Optical filtering with axial whispering gallery modes on the surface of tapered optical fibers

We study the Q-factors of whispering gallery modes on the surface of standard optical fibers using both spectral and ring-down measurements and show that annealing by the fire torch yields Q-factors higher than 10⁷. In this case, however, variations of the effective radius are introduced, that make the mode spectrum complicated as a number of modes with a nonzero axial component of the wave vector appear. We demonstrate that tapering the fiber sample allows to evade this complexity and to build up an efficient filter operating in transmission. (letter)