The continuous wave (CW) and passively Q-switched performances of Nd:CaYAlO4 crystal with both a- and c-cut were demonstrated. The CW output powers of 1.15 W and 1.26 W were obtained under the pump power of 8.96 W with slope efficiencies of 15.2% and 16.8% for a- and c-cut samples, respectively. As a result, new dual-wavelength all-solid-state lasers at 1080 nm and 1081 nm were achieved with c-cut crystal. By using Cr4+:YAG wafer as saturable absorber, we performed Q-switching experiments. The highest average output powers and shortest pulse widths were measured to be 0.798 W, 10.6 ns and 0.537 W, 9.6 ns for a- and c-cut samples, respectively.
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Recent research indicates that, the disordered crystals, such as Nd:CNGG and Nd:CLNGG, have distinguished performance in Q-switching and mode-locking regimes due to smaller stimulated emission cross section and broader emission band [1–6]. Yu et al.  demonstrated Q-switched laser performance, from a Nd:CNGG laser, with the pulse width, pulse energy and peak power of 12.9 ns, 173.16 μJ and 12.3 kW, respectively. Diode-pumped passively mode-locked Nd:CLNGG allowed the shortest pulses, up to now, ever produced with a Nd-doped crystal with the production of 900-fs pulses .
The structure of Nd:CaYAlO4 is also disordered. It crystallizes with tetragonal K2NiF4 structure. In this structure, between the AlO6 octahedral layers, the divalent Ca and the trivalent Y cations statistically occupy the ninefold co-ordinated sites which have distorted C4v symmetry and Nd3+ ions can randomly substitute Ca or Y ions . The disordered structure leads to considerable inhomogeneous broadening of the optical spectra, which permits possible generation of ultrashort pulses. Despite the promising properties of this crystal, so far only a little effort has been given to its study [8–10]. In 1989, H. R. Verdún  reported its continuous-wave (CW) performance with the maximum output power of only about 80 mW.
In this work, the diode-pumped CW and passively Q-switched laser performance of both a- and c-cut Nd:CaYAlO4 crystals were investigated. Over 1 W CW output power was obtained. By spectral analysis it has been found that the output laser has dual wavelengths with c-cut sample.
2. Spectral properties of Nd:CaYAlO4 crystal
The Nd:CaYAlO4 crystal used in the experiments was grown by traditional Czochralski method, with a Nd3+-ion doping concentration of 1 at.%. The absorption spectrum was recorded using a Lambda 900 spectrophotometer (Perkin-Elmer Company). The fluorescence spectrum was recorded by a spectrofluorometer (Fluorolog-3, Jobin Yvon, Edision, USA) equipped with a R5509-72 photomultiplier tube. An 808 nm continuous wave diode laser was used as the excitation source. All the measurements were performed at room temperature.
The absorption and emission cross section spectra are shown in Fig. 1 . From the absorption spectrum, the transitions 4 I 9/2→4 G 5/2 + 2 G 7/2, 4 I 9/2→4 F 7/2 + 4 S 3/2, and 4 I 9/2→4 F 5/2 + 2 H 9/2 around 587, 752, and 807 nm are prominent. The peak absorption cross section at 752 nm is 10.7 × 10−20 cm2, which is comparable to that at 807 nm (10.9 × 10−20 cm2). But corresponding to the emission band of AlGaAs laser diodes generally employed as the pump source for Nd3+ lasers, the absorption properties around 808 nm is most valuable. From the fluorescence spectrum, three emission bands corresponding to the 4 F 3 / 2 → 4 I 9 / 2, 4 I 11 / 2 and 4 I 13 / 2 transitions are observed at 870-940 nm, 1040-1160 nm and 1320-1450 nm, respectively. For 4 F 3 / 2 →4 I 11 / 2 transition, the peak emission cross section is 10.4 × 10−20 cm2 at 1080 nm. The FWHMs for the absorption (807 nm) and emission (1080 nm) bands are 5 and 20 nm, respectively. This is the evidence of the dominant contribution of disordered structure to the inhomogeneous broadening of the optical spectra. Broad absorption band is propitious to increase the diode-pumping efficiency, while broad emission band is favorable for ultrashort pulse laser process. So Nd:CaYAlO4 should be an excellent gain medium for a high efficient all-solid state ultrafast laser.
3. Laser experiments
Samples used in the laser experiments were cut along a and c axes with the same dimensions of 3 mm × 3 mm × 6 mm. The 3 × 3 mm2 faces were polished and anti-reflection (AR) coated at 808 nm and 1.08 μm. The CW and Q-switched laser experiments were carried out in a simple plano-plano resonator, as shown in Fig. 2 . The pump source employed in the experiments was a fiber-coupled LD with a central wavelength around 808 nm. Its output beam was focused into the laser crystal through the focusing optics (N. A. = 0.15) with a spot radius of 0.2 mm. M1 was a flat mirror with AR coated at 808 nm and 1.08 μm on the pump face and high-transmission (HR) coated at 808 nm on the other face. The output coupler (OC) M2 was also a flat mirror with 6% transmission. Two Cr4+:YAG samples were used as the saturable absorbers with the initial transmissions (T0) of 90% and 83%, respectively. Their end faces were HR coated at 1.08 µm. The laser crystal, positioned close to M1 mirror in the resonator, was wrapped in indium foil and water-cooled by a copper micro-channel heat-sink to dissipate the heat, and the cooling water was kept at 22 °C in our experiments.
4. Results and discussions
The length of the cavity was tuned to be about 50 mm, and the CW laser operation was tested first. Figure 3 shows the CW output performances of a- and c-cut Nd:CaYAlO4 crystals using the output coupler with transmission of 6%. The highest output powers of 1.15 W and 1.26 W were obtained under the pump power of 8.96 W, the thresholds were measured to be 1.43 W and 1.62 W, and the slope efficiencies were linearly fitted to be 15.2% and 16.8% for a- and c-cut samples, respectively. There was no pump saturation, which indicates the output power can be further scaled with high pump power. To avoid damage on the crystals, the pump power has been limited below 9 W. N. Mermilliod  has proved that in the CW model the pump power at threshold is inversely proportional to the product of the emission cross section and lifetime. For a disordered structure, in connection with the broader emission spectrum, smaller cross section can be expected. As shown in Fig. 1, the peak emission cross section is 10.4 × 10−20 cm2 at 1080 nm. And we measured the fluorescence lifetime to be only 129 µs. Therefore, the lasing threshold is high for Nd:CaYAlO4. With an optical spectrum analyzer (0.2 nm spectral resolution HR2000, Ocean Optics Inc.), the CW laser spectrum at threshold was recorded. The laser spectra of a- and c-cut crystals are shown in Fig. 4 . It can be observed that the a-cut laser emission is centered at 1080 nm, while c-cut laser emits at 1080 and 1081 nm. Both of them were linearly polarized. As Nd:CaYAlO4 belongs to the tetragonal system, it has two polarizations (σ and π). It can be seen the lasing properties are polarization dependent. As a result, the Nd:CaYAlO4 crystal is a new candidate for dual-wavelength all-solid state laser. And the 1.08 μm emission can be used to optically pump metastable helium atoms.
With the Cr4+:YAG absorber inserted into the cavity, stable passively Q-switched operation was achieved. Figure 5 shows the average output power of a- and c-cut crystals when the Cr4+:YAG saturable absorbers with initial transmissions of T0 = 90% and 83% were inserted into the cavity. It indicates that with the increase of Cr4+:YAG’s initial transmission, the maximum output power raises. For T0 = 90% the maximum output powers of 0.798 W and 0.537 W were obtained for a- and c- cut crystals, with thresholds of 3.07 W and 3.47 W, respectively, and the optical conversion efficiencies were 9.6% and 6.5%, while the slope efficiencies were 15.3% and 11.1%, respectively. The optical conversion efficiency can be further increased if adopting a Cr4+:YAG crystal with high initial transmission. However, the pulse width will be increased. Based on the analysis by J. J. Zayhowski , it is possible to further optimize the Q-switching performance by balancing the pulse width and output power. The pulse width and repetition rate versus the pump power are shown in Fig. 6 and Fig. 7 . It can be found that the frequency raises with the increase of pump power and using the saturable absorber with lower transmission the repetition rate was lower but the pulse width was shorter. The highest repetition rates of 40.7 KHz and 31.0 KHz were achieved for a- and c-cut crystals, respectively, using the T0 = 90% saturable absorber. As shown in Fig. 8 , the shortest pulse widths of 10.6 ns and 9.6 ns were obtained for a- and c-cut crystals, respectively, with the T0 = 83% saturable absorber. From the average output power, repetition rate and pulse width, the pulse energy and peak power can be calculated. The largest pulse energy values of 38.2 µJ and 34.4 µJ and highest peak powers of 3.4 kW and 3.2 kW were achieved for a- and c-cut crystals, respectively, using the T0 = 83% saturable absorber. It can be found that compared with a- cut sample, c-cut Nd:CaYAlO4 is optimal to be applied in high power operation with high efficiency and dual-wavelength lasing in CW mode and preferable to generate shorter pulses in Q-switching operation. And it is believed that the better passively Q-switched laser performance can be achieved if adopting proper saturable absorber and output coupler.
In conclusion, the optical properties of Nd:CaYAlO4 crystal were investigated and it has been discovered that the absorption and emission spectra are broader with FWHMs of 5 and 20 nm, respectively, due to the disordered crystal structure. Continuous wave and passively Q-switching operations have been demonstrated. 1.15 W and 1.26 W CW output powers were obtained for T = 6% under the pump power of 8.96 W with slope efficiencies of 15.2% and 16.8% for a- and c-cut samples, respectively. Dual-wavelength laser oscillation at 1080 nm and 1081 nm was achieved with c-cut crystal. By using Cr4+:YAG crystal as saturable absorber, we conducted Q-switching experiments. The highest average output powers, shortest pulse widths, largest pulse energy values and highest peak powers were measured to be 0.798 W, 10.6 ns, 38.2 µJ, 3.4 kW and 0.537 W, 9.6 ns, 34.4 µJ, 3.2 kW for a- and c-cut samples, respectively. It is believed that the better passively Q-switched laser performance can be achieved if adopting proper saturable absorber and output coupler. It can be concluded that Nd:CaYAlO4 crystal is a new candidate for dual-wavelength all-solid state laser, and the 1.08 μm emission can be used to optically pump metastable helium atoms. With broad emission bandwidth, Nd:CaYAlO4 crystal can also be a promising ultrafast laser material.
This work was supported by National Natural Science Foundation of China (No. 60938001).
References and links
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