1. Fiber-Optic Communications
James N. Downing

2. Characteristics of Optical Fibers
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.