Study On Multiplexing Of Orthogonally Polarized Dual-frequency Fiber Laser Sensors

In the recent years, the demand for large-scale sensor network has been greatly increaseddue to the requirements of environmental monitoring, multi-point measurement and control andinfrastructure health. The dual-frequency fiber grating laser sensor has presented manyadvantages, including narrow bandwidth, high resolution, high signal-to-noise ratio. Comparedwith the wavelength-encoded fiber Bragg grating sensor, it is completely frequency encoding,and the cost for the frequency detection equipment is much lower than the wavelengthinterogators. Therefore, the sensor has great application potential in the further sensing networksystem. The multiplexing of this kind of sensor is required in both wavelength and frequencydomains. We have proposed two methods to permanently change the output beat frequency of thelaser, and the implementation of a sensor array consisting of16sensing elements is demonstrated.The content of the thesis includes:(1) The research on single-longitudinal-mode of dual-frequency fiber laser. The laser isfabricated by inscribing two wavelength-matched Bragg gratings in an Er-doped fiber by use of a193nm excimer laser and phase mask method. The UV illuminationis controlled during thegrating inscription to control the lasing wavelength with respect to the grating transmission band,to achieve single longitudinal mode output.. The signal-to-noise ratio of laser output is above60dB and the beat signal has a signal-to-noise ratio of above40dB.(2) We proposed two methods to permanently change the beat frequency of distributedBragg fiber laser: CO2laser heat treatment method and rotation-fusion method. By controllingthe exposure between the two fiber gratings, the CO2laser heat treatment method can effectivelychange the birefringence of fiber, so as to change the beat frequency. The rotation-fusion methodis carried out by slicing the laser cavity into two sections and then aligning them with a rotatedangle. It can partially offset the optical path difference between two polarization, so as to reducethe beat frequency. The two methods respectively realized the frequency tuning range from200MHz to289MHz and from1.7GHz to2.06GHz. It greatly extends the tuning range of outputbeat frequency, based on which frequency division multiplexing can be realized.(3) Based on the heat treatment of the CO2laser, we demonstrate the implementation of asensor array consisting of10sensors pumped by a single980nm laser diode and an array with 16sensors pumped by two980nm laser diodes. Wavelength-and frequency-divisionmultiplexing of heterodyning fiber grating laser sensors are realized simultaneously. We alsoanalyze and discuss the factors limiting the ability of multiplexing, including the pump powerdepletion, wavelength separation, and the stability of the beat signal.