Notch increases absorption length in photodiode

Integrating an angled total-internal-reflection mirror into a high-speed unitraveling-carrier photodiode (UTC-PD) has enabled researchers at NTT Photonics Laboratories (Kanagawa, Japan) to raise the efficiency of the device by 50%.

Integrating an angled total-internal-reflection mirror into a high-speed unitraveling-carrier photodiode (UTC-PD) has enabled researchers at NTT Photonics Laboratories (Kanagawa, Japan) to raise the efficiency of the device by 50%. The mirror consists of an air-filled notch in the back surface of the indium phosphide (InP) substrate; the photodiode itself is fabricated on the substrate's back surface off to one side of the notch. Light passes through the InP, bounces off the notch, and strikes the photodiode at an angle of 56° to the normal. The photodiode itself has an absorption layer 4700Å thick for normal-incidence light. The oblique incidence angle results in an absorption length increased by a factor of 1.8.

The fabricated device simultaneously exhibits a responsivity of 0.65 A/W, a 3-dB bandwidth of up to 58 GHz, and an output voltage of 5 V at a 1.55-µm wavelength. External quantum efficiency tops 50%. Useful for telecommunications and ultrafast measurement, the device has a high enough output voltage that it can directly drive lithium niobate and electroabsorption modulators, say the researchers. Contact Hiroshi Ito at ito@nuap.nagoya-u.ac.jp.

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