A hollow-core microstructured polymer optical fiber is fabricated from polycarbonate material and guidance by inhibited coupling in a two-layer structure is demonstrated in two strong transmission bands with minimum losses of at and at . The latter corresponds to a loss well below the polycarbonate material loss at this wavelength, and to our knowledge it is the lowest loss hollow-core polymer fiber reported to date. The short-term operational temperature limit of the fiber is shown to be , significantly higher than that of conventional polymer optical fibers made of other polymers.
© 2008 Optical Society of America
Short distance, high-data-rate communication links based on a polymer optical fiber (POF) are increasingly being employed to access digital services and devices in local networks, cars, airplanes, hospitals, apartments, and office spaces. The physical characteristics of POFs offer a number of advantages over conventional cables; a POF is lightweight, flexible, and cheap, while offering vastly superior data rates over copper cable . A persistent problem with POF, however, is the poor performance in relatively humid, high-temperature environments. A key requirement for the use of fibers in the automobile industry is operation at temperatures between and and an attenuation below at either 650 or .
The range of highly transparent thermoplastic materials available for POF is limited, most having a glass-transition temperature below , such as the most commonly used polymethylmethacrylate (PMMA). One notable exception is polycarbonate (PC) with , which was viewed as the most likely candidate for high-temperature POF applications , but cannot meet the loss limit [2, 3, 4]. Other thermoplastics with much higher do exist but have even lower transparency.
A few years ago, light guidance in polymer fibers was demonstrated using microstructure [5, 6]. Light was guided in a hollow-core POF through the photonic bandgap effect , or the inhibited coupling mechanism [8, 9]. It was estimated that in such fiber, the effect of the material loss on the transmission can be reduced by almost 4 orders of magnitude by having only 0.025% overlap between the core mode and the material . Demonstrated loss values for hollow-core silica fibers are as low as for fibers guiding by photonic bandgaps  and for inhibited-coupling guidance .
In this Letter we report the fabrication and characterization of a high-temperature-resistant hollow-core microstructured polymer optical fiber (HC-mPOF) with a simple two-layer kagome-like structure, made from a low-optical-grade, high- material. This demonstrates both that inhibited-coupling guidance can be achieved with a simple two-layer structure, and that this guidance mechanism can potentially provide an alternative route toward meeting the key requirements for POF in high-temperature applications.
The fiber was made using the capillary stacking method  using -diameter low-grade PC capillaries with a wall thickness. During the fiber draw a overpressure was selectively applied to the core only. The draw conditions created a lattice in the cladding similar to a kagome lattice.
A scanning-electron mircoscope image of the fiber is shown in Fig. 1a . The visible deformations of the structure are caused by the razor-blade cutting method, which drags the thin bridges off to one side [cutting direction is left to right in (a) and bottom left to top right in (b)]. Cutting PC does not seem to involve crack propagation as it does for PMMA fibers . Figure 1b shows the transmitted colored spot guided in the core when the fiber is illuminated from below in the microscope using a white-light source. The guided colors depend on the dimensions. Note that the guided-mode profile in the core is perfectly hexagonal with straight, dark lines outlining it, confirming that the deformations are at the end face only. The fiber core is measured to be in diameter, and the struts in the lattice in this fiber are estimated to range from in thickness.
A broadband supercontinuum source was launched into two pieces of in length of fiber of and diameter, and the transmission spectrum was recorded. Two transmission bands are observed as shown in Fig. 2 : (1) -wide band around and (2) a -wide band around . The difference in outer diameter (of 3.5%) causes the transmission bands of the two fibers to be centered on 581 and (a 5.5% shift) and in the near-IR (NIR) on 1295 and (a 3.5% shift). The high-loss regions surrounding these transmission bands can be estimated by considering the struts in the cladding to be planar waveguides and calculating their mode cutoffs [13, 9]. Estimating the strut thickness to be on average for these fibers gives high-loss regions near 540, 810, and in agreement with the observed spectra. The width of the high-loss regions is mostly a consequence of variations in strut thickness in the fiber cross section, which increasingly affects shorter wavelengths, and the large refractive index of PC that widens the high-loss regions through vector effects . The transmission windows of the fiber could be easily adjusted to match the 650 or automotive specifications through the structure size.
The inhibited coupling guidance mechanism has been observed in calculations on kagome-structured photonic crystal fibers [8, 13, 14]. A characteristic of this mechanism is that increasing the number of cladding layers around the core does not reduce the loss of the fiber [8, 13], in contrast to the case of photonic bandgap guidance. We note that the two-layer structure reported here is to our knowledge the first experimental demonstration of clear transmission through such a simple structure by inhibited coupling.
In the visible, the lowest loss fiber was drawn at core pressure. The cut-back loss measurement on of this fiber is shown in Fig. 3 . Owing to the multimode nature of the guidance in these fibers, the loss measurements show oscillations due to modal interference, and the dashed curves (polynomial fit curves) are added to provide a conservative estimate of the loss minimum. The lowest loss in the visible was found to be in the middle of a transmission window.
In the NIR, the lowest loss fiber was drawn at core pressure (Fig. 1). Owing to the larger core size, noise in the spectrum arising from multimode speckle did not allow the loss as a function of wavelength to be measured accurately . Therefore, the power was integrated over the range , and this was used to calculate the loss (note that this will generally overestimate the loss). The lowest loss in the NIR was thus found to be , as is indicated by the horizontal line in Fig. 3. This value is an average of three similar fibers, which showed average losses of 4.17, 2.33, and , respectively. Improved fabrication methods and the large core is most likely responsible for the low loss of this fiber. The simple-two layer structure may also contribute as it reduces the risk of deformations in the cladding.
It is interesting to compare the loss values of the PC HC-mPOF to the material loss. The lowest loss of a solid-core PC POF made from high-purity POF-grade PC was reported to be at . In the NIR, however, strong C–H molecular vibration absorptions dominate. For amorphous PC used in polymer planar waveguides, the loss at is reported to range from [16, 17]. For PC POF the loss was calculated up to , and very conservatively extrapolating this data to gives [2, 4] (c.f. the PMMA loss at is ).
The material we used for the fiber presented here was low-grade extruded PC, being lower cost and easier to obtain, which has a high level of contamination and dust particle count (black specks can be observed by eye). Its loss was measured to be at . Whether comparing with this measured value or the reported values of high-grade materials in the literature, we have clearly achieved guidance far below the material losses in the NIR. The measured loss is a factor of 69 lower than the bulk material, which indicates that only 1.4% of the mode field is propagating in the PC (since confinement loss and other losses are neglected, the actual fraction will be lower). We have no doubt the observed losses can be reduced further by improvements in fabrication and design.
We compare the temperature characteristics of four fibers: (1) the PC HC-mPOF, (2) a square-lattice PMMA HC-mPOF , (3) a solid-core PMMA mPOF (design 2 in ), and (4) a -diameter solid PC core inside a outer diameter PMMA cladding . The temperature resistance was measured by placing the fiber in a -long deep groove in an aluminium plate that was heated, and the temperature measured with a thermocouple from the groove. While increasing the temperature over , the transmission was integrated from and the change in loss (Fig. 4 ) was determined by comparing with the reference measurement.
Sharp increases in the transmission loss are observed near for PMMA and for PC; values corresponding to the of these materials ( and , respectively). Inspection shows that the heating caused a 20% expansion of the fiber due to a release of frozen-in stress from the fiber draw, despite annealing for at least (at for PMMA and for PC). For the solid-core fibers the original transmission signal is recovered after cooling, but for the HC fibers the increased loss is permanent, suggesting that the microstructure has been deformed and no longer guides light.
Polymer optical fibers can be made more temperature resistant by using a material with a higher glass-transition temperature, though this is usually associated with a higher material loss. In the case of hollow-core POF, the effect of the material loss is greatly reduced. A loss of was achieved, 69 times below the material loss. Utilizing materials with even higher such as a cyclo olefin copolymer and fluoroethylene propylene will potentially provide a cheap method of making high-temperature-resistant low-loss polymer fibers.
We thank the Australian Research Council, Richard Lwin, and the University of Sydney Electron Microscope Unit.
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