Pedro Bernardes and Dawei Liang
CEFITEC, Dept. of Physics, F. C.T, The New University of Lisbon,

Quinta da Torre, 2825 Campus de Caparica, Portugal

A new pumping configuration by optical fibers was used to produce a Nd :YAG solar laser power. The sunlight was concentrated by a primary parabolic mirror. The solar energy from the focus was transmitted to a pump cavity by means of a two- stage transmission system. The solar power of 355W was transmitted by a fused silica angle transformer with circular input and hexagonal output cross-sections. Both angle reduction and beam uniformity were achieved, suitable for its light coupling to a compact 37 optical fiber bundle with NA=0.4. In order to cover all the output area of the angle transformer, each fiber with 1.5mm diameter was hexagonally polished at its input end. The total output solar power of 184W was measured from the 37 optical fiber bundle. The optical fibers were mounted in a semi-cylindrical pattern around the flow-tube, by means of an aluminium part that provided 4×9 matrix fiber distributions. To concentrate efficiently the light energy from the optical fibers to the laser crystal, 2-D curve polishing at the output ends of the optical fibers were done. The diameters of the Nd:YAG laser rod (4mm) and of the flow tube (8mm) were dimensioned to achieve the maximum energy flux inside the active medium. To ensure maximum absorption, a double-pass pumping scheme was accomplished by applying a gold reflector onto half of the internal wall of the flow tube. Using an output coupler of 94% reflectivity, Nd:YAG laser operation was achieved, resulting in a maximum output power of 2.46W. The proposed configuration is scalable trough application of more optical fibers along the flow — tube of a longer laser rod. Further improvements in homogeneity of the absorbed pumping power can be obtained by using large numerical aperture optical fibers mounted around laser rod.

1. Introduction

By converting directly incoherent and broadband sunlight into monochromatic laser radiation, sun-pumped solar lasers find many applications in space ranging from power transmission, propulsion to earth and atmospheric sensing. Due to the complete elimination of the electrical power supply unit and the simple optical to optical pumping scheme, the solar laser is more reliable in some specific applications. The first Nd:YAG solar laser was reported by Young1, who obtained a laser output of 1W for an efficiency of 0.57%. Improvement in higher output power was achieved by Arashi2, Weksler3 and more recently Cooke4. The actual research in solar pumped laser is still mainly devoted to achieving higher output power and higher laser efficiency while other aspects like the beam quality, uniformity of the pumping and flexibility (the flexible separation of a primary and a secondary concentrator) were not much stressed.

A solar-pumped laser utilizes a two-stage system that incorporates a focusing first-stage primary parabolic mirror that tracks the sun and a second-stage, usually a non-imaging concentrator. The laser head and its associated optics are placed near or directly at the focus of the collector. The impossibility to take apart primary and secondary concentrator penalizes the flexibility, and turns the two-stage system unsuitable for certain applications. The efficient transport of concentrated sunlight to a remote target by solar fiber-optic mini­dishes scheme with high efficiencies was reported recently by Gordon5, which may
constitute an important advance for solar-pumped laser research due to utilization of large numerical (NA=0.66) optical fibers. Non-imaging optics plays an important role in solar lasers by providing means for concentrating sunlight to intensities approaching the theoretical limit. Based on the edge-ray principle, the compound parabolic concentrator (CPC) that gives the maximum concentration for a two dimensional cavity, is the most commonly used for side-pumping solar laser. Although the non-imaging pump cavity provides a large amount of pump power, it does not give a Gaussian absorption pumping profile, affecting the laser output beam quality6. In side-diode pumped laser, the close — coupled fiber optic or glass plate pumping geometry approaches the ideal TEM00 mode — matched absorption distribution7,8, which, may be beneficial to designing sun-pumped lasers.

The experimental results of a fiber optic solar-pumped laser are presented. The new side­pumping scheme allows the separation for many meters of the radiant source at the focus and the active medium by means of a bundle of 36 optical fibers. The output ends of the 36 fibers are displayed in a close-coupled geometry half way around the medium in order to achieve pumping homogeneity. A double pass pumping scheme allows an efficient absorption of the pumping energy.

The fiber optic solar laser system is given in Fig.1. A primary parabolic mirror with 150cm diameter, 67 cm of focal length, 85% reflectivity and a small centred hole of 8cm diameter was used. A plane mirror of 14cm diameter was used to invert the incoming concentrated solar light to the input end of a 2 meter light guide-optical fiber bundle assembles, by which concentrated solar light was transmitted to a convenient place for flexible pumping of the small solar laser rod. Due to the flexibility of the fiber bundle, constant solar power was obtained while the primary mirror was working in the direct tracking mode. Flexibility in solar energy transmission, allows the location of the laser head outside the focal area of the primary parabolic mirror.

Some descriptions of the technical parameters for the solar laser system by optical fibers ranging from the flux distribution at the focus to the light-coupling scheme to the laser crystal will be given in the following sections. Angle-dependent light interception and transmission efficiencies will also be discussed.