Measurement of the quantum mechanical fluctuation of a laser and the vacuum field by an effective time-delayed homodyne detector

The authors have measured the quantum mechanical noise of a coherent light source and the fluctuation of the vacuum field simultaneously, using a balanced homodyne detector with an effective time delay. Without the conventional coaxial delay line, they have achieved a time delay by shifting the digitized data stored in a personal computer. With this method, they have obtained noise power spectra from the light from a laser diode and an Ar+ laser, without any distortion, comparable to those obtained with a coaxial delay line.