【正文】 a stepindex multimode fiber, rays of light are guided along the fiber core by total internal reflection. Rays that meet the corecladding boundary at a high angle, greater than the critical angle for this boundary, are pletely reflected. The critical angle (minimum angle for total internal reflection) is determined by the difference in index of refraction between the core and cladding materials. The critical angle determines the acceptance angle of the fiber, often reported as a numerical aperture. In gradedindex fiber, the index of refraction in the core decreases continuously between the axis and the cladding. This causes light rays to bend smoothly as they approach the cladding, rather than reflecting abruptly from the corecladding boundary. The resulting curved paths reduce multipath dispersion because high angle rays pass more through the lowerindex periphery of the core, rather than the highindex center. The index profile is chosen to minimize the difference in axial propagation speeds of the various rays in the fiber. This ideal index profile is very close to a parabolic relationship between the index and the distance from the axis. Singlemode fiber Fiber with a core diameter less than about ten times the wavelength of the propagating light cannot be modeled using geometric optics. Instead, it must be analyzed as an electromagic structure, by solution of Maxwell39。 optical fiber sensor 1. Introduction Optical fiber sensor is developed at the end of 70s as a new type of sensor, it has not affected by electromagic field effect, in essence safety explosionproof, small size, corrosion resistance, the advantages of high sensitivity. Used in traditional sensor is difficult to set foot in the extreme environment, so in the military, aviation, biological medicine, building construction field is popular. So to optical fiber sensor research has very important practical significance. Sensing technology is popular in recent years, the application of the technology in the sensitive, accurate sensor, strong adaptability, cabi and wisdom of the direction of development. In this process, optical fiber sensors the sensor to the new members of the family is extremely popular. Optical fiber has many excellent properties, such as: the electromagic interference and atomic radiation performance, diameter is fine, soft and light weight of mechanical properties, insulation, without induction electrical properties, water resistant, high temperature resistant and corrosion resistance of the chemical properties, it can not reach people places (such as high), or harmful to the person of the area (such as nuclear area), plays the role of the refreshing, and still can transcend boundaries, receiving of physiological the senses of not feeling outside information. Optical fiber sensor is a new technology in recent years, can be used to measure physical quantities, such as sound field, electric field, pressure, temperature, angular velocity, acceleration, etc, but also can plete the existing measuring technology difficult to plete measuring task. In the narrow space, in strong electromagic interference and high voltage environment, optical fiber sensors shows the unique charm. Therefore, the optical fiber sensing technology application research has the very good prospects. Optical fiber sensor light sensitive information carrier, as the optical fibre as sensitive information transmission medium. Therefore, it also has the characteristics of optical fiber and optical measurement. (1) the electric insulation performance is good. (2) antielectromagism interference ability. (3) the noninvasive. (4) the high sensitivity. (5) to be easy to implement the measured signal remote monitoring 2. Basic knowledge of transducers A transducer is a device which converts the quantity being measured into an optical, mechanical, ormore monlyelectrical signal. The energyconversion process that takes place is referred to as transduction. Transducers are classified according to the transduction principle involved and the form of the measured. Elements Although there are exception, most transducers consist of a sensing element and a conversion or control element. For example, diaphragms, bellows strain tubes and rings, bourdon tubes, and cantilevers are sensing elements which respond to changes in pressure or force and convert these physical quantities into a displacement. This displacement may then be used to change an electrical parameter such as voltage, resistance, capacitance, or inductance. Such bination of mechanical and electrical elements form electromechanical transducing devices or transducers. Similar bination can be made for other energy input such as thermal. Magic and chemical, giving thermoelectric, electromaaic, and electrochemical transducers respectively. Sensitivity The relationship between the measured and the transducer output signal is usually obtained by calibration tests and is referred to as the transducer sensitivity K1= outputsignal increment / measured increment . In practice, the transducer sensitivity is usually known, and, by measuring the output signal, the input quantity is determined from input outputsignal increment / K1. of an ideal transducer 1) high fidelitythe transducer output waveform shape be a faithful reproduction of the measured。 there should be minimum distortion. 2) There should be minimum interference with the quantity being measured。s equations as reduced to the electromagic wave equation. As an optical waveguide, the fiber supports one or more confined transverse modes by which light can propagate along the fiber. Fiber supporting only one mode is called singlemode or monomode fiber. The behavior of largercore multimode fiber can also be modeled using the wave equation