Hollow-Core Holey Fiber Is Fabricated with Polymer

Polymer fibers generally are less expensive than silica fibers and often enable applications that would be economically impractical with silica. But polymers absorb strongly at wavelengths above 850 nm, primarily because of resonances with C-H vibrational bands, and thus are not suitable for infrared applications.

Hollow-core polymer fibers, however, which transmit light in the air of their hollow cores rather than in the polymer, would be expected to transmit in the infrared. Recently, researchers at the Optical Fibre Technology Centre of the University of Sydney in Australia fabricated what they believe are the first hollow-core polymer fibers and demonstrated that they indeed are capable of transmitting infrared wavelengths that are highly absorbed in polymer.

The hollow-core fibers are based upon the photonic bandgap effect, in which the arrangement of the holes in the cladding is such that certain wavelengths cannot propagate. Therefore, these wavelengths are confined by default in the fiber core. The fibers are similar to one-dimensional, ring-structured Bragg fibers in that they are composed of alternating rings of high-index polymer and low-average-index polymer and air (Figure 1).

Figure 1. Scanning electron micrographs show the cross sections of three fibers. The visible cracks and the apparent variation of the thickness of the core boundary are caused by cleaving the fiber. Images ©OSA.

The investigators fabricated the fibers with a two-stage process frequently used for polymer fibers. First, they constructed preforms by drilling holes into 70-mm-diameter polymethyl methacrylate blanks, and they drew the preforms into 6-mm canes. Then they fitted the canes into snug 12-mm-diameter sleeves and drew the resulting assemblies down to fibers 120 to 330 µm in diameter.

Although this process works well with conventional, solid polymer fibers, they encountered difficulty with hollow-core fibers because the large central hole — the fiber core — tended to expand during the drawing process. Although the ring-structured Bragg fibers are tolerant of such deformations, the researchers nonetheless found it helpful to control the expansion by applying external pressure while the fibers were being drawn.

Figure 2. The transmission peaks occurred at fixed ratios of wavelength to fiber diameter, as indicated by the top scale.The visible transmission through the fiber shifted from yellow to violet to blue to green to red (top to bottom) as the fiber size decreased.

To measure the transmission of the fibers, they launched light from a supercontinuum source into them and observed the transmitted light with a spectrum analyzer (Figure 2). Distinct transmission windows appeared in the fibers’ output, and the windows scaled with the fiber diameter as a result of the harmonic nature of the interference effect. There were five windows, and the lowest loss — ~31 dB/m — occurred at 1300 nm.

Figure 3. The transmission of the hollow-core fiber (black curve) and the transmission of polymethyl methacrylate (gray curve) are shown, with the polymer’s absorption peaks highlighted in gray. The transmission of the fiber is much better in the infrared than the transmission of the material itself.

The proof of the pudding, however, lay in a comparison of the fibers’ transmission to that of the polymer itself (Figure 3). For wavelengths longer than 1120 nm, the transmission of the fibers generally was significantly higher than the material’s. In particular, at ~1390 nm, the fibers’ loss of 40 dB/m was much better than that in polymer, 420 dB/m. For wavelengths beyond 1600 nm, where the polymer material essentially is opaque (with loss of ~3000 dB/m), the hollow-core waveguide still functioned, with a loss of approximately 60 dB/m.