Low-latency FPGA-based TDC phase detection scheme for optical frequency comb locking

Precise phase locking of optical frequency combs requires high-bandwidth feedback to suppress fast phase noise. We present a low-latency digital phase detection scheme based on a high-resolution time-to-digital converter (TDC) implemented in a field-programmable gate array (FPGA). By directly converting signal arrival time into phase error, the total system latency is 115 ns. A digital servo system based on this approach achieves a 1 MHz control bandwidth in voltage-controlled oscillator (VCO) locking. Applied to an erbium-doped fiber frequency comb, the f_beat locking bandwidth reaches 500 kHz, with a modified Allan deviation of 6.97×10^-18 at 1 s. These results indicate that FPGA-based TDC phase detection enables high-bandwidth and stable optical frequency comb phase locking.