Fiber Optics

Comms 4 Fiber Optics

Fiber Optic Communication System

A communication system wherein an electrical signal is converted to an optical signal, sent over an optical fiber, and then converted back to an electrical signal

Dr

Narinder Singh Kapany

Widely acknowledged as the father of the optical fiber; he coined the term "fiber optics" in 1956

Visible Light Spectrum

The range of wavelengths from approximately 0.77 micrometers (Red) to 0.4 micrometers (Violet)

Reflection

Phenomenon occurring when a wave strikes an object and bounces back toward the source

Refraction

Phenomenon occurring when a light wave passes from one medium into a medium having a different velocity of propagation, causing a change in direction

Refraction Condition (n1 < n2)

The angle of refraction is less than the angle of incidence (bends towards normal)

Condition (n1 > n2)

The angle of refraction is greater than the angle of incidence (bends away from normal).

Core

The part of an optical fiber where light travels

Cladding

The part of an optical fiber that reduces loss of light, reduces scattering loss, protects the core from contaminants, and adds mechanical strength

Buffer

The part of an optical fiber that protects from physical damage and prevents scattering losses due to microbends

Total Internal Reflection (TIR)

The principle by which light propagates down the fiber; occurs when the angle of incidence is large enough to force the refracted ray to bend 90 degrees relative to the incident ray

Conditions for TIR

1. Core index must be greater than cladding index (n1 > n2). 2. Angle of incidence must be greater than the critical angle.

Critical Angle

The minimum angle of incidence at which a light ray striking the interface results in an angle of refraction of 90 degrees

Critical Angle Formula

theta_c = inverse sin (n2 / n1)

Numerical Aperture (NA)

The figure of merit used to measure the magnitude of the acceptance angle; describes the light gathering ability of an optical fiber

Numerical Aperture Formula

NA = square root of (n1^2 - n2^2)

Acceptance Angle

The maximum angle in which external light rays may strike the air/fiber interface and still propagate down the fiber

Acceptance Angle Formula

theta_max = inverse sin (NA)

Mode

The path or way light may travel into the optical fiber; classified as Single Mode or Multimode

Maximum Core Radius (Single Mode)

Formula: r_max = (0.383 * lambda) / NA

V Number (Normalized Frequency)

A quantity that determines how many modes a fiber can support

Single Mode V Number

Less than or equal to 2.405

V Number Formula

V = (pi * d * NA) / lambda

Number of Modes (M) Formula

M is approximately (1/2) * V^2

Modal Dispersion (Pulse Spreading)

The phenomenon where modes arrive at the fiber end at slightly different times; occurs in multimode fibers

Index Profile

Describes the value of the fiber's refractive index as a function of axial distance at any fiber diameter

Step-Index Optical Fiber

A fiber with a core of uniform refractive index surrounded by a cladding of uniform refractive index less than the core; has an abrupt change at the boundary

Graded-Index Optical Fiber

A fiber where the refractive index is highest at the center of the core and decreases gradually with distance toward the outer edge

Fiber Optic Splice

A permanent fiber joint used to establish an optical connection between two individual optical fibers

Cleaving

The process of cutting the end of the fiber in preparation for connecting two fibers

Photon Energy Formula

E = h * f = (h * c) / lambda; where h is Planck's constant