Abstract

Accurate knowledge of the laser linewidth is essential for many applications in coherent optical communication, optical metrology and high-resolution spectroscopy. The self-heterodyne/homo-dyne measurement technique is an established method for measuring the linewidth of semiconductor lasers [1, 2]. The basic idea of the technique is to convert the optical phase or frequency fluctuations of the laser into variations of light intensity in a fiber-optic interferometer. In the interferometer, the optical signal is compared with a delayed replica of itself and detected with a fast photodiode. In the heterodyne case, an acousto-optic modulator is used to shift the spectrum in order to reject the delected DC signal in the photodiode and to allow use of a standard RF spectrum analyzer to measure the spectrum. The optical losses in the acousto-optic modulator arc, however, substantial and reduce the overall sensitivity of the setup, typically by several dB.

© 1996 IEEE