This paper presents a nonmechanical method of measuring temporally coherent light which may be dominated by incoherent background radiation. To obtain detection in the presence of high levels of incoherent radiation, the detector must reject constant and fluctuating contributions due to incoherent light. Extreme sensitivity of the method derives from preferentially modulating the coherent light in such a way that the exact periodicity of the resulting detected signal component is known, permitting, in principle, processing gains which are almost arbitrarily large. Wavelength estimates are also obtained, even when the coherent source power is orders of magnitude less than the power of background radiation. The approach lends itself to rugged and easily constructed implementation. This paper then provides an in-depth noise analysis of the general approach. Numerical examples are also given. Results of a crude laboratory test are presented. Although limited by shortcomings of the components at hand, the experim ent demonstrated easy measurement of coherent radiation >40 dB below the ambient incoherent light and discernible output at a signal-tointerference level of –53 dB.
Jonathan D. Cohen, "Electrooptic detector of temporally coherent radiation," Appl. Opt. 30, 874-883 (1991)