Photon Correlation Spectroscopy for Observing Natural Lasers

Natural laser emission may be produced whenever suitable atomic energy levels become overpopulated. Strong evidence for laser emission exists in astronomical sources such as Eta Carinae, and other luminous stars. However, the evidence is indirect in that the laser lines have not yet been spectrally resolved. The lines are theoretically estimated to be extremely narrow, requiring spectral resolutions very much higher (R approx.= 10**8) than possible with ordinary spectroscopy. Such can be attained with photon-correlation spectroscopy on nanosecond timescales, measuring the autocorrelation function of photon arrival times to obtain the coherence time of light, and thus the spectral linewidth. A particular advantage is the insensitivity to spectral, spatial, and temporal shifts of emission-line components due to local velocities and probable variability of 'hot-spots' in the source. A laboratory experiment has been set up, simulating telescopic observations of cosmic laser emission. Numerically simulated observations estimate how laser emission components within realistic spectral and spatial passbands for various candidate sources carry over to observable statistical functions.