MONODIP SINGHA ROY
ROLL NO: 12013515004
Optical communication is a communication at a distance using light to carry
Optical communications can be said , in combination with microwave and
wireless technologies, are enabling the construction of high-capacity networks
with global connectivity.
The most common wavelengths used for optical communication fall between
0.83 and 1.55 microns. Other wavelengths are also used but this range
encompasses the most popular applications. A wavelength of 1 micron
corresponds to a frequency of 300 THz (300,000 GHz).
Optical Communication is mainly divided into two forms , they are :
Analog Optical Communication.
Digital Optical Communication.
Analog Optical Communication: Fiber Optics.
Digital Optical Communication: Free Space Optics.
Fiber-optic communications is the method of transmitting data from one point to
another by sending pulses of light through an optical fiber. Optical fiber is a
waveguide made of very thin tubes of glass whose diameter is of the order of a few
micrometers. Here the optical fiber works on the principle of ‘Total Internal
The process of communicating using an optical fiber involves the following -
a) Converting electrical signal to optical signal at the transmitter.
b) Transmission of optical signal to the cable.
c) Relaying the signal through the optical-fiber.
d) Receiving the optical signal at the detector.
e) Converting the optical signal back to the electrical signal at the receiver.
A block diagram of a fiber-optic communication system is
When a light ray is incident on a boundary separating two different media, part of
the ray is reflected back into the first medium and while the other part is bent and
enters into the second medium. The bending in the second medium known
as refraction and depends on the refractive index of the two media.
By Snell’s Law
n1sinφ1 = n2sinφ2
sinφ1/sinφ 2 = n2/n1
where n1 and n2 are the refractive indices of the medium 1 and 2 respectively and
φ1 and φ2 are the angle of incidence and angle of refraction respectively.
For n1>n2, if sinφ1=n2/n1 then φ2=90°.(Here, φ1 is known as the ‘Critical Angle’). If
we further increase φ1 from this angle there will be no refracted wave and the light
ray will be completely reflected into the first medium. This phenomenon is known
as the Total Internal Reflection.
An optical transmitter is needed to convert an electrical signal to
a light pulse for transmission in the optical medium i.e. the fiber-optic
cable. The most commonly used optical transmitters are semiconductor
devices such as Light-Emitting Diodes (LEDs) and Laser Diodes. In our
experiment, we have used an LED transmitter.
An optical receiver is needed to convert light pulses to electrical
signals. The main component of an optical receiver is a photo detector, which
converts light into electricity using the photoelectric effect. In our experiment
the photo detector is a semiconductor based photodiode.
Types of Transmission
There are two types of transmission schemes may be
present in fiber-optic transmission system.
As the name implies, single mode optical fiber is designed to propagate
only one light ray. It is used in high speed long distance communication.
A multi mode optical fiber is designed to propagate more than one light
wave at a time. A larger diameter of the core is required to accommodate more
light rays facilitate the transmission. It is typically used in short distance
communication. In this particular experiment, we have used a multi mode fiber.
Used in telecommunication and networking because it is flexible and can be
bundled as cables. Although fibers can be made out of either transparent plastic
(POF = plastic optical fibers) or glass, the fibers used in long-distance
telecommunications applications are always glass, because of the lower optical
absorption. The light transmitted through the fiber is confined due to total internal
reflection within the material. This is an important property that eliminates
signal crosstalk between fibers within the cable and allows the routing of the cable
with twists and turns. In telecommunications applications, the light used is
typically infrared light, at wavelengths near to the minimum absorption wavelength
of the fiber in use.
Fibers are generally used in pairs, with one fiber of the pair carrying a signal in each
direction, however bidirectional communications is possible over one strand by
using two different wavelengths (colors) and appropriate coupling/splitting devices.
Lower cost in the long run.
Low loss of signal (typically less than 0.3 dB/km), so repeater-less transmission over
long distances is possible.
Large data-carrying capacity (thousands of times greater, reaching speeds of up to
1.6 Tb/s in field deployed systems and up to 10 Tb/s in lab systems).
Immunity to electromagnetic interference, including nuclear electromagnetic
pulses (but can be damaged by alpha and beta radiation).
No electromagnetic radiation; difficult to eavesdrop.
High electrical resistance, so safe to use near high-voltage equipment or between
areas with different earth potentials.
Signals contain very little power.
No crosstalk between cables.
No sparks (e.g. in automobile applications).
Difficult to place a tap or listening device on the line, providing better physical
High investment cost.
Need for more expensive optical transmitters and receivers.
More difficult and expensive to splice than wires.
At higher optical powers, is susceptible to "fiber fuse" wherein a bit too much light
meeting with an imperfection can destroy as much as 1.5 kilometers of wire at several
meters per second. A "Fiber fuse" protection device at the transmitter can break the
circuit to prevent damage, if the extreme conditions for this are deemed possible.
Cannot carry electrical power to operate terminal devices. However, current
telecommunication trends greatly reduce this concern: availability of cell phones and
wireless PDAs; the routine inclusion of back-up batteries in communication devices; lack
of real interest in hybrid metal-fiber cables; and increased use of fiber-based intermediate
Almost all these disadvantages have been surmounted or bypassed in contemporary
telecommunications usage, and communication systems are now unthinkable
without fiber optics. Their cost is much more economic than old coaxial cables
because the transmitters and receivers (laser and photodiodes) turn out cheaper
than electric circuitry as their capacity is much superior. The cost of regeneration in
electrical long distance transmission systems is completely impractical for modern
Free-space optics FSO communication uses modulated optical beams, usually generated
by laser sources or light emitting diodes LEDs, to transmit information line of sight
through the atmosphere. Recently, there has been an exponential increase in the use
of FSO technology, mainly for “last mile” applications, because FSO links provide the
transmission capacity to overcome bandwidth bottlenecks. The desire to develop
high-speed Internet access has stimulated much of this growth and as a result, the
major focus of most FSO research and development has been toward the
transmission of digital signaling formats. Fiber optics has been traditionally used for
transmission of both digital and analog signals. The transmission of rf intensity
modulated signals over optical fibers is well established. FSO can transmit data,
voice or video at speeds capable of reaching 2.5 Gbps.
The advantages of transmitting modulated rf signals over FSO links are as follows:
FSO transmission links can be deployed quicker, and in some instances more
economically, than optical fiber links.
FSO is highly invulnerable to interference from other sources of laser radiation.
FSO can be implemented for portable applications, e.g., movable radar dish
FSO provides a viable transmission channel for transporting IS-95 CDMA signals to
base stations from macro- and microcell sites and can decrease the setup costs of
temporary microcells deployed for particular events, e.g., sporting events, by
eliminating the need for installing directional microwave or connecting cable.
FSO introduces a viable transmission medium for the deployment of cable
television CATV links in metropolitan areas where installing new fiber infrastructure
can be relatively expensive.
Analog FSO can reduce the cost of transmission equipment as compared to a
When compared with wireless rf links, FSO requires no licensing and provides
better link security and much higher immunity from electromagnetic interference
EDFA: Erbium Doped Fiber Amplifier is a optical repeater used to boost the
optical signals carried out through optical cables.
APD: Avalanche Photodiode used for converting light signals into electrical
The electrical signal is converted into optical power and the transmitted through air.
After undergoing the influences of time-dispersive channel and ambient light, the
optical signal is directly translated into a photocurrent as the detector.
The electrical SNR in the optical links depends on the square of the optical power,
which has a deep impact in both the designs and performance of the optical wire
One or more Laser Diode (LD) or Laser Emitting Diode (LED) are used. The
choice between LED or LD is determined by various factors that influence price
and performance as known from traditional optical communication.
An optical concentrator (collect and concentrate on incoming radiation) and an
optical filter (to reject ambient light) , a photo detector (to convert optical power
into photocurrent) and an electrical front-end (performing amplification, filtering,
No licensing required.
Installation cost is very low as compared to laying Fiber.
No sunk costs.
No capital overhangs.
Highly secure transmission possible.
High data rates, up to 2.5 Gbps at present and 10 Gbps in the near future.
High launch power represents eye hazards.
Signals scattering results in multiple impairments.
Blockage leads to design challenges.
Low power source requires high sensitive receivers.
Light interference negatively affects systems performance.
If the signal is detected by a receiver that requires a minimum average power of bit
rate B, the maximum transmission distance is limited.
The system requirements typically specified in advance are the bit rate B and the
transmission distance L.
The performance criterion is specified through bit-error rate, a typical requirement
being BER < 10-9.
• Ptr = Average transmitted power
• Prec = Average received power
• αf = Net loss, fiber connectors, splices
• B=bit rate
• Np = Minimum bits
• L = Distance
The equation for loss limited light wave system is given as:
At 1550nm, wide region of low-loss wavelengths is irresistible for
WDM systems even with high dispersion.
1250 1350 1450 1550 1650
Long-Haul it means long distance, Long-haul optics refers to the transmission of
visible light signals over optical fiber cable for great distances, especially without or
with minimal use of repeaters.
In advance in fiber optic technology have made long-haul communications systems
reach distances that were once unheard of. Today's fiber optic transmission links
transmit multiple channels of video and audio signals over worldwide distances, and
can reach high traffic volumes. This distance is made possible by a number of devices
that amplify optical signals and combine larger and larger numbers of signals for
transmission over a single optical fiber.
Currently, there are three systems in use for long-haul applications, namely, intensity
modulation and direct detection (IM-DD), wavelength division multiplexing (WDM), and
coherent systems. IM-DD can be enhanced by optical amplification using Er-doped
fiber amplifiers (EDFA). To increase the capacity of existing systems, WDM is used.
Using 50 wavelengths around 1.55 mm, a fiber can offer an information bandwidth
approaching 1 Tb/s. All channels can be simultaneously amplified by EDFA.
The long-distance systems can be divided into terrestrial and submarine applications.
For submarine system, reliability is the most important consideration because of the
high cost of repair if it is needed. Redundancy often is built into the system. The
terrestrial systems are subject to weather conditions, e.g., extreme temperature
Dispersion is defined as pulse spreading in an optical fiber. As a pulse of light
propagates through a fiber, elements such as numerical aperture, core diameter,
refractive index profile, wavelength, and laser line width cause the pulse to
broaden. Dispersion increases along the fiber length. The overall effect of
dispersion on the performance of a fiber optic system is known as Intersymbol
Interference (ISI). ISI occurs when the pulse spreading caused by dispersion
causes the output pulses of a system to overlap, rendering them undetectable.
Dispersion is generally divided into three categories:
Polarization mode dispersion.
Modal Dispersion: Modal dispersion is defined as pulse spreading caused by the time
delay between lower-order modes and higher-order modes. Modal dispersion is
problematic in multimode fiber, causing bandwidth limitation.
Polarization Mode Dispersion: Polarization Mode Dispersion (PMD) occurs due to
birefringence along the length of the fiber that causes different polarization modes to
travel at different speeds which will lead to rotation of polarization orientation along
Chromatic Dispersion: Chromatic Dispersion (CD) is pulse spreading due to the fact
that different wavelengths of light propagate at slightly different velocities through the
fiber because the index of refraction of glass fiber is a wavelength-dependent
quantity; different wavelengths propagate at different velocities. Chromatic dispersion
consists of two parts:
After fiber transmission
40 Gb/s optical signal
Dispersion compensating fiber (DCF)
After dispersion comp.
Longer wavelength Slow (Fast)
Shorter wavelength Fast (Slow)
Longer wavelength Fast (Slow)
Shorter wavelength Slow (Fast)
DISPERSION COMPENSATION EXAMPLE
Large negative dispersion coefficient
Minimal nonlinear contributions
Corrects dispersion slope as well
In order to remove the spreading of the optical or light pulses, the dispersion
compensation is the most important feature required in optical fiber communication
system. The most commonly employed techniques for dispersion compensation
are as follows:
Dispersion Compensating Fiber.
Electronic Dispersion Compensation.
Fiber Bragg Grating.
Dispersion Compensation Fibers: DCF is a loop of fiber having negative dispersion
equal to the dispersion of the transmitting fiber. It can be inserted at either beginning
(pre-compensation techniques) or end (post-compensation techniques) between two
optical amplifiers. But it gives large footprint and insertion losses.
Electronic Dispersion Compensation: Electronic equalization techniques are used in
this method. Since there is direct detection at the receiver, linear distortions in the
optical domain, e.g. chromatic dispersion, are translated into non linear distortions
after optical-to-electrical conversion. It is due to this reason that the concept of
nonlinear cancellation and nonlinear channel modeling is implemented. For this
mainly feed forward equalizer (FFE) and decision feedback equalizers (DFE)
structures are used. EDC slows down the speed of communication since it slows
down the digital to analog conversion
Fiber Bragg Grating: Fiber Bragg Grating (FBG) has recently found a practical
application in compensation of dispersion-broadening in long-haul communication. In
this, Chirped Fiber Grating (CFG) is preferred. CFG is a small all-fiber passive device
with low insertion loss that is compatible with the transmission system and CFG’s
dispersion can be easily adjusted. CFG should be located in-line for optimum results.
This is a preferred technique because of its advantages including small footprint, low
insertion loss, dispersion slope compensation and negligible non-linear effects. But
the architectures using FBG is complex.
Digital filters: Digital filters using Digital Signal Processing (DSP) can be used for
compensating the chromatic dispersion. They provide fixed as well as tunable
dispersion compensation for wavelength division multiplexed system. Popularly
used filter is lossless all-pass optical filters for fiber dispersion compensation, which
can approximate any desired phase response while maintaining a constant, unity
amplitude response. Other filters used for dispersion compensation are band pass
filter, Gaussian filters, Super-Gaussian filters, Butterworth filters and microwave
Various techniques for Dispersion Compensation:
Power budget :The purpose of the power budget is to ensure that enough power will
reach the receiver to maintain reliable performance during the entire system lifetime
PB : PRX > PMIN
PRX = Received Power
PMIN = Minimum Power at a certain BER
PRX = PTX – Total Losses + Total Gain - PMARGIN
PTX = Transmitted Power
PMARGIN ≈ 6 dB
Start with BER and bit rate, determine B based on coding method
B = 1/2RC gives the maximum load resistance R based on B and C
Based on R and M, determine detector sensitivity (NEP), multiply by B1/2
Add system margin, typically 6 dB, to determine necessary power at
Add power penalties, if necessary, for extinction ratio, intensity noise
(includes S/N degradation by amplifiers), timing jitter
Add loss of fiber based on link distance
Include loss contributions from connections and splices
End up with required power of transmitter, or maximum length of fiber for a
given transmitter power
Imagine we want to set up a link operating at 1550 nm with a bit rate of 1 Gb/s using
the RZ format and a BER of 10-9. We want to use a PIN photodiode, which at
this wavelength should be InGaAs. The R0 for the diode is 0.9 A/W.
Bit rate and coding format determine upper limit of rise time
Rise time of transmitter (from manufacturer; laser faster than LED)
Pulse spread due to dispersion
Rise time of receiver (from manufacturer; PIN faster than APD)
Rise time components are combined by taking the square root of sums of squares.
For this example, tMD=0, tTR=100 ps, tRC=0.5 ns, and tGVD= 21.8 ps as before.
tr is therefore 510 ps, and the rise time budget does not meet the limit.
Can use NRZ format
Use faster detector or transmitter
Use graded-index fiber for less dispersion