A proposed wavelength-division-multiplexing (WDM) optical communication system would feature a single optical fiber carrying eight channels of digital data signals. In the original application, the signal in each channel would be a serial digital video-camera output, but the system could just as well be used to convey other serial data streams. The data rate in each channel could be as high as 2.5 Gb/s.
The center wavelengths in the eight channels would range from 1,535.04 to 1,557.37 nm, and are chosen to obtain an interval of 400 GHz between the center frequencies of adjacent channels. The transmitter for each channel would be a distributed-feedback laser with a modulation frequency band of 50 kHz to 2.5 GHz. Each transmitter would accept input data signals at the emitter-coupled-logic (ECL) level. Because the laser transmitters would be subject to thermal wavelength drift, a feedback-controlled thermoelectric cooler would be used to maintain constant temperature and thereby prevent the middle frequency of the laser of each channel from drifting into the adjacent channel. The channel separation of 400 GHz would provide a margin against any drift due to a harsh environment, such as the space shuttle launch pads.
The optical fiber would be of the single-mode type. The outputs of the transmitters would be wavelength-multiplexed and coupled into the optical fiber by one of several alternative devices: a simple power coupler, a diffraction-grating-based coupler, an arrayed waveguide grating, or an interference-filter-based coupler. In choosing one of these devices for a specific application, one would have to consider the following characteristics, among others: A diffraction-grating-based coupler would offer minimum insertion loss for each channel, but would be less thermally stable than an interference-filter-based coupler would be. On the other hand, in an interference-filter-based coupler, the insertion loss in each successive channel would be additive.
At the receiving end of the optical fiber, the signals would be wavelength-demultiplexed. With the exception of the simple power coupler, any of the three wavelength-multiplexing devices mentioned above could be used as the demultiplexer. The wavelength-demultiplexed optical signals would be fed to separate receivers for conversion to electrical data signals. The receivers would be capable of detecting infrared radiation at wavelengths from 1,200 to 1,600 nm. The electrical outputs of the receivers would be at the ECL level.
This work was done by William T. Toler of Kennedy Space Centerand Robert W. Swindle and F. Houston Galloway formerly of I-NET.
Inquiries concerning rights for the commercial use of this invention should be addressed to
the Technology Programs and Commercialization Office
Kennedy Space Center
Refer to KSC-11974.