信息科学与电子工程专业英语 教学课件 Unit 8 Optical Communications.pptVIP

* 14 Principle of operation An optical fiber is a cylindrical dielectric waveguide that transmits light along its axis, by the process of total internal reflection. The fiber consists of a core surrounded by a cladding layer (Figure 8.1). To confine the optical signal in the core, the refractive index of the core must be greater than that of the cladding. 光纤是一个圆柱形的电介质波导，通过全内反射沿其轴传播光 光纤由一个被包层包围的纤芯组成 * 14 The boundary between the core and cladding may either be abrupt, in step-index fiber, or gradual, in graded-index fiber. Figure 8.1 A typical optical fiber consisting of a transparent material of refractive index n1 and surrounded by a cladding of a slightly lower refractive index n2. Most of the propagating energy is confined to the core region and the field decays exponentially in the cladding. Cladding (n2) Core (n1) * 15 Multimode fiber Fiber with large (greater than 10 μm) core diameter may be analyzed by geometric optics. Such fiber is called multimode fiber, from the electromagnetic analysis. In a step-index multimode fiber (Figure 8.2), rays of light are guided along the fiber core by total internal reflection. 由于全内反射光线沿光纤纤芯传播 * 15 Figure 8.2 A step-index multimode fiber in which the refractive index in the core is constant. Light rays impinging on the core-cladding interface at an angle greater than the critical angle are trapped inside the core of the waveguide. In such a fiber rays traveling at larger angles to the axis have to traverse a larger path and hence take a longer time than those rays which propagate with lesser angles to the axis. This leads to a substantial amount of broadening in a pulse propagating through the fiber. x n(x) q Core: n1 Cladding: n2(n1) z * 15 Rays that meet the core-cladding boundary at a high angle (measured relative to a line normal to the boundary), greater than the critical angle for this boundary, are completely reflected.9 当光线射到纤芯和包层之间界面的角度（与垂直于边界的直线之间的夹角）大于临界角时，光线被完全反射。 * 15 The critical angle (minimum angle for t