Mode‐field‐converting fiber for coupling to photonic devices

Abstract

A mode‐field‐converting (MFC) fiber is proposed to reduce mode‐field mismatch loss which is produced when an optical fiber and a photonic device chip are butt‐coupled to each other. an MFC fiber is fabricated by fusion‐splicing a specially designed device‐compatible fiber with an ordinary transmission‐line fiber and by heating the spliced point to form an index‐and‐dimensional taper. Also proposed is an elliptic‐mode‐field (EMF) fiber which is composed of an elliptic core and two claddings to yield a highly elliptic mode field. An EMF fiber with an ellipticity of 2 and MFC fibers with the EMF fiber were fabricated experimentally and the lowest conversion loss of 0.09 dB was obtained. the MFC fiber was designed for coupling to a semiconductor optical switch and achieved shape conversion. Arrays of eight MFC fibers also were fabricated, and the lowest loss was 0.11 dB with a size conversion ratio of 2. the MFC fiber array was designed for coupling to a high‐Δ silica waveguide. Those loss values around 0.1 dB were sufficiently lower than the corresponding initial mode‐field mismatch losses of 0.7 and 1.7 dB; and, thus, the loss reduction effect of an MFC fiber was confirmed experimentally.