It helps all engineering student.

1. Submittedby,
ADITYA GHOSH
ARGHAYA GOSWAMI
DEBOJYOTI MUKHERJEE
PRITAM SARDAR
PROHLAD MONDAL
6th SEMESTER
DEPT OF E.C.E
SWAMI VIVEKANANDA INSTITUTE OF SCIENCE & TECHNOLOGY

2. PRESENTATION
LAYOUT:
 Introduction
 Why LASER Communication?
 Why not Fiber Optics?
 Why not Microwave?
 Why Laser Instead of RF?
 Types of Laser
 FEATURES OF LASER COMMUNICATIONS SYSTEM
 Modern era
 Advantage
 Disadvantage
 Conclusion

3. INTRODUCTION
 What is Laser Communication ?
 Laser: A laser is a device that emits light through a
process of optical amplification based on the stimulated
emission of electromagnetic radiation .
 Communication : Exchange the information from one
place to another place .
 Laser Communication : It’s system used as a wireless
connection whose exchange information through laser beam
which is transmitted through free space .

4. Invention of Laser Communication
 From the earliest days of laser development,
researchers realized that light could outperform
radio in terms of information speed and density. It
came down to physics. Light wavelengths are
packed much more tightly than sound waves, and
they transmit more information per second, and
with a stronger signal .

5. •In November 2001 the worlds
first laser data connection was
achieved in space between
the European Space
Agency Artemis satellite.
In November 2014 the first
ever gigabit laser-based
communication was achieved
by ESA and called
the European Data Relay
System (EDRS).

6. Why LASER Communication?
 Current mostly used high speed
technologies:
 Fiber optics
 Microwave

7. Why not Fiber Optics?
 Not always possible to lay fiber lines
 Satellites
 Combat zones
 Physically / Economically not practical
 Emergencies

8. Why not Microwave?
 Beam width : Laser beam
width is narrower than that of
Microwave antennas.
Bandwidth : very much larger
for Laser than microwave.
Security : Laser is more secure
than microwave.
 Power : Low power needed
for laser compared to microwave.
Antenna size for Laser
as compared to microwave.

9. Why Laser Instead of RF?
 Bandwidth
 for Laser Communication (LC) is 100 times greater
than for RF.
 Power
 in LC is directed at target, so much less transmission
power required. Also the power loss is less.
 Size / Weight
 LC antenna is much smaller than RF.
 Security
 Due to low divergence of laser beam, LC is more
secure than RF.

10. Types of laser
 Solid-state lasers
 Gas lasers
 Dye lasers
 Semiconductor lasers
10

11. Solid-state lasers
Fig4: Basic parts of Solid State Laser System
11

12. Gas lasers
12
Fig5: Diagram of He-Ne gas laser

13. Dye laser
13
Fig6: Energy band diagram of dye lasers

14. Semiconductor lasers
14
Fig7: simplified diagram of p-n junction
under no bias condition
Fig8: simplified diagram of p-n
junction under forward bias condition
Fig9: Semiconductor lasers diagram

15. Laser Diode
15
Fig10: block diagram of Laser Diode

16. FEATURES OF LASER
COMMUNICATIONS SYSTEM
 parameters are grouped into five major
categories: link, transmitter, channel,
receiver, and detector parameters.
Laser communication parameters are grouped into five major
categories: link, transmitter, channel, receiver, and detector
parameters.

17. Transmitter Design

18. Receiver Design

19. LINK PARAMETERS
 Types of link:
 Acquisition
○ Acquisition time, false alarm rate, probability
of detection
 Tracking
○ Amount of error induced in the signal circuitry
 Communications
○ Bit error rates

20. TRANSMITTER
PARAMETERS
 Laser characteristics, losses incurred in
the transmit optical path, transmit
antennae gain, transmit pointing losses.
 Laser characteristics
 peak and average optical power
 pulse rate
 pulse width

21. CHANNEL PARAMETERS
 Consists of
 Range, associated loss
 background spectral radiance
 spectral irradiance

22. RECEIVER PARAMETERS
 The receiver parameters are the
 Receiver antenna gain
 Receive optical path loss
 Optical filter bandwidth
 Receiver field of view

23. DETECTOR
PARAMETERS
 The detector parameters are the type of
• detector, gain of detector,
• quantum efficiency,
• heterodyne mixing efficiency,
• noise due to the detector,
• noise due to the following pre amplifier and
angular sensitivity.

24. BEAM ACQUISITION,
TRACKING AND POINTING
 The transmitting
satellite should
transmit the
narrowest possible
beam for
maximum power
concentration.
 The minimal band
width is limited by
the expected error
in pointing the
beam to the
receiver.

25. The Atmospheric Channel: Scattering
• caused when wavelength collides with scattering particle
• no loss of energy, only directional redistribution
• physical size of particle determines type of scattering:
 particle    Rayleigh scattering (symmetric)
 particle    Mie scattering (forward direction)
 particle    extreme forward scattering
Transmittance (scattering + absorption): 





 
z
0o
dzexp
I
I(z)

No smoke
BER 10-8
Weak smoke
BER 10-4
Heavy smoke
BER 10-3
Communication
Transmitter (155Mb/s)
Transmitter

26. The result for the scatter attenuation depends on the visibility, V in Km and the
wavelength  given in m. Visibility V is that distance within which the naked
eye can still recognize larger buildings. If mist or fog is in the atmosphere,
visibility decreases. From the above equation we can generate the following
Table:
Weather Fog Medium Fog
Extreme rain up to
180 mm/h, hail
storm
Haze
Rain with 100 medium rain light to
mm/h, medium to 45 mm/h, medium
snow fall, light fog light snow rain
fall, mist
Clear
Visibility in Km 0.05 0.2 0.5 1 2 4 10 23
Atten.dB/Km @800
nm
345 88 33 16 7.5 3.1 1.05 0.5
Atten,dB/Km@1550
nm
345 87 34 10.5 4.5 2.1 0.4 0.2

27. Basic Free-Space Laser
Communications System

28. Free-Space Laser Communications Link Analysis
Consider a transmitter antenna with gain GT transmitting a total power
PT Watts for a communication range, L.

29. Free-Space Laser Communication Link Equation,
Link Margin and Data Rate
 Received Power
Link equation combines attenuation and
geometrical aspects to calculate the received
optical power as a function of range, telescope
aperture sizes and atmospheric transmissions.
The link equation can be used to generate
power detection curves as a function of range.
Figure shows the calculated received power as
a function of range for the case of a 10 Mbit/s
bandwidth, using a LED operating at 0.85- μm
wavelength, 40 mW power, 13-cm receiver,
atmospheric transmission r3eceiver4 optical
efficiency of 0.2, transmitter divergence angle
of 1 degree =0.0175 radians, and NEP (noise
equivalent power) of the Si detector of 300 nW
for daytime operation.

30. Link Margin
Link margin describes how much margin a given system has at a given range to
compensate for scattering, absorption and turbulence losses. The link margin
is defined as: M = (Received Power Available)/ (Required Received Power)
Required Received power for a given data rate and receiver sensitivity is:
Preq = Nb.r.(hc/λ) where Nb is the receiver sensitivity (Photons/Bit), r is the data rate, h =
Planck’s constant, c = velocity of light
The Margin, M is then given by:
M = PT/[r.(hc/λ) ].(dR
2/θT
2L2)τatm τ TτR.(1/ Nb)
Data Rate
The data rate is given by: r = (PT τatm τ TτR..A)[π(θT/2)2L2.Ep. Nb.] where Ep is the
laser photon energy=hc/ λ.
Example: For a 10 cm telescope, diffraction limited divergence = 14 μrad,
transmitter peak power =200 mW, transmitter efficiency =o.5, receiver
efficiency = 0.5, and using an avalanche photo-detector with sensitivity of 60
photons/bit for 10-8 BER , the Figure shows the data rate as a function of range, L.

32. Table 1. Link Analysis Example of a Satellite-to-Ground Laser
Communication System
Parameter Value/Factor dB
Wavelength ()
Range (L)
Data Rate
Receiver Diameter (D)
Transmitter Divergence
Angle (T)
Transmitter Antenna
Gain (GT = 16/ (T)2
)
Transmitter Optical Loss
Space Loss ( S = (/4L)2
)
Receiver Antenna
Gain ( GR = (D/)2
)
Receiver Optical Loss
SYSTEM LOSS
Atmospheric Turbulence
Margin
Clear Air Transmission
Loss
TOTAL LINK LOSS
LINK MARGIN
DESIGN LOSS
Required Received Signal
at 3 Gbps
Required Laser Power at 3
Gbps = Required received
signal – Design Loss
0.635 micrometer
4.83 x 105
meter
3 Gbps
1.4 meter
2.07 x 10-4
radians
3.73 x 108
0.1
1.09 x 10-26
47.974 x 1012
0.1
9.36 x 10-8
Watt
4.14 Watt (= 10 6.17/10
)
+85.72
-10.0
-259.61
+136.81
-10.0
-57.08
-11.30
-2.08
-70.46
-6.00
-76.46
-70.29 (=10 log10 9.36x10-8
)
-70.29+76.46 = 6.17

33. RELIABILITY OF LASER COMMUNICATION LINKS
 Consider the link power budget. It includes all average losses of optical
power P [dBm], which arise between the laser source and the receiving
photo-detector.
 Pt [dBm] = transmitter power, Prec [dBm] = received power, P0 [dBm]
= receiver sensitivity and Lp [dBm] = propagation loss. LM is an initial link
parameter that serves to express the reliability of the lasercom system.
 LM = Pt - Lp - P0
 The link availability is a percentage of time Tav[%], when the data
transmission bit error rate is less than its defined value. The link availability
can be expressed as by a probability that additional optical power losses LA
[dB] caused by atmospheric effects are less than link margin LM. The
attenuation of radiation in the atmosphere has a dominant share among all
losses.
 The link availability can be expressed by means of a probability density
p(A) of an attenuation coefficient A [dB/km] from the following equation:
 where A is the limiting attenuation coefficient value, which is given by
 A = [LM(D)/D].1000, D being the range.
 
A
av AdApT


0
)()(%100

34. One of the possible ways to determine the distribution of p(A) is based on long-time
monitoring of of a received signal level of a real measuring link. Another way consists in
utilizing data that was collected in the past. Visibility V[km] is the quantity to be monitored
and it serves to determine the attenuation coefficient.
Statistical distribution functions F(A< ) can be created, which represents statistical link
models. The values of the above integral can be determined from these functions for given
limiting attenuation coefficients. An example of statistical link model is shown in the following
figure.
Note that for two limiting attenuation coefficient values A= 21 dB/km, and A= 8 dB/km, the
corresponding link availabilities are Tav = 93% and Tav = 91% .

35. PROBABILITY DENSITY FUNCTIONS OF IRRADIANCE
FLUCTUATIONS
Scintillation can lead to power losses at the receiver: eventually can cause
fading of the received signal below a prescribed threshold value. Therefore
we need to know the form of the PDF to evaluate lasercom system
performance.
Some of the PDFs:
Lognormal distribution:
,
),(2
),(
2
1
),(
ln
exp
2),(
1
)( 2
2
2




























Lr
Lr
LrI
I
LrI
Ip
I
I
I 


I>0 (nonnegative
irradiance)
K Distribution: ),2()(
)(
2
)( 1
2/)1(
IKIIp 







 I > 0
Lognormal-Rician Distribution:
z
r
er
Ip
2
)1(
)(























 

0
22
22
0 ,
2
)
2
1
(ln
)1(
exp
)1(
2
z
dz
z
z
Ir
z
rIr
I
z
z


I > 0
Gamma-Gamma Distribution: dxxpxIpIp xy )()()(
0



=  ,2
)()(
)(2 1
2/)(
2/)(
IKI 










I > 0

36. The Probability of Error, Bit Error Rate (BER)
pI(s) = probability distribution of irradiance
Is= instantaneous signal current with mean
value
<Ps> = mean signal value
<SNR> is the mean SNR in presence of
turbulence

37. Effect of Atmospheric Turbulence on Bit
Error Rate
• Atmospheric turbulence significantly impacts BER
• Even with aperture averaging, reduction in BER is several orders of magnitude
• As atmospheric turbulence strength and path lengths increase, so does the BER
Weak turbulence:
PCB reduces BER by 3 orders of magnitude
Moderate turbulence:
PCB reduces BER by only 1 order of magnitude
-50.0 -45.5 -41.0 -36.5 -32.0 -27.5 -23.0 -18.5 -14.0
Receiver Power (dBm)
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
100
BitErrorRate
L = 1000 m
D = 4 cm, Cn2 = 10e-14
D = 8 cm, Cn2 = 10e-14
D = 4 cm, Cn2 = 5x10e-14
D = 8 cm, Cn2 = 5x10e-14
no turbulence
-50.0 -45.5 -41.0 -36.5 -32.0 -27.5 -23.0 -18.5 -14.0
Receiver Power (dBm)
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
100
BitErrorRate
L = 2000 m
D = 4 cm, Cn2 = 10e-14
D = 8 cm, Cn2 = 10e-14
D = 4 cm, Cn2 = 5x10e-14
D = 8 cm, Cn2 = 5x10e-14
no turbulence

38. Uplink Slant Path Laser Communication
Link
Note that the atmospheric model for Cn
2
is to be taken from Hufnagel-Valley (H-
V) model, described earlier. This model
shows the variation of Cn
2
as a function
of height taking into account of the
zenith angle. The probability of fade for
an uplink spherical wave to a geo-
stationary satellite under various
atmospheric conditions is shown in the
following figure.
Downlink Slant Path Laser Communication
Link
The plane wave model can be used to
calculate the irradiance variance and then
probability of fade. The figure shows the
probability of fade for a downlink path
from a satellite in geo-stationary orbit.

39. Probability of Fade for Uplink and Downlink

40. Modern era
 Defense and sensitive areas
• Laser Range Finder
• Underwater Laser
• Laser Radar (Lidar)
Satellite – satellite communication
• Telephony
• Television and radio
• Mobile Satellite technology

41. Laser Range Finder
 The laser range finder works on the principle
of a radar & use to knock down an enemy
tank.

42. Underwater Laser
 Lasers can also be used as a source of
underwater transmission. At present, the
submarines have to rely on a sonar to find the
enemy crafts and to avoid the underwater
objects.

43. Laser Radar (Lidar)
 Besides, the laser beam can be focused
with lenses an mirrors easily whereas
microwaves need huge antenna for
focusing.
 The great advantage of the use of
carbon dioxide lasers for radar
application is their capacity to produce
high power output with requisite The
spectral purity.

44. Satellite to satellite
communication
Satellite to satellite communications
are comprised of 2 main
components:
 The Satellite :The satellite itself is
also known as the space segment,
and is composed of three separate
units, namely the fuel system, the
satellite and telemetry controls,
and the transponder.
 The Ground Station : This is the
earth segment. The ground
station's job is two-fold. In the case
of an uplink, or transmitting station,
terrestrial data in the form of
baseband signals, is passed
through a baseband processor, an
up converter, a high powered
amplifier, and through a parabolic
dish antenna up to an orbiting
satellite.

45. Telephony
 The first and still, arguably, most
important application for communication
satellites is in international telephony .

46. Television and Radio
There are two types of satellites used for
television and radio:
 Direct Broadcast Satellite (DBS)
 Fixed Service Satellite (FSS)

47. Mobile satellite technology
Initially available for broadcast to
stationary TV receivers, popular mobile
direct broadcast applications made their
appearance with that arrival of two
satellite radio systems : Sirius and XM
Satellite Radio Holdings. Some
manufacturers have also introduced
special antennas for mobile reception of
DBS television.

48. Advantages of Laser
Communication
 Less frequency restrictions
 Smaller aperture dimensions and thus reduced size and
mass
 Autonomous alignment agility resulting in less platform
man oeuvres .

49. Disadvantages of Laser
Communication
 For terrestrial applications, the principal limiting factors are:
 Beam dispersion
 Atmospheric absorption
 Rain
 Fog (10..~100 dB/km attenuation)
 Snow
 Scintillation
 Interference from background light sources (including the
Sun)
 Shadowing
 Pointing stability in wind

50. IMPROVEMENT OF LASER
COMMUNICATIONS
PERFORMANCE
 Still need more accurate theory of laser
propagation through atmospheric
turbulence , validity of the stationery
process to be investigated existence of
non-stationery turbulence in some
regime of propagation path .
 Multiple arrys of detectors can mitigate
some fluctuation effects due to
turbulance –how many and where do we
place them ?

51. Conclusion
 In spite of the fact that the dream of the
communication engineers to have a multimillion
channels operation on a single laser beam is not
realizable in the near future , firstly , because of the
poor characteristics of propagation in atmosphere .
 One thing is certain , the tremendous potential of the
laser as the high density communications system
has not yet materialized . A great deal of work has
been done and many obstacles have been overcome
, but the basic problem of devising a suitable
transmission technique is a dilemma awaiting a
solution .