Monolithic optical waveguide filters based on Fourier expansion

DETAILED DESCRIPTION In accordance with the invention, a new type of monolithic optical waveguide filter comprises a chain of optical couplers of different effective lengths linked by different differential delays.
The transfer function of the chain of couplers and delays is the sum of contributions from all possible optical paths, each contribution forming a term in a Fourier series whose sum forms the optical output.
A desired frequency response is obtained by optimizing the lengths of the couplers and the delay paths so that the Fourier series best approximates the desired response.
The filter is advantageously optimized so that it is insensitive to uncontrolled fabrication errors and is short in length.
The wavelength dependence of practical waveguide properties is advantageously incorporated in the optimization.
Consequently, the filter is highly manufacturable by mass production.
Such filters have been shown to meet the requirements for separating the 1.
3 and 1.
55 .
mu.
m telecommunications channels and for flattening the gain of Er amplifiers.