Lightwave frequency tracking with a tunable DBR laser

Described are the construction and noise performance of a lightwave frequency tracking loop, which is indispensable for sub-megahertz frequency-control of tunable laser diodes (LDs). 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 used 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 Δv and frequency fluctuations σ_v, respectively, and the increase in the flicker component with tuning is reported. A PLL analysis based on the measured values of Δv=29 MHz and σ_v=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