Monolithic integrated optical circuit for coherent detection

This paper explains the configuration and fabrication process of the monolithic integrated optical circuit for coherent detection. After the characteristics of each constituent element of the circuit are presented, the confirmation of the basic operation of the present integrated circuit as an optical heterodyne receiver is described. This circuit consists of a wavelength tunable multielectrode DFB laser as the local oscillator, a 3-dB directional coupler for superposition of the external signal light and the local oscillator light, and a waveguide-type pin photodetector for signal detection. Optical connection among the components and the propagation of the external signal light are realized by the semiconductor waveguides. The present integrated optical circuit is fabricated through three-step MOVPE growth processes and one etching process. The local oscillator DFB laser or the pin photodetector is connected to the optical waveguide via a butt joint. Characteristics of each component of the integrated optical circuit actually fabricated are measured and then the present monolithic integrated optical circuit is measured as an optical heterodyne receiver. When the output of the DFB laser modulated with a sinusoidal current superposed on the dc injection current is coupled to the present integrated optical circuit, a clear beat signal including the modulating sidebands is confirmed. From the results, the basic operation of the present integrated optical circuit as a heterodyne receiver is confirmed experimentally.