We describe a simple method for generating known optical aberrations dynamically, using a ferroelectric liquid-crystal spatial light modulator. Aberrations inherent in the optical system are measured and corrected, and as an example Kolmogorov turbulence is simulated for aperture sizes D/r(0) from 0 to 30, varying at frame rates up to 2.5 kHz. A measure of wave-front generation efficiency is introduced and is shown to be better than 86% for Kolmogorov phase screens with D/r(0) in the range from 0 to 30.
SummaryWe consider various strategies for confocal imaging of human skin which seek to reduce the effects of the specimen-induced aberrations. We calculate the spherical aberration introduced by the stratified structure of skin and show how the confocal signal is affected when attempting to image at various depths within the dermis. Using simple methods it is shown how images might be improved by compensating for the induced aberration. The methods include the use of an iris to reduce the pupil area, changing the refractive index of the immersion medium and using a lens with variable coverglass correction.
Direct laser writing is widely used for fabrication of subsurface, three dimensional structures in transparent media. However, the accessible volume is limited by distortion of the focussed beam at the sample edge. We determine the aberrated focal intensity distribution for light focused close to the edge of the substrate. Aberrations are modelled by dividing the pupil into two regions, each corresponding to light passing through the top and side facets. Aberration correction is demonstrated experimentally using a liquid crystal spatial light modulator for femtosecond microfabrication in fused silica. This technique allows controlled subsurface fabrication right up to the edge of the substrate. This can benefit a wide range of applications using direct laser writing, including the manufacture of waveguides and photonic crystals.
We present a technique to measure the refractive index profile of direct laser written waveguides. This method has the potential for straightforward implementation in an existing laser fabrication system. Quantitative phase microscopy, based on the Transfer of Intensity equation, is used to analyse waveguides fabricated with an ultrashort pulsed laser embedded several hundred micron below the surface of fused silica. It is shown that the cumulative phase change induced by the waveguide perpendicular to its axis may be monitored in real-time during the fabrication process. Results are verified through comparison with interferometry. Tomographic measurements using illumination from a high numerical aperture condenser lens are used to infer the waveguide cross-section. Results are compared with measurements of the waveguide cross-section from a third harmonic generation microscope.
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