Design and fabrication of cavity reflector for enhanced LED-fiber coupling
Abstract
Stimulation of brain cells by means of optical illumination is a method used by neuroscientists to study the influence of specific brain cells on certain physiological activities. When the stimulation is done using a wireless implantable device to aid the mobility of the subject, it imposes the constraints of size and very limited power input to the stimulating circuitry. Since most of the power is consumed by the light source (LED), the overall efficiency of the illuminating system, consisting of the LED and the coupled optical fiber leading to the target neuron, is critical for proper functioning of the implanted device. Usually a significant portion of the LED output is lost during the LED-fiber coupling due to the omnidirectional nature of the LEDs. The goal of this thesis is to minimize this power loss by concentrating the light using cavity reflectors. In this thesis, we first determine the optimal geometry of a cavity reflector using a Monte-Carlo based simulation. We find that smaller the size of the cavity and larger the height, better is the concentration of light. Such an optimal cavity has slope between 60° and 75° and can concentrate 70% of light within ±20° from normal. We also find that a vertical cavity stacked on top of a 54° inclined cavity can concentrate 91.69% of light within ±20°. We fabricate a 54° inclined cavity reflector by wet anisotropic etching of silicon using KOH etchant. This anisotropic etching occurs along the atomic planes and we are able to get smooth shiny reflector surface. We also use a simple characterization technique to determine the enhancement of optical directionality using a photographic method. As predicted from the simulations, optical power from the smallest cavity showed the highest peak in the intensity curves. We also compare the optical power coupled to the fiber tip using two sphere lenses and the cavity reflector only. We found that double sphere lens gives better light coupling to fiber than the cavity reflector only. Finally, based on this work, we provide future directions for effective enhancement of the LED-fiber coupling.
Degree
M.S.E.C.E.
Advisors
Irazoqui, Purdue University.
Subject Area
Electrical engineering
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