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Precision Fiber Optic Links for Transporting Signals Off Cryogenic Infrared Focal Planes

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The high performance Infrared Focal Plane Array IRFPA detectors used by surveillance sensors for data acquisition, discrimination and track and on hit to kill interceptors produce vast amounts of data of wide dynamic range 80 dB and high bandwidth 10 MHz. Signal processing requires these data be digitized so that linearization, temporal and spatial filtering, coincidence rejection and discrimination algorithms may be applied. Fortunately, all of these functions can readily be performed with existing room temperature digital hardware, in the form of Analog to Digital converters and Digital Signal Processor DSP chips. One difficulty in carrying out the signal processing lies in the fact that since the FPA produces picoamp or nanovolt signals, they are highly susceptible to the effects of Electro-Magnetic Interference EMI in their transmission from the FPA cold finger to the warm off dewar signal processor. A solution to the problem of EMI contamination is to be found in placing the digital processors near the cold FPA focal plane. However, this method of signal processing introduces a sizable heat load to the cryogenic environment which makes it less attractive. An alternate approach to the processing of the digital signal at the FPA is to transport the FPA signal via an optical link to a remote processor. Visidyne has designed and tested such a link. The link is based on converting the FPA voltage to a phase modulated optical signal by means of a passive optical waveguide. The conversion is made by modulating the optical path length of two guided optical waves through the Pockels effect, a process that is extremely linear and has a large frequency GHz bandwidth response. The converted signal can then be transmitted by means of optical fiber to the environment outside the dewar. Because the signal is transmitted optically, it remains immune to degradation through EMI and therefore maintains its fidelity.

Subject Categories:

  • Fiber Optics and Integrated Optics
  • Non-Radio Communications

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