FIBEROPTIC SENSORS: Self-calibrated fiberoptic sensor measures strain

A fiberoptic white-light interferometric sensor has been developed to monitor structural strain in real time. The absolute extrinsic Fabry-Perot interferometric (AEFPI) sensor uses a broadband source and a spectrum analyzer to provide high-resolution, real-time strain assessment without calibration or reference data. Such sensors have applications in smart structures.

In a conventional extrinsic Fabry-Perot interferometric (EFPI) sensor system, the ends of a single-mode input fiber and a reflecting single- or multimode sensor fiber are aligned in a hollow-core silica fiber such that a Fabry-Perot cavity is formed (see figure). Output from a diode laser is coupled into the input fiber. When the light reaches the endface of the input fiber, part of it reflects from the surface, forming a reference beam. The transmitted portion of the beam travels across the cavity to reflect from the endface of the multimode fiber. If the cavity length is less than half the coherence length of the laser, the two beams will interfere; detected intensity varies as a function of cavity length. Because the fibers are embedded in or attached to the structure, applied strain changes the cavity length, and the instrument can thus measure strain.

Conventional EFPI sensors have several limitations. The processing method requires sophisticated fringe-counting techniques, calibration values, and reference data. Because the transfer function is sinusoidal, output intensity as a function of parameter magnitude (strain, vibration) is nonlinear. In these nonlinear regions, the system cannot detect changes in direction of applied strain.

Absolute EFPI sensors

In an effort to overcome the drawbacks of these systems, researchers at the Fiber & Electro-Optics Research Center of Virginia Polytechnic Institute and State University (VPISU, Blacksburg, VA) and Fiber and Sensor Technologies (Blacksburg, VA) have collaborated to develop an AEFPI based on white-light interferometry. A broadband source such as a superluminescent diode (SLED) is used with a standard extrinsic Fabry-Perot design. A diffraction grating disperses the optical signal, which is relayed to a charge-coupled-device detector.

The sensor does not require fringe-counting, calibration, or reference data. For any two signal wavelengths, l1 and l2, 2 µϵ out of phase, the Fabry-Perot cavity length or gap separation can be determined by computer using a simple algorithm.

The device has a dynamic range of more than 10,000 µϵ (microstrain units) and can detect a minimum axial strain of 100 µϵ. Currently, demodulation electronics limit sensor application to static and quasi-static measurements, but the group expects this to change in upcoming months. Says VPISU researcher Vikram Bhatia, "We're trying to enhance the refresh rate of the optical spectrum analyzer, and, by improving that, we can increase the frequency response of the system."

The sensor was used to measure strain in prestressed concrete as part of a project for the Federal Highway Administration; in the course of the experiment the sensor outperformed conventional mechanical gauges. Notes Bhatia, "The mechanical strain gauges were damaged due to the high strain, while the AEFPI sensor kept functioning well past 10,000 µϵ." The device has been installed to monitor aircraft structural integrity at an Air Force base and to measure strain in polymer composite matrices. Bhatia described these results at Photonics East last October in Philadelphia (paper #2594-18). Fiber and Sensor Technologies is currently commercializing the strain-measuring technology.