Study Of Optic Fiber Filter Based On Mode Interference

With the development of optical fiber technology, the fiber filter has not only become an essential optical communications device, but also has been widely used in the field of optical fiber sensor. While the length and refractive index of the fiber are modulated by the external physical circumstances, the optical spectrum of the filter will shift. According to this, the external physical circumstances changes can be detected.Compared to the conventional sensors,because the fiber optic sensors own the characteristics with high sensitivity, anti-electromagnetic interference, small size and compact structure,they have been in many applications, such as large bridges, dams, strain measurements, petroleum, petrochemical industry, temperature, pressure safety warning, special materials within the structure of the strain, stress, aging surveillance, etc.In the first chapter, the development of optical fiber filters are introduced, and the main parameters of optical fiber filters are classified and discussed. Besides these, research progress of the optical fiber filters is summarized and analysied in detail.At the last of the chapter, the main contents and innovation of this paper are listed.The interference theory of of a specific mode in the fiber is given based on the basic theory of interference in the second chapter.At the first of this chapter, the characteristics of the single arm and two arms interferometer sensors are compared, and the typical advantages and disadvantages of the two kinds of interferometer sensors are summarized. According to electromagnetic theory and Maxwell equations, theoretical model of mode interference is derived based on an step index fiber. Take single mode-multimode-singlemode (SMS) structure for example, mode theory of fiber and the formula of maximum free spectral range was given. Finally, the conditions and princlples of optical fiber polarization mode interference are analysed, which will provide the theoretical basis for the subsequent chapters.In the third chapter, all fiber reflective Solc-type filters is designed based on the traditional Solc-type filter.Moreover, theoretical analysis is done in the method of Jones matrix. Compared to the fiber bragg grating sensors, the sensitivity of the temperature sensor based on reflective Solc-type filter can be improved 2.5 times, and the wavelength shift induced by temperature variety is 0.03nm/℃. If the length and Bragg wavelength is optimized, high-resolution measurements of dynamic small-signal are achieved based on this filter. This design is the characteristics with simplicity and high resolution.Besides these, the ability to measure the low-frequency signals (< 1Hz) have been improved significantly through the common mode rejection. By the use of intensity ratio between the signal light and reference light, the fluctuations caused by light source are eliminated. The system is particularly suitable for small-signal measurement, especially in the sound sensor.A novel filter with cascade structure was designed in chapter 4.It is composed of fiber polarizer, polarized-maintaining photonic crystal fiber and Sagnac fiber loops. According to the theoretical result which is deduced by the Jones matrix, the filter has the function of tunablility and flat top.In the experiment, based on the characteristics of photonic crystal fiber and the loss caused by splice the Ericsson 995, the splice program is optimized to achieve low-loss splice between photonic crystal fiber and the ordinary fiber. The filter experimental setup is built on the basis of low-loss splice. The experimental results show that the bandwidth at 0.5dB reaches 4nm, which exceed 50 percent of free spectral range of 7.52nm.The channel isolation achieved 14dB,and the peak wavelength with temperature variation is less than 1.2pm/℃. Because the birefringence effect changes caused by the temperature in the photonic crystal fiber is very small, the temperature stability of filter can be improved greatly.A cascaded structure is designed based on two Sagnac loops using polarization beam splitter (PBS) and 3dB coupler in chapter 5. According to the theoretical analysis, if the length of photonic crystal fiber in the two rings is equal, interference extinction ratio will double compare with the single ring. Using Ericsson 995PM splicer and extinction ratio meter to realize online inspection, the photonic crystal polarization maitaining fiber and conventional polarization maitaining fiber can splice at an angle of 45 degrees between the two fast or slow axes. Using the above reflective Solc-type filter, a strain measurement independent of temperature based on the photonic crystal fiber is achieved, and the measurement sensitivity is the 0.3pm/με. Using splice technology mentioned earlier, dual-Sagnac loop cascaded structure is built in experiment. Experiment results show that the interference extinction ratio reaches 43dB. By using this characteristic, high-resolution demodulation is accomplished successfully in two different kind of fiber bragg grating sensors. Static strain resolution is 0.33με, and dynamic strain resolution is 0.47 nε/(?), respectively.The resolution of this kind of demodulation system, compared with report in the previous literature, has been significantly improved. Therefore, this filter is very appropriate for the high-precision measurement.