Concept And Design Of An Integrated Wavelength Demultiplexer

Design of a hybrid integrated optical/bulk device that spatially separates wavelength multiplexed channels on a ferroelectric substrate is discussed. Parameter optimization yields a 2 millimeter long structure with 50 channels at crosstalk below 60 db and with 0.2% wavelength resolution. It is well understood that development of optoelectronic systems that combine optical and electronic signal processing functions on a single substrate can serve to advance optical communications technology. Recent work demonstrating photodetection in laser recrystallized silicon films on ferroelectric substrates represents a potential contribution to that development. In this paper we discuss characteristics of a demultiplexer that is designed for integration on a ferroelectric substrate as one of several coplanar signal processing devices in an optical wavelength encoded multiple channel communication system. Related though different devices have been previously studied. We have analyzed the properties of the structure shown in Figure 1 for the demultiplexer application. The device consists of a tapered diffused waveguide with a chirped grating that produces radiation out of the plane of the waveguide and into the substrate. The grating period and chirp rate are selected so that the emerging beams focus on either the crystal face opposite the grating or on the same face as the grating. Reflection from the opposite surface is required for the latter case. The chirped grating serves to focus the beams and to spatially disperse beams of different wavelength. Aspects of the design are dictated by fabrication considerations for the silicon detectors that are shown in the figure.