Transcript ch 3 - Nmsu
Fiber-Optic Communications James N. Downing Chapter 3 Characteristics of Optical Fibers Chapter 3 Characteristics of Optical Fibers 3.1 Light Propagation in Optical Fibers Acceptance Angle and Numerical Aperture – Acceptance angle is the angle cone of light transmitted down the fiber. – Numerical aperture is the sine of ½ of the acceptance angle. Chapter 3 Characteristics of Optical Fibers 3.1 Light Propagation in Optical Fibers Fiber Modes – Fiber mode refers to the way waves propagate down a fiber. – The geometry of the fiber as well as the existence of waves traveling forward and backward allows only certain ray angles to propagate. – Bessel functions describe which modes yield numerical results: V-number Chapter 3 Characteristics of Optical Fibers 3.1 Light Propagation in Optical Fibers V-number 2 aNA V where – N is the number of modes – a is the radius of the fiber – λ is the wavelength of light For single-mode fiber, V < 2.405 N V 2 2 Chapter 3 Characteristics of Optical Fibers 3.1 Light Propagation in Optical Fibers Modal Properties – Ideally all angles carry equal amounts of energy. – Actual mode distribution differs due to launch conditions, coupling, and leaky modes. – Mode coupling describes how energy is transferred between modes. – Leaky modes are the highest order modes that transmit into the cladding or transmitted back into the core. Chapter 3 Characteristics of Optical Fibers 3.1 Light Propagation in Optical Fibers Modal Properties – Mode distribution describes how evenly the energy is distributed across all modes. – Mode scrambler is used to achieve steady state for measurement purposes on short fibers. – Cutoff wavelength is the minimum propagation wavelength that can be transmitted. – Mode-field diameter (output spot size) is approximately the core diameter for multimode fibers. Chapter 3 Characteristics of Optical Fibers 3.2 Fiber Dispersion – Dispersion is the spreading of a light pulse as it propagates down the fiber. – Dispersion may be either modal or chromatic. Chapter 3 Characteristics of Optical Fibers 3.2 Fiber Dispersion Modal Dispersion – The temporal spreading of a pulse in an optical waveguide caused by modal effects – Intermodal, or modal, dispersion occurs only in multimode fibers. – Contributes to pulse broadening Chapter 3 Characteristics of Optical Fibers 3.2 Fiber Dispersion Material Dispersion – Material dispersion occurs because the spreading of a light pulse is dependent on the wavelengths' interaction with the refractive index of the fiber core. – Material dispersion is a function of the source spectral width, which specifies the range of wavelengths that can propagate in the fiber. – Material dispersion is less at longer wavelengths. Chapter 3 Characteristics of Optical Fibers 3.2 Fiber Dispersion Waveguide dispersion – Waveguide dispersion occurs because the mode propagation constant is a function of the size of the fiber's core relative to the wavelength of operation. – Waveguide dispersion also occurs because light propagates differently in the core than in the cladding. Chapter 3 Characteristics of Optical Fibers 3.2 Fiber Dispersion Polarization Mode Dispersion – Polarization mode dispersion (PMD) occurs when different planes of light inside a fiber travel at slightly different speeds, making it impossible to transmit data reliably at high speeds. Chapter 3 Characteristics of Optical Fibers 3.2 Fiber Dispersion Total Dispersion – Total dispersion is due to all types of dispersion t tot t 2 mod t 2 chrom t 2 pol Chapter 3 Characteristics of Optical Fibers 3.3 Fiber Losses – Absorption loss occurs at wavelengths greater than 1.55µm due to infrared vibration. – Scattering can be significant at shorter wavelengths. – Attenuation describes the total loss of a optical fiber system – Bending loss occurs when total internal reflection deteriorates because of installation procedures. Chapter 3 Characteristics of Optical Fibers 3.4 Types of Fiber Multimode Fiber – – – – – Can transmit more than a single mode Relatively inexpensive Easy to couple with LEDs and detectors Large bandwidth NA ~ 0.20 Chapter 3 Characteristics of Optical Fibers 3.4 Types of Fiber Single-Mode Fiber – – – – – – – Allows only a single mode to propagate Difficult to handle and couple More expensive Requires a laser source Large bandwidth High speed/large bandwidth systems NA ~ 0.12 Chapter 3 Characteristics of Optical Fibers 3.4 Types of Fiber Step-Index Fiber – Most common – Two distinct refractive indices – Core refractive index constant Chapter 3 Characteristics of Optical Fibers 3.4 Types of Fiber Graded-Index Fiber – Refractive index varies between the central core and the cladding – More expensive – Dispersion and bandwidth improved – Works best for multimode fiber – Rays refract continuously Chapter 3 Characteristics of Optical Fibers 3.5 Special Fiber Types Plastic Fiber – – – – – High attenuation Less expensive than glass Easy to work with Step-index fibers Used in automobiles, consumer products, industrial control, and small LANs Chapter 3 Characteristics of Optical Fibers 3.5 Special Fiber Types Dispersion-Shifted Fiber – Adjusts for pulse spreading caused by material and waveguide dispersion Chapter 3 Characteristics of Optical Fibers 3.5 Special Fiber Types Polarization Maintaining Fiber – Used in lithium niobate modulators and Raman amplifiers – Maintains polarization of the incoming light – Minimizes cross-coupling between polarization modes Chapter 3 Characteristics of Optical Fibers 3.5 Special Fiber Types Photonic Crystal (Holey) Fibers – – – – – – Dispersion can be controlled Nonlinear properties Single-mode Wide wavelength Cladding region consists of air holes Two categories: High-index and low-index guiding fibers Chapter 3 Characteristics of Optical Fibers 3.5 Special Fiber Types Other Fibers – Low OH Fiber—low water content – Rare-Earth Doped Fiber—gain media fro amplifiers and lasers. Erbium doped fiber amps used for over C- and Lbands – Reduced Cladding Fibers—cladding has been reduced from 125µm to 80µm Chapter 3 Characteristics of Optical Fibers 3.5 Special Fiber Types Other Fibers – High-Index Fibers—used in couplers and DWDM components – Photosensitive Fibers—change their refractive index permanently when illuminated with UV radiation – Lensed Fibers—used to launch light from transmitters into fibers. May add curvature to an end and be more cost effective than wasting energy due to mismatches.