light sources – ii the laser and external modulation optical communication systems -xavier...
TRANSCRIPT
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Light Sources – IIThe Laser and External
Modulation
Optical Communication Systems -Xavier Fernando
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The LASERLight Amplification by ‘Stimulated Emission’ and Radiation
• Laser is an optical oscillator. It comprises a resonant optical amplifier whose output is fed back into its input with matching phase. Any oscillator contains:
1- An amplifier with a gain-saturated mechanism 2- A positive feedback system 3- A frequency selection mechanism 4- An output coupling scheme
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LASER
• In laser, the amplifier is the pumped active medium (biased semiconductor region).
• Feedback can be obtained by placing the active medium in an optical resonator, such as Fabry-Perot structure.
• Frequency selection is achieved by the resonators, which admits only certain modes.
• Output coupling is accomplished by making one of the resonator mirrors partially transmitting.
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Fundamental Lasing Operation
• Absorption: An atom in the ground state might absorb a photon emitted by another atom, thus making a transition to an excited state.
• Spontaneous Emission: random emission of a photon, which enables the atom to relax to the ground state.
• Stimulated Emission: An atom in an excited state might be stimulated to emit a photon by another incident photon.
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Stimulated Emission
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In Stimulated Emission incident and stimulated photons will have
Attribute Result
Identical Energy Narrow line width
Identical Direction Narrow beam width
Identical Phase Temporal Coherence
Identical Polarization Coherently polarized light
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Lasing in a pumped active medium• In thermal equilibrium the stimulated emission is
essentially negligible, since the density of electrons in the excited state is very small, and optical emission is mainly because of the spontaneous emission. Stimulated emission will exceed absorption only if the population of the excited states is greater than that of the ground state. This condition is known as Population Inversion. Population inversion is achieved by various pumping techniques.
• In a semiconductor laser, population inversion is accomplished by injecting electrons into the material to fill the lower energy states of the conduction band.
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Fabry-Perot Laser (resonator) cavity
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Fabry-Perot Resonator
A
B
L
M1 M2 m = 1
m = 2
m = 8
Relative intensity
m
m m + 1m - 1
(a) (b) (c)
R ~ 0.4
R ~ 0.81 f
Schematic illustration of the Fabry-Perot optical cavity and its properties. (a) Reflectedwaves interfere. (b) Only standing EM waves, modes, of certain wavelengths are allowedin the cavity. (c) Intensity vs. frequency for various modes. R is mirror reflectance andlower R means higher loss from the cavity.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall) [4-18]
R: reflectance of the optical intensity, k: optical wavenumber
1,2,3,.. :modesResonant mmkL
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Fabry-Perot Lasing CavityA Fabry-Perot cavity consists of two flat, partially reflecting mirrors that establish a strong longitudinal optical oscillator feedback mechanism, thereby creating a light-emitting function.
The distance between the adjacent peaks of the resonant wavelengths in a Fabry-Perot cavity is the modal separation. If L is the distance between the reflecting mirrors & the refractive index is n, then at a peak wavelength λ the MS is given by
nL2 Separation Modal
2
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How a Laser Works
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Laser Diode Characteristics• Nanosecond & even picosecond response time (GHz BW)• Spectral width of the order of nm or less• High output power (tens of mW)• Narrow beam (good coupling to single mode fibers)
• Laser diodes have three distinct radiation modes namely, longitudinal, lateral and transverse modes.
• In laser diodes, end mirrors provide strong optical feedback in longitudinal direction, so by roughening the edges and cleaving the facets, the radiation can be achieved in longitudinal direction rather than lateral direction.
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Laser Operation & Lasing Condition
• To determine the lasing condition and resonant frequencies, we should focus on the optical wave propagation along the longitudinal direction, z-axis. The optical field intensity, I, can be written as:
• Lasing is the condition at which light amplification becomes possible by virtue of population inversion. Then, stimulated emission rate into a given EM mode is proportional to the intensity of the optical radiation in that mode. In this case, the loss and gain of the optical field in the optical path determine the lasing condition.
• The radiation intensity of a photon at energy varies exponentially with a distance z amplified by factor g, and attenuated by factor according to the following relationship:
)()(),( ztjezItzI
h
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zhhgIzI )()(exp)0()( [4-20]
1R 2R
Z=0 Z=L
)2()()(exp)0()2( 21 LhhgRRILI [4-21]
2
21
21 t,coefficien absorption effective :
tcoefficiengain :g factor,t confinemen Optical :
nn
nnRα
1n
2n
Lasing Conditions:
1)2exp(
)0()2(
Lj
ILI
[4-22]
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Threshold gain & current density
21
1ln
2
1
RRLgth
thgg :iff lase"" tostartsLaser
For laser structure with strong carrier confinement, the threshold current Density for stimulated emission can be well approximated by:
thth Jg
onconstructi device specificon dependsconstant :
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Laser Resonant Frequencies• Lasing condition, namely eq. [4-22]:
• Assuming the resonant frequency of the mth mode is:
,...3,2,1 ,2L2 1)2exp( mmLj
n2
1,2,3,... 2
mLn
mcm
LnLn
cmm 22
2
1
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Spectrum from a laser Diode
widthspectral: 2
)(exp)0()(
20
gg
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Semiconductor laser rate equations• Rate equations relate the optical output power, or # of photons per unit
volume, , to the diode drive current or # of injected electrons per unit volume, n. For active (carrier confinement) region of depth d, the rate equations are:
emission stimulatedionrecombinat sspontaneouinjectionrateelectron
lossphoton emission sspontaneouemission stimulatedratePhoton
Cnn
qd
J
dt
dn
RCndt
d
sp
phsp
densitycurrent Injection
timelifephoton
mode lasing theintoemission sspontaneou of rate
process absorption &emission optical theofintensity theexpressingt Coefficien
:
:
:
:
J
R
C
ph
sp
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Threshold current Density & excess electron density
• At the threshold of lasing:
• The threshold current needed to maintain a steady state threshold concentration of the excess electron, is found from electron rate equation under steady state condition dn/dt=0 when the laser is just about to lase:
0 ,0/ ,0 spRdtd
thph
ph nC
nCn
10/ 25]-[4 eq. from
sp
thth
sp
thth nqdJ
n
qd
J
0
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Laser operation beyond the threshold
• The solution of the rate equations [4-25] gives the steady state photon density, resulting from stimulated emission and spontaneous emission as follows:
thJJ
spphthph
s RJJqd
)(
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External quantum efficiency
• Number of photons emitted per radiative electron-hole pair recombination above threshold, gives us the external quantum efficiency.
• Note that:
)mA(
)mW(]m[8065.0
)(
dI
dP
dI
dP
E
q
g
g
g
th
thiext
%40%15 %;70%60 exti
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Laser P-I Characteristics (Static)
Threshold Current
External Efficiency Depends on the slope
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Laser Optical Output vs. Drive CurrentSlope efficiency = dP/dIThe laser efficiency changes with temperature:
20° C 30° C
40° C
50° COp
tica
l ou
tpu
tEfficiencydecreases
Relationship between optical output and laser diode drive current. Below the lasing threshold the optical output is a spontaneous LED-type emission.
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Modulation of Optical Sources
• Optical sources can be modulated either directly or externally.
• Direct modulation is done by modulating the driving current according to the message signal (digital or analog)
• In external modulation, the laser is emits continuous wave (CW) light and the modulation is done in the fiber
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Why Modulation
• A communication link is established by transmission of information reliably
• Optical modulation is embedding the information on the optical carrier for this purpose
• The information can be digital (1,0) or analog (a continuous waveform)
• The bit error rate (BER) is the performance measure in digital systems
• The signal to noise ratio (SNR) is the performance measure in analog systems
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Direct Modulation
• The message signal (ac) is superimposed on the bias current (dc) which modulates the laser
• Robust and simple, hence widely used• Issues: laser resonance frequency, chirp, turn on
delay, clipping and laser nonlinearity
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Light Source LinearityIn an analog system, a time-varying electric analog signal modulates an optical source directly about a bias current IB. •With no signal input, the optical power output is Pt. When an analog signal s(t) is applied, the time-varying (analog) optical output is: P(t) = Pt[1 + m s(t)], where m = modulation index
For LEDs IB’ = IB
For laser diodes IB’ = IB – Ith
LED Laserdiode
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Modulation of Laser Diodes
• Internal Modulation: Simple but suffers from non-linear effects.• External Modulation: for rates greater than 2 Gb/s, more complex,
higher performance.• Most fundamental limit for the modulation rate is set by the photon
life time in the laser cavity:
• Another fundamental limit on modulation frequency is the relaxation oscillation frequency given by:
thph
gn
c
RRLn
c
21
1ln
2
11
2/1
11
2
1
thphspI
If
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Laser Digital Modulation
Current (I)I(t)
Ith
I1
t
P(t)
t
I2
th
spd II
IIt
2
12ln
Optical Power
(P)
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• Input current– Assume step input
• Electron density– steadily increases
until threshold value is reached
• Output optical power – Starts to increase
only after the electrons reach the threshold
Turn on Delay(td)
Resonance Freq.(fr)
I1
I2
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Turn on Delay (lasers)• When the driving current suddenly jumps from
low (I1 < Ith) to high (I2 > Ith) , (step input), there is a finite time before the laser will turn on
• This delay limits bit rate in digital systems• Can you think of any solution?
th
spd II
IIt
2
12ln
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Relaxation Oscillation• For data rates of less than approximately 10 Gb/s (typically 2.5
Gb/s), the process of imposing information on a laser-emitted light stream can be realized by direct modulation.
• The modulation frequency can be no larger than the frequency of the relaxation oscillations of the laser field
• The relaxation oscillation occurs at approximately
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The Modulated Spectrum
Two sidebands each separated by modulating frequency
Twice the RF frequency
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Limitations of Direct Modulation
• Turn on delay and resonance frequency are the two major factors that limit the speed of digital laser modulation
• Saturation and clipping introduces nonlinear distortion with analog modulation (especially in multi carrier systems)
• Nonlinear distortions introduce higher order inter modulation distortions (IMD3, IMD5…)
• Chirp: Laser output wavelength drift with modulating current is also another issue
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Laser Noise
• Modal (speckel) Noise: Fluctuations in the distribution of energy among various modes.
• Mode partition Noise: Intensity fluctuations in the longitudinal modes of a laser diode, main source of noise in single mode fiber systems.
• Reflection Noise: Light output gets reflected back from the fiber joints into the laser, couples with lasing modes, changing their phase, and generate noise peaks. Isolators & index matching fluids can eliminate these reflections.
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Temperature Dependency
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External ModulationWhen direct modulation is used in a laser transmitter, the process of
turning the laser on and off with an electrical drive current produces a widening of the laser linewidth referred to as chirp
The electro-optical (EO) phase modulator (also called a Mach-Zehnder Modulator or MZM) typically is made of LiNbO3.
The optical source injects a constant-amplitude light signal into an external modulator. The electrical driving signal changes the optical power that exits the external modulator. This produces a time-varying optical signal.
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Mach-Zehnder Principle
phase) (opposite ceinterferenen destructiv --even is If
(inphase) ceinterferen veconstructi -- odd is If
isoutput lower In the
isoutput upper In the
:signals ginterferin twoebetween th difference phase relative Total•
m
m
L
mL
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External Modulated Spectrum
• Typical spectrum is double side band• However, single side band is possible which is
useful at extreme RF frequencies
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Traveling Wave Phase Modulator
• Much wideband operation is possible due to the traveling wave tube arrangement (better impedance matching)
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Distributed Feedback Laser (Single Mode Laser)
The optical feedback is provided by fiber Bragg Gratings Only one wavelength get positive feedback
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Fiber Bragg GratingThis an optical notch band reject filter
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DFB Output Spectrum
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Nonlinearity
...2coscos)(
cos)(
210
tAtAAty
tAtx
x(t) Nonlinear function y=f(x) y(t)
Nth order harmonic distortion:
1
log20A
An
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Intermodulation Distortion
nmmn m,ntnmBty
tAtAtx
,21
2211
2,...1,0, )cos()(
coscos)(
Harmonics:
21 , mn
Intermodulated Terms:
,...2,2, 212121
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Transmitter Packages• There are a variety of transmitter packages for different applications. • One popular transmitter configuration is the butterfly package.• This device has an attached fiber fly lead and components such as the
diode laser, a monitoring photodiode, and a thermoelectric cooler.
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Transmitter PackagesThree standard fiber optic transceiver packages