407 W End-pumped Multi-segmented Nd:YAG Laser - OSA Publishing

407 W End-pumped Multi-segmented Nd:YAG Laser Dietmar Kracht, Ralf Wilhelm, Maik Frede Laser Zentrum Hannover e.V., Hollerithallee 8, D-30419 Hannover, Germany [email protected]

Klaus Dupré, Lothar Ackermann FEE GmbH, Struthstr. 2, D-55743 Idar-Oberstein, Germany [email protected]

Abstract: A composite crystalline Nd:YAG rod consisting of 5 segments with different dopant concentrations for high power diode end-pumping is presented. A maximum laser output power of 407 W with an optical-tooptical efficiency of 54 % was achieved by longitudinal pumping with a high power laser diode stack. ©2005 Optical Society of America OCIS codes: (140.3580) Lasers, solid-state; (140.3480) Lasers, diode-pumped

References and links 1.

S.C. Tidwell, J.F Seamanns, M.S. Bowers, A.K. Cousins, "Scaling CW Diode-End-Pumped Nd:YAG Lasers to High Average Powers," IEEE J. Quantum Electron.. 28, p.997 (1992). 2. F. Hanson, "Improved laser performance at 946 and 473 nm from a composite Nd:Y3Al5O12 rod," Appl. Phys. Lett. 66, 3549 (1995). 3. M. Tsunekane, N. Taguchi, and H. Inaba, "High power operation of diode-end-pumped Nd:YVO4 laser using composite rod with undoped end," Electron. Lett. 32, 40 (1996). 4. M. Tsunekane, N. Taguchi, T. Kasamatsu, and H. Inaba, "Analytical and experimental studies on the characteristics of composite solid-state laser rods in diode-end pumped geometry," IEEE J. Sel. Top. Quantum Electron. 3, 9 (1997). 5. C. Bibeau, R. J. Beach, S. C. Mitchell, M. A. Emanuel, J. Skidmore, C. A. Ebbers, S. B. Sutton, and K. S. Jancaitis., "High-Average-Power 1 µ m Performance and Frequency Conversion of a Diode-End-Pumped Yb:YAG Laser," IEEE J. Quantum Electron. 34, 2010 (1998). 6. E. Honea, R. J. Beach, S. C. Mitchell, J. Skidmore, M. A. Emanuel, S. B. Sutton, S. A. Payne, P. V. Avizonis, R. S. Monroe and D. G. Harris, "High-power dual-rod Yb:YAG laser," Opt. Lett. 25, 805 (2000). 7. D. Kracht, M. Frede, R. Wilhelm, C. Fallnich, "Comparison of crystalline and ceramic composite Nd:YAG for high power diode end-pumping," Opt. Express 13, 6212 (2005), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-13-16-6212. 8. M. Frede, R. Wilhelm, R. Gau, M. Brendel, I. Zawischa, C. Fallnich, F. Seifert and B. Willke, "High-power single-frequency Nd:YAG laser for gravitational wave detection," Class. Quantum Grav. 21, 895 (2004). 9. M. Frede, R. Wilhelm, M. Brendel, C. Fallnich, F. Seifert, B. Willke, K. Danzmann, "High power fundamental mode Nd:YAG laser with efficient birefringence compensation," Opt. Express 12, 3581 (2004), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-15-3581. 10. W. Koechner, Solid-State Laser Engineering, 4th edition, (New York, Springer, 1995) 11. K. Contag, Modellierung und numerische Auslegung des Yb:YAG-Scheibenlasers, (Munich, Herbert Utz Verlag, 2002)

1. Introduction In end-pumped solid-state lasers with homogeneous dopant distribution the exponential decay of the pump light distribution causes high temperature gradients and mechanical stress peaks limiting the maximum incident pump power for a given rod length [1-4]. Nd:YAG laser rods consisting of multiple segments with different dopant concentrations allow the minimization

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Received 18 October 2005; revised 17 November 2005; accepted 22 November 2005

12 December 2005 / Vol. 13, No. 25 / OPTICS EXPRESS 10140

of the longitudinal temperature and stress gradient in diode end-pumped lasers. In order to smooth these peak values, long absorption paths can be achieved by applying wing pumping as well as low dopant concentrations both optionally with a pump light double pass [5-7]. As a continuous increase of the dopant concentration, which would result in a homogeneous absorbed pump power density, has not been realized so far, applying multiple segments with increasing dopant concentrations represents a practicable trade-off. In this paper we report on the evaluation of a composite Nd:YAG laser rod design for high power end-pumping with a laser diode stack. 2. Experimental setup In our setup, shown in Fig. 1, the multi-segmented laser rod was longitudinally pumped with a stack consisting of 8 laser diodes (nLight, type NL-VSA-8-750-808-F) with a nominal output power of 100 W each resulting in a maximum available pump power of 800 W, or 750 W after integrated fast-axis collimation, respectively. The pump radiation from the diode stack was focused into the laser rod with a system of three lenses. The composite Nd:YAG laser rod (from FEE GmbH) was 5 mm in diameter and 62 mm long. The barrel surface of the laser rods was polished to optical quality, and therefore acts as a waveguide for the pump light. This is due to total internal reflection because of the difference in refractive indices between YAG and cooling water [8,9]. The pump side of the laser rod was antireflection coated for the pump wavelength of 808 nm and highly reflective for the laser wavelength of 1064 nm. Therefore the end-facet of the rod acted as the laser resonator mirror. The opposite side was antireflection coated for the laser wavelength of 1064 nm. A plane mirror was used as the output coupler for the resonator. Using this setup the laser properties of the multi-segmented Nd:YAG laser rod were examined.

Fig. 1. Schematic set-up of the laser system.

2. Rod design The multi-segmented Nd:YAG laser rod (see Table 1 for the lengths and dopant concentrations) was 5 mm in diameter and 62 mm long with a 42 mm long active region with attached undoped end-caps of 10 mm. Table 1. Length and dopant concentration of the segments of the laser rod.

Segment

1

2

3

4

5

Length

10 mm

22 mm

10 mm

10 mm

10 mm

Dopant

0%

0.1 %

0.23 %

0.6 %

0%

To illustrate the thermal properties of the rod design, the heat generation rate, the temperature on the rod axis, and the stress on the rod surface were calculated, as shown in Fig. 2.

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Fig. 2. Heat generation rate distributions (a), numerically calculated temperature profiles (b), and von-Mises equivalent stress on rod surface (c) of the different designs; pump wavelength: 807 nm, spectral width (FWHM): 2.5 nm, pump power: 750 W.

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The on-axis heat generation rate was calculated by geometrical raytracing of the pump light using standard software (Zemax) involving 2 million rays. The crystal was divided into about 800.000 volume elements. The effect of pump light guidance based on total internal reflection is responsible for the non-exponential shape of the distribution. The temperature distributions were calculated by a 3-D finite element model using standard software (Ansys) involving roughly 35.000 nodes to describe one quarter segment of the crystal. The finite element model takes the temperature dependence of the heat conductivity into account and assumes a heat generation efficiency of η h = 0.3 and a heat transfer coefficient between the rod barrel surface and the cooling water of h = 1 W/cm 2 K . In the multi-segmented rod a significant reduction of temperature peak values was achieved compared to a rod with only one doped segment as can be seen in Fig. 2(a). A dopant concentration of 0.245 % in the crystal with one 42 mm long doped segment corresponds to the same total absorbed power of 95 %. The temperature distribution along the optical axis is smoothed using more segments, and therefore a higher pump power density is possible before the fracture limit of the crystal is reached, as shown in Fig. 2(b). Fracture limit values for the tensile stress on the rod surface in the range between 130 and 260 MPa for Nd:YAG are reported in literature, depending on the surface quality [10]. The surface stress is determined by the vector sum of the main components of the stress tensor (von-Mises equivalent stress), which has been calculated using a finite element model similar to the thermal model mentioned above. The model permits free expansion of the crystal in all directions, and takes the temperature dependence of the expansion coefficient as given by Contag [11] into account. As can be deduced from Fig. 2(c), the segmented design does not exceed the rupture stress, even if the lower tensile stress limit is assumed, while an equivalent conventional end-pumped design would exceed these values by between 40 to 180 %. 3. cw Laser power In order to evaluate the laser properties of the multi-segmented Nd:YAG rod, the cw multimode laser output was measured for optimized output coupler transmissions of 20 %. The short resonator with 80 mm length was built by a plane output coupler and the highly reflective coated end-facet of the laser rod. The results are shown in Fig. 3. A maximum laser output power of 407 W was realized with a launched pump power of 750 W. A slope efficiency of 62 % and an optical to optical efficiency of 54 % was achieved. 450 400

Output Power [W]

350 300 250

slope efficiency: 62%

200 150 100 50 0 100

200

300

400

500

600

700

800

Pump Power [W]

Fig. 3. Laser output vs. diode pump power for the multi-segmented Nd:YAG laser.

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4. Summary In summary, a high-power, highly efficient laser using a multi-segmented Nd:YAG rod in an end-pumping configuration was presented. With a pump power of 750 W an output power of 407 W was achieved, corresponding to an optical-to-optical efficiency of 54 % by longitudinal pumping with a high power laser diode stack. In principle these multi-segmented laser rods enable the power scaling of the end-pumping configuration to the same values than transversally pumped systems while preserving the advantages of better pump light distribution and efficiency. Acknowledgements The work was partly funded by the German Ministry of Education and Research under contract 13N8299.

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