A polishing system includes a platen, a carrier head to hold a surface of a substrate against a polishing pad on the platen, a monitoring module including a light source and a detector, an optical fiber having a proximate end coupled to the monitoring module and a distal end, and an optical head removably mounted in the platen. The optical head holds the distal end of the optical fiber to direct light through a window in the polishing pad to the surface of the substrate and receive reflected light from the surface of the substrate, and the optical head is configured to adjust a distance from the distal end of the fiber to the window.