Apparatus include a microscope including an objective and a stage for positioning a test object relative to the objective, the stage being moveable with respect to the objective, and a sensor system, that includes a sensor light source, an interferometric sensor configured to receive light from the sensor light source, to introduce an optical path difference (OPD) between a first portion and a second portion of the light, the OPD being related to a distance between the objective lens and the stage, and to combine the first and second portions of the light to provide output light, a detector configured to detect the output light from the interferometric sensor, a fiber waveguide configured to direct light between the sensor light source, the interferometric sensor and the detector, a tunable optical cavity in a path of the light from the sensor light source and the interferometric sensor, and an electronic controller in communication with the detector, the electronic controller being configured to determine information related to the OPD based on the detected output light.