Wrinkles Improve Fiber Optic Strain Sensor

Breck Hitz

Fiber sensors are excellent detectors of both strain and temperature — and that can be a problem. They respond to both environmental variables, making it difficult to tell which one is changing. To compensate, fiber optic strain sensors often are deployed with separate temperature sensors nearby, so the effect of strain on the fiber sensor can be isolated.

Figure 1. The scientists wrote the long-period fiber grating (LPFG) with a CO₂ laser while monitoring its transmission spectrum in real time (a). A micro-photograph of the resulting grating shows the small channels ablated in the side of the fiber (b). Images reprinted with permission of Optics Letters.

Recently, researchers at Hong Kong Polytechnic University demonstrated a fiber optic sensor that is far more sensitive to strain than conventional sensors, but no more sensitive to temperature. Wrinkles are the secret of its success.

Figure 2. The fiber in Figure 1(b) is diagrammed schematically (a). The dimensions are D = 15 μm, W = 50 μm and Λ = 410 μm. When a longitudinal strain is applied to the fiber, it tends to become wrinkled because of the uneven restoring force along its length (b).

Long-period fiber optic gratings are fabricated by creating periodic index perturbations along the fiber’s length. The “long period” designation indicates that the periodicity of the perturbations is much longer than the wavelength of light guided in the fiber. These perturbations form a grating that couples light at resonant wavelengths out of the core, so one or more narrow loss peaks are imposed on the fiber’s transmission spectrum. Long-period gratings make good sensors because both strain and temperature alter the grating’s effective periodicity and, therefore, change the wavelength of the loss peaks.

Figure 3. The notched fiber grating (blue) showed 25 times the sensitivity to strain as the normal (unnotched) grating (red) (a). But both gratings showed the same temperature sensitivity (b).

A common technique for creating the index perturbations is to write them thermally with a laser. In this case, the scientists went one step further, hitting the side of a photonic crystal fiber with enough energy from a Synrad Inc. CO₂ laser not only to create an index change, but also to ablate some of the glass from the side of the fiber to create small notches (Figure 1). When a wrinkled piece of cloth is stretched, its wrinkles tend to disappear, but when the straight fiber in Figure 1 is stretched, it becomes wrinkled (Figure 2). This wrinkling, or microbending, creates a greater change in the fiber’s refractive index than that created when a normal (unnotched) fiber grating is stretched. Thus, the notched fiber grating is more sensitive to strain than an unnotched one.

To demonstrate the enhanced sensitivity of their notched long-period grating, the investigators compared it with a similar grating that they fabricated using less energy from the CO₂ laser, so that index perturbations — but no notches — were created in the fiber. The two gratings had similar optical characteristics when no strain was applied, but the notched grating exhibited 25 times the sensitivity to strain as the unnotched grating (Figure 3, top). Meanwhile, both gratings showed the same sensitivity to temperature change (Figure 3, bottom). In other words, the notched fiber would be 25 times superior to the unnotched one at sensing strain in the presence of simultaneous temperature change.

Optics Letters, Dec. 1, 2006, pp. 3414-3416.