TDM receiver technology continues to follow an evolutionary path toward higher bandwidths as the transmission requirements go up. Benefits of integrating a preamplifier with the photodetector increase as the required speed increases, because parasitics become intolerable. Thus, work on OEIC receivers has intensified in recent years. Japanese participants include NTT, NEC, Sumitomo, and Fujitsu. The widest published bandwidth of 23 GHz was obtained using a pin/double-heterojunction bipolar transistor technology (Sano et al. 1994). Competitive U.S. efforts for high-speed receivers exist at AT&T, and there are smaller efforts at Rockwell and Hughes. In the lower-speed arena previous work at IBM on integrated MSM-MESFET receivers still would appear to be state of the art; this technology is being manufactured at Vitesse. Thus, generally, the panel sees the United States and Japan about even in this area.
In Japan there is also active development of a standardized access module specified by NTT that incorporates 1.55 micron broadband receivers as well as 1.3 micron transceivers. Generally, a passive splitter is used at 1.3 micron where relatively low data rates are involved, but a wave-length selective coupler or splitter is needed to select out the 1.55 micron channel with low loss. NTT continues to do research in this area, as do numerous other Japanese companies. Integrated WDM receiver technology is minimal in Japan. Efforts at AT&T and Bellcore in the United States would seem to put the United States in the lead in this technology. For example, workers at AT&T recently demonstrated a rather complex WDM receiver using on-chip gain, a filter, and a fast photodetector on the same chip (Verdiell et al. 1994).