Certain aspects relate to systems and techniques for optical strain sensing in medical instruments. In one aspect, a medical instrument includes an elongated shaft, and at least one pull wire extending from a proximal end of the elongated shaft to the distal end of the elongated shaft. The at least one pull wire is configured to cause actuation of the medical instrument in at least one degree of freedom. The at least one pull wire includes an optical fiber configured to provide an indication of strain along the at least one pull wire.