1. Optical Fibre Communication
Systems
Lecture 5 – Optical Amplifier
Professor Z Ghassemlooy
Optical Communications Research Group
Northumbria Communications Research Laboratory
School of Computing, Engineering and
Information Sciences
The University of Northumbria
U.K.
http://soe.unn.ac.uk/ocr
Prof. Z Ghassemlooy 1

2. Contents
 Why the need for optical amplifier?
 Spectra
 Noise
 Types
 Principle of Operation
 Main Parameters
 Applications
Prof. Z Ghassemlooy 2

3. Signal Reshaping and Amplification
In long distance communications, whether going
through wire, fibre or wave, the signal carrying the
information experience:
- Power loss
- Pulse broadening
which requires amplification and signal reshaping.
m In fibre optics communications, these can be done in
two ways:
– Opto-electronic conversion
– All optical
Prof. Z Ghassemlooy 3

4. Signal Reshaping and Amplification
Depending on its nature, a signal can also be regenerated.
A digital signal is made of 1's and 0's: it is possible to
reconstruct the signal and amplify it at the same time.
s An analog signal however, cannot be reconstructed
because nobody knows what the original signal looked
like.
Prof. Z Ghassemlooy 4

5. Why the Need for Optical Amplification?
Semiconductor devices can convert an optical signal into an
electrical signal, amplify it and reconvert the signal back to
an optical signal. However, this procedure has several
disadvantages:
– Costly
– Require a large number over long distances
– Noise is introduced after each conversion in analog signals
(which cannot be reconstructed)
– Restriction on bandwidth, wavelengths and type of optical
signals being used, due to the electronics
e By amplifying signal in the optical domain many of these
disadvantages would disappear!
Prof. Z Ghassemlooy 5

6. Optical Amplification
a Amplification gain: Up to a factor of 10,000 (+40 dB)
i In WDM: Several signals within the amplifier’s gain (G)
bandwidth are amplified, but not to the same extent
p It generates its own noise source known as Amplified
Spontaneous Emission (ASE) noise.
Weak signal Amplified signal
Pin Optical Pout
Amplifier
ASE ASE
(G)
Pump Source
Prof. Z Ghassemlooy 6

7. Optical Amplification - Spectral
Characteristics
(unamplified signal)
(amplified signal)
Single channel
Power
Power
ASE
Wavelength Wavelength
WDM channels
(unamplified signal)
(amplified signal)
Power
Power
ASE
Wavelength Wavelength
Prof. Z Ghassemlooy 7

8. Optical Amplification - Noise Figure
a Required figure of merit to compare amplifier noise
performance
o Defined when the input signal is coherent
Input signal − to− noise ratio ( SNRi )
Noise Figure (NF) =
Output signal − to− noise ratio ( SNRo )
NF is a positive number, nearly always > 2 (I.e. 3 dB)
Good performance: when NF ~ 3 dB
: NF is one of a number of factors that determine the
overall BER of a network.
Prof. Z Ghassemlooy 8

9. Optical Amplifiers - Types
There are mainly two types:
Semiconductor Laser (optical) Amplifier (SLA) (SOA)
s Active-Fibre or Doped-Fibre
– Erbium Doped Fibre Amplifier (EDFA)
– Fibre Raman Amplifier (FRA)
– Thulium Doped Fibre Amplifier (TDFA)
Prof. Z Ghassemlooy 9

10. SLA - Principle Operation
i Remember diode lasers?
Suppose that the diode laser has no mirrors:
- we get the diode to a population inversion condition
- we inject photons at one end of the diode
o By stimulated emission, the incident signal will be amplified!
– By stimulated emission, one photon gives rise to another photon: the total
is two photons. Each of these two photons can give rise to another
photon: the total is then four photons. And it goes on and on...
Problems:
) Poor noise performance: they add a lot of noise to the signal!
r Matching with the fibre is also a problem!
e However, they are small and cheap!
Prof. Z Ghassemlooy 10

11. SLA - Principle Operation
Excited state
Pump
Pump signal signal Energy Absorption
@ 980 nm @ 980
nm
Electrons in ground state
Excited state
Tra
nsi
tion
Pump signal Metastable
@ 980 nm state
Ground state
www.cisco.com
Prof. Z Ghassemlooy 11

12. SLA - Principle Operation
Excited state
Tra
nsi
tion Metastable state
ASE Photons
Tra
Pump signal
@ 980 nm 1550 nm
ns
itio
n
Ground state
Excited state
Tra
nsi
tion Metastable state
Pump signal Stimulated
@ 980 nm Signal photon emission
1550 nm 1550 nm
Ground state
Prof. Z Ghassemlooy 12

13. Erbium Doped Fibre Amplifier (EDFA)
r EDFA is an optical fibre doped with erbium.
– Erbium is a rare-earth element which has some interesting properties for fibre
optics communications.
– Photons at 1480 or 980 nm activate electrons into a metastable state
– Electrons falling back emit light at 1550 nm.
– By one of the most extraordinary coincidences, 1550 nm is a low-loss
wavelength region for silica optical fibres.
– This means that we could amplify a signal by 540
using stimulated emission. 670
820
980
EDFA is a low noise light Metastable
amplifier. 1480
state
1550 nm
Ground state
Prof. Z Ghassemlooy 13

14. EDFA - Operating Features
Amplifier length
Input signal 1-20 m typical
Amplified signal
Pump from an Cladding
Erbium doped core
external laser
1480 or 980 nm
• Available since 1990’s:
• Self-regulating amplifiers: output power remains more or less constant
even if the input power fluctuates significantly
• Output power: 10-23 dBm
• Gain: 30 dB
• Used in terrestrial and submarine links
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15. EDFA – Gain Profile
+10 dBm
ASE spectrum when no
• Most of the pump power appears input signal is present
at the stimulating wavelength
• Power distribution at the
Amplified signal spectrum
other wavelengths changes (input signal saturates the
with a given input signal. optical amplifier) + ASE
-40 dBm
1575 nm
1525 nm
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16. EDFA – Ultra Wideband
Ultra-Wideband EDFA
30
C-Band L-Band
15
Noise Figure (dB)
40.8 nm 43.5 nm
20
Gain (dB)
Total 3dB Bandwidth = 84.3 nm
10
10
Noise ≤ 6.5 dB 5
Output Power ≅ 24.5 dBm
0
1525 1550 1575 1600
Alastair Glass Photonics Research Wavelength (nm)
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17. Optical Amplifiers: Multi-wavelength
Amplification
www.cisco.com
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18. Optical Amplifier - Main Parameters
r Gain (Pout/Pin)
u Bandwidth
u Gain Saturation
u Polarization Sensibility
s Noise figure (SNRi/SNRo)
R Gain Flatness
R Types
– Based on stimulated emission (EDFA, PDFA, SOA)
– Based on non-linearities (Raman, Brillouin)
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19. Optical Amplifier - Optical Gain (G)
r G = S Output / S Input (No noise)
a Input signal dependent:
– Operating point (saturation) of
EDFA strongly depends on
power and wavelength of
incoming signal
Gain (dB)
EDFA
40
P Input: -30 dBm
• Gain ↓ as the input power ↑
Pin Gain Pout 30 -20 dBm
-20 dBm 30 dB +10 dBm -10 dBm
-10 dBm 25 dB +15 dBm 20 -5 dBm
Note, Pin changes by a factor of ten
10
then Pout changes only by a factor of 1520 1540 1560 1580
three in this power range.
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20. Optical Amplifier - Optical Gain (G)
r Gain bandwidth
– Refers to the range of frequencies or wavelengths over which the
amplifier is effective.
– In a network, the gain bandwidth limits the number of wavelengths
available for a given channel spacing.
• Gain efficiency
- Measures the gain as a function of input power in dB/mW.
• Gain saturation
- Is the value of output power at which the output power no longer increases
with an increase in the input power.
- The saturation power is typically defined as the output power at which
there is a 3-dB reduction in the ratio of output power to input power (the
small-signal gain).
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21. Optical SNR
 For BER < 10-13 the following OSNRs are required:
 ~ 13 dB for STM-16 / OC-48 (2.5 Gbps)
 ~ 18 dB for STM-64 / OC-192 (10 Gbps)
 Optical power at the receiver needs to bigger than receiver
sensitivity
 Optical Amplifiers give rise to OSNR degradation (due to
the ASE generation and amplification)
– Noise Figure = OSNRin/OSNRout
 Therefore for a given OSNR there is only a finite number
of amplifiers (that is to say a finite number of spans)
 Thus the need for multi-stage OA design
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22. Optical Amplifiers: Multi-Stage
Er3+
Doped Fiber
Input Signal Output Signal
Optical
Isolator
Pump Pump
1st Active stage co-pumped: 2nd stage counter-pumped:
optimized for low noise figure optimized for high output power
NF 1st/2nd stage = Pin - SNRo [dB] - 10 Log (hc2∆λ / λ3)
NFtotal = NF1+NF2/G1
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23. System Performance: OSNR Limitation
 5 Spans x 25 dB
 32 Chs. @ 2.5Gb/s with 13 dB OSNR
 BER < 10-13
Channel Count / Span Loss Trade-Off:
 5 spans x 22 dB
 64 chs @ 2.5Gb/s with 13 dB OSNR
 BER < 10-13
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24. Raman Amplifier
Transmission fiber Transmission fiber
Er
Amplifier
1450/ 1550 nm
WDM
1550 nm signal(s)
1453 nm
pump
Cladding pumped
fiber laser
Raman fiber laser
•Offer 5 to 7 dB improvement in system performance
•First application in WDM
P. B. Hansen, et. al. , 22nd Euro. Conf. on Opt. Comm., TuD.1.4 Oslo, Norway (1996).
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25. Optical Amplifiers - Applications
• In line amplifier
-30-70 km
-To increase transmission link
• Pre-amplifier
- Low noise
-To improve receiver sensitivity
• Booster amplifier
- 17 dBm
- TV
• LAN booster
amplifier
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