1. Introduction

Diode-pumped solid-state lasers are very attractive owing to their simplicity, high conversion efficiency and good beam quality, however, most of these lasers operate at 1.06μm. On the other hand, the laser sources operating at 1.3μm, especially those pulsed sources have wide applications in many fields, such as medicine, fiber optics, spectroscopy, etc.

As we know, laser materials play important roles in diode-pumped laser system. Nd doped crystals, such as Nd:YAG, Nd:YVO₄, Nd:GdVO₄, are usually used as laser crystals. Recently, Nd:LuVO₄ crystal has attracted much attention since it was reported by C. Maunier in 2002 [1-3]. It has high absorption cross section of 69×10^-20cm² at 808nm and high thermal conductivity of 7.96Wm^-1K^-1 and 9.77Wm^-1K^-1 along a- and c-axis, respectively [4]. Using the Nd:LuVO₄ crystal as the gain medium, Liu et al have reported continuous-wave (CW) output power of 12.55W at 1.06μm with a slope efficiency of 52.3% [5], and the laser pulse as short as 7.1ps has been obtained by Yu et al with GaAs wafer in 2008 [6]. So, Nd:LuVO₄ has been regarded as a promising laser crystal for solid-state lasers, however, the research work on its characteristics at 1.34μm has been rarely performed. As is now well known, Nd:LuVO₄ has high fluorescence branching ratio and emission cross section at 1.34μm [7], thus, it is also suitable for 1.34μm laser operation, just like Nd:YVO₄ and Nd:GdVO₄ crystals.

For passive Q-switching operation of solid-state lasers at 1.3μm, Co-doped crystal [8], semiconductor saturable absorber mirror (SESAM) [9], and V³⁺:YAG [10] have ever been employed as saturable absorbers. The experimental results have shown that pulse width obtained by Co-doped crystal, such as Co:LMA, is usually wider than the others [11], while SESAM has higher loss and lower damage threshold, which limits its application. V³⁺:YAG has an absorption band ranging from 0.75μm to 1.44μm [12], which makes it suitable for passive Q-switching at 1.06μm, 1.34μm and 1.44μm. In addition, it has higher damage threshold and ground state absorption cross section (7.2×10^-18cm²), shorter absorption recovery time, lower saturable energy intensity (0.05J/cm²) and residual absorption at 1.3μm [13], which makes it an effective passive Q-switcher for 1.34μm laser.

This is the first time, the best to our knowledge, to report a diode-pumped passively Q-switched Nd:LuVO₄ laser at 1.34μm with V³⁺:YAG as a satuarable absorber. The characteristics of CW operation with a-cut and c-cut Nd:LuVO₄ crystals were also studied in this paper.

2. Experimental setup

The schematic of the laser setup was shown in Fig. 1, in which a simple plano-concave cavity was employed. The pump source was a 30-W fiber coupled semiconductor laser with a center wavelength of 808nm and a numerical aperture of 0.22. The output beam was focused into the Nd:LuVO₄ crystal with a spot size of 400μm by a coupling optics system. M₁ is a concave mirror with a curvature of 250mm, coated with anti-reflection (AR) of 808nm on one side, high reflection (HR) of 1.34μm and high transmission (HT) of 808nm on the other side. M₂ is a plane mirror coated with part transmission at 1.34μm. For 1.3μm operation, both mirrors are coated with transmission higher than 95% at 1.06μm to suppress the 1.06μm emission. Two a-cut and c-cut Nd:LuVO₄ crystals with the same Nd³⁺ doping level of 0.5.at% and dimensions of 3×3×8mm³ were used as laser medium, respectively. Both faces of the Nd:LuVO₄ crystals were AR coated at 808nm and 1.34μm. It was wrapped by indium foil and mounted in a copper block which temperature was controlled at 18°C. A 0.5mm-thick V³⁺:YAG crystal with initial signal transmission of 94% was used. It was AR coated at 1.34μm on both sides. The cavity length was fixed as short as 20mm to prevent thermal lensing effect from affecting the laser performance under high pump power level. The temporal profile and repetition rate of the laser pulses were recorded by a fast photodiode detector (NEW FOCUS 0901) and a 1-GHz oscilloscope (Tektronix TDS5104).

 

Fig. 1. Schematic of experimental laser setup.

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3. Results and discussions

The CW operation of the Nd:LuVO₄ laser at 1.34μm was realized first. The dependence of output power on the pump power was given in Fig. 2. Figures 2(a) and 2(b) showed the characteristics of c-cut and a-cut Nd:LuVO₄ lasers, respectively. It can be seen from Fig. 2(a), the output power of 1.72W was obtained from c-cut Nd:LuVO₄ laser at the pump power of 12.88W with output mirror of T=8%, giving an optical-optical conversion efficiency of 13%. The threshold power was 1.13W, corresponding to the slope efficiency of 15%. By using T=3%, the output power of 1.59W was obtained at pump power of 10.30W, corresponding to the optical conversion efficiency of 15%. The threshold power was 0.53W, and the slope efficiency was 16%. However, the output power began to saturate when the pump power exceeded 10.30W. In Fig. 2(b), the output power of 1.20W (pump power of 12.0W) and 1.27W (pump power of 9.45W) was obtained, giving the optical conversion efficiency of 10% and 13%, and the threshold powers were 1.59W and 0.76W, respectively, for T=8% and T=3% of a-cut Nd:LuVO₄ lasers. Comparing Figs. 2(a) and 2(b), we can see that the output power of c-cut Nd:LuVO₄ laser was higher than that of a-cut Nd:LuVO₄ laser with the same output coupler and pump level. Furthermore, the threshold power of c-cut Nd:LuVO₄ laser was lower than that of a-cut with the same output coupler.

 

Fig. 2. CW and Q-switched output power for c-cut (a) and a-cut (b) Nd:LuVO₄ laser.

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This mainly resulted from that the net π-polarized emission cross section is lower than that of σ-polarized at 1.34μm due to the excited state absorption (ESA) at 1.34μm. The ESA peak of Nd:LuVO₄ is located at 1342nm. In our experiments, the emission wavelength was measured to be 1341.7nm and 1340.6nm for a-cut and c-cut Nd:LuVO₄ lasers, which resulted in the ESA at a-cut Nd:LuVO₄ crystal was higher than that of c-cut. The reported net π-polarized and σ-polarized emission cross sections are 1.5×10^-19 and 1.9×10^-19cm², respectively [14]. Additionally, the quantum defect is about 0.40 for 1.34μm, if considering the effect of ESA, the thermal focusing effect is very serious at 1.34μm for the Nd:LuVO₄ laser. The thermal conductivity of Nd:LuVO₄ crystal along c-axis is higher than that of a-axis, thus, the c-cut Nd:LuVO₄ laser will have better laser operation, which is in agreement with our experiment results.

Then the passively Q-switching performance of Nd:LuVO₄ laser was studied. From Fig. 2 (a) and 2(b), it is found that the average output power of passive Q-switching operation was lower than CW regime because of the inserted loss of V³⁺:YAG. The best laser performance occurred in c-cut Nd:LuVO₄ laser with T=8%. The maximum average output power of 1.02W was obtained and the optical conversion efficiency was 8% at T=8% using c-cut Nd:LuVO₄. This power reached 60% of the CW output power. The pump threshold was 2.85W, giving a slope efficiency of 10%.

In our passively Q-switched system, the ratio of cross sections of V³⁺:YAG and Nd:LuVO₄, σ_gsa/σ_em, is 48 for a-cut and 38 for c-cut Nd:LuVO₄. The second threshold condition is easy to be satisfied in Nd:LuVO₄ passively Q-switched lasers. Consequently, a very short cavity can guarantee that saturation in the absorber occurred before the gain saturated in the laser medium and can realize efficient Q-switching performance.

 

Fig. 3. Pulse width versus pump power.

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Fig. 4. Pulse repetition rate versus pump power.

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Based on analysis of the coupled rate equations [15], at the lower pump power, i.e. at the lower pump rate, the pulse width decreased dramatically when the pump power increased. However, in the higher pump rate, the pulse width deceased slowly, or unnoticeably with the pump power, because the population inversion density of the gain medium depleted at a very short time and the pulse width was saturated. From Fig. 3, it can be observed that the pulse width varied little when pump power increased from 3W to 13W. It also can be found that the pulse width of a-cut Nd:LuVO₄ laser is a little narrower than that of c-cut. The average pulse durations were roughly to be 24.4ns with T=3% and 23.7ns with T=8% for c-cut Nd:LuVO₄ lasers. For a-cut one, they were 21.9ns and 21.5ns, respectively. According to the passively Q-switched theory, the emission cross section affects the pulse width [16]. The higher the emission cross section was, the wider the pulse width was. Moreover, when the higher transmission of output coupler was used, always the narrower pulse width was obtained, which coincided with the lowest pulse width of 21ns obtained in a-cut Nd:LuVO₄ laser with T=8%.

The pulse repetition rate versus the pump power was given in Fig. 4. It can be seen that the pulse repetition rate increased linearly with the augment of pump power. The relationship between pulse repetition rate and pump power can be written as Eq. (1) [17]:

where, P is the pump power, τ is the fluorescence time of the laser medium, and P_thres is the threshold of the pump power. Equation (1) shows that higher repetition rate can be realized at lower threshold power and higher pump power. The threshold powers of the Q-switching were 2.22W at T=3% and 2.85W at T=8% for c-cut Nd:LuVO₄ lasers. For a-cut Nd:LuVO₄ lasers, these values increased to 3.56W and 3.64W at T=3% and T=8%. So the maximum repetition rate of 114kHz was obtained at pump power of 10.30W with T=3% in c-cut Nd:LuVO₄ laser. These results also indicate that the higher repetition rate can be obtained with the lower transmission of the output mirror. Figure 5 gave a typical pulse shape at 1.34μm with pulse width of 22ns and Fig. 6 showed a pulse train with pulse repetition rate of 31kHz in our experiments.

 

Fig. 5. A pulse with pulse width of 22ns.

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Fig. 6. A pulse train of 31kHz repetition frequency.

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Through calculation, the maximum pulse energy and peak power of 17.6μJ and 820W were achieved in a-cut Nd:LuVO₄ laser with T=3%. Because the average output power varied slowly and pulse width varied little with the pump power, the pulse energy and peak power mainly depended on the pulse repetition rate. As a result, the higher pulse energy and peak power could be obtained in the a-cut Nd:LuVO₄ laser.

4. Conclusions

We have demonstrated a diode-pumped passively Q-switched Nd:LuVO₄ laser at 1.34μm with V³⁺:YAG crystal for the first time. The maximum CW and Q-switched output power of 1.72W and 1.02W were obtained with c-cut Nd:LuVO₄ laser at T=8%. In addition, the output power of a-cut was lower than that of c-cut Nd:LuVO₄ laser. When pump power and transmission of output coupler increased, pulse width had no evident variation. It was measured to be about 22 ns. The pulse repetition rate ranging from 8.22kHz to 114kHz depended on the pump power and transmission of output mirror. At last, the maximum pulse energy of 17.6μJ and peak power of 820W were obtained with a-cut Nd:LuVO₄ laser. Shorter pulse width, higher pulse energy and peak power will be obtained if a lower small signal transmission of V³⁺:YAG crystal is used.