HIGH-SPEED III-V SEMICONDUCTOR INTENSITY MODULATORS

This paper describes a GaAs-AlGaAs, loaded-line traveling-wave modulator which has achieved bandwidths up to 36 GHz to date with low (< 6 V) drive voltage into a 50 OMEGA device. The loaded-line design concept is able to combine the high efficiency of vertical p-i-n-type phase modulators with a velocity-matched 50 OMEGA-structure to obtain very high bandwidth-voltage ratios. Electro-optic and microwave design considerations are largely decoupled in this design, and its evolution is here traced through lumped precursor devices which, using the same processes and waveguide optimizations, developed and proved the trench-isolation and series push-pull technologies required for the traveling-wave modulator. At 1150 nm, a bandwidth of 25 GHz for V-pi = 4.85 V typically represents the performance. At 1300 nm, the bandwidth is unchanged but the drive voltage is higher by the expected wavelength-ratio factor. Reduced drive voltage can be obtained without bandwidth sacrifice by choosing a wavelength close to the material band edge. Threefold enhancement in AlGaAs at short wavelength is demonstrated (lumped only, to date). Even greater gains are possible using band edge-engineered quantum-well materials. Modeling shows that bandwidths of 40-60 GHz are possible with this traveling-wave scheme.