Unit 8: Optical Amplifiers and Networks

All optical amplifiers increase the power level of incident light through a stimulated emission or an

optical power transfer process.

The basic operation of a generic optical amplifier is shown in Fig. below

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.

Basic Applications and Types of Optical Amplifiers:

Optical amplifiers have found widespread use in diverse application ranging from ultra-long

undersea links to short links in access networks.

General Applications:

Fig. below shows the following three general applications

In-line optical amplifiers

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

o In such a link complete regeneration of the signal is not necessary and simple amplification

of the optical signal is sufficient.

o Thus, an in-line amplifier can be used to compensate for transmission loss and increase the

distance between regenerative repeaters.

Pre-amplifier

o It is a front end amplifier for an optical receiver.

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

o 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

o Also called booster amplifier includes placing the device immediately after an optical

transmitter to boost the transmitted power.

o This increases the transmission distance by 10-100km and compensate for coupler insertion

loss and power splitting loss.

o Fig. shows an example for boosting the optical signal in front of a passive star coupler.

Amplifier Types

Semiconductor Optical Amplifiers (SOAs)

o Alloys of semiconductor materials from groups III and V make up active medium

for SOAs.

o They work in O-band and C-band and can be integrated on the same substrate as

other optical devices.

o Compared with DFAs they consume less electrical power, have fewer components

and are more compact.

o The rapid gain response give rise to crosstalk effects when a broad spectrum of

wavelengths must be amplified.

Active fiber or doped fiber amplifiers (DFAs)

o The active medium is created by lightly doping silica or tellurite fiber core with rare

earth elements such as thulium, erbium.

o They operate in S, C, L bands.

o They have the ability to pump devices at several wavelengths and possess low

coupling loss for the given fiber transmission medium.

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

Raman amplifiers.

o A fiber based Raman amplifier uses stimulated Raman scattering (SRS) occurring in

silica fibers when an intense pump beam propagates through it.

o 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.

o Fig. shows how a fiber can be used as a Raman amplifier.

o The pump and the signal beams at frequencies s and p are injected into the

fiber through a fiber coupler.

o The energy is transferred from the pump beam to signal beam through SRS as

the two beams co-propagate inside the fiber.

o The pump and the signal beams counter-propagate in the backward pumping

configuration commonly used in practice.

Erbium doped fiber amplifiers (EDFA):

A silica fiber doped with erbium, is known as erbium doped fiber amplifier and is popular material

for long haul telecommunication applications.

The operation of a standard EDFA normally is limited to the 1530 to 1560nm region.

Application mechanism:

Simplified energy level diagram and various transition processes of Er3+

ions in silica is shown in

Fig. below

The various energy transitions and photon emission ranges are as follows-

o The pump band shown in the top left of Fig. exists at 1.27ev separation from the bottom of

the 4I15/2 ground state. This energy corresponds to a 980nm wavelength.

o The top of the 4I13/2 metastable band is separated from the bottom of the

4I15/2 ground state

band by 0.841eV. this energy corresponds to 1480nm wavelength.

o The bottom of the 4I13/2 metastable band is separated from the bottom of the

4I15/2 ground

state band by 0.814eV. This energy corresponds to 1530nm wavelength.

o The bottom of the 4I13/2metastable band is separated from the top of the

4I15/2 ground state

band by 0.775eV. This energy corresponds to 1600nm wavelength.

The pump wavelengths are 980nm and 1480nm. The photons emitted during transitions of electrons

between possible energy levels in the metastable and ground state bands can range from 1530nm to

1600nm.

The operation is as follows-

o Process 1: A pump lase emitting 980nm photons is used to excite ions from the ground state

to the pump level.

o Process 2: These excited ions decay very quickly from the pump band to the metastable

band. During this decay the excess energy is released as phonons.

o Process 3: Another possible pump wavelength is 1480nm. The absorption of a 1480nm pump

photon excites an electron from the ground state directly to the lightly populated top of

metastable level.

o Process 4: These electrons then tend to move down to the more populated lower end of the

metastable level.

o Process 5: Some of the ions sitting at the metastable level can decay back to the ground state

in the absence of externally stimulating photon flux. This decay is known as spontaneous

emission.

o Process 6: When a flux of signal photons that have energies corresponding to band-gap

energy between ground state and metastable level passes through the device a small portion

of external photons will be absorbed by ions in the ground state, which raises these ions to

the metastable level.

o Process 7: In the stimulated emission a signal photon triggers an excited ion to drop to the

ground state, thereby emitting a new photon of same energy, wave-vector and polarization as

the incoming signal photon. The emission range is 1530-1560nm.

Fig above gives an example for Ge-doped silica glass that has Al co-dopants. The Al helps to absorb

Er ions in the glass and they broaden the amplifier gain spectrum.

Beyond 1560nm the gain decreases steadily until it reaches 0 dB at around 1616nm.

EDFA Architecture:

a. Co-directional pumping:

o The pump light is injected from the same direction as the signal flow.

o This gives better noise performance.

b. Counter-directional pumping:

o The pump light is injected in the opposite direction to the signal flow.

o It allows higher gains.

c. Dual pumping:

o Pump Light is injected from both directions of the signal flow.

o The typical gain is +35dB.

SONET/SDH:

In digital time division multiplexing (TDM) scheme, a standard signal format called synchronous

optical network (SONET) in North America and synchronous hierarchy (SDH) in other parts of the

world was introduced.

Transmission Formats and Speeds:

Fig. below shows the basic structure of a SONET frame.

This is a two dimensional structure consisting of 90 columns by 9 rows of bytes.

In standard SONET technology, a section connects adjacent pieces of equipment, a line is a longer

link that connects two SONET devices, and a path is a complete end to end connection.

The first three columns comprise transport overhead bytes that carry network management

information.

The remaining field of 87 columns is called the synchronous payload envelope (SPE) and carries

user data plus nine bytes of path overhead (POH). The POH supports performance monitoring by end

equipment, status, signal labeling, etc. The POH are always in one column and can be located

anywhere in SPE.

The fundamental SONET frame has 125 s duration. Thus, the transmission bit rate of the basic

SONET signal is 51.84Mbps. This is called STS-1 signal where STS stands for synchronous

transport signal.

An STS-N signal has a transmission rate equal to Nx51.84Mbps.

In SDH the basic rate is equivalent to STS-3 or 155.52Mbps. This is called synchronous transport

module-level 1 (STM-1). Higher rates are designated by STM-M.

For values of N greater than 1, the columns of the frame becomes N times wider, with the number of

rows remaining at nine.

STS-3 frame is 270 columns wide with first nine columns containing overhead information and the

next 261 columns being payload data.

An STM-N frame has 125micro-sec duration and consists of nine rows, each of which has length

270xN bytes.

SONET/SDH Rings:

A key characteristic of SONET/SDH is that they are configured as either ring or mesh architecture.

This is done to create loop diversity for un-interrupted service protection purposes in case of link or

equipment failures. They are also called self-healing rings.

The following two architectures have become popular for SONET and SDH networks

o Two fiber, unidirectional, path switched ring ( two fiber UPSR)

o Two fiber or four fiber, bi-directional, line switched ring ( two or four fiber BLSR)

Fig. below shows a two fiber unidirectional path switched ring network.

In a unidirectional ring the normal traffic travels clockwise around the ring, on the primary path. For

e.g. the connection from node 1 to node 3 uses link 1 and 2, whereas the traffic from node 3 to node

1 traverses links 3 and 4.

The counterclockwise path is used as an alternate route for protection against link or node failures.

This protection path is indicated by dashed lines.

To achieve protection, the signal from a transmitting node is dual-fed into both primary and

protection fibers. This establishes a designated protection path on which traffic flows

counterclockwise: namely, from node 1 to node 3 via links 5 and 6 as shown in Fig. above.

Fig below illustrates the architecture of a four fiber bidirectional line switched ring.

Here, the two primary fiber loops are used for normal bidirectional communication and the two other

secondary fiber loops are standby links for protection purposes.

The four-fiber BLSR has a capacity advantage since it uses twice as much fiber cabling and because

traffic between two nodes is sent only partially around the ring. To see this consider the connection

between nodes 1 and 3. The traffic from node 1 to 3 flows in a clockwise direction along links 1p

and 2p. In the return path the traffic flows counterclockwise from node 3 to node 1 along links 7p

and 8p.

To see the function of the standby link in the four-fiber BLSR, consider first the case where the

transmitter or receiver circuit used on the primary link fails in either node 3 or 4. In this situation, the

affected nodes detect a loss of signal condition and switch both primary fibers connecting these

nodes to the secondary protection pair.

The exact same reconfiguration scenario will occur when the primary fiber connecting nodes 3 and 4

breaks.

High Speed Light-wave Links:

A new challenge emerge in the development of high speed optical transceivers operating at higher

rates such as 10, 40, and 160 Gbps

I. Links operating at 10Gbps:

These include Fiber Channel connections for storage area networks, 10-gigabit Ethernet lines for

local area and metro networks and SONET/SDH terrestrial and undersea long haul lines and metro

links.

As a result of product development efforts several multimode fibers with different bandwidth grades

exists for 10-Gbps use. They are

o OM1 grade fiber is used with LEDs. The bandwidth of these fibers is 200 MHz-km at

850nm and 500MHz-km at 1310nm.

o OM2 grade fiber is used to send 1-Gbps signals at 850nm over about 750m distances and

10Gbps over 82m.

o OM3 grade fiber has the highest bandwidth and can support 10Gbps data rates over distances

up to 300m

II. Links operating at 160 Gbps

Systems that can operate at 160Gbps over a single wavelength were installed using standard G.652

single mode fibers.

These links use the concept of optical time division multiplexing to form 160Gbps data stream.

The bit interleaved OTDM multiplexing technique is similar to WDM.

III. Links operating at 40 Gbps

New challenges in terms of transceiver response characteristics, chromatic dispersion control and

polarization mode dispersion compensation arise when transitioning to higher capacity links such as

40Gbps data rates.

For example, compared to 10Gbps system, using a conventional on-off keying modulation format a

link operating at 40Gbps is sixteen times more sensitive to chromatic dispersion, four times more

sensitive to polarization mode dispersion.

Therefore alternate modulation schemes such as binary differential phase shift keying (DPSK) are

employed.