Abstract

Monolithic integration of semiconductor optoelectronic devices including waveguides on a single small chip is focused now.^1,2 We initiated the work with a sketch of monolithic structures for heterodyne detection called optical coherent block,³ and it has been followed by refining single-mode semiconductor waveguide devices such as the Mach-Zehnder interferometer.⁴ Then a prototype of a monolithic integrated coherent receiver including a local oscillating (LO) laser, a 3-dB waveguide directional coupler, and pin photodetectors (PINs) has been successfully fabricated with preliminary heterodyning detection experiments.¹ Figure 1 shows a schematic of the first reported device. The size is small: 0.25 mm x 4.15 mm. The device applicability seems to be fairly high on the basis of the several advantages. First, the LO laser coupled to the monolithically jointed waveguide has normally operated with reasonable linewidth of a few tens of megahertz without an isolator between them for the ordinary system experimental setup. Then the device compactness and mechanical stability could be potentially superior to the hybrid device combination up to a certain level of the coherence. However, at present, dual PINs on the chip are not operating with the balanced mixing mode because of the technological immaturity, i.e., the PINs are electrically common on the conductive n-substrate. Namely, the integration of those components on a semi-insulating substrate is a final goal for the device completion. On the other hand, high performances of the coherent transmission experiments are reported in the system when conventional discrete devices are combined. Also, a silica (or glass) waveguide coupler instead of the fiber-type coupler will emerge in near future. Consequently, three sets of the components combination are likely to be candidates for the future system implementation.

© 1990 Optical Society of America