The solar power of 184W was successfully transmitted away from the focal area of the primary mirror to a laser head by 36 flexible optical fibers. Power transmitted by each fiber shows variation less than 0.5%, facilitating hence the further uniform pumping of a 4mm diameter Nd:YAG laser crystal. The optical fibers were mounted around the flow tube via an aluminium part that provided a half cylindrical 4 x 9 matrix fiber distribution as shown in Fig. 7a) and 7b).

Fig. 7(b) — Optical fiber bundle output end view

The output light characteristics of the presented two stage transmission system, allowed the use of thin laser rod with efficient light coupling which is advantageous as the cooling is more efficient, reducing both thermal gradient and thermal lensing effects.

To maximize the flux energy in the active medium, curve polishing at the output end of the optical fibers was done. This allows a better match of the spot-size in the focal region. Theoretical analysis was confirmed by performing the Zemax ray-tracing program.

Figure 8 — Zemax ray-tracing. a) Plane extremity, b) Curved extremity

To maximize the flux energy in the active medium, it was necessary to use a double pass scheme. This was accomplished by depositing a cylindrical reflector on half of the internal wall of the flow-tube.

The Nd:YAG rod, with 1.1at.% Nd3+, 4mm in diameter and 25mm in length was inserted in the center of flow-tube center, with 8mm external diameter and 12mm in length.

(a) (b)

Figure 9: (a) — Pumping scheme; (b) — Optical fibers coupling.

The resonant cavity was formed by a 94% reflectivity output coupler of -1.0m radius of curvature and a 100% mirror of the same curvature. The cavity length was 650mm and the Nd: YAG rod was positioned at the centre of the resonator.

In order to prevent laser rod from becoming solarized and to reduce unwanted temperature rise, a doped flow tube was used. Demineralised water was used as a coolant and the flow rate was 2.2l/min.

The laser output power was measured as a function of the input power (Figure 11). Here the input power is taken to mean the power collected at the entrance of the light guide. The variation of the input power was achieved by masking the primary mirror with ring pieces of non-reflecting material, which affected the collecting radius of the primary concentrator and the flux distribution on the focus.

The maximum output power obtained was 2.46W, for an overall efficiency of 0.69% and slope efficiency greater than 1.6%. This is a reasonable value for the conversion efficiency of a solar-pumped Nd:YAG laser. The output power was stable to within 3% for periods of several minutes. The threshold was determined to be approximately 220W of the power collected by the light guide.

8. Conclusion

The low efficiency and high lasing threshold was attributed to the low transmission efficiency of the fiber bundle due to the angle-dependent transmission and also to coupling loss between fibers and light guide. Only 50% of solar radiation at the light guide entrance was transmitted to the laser crystal, in comparison with the 85% obtained by Weksler and

J. Shwartz3 with a CPC.

Further improvements in homogeneity of the absorbed pumping power can be achieved by using large numerical aperture (NA=0.66) optical fibers mounted around the laser rod as shown in Fig. 12. To guarantee that the optical energy is focused on the rod due to the highest NA, the output extremity of fibers must be close to the rod.

01

9. References

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(3) M. Weksler, J. Shwartz, “Solar-pumped Solid-State Lasers”, IEEE Journal of quantum electronics, vol. 24, No. 6, June 1988

(4) D. Cooke, “Sun-pumped Lasers: revisiting an old problem with nonimaging optics,” Applied Optics, vol.31, No 36, 7541-7546, 20 december 1992

(5) D. Feuermann, J. M. Gordon, M. Huleihil, “Solar Fiber-optic mini-dish concentrators:first experimental results and field experience,” Solar Energy Vol.72,No.6., pp. 459-472,2002.

(6) R. John Koshel and I. A. Walmsley, “Modeling of the gain distribution for diode pumping of a solid-state laser rod with nonimaging optics,” Applied Optics, vol.32, No. 9, 1517-1527, 20 march 1993.

(7) N. Pavel, Y. Hirano, S. Yamammoto, Y. Koyata, T. Tajime, “Improved pump-beam distribution in a diode side_pumped solid-state laser with highly diffuse, cross-axis beam delivery system,” Applied Optics, vol.39, No 6, 986-992, 20 february 2000.

(8) L. R. Mashall, A. Kaz, and R. L. Burnham, “ Highly efficient TEM00 operation of transversely diode-pumped Nd. YAG lasers,” Opt. Lett. 17, 186-188 (1992).

(9) D. L. Evans, “On the performance of cylindrical parabolic solar concentrators with flat absorbers,” Solar Energy, Vol. 18, pp. 379-385 (1977).

(10) D. Liang, S. Duarte, J. Trindade, D. Ferreira and L. F. Monteiro, “High power solar energy transmission by solid-core fused silica light guides,”

(11) J. Harris and W. S. Duff, “Focal plane flux distributions produced by solar concentrating reflectors,” Solar Energy, Vol. 27, pp. 403-411 (1981).