Micromachined two-chip, low-cost tunable filters for WDM

Abstract

Wavelength Division Multiplexing has become a leading technology for long haul transmission systems which operate at 1550 nm wavelength. One of the key components of such systems are tunable filters. Beside low insertion loss, polarisation insensitivity and large tuning range there is a strong demand for cost effectiveness and reliability. Two-chip micromachined filters are very promising candidates to fulfil these demands. In this paper we present and discuss a tunable optical filter structure which uses a simple bulk-micromachining process based on low-cost dielectric Bragg mirrors. The tuning is achieved by current induced thermal heating of the membrane suspensions. Common micromachined tunable optical filters either employ semiconductor Bragg mirrors with current induced heating or dielectric membrane mirrors with electrostatic actuation. The new concept combines the advantages of both types, the low-cost dielectric material and the simple actuation principle by current flow to create a best-of-breed two-chip solution. The alignment process of the two-chip cavity has been simplified to the point where a simple place-and-fix strategy can be applied. By matching the exciting Gaussian input beam to the stable half-symmetric cavity a fiber coupled and packaged tunable optical filter has been realized based on this concept. These micromachined tunable membranes are in general applicable to a wide variety of tunable components for wavelength division multiplexing systems, such as tunable optical filters, receivers and vertical cavity surface emitting lasers (VCSEL).