Fiber Nonlinearities.pptx

7/28/2019 Fiber Nonlinearities.pptx

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OPTICAL FIBER NONLINEARITIES

AND ITS IMPACT ON HIGH

BIT RATE LONG HAULOPTICAL FIBER COMMUNICATIONS

SYSTEMS

Kishori Sharan Mathur

Research Scholar, JJT University, Jhunjhunu

333001, Rajasthan, India

Tel :+91- 9971652846, Email: [email protected]


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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

2


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NONLINEAR EFFECTS IN OPTICAL FIBERAROSE DUE TO:

INCREASE IN OPTICAL POWER LEVELS

INCREASE IN NUMBER OF TRANSMITTED

WAVELENGTHS(DWDM SYSTEMS)

INCREASE IN DATA RATEINCREASE IN TRANSMISSION DISTANCES

FIBER NONLINEARITIES REPRESENTSFUNDAMENTALS LIMITATIONS TO AMOUNT

OF DATA THAT CAN BE TRANSMITTED ON A

SINGLE OPTICAL FIBER CABLE


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Transmission of multiple channels using WDM systems

with 8, 16 or 32 channels (multiplexing of 2.5 Gbit/s signals)

1,2,3--n


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FIBER NONLINEARITIES CAN BE CLASIFIED IN TWO CATEGORIES:

Single channel Multichannel

Refractive indexrelated

Intensity

dependent

variations inrefractive index of

silica fibers-Kerr

effects

Self phase modulation(SPM)

Cross phase modulation(XPM), Four wave

mixing (FWM)

Scattering relatedFrequency of

scattered light

shifted downwards

Stimulated brillouinscattering (SBS)

Stimulated Ramanscattering(SRS)


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Figure 1 shows relationship of refractive index of silica fiber versus optical power.


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MINIMIZING THE AMOUNT OF POWER P LAUNCHED

AND MAXIMIZING THE EFFECTIVE AREA i.e., MODEFIELD DIAMETER (MFD) OF THE FIBER i.e., (AEFF)

ELIMINATES THE NONLINEARITIES PRODUCED BY

REFRACTIVE INDEX POWER DEPENDENCE.

BUT NORMALLY, MINIMIZING THE POWER LEVELS

GOES AGAINST THE CURRENT APPROACH TO MINIZE

THE NUMBERS OF OPTICAL AMPLIFIERS WHICH HAS

DETRIMENTAL EFFECTS ON COST.

HOWEVER MAXIMISING THE EFFECTIVE AREA (MFD)

REMAINS THE MOST COMMON APPROACH IN THE

LATEST FIBER DESIGNS.


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STIMULATED BRILLOUIN SCATTERING (SBS)

SBS ARISES WHEN STRONG OPTICAL SIGNAL

GENERATES AN ACOUSTIC WAVE WHICH PRODUCESVARIATIONS 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 DENSE WAVELENGTH DIVISION

MULTIPLEXING (DWDM) SYSTEM

SBS SETS AN UPPER LIMIT ON THE AMOUNT OFOPTICAL POWER THAT CAN BE LAUNCHED INTOAN OPTICAL FIBER.


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IT IS PARTICULARLY IMPORTANT TO CONTROL SBS IN HIGHSPEED TRANSMISSION SYSTEMS USING EXTERNALMODULATORS 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 ANOPTICAL SOURCE RESULTS IN MINIMIZING THE SBS, BUTBROADENING OF LINE WIDTH OF TRANSMITTER INCREASESTHE DISPERSION SUSCEPTIBILITY OF THE TRANSMITTER,PRIMARILY A CONCERN WHEN OPERATING AT 1550 NM OVERNON 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. THISCAN 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 LASERO/PIN FREQUENCY,ROUGHLY AT 100 TO 200 MHZ TO RAISE THE BRILLOUINTHRESHOLD.


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STIMULATED RAMAN SCATTERING (SRS)

STIMULATED RAMAN SCATTERING IS AN INTERACTIONBETWEEN 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 SUCHOPTICAL 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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STIMULATED RAMAN SCATTERING (SRS)TO UNDERSTAND THE MECHANISM OF SRS LET US CONSIDER

A PHOTON OF ENERGY h1 IS INCIDENT ON A MOLECULEHAVING A VIBRATIONAL FREQUENCYM, THIS MOLECULE CAN

ABSORB SOME ENERGY FROM PHOTON. IN THIS INTERACTION,THE PHOTON IS SCATTERED THEREBY ATTAINING THE LOWERFREQUENCYv 2AND A LOWER ENERGY hV2.

THE MODIFIED PHOTON IS CALLED ASTOKES PHOTON.

THE OPTICAL SIGNAL WAVE THAT IS INJECTED INTO A FIBERIS OFTEN CALLED PUMP WAVE, SINCE IT SUPPLIES POWER TOTHE GENERATED WAVE. THIS PROCESS GENERATES SCATTERED

LIGHT AT A WAVELENGTH LONGER THAN THAT OF THEINCIDENT LIGHT.

IF ANOTHER SIGNAL IS PRESENT AT THIS LONGERWAVELENGTH, THE SRS PHENOMENON WILL AMPLIFY IT AND

THE PUMP WAVELENGTH SIGNAL WILL DECREASE IN POWER.


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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 mwNEAR 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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EDFA

1455nm pump

1.1w

1480nm pump

1.3w

EDFA Rx

16*10Gbit/s

Transmitter

+28.5dbm 310km

120km

120km

Remote amplifier box

EDFA

Optical mux Optical isolator Optical mux

REPEARTERLESS UNDER SEA LINK WITH RAMAN PREAMPLFICATION AND EDFA

AMPLIFICATION


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SELF PHASE MODULATION (SPM)

THE REFRACTIVE INDEX OF MANY OPTICAL MATERIALS CAN BE

GIVEN BY

N = NO+N2 I = NO+N2 P/AEFFWHERE, 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 m2/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 PROPAGATING 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.


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THE COMBINATION OF SPM AND DISPERSIONRESULTS IN TWO PHENOMENONS 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 AFREQUENCY 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 LONGERWAVELENGTH 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 POSITIVETHE 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.

SOLITION ?


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SOLITION ?

Linear systems suffer from chromatic dispersion. Typically 10Gbps

systems require periodic dispersion compensating fiber. As bit rates

increases 40Gbps & beyond, and as distances increases up to

10,000km, the effect of dispersion becomes severe.

Also, as bit rate increases, pulses get smaller & their instantaneous

power gets higher (i.e. the energy has to be squeezed into a shorter

pulse) thus producing nonlinearties.

Also, higher power is required to combat noise as distances increases.

Thus, it becomes increasingly difficult to manage dispersion & to

limit nonlinearties in high bit rate long distance communications. Non

linear, or solition, system change the game by accepting & using non

linearity to combat the dispersion, solving two problems at once.

Experiment carried out over 10,000 km fiber at a data rate of

10Gbps. The result shows no change in the shape of the pulse,

resulting in limitless possibility for data transmission.


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Characteristics of a high-intensity sharply

peaked solition pulsed that is subject to the

Kerr effect as it travels through a nonlinear

dispersive fiber


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CROSS PHASE MODULATION (XPM)

IN CASE OF CROSS PHASE MODULATION REFRACTIVE

INDEX NONLINEARITIES CONVERTS OPTICAL INTENSITYFLUCTUATIONS 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 THEPENALTY 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.


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FOUR WAVE MIXING (FWM)

GENERALLY SYSTEMS THAT CARRY A NUMBER OF

SIMULTANEOUS WAVELENGTHS, SUCH AS DWDMSYSTEMS, EXHIBIT FOUR WAVE MIXING.IT OCCURS DUE TO HIGH LAUNCH POWER ANDLOW DISPERSION IN DWDM CHANNELS.FWM IS CLASSIFIED AS THIRD ORDER DISTORTION

PHENOMENON.THIS THIRD ORDER DISTORTION MECHANISMGENERATES THIRD ORDER HARMONICS IN THESYSTEMS WITH ONE CHANNEL.

IN MULTI CHANNEL SYSTEMS, THIRD ORDERMECHANISMS GENERATE THIRD ORDER HARMONICSAND A GAMUT OF CROSS PRODUCTS. THESE CROSSPRODUCTS RESULTS IN CROSS TALK WHEN THEY FALLNEAR OR ON TOP OF THE DESIRED SIGNALS.


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THESE CROSS PRODUCTS ARE KNOWN AS GHOST CHANNELS SOME

OF WHICH OVERLAP THE ORIGINAL INPUT SIGNAL CHANNELDEPENDING ON THE NUMBERS OF ACTUAL CHANNELS AS SHOWN INFIGURE


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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 POWERMIXING 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 G652SINGLE 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 WEREINTRODUCED HAVING CHROMATIC DISPERSION OF 3 TO 9

PS/NM/KM WHICH IS SUFFICIENT TO SUPPRESS FWM

EFFECTS.



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SIGNIFICANCE OF INTER- AND INTRACHANNEL NONLINEAR IMPAIRMENT IN

WDM SYSTEMS OF DIFFERENT PER-CHANNEL BITRATES.

FOR HIGH-SPEED TDM SYSTEMS EXCEEDING 10 GB/S PER CHANNEL, THE

DOMINANT NONLINEAR INTERACTIONS ARE INTRACHANNEL CROSS-PHASE

MODULATION AND INTRACHANNEL FOUR-WAVE MIXING.


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CONCLUSION

TWO SIGNIFICANT TECHNOLOGICAL ADVANCES DENSE WAVELENGTH DIVISION MULTIPLEXING SYSTEM AND ERBIUMDOPED FIBER AMPLIFIERS (OPTICAL AMPLIFIERS) ISRESPONSIBLE FOR ADVANCES IN OPTICAL COMMUNICATIONSFIELD.

BUT DWDM SYSTEMS CAME WITH A PRICE. THERE WASSEVERE RESTRICTION ON BIT RATE.

ALSO THE HIGH POWER LAUNCHED BY EDFAS INCONJUNCTION WITH SIMULTANEOUS TRANSMISSION OF MANYCHANNELS RESULTED IN NEW PROBLEMS.

SUCH AS FOUR WAVE MIXING PHENOMENON.

ALSO THE TREND TO DECREASE SPACING BETWEENCHANNELS IN DWDM SYSTEMS AGGRAVATED THE SITUATION.

TO COUNTER THESE TECHNOLOGICAL PROBLEMS NON ZERODISPERSION SHIFTED FIBER (NZ-DSF) WERE DEVELOPED.


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CONCLUSION

TO INCREASE THE REPEATER LESS DISTANCE BETWEEN EDFAS &

LAUNCHING OF MORE OPTICAL POWER IS REQUIRED IN THEFIBERS. THIS TOGETHER WITH DWDM TECHNOLOGY INCREASES

THE NON LINEAR EFFECTS IN THE FIBERS.

SPM AND XPM RESULT IN PULSE SPREADING, WHILE SRS AND

SBS BRING ON ATTENUATION.

FORTUNATELY A SOLUTION TO THESE PROBLEMS IS FOUND INTHE FORM OF FIBER WITH LARGE EFFECTIVE AREA(A eff ). THE DEVELOPMENT THAT VAULTED FIBER OPTICS

COMMUNICATIONS TO NEW HEIGHTS. ONE SUCH EXAMPLE ISCORNINGS NZ-DSF LEAF (LARGE EFFECTIVE AREA FIBER) WHICHHAS A TYPICAL AEFF OF 72M

2 IN CONTRAST TO 55 M2 FORREGULAR NZ DSF FIBER.

MANAGEMENT OF FIBER NON LINEARITIES FOR


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MANAGEMENT OF FIBER NON LINEARITIES FOR

INCREASING SYSTEM CAPACITY AND REACH :

In Shannons channel capacity limit study fiber nonlinear

coefficient is of 1.27/w/km, a value typical of SSMFs, with lowernonlinear coefficient, high nonlinear limit and thus higher capacity

can be allowed. Lower nonlinear coefficient can be realized by

using larger effective fiber core area (Aeff). In fact, pure silica

core fibers (PSCF) have Aeff of 118 m , which is about 50%larger than that of SSMF (80 m). Theoretical study has recently

shown that Aeff as large as 160 m can be achieved with PSCF.

Increasing the fiber effective area improves the spectral efficiency

and is most valuable for long haul transmission system.Next

Figures shows the difference between traditional NZ-DSF fibers

and LEAF fibers.


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Mode Field Diameter of a Single Mode Fiber

SMALLER THE EFFECTIVE AREA (MFD), HIGHER THE INCIDENCE OF NONLINEAR EFFECTS


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CONCLUSION

THE MAJOR BENEFIT OF THE NEW FIBER IS IT IS ABILITY TOHANDLE MORE POWER WITHOUT BEING AFFECTED BYNONLINEARITIES, ESSENTIALLY INCREASING IT ISINFORMATION CARRYING CAPACITY.

THE VTT TECHNICAL RESEARCH CENTRE OF FINLAND HASALSO REPORTED A FIBER WITH RIB SHAPED CORE AND NONCYLINDRICAL OUTER FORM HAVING ULTRA LARGE MODEDIAMETER (> 50 M2) CAPABLE OF TRANSMITTING VERY HIGHOPTICAL POWERS WITH LARGE NUMBERS OF WAVELENGTH

CHANNELS WITH SMALLER NONLINEARITIES.


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Fiber Nonlinearities.pptx