. In such a case, the signal dependence will cause fatal errors. Experimental and simulation results are provided to describe the defects of this method. An analysis of the restrictions on the use of the PGC method is also provided.
This article firstly gives out when the phase modulation amplitude is at best value of homodyne demodulation using phase generated carrier, and then presents three methods that can be realized to get the best value. Some differences based on practicality are made among them, and a method which is easy to be implemented by digital circuits, is chosen to be carried out by digital circuits. Also this paper gives out simulation analysis and real experiment results. The error range is from -3.80% to 2.11% with real system. The origins of the error limiting the accuracy are discussed.
In this paper, a theoretical model of the interferometer using the coaxial structure was established to investigate the temperature effect in measuring the acoustic wave. The experimental result shows that the ratio of the phase shift is 312.5 rad/°C when the Michelson interferometer was used with the 3 meters fiber and the Aluminum as the cylinders. If the maximum amplitude of the signal that can be measured by the system is 10 rad, the temperature variation should be smaller than 0.032°C to keep the system from out of measurement range. Then in a system with measuring time period is 6s for seismic detection, the temperature variation should be smaller than 0.0054°C/s within the measuring period.
This paper reports, on the use of a tapered single mode fiber as a sensing element for the detection of acoustic emission (AE) and ultrasound. When an acoustic wave impinges on the mode-coupling region of a fiber, the coupling coefficient is modulated via the photo-elastic effect. Therefore, the transfer function of the fiber is modulated by an acoustic wave. The sensitivity of the sensor at 156 kHz was approximately 1.2 V/mbar. Because of the resonant condition of the coupled-mode theory, this sensor is almost immunity of the environment perturbations, and the output is a DC signal instead of AC, so it is very suitable for those situations such as faulty protection which need very fast responsibilities.