EE 230: Optical Fiber Communication Lecture 7

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EE 230 Optical Fiber Communication Lecture 7
Optical Amplifiers-the Basics
From the movie Warriors of the Net
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Amplifier Types and Applications
Amplifiers are used to overcome fiber loss They
are used in 4 basic applications In-line
amplifiers for periodic power boosting Power
Amplifier to increase the power to greater
levels than possible from the source Pre-ampli
fier to increase the received power
sensitivity Distribution loss compensation in
local area or cable networks
Fiber Optics Communication Technology-Mynbaev
Scheiner
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Characteristics of all amplifiers

• They operate by creating a population inversion,
  where there are more individuals in a high energy
  state than in a lower one
• The incoming pulses of signal on the fiber induce
  stimulated emission
• They saturate above a certain signal power
• They add noise to the signal

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Comparison of Real and Ideal Amplifier
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Inhomogeneous Gain Broadening
Inhomogeneous broadening The individual atomic
responses within and inhomogeneously broadened
transition all add up to yield the measured
lineshape
A Gaussian inhomogeneously broadened atomic
lineshape such as produced by doppler broadening
in atoms
Lasers-Siegman
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Interaction of Atoms with Light
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Rate Equations and Populations
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Unstimulated Population densities in 2 level
atom
Energy levels 1 and 2 and their decay times. By
means of pumping, the population density of level
2 is increased at the rate R2 while that of level
1 is decreased at the rate R1
Idealy t21tspltltt20 so t2tsp
For large DN or No (also called inversion
density) We want t2 long, but t21 not too small,
t1 and R1 large
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Population densities with a strong resonant signal
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Ideal Amplifier System
Third excited state with very short lifetime, no
fluorescence
Second excited state with very long lifetime and
high cross section for stimulated emission
Pump process with large cross section
Energy gap between first and second excited
states matches telecommunication frequencies
First excited state with very short lifetime
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Amplified Spontaneous Emission
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Noise Figure Measurement
Fiber Optics Communication Technology-Mynbaev
Scheiner
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Noise Figure
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3 main types and 3 Big Ideas

• The main types of optical amplifiers are
• Semiconductor amplifiers (lasers that arent
  lasing)
• Doped fiber amplifiers
• Raman and Brillouin Amplifiers
• The three big ideas
• Gain and gain bandwidth
• Gain saturation
• Noise and noise figure

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Laser Amplifiers
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Semiconductor Optical Amplifiers
Fiber Optics Communication Technology-Mynbaev
Scheiner
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Types of SOA
Fabry-Perot Amplifier High gain but non-uniform
gain spectrum
Traveling wave amplifier Broadband but very low
facet reflectivities are needed
Gain as a function of frequency Ripples are
caused by the cavity modes The overall gain curve
is due to the width of the atomic transition in
the semi-conductor
Fundamentals fo Multiaccess Optical Fiber
Networks Dennis J. G. Mestgagh
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Amplifier Bandwidths
Comparison of the bandwidths of Fabry Perot and
Traveling wave amplifiers
Fiber Optics Communication Technology-Mynbaev
Scheiner
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Traveling Wave SOA
To make a traveling wave Semiconductor Optical
Amplifier the Fabry-Perot cavity resonances must
be supressed. To accomplish this the
reflectivity must be reduced. Three approaches
are commonly used Anti-reflection
coating Tilted Active Region Use of
transparent window regions
Fiber Optics Communication Technology-Mynbaev
Scheiner
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Saturation Power
Semiconductor Optical amplifiers saturate
silmilarly to a 2 level atom The typical
saturation output power for SOAs is around 5-10
mW
Gain saturation and saturation power
Fiber Optics Communication Technology-Mynbaev
Scheiner
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Crosstalk in Semiconductor Amplifiers

• Rate equation for pump current
• If F suddenly goes to zero, as in 1-0 sequence,
• Time constant is (ns)
• If F suddenly turns on,
• which is smaller

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Parameters on previous slide

• Ncarrier density (cm-3)
• Ipump current (ampcoul/s)
• qcharge on electron (coul)
• L,w,dcavity dimensions (cm3)
• ?recombination lifetime (s)
• ?confinement factor (unitless)
• ?photon density (cm-3)
• again coefficient (cm-1)

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Crosstalk in semiconductor amplifiers

• If time constant for spontaneous decay of excited
  state is shorter than the bit duration, the
  population of the excited state will vary sharply
  with the optical power in the fiber, and gain
  will depend on the fraction of 1s and 0s in the
  data stream.
• If time constant is long, then the population in
  the excited state will be constant, dependent
  upon the pump power but not the signal power.

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Reduction of Polarization Dependence
Three main approaches Connect the amplifiers in
series Residual facet reflectivity can cause
undesired coupling between amplifiers resulting
in poor noise and dynamic performance Connect
them in parallel Good solution but
complex Double pass with polarizaion
rotation Automatic 6 db loss due to coupler
Fiber Optics Communication Technology-Mynbaev
Scheiner
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Undesired effects in an SOA

• Cross saturation can cause undesired coupling
  between channels
• This can be used for wave length conversion and
  controlling light with light
• If used for multiple channels in a switched
  network gain must be adjusted as channels are
  added and dropped
• Four wave mixing is also quite pronounced in SOAs
• Causes undesired coupling of light between
  channels
• Can however also be used to advantage in
  wavelength converters.
• High coupling loss
• Polarization sensitive gain

Fiber Optics Communication Technology-Mynbaev
Scheiner
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Short Pulse Amplification in SOAs
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Semiconductor amplifier advantages

• Are the right size to be integrated with
  waveguide photonic devices (short path length
  requirement)
• Can easily be integrated as preamplifiers at the
  receiver end
• Use same technology as diode lasers
• Gain relatively independent of wavelength
• Are pumped with current, not another laser

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Semiconductor amplifier disadvantages

• Polarization dependence
• Self-phase modulation leading to chirp
• Cross-phase modulation
• Four-wave mixing and crosstalk
• Extremely short (ns) excited state lifetimes