The present invention relates to fiber optic devices. In particular, it relates to a flexible microbend device that may be attached to an optical fiber and used in a sensor arrangement for measuring internal strain in a structure.
Optical fibers are used to convey light between a light source and a light detector. Light in the fiber can be modulated by bending or otherwise distorting the fiber. This produces a modulated signal which can be picked up and processed by the light detector.
Microbend devices may be used to induce distortions in a fiber causing the light passing through the fiber to be modulated at a frequency corresponding to the force applied to the device. Typically, the device is applied to the fiber such that it surrounds the fiber.
One known device is a two jaw corrugated arrangement which squeezes an optical fiber to modulate the light signal passing therethrough. Macedo et al. (U.S. Pat. No. 4,342,907) describes such a device where an upper piece is firmly connected to an enclosure by a support. A lower piece was attached to an elastic membrane. When either a static pressure or dynamic time varying pressure reaches the membrane, it deflects pushing the lower piece, toward the upper piece, thus deforming the optical fiber which is held against the ridges of the device. They also describe another approach in U.S. Pat. No. 4,443,700 where two steel pieces having corrugated like surfaces with interleaving ridges form a vise which would be used to squeeze and thus deform the fiber. Such devices fail to have the compactness and flexibility desired for measuring forces internally in a structure.
Kramer (U.S. Pat. No. 5,193,129) introduced microbends by interweaving an optical fiber through rungs of a ladder-like structure, encapsulating it in a foam-like material, and surrounding it with a sheath. Light transmitted through the optical fiber is diminished to a value less than a threshold value upon the occurrence of microbending caused by pressure applied at any location along the length thereof. The rungs of the ladder structure are sized and spaced to provide a proper locus about which microbending may be produced. Sansone (U.S. Pat. No. 5,694,497) points out one of the deficiencies of this sensor by stating that it must use part of the structure it is embedded in to complete the sensor design. In fact, the utility of this type of sensor is limited in that it must be woven into the substrate or structure being measured and cannot be later repositioned as needed.
Udd et al. (U.S. Pat. No. 5,118,931) introduced microbends into their sensor system by melting the optical fiber and pulling simultaneously to give the fiber a smaller diameter. Lengths of this fiber were spliced into the unmelted fiber to produce the sensor. Deformation of the fiber affects propagation of light therethrough, permitting detection based upon detected changes in light throughput. As with the Kramer device, the deficiency in this design is that once spliced into the fiber, there is no way to later reposition the structure.
Sansone (U.S. Pat. No. 5,694,497) describes an intrinsically self-deforming microbend deformer. In this device, the fiber is twisted about itself. At least one twisted section acts as an intrinsically self-deforming microbend deformer. The problem with this device is that there is no way to obtain a fixed period and/or thus be able to adjust the sensitivity of the sensor.
An object of the present invention is to provide a microbend device that may be removed and reattached to an optical fiber along its length.
Another object of the present invention is to provide a microbend device that is flexible.
Another object of the present invention is to provide a sensor that employs a flexible microbend device, such that the sensor is intensity-based.
Another object of the present invention is to provide a method for using a sensor that employs a flexible microbend device, such that the method is used to measure either radial or linear forces.
By the present invention, a flexible microbend device for attachment to an optical fiber is provided. The device comprises an upper bending element grid having at least one flexible element and a lower bending element grid having at least one flexible element. Each upper bending element grid alternately engages each lower bending element grid.
When the device is attached to an optical fiber, it forms a fiber optic microbend sensor. The optical fiber is positioned between each flexible element of the upper bending element grid and each flexible element of the lower bending element grid such that each flexible element alternates between the upper and the lower bending element grids.
In use, the sensor is either attached to a surface of a structure or embedded in a host material. Optical power is introduced to the microbend sensor and the reflected light is monitored with a detector.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be obtained by means of instrumentalities in combinations particularly pointed out in the appended claims.