Optical Fiber Strain Sensor

982

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 9, NO. 7, JULY 1997

A Novel Optical Fiber-Based Strain Sensor
A. Martin, R. Badcock, C. Nightingale, and G. F. Fernando
Abstract— This communication reports on a novel intensitybased optical fiber strain sensor. A conventional multimode fiber was drawn-down to give a reduced cross section that resembled a symmetrical taper. The sensors wereembedded into glass fiber reinforced epoxy composites and the light transmission characteristics through the profiled optical fiber was measured as a function of applied stress. A linear response was observed. Index Terms—Optical fiber, paper, strain sensor composites.

I. INTRODUCTION

O

PTICAL fiber-based strain monitoring of engineering structures has received significant coverage in recent yearsbecause the structural integrity can be inferred from strain measurements. This communication reports on a novel intensity-based strain sensor which can be embedded into materials to provide in situ strain data. Unlike conventional micro-bend [1] and etched strain sensors [2], the sensor design proposed here is simple and cost-effective to manufacture. II. SENSOR DESIGN, MANUFACTURE, AND EMBEDMENTFig. 1. Photograph of the profile strain sensor.

A sensor has been designed where a taper is introduced into a defined region of an optical fiber. This affects the effective numerical aperture (NA) and hence the light transmission characteristics of the optical fiber. When this tapered region is strained, the geometry of the taper is altered causing a change in the effective NA and therefore achange in the light transmission characteristics. A micrograph of the “profile” strain sensor is presented in Fig. 1. The profile was introduced by securing a 50/125 m multimode optical fiber in a fusion splicer (BIT BFS-60 fusion splicer) and then activating the arcing process followed by drawing the fiber by a pre-set distance. The profile could be controlled via the draw length (0–995 m), draw-time(0.1–9.9 s), arc current (5–25 mA), and draw-speed (50, 100, 200 m/s ). Six profile sensors were fabricated using identical fabrication parameters and then embedded within the mid-layer of six individual cross-ply (16 ply) glass fiber reinforced epoxy prepregs. The prepregs were cured in a custom-made mold [3] using the cure schedule recommended by the manufacturer. The composite specimens wereprepared for tensile testing using conventional procedures. The tensile tests were carried out on an Instron 1195 mechanical testing machine at an extension rate of 5 mm/min. Surface mounted
Manuscript received January 31, 1997; revised April 2, 1997. A. Martin, R. Babcock, G. F. Fernando, and C. Nightingale are with the Department of Materials Engineering, Brunel University, Uxbridge UB8 3PH, U.K.Publisher Item Identifier S 1041-1135(97)05047-7.

Fig. 2. Comparison of the output from 1) an embedded profile sensor, 2) an optical fiber (no sensing element), and 3) a surface-mounted strain gauge to applied stress. The material used was a 16-layered cross-ply glass fiber reinforced composite. The sensor and the optical fiber were located between ply numbers 8 and 9. The solid-line represents thepredicted response for the profile sensor.

electrical resistance strain gauges were used to obtain independent strain measurements. The profile sensor was illuminated with a 850-nm laser source (Profile LDS1310) and an InGaAs photodiode/amplifier (Profile PDA 200) was used to measure the transmitted light intensity as a function of applied stress. III. RESULTS
AND

DISCUSSION

Fig. 2 illustratesthe relative responses from the embedded profile sensor, an optical fiber (without any sensing element) and the surface mounted strain gauge to applied stress. On comparing the relative responses from the embedded profile sensor and the optical fiber, the profile sensor showed a strain sensitivity that was two orders of magnitude greater. The

1041–1135/97$10.00 © 1997 IEEE

MARTIN et al.: A...