1. Introduction

In recent years, multifunctional laser crystals, such as the self-frequency-doubling, self-Raman conversion, self-mode locked, and self-Q-switched laser crystals, have been paid a lot of attention, because of their compactness, low loss, and simplicity in the laser design and application. In the self-Q-switching regime [1-6], neodymium (Nd³⁺) and chromium (Cr⁴⁺) co-doped YAG crystals has been investigated in detail and identified to be an excellent self-Q-switched laser material. However, due to the substitution of Cr⁴⁺ ions for a fraction of Al³⁺ ions at Al-ion sites in this crystal, Ca²⁺ or Mg²⁺ ions should also be co-doped to keep the balance of the charge, which brings some complexities in the crystal growth and problems in the applications. So it is significant to search for new self-Q-switched laser materials. Recently, the polarized absorption spectra of Cr⁵⁺ doped YVO₄ [7] and GdVO₄ [8] have been studied and found that the Cr⁵⁺ doped vanadates have polarized absorption properties and their π polarized (E//c) absorption bands located at 900 nm to 1300 nm can be used as a passive Q-switch for about 1 µm lasers. Because Cr⁵⁺ substitute for V⁵⁺ ions in those Cr⁵⁺ doped vanadates, no balancing charge is needed. With Cr:YVO₄ and Cr:GdVO₄ as saturable absorbers, passively Q-switched laser performance has been demonstrated at 1.02 and 1.06 µm by using Yb:YVO₄ and Nd:Lu_0.5Gd_0.5VO₄ as laser crystals, respectively. Previous laser experiments have proved that the Nd doped vanadate family of laser materials has excellent properties [9-12]. Among the vanadates, Nd:GdVO₄[10, 11] is the one with wonderful thermal properties and has been widely studied. With respect to Nd:YAG, Nd:GdVO₄ has the broader absorption bandwidth, larger absorption and emission cross-sections, polarized emission, and comparable thermal conductivity. It can be proposed that Nd³⁺ and Cr⁵⁺ co-doped GdVO₄ should be a promising passively self-Q-switched laser material. In this letter, we report the polarized absorption and passively self-Q-switched laser properties of Nd, Cr:GdVO₄. The results show that Nd, Cr:GdVO₄ is a new potential self-Q-switched laser material.

2. Experiments

By the Czochralski method, the Nd, Cr:GdVO₄ crystal was grown from the melts of mixed polycrystalline material NdVO₄, GdVO₄ and GdCrO4 under a nitrogen atmosphere containing 2% oxygen (v/v) in an iridium crucible. The growing process was the same as that of Nd:GdVO₄. Figure 1 shows the as-grown Nd, Cr:GdVO₄ crystal boule with dimensions of about Φ30 mm×20 mm. Observed under He-Ne laser, no light scattering is obtained, which means that the as-grown crystal has excellent quality and is suitable for the application in the laser experiment. Both of the Nd and Cr concentrations in Nd, Cr:GdVO₄ crystal were measured to be 1 at.% by the X-ray fluorescence method. The crystal was cut along its a-axis with dimensions of 3×3×4 mm³, and its two 3×3 mm² end-faces were polished.

The polarized absorption spectra of Nd, Cr:GdVO₄ were measured with Hitachi U-3500 spectrophotometer in the wavelength of 400 nm-1500 nm at room temperature. The incident light was perpendicular to the face of the crystal during the absorption spectra measurement. The ground absorption cross-section σ_GSA of Nd, Cr:GdVO₄ at 1.06 µm was estimated by using the mode-locked Nd:YAG laser 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) [7].

The pulsed laser experimental setup is based on a plano-concave resonator with the length of about 30 mm. The pump source employed in the experiment was a fiber-coupled laser-diode with the central wavelength around 808 nm. Through the focusing optics (N. A.= 0.22), the output of the source was focused into the crystal with a spot radius of 0.256 mm. The pump mirror M1 was a concave mirror with a curvature radius of 500 mm, AR coated 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 flat mirrors M2 were output couplers with different transmissions of OC=40% and 30% at 1.06 µm. The average output powers were measured by the power meter (EPM 2000. Melectron Inc.). To remove the heat generated from Nd, Cr:GdVO₄ under high pump power levels, the crystal was wrapped with indium foil and mounted in a water-cooled copper block with cooling water controlled to be 18 °C.

 

Fig. 1. As-grown crystal Nd, Cr:GdVO₄ boule.

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Fig. 2. Polarized absorption spectra of Nd:Cr:GdVO₄ at room temperature.

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3. Results and discussions

The polarized absorption spectra are shown in Fig. 2. With respect to the spectra of Cr:GdVO₄ [8] and Nd:GdVO₄ [10, 11], it can be noted that the spectra of Nd, Cr:GdVO₄ are the combination of those of Cr:GdVO₄ and Nd:GdVO₄. Due to the strong absorption of Nd³⁺ ions at about 680 nm (transmission of ⁴I_9/2 to ⁴F_9/2) for π polarization, that of Cr⁵⁺ ions at 642 nm (²A₁ to ²E), which is only allowed for σ polarization (E⊥c), is not evident. The inset of Fig. 2 shows the absorption spectra in the wavelength from 780 nm to 900 nm. The full width at half-maximum (FWHM) at 808 nm (⁴I_9/2 to ⁴F_5/2+²H_9/2) is 10 nm, which is much larger than that of Nd:GdVO₄ (4 nm [11] or 1.6 nm [10, 12]). It can be believed that the wider absorption line of Nd³⁺ ions is ascribed to the inhomogeneous broadening which is caused by the variation in the crystal field experienced by the Nd ions due to the random distribution of Cr and V ions at the V-ion sites (with respect to the pure Nd:GdVO₄) neighboring the Nd ions. The absorption band of Cr⁵⁺ ions at 1110 nm, with FWHM of about 250 nm, is generated by the transmission of ²A₁ to ²B₂ [8] which is electric dipole allowed only for p polarization, and is not overlapped with that of Nd³⁺ ions at 808 nm. From the spectra, it can be proposed that Nd, Cr:GdVO₄ should be a potential highly-efficient self-Q-switched laser material at 1.06 µm. Using the same measuring method as the ground absorption cross-section of Cr:GdVO₄, the σ_GSA of Nd, Cr:GdVO₄ was estimated, which is almost the same as that of Cr:GdVO₄ (about 1.5×10^-18 cm2) [8].

 

Fig. 3. Variation of the output power versus incident pump power.

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Fig. 4. Variations of PRF and pulse width versus incident pump power.

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Fig. 5. Variation of pulse energy versus incident power.

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Fig. 6. Pulse train with the repetition rate of 60.2 kHz

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Figure 3 presents the variation of the average output power (P_av) with the increase of incident pump power. The maximum output powers of 243 and 265 mW were obtained with OC=40% and 30%, respectively, at the incident pump power of 3.2 W, with the respective corresponding optical conversions of 7.6% and 8.3%. The thresholds were achieved to be 1.87 and 1.43 W, and the slope efficiencies were 18.3% and 15.3%, with OC=40% and 30%, respectively.

The pulse repetition frequency (PRF), pulse width (t), and pulse energy (E) versus incident pump power are shown in Figs. 4 and 5, respectively. The PRF increased from 60.2 kHz to 250 kHz and 59 to 366 kHz with the increase of incident pump power by using OC=40% and 30%, respectively. When the incident pump power was increased to be larger than 3.2 W, the laser output became almost continuous-wave (cw), which is due to the small modulation depth of the Nd, Cr:GdVO₄ crystal at 1.06 µm and can be overcome by increasing the Cr⁵⁺ concentrations in the crystal. We believe that, for a low Cr-doped crystal which has the small modulation depth, it is easy to be bleached by the intracavity power. Thus the saturable absorber acts as a linear absorption and the laser operates in cw rather than Q-switching mode, when the intracavity power becomes high. The minimum pulse widths of 230 and 362 ns were obtained with OC=40% and 30%, respectively, under the incident pump power of 2.92 W. The maximum pulse energies were gotten to be 1.12 µJ and 0.9 µJ with the two output couplers, respectively, under the respective corresponding incident pump powers of 2.6 W and 2.33 W. By the pulse energy and width, the peak power can be calculated. The highest peak power of 3.82 W was achieved with OC=40%. The pulse train with 60.2 kHz is shown in Fig. 6. The inset of this figure presents the pulse profile with a pulse width of 230 ns.

Comparing to the previous result (P_av=122 mW, t=361 ns, and E=0.77 µJ) obtained with Cr:GdVO₄ as the saturable absorber and Nd:Lu_0.5Gd_0.5VO₄ as the laser crystal [8], we have achieved the much better pulsed laser operation (P_av=265 mW, t=230 ns, and E=1.12 µJ) with Nd and Cr co-doped GdVO₄. It is believed that the better Q-switched laser performance is generated by lower surface reflection loss of only one crystal (Nd, Cr:GdVO₄) than that of two-crystal configuration (Cr:GdVO₄ and Nd:Lu_0.5Gd_0.5VO₄). Because Nd, Cr: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 believed that the purity of the crystal and the pulsed laser performance can be improved if Nd, Cr:GdVO₄ is annealed. The results can also be better if the Nd, Cr:GdVO₄ crystal is AR coated at 1.06 µm to reduce intracavity reflection loss and the Cr concentrations in the crystal is increased.

4. Conclusion

In conclusion, the Nd³⁺ and Cr⁵⁺ co-doped GdVO₄ crystal was grown by the Czochralski method for the first time to our knowledge. Polarized absorption spectra of the crystal were measured at room temperature and show that Nd, Cr:GdVO₄ has the polarized absorption property. The absorption band of Cr⁵⁺ ions at 1110 nm, with FWHM of about 250 nm, is only electric dipole allowed for π polarization and enables the crystal to be a passively self-Q-switched laser material at 1.06 µm. With the Nd, Cr:GdVO₄ crystal, the self-Q-switched laser performance was demonstrated. The maximum output power, shortest pulse width, and largest pulse energy were obtained to be 265 mW, 230 ns, and 1.12 µJ, respectively, which are better than the previous results with Cr:YVO₄ and Cr:GdVO₄ as saturable absorbers. The laser performance can be better if the crystal is annealed and AR coated at 1.06 µm, and Cr concentrations in the crystal is increased. If the relaxation time of the excited state ²B₂ of Cr⁵⁺ ions in the Nd:GdVO₄ environment, which will be measured in the future, is comparable with that of Cr⁵⁺:YVO₄ (3.5±1.5 ns) [7], this crystal should also have promising applications used as a self-mode locked laser material. All the results and discussions show that Nd, Cr:GdVO₄ is a new potential passively self-Q-switched laser material.