Cat-eye retroreflectors based large-dynamic-range alignment-free laser

Abstract

Lasers with cavities consisting of retroreflecting elements can give the potential for large-dynamic-range alignment-free operation, which makes the important applications in adaptive wireless laser power transfer/communication possible. In such an emerging approach based on resonant laser beam in the cavity, the laser is delivered to the photovoltaic cell for charging application (or photodiode for communication application) at the receiver automatically, without the necessity of positioning and aiming the receiver in conventional laser wireless power transfer techniques. The laser capable of operating alignment-free efficiently across large-dynamic-range is essential for the application. In this work, the requirements for the dynamic range of alignment-free operation are summarized. An alignment-free laser with a cavity consisting of cat-eye retroreflectors is designed, and a large alignment-free dynamic range as never before is experimentally demonstrated. Telescope system in the laser cavity is adopted to suppress the beam expansion to enhance the working distance between the laser transmitter and the receiver. Coupled cavity scheme is used to reduce the laser intensity between the transmitter and the receiver for laser safety. By calculating the stability zone of the laser cavity, it is found that the stability zone of the receiver cat-eye distance is quite narrow. Hence, the laser operation is very sensitive to the defocusing of the cat eye defocusing. Moreover, the cat eye defocusing induced by optical aberrations of spherical aberration and field curvature can be rather serious, when the long working distance results in a large beam size and the angle of incidence is large, hence limiting the effective working distance and the field of view of the alignment-free laser significantly. In the experiment, the improved optical designs with the aberrations compensation are adopted for large-dynamic-range alignment-free operation. The end-pumped Nd:GdVO<sub>4</sub> laser at 1063 nm can deliver over 5-W output within a working distance range of 1–5 m, and a receiver field of view of ±30°, without cavity realignment. The transmitter field of view reaching 4.6° (full width at half maximum) at a working distance of 5 m is also realized, with a corresponding receiver transverse movement range of 40 cm. Our work clarifies the optimizing criteria of the large-dynamic-range alignment-free laser based on cat-eye retroreflectors.