Optical cavities employing photonic bandgap structures in a taper fiber

Summary form only given. As is known, a singularity in a one-dimensional bandgap structure can be used as a resonator. It turns out that a resonator can be formed without any singularity in a homogeneous periodical structure if the conditions of propagation of the wave along the structure are changing gradually. For example, the gradual change in propagation constant takes place for a whispering gallery (WG) light wave propagating in a tapered fiber with the refraction index or/and diameter modulated with the space period of tens micrometers. The formation of the resonator can be understood because the propagation constant becomes negative in the narrow fiber side and periodically changes in the wide side. These periodical changes form DBR in the fiber wide side. Since the propagation constant gradually decreases gradually along the taper fiber, the DBR has a limited length and its reflective property depends on the propagation constants. The quality factor Q of the resonator depends on the same parameters.