Electrooptic Tunable Filters for WDM Networks

Abstract

An attractive component for wavelength division multiplexing (WDM) in fiber optic networks is a four-port, tunable channel-dropping filter [1]. Such components are important to increase the bit rate for high capacity optical networks by better utilization of the fiber bandwidth, and also in switching functions for efficient signal routing. Polarization independence is a key requirement, and high speed (<1 μs) tuning is needed for packet-switching. Tunable wavelength filters have been produced in a number of materials using acoustooptic and electrooptic tuning schemes as well as movable gratings [2], [3]. This paper discusses the development of electrooptically tunable filters in ferroelectric LiTaO3 crystals. These filters make use of the static strain-optic (SSO) effect for phase matched polarization coupling in diffused channel waveguides. The use of the SSO effect in producing low-loss channel waveguides in LiNbO3 and LiTaO3 substrates, as well as in tungsten bronze Sr0.6Ba0.4Nb2O6 (SBN:60) and BSTN crystals [4] is also presented. The tungsten bronzes are attractive materials for making channel-dropping filters due to their larger r33 electrooptic coefficients [5], which could make it possible to increase the number of accessible wavelength channels and reduce electrooptic tuning voltages.