Extruded holey fibers give higher nonlinearity

By Yvonne Carts–Powell

A single–mode nonsilica glass holey fiber made from an extruded preform proves highly nonlinear—it has about a 500 times larger effective nonlinearity than standard silica fiber. Nonlinear fibers can be used to make compact amplifiers and other devices that operate at low powers.

Usually, holey fiber preforms are made by stacking capillaries around a solid rod to form the pattern of holes in the cladding region. Tanya Monro and others within the Optoelectronics Research Centre (ORC) at University of Southampton (UK) instead used extrusion, which is reproducible, controllable, and can make complex structured preforms with good surface quality. Because fewer interfaces are involved than in stacking approaches to making holey fibers, extrusion can theoretically offer lower losses. Extrusion can also be used to make structures that can't be made with the stacking method.

The Southampton group also diverged from the norm by drawing a fiber of compound glass, rather than pure silica. It is difficult to create conventional single–mode fibers from compound glasses without high losses, but the superior confinement available using holey fibers makes this possible. The group measured an effective nonlinearity of 550 W–1 km–1 in a holey fiber they made from SF57, a glass with a high lead concentration and relatively low softening temperature.1 SF57 has a refractive index of 1.83 at 1.53 μm and nonlinear refractive index of 4.1 × 10–19 m2/W at 1.06 μm.

The nonlinearity of the fiber is more than 500 times that of a standard single–mode fiber, more than 15 times that of the highest previous small–core silica holey fiber, and comparable to the previously reported high nonlinearity fibers based on chalcogenide glass. The researchers suggest that other compound glasses, with higher nonlinearities than SF6, and with smaller cores, could be used to create highly nonlinear optical devices.

Monro's group extruded a preform with a central core supported by three long thin membranes. The preform, with an outer diameter of 16 mm, was caned in a fiber–drawing tower down to an outer diameter of 1.6 mm, while retaining the same geometry. The cane was then inserted into an extruded jacket tube, and this assembly was drawn down to a fiber with an outer diameter of 120 mm.

The core is 2 μm in diameter and is suspended by three 2–μm–long supports that are less than 400 nm thick. The air around the core, everywhere except in the three supports, acts to confine light within the core. This allows the fiber to effectively transmit single–mode at a number of wavelengths.

They observed robust single–mode guidance at both 633 and 1500 nm. Loss in the fiber was 4 dB/m at 1550 nm, which the researchers believe is due mostly to light leaking out from the struts. The loss could be decreased, they expect, by lengthening the struts, which can be achieved using the same caning and drawing process.

For more information contact: Tanya Monro at tmm@orc.soton.ac.uk.

1. T. M. Monro et al., OFC 2002, Postdeadline Paper FA1.

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