Optical Filters for Discrimination Between Coherent/Incoherent Light

The development of manufacturing techniques necessary to produce a new type of optical filter is proposed. The filter is made by coating a transparent substrate with optical films in such a manner as to produce broadband interference effects. The optical thickness of each film layer of the coating exceeds the coherence length of sunlight and many artificial sources, but not the coherence length of most laser sources, resulting in a thick-film filter that discriminates between coherent and incoherent light over broad wavelength ranges. Rudimentary calculations show that by using this type of filter, a contrast ratio in excess of 1000 between coherent and incoherent light is possible using readily available materials. The objectives of this proposal are to develop the technology necessary to fabricate such a filter and to test the fabricated filter under a variety of circumstances. The immediate benefit to BMDO is seen in the area of protection of sensors from laser attack. Other benefits to having a coherence filter include: the operation of a laser-based communication system in direct sunlight and protection from laser radiation for any type of optical sensor, including the human eye. In addition to the military applications, these filters have enormous potential for the civilian markets in the area of laser safety and laser communications.