This dissertation describes the mode coupling in tapered fiber structure. The modal characteristics of tapered higher-order-mode fiber (HMF) are analyzed considering cladding-air boundary. As its application, we demonstrate novel fused-type mode selective couplers (FMSCs) that operate unique function in two-mode fiber (TMF) devices. Mode selective couplers (MSCs) are passive fiber-optic components that couple the $LP_{01}$ mode of a single-mode fiber (SMF) and the $LP_{11}$ mode of a TMF. They have been fabricated in the form of a polished coupler with a pair of a SMF and a TMF, where the phase-matching condition is satisfied between the $LP_{01}$ mode of the SMF and the $LP_{11}$ mode of the TMF. When compared to the polished-type MSCs, FMSCs have advantages of temperature-stability, compactness, and simple fabrication process. They don’t need special fiber pair to satisfy the phase-matching condition because SMF arm is simply prepared by tapering a portion of TMF. Additionally, novel directional couplers - $LP_{02}$ mode FMSCs - are demonstrated. Modeling, numerical calculation, fabrication, and characterization are described respectively.
$LP_{11}$ mode FMSCs are fabricated using circular core TMF at the wavelength of 1064 nm and elliptical core TMF at 1550 nm. The modal properties in the waist of the coupler are calculated to find optimum initial condition to satisfy the phase-matching condition in the coupling region. SMF arm and TMF arm are prepared by way of pre-tapering and etching portions of TMF respectively. They are fused and pulled together to form a fused coupler using flame-brushing method. The coupling efficiency of 80% and the mode extinction of 20 dB are achievable in each case, and the couplers show high temperature-stability over the range of about 80 degrees. The operating bandwidth measures from several tens of nm to over 100 nm.
Directional couplers that couple the $LP_{01}$ mode of SMF and the $LP_{02}$ mode of HMF are demonstrated for the first time. The $LP_{02}$ mode FMSCs are fabricated with circular core HMFs at the wavelength of 633 nm and 1550 nm respectively. Although the modeling and simulation schemes are similar to that of $LP_{11}$ mode FMSC, the fabrication condition of the $LP_{02}$ mode FMSC is more rigorous to suppress the unwanted mode coupling to $LP_{21}$ mode which is another higher-order mode guided in HMF. The coupling efficiency of 70% is achieved in each case and the mode extinction is measured to 13 dB at 633 nm and 8 dB at 1550 nm. The fabricated LP02 mode FMSC has potential application in broadband dispersion compensation using $LP_{02}$ mode if further research is followed to improve its mode extinction.
The broad operating bandwidth and the unique function of proposed FMSCs will be useful in WDM optical communication and sensor application.
