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Photoconductive Optical Detection Scheme for Integrated Optic Applications

IP.com Disclosure Number: IPCOM000036260D
Original Publication Date: 1989-Sep-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 3 page(s) / 33K

Publishing Venue

IBM

Related People

May, P: AUTHOR

Abstract

By making use of a photoconductive resonant transmission line structure, a monolithic photodetector compatible with advanced silicon bipolar technology is described which has a responsivity equivalent to p-i-n diode structures. A novel demultiplexing technique that utilizes this method is also described.

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Photoconductive Optical Detection Scheme for Integrated Optic Applications

By making use of a photoconductive resonant transmission line structure, a monolithic photodetector compatible with advanced silicon bipolar technology is described which has a responsivity equivalent to p-i-n diode structures. A novel demultiplexing technique that utilizes this method is also described.

For monolithic integration of silicon photodetectors and amplifiers in an all-silicon integrated optic receiver, p-i-n diodes have been the detector of choice (see, for instance, 1) despite their large vertical profiles (at least 5 microns for reasonable efficiency - with absorption depth at 800 nm of 10 microns). Photoconductive detectors are more naturally integrated monolithically but suffer from poorer responsivity. However, advantage can be taken of Q-switched resonant transmission line cavities to produce a detector scheme with photoconductive gaps that has the sensitivity of a p-i-n detector. The figure shows the basic principle of the device. A laser diode is driven sinusoidally at some high frequency, say, 20 GHz. Information is impressed on the diode by external modulation at a lower frequency, say, 1 GHz. This light then impinges on a Q-switched photoconductive transmission line cavity that is resonant at 20 GHz (approximately 1 cm long). The Q switch is then operated at the lower modulation frequency producing an electrical signal that follows the information. Although photoconductive gaps are generally less sensitive than p-i-n structures, here the modulated signal can be built up by superposition within the resonant cavity (note that there is no gain) over 20 cycles of the cavity before it is switched out. Essentially, advantage is taken of the high frequency driving rates of the laser diode compared to the typical external modulation rates utilized in optoelectronic circuits. Because the transmission line structure can be compatible with silicon processing technology (aluminum lines on polysilicon on silicon substrates [2]), this is a truly integratable scheme. If a 5 mW laser produces a 50 mV signal on a 100-ohm transmission line with 5 V across it, then after 20 cycles one should be able to Q-switch 500 mV from the cavity. This corresponds to a sensitivity (at 1 GHz) of 1 A/W. A...