Laser injection locking and nanophotonic spectral translation of electro-optic frequency combs

High-resolution electro-optic frequency combs (EO combs) consisting of thousands to millions of comb teeth across a bandwidth between 1 and 500 GHz are powerful tools for atomic, molecular, and cavity-based spectroscopy, including in the context of deployable quantum sensors. However, achieving sufficiently high signal-to-noise ratio (SNR) EO combs for use across the broad range of wavelengths required in the aforementioned applications is hindered by the corresponding unavailability of relevant components such as narrow-linewidth lasers, electro-optic phase modulators with adequate optical power handling, and low-noise optical amplifiers. Here, we address the latter two points by showing that optical injection locking of commercial Fabry–Perot (FP) laser diodes can help enable high-SNR EO combs. We injection-lock a 780 nm FP laser diode to more than 10⁶ comb teeth at injected comb powers as low as 1 nW and produce a high-SNR replica of the EO comb. In comparison to a commercial semiconductor optical amplifier, injection locking achieves ≈100× greater SNR for the same input power (when <1 µW) and equal SNR for >35× lower input power. Such low-power injection locking is of particular relevance in conjunction with nanophotonic spectral translation, which extends the range of wavelengths available for EO combs. We show that the usable wavelength range of a 780 nm EO comb produced by photo-induced second harmonic generation of a 1560 nm EO comb in a silicon nitride resonator is significantly increased when combined with optical injection locking. Our results demonstrate that optical injection locking provides a versatile and high-performance approach to addressing many different scenarios in which the EO comb SNR would be otherwise limited.