Abstract—Fiber optic sensor technology offers the possibility of sensing different parameters like strain, temperature, pressure in harsh environment and remote locations. these kinds of sensors modulates some features of the light wave in an optical fiber such an intensity and phase or use optical fiber as a medium for transmitting the measurement information. The advantages of fiber optic sensors in contrast to conventional electrical ones make them popular in different applications and now a day they consider as a key component in improving industrial processes, quality control systems, medical diagnostics, and preventing and controlling general process abnormalities. This paper is an introduction to fiber optic sensor technology and some of the applications that make this branch of optic technology, which is still in its early infancy, an interesting field. Keywords—Fiber optic sensors, distributed sensors, sensor application, crack sensor. I. INTRODUCTION VER the past decades many product revolutions have taken place due to the growth of the optoelectronics and fiber optic communications industries. In the process of fiber optic developments, different researches had been done which are focused on suitable design of fibers. An offshoot of this observation was a new thoughts directed to use optical fibers in designing sensing systems, which led to the fiber based sensing devices and components. A high volume fiber requirement in the telecommunication industry has brought down substantially the material cost of fiber sensor and the performances of the fiber improve dramatically over the years [1]. As a result, the ability of fiber optic sensors to displace traditional sensors for rotation, acceleration, electric and magnetic field measurement, temperature, pressure, acoustics, vibration, linear and angular position, strain, humidity, viscosity, chemical measurements, and a host of other sensor applications has been enhanced. The rapidly growing interest, with fiber-based sensing owed to some attractive reasons like small size, light weight, immunity to electromagnetic interference (EMI), passive (all- dielectric) composition, high temperature performance, large bandwidth, high sensitivity, environmental ruggedness, and the ability of distributed sensing [2]. B. Gholamzadeh is with Sadjad Institute of Higher Education, Mashhad, Iran (phone: +98-915-5051041; e-mail: baharehgholamzadeh@sadjad.ac.ir ). H. Nabovati, Department of Electrical Engineering, Sadjad Institute of Higher Education, Mashhad, Iran (e-mail: nabovati@sadjad.ac.ir ). Fiber optics for sensing applications are used to communicate with a sensor device or use a fiber as the sensor itself to conduct continuous monitoring of physical, chemical, and biological changes in the subject or object of study. In fiber-optic sensors, information is primarily conveyed in all optical sensors by a Change in either phase, polarization, frequency, intensity or a combination thereof. But the photo detector, being a semiconductor device, only senses intensity of light at the detector surface. So the art of sensing with polarization, phase or frequency modulation involves interferometric or grating based signal processing optical circuits [3]. The resulting signals can be then telemeter to places other than the location of the sensor by means of a fiber optic signal transmission system. In the following sections the basic types of fiber optic sensors which are being developed are reviewed followed by a discussion of how these sensors will be applied. II. FIBER OPTIC SENSORS The technology and applications of optical fibers have progressed very rapidly in recent years. Optical fiber, being a physical medium, is subjected to perturbation of one kind or the other at all times. It therefore experiences geometrical (size, shape) and optical (refractive index, mode conversion) changes to a larger or lesser extent depending upon the nature and the magnitude of the perturbation. In communication applications one tries to minimize such effects so that signal transmission and reception is reliable. On the other hand in fiber optic sensing, the response to external influence is deliberately enhanced so that the resulting change in optical radiation can be used as a measure of the external perturbation. In communication, the signal passing through a fiber is already modulated, while in sensing, the fiber acts as a modulator. It also serves as a transducer and converts measurements data like temperature, stress, strain, rotation or electric and magnetic currents into a corresponding change in the optical radiation. Since light is characterized by amplitude (intensity), phase, frequency and polarization, any one or more of these parameters may undergo a change. The usefulness of the fiber optic sensor therefore depends upon the magnitude of this change and our ability to measure and quantify the same reliably and accurately. The advantages of fiber optic sensors are freedom from EMI, wide bandwidth, compactness, geometric versatility and economy. In general, FOS is characterized by high sensitivity when compared to other types of sensors. It is also passive in nature due to the dielectric construction. Specially prepared Fiber Optic Sensors Bahareh Gholamzadeh, and Hooman Nabovati O World Academy of Science, Engineering and Technology International Journal of Electrical, Computer, Energetic, Electronic and Communication Engineering Vol:2, No:6, 2008 1107 International Scholarly and Scientific Research & Innovation 2(6) 2008 scholar.waset.org/1999.5/10905 International Science Index, Electronics and Communication Engineering Vol:2, No:6, 2008 waset.org/Publication/10905