The document summarizes Raman fibre amplifiers. It discusses the working principle of Raman amplification using stimulated Raman scattering. It also covers design considerations for Raman amplifiers like optimizing gain, pump power thresholds, different pumping schemes to broaden bandwidth, and the impact of Raman scattering on WDM systems. Literature on Raman amplifier performance evaluation, gain flattening using hybrid Raman-EDFA configurations, and efficiency enhancement is also reviewed.

1. Raman Fibre Amplifier
Presented By: Prem Babu
Roll no. MT17ECE008
Supervised By: Dr. Sarika Pal
Co-Supervised By : Mr. Tejas Laheri
Department of Electronics and Communication Engineering
National Institute of Technology Uttarakhand

2. Outline
 Review of Last Presentation
 Introduction
 Design consideration
 References

3. Review of last presentation
 Introduction
 Application of Raman Amplifier
 Working principle
 Advantages and Disadvantages
 Difficulties with Raman Amplifiers
 Present scenario: Real World Raman Amplifier Application

4.  Introduction
• Raman amplifier is a type of Optical amplifiers where as Erbium
doped fibre amplifier and Semiconductor optical amplifier are other
two types.
History :
• In 1971 Stolen et al experimentally observed the stimulated Raman
emission in a single-mode optical fibre.
• In 1980 the Raman amplifier was started.
• From 1990 we are practically using these devices for
communication.

5. Figure 1: Working of Erbium doped amplifiers[5]

6. Raman Amplifier
 For optical amplification we need stimulated Raman scattering.
 The frequency difference between fp and fs has to match a relationship in order to
fully use of this non linear effect.
 If the intensity of the incident field is below a threshold, spontaneous scattering
occurs.
Figure 2:Show Spontaneous and stimulated scattering [4]

7. Figure 3: working of Raman amplifier[5]

8.  Literature Survey
• By R.S. Kalera, 2013, [1] Performance evaluation of EDFA,
RAMAN and SOA optical amplifier for WDM systems
 compared on the basis of transmission distance (40–200 km) and
dispersion (2–10 ps/nm/km) with and without nonlinearities.
 when the dispersion is 2 ps/nm/km and the number of channels are less,
then SOA provide better results
 When dispersion is increased from 2 to 10 ps/nm/km, EDFA provides better
results than SOA in the term of BER and output power, but it shows non-
uniform gain spectrum.
 RAMAN amplifier provides better results for L band amplification and
gain flatting issue because it can substantially reduce the impact of fiber
nonlinearity.

9. Literature Survey
• By Mohammed N. Islam,2002[7] Raman Amplifiers for
Telecommunications
 Distributed Raman amplifiers improve the noise figure and reduce the
nonlinear penalty of fiber systems, allowing for longer amplifier spans,
higher bit rates, closer channel spacing, and operation near the zero-
dispersion wavelength.
 Lumped or discrete Raman amplifiers are primarily used to increase the
capacity of fiber-optic networks, opening up new wavelength windows for
wavelength-division multiplexing such as the 1300 nm, 1400 nm, or short-
wavelength -band.

10. Figure 4: Distributed Raman Amplifier [5]

11. For Design/Implimentaion consideration of
Raman Amplifier
• Gain
• Pump power threshold and its limitations
• Pumping Schemes for Raman Amplifier
• Broad Banding of Raman Amplifier
• Impact of Raman Scattering on WDM system

12. 1.Optimizing gain
• The energy conversion process between the pump and the Stokes is
characterized by parameter called the Raman Gain ( gR ). and depend on
material composition of fiber core and its dopants .
• Raman gain is inversely proportional to the wavelength of the pump &
depends on the polarization of the wave.
• The Raman gain is much higher for parallel polarization (when the pump and
the stokes waves have same polarization) compared to that for the
perpendicular polarization (i.e. when the pump and the Stokes waves have
orthogonal polarization).
• The maximum gain is practically constant over a bandwidth from 9THz to
16THz. The mean of the high gain region is around 13THz ( which at 1550nm
wavelength corresponds to change in λ =112nm. )

13. 1.Optimizing gain
 The photon of pump beam fp is scattered by molecules in the fibre medium
and become the lower energy photon fs .
Figure 5:frequencies presence after scattering [4] Figure 6: gain vs shifted freq curve [4]

14. The signals levels on a long haul optical link with EDFA
and with Raman amplification.
Figure7:signal power distribution with its length in presence of EDFA and Raman amplifiers [4]

15. 2.Pump power threshold
• The threshold for stimulated scattering is defined as input intensity Ipth
value of the pump for which Stoke wave shows gain in the fiber.
• the length of the fiber =Leff ,
• threshold intensity for Stimulated Raman Scattering (SRS) = Ipth >> 𝛼 𝑠 𝑔 𝑅
• For a SM fiber ,
Let , core effective area = 80µm ; α=0.2dB/Km
Raman gain = 7*1014 m/W
then Raman threshold power,Pth = IpthAeff >> 53mW
• The threshold given above just tells that above this pump power the there
will be gain for any signal above the noise. However still the Stokes power
is orders of magnitude smaller than the Pump.

16. 2.Pump power threshold
• If we define the threshold as the input power for which the output powers
of pump and Stokes are equal,
then its value is approximately
Ith =
16
𝑔 𝑅
𝐿 𝑒𝑓𝑓
• Typical power to achieve this threshold intensity in a SM fiber is about 1
W.

17. 3.Pumping scheme
• Forward pumping
Disadv: Double Rayleigh back
scattering (DRBS)
Superposition between the signal and
the time delayed double back-scattered
light leads to time dependent noise.
• Backward pumping
Mostly used pumping scheme is
counter pumping.
Figure 8: Pumping Scheme for Raman Amplifier [4]

18. Literature Survey
• By Emami, F., & Jafari, A. H. (2008).[6] Analysis and
comparison of multiwavelength Raman amplifiers with
different configurations.
 The results show that amplitude spontaneous emission (ASE) in backward
pumping is less than bi-directional pumping with improved noise figure.

19. 4.Broad Banding of Raman Amplifier
Why ?
• The Raman amplifier with single pump gives a bandwidth of about
7 THz which is approximately 60nm.
• The trans mission window of the fiber is about 400nm (1200nm
to1600nm).
• A broad band amplifier therefore is very desirable.

20. Literature Survey
• By Felinskyi, G. S., & Korotkov,2005[7] ACTUAL
BAND MODEL FOR DESIGN OF OPTICAL FIBER
RAMAN AMPLIFIER WITH MULTIWAVE PUMPING
 Describe the wavelength dependence of Raman gain in optical fibers and
are very useful for the estimation of the gain bandwidth, Raman lasing,
noise performance, and amplification processes in Raman amplifiers.
 Proposed modeling allows us to analyze fiber Raman amplifier with
combined multiwavelength pumping source for the extension of
amplification banrwidth to L-band, which has the broad bandwidth over 80
nm and low gain ripple less than 0.5 dB.

21. How it is done…
• Using multiple pumps, wide band amplifiers with a very small gain ripple
can be designed.
• It should be kept in mind however, that in a multiple pump scheme, there
is exchange of power between the pump themselves due to Raman
process.
Figure 9: Method to increase bandwidth [5]

22. 5.Impact of Raman Scattering on
WDM system
If number of channels is small,
the maximum power per channel
decreases as 1/n,
but
If the number of channel is large,
the power decreases as 1/n2.
Figure 10: maximum power per channel [4]

23. 5.Impact of Raman Scattering on WDM system
• For DWDM system the Raman interaction is very complicated.
• Every wavelength acts as a pump for wavelengths longer than it, and as
Stokes wave for a wavelength shorter than it.
• Hence Due to Raman scattering every channel receives power and every
channel loses power.
• There is systematic flow of power from higher frequency channels to the
lower frequency channels.
• So to start with if all channels had equal power, at the end the spectrum
will be as shown in Fig.

24. Figure 11:Effect of Raman Scattering on WDM channel [4]

25. Literature Survey
• By Simranjit Singh and R. S. Kaler, 2014, [2] Flat-Gain L-
Band Raman EDFA Hybrid Optical Amplifier for Dense
Wavelength Division Multiplexed System
 An efficient gain-flattened L-band optical amplifier is demonstrated using a
hybrid configuration with a distributed Raman amplifier (DRA) and an
erbium-doped fiber amplifier(EDFA) for 160×10-Gb/s dense wavelength
division multiplexed at 25-GHz interval
 It is observed that as we increase the input power, the gain variation over
the bandwidth increases. With an input signal power of 3 mW, a flat gain of
>10 dB is obtained for the frequency region 187 to 190.975 THz with a
gain variation of less than 4.5 dB. It is also observed that the smooth output
power spectrum is obtained when the input power of all channels is fixed at
3 mW.

26. It can be seen that some part of the wavelength band is efficiently amplified by EDFA
with a high gain and the other is amplified by Raman, which means that over the whole
wavelength grid, a single amplifier shows a large variation. But if the Raman amplifier
is combined with EDFA in any configuration (cascaded or parallel), then the large gain
flatness can be achieved even with the highest possible gain.

27. Figure12: Architectures of inline optical and hybrid optical amplifiers

28. Literature Survey
• By Ju Han Lee 2005[8]. Dispersion-compensating
Raman/EDFA hybrid amplifier recycling residual
Raman pump for efficiency enhancement.
 The proposed dispersion-compensating hybrid amplifier system has only
one pump source for Raman amplification in the dispersion-compensating
fiber (DCF) and the residual pump power after the DCF is recycled for
secondary signal amplification in an erbium-doped fiber cascaded to the
DCF.
 Using the proposed scheme, we achieve the significant enhancement of
both signal gain and effective gain-bandwidth by 15 dB (small signal gain)
and 20 nm, respectively, compared to the performance of the Raman-only
amplifier

29.  References
1.Singh, Simranjit, Amanpreet Singh, and R. S. Kaler. "Performance
evaluation of EDFA, RAMAN and SOA optical amplifier for WDM
systems." Optik-International Journal for Light and Electron Optics 124.2
(2013): 95-101.
2.Singh, Simranjit, and R. S. Kaler. "Novel optical flat-gain hybrid amplifier
for dense wavelength division multiplexed system." IEEE Photonics Technol.
Lett 26.2 (2014): 173-176.
3.Basics and principle of Raman Spectroscopy - Learn under 5 min - Stokes
and Anti-Stokes - AI 09 (2).mp4
4.https://nptel.ac.in/courses/117101054/downloads/lect21.pdf
5. https://www.youtube.com/watch?v=6gDPk2JzeY0
6.Emami, F., & Jafari, A. H. (2008). Analysis and comparison of
multiwavelength Raman amplifiers with different configurations. 2008 9th
International Conference on Laser and Fiber-Optical Networks
Modeling.doi:10.1109/lfnm.2008.4670353

30. References
7. Felinskyi, G. S., & Korotkov, P. A. (n.d.). Actual band model for design of
optical fiber raman amplifier with multiwave pumping. Proceedings of CAOL
2005. Second International Conference on Advanced Optoelectronics and
Lasers, 2005. doi:10.1109/caol.2005.1553948
8. Ju Han Lee, You Min Chang, Young-Geun Han, Sang Hyuck Kim, Haeyang
Chung, & Sang Bae Lee. (2005). Dispersion-compensating Raman/EDFA
hybrid amplifier recycling residual Raman pump for efficiency enhancement.
IEEE Photonics Technology Letters, 17(1), 43–
45. doi:10.1109/lpt.2004.837264