Research On Optical Comb Filters Based On Optical Fiber Sagnac Loop

As one of the key technologies in the twentieth century, the maturity of Dense WavelengthDivision Multiplexing (DWDM) has greatly promoted the development of opticalcommunication and it has been the mainstream method for capacity increasing in optical fibercommunication network. In the technique, with the increasing of the channel density, variousoptical comb filtering techniques have gradually become a hot research field. Among them, fiberSagnac loop filter has received great attention because of its tunableness, low insertion loss andsimple configuration. So far, fiber Sagnac loop comb filters have been widely used in realizingmulti-wavelength source, wavelength multiplexing, wavelength demultiplexing, high-speedimpulse sequence, multi-channel dispersion compensation, fiber sensors and so on.Firstly, basic theory and analysis methods for High birefringence fiber (HBF) Sagnacloop and fiber bragg grating (FBG) Sagnac loop are introduced. Based on coupled-modetheory, we specially deduce the transfer matrix method regarding points about nonuniformfiber gratings. And Jones calculus is introduced to analyze two kinds of fiber Sagnac loop.Secondly, symmetric chirped fiber grating is proposed to increase the reflection bandwidthof the fiber grating. Compared with uniform fiber grating Sagnac loop, symmetric chirped fibergrating Sagnac loop has advantages such as larger bandwidth, more numbers of pass band andequal channel intervals. However, intensive modulation phenomenon exists in the first schemebecause the reflectivity of fiber bragg gratings can’t reach a hundred percent. In order to mitigatethe phenomenon, we put forward a new type structure with piecewise uniform fiber gratinginserted into a Sagnac loop. Based on analysis and numerical simulations, it is proven thathigh-performance comb filters can be realized with the new type structure. Meanwhile,multi-stage fiber grating Sagnac loops is the focus of the investigation, which is appropriate forrealizing narrow-band comb filtering because it could decrease the line width and increase theisolation effectively.Finally, the principle of HBF Sagnac loop is analyzed theoretically. We alsodemonstrate one-stage and multi-stage HBF Sagnac loop theoretically and experimentally.And some relevant factors about the transmission performance of HBF Sagnac loop areinvestigated. On the basis of the simulations, we design a narrow-band filter, which couldincrease the isolation and decrease the line width effectively. And based on the experimentalresults, we get the narrowband spectrum with the structure of two-stage Lyot-Sagnac loop.