Insertion apparatus

1. An insertion apparatus comprising:

a flexible insert section configured to be inserted into an insertion target; and

a curved-shape detection sensor configured to detect a curved shape of the insert section,

wherein the curved-shape detection sensor comprises an optical fiber including at least a core, a cladding covering the core, a coating covering the cladding, and a sensing material mechanically attached to the core, the curved-shape detection sensor being configured to detect the curved shape,

the optical fiber is disposed at least in the insert section, and

the optical fiber includes at least one displacement restraint configured to directly or indirectly restrain a displacement of the sensing material relative to the insert section,

wherein the displacement restraint is formed in an opening formed by partly removing the coating and the cladding, and

the displacement restraint directly restrains a displacement of the core relative to an internal component of the insert section along curvature of the insert section.

2. The insertion apparatus according to claim 1 , wherein the sensing material includes a light absorber that absorbs part of light guided through the core of the optical fiber,

the light absorber has an adhesive force, and

the sensing material also serves as the displacement restraint.

3. The insertion apparatus according to claim 1 , wherein

the displacement restraint restrains a displacement of the sensing material relative to the insert section indirectly through the cladding by restraining a displacement of the cladding relative to an internal component of the insert section along curvature of the insert section.

4. The insertion apparatus according to claim 3 , wherein the displacement restraint restrains a displacement by adhering to the internal component.

5. The insertion apparatus according to claim 3 , wherein the displacement restraint is disposed in a same position as the sensing material in a longitudinal direction of the optical fiber or close to the sensing material in the longitudinal direction of the optical fiber.

6. The insertion apparatus according to claim 3 , wherein the displacement restraint is configured to contact the core and has a material whose refractive index is lower than that of the core.

7. The insertion apparatus according to claim 3 , wherein the displacement restraint comprises a plurality of displacement restraints, and

the optical fiber in a range between the plurality of displacement restraints is slackened to restrain a displacement of the core relative to an internal component of the insert section along curvature of the insert section.

8. The insertion apparatus according to claim 1 , wherein the displacement restraint restrains a displacement by adhering to the internal component.

9. The insertion apparatus according to claim 1 , wherein the displacement restraint is disposed in a same position as the sensing part in a longitudinal direction of the optical fiber or close to the sensing part in the longitudinal direction of the optical fiber.

10. The insertion apparatus according to claim 1 , wherein the displacement restraint is configured to contact the core and has a material whose refractive index is lower than that of the core.

11. The insertion apparatus according to claim 1 , wherein the displacement restraint comprises a plurality of displacement restraints, and

the optical fiber in a range between the plurality of displacement restraints is slackened to restrain a displacement of the core relative to an internal component of the insert section along curvature of the insert section.