The attached narrated power point presentation attempts to explain the methods for amplification of light, It throws light into the different types of optical amplifiers such as semiconductor optical amplifiers and fiber amplifiers. The material will be useful for KTU final year B Tech students who prepare for the subject EC 405, Optical Communications.

1. 1
Optical Amplification
MEC

2. 2
Contents
• Amplification of Light.
• Optical Regeneration.
• Optical Amplification.
• Types of Amplifiers.
- Semiconductor Optical Amplifiers.
- Rare Earth Doped Fiber Amplifiers.
- Raman Fiber Amplifiers.
• Applications.

3. 3
Glossary
• Gain - ratio of output power to input power
(in dB).
• Gain efficiency - gain as a function of input
power (dB/mW).
• Gain saturation - maximum output power
of the amplifier, beyond which it cannot
increase despite the input power increase.
• Gain bandwidth – range of frequencies
over which the amplifier is effective.

4. 4
Glossary
• Polarization sensitivity - gain dependence
of optical amplifiers on the polarization of
the signal.
• Quantum yield - measure of efficiency of a
photon at a given wavelength for a given
reaction and for a given period of time.
• In optical amplifiers, noise is due to
spontaneous light emission of excited
ions.

5. 5
Light Amplification
Domain Conversion / Repeaters – Light to
Electricity, 3R/4R and back.
Direct Amplification – No Conversion.
* Semiconductor Optical Amplifiers.
* Fiber Amplifiers.
- Raman Fiber Amplifiers.
- Rare Earth Doped Fiber Amplifiers.
- Brillouin Fiber Amplifiers.

6. 6
Optical Regeneration
• Letter R with a number classify type of
regeneration as per features & complexity.
(a) 1R (Reamplification)
(b) 2R (1R+R, Reamplification + Reshaping)
(c) 3R (2R+R, Reamplification + Reshaping +
Retiming)
(d) 4R (3R+R, Reamplification + Reshaping +
Retiming + Reallocation of wavelengths)
• ‘Re’ – requirement for action to enhance signal
quality.

7. 7
Optical Regeneration
• Add repeaters when conventional path
loss exceeds available power margin.
• Conventional Repeater to perform:
- Photon to Electron conversion.
- Electrical Amplification.
- Retiming.
- Pulse Shaping.
- Electron to Photon conversion.
• Electronic Circuitry required.

8. 8
Optical Regeneration

9. 9
Optical Regeneration
Signal + Noise
Amplification Reshaping,
Retiming
Wavelength
Reallocation
AND Gate

10. 10
Optical Regeneration

11. 11
Optical Amplifiers
• No Domain Conversion.
• Placed at intervals along fiber link, provide
linear amplification.
• A single in-line component for any kind of
modulation at any transmission rate.
• Bidirectional, permit multiplexing.
• Use as linear repeaters, optical gain blocks,
wavelength converters, optical receiver
preamplifiers.
• Nonlinear mode as optical gates, pulse
shapers and routing switches.

12. 12
Optical Amplifiers

13. 13
Semiconductor Optical Amplifiers
• Utilize stimulated emission from injected
carriers.
• Smaller size, can be integrated to produce
subsystems.
• High gain, low power consumption.
• Appropriate for use with single mode fibers.
• Major Types based on facet reflectivities:
- Travelling Wave Amplifier – non resonant.
- Fabry Perot Amplifier – resonant.

14. 14
Semiconductor Optical Amplifiers
• Polarization dependent, require a
polarization-maintaining fiber (polarization
sensitivity in the range 0.5 - 2 dB).
• Relatively high gain (20 dB) and large
bandwidth.
• Output saturation power in the range of 5-
10 dBm.
• Wavelength regions of 0.8, 1.3, and 1.5
μm.

15. 15
Semiconductor Optical Amplifiers
• Compact semiconductors easily
integrable with other devices, can be
used as wavelength converter.
• Several SOAs may be integrated into
an array.
• Nonlinear phenomena (four-wave
mixing) - SOAs have a high noise
figure and high cross-talk level.

16. 16
Semiconductor Optical Amplifiers
R1, R2 – Output laser facet reflectivities

17. 17
Semiconductor Optical Amplifiers

18. 18
Semiconductor Optical Amplifiers
• TWA – widely used, large optical
bandwidth, high saturation power, low
polarisation sensitivity.
• TWA – low facet reflectivity (0.1 - 0.2 %),
lower signal gain, higher spontaneous
emission.
• TWA – difficult to fabricate, need for high
quality facet antireflection coatings,
superior than FPA in terms of noise and
saturation of signal gain.

19. 19
Semiconductor Optical Amplifiers
• Injection current delivers external energy
to pump electrons to conduction band.
• Input signal stimulates electron transition
down to the valence band.
• Photon emission of same energy at the
same wavelength as the input signal.

20. 20
Semiconductor Optical Amplifiers
• Signal Gain – ratio between output and
input optical signal power.
• For TWA,
• Single Pass Gain:
Г – Optical confinement factor.
gm – material gain coefficient.
go – unsaturated material gain coefficient.
I – Optical signal intensity.
Is – unsaturated optical signal intensity.
α – internal loss, L – length.

21. 21
Semiconductor Optical Amplifiers
• Single pass phase shift:
ν – incident signal frequency.
ν0 – amplifier resonant mode frequency.
Ø0 – nominal phase shift.
b – linewidth broadening factor.

22. 22
Amplifier Noise Figure
• Noise power limits repeater cascading.
• Mean noise power due to spontaneous
emission accumulates with number of
repeaters.
• Gain saturation when accumulated noise
power = signal power.
• Noise Figure - signal to noise degradation
due to amplification.

23. 23
Amplifier Noise Figure
• Noise Figure depends on population
inversion, excess noise factor, number of
transverse modes in the cavity, optical
bandwidth for amplified spontaneous
emission, number of incident photons etc.
• Typical value = 4.5 dB

24. 24
Amplifier Noise Figure

25. 25
3-dB Optical bandwidth
• 3-dB Optical Bandwidth is a function of
amplifier gain and facet reflectivities.
• For single longitudinal mode,
n – active layer refractive index, L – cavity
length.

26. 26
Fiber Amplifiers
• Fiber amplifiers - gain provided by either
stimulated Raman or Brillouin scattering or
by rare earth dopants.
• Provide high gain over wide spectral
bandwidths.
• Brillouin Amplifiers have narrow spectral
width, used for channel selection within
WDM, as channel amplifiers.
• In-line interconnect compatibility issues
within fiber optic links.

27. 27
Optical Amplifiers
Narrow
spectral
bandwidth

28. 28
Fiber Amplifiers
WDM Coupler provides
low insertion loss.
Rare earth doped / Raman /
Brillouin Amplifiers
High signal gain.
High saturated output
power.
Minimal noise.
Ultra-wide bandwidth.

29. 29
Fiber Amplifier Applications
Power Amplifier
In-line Optical Repeater
Pre-amplifier

30. 30
Basic Optical Amplifier Applications
To reduce transmission losses, Increase repeater distances
Front end pre-amplifier
Booster Amplifier, Linear Gain Blocks
Compensation
for coupler
insertion loss
and power
splitting loss
Improve Gain
& SNR
LAN

31. 31
Thank You