FIBEROPTICS: Fabrication of long holey fibers gets practical

Holey optical fibers are microstructured silica fibers with an array of microscopic air holes that run down the full fiber length. If the size, spacing, and geometric arrangement of holes are chosen appropriately, these structures can be made to guide light. Guidance can be obtained by either of two distinct mechanisms: through photonic-bandgap effects related to certain periodic arrangements of large air holes or through volume average refractive-index effects that do not intrinsically rely on air-hole periodicity.

The optical properties of such fibers are quite unlike those of conventional fibers and are causing great excitement within the optics community. For example, holey fibers can produce unique dispersion properties, exhibit large mode areas, and provide single-mode operation over an extended range of operating wavelengths.

Even with such interesting features, little has been known about the most basic properties of holey fibers until recently. Reasons for this include difficulty in modeling such complex optical systems and problems related to the fabrication and ease of handling sufficiently long lengths of the fiber, which have hindered the reliable measurement of even the most basic properties such as loss and dispersion.

Technological stretch

Peter Bennett, Tanya Monro, and David Richardson of the Optoelectronics Research Centre at the University of Southampton (Southampton, England) report they have fabricated mechanically robust holey fiber greater than 50 m in length. They have also demonstrated that the fiber can be spliced to conventional fiber types—an important issue for practical application of the technology. These technological advances permitted a detailed characterization of a holey fiber near 1.5 µm. The results may represent the first report of loss, nonlinearity, and dispersion for holey fiber in the 1550-nm telecommunications window.¹

To produce robust fibers, the researchers applied a modification of the fabrication process used to make the first photonic crystal fibers.² Pure silica capillaries were stacked in a hexagonal pattern with a solid silica rod as the core. The stack was then drawn in a two-stage process, with a borosilicate glass outer cladding added after the first draw. This structure was then pulled to a glass-clad holey fiber of approximately 250-µm outer diameter and coated with a conventional polymer coating to further strengthen the fiber. Bennett and colleagues believe the two-coating fabrication process should be capable of producing kilometer-length fibers without any modifications to the procedure.

A scanning electron micrograph of the holey fiber characterized by the researchers showed that its initial periodic structure becomes slightly deformed during the pulling process (see figure on p. 20). The spacing of the holes is approximately 1.8 µm, with a hole size of 0.34 µm. The air-filling fraction is around 20%. The fiber is single-mode at 1550 nm.

The loss determined by the standard cutback method was 0.24 dBm^-1, a value that the researchers claim is not unreasonable at such an early stage of a fiber's development. Ultimately, they believe that significantly lower losses will be obtainable.

When the holey fiber was spliced to conventional dispersion-shifted fiber, the resulting loss across the splice was roughly 1.5 dB at 1.55 µm, a value that could be almost entirely attributed to the large mode mismatch between the two fiber types. Although such splice losses are not prohibitive, Bennett and colleagues believe it should be possible to reduce them using a buffer fiber of intermediate spot size.

The research team measured the group-velocity dispersion of a 6.5-m length of the holey fiber over a range of 1480-1590 nm using a conventional electronic phase-measurement technique that includes a tunable narrow-linewidth laser and a network analyzer. From measurements of the Kerr nonlinearity and Brillouin threshold they also found that the small effective-mode area meant that the effective nonlinearity of the fiber was approximately four times larger than that of standard dispersion-shifted fiber. The dispersion and nonlinearity measurements were in agreement with predictions made from a numerical model developed at Southampton. This model can be used to accurately predict the properties of holey fibers prior to fabrication.

The properties of holey fibers are interesting enough that they can be expected to find use in a broad range of application areas including ultrafast optics, fiber lasers, optical sensors, and telecommunications.

REFERENCES

1. P. J. Bennett, T. M. Monro, and D. J. Richardson, Opt. Lett. 24, 1203 (Sept. 1, 1999).
2. P. J. Bennett, T. M. Monro, N. G. R. Broderick, and D. J. Richardson, Proc. European Conference on Optical Communications, Nice, France, paper MoB1.2 (Sept. 1999).