Tunable optical waveguide coupler

Abstract

Various optical functions are generated in accordance with the present invention of a novel broadly tunable monolithic wavelength selective coupler which can be integrated with a gain medium to form a broadly tunable laser. The tunable wavelength selective coupler supports a pair of asynchronous waveguides, an upper waveguide and a lower waveguide, in combination with a phase match course grating for coupling optical energy between said waveguides. One end of the lower waveguide terminates at an output facet. The corresponding end of the upper waveguide terminates in an optical signal absorbing medium. The other end of the lower waveguide is terminated to prevent optical energy from entering the waveguide; and, the corresponding end of the upper waveguide terminates at an input facet. The combination of a gain section and the monolithically tunable wavelength selective coupler forms a broadly tunable laser which is an important source of optical energy for a number of applications such as wavelength division multiplexed networks and switching systems. The laser frequency is determined by that wavelength .lambda.o which satisifies the forward coupling phase match condition, .lambda.g=.LAMBDA..vertline.N.sub.2 -N.sub.1 .vertline. of the coupler where .LAMBDA. is the coarse grating period and N.sub.1, N.sub.2 are the effective indices of the two waveguides. Wavelengths which are not coupled from the upper waveguide to the lower waveguide are attenuated in the optical signal absorbing means. Tuning of the laser wavelength is achieved by either injecting current into or applying a reverse bias voltage to the upper waveguide to decrease or increase its index respectively and change the coupled wavelength.