An optical add/drop multiplexer is a key device in wavelength-division multiplexing communication systems, that adds/drops a few channels in a communication line. This dissertation describes an all-fiber optical add-drop multiplexer based on spatial-mode coupling in a two-mode fiber(TMF) by a tilted fiber Bragg grating and a mode-selective coupler. Since it is an all-fiber device using a fiber Bragg grating, it has potential advantages of low insertion loss and excellent spectral profile. The principle, fabrication methods, and the optical characteristics of the device are described. And several novel fabrication procedures are proposed and demonstrated as an effort to make a more efficient device than previously demonstrated examples.
The present device comprises two mode-selective couplers and one tilted fiber Bragg grating between them. The mode-selective coupler couples the fundamental $LP_01$ mode of a single-mode fiber to a higher order $LP_11$ mode of a TMF. The tilted fiber Bragg grating in this device reflects light at a specific wavelength accompanied by the conversion of the propagating modes. To reduce coupling loss in the mode-selective couplers, the TMF was designed to have high numerical aperture and an elliptical core with odd $LP_11$ mode cut off. In addition, by tapering one of fibers properly, the coupling ratio of the coupler was improved further based on better phase matching.
The tilt angle of the grating was adjusted for an efficient mode conversion without unwanted mode coupling which causes a loss and an interchannel crosstalk. The fine adjustment was performed by using a novel fabrication method which enabled in-situ measurement of the grating reflection spectrum along the tilt angle. Improved suppression of the unwanted reflection of the $LP_11$ mode to the $LP_11$ mode itself was demonstrated. Moreover, it was shown that losses of the grating due to cladding mode coupling, which is common in fiber Bragg gratings, are very small in the case of this tilted grating using the high NA TMF.
