Abstract
We have produced high transmission sub-wavelength tapered optical fibers for the purpose of whispering gallery mode coupling in fused silica microcavities at 780 nm. A detailed analysis of the fiber transmittance evolution during tapering is demonstrated to reflect precisely the mode coupling and cutoff in the fiber. This allows to control the final size, the number of guided modes and their effective index. These results are checked by evanescent wave mapping measurements on the resulting taper.
Highlights
Tapered optical fibers have attracted a large interest in the two last decades, due to their numerous applications as directional fiber-fiber couplers, filters for wavelength multiplexing[1], biosensors and more recently for the production of broad-spectrum-light from femtosecond laser impulsions[2]
Efficient coupling to microcavities whispering gallery modes (WGM) implies interaction lengths reduced to a few micrometers which in turn requires to work with smaller taper diameters and to control the effective index of the taper modes
The observed oscillations originate from the beating of different local modes, which propagate along the taper with different propagation constants βi ≡ 2π/λ × Neff, i (Neff, i is their effective index), and recombine at the output with a relative phase which depends on the effective length of the taper
Summary
Tapered optical fibers have attracted a large interest in the two last decades, due to their numerous applications as directional fiber-fiber couplers, filters for wavelength multiplexing[1], biosensors and more recently for the production of broad-spectrum-light from femtosecond laser impulsions[2]. A. Birks, “Phase-matched excitation of whispering-gallery-mode resonances by a fiber taper,” Opt. Lett. Haroche, “Characterizing whispering-gallery modes in microspheres by direct observation of the optical standing wave in the near field,” Opt. Lett.
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