WDMs have been used to separate a plurality of signals spatially by the difference of wavelengths in prior bidirectional multiwavelength optical communication network. For excluding WDMs, linear PD/LD modules adopt eigen-wavelength photodiodes sensing only the light of an eigen wavelength but...http://www.google.com/patents/US5787215?utm_source=gb-gplus-sharePatent US5787215 - Linear PD/LD module, linear PD/LED module, linear LD/PD module, linear LED/PD module and linear PD module 
