Matchmaking: Holey Fiber Meets Conventional Fiber

Breck Hitz

Holey fibers — or photonic crystal fibers — possess many characteristics that make them potentially useful in a host of applications. They can be designed with customized dispersion characteristics, highly nonlinear properties, or to propagate only a single mode in an arbitrarily large core. But their usefulness is significantly eclipsed by the difficulty of splicing holey fibers to conventional fibers.

Figure 1. These scanning-electron microphotographs of the end face of the LMA-5 holey fiber show a gradually increasing collapse of the airholes with two, five, seven and nine (A to D, respectively) low-voltage arcs from the fusion splicer. Images reprinted with permission of Optics Letters.

The debilitating high losses occurring at splices between holey fibers and conventional fibers typically have two causes: an inherent mode mismatch between the two, and a collapse of the airholes in the holey fiber resulting from heat applied in creating the splice. When the airholes in the vicinity of the splice collapse, the fiber loses its ability to guide light, and the light that should be coupled into the other fiber escapes.

Recently, scientists at Hong Kong Polytechnic University demonstrated a technique using a conventional fusion splicer to create low-loss splices between a small-core holey fiber and a standard single-mode fiber without any intermediate fiber.

Figure 2. To create the appropriate thermal gradient in the holey fiber and fuse the fibers at the interface, the scientists offset the discharge location from the interface between the two fibers and applied repeated low-current discharges to the splice (A). The gradual collapse of the airholes in the holey fiber allowed the mode to expand adiabatically as it moved from the holey fiber into the single-mode fiber (B).

They applied repeated low-current discharges to the splice — deliberately creating a gradual collapse of the airholes — so that the mode in the holey fiber expanded adiabatically to match the mode of the single-mode fiber. Collapsing of airholes of the holey fiber during splice is generally considered a lethal drawback; however, here the scientists used it to facilitate low-loss splicing of small-core holey and conventional fibers.

They spliced two kinds of commercial holey fiber — Crystal Fibre A/S’ LMA-5 and NL-1550-POS-1 — to Corning Inc.’s SMF-28. The holey fibers had core diameters of 4.1 and 2.1 μm, respectively, and the SMF mode-field diameter was ~10.4 μm at 1550 nm. Before making the low-loss splices, the researchers measured the butt-coupling loss of splices between LMA-5 and SMF-28 at 3.62 dB, and between NL-1550-POS-1 and SMF-28 at 6.30 dB.

To illustrate the gradual collapse of airholes, they imaged the face of the LMA-5 fiber after different numbers of low-current discharges (Figure 1). The single-mode fiber was not in contact with the holey fiber for these photographs.

To create the low-loss splices, they offset the arc-discharge location 50 μm from the interface between the two fibers (Figure 2A). They applied a series of low-current discharges, producing a thermal gradient that gradually collapsed the airholes of the holey fiber, allowing an adiabatic expansion of the holey-fiber mode to match the larger mode in the single-mode fiber (Figure 2B).

Figure 3. The loss for a splice between LMA-5 holey fiber and conventional SMF-28 single-mode fiber initially decreased with an increasing number of low-current discharges (A). Similar results occurred with NL-1550-POS-1 holey fiber and SMF-28 (B). In both cases, the minimum loss was much less than the butt-coupling loss.

For both holey fibers, the splice loss decreased with the number of discharges to a minimum value and thereafter increased (Figure 3). In both cases, the minimum loss was significantly less than the butt-coupling loss.

Optics Letters, Jan. 15, 2007, pp. 115-117.