High-damage-resistant tungsten disulfide saturable absorber mirror for passively Q-switched fiber laser

1. Introduction

Q-switched fiber lasers are significantly important in applications such as pumping of optical parametric oscillators, medicine, environmental sensing, laser fabrication and optical communication [1,2]. The pulse trains with large pulse energy (at nJ-mJ level), tens to hundreds of kHz repetition rates and short pulse durations (at ns-μs level) can be generated by Q-switched fiber lasers [3–10]. In order to achieve pulsing operation in fiber lasers, an optical modulator is necessary to modulate the quality factor (the ratio between the energy stored in the gain media and the lost per oscillation cycle) of the laser cavity [11–13]. Generally, the saturable absorber (SA) is an efficient optical modulator for passively Q-switched fiber lasers to generate stable pulses. As one of the most widely used SAs in commercial laser systems, the semiconductor saturable absorber mirror (SESAM) generally employs hetero-structure quantum wells as an absorber deposited on a distributed Bragg grating (DBG) by the technology of metal-organic chemical vapor phase deposition (MOCVD) or molecular beam epitaxy (MBE) [3]. However, SESAMs suffer from some inherent drawbacks such as complex fabrication process and package as well as narrow operation bandwidth (typically several tens of nm [3]) owning to the limited energy bandgap of semiconductors and narrow bandwidth of DBGs.

Recently, a vibrant research area on low-dimensional nanomaterials has emerged due to their remarkable optical properties such as high third-order nonlinear susceptibility and ultrafast carrier dynamics [14–30]. The low-dimensional nanomaterials utilized in fiber lasers mainly include carbon nanotubes (CNTs), graphene, transition mental dichalcogenides (TMDs), topological insulators (TIs, such as bismuth telluride, bismuth selenide and antimony telluride) and black phosphorus (BP), making pulsed fiber lasers more compactness, easy implementation and low cost. Furthermore, passively Q-switching operation with remarkable performance have been intensively investigated by embedding SAs based on those nanomaterials into laser cavities [31–49]. As one member in TMDs family, tungsten disulfide (WS₂) has attracted great research interest due to their remarkable nonlinear optical properties. The monolayer of WS₂ consists of a hexagonal layer of tungsten sandwiched between two layers of sulphur. Due to quantum confinement and surface effects, the bandgap of WS₂ is demonstrated to strongly depend on the number of layers [52,53]. For instance, the bulk of WS₂ is indirect semiconductor with the bandgap value of 1.3 eV while the monolayer has a direct bandgap with the value of 2.1 eV [54]. The recent observation also testified that the monolayer WS₂ has a large third-order nonlinear optical susceptibility [55]. In addition, many research employing SA based on WS₂ has been carried out to achieve passively Q-switching operation. However, the previous reports show that the Q-switched fiber lasers based on WS₂ have a small range of repetition rates (typically several tens kHz) and a large pulse duration (a few μs) [33–36]. The main reason is that the optical performance of SAs is unstable when the pump power is increased over their ability to bear. Generally, WS₂ nanosheets are embedded into polyvinyl alcohol (PVA) to form free-standing WS₂-PVA SA composite in early research. This approach is easy to implement and low-cost. But those SAs suffer from optical damage in the high pump power, resulting from the low heat resistance of PVA. Therefore, in order to make Q-switched fiber lasers have larger changing range of repetition rates, shorter pulse duration and higher single pulse energy, there is a strong motivation for better damage-resistant saturable absorber.

Here, we report a fiber-integrated WS₂-SAM with high damage resistance for passively Q-switched fiber lasers. The WS₂ is deposited on the tip of polarization-maintaining fibers (PMF, PM980) using magnetron sputtering technique, and then covered with a dense gold film. The gold film acts as an end mirror with high reflectivity and a protective medium to isolate WS₂ from surrounding air. Moreover, compared with PVA, the gold film with a high heat resistance can increase the tolerance of WS₂-SAM to optical damage, resulting in an improvement of the stability for Q-switching operation in a high pump power. The stable Q-switching operation is achieved by employing the WS₂-SAM in a linear-cavity EDFL. When the pump power is increased from 20 mW to 600 mW, the repetition rates is changed from 29.5 kHz to 367.8 kHz, the average output power is increased from 0.5 mW to 25.2 mW and the pulse duration is decreased from 1.269 μs to 154.9 ns. At highest pump power, the pulse energy and signal-noise-ratio (SNR) are 68.5 nJ and 42 dB, respectively. Our results demonstrate that WS₂-SAMs exhibit remarkable performance including of high-damage-resistance, robust in configuration, as well as suitable for mass production at a low cost and good reproducibility, thus paving a new way for ultrafast photonic device in the field of pulsed fiber lasers.

2. WS₂-SAM fabrication and characterization

The WS₂ is deposited on the tip of a Panda-style polarization maintaining fiber (PMF) using magnetron sputtering technique. Considering that PMFs could suppress one polarization mode, this design of SAM would improve the environmental ability of laser operation. Prior to deposition, vacuum pressure of the reaction chamber is cryopumped to ~1 × 10⁻³ Pa. The radio-frequency (RF) power is settled to 100 W during the deposition process. The tip of PMF is deposited with a dense layer of WS₂ nanomaterial by 1-hour magnetron sputtering technological processing firstly. This thin WS₂ nanomaterials would function as a saturable absorber. Subsequently, the gold film with ~300 nm thickness is deposited by direct-current magnetron sputtering at the power of 80 W. The gold film coated on the fiber tip acts not only as a broadband reflection mirror, but also as a protective medium to avoid WS₂ polluted and oxidized by the environment. Thus, the fiber integrated WS₂-SAMs have following advantages: 1) overcoming the thermal problems which have limited the repetition rates and pulses energy of Q-switched fiber lasers at high pump power; 2) reducing the device insertion loss effectively; 3) owning higher reliability, and suitable for mass production at low cost.

In order to observe the morphology of WS₂ deposited on the tip of PMF, we produce several samples that are only deposited WS₂ with the same specifications. Figures 1(a) and 1(b) show the scanning electron microscopy (SEM) images of WS₂ with different resolution. It is observed that a dense layer of WS₂ is coated on the tip of PMF at a low resolution. The WS₂ nanoparticles are also observed with diameters from several nanometers to ~50 nm while further utilizes a higher resolution of SEM. The image of WS₂-SAM by optical microscopy is indicated that the device has compact size, as shown in Fig. 1(c). Raman spectroscopy is utilized to confirm the atomic structural arrangement of WS₂. The Raman spectra is measured by a Raman spectrometer (LabRAM HR Evolution) with a laser wavelength at 488 nm. As shown in Fig. 1(d), there are two main peaks in the region of 300-450 cm⁻¹. The characteristic bands observed at 356 cm⁻¹ and 417.5 cm⁻¹ are corresponding to $E_2^1{}_g$mode (an in-plane motion of tungsten and disulfide atoms) and A_1g mode (an out-of-plane motion of tungsten and disulfide atoms). The results are also in good agreement with the earlier reported Raman spectrum of WS₂ [54].

 

Fig. 1 (a) SEM image and (b) zoomed image of WS₂ on the tip of PMF with different resolutions; (c) Optical image of WS₂-SAM; (d) Raman spectra analysis.

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The nonlinear optical absorption property of WS₂-SAM is characterized by balanced twin-detector method. The schematic of measurement is shown in Fig. 2(a). The laser operates at central wavelength of 1561 nm with 912 fs pulse duration and 18.7 MHz repetition rate. The result is shown in Fig. 2(b). The measured curve could be divided into two parts. The first part (intensity: < 609.7 MW/cm²) is a typical saturable absorption curve that the reflectivity of WS₂-SAM is increased while increasing incident light intensity. The modulation depth, saturable intensity, and nonsaturable loss of the WS₂-SAM are measured to be 7.7%, 342.6 MW/cm², and 4.4%. Here, the small value of nonsaturable loss could be attributed to the fiber integrated structure with high reflectivity of gold film acting as reflector, which would reduce the threshold for Q-switching operation. Interestingly, after the intensity of incident light over 609.7 MW/cm², the reflectivity of WS₂-SAM is decreased while further increasing incident light intensity. It could be explained by optical limiting effect resulted from two-photon absorption (TPA) effect and nonlinear scattering.

 

Fig. 2 (a) Schematic of the power-dependent characteristics measurement; (b) Nonlinear optical absorption properties of the WS₂-SAM.

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3. Experimental setup of Q-switched fiber laser using WS₂-SAM

Figure 3 shows the diagrammatic sketch of the Q-switched fiber laser. A simple linear cavity is constructed to achieve Q-switching operation employing the WS₂-SAM. The cavity consists of 20.4 cm erbium-doped fiber (EDF, Liekki 110-4/125) as gain media with absorption coefficient of 250 dB/m at 980 nm and 11.5 cm single mode fiber (SMF). The total length of linear cavity is 31.9 cm. The pump source is a laser diode with 976 nm wavelength and 600 mW maximum output power. Pump light is injected into the laser cavity by one port of a 980/1550 nm wavelength division multiplexer (WDM) and the laser output is extracted via another port of WDM. A polarization independent isolator (ISO) is employed to prevent undesired feedback from outside cavity connections. The WS₂-SAM is spliced directly with EDF. A fiber Bragg grating (FBG) with 3-dB bandwidth of 0.6 nm is used in laser cavity, serving as a high reflectivity mirror (88.52% reflectivity) for 1560 nm wavelength light and an output device. The optical spectrum is monitored by an OSA (Yokogawa153 AQ6370B) and a real time oscilloscope with a bandwidth of 1 GHz (Tektronix DPO7104C) is utilized for monitoring the temporal evolution of the output pulse train. A 3 GHz RF spectrum analyzer (Agilent N9320A) coupled with a 15 GHz photo-detector (EOT ET-3500FEXT) is also employed to monitor the output pulse trains in frequency domain.

 

Fig. 3 Schematic of Q-switched fiber laser using WS₂-SAM.

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4. Experimental results and discussion

4.1 Q-switched EDFL characterization and discussion on the high-damage resistance of WS₂-SAM

The self-starting Q-switching operation is achieved from an all-fiber integrated linear cavity laser as soon as the pump power exceeds the threshold of 10 mW, generating pulse trains centered at 1560 nm (shown in Fig. 4(d)). The low threshold of Q-switched fiber laser is mainly benefited from the little nonsaturable loss of WS₂-SAM. Figure 4(a) shows the measured pulse trains at pump power of 30 mW, 120 mW, 220 mW, 280 mW, 440 mW, and 540 mW, respectively. When the pump power is at 600 mW, the full width at half maximum (FWHM) of the Q-switched pulse is 154.9 ns, as shown in Fig. 4(b). The RF spectrum in a 6 MHz span is measured with the resolution bandwidth (RBW) of 1 kHz, as presented in Fig. 4(c). The insert figure in Fig. 4(c) illustrates that the fundamental frequency is 367.8 kHz corresponding to a period of 2.72 μs and the SNR is ~42 dB at the RBW of 10 Hz. Those results demonstrate that Q-switched pulse trains are stable in the cavity.

 

Fig. 4 (a) The pulse trains under different pump powers; (b) Single pulse envelope, insert is the single pulse -picture of oscilloscope; (c) RF spectra in span of 6 MHz and insert is fundamental repetition -rate with RBW of 10 Hz; (d) Spectrum of Q-switching operation.

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When a continuous-wave pump source is used in a Q-switched fiber laser, output pulse properties are mainly determined by the nonlinear dynamics of gain media and SA. As a result, the pulses duration and repetition rates are depended on the pump power [35]. Figure 5(a) shows the measured average output power and pulse repetition rate as a function of the pump power, while 5(b) shows the variable curves of the measured pulse duration and calculated pulse energy with pump power. Due to the stable Q-switching operation can be obtained under a large adjustable range of pump power from 20 mW to 600 mW, leading to a wide variation range of the repetition rate and pulse duration. It is observed experimentally that the repetition rate is changed from 29.5 kHz to 367.8 kHz and the pulse duration is decreased from 1.269 μs to 154.9 ns when the pump power is increased from 20 mW to 600 mW. It should be noted that the average output power of Q-switched fiber laser shows a linear relationship when the pump power is increased and the trend is no changed even at pump power of 600 mW, owing to the high-damage resistance and remarkable nonlinear optical properties of WS₂-SAM in high pump power. It also means that the average output power and repetition rates can be further increased by using higher pump power. Interestingly, when the pump power is over 300 mW, there is no longer significant change in the pulse duration while the pump power is further increased. It is understood that the WS₂-SAM is fully saturable after pump power exceeded 300 mW. That phenomenon is also reported in Ref [4] by employing a SESAM as SA in a Q-switched laser. After eliminating the residue of pump power at output port, the maximum average output power of Q-switched pulses is measured to be 25.2 mW, corresponding to the pulse energy of 68.5 nJ. To the best of our knowledge, this is the largest average output power and pulse energy in an EDFL with a linear cavity structure. In addition, we testify the stability of Q-switching operation several times in same conditions per 2 hours with the pump power adjusted from 20 mW to 600 mW. The results show that the Q-switched laser still works well, indicating remarkable performance of WS₂-SAM.

 

Fig. 5 (a) Output power and repetition rates of Q-switched fiber laser as functions of the pump power; (b) Variable curves of the pulse energy and duration with pump power.

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To further demonstrate the high damage resistant property, the fluence of WS₂-SAM is calculated by the formula: F = E/S, where F is the fluence, E is the pulse energy, and S is the mode area [4]. The intracavity pulse energy is calculated to be 0.53 μJ considering that the FBG utilized in laser cavity has a reflectivity of 88.52% for operation wavelength at 1560.1 nm and the output pulse energy is 68.5 nJ. Those result in the fluence of WS₂-SAM of 2.2 J/cm². Surprisingly, compared with commercial SESAMs (BATOP, SAM-1550-14-5ps-x, λ = 1550 nm) with the similar modulation depth (8%), the damage resistance of WS₂-SAM fabricated in our experiment is at least more than that of SESAM (500 μJ/cm²) by three orders of magnitude [56]. Due to the common method to couple the SESAM with the end face of the FC connector is utilizing perpendicularly space butt coupling and the SESAM is exposed to the air, resulting in saturable absorption materials to be oxidized and destroyed easily at high pump power. Contrary to the SESAM, the WS₂-SAM has high damage resistance, benefited from two reasons: 1) the WS₂ is coated by a dense gold film acting as a protective medium for WS₂ to isolate air absolutely from the environment, thus avoiding WS₂ being oxidized at high pump power; 2) the gold film has excellent heat dissipation characteristics so as to hold remarkable performance of the WS₂-SAM at a high pump power. In addition, benefited from the all fiber-integrated structure and high reflectivity of gold film, the nonsaturable loss of WS₂-SAM (4.4%) is also lower than that of SESAM (6%), resulting in a low threshold for Q-switching operation in our experiment. Thus, those results demonstrate that the WS₂-SAM with high damage resistance and remarked nonlinear optical properties is a promising candidate to generate short pulses in high power fiber lasers. In addition, replacing the FBG by a linearly chirped fiber Bragg grating (LCFBG) and further optimizing the laser cavity, it is believed that passively mode locking operation can be achieved based on the WS₂-SAM with a linear cavity structure [32].

Table 1 give performance comparisons of passively Q-switched fiber lasers based on different low-dimensional materials. Compared with previous works using such materials (CNT, graphene, Bi₂Te₃, BP and WS₂) as SA materials to achieve Q-switching operation in EDF lasers, our work has the largest average output power and greater repetition rate adjustment range. It is notable that the pulse width of 154.9 ns in our work is the shortest duration in following comparisons. In our experiment, the maximum pump power is 600 mW. We believe that the larger repetition rate and average output power with shorter pulse duration could be achieved via further increasing pump power.

Table 1. Comparison of Q-switched fiber lasers with different saturable absorbers

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4.2 Comparison of Q-switching operation performances with different lengths of EDF

The performance of Q-switching operation utilizing WS₂-SAM with different EDF lengths are investigated and compared, as shown in Table 2. In compared experiments, the only changed factor is the length of EDF, resulting in changing the gain for the laser cavity. The 12.5 cm and 30 cm length EDFs are employed in same linear cavity, respectively. Although all of them can work at Q-switching operation, there are several differences: 1) the linear cavity with 20.4 cm EDF as gain media has optimized Q-switching performances, including largest changing range of repetition rate, maximum pulse energy, shortest pulse duration, and best stability. 2) while using 12.5 cm EDF as gain media in laser cavity, the Q-switching threshold is 25 mW and the stability is not as good as employing 20.4 cm EDF as gain media. The phenomenon could be understood that the optical devices used in the laser cavity could bring some loss and if the gain media in laser cavity is too short, there requires more pump power to exceed the threshold of Q-switching operation. While the pump power is over 460 mW, the Q-switched pulse trains is a little unstable in oscilloscope and the SNR is ~36.3 dB measured at pump power of 600 mW by RF spectrum analyzer. It is explained that if the pump power is too high, some pump light is not absorbed sufficiently by gain media so as to influence the stability of Q-switching operation. 3) as for the linear cavity consisted with 30.1 cm EDF acted as gain media, the threshold of generating Q-switched pulses is 18.1 mW and the output pulses is quasi-stable. In the beginning of generating Q-switched pulses, since the gain media in laser cavity is too large, some section of EDF is not pumped enough, resulting in increasing the cavity loss, resulting in the fiber laser needs higher pump power to exceed the threshold. Passively Q-switching operation is achieved when the pump power ranged from 18 mW to 470 mW. However, the unstable Q-switched pulse trains appear when the pump power is further increased. In order to confirm whether the WS₂-SAM is destroyed under higher pump power, the pump power is decreased below 470 mW and stable Q-switched pulse trains appear again. It is believed that the unstable Q-switching is mainly resulted from the over-saturation of the SA at high pump power.

Table 2. The performances of Q-switching operation with different lengths of EDF

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5. Conclusion

In conclusion, we fabricate a WS₂-SAM with an all-fiber-integrated configuration by deposited WS₂ on the tip of fiber as SA layer and then covered with a dense gold film acted as a high reflection mirror and a protective medium for WS₂ layer so as to avoid WS₂ being oxidized and destroyed at high pump power. By placing the WS₂-SAM into an EDFL, we obtain stable Q-switched pulse trains with repetition rates range from 29.5 kHz to 367.8 kHz and the pulse duration range from 1.269 μs to 154.9 ns. The generated pulses have average output power of 25.2 mW, pulse energy of 68.5 nJ and SNR of 42 dB at highest pump power. In the future works, we believe that using all-polarization maintaining structure and optimization of the reflectivity of FBGs in fiber lasers are essentially required for further enhancement of the performances of Q-switching operation. Our results provide a practical solution to fabricate SA devices with high damage resistance, paving the way for the development of new photonic devices for pulsed fiber lasers with high output energy and wide tunable frequency.