Tensile Behavior of photonic crystal fibres

The mechanical strength and failure behavior of two photonic crystal silica optical fibers with different diameters were investigated using tensile test. The effect of polymer coating on the failure behavior was also studied. The results indicated that all fibers failed in a brittle manner and the failure normally initiated from fiber surfaces. The failure loads observed in the coated fibers are higher than that in bare fibers and the reason is explained by the apparent delamination between the fiber and the polymer coating when loaded on the fiber surfaces. The relationship between a characteristic parameter measured on the fracture surfaces and the failure stress was examined. Introduction Photonic crystal optical fibers (also referred as microstructured and holey fibers) are of interest since they offer a simple alternative to controlling the index profile of optical waveguides other than using expensive dopants [1-4]. They are also featured by some interesting characteristics, such as unique dispersion properties, as well as single-mode operation over an extended range of operating wavelengths [3]. The potential applications include gas-based nonlinear optics, sensing, lasers, high harmonic generation, ultrahigh nonlinearities and even guidance of atoms and particles [4]. On the other hand, the brittle nature, mechanical damage and failure of silica fibers, remain the key material issues. The flaws on the surface of fibers caused by processing, cleaving or subsequent assembling make the situation more complex. Very limited investigation has been directed to the strength of photonic crystal fiber as its development is still at the infancy stage. Without a basic understanding of mechanical reliability, it is difficult to imagine an extensive application of these novel fibers in telecommunication industry, even for a local network. In this work, the failure behavior of silica photonic crystal fibers was examined using tensile test. Experimental Procedure Two silica photonic crystal fibers with a similar holes arrangement but different diameters were fabricated using capillary stacking technique. The arrangements of air holes are shown in Fig. 1. The two fibres have diameters of 100 μm and 125 μm, respectively, and are in-line coated with 60 μm acrylate polymer. The tensile test was carried out in accordance with the American Standard for Test and Measurement (ASTM) D 3379-75. An Instron 5567 with a load cell of 10 and 100 N and pneumatic grips was used. Samples were prepared by mounting a single fiber on a pre-prepared paper frame. The length of the cut-out is equal to the gauge length, i.e. the length over which the strain is measured. A 25 mm gauge length was used for the 125 μm fibre. The fiber was mounted on the paper frame using epoxy in two different ways, i.e., mounting on the coating directly and on the bare fiber after removing the coating by immersing the fibers into acetone for a few minutes. To investigate the effect of gauge length on failure behaviour, three gauge lengths, i.e., 10, 25 and 50 mm were applied to the fibre with 100 μm in diameter after removing the polymer coating. The failure process of the fibers during the tensile test was continuously monitored and recorded using a CCD camera connected with a video recorder. The fracture surfaces of the fibers were observed using scanning electron microscopy (SEM). Fig. 1 End face of the holey fibres. Results and discussion The average failure (maximum) loads for the 125 μm fibre with and without the polymer coating are 26.8 N and 6.1 N, respectively. A typical elastic load-displacement curve is observed in the bare fibre, indicating brittle failure behaviour. For the coated fibre, however, the load increases linearly until the peak value and then drops gradually, as shown in Fig. 2. Displacement, mm 0 2 4 6 8 10 Lo ad , N