Laser linewidth measurements using self-homodyne detection with short delay

We compare delayed self-homodyne and self-heterodyne detection in the case of a time delay of the order of the coherence time of the laser. Both methods are found to yield similar results but the homodyne method is simpler to set up and gives a markedly better signal-to-noise ratio. With the short delay the low-frequency 1/f-noise of the laser is effectively filtered out and the measurement provides a value for the Lorentzian contribution to the laser linewidth which is often of prime interest in coherent optical communication systems. (C) 1998 Elsevier Science B.V. All rights reserved.