Modeling and optimizing output characteristics of intensity modulated optical fiber-based displacement sensors

Optical-fiber-based contactless sensing techniques are used in a wide range of sensing applications in many industries to measure physical parameters such as displacement, torque, strain, and vibration. Due to extremely small diameters, flexibility, and the light weight of optical fibers, these sensors can be employed to provide accurate contactless sensing in areas where space is a major limitation. Although many different modulation schemes are possible through optical fibers, the intensity modulation (IM) scheme is very popular due to low cost, simplicity of the sensing structure, and its compatibility with multimode fibers. However, in comparison with other optical modulation schemes, the main drawback of IM is the limited sensitivity. Further, the optical fiber tips must be mounted very close to the reflective element (required to modulate the light intensity as a function of the measured physical quantity) to achieve the highest sensitivity and linearity. The aim of this paper is to mathematically model the output characteristics of IM optical fiber sensors under a number of different sensing configurations. These include IM sensors, utilizing planar, concave or convex reflective elements. Furthermore, a novel sensing configuration is proposed: an IM sensor employing a converging lens in between the fiber tips and the reflective element. In this configuration, the output sensitivity can be increased significantly simply by altering the gap between the lens and fiber tips. A nonlinear mathematical expression is developed for each configuration, and with that, once the output response for a planar reflector is known, sensing characteristics for other sensor configurations can be derived. Hence, this paper is a useful tool for predicting sensor characteristics and optimizing sensor design.