Denise Valente

Wavefront sensing using a liquid-filled photonic crystal fiber

A novel wavefront sensor based on a microstructural array of waveguides is proposed. The method is based on the sensitivity in light-coupling efficiency to the wavefront gradient present at the entrance aperture of each waveguide in an array, and hence the amount of incident light that couples is influenced by wavefront aberrations. The concept is illustrated with wavefront measurements that have been performed using a liquid-filled photonic crystal fiber (LF-PCF) working as a coherent fiber bundle. The pros and cons of the LF-PCF based sensor are discussed.

Surface-plasmon-based wavefront sensing

The reduction of wavefront aberrations is essential in a number of fields, including astronomy, microscopy, photography, vision science, lithography, and lasers. Aberrations may be determined either directly with wavefront sensors or indirectly with signal- or image-based optimization algorithms. Here, we introduce a novel wavefront-sensing methodology that employs intensity differences across a beam of light to encode local wavefront slopes via attenuated total internal reflection following surface-plasmon excitation at the surface of a thin gold film. This method excels due to the dense spatial sampling of the wavefront and the fact that the wavefront itself can be determined by straightforward integration of two sets of images captured in orthogonal directions without time-consuming optimization, deconvolution, or spot centroiding.

Volumetric integration model of the Stiles-Crawford effect of the first kind and its experimental verification

The integrated Stiles-Crawford function is commonly used as apodization model for vision through the natural eye pupil. However, this method does not account for possible effects related to the retinal thickness, the large length-to-diameter aspect ratio of the photoreceptors, or the use of nonMaxwellian illumination. Here, we introduce a geometrical optics model to calculate the fraction of overlap between light at the retina and the photoreceptor outer segments where absorption triggers vision. The model, which does not account for photoreceptor waveguiding, is discussed for both Maxwellian and nonMaxwellian illumination. The integrated Stiles-Crawford effect is analyzed experimentally with a uniaxial pupil-size flicker methodology and we find that the psychophysical measurements match better to the geometrical optics predictions than direct integration of a Stiles-Crawford function.

Retina-simulating phantom produced by photolithography

Cone photoreceptors have a narrow acceptance angle that is well matched to the size of the eye pupil and dampens the visual impact of aberrations and scattering. However, the structure of the human retina is not replicated in existing eye models used to test refractive designs or retinal implants that restore partial vision to the blind. Here, we report on an artificial waveguide-based retinal phantom manufactured by photolithography in photoresist film with dimensions and refractive index contrast similar to the retinal receptor layer. The optical performance of the waveguide array is analyzed in terms of angular coupling efficiency, and it is experimentally verified that the structure leads to improved resolution and contrast of optical images transmitted through the layer when defocus is present.

Sensing Wavefront Slopes using Intensity Gradients

Aberrations can be mapped into intensity variations and quantified with cameras. Here, we report on two such methods using, respectively, attenuated total internal reflection associated with surface-plasmon excitation and photonic crystals with arrays of waveguides.