A new passively self-Q-switched Nd, Cr:GdVO4 laser crystal was grown by Czochralski method for the first time to our knowledge. Polarized absorption spectra were measured at room temperature. The absorption bands display polarization character and an absorption band of Cr5+ ions at 1110 nm enables the crystal to be a self-Q-switched laser material at 1.06 µm. In the passive self-Q-switching operation, the maximum output power, shortest pulse width, and largest pulse energy were obtained to be 265 mW, 230 ns, and 1.12 µJ, respectively.
©2008 Optical Society of America
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  and GdVO4  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:Lu0.5Gd0.5VO4 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.
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 σGSA 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) .
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.
3. Results and discussions
The polarized absorption spectra are shown in Fig. 2. With respect to the spectra of Cr:GdVO4  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 4I9/2 to 4F9/2) 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 (4I9/2 to 4F5/2+2H9/2) is 10 nm, which is much larger than that of Nd:GdVO4 (4 nm  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  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 σGSA of Nd, Cr:GdVO4 was estimated, which is almost the same as that of Cr:GdVO4 (about 1.5×10-18 cm2) .
Figure 3 presents the variation of the average output power (Pav) 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 (Pav=122 mW, t=361 ns, and E=0.77 µJ) obtained with Cr:GdVO4 as the saturable absorber and Nd:Lu0.5Gd0.5VO4 as the laser crystal , we have achieved the much better pulsed laser operation (Pav=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:Lu0.5Gd0.5VO4). 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.
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) , 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.
This work is supported by the National Basic Research Program of China under Grand 2004CB619002.
References and links
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