Tunable filtering characteristics of cascaded photonic crystal Mach-Zehnder interferometer

Based on the electrically controlled birefringence of the liquid crystal and the photonic crystal self-collimation effect, a tunable Mach-Zehnder interferometer filter structure was designed by introducing complex defects in the two-dimensional photonic crystal. The mathematical model was established between the applied voltage and the effective refractive index of the liquid crystal with applying free energy theory of the liquid crystal. Based on the phase modulation principle of Mach-Zehnder interferometer and the theory of the equivalent refractive index of the photonic crystal, the effect of the electrically controlled birefringence of the liquid crystal on output spectra of Mach-Zehnder interferometer filter was theoretical analyzed, and the output spectra of the filter was simulated with finite difference time domain method. And according to the simulation results, the cascaded optimization of the structure was designed. The results show that by controlling the applied voltage can change the transmission wavelength of output and achieve the effect of tunable filter. After level cascaded, the filter effect has a significant improvement. The filter bandwidth of a half-wave reduced from 20 nm to 7 nm, and the tuning range was increased from 15 nm to 40 nm. Therefore, the filter can more optimization by means of further cascading of the structure, in order to used in the optical wavelength division multiplexing system.