Technical Insight
Low cost taper photodiodes with more than 50 GHz bandwidth (Research Review)
Detectors employed in very high data rate optical networking applications feature side-illuminated photodiodes to increase the efficiency-bandwidth product (see ). This represents an improvement compared to top illuminated devices, but at the cost of expensive, sophisticated packaging. In a recent paper (Elec. Lett. 2001 37(5) 516), S Demiguel et al. describe a new side-illuminated detector that employs a double-stage taper photodiode. The device integrates etched waveguide input facets and anti-reflection coating on wafer. Conventional contact lithography is used, and the photodiode mesa and tapers are fashioned with RIE. The AR coating is applied on-wafer and patterned with simple chemical etching, which allows on-wafer testing for improved yields and lower fabrication costs. The device and tapers are grown on SI substrates by MOVPE. The photodiode consists of a monomode input waveguide with three thin InGaAsP layers (1.05 m bandgap wavelength) embedded with a 700 nm thick InP layer. The impinging light is first coupled to a monomode taper where the power is confined to an InGaAsP (1.5 m bandgap) layer, before being directed to the second multimode InGaAsP taper which has a bandgap of 1.4 m. The higher bandgap material acts both as an optical matching layer and n-type contact. At an input wavelength of 1500 nm, the responsivity as a function of the first and second taper length (Lt1 and Lt2) was 0.46 A/W for Lt1 = 500 m and Lt2 = 250 m. This combination led to a polarization figure of less than 0.1 dB for the device. At 50 GHz, the taper photodiode reaches the 1 dB power compression point above +12 dBm.
