APL Photonics (May 2020)

Low-loss, high-bandwidth fiber-to-chip coupling using capped adiabatic tapered fibers

  • Saeed Khan,
  • Sonia M. Buckley,
  • Jeff Chiles,
  • Richard P. Mirin,
  • Sae Woo Nam,
  • Jeffrey M. Shainline

DOI
https://doi.org/10.1063/1.5145105
Journal volume & issue
Vol. 5, no. 5
pp. 056101 – 056101-8

Abstract

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We demonstrate adiabatically tapered fibers terminating in sub-micron tips that are clad with a higher-index material for coupling to an on-chip waveguide. This cladding enables coupling to a high-index waveguide without losing light to the buried oxide. A technique to clad the tip of the tapered fiber with a higher-index polymer is introduced. Conventional tapered waveguides and forked tapered waveguide structures are investigated for coupling from the clad fiber to the on-chip waveguide. We find the forked waveguide facilitates alignment and packaging, while the conventional taper leads to higher bandwidth. The insertion loss from a fiber through a forked coupler to a sub-micron silicon nitride waveguide is 1.1 dB and the 3 dB bandwidth is 90 nm. The coupling loss in the packaged device is 1.3 dB. With a fiber coupled to a conventional tapered waveguide, the loss is 1.4 dB with a 3 dB bandwidth extending beyond the range of the measurement apparatus, estimated to exceed 250 nm.