a metallized glass fiber optic is braze - bonded or solder - bonded to a metal shim to form a sensor unit that may be spot welded to a structure for monitoring structural strain . preferably shim thermal characteristics are selected to match those of the structure being monitored , and a cover plate protects the fiber after attachment to the shim . alternatively , a non - metallized optical fiber is attached to the metal shim using a special ceramic adhesive coat whose metal constituents thermally match the thermal expansion co - efficient of the shim . after adhesive curing , a protective cover plate is attached . an embodiment for use with an extrinsic fabry - perot type sensor provides a metal plate with a fiber - sized groove that guides the target and the transmit / receive fiber optic elements together to form an air gap . another embodiment metallizes the fiber optic elements with a mask ring having a depressed region into which a cover plate is attached . each of the embodiments may be used with a white light interferometer to provide long term strain measurements at temperatures up to about 1 , 000 ° f ., and may be used in radiation environments .