High-speed electro-optic polarization modulator using a buried channel optical waveguide

A new lithium tantalate electro-optic polarization modulator using a buried channel optical waveguide is proposed. Utilizing a precisely designed buried channel waveguide, complete phase matching between TE and TM modes is obtainable. In addition, using a periodic poling structure, quasi-velocity matching between lightwaves and a modulation microwave is obtainable at modulation frequencies over 10 GHz.

I. Introduction

An optical polarization modulator/converter is a basic and important device in many photonic/quantum electronic systems and has many potential applications in advanced optical communications including optical vector modulators (OVM) and polarization-mode dispersion (PMD) compen-sators. Several types of polarization modulator/converter have been proposed[1]–[4]. Optical polarization modulators based on the electro-optic (EO) effect are very attractive since they can operate at high-frequency ranges over 10 GHz, unlike mechanical type devices. We focus on LiNbO₃ and LiTaO3 since they have a large Pockels coefficient of pm/V applicable for polarization modulation/conversion. A key point for efficient polarization conversion using the Pockels coefficient is the phase matching between two orthogonally polarized modes with birefringence; one mode corresponds to an ordinary ray in the crystal, and the other corresponds to an extra-ordinary ray. For phase matching, LiTaO3 has about 10 times lower birefringence than that of LiNbO₃ at 1550 nm of wavelength.