The attached narrated power point presentation attempts to explain the working principle, types, classifications, merits, demerits, applications,safety and deployment issues related to Raman Amplifiers. The material will be useful for KTU final year B Tech students who prepare for the subject EC 405, Optical Communications.

1. 1
Raman Fiber Amplifiers
MEC

2. 2
Contents
• Introduction.
• Stimulated Raman Scattering.
• Raman Amplifiers and Classifications.
• Raman Gain.
• Noise and Interference.
- ASE Noise
- Double Rayleigh Scattering Reflections.
- Relative Intensity Noise.
• Deployment Issues and Applications.

3. 3
Introduction
• Ability of light to scatter inelastically on a
molecular structure (gas, liquid or solid)
and exchange energy with material
discovered by Sir. C. V. Raman in 1928.
• Effect known as ‘Raman scattering’.
• Incoming photon either red-shifted (Stokes
shift) or blue-shifted (anti-Stokes shift) by
interaction with the medium.

4. 4
Introduction
• Fraction of photon energy absorbed or
emitted by the material as molecular
vibrations (heat) - phonons.
• Raman scattering occur in all materials,
dominant in silica glass.
• Raman transitions due to bending motion of
Si-O-Si bond.
• Raman scattering is inelastic, molecule will
decay to a vibrational level different from
initial state.

5. 5
Introduction
• Stokes Raman scattering - final energy level
of the molecule higher than the initial level.
• Anti-Stokes Raman scattering - final energy
level lower than the starting level.
• Stokes scattering is more common.
• For Stokes shift,
νp,νs - frequency of pump and Stokes photon,
Ephonon – phonon energy.

6. 6
Introduction

7. 7
Raman Amplifiers
• Nonlinear effects within optical fiber used
to provide optical amplification.
• Amplification achieved using Stimulated
Raman Scattering (SRS).
• Self-phase matching between the pump
and the signal.
• Broad gain–bandwidth and fast response,
attractive for WDM systems.

8. 8
Raman Amplifiers
• Provide gain over the entire fiber band (i.e.
0.8 to 1.6 μm).
• Pump signal optical wavelengths typically
500/cm higher in frequency than signal to
be amplified.
• Multiple pump wavelengths for flat gain,
reduce polarisation dependent gain.
• Broad spectral bandwidth of up to 100 nm
with suitable fiber doping.
• Continuous-wave Raman gains exceed 20
dB.

9. 9
Raman Amplifier with Bidirectional
Pumping

10. 10
Stimulated Raman Scattering
• Nonlinear scattering observed at high optical
power densities in long single-mode fibers.
• Provide optical gain with frequency shift.
• Useful for optical amplification.
• Modulates light through thermal molecular
vibrations within the fiber.
• Scattered light appears as upper and lower
sidebands separated from incident light by
the modulation frequency.

11. 11
Stimulated Raman Scattering
• High-frequency optical phonon generated
in the scattering process.
• Bidirectional - forward and backward
directions.
• Threshold optical power for SRS:
d - fiber core diameter, λ - operating
wavelength, αdB - fiber attenuation, dB/km.

12. 12
Raman Amplifiers
• Raman gain > 40 dB
with fluoride glass
fiber and Raman
shift of 590/cm.
• Forward & backward
pumping, high pump
powers needed.
• Raman gain
depends on fiber
length, attenuation
and core diameter.

13. 13
Raman Gain
• Raman Gain
gR - Power Raman gain coefficient, Aeff and
Leff - effective core cross-sectional area and
length, k - numerical factor that accounts for
polarization scrambling between optical pump
and signal, reff - effective core radius, αp -fiber
transmission loss at pump wavelength, L -
actual fiber length, Pp – pump power.

14. 14
Raman Gain
• k = 2 for complete polarization scrambling,
as in conventional single-mode fiber.
• Raman gain efficiency (gR/Aeffk) measured
in W−1 km−1.
• Raman gain becomes larger as fiber
lengths increase up to around 50 km,
asymptotically it reaches a constant value.
• Higher Raman gains obtained with lower
loss fibers and lower core diameters.

15. 15
Raman Gain
10 μm core single-mode fibers.
Pump input power = 1.6 W

16. 16
On-Off Gain
• On-off gain of Raman amplifier - increase in
signal output power when pumps are turned
on.
• Distributed Amplification - gain in the
transmission fiber itself, reduces system
signal-to-noise ratio (SNR) degradation,
compared with discrete amplifier (only)
systems.

17. 17
Raman Amplifier Classifications
• Main categories – discrete, distributed and
hybrid.
• Discrete Raman amplifiers/Lump Raman
amplifiers used as lumped elements
inserted into transmission line for gain.
• Discrete - pump power is confined to the
lumped element (~20 km long fiber).
• Distributed Raman amplifier extends pump
power into transmission line fiber.

18. 18
Raman Amplifier Classifications
• Discrete amplifiers - amplification takes
place at a single point at the end of the
link.
• Distributed amplifiers - amplification takes
place along the fiber, avoids low power at
the end of the link, allows lower power to
be launched at the starting of the link.
• DRA prevents signal attenuation to very
low powers, improves SNR.

19. 19
Raman Amplifier Classifications
• Distributed - amplification takes place
along several kilometers of fiber(~100 km).
• Hybrid Raman Amplifiers - Lumped and
distributed Raman fiber amplifiers
combined for wideband applications.
• Combined amplification increases overall
amplified spectral bandwidth.
• Amplified Spectral Emission (ASE) noise &
Double Rayleigh scattering reflections.

20. 20
Raman Amplifier vs EDFA

21. 21
Noise and Interference
• Selection & number of pump signals
influence amplifier noise.
• ASE contributes most of the noise.
• Common sources of noise include:
- Beating of the signal with ASE due to
double Rayleigh scattering reflections or
multipath interference.
- Non Linear Effects - four-wave mixing,
self-phase modulation, cross-phase
modulation.
• Relative Intensity Noise.

22. 22
Double Rayleigh Scattering
Reflections
• Fiber length influences noise within
Raman amplifiers.
• Magnitude proportional to fiber length.
• ASE noise reflected together with signal,
cause it to increase several times.
• Effect of double Rayleigh scattering
reflections reduced if multiple stages of
amplification over full fiber length.

23. 23
Relative Intensity Noise
• Pump signal and input signal interact for a
longer time over several fiber kilometers.
• Fluctuations in pump power (pump noise)
transferred to transmitted signal.
• Pump noise also called Relative Intensity
Noise (RIN).
• Severe if multiple pump signals used to
achieve wideband amplification.

24. 24
Minimising RIN
• Reduce interaction time between pump
and input signal.
• Achieved by backward pumping – counter-
propagation of signals where interaction
time for pump and signal is very short.

25. 25
Other Limitations
• Low power efficiency.
• Safety issues due to high optical powers in
the fiber.
• Enhanced problems with nonlinearities,
due to high path-average power in fibers.
• Gain limited by available pump powers
and wavelengths.

26. 26
Laser Safety
• Output power of Raman pump modules
higher than typical power levels in EDFA
systems.
• DRA generate ASE along transmission line.
• Even in case of fiber break, ASE power within
C - band can propagate along the system.
• Reducing power to a safe level in case of
accidental connection opening/fiber break.
• Detection of fiber break/open connector,
allow automatic shut down of Raman pump
module.

27. 27
Gain Measurement
• Achievable Raman gain and shape of gain
spectrum depends on fiber type and
quality of fiber line.
• Achievable gain vary from spool to spool
due to manufacturing variations.
• Ability for accurate real time Raman gain
measurement.
• Adjust pump powers to achieve desired
average gain and gain shape.

28. 28
System Integration
• Integrating DRA modules into existing
system architecture time consuming and
costly.
• Tight integration of Raman and EDFA
modules allow module parameter
optimization, enhanced gain flatness.
• Stand alone Raman amplifiers to extend
existing capabilities.

29. 29
Applications
• Long distance single span links
- undersea links between islands, remote
coastal locations, oil rigs etc.
- locations separated by mountain ranges
or desert.
- where commercial, legal or security
constraints render amplification sites
impractical.

30. 30
Applications
• Long spans within multispan links.
- one or more spans longer than others.
- hut skipping : spans made intentionally
longer, skip repeaters to reduce capital
and operating expenditure.
• High capacity long distance systems.
• Optical amplifiers and waveguides in
general.

31. 31
Thank You