Measurement of the spectral linewidth of semiconductor lasers for use with coherent optical communication systems

Semiconductor laser diodes with narrow linewidths (10 kHz–10 MHz) are essential for coherent fiber optical communication systems. Linewidth measurement techniques fall into three groups. We discuss each group and then focus our attention on the unbalanced interferometric method. We have developed, theoretically, the correlated unbalanced interferometric method and show that the envelope function of the beat signal is the Fourier transform of the original laser lineshape. We also propose novel approaches to the linewidth measurement techniques, such as serial use of two acoustooptic modulators for cross-correlation measurement and combination of Mach-Zehnder and Sagnac interferometers.