Watt-Level Dual-Wavelength Q-Switched Mode-Locked All-Solid-State Tm:CYA Laser

A passively Q-switched mode-locked all-solid-state Tm:CaYAIO4 (Tm:CYA) laser with a MoS2 saturable absorber is demonstrated for the first time. Employing a typical X-type cavity and output coupling mirror of 9%, when the laser diode pump power is higher than 8.5 W, the laser enters into a stable Q-switched mode-locked operation, further increasing the pump power into a dual-wavelength operating state; the corresponding typical center wavelengths are 1,863 and 1,877 nm. When the pump power reaches 20 W, the maximum output power is 1.15 W, the frequency of mode-locked pulse is 103.7 MHz, and the modulation depth of mode-locked pulses in the Q envelope is close to 100%. The results prove that MoS2 saturable absorber is suitable for 2-μm high-power mode locking.


INTRODUCTION
The 2-µm band is located at the absorption peak of water and the window band of atmosphere, which is a safe area for the human eye. Based on the above characteristics, 2 µm band laser is widely used in high-precision surgery [1], free space communication [2], and other fields. In particular, high-power dual-wavelength mode-locked lasers can generate coherent terahertz radiation by difference frequency [3,4], which is widely used in differential radar and remote space detection [5].
Passive mode locking is one of the common methods to obtain ultrashort pulse. In general, the mode-locked technique needs to have saturable absorber (SA), non-linear loop mirror, or non-linear polarization rotating [6]. At present, many materials have been employed as SA for passively mode-locked lasers, such as semiconductor saturable absorption mirrors, carbon nanotubes, graphene, and black phosphorus [7,8]. An excellent SA should possess the properties of higher damage threshold, ultrafast recovery time, and lower saturation intensity [9,10]. Among them, due to the complex process, narrow working bandwidth, and high cost of semiconductor saturable absorption mirrors, we need to look for other materials. Carbon nanotubes are widely used in fiber lasers [11,12], but their performance is poor in 2 µm solid-state lasers due to their unique structures and properties. Graphene is widely used in 1 µm, but its absorption efficiency at 2 µm is weak, which limits its ability to adjust light, and black phosphorus is easily oxidized in the presence of oxygen and water, leading to its instability in the experiment [13]. As such, MoS 2 has attracted our attention with its universality and good stability. MoS2, as a SA, was first widely used in fiber lasers [14][15][16]. Later, it is slowly used in solid-state lasers to realize Q-switching or mode-locked operation. In 2015, a passively Q-switched mode-locked (QML) Tm:LiLuF4 (LLF) laser with a MoS2 SA is demonstrated for the first time [17]. In 2017, the maximum peak power of 4.33 W is obtained using MoS 2 as SA in Er:LUAG laser [18]. Next year, Li et al. first used MoS 2 to realize Q-switched operation and obtained an average output power of 3.3 W in Tm,Ho:YAP lasers [19]. We also use MoS 2 SA to realize QML operation in Tm,Ho:LLF [20] and Tm:LuAG [21] lasers, respectively. In short, MoS 2 has great potentiality according to the current experimental progress.
CYA crystals are widely used because of their excellent mechanical strength, chemical stability, and high thermal conductivity [22]. As early as 1997, Tm:CYA crystal was pumped by Ti:Sapphire laser to realize continuous wave (CW) operation, in which the output power and slope efficiency were 50 mW and 18%, respectively [23]. In 2015, Kong et al. demonstrated a CW mode-locked Tm:CYA laser with a maximum output power of 830 mW and obtained dual-wavelength output of 1,958.9 and 1960.6 nm [24]. The next year, Yao et al. pumped Tm:CYA with a Raman fiber laser and obtained output of up to 6.8 W under CW operation [25]. Recently, Lan et al. used MoS 2 for the first time to achieve Q-switched operation in Tm:CYA laser, with the maximum output power of 0.49 W [26].
In this paper, we first use MoS 2 as SA to realize a Watt-level dual-wavelength QML operation in Tm:CYA laser. The output power of QML laser is 1.153 W, and the dual-wavelength is 1,863 and 1,877 nm, respectively. The dual-wavelength laser will have a broad application prospect in the field of optical communication and sensing.

PREPARATION OF MOS 2 SA
In this experiment, MoS 2 SA is prepared by liquid phase stripping method. First, the powdered MoS 2 is added to the water-based solution with a concentration of ∼1 mg/ml. Then, a glass substrate is prepared by the following procedure. First, the glass substrate is immersed in deionized water. Second, it is treated with ultrasonic cleaning apparatus for ∼10 min. Third, the surface dust of substrate is removed by ultrasound in alcohol for 10 min. Fourth, the treated glass piece is placed in a mixed solution of sulfuric acid and hydrogen peroxide for ∼1 h before hydrophilic treatment. Finally, the glass piece is soaked in the glass in clean deionized water for ∼10 min. Then, the glass substrate is prepared, and then the prepared MoS 2 solution was dropped on it directly. Figure 1a is the electron micrograph of MoS 2 , which proves that MoS 2 has been spun on the glass substrate. Figure 1b shows the Raman spectrum of MoS 2 . The phonon vibration modes of MoS 2 mainly include E 1 2g (in plane) and A 1g (out of plane), and the two main vibration modes change with the thickness. Specifically, the A 1g mode shifted blue, while the E 2g mode shifted red. In the figure, E 2g and A 1g are located at 383 and 402 cm −1 , respectively. It can be seen that MoS 2 is a small layer structure indeed. The prepared MoS 2 SA material is shown in Figure 1c.

EXPERIMENTAL SETUP
The experiment setup of dual-wavelength QML Tm:CYA laser is shown in Figure 2. In this experiment, a 790 nm laser diode (LD) with a maximum output power of 30 W is used as the pump source. The Tm:CYA crystal is cut at the Brewster angle, and the two end faces are polished. The doped concentration of Tm 3+ is 4%, and the size is 3.8 × 4 × 4 mm. The crystal was wrapped with indium foil and tightly mounted in a water-cooled copper block system with stable temperature holding at 11 • C. L is the pump focusing and collimating system with a ratio of 1:0.8 and a transmittance of 95% for 790 nm. Its working distance is 49.2 mm, and external diameter is 37.5 mm. In the laser cavity, M1 and M2 are flat concave mirrors with curvature radius of 75 and 100 mm, respectively. Both have high transmissivity to pump light and highly reflective to 1,850-2,100 nm. M3 is a flat-concave mirror with a curvature radius of 100 mm and has a reflectivity of more than 99.9% for wavelength from 1,850 to 2,100 nm, whose main function is to focus the oscillating light to the SA and start QML operation. M4 is a flat mirror with the same reflective film as M3. In the experiment, plane mirrors with 3, 5, and 9% transmissivity are used as the output couplers (OC).

EXPERIMENTAL RESULTS AND DISCUSSION
The output power as a function of the pump power is plotted in Figure 2. Contrast experiments are carried out with OCs with different transmissivities of 3, 5, and 9%, respectively. At first, the CW laser performance is discussed without MOS 2 SA in the cavity. The threshold power of 3% OC is 2.4 W, the slope efficiency is 11%, and the maximum output power of 1,857 mW is obtained at 20 W pump power. The threshold power of 5 and 9% OC are 2.6 and 2.8 W, respectively. When pump power is 20 W, the maximum output power is increased to 2,380 and 2,600 mW, respectively. To obtain high power output, we theoretically simulate the optimal transmissivity parameters of the OC.
The relationship between the best transmissivity of the cavity and the pump power is shown in Equation (1), where σ is the crystal emission cross-section, τ f is the emission life, λ P is the pump wavelength, P in is the pump power, α p is absorption coefficient, L is the crystal length, δ O is the cavity loss, W p is the average pump spot radius, and W O is the oscillating spot radius. Through the simulation Equation (1), the curve of Figure 3 is obtained. It can be seen from the figure that when the pump power is 20 W, the optimal transmissivity T of the cavity is ∼0.088. Considering the actual situation of optical glass coating in processing, it is more appropriate to choose an output mirror with a transmittance of 0.09. When the MoS 2 SA is inserted into the cavity, the laser oscillation is obtained with the threshold power of 3, 3.2, and  3.5 W under 3, 5, and 9% OC, respectively. After entering the stable QML operation, the corresponding maximum output power is 506, 894, and 1,153 mW. The relationship between average output power and pump power in CW and QML operation is shown in Figures 4, 5, respectively. Through the comparison of the different OCs mentioned above, it is found that the QML output power of 9% OC is up to 1.15 W, which is the highest output power among all OCs. This is also consistent with the theoretical simulation above. This kind of QML pulse has potential application in remote detection.
The spectrum of the QML pulse is measured using an optical spectrum analyzer (AvaSpec-NIR256-2.5 TEC). As shown in Figure 6, the dual-wavelength synchronous output is obtained with the center wavelength of 1,863 and 1,877 nm and the  spectrum of half-width of 11 and 8 nm, respectively. It is found that the central wavelength and the spectral width of the spectrum do not change with the output power, and the output spectrum is stable.
The pulses of QML laser were recorded by a fast photodiode (ET-500) with a digital oscilloscope (RIGOL, DS4034). The typical QML pulse trains in 500 µs, 10 ns, and 2 ns timescales are shown in Figure 7. The QML pulses have a repetition frequency of 103.7 MHz, corresponding to the laser cavity length of 1.45 m.
Since the autocorrelator is only suitable for measuring the pulse width of CW mode-locked pulse, the Q-switched modelocked pulse cannot obtain autocorrelation envelope because of the envelope modulation of kilohertz, so we can only estimate the pulse duration roughly in theory. Here, Equation (2) is used to estimate the width of the QML pulse [27]. t m = t r 2 + t p 2 + t o 2 (2) Here, t m is the rising edge time of the measured mode-locked pulse, t r is the rising edge time of the actual mode-locked pulse, t p is the rising edge time of the photodetector, and t o is the rising edge time of the oscilloscope. In the experiment, the rising edge time of the mode-locked pulse is ∼2.06 ns, and the rising edge time of the photodetector is ∼35 ps. t o can be estimated as 1,900 ps using the Equation (2) as follows.
Among them, W b is the bandwidth of the oscilloscope, which is 200 MHz in the experiment. Thus, it can be calculated that the rise time of the mode-locked pulse is ∼795 ps. Since the actual mode-locked pulse width is ∼1.25 times of the rising edge time, it is calculated as 994 ps.

CONCLUSION
In conclusion, we first use MoS 2 SA to realize a Watt-level dual-wavelength Q-switched mode locked operation in the Tm:CYA laser. Using a typical "X" type five mirror folded cavity, a maximum CW output power of 2.6 W is obtained under 9% output coupling mirror. After the laser enters into stable dual-wavelength QML operation, the maximum output power reaches 1.15 W with central wavelengths of 1,863 and 1,877 nm. The repetition frequency of QML is 103.7 MHz, and the modulation depth of mode-locked pulses in the Q envelope is close to 100%. The results show that MoS 2 SA is suitable for 2 µm high-power mode locking and has potential application value in high-power mode locking. Next step, we will strive to achieve high-power CW mode-locked operation by increasing the pump power and reducing the loss of MoS 2 . The LD pumped high-power mode-locked laser has the advantage of low cost and will be widely used in industrial production.

DATA AVAILABILITY STATEMENT
All datasets generated for this study are included in the article/supplementary material.

AUTHOR CONTRIBUTIONS
WL and QX designed the overall scheme. CC and RS carried out specific experiments. YZ provided experimental guidance.