Design of variable link length indoor visible light communication system using focus tunable liquid lens

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Indoor visible light communication (VLC) systems commonly make use of non-tunable, fixed optical components to achieve best communication performance at fixed transmitter and receiver separation distance. In practice suitable tuning mechanism to achieve maximum throughput for user equipment held at variable link length is desirable. This becomes more relevant when optical illumination spot with narrow beam profile are used as VLC transmitters. In this paper, we simulate the effect of a focus tunable liquid lens-based transmitter system for variable link length collection efficiency optimization in phosphor down-converted white light VLC link. We use a remote phosphor as a secondary source, which down-converts the narrow-angle incident blue laser light into wide-angle white light with Lambertian mode number 1.54. We perform optical simulations using non-sequential ray-tracing to calculate the overall light collection efficiency at the detector plane as a function of varying liquid lens radius of curvature. Optical rays at the blue wavelength at the phosphor plane are directed to the receiver using a focus-tunable lens followed by a fixed 100 mm focal-length aspherical-lens. At the receiver side a single aspherical-lens of 32 mm focal-length is used in front of photodetector. The liquid lens focal length is tuned to vary the position of the peak of the optical throughput along the optical path relative to the transmitter. The tuning of the liquid lens from -10 to 0 dioptres results in maximum optical throughput to shift from ~2 m to 30 cm, without moving any optical component in the path. Liquid lenses are promising addition to indoor VLC systems to achieve focus tunable optical link with best optical throughput for communication at varying link lengths..