TAPERED RIB ADIABATIC FOLLOWING FIBER COUPLERS IN ETCHED GAAS MATERIALS FOR MONOLITHIC SPOT-SIZE TRANSFORMATION

Design details and demonstration data are presented for an (Al,Ga)As m onolithic tapered rib waveguide achieving modal spot-size transformati on. The tapered rib adiabatic following fiber coupler structure (TRAFF IC) achieves two-dimensional (2-D) expansion of the output optical mod e of single-transverse-mode semiconductor waveguide modulators and las ers using a one-dimensional (1-D) taper between noncritical initial an d final taper widths which are compatible,vith optical lithographic te chniques, Measurements are presented of total mode expansion losses be tween similar to 1.5-2.0 dB and semiconductor to single-mode-fiber wav eguide coupling losses of similar to 0.5-1.0 dB for doped pin optical- modulator-type waveguides using the TRAFFiC waveguide, A semiconductor laser with a TRAFFiC tapered-rib mode-expansion section and measured coupling loss between the laser output and single-mode fiber of only 0 .9 dB is described, Finally, a TRAFFiC spot-size transformer for undop ed waveguide modulators with total mode expansion losses of 1.84 dB an d excellent modal behavior at 1.32-mu m wavelength is presented, The T RAFFiC structure is particularly well suited for integration with both active and passive etched rib waveguide devices, Fabrication is relat ively simple, requiring only patterning and etching of the tapered wav eguide and uniform-width outer mesa waveguide without any epitaxial re growth.