Measurement of mechanical properties of GRIN rod lens

The GRIN rod lenses are favorable optical elements for the image transmission in applications where transverse dimensions of the optical system are limited such as rigid borescopes or endoscopes. Flexible versions of these instruments usually use imaging bundles of optical fibers to assure both the mechanical elasticity and the optical transmission. The image resolution of this solution suffers from being limited by minimum fiber core diameter allowable for imaging and total number of fibers in a given cross-section area. GRIN rod lens allows for a higher image resolution for small diameters. While the optical properties of the GRIN rod lens are well known, there is no information about its allowed mechanical stress. In this paper we present a simple system for load and deformation measurement of a GRIN rod lens aimed to be used for a miniature endoscope for eye surgery operation. The system allows for loading force measurement with a deformation strain gauge cell and deformation measurement with an optical triangulation sensor. The paper presents deformation limits of the tested GRIN rod lens loaded by a three-point bending test. We also evaluated the Young modulus of the GRIN rod lens material from the measured data. Our data shows that tested GRIN lenses can be statically deformed by bending down to minimum radius 105 mm without braking.