Optical Multiplexers

Presented by: Aizaz Ahmed Sahito

Multiplexing

Multiplexing is an essential part in a communication system where

multiple users transmit data simultaneously through a single link,

whether the link is a coaxial cable, a fiber, radio or satellite.

Multiplexing is widely employed in communication systems due to

its capability to increase the channel utilization or the transmission

capacity and decrease system costs.

Multiplexing in Fiber Optics

The bandwidth properties of optical fiber are well known and make it the media of choice for high-speed data and video applications. However, various forms of multiplexing are required to take advantage of this bandwidth.

Optical multiplexer and de-multiplexer are basically passive optical filter systems, which are arranged to process specific wavelengths in and out of the transport system (usually optical fiber).

Process of filtering the wavelengths can be performed using:

Prisms

Thin film filter

Dichroic filters or interference filters

There are different techniques in multiplexing light signals onto a single

optical fiber link.

Optical Multiplexing Techniques

Optical Time Division Multiplexing (OTDM)

Separating wavelengths in time

Wavelength division multiplexing (WDM)

Each channel is assigned a unique carrier frequency

Channel spacing of about 50GHz

I. Coarse Wavelength Division Multiplexing (CWDM)

II. Dense Wavelength Division Multiplexing

Uses a much narrower channel spacing, therefore, many more wavelengths are supported.

Optical Time Division Multiplexing

Optical time division multiplexing (OTDM) has a similar concept to

electrical TDM, only that it is implemented in optical domain.

OTDM (Optical Time-Division Multiplexing) is a very powerful optical

multiplexing technique that deliveries very high capacity of data

over optical fiber.

The basic principle of this technology is to multiplex a number of

low bit rate optical channels in time domain.

Optical Time Division Multiplexing

The overall OTDM system can be viewed as three big blocks

1. transmitter block

2. line system,

3. receiver block.

The transmitter block is consist of Laser sources, modulators, channel

alignment systems, and multiplexer.

The line system contains optical amplifiers and transmission fibers.

The receiver block is made of synchronization/timing extraction circuit and

channel De-multiplexer.

Wavelength division Multiplexing

Wavelength division multiplexing (WDM) is used to transmit more than one

high-speed digital data stream on a single optical fiber.

Different wavelengths of light, (different colors) propagate in a single fiber

without interfering. The devices that do the optical combining and

separation are referred to as WDMs.

These are passive optical devices that typically employ optical filters

In wavelength-division multiplexing (WDM) systems, different independent

users transmit data over a single fiber using different wavelengths

At the transmitter side, n independent user’s data are modulated onto n

high frequency carriers, each with a unique wavelength (λ).

A wavelength multiplexer combines these optical signals and couples

them into a single fiber.

At the receiving end, a De-multiplexer is required to separate the optical

signals into appropriate channels.

This is done with n optical filters,

Coarse wavelength division multiplexing

uses a relatively small number of channels, e.g. four or eight, and a large

channel spacing of 20 nm

The resulting total data rates are useful e.g. within metropolitan areas, as

long as broadband technologies are not widespread in households

Dense wavelength division multiplexing

It extended method for very large data capacities, as required e.g. in the

Internet backbone. It uses a large number of channels (e.g. 40, 80, or

160), and a correspondingly small channel spacing

Code Division Multiplexing

Also used in microwave transmission.

Spectrum of each wavelength is assigned a unique spreading

code.

Channels overlap both in time and frequency domains but the

code guide each wavelength.

optical add-drop multiplexer (OADM)

It is a device used in wavelength division multiplexing systems for

multiplexing and routing different channels of light into or out of a single

mode fiber (SMF).

"Add" and "drop" here refer to the capability of the device to add one or

more new wavelength channels to an existing multi-wavelength WDM

signal. and/or to drop (remove) one or more channels, passing those

signals to another network path.

An OADM may be considered to be a specific type of optical cross-

connect.

Applications

The major scarce resource in telecommunication is bandwidth – users

want transmit at more high rate and service providers want to offer more

services, hence, the need for a faster and more reliable high speed

system.

Reducing cost of hardware, one multiplexing system can be used to

combine and transmit multiple signals from Location A to Location B.

Each wavelength, λ, can carry multiple signals.

Mux/De-Mux serve optical switching of signals in telecommunication and

other field of signal processing and transmission.

Future next generation internet.