Mapping of index of refraction profile for polymer gradient index optics using confocal Raman spectroscopy

Nicholas S Kochan,Duncan T Moore,Greg R Schmidt,Andrew J Berger

doi:10.1117/1.oe.59.11.112605

Nicholas S Kochan, Duncan T Moore + Show 2 more

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https://doi.org/10.1117/1.oe.59.11.112605

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Abstract

Scanning confocal Raman microscopy is proposed to measure a gradient index (GRIN) profile at an optical surface. The Raman microscope is calibrated to index of refraction for a binary copolymer GRIN material, and then the index of refraction is mapped on the plano surface of a GRIN polymer lens. The measurement deduces axial shift of 680 μm and identifies lateral tilt or decenter with respect to the nominal position of the GRIN profile. Results suggest that the mapping method is a nondestructive way to measure the GRIN profile of a GRIN lens and its positioning within the lens geometry, to within the sampling precision of the Raman microscope.

Highlights

Gradient index (GRIN) optics offer many advantages in optical design but often pose unique optical metrology challenges
Raman spectroscopy is superior to interferometry for mapping the GRIN profile in the following ways: it can nondestructively measure large differences in index of refraction and can map the absolute index of refraction rather than difference in index of refraction across the optic
Three data points at different locations on each homogeneous sample are taken with the Raman microscope to account for unwanted material variation within a reference measurement

Summary

Introduction

Gradient index (GRIN) optics offer many advantages in optical design but often pose unique optical metrology challenges. Direct measurements of imaging performance at the design wavelengths, such as with interferometry,[1] can be used for measuring effects of manufacturing errors in a GRIN lens system. These effects on imaging performance cannot diagnose all errors of GRIN fabrication. To this end, Raman spectroscopy can provide material distribution information, and index of refraction information, to measure the optic nondestructively. Raman spectroscopy is superior to interferometry for mapping the GRIN profile in the following ways: it can nondestructively measure large differences in index of refraction and can map the absolute index of refraction rather than difference in index of refraction across the optic.

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