  The detection of laser radiation originating from space is a positive
indicator of Extra Terrestrial Intelligence (ETI). Thus far the optical search
for ETI (OSETI) has looked for enhanced brightness in the form of either
narrow-band spectral emission or time correlated photons from laser pulses. In
this paper it is proposed to look for coherence properties of incoming light in
a manner that can distinguish against atomic emission lines. The use of photon
sensitive detectors and a modulating asymmetric interferometer has been
modelled. The results suggest that the sensitivity of detection for continuous
laser sources would be better than current spectroscopic approaches, with
detection thresholds of $<$1 photon s$^{-1}$m$^{-2}$ against a background with
a spectral bandwidth of 0.1nm over an observation time of 750s.
