1. Optical Amplifiers

2. Necessity of Optical amplifiers?
To Transmit a signals over long distances
(>100km), to compensate attenuation
losses.
Initially this was accomplished with an
optoelectronic module consisting of
optical RX, regenerator, equalizer, & an
optical TX to send the data.
Although functional this arrangement is
limited by optical to electrical & electrical
to optical conversions.

3. Introduction
An optical amplifier is a device which
amplifies the optical signal directly
without ever changing it to electricity. The
light itself is amplified.
Reasons to use the optical amplifiers:
Reliability
Flexibility
Wavelength Division Multiplexing
(WDM)
Low Cost

4. Basic Concepts
Most optical amplifiers use stimulated emission
An optical amplifier is basically a laser without
feedback
Optical gain is realized when the amplifier is pumped
optically (or electrically) to achieve population
inversion
Gain depends on wavelength, internal light intensity
and amplifier medium
Three types: semiconductor optical amplifiers,
Raman Amplifiers and fiber doped amplifiers

5. Applications
Power Amp
Configurations

6. Selecting Amplifiers
Maximum
Type Gain Noise figure
Output power
Power High output Not very
High gain
Amplifier power important
Medium Good noise
In-line Medium gain
output power figure
Preamplifi Low output Low value < 5
Low gain
er power dB essential

7. Generic optical amplifier
Continuous Wave
(Constant)
Energy is transferred from the pump to signal

8. Coherence
Incoherent light waves Coherent light waves

9. Atomic Transitions
Stimulated absorption

10. Stimulated emission

11. Condition for Amplification
by Stimulated Emission
Population Inversion:
More Electrons in higher energy level
Pumping:
Process to achieve population inversion
usually through external energy source
In general if N2 > N1 then MEDIA IS SAID TO
BE ACTIVE

13. Semiconductor Optical Amplifiers
Similar to Laser diodes but the emission is triggered
by input optical signal
Work in any wavelength (+)
Have high integration, compact and low power
consumption (+)
Gain fluctuation with signal bit rate (-)
Cross talk between different wavelengths (-)
Two types: Fabry-Perot or Traveling Wave Amp.

14. Solid State Amplifier
Gain VS Power

15. Distributed Fiber Amplifiers
The active medium is created by lightly
doping silica fiber core by rare earth
element Ex: Erbium (Er)
Long fiber length (10-30 m)
Low coupling loss (+)
Transparent to signal format and bit rate
No cross talk
Broad output spectrum (1530 – 1560 nm)
Works only in specific Wavelengths

16. Amplification Process of EDFA
N3 N3
Radiationless
Decay
980 nm N2 N2
Pump
N1 N1
Optical Pumping to Higher Energy levels Rapid Relaxation to "metastable" State
N3
~1550 nm
~1550 nm N2
Signal
N1
Output
Stimulated Emission and Amplification

17. Fig. 11-4: Erbium energy-level diagram

18. EDFA Co-Directional Pumping
configurations
Counter Directional
Dual Pumping

19. Gain versus EDFA length
There is an
optimum length
that gives the
highest gain
Negative gain if
too long

20. Gain versus pump level
Gain decreases
at large signal
levels
Signal dependant
gain
This increases
with the pump
power

21. Amplified Spontaneous Emission (ASE) Noise

22. EDFA Noise Figure
= (Input SNR)/(Output SNR)

23. SNR degradation due to amplification

24. Fig. 11-12a: Gain-flattened EDFA-A

25. Fig. 11-12b: Gain-flattened EDFA-B

26. Raman Amplifiers
 Raman Fiber Amplifiers (RFAs) rely on an
intrinsic non-linearity in silica fiber
 Variable wavelength amplification:
Depends on pump wavelength
For example pumping at 1500 nm produces
gain at about 1560-1570 nm
 RFAs can be used as a standalone amplifier or
as a distributed amplifier in conjunction with
an EDFA Source: Master 7_5

27. Stimulated Raman Scattering
Stimulated Raman Scattering (SRS) causes a
new signal (a Stokes wave) to be generated
in the same direction as the pump wave
down-shifted in frequency by 13.2 THz (due
to molecular vibrations) provided that the
pump signal is of sufficient strength.

28. Distributed Raman Amplification
(I)
 Raman pumping takes place backwards over the fiber
 Gain is a maximum close to the receiver and decreases in
the transmitter direction
Long Fiber Span
Optical
Transmitter EDFA
Receiver
Raman
Pump
Laser

29. Distributed Raman Amplification (II)
 With only an EDFA at the transmit end the optical power
level decreases over the fiber length
 With an EDFA and Raman the minimum optical power level
occurs toward the middle, not the end, of the fiber.
EDFA
+
Raman
Optical Power
EDFA
only
Distance
Source: Master 7_5
Animation

30. Broadband Amplification using Raman
Amplifiers
 Raman amplification can provides very broadband
amplification
 Multiple high-power "pump" lasers are used to
produce very high gain over a range of wavelengths.
 93 nm bandwidth has been demonstrated with just
two pumps sources
 400 nm bandwidth possible?
Source: Master 7_5

31. Advantages and Disadvantages of Raman
Amplification
 Advantages
 Variable wavelength amplification possible
 Compatible with installed SM fiber
 Can be used to "extend" EDFAs
 Can result in a lower average power over a span, good for lower
crosstalk
 Very broadband operation may be possible
 Disadvantages
 High pump power requirements, high pump power lasers have only
recently arrived
 Sophisticated gain control needed
Source: Master 7_5
 Noise is also an issue

32. Conclusion
Optical amplifiers perform a critical
function in modern optical networks,
enabling the transmission of many
terabits of data over long distances of up to
thousands of kilometers.

33. EDFAs

34. SOA

35. Raman Amplifiers

36. Latest technology in Optical
Amplifiers

38. References