Lightwave frequency tracking with a tunable DBR laser

Abstract

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 conven tional phase-locked loop (PLL), and a wide-dynamic-range dig ital 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 LO are estimated from measured Iinewidth Ll v and frequency fluctuations a" 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 Llv = 29 MHz and a, = 2.4 MHz predicts that the phase error in the tracking loop is mainly caused hy the flicker frequency-noise component not by the white frequency-noise component. A tracking experi ment with the OBR LD demonstrates that measured noise per formance agrees with theoretical predictions. Guidelines for tracking-loop designs are also described.