A comparison between composite crystalline and ceramic composite Nd:YAG rods for high power diode end-pumping is presented. Laser output power characteristics as well as the thermal lensing properties of the composite laser rods were evaluated. A maximum laser output power of 121 W and an optical-to-optical efficiency of 48 % were achieved by longitudinal pumping with fiber-coupled laser diodes.
©2005 Optical Society of America
Polycrystalline ceramic Nd:YAG laser material enables new possibilities in designing the laser medium with respect to dopant, size and geometry [1,2].To evaluate the ceramic laser rods for high power diode end-pumping we compared the laser as well as the thermal lensing properties to segmented crystalline rods.
2. Experimental setup
In our setup, sketched in Fig. 1, composite laser rods were longitudinally pumped with a bundle of 10 fiber-coupled laser diodes (Jenoptik Laserdiode GmbH, type JOLD-30-CPXF-1L) with a nominal output power of 30 W each resulting in a maximum available pump power of 300 W. Each diode was stabilized to an individual temperature by thermo-electrical cooling for emission wavelength tuning to meet maximum absorption in the laser rod. Two different bundles of ten fibers with 600 μm core diameter or 800 μm respectively and a NA of 0.22 were used. The pump radiation from the fiber bundle was focused into the laser rod with an appropriate system of three plano-convex lenses. The composite Nd:YAG laser rods were 3 mm in diameter and consisted of a 40 mm long 0.1 % doped active region and 7 mm long undoped end-caps. Polycrystalline ceramic rods (from Baikowski Chimie, France) as well as crystalline rods (from FEE GmbH, Germany) were used. The barrel surface of the laser rods was polished to optical quality and therefore acts as a waveguide for the pump light due to total internal reflection because of the difference in refractive indices between YAG and cooling water [3,4]. Both sides of the laser rod were antireflection coated for the laser wavelength of 1064 nm. The pump side of the rod was antireflection coated for the pump wavelength of 808 nm while the other side is highly reflective to achieve a longitudinally homogeneous pump light distribution in the rod [5,6]. The total absorption of the pump light was estimated to be 95 %. With this setup we investigated the laser and thermal lensing properties of crystalline and polycrystalline ceramic Nd:YAG with the two different fiber bundles.
3. cw Laser power
In order to evaluate the composite ceramic Nd:YAG rods for high power end-pumping we compared the laser output to results with segmented crystals which are already well established. The cw multimode laser output was measured with two fiber bundles consisting of 10 fibers with 600 μm or 800 μm diameter each for different output coupler transmissions of 10 %, 18 %, and 22 % (25 % respectively). The short resonator was built by a plane output coupler and plane dichroic pump mirror, highly reflective for the laser and antireflective for the pump beam. The results with the 600 μm bundle are shown in Fig. 2(a) and 2(b). We achieved a maximum output power of 112 W with an optical to optical efficiency of 48 % and a maximum slope efficiency of 55 % with the diffusion bonded crystal and a pump power of 232 W. The slope efficiencies measured with the composite ceramic rod were nearly identical although we obtained a slightly different maximum output power of 110 W and the maximum output was reached with the higher transmission of 22 % instead of 10 % for the crystal which indicates slightly higher internal parasitic losses in the ceramic rod. To verify these results we also tested different rod samples with the 800 μm fiber bundle as shown in Fig. 2(c) and 2(d). The better coupling efficiency from the pump diodes to the 800 μm fibers resulted in a maximum pump power of 275 W. We generated a maximum output power of 121 W with an optical to optical efficiency of 45 % at a mirror transmission of 12 % with the segmented crystal and with the composite ceramic the maximum output power was 113 W at a mirror transmission of 18 %. So again the maximum output was slightly less than for the crystal and the optimal transmissions moved to higher values indicating higher losses in the ceramic rod. But the difference in output power was less than 6 %.
In order to demonstrate the effect of different optimal transmissions the results with the 800 μm fiber bundle for the different output couplers are shown in a Rigrod plot in Fig. 3(a) [7,8]. One can see that the maximum power for the ceramic is achieved at higher transmissions than for the crystal. To quantitatively analyze these effect we made a Findlay-Clay Analysis  with our data which resulted in one way losses of 1.9 % for the crystal and 2.9 % for the ceramics as shown in Fig. 3(b).
4. Thermal lensing
In order to verify the thermal lensing properties of the composite laser rods we investigated the stability of the plane-plane resonator by varying the distances of the output coupler to the laser rod. In order to determine the focal length of the thermal lens the end of the stability range of the plane-plane resonator was measured for different pump powers and the ABCD Matrix formalism was used to calculate the focal length of the thermal lens . We investigated the stability range systematically for the ceramic and the crystal as shown in Fig. 4(a). We found the expected linear increase of refractive power with increasing pump power. A linear fit gave very similar results for the thermal lensing values for the crystal and the ceramic of 9.3 Dpt/kW and 9.5 Dpt/kW respectively. These results were measured with the 800 μm fiber bundle. With the 600 μm bundle the different pump light distribution in the laser rod resulted in different thermal lensing properties [11,12]. Therefore the refractive power of the composite ceramic YAG rod was compared for the two different fiber bundles as shown in Fig. 4(b). The tighter focus of the smaller fibers led to increased refractive power of 13.7 Dpt/kW instead of 9.5 Dpt/kW for the 800 μm fibers. The refractive power per pump power was consistent with the typical value range from 10–15 Dpt/kW for high-power transversally diode pumped systems [13–15].
In summary we presented a comparison of polycrystalline and crystalline composite Nd:YAG laser rods for high power diode end-pumping. A maximum cw laser output power of 121 W and a maximum optical to optical efficiency of 48 % were achieved for the composite laser crystal and 113 W and 47 % for the ceramic rod respectively. We also investigated the refractive power of the thermal lens for both types of composite laser rods which showed similar values of 9.3 Dpt/kW and 9.5 Dpt/kW. In conclusion we demonstrated that the properties of composite ceramic Nd:YAG laser rods are nearly identical to those of segmented crystals and therefore polycrystalline ceramic Nd:YAG is an appropriate candidate for high power end-pumping.
The work was funded by the German Ministry of Education and Research under contract 13N8299.
References and links
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