GHz ultrahigh-order harmonic laser output based on nonlinear multimode interference

Optical frequency combs (OFCs) with GHz repetition rates are being widely used. They are applied in high-speed fiber communications, microwave photonics, and precision spectroscopy. However, achieving stable ultrahigh-order harmonic generation at low pump power remains a major challenge in mode-locked fiber lasers. A harmonic mode-locking (HML) fiber laser based on nonlinear multimode interference is proposed and demonstrated. Theoretical analysis indicates that splicing a graded-index multimode fiber with a highly nonlinear fiber (HNLF) not only enhances mode purity through multimode interference filtering but also substantially reduces the soliton area due to the large nonlinear coefficient. Consequently, the threshold for harmonic generation is being reduced. Experimental results confirm a stable 571st harmonic output. A 5 m HNLF under 1 W pump power, corresponding to a repetition rate of 7.006 GHz, with excellent frequency stability and a signal-to-noise ratio of 40 dB. This HML structure is supported by theoretical and experimental investigations. It offers a low-cost and scalable approach to the development of stable OFCs with GHz repetition rates.