Laser Linewidth and Phase Noise Evaluation Using Heterodyne Offline Signal Processing

Abstract

A simple heterodyne scheme with digital data acquisition and offline signal processing is introduced to obtain phase noise spectra and linewidths of monomode lasers. The data are analyzed using time-domain memorized signals, enabling the evaluation of parameters such as coherence time, phase noise, laser linewidth as a function of bias current, and the identification of points where loss of coherence occurs. Results are presented for lasers with linewidths from $\text{20}\, \text{kHz}$ up to $\text{2}\, \text{MHz}$ .