Measurement of the frequency linewidth of single-mode lasers by means of a quadrature signal interferometer

Numerous measurement methods require information about the frequency linewidth of laser light. The frequency linewidth is often measured by superposing the emitted fields of two equal lasers and evaluating the beat frequency. However, this method reveals no information about effects that influence both lasers simultaneously in the same manner, e.g. through electromagnetic disturbance. Other methods involve high-finesse Fabry-Perot cavities as frequency detectors. In order to measure the linewidth of single-mode lasers independently, we use a quadrature signal interferometer. We formulate conditions, under which there is an unambiguous relation between the interferometric phase and the laser frequency. In contrast to conventional methods, the frequency distribution can be determined even if the laser frequency is scanning or drifting. Results obtained from measurements on different laser diodes show the usefulness of this method.