Investigation of an electrooptic tunable filter in lithium niobate

Abstract

A polarization independent electrooptic add/drop tunable filter fabricated on a LiNbO3 substrate with a narrow -3 dB bandwidth (FWHM) of 1.56 nm operating in the 1.55 μm wavelength regime has been developed to meet the demands of fast tuning speed and increased channel capacity for dense wavelength division multiplexed (WDM) networks. The operation of the filter is based on passive polarization beam splitters and strain-induced phase-matched TE↔TM polarization mode converters. Extinction ratios as high as 20 dB for polarization beam splitters were achieved using zero-gap two-mode interference directional couplers with an opening angle of 0.55°. A tunable TE↔TM polarization mode converter with 98.2 % conversion efficiency was obtained using a strain-induced refractive index grating consisting of 765 parallel strips 10.5 μm wide in a strained SiO2 surface film with a spatial period of 21 μm. Thermal and electrooptic tuning of the polarization mode converters were examined. A polarization independent electrooptic add/drop tunable filter in which the fabrication parameters of the splitter and the mode converter were optimized was produced. Fiber-to-fiber insertion loss less than 6.3 dB was measured on a 62 mm long filter device. The spectral characteristics reveal a -3 dB bandwidth of 1.56 nm and nearest sidelobes about 12 dB below the center peak. A thermal tuning rate of -0.903 nm/°C was realized. Electrooptic tuning was also demonstrated. A tuning range of 14.08 nm with applied DC voltages from -80 V to +80 V was achieved indicating an electrooptic tuning rate of 0.086 nm/V. The filter performance for both TE and TM modes was examined and polarization independence of the spectral characteristics was confirmed.