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, a GHz figure-9 Er-doped OFC based on a nested fiber ring resonator is presented. By matching the free-spectral range (FSR) of external and internal optical resonators, an 11-fold increase in repetition rate from 82 MHz to 907 MHz is achieved. Notably, it is demonstrated that a 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 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. This work provides a valuable contribution to the field of ultrafast laser and optical metrology, which demonstrates the potential of nested-resonators for generating low noise OFCs with boosted repetition rate. A pioneering demonstration of a GHz optical frequency comb (OFC) based on nested fiber resonators is presented. This approach overcomes the limitations of short cavity designs, offering enhanced design flexibility, good noise performance, and long-term stability. It is believed that this work opens new possibilities for the development of high-repetition-rate OFCs and contributes to the advancement of OFC technology.image