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Tutorial Note #4
Basics of OpticalCommunication

Basics of Optical Communication – V002 – 12/NOV/2003 (edited 11/04/2019)

This Application note describes the basics of
optical communication. A brief overview
about generating and detecting opticalsignals is given. Important parameters of ahigh-speed optical transmission system are
introduced, followed by an overview of thedifferent modulation formats.

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In a PIN diode an electron–hole pair isgenerated when a photon with an energy
above the bandgap energy is absorbed.
For an ideal PIN diode each photon would
generate one electron–hole pair.
Real world PIN diodes have less efficiency;
the quantum efficiency takes this into
account. A common detector parameter isknown as responsivity, r, in terms ofphotocurrent per optical power (A/W).

A photodetector generates a photocurrent
according to the amount of incoming lightand its responsivity. The photocurrent isconverted into a voltage by a amplifier or aresistor.
It is important to point out that the optical
input power is the sum of the optical average
power and the optical modulation power.

This example explains why the optical
average power and the optical modulation
power are of interest:
An optical signal with an optical power of
100 μW when a “0” is transmitted and an
optical power of 1000 μW when a “1” istransmitted are detected by a detector with a
responsivity of 20 V/W. The electrical signal
will therefore be between 2 mV (for “0”) and
20 mV (for “1”). If this signal is AC coupled,
the DC term (corresponding to the opticalaverage power) will be filtered off.
The remaining signal corresponds to thedetected optical modulation power.

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Laser is the abbreviation for:Light Amplification by Stimulated Emissionof Radiation
There are two mechanisms for producing
photons:1) Spontaneous Emission:
Stochastic recombinations of electron – holepairs generate photons, this is the principle ofan LED
2) Stimulated Emission:
A photon can cause further electron-hole
recombinations, and thereby produce more
photons. These photons have the same phaseand wavelength as the stimulating photon.
This mechanism is the basis of the laser.

In a laser diode, two semi-transparent mirrors
form an optical resonator.
As the resonator length is large compared to
the wavelength several modes (integers of awavelength) are generated.

In a Distributed Feedback laser, distributed
feedbacks (reflections) inside the resonator
suppress all possible modes but one.
As the good side mode suppression isachieved by internal reflections that are
carefully designed, the DFB laser is very
sensitive to external reflections; this means ahigh optical isolation is necessary.

4As we have already seen, the quality of an
optical transmission is not only given by thereceived power but also by the ratio in powerof the on/off state.
The extinction ratio is the parameter
describing this ratio.
The ER can be expressed in linear terms(without any unit), logarithmic (in dB) or inpercent.

Lets go back to the example on page 3:
The optical signal had an optical power of
100 μW when a “0” was transmitted and an
optical power of 1000 μW when a “1” wastransmitted, this translates to an extinction
ratio of 10.

What will happen to the electrical output
signal if we increase the average opticalpower?
The answer is: We don’t know ………
unless we don’t specify the extinction ratio
as well.
In this example we have increased theaverage optical power but degraded theextinction ratio, therefore the electrical signal
is smaller than before.

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When an optical signal is intensity modulated
an unwanted incidental frequency or phasemodulation often occurs. This angularmodulation causes a significant broadeningof the spectrum and therefore possible
problems due to chromatic dispersion.
Chirp can also make phase modulation
impossible.

The orientation of the electromagnetic fieldalong the propagation is described by thepolarization:
There are unpolarized and polarized waves.
For polarized waves the state of polarization
can be defined.

Polarized waves can be linear or ellipticalpolarized.
Linear polarization can be horizontal, verticalor of any angle in between.
Elliptical polarization can be left handed orright handed.

6Linear Polarization is shown here: Note thatthe amplitude of Ex and Ey can vary, thepolarization will stay linear.
If the phase relation of Ex and Ey is changed
we will get elliptical polarization.

Here you see circular polarization (a special
case of elliptical polarization). We have toadmit, this is a diagram of artistic merit, but
to judge the state of polarization it is not very
helpful.
Therefore to display the state of polarization
the Poincare sphere is used.

At the Poincare Sphere the equator represents
all possible linear polarizations.
The northern hemisphere represents all righthand elliptical polarizations and the southernhemisphere represents all left-hand ellipticalpolarizations.
South and North Pole represent circularpolarization.

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The state of polarization is described by theStokes parameter:(1,0,0) is horizontal polarization; (-1,0,0) is
vertical polarization(0,1,0) is +45° linear polarization; (-1,0,0) is- 45° linear polarization(0,0,1) is right hand circular and (0,0,-1) isleft hand circular

Dispersion means the velocity of propagation
is not constant; dispersion causes pulse
broadening.
Within a multimode fiber the different modes
travel with different speeds causing mode
dispersion.
In a single mode fiber two dispersion effects
occur:
Chromatic dispersion and polarization mode
dispersion.
Chromatic dispersion means that different
colors (wavelengths) of the optical signal
travel at different speeds.

Due to birefringence different states ofpolarization travel with different speed, thiscauses pulse broadening.

8Non-linear effects are sometimes helpful but
mostly unwanted.
It is beyond the scope of this seminar to givea detailed overview on non-linear effects infibers, but as the tolerance against nonlinearity is one reason why new modulation
schemes are evaluated, we will cover somebuzzwords.

The two principal effects causing non-linear
behavior are:-Kerr Effect
Scattering Effects

We now know the limiting factors for opticalhigh-speed data transmission:- Dispersion- Fiber non- linearity- Polarization mode dispersion
New fibers - like dispersion-shifted fibers orlarge effective area fibers - are beeing
developed to improve the characteristic of
the transmission channel.
Another research area is the development ofnovel modulation formats that are morerobust against transmission imperfections.

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