Optical fibers are excellent communication media with broad bandwidth, low loss, light weight, low cross talk, and immunity to electromagnetical interferences. With high speed lasers and photodetectors, optical fibers are also attractive for processing broadband signals. Many noncoherent optical fiber signal processing devices reported in literature were constructed to perform various functions such as frequency filtering, high speed pulse generation, encoding, and decoding. The basic components of an optical fiber signal processing device include light sources, delay lines, attenuators, directional couplers, and photodetectors. 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 noncoherent optical fiber signal processing devices, the traditional signal processing design methods and 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 controlled without complicating the device structure. In this paper, we investigate the possibility of 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 employs window 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.