Optimal Sphere-Sensor Ratio for Microsphere-Enhanced Photodetector Sensors in Cameras/Telescopes With and Without Central Obscuration

A microsphere lens on top of a photodetector sensor increases its sensitivity and reduces its Noise-to-Signal Ratio (NSR), as shown in our previous work. This paper discusses a methodology to determine the optimal sphere-sensor ratio, in terms of increased current sensitivity, for microsphere-lens-enhanced photodetectors, using computational analysis including a modeling, with an experimentally derived photo-detector sensor responsivity and simulation setup that emulates conditions generally found within cameras and telescopes. In particular, we study and analyze the effect that several variables have on such ratios. We focus on variables such as microsphere material type, photodetector hardware technology type, light source temperature, and in the case of reflective telescopes, central obscurations from a secondary mirror. Results of the effect of microsphere-sensor misalignment on such ratios are also presented. As we show in this paper, since the optimized ratio is dependent on a given wavenumber considered, therefore the proposed methodology uses an integral optimization over a given spectral band to derive the optimized ratio over that band. Some of our findings show, for the first time, that the optimal ratio is virtually independent of a given central obscuration. Finally, we demonstrate here the flexibility of the proposed methodology in showing performances of microsphere-enhanced photodetector sensors.

I. Introduction

As described in [1]–[3], a microsphere lens was experimentally applied to a Strained Layer Super-lattice (SLS) detector which increased sensitivity [1] and reduced noise [2]. Several sphere sizes and materials were tried and it was determined that a sapphire microsphere on a photodetector sensor had the highest intensity [1] and lowest noise to signal ratio [2] of those tested. For those tests, effectively a sapphire microsphere with a diameter 7.5 times the SLS sensor diameter was placed in an f/19.7 camera obscura, as shown in Fig. 1. As shown below, this is not the optimum set of ratios. In this paper we propose a methodology to determine, for the first time, what the optimum sets of microsphere-sensor ratios can be for cameras and telescopes with refractive lenses as well as with reflecting mirrors, where central obscuration from a secondary mirror needs to be taken into account. Fig. 1.

Photodetector sensor setup.