Zero-Offset Frequency Locking of Lasers at Low Optical Powers With an Optical Phase Locked Loop

Frequency locking of lasers is fundamental to a vast number of applications within the field of optics. Usually, when locking a laser to an optical reference wave, it is imperative that locking can be maintained in spite of low reference powers. Previous solutions to frequency locking involve injection locking and/or optical phase locked loops. While previous works have shown locking to weak waves, we extend the lowest demonstrated optical power locked to by approximately 20 dB, realizing locking down to −90 dBm, using a novel digital dither optical phase-locked loop. Measurements of the locked laser phase error verify the performance. The loop design circumvents the presence of a dither on the locked laser light, hence avoiding dither penalties, and low-power locking is realized via coherent detection gain without any optical amplifiers. Low phase noise standard deviations of less than 20°at −80 dBm optical power and Allan deviation of
$3\cdot 10^{-16}$
at 1 s averaging time indicate great potential for a variety of applications within optical sensing, communications, and metrology.