Low-visibility optical communications: received signal level as a function of receiver field of view

Experiments were conducted with an optical transmitter which provided accurately timed pulses from both a coherent He-Ne laser at 632.8 nm and an incoherent GaAs LED at wavelengths in the range from 900 to 940 nm. Suitable optical equipment was used to combine the radiation from each source into a single, collinear beam initially 15 cm in diameter. The two light sources were intensity-modulated by different waveforms. A PIN silicon photodiode was employed as a receiver. The investigation shows that forward scattered light in low visibility conditions can contribute significantly to the usable signal level in an optical receiver. To exploit the scattered light, the receiver must have a sufficiently large field of view.