Pulsed fiber laser operated at the 2 µm region has attracted tremendous interest and obtained significant development over the past decade, since it exhibits ideal applications in medical treatment, material processing, infrared spectroscopy, and remote sensing^[1–3]. Recently, the noise-like (NL)-named pulse laser has obtained a lot of attention and been widely reported. Different from the regular pulse, in the time domain, the NL pulse is a pulse envelope with a width of nanosecond- or picosecond-level, which is constructed by numerous random femtosecond-level subpulses. In the frequency domain, the NL pulse has a wide and smooth spectrum, which is the result of ensemble averaging of countless highly structured elementary spectra^[4]. Therefore, the NL pulse shows a low coherence. These inherent features provide the NL pulse with many valuable applications. The ultrashort subpulses and wide spectrum make it an ideal candidate for flat mid-infrared supercontinuum generation^[5–7]. Moreover, the NL pulse is an excellent laser source for material surface processing, due to the high frequency of the subpulses in an envelope and the inherent low-coherence property (absence of interference between incident and reflected laser)^[8].

For the generation of NL pulses at 2 µm, passive mode locking is the commonly used and effective technique. Different mode-locked mechanisms include nonlinear polarization evolution (NPE)^[9–11] and nonlinear amplifying/optical loop mirrors (NALMs/NOLMs)^[3,6,12,13]. Real saturable absorbers (SAs) such as SESAM and graphene^[14,15] were reported for the mode-locked NL pulse generation. Nevertheless, most of the reported studies are based on the non-polarization-maintaining (non-PM) fiber oscillator. Although such a fiber laser system has the advantage in controllability, it suffers from poor environmental stability and weak repeatability. Moreover, the polarization state of the laser is random.

Recently, in the 2 µm spectral region, several works about the mode-locked NL pulse with an all-PM fiber configuration have been reported. Michalska et al. reported mode-locked NL pulses at a central wavelength of ∼1983nm with maximum output power of ∼189mW and pulse energy of ∼43.4nJ, relying on an NALM^[16]. Mode-locked NL pulses based on NOLM also have been reported. The maximum output power of ∼80.5mW and pulse energy of ∼13.5nJ were obtained at a central wavelength of ∼1943nm by Zhang et al.^[17]. In our previous works, with the NOLM and NALM techniques, NL pulses with the maximum output power of ∼39.5mW (pulse energy of ∼40.3nJ) and ∼82.6mW (pulse energy of ∼25.6nJ) were obtained from the all-PM fiber oscillator, respectively^[18,19]. By the aforementioned works, the output power of the mode-locked fiber oscillator was mainly limited at the milliwatt level. With a high laser power requirement, the weak seed pulse laser will significantly add expense and complexity to the amplification system. In addition, with the multistage fiber amplification system, the mismatching of the gain spectrum between the commercially used single-clad Tm-doped fiber (TDF) and double-clad TDF might deform the shape of the laser spectrum and lower the efficiency^[18], since the NL pulse always exhibits a wide spectrum with several tens of nanometers.

In this paper, we have presented a stable watt-level mode-locked NL pulse fiber oscillator at a central wavelength of ∼1995nm, relying on the NALM. The all-PM-fiberized oscillator can directly deliver the maximum output power of ∼1.017W and pulse energy of ∼0.61µJ with a longtime power fluctuation of ∼0.1%, which is the highest output power of mode-locked NL pulse from any fiber oscillators at the 2 µm region, to the best of our knowledge.

The schematic diagram of the all-PM fiber mode-locked NL pulse oscillator is depicted in Fig. 1. The NALM-based mode-locked fiber oscillator is established with a figure-8 cavity. In the left loop, a section of ∼3m PM double-clad TDF (10/130, Coherent) serves as the gain fiber, pumped by a multimode diode laser at ∼793nm (BWT) via a PM-fiber combiner. The residual pump power is removed by a commercial stripper (LIGHTCOMM) with a stripping efficiency of ∼23.66dB behind the PM-TDF. A long piece of PM-1550 fiber (∼110m) is employed for the accumulation of nonlinear phase shift between the counterpropagating lights in the loop. In the other side, the loop contains a PM isolator with a fast axis blocked and a PM fiber coupler with an output ratio of 70%, which ensure the unidirectional propagation and the high-power output, respectively. The two loops are connected by a 2×2 PM-fiber coupler with coupling ratio of 20:80, in which the 80% port is connected to the combiner and the 20% port is connected to the PM-1550 fiber. Such a strongly imbalanced NALM design will result in a small switching power, according to the relationship of^[19]Ppeak=λAeff2n2(ρG+ρ−1)L,where λ is the laser wavelength, Aeff is the effective mode area, G is the value of TDF gain, ρ is the splitting ratio (∼0.8 in our experiment), and L is the length of the loop mirror. The total cavity length is ∼124.5m, and the net dispersion is ∼−11.3ps2.

Stable mode-locked NL pulse trains can self-start by directly increasing the incident pump power to ∼3.47W. The central wavelength of the mode-locked NL pulse is ∼1995nm, measured by an optical spectrum analyzer (AQ6375, Yokogawa), as shown in Fig. 2(a). With the enhancement of the incident pump power from ∼3.47W to ∼7.85W, the spectral width is slightly broadened from ∼14.7nm to ∼16.2nm, and the output power is linearly increased from ∼0.286W to ∼1.017W with a slope efficiency of ∼16.7% [Fig. 2(b)]. The pulse performances were characterized by a fast photodetector (ET-5000F) together with a high-speed (20 GS/s) oscilloscope and an RF analyzer (N9020B, Keysight), as exhibited in Fig. 3. The NL pulse with a rectangle shape envelope is gradually broadened, as depicted in Fig. 3(a). The widths of the NL pulse envelope are ∼4.5ns, ∼9.7ns, ∼13.4ns, and ∼16ns at the output powers of ∼0.286W, ∼0.617W, ∼0.86W, and ∼1.017W, respectively. Figure 3(b) gives the corresponding RF spectra with the modulation frequencies of ∼222MHz, ∼105MHz, ∼75MHz, and ∼62MHz, which well satisfy the relationship of τ=1/f. The wide and smooth spectrum, and nanosecond-level pulse envelope width are the typical characters of the mode-locked NL pulse.

For further clarifying the properties of the mode-locked NL pulse, the autocorrelation traces were measured by an autocorrelator (pulseCheck 600, APE), as shown in Fig. 4. According to the typical character of a mode-locked NL pulse, a spike rides on a broad pedestal will be expected. Nevertheless, due to the nanosecond-level pulse envelope and the limitation of the scanning range of the utilized autocorrelator, only the coherent spike was recorded. Contrary to the pulse envelope, the width of coherence spike is decreased with the enhancement of the pump power. The widths of the spike are ∼364fs, ∼335fs, ∼330fs, and ∼323fs, corresponding to the output power of ∼0.286W, ∼0.617W, ∼0.86W, and ∼1.017W, respectively, with the sech2-pulse profile assumed.

The fundamental frequency of the mode-locked NL pulse was also recorded with the resolution of ∼10Hz, as given in Fig. 5(a). The fundamental repetition rate of ∼1.662MHz, in accordance with the total cavity length of ∼124.5m, has a signal-to-noise ratio (S/N) of ∼83dB, which is much higher than the typical results (∼60dB) in previously reported works^{[10–13,16–18,20]}, and indicates the high stability of the NL pulse mode-locked operation. Moreover, the long-time power stability of the mode-locked NL pulse is measured and plotted in Fig. 5(b). At the maximum output power of ∼1.017W, the power fluctuation is only ∼0.1% during the 8 h of monitoring, which exhibits a robust power stability of the all-PM fiber oscillator.

For better understanding the research actuality of the mode-locked NL pulse, the representative results of the mode-locked NL pulse fiber laser in the 2 µm region are summarized and illustrated in Table 1. In our work, it is the first report of the mode-locked NL pulse with watt-level output power directly from the fiber oscillator, and both the average output power and pulse energy are the highest values, to the best of our knowledge. The stable watt-level all-PM fiber oscillator could act as the ideal seed source for the generation of high-power NL pulse laser, since it will effectively simplify the follow-up fiber amplified system by dispensing with the pre-amplifier stage. In addition, the absence of the single-clad TDF-based pre-amplifier chain will avoid the reshaping of the spectrum of the NL pulse during the power amplification, caused by the mismatching of the gain spectrum between the single-clad and double-clad TDFs.

Theoretically, a higher output power can be expected by further increasing the pump power, due to the high damage threshold of the NALM-based mode-locked oscillator. Meanwhile, the mode-locked NL pulse can tolerate a high pulse energy by broadening the envelope width and fixing the peak power to maintain the mode-locked operation. Moreover, for the higher power operation, the appropriate cooling system should be adopted, considering the high pump power and the mismatched splicing points between the double-clad fiber and PM-1550 fiber.

In summary, we have experimentally demonstrated a watt-level mode-locked NL pulse fiber laser at a central wavelength of 1995 nm. The oscillator has an all-PM fiber configuration, which can directly deliver the maximum output power of ∼1.017W and pulse energy of ∼0.612µJ with a slope efficiency of ∼16.7% and a longtime power fluctuation of ∼0.1%. Moreover, the mode-locked NL pulse exhibits a good tunability in envelope and coherence spike widths within the ranges of ∼4.5–16ns and ∼346–323fs, respectively. Thus, the NL pulse oscillator is the ideal candidate for the implementation of a high-power NL pulse laser and will have potential significant applications in mid-IR spectroscopy, low-coherence spectral interferometry, and materials processing.