Inclinometer Shows Versatility of Fiber Optic Sensors

Breck Hitz

There are a multitude of techniques with which to measure temperature, strain, pressure and other parameters with fiber optic sensors. In yet another example of the versatility of optical fibers as measurement devices, a collaboration of scientists from INESC-Porto and Universidade do Porto, both in Portugal, and Universidade Tecnológica Federal do Paraná in Curitiba, Brazil, has designed and demonstrated a fiber optic inclinometer based on a fiber’s taper and a long-period grating in the same fiber. A classic fiber optic Mach-Zehnder interferometer divides the incoming light between two fibers and recombines it at a second junction (Figure 1, top).

Figure 1. In both a classic fiber optic Mach-Zehnder (top) and a single-fiber Mach-Zehnder (middle), the incoming light is split into two parts and subsequently recombined. A variation of the single-fiber scheme replaces one of the long-period gratings with a nonadiabatic taper (bottom). Images reprinted with permission of Optics Letters.

In a single-fiber Mach-Zehnder, a long-period grating diffracts light at its resonant wavelengths from the fiber core into the cladding, and a second long-period grating couples light from the cladding back into the core (Figure 1, middle). In both cases, interference occurs when the separated beams are recombined. The nature of the interference — constructive or destructive — depends upon the optical path difference between the interferometer’s two arms.

Figure 2. The magnitude of the coupling provided by the taper changed when the fiber was bent.

Wishing to fabricate a simpler device that required only a single long-period grating, the scientists experimented with softening a length of Corning SMF-28 fiber in a splicing machine and pulling an ∼500-μm-long taper into the softened section. They reasoned, correctly, that the taper would expand the core field so that part of its light would be coupled into the cladding.

Having demonstrated that the taper satisfactorily coupled light from the core into the cladding, the scientists next constructed a single-fiber Mach-Zehnder by combining the taper with a single long-period grating (Figure 1, bottom). When the light traveling in the cladding rejoined the light in the core, the resulting interference was a measure of the optical path difference between the two arms of the interferometer.

Figure 3. The visibility of the interferometer’s fringes provided a measurement of the angular displacement.

To demonstrate the utility of such an interferometer, the scientists designed an inclinometer — an instrument for measuring angular displacement — based on the interferometer. They sheathed the fiber on either side of the taper in capillary tubes and bent the fiber precisely at the taper (Figure 2). The bend changed the magnitude of the coupling into the cladding, but not the phase of the light coupled back into the core at the grating. Therefore, the bend affected the fringe visibility, but not the location of the fringes (Figure 3).

Optics Letters, Oct. 15, 2006, pp. 2960-2962.