Fiber-Optic Communications James N. Downing

Characteristics of Optical Fibers 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 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

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

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 L-bands 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.