Optical Amplifiers and WDM - Optical Communication

Optical Amplifier

1. 1
Optical Amplifier
CHAPTER 7

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

3. Why the Need for Optical Amplification?
3
•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
4
•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
P
in
Amplified signal
P
out
ASE ASE
Pump Source

5. Optical Amplification - Spectral Characteristics
5
Wavelength
P
o
w
e
r

(
u
n
a
m
p
l
i
f
i
e
d
s
i
g
n
a
l
)
Wavelength
P
o
w
e
r

(
a
m
p
l
i
f
i
e
d

s
i
g
n
a
l
)
ASE
Wavelength
P
o
w
e
r

(
u
n
a
m
p
l
i
f
i
e
d
s
i
g
n
a
l
)
Wavelength
P
o
w
e
r

(
a
m
p
l
i
f
i
e
d
s
i
g
n
a
l
)
ASE
Single channel
WDM channels

6. Optical Amplification - Noise Figure
6
•Required figure of merit to compare amplifier
noise performance
•Defined when the input signal is coherent
)(rationoisetosignalOutput
)(rationoisetosignalInput
(NF)FigureNoise
o
i
SNR
SNR



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
7
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
8
•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)
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 0.001.
where
R
1 = input facet
reflectivity
R
2
= output facet
reflectivity

10. 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


  
2
21
2
21
21
sin41
11
SS
S
FP
GRRGRR
GRR
fG



where R
1 = input facet reflectivity
R
2 = output facet reflectivity
Single pass phase shift
 
f
ff
m





LgG
S
expThe single pass gain is given
by
f
m
= cavity resonance frequencies
f = longitudinal-mode spacing

11. OPTICAL AMPLIFIERS
 G
FP
reduce to G
S
when R
1
=R
2
=0.
 G
FP 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
 
 










21
21
211
2
1
sin
2
2
S
S
mFPA
GRR
GRRf
ffB




  
2
21
2
21
21
sin41
11
SS
S
FP
GRRGRR
GRR
fG



 
f
ff
m






12. Question
12

13. Answer
13

14. 14
Answer

15. SLA - Principle Operation
15
Electrons in ground state
Pump signal
@ 980 nm
Energy Absorption
Excited state
Pump
signal
@ 980
nm
T
r
a
n
s
itio
n
Metastable
state
Excited state
Ground state
Pump signal
@ 980 nm
www.cisco.com

16. SLA - Principle Operation
16
ASE Photons
1550 nm
Ground state
Excited state
Metastable state
T
r
a
n
s
itio
n
T
r
a
n
s
i
t
i
o
n
Pump signal
@ 980 nm
Excited state
Metastable state
T
r
a
n
s
itio
n
Pump signal
@ 980 nm
Stimulated
emission
1550 nm
Signal photon
1550 nm
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.
17
1480
980
820
540
670
Ground state
Metastable
state
1550 nm
EDFA is a low noise light
amplifier.

18. EDFA - Operating Features
18
• 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
Input signal
Pump from an
external laser
1480 or 980 nm
Erbium doped core
Cladding
Amplifier length
1-20 m typical
Amplified signal

19. 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
22
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
23
0
10
20
30
1525 1550 1575 1600
5
10
15
Noise  6.5 dB
Output Power  24.5 dBm
L-BandC-Band
Total 3dB Bandwidth = 84.3 nm
43.5 nm40.8 nm
Wavelength (nm)
G
a
in

(
d
B
)
N
o
is
e

F
ig
u
r
e

(
d
B
)
Ultra-Wideband EDFA

24. Optical Amplifiers: Multi-wavelength
Amplification
24
www.cisco.com

25. Optical Amplifier - Main Parameters
25
• 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)
•G = S
Output
/ S
Input
(No noise)
•Input signal dependent:
–Operating point (saturation) of
EDFA strongly depends on
power and wavelength of
incoming signal
26
Gain (dB)
1540 1560 1580
10
1520
20
40
30
-5 dBm
-20 dBm
-10 dBm
P
Input:
-30 dBm
• Gain ↓ as the input power ↑
P
in
Gain P
out
-20 dBm 30 dB+10 dBm
-10 dBm 25 dB+15 dBm
Note, P
in
changes by a factor of ten
then P
out
changes only by a factor of
three in this power range.
EDFA

27. Optical Amplifier - Optical Gain (G)
27
•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
28
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
29
NF
total
= NF
1
+NF
2
/G
1
NF
1st/2nd stage = P
in - SNR
o [dB] - 10 Log (hc
2
 / 
3
)
Er
3+
Doped Fiber
Pump 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

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

31. 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. 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).
opsp   
The frequency difference,
known as the Stokes shift.
spR 
 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. OPTICAL AMPLIFIERS
Gain in Raman Fiber Amplifier
Raman gain as a function of the optical pump power is given as









kA
LPg
G
eff
effpR
Rexp
The effective fiber core area
2
effeff
rA
where g
R
= Raman gain coefficient
k = a numerical factor that accounts for polarization scrambling between the
optical pump and signal. (k = 2 for complete polarization scrambling)
 
P
P
effL



exp1
The effective fiber length
r
eff
is the effective core radius.

P
= fiber transmission loss
at the pump wavelength
 is the actual fiber length.
G
R dependence on  and 
P for a pump
input power of 1.6 W and fiber core
diameter of 10 m.

34. Optical Amplifiers - Applications
34
• 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