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Progress made in optical references, including ultrastable Fabry&#x2013;Perot cavities, optical frequency combs, and optical atomic clocks, has driven the need for ultrastable optical fiber networks. Telecom-wavelength ultrapure optical signal transport has been demonstrated on distances ranging from the laboratory scale to the continental scale. In this article, we present a Doppler-cancellation setup based on a digital phase-locked loop (PLL) for ultrastable optical signal dissemination over fiber. The optical phase stabilization setup is based on a usual heterodyne Michelson-interferometer setup, while the software-defined radio (SDR) implementation of the PLL is based on a compact commercial board embedding a field-programmable gate array and analog-to-digital and digital-to-analog converters. Using three different configurations, including an undersampling method, we demonstrate a 20-m-long fiber link with residual fractional frequency instability as low as 10<sup>&#x2212;18</sup> at 1000 s and optical phase noise of &#x2212;70 dBc/Hz at 1 Hz with a telecom frequency carrier.

Digital Doppler-Cancellation Servo for Ultrastable Optical Frequency Dissemination Over Fiber