Heterodyne detection offers the possibility of greatly improved detection sensitivity relative to direct detection techniques. As has often been cited, the possibility of sensitivities down to single photon detection is, in principle, achievable. This is primarily because it is possible to narrow the effective bandwidth of the optical signal down to ～ 1 MHz, or less, by standard RF filtering techniques on the intermediate frequency (IF) signal. Achieving such narrow bandwidths would be an extremely difficult task by strictly optical techniques. As a result, the total noise can be restricted to just the shot-noise in the cw local oscillator (LO) current. Since the heterodyne signal is also proportional to the LO intensity, the carrier-to-noise ratio (CNR) becomes independent of the LO intensity when the noise is completely LO shot-noise limited.
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