Nonlinearities in Optical Fiber Networks and It is Remedial Measures

download Nonlinearities in Optical Fiber Networks and It is Remedial Measures

of 39

  • date post

    13-Apr-2015
  • Category

    Documents

  • view

    18
  • download

    2

Embed Size (px)

description

Nonlinearities in Optical Fiber Networks and It is Remedial Measures

Transcript of Nonlinearities in Optical Fiber Networks and It is Remedial Measures

Kishori Sharan Mathur Research Scholar, JJT University, Jhunjhunu 333001, Rajasthan, India kishorimathur@hotmail.com

1

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

FIBER NONLINEARITIESNonlinearities 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.3

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

- 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)4

FIBER NONLINEARITIESTable 1 Single chanel Multichanel Self phase modulation (SPM) Cross phase modulation (XPM), Four wave mixing (FWM) Stimulated brilloun scattering Stimulated raman (SBS) scattering(SRS)

Refractive index related

Scattering related

5

6

KERR EFFECTSKERR 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.7

SCATTERING EFFECTSTHERE 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

8

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

STIMULATED BRILLOUIN SCATTERING (SBS)

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 effects10

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.11

STIMULATED BRILLOUIN SCATTERING (SBS)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.12

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.

13

STIMULATED RAMAN SCATTERING (SRS)

SIX CHANNEL DWDM TRANSMITTED OPTICAL SPECTRUM

14

STIMULATED RAMAN SCATTERING (SRS)

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.

15

STIMULATED RAMAN SCATTERING (SRS)TO UNDERSTAND THE MECHANISM OF SRS LET US CONSIDER A PHOTON OF ENERGY h1 IS INCIDENT ON A MOLECULE HAVING A VIBRATIONAL FREQUENCY M, THIS MOLECULE CAN ABSORB SOME ENERGY FROM PHOTON. IN THIS INTERACTION, THE PHOTON IS SCATTERED THEREBY ATTAINING THE LOWER FREQUENCY 2 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 LONGERWAVELENGTH, THE SRS PHENOMENON WILL AMPLIFY IT AND THE PUMP WAVELENGTH SIGNAL WILL DECREASE IN POWER.16

STIMULATED RAMAN SCATTERING (SRS)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

Transmission Fiber

17

f

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 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 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

SELF PHASE MODULATION (SPM)THE COMBINATION OF SPM AND DISPERSION RESULTS 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.19

SELF PHASE MODULATION (SPM)

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

20

SELF PHASE MODULATION (SPM)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.21

22

EFFECTS OF NONLINEARITES

23

NON LINEAR EFFECTS: CROSS PHASE MODULATION (XPM) XPM acts as a crosstalk penalty, which increases with increasing channel power level and system length and wit