1. Introduction

Diode-pumped solid-state lasers (DPSSLs) based on Nd-doped crystals, such as Nd:YAG, Nd:YVO₄, etc, have been extensively developed due to their promising practical applications in medical treatment, material processing, military and so on [1–3]. In recent years, considerable efforts have been devoted to searching for new single crystals of high quality and optimized performance and new technology for DPSSLs [4–8]. Yu et al. [7] demonstrated Q-switched laser performance, from an Nd:CNGG laser, with the pulse width, pulse energy and peak power of 12.9 ns, 173.16 µJ and 12.3 kW, respectively. And Tan et al. [8] have produced transform-limited 4 ps pulses with ceramic Nd:YAG using graphene as saturable absorber which would be a potential candidate as a mode locker for DPSSLs.

Nd-doped vanadate mixed crystals, such as Nd:Lu_xY_1-xVO₄, Nd:Gd_xY_1-xVO₄, Nd:Lu_xGd_1-xVO₄, etc, have shown their energy storage capacities and more-efficient Q-switched laser properties due to inhomogeneous broadening of fluorescence [9–11]. LuYAG is considered as a solid solution of LuAG and YAG, and the concentration of the solid solution can be indicated as percentage of the Y/Lu ratio [12]. Similar with YAG and LuAG, LuYAG crystallizes in cubic, and it has the advantages of excellent thermal and mechanical properties. As is called a kind of mixed crystal, which can enhance the structure disorder, it can induce broadened optical spectra, and hence, can generally improve the laser performance in Q-switched regimes.

2. Spectral properties

The mixed Nd:LuYAG laser crystal was obtained by the Czochralski method. The Nd-doped concentration in the mixed crystal was 1 at.%. A sample was cut from the as-grown crystal and two surfaces perpendicular to the <111>-growth axis were polished for spectral measurements. Low and room temperature absorption spectra were recorded with a Varian Model 5E UV-VIS-NIR (UV–visible–near-IR) absorption spectrophotometer. The crystal was placed into an Oxford Model CF 1204 continuous-flow liquid helium cryostat equipped with a temperature controller. The absorption spectrum of Nd:LuYAG crystal between 300 nm and 900 nm recorded at 10 K is presented in Fig. 1 .

 

Fig. 1 Low-temperature optical spectra of Nd³⁺ ions in the LuYAG crystal. Panel (a) shows absorption of Nd³⁺ ions; Panel (b) corresponds to the Nd³⁺ emission acquired under the 808 nm excitation.

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Eight groups of bands between 300 nm and 900 nm are characteristic for transitions from the ground ⁴ I _9/2 level to the excited levels. Low-temperature emission spectrum corresponding to the ⁴ F _{3 / 2}→⁴ I _{9 / 2} and ⁴ F _{3 / 2}→⁴ I _{11 / 2} transitions is also shown in Fig. 1. The excitation source was a diode laser emitting at 808 nm, coincident with the Nd³⁺ pump band related to the ⁴ I _9/2→⁴ F _{5 / 2} transition. The energies of crystal field levels of Nd³⁺ multiplets relevant for laser operation can be determined from the low-temperature absorption and emission spectra. The electronic energy levels for ⁴ F _3/2 multiplet of Nd³⁺ in LuYAG lie at 11430 and 11505 cm⁻¹. The overall splittings of the ⁴ I _{9 / 2} and ⁴ I _{11 / 2} manifolds are 870 cm⁻¹ and 520 cm⁻¹, respectively. The ⁴ F _3/2 fluorescence lifetime was measured to be 263 µs at 10 K. Inspection of low temperature optical spectra reveals that the widths of lines related to transitions between individual crystal field levels of Nd³⁺ ions in LuYAG crystal are considerably larger than those reported for Nd³⁺ in YAG crystal. In particular the linewidth of transition from the lowest crystal field component of the ⁴ F _3/2 multiplet to the lowest crystal field component of the ⁴ I _{11 / 2} multiplet of Nd³⁺ in LuYAG amounts to a FWHM of 0.72 nm (6.4 cm⁻¹) at 8 K. This value is roughly four times larger than that recorded at 77 K for 1 at.% Nd:YAG crystal [13].

As the laser experiments were conducted at room temperature, we also tested the room temperature spectral properties. Figure 2 shows the room temperature absorption spectrum of ⁴ I _9/2→² H _9/2 + ⁴ F _5/2 transition corresponding to AlGaAs LD emission region and the room temperature fluorescence spectrum corresponding to ⁴ F _3/2→⁴ I _11/2 transiton excited by an 808 nm laser diode. The peak absorption cross section at 809 nm is about 8.3 × 10⁻²⁰ cm² with a FWHM of 9 nm. Broad absorption is suitable for efficient diode-pumping.

 

Fig. 2 Room temperature absorption spectrum of ⁴ I _9/2→² H _9/2 + ⁴ F _5/2 transition and fluorescence spectrum corresponding to ⁴ F _3/2→⁴ I _11/2 transiton.

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3. Laser experiments

The Nd:LuYAG crystal used for the laser experiments was cut along its <111> direction with the dimensions of 3 × 3 × 10 mm³, and its end faces were polished and antireflection (AR) coated at 808 nm and 1.06 μm. During the experiments, the sample was wrapped with indium foil and held in a water-cooled aluminum block to maintain a temperature of 22 °C. The CW and Q-switched laser experiments were carried out in the plano-plano resonator shown in Fig. 3 . The pump source was a fiber-coupled diode laser, emitting at the wavelength range of 808 nm. Through the focusing optics (N.A. = 0.15), the output beam of the source was put into the laser crystal with a spot radius of 0.2 mm. M1 was a plano mirror, anti-reflection coated at 808 nm on the entrance face, high reflection coated at 1.06 μm and high transmission coated at 808 nm on the other face. M2 was a flat mirror with the transmission at 1.06 μm of 6%. The length of the cavity was 50 mm. The focal length of thermal lens was calculated to be from 0.2 m to 0.05 m with the absorbed pump power increased from 1 W to 5 W. Considering the thermal lenses, the cavity would be a stable cavity with g₁g₂ changing from 0.4 to 0.8, and the radius of beam waist in cavity changing from 0.2 mm to 0.3 mm. So the pump power could be superposed well with the cavity mode. In our experiment, the CW laser operation was studied first. And the pulsed laser operation was carried out by inserting the absorbers into the cavity. Three different Cr⁴⁺:YAG saturable absorbers were used, with transmissions of 90%, 83% and 70%, respectively.

 

Fig. 3 Experimental configuration of the Nd:LuYAG laser.

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4. Results and discussions

The output characteristics of Nd:LuYAG at 1064 nm in the CW regime are depicted in Fig. 4 . The laser reached threshold at an absorbed pump power of 0.75 W. A maximum output power of 1.67 W was obtained at an absorbed pump power of 4.71 W, corresponding to an optical conversion efficiency of 35.5% and the slope efficiency of 39.8%. With the Cr⁴⁺:YAG saturable absorbers placed close to the laser crystal in the cavity, stable passively Q-switched operation was achieved. The generated average output powers as a function of absorbed pump powers are also shown in Fig. 4. The threshold powers of the Q-switched laser are much higher than that of the CW laser, and the oscillating thresholds were 1.38, 1.6 and 2.49 W with the saturable absorber initial transmission of 90%, 83% and 70%, respectively. The maximum average output power of 0.71 W was obtained with the saturable absorber of T₀ = 90%, and accordingly the maximum slope efficiency and optical conversion efficiency were determined to be 21.9% and 17.3%, respectively. The pump power has not been increased to higher levels in order to avoid damage of the optical components.

 

Fig. 4 Continuous-wave output power of Nd:LuYAG microchip laser as a function of absorbed pump power and average output power as a function of the absorbed pump power for Q-switched Nd:LuYAG lasers with different Cr⁴⁺:YAG as saturable absorber.

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With a digital oscilloscope, the pulse width and repetition rate of the passively Q-switched laser were measured, which are presented in Fig. 5(a) . From this figure, it can be found that the pulse width was almost constant under all the pump powers, but decreased with the decrease of the saturable absorber initial transmission. The minimum pulse width of 9.6 ns was obtained under the absorbed pump power of 3.59 W with the saturable absorber of T₀ = 70%. With the saturable absorber initial transmission decreasing, the duration of the generated laser pulse can be further reduced, at the expense of lowering the achievable output power. As shown in Fig. 5(b), we can see that the repetition rate increased with the augmentation of the absorbed pump power and saturable absorber initial transmission under the same pump power. With the saturable absorber initial transmission of 90%, repetition rate increases linearly from 5.4 kHz to 40.0 kHz. The pulse width remains nearly constant of 17.4 ns independent of the absorbed pump power.

 

Fig. 5 (a) The pulse width versus the absorbed pump power with different saturable absorber initial transmission; (b) The pulse repetition rate versus the absorbed pump power with different saturable absorber initial transmission.

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Figure 6 shows a typical pulse profile with a width of 9.6 ns. It can be seen that the laser pulse was not symmetrically shaped and the falling edge was slower than the rising edge, implying that the transmission of output coupling used in the experiment was a bit low [14]. But if the output coupler with a high transmission is used in the Q-switched operation, the average output power would be much lower [3].

 

Fig. 6 Typical pulse profile of the Q-switched Nd:LuYAG laser with the width of 9.6 ns

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With the repetition rate and average output power, the pulse energy can be calculated. We found that the pulse energy varied in the range from 17.8 µJ to 26.5 µJ with T₀ = 90%, 29.7 µJ to 34.0 µJ with T₀ = 83%, and 57.5 µJ to 61.7 µJ with T₀ = 70%. The maximum pulse energy was 61.7 µJ under the absorbed pump power of 3.59 W with the saturable absorber of T₀ = 70%. Combining the pulse width, the highest peak power is calculated to be 6.4 kW under the same pump power. It is believed that the passively Q-switched laser output could be better if proper saturable absorbers and output coupler are used.

5. Conclusion

In conclusion, an Nd:LuYAG crystal was grown by Czochralski method. Absorption and emission spectra of Nd:LuYAG crystal at low and room temperature were investigated, and a number of material parameters relevant to laser operation were determined. The CW and passively Q-switched laser performances of Nd:LuYAG crystal at 1.06 μm were reported for the first time to our knowledge. A CW output power of 1.67 W was achieved under the absorbed pump power of 4.71 W. In passively Q-switched operation, the shortest pulse width of 9.6 ns, largest pulse energy of 61.7 µJ, and highest peak power of 6.4 kW were obtained, with Cr⁴⁺:YAG crystals as the saturable absorbers.