Tham khảo tài liệu 'fiber optic telecommunication phần 2', kỹ thuật - công nghệ, kĩ thuật viễn thông phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Fiber Optic Telecommunication or by Equation 8-6. V 2n a X . 8-5 V - ia X n1 X 2 xA 1 A 8-6 In either equation a is the fiber core radius A is the operating wavelength . is the numerical aperture n1 is the core index and A is the relative refractive index difference between core and cladding. The analysis of how the V-number is derived is beyond the scope of this module but it can be shown that by reducing the diameter of the fiber to a point at which the V-number is less than higher-order modes are effectively extinguished and single-mode operation is possible. Example 4 What is the maximum core diameter for a fiber if it is to operate in single mode at a wavelength of 1550 nm if the . is From Equation 8-5 V 2n a A X . Solving for a yields a V A . For single-mode operation V must be or less. The maximum core diameter occurs when V . So plugging into the equation we get amax 1550 nm 2n pm or dmax 2 X a pm The core diameter for a typical single-mode fiber is between 5 pm and 10 pm with a 125-pm cladding. Single-mode fibers are used in applications in which low signal loss and high data rates are required such as in long spans where repeater amplifier spacing must be maximized. Because single-mode fiber allows only one mode or ray to propagate the lowest-order mode it does not suffer from modal dispersion like multimode fiber and therefore can be used for higher bandwidth applications. However even though single-mode fiber is not affected by modal dispersion at higher data rates chromatic dispersion can limit the performance. This problem can be overcome by several methods. One can transmit at a wavelength in which glass has a fairly constant index of refraction 1300 nm use an optical source such as a distributed-feedback laser DFB laser that has a very narrow output spectrum use special dispersion- 2000 University of Connecticut 303 Fundamentals of Photonics compensating fiber or use a combination of all .
