A force sensor apparatus includes a tube portion having a plurality of radial ribs and at least one fiber optic strain gauge positioned over each of the plurality of radial ribs. The strain gauges comprise a negative thermo-optic coefficient optical fiber material. A proximal end of the tube portion is operably couplable to a shaft of a surgical instrument and a distal end of the tube portion is proximally couplable to a wrist joint. Optionally, a thermal shunt shell is over an outer surface of the tube portion. Adjacent fiber optic strain gauges with differing thermal responses are used to solve simultaneous equations in strain and temperature to derive strain while rejecting thermal effects.