Efficient polarized laser oscillation along the Ng principal optical axis is demonstrated at room temperature with a Np-cut Yb:KLu(WO4)2 crystal in a three-mirror folded resonator pumped by a Ti:sapphire laser at 981 nm. To our knowledge, this is the first laser study of this polarization for bulk crystals of this type. A continuous-wave output power of 0.55 W was obtained at 1044 nm for an absorbed pump power of 1.44 W, leading to optical-to-optical and slope efficiencies of 38% and 52%, respectively.
© 2008 Optical Society of America
In the past decade, KT(WO4)2 (T=Y, Gd, and Lu), belonging to the family of monoclinic potassium double tungstates, have become a very important class of laser host crystals for the trivalent ytterbium (Yb) ion, since they offer unique advantages including exceptionally large absorption and emission cross sections, wide absorption and emission bands, and weak concentration quenching. Having a low symmetry of space group C2/c (point group 2/m), these Yb-doped tungstates are also characterized by very strong anisotropy in their absorption as well as emission spectra . As a result, their laser performance usually depends largely on the crystal orientation, which, however, has not been studied in detail for a long period. It was not until very recently that such an investigation was conducted by comparing experimentally the laser performance with Yb:KLu(WO4)2 (Yb:KLuW) crystals cut along the Np, Nm, and Ng principal optical axes, revealing the superiority of the Ng-cut crystal in both pump power utilization and output power scaling .
The strong anisotropy in emission cross section (σ em) also leads to polarized laser oscillation when the crystal used is cut along one of the principal optical axes. Most of the laser actions achieved in the past with the three Yb:KT(WO4)2 crystals utilized samples cut along the Np axis (// b crystallographic axis), with laser polarization parallel to the Nm axis. Laser operation with polarization along the Np axis was also obtained by using a Nm-cut Yb:KLuW crystal . Up to now, however, little attention has been paid to realizing laser oscillation of polarization along the Ng axis, probably due to the small σ em for this configuration being less interesting for efficient continuous-wave (cw) operation. On the other hand, however, the small σ em for E//Ng axis will be advantageous for operation in the Q-switched regime, since higher pulse energy and peak power can be expected as the pulse energy generated by Q-switching action usually is inversely proportional to the magnitude of σ em. This is crucial for achieving efficient frequency conversion through self stimulated Raman scattering, as all of the three Yb:KT(WO4)2 crystals have proved as promising self-Raman frequency converters [3–6]. Only very recently was such oscillation demonstrated using diffusion-bonded composite Yb:KY(WO4)2 at relatively low power levels due to the low absorption .
In this paper, we report on efficient cw laser oscillation of polarization parallel to the Ng principal optical axis, achieved at room temperature with a Np-cut Yb:KLuW crystal which was orientated at Brewster angle and pumped by a cw Ti:sapphire laser. To our knowledge, this is the first study of laser operation with linear polarization along the Ng axis with a crystal belonging to this class for any kind of rare-earth doping.
2. Experimental laser setup
As illustrated in Fig. 1, an astigmatically compensated three-mirror folded resonator was employed in the laser experiment. The end mirror M1 was a concave one of radius of curvature of 50 mm. The folding mirror, M2, was also concave, having a radius of curvature of 100 mm. Both mirrors were coated highly reflecting for 1020–1240 nm, and highly transmitting for the pump wavelength of 981 nm. M3 was a plane mirror, it served as the output coupler. For laser optimization, several couplers were used with different transmission ranging from 1% to 5%. The total physical cavity length was 650 mm.
The laser crystal, a 3 mm thick uncoated Np-cut Yb:KLuW sample with an aperture of 4 mm×4 mm (Yb concentration of 5.24 at. % in the crystal), was located at the waist position of the M1M2 arm, and orientated under Brewster angle condition. A cw Ti:sapphire laser, capable of generating 2 W of output power at 981 nm, was utilized as the pump source. The pump beam was focused onto the laser crystal with a spot radius of ~22 µm through M2 by a focusing lens L (f=62.8 mm), see Fig. 1.
3. Results and discussion
In order to achieve laser oscillation polarized along the Ng axis (Ng-polarized) with the Np-cut Yb:KLuW crystal, some special measures must be taken to suppress the predominant Nm-polarized oscillation, since σ em is much higher for E//Nm than for E//Ng axis . By placing the Np-cut Yb:KLuW crystal at Brewster angle of 64.37° for E//Ng (ng=2.084 at λ=1.0 µm ) with Ng axis lying in the horizontal plane, the laser oscillation with E//Ng will see little or no Fresnel losses; on the other hand, however, the oscillation with E//Nm will suffer substantial reflection losses amounting to ~62% for a single pass (nm=2.03 at λ=1.0 µm ) which does not allow oscillation of this polarization.
A similar situation occurs also for the polarized pump beam from the Ti:sapphire laser. To avoid the significant reflection losses, the polarization direction of the pump beam was aligned to be parallel to the Ng axis. From the polarized absorption spectra shown in Fig. 2, which were measured for the 5.24 at. % Np-cut Yb:KLuW crystal, one notes that although the absorption coefficient for E//Ng is generally much lower than for E//Nm, it still amounts to 5.6 cm-1 at the peak absorption wavelength of 981 nm, corresponding to a small-signal absorption of 0.81 for the 3 mm thick crystal.
Efficient Ng-polarized cw oscillation was achieved with the Yb:KLuW laser formed employing the three-mirror folded resonator under Ti:sapphire laser pumping at 981 nm. The laser was operated at room temperature, with the Yb:KLuW crystal clamped simply between two copper blocks without any active cooling. Figure 3 shows the relations between the output power and the absorbed pump power (P abs) for different output couplings (T). The laser reached threshold at P abs=0.36, 0.40, and 0.48 W for T=2%, 3%, and 5%, respectively. Just as expected, the smallest output coupling led to not only the lowest threshold, but also to the optimal operation in the operational region slightly above threshold (P abs<~0.6 W). Beyond this region, however, the most efficient operation was obtained with an output coupler of T=3%, generating a maximum output power of 0.55 W at 1044 nm for the highest available pump power corresponding to P abs=1.44 W. For this case, the optical-to-optical and slope efficiencies were 38% and 52%, respectively. In the case of T=5%, an equally high slope efficiency of η=52% was measured, but the maximum output power of 0.47 W generated at 1042 nm was lower in comparison with that produced with the output coupler of T=3%.
It is noted from Fig. 3 that the highest available P abs, which was limited by the maximum incident pump power of 1.86 W provided by the Ti:sapphire laser, was not the same for different output couplings. In fact, the absorption (the fraction of incident pump power absorbed by the crystal) was dependent on the operational conditions of the laser. Figure 4 depicts the variation of the absorption with the incident pump power (P in), determined experimentally by measuring the residual pump power passing through the end mirror M1. In the absence of laser action, the absorption at P in=0.44 W was 0.84, comparable with the calculated small-signal absorption (0.81) using the measured spectra data. With increasing P in, the absorption was bleached (from 0.84 to 0.54) by the reduction of the ground-level population. Such an absorption bleaching was greatly mitigated under lasing conditions by the intracavity intensity, which counteracted the bleaching effect efficiently through the “population recycling” process, leaving the real absorption unchanged or only slightly lower than the low-power level. As expected, the absorption decreased with increasing output coupling (0.84–0.80 for T=2%, 0.81–0.78 for T=3%, and 0.78–0.77 for T=5%) which is related to the decreasing intracavity intensity.
Figure 5 illustrates a direct comparison of the output characteristics between the Ng- and Nm-polarized laser oscillations. The Nm-polarized laser oscillation was obtained with the Yb:KLuW crystal rotated by 90° around the Np axis, making the Nm axis lie in the horizontal plane. One notes firstly that the threshold for the Nm-polarized lasing, P abs=0.28 W, was considerably lower than the corresponding value for the Ng-polarized oscillation. This is attributed to the significant difference in σ em for the two polarization directions at the actual lasing wavelength (1044–1045 nm): ~0.7×10-20 cm2 for E//Nm versus ~0.3×10-20 cm2 for E//Ng . The larger σ em for E//Nm also resulted in more efficient laser operation, with a slope efficiency determined to be 63%, the same as that measured in the case of an epitaxial Yb:KLuW crystal under the same operational conditions .
In cw operation regime, the Ng-polarized laser oscillation is inferior to the Nm-polarized one with respect to the threshold, efficiency, and power scaling potential. Nevertheless, the demonstration that oscillation of this polarization is possible is important because for Q-switched operation, the moderate σ em for E//Ng will be advantageous due to the enhanced energy storage capacity. With an intracavity polarization selecting element or surface the Ng-polarized laser oscillation can be forced to occur with a Np- or Nm-cut Yb:KLuW crystal. From the Q-switched Nm-polarized laser performance achieved with a Np-cut Yb:KLuW crystal under diode pumping , taking into account the ~2 times lower σ em, one expects that such a device with diode pumping is potentially capable of producing laser pulses of >0.4 mJ in energy, ~1 ns in duration, and >400 kW in peak power. This may be particularly attractive for the improvement of the self-Raman conversion efficiency in Q-switched lasers based on Yb-doped KT(WO4)2 crystals.
In summary, efficient continuous-wave laser oscillation with polarization parallel to the Ng principal optical axis has been demonstrated at room temperature with a Np-cut Yb:KLuW crystal, employing a three-mirror folded resonator under Ti:sapphire laser pumping at 981 nm. An output power of 0.55 W was generated at 1044 nm with an optical-to-optical efficiency of 38%. The slope efficiency was determined to be 52%, amounting to 83% of that obtained in the case of Nm-polarized oscillation.
This work was supported by the National Natural Science Foundation of China (No. 60778013, No. 10744003, No. 50590401, and No. 50721002), and the EU project DT-CRYS, NMP3-CT-2003-505580.
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