1. Introduction

Passive Q-switching of lasers has resulted in many practical applications in areas such as industry, medicine, military applications, and basic scientific research, where compact, reliable, and cost-effective nanosecond-pulsed lasers are required. A saturable absorber is one of the most important parts of a pulsed laser. Nowadays, the crystals doped with 3d^N-ions in the arrangement of tetrahedral symmetry have found potential applications as saturable absorbers for near- and mid-infrared lasers [1]. With the electronic configuration of 3d¹, the Cr⁵⁺ ion with a vanadate crystal as the host material should also have broad absorption in this waveband. The Cr⁵⁺-doped YVO₄ crystal has been studied and has proved to be a suitable passive saturable absorber Q-switch laser of about 1 µm [2]. As an isomorph of YVO₄, GdVO₄ has similar properties with that of YVO₄ and excellent thermal properties [3, 4], and it can be proposed that Cr⁵⁺:GdVO₄ should also be a promising material as a saturable absorber. In this study, we report the polarized absorption spectra of a new Cr⁵⁺:GdVO₄ crystal and the passive Q-switching laser performance demonstrated with Nd:Lu_0.5Gd_0.5VO₄ as the laser crystal and Cr⁵⁺:GdVO₄ as the saturable absorber.

 

Fig. 1. Schematic diagram of experimental laser setup.

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2. Experiments

A Cr⁵⁺:GdVO₄ crystal was grown along its a-axis by the Czochralski method in a nitrogen atmosphere containing 2% oxygen (v/v) in an iridium crucible. The Cr concentration was 1% in the GdCrO₄ and GdVO₄ polycrystalline materials. The growing process is same as that for Nd:GdVO₄. The crystals were cut along the a-axis with dimensions of 3×3×0.5 mm, 3×3×1 mm, 3×3×1.6 mm, and 3×3×3.2 mm (a×c×a). All of the 3×3 mm faces were polished.

The polarized absorption spectra of Cr⁵⁺:GdVO₄ were measured with a Hitachi U-3500 spectrophotometer in the wavelength of 200~2000 nm at room temperature and with a crystal 3.2 mm long. The incident light was perpendicular to the wafers in the absorption spectra measurements. The ground absorption cross-section σ_GSA of Cr⁵⁺:GdVO₄ at 1.06 µm was obtained by using the mode-locked Nd:YAG laser (PY61 type) with a repetition rate of 10 Hz and a pulse duration of 40 ps, which is very short compared to the Cr5+ excited-state lifetime (about 3.5 ns) [2], and with Cr⁵⁺:GdVO₄ 3.2 mm long. The polarized direction of the laser was parallel to the c-axis of Cr⁵⁺:GdVO₄. The pulsed-laser operation was demonstrated in a plano-concave resonator shown in Fig. 1. The pump source employed in the experiment was a fiber-coupled laser diode with a central wavelength around 808 nm. Through the focusing optics (N. A.=0.22), the output of the source was put into the laser crystal with a spot radius of 0.256 mm. M1 was a concave mirror with a curvature radius of 200 mm. It was antireflection- coated (AR) at 808 nm on the flat face, high-reflection coated at 1.06 µm, and high-transmission coated at 808 nm on the concave face. The laser crystal was a Nd-doped Lu_0.5Gd_0.5VO₄ crystal with a Nd concentration of 0.5% and dimensions of 3×3×6 mm (a×c×a). The two faces were also polished and AR-coated at 808 nm and 1.06 µm. The output coupler (OC) M2 was a flat mirror. Based on the analysis of Degnan [5] and X. Zhang [6], greater pulse energy can be obtained by using the OC with larger transmission at an oscillating wavelength.

 

Fig. 2. Polarized absorption spectra of Cr⁵⁺:GdVO₄. The inset shows the energy level diagram with the selection rules for Cr⁵⁺ (3d¹ ion) in a tetrahedral crystal field of symmetry D_2d comparing with that shown in Ref. [8].

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Fig. 3. Output power versus the increase of incident pump power.

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An OC with transmission at 1.06 µm at 40% was used in the pulsed-laser experiment. Both the laser crystal and the saturable absorber were held in the water-cooled copper block with cooling water at 18°C. The laser crystal was placed close to M1, and Cr⁵⁺:GdVO₄ was put near M2 with its c-axis parallel to that of the mixed crystal. The length of the cavity was tuned to be as short as 30 mm. The laser output powers were measured by a power meter (EPM 2000. Melectron Inc.). Temporal behaviors of the Q-switched laser were recorded by a TED620B digital oscilloscope (500-MHZ bandwidth and 2.5-Gs/s sampling rate, Tektronix Inc.).

 

Fig. 4. Pulse Repetition Frequency (PRF) versus the incident pump power.

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Fig. 5. Pulse width versus the incident pump power.

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3. Results and discussion

Polarized absorption spectra at room temperature were shown in Fig. 2. Similar to Cr⁵⁺:YVO₄ [2, 7], Cr⁵⁺ ions substitute for tetrahedrally coordinated V⁵⁺ in the GdVO₄, and the site symmetry of CrO₄ ^3- is D_2d. The energy level diagram was shown in the inset of Fig. 2 compared with that for the Cr⁵⁺ ion in the YVO₄ [2, 8]. The absorption band at 1110 nm is generated by transmission of ²A₁ to ²B₂, which is the only electric dipole allowed for π polarization (E//c), and the band at 642 nm is due to that of ²A₁ to ²E, which is only allowed for σ polarization (E⊥c). The crossed-out arrows in the inset of Fig. 2 mean that the transmission cannot be realized owing to the selection rules [8]. Just like the polarized absorption property of Cr⁵⁺:YVO₄, the absorption band at 1110 nm should have potential application as a saturable absorber for lasers at about 1µm. The transmission of the crystal at 1.06 µm was measured to be 56.6%.

 

Fig. 6. Pulse energy versus incident pump power.

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Fig. 7. Pulse train with repetition rate of 131.6 kHz.

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Based on the model for the saturable absorber developed by Y. Shimony et al. [9], the σ_GSA of (1.5±0.5)×10^-18 cm² was obtained, which is comparable with that of Cr⁵⁺:YVO₄ (about 2.0×10^-18 cm²) [2]. The excited-state absorption cross-section σ_ESA, which is much smaller than σ_GSA (σ_ESA/σ_GSA<0.05) [2], was not considered during this calculation.

The continuous-wave (cw) and passive Q-switching laser performance of Nd:Lu_0.5Gd_0.5VO₄ has been studied with Cr⁴⁺:YAG as a saturable absorber in Ref. [10] in the same resonator, and it was found that its output energy is greater than that obtained with Nd:GdVO₄. The cw characterization was shown in Fig. 3(a) with the optimized OC at 14%. At the incident pump power of 3.79 W, the cw output power of 1.92 W was obtained, giving optical conversion of 50.6% with a threshold of 0.33 W. The Q-switched laser experiments were processed with four kinds of saturable absorbers. By using saturable absorbers with a length of 0.5, 1, and 1.6 mm, the output lasers were almost a cw when the pump power was larger than the threshold. Owing to its largest change in nonlinear absorption, the best pulsed results were obtained by using the saturable absorber with a length of 3.2 mm. The average pulsed laser output operation was shown in Fig. 3(b) by using the Cr⁵⁺:GdVO₄ with a length of 3.2 mm and OC of 40%. At the same incident pump power, the average output power (P_av) of 122 mW was achieved with a slope efficiency of 11.3%. The threshold of the pulsed laser was measured to be 2.77 W. The pulse repetition frequency (PRF), pulse width (t), and pulse energy (E) versus the incident pump power were shown in Figs. 4, 5, and 6, respectively, by using the saturable absorber with a length of 3.2 mm. It can be found that the PRF rose with the increase of the pump power. The maximum PRF was found to be 222.7 kHz at the incident pump power of 3.79 W. The minimum pulse width of 361 ns and maximum pulse energy of 0.77 µJ were obtained at the incident pump power of 3.5 W with a corresponding PRF of 131.6 kHz. The pulse train with 131.6 kHz is shown in Fig. 7. The inset of this figure presents the pulse profile with a pulse width of 361 ns.

Because Cr⁵⁺:GdVO₄ was grown in the atmosphere with little O₂, parts of Cr and V ions with lower valences existed in the crystal. It can be proposed that the purity of the crystal can be improved and the pulsed-laser results may be better if Cr⁵⁺:GdVO₄ was annealed. The results can also be improved if the saturable absorber was AR-coated at 1.06 µm to reduce intracavity reflection loss.

Comparing the pulsed-laser performance (P_av=22 mW, t=600 ns, and E=0.1 µJ) [2] with Cr⁵⁺:YVO₄ as a saturable absorber, we have demonstrated a better result (P_av=122 mW, t=361 ns, and E=0.77 µJ) with Cr⁵⁺:GdVO₄. Cr⁵⁺:GdVO₄ should also be a promising material for use as a passive shutter for mode-locked solid-state lasers if its relaxation time, which will be measured in the future, is comparable to that of Cr⁵⁺:YVO₄ (3.5±1.5 ns) [2].

4. Conclusion

The polarized-absorption spectra of the new Cr⁵⁺:GdVO₄ crystal were measured at room temperature. The passive Q-switching laser was demonstrated with Nd:Lu_0.5Gd_0.5VO₄ as a laser crystal and Cr⁵⁺:GdVO₄ as a saturable absorber. Compared with the previous results with Cr⁵⁺:YVO₄, we have obtained a better pulsed laser performance with Cr⁵⁺:GdVO₄. It was also proposed that the result should be better if the Cr⁵⁺-doped crystal was annealed and AR-coated at 1.06 µm.