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A compact dual-wavelength passively Q-switched green laser by intra-cavity frequency doubling of a Yb:YAG/Cr4+:YAG/YAG composite crystal was demonstrated for the first time to our best knowledge. The maximum green laser output power of 1.0 W was obtained under the pump power of 9.7 W, and the corresponding slope efficiency is 15.2%. The shortest pulse width, largest pulse energy, and highest peak power were achieved to be 5.54 ns, 246.1μJ, and 40.76 KW, respectively. Dual-wavelength laser oscillation simultaneously at 515 nm and 524.5 nm has been achieved. This passively Q-switched dual-wavelength green laser can be used as a laser source for Terahertz generation.
© 2017 Optical Society of America
1. Introduction
In recent years dual-wavelength lasers have attracted more and more attention on account of the potential applications in the generation of terahertz (THz) radiations. There are several techniques for the generation of terahertz radiation, including optical rectification of femtosecond pulses [1], quantum cascade lasers [2], and difference frequency generation (DFG) of dual-wavelength lasers [3]. The difference-frequency generation (DFG) from optical pump sources represents one of the most promising methods. Usually, THz generation based on optical parametric processes in a nonlinear optical (NLO) crystal, such as ZnGeP2 (ZGP), GaSe, GaP [4–6].The key of generation of dual-wavelength lasers is the net-gain equalization of two particular spectral lines with one sufficiency broad-gain bandwidth [7]. Yb-ion and Nd-ion have a broad-gain bandwidth for the dual-wavelength radiation. Dual-wavelength lasers has been reported in a number of trivalent neodymium ions (Nd3+) doped laser materials [8, 9]. Compared with Nd-ion, the Yb-ion has several merits, such as a small quantum defect, a high quantum efficiency and a broad absorption bandwidth [10]. The dual-wavelength lasing from the Yb-ion was demonstrated based on laser crystals such as Yb:GdVO4 [11], Yb:KLu(WO4)2 [12], Yb:KGd(WO4)2 [13]. In contrast, the reports on the dual-wavelength Yb:YAG laser are fewer. Figure 1 shows the energy levels of Yb:YAG crystal and energy gap of each Stark levels from the ground state. The trivalent Yb3+ ion possess only two electronic states: the ground 2F7/2 manifold and the excited 2F5/2 manifold. The Boltzmann distribution at the upper level and the terminated laser levels (612 cm−1 for 1030 nm oscillation and 785 cm−1 for 1049 nm oscillation) of Yb:YAG crystal are also shown in Fig. 1.
Continuous-wave dual-wavelength laser has been demonstrated in a diode-pumped Yb:YAG laser [14]. The experimental results show that Q-switched dual-wavelength laser with high peak power possesses a balanceable and stable proportion of dual-wavelength output [15]. Yb:YAG crystal has a relatively large emission cross section [16], which is suitable for Q-switching operation. Compared to actively Q-switched lasers, passively Q-switched lasers have the advantages of compactness and simplicity. Passively Q-switched Yb:YAG crystal or ceramic lasers have been investigated using different kinds of saturable absorbers, including Cr4+:YAG [17], GaAs [18], graphene [19], single-walled carbon nanotubes (SWCNTS) [20]. Among these saturable absorbers, Cr4+:YAG crystal is widely used due to its high damage threshold and great thermal conductivity [21]. With the advent of Cr4+:YAG passively Q-switched lasers, composite crystals have been widely used in the passively Q-switched laser [22]. In addition, monolithic composite crystal bonding with an undoped segment has been proven effective in relieving the thermal effect and improving laser performance by reducing the spatial of the temperature and the thermally induced stress [23, 24]. Yb:YAG composite crystals have been used for generating the fundamental laser [25, 26], and the frequency doubling green laser [27]. However, the Q-switched dual-wavelength green laser in a diode-pumped Yb:YAG composite crystal has not been reported previously, to the best of our knowledge.
In this paper, we firstly reported a dual-wavelength green pulsed laser that is passively Q-switched by a Yb:YAG/Cr4+:YAG/YAG composite crystal and intra-cavity frequency-doubled by a LBO crystal. Under the pumped power of 9.7 W, the maximum total average output power of 1.0 W was obtained with the slope efficiency of 15.2%. A peak power of 40.76 kW was achieved with a pulsewidth of 5.54 ns, and the largest pulse energy was 246.1 μJ. A dual-wavelength green-pulse laser radiation at 515 nm/524.5 nm has been realized. We believe that this passively Q-switched dual-wavelength green laser provides a possible approach for developing visible laser sources for the generation of THz radiation.
2. Experimental setup
The dual-wavelength lasing was observed in an experimental setup shown in Fig. 2. A short linear resonant cavity is adopted to make the cavity compact. The resonant cavity consists of two mirrors (M1 and M3) with a total length of 48.5 mm. The distance from input mirror M1 to the end side of composite crystal (L1) was approximately 14 mm, and the distance from the end side of composite crystal to the output coupler M3 was 34.5 mm. The input mirror M1 is a plane mirror, it was high-transmission (HT) coated at 940 nm (R<0.25%) and high-reflection (HR) coated at 1030 and 1049 nm (R>99.8%). The output coupler M3 is a plane-concave mirror with a curvature radius of 300 mm. It was HR coated at 1030 nm (R>99.5%) and 1049 nm (R>99.4%), and HT coated at 515 and 524.5 nm (T = 99.5%). Besides, M2 is a plane mirror that is inserted between the composite crystal and LBO crystal. It was HT coated at 1030 and 1049 nm, and HR coated at 515 and 524.5 nm for preventing the frequency doubled laser passing through the composite crystal.
Fig. 2 Experimental configuration of the dual-wavelength passively Q-switched green laser with a Yb:YAG/Cr4+:YAG/YAG composite crystal.
The composite crystal with a total length of 9 mm contains a 5.0 at. % Yb-doped YAG crystal, a Cr4+:YAG crystal with an initial transmission of 90% and a nondoped YAG crystal. The dimensions of Yb:YAG and Cr4+:YAG crystal are 4 × 4 × 5 mm3 and 4 × 4 × 2 mm3, and the nondoped YAG crystal has the same dimension with the saturable absorber. The two surfaces of composite crystal are coated with HT film at 940 nm and 1030 nm. A LBO crystal with dimensions of 3 × 3 × 10 mm3 is used to frequency doubling of the fundamental wave. It is cut for a type I phase matching (θ = 90°, φ = 13.6°). It was HT coated at 1030 and 1049 nm, and HT coated at 515 and 524.5 nm. Intra-cavity frequency doubling is applied to reach the aim of the high peak power inside the cavity, the high conversion efficiency and stable laser output [28]. The composite crystal and LBO crystal are wrapped with the indium foil and mounted in a water-cooled copper block, and cooled at 12 and 25°C, respectively. The pump source is a 20 W fiber-coupled 940 nm laser diode with a core diameter of 200 μm and numerical aperture of 0.22. A pair of the plano-convex coupling lenses with 1:1 imaging ratio focused the pump beam to a waist of 200 μm in diameter inside the composite crystal. The longitudinal position of the pump image inside the composite crystal was adjusted experimentally to maximize laser output power.
3. Experimental results and discussion
The output pulse characteristics of the dual-wavelength green laser were investigated, including the average output power, pulse width, pulse repetition rate. Figure 3 depicted the relationship between the average output power and the pump power, which indicated that the average green output power increased almost linearly with the increase of the pump power. A dual-wavelength green laser operation was realized at 515 nm and 524.5 nm with maximum total average output power of 1.0 W at 9.7 W of pump power, corresponding to the slope efficiency of 15.2% and the threshold pump power of 4.14 W. The inset is the laser spectrum of the output beams of 514.9 nm and 524.5 nm at the pump power of 5.34 W. The laser system with Yb:YAG/Cr4+:YAG/YAG composite crystal was stable when the pumped power was lower than 10 W. The saturable phenomenon of average output power did not appear when the pumped power under 10 W. This may be caused by the undoped YAG crystal part bonded to the Cr4+:YAG crystal, which speeded up thermal diffusing to increase the effective focal length of thermal lens.
Fig. 3 Average output power versus pump power for dual-wavelength green laser. Insets: the laser spectrum at the pump power of 5.34 W.
Figure 4 showed the pulse width, repetition rate, pulse energy and peak power of the passively Q-switched green lasers as functions of pump power. From Fig. 4(a) it can be found that the pulse width decreased with the increase of the pump power from 8.88 ns down to 5.54 ns. The green pulse repetition rate monotonically increased as the pump power increased, and the maximum repetition rate of 4.87 kHz was obtained under incident pump power of 9.7 W. Figure 4(c) showed the train of laser pulses under the pumped power of 9.7 W, and the pulse profile with the width of 5.54 ns was shown in Fig. 4(d). According to the repetition rate, the pulse energy and the peak power of the output laser can be calculated. The function between them and the pump power has been shown in Fig. 4(b). The maximum pulse energy and the maximum peak power were 246.10 μJ and 40.76 kW, respectively. Besides, the far field spot of the passively Q-switched dual-wavelength green laser was measured by a CCD camera and illustrated in Fig. 5. As it shows, the output of passively Q-switched green laser was a high-order mode.
Fig. 4 (a) Pulse width and repetition rate of dual-wavelength green laser varying with increasing pump power. (b) Pulse energy and peak power versus incident pump power. (c) The train of laser pulses under the pump power of 9.7 W. (d) Profile of the frequency-doubling pulse with the width of 5.54 ns under the pump power of 9.7 W.
The spectrum of Q-switched dual-wavelength green laser was recorded by the spectrometer with a spectral resolution of 0.3 nm. Figure 5 showed the evolution of the lasing spectrum with the increase of the pump power. The central wavelength of dual-wavelength green laser was around 515 nm and 524.5 nm, and the two modes have almost the same threshold. A 4:5 spectral intensity ratio of 515nm and 524.5 nm happened at the pump power of 5.34 W. The spectral intensity ratio of 515 nm and 524.5 nm was stable at 2:1 as the pump power was increased. It may be attributed to the Q-switched operation with high peak power. There was a slight shift of the frequency doubling dual-wavelengths with the increasing of the pump power, which was shown in Table 1. There were two reasons for this behavior. On the one hand, the emission spectra of Yb:YAG crystal shifted to longer wavelength with the increasing of the temperature due to more heat generated inside the gain medium with the pump power [29]. On the other hand, the central wavelength of the laser emission shifted with the increasing of intra-cavity light intensity, which derived from the increasing of the lower laser level population [30]. When a Yb3+ ion jumped back to the lower energy level, it always relaxed to the other even lower energy levels. This relaxation phenomenon would increase the reabsorption losses.
Table 1. Variations of frequency doubling dual-wavelengths with incident pump powers
Stable coexistence of 515 nm and 524.5 nm lasing was observed in the experiment, which was attributed to the competition and equalization of their net gain. Recently, there was much research attention devoted to the dual-wavelength laser for the generation of THz radiation with diode-pumped Yb-doped crystals. An acceptable single pulse energy and peak power was obtained, which is crucial in nonlinear optical frequency conversion. It was believed that this passively Q-switched simultaneous dual-wavelength green laser should be used as a visible laser source for the generation of THz radiation (Fig. 6).
4. Conclusions
In conclusion, a dual-wavelength green-pulse laser operation of intra-cavity frequency doubling Yb:YAG/Cr4+:YAG/YAG composite crystal was firstly demonstrated. The maximum green laser output power of 1.0 W was measured at the incident pump power of 9.7 W, and the corresponding slope efficiency is 15.2%. The shortest pulse width, largest pulse energy, and highest peak power were achieved to be 5.54 ns, 246.1 μJ, and 40.76 kW, respectively. A dual-wavelength green laser stable oscillation at 515 nm and 524.5 nm was obtained. The experiment shows that a high-order mode was exported. The conversion efficiency of dual-wavelength green laser could be improved by enhancing the power density and improving the beam quality. Applications to frequency-difference generation in the THz range could then be expected.
Funding
National Natural Science Foundation of China (NNFC) (61475067); Fundamental Research Funds for the Central University (FRFCU) (11615454); Guangdong Project of Science and Technology Grants (GPSTG) (2014B090903014, 2014B010131004, 2014B010124002, 2015B090901014, 2016B090917002, 2016B090926004).
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