Noise-Like Rectangular Pulses With Asymmetric Spectral Stripes in a 1.7-μm Ultrafast Fiber Laser

In this study, we have observed and characterized distinct noise-like rectangular pulses (NLRPs) in a passively mode-locked fiber laser at 1.7 μm. The corresponding optical spectrum features unique asymmetric stripes, similar to asymmetric spectral modulation. Additionally, this pulse regime possesses the temporal characteristics with free of pulse breakup even under the increased pump power. Furthermore, the time-stretch dispersive Fourier transformation measurements reveal substantial shot-to-shot spectral fluctuations alongside smooth averaged spectrum without spectral stripes, thereby confirming that these NLRPs originate from an ensemble of stochastic sub-pulses with random intensities. Numerical simulations reproduced similar experimental observations and revealed that spectral filtering is an important factor influencing the spectral stripe features. These findings not only offer a new perspective for the design and optimization of ultrafast fiber lasers but may also provide insightful implications for high-resolution distance metrology in practical applications.