Researchers at California Institute of Technology in Pasadena have filled the holes of a holey fiber and propose that such a device could prove useful for nonlinear optics, for spectroscopy and for other applications. They have demonstrated efficient two-photon fluorescence in their experimental fiber.There are two types of holey fiber, each based on its own optical principle. In a photonic crystal fiber, light is guided in the core by total internal reflection because the airholes in the cladding make its effective index lower than that of the core. In a photonic bandgap fiber, light is guided in the core by Bragg reflection caused by the spatial periodicity of the holes in the cladding. The core index of a photonic bandgap fiber is usually less than that of the cladding. (The terminology used here is not universally accepted: Sometimes "photonic crystal fiber" means any holey, or "microstructured," fiber. For the purposes of this article, we will use the definitions in this paragraph.)Conceptually, the scientists wanted to fill the core of a hollow-core photonic bandgap fiber with a working fluid, thereby converting it into a photonic crystal fiber in which light would be guided in the working fluid. By properly choosing the fluid -- a water solution of a material of spectroscopic interest, for example -- they could capitalize on the fluid's long interaction length with the guided light.But to fill only the core and not the cladding with a working fluid, they needed to resort to a multistep fabrication scheme (Figures 1a to 1d). First, they connected a syringe to the end of the fiber and forced a UV-curable adhesive into all the fiber's holes. Figure 1. The core is plugged with an adhesive. The cladding airholes and the core then are filled with a working fluid.Because the air core was significantly larger than the cladding airholes, the viscous adhesive flowed more easily -- and farther -- into the core than into the cladding (Figure 1a). The scientists cured the adhesive and cleaved the fiber so that a plug of adhesive remained in the core but that the airholes in the cladding were open.They again connected the syringe to the end of the fiber and forced adhesive into the holes. But now the core was plugged, so the adhesive flowed only into those airholes in the cladding.The scientists continued until these holes were filled to a point past the plug in the core (Figure 1b). They again cured the adhesive and cleaved the fiber, this time yielding plugs in all the cladding airholes (Figure 1c). At last, they could force a working fluid into the core alone because all the cladding airholes were plugged. Once the core was satisfactorily filled, they cut off the end of the fiber with the plugged cladding airholes (Figure 1d).Figure 2. Because the airholes occupy such a large portion of the cladding, light can be guided in even a low-index core such as water. This model shows the profile of a guided mode in a water-core fiber.Because the fiber's cladding is approximately 90 percent air, light is guided in even a low-index core fluid such as water (Figure 2). Accordingly, water solutions of chemicals could be sensitively analyzed spectroscopically in the fiber. Teflon-coated hollow fibers similarly confine light in a working fluid over long distances, but the core-filled holey fiber offers a much larger numerical aperture and a much smaller core diameter, both significant advantages in spectroscopic applications.Another potential application is in nonlinear optics. If the holey fiber core is filled with highly nonlinear optical materials, the resulting fiber can generate a much greater nonlinear response than a traditional silica fiber.Figure 3. To demonstrate the potential of the fluid-filled fiber, the scientists excited two-photon fluorescence in a fiber core doped with rhodamine 640.To demonstrate the potential of the fibers, the scientists filled the core with an optical adhesive doped with rhodamine 640 and illuminated it with 890-nm, 200-fs laser pulses (Figure 3). The resulting two-photon fluorescence, collected in the backward direction, was generated more efficiently than with conventional techniques.