Optical networks using wavelength division multiplexing (WDM) have bee
n extensively investigated in order to actualize all optical networks
in the future. WDM networks have great potential for realizing ultra-h
igh capacity both for transmission and optical routing. We have to wai
t, however, for device technologies to mature before fully utilizing t
his potential. This paper reviews recent progress in research and deve
lopment into optical devices for WDM networks.