1. Introduction

Nd-doped gadolinium yttrium vanadate crystals (Nd:Gd_xY_1-xVO₄) have the altered material parameters by changing the Gd/Y ratio [1,2], such as the reduced stimulated emission cross-section, increased upper-level lifetime and higher specific heat than that of the Nd:YVO₄ crystals. Liu et al. have reported the improvement of passive Q-switching performance by use of Nd:Gd_xY_1-xVO₄ compared with that achieved with Nd:YVO₄ and Nd:GdVO₄ [2,3]. On the other hand, the randomly distributed Y and Gd ions in the crystals modify the local crystal field experienced by the Nd ions. Therefore the new vanadate crystals have an inhomogeneously broadened gain, which have been applied in the mode-locked lasers [4, 5]. However, more attention have been paid to the operation of the Nd:GdYVO₄ lasers at 1.06μm [2–7]. Recently, diode pumped continuous-wave laser performance at 1.34μm with series a-and c-cut Nd:GdYVO₄ crystals were demonstrated in Ref [8]. Peng Li et al. have also reported a flash-lamp pumped passively Q-switched Nd:GdYVO₄ laser at 1.34μm with Co²⁺:LaMgAl₁₁O₁₉ being the saturable absorber [9]. As far as we know, there has not been any report on the passive Q-switching operation of the Nd:GdYVO₄ laser at 1.34μm under the diode pumping.

As for the pulsed laser operation at 1.34μm, several saturable absorbers such as Co²⁺-doped crystals [10,11], semiconductor saturable absorbers (SESAs) [12–14], and V³⁺-doped crystals [15–17] have been employed as the passive Q-switches. The reported experimental results show that the Q-switching efficiency obtained by Co-doped crystals such as Co²⁺:LMA are usually low [10,11], while a SESA has a lower damage threshold, which limits its application. V³⁺:YAG crystals have attracted great interest due to their excellent physical and optical performance at 1.34μm, such as high damage threshold, short absorption recovery time of about 5 ns, high ground state absorption cross-section of 7.2×10^-18cm², low saturable energy density of 0.05J/cm² and low residual absorption at 1.34μm [15], which makes them an effective passive Q-switch for 1.34μm radiation.

In this paper, the performance of a diode-pumped passively Q-switched Nd:Gd_0.5Y_0.5VO₄ laser at 1.34μm with V³⁺:YAG as the saturable absorber was demonstrated for the first time to the best of our knowledge. The maximum average output power of 0.96W was achieved under the pump power of 7.28W, corresponding to optical-to-optical conversion and slope efficiency of 13.2% and 17.6%, respectively. The minimum pulse width attained was 47.8ns with the pulse repetition frequency of 76kHz, with the single pulse energy and peak power estimated to be 8.7μJ and 182W, respectively.

2. Experimental setup

The experimental arrangement of the diode-pumped passively Q-switched Nd:Gd_0.5Y_0.5VO₄ laser was shown schematically in Fig. 1. The pump source was a fiber-coupled diode laser with a core diameter of 0.4mm and numerical aperture of 0.22. Its radiation was coupled into the laser crystal by a focusing optical system with a 25mm focal length and 95% coupling efficiency. The radius of the pump beam within the laser crystal was around 200μm. M1 was a concave mirror with a 200mm radius of curvature. It had antireflection coatings at 808nm on one surface, high-reflection coated at 1.34μm and high-transmission coated at 808nm on the other surface. The output coupler M2 was with partial transmissions at 1.34μm on one surface (T=8% and 15% were available). The other surface was antireflection coated at 1.34μm to minimize the Fresnel losses. The a-cut and 0.5 at.% Nd:Gd_0.5Y_0.5VO₄ crystal with the dimensions of 4×4×7mm³ had the antireflection coatings at 1.34μm on both surfaces. It was wrapped with indium foil and mounted in copper block cooled by water at a temperature of 20°C. The V³⁺:YAG absorber, with the dimensions of 5×5×0.5mm³ and the initial transmission of 94% at 1.34μm, was placed next to the output mirror M2 to realize the Q-switching operation at 1.34μm. The laser pulse was recorded by a Tektronix DPO7104 digital oscilloscope (1GHz bandwidth, 5Gs/s sampling rate) and a photodetector (New focus, model 1611).

 

Fig. 1. The schematic diagram of the diode-pumped passively Q-switched laser.

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3. Experimental results and analysis

The thermal lensing effect of diode-end-pumped Nd-doped laser at 1.3μm was stronger than that at 1.06μm due to the larger quantum defect as well as strong excited state absorption for the 1.3μm transition [18]. The focal lengths of thermal lens in diode-end-pumped continuous-wave Nd:Gd_0.5Y_0.5VO₄ laser at 1.34μm were determined by referring the methods adopted in Ref. [19]. The input mirror M1, as shown in Fig. 1, would be replaced by a plane mirror with the identical coatings mentioned above. The output power was optimized near the pump threshold at a given cavity length. While the pump power increased to a critical value, the strong thermal lensing effect caused the laser cavity to become unstable, which would result in a rapid decrease of the continuous-wave output power. The corresponding cavity length could be regarded as the approximate thermal focal length under the corresponding critical pump power. Furthermore, the relationship between the thermal focal length f_th and input pump power P_in could be expressed by f_th=Cw_p²/P_in [19], where C was a proportional constant; w_p was the pump-beam radius in the active medium. Therefore, the approximate constants C could be obtained by fitting the experimental data with the above formula. Figure 2a showed the corresponding experimental data and theoretical fitted curve, with the C estimated to be ~ 1.5×10⁴W/mm.

 

Fig. 2. (a). The relationship between the thermal focal length and the inverse of input pump power for the Nd:Gd_0.5Y_0.5VO₄ laser at 1.34μm; (b) the output power including the continuous-wave and pulsed operation versus the incident pump power.

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The continuous-wave operation of the Nd:Gd_0.5Y_0.5VO₄ laser at 1.34μm was performed firstly, with the cavity length adjusted to be 40mm and the concave mirror with 200mm radius of curvature exploited, as shown in Fig. 1. The dependence of output power on the pump power for different output couplers was given in Fig. 2b. As could be seen, when the output couplers T=8% and T=15% were exploited, the maximum output powers of 2.42 and 1.94W were respectively obtained under the pump power of 7.28W, with the threshold pump powers of 0.4 and 0.85W, respectively. The corresponding optical-to-optical conversion efficiency for the said two situations were estimated to be respectively 33.2% and 26.5%, with the slope efficiency estimated to be 35.2% and 30.2%, respectively.

To generate efficient passive Q-switching, it is essential that the saturation in the absorber occurs earlier than that in the gain medium, which is sometimes termed the second threshold condition [20]. Usually it can be achieved by augmenting the ratio of the effective mode area in the gain medium to that in the absorber (A/A_s) for the passively Q-switched Nd-doped lasers. For this passively Q-switched Nd:Gd_0.5Y_0.5VO₄/V³⁺:YAG laser at 1.34μm, the second threshold condition can be easily satisfied due to three contributing factors, which are the relatively smaller stimulated emission cross-section of Nd:Gd_0.5Y_0.5VO₄ at 1.34μm (~1.0×10^-19cm² [8]), the larger ground state absorption cross-section (7.2×10^-18cm² [15]) and smaller excited state absorption cross-section (7.4×10^-19cm² [15]) of V³⁺:YAG at 1.34μm. This looses control on the ratio of A/A_s. According to the measured thermal lens, the beam radius variation versus the pump power for the cavity exploited in our experiment is shown in Fig. 3. As can be seen, the beam radius in the laser crystal is insensitive to the thermal lens at the pump power range below about 8W, while it increases quickly with the pump power exceeding 8W due to the severe thermal lensing effect.

By inserting the V³⁺:YAG into the cavity adopted in the continuous-wave operation, the passive Q-switching operation of the Nd:Gd_0.5Y_0.5VO₄ laser at 1.34μm was realized, with the relationship between the average output power and incident pump power also shown in Fig. 2b. The best performance was obtained with the T=15% output coupler exploited. The maximum average output power of 0.96W was achieved under the pump power of 7.28W, corresponding to optical-to-optical conversion and slope efficiency of 13.2% and 17.6%, respectively. The fluctuation at the average output power 0.96 W over hours-long operation was found to be less than 1.0%. The output beam quality factor in this situation was estimated to be <1.5. For the other situation, 0.78W average output power was obtained under the same pump power, with the threshold pump power of 1W. The Q-switching efficiency (ratio of the Q-switched output power to the continuous-wave power at the maximum pump power) were found to be 49.5% and 32.2% for the two situations of T=15% and T=8%, respectively. The relationship between the pulse repetition frequency (PRF) and the incident pump power was displayed in Fig. 4a. It was found that the PRF increased with the pump powers. When the output couplers with the transmissions of 8% and 15% were exploited, the PRF respectively presented the variations of 42.5–167 kHz and 23–150 kHz with the pump power increasing from 2.24 to 7.28W. Smaller output coupling would lead to higher PRF which coincided with the experimental results shown in Ref [17].

 

Fig. 3. The beam radius variation versus the pump power.

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Fig. 4. (a). The pulse repetition frequency versus the pump power; (b) the pulse widths versus the pump power.

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Figure 4b depicted the pulse widths versus the incident pump powers for the two output couplers. As could be seen, the pulse widths for the two output couplers presented the similar variation trend. They firstly decreased with increasing the pump powers to a certain value, and then slightly increased with pump power. For the Q-switched lasers, when the PRF achieved such a high value that the initial inversion population could not be fully consumed during the interval when the Q-switch was turned on, the rise time of a pulse would be extended, resulting in the generation of stretched pulses. So the slight increase of the pulse widths was observed in our experiment. The minimum pulse width of 47.8ns with the PRF of 76kHz was attained with the T=15% output coupler under the pump power of 5.12W, with the maximum single pulse energy and peak power estimated to be 8.7μJ and 182W, respectively. The temporal pulse profiles for the above situation, along with the corresponding train of pulses, were shown in Figs. 5a and 5b, respectively.

 

Fig. 5. (a). The temporal pulse profile taken at the pump power of 5.12W with T=15% output coupler exploited; (b) the corresponding pulse train.

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4. Summary

In summary, a diode-pumped passively Q-switched Nd:Gd_0.5Y_0.5VO₄ laser at 1.34μm with V³⁺:YAG as the saturable absorber was demonstrated in this paper. The focal lengths of thermal lens in the diode-end-pumped Nd:Gd_0.5Y_0.5VO₄ laser for the 1.34μm transition was experimentally investigated, with the corresponding proportion constant estimated to be ~ 1.5×10⁴W/mm. The maximum continuous-wave output power of 2.42W was achieved with the T=8% output coupler under the pump power of 7.28W. As for the passive Q-switching operation under the same pump power, the maximum average output power of 0.96W was attained with the T=15% output coupler, corresponding to optical-to-optical conversion efficiency of 13.2%, higher than that of the previously published results [10–13, 17]. The minimum pulse width attained was 47.8ns with the pulse repetition frequency of 76kHz, with the single pulse energy and peak power estimated to be 8.7μJ and 182W, respectively.