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 ApertureThe Numerical Aperture of the fiber is the sine of the maximum angle of an incident beam that can be guided in the core

2 2core cladNA n n ;= −

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 signalfrom laser or from LED.

Modern optical communication systems require ultra-fast (10-100 Gb/s) modulation

Direct laser current modulation

E

RB

Simplest way to impress the information onto the optical beam –direct laser current modulation

Current

Time

Optical power

TimeIdeal laser response to the

current modulation

Direct laser current modulation issues:relaxation oscillations frequency

Laser high-frequency equivalent circuit

Electron – Photon interaction in the cavity

Laser response to a step-like bias

Electron concentration

Optical power

Threshold concentration

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

Signal(information)

Laser(optical carrier)

Direct modulation scheme

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

Electro-absorption in QW structure in electric field

Eabs

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 electro-optic coefficient

Mach – Zehnder modulator

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 /cThe 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)

l

Mach – Zehnder modulator (continued)

V

The required index change ∆nπ = λ/(2l)For the Pockels effect, the difference between top and bottom indices:

30r ijn n r 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)

l

( )30 2 2r ijn n r V d l/ /( )π π λ∆ = = ( )3

0ij rV d l r nπ λ=

Mach – Zehnder interferometer:electro-optical RF modulator

Example: Mach – Zehnder modulator Vp calculation

Vl

Interferometer waveguide is made of LiNbO3n = 2.3; rij = 10.8 *10-12 m/V;

The waveguide thickness d = 1 µmThe modulator length, l = 1mm;

The optical beam wavelength, λ = 1.3 µm

( )30ij rV d l r nπ λ=

V = 9.9 V