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Fast Optical Communication Components
Fiber optics
In optical communications, fiber replaces copper coaxial
cables used in wired networks
Fiber optics
Fiber optic telephone communication system
Optical Fiber
An optical fiber is a cylindrical dielectric waveguide that transmits light along its axis,
by the process of total internal reflection.
The fiber consists of a core surrounded by a cladding layer. To confine the optical
signal in the core, the refractive index of the core must be greater than that of the
cladding.
Optical Fiber Numerical Aperture
The Numerical Aperture of the fiber is the sine of the maximum angle of an incident
beam that can be guided in the core
2
2
NA = ncore
− nclad
;
For example, taking ncore=1.62 and nclad=1.52, we find the NA
to be .56.
The corresponding angle, Θ = arcsin(.56) = 34 deg
The “acceptance angle” = 2Θ = 68 deg
Dispersion in optical fibers
Optical Fiber Attenuation
For long distance communications optical sources and detectors operating at 1300 nm
and 1600 nm are needed.
Dispersion in optical fibers
Dense Wavelength Division Multiplexing
Nortel has demonstrated 6 Tb/s 1000 km DWDM
using 160 different wavelengths 40 Gb/s each
Optoelectronic modulation
The role of modulator is to impress the information onto the optical signal
from laser or from LED.
Modern optical communication systems require ultra-fast (10-100 Gb/s) modulation
Direct laser current modulation
RB
Current
E
Time
Simplest way to impress the information onto the optical beam –
direct laser current modulation
Optical power
Ideal laser response to the
current modulation
Time
Direct laser current modulation issues:
relaxation oscillations frequency
Electron – Photon
interaction in the cavity
Laser high-frequency
equivalent circuit
Electron concentration
Laser response to
a step-like bias
Threshold concentration
Optical power
Direct laser current modulation issues:
Dispersion and chirp
Chirp is the shift of the laser’s center wavelength
during single pulse durations
Chirp in directly modulated laser
Effective pulse broadening due to chirp and
fiber dispersion
different levels of fiber dispersion
External optical modulation
Direct modulation scheme
Signal
(information)
Laser
(optical carrier)
Modulator
Channel (Fiber)
External modulation scheme
Franz-Keldysh Effect
In strong electric field the band edges get
tilted
There is a probability for the electron, which
absorbed the photon with the energy less than
the bandgap to transfer into the conduction
band
Quantum –confined Stark Effect:
the analogous of Franz-Keldysh effect in QWs
Electron wave function compression in the
quantum well structure
Eabs
Electro-absorption in QW structure in electric field
MQW Electro-absorption modulator
Electro-Optical Modulators
The principle of operation is based on the linear electro-optic effect, or the
Pockels effect: a change of optical refractive index in the waveguide due to
application of an electric field.
Pockels effect:
rij is the electrooptic coefficient
Mach – Zehnder modulator
l
V
The light velocity in the waveguide: v = c/neff ;
The time to travel the distance l (the length of the interferometer):
t = l/v = l neff /c;
When the electric field is applied, the refractive index changes by ∆n;
The travel time change:
∆t = l ∆n /c
The electric field causes the beam to delay by π if ∆t =T/2
(where T is the wave period);
For the light beam, T = λ/c;
T/2 = λ/(2c) = l ∆n /c;
The required index change ∆nπ = λ/(2l)
Mach – Zehnder modulator (continued)
l
V
The required index change ∆nπ = λ/(2l)
For the Pockels effect, the difference between top and bottom indices:
∆ n = nr30 rij E
If the thickness is d, then E = (V/d)/2
- the applied voltage is divided between the two arms.
From this, we find the voltage Vπ needed to delay the beam by π (i.e. by λ/2)
∆ nπ = nr30 rijVπ / ( 2d ) = λ /( 2l )
Vπ = d λ
(
3
l rij nr 0
)
Mach – Zehnder interferometer:
electro-optical RF modulator
Example: Mach – Zehnder modulator Vp calculation
l
Vπ = d λ
(
V
3
l rij nr 0
)
Interferometer waveguide is made of LiNbO3
n = 2.3; rij = 10.8 *10-12 m/V;
The waveguide thickness d = 1 µm
The modulator length, l = 1mm;
The optical beam wavelength, λ = 1.3 µm
V = 9.9 V