Fiber-Optic Communications

1. Fiber-Optic Communications James N. Downing

2. Chapter 3 Characteristics of Optical Fibers

3. 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.

4. 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

5. 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

6. 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.

7. 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.

8. 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.

9. 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

10. 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.

11. 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.

12. 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.

13. Chapter 3 Characteristics of Optical Fibers 3.2 Fiber Dispersion Total Dispersion • Total dispersion is due to all types of dispersion

14. 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.

15. 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

16. 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

17. Chapter 3 Characteristics of Optical Fibers 3.4 Types of Fiber Step-Index Fiber • Most common • Two distinct refractive indices • Core refractive index constant

18. 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

19. 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

20. Chapter 3 Characteristics of Optical Fibers 3.5 Special Fiber Types Dispersion-Shifted Fiber • Adjusts for pulse spreading caused by material and waveguide dispersion

21. 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

22. 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

23. 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

24. 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.