Intrinsic polymer optical fibers for large deformation strain sensors

Large deformation strain sensors have application in concrete and steel structures designed to resist earthquake loadings, as well as other potential uses, including morphing aircraft structures and highly flexible space structures. Research indicates that sensors comprising polymer optical fibers (POFs) have several advantages over more commonly used silica fibers. They exhibit high failure strain, are more flexible, and do not require a protective coating to prevent moisture ingress.

Previous research has demonstrated that the inherent displacement phase sensitivity of a single-mode POF is 14% greater than that of comparable and commonly used silica single-mode optical fibers.¹ Additionally, the fabrication of relatively low attenuation single-mode POFs designed to operate at near infrared wavelengths has permitted the writing of high deformation Bragg grating sensors and filters with a large tuning range.² For these studies, the maximum strain range achieved was on the order of 6%. We have found that the maximum usable strain range in a POF without a fiber Bragg grating is in fact much greater than the yield strain of the optical fiber.³

We recently demonstrated that single-mode PMMA (polymethylmethacrylate) optical fibers can be used to measure strain up to approximately 15% elongation of the fiber. The PMMA fiber that we used (from Paradigm Optics) was designed for single-mode operation at red light wavelengths. Its mechanical behavior varies strongly with strain rate, as might be expected for polymeric materials, with yield strain of 3–5%.⁴ To predict the phase sensitivity of the POF sensor to applied axial strain in both the small and large deformation regions, the classic displacement phase sensitivity formulation was extended to include both the finite deformation of the optical fiber and nonlinear strain optic effects.⁴ While it was not known beforehand whether both these effects were important, subsequent testing has shown them to be on the same order of magnitude.⁵

Using a Mach–Zehnder in-fiber interferometer (see Figure 1), we were able to successfully measure the phase shift up to 15.8% elongation of the POF, as plotted in Figure 2.³ At this point it was not known whether the coherent propagation strain limit was due to induced birefringence in the fiber, other polarization issues, changes in the core mode field diameter, or whether it genuinely represented the end of coherent light propagation. It was observed, however, that all light propagation through the fiber ceased shortly above this strain limit. The mean value of phase shift sensitivity was measured to be 1.39(×)10⁷rad m⁻¹.³

Figure 1. Mach–Zehnder interferometer for the measurement of POF phase sensitivity. He-Ne: Helium-neon.

Figure 2. Calculated phase shift sensitivity from experimental data. Mean value is indicated by the horizontal line.

Inasmuch as the final goal of this project was to embed the POF sensor in structural components for monitoring of infrastructural systems, its compatibility with various sentence materials was tested. Fiber pull-out tests demonstrated that cementious materials provided the strongest bond to the POF and did not apply enough transverse compressive stress during cure to prevent light transmission through the fiber. Additionally, a method of precasting the POF sensor into smaller concrete samples was developed to permit embedment in full-scale structural members (see Figure 3).

Figure 3. Photograph of precast small concrete sample with embedded sensor placed in structure prior to casting of concrete.

The practical challenges of implementing POF large deformation sensors include high material attenuation of PMMA (see Figure 4). Therefore, for large structural applications the sensor must be spliced to silica optical fibers to act as the ingress and egress channels. The fact that these single-mode POFs are still experimental means that new coupling strategies had to be developed together with new preparation techniques based on high-temperature cutting. For the practical application of a single-mode POF interferometer as a strain sensor, further investigations are under way to measure its response to cyclic loading.

Figure 4. POF sample during loading with visible light attenuation. Bright spots are locations where sensor is glued to supports.

These results indicate that interferometry-based single-mode POF sensors can be used for strain ranges over twice that of POF Bragg grating sensors and three times that of silica in-fiber interferometers. This work also demonstrates that coherent light waves can be propagated in a polymer optical fiber well beyond the yield strain limit.

This work was supported by the National Science Foundation through grant CMS 0428301 (Shih-Chi Liu, program manager).