Using an analytical model for power distribution from a series of radiative taps on multimode optical fibers we show how to design a low-cost distributed strain sensor.

It is possible to fabricate radiative taps on optical fibers by using a variety of methods such as

1. laser micro machining,
2. chemical etching,
3. macro bending and polishing
4. cutting and polishing, and
5. making radiative Bragg gratings, etc.

A cut or indentation is made by one of these fabrication methods on the cladding of an optical fiber and a fraction of the evanescent optical field in the cladding and the cladding modes are radiated out from this cut. Thus we can sample a small portion of the optical fields inside the fiber from such a tap. Radiative taps have several uses in making fiber optic components, sensors, tapped delay lines, signal processors, and add-drop multiplexers. A series of such taps distributed along an optical fiber can be used to build devices and subsystems such as filters, equalizers long period Bragg gratings , distributed fiber optic sensors , or spectrum analyzers.

The distributed sensor we discuss is a low-cost intensity modulated sensor consisting of a number of identical taps on a large core multimode fiber. A fraction of the light coupled to the fiber is radiated out from each tap and detected by a photodetector. The output from each photodetector are monitored by a central processor. When stress or pressure is applied a few taps in the vicinity are strained. The optical power radiated by those taps changes as a result and the central processor determines the magnitude of the strain from the change. Thus unlike 1 more sophisticated sensors [12, 13] our distributed strain sensor requires neither an optical-time-domain-reflectometry equipment nor the measurement of Brillouin scattered fields.