LIGHTWAVE FREQUENCY TRACKING WITH A TUNABLE DBR LASER

This paper describes the construction and noise performance of a lightwave frequency tracking loop, which is indispensable for submegahertz frequency-control of tunable laser diodes (LD's). The tracking loop is constructed with a tunable distributed-Bragg-reflector (DBR) LD as a conventional phase-locked loop (PLL), and a wide-dynamic-range digital phase detector is employed to overcome the extremely large phase noise of the DBR LD. The white and flicker frequency-noise components of the phase noise in an experimental DBR LD are estimated from measured linewidth-DELTA-nu and frequency fluctuations-sigma-nu, respectively, and the increase in the flicker component with tuning is reported for the first time. A PLL analysis based on the measured values of DELTA-nu = 29 MHz and sigma-nu = 2.4 MHz predicts that the phase error in the tracking loop is mainly caused by the flicker frequency-noise component not by the white frequency-noise component. A tracking experiment with the DBR LD demonstrates that measured noise performance agrees with theoretical predictions. Guidelines for tracking-loop designs are also described.