A phased-array waveguide multiplexer/demultiplexer, developed at the University of Maryland (Baltimore), is compact, has low losses, and is compatible with both frequency- and wavelength-based spectral channel-spacing standards for wavelength division multiplexing (WDM). Researchers say that the 24 × 34-mm demonstration chip, and chips based on it, should provide those working on WDM systems with reliable components that are less expensive because of their high production yields. The device consists of silica waveguides on a silicon substrate and uses a new configuration of phased-array waveguide grating architecture.
Light enters the device through a single waveguide, propagates freely for short distances, and is coupled into a one-dimensional waveguide array.1 This array is designed so that each waveguide has a path length that differs from the next by a specific amount, thereby creating a phase profile at the array output that is very different from that at the input. At the array output interference causes an effect similar to that from a diffraction grating or lens—different colors are focused at different points in the output image plane. In this way, multiplexed channels can be separated or detected. To multiplex (MUX) instead of demultiplex (DMUX) signals, the same process is carried out in reverse—the same component is effectively used backwards.
Meeting a double standard
The design architecture exploits the symmetry of the system, although it is itself asymmetric. The central "waveguide grating" that performs the MUX/DEMUX function remains the same, but instead of a single waveguide input and waveguide (or detector) array output, there is a one-dimensional slab waveguide array on each side of the grating.2
One of the slab waveguides is configured as a demultiplexer receiver for constant wavelength channel spacing (typically 1 or 2 nm). The angle of divergence from the grating is linear with changing wavelength, so the waveguides in this array are equally spaced (angularly) from each other. The other slab waveguide is designed for use with the constant frequency standard, and the waveguide spacing is variable.
Conforming to both WDM channel-spacing standards enables the central wavelength or frequency of the spectrum to be changed. This is possible because there are several different waveguides that can carry the input (or output) of the multiplexed signal, and the waveguide can be chosen to optimize the system. Given the difficulty of fabricating waveguides to very high precision and the lack of ideal light sources, such flexibility gives the device a new level of practicality. In the demonstration chip, the central wavelength and frequency could be varied in steps of approximately 0.4 nm and 50 Hz, respectively.
Researcher Wenhua Lin says that her group has developed a self-alignment technique to optimize the coupling of each separated wavelength to each receiving channel. The need for highly uniform fabrication processes is thereby further reduced and, says Lin, should allow the devices to be made on a full wafer with high yield. A chip that allows switching as well as MUX/DEMUX was described at OFC '97 (Dallas, TX) last month. All these devices, say researchers, exhibit low loss, good crosstalk rejection, and easy integration with other optoelectronic systems.