Detection and Intercept of FSO Interplane Communications Using Long-distance Transmission (DIFICULT)

It is inherently difficult to derive information from a free space optical (FSO) communication link unless you have access to the beam path. We will work to develop a means of measuring a laser beam direction from outside the beam path that exploits single photon detectors (SPDs). SPDs are the most sensitive detectors possible, especially when multi-spatial mode receivers are employed to capture the maximum amount of scattered light. Prototype systems will be built and tested. In addition to the beam directionality these systems should also be capable of deriving information about the data transmitted such as the data modulation rate or even in some conditions the actual data bits.