High-dynamic-range heterodyne measurement of optical spectra.

The dynamic range of a conventional heterodyne spectroscope is limited by the intensity noise and the shot noise of the optical spectrum under test. We propose to improve the dynamic range by several orders of magnitude by using phase-sensitive detection. We demonstrate the feasibility of our method by measuring the tails of a distributedfeedback laser line 60 dB below maximum, which gave an improvement of 35 dB compared with that of the conventional technique: 15 dB by using a balanced receiver and another 20 dB by the phase-sensitive detection.