1. Introduction

Tetragonal sodium double tungstates, represented by NaT(WO₄)₂ (T = Gd, Y, La, Lu, etc.), are usually referred to as disordered crystals in that Na⁺ and T³⁺ are randomly distributed at the same cationic lattice sites. Capable of accommodating active trivalent rare earth ions, these disordered tungstates are potential hosts for Nd³⁺, Yb³⁺, Tm³⁺, etc. Since 2004, Yb-doped NaT(WO₄)₂ crystals have started to attract more and more attention [1–6], the hosts studied include NaGd(WO₄)₂ [1–4], NaLu(WO₄)₂ [5], and NaLa(WO₄)₂ [6]. Due to their disordered character, the emission spectrum of Yb ions in these crystals is inhomogeneously broadened, which is favorable for generating femtosecond pulses by mode-locking techniques. On the other hand, comparing with their monoclinic potassium double tungstate counterparts with the chemical formula KT(WO₄)₂, the sodium tungstates are considered to be more promising for power scaling due to the uniaxial nature which facilitates processing of thin disks and in general exhibits less anisotropy of the thermo-mechanical properties [2]. Weak anisotropy in thermal expansion has been confirmed in sodium tungstates compared with potassium tungstates [7], this is advantageous for reducing the risk of thermal fracture in high-power applications. It has also been indicated, however, that the thermal conductivities of sodium tungstates are only about one third of those for potassium tungstates [7]. As a consequence, efficient cooling is essential for power scaling with these crystals.

Among the Yb-doped sodium double tungstates, Yb:NaGd(WO₄)₂ (Yb:NaGdW) was the first with which laser action was realized at room temperature [1]. So far, it has received more attention and proved to be the most promising in generating continuous-wave (cw) and pulsed laser output power [2–4]. Another member of the family of sodium tungstates, NaY(WO₄)₂ (NaYW), which, like NaGdW, can be grown in large sizes by the Czochralski method, is also interesting to be studied as a potential host for the Yb ion because doping is expected to introduce much weaker crystal distortion in this case due to the closer ionic radii of Y and Yb. However, much less efforts were devoted to the Yb:NaYW crystal. Only very recently, was laser operation achieved with Yb:NaYW by Ti: sapphire laser pumping, producing a cw output power of 0.46 W, and an average power of 0.09 W in the mode-locked regime with a pulse duration of 53 fs [8].

In this paper, we report, for the first time to our knowledge, the laser performance of Yb:NaYW crystal end pumped by a high-power diode. Employing a simple plano-concave resonator, efficient cw laser oscillation was obtained at room temperature; with a Cr⁴⁺:YAG crystal used as a saturable absorber, passively Q-switched operation was also demonstrated.

2. Experimental laser setup

The resonator used to construct the Yb:NaYW laser was a plano-concave one. The plane mirror was coated highly reflecting for 1015–1230 nm (> 99.8%) and highly transmitting for 880–990 nm (> 97%). As the output coupler, several concave mirrors of radius-of-curvature of 50 mm were used, with output transmission in the range of T = 0.5%–10%. The uncoated Yb:NaYW crystal was a-cut, 3 mm thick with an aperture of 3.3 mm × 3.3 mm. The Yb concentration in the crystal was measured to be 4.8 at. %. The crystal was held in a copper block maintaining a temperature of 12 °C by cooling water, and was positioned close to the plane mirror in the resonator. A 0.3 mm thick Cr⁴⁺:YAG crystal cut along [111] direction, having an initial transmission of T ₀ = 98% (measured at 1.06 μm), was used as a saturable absorber for passive Q-switching. It was antireflection (AR) coated for 1.06 μm on both end faces. The pump source used was a 50 W fiber-coupled diode (S50-980-2, Apollo Instruments, fiber core diameter of 200 μm and NA of 0.22) emitting infrared radiation at 974–981 nm depending on the output level. Its output beam was focused by a 1:1 reimaging unit and delivered onto the Yb:NaYW crystal through the plane mirror. During the experiment, no measures were taken to tune the emitting wavelength of the diode to coincide with the absorption peak of the Yb:NaYW which is located at 975 nm [8]. The small-signal absorption of the 3 mm thick Yb:NaYW crystal for the unpolarized pump light at ∼974 nm was measured to be ∼0.7, corresponding to an absorption coefficient of ∼4.0 cm^-1. This value is lower than the calculated one using the polarized absorption cross sections [8], which amounts to ∼5.3–6.5 cm^-1 for 972–978 nm.

3. Results and discussion

In the cw regime, the Yb:NaYW laser was optimized by adjusting the cavity length to be ∼ 49 mm, making the resonator in a near hemispherical configuration. Figure 1 shows the relations between the output power and the absorbed pump power (P _abs), achieved at room temperature by using different output couplings. The laser threshold was reached at P _abs = 0.68, 1.20, and 1.74 W in the cases of T = 0.5%, 2%, and 5%, respectively. In terms of the generated output power and efficiency, the output coupler of T = 2% was the optimal; a slope efficiency of 50% was determined for P _abs < ∼9.0 W. Above this pump level, the laser operation became less efficient, due to a thermal rollover and thus thermally enhanced reabsorption losses. The highest output power generated by the laser was 5.2 W at an emission wavelength of 1045 nm, reached at P _abs = 14.1 W, resulting in an optical-to-optical efficiency of 37%. The maximum incident pump power was 21.7 W, and the fraction of the pump power absorbed in the first pass through the crystal was determined to be 0.63 (the pump power absorbed in the second pass was ∼3% of that in the first one). The polarization state of the laser beam was examined by use of a polarizer, indicating a linear polarization along the c axis (π polarization), which was independent of both the output coupling used and the power level. These results are in good accordance with the gain cross section calculations in Ref [8].

 

Fig. 1. Output power of the Yb:NaYW laser versus absorbed pump power for different output couplings when operated in the cw regime.

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The laser efficiency obtained with the Yb:NaYW was considerably lower compared to that for Yb:NaGdW [4], this is unexpected considering the fact that the emission cross section is even greater for Yb:NaYW than for Yb:NaGdW [8]. The main reason is probably due to the Yb concentration and thickness of the Yb:NaYW crystal used which could be far from optimum.

Passively Q-switched operation was obtained by inserting the Cr⁴⁺:YAG crystal between the laser crystal and the output coupler. The cavity length in this case was shortened to ∼22 mm to take advantage of a short cavity lifetime. To avoid possible damage to the intracavity elements, a larger output coupling of T = 10% was utilized.

Figure 2 shows the Q-switched average output power as a function of P _abs, for comparison, the cw output power, generated with the Cr⁴⁺:YAG crystal removed from the cavity, is also presented. The passive Q-switching laser action reached the threshold at P _abs = 4.3 W. Above the threshold, the average output power increased approximately linearly with P _abs, giving a slope efficiency of 35%. At P _abs = 10.1 W, the Q-switched laser produced an average output power of 2.0 W at 1027 nm, the optical-to-optical efficiency was about 20%.

 

Fig. 2. Q-switched and cw output power as a function of absorbed pump power for T = 10%.

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For the same pump power, the cw output power reached 2.9 W at 1031 nm. The fraction converted to Q-switched output was ∼ 70%. By employing a Cr⁴⁺:YAG crystal of higher initial transmission (T ₀ > 98%), the fraction of conversion might be increased, yielding more average output power, at the expense of lengthening the laser pulse duration. The emission wavelength shift from 1031 nm in the cw operation to 1027 nm in the Q-switched operation was due to the additional losses introduced by the Cr⁴⁺:YAG absorber, just as in the case of Yb:NaGdW laser [4]. The emission line of both the cw and Q-switched operations consisted of several longitudinal modes.

In passively Q-switched operation, the pulse repetition frequency (PRF) is dependent on the operational power level. For the present Q-switched Yb:NaYW laser, the PRF was found to vary approximately linearly with Pabs, increasing from 2.8 kHz (P _abs = 5.4 W) to 13.9 kHz (P _abs = 10.1 W). From the average output power and the corresponding PRF, one can calculate the energy in a single pulse. The pulse energy determined was roughly independent of P _abs, with an average value of 145 μJ.

With a 2 GHz oscilloscope, we monitored the generated pulse profile. The pulsed laser signal was detected by a fast photodiode. The laser pulse remained unchanged in duration in the entire operational range. Figure 3 depicts a typical pulse profile, which was taken at P _abs = 6.1 W. A FWHM duration of 26 ns was measured. The peak power of the laser pulse, determined from the pulse energy and duration, amounted to 5.6 kW. Figure 4 gives a recorded pulse train for P _abs = 6.1 W, the PRF was 4.2 kHz, with time jitter less than 10%. The amplitude fluctuations from pulse to pulse were less than 5%.

 

Fig. 3. Typical pulse profile of the Q-switched Yb:NaYW laser taken at P _abs = 6.1 W.

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Fig. 4. A pulse train of the Q-switched Yb:NaYW laser recorded at P _abs = 6.1 W. The horizontal time scale is 0.2 ms/div.

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The quality of the cw output beam of the Yb:NaYW laser was evaluated by using the knife-edge scanning method, with the M2 factor estimated to be 2.0–2.5 for both the horizontal and vertical directions, indicating the existence of higher-order transverse modes. It was not feasible to do this with the Q-switched output beam, as the knife-edge would be burned quickly, preventing any reliable measurement of the beam size.

Yb-doped crystals are usually characterized by lower emission cross sections and longer fluorescence lifetimes in comparison with their Nd-doped counterparts, making them more desirable for applications in Q-switched lasers, because of the enhanced energy storage capacity. A clear evidence for this point is provided by making a comparison between the passively Q-switched laser performance of Yb:NaYW and Nd:NaYW crystals: a diode pumped Nd:NaYW laser formed with a similar cavity which was also passively Q-switched by using a Cr⁴⁺:YAG crystal (T ₀ = 91%) produced a pulse energy of 27 μJ and a peak power of 553 W [9], which is respectively 5 and 10 times lower than achieved with Yb:NaYW in the present work. This is explained by the significant difference in stimulated emission cross section and fluorescence lifetime: ∼0.8×10^-20 cm²(at 1027 nm), 392 μs for Yb:NaYW [8] versus 6.0×10^-20 cm², 180 μs for Nd:NaYW [9].

4. Conclusions

In summary, continuous-wave and passively Q-switched laser operation of the disordered Yb:NaYW has been demonstrated under diode pumping. A cw output power of 5.2 W was obtained with an optical-to-optical efficiency of 37%. The slope efficiency in the absence of thermal losses was 50%. Passively Q-switched by a Cr⁴⁺:YAG saturable absorber, the laser yielded an average output power of 2.0 W at a PRF of 13.9 kHz with a slope efficiency of 35%. The generated pulse energy, duration, and peak power were 145 μJ, 26 ns, and 5.6 kW, respectively.