Polymer optical fibers (POFs) have significant advantages for many sensing applications, including high elastic strain limits, high fracture toughness, high flexibility in bending, high sensitivity to strain and potential negative thermo-optic coefficients. The recent emergence of single-mode POFs has enabled high precision, large deformation optical fiber sensors. This article describes recent advances in both multi-mode and single-mode POF based strain and temperature sensors. The mechanical and optical properties of POFs relevant to strain and temperature applications are first summarized. POFs considered include multi-mode POFs, solid core single-mode POFs and microstructured single-mode POFs. Practical methods for applying POF sensors, including connecting and embedding sensors in structural materials, are also described. Recent demonstrations of multi-mode POF sensors in structural applications based on new interrogation methods, including backscattering and time-of-flight measurements, are outlined. The phase-displacement relation of a single-mode POF undergoing large deformation is presented to build a fundamental understanding of the response of single-mode POF sensors. Finally, this article highlights recent single-mode POF based sensors based on polymer fiber Bragg gratings and microstructured POFs.

ABSTRACT4This paper investigates the use of embedded optical fiber Bragg gratings to measure strain near a stress concentration within a solid structure. Due to the nature of a stress concentration (i.e., the strong nonuniformity of the strain field), the assumption that the grating spectrum in reflection remains a single peak with a constant bandwidth is not valid. Compact tension specimens including a controlled notch shape are fabricated, and optical fiber Bragg gratings with different gage lengths are embedded near the notch tip. The form of the spectra in transmission varies between gages that are at different distances from the notch tip under given loading conditions. This variation is shown to be due to the difference in the distribution of strain along the gage length. By using the strain field measured using electronic speckle pattern interferometry on the specimen surface and a discretized model of the grating, the spectra in transmission are then calculated analytically. For a known strain distribution, it is then shown that one can determine the magnitude of the applied force on the specimen. Thus, by considering the nonuniformity of the strain field, the optical fiber Bragg gage functions well as an embedded strain gage near the stress concentration.KEY WORDS--Optical fiber sensor, Bragg grating, embedded sensor, strain distribution, nondestructive evaluationThe use of optical fiber gages to measure temperature, strain, or even detect fracture in a material has great potential due to their relatively small size, sensitivity and immunity to electrical fietds.l Surface-mounted optical fibers, for example, have been used in a variety of strain gage applications. 2 Furthermore, due to their dimensions, optical fiber gages can be embedded unobtrusively into materials, particularly composites already containing fiber reinforcements. However, once an optical fiber is embedded, the interpretation of the gage response becomes more complex due to the effects of May 20, 1999. Final manuscript received: October 10, 2000 interface between the fiber and the material, as well as the multiple components of strain applied to the fiber. 3-6 The problem is further complicated when the strain field surrounding the gage is not sufficiently uniform with respect to the scale of the gage length.As an example, the optical fiber Bragg grating (OFBG) sensor permits the localized measurement of axial strain in an optical fiber. In comparison with a simple optical fiber displacement gage, the response of the OFBG sensor is only affected by the strain (or temperature) field at the location of the grating and not along other portions of the opticai fiber. This property makes the OFBG sensor especially useful for measuring localized phenomena such as the strain conditions near a region of fracture. 7The conventional treatment of the OFBG as a strain gage assumes that the reflected spectrum is a single distinct peak whose shift is linearly proportional to the applied strain. Naturally, this assumption is only valid if the gage le...

This paper derives the phase response of a single-mode polymer optical fibre for large-strain applications. The role of the finite deformation of the optical fibre and nonlinear strain optic effects are derived using a second order strain assumption and shown to be important at strain magnitudes as small as 1%. In addition, the role of the core radius change on the propagation constant is derived, but it is shown to be negligible as compared to the previous effects. It is shown that four mechanical and six opto-mechanical parameters must be calibrated to apply the sensor under arbitrary axial and transverse loading. The mechanical nonlinearity of a typical single-mode polymer optical fibre is experimentally measured in axial tension and is shown to be more significant than that of their silica counterpart. The mechanical parameters of the single-mode polymer optical fibre are also measured for a variety of strain rates, from which it is demonstrated that the strain rate has a strong influence on yield stress and strain. The calibrated constants themselves are less affected by strain rate.

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