A New Approach Of Designing Optical Fiber Signal Processing

Abstract

A New Approach of Designing Optical Fiber Signal ProcessingJingshown Wu and Ing -Jiunn SuDepartment of Electrical Engineering, National Taiwan UniversityTaipei, Taiwan, R.O.C. 107AbstractOptical fibers are excellent communication media with broad bandwidth, low loss, lightweight, low cross talk, and immunity to electromagnetical interferences. With high speedlasers and photodetectors, optical fibers are also attractive for processing broadbandsignals. Many noncoherent optical fiber signal processing devices reported in literaturewere constructed to perform various functions such as frequency filtering, high speed pulsegeneration, encoding, and decoding. The basic components of an optical fiber signal pro-cessing device include light sources, delay lines, attenuators, directional couplers, andphotodetectors. Recently the lattice optical fiber structures which are consisted of two -fiber directional couplers and various length of delay lines are investigated intensively.This structures can provide some good features such as complete mathematical formulation,straight forward implementation, modularity, and convenience for filter synthesis. However,due to the positive system property, i.e. non -negative coefficients of the noncoherentoptical fiber signal processing devices, the traditional signal processing design methodsand procedures can not be applied. For example, in the filter design, the passband band-width, transition band roll -off, and side lobe response can not be completely controlledwithout complicating the device structure. In this paper, we investigate the possibilityof using optical fiber to design the filters with the traditional properties such as maxi-mally flat or equal. -ripple responses. Then we will report a new approach, which employswindow functions and uses two -fiber as well as multi -fiber directional couplers, to synthe-size filters with desired responses. The mathematical derivation, synthesis procedures,and frequency responses of various filters will also be presented.IntroductionOptical fibers are used in many areas of science and engineering, such as communication,environmental sensing, and signal processing. Recently, the availability of large bandwidthfibers, directional couplers, optical attenuators, high speed lasers and photodetectors hasstimulated a lot of research topics in optical fiber signal processing.' Many optical fibersignal processing devices with various functions such as frequency filtering