Optical fiber-type optical element, laser diode module, and fiber laser

An optical fiber-type optical element, comprising:na core section provided so as to extend in a light-guiding direction in which incident light propagates;a photosensitive layer provided so as to extend in the light-guiding direction and peripherally enclose the core section, the photosensitive layer including a refractive index grating formed therein by irradiation with ultraviolet light having a predetermined wavelength; anda cladding section provided between the core section and the photosensitive layer, the cladding section having a lower refractive index than the core section and a lower photosensitivity than the photosensitive layer, the photosensitivity being a property in which a refractive index changes in response to irradiation with the ultraviolet light,wherein the core section has a lower photosensitivity than the photosensitive layer,wherein the photosensitive layer is present in a region where a light-guiding mode in which a larger light amplitude part is mainly within the core section has a light intensity which is lower than 1/e2 times a maximum intensity,wherein the optical fiber-type optical element is a multi-mode fiber, andwherein the optical fiber-type optical element further comprises another cladding section provided so as to extend in the light-guiding direction and peripherally enclose the photosensitive layer.