Download EE 230: Optical Fiber Communication Lecture 7

EE 230: Optical Fiber Communication Lecture 7
Optical Amplifiers-the Basics
From the movie
Warriors of the Net
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-amplifier to increase the received power
sensitivity
Distribution loss compensation in local area
or cable networks
Fiber Optics Communication Technology-Mynbaev & Scheiner
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
Comparison of Real and Ideal
Amplifier
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
Interaction of Atoms with Light
Rate Equations and Populations
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
For large DN or No (also called inversion density)
We want t2 long, but t21 not too small, t1 and R1 large
Idealy t21~tsp<<t20 so t2~tsp
Population densities with a strong
resonant signal
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
Amplified Spontaneous Emission
Noise Figure Measurement
Fiber Optics Communication Technology-Mynbaev & Scheiner
Noise Figure
Noise Figure º
SNRin
SNRout
A perfect amplifier would have a
Noise figure of 1 or 0 dB
Noise figure of an amplifier cascade
Fsystem = Fn1 +
F
Fn 2
Fnk
+ n3 + ..... +
G1 G1G2
G1G2G3...Gk- 1
For lowest overall noise figure you should put
the lowest noise amplifier first
3 main types and 3 Big Ideas
The main types of optical amplifiers are:
•Semiconductor amplifiers (lasers that aren’t lasing)
•Doped fiber amplifiers
•Raman and Brillouin Amplifiers
The three big ideas
•Gain and gain bandwidth
•Gain saturation
•Noise and noise figure
Laser Amplifiers
Semiconductor Optical Amplifiers
Fiber Optics Communication Technology-Mynbaev & Scheiner
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
Amplifier Bandwidths
Comparison of the bandwidths of Fabry Perot and Traveling wave amplifiers
Fiber Optics Communication Technology-Mynbaev & Scheiner
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
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
Crosstalk in Semiconductor Amplifiers
Rate equation for pump current
dN
I
N (t ) ac
N (t )  N0 (t )



dt qLWD
t
n
If Φ suddenly goes to zero, as in 1-0 sequence,
It
N (t ) 
1  e t / t
qLWD
Time constant is
(ns)
Tdown  t


If Φ suddenly turns on,
which is smaller
 1 ac 
Tup   

n 
t
1
Parameters on previous slide
•
•
•
•
•
•
•
•
N=carrier density (cm-3)
I=pump current (amp=coul/s)
q=charge on electron (coul)
L,w,d=cavity dimensions (cm3)
t=recombination lifetime (s)
=confinement factor (unitless)
=photon density (cm-3)
a=gain coefficient (cm-1)
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.
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
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
Short Pulse Amplification in SOAs
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
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