Optical filtering characteristics of planar distributed feedback (DFB) guiding structures which have L/4 phase-shift region, are evaluated by using the equivalent network based on a newly developed concept of Floquet’s theorem and bisection principle (Babinet’s theorem). The numerical results reveal that this method offers a simple and convenient algorithm to analyze the filtering and oscillating characteristics of L/4 phase-shifted DFB configurations as well as decreases dramatically the computational time.