Using a retro-reflecting echelle grating to improve WDM demux efficiency

The use of uncoated facets on the echelle grating in a monolithic InP WDM demux leads to a reflection loss of 5.65 dB for an index of refraction of approximately 3.18. This loss can be largely eliminated by coating the out of plane facet with gold, but it involves another processing step. An alternative is to use total internal reflecting (retro-reflecting) facets. We have fabricated a WDM demultiplexer in InP for use at 1550 nm using an echelle with retro-reflecting facets and have compared its performance to another demux on the same wafer using an echelle with flat facets.