Objective In recent years, 1.5 mu m ultrafast fiber lasers have attracted wide attention due to their critical roles in many fields. Fiber mode- locked lasers mainly include active and passive mode- locked fiber lasers. The passive mode- locked fiber laser is a simple and economical method to realize ultrafast pulse generation. Saturable absorbers play an important role in passive mode- locked fiber lasers. In recent years, different kinds of saturable absorbers (e.g., carbon nanotubes, graphene, topological insulators, transition metal chalcogenides, and black phosphorus) have been used to generate ultrafast pulses. In this paper, we use the magnetron sputtering deposited WTe2 as a saturable absorber to achieve ultrafast pulse generation in an erbium- doped fiber laser. With the wavelength of 1559.31 nm, the measured 3 dB spectral bandwidth is 11.54 nm and the pulse duration is 231 fs. The ultra- short pulse with the maximum average output power of 58 mW and the pulse energy of 2.18 nJ is obtained at the fundamental frequency of 26.6 MHz. The results indicate that the WTe2 saturable absorber can be used as an excellent photonic device to generate ultra- short pulses at 1.5 mu m band. Methods To study the nonlinear properties of the WTe2 saturable absorber based on micro-nanofiber, we apply the device in an erbium- doped fiber laser to generate ultrafast pulses at a 1.5 mu m band. The pump source is a commercial 980 nm laser diode (LD) with a maximum output power of 400 mW. A 1.5 m long erbium- doped fiber is used as the gain medium. The optical coupler has a spectral ratio of 50:50. The length of the entire resonant cavity is about 9.3 m. A polarization controller is utilized to adjust the polarization state of the oscillating beam in the cavity, while a polarization- independent optical isolator is used to ensure unidirectional operation. In addition to the erbium- doped fiber, the other fibers including the pigtails of each component are SMF-28e fibers. In fiber laser systems, all components are polarization- independent, which avoids self-starting mode- locking operation caused by nonlinear polarization evolution. It is worth noting that in erbium- doped fiber laser systems, the direction of the laser is opposite to that of the pumping laser, avoiding the output of residual pump power. The output characteristics of the laser are measured by spectrometers (Yokogawa AQ63700), oscilloscopes (ROHDE &SCHWARZ RTO2024), and photodetectors (EOT ET- 3500F). We use an emission spectrum analyzer (ROHDE &SCHWARZ FSV13) to record the emission spectrum during mode- locked operation, and a commercial autocorrelator (APE Pulsecheck) to measure the pulse width. Results and Discussions With the increase in pump power, the cavity is initially in continuous operation. When the pump power reaches 40 mW, we observe a self-starting stable mode- locking. When the pump power reaches a maximum of 400 mW, it still maintains a stable mode- locked operation state, with the maximum average output power of 58 mW and the corresponding pulse energy of 2.18 nJ. The measurement results of the spectrum of the output pulse ranging from 900 to 1700 nm show no pump signal at 980 nm, indicating no residual pump power. When the pump power reaches the maximum, the output power basically still maintains a linear increase, with a slope of 15.3%. The mode- locking performance is characterized at the maximum pump power. The mode- locking spectrum centered at 1559.31 nm is obtained with a 3 dB spectral bandwidth of 11.54 nm. The Kelly sideband on the spectrum confirms that the mode- locked laser operates in the soliton region. The autocorrelation curve of the mode- locked pulse with a full width at half maximum of 356 fs is obtained, and can be well fitted by the sech2(center dot) curve. With a de- correlation factor of 0.648, the actual pulse width is calculated to be 231 fs. In addition to the excellent nonlinear properties of WTe2 materials, erbium- doped fibers feature large dispersion compensation, with a dispersion parameter of 36 ps(2)/km at 1560 nm, which also helps realize ultra- short pulses. Since the dispersion parameter of the single- mode fiber at 1560 nm is-22 ps(2)/km, the total group velocity dispersion is about-0.12 ps(2). In addition, the time- bandwidth product is 0.33, indicating slight pulse chirps. Further compression with certain methods is possible. Conclusions The saturable absorber of WTe2 based on micro-nanofiber is prepared by magnetron sputtering technology, and its nonlinear optical properties are studied. The micro-nanofiber WTe2 saturable absorber is embedded in an erbium- doped fiber laser system with a ring cavity structure. At 1559.31 nm, the measured 3 dB spectral bandwidth is 11.54 nm and the pulse width is 231 fs. The mode- locked laser output with the highest average output power of 58 mW and the pulse energy of 2.18 nJ is obtained at the fundamental frequency of 26.6 MHz. The results show that WTe2 saturable absorber can be used as an excellent photonic device to generate ultrafast pulses at 1.5 mu m band. In addition, magnetron sputtering deposition technology is expected to prepare high-performance saturable absorbers with large modulation depth, low nonsaturable loss, and low saturable intensity, which is conducive to the generation of ultrafast pulses with high stability and high power.
