Directional coupler type optical function element

Abstract

A directional coupler type optical function element with a high extinction ratio, in which a junction of a 2-input/2-output directional coupler or 1-input/2-output directional coupler, formed of a semiconductor or dielectric, is formed by successively optically connecting, from the input side to the output side, a front-stage partial junction, front-stage partial junction with electrode, central partial junction, rear-stage partial junction with electrode, and rear-stage partial junction, each having a predetermined length. The connection state at the front-stage partial junction and an incidence-side lead section optically connected thereto and the connection state at the rear-stage partial junction and an emergence-side lead section optically connected thereto cancel each other, thereby equivalently providing a symmetrical connection state and preventing the extinction ratio for a cross mode from lowering. Moreover, by forming the central partial junction with a proper length, the extinction ratio for a through mode can be kept high enough. Thus, high extinction ratio characteristics can be enjoyed for both of the cross and through modes.