Holey Fibers Connect to Conventional Fibers with Low Loss
Holey fibers have several characteristics that make them useful in numerous applications, including the ability to manipulate dispersion. But an impediment to their widespread utilization has been the difficulty in splicing holey fibers to conventional fibers. Even when the modes propagating in the two fibers match -- which is rarely the case -- the splice is problematic because the air holes in the holey fiber can be corrupted during the splicing process.
Recently, researchers at University of Bath in the UK demonstrated that holey fibers can be fabricated with a conventional fiber embedded in them, so that light propagating from the conventional fiber into the holey fiber (or vice versa) experiences loss of less than 1 dB, even when there is a significant mode mismatch between the two. This is better performance than can be obtained with a conventional splice between the two. The drawback of the technique is that it is not applicable to fibers that have already been fabricated.
Figure 1. To create a low-loss connection between a conventional single-mode fiber and a holey fiber, the single-mode fiber is threaded into a void in the preform from which the holey fiber is subsequently drawn (a). Seen in a cutaway view from the side, the core of the holey fiber is formed from the width of the single-mode fiber, plus a little silica from the preform (b). Images ©OSA.
Holey fibers are fabricated by first assembling a preform, a stack of hollow tubes and rods with the geometry desired in the final fiber. The preform is heated and softened, and it is stretched into an intermediate preform that retains the geometry of the original. The intermediate preform is then placed at the top of a drawing tower, again heated and softened, and drawn into the fiber itself, which also retains the original geometry.
The Bath researchers modified this process by omitting from the preform the solid rod that ultimately forms the core of the holey fiber. They stretched the preform to create the intermediate preform, which they called a ferrule. The ferrule had a central void large enough for a single-mode fiber to fit into it (Figures 1 and 2).
Figure 2. The ferrule is 3 mm in diameter, and the single-mode fiber can be seen threaded into the central void (a). A close-up view of the central void shows sufficient clearance to thread the single-mode fiber easily into the ferrule (b). The holey fiber drawn from the ferrule is endlessly single-mode (c).
They threaded a single-mode fiber into the entire length of the ferrule and drew the holey fiber from it. But they stopped the drawing process after several meters of holey fiber had been drawn, preserving the end of the ferrule and the single-mode fiber. The core of the new holey fiber was formed from the single-mode fiber and from part of the material of the ferrule.
The silica of the ferrule collapsed against the single-mode fiber, and the mode expanded as the ferrule narrowed to become guided by the core of the holey fiber. So long as it is gradual enough to be adiabatic, the transition from one fiber to the other can be low-loss.
Figure 3. To connect two single-mode fibers to the two cores of a dual-core holey fiber, the researchers omitted two rods from the holey fiber's preform (a). They then threaded a single-mode fiber into each of the voids (b) and drew the dual-core holey fiber (c). When illuminating both or either of the single-mode fibers, the light emerging from the dual-core fiber showed crosstalk extinction better than −34 dB (d, e, and f).
The researchers demonstrated their technique by creating low-loss connections between single-mode fibers and three types of holey fiber: an endlessly single-mode fiber, a highly nonlinear (i.e., small-core) holey fiber and a dual-core holey fiber. In all cases, the loss across the connection was 0.8 dB or less. In the case of the dual-core fiber, the connection also was accomplished with no observable crosstalk between the cores (Figure 3).
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