Kishori Sharan Mathur

Research Scholar, SHRI JJT University,

Jhunjhunu – 333001, Rajasthan, India

kishorimathur@hotmail.com

OPTICAL AMPLIFIERS FOR

INCREASING REPEATERLESS

DISTANCE OF A OPTICAL FIBER

LINK

OPTICAL AMPLIFIERS In long haul communication systems repeaters are at 100km distance Conventional repeaters performs optical to electrical conversion carryout electrical amplification,retiming,reshaping and then electrical to optical conversion (3R systems) This increases cost and complexity of an optical communication system and also reduces the operational bandwidth of a system Today it is done by erbium doped optical fiber amplifiers(EDFA) by inserting a length of 10 meter fiber for every 100 km length of main fiber.Due to operation in optical domain S/N ratio greatly improves Gain as large as 45to 50 db can be achieved Operation of EDFA is in the region of 1530nm to 1560nm range To achieve higher gain and wider BW of operation Raman fiber amplifier is used combined with EDFA to amplify signal in the range of 1531nm to 1616nm

EDFA AND RAMAN AMPLIFICATION

BASIC OPERATION OF AN OPTICAL AMPLIFIER

The device absorbs energy supplied from an external source

Called the 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,

Thereby producing an amplified signal

For an optical signal amplification at 1550nm InGaAsP laser

Diode operating at a wave length of 1480nm or 980nm is used

As a pumping source for EDFA amplifiers

Currently EDFAs are limited to 1550nm systems only

“Simple” device consisting of four parts: Erbium-doped fiber An optical pump (to invert the population). A coupler An isolator to cut off back propagating noise

Isolator Coupler IsolatorCoupler

Erbium-Doped

Fiber (10–50m)

Pump

Laser

Pump

Laser

THE 980 NM LASER PUMP BLOWS THE ERBIUM IONS INTO A MUCH

HIGHER STATE OF EXCITEMENT THAN THE LITTLE 1480 NM PUMP. BUT

THE HIGHER THE IONS ARE SHOT INTO THE SKY, THE LESS TIME THEY

WILL SPEND AT THAT ALTITUDE. THEY WILL STAY AT THAT EXCITED

STATE FOR ONLY NANOSECONDS BEFORE THEY DROP DOWN TO THE

NEXT STATE. ONCE THEY REACH THE NEXT STATE, THEY STAY AT THAT

LEVEL MUCH LONGER THAN THE IONS THAT WERE EXCITED BY THE

1480.NM LASER PUMP—MILLISECONDS, THIS TIME. AND THE LONGER

THEY STAY AT THE EXCITED STATE, THE MORE LIKELY THAT THE SIGNAL

WILL COME ALONG AND GENERATE STIMULATED EMISSION. ERBIUM

IONS GET EXCITED WHEN A 1480 NM PUMP LASER STRIKES THEM.

FURTHERMORE, THIS REDUCES UNWANTED SPONTANEOUS EMISSIONS

THAT CAUSE NOISE TO BE INJECTED INTO THE PHOTON STREAM.

BECAUSE OF THIS BEHAVIOR, A 980 NM PUMP PROVIDES GREATER

AMPLIFICATION EFFICIENCY AND IS THE FAVORED PUMP WAVELENGTH

FOR MOST EDFAS. EDFA IS USED MOST OFTEN IN SUBMARINE SYSTEMS,

WHERE OPTICAL SIGNALS MUST TRAVEL FOR THOUSANDS OF MILES IN

THE BRINY DEEP. EDFAS CAN BE MANUFACTURED AND PLACED IN

WATERPROOF BOXES AT 50-MILE INTERVALS. TO ASSURE RELIABILITY,

THESE EDFAS INCORPORATE A SIMPLE DESIGN. THOUGH POPULAR ON

THE OCEAN FLOOR, EDFAS ARE ALSO GAINING POPULARITY ON DRY

LAND.

A 980 NM PUMP LASER GETS THE ERBIUM

IONS EXCITED, BUT JUST FOR A BRIEF TIME

OPTICAL AMPLIFIER

ADVANTAGES OF EDFA

Operate in optical domain hence less noise

It is bit rate transparent i.e., amplifier can work efficiently

at any (variable) bit rate, where as electronic amplifiers

can work at the designed bit rate only

EDFAs have large gain BW (40nm spectral BW).Hence it

is possible to use only one amplifier in WDM multiplexed

system for amplification of entire wavelengths

Fiber amplifiers can be easily spliced to optical fiber link

with Minimum insertion loss

Gain provided by EDFAs is polarization insensitive

472

APPLICATIONS OF EDFAS

In line optical amplifiers

When SMF links are only attenuation limited

i.e. with minimum dispersion, amplification of optical signal

is sufficient to increase repeater less transmission distance

Pre amplifiers

It amplifies the weak optical signal before photo- detection

so that S/N ratio degradation due to thermal noise

in the receiver can be suppressed

Power amplifiers

By placing the device immediately after an optical

transmitter boost the transmitted power resulting longer

transmission distance

Booster amplifiers

In a LAN it can work as a booster amplifier to

compensate for coupler insertion loss and power splitting loss

473

TX Rx

TX

Rx TX

Rx

TX

Rx

Rx

Rx

IN LINE AMPLIFIER

PREAMPLIFIER

POWER(BOOSTER)AMPLIFIER

LAN BOOSTER AMPLIFIER

STAR COUPLER

Fiber lines

474

RAMAN AMPLIFICATION•Non linear stimulated raman scattering (SRS)

•Scattering of light by molecules, in which scattered light is shifted

from the incoming light by a frequency characteristic of the

molecules.

•In the raman amplifier, optical carrier signal interacts coherently with

the silica molecules of an optical fiber that has been excited by a high

power pump laser.

•This results in optical amplifications of the optical carrier.

MAJOR ADVANTAGES OF SRS (RAMAN AMPLIFICATION)

•Can reduce the amplifier noise in a network.

•Longer distances between optical amplifiers & OEO regenerators.

•Tighter channel spacing or higher data rate in wdm systems.

RAMAN AMPLIFICATION

Counter propagating Raman pumps are more efficient than co-propagating pumps.

In co-propagating i.e. pumping from the beginning of span a large increase in pump power is needed to produce small increase.

In counter propagating where the Raman pump is fed from the end of the span Raman gain can build up much higher without saturating the amplifier. Hence, less pump power for higher gain.

Raman gain can be generated in all types of fibers.

Raman amplifiers allow for gain across a wide spectrums of wavelength .

Practical example in submarine optical network unrepeated link of 520km with gain of 25db by Raman effect. In spectral band of 1535nm to 1565nm.

Possible to deploy unrepeated links capable of transmitting 15Tbps over a distance of 200km.

STIMULATED RAMAN SCATTERING RAMAN GAIN SPECTRUM FOR SILICA FIBERS

RAMAN AMPLIFICATION IS DIFFERENT FROM EDFA, BECAUSE THERE IS

NO NEED TO SPECIALLY DOPE THE FIBER. AMPLIFICATION USUALLY

OCCURS WITHIN THE ENTIRE LENGTH OF THE FIBER, RATHER THAN IN

ONE PARTICULAR PLACE, WITHIN A SPECIAL DEVICE, LIKE EDFA.

AMPLIFICATION WITHIN THE LENGTH OF FIBER IS KNOWN AS

DISTRIBUTED AMPLIFICATION. TO THE FIBER CARRYING YOUR SIGNAL,

YOU ADD A PUMP LASER, WHICH WILL AMPLIFY THE SIGNAL ALONG THE

FIBER’S RUN UNTIL THE PUMP SIGNAL ULTIMATELY FADES AWAY. YOU

CAN APPLY A PUMP LASER IN DIFFERENT LOCATIONS TO GENERATE

DIFFERENT KINDS OF RAMAN AMPLIFICATION.

▼ APPLYING THE PUMP AT THE BEGINNING OF THE FIBER IS CALLED CO-

PUMPING.

■ APPLYING THE PUMP AT THE END OF THE FIBER IS CALLED COUNTER-

PUMPING. THIS PLACEMENT GENERALLY YIELDS BETTER RESULTS THAN

CO-PUMPING.

▲ PUMPS CAN BE APPLIED AT BOTH ENDS, IN WHAT IS CALLED CO-

COUNTER PUMPING.

RAMAN AMPLIFICATION

A PUMP LASER CAN BE APPLIED IN

ONE OF THREE LOCATIONS

A PUMP LASER EMITTING A SINGLE WAVELENGTH IS USEFUL FOR

TRANSMISSIONS SENT ACROSS SINGLE-MODE FIBER. HOWEVER, IF

YOU ARE USING A WAVELENGTH DIVISION MULTIPLEXING

(WDM)SYSTEM, SEVERAL DIFFERENT PUMP WAVELENGTHS WILL

NEED TO BE GENERATED AND USED TOGETHER TO AMPLIFY EVERY

WAVELENGTH.

DISTRIBUTED Vs. LUMPED AMPLIFICATION

THE USE OF MULTIPLE PUMP WAVELENGTHS TO ACHIEVE FLAT BROADBAND GAIN

A SIMPLIFIED BLOCK DIAGRAM OF RAMAN AMPLIFIER DEPLOYED IN BACKWARD PUMPING CONFIGURATION

EDFA

1455nm pump

1.1w

1480nm pump

1.3w

EDFA Rx

16*10Gbit/s

Transmitter

+28.5dbm 310km

120km

120km

Remote amplifier box

EDF

Optical mux Optical isolator Optical mux

REPEARTERLESS LINK WITH RAMAN PREAMPLFICATION AND EDFA

AMPLIFICATION

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