Q-switching is advantageous in the production of giant pulses with extremely high pulse energy.^[1] Higher output power is possible in Q-switched solid state lasers (SSLs)^[2] than that in Q-switched fiber lasers. Therefore, Q-switched SSLs, especially at 1 m wavelength, have extensive applications in the fine processing, military and medical fields, etc.^[3–5] In recent years, many military applications, such as optoelectronic countermeasure and precision laser radar, require high power lasers that emit sub-microsecond pulses with a high repetition rate.^[6,7] With the power and pulse width fixed, the higher the repetition rate of laser pulse, the better the interference effect is and the wider the expanded scope of its military applications.^[7] Consequently, high repetition rate Q-switching has obviously become a crucial technology in the military field.

Compared with active Q-switching,^[4] passively Q-switching does not necessarily rely on the high power consumption of external modulating components such as electro-optic or acousto–optic modulators, so that it can completely satisfy the requirements for low cost and equipment miniaturization.^[8] The generation of passively Q-switched pulse is due to the energy mutation caused by the nonlinear saturable absorption of intracavity saturable absorber (SA), such as the semiconductor saturable absorber mirror (SESAM)^[9,10] or Cr:YAG.^[11–13] Considering the expensive cost, complex fabrication, narrow wavelength sensibility from the above SA, carbon nanotubes (CNTs),^[14–16] graphene,^[17–19] and some new low-cost two-dimensional (2D) graphene-like materials like topological isolators (TIs),^[20–24] transition metal dichalcogenides (TMDs) including WS₂, molybdenum disulfide (MoS , etc.,^[25–33] black phosphorus (BP)^[34–38] are made into the effective SAs through a simple fabrication process and used for Q-switching technology in SSLs of various wavelength bands.^[14–38] As shown in Table 1, these emerging SAs are (SWCNT, single-walled carbon nanotube; DWCNT, multi-walled carbon nanotude; SAM, saturable absorption mirror; PVA, polyvinyl alcohol; PMMA, polymethyl methacrylate) mostly made by growing nanomaterials through chemical vapor deposition and depositing them on a quartz substrate or a mirror by the spin-coating method. Meanwhile, these materials possess excellent nonlinear optical responses^[39] and ultrafast relaxation times,^[40] which are appropriate to the generation of a high repetition rate and short pulse duration lasers. Based on the relatively mature CNTs and graphene SAs, the repetition rate and the pulse width of passively Q-switching SSLs reported have been respectively optimized to super-500 kHz and sub-100 ns, and the average output power has reached the watt-level. However, even if numerous research results have proved that graphene-like 2D materials are the excellent SAs in Q-switched SSLs, the repetition rate and the average output power of reported pulsed lasers are very low. The maximum repetition rate of 333 kHz and the shortest pulse width of 182 ns are obtained in a Yb:LGGG laser Q-switched by MoS₂ SAM, and the corresponding average output power is only 600 mW.^[29]

Table 1.

Table 1.

Table 1. Passively Q-switching SSLs based on new materials. . SA Gain medium Wavelength/nm Repetition rate/kHz Pulse width/ns Output power/W Reference SWCNT-SA Nd:LuYGdVO4 1060 450 52 1.56 14 Nd:LuVO4 630 95 1.8 DWCNT Nd:YAG 1064 316 90 0.64 15 Graphene SAM Nd:GdVO4 1063 704 105 2.3 17 Graphene K9 glass substrate Nd:LYSO 1079 159 96 1.8 18 Graphene oxide/PVA Nd:GdVO4 1063 600 104 1.22 19 Bi2Se3 Nd:YVO4 1064 135 250 0.074 20 Bi2Se3 quartz plate Nd:GdVO4 1063 547 666 0.032 21 Bi2Te3 quartz substrate Yb:GAB 1043 110 370 0.04 22 Bi2Se3 film Yb:KGW 1030 174 1500 0.82 23 MoS2-BK7 Nd:YAlO3 1079 232.5 227 0.26 25 MoS2 mica Tm:GdVO4 1902 48 800 0.1 26 PMMA/MoS2 Yb:Ca3Y2(BO 1030–1050 140.4 420 0.105 27 WS2-BK7 Pr:LiYF4 640 88 630 0.022 28 MoS2 substrate Yb:LGGG 1025–1028 333 182 0.6 29 605 69.1 278 0.017 MoS2 quartz glass Pr:GdLiF4 639 140.3 403 0.009 30 721 74 382 0.008 MoS2 golden mirror Tm:CLNGG 1979 112 4840 0.062 31 Nd:GdVO4 1060 732 970 0.227 MoS2 quartz glass Nd:YGG 1420 77 729 0.052 32 Tm:Ho:YGG 2100 149 410 0.206 BP quartz glass Yb:ScBO3 1063 30 495 0.167 34 BP SAM Cr:ZnSe 2400 176 189 0.036 35 Pr:GdLiF4 639 172 189 0.018 BP quartz glass Nd:GdVO4 1064 312 495 0.022 36 Tm:Ho:YGG 2100 122 636 0.027 Yb:LuYAG 1030 63.9 1730 0.006 BP gold-film mirror Tm:CaYAlO4 1930 17.7 3100 0.012 37 Er:Y2O3 2720 12.6 4470 0.006 BP SAM Yb:CYA 1046 113.6 620 0.037 38

Table 1. Passively Q-switching SSLs based on new materials. .

In this paper, we demonstrate WS₂ as SA for the Q-switched Nd:YVO₄ laser. The WS₂-based SA is fabricated by the vertical evaporation method. To achieve a higher repetition rate and shorter pulse width, an only 40-mm length plano-concave laser cavity is designed. Eventually, the 153-ns pulses with a repetition rate of 1.578 MHz and an average output power of 1.19 W are obtained in the 1064-nm Q-switching operation. The results are much better than previous ones from the SSLs Q-switched by TMDs-based absorbers.

2. Preparation and characterization of WS

Firstly, the few-layer WS₂ is prepared by using a liquid phase exfoliation method.^[41] The proper quantity of bulk WS₂ powders is added into a water solution. After a 5-hour high-power sonication, the bulk WS₂ is exfoliated into few-layer nanosheets which are dispersed in the water solution. Following the synthesis of the few-layer WS₂ suspension, the WS₂ nanosheets are transferred onto uncoated quartz substrate by the vertical evaporation method.^[42] We pour the WS₂ suspension in a square box. Then a hydrophilic quartz plate (1-mm thick) is inserted vertically into the box. Exposing them to air until all the solvent has evaporated, the WS₂ nanosheets are deposited on the surface of the quartz substrate. Finally, the quartz coated with WS₂ nanosheets acts as a transmission-type absorber of the high repetition rate Q-switched operation. Unlike the previous method, the vertical evaporation method not only controls the transmission rate of WS₂ SA, but also is a low-cost fabrication method.

The Raman spectrum of the WS₂ SA excited by a 532-nm laser source is measured by a Raman spectrometer. As displayed in Fig. 1(a), two characteristic vibration modes E (in-plane) and A (out-of-plane) are located at 352.2 cm and 415.9 cm, respectively. It should be noted that the frequency difference between two modes is in agreement with the case of few-layer WS₂.^[43,44] The linear absorption of the WS₂ SA is investigated in a wavelength range from 300 nm to 1100 nm by using an optical spectrometer scanning as depicted in Fig. 1(b). Considering the interference from the absorptions of quartz substrate, we prepare the pure quartz substrate as a reference. In comparison with a very flat profile at a level of 93%±0.5% of pure quartz plate (red line), the transmission of WS₂ quartz plate (blue dotted line) varies nonlinearly with wavelength and shows a transmission of 72.7% at our laser wavelength of 1064 nm. Ruling out the interference from the substrate, WS₂ SA shows an absorption of 20.7% at 1064 nm.

Fig. 1.

	Figure Option ViewDownloadNew Window
	Fig. 1. (color online) (a) Raman spectrum; (b) Transmission spectra.

Figure 2 shows the schematic of passively Q-switched Nd:YVO₄ laser with our WS₂ SA. The pump source is a 30-W fiber-coupled laser diode (LD) with emission centered at 808 nm. The core diameter is 200 m and the numerical aperture is 0.22. Through a 1:2 optical focusing system, the pump laser is focused onto the gain crystal with a spot radius of about 200 m. A 3 3 15 mm a-cut Nd:YVO₄ crystal with an Nd concentration of 0.7 at.% is employed as the gain medium, and both light-passing faces of the crystal are coated for 808-nm and 1064-nm anti-reflection (AR) films. In order to ensure excellent heat dissipation, the Nd:YVO₄ crystal is wrapped with indium foil and mounted in a water-cooled copper block maintained at 20 C by a cool-water machine. The Q-switched laser is based on a standard plano-concave cavity with a length of about 40 mm. M₁ is a dichroic mirror, which is coated for 808-nm AR and 1064-nm high-reflection (HR) films. The concave mirror M₂ with a radius of curvature of 100 mm acts as an output coupler (OC), which has a transmittance of 15% at a laser wavelength of 1064 nm. During Q-switching operation, the as-fabricated WS₂ SA is inserted into the laser cavity and located as close to the gain crystal as possible. A fast photodetector (DET10A/M, Thorlabs, Inc., USA) with a specified rise time of 1 ns and a digital oscilloscope (TDS5000B, Tektronix, Inc., USA) with 1-GHz bandwidth are employed to detect and record the laser pulse in the experiment.

Fig. 2.

	Figure Option ViewDownloadNew Window
	Fig. 2. (color online) Schematic diagram of the experimental setup.

Initially, in order to confirm the function of the WS₂ SA we investigate the performance of continuous wave (CW) Nd:YVO₄ laser without SA as a reference. The CW laser operation is realized at a threshold pump power of 2.2 W. The relationship between the CW output power and the pump power is plotted in Fig. 3(a). The blue dots and the red line represent output power and its fitting line, respectively. An average output power of 4.7 W is obtained under a pump power of 11 W, resulting in an optical-to-optical efficiency of 42.7% and a high slope efficiency of 56%. Any self Q-switching has not been observed during the CW laser experiment. When WS₂ SA is inserted into the laser resonator, the pulsed laser oscillation is achieved by finely tuning the position of SA in the laser cavity and adjusting the pump power. As soon as the pump power exceeds the threshold of 8.5 W, the stable passively Q-switched operation (QS) is observed. Such a QS can be maintained until the pump power rises up to 10.5 W. However, the absorber would be damaged by high power laser irradiation if we continue to increase pump power. The average output power of QS mode with respect to pump power is demonstrated by blue pentagram notations and the purple fitting line of Fig. 3(a). Under the restriction of thermal damage, the maximum average output power can reach 1.19 W under a pump power of 10.5 W, corresponding to the slope efficiency of 30% and optical-to-optical efficiency of 11.3%. Such a high output power and slope efficiency are attributed to two factors. One is the sufficient spatial mode-matching between the volume of the pump and laser beams in the Nd:YVO₄ gain crystal exploited in our experiment. The other one is the improved performance that will be realized with a good balance between suitable transmissions of WS₂ SA and OC. However, it can be found that the slope efficiency of the QS laser has a relative drop compared with that of the CW laser and the QS pump threshold is higher, which mainly is caused by the insertion loss of the WS₂ SA.

Fig. 3.

	Figure Option ViewDownloadNew Window
	Fig. 3. (color online) (a) Output power versus the pump power. (b) Evolutions of the pulse repetition rate and the pulse width with the pump power. (c) The pulse trains and single pulse profile of the shortest pulse. (d) Evolutions of the pulse peak power and the pulse energy with pump power.

The pulse repetition rate and the pulse width under different incident pump powers are recorded in Fig. 3(b). From the figure we can see that the pulse width has a rapid drop from 283 ns to the shortest 153 ns as the pump power increases from 8.5 W to 10.5 W, while the repetition rate increases continuously from 1.176 MHz to 1.578 MHz. Our maximum repetition rate of 1.578 MHz is nearly twice as high as that reported in Q-switched SSLs based on low-dimensional nanomaterials,^[17] even at least five times as high as that of the previous TMD Q-switched SSLs.^[29] The higher repetition rate of passively Q-switched SSLs may be attributed to the ultrafast relaxation time of WS₂ (∼30 fs),^[43] relatively large stimulated emission cross section of Nd:YVO₄^[44] and an appropriate short cavity.

The pulse duration of 153 ns with a pulse repetition rate of 1.578 MHz is obtained under an output power of 1.19 W. The pulse trains and single pulse profile are displayed in Fig. 3(c). The shortest pulse width of 153 ns is slightly shorter than 182 ns of the previous passively Q-switched SSLs with TMDs SA.^[29]

According to the recorded average output power, pulse repetition rate and pulse width, we can approximately calculate the pulse peak power and pulse energy (Fig. 3(d)). The maximum single pulse energy is estimated to be 0.75 J and corresponding peak power is 4.9 W. As shown in Fig. 4(a), the optical spectrum of QS laser is centered at 1064.383 nm and the full width at half maximum (FWHM) is measured to be 0.107 nm. Finally, the beam quality of the Q-switched Nd:YVO₄ laser at a pump power of 10.5 W is measured by the 90/10 knife-edge method and shown in Fig. 4(b). The M² factors are calculated to be and .

Fig. 4.

	Figure Option ViewDownloadNew Window
	Fig. 4. (color online) (a) Optical spectrum. (b) Laser beam quality.

Comparing with the results previously reported on passively QS laser based on TMDs SA, we obtain here short pulses with significantly large output power and higher than the 1-MHz repetition rate. Even, a maximum repetition rate of 1.578 MHz can be considered as being superior among the QS lasers based on other nanomaterials like CNTs, graphene, and TIs (see Table 1). However, compared with the results obtained with the QS laser by SESAMs or Cr:YAG,^[8–13] our Q-switched laser results, the pulse width and output power lag behind. It is mainly due to the immature fabrication technology of 2D materials-based SAs. Thus future design of new emerging SAs for the generation of high energy ultrashort Q-switched pulses should focus on the optimization of the modulation depth and the SA transmittivity. Besides, the present results can still be certainly improved by shorting the laser cavity, by optimizing mode-matching between pump and laser beams and by removing the generated heat in WS₂ SA.

In this article, the efficient performance of a transmission-type WS₂ SA on the high repetition rate Q-switched Nd:YVO₄ laser is demonstrated. The minimal duration of output pulses is 153 ns at pulse repetition rate 1.578 MHz, average output power 1.19 W, and wavelength of nm. The results are the best among the results obtained by Q-switched solid-state lasers with TMDs-based absorbers. Nevertheless, it is the first time that the repetition rate has been above 1 MHz for Q-switched solid state laser based on one-dimensional or 2D materials. The maximum single pulse energy is 0.75 J and the highest peak power is 4.9 W. The results indicate that the WS₂ SA made by the vertical evaporation method can be used to generate high-output-power hundreds of nanosecond laser pulses at a repetition rate at an MHz-level.