Application of finite impulse response network concept in wavelength-tunable electro-optic filter

We present a model of wavelength-tunable electro-optic filter on x-cut, y-propagation Ti-diffused lithium niobate. The filter is composed of two polarization beam splitters and a polarization converter which is controlled by an N-stage cascade of alternating coupled-mode and phase-shifted electrodes. The concept of finite impulse response digital filtering is used to calculate the voltage matrices V-c and V-p in the z field, which control the mode coupling and phase shift respectively. By changing the input voltages, a wavelength-tunable bandpass filter or a non-periodic multi-wavelength selector can be obtained. For the bandpass filter, the rectangular degree of passband can be improved by increasing the number of cascading stages N. If N is equal to 24, the side mode suppression ratio (SMSR) could reach 25 dB. And the passband can be tuned by simply changing the voltage matrix V-p. For the multi-wavelength selector, the design process can be fulfilled by the discrete Fourier transform method. The bandwidth of each selected wavelength increases but the number of selected wavelengths decreases when the number of cascading stages N decreases. If N is equal to 24, a bandwidth of 0.29 nm with a SMSR of 12 dB is achieved within a 8 nm range centered at 1550 nm.