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Resonant Photoconductive Cavity

IP.com Disclosure Number: IPCOM000035035D
Original Publication Date: 1989-May-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

This article describes a photoconductive resonant cavity in which a transmission line cavity, defined by impedance discontinuities, has a cavity length matched to the repetition rate of an exciting optical source, a semiconductor diode, for instance, which photoconductively produces picosecond electrical pulses at a particular point on the line. The intracavity signal is much larger than in the non-resonant case and the output is also larger. The generation of picosecond electrical pulses using photoconductive gaps excited by ultrashort light pulses [1] has become a standard technique. Various geometries have been used but none employing resonant structures.

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Resonant Photoconductive Cavity

This article describes a photoconductive resonant cavity in which a transmission line cavity, defined by impedance discontinuities, has a cavity length matched to the repetition rate of an exciting optical source, a semiconductor diode, for instance, which photoconductively produces picosecond electrical pulses at a particular point on the line. The intracavity signal is much larger than in the non-resonant case and the output is also larger. The generation of picosecond electrical pulses using photoconductive gaps excited by ultrashort light pulses [1] has become a standard technique. Various geometries have been used but none employing resonant structures. The cavity is defined by impedance discontinuities in the transmission line (formed by changing the geometry of the transmission line pair or by using lumped components, such as chip resistors and capacitors) and is chosen to have an electrical pulse round trip time equal to (or multiple or sub-multiple of) the period of the laser oscillations. It has been shown that in transmission line structures consisting of aluminium lines on polysilicon (damaged for fast recombination) on quartz substrates a 3- picosecond (ps) electrical pulse broadened to only 5 ps over a centimeter of travel. The dispersion for polysilicon on silicon substrates was only slightly larger. For reasonable cavity lengths of order a few centimeters (compatible with diode lasers modulated at GHz frequencies) then bandwidths for the oscillator of about 40 GHz should be readily achievable. For superconductive lines, the bandwidth would be several hundred GHz. The figure shows a possible linear geometry. For a 5 GHz rep. rate laser then the cavity length is adjusted to have a round trip time of 400 ps. The figure shows a photoconductive gap in the center of the cavity (or more practically sliding contact is made by the exciting beam at the center of the cavity [2] and one discontinuity as infinite (100% mirror). A pulse therefore sees 'gain' twice every round trip, there are two pulses in the cavity at any particular time and the output from the cavity is at the same rep. rate as the laser. Note that 'gain...