Automating interferometric focal length measurement techniques with varifocal optics

Abstract

Characterizing the focal length of a sample lens is one of the most fundamental and ubiquitous of measurements in optical metrology. In this paper, we eliminate bulk mechanical motion of optical components in various popular interferometric shop testing methods for sample focal length measurement. While most existing interferometric focal length measurement techniques require precise positioning and repositioning of bulk optical components according to the sample lens focal length, we show that repeatable and reliable focal length measurements are possible without a need of bulk motion of optical elements. We demonstrate that elimination of bulk mechanical motion is achieved via the introduction of varifocal optics such as focus tunable optic lenses in measurement testbeds. This improves the repeatability and compactness of these methods. Interferometric techniques which we automate are ones that use a standard double frequency grating and a Ronchi ruling. Moreover, we also upgrade a traditional Twyman–Green (TG) interferometer to an actively tunable version which can characterize focal length without requiring bulk motion of optical components. The presented techniques are validated with results from carefully designed experiments.