1. CHAPTER 7
Optical Amplifier

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

3. 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
By amplifying signal in the optical domain many of these disadvantages would disappear!

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

5. Optical Amplification - Spectral Characteristics
Wavelength
(unamplified signal)
Power
Wavelength
(amplified signal)
Power
ASE
Single channel
WDM channels
Wavelength
(unamplified signal)
Power
Wavelength
(amplified signal)
Power
ASE

6. Optical Amplification - Noise Figure
Required figure of merit to compare amplifier noise performance
Defined when the input signal is coherent
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.

7. Optical Amplifiers - Types
There are mainly two types:
Semiconductor Laser (optical) Amplifier (SLA) (SOA)
Active-Fibre or Doped-Fibre
Erbium Doped Fibre Amplifier (EDFA)
Erbium (active medium)
Fibre Raman Amplifier (FRA)
Thulium Doped Fibre Amplifier (TDFA)
Thulium(active medium)

8. SLA - Principle Operation
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
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!
Matching with the fibre is also a problem!
However, they are small and cheap!

9. OPTICAL AMPLIFIERS
Semiconductor Laser Amplifiers (SLA)
Two major types: -The resonant Fabry-Perot amplifier (FPA)
-The nonresonant (single pass) traveling-wave amplifier (TWA)
where
R1 = input facet reflectivity
R2 = output facet reflectivity
SLA is based on the conventional semiconductor laser structure (gain- or index-guided).
In FPA, the reflectivities of the facets are between 30% to 35% whereas in TWA the reflectivities are less than

10. Fabry-Perot amplifier (FPA)
OPTICAL AMPLIFIERS
Fabry-Perot amplifier (FPA)
For operation, FPA is biased below the normal lasing threshold current.
When an optical signal enters the FPA, it gets amplified as it reflects back and forth between the mirrors until it is emitted at a higher intensity.
Easy to fabricate but the optical signal gain is very sensitive to variations in amplifier temperature and input optical frequency.
Used within nonlinear applications such as pulse shaping and bistable elements.
Gain and bandwidth of an FPA
Using the standard theory of FP interferometers, the cavity gain of SLA as a function of signal frequency f is
The single pass gain is given by
Single pass phase shift
where R1 = input facet reflectivity
R2 = output facet reflectivity
fm = cavity resonance frequencies
f = longitudinal-mode spacing

11. GFP reduce to GS when R1=R2=0.
OPTICAL AMPLIFIERS
GFP reduce to GS when R1=R2=0.
GFP peaks whenever f coincides with one of the cavity-resonance frequencies and drop sharply in between them.
Amplifier bandwidth is determined by the sharpness of the cavity resonance.
The ±3 dB single longitudinal mode bandwidth is

12. Question

13. Answer
Pls refer page 306: Optical Communications Principles and practise, John M. Senior.

14. Answer

15. SLA - Principle Operation
Energy Absorption
Excited state
Pump signal
@ 980 nm
Electrons in ground state
Pump signal
@ 980 nm
Transition
Metastable
state
Excited state
Ground state
Pump signal
@ 980 nm

16. SLA - Principle Operation
ASE Photons 1550 nm
Ground state
Excited state
Metastable state
Transition
Pump signal
@ 980 nm
Excited state
Metastable state
Transition
Pump signal
@ 980 nm
Stimulated
emission
1550 nm
Signal photon
Ground state

17. Erbium Doped Fibre Amplifier (EDFA)
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
using stimulated emission.
1480
980
820
540
670
Ground state
Metastable
state
1550 nm
EDFA is a low noise light amplifier.

18. EDFA - Operating Features
Input signal
Pump from an
external laser
1480 or 980 nm
Erbium doped core
Cladding
Amplifier length
1-20 m typical
Amplified signal
Available since 1990’s:
Self-regulating amplifiers: output power remains more or less constant even if the input power fluctuates significantly
Output power: dBm
Gain: 30 dB
Used in terrestrial and submarine links

19. Amplification in Erbium-doped Fiber Amplifiers
OPTICAL AMPLIFIERS
Amplification in Erbium-doped Fiber Amplifiers
Amplification in an EDFA occurs through the mechanism of stimulated emission. The pumping light is absorbed by the erbium ions, raising them to excited states and causing population inversion.
Two ways to attain population inversion in EDF:
Indirect pumping at 980 nm wavelength - Er ions are excited to upper level (3) and
they non-radiatively fast decay to the intermediate energy (metastable) level (2).
Direct pumping at 1480 nm wavelength -Er ions are excited directly to the level (2).
The signal (to be amplified) stimulates transition of the excited Er ions from level 2 to level 1 and results in radiation of photons with same wavelength, direction, and phase to the signal photons.
This gives rise to a coherent (amplified) output with respect to signal input.

20. OPTICAL AMPLIFIERS
A pumping signal can co-propagate with an information signal or it can counter-propagate.
Co-propagating
pump
Counter-propagating pump
Bi-directional pump

22. EDFA – Gain Profile Most of the pump power appears
ASE spectrum when no input signal is present
Amplified signal spectrum
(input signal saturates the optical amplifier) + ASE
1575 nm
-40 dBm
1525 nm
+10 dBm
Most of the pump power appears
at the stimulating wavelength
Power distribution at the
other wavelengths changes
with a given input signal.

23. EDFA – Ultra Wideband

24. Optical Amplifiers: Multi-wavelength Amplification

25. Optical Amplifier - Main Parameters
Gain (Pout/Pin)
Bandwidth
Gain Saturation
Polarization Sensibility
Noise figure (SNRi/SNRo)
Gain Flatness
Types
Based on stimulated emission (EDFA, PDFA, SOA)
Based on non-linearities (Raman, Brillouin)

26. Optical Amplifier - Optical Gain (G)
Gain (dB)
1540
1560
1580 10
1520 20 40 30
-5 dBm
-20 dBm
-10 dBm
P Input: -30 dBm
G = S Output / S Input (No noise)
Input signal dependent:
Operating point (saturation) of EDFA strongly depends on power and wavelength of incoming signal
EDFA
Gain ↓ as the input power ↑
Pin Gain Pout
-20 dBm dB +10 dBm
-10 dBm dB +15 dBm
Note, Pin changes by a factor of ten then Pout changes only by a factor of three in this power range.

27. Optical Amplifier - Optical Gain (G)
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).

28. 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

29. Optical Amplifiers: Multi-Stage
Doped Fiber
Pump
Input Signal
Output Signal
Optical
Isolator
1st Active stage co-pumped:
optimized for low noise figure
2nd stage counter-pumped:
optimized for high output power
NF 1st/2nd stage = Pin - SNRo [dB] - 10 Log (hc2 / 3)
NFtotal = NF1+NF2/G1

30. Raman Amplifier Offer 5 to 7 dB improvement in system performance
Transmission fiber
1550 nm signal(s)
Cladding pumped
fiber laser
1450/ 1550 nm
WDM
1453 nm
Pump
(raman pump) Er
Amplifier
Raman fiber laser
Offer 5 to 7 dB improvement in system performance
First application in WDM

31. Advantages of EDFA over SLA
OPTICAL AMPLIFIERS
Advantages of EDFA over SLA
The doped-fiber amplifiers have some advantages over semiconductor laser amplifiers:
Wider spectral bandwidth which allows more number of signal channels to be amplified simultaneously.
Flat gain characteristic over the practical range of wavelengths; appropriate for optical fiber links.
Compatibility for in-line interconnection within optical fiber links.
Suitable for use in dense wavelength division multiplexed transmission.

32. Fiber Raman Amplifiers
OPTICAL AMPLIFIERS
Fiber Raman Amplifiers
A fiber Raman amplifier uses stimulated Raman scattering (SRS) occurring in silica
fibers when an intense pump beam propagates through it.
SRS - The incident pump photon gives up its energy to create another photon of reduced energy at a lower frequency (inelastic scattering); the remaining energy is absorbed by the medium in the form of molecular vibrations (optical phonons).
The frequency difference,
known as the Stokes shift.
Because of amorphous nature of glass, the vibrational energy levels of silica molecules merge together to form a band and allows s to differ from p over a wide range (~20 THz).
Raman amplification exhibits self-phase matching between the pump and signal.
The pump signal optical wavelengths in Raman fiber amplifiers are typically 500 nm lower than the signal to be amplified, and the pumping signal can propagate in either direction along the fiber.

33. Gain in Raman Fiber Amplifier
OPTICAL AMPLIFIERS
Gain in Raman Fiber Amplifier
Raman gain as a function of the optical pump power is given as
where gR = Raman gain coefficient
k = a numerical factor that accounts for polarization scrambling between the
optical pump and signal. (k = 2 for complete polarization scrambling)
The effective fiber core area
reff is the effective core radius.
The effective fiber length
P = fiber transmission loss
at the pump wavelength
 is the actual fiber length.
GR dependence on  and P for a pump input power of 1.6 W and fiber core diameter of 10 mm.

34. 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