The attached narrated power point presentation attempts to explain the basic working principle , different types and characteristics, gain and noise performance . drawbacks and applications of semiconductor optical amplifiers. The material will be beneficial to KTU final year B Tech students who prepare for the subject EC 405, Optical Communications.

1. 1
Semiconductor Optical
Amplifiers
MEC

2. 2
Contents
• Introduction.
• SOA Structure and Characteristics.
• SOA Types.
• Gain and Spectral Bandwidth.
• SOA Performance.
• Facet Reflectivity.
• Gain Clamping and Crosstalk.
• Vertical Cavity SOA.

3. 3
SOA Structure

4. 4
Introduction
• Based on conventional semiconductor laser
structure.
• Stimulated emission, electron transition to
ground level emitting photons.
• Output facet reflectivities between 30 and
35%.
• Used in nonlinear and linear modes of
operation.

5. 5
Introduction
• Fabry-Perot Amplifiers - resonant.
• Traveling-wave (TW) and near-traveling-
wave (NTW) amplifiers - single-pass devices.
• Injection-locked laser amplifier - laser
oscillator to oscillate at incident signal
frequency.
• High internal gain (15 to 35 dB), low power
consumption.
• Single-mode waveguide structure - suitable
for use with single-mode fiber.

6. 6
Light Output vs Current

7. 7
Classifications
• Two main groups - Fabry–Pérot amplifiers
(FPAs) and traveling-wave amplifiers
(TWAs) based on facet reflectivities.
• FPA facet reflectivities 0.01 to 0.3.
• FPA facet reflectivity large, highly resonant
amplifier, transmission characteristic
comprise of very narrow passbands.
• FPA biased below normal lasing threshold
current.
• Light enters one facet, amplified at other
facet along with inherent noise.

8. 8
Fabry Perot Amplifier
• Fabry–Pérot amplifier - oscillator biased
below oscillation threshold - resonant.
• FPA very sensitive to fluctuations in bias
current, temperature & signal polarization.
• Resonant nature of FPAs, high internal
fields.
• Used in nonlinear applications – to provide
pulse shaping & bistable elements.
• True TWA - limiting case with facets
exhibiting zero reflectivity.

9. 9
FPA Pass Band
Peak gain wavelength
δλ
2
2 L




Mode Spacing

10. 10
Travelling Wave SOA
• Fabry–Pérot resonance suppressed by
reduction in facet reflectivity.
• Antireflection coatings applied to laser
facets reduce/eliminate end reflectivities.
• Thin layer of silicon oxide, silicon nitride or
titanium oxide on end facets, reflectivities
reduced to 1 x 10−4 or less.
• Increased amplifier spectral bandwidth.

11. 11
Travelling Wave SOA
• Transmission characteristics less
dependent on fluctuations in bias current,
temperature and input signal polarization.
• Superior to FPA for linear applications.
• Advantages in relation to both signal gain
saturation and noise characteristics.
• Antireflection facet coatings help increase
lasing current threshold.
• Operating currents far beyond normal lasing
threshold current.

12. 12
Near TWA
• True TWA - facets exhibit zero reflectivity.
• Residual reflectivity remains even for best
antireflection coatings.
• Low reflectivity of 1 x 10-5 observed at 1.5
μm.
• Also referred to as near travelling wave
amplifiers.

13. 13
Cavity Gain of SOA
• Function of signal frequency f.
R1, R2 - input & output facet reflectivities,
Gs – singlepass gain, ø - single-pass
phase shift.
fo - Fabry–Pérot resonant frequency,
δf - free spectral range.

14. 14
3 dB Spectral Bandwidth
• 3 dB spectral bandwidth of an FPA or ±3 dB
single longitudinal mode bandwidth defined
by FWHP points:
δf – mode separation frequency interval, n –
refractive index of amplifier medium, L –
amplifier length.

15. 15
3 dB Spectral Bandwidth
• As a function of cavity gain G, 3dB
spectral bandwidth:
• Single pass gain
• Net gain coefficient per unit length
- optical confinement factor, - effective
loss coefficient per unit length.

16. 16
Single Pass Gain
• Material gain coefficient
• go- unsaturated material gain coefficient
without input signal, Is - saturation intensity
I - signal intensity.
• Single-pass gain

17. 17
Single Pass Phase Shift
• Total phase shift
where nominal phase shift
b - linewidth broadening factor, n - material
refractive index.
• Both single- pass gain and phase are
functions of optical intensity.

18. 18
Single Pass Parameters
• For a constant signal intensity (i.e. with
frequency modulation) there is no inherent
signal distortion.
• With time-varying intensity gain and phase
may change with time, cause signal
distortion.
• Gs and øs depend on input signal intensity.
• SOA will exhibit nonlinear and bistable
characteristics at high input powers.

19. 19
TWA 3 dB Bandwidth
• 3 dB bandwidth of a TWA is three orders
of magnitude larger than that of an FPA.
• Passband peak and trough relative
amplitudes depend on facet reflectivities,
single-pass gain (hence applied bias
current) and input intensity.
• Gain undulation / peak–trough ratio of
passband ripple : difference between
resonant and nonresonant signal gain.

20. 20
Passband
Characteristics
Passband ripple

21. 21
Gain Undulation of
Passband Ripple
• Gain undulation or peak–trough ratio of
the passband ripple:

22. 22
Peak Trough Ratio
• For wideband operation, peak–trough ratio
to be small.
• Normally considered to be less than 3 dB
for TWAs over their signal–gain spectrum.
• An amplifier with gain ripple significantly
exceeding 3 dB categorized as an FPA.
• A gain ripple of zero would correspond to a
pure TWA.

23. 23
Cavity Gain
• Active cavity gain of the device - ratio of
output signal power to input signal power
from the cavity.
• With fiber coupling losses included at both
ends of the device, gain called as fiber-to-
fiber gain of the device.
• Gain linear for small output signal powers
(small signal gain), saturates at higher
output signal power levels.

24. 24
Saturation Output Power
• Output signal power at the point of
intersection where linear gain region
meets nonlinear gain region.
• After this point gain will decrease with
increasing input signal power.
• High value of saturated output power
preferred for SOA.
• Gain saturation – amplifier saturated
output power defined at the point where
amplifier gain is reduced by 3 dB.

25. 25
Saturation Output Power
• Depends on both the structure and the
type of material used in the device.
• Typical values lie in the range of 5 to 20
dBm.
• Gain of SOA depends intensity of signal(s)
present in the active cavity medium,
frequency (or wavelength) of the signal(s)
and reflections at mirror facets.

26. 26
SOA Material Systems
• Direct bandgap III–V compounds cover full
optical fiber communication wavelength
range from 0.8 to 1.7 μm.
• III–V compounds used for SOA
construction.
• Wide spectral bandwidths (TWA - 50 to 70
nm) using high-quality antireflection facet
coatings.

27. 27
SOA SNR Performance
• Narrow spectral bandwidth of FPAs
provides inherent noise filtering.
• Not obtained with TWAs, subject to
increased levels of noise.
• Residual facet reflectivity in TWAs due to
backward gain.
• Gain of backward travelling signal - ratio of
power in the backward-travelling signal to
the input signal power into the amplifier.

28. 28
SOA SNR Performance
• Gain of the backward-travelling signal:
Pb - power in the backward-travelling
signal, Pin - input signal power.

29. 29
SOA SNR Performance
R1 = R2 for Gs = 25 dB

30. 30
Reducing Facet Reflectivity
angled facet
angled facet–flared waveguide
window-facet waveguide
Active region is inclined away from facet cleavage plane.
Broaden the end facets of the waveguide
Window contains a transparent region
W
W↑ R↓
ӨP - Tilt
Coupling Efficiency ↑

31. 31
Gain Clamping
• To maintain/clamp carrier concentrations to
a fixed level in the SOA active cavity
medium.
• To avoid the situation where gain of SOA
can change due to variation of input signal
power.
• Two optical signals at different wavelengths
applied to the device, gain produced by one
optical signal can modify the response of
the other due to nonlinear effects -
crossgain modulation/four-wave mixing.

32. 32
Gain Clamping
• Active cavity gain of the device to be at a
constant level.
• Achieved by incorporating lasing action
into the amplifier.
• Reduces crosstalk.
• To facilitate gain-clamped SOA, mirrors
placed at either end of the device.
• Creates a resonant cavity, similar to the
Fabry–Pérot laser, used to stabilize gain of
the optical amplifier.

33. 33
Gain Clamping
• Gain clamped SOA has two distributed
Bragg reflectors used as mirrors.
• Highly reflective DBR mirrors needed to
stabilize the gain of the device.
• Lasing field is longitudinal, parallel to the
direction of optical signal.
• Active DBR region compensates for the
loss at the input to the amplifier.

34. 34
Gain Clamping
• One or both DBR sections chosen as the
active region, apply an injection current to it.
• Employ a DBR mirror structure, laser field
perpendicular to the signal and vertical
within the amplifier structure.
• Optical signal to be amplified passes
horizontally through active medium, directly
through the field from the laser which is
pumping photons vertically in the active
medium.

35. 35
Gain Clamping
• Circulating optical power from the vertical
cavity laser overlaps with the amplifier
waveguide, creates optical feedback to
maintain a constant local gain in the
amplifier.
• Vertical laser action linearizes amplifier
gain, provides ultrafast optical feedback.

36. 36
Gain Clamped SOA

37. 37
Vertical Cavity SOA
• Active cavity orientation
modified to enable the
SOA to emit vertically
from the surface.
• Most of the light emitted
can more easily be
coupled to other
interfaced or integrated
devices.
• Gain confined to small
active region.
Vertical cavity surface-emitting laser

38. 38
Crosstalk
• Gain seen by the signal in one channel
varies with the presence or absence of
signal in other channels.
• To reduce crosstalk:
- operate in small signal region.
- gain clamping.
• Crosstalk used in wavelength converters.

39. 39
SOA Applications
Booster/Post Amplifier
In-Line Amplifier
Pre-amplifier
Wavelength Converter

40. 40
Thank You