An electro-optically Q-switched pulsed laser at 1.34 μm with a repetition rate of 100 kHz applying optically active langasite (La3Ga5SiO14) crystal has been reported. With Nd:YVO4 as laser crystal, the electro-optically Q-switched pulsed lasers were obtained with the maximum repetition rate of 100 kHz, maximum average output power of 2.42 W, and a minimum pulse width of 2.4 ns. Based on the theory of rate equations, the optimal pulse energy of the electro-optical Q-switching could be calculated. The experimental results have been found to be matched well with the theoretical calculations. To the best of our knowledge, this work presents the highest repetition rate and shortest pulse width which are achieved by an electric-optic LGS Q-switching at the wavelength of 1.34 μm, and it enriches the material categories for generating the high repetition rate pulsed laser.
© 2017 Optical Society of America
The pulsed laser at 1.34 μm which possesses high repetition rates and a short pulse width has extensive applications in fiber optics, military, spectroscopy, laser ranging, materials processing, medicine and frequency conversion, etc [1, 2]. For instance, the laser at 1.34 μm is suitable for fiber communication because that it coincides very well with the low loss spectrum and low dispersion of the silica fiber, and the short high repetition rate pulsed lasers are desired for material processing in which the processing speed depends upon the repetition rate of the laser source. Generally speaking, the pulsed laser with high repetition rates can be achieved by active or passive Q-switching . Compared with passive Q-switching, active Q-switching can be controlled much easier, and has more stabilized pulse energy, larger peak power and lower temporal jitter at high repetition rates . There are two representative active Q-switched modes which are electro-optic and acousto-optic Q-switching [5–7]. The electro-optic Q-switching has lots of advantages applying in many fields because of its better hold-off ability , larger pulse energy and more controllable repetition rates [2, 8]. However, when the electro-optic Q-switched processing under high repetition rate, the pulsed laser performance is constrained by the piezoelectric ring effect [9, 10]. Generally, the piezoelectric ring effect depends on the piezoelectric properties of electro-optic crystals and has a strong impact on the properties of pulsed lasers with high repetition rates. Therefore, the acousto-optic Q-switched mode is widely used among the active Q-switching, and there have been many reports about the acousto-optic Q-switched lasers at 1.34 μm in recent years [11, 12]. However, the acousto-optic Q-switching has some problems in the high energy of high repetition rates duo to the diffraction loss . The passive Q-switching can obtain a short pulse width because of its simplified cavity configuration, and many results about achieving 1.34 μm passive Q-switched lasers have been published [13, 14], but it is only suitable for medium and low power lasers because of the physical property of the saturable absorber and large losses. Therefore, the electro-optical Q-switcher with small piezoelectric ring effect should be still favorable for the generation of efficient Q-switched laser with high repetition rate. Nowadays, there are several representative electro-optical crystals which have been discovered, for example, KD2PO4 (KD*P), LiNbO3 (LN), β-BaB2O4 (BBO), Rb6TiOPO4 (RTP) and La3Ga5SiO14 (LGS) . However, these electro-optic crystals also have some intrinsic problems. Respectively, the deliquescence of KD*P and the crystals could only be operated under 10 kHz due to its high piezoelectric coefficient (23.2 × 10−12 C/N) . Besides, because of the photorefractive effect, the optical damage threshold of LN crystal is only 100 MW/cm2 . RTP must be used as pairs owing to its natural birefringence and low symmetry structure (orthorhombic). The growth of large longitudinal length BBO crystal along c axis is still difficult. Recently, the LGS crystal has attracted a great deal of research attention, and it is an comprehensive electro-optical crystal with an electro-optical coefficient 2.3 × 10−12 m/V , broad transmission spectra ranges  and high optical damage threshold of 950 MW/cm2 . Besides, the small piezoelectric coefficient (6 × 10−12 C/N)  of LGS reveals the possibility for the Q-switching under high repetition rates without piezoelectric ring effect. However, LGS has the actively optical effect which limits the laser cavity using a longer configuration. Recently, by studying the interaction between the optical activity and electro-optics, we found that the influence of the optical activity can be ignored if a quarter wave plate was rotated as the rotation of light polarization when the light propagates the optical active materials . Therefore, we can propose that the LGS electro-optic Q-switching with high-repetition rates may be achievable at 1.34 μm.
In this work, we demonstrate an active electro-optical Q-switching at 1.34 μm modulated by langasite (LGS) crystal, meanwhile, we use Nd:YVO4 as the laser crystal for generating lasers at 1.34μm due to its large stimulated emission cross section and short fluorescence lifetime. The LGS electro-optic Q-switched lasers have the maximum repetition rate of 100 kHz with output power of 2.42 W, the pulse energy could be 24.2 μJ, and the pulse width of 2.4 ns. Based on the theory of active Q-switching [10, 21–24], the optimal pulse energy with a repetition rate of 100 kHz in the langasite (LGS) electro-optical Q-switching could be calculated. The theoretical calculations of the pulse energy have been found to be well agreed with the experimental data. The repetition rate of 100 kHz and the pulse width of 2.4 ns in this langasite (LGS) electro-optical operation represents the highest repetition rate and narrowest pulse width among the active electro-optic Q-switching.
2. Experiments and results
An electro-optical Q-switcher was designed to be a plane-concave laser cavity according to the theoretical analysis. In our previous work, an “odd transit time” design had been presented with only rotating a quarter wave plate, which simplified the laser cavity configuration . The detailed experimental setup was performed in Fig. 1 as follows. The pump source was a fiber coupled LD at 808 nm. The pump light was focused onto the laser crystal with a focusing system with a compression ratio of 1:1 and the fiber radius was 100 μm with a numerical aperture of 0.22. Two a-cut Nd:YVO4 laser crystals were used as the gain media with the doping concentration of 0.5 at% and 0.27 at%, respectively. The dimensions for the 0.5 at% Nd:YVO4 are 4 × 4 × 7 mm3, and those of the other one are 3 × 3 × 9 mm3. The 4 mm × 4 mm transitive surfaces of the laser crystal have been polished, and the 3 mm × 3 mm surfaces of 0.27 at% Nd:YVO4 was polished and AR coated at 1064 nm and 1342 nm. The input mirror M1 was plane with AR coating at 808 nm, 1064 nm and high reflective (HR) coating at 1342 nm. The output mirror M2 was concave with a curvature of 100 mm and the transmission (T) at 1342 nm were 10% and 20%. The power was measured by a power meter (1918-R, Newport, Inc.). The length of laser cavity was set to be 75 mm because of the influence of thermal lens effect. The polarizer, LGS electro-optical crystal and quarter wave plate (QWP) were inserted the laser cavity in order. The LGS electric-optical crystal was cut along Z axis with dimensions of 4 × 4 × 20 mm3 (X × Y × Z). The transmission surfaces (4 mm × 4 mm) of LGS crystal have been polished and coated AR films at 1064 nm and 1342 nm, and the Y-Z surfaces were coated with Au. The quarter wave voltage was 4600 V according to the theoretical calculation with an aspect ratio of 1:5. The laser propagated in the LGS crystal along Z axis with the light polarization direction at the angle of 45° to X axis. Because of the optical activity, the light polarization was rotated to be 22° after propagating the electric-optical crystal along the Z axis, which leads that the QWP will rotate the angle of 22°. The voltage driver has the maximum output voltage of 5000 V with a repetition rate of 100 kHz and a rising time of 8 ns. The temporal pulse behaviors of the LGS electric-optically Q-switched laser were measured by a MSO72504DX digital oscilloscope (23 GHz bandwidth and 50 GS/s sample rate, Tektronix, Inc.).
The continuous-wave (cw) Nd:YVO4 crystal lasers were achieved by removing the polarizer, the LGS Q-switcher and QWP out of the cavity, and the results are shown in Fig. 2(a). From the figure, it can be found that for the 0.5 at% Nd:YVO4 crystal, when the absorbed pump power was 16 W and T = 10%, the maximum cw laser power could be achieved 4.58 W with a slope efficiency of 35%. And with the T = 20% output coupler, the maximum cw output power was reduced to 3.88 W, and the slope efficiency was also drop down to 27.8%. However, for the 0.27 at% Nd:YVO4, the maximum cw output power was 4.28 W with a slope efficiency of 29.4% under the absorbed pump power of 16 W when T = 10%. When the transmittance of output mirror was 20%, the maximum cw output power was 4.02 W with a slope efficiency of 27.3%.
For the Q-switching, the polarizer, a LGS Q-switched crystal and the QWP should be placed in order in the cavity. The polarization of the output laser was chosen to be π-polarized using a polarizer, and the laser can be switched off when we inserted the LGS Q-switched crystal and rotated the QWP, and the Q-switched laser could be achieved by applying a driven voltage with different repetition rates. Figure 2(b) shows the average output power versus absorbed pump power with a repetition rate of 100 kHz. It should be noted that if the doping concentration of gain media was 0.5 at%, and the transmittance of output mirror was 10%, the damage appeared on the surface of the laser crystal under the absorbed pump power over than 10 W. For avoiding the damage by the intracavity high peak power, the output coupler with T = 20% was used, and the maximum average output power was 2.21 W with a slope efficiency of 17.7%. However, for the 0.27 at% Nd:YVO4, the maximum average output power could be achieved of 2.14 W with a slope efficiency of 15.5% by using the T = 20% output coupler. With T = 10%, the maximum average output power was achieved of 2.42 W with a slope efficiency of 16.8%, and the surface of laser crystal was not been damaged. The relative higher damage threshold at surfaces should be contributed that the lower doping concentration which may be related by the distorted crystal lattices due to the larger radius of Nd3+ than that of Y3+ [25, 26]. Applying a driven voltage, the Q-switched laser could be achieved with different repetition rates, and the average output power was given in Fig. 3(a). From this figure, the Q-switched pulsed lasers could be obtained in different repetition rates, 40 kHz, 60 kHz, 80 kHz and 100 kHz. Meanwhile, using a gain media with a doping concentration of 0.5 at%, the corresponding maximum average output power was 1.61 W, 1.83 W, 2.03 W and 2.21 W under the absorbed pump power of 16.34 W. However, if the gain media was 0.27 at% dopant, with a repetition rate of 100 kHz, the achieved maximum average output power 2.42 W was higher than that achieved with 0.5 at% Nd:YVO4 (2.21 W). The single pulse energy could be calculated with the average output powers and repetition rates, and the results were shown in Fig. 3(b). The single pulse energy increased with the decreasing of the repetition rates. For the 0.5 at% Nd:YVO4, the maximum single pulse energy was 40 μJ, 30 μJ, 24.7 μJ, 22.1 μJ with the repetition rates of 40 kHz, 60 kHz, 80 kHz and 100 kHz, respectively, under the absorbed pump power of 16.34 W. For 0.27 at% Nd:YVO4, the achieved maximum single pulse energy 24.2 μJ was under the repetition rate of 100 kHz. Using the digital oscilloscope, the pulse widths of the Q-switched lasers were recorded. The pulse width versus absorbed pump power was shown in Fig. 3(c). From this figure, most of the pulse widths were stabled between 4 ns and 8 ns with the increasing absorbed pump powers under the different repetition rates, and the narrowest was 2.4 ns. However, with 0.27 at% Nd:YVO4, most of the pulse widths were in the range from 3 ns and 5 ns under a repetition rate of 100 kHz. Figure 4 gave the pulse trains with the repetition rate of 100 kHz and indicated that the pulses of the Q-switched laser were stable without the piezoelectric ring effect, which revealed that the LGS crystal could be used as a high rate Q-switcher at 1.34 μm with a repetition rate of 100 kHz. The inset of Fig. 4 is the pulse profile with the pulse width of 2.4 ns. The corresponding peak power could be achieved with the single pulse energy and the pulse width. The peak power vs the absorbed pump power was shown in Fig. 3(d). The maximum peak powers were 7.8 kW and 5.7 kW for the 0.27 at% and 0.5 at% Nd:YVO4, respectively, under the repetition rate of 100 kHz.
3. Theoretical analysis
According to the optimal energy output equations introduced by Degnan for the Q-switched lasers , the equation can be described in the following form,24], б is the stimulated emission cross section of the laser crystal.
Based on the solution of rate equations, the initial inversion density no and the residual inversion density nƒ can be written as ,24].
The resonator loss factor δ can be calculated using the relationship between the threshold pump power Pth and the transmittance of the output mirror T [27, 28],Eq. (4), obviously K2 is the slope efficiency. Therefore, a constant of the resonator loss δ could be obtained with a linear equation fitting from the threshold powers Pth measured with different output mirrors such as 10%, 15% and 20%.
With parameters used in theoretical calculations in Table 1, the optimal output single pulse energy can be calculated under different repetition rates and the transmittance of the output mirrors. Although many theoretical analysis of active or passive Q-switching have been reported for long years, a reliable theoretical calculation of electric-optic Q-switched is described. Based on this theory, the single pulse energy can be obtained with different repetition rates and transmittance of output mirrors. The results of theoretical calculations were given in Table 2. Finally, the theoretical values were well agreed with the experimental results, which indicated the cavity design of the electric-optic Q-switching was reasonable and optimal.
In conclusion, using a simple and short Q-switched laser cavity with an optical active crystal as the electric-optical Q-switcher, the LGS electric-optical Q-switched Nd:YVO4 laser was achieved with the maximum repetition rate of 100 kHz, the maximum average output power of 2.42 W, the pulse energy of 24.2μJ and the narrowest pulse width of 2.4 ns. All of these results were free piezoelectric ring effects, which identified that the LGS can be used as high repetition rates Q-switcher at the wavelength of 1.34 μm at least under the repetition rate of 100 kHz. Based on the theoretical calculation of the active electric-optical Q-switching, the calculated values of the pulse energy were almost consistent with the experimental values. The above results confirmed the LGS could provide a practical and tunable Q-switched laser with high repetition rates for many applications such as fiber optics, military applications, spectroscopy, laser ranging, materials processing and medicine, etc.
National Natural Science Foundation of China (NSFC) (51422205, 51372139, 51632004); Natural Science Foundation for Distinguished Young Scholars of Shandong Province (JQ201415); Taishan Scholar Foundation of Shandong Province, China (Tspd20150202).
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