Long-period fiber gratings have been widely used as wavelength filters in optical fiber communication and fiber optic sensor systems.
In this thesis, we demonstrate a long-period fiber grating that couplies core-mode to cladding-modes in the infrared region, which is written by the modal-interference pattern of guided light of 488 nm wavelength. The modal-interference pattern along the fiber induces permanent refractive index change in the Ge-doped photosensitive fiber. Theoretical calculation has been carried out and showed that the grating has resonant coupling at multiple wavelengths as well as the writing wavelength, where the phase matching condition is satisfied.
In the experiment, elliptical-core fiber with cutoff wavelength of 630 nm was used. The fiber was hydrogen-loaded for high photosensitivity. The grating was written using interference pattern of $LP_01$ and $LP_11$ core modes at 488 nm. The fabricated grating showed 99% coupling efficiency between the two core modes for the grating length of 50 cm at the writing wavelength. The coupling between $LP_01$ core-mode and cladding mode was observed in infrared region, at the wavelengths of 867.25, 869.65, 873.25, 875.5 nm. At each wavelength, the coupling efficiency and the 3-dB spectral bandwidth was about 60% and 1.5 nm, respectively. The four wavelengths correspond to coupling to two different cladding modes for two orthogonal polarization states of the input light. This method makes it possible to fabricate a long-period grating with total length of more than tens of centimeter using blue-green laser wavelengths, that can be used at infrared wavelength. The coupling wavelengths can be shifted to the communication wavelengths of 1.3 or 1.55㎛, if a fiber with proper paramenter for the required phase matching condition is used.
