Optical Amplifiers and

Networks Prepared by

Jagadish. M, Assistant Professor,

Dept. of ECE,

PES Institute of Technology and Management, Shivamogga

Optical Amplifier

• All optical amplifiers

increase the power level

of incident light through

a stimulated emission or

an optical power

transfer process

Optical Amplifier

• The device absorbs energy supplied from an external source called pump

• The pump supplies energy to electrons in an active medium which raises them to higher energy levels to produce population inversion

• An incoming signal photon will trigger these excited electrons to drop to lower levels through a stimulated emission process and emits photons of equal energy. The result is an amplified optical signal

Optical Amplifiers

- Applications

• In-line optical

amplifiers

• Pre-amplifier

• Power Amplifier

Optical Amplifiers

- Applications • In-line optical amplifiers

– In a single mode link the effects of fiber dispersion is small.

– In such a link complete regeneration of the signal is not necessary and simple amplification of the optical signal is sufficient.

– Thus, an in-line amplifier can be used to compensate for transmission loss and increase the distance between regenerative repeaters.

• Pre-amplifier – It is a front end amplifier for an optical receiver.

– A weak optical signal is amplified before photodetection to increase signal to noise ratio.

– Compared with other front-end devices such as APD or heterodyne detectors, an optical pre-amplifier provides a large gain factor and broader bandwidth.

• Power Amplifier – Also called booster amplifier includes placing the device immediately after an

optical transmitter to boost the transmitted power.

– This increases the transmission distance by 10-100km and compensate for coupler insertion loss and power splitting loss.

Optical Amplifier - Types

• Alloys of semiconductor materials from groups III and V make up active medium for SOAs.

• They work in O-band and C-band and can be integrated on the same substrate as other optical devices.

• Compared with DFAs they consume less electrical power, have fewer components and are more compact.

• The rapid gain response give rise to crosstalk effects when a broad spectrum of wavelengths must be amplified.

Semiconductor Optical Amplifiers

(SOAs)

• The active medium is created by lightly doping silica or tellurite fiber core with rare earth elements such as thulium, erbium.

• They operate in S, C, L bands.

• They have the ability to pump devices at several wavelengths and possess low coupling loss for the given fiber transmission medium.

• They are immune from interference effects between different optical channels.

Active fiber or doped fiber

amplifiers (DFAs)

• A fiber based Raman amplifier uses stimulated Raman scattering (SRS) occurring in silica fibers when an intense pump beam propagates through it.

• The incident pump photon gives up its energy to create another photon of reduced energy at lower frequency. The remaining energy is absorbed by the medium in the form of molecular vibrations. Thus Raman amplifiers must be pumped optically to provide gain.

Raman amplifiers

Erbium Doped Fiber Amplifier

(EDFA)

• A silica fiber doped with Erbium and hence the

name EDFA

• The operation of EDFA is limited to 1530nm

to 1560 nm region

EDFA –

Amplification Mechanism • Two principal levels

– Metastable state 4I13/2

– Pump level 4I11/2

• The pump band exists at 1.27 eV separation from bottom ground state. The energy corresponds to 980 nm wavelength

• The top metastable state is separated from bottom ground state by 0.841 eV. This energy corresponds to 1480nm wavelength

• The bottom metastable state is separated from bottom ground state by 0.814 eV. This energy corresponds to 1530 nm wavelength

• The bottom metastable state is separated from top ground state by 0.775 eV. This energy corresponds to 1600nm wavelength.

Fig. Simplified energy level diagram

EDFA Architecture

Tap Couplers

One or more pump lasers

isolators

• Prevents amplified signal from reflecting back into device

Doped fiber

Used both sides to compare incoming signal

with amplified output

EDFA Architecture

a. Co-directional Pumping – Pump light injected in

same direction as signal flow

– Better noise performance

b. Counter directional pumping – Pump light injected in

opposite direction as signal flow

– Higher gains

c. Dual pumping – Gain – 35dB

Optical Networks –

SONET/SDH

• Standard signal format

• SONET – Synchronous Optical Network

– Used in North America

• SDH – Synchronous digital Hierarchy

– Other parts of the world

Basic structure of

SONET frame • Two dimensional structure

– 90 col, 9 rows of bytes

• First three columns – Transport overhead bytes

– Network management information

• 4 to 87 – synchronous payload envelope (SPE) – User data + nine bytes of path

overhead (POH)

– POH supports performance monitoring, signal labeling etc. and can be located anywhere in SPE.

Basic structure of

SONET frame • Section – adjacent pieces of equipment

• Line – connects two SONET devices

• Path – complete end to end connection

• SONET frame – 125 micro sec

• Transmission rate– 51.84Mbps

• STS-N : – bit rate – N x 51.84Mbps

• In SDH basic rate is equivalent to STS-3. This is called synchronous transport module level 1 STM level 1.

SONET/SDH Rings

• Commonly called self healing rings

• This is done for non interrupted service protection purposes case of link or equipment failures

• Classification based on three main features

– Either two or four fibers running between nodes

– Operating signals can travel in clockwise only or in both directions

– Protection switching can be performed either

SONET/SDH Rings

• Traffic travels clockwise around the ring – primary path

– E.g. Connection from node 1 to node 3 uses link 1 & 2

• Counter-clockwise path is used as an alternate route for protection against link or node failures

SONET/SDH Rings

• Four fiber bidirectional

line switched ring

– Two primary fiber loops

– bidirectional

communication

– Two standby links are for

protection purposes

• Advantage over two

fiber

– Capacity increases

High Speed

Light-wave Links

• Links operating at 10Gbps

– OM1 grade fiber: 100 Mbps

– OM2 grade fiber: 1Gbps over 750m

– OM3 grade fiber: 10Gbps

• Links operating at 40Gbps

– Differential phase shift keying

• OTDM links operating at 160Gbps

– Bit interleaved multiplexing technique

Any Questions???