Tapered single-mode fibres and devices. Part 1: Adiabaticity criteria

Heat-and-pull rig for fiber taper fabrication

“…23 This criterion requires the taper profile to be such that the change in taper angle is small enough to prevent light propagation being either coupled from the fundamental mode, HE 11 , to higher order parasitic modes in the fiber or being lost as radiation. The criterion can be stated as ͉dr / dz͉ ഛ ͑z͓͒␤ 1 ͑z͒ − ␤ 2 ͑z͔͒ /2 , where dr / dz defines the local taper angle, ⍀͑z͒, by the trigonometric expression ⍀͑z͒ = tan −1 ͉dr / dz͉, ͑z͒ is the local core radius, ␤ 1 ͑z͒ and ␤ 2 ͑z͒ are the local propagation constants of the fundamental mode and the next closest mode, respectively, and z is along the fiber axis.…”

“…The criterion can be stated as ͉dr / dz͉ ഛ ͑z͓͒␤ 1 ͑z͒ − ␤ 2 ͑z͔͒ /2 , where dr / dz defines the local taper angle, ⍀͑z͒, by the trigonometric expression ⍀͑z͒ = tan −1 ͉dr / dz͉, ͑z͒ is the local core radius, ␤ 1 ͑z͒ and ␤ 2 ͑z͒ are the local propagation constants of the fundamental mode and the next closest mode, respectively, and z is along the fiber axis. 15,23 The transcendental equation for the propagation constants has been solved numerically as a function of local taper diameter. Figure 3 shows a plot of this criterion and the core taper angle of a typical adiabatic taper as produced by the abovedescribed method.…”

“…An approximation for the core guidance curve is given in Ref. 23 and is based on the assumption of an infinite cladding diameter. Gradually, the diameter and core guidance parameter decrease until the core effectively disappears and the propagating light becomes cladding-air guided.…”

“…For the cladding guidance curve, an approximation is also given in Ref. 23 and is based on the assumption that the core diameter is vanishing. Figure 4 shows a typical taper angle decreasing when approaching the taper waist.…”

“…This article describes the procedure and requirements for pulling adiabatic fiber tapers 15,23 with typical losses in the range of 0.1-0.3 dB at 980 nm. We also report on successful attempts at fabricating bottle resonators to suitable dimensions using a microtapering technique.…”