In this paper we report results for an intrinsic evanescent field sensor based on not-regular plastic optical fiber with
polymer film containing Malachite Green MG<sup>+</sup>([PhC(C<sub>6</sub>H<sub>4</sub>NMe<sub>2</sub>)<sub>3</sub>]<sup>+</sup>) as an absorption reagent, which coats the fiber's
imperfected area. A theoretical model was developed which shows that changes of light in such structure result from the
attenuation of light in the strait and bent imperfected fiber. In this model, the imperfected area with malachite green
polymer film is replaced by a uniform layer with a complex refractive index. The changes in color and absorption
characteristics of the polymer film depend on the acidic and basic environmental properties in the sensing area.
Additional increase of the evanescent field interaction can be achieved by decrease the bending radius of the fiber with
the coated imperfection area at the middle of the bent fiber. An imperfected plastic optical fiber with Malachite Green
coating has been presented for the detection of ammonia vapor. The initial results show that depending on the sensing
application demands, it is possible to design a high sensitive sensor with a relatively long response time, while when the
demands require fast response times the sensor with less sensitivity can be used. In addition, the sensors' sensitivity can
be calibrated in real-time by changing the bending radius.
The concept of highly sensitive fiber optic displacement sensors is presented. It is based on macrobending plastic fiber with structural imperfections on the outer side. The theoretical principles of optical power losses in such fibers due to multiple curvatures are based on a surface that serves as a model for an imperfect layer and has the same optical properties. The sensor was tested using two different types of roughness topologies. We discuss some experimental results that confirm our prediction of considerable dependence of the output numerical aperture (the output signal) on the changing imperfections caused by bending the fiber.