Nonlinearities in Optical Fiber Networks and It is Remedial Measures

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Kishori Sharan Mathur

Research Scholar, JJT University,

Jhunjhunu – 333001, Rajasthan,

India

[email protected]

Fiber Nonlinearities

• As long as optical power within an optical fiber is small, the fiber can be treated as a linear medium; that is the loss and refractive index are independent of the signal power

• When optical power level gets fairly high, the fiber becomes a nonlinear medium; that is the loss and refractive index depend on the optical power

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Nonlinearities effects in optical fiber arose due to (i) Increase in optical power levels. (ii) Increase in transmitted wavelengths (DWDM systems) (iii) Increase in data rate. (iv) Increase in transmission distances.

FIBER NONLINEARITIES

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Nonlinear effects

- Change of Refractive index :

Self-Phase Modulation (SPM)

Cross-Phase Modulation (XPM)

Four-Wave Mixing (FWM)

- Stimulated Scattering:

Stimulated Brillouin Scattering (SBS)

Stimulated Raman Scattering (SRS)

Response of fiber to optical power is nonlinear.Nonlinear effects appear when the power launched into fiber is high.

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Table 1

Single chanel Multichanel

Refractive index related Self phase modulation (SPM) Cross phase modulation

(XPM), Four wave mixing

(FWM)

Scattering related Stimulated brilloun scattering

(SBS)

Stimulated raman

scattering(SRS)

FIBER NONLINEARITIES

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KERR EFFECTS

KERR EFFECTS CONSIST OF THREE DIFFERENT PHENOMENA. IN

AN OPTICAL FIBER, THE CORE HAS A SPECIFIC REFRACTIVE

INDEX THAT DETERMINES HOW LIGHT TRAVELS THROUGH IT.

DEPENDING ON HOW INTENSE IS THE LIGHT TRAVELING

THROUGH THE CORE, THIS INDEX CAN CHANGE. THIS

INTENSITY-DEPENDENCE, KNOWN AS THE KERR EFFECT, CAN

CAUSE THE FOLLOWING ISSUES:

▼ SELF PHASE MODULATION THIS OCCURS WHEN A

WAVELENGTH CAN SPREAD INTO ADJACENT WAVELENGTHS ON

ITS OWN.

■ CROSS PHASE MODULATION THIS OCCURS WHEN SEVERAL

DIFFERENT WAVELENGTHS IN A WDM SYSTEM CAN CAUSE EACH

OTHER TO SPREAD OUT.

▲ FOUR WAVE MIXING THIS OCCURS WHEN TWO OR MORE

WAVELENGTHS CAN INTERACT TO CREATE AN ENTIRELY NEW

WAVELENGTH.

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SCATTERING EFFECTS

THERE ARE TWO TYPES OF NONLINEAR SCATTERING EFFECTS

TO BE AWARE OF IN OPTICAL NETWORKS.

▼ STIMULATED RAMAN SCATTERING THIS OCCURS WHEN

LIGHT LOSES ENERGY TO MOLECULES IN THE FIBER AND IS

REEMITTED AT A LONGER WAVELENGTH. THIS IS DUE TO THE

LOSS OF ENERGY.

▲ STIMULATED BRILLOUIN SCATTERING THIS OCCURS WHEN

LIGHT WITHIN THE FIBER CREATES ACOUSTIC WAVES. THIS

CAN SCATTER THE LIGHT INTO DIFFERENT WAVELENGTHS

AND DISRUPT THE SIGNAL.

BECAUSE OF NONLINEAR EFFECTS, LIKE SCATTERING AND

KERR EFFECTS, DATA CAN BE LOST OR CORRUPTED

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STIMULATED BRILLOUIN SCATTERING (SBS)

SBS ARISES WHEN STRONG OPTICAL SIGNAL

GENERATES AN ACOUSTIC WAVE WHICH PRODUCES

VARIATIONS IN THE REFRACTIVE INDEX.

THESE PERIODIC VARIATIONS IN REFRACTIVE INDEX,

CAUSED BY HIGH POWER INCIDENT LIGHT WAVE,

CAUSES BACK REFLECTIONS SIMILAR TO THE EFFECT

OF BRAGG GRATINGS .

THE BACK SCATTERING CAUSES LOSS OF SIGNAL

POWER.

THE SBS EFFECT IS CONFINED WITHIN A SINGLE WAVELENGTH CHANNEL IN A DENCE WAVELENGTH DIVISION MULTIPLEXING (DWDM) SYSTEM

SBS SETS AN UPPER LIMIT ON THE AMOUNT OF OPTICAL POWER THAT CAN BE LAUNCHED INTO AN OPTICAL FIBER. 9


IT IS PARTICULARLY IMPORTANT TO CONTROL SBS IN HIGH SPEED TRANSMISSION SYSTEMS USING EXTERNAL MODULATORS AND CONTINUOUS WAVE (CW) LASER SOURCES.

The phenomenon of SBS threshold effects

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STIMULATED BRILLOUIN SCATTERING

(SBS) THE SBS THRESHOLD IS STRONGLY DEPENDENT ON THE OPTICAL SOURCES LINE WIDTH

FIG SHOWS HOW THE SBS THRESHOLD INCREASES PROPORTIONALLY AS THE OPTICAL SOURCE LINE WIDTH INCREASES.

BROADENING THE EFFECTIVE SPECTRAL WIDTH OF AN OPTICAL SOURCE RESULTS IN MINIMIZING THE SBS, BUT BROADENING OF LINE WIDTH OF TRANSMITTER INCREASES THE DISPERSION SUSCEPTIBILITY OF THE TRANSMITTER, PRIMARILY A CONCERN WHEN OPERATING AT 1550 nm OVER NON DISPERSION SHIFTED SINGLE MODE FIBERS.

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VARIOUS SCHEMES ARE AVAILABLE FOR REDUCING THE POWER

PENALTY EFFECTS OF SBS AS FOLLOWS:


(I) KEEPING THE OPTICAL POWER OF WDM CHANNELS

BELOW THE SBS THRESHOLD. FOR LONG HAUL

COMMUNICATION SYSTEMS, THIS MAY REQUIRE A REDUCTION

IN No. OF OPTICAL AMPLIFIER .

(ii) INCREASING THE LINE WIDTH OF THE SOURCE. THIS

CAN BE ACHIEVED THROUGH DIRECT MODULATION OF

SOURCE (AS OPPOSED TO EXTERNAL MODULATION) SINCE

THIS CAUSES THE LINE WIDTH TO BROADEN BECAUSE OF

CHIRPING EFFECTS. BUT IT MAY RESULT IN LARGE

DISPERSION PENALTY.

(III) SLIGHTLY DITHERING THE LASER O/P IN FREQUENCY, ROUGHLY AT 100TO 200 MHZ TO RAISE THE BRILLOUIN THRESHOLD.

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STIMULATED RAMAN SCATTERING (SRS)

STIMULATED RAMAN SCATTERING IS AN INTERACTION

BETWEEN LIGHT WAVES AND THE VIBRATIONAL MODES OF

SILICA MOLECULES.

BUT SINCE THE THRESHOLD OF SRS IS CLOSE TO 1 WATT I.E.

NEARLY THOUSAND TIMES HIGHER THAN SBS IT IS MUCH LESS

A PROBLEM THAN SBS.

BUT THE THRESHOLD LIMIT DROPS PROPORTIONALLY BY

THE NUMBER OF OPTICAL AMPLIFIERS IN SERIES.

HENCE A FIBER OPTICAL LINK THAT INCLUDE THREE SUCH

OPTICAL AMPLIFIER WILL REACH THIS LIMITS AS EDFAS GIVES

OPTICAL POWER OUTPUT OF 500 mw (27dbm) AND IN FUTURE

THIS OUTPUT WILL GO EVEN HIGHER.

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SIX CHANNEL DWDM TRANSMITTED OPTICAL SPECTRUM

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SRS EFFECT ON SIX CHANNEL DWDM TRANSMITTED OPTICAL SPECTRUM

FOR A SINGLE CHANNEL SYSTEM THRESHOLD IS AROUND 500 mw

NEAR 1550 nm

FOR A 20 CHANNEL SYSTEM THRESHOLD PTH EXCEEDS 10 mw AND IT IS AROUND 1 mw FOR A 70 CHANNEL SYSTEM.


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TO UNDERSTAND THE MECHANISM OF SRS LET US CONSIDER A PHOTON OF ENERGY hᵥ1 IS INCIDENT ON A MOLECULE HAVING A VIBRATIONAL FREQUENCY ᵥM, THIS MOLECULE CAN ABSORB SOME ENERGY FROM PHOTON. IN THIS INTERACTION, THE PHOTON IS SCATTERED THEREBY ATTAINING THE LOWER FREQUENCY ᵥ2 AND A LOWER ENERGY hV2. THE MODIFIED PHOTON IS CALLED A STOKES PHOTON. THE OPTICAL SIGNAL WAVE THAT IS INJECTED INTO A FIBER IS OFTEN CALLED PUMP WAVE, SINCE IT SUPPLIES POWER TO THE GENERATED WAVE. THIS PROCESS GENERATES SCATTERED LIGHT AT A WAVELENGTH LONGER THAN THAT OF THE INCIDENT LIGHT.

IF ANOTHER SIGNAL IS PRESENT AT THIS LONGER

WAVELENGTH, THE SRS PHENOMENON WILL AMPLIFY IT AND THE PUMP WAVELENGTH SIGNAL WILL DECREASE IN POWER.

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1) Effect and consequences • SRS causes a signal wavelength to behave as a “pump” for longer wavelengths,

either other signal channels or spontaneously scattered Raman-shifted light. The shorter wavelengths is attenuated by this process, which amplifies longer wavelengths

• SRS takes place in the transmission fiber

2) SRS could be exploited as an advantage • By using suitable Raman Pumps it is possible to implement a Distributed Raman

Amplifier into the transmission fiber. This helps the amplification of the signal (in co-operation with the localized EDFA). The pumps are depleted and the power is transferred to the signal

f f Transmission Fiber 17

SELF PHASE MODULATION (SPM)

THE REFRACTIVE INDEX OF MANY OPTICAL MATERIALS CAN BE

GIVEN BY

N = NO+N2 I = NO+N2 P/AEFF

WHERE, NO IS THE ORDINARY REFRACTIVE INDEX OF THE MATERIAL

AND N2 IS THE NONLINEAR INDEX COEFFICIENT. FOR SILICA, THE

FACTOR N2 IS ABOUT 2.6 X 10-8 μm2/w.

THIS NONLINEARITY IN THE REFRACTIVE INDEX IS KNOWN AS KERR

NONLINEARITY.

THE NONLINEARITY PRODUCES A CARRIER BASED PHASE

MODULATION OF THE PROPAGATING WAVE WHICH IS CALLED KERR

EFFECT.

IN SINGLE WAVELENGTH LINKS, THIS GIVES RISE TO SELF PHASE

MODULATION (SPM) WHICH CONVERTS OPTICAL POWER

FLUCTUATIONS IN A PROPOGATING LIGHT WAVE TO SPURIOUS

PHASE FLUCTUATIONS IN THE SAME WAVE. SPM RESULTS IN

DIFFERENT WAY IF ACTING ALONE OR WHEN COUPLED WITH

DISPERSION OF THE FIBER. 18


THE COMBINATION OF SPM AND DISPERSION

RESULTS IN TWO PHENOMENON’S WITH MANY

CONSEQUENCES FOR REAL TRANSMISSION

SYSTEMS.

(I) IT RESULTS IN MODULATION INSTABILITY.

(II) SOLITONS

THE SPM EFFECTS CAN BE NEGLIGIBLE WHEN

THE PEAK POWER IS BELOW 166 mW OR 18 dbm

AVERAGE POWER.

BY USING DISPERSION COMPENSATING FIBERS

(DCF), SPM CAN BE REDUCED.

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AS AN OPTICAL PULSE TRAVELS DOWN THE FIBER, THE

TRAILING EDGE OF THE PULSE CAUSES THE REFRACTIVE

INDEX OF THE FIBER TO RISE, RESULTING IN BLUE SHIFT IN

FREQUENCY (TOWARDS HIGHER FREQUENCIES OR SHORTER

WAVELENGTHS). THE LEADING EDGE OF THE PULSE

DECREASES THE REFRACTIVE INDEX OF THE FIBER CAUSING A

RED SHIFT (TOWARDS LOWER FREQUENCIES OR LONGER

WAVELENGTHS). THESE RED AND BLUE SHIFTS INTRODUCE A

FREQUENCY CHIRP ON EACH EDGE WHICH INTERACTS WITH

FIBER'S DISPERSION TO BROADEN THE PULSE AS SHOWN IN

FIG


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IN FACT IN CASE OF NORMAL DISPERSION REGION OF THE FIBER WHERE

CHROMATIC DISPERSION IS NEGATIVE THE RED LIGHT WHICH HAS LONGER

WAVELENGTH AND SEES LOWER REFRACTIVE INDEX RESULTS IN RED LIGHT

TRAVELLING FASTER THAN BLUE LIGHT SEEING HIGHER REFRACTIVE INDEX.

HENCE BOTH RED AND BLUE MOVES AWAY FROM THE CENTRE OF PULSE.

HENCE CHIRPING RESULTS IN PULSE BROADENING.

BUT IN ANOMALOUS REGION WHERE CHROMATIC DISPERSION IS POSITIVE

THE RED SHIFTED LEADING EDGE OF THE PULSE TRAVELS SLOWER THAN

TRAILING EDGE.

THUS BOTH MOVES TOWARDS THE CENTRE OF THE PULSE.

IN THIS CASE SPM CAUSES THE PULSE TO NARROW, HENCE PARTLY

COMPENSATING FOR CHROMATIC DISPERSION AND UNDOING THE

FREQUENCY CHIRP.

IN ADVANCE NETWORK DESIGNS, SPM CAN BE USED TO PARTLY

COMPENSATE FOR THE EFFECTS OF CHROMATIC DISPERSION. THIS

PHENOMENON ALSO RESULTS IN FORMATION OF SOLITON PULSES.

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EFFECTS OF NONLINEARITES

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NON LINEAR EFFECTS: CROSS PHASE MODULATION (XPM)

• XPM acts as a crosstalk penalty, which increases with increasing channel power level and system length and with decreasing channel spacing

• XPM causes a spectral broadening of the optical pulses and thus reduces the dispersion tolerance of the system

• At 10 Gbps, its penalty is minimized by distributing dispersion compensation at each line amplifier site

• If dispersion is compensated only at the terminal ends, there will be a residual penalty due to XPM

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CROSS PHASE MODULATION (XPM)

IN CASE OF CROSS PHASE MODULATION REFRACTIVE

INDEX NONLINEARITIES CONVERTS OPTICAL INTENSITY

FLUCTUATIONS IN A PARTICULAR WAVELENGTH CHANNEL TO

PHASE FLUCTUATIONS IN ANOTHER CO PROPAGATING

CHANNEL.

IN FACT, SPM IS ALWAYS PRESENT WHEN XPM OCCURS.

TO AVOID THE XPM FOR TWO CHANNEL SYSTEM THE

LIMITING CHANNEL POWER IS AROUND 56 mw (17.5 dbm). FOR

A TEN CHANNEL WAVELENGTH SYSTEM THE LIMIT IS AROUND

10 mw.

IN FACT SEPARATION BETWEEN DWDM CHANNELS ALSO

AFFECTS THE XPM.

AN INCREASE IN THE SEPARATION WILL DECREASE THE

PENALTY OF POWER DUE TO XPM.

FOR DIRECT DETECTION OPTICAL FIBER SYSTEMS THE

IMPACT OF XPM IS LESS WHEREAS THE XPM COULD BE A

PROBLEM FOR HIGH RATE DWDM SYSTEMS AND WHEN

COHERENT DETECTION SCHEMES ARE USED. 25

FOUR WAVE MIXING (FWM)

GENERALLY SYSTEMS THAT CARRY A NUMBER OF SIMULTANEOUS WAVELENGTHS, SUCH AS DWDM SYSTEMS, EXHIBIT FOUR WAVE MIXING. IT OCCURS DUE TO HIGH LAUNCH POWER AND LOW DISPERSION IN DWDM CHANNELS. FWM IS CLASSIFIED AS THIRD ORDER DISTORTION PHENOMENON. THIS THIRD ORDER DISTORTION MECHANISM GENERATES THIRD ORDER HARMONICS IN THE SYSTEMS WITH ONE CHANNEL. IN MULTI CHANNEL SYSTEMS, THIRD ORDER MECHANISMS GENERATE THIRD ORDER HARMONICS AND A GAMUT OF CROSS PRODUCTS. THESE CROSS PRODUCTS RESULTS IN CROSS TALK WHEN THEY FALL NEAR OR ON TOP OF THE DESIRED SIGNALS.

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NON LINEAR EFFECTS: FOUR WAVE MIXING (FWM)

1) Effect and consequences • FWM is the generation of new optical waves (at frequencies which are

the mixing products of the originator signals). This is due to interaction of the transmitted optical waves. The created mixing products interfere with the signal channels causing consequent eye closing and BER degradation Decreasing channel spacing and chromatic dispersion will increase FWM

• N channels N2(N-1)/2 side bands are created, causing – Reduction of signals – Interference – Cross talk

2) Counteractions • Avoid use of ITU-T G.653 (DSF) fiber, Use of ITU-T G.652 (SMF) fiber

and ITU-T G.655 (NZDSF) fiber • Unequal channel spacing will cause the mixing products to be created

at different frequencies which do not interfere with the signal channels

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THE MAGNITUDE OF FWM PRODUCTS, REFERRED

TO AS THE FWM MIXING EFFICIENCY IS AFFECTED

BY FOLLOWING MAJOR FACTORS.

CHANNEL SPACING

FIBER DISPERSION SIGNAL POWER MIXING EFFICIENCY INCREASES DRAMATICALLY AS THE CHANNEL SPACING BECOMES CLOSER AND CLOSER. IN CASE OF FIBER DISPERSION, MIXING EFFICIENCY IS INVERSELY PROPORTIONAL TO TO THE FIBER DISPERSION, BEING STRONGEST AT THE ZERO DISPERSION POINT. FWM EFFICIENCY IS EXPRESSED IN dB AND MORE NEGATIVE VALUES ARE PREFERRED. SINCE THEY INDICATE LOWER MIXING EFFICIENCY.

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FWM EFFICIENCY IN SINGLE MODE FIBERS

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IN AN OPTICAL DWDM SYSTEM DESIGN USES NON

DISPERSION SHIFTED FIBER (NDSF) E.G., STANDARD G652

SINGLE MODE FIBERS WITH DISPERSION OF 17 PS/NM/KM AND

THE MINIMUM RECOMMENDED INTERNATIONAL

TELECOMMUNICATION UNION (ITU) DWDM SPACING OF 0.8 NM,

THEN MIXING EFFICIENCY WILL BE ABOUT - 48 DB AND WILL

HAVE LITTLE EFFECT ON THE SYSTEM.

BUT FOR HIGH DATA RATE SYSTEM HIGH CHROMATIC

DISPERSION WILL RESULT IN HIGHER DISPERSION

PENALTIES.

TO AVOID HIGH DISPERSION PENALTIES G 655 FIBERS WERE

INTRODUCED HAVING CHROMATIC DISPERSION OF 3 TO 9

PS/NM/KM WHICH IS SUFFICIENT TO SUPPRESS FWM

EFFECTS.


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Four wave mixing (FWM) is one of the most troubling issues

Three signals combine to form a fourth spurious or mixing

component, hence the name four wave mixing, shown below in

terms of frequency w:

Spurious components cause two problems:

Interference between wanted signals

Power is lost from wanted signals into unwanted spurious

signals

The total number of mixing components increases dramatically

with the number of channels

Four Wave Mixing

Non-Linear

Optical Medium

w1

w3

w2

w4 = w1 + w2 - w3

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NON LINEAR EFFECTS: FOUR WAVE MIXING (FWM) CONTD.

• Consider a simple three wavelength (l1, l2 & l3)

• Let’s assume that the input wavelengths are ll = 1551.72 nm, l2 = 1552.52 nm & l3 = 1553.32 nm. The interfering wavelengths that are of most concern in our hypothetical three wavelength system are:

– l1 + l2 - l3 = 1550.92 nm

– l1 - l2 + l3 = 1552.52 nm

– l2 + l3 . l1 = 1554.12 nm

– 2l1 - l2 = 1550.92 nm

– 2l1 - l3 = 1550.12 nm

– 2l2 - l1 = 1553.32 nm

– 2l2 - l3 = 1551.72 nm

– 2l3 - l1 = 1554.92 nm

– 2l3 - l2 = 1554.12 nm 32

Traditional non-multiplexed systems have used dispersion shifted fiber at

1550 to reduce chromatic dispersion

Unfortunately operating at the dispersion minimum increases the level of

FWM

Conventional fiber (dispersion minimum at 1330 nm) suffers less from FWM

but chromatic dispersion rises

Solution is to use "Non-Zero Dispersion Shifted Fiber" (NZ DSF), a

compromise between DSF and conventional fiber (NDSF, Non-DSF)

ITU-T standard is G.655 for non-zero dispersion shifted single mode fibers

REDUCING FWM USING NZ-DSF

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One way to improve on NZ-DSF is to increase the effective area of the fibre

In a singlemode fibre the optical power density peaks at the centre of the fibre core

FWM and other effect most likely to take place at locations of high power density

Large effective Area Fibres spread the power density more evenly across the fibre

core

Result is a reduction in peak power and thus FWM

REDUCING FWM USING A LARGE EFFECTIVE AREA FIBRE NZ-DSF

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Corning LEAF has an effective area 32% larger than conventional NZ-DSF

Claimed result is lower FWM

Impact on system design is that it allows higher fibre input powers so span

increases

Section of DWDM

spectrum

NZ-DSF shows

higher FWM

components

LEAF has lower

FWM and higher per

channe\l power

DWDM

channel

FWM

component

CORNING LEAF

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Provides small amount of dispersion over EDFA band

Non-Zero dispersion band is 1530-1565 (ITU C-Band)

Minimum dispersion is 1.3 ps/nm-km, maximum is 5.8 ps/nm-km

Very low OH attenuation at 1383 nm (< 1dB/km)

Dispersion

Characteristics

LUCENT TRUEWAVE NZDSF

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LUCENTS ALL WAVE BROADBAND FIBER

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ZERO WATER PEAK SINGLE MODE FIBER

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Nonlinearities in Optical Fiber Networks and It is Remedial Measures

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