The characteristics of high-sensitivity photodetectors suitable for wide bandwidth optical communication systems are summarized. Photodiodes, photomultipliers, and photoconductive detectors for wavelengths from 0.3 µm to 10.6 µm are covered. The use of internal current gain by means of avalanche and electron multiplication and by means of optical heterodyne detection to increase sensitivity of high speed photodetectors is discussed. The application to visible and infrared laser communication systems is reviewed.

Photodetectors for optical communication systems

The design of a high-sensitivity wide-bandwidth 10.6-μm heterodyne receiver has been established for space and ground operational use, and key elements for a satellite-to-earth communications link were demonstrated. The receiver includes a cooled HgCdTe infrared mixer diode, a conical scanner for spatial tracking, an acquisition channel for spatial search and station alignment, and an automatic frequency-control channel to maintain a fixed laser frequency offset. The infrared mixer is designed to provide nearly quantum-noise-limited operation over an extended range of mixer temperature, bias voltage, and IF. The resultant experimental receiver has a measured noise equivalent power of less than 10-19W/Hz over the 15- to 40-MHz IF band for mixer temperatures from 85 to 115 K, and less than 2 \times 10^{-19} W/Hz up to 140 K. Mixer 3-dB cutoff frequencies as high as 420 MHz were measured at a mixer temperature of 125 K. An analysis and engineering equations are given for receiver noise components, noise equivalent power, available mixer conversion gain, mixer transducer gain, and quantum-noise factor in terms of such factors as mixer parameters, quantum efficiency, mixer temperature, dynamic conductance, bias voltage, LO power, and IF amplifier characteristics.

High-sensitivity receiver for infrared laser communications

A large variety of methods for the detection of infrared radiation have been developed primarily over the last ten to twenty years. These include thermal detection using bolometers, thermocouples and pyroelectric detectors, photon detection by extrinsic and intrinsic semiconductor photoconductors, photodiodes and photoemitters, and infrared imaging using these devices. These methods also include the use of devices with internal gain such as avalanche photodiodes and infrared photomultipliers as well as various unconventional detector schemes. General considerations of infrared imaging and high speed detection are discussed and many of these methods are reviewed and compared, with particular attention given to their applications and limitations.

Infrared detectors and applications

Wideband heterodyne receiver provides detection and demodulation of incident frequency modulated laser signal; search and acquisition circuitry to align two stations; tracking circuitry to maintain spatial alignment; and laser frequency monitor to frequency lock the transmit and local oscillator lasers.

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High-sensitivity receiver for CO 2 laser communications

Expressions are given for heterodyne noise equivalent power, conversion gain, and frequency response in terms of diode parameters, local oscillator power, and IF amplifier noise factor. Experimental results are reported on heterodyne detection at 10.6 µm in a reverse-biased HgCdTe photodiode.

E. Sard

Papers

Infrared 10.6-µm photodiode heterodyne detection

E. Sard

Papers

Infrared 10.6-&#181;m photodiode heterodyne detection

Infrared 10.6-µm photodiode heterodyne detection