Optical Interferometer based on Gradient-Index Lens for Cryogenic Scanning Probe Microscopy

Optical-fiber-based interferometer has been widely used in the operation of cryogenic scanning probe microscopy to detect minute deflections of the cantilever probe. Achieving a high contrast of interference signals yields high force sensitivity. One approach to do so is to implement a Fabry-Perot cavity in which multiple reflections between the fiber and the cantilever produce sharp interference patterns. However, this method requires positioning the fiber very close to the cantilever surface, posing a challenge on alignment under cryogenic conditions. In this work, we present an interferometric setup utilizing a Gradient-Index (GRIN) lens to focus the output beam onto the cantilever surface. This design minimizes optical loss and improves the collection efficiency of reflected beam, enabling the formation of a Fabry-Perot interferometer with larger cavity lengths. We will compare the performance of this GRIN-lens-based system with that of the conventional fiber-only approach.