Passive mode-locked fiber lasers have potential applications in fiber optic communication, fiber optic sensing, optical frequency measurement, and aerospace due to their good beam compactness, small size, low fabrication cost, tunability, and easy generation of ultrashort pulses. In recent decades, several types of mode-locked pulses can be generated using passive mode-locked fiber lasers, such as Gaussian pulses, self-similar pulses, soliton pulses, noise-like pulses, and so on. Among them, noise-like pulses is a special pulse generated by mode-locked lasers under certain conditions, which is widely used in low-coherence spectral interferometry, micromachining, nonlinear frequency conversion and supercontinuum spectrum generation due to its wide pulse width, high energy, and low time-domain coherence. In light of these applications, broadband noise-like pulse generation in erbium-doped fiber lasers has attracted considerable interest. The study of noise-like erbium-doped fiber lasers based on passive mode-locking technique has been reported extensively. Researchers have mostly used longer gain fibers or single-mode fibers to adjust intracavity dispersion and accumulate nonlinearity as a way to obtain the output of noise-like mode-locked pulses. So far, it has not been reported that using highly nonlinear fiber based on nonlinear polarization rotating mode-locking mechanism to manage the nonlinear in laser cavity to realize the mode-locked fiber laser with wide spectrum. In this work, we experimentally report the noise-like pulses generation in an anomalous dispersion erbium-doped fiber laser based on nonlinearity management technique. The erbium-doped mode-locked fiber laser adopt nonlinear polarization rotation technique. In the experiment, by introducing dispersion compensating fiber and highly nonlinear fiber into the nonlinear polarization rotation mode-locked erbium-doped fiber laser resonator, intracavity dispersion and non-linearity management is achieved, resulting in a stable mode-locked pulse output. When the intracavity highly nonlinear fiber length is 6 cm, corresponding to a net cavity dispersion of about - 0.019 ps(2). An ultrashort pulse output with a central wavelength of 1 534 nm, a pulse width of 1.9 ps, a repetition frequency of 20.1 MHz, and a 40 dB spectral bandwidth of about 100 nm can be obtained. Based on this, the length of the highly nonlinear fiber in the cavity is increased to 30 cm, and the total dispersion in the cavity is - 0.021 ps(2). This laser system can achieve noise-like pulse operation by properly adjusting the state of the wave-plate when the pump power is 1 100 mW. The output spectral coverage range of noiselike pulse is 1 280-1 850 nm, the bandwidth of 40 dB is 500 nm, the peak pulse width is as short as 70.9 fs, the base pulse width is 26.6 ps, and the repetition rate is about 19.7 MHz. The maximum output power is 2.08 mW at pump power of 1 100 mW , corresponding to an optical conversion efficiency of 0.18%. In order to verify the power stability of the noise-like mode-locked fiber laser, we monitor its output power for 2 hours, and the monitoring results showed that the output power is always maintained at about 1.01 mW , and the root mean square is calculated to be 1.14% for 2 hours, indicating that it has good environmental stability. After obtaining noise-like mode-locking pulses, we investigate the output spectra and the corresponding autocorrelation curves of noise-like pulses at different pump powers. It is found that the duration of the spike pulses decreased slightly with increasing pump power, a feature that mainly stems from the fact that the spectral bandwidth becomes progressively wider. Conversely, as the pump power increases, the pulse width of the base becomes progressively wider. And as the pumping power increases, the shape of the spectrum remains essentially the same and the output spectrum broadens in both the short and long wavelength directions, gradually increasing the spectral coverage. This is mainly due to the increasing power coupled into the highly nonlinear fiber as the pumping power increases. The present experimental study will allow subsequent optimisation of the fusion loss of the highly nonlinear fiber by tailoring the fibre device for wide bandwidth operation and selecting a higher power pump laser, resulting in a wider spectrum of noise-like mode-locked pulse output. The research in this paper provides a feasible solution for preparing a broad-spectrum noise-like mode-locked laser light source, which has great potential for applications due to its compactness, stable output, and ease of fabrication.
