An optical fiber is positioned in a lumen of a sheath so a gap exists between the sheath and optical fiber. A filling member fills part of the longitudinal extent of the gap and fixes the optical fiber. The gap is devoid of the filling member over a part of the longitudinal extent of the of the optical fiber so that an air gap exists between the optical fiber and the sheath. In the event bending, expansion and/or contraction are applied to the sheath, the stress is inhibited from being transmitted to the optical fiber. If the sheath is expanded and contracted, one end of the optical fiber is open and so the optical fiber is not expanded/contracted like the sheath expansion and contraction. Consequently, stress is not likely to be transmitted to the optical fiber and so it is possible to maintain a constant length of the optical fiber.
