Many applications of optical frequency combs (OFCs) require manipulation and amplification of individual comb modes, e.g., arbitrary waveform generation, terahertz generation and telecommunications. Extracting individual comb modes can be a challenging task for OFCs with narrow comb mode spacings (100 MHz to 10 GHz) due to the limitations of conventional optical filters. Optical injection locking can address this problem, but—due to the relatively large bandwidth (1 to 10 GHz) required for simple (i.e., without the need for additional feedback loops) and stable locking—can struggle when processing OFCs with sub-GHz comb mode spacings. Here, we present an approach to optical injection locking which incorporates a dither-free phase locked loop that allowed for long-term locking to OFCs with comb spacings below the high power injection locking bandwidth. As a result, we achieved robust injection locking directly to a sub-GHz OFC (250 MHz in our experiments). Optimization of the optical injection power is carried out using detailed phase noise characterization. We achieved an Allan deviation for the frequency variation of the slave laser with respect to the injected comb mode (1 s gate time) down to 9.7 × 10<sup>-17</sup> and 4.4 × 10<sup>-19</sup> at 1 s and 1000 s averaging times respectively, and a phase error variance of 0.02 rad<sup>2</sup> (integration bandwidth of 100 Hz to 500 MHz).
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