GHz figure-9 Er-doped optical frequency comb based on nested fiber ring resonators

The field of optical-frequency-combs (OFCs) has seen significant progress in recent years due to the advance of mode-locked fiber lasers with low-noise performance and long-term reliability. With the emergence of applications, OFCs with repetition rates close to GHz have become highly sought after. However, generating GHz fiber-based combs with low noise remains a significant challenge. In this study, we present a GHz figure-9 Er-doped OFC based on nested fiber ring resonator. By matching the free-spectral-range (FSR) of external and internal optical resonator, we achieved an 11-fold increase in repetition rate from 82 MHz to 907 MHz. Notably, we demonstrate that single pulse operation with long-term FSR matching can be achieved without any active feedback control. The comb offset frequency is detected using f-to-2f technique to ensure the high coherence between pulses, which is distinguished from harmonic mode-locking. The results show that the nested-resonators are capable of generating GHz ultrafast OFC pulses with low phase noise via a more flexible cavity design than conventional short-cavity solutions. We think that this work provides a valuable contribution to the field of ultrafast laser and optical metrology, which demonstrates the potential of nested-resonator for generating low noise OFCs with boosted repetition rate.