1. Introduction

In recent years, researchers have paid a lot of attention on the multifunctional laser crystals, such as the self-frequency-doubling, self-Raman conversion, self-mode locked, and self-Q-switched laser crystals, because of their compactness, low loss, and simplicity in the laser design and application. In the self-Q-switching regime [1, 2], neodymium (Nd³⁺) and chromium (Cr⁴⁺) co-doped YAG crystal 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 growth and self-Q-switched laser output of Nd:Cr:GdVO₄ [3] crystal have been studied. Because Cr⁵⁺ substitutes for V⁵⁺ ions in the Cr⁵⁺ doped vanadates, no balancing charge is needed. Thus, some complexities in the crystal growth and problems in the applications could be avoided. In the self-Q-switched operation of Nd:Cr:GdVO₄ crystal, the maximum output power, shortest pulse width, and largest pulse energy were obtained to be 265 mW, 230 ns, and 1.12 μJ, respectively. The results show that Nd:Cr:GdVO₄ is a potential self-Q-switched laser material. As an isomorph of Nd:GdVO₄, Nd:YVO₄ has similar properties with and possesses much larger absorption and emission cross-sections [4]. Therefore, it can be proposed that Nd:Cr:YVO₄ should also be an excellent self-Q-switched laser material and possess much better self-Q-switched laser performance in some aspects than that of Nd:Cr:GdVO₄. In this paper, a new kind of self-Q-switched laser crystal Nd:Cr:YVO₄ is systematically studied. The polarized spectra and laser properties were reported and the results show that Nd:Cr:YVO₄ is a new potential self-Q-switched laser material.

2. Experiment details

A Nd:Cr:YVO₄ single crystal was grown by the Czochralski method under a nitrogen atmosphere containing 2% oxygen (v/v) in an iridium crucible. The as-grown Nd:Cr:YVO₄ crystal has an excellent quality (no scatter pellets can be observed under 5 mW He-Ne laser). Figure 1(a) is the photograph of the as-grown Nd:Cr:YVO₄ crystal boule grown along the a direction. Its dimensions are about Φ32×20mm². The Nd and Cr concentrations in Nd:Cr:YVO₄ crystal were measured to be 0.79 at.% and 1.40 at.% by the X-ray fluorescence method, respectively.

 

Fig. 1 (a) As-grown crystal Nd:Cr:YVO₄ boule; (b) XRPD patterns of the Nd:Cr:YVO₄ and standard data.

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Several samples with different sizes were prepared from this crystal boule for the experiments. The structure of the as-grown crystal was studied by X-ray powder diffraction (XRPD), and results are shown in Fig. 1(b). The as-grown crystal consists of an YVO₄ single phase belonging to the I4₁/amd space group. According to the peak 2θ values in the XRPD pattern, the unit cell parameters are calculated to be a = b = 7.1211Å, c = 6.2904Å. These results are in good agreement with the data given in the JCPDS for YVO₄ (a = b = 7.1209Å, c = 6.2902Å).

The rocking curves were measured along the c-axis by using the high resolution X-ray diffraction method to test the lattice integrity of the as-grown crystals. The result is shown in the upper right of Fig. 1(b), the full width at half-maximum (FWHM) values is only 14.4”. The diffraction peaks show good symmetry without any splitting, which indicates that the Nd:Cr:YVO₄ single crystal has highly excellent quality and is suitable for laser applications. In order to investigate the influence of annealing, half of the crystal was cut and annealed. Several samples were processed from the annealed crystal.

The polarized absorption spectra of Nd:Cr:YVO₄ crystal were measured at room temperature with two samples before and after annealing. The experimental instrument used in the experiments was a V-570 JASCO ultraviolet/visible/near infrared (UV/Vis/NIR) spectrophotometer. The incident light was perpendicular to the a-axis of the crystal during the absorption spectra measurement.

The self-Q-switched laser experimental setup is based on a plano-plano resonator with the length of about 20 mm. The pump source used in the experiment was a fiber-coupled LD (OPC-D030-FCHS, Opto Power Corporation) with the emission wavelength centered at 808 nm. The output beam of the LD was focused into the Nd:Cr:YVO₄ crystal sample with a spot radius of about 0.256 mm and a numerical aperture of 0.22 achieved by using a focusing system. The pump mirror M1 was a plano mirror, antireflection (AR) coated at 808 nm on the pump face, high reflectance (HR) coated at 1.06 μm and high-transmittance (HT) coated at 808 nm on the other face. The flat mirrors M2 were output couplers with different transmissions of OC = 40% and 10% 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:YVO₄ under high pump power levels, the crystal sample 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

3.1 Spectral properties

Figure 2(a) shows the polarized absorption spectra of the Nd:Cr:YVO₄ crystal before annealing. The Cr⁵⁺ ions substitute for tetrahedral coordinated V⁵⁺ in the YVO₄ which has a zircon structure. In the zircon structure, the tetrahedron is tetragonally elongated along the c-axis so that the CrO₄^3- site symmetry is D_2d [5, 6]. From Fig. 2(a), it can be seen that the absorption spectra of the crystal showed polarized absorption character, the absorption for π polarized direction is stronger than the σ polarized direction. There is a broad absorption band near the 1100 nm for π polarization, which enabled the crystal to be a self-Q-switched laser matrix. Its absorption coefficient is 2.268 cm⁻¹ and the full width at half maximum (FWHM) of about 240 nm. Combining with the energy level diagram of Cr⁵⁺ (3d¹) ion, we can conclude that the absorption band near 1100 nm is due to the ²A₁→ ²B₂ [7] transition which is electric dipole allowed only for E // c (π) polarization. Additionally, there has a strong absorption peak centered at 808 nm with FWHM of 11 nm almost similar to that of Nd:Cr:GdVO₄ [3], which is much larger than that of Nd:YVO₄ (2 nm) [8]. It can be believed that the wider absorption line of Nd³⁺ ions is attributed 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:YVO₄) neighboring the Nd ions [3]. This broad bandwidth means that Nd:Cr:YVO₄ is more suitable for diode pumping.

 

Fig. 2 Polarized absorption spectra of Nd:Cr:YVO₄ crystal before (a) and after (b) annealing.

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Because Nd:Cr:YVO₄ was grown in the atmosphere with little O₂, parts of Cr and V ions with lower valences existed in the crystal. Therefore, annealing can induce the Cr and V ions to become those with high valences [9]. The sample that cut from the annealed crystal was used to measure the polarized spectra for comparison with the sample before annealing. Figure 2(b) shows the polarized absorption spectra of the Nd:Cr:YVO₄ crystal after annealing. It can be seen that the absorption intensity of the wide band centered at 1100nm for π polarization increased, and its absorption coefficient became 3.22 cm⁻¹, larger than 2.27 cm⁻¹ (before annealing). The results showed great affect of annealing on the saturable absorbing of Nd:Cr:YVO₄, which will be discussed in the laser experiments.

3.2 Self-Q-switched laser performance

Using the experimental setup mentioned above, the samples before and after annealing were designed for the self-Q-switched laser performance. The samples were cut to the dimensions 3 × 3 × 7 mm³ and the 3 × 3 mm² faces perpendicular to its a direction were polished. Figure. 3(a) presents the variation of the average output power (P_av) with the increase of incident pump power. The sample used here was not annealed, its dimensions is 3 × 3 × 7 mm³. The pumping threshold of Nd:Cr:YVO₄ crystal is 1.35 W, and the maximum output power of 196 mW was obtained with OC = 40%, at the incident pump power of 5.21 W. At the maximum pump power the Q-switched pulse of 319 ns pulse width and the repetition rate of 344.8 kHz were obtained. The energy of single pulse was about 0.57 μJ. By the pulse energy and width, the peak power can be calculated. The highest peak power of 1.67 W was achieved with OC = 40%. The pulse train with 344.8 kHz is shown in Fig. 3(b). The inset of this figure presents the pulse profile with a pulse width of 319 ns. Based on the analysis on the passive Q-switching [10, 11], the wide pulse width and small pulse energy are induced by the small modulation depth of the saturable absorber, and it can be proposed that with much larger modulation depth, the laser performance can be improved.

 

Fig. 3 (a) Variation of the output power versus incident pump power with OC = 40% before annealing;(b) Pulse train with the repetition rate of 344.8 kHz.

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With a annealed Nd:Cr:YVO₄ crystal sample, the self-Q-switched laser operation was achieved. Figure. 4(a) presents the average output powers (P_av) under various incident powers with OC = 40%. The results show that the maximum average output power (P_av) of 120 mW was achieved with the incident pump power of 6.23 W. The threshold of the pulsed laser was measured to be 3.94 W much higher than that with un-annealed one, which is generated by the larger saturable absorption in the Nd:Cr:YVO₄ due to its larger absorption of Cr ions. At the maximum pump power, the Q-switched pulse with the width of 173.5 ns and repetition rate of 151.4 kHz were obtained. The energy of single pulses was 0.79 μJ. Using the pulse energy and width, the peak power was calculated to be 4.57 W. The pulse train with 151.4 kHz is shown in Fig. 4(b). The inset of this figure presents the pulse profile with a pulse width of 173.5 ns. Compared with the un-annealed crystal, it can be concluded that the annealed crystal possesses much more Cr⁵⁺ ions and larger modulation depth which is favorable for a self-Q-switched laser.

 

Fig. 4 (a) Variation of the output power versus incident pump power with OC = 40% after annealing; (b) Pulse train with the repetition rate of 151.4 kHz.

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The self-Q-switched laser operations with OC = 10% were also achieved. The experimental results are shown in Fig. 5(a) and Fig. 5(b). Figure 5(a) presents the variation of the average output power (P_av) with the increase of incident pump power. The pumping threshold is 1.53 W and the maximum output power of 103 mW was obtained at the incident pump power of 3.47 W. At the maximum pump power, the Q-switched pulse with width of 85.8 ns and the repetition rate of 293 kHz were obtained. The energy of single pulse was about 0.35 μJ much smaller than that with OC = 40% shown above. By the pulse energy and width, the peak power was calculated to be 4.09 W. The pulse train with 293 kHz is shown in Fig. 5(b). The inset of this figure presents the pulse profile with a pulse width of 85.8 ns. Comparing to the previous result (P_av = 22 mW, t = 600 ns, and E = 0.1 μJ) obtained with Cr:YVO₄ as a saturable absorber and Yb:YVO₄ as a laser crystal, we have achieved the much better pulsed laser operation (P_av = 120 mW, t = 85.8 ns, and E = 0.79 μJ) with Nd and Cr co-doped YVO₄. The results show that the energy is a little smaller than that of Nd:Cr:GdVO₄ (1.12 μJ). However, the pulse width of Nd:Cr:YVO₄ is much narrower than that of Nd:Cr:GdVO₄ (230 ns) [3], which identified that the Nd:Cr:YVO₄ crystal is also a potential self-Q-switched laser material. It can be believed that the results can be improved if the Nd:Cr:YVO₄ crystal is AR coated at 1.06 μm to reduce intracavity reflection loss and the Cr concentrations in the crystal is increased.

 

Fig. 5 (a) Variation of the output power versus incident pump power with OC = 10% after annealing; (b) Pulse train with the repetition rate of 293 kHz.

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4. Conclusion

The high-quality Nd³⁺ and Cr⁵⁺ co-doped YVO₄ crystal was grown by the Czochralski method. The space group and effective segregation coefficient of Nd³⁺ and Cr⁵⁺ are determined. Polarized absorption spectra of the crystal were measured at room temperature and show that Nd:Cr:YVO₄ has the polarized absorbing property. The absorption band of Cr⁵⁺ ions at 1110 nm, with FWHM of about 240 nm, is only electric dipole allowed for π polarization and enables the crystal to be a self-Q-switched laser material at 1.06 μm. With the Nd:Cr:YVO₄ crystal, the self-Q-switched laser performance was demonstrated. The maximum output power, shortest pulse width, and largest pulse energy were obtained to be 120 mW, 85.8 ns, and 0.79 μJ, respectively. Therefore, all the results and discussions show that Nd:Cr:YVO₄ is a new potential self-Q-switched laser material.