Effect of an Ambient Index Change on the Transmission Characteristics of Versatile D-Shaped Fibers

The effect of an ambient index change on the transmission characteristics of versatile D-shaped fibers, such as single mode fibers (SMFs) and photonic crystal fibers (PCFs) for various coupling strengths of the evanescent field is investigated. The coupling strength of the evanescent field is changed by using the remaining length of the cladding region (h) in the D-shaped fiber, which is defined as the distance between the core and the cladding. The transmission loss of the D-shaped fibers changes in the range between 0 to −62 dB as the ambient index is increased in the range from 1.3 to 1.8 because the evanescent field coupling between the core region and the cladding of the D-shaped fiber is enhanced by the changes in the ambient index. A maximum transmission loss of −62 dB is induced when the ambient index is equal to the effective index of the core mode. As the ambient index becomes higher than the effective index of the core mode, the transmission loss is decreased to −8 dB because the evanescent field is coupled back into the core region. The sensitivity of the transmission loss to the ambient index change strongly depends on the type of fiber, such as a SMF or a PCF, and on the coupling strength of the evanescent field. The remaining length of the cladding region (h) can be determined by using the coupling strength of the evanescent field.