Mode Dispersion

1
???????????
2
??????????,????????

• ????(Mode Dispersion)
• ????(Cromatic Dispersion)
• ????(Polarization Mode Dispersion)

3
????????
4
(No Transcript)
5
Material (Chromatic) Dispersion

• This is caused by the fact that the refractive
  index of the glass we are using varies (slightly)
  with the wavelength. Some wavelengths therefore
  have higher group velocities and so travel faster
  than others. Since every pulse consists of a
  range of wavelengths it will spread out to some
  degree during its travel.

6
Group Velocity Dispersion (GVD)

• Normal Dispersion Regime the long wavelengths
  travel faster than the short ones! Thus after
  travelling on a fibre wavelengths at the red end
  of the pulse spectrum will arrive first. This is
  called a positive chirp!
• Anomalous Dispersion Regime the short
  wavelengths (blue end of the spectrum) travel
  faster than the long wavelengths (red end). After
  travel on a fibre the shorter wavelengths will
  arrive first. This is considered a negative chirp.

7
Polarisation Mode Dispersion (PMD)

• There is usually a very slight difference in RI
  for each polarisation. It can be a source of
  dispersion, usually less than .5 ps/nm/km.
• The effect is to cause a circular or elliptical
  polarisation to form as the signal travels along
  the fibre.
• Dispersion resulting from the birefringent
  properties of fibre is called Polarisation Mode
  Dispersion (PMD).

8
Waveguide Dispersion

• The shape (profile) of the fibre has a very
  significant effect on the group velocity. This is
  because the amount that the fields overlap
  between core and cladding depends strongly on the
  wavelength. The longer the wavelength the further
  the the electromagnetic wave extends into the
  cladding.
• since a greater proportion of the wave at shorter
  wavelengths is confined within the core, the
  shorter wavelengths see a higher RI than do
  longer wavelengths. Therefore shorter wavelengths
  tend to travel more slowly than longer ones.

9
????????????????
G.652????(NDSF)
G.653????(DSF)
G.655????(NZ-DSF)
??G.655 ?????G.655
10
G.652????(NDSF)
????????? ??? ????? ????? ??????? 2.5Gb/s?????
??????600km 10Gb/s???????????34km G.652DCF???????
?? ?? ????10Gb/s??????,?????
2.5Gb/s?????DWDM?
11
G.653????(DSF)
??? ??? ????? ??????????EDFA?? ????(FWM)?????
?,???DWDM?? ?? ???10Gb/s?????????,?????
DWDM??,???????????
12
G.655????(NZ-DSF)

• ?1530-1565nm????????
• ?????????,?????????????(????)????
• ????????????????
• ???SPM??????,???SPM???????
• ?DWDM?????????

?? ???10Gb/s????DWDM??, ????????,DWDM????????
???
13
??????????
?????
?????
14
Calculating Dispersion

• in a typical single-mode fibre using a laser with
  a spectral width of 6 nm over a distance of 10 km
  Dispersion 17ps/nm/km 6 nm 10 km 1020
  ps
• At 1 Gbps a pulse is 1 ns long. So the system
  would not work. (20 is a good guideline for the
  acceptable limit.) But it would probably work
  quite well at a data rate of 155 Mbps (a pulse
  length of 6.5 ns).
• A narrow spectral width laser might produce only
  one line with a linewidth of 300 MHz. Modulating
  it at 1 Gbps will add 2 GHz. 2,300 MHz is just
  less than .02 nm (at 1500 nm). So now
• Dispersion 17ps/nm/km .02 nm 10 km 3.4 ps
• In this case, dispersion just ceased to be a
  problem.

15
??????

• ??????
• ??????
• ??????
• ?????
• ??????

16
Control of Spectral Width

• Simple FP laser over 5 nm
• External cavity DBR laser lt .01 nm
• Modulation adds to the bandwidth of the signal,
  by twice the highest frequency present in the
  modulating signal (1 Gbps, .04 nm)!
• Using more complex signal coding rather than
  simple OOK.
• Using WDM(a 2.5 Gbps signal has 1/4 of the
  problem with dispersion as a 10 Gbps signal).

17
Dispersion Shifted Fibre

• dispersion shifted fibre is designed with a
  dispersion zero point at around 1550 nm.
• However, it is not always possible or indeed
  desirable
• In many cases we can't have DSF because the fibre
  we must use is already installed.
• four-wave mixing effectively prohibit the use of
  DSF.

18
Dispersion Compensating Fibre
19
Balancing Dispersion on a Link
20
DCF?????

• ???(0.5dB/km)
• ????(DCF 20mm2 G-652 80mm2 ), ????????????
  2-4????
• ??????3-6dB
• ???????(DCF-15 -20 ps/nm2/kmG-652 0.09ps/
  nm2/km).
• ??????,???????

21
Mid-Span Spectral Inversion

• The concept here is to use a device in the middle
  of the link to invert the spectrum. This process
  changes the short wavelengths to long ones and
  the long wavelengths to short ones. When the
  pulse arrives it has been re-built exactly -
  compensated for by the second half of the fibre.

22
Principle

• This spectral inversion is performed by a process
  called optical phase conjugation. Devices that
  change the wavelength using either 4-Wave Mixing
  or Difference requency Generation invert the
  spectrum as a biproduct of their wavelength
  conversion function. These can be used as
  spectral inverters if we can tolerate the
  wavelength shift involved.

23
Chirped Fibre Bragg Gratings
To compensate for 100 km of standard (17
ps/nm/km) fibre the chirped grating needs to be
17 cm long for every nm of signal bandwidth! In
this instance a WDM system with channels spread
over (say) 20 nm would need a chirped FBG (20 x
17) 340 cm long!
24
??????????

• ??????? (2neffL/c) (1/Dlc)
• neff ????? c ??
• Dlc ???????????.
• 5 cm ????????????300 km?10Gb/(????0.1nm)??????(
  5100ps/nm)

25
????????


26
What happen

• Increase in significance exponentially with the
  level of optical power in the fibre.
• Elastic effects no energy exchange between the
  optical wave and the matter (four-wave mixing).
• Inelastic Scattering there is an energy
  transfer between the matter involved and the
  optical wave.

27
????????

• ?????,????lt10mw,?????2.5Gb/s?,????????????,??????
  ????????????
• WDM???,??????????????,??????????
• ??????????????????(??)????????????
• ???????????????????????

28
????????

• ???????????
• nn0 n2P/Ae
• ??P ????, Ae ????????
• ?????????????,??????? ,?? SPM, XPM and FWM
• ????????????????,??????

29
Carrier-Induced Phase Modulation

• The presence of light in a fibre causes a (tiny)
  change in the refractive index of the fibre. This
  is because the electromagnetic field that
  constitutes the light acts on the atoms and
  molecules that make up the glass. This is called
  the Kerr Effect.
• At low intensities the effect is linear that is,
  the amount of RI change varies linearly with the
  intensity of the light. At high intensities the
  effect is highly non-linear. This is called the
  Nonlinear Kerr Effect.

30
How it works

• At very high powers Kerr nonlinearities can be
  used to balance the effects of chromatic
  dispersion in the fibre and a soliton is
  formed.
• At medium power levels (below the level needed to
  form solitons) Kerr effect has been used to
  construct devices that compress and re-form
  pulses and hence undo the effects of chromatic
  dispersion.
• At low power levels the results of Kerr effect
  are self-phase modulation and cross-phase
  modulation.

31
?????(SPM)

• ?????(SPM)???????????????????????,????????????????
  ???????,???????????????,??????????
• SPM??????????????????????
• SPM???????????????????????????

32
SPM???

• E(Z,t)Ecos(wot-Boz)
• ?????(SPM)??E(z,t)????E2z??,?SPM??????????????E2
  ?????z?

33
??????(XPM)

• ??????(XPM)???????????????????????,??????
• ????? ,????????SPM,??????????(XP
  M)?
• ?E1E2 ?XPM?????SPM??????XPM???WDM???SPM??????????
  ????
• ??????????XPM
• DSF??(10Gb/s)WDM???,XPM???????????

34
????(FWM)

• ???????????,??????????,????????????,??????????(FWM
  )
• ????? w4 w1w2w3
• ????? w4w3 w1w2
• ????? w4w3 2wp (wpw1w2)
• ???????????
• ?????? wS 2w1- w2
• ??????? wA 2w2- w1

35
Four Wave Mixing Effects
36
???????

• FWM???????????????????????????????????????(???????
  ???),?FWM?????,????,??????,??????????????,????????
  ?????????????,??????????????????????,??????,??????
  ????
• ?????????????,FWM???????
• ????????,??????,??????FWM?
• ???????,??????,FWM????
• ??????,FWM?????

37
??FWM???

• ??????????,???????????????????????????WDM??????,??
  ?????????????
• ??????,???????????????????????????,???????????????
  ????????,??????SRS??????
• ?????????,???????????
• ??DSF????1560nm????????????????DSF,??????????????
  ,??????FWM????????L-band?EDFA?
• ?????????????,????????????????

38
???????(SBS)

• ???????(SBS)??????????????????,???????????????
• SBS????,??????????,?1.55mm?,WB11.1GHZ?
• ???????????????????????20MHZ???(???????),SBS??????
  ?????????
• SBS?????????????,????????
• SBS?????(?????9dBm). ??????????SBS????
  (100MHz??16 dBm )
• SBS?????gB??410-11m/W,???????

39
SBS Threshold Variation with Wavelength
40
Major problem with SBS

• In long distance systems where the span between
  amplifiers is great and the bit rate low (below
  about 2.5 Gbps).
• In WDM systems (up to about 10 Gbps) where the
  spectral width of the signal is very narrow.
• In remote pumping of an erbium doped fibre
  amplifier (EDFA) through a separate fibre. EDFA
  pumps typically put out about four lines of
  around only 80 MHz wide. Each of these lines is
  limited by SBS in the amount of power
• that can be used.

41
??SBS???

• ?????????SBS?????
• ???????,??100MHz(0.1nm)?
• ???????

42
??????(SRS)

• SRS????????????????SRS????????????????????
• ???SBS?3????,??100nm????
• SRS ?? DWDM??????????,?????
• ?????????????,?????????
• ????????????????????SRS.
• ??????SRS????????????????????,????????????????????
  ??????
• ?????????SRS

43
Stimulated Raman Scattering
44
??SRS???

• ???????
• ???????SRS?????
• ???????

45
Raman Effect Amplifiers

• The signal to be amplified must be longer in
  wavelength than the pump.
• Optimal amplification occurs when the difference
  in wavelengths is around 13.2 THz.
• At very high power it is possible for all of the
  signal power to be transferred to the Stokes
  Wave.
• In regular Ge-doped fibre the effect is very
  small and it takes a relatively long length of
  (about 1 km).

46
Wavelength Changing with SRS
47
A 1310 nm Band SRA

• Signal light and pump light enter the device
  together through a wavelength selective coupler.
• The pump light at 1064 nm is shifted to 1117 nm
  and then in stages to 1240 nm.
• The 1240 nm light then pumps the 1310 band signal
  and amplification is obtained.
• A high level of Ge dopant is used (around 20) to
  increase the SRS effect.

48
SRFA
49
Comparison of the OAs
50
?????

• (CL)??EDFA
• C??EDFASRFA
• TDFA(Thulium-doped fluoride fiber amplifier)SRFA
• ???SRFA
• SOA