An apparatus and method for diagnosis or treatment of a vessel or organ. The apparatus includes a deformable body such as a catheter having a tissue ablation end effector and an irrigation channel in fluid communication therewith. At least two sensors are disposed within a distal extremity of the deformable body, the sensors being responsive to a wave in a specified range of frequency to detect deformations resulting from a contact force applied to the distal extremity. A microprocessor can be operatively coupled with the sensors to receive outputs therefrom, the microprocessor being configured to resolve a multi-dimensional force vector corresponding to the contact force. In one embodiment, the sensors are fiber Bragg grating sensors, and the wave is injected into the fiber Bragg grating strain sensors from a laser diode.