Sensitivity of various D-shaped fibers to external refractiveindex change depending on coupling strength ofevanescent field

摘要

The effect of an ambient index change on the transmission characteristics of versatile D-shaped fibers such as singlemode fibers (SMFs) and photonic crystal fibers (PCFs) is investigated, which depend on the coupling strength of theevanescent field. The transmission loss of the D-shaped fibers increases as the ambient index increases because theevanescent field coupling from the core region can be enhanced by the increase of the ambient index. The couplingstrength of the evanescent field can be changed by the remaining length of the cladding region in the D-shaped fiber,which can be defined as the distance between the core and the cladding. The transmission loss becomes extremely highwhen the ambient index is equal to the effective index of the core mode. When the ambient index is higher than that ofthe effective index of the core mode, the transmission loss decreases because the evanescent field is coupled back intothe core region. The sensitivity of the transmission loss to the ambient index change is strongly depending on types, theremaining length of the cladding, and the polished length of fibers such as SMFs and PCFs. The remaining length of thecladding region can be determined by the coupling strength of the evanescent field.