Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1998-05-04
2002-02-19
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
Reexamination Certificate
active
06348683
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to the field of high frequency transceivers. In particular, the invention relates to RF and quasi-optical transceivers and to methods of transmitting and receiving RF and quasi-optical radiation for applications that include spectroscopy, imaging, network analysis, radar and communications.
BACKGROUND OF THE INVENTION
Photoconductive “Auston” switches that convert femtosecond optical pulses into high frequency electrical pulses have been used to generate quasi-optical signals. These switches are constructed from undoped semiconducting materials such as GaAs, InP, and silicon, which are normally insulating. In operation, the semiconductors are irradiated by femtosecond laser pulses which cause electrons and holes to be injected into the conduction and valence bands of the semiconductors thereby making the semiconductors highly conductive. The “Auston” switch has been used in systems to perform high-resolution spectroscopy. These systems, however, are large and expensive and have relatively low power per unit spectral bandwidth.
Another method of generating quasi-optical signals is to use an optical-heterodyne converter or photomixer source. Photomixer sources are compact solid-state sources that use two single frequency tunable lasers, such as diode lasers, to generate a terahertz difference frequency by photoconductive mixing in a photoconductive material. Photomixer sources using low-temperature-grown (LTG) GaAs have been used to generate coherent radiation at frequencies up to 5 THz.
Photomixer sources have been used in conjunction with cryogenic detectors, such as bolometers, to construct local oscillators and high-resolution gas spectrometers. These devices, however, suffer from having to use cryogenic materials. Other sources such as backward-wave-oscillators used in conjunction with Schottky mixers or thermal detectors are physically large and expensive. Other sources such as molecular gas lasers are discrete frequency sources that are also large and expensive.
SUMMARY OF THE INVENTION
It is a principal object of this invention to provide a quasi-optical transceiver that can be constructed with commercially available components and that is compact, inexpensive, and does not require the use of cryogenics. It is another principal object of this invention to provide a quasi-optical transceiver that has high spectral brightness. It is another principal object of this invention to provide a quasi-optical transceiver that is frequency agile, continuous tuning, and relatively insensitive to source frequency drift. It is another principal object of this invention to provide a quasi-optical high resolution spectrometer with a high signal-to-noise ratio that can be constructed from a photomixer transceiver. Other principal objects of this invention are to provide an imaging system, network analyzer, radar, and a communication system that can be constructed from a photomixer transceiver.
A principal discovery of this invention is that a high frequency transceiver can be constructed from two photomixers pumped by the same optical sources and that such a transceiver has numerous advantages over the prior art. These advantages include high spectral brightness, frequency agility, continuously tuning, and relative insensitivity to source frequency drift. Another principal discovery of the present invention is that such a transceiver may be used to perform high-resolution spectroscopy with a state-of-the-art signal-to-noise ratio, but with reduced physical size and cost.
Accordingly, the present invention features a transceiver for transmitting and receiving high frequency radiation. The transceiver includes a first light source that generates radiation at a first frequency and a second light source that generates radiation at a second frequency. The first and the second light source have a difference frequency that is approximately equal to the difference between the first and the second frequencies. A transmitter includes a first photomixer that is optically coupled to the first and the second light source. A first antenna is electrically coupled to the first photomixer. In operation, the first antenna radiates a signal generated by the first photomixer at the difference frequency.
A receiver includes a second antenna positioned to receive the signal radiated by the first antenna. The second antenna generates a time varying voltage in response to the signal. A second photomixer is electrically coupled to the second antenna and is optically coupled to the first and the second light source. The second photomixer generates a current signal in response to the time varying voltage generated by the second antenna.
The transceiver of the present invention has numerous applications including spectroscopy, imaging, network analysis, radar and communications. The present invention also features a real time spectrometer, an imaging system, a network analyzer, a radar and a communications system. In addition, the present invention features methods of performing real time spectroscopy, imaging an object, performing network analysis, forming a Doppler radar image and communicating.
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McIntosh K. Alexander
Verghese Simon
Le Que T.
Massachusetts Institute of Technology
Testa Hurwitz & Thibeault LLP
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