To test the high power solar energy transmission capacity of light guide assembly, an experimental set-up was built. Owing to the difficulty in measuring directly the total solar power from the light guide assembly, each individual light guide was mounted on its correct light coupling position. A Moletron power meter was used to measure the output power from each straight, curved or curved and twisted light guide. A total solar power of about 880W was then deduced. The output light divergence from each light guides was less than 300, which ensured the future light coupling to the laser crystals.

Nd:YAG End-Side Laser Pumping

With the curve polishing at the output end of the light guides, a spot-size in the focal region of 4mm was achieved. Theoretical analysis confirmed that 80% of the sunlight transmitted by the light guide matched the rod cross section. To achieve maximum flux energy in the active medium, a 5mm diameter Nd:YAG rod was chosen. The crystal with a nominal neodymium concentration of 1.1.% and 20mm in length, was coated at one end to be
highly reflection for A=1.064pm. The other end was plane and antireflection coated. The Nd:YAG rod was inserted in the flow-tube (see Fig. 9) of 12mm external diameter and 30mm in length.

The resonant cavity was formed by a 94% reflectivity output coupler of -1.0m radius positioned at 300mm from the Nd:YAG rod. To prevent laser rod damage from the UV radiation and to reduce unwanted temperature rise, a doped flow tube was used. Demineralised water was used as a coolant and the flow rate was 4l/min. The equilibrium temperature of the coolant for cw-Sun pumped laser operation was 28°C.

The continuous laser output power was measured as a function of the power transmitted by the light guide assembly (Fig.10). To achieve variation of the output power, the primary mirror was masked with ring pieces of non-reflecting material, affecting the collecting radius of the primary concentrator. Besides controlling the incident sunlight power in the active medium, this practice had an effect on the flux distribution within the Nd:YAG rod.

For a solar irradiance of 800W/m2, the maximum measured cw-laser output power was 8.6W, corresponding to an overall efficiency of 0.97% and 1.8% slope efficiency. The threshold was determined to be approximately 300W of the power collected by the light guide. Considering the total solar power at the focus, the total efficiency was 0.68%.

CONCLUSIONS

Due to the symmetric end-side pumping of a Nd:YAG laser crystal by a fused silica light guide assembly, satisfactory solar laser efficiency was obtained. The new light guide assembly permitted tailoring the pumping flux distribution within the active medium at the cost of slightly lower solar laser efficiency, due to the transmission loss of the light guides. Much experimental work was done in finding the optimum light guide shape and in achieving the minimum focused end-side pumping zone with the flow tube. The ZEMAX ray-tracing program confirmed some of the experimental research.