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
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
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
12.
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.
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
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.