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Proceedings of the 2011 ASEE North Central & Illinois-Indiana Section Conference Copyright © 2011, American Society for Engineering Education
Free Space Optical Communication System
Kasey Hixson, Paul Trader, Chris Romanowski, Adam Mock
College of Engineering
Central Michigan University
Mount Pleasant, MI 48859
Email: [email protected] and [email protected]
Abstract
Using light to transmit audible information from one point to another was first
demonstrated by Alexander Graham Bell in 1880 with the “photophone.” [1]
However, light
based telephony is not seen in present day products. On the other hand, the use of light confined
to thin glass fibers to transmit information has revolutionized long distance communication
(>100mi). In this electrical engineering senior design project, “free space” light-based
communication is the focus. The main goal is to design and create a system involving a visible
laser and a detector in order to transmit information through “free space” (vacuum, air, or water)
without the use of a glass fiber. Novel aspects of this Free Space Optical Communication
System (FSOC) include tracking capabilities and additional pathways for the channel. The final
design will be a versatile communication system for the transmission of both analog and digital
data, such as audio and video signals respectfully. In addition to the general novelty of
transmitting data through free space with the use of a laser, this project has several important
applications including under water equipment communication, secure channel military systems
that are non-weapon related, and outer space communication networks.
Introduction
As a society we are constantly in the need for an increase in communication bandwidth
for applications such as high definition music and video and real-time video conferencing.
Lightwave communication systems have enormous bandwidth due to their relatively large carrier
frequencies (100 THz). Though there is a finite limitation to the amount of bandwidth available
for communication, FSOC has a very high bandwidth that is not being utilized on a large scale at
this time. An important application for FSOC is rapid communication system deployment in
times of disaster. In most natural disaster situations, it takes a long time to re-set communication
for businesses and relief organizations. Free space systems would allow for very quick response
and set up times for these problems. Another application for laser communication channels is
communication between underwater equipment such as submarines where, unlike radio
frequency (RF) communication, lasers are more easily transmitted under water. [2]
The security
of free space systems, due to the necessity of a direct path between transmitter and receiver,
make them very attractive for military use. A command center would be able to send secure
information to distant vehicles/soldiers as long as they were in sight. Since the communication
channel must be a line-of-sight path between the transmitter and receiver, the channel will
remain secure as long as it is visually checked for any type of interception technology. It should
be noted that the military applications of this FSOC system are not weapons related. Finally,
FSOC has great possibilities for communication in space due to the lack of atmospheric
absorption. [3]
The possibility for high bandwidth communication between planets, spacecraft,
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Proceedings of the 2011 ASEE North Central & Illinois-Indiana Section Conference Copyright © 2011, American Society for Engineering Education
and satellites may be a lot more feasible with free space communication. These are a few
applications of the many more that are sure to be defined as the technology becomes more
refined and complex. Along with various applications, there is no need to bury fiber optic or
other forms of cables for transmission. This makes FSOC more environmental friendly than
traditional forms of communication in that there would be no destruction of habitats, while
requiring less materials and space for instillation.
It is important to investigate the various implications of the project on society and the
world in general. Specifically, our project touches on a few socio-political concerns. The first
concern of our product is the ethical concern of product safety. The device used for
communication in our project is a high-powered laser. It has the possibility of burning the
corneas or retinas due to the sensitivity to light of the eyes. [4]
It is also possible to burn skin
with a laser of power over 1 W. Clearly, these dangers need to be accounted for in any product
design that could be for public use. The specific wavelength of the laser is important in that only
certain wavelengths (between [0.4-1.4] micrometers) are seen by the human eye. Eye response
and skin response both drastically decrease with longer wavelengths (>1.4 micrometers) and,
therefore, would optimize this project‟s product safety. Budget limitations do not allow for a
laser wavelength that is invisible to the human eye, but if our product was commercially
developed, a laser wavelength above 1.4 micrometers would be used to ensure safety. Knowing
that this product may be harmful to others‟ eyes, we will choose to work and test our product in a
closed environment where all persons are aware of the product, its dangers, and the proper safety
measures to follow.
Goals and Customer Needs
Potential customers can be extrapolated from the possible applications of this project.
Clearly the US government/other governments would be interested in FSOC because of the
applications in regards to space and safe/secure channels. For this reason NASA and U.S.
military organizations like the Army and Navy are possible customers. In addition to military
and space organizations, relief organizations such as the Red Cross could also be possible
customers. Setting up infrastructure in disaster zones is of high importance and a free space
design would allow for a high bandwidth communication system to be implemented quickly.
The projects‟ customer needs along with motivation for a more advanced system when compared
to other university free space projects ultimately shaped the FSOC goals as follows:
1. Create the communication channel
2. Demonstrate analog communication
3. Demonstrate digital communication
4. Characterize the communication link's performance (bandwidth, robustness in
fog, rain, performance vs. distance between point A and B, etc)
5. Create a targeting system, so that the free space optical communication system
can track a moving detector and maintain the link
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Proceedings of the 2011 ASEE North Central & Illinois-Indiana Section Conference Copyright © 2011, American Society for Engineering Education
6. Implement path redundancy or other mechanisms to maintain the link even if
line of sight is blocked
Previous free space communication projects from universities such as Georgia Institute of
Technology and the University of Brussels both successfully transmitted data through free space
using a laser or LED. [5,6]
With goals like number five and six as listed above, this project sets
itself apart from other previous work. The advancements of multiple paths and a tracking system
to track a moving detector, evolves this system into a more technical, useful, and applicable
product to customers and society as a whole.
Transmitter
To transmit an analog signal all that is necessary is to connect the analog signal and the
power supply of the laser together using an adder circuit as seen in Figure 1. The adder circuit
uses three 1Kohm resistors to add the two voltages together. With an off the shelf laser pointer
operating at 4.5 volts, no signal amplification needs to be done for this method to work. The
output of a basic CD player is around 0.5 volts. This method of modulation results from the
linear relationship between the input voltage to the laser and output power of the laser. Visually
this can be seen using a laser pointer and a DC voltage source. With the laser pointer connected
to the power supply, as the voltage increases past a threshold amount (for a laser pointer this
value is about two volts) the intensity of the laser also increases. With this in mind, if the input
voltage to the laser is set a little below the operating voltage (4.5V for the pointer) then the
output voltage of a music signal can be added to the laser voltage. This modulation of the laser
input voltage will then modulate the output power of the laser.
Figure 1: FSOC Transmission/Detection Schematic
The development of a digital transmission system is fairly similar to the analog version.
To transmit a digital signal, the signal simply needs to be put in an adder circuit with the input
voltage to the laser. This design requires a constant voltage set at or just under the threshold
voltage for the laser being used. The digital signal will then be added to this to turn the laser on.
For example, a laser pointer has a threshold voltage at around 2V and an operating voltage
around 4.5V. If this were the case, the constant input voltage would be set at just under 2V.
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Proceedings of the 2011 ASEE North Central & Illinois-Indiana Section Conference Copyright © 2011, American Society for Engineering Education
This means that the digital signal would need to add an additional 2.5V to get a „high‟ signal and
0V to get a „low‟ signal.
With this in mind, the design of digital transmission circuit is almost complete. The
difference between analog and digital transmission lies within the need for an analog-to-digital
converter (ADC). An analog message signal will be input to an ADC. The output of the ADC
will be inputted to the adder circuit with the laser's power supply. The ADC converts the
continuous analog signal into discritized samples. It will encode the analog information into bits
of digital “high” and “low” data. The signal will need to be amplified so that the laser will be in
its operation range at “high”. Using digital communication allows for more channels of data to
be sent with one system, allowing a much larger range of communication information to be sent
over the channel. Digital transmission is very important in that computer data could also be sent
over the channel. All of the applications and customers of FSOC have motive to transmit
computer signals and therefore digital transmission is significant.
Receiver
The receiver design includes a photo detector, a signal amplification circuit, and an
output device (speaker, computer, etc). A photo resistor and its own input voltage will be used
as the detector (see Figure 1). The input voltage is needed for the photo resistor because as the
intensity of light increases, the resistance across the device decreases. This means that the photo
resistor needs to be in series with a resistor to create a voltage divider circuit. As the resistance
of the photo resistor changes, the voltage across it will also change. The changing resistance due
to the variable light intensity modulates the voltage source and creates an output voltage signal.
Photo resistors are inherently very small and have minimal surface area (see Figure 2). In
order to have a more stable and accurate receiver system, the surface area of the photo resistor
needs to be significantly increased. The larger size of the photo resistor allows the system to
maintain the communication link in times of laser/detector movement. In order to accomplish
this increased surface area and more efficient receiver system, a parallel connection of large
photo resistors will be implemented (see Figure 3). This allows for the laser to move across
many photo resistors, all the while keeping the communication channel intact.
0.70cm 10.0cm
Figure 2: Traditional Photo Resistor Figure 3: Parallel Photo Resistor Array
As with the digital transmission system, the digital reception is similar to its analog
counterpart. All that is needed for the digital receiver is a digital-to-analog converter (DAC) in
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Proceedings of the 2011 ASEE North Central & Illinois-Indiana Section Conference Copyright © 2011, American Society for Engineering Education
M1
series with any analog output component such as a speaker. The DAC converts the “high” and
“low” coded bit information back into the continuous analog signal, such as an audio signal, in
order to be properly communicated. As with the analog receiver, the digital receiver‟s photo-
detector will also be in a voltage divider configuration with a standard resistor and a power
supply.
Path Redundancy
Path redundancy is examined in order to maintain the communication link when the
direct line of sight is blocked. The most practical and achievable form of path redundancy
comes from the use of beam splitters. A beam splitter is an optical device that can split an
incident light beam into two or more beams. Part of the beam is transmitted and the other part is
reflected, allowing for a solution in the case of a blocked line of sight. The performance of beam
splitters is dependent on the coating specifications and for this project a 50/50%
transmission/reflection beam splitter would optimize path redundancy. The conceptual solution
to the case of a blocked line of sight can be seen in Figure 4.
Laser
10mW Beam
BS1 (50/50%) M1
5mW
M2 2.5mW BS2 (50/50%)
2.5mW 2.5mW 5 mW
Receiver
Figure 4a: Path Redundancy Concept Figure 4b: Path Redundancy Implemented
[Beam Splitter (BS) and Mirror (M)]
The beam splitters create three different transmission beams, all of which carry the
original data-information, however, one beam has half the power of the original signal and the
Laser
Reciever
M2
BS1
BS2
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Proceedings of the 2011 ASEE North Central & Illinois-Indiana Section Conference Copyright © 2011, American Society for Engineering Education
other two beams have a quarter of the original signal. Using this system, path redundancy is
achieved as the detector is aligned to all three beams. When the line of sight is blocked from one
beam, there are two other beams that are able to maintain the communication channel. This is a
good design, in that the communication channel is still upheld through a maximum of two
blocked paths. It should be noted that this application of beam splitters has limitations in regards
to a moving detector. The beam splitter system shown in Figure 4 applies specifically to a
communication channel with a non-moving receiver. This system would be particularly useful
for applications such as building to building communication where a tracking system is not
needed, yet any one direct path can be obscured by an object.
Tracking
With the transmission and reception systems in place, the tracking system becomes the
cornerstone of the design. Though FSOC systems have already been designed, implemented,
and are commercially available, the tracking system is unique to this design and allows for
secure communication between a moving transmitter and receiver. In particular, this design is
ideal for secure non-weapons military operations. A reception system placed on a backpack
would allow for secure communication between two separate moving groups of soldiers in the
field (see Figure 5). The direct line of sight needed for communication provides security of the
information because anything that could intercept the communication would be seen and the
communication link could be severed. The secure line would also allow for any type of data to
be sent between the two groups (audio, video, text). In addition to use between groups of people,
the tracking system could also be used for secure communication between people/vehicles on the
ground and aircraft. Clearly, the tracking system is an important aspect of the design and allows
for some fairly exciting and important results.
(Safe and Distant Transmitter Location)
Figure 5: Secure Military Communication
The tracking system will be based around image processing software and a small
computer camera. The complete system will have three main components including two
servomotors, a digital camera, and a microcontroller. The transmission base will have the
camera, laser, and servo motors connected on one base (see Figure 6). Acting as a turret, the two
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Proceedings of the 2011 ASEE North Central & Illinois-Indiana Section Conference Copyright © 2011, American Society for Engineering Education
servomotors allow the transmitter to move in 2 dimensions with 2 degrees of freedom, which
covers all the possibilities of motion for the receiver. The top servomotor allows camera/laser
lever arm to move along an arc path at various phi (Ø) angles (-90 degrees to 90 degrees), which
ultimately controls the vertical alignment of the laser/camera. The bottom servomotor essentially
controls the left and right positioning of the laser/camera by moving along theta (θ) angles (0
degrees to 360 degrees). The webcam is used to capture the motion of the detector. An image-
processing program is then used to calculate the X and Y coordinates of the center of the
reception array. Finally, the microcontroller is used to interface between the computer image-
processing software and the servomotors. Since the laser is directed at the middle of the
camera‟s view, the laser will in turn always be on the receiver.
Figure 6: Transmission Base with Tracking Capabilities
The image processing software Roborealm will be used to track the position of the photo-
resistor-array. The software allows for a specific color and geometry to be tracked by the
camera. Knowing this, the FSOC tracking system will be constructed with an orange square
frame to enclose the array (see Figure 5). The software and microcontroller interface can then
track the position of the orange square and properly move the transmission base to always have
the laser in contact with the receiver. Figure 7 shows the flow chart for the tracking of a moving
receiver target.
Z
C
Y X
Ø
θ
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Proceedings of the 2011 ASEE North Central & Illinois-Indiana Section Conference Copyright © 2011, American Society for Engineering Education
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Figure 7: Tracking System Flow Chart
In the process of developing the concepts for the tracking system, the physical limitations
became a matter of concern. Specifically, every tracking system has limitations in the response
time between the motion of the receiver and the motion of the laser. Additionally, high accuracy
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Proceedings of the 2011 ASEE North Central & Illinois-Indiana Section Conference Copyright © 2011, American Society for Engineering Education
is also needed to maintain a stable communication link. A simple solution is to increase the area
of reception on the receiver with the use of a photo resistor array. The proposed solution
decreases the amount of accuracy needed because the laser‟s target would be larger. In addition
to these problems, the small computer mounted camera used in this project limits the tracking
abilities to a short range. The range could be increased with an advanced camera and with
optical lenses, however these advancements are too expensive for the budget of the project and
therefore could be implemented in future designs.
Characterization of System Performance
The task of characterizing the project‟s performance will be an ongoing procedure
throughout the duration of constructing the FSOC system. The goal is to provide as much data
as possible on the final product‟s performance such as bandwidth, bit-rate, performance versus
distance, and robustness in fog, rain, and other weather conditions. One parameter that is of
significant interest within this project is the distance of the communication channel. This is a
parameter that can be estimated for the project‟s concept using the Beer-Lambert law, and Kim‟s
visibility equation [Equations (1) and (2), respectively] that defines the attenuation coefficient. [7]
Specifically, the Beer-Lambert Law allows one to obtain the range of the communication
channel as a function of the attenuation. The attenuation coefficient is a result of the molecular
absorption, molecular scattering, and particle scattering of the medium in which the laser is
traveling. [7]
This results in the energy loss of a laser beam and is of particular importance to the
performance and distance of the system. The power of the laser will be 100mW in order to
accomplish the goals set for the FSOC system, while keeping cost at feasible value. Once the
final product‟s receiver power is known, the attenuation coefficient is the only parameter needed
to solve for the range in the Beer-Lambert law.
Beer Lambert
……………………………………………………………………………..(1)
τ(R) = Transmittance at range R
P(R) = Laser power at R
P(0) = Laser power at source
σ = attenuation
Kim‟s Visibility
……………………………………………………………………………(2)
σ = atmospheric attenuation
V = visibility in Kilometers
λ = wavelength in nanometers
q = size distribution of the scattering particl
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Proceedings of the 2011 ASEE North Central & Illinois-Indiana Section Conference Copyright © 2011, American Society for Engineering Education
This is where Kim‟s visibility equation comes into play, under which the attenuation
coefficient can be found as a function of visibility and wavelength. The laser wavelength was
chosen to be 490 nm-560 nm (green) based on the application of underwater communication.
Green wavelengths have a very deep transmission through water when compared to other
wavelengths such as red. [2]
Once the wavelength is known (in our case approximately 560nm)
the equation for the attenuation confection becomes a function of visibility alone. Visibility is
defined as the atmospheric distance for a transmission of 2%. The value of visibility is
completely dependent on the weather conditions and can be found on any local weather channel.
Knowing the power ratio, wavelength, and visibility, the range of a communication
channel can then be found using the Beer-Lambert Law. Using a wavelength of 560nm, various
visibility values, and different weather conditions, the expected attenuation coefficient and
communication range of the project was calculated using Kim‟s visibility equation and the Beer
Lambert Law respectively (see Table 1). As visibility decreases due to poor weather conditions,
it causes the attenuation coefficient to increase and the communication range to decrease. It
should be noted that the power ratio of the receiver power to that of the transmitted laser power
was selected to be (1/5) although it has yet to be characterized. From the Beer Lambert Law it
can be seen that the power ratio drastically affects the range of communication in that the lower
the ratio, the higher the channel range. This quantitative analysis of communication range is
meant to later be compared with the actual results after tests have been conducted.
Table 1: Visibility vs. Communication Range
The project‟s analog transmission range was characterized from 1m to 35m using a 5mW
red laser pointer and an audio signal under safe and closed building conditions. It should be
noted that the communication channel‟s maximum range was not reached in this experiment.
Table 2 and Figure 8 show the relationship of the received audio signal‟s Vrms and channel
length. The ideal constant relationship was kept up to 25 meters. Beyond this distance, the
Visibility (km)
Range (m)
0.05 21
0.2 82
0.5 206
1 415
2 833
4 1676
10 4214
23 9692
Weather
Dense Fog
Moderate Fog
Light Fog
Rain
Haze
Haze
Fair
Clear
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Proceedings of the 2011 ASEE North Central & Illinois-Indiana Section Conference Copyright © 2011, American Society for Engineering Education
Vrms drops slightly due to the optical distance of the laser beam. As Table 2 shows, the
diameter of the beam increased by a factor of five from 1m to 35m. The increasing beam
diameter can be controlled using lenses to refocus the beam. The increasing beam diameter also
accounts for a majority of the drop in Vrms as opposed to solely atmospheric absorption. This is
because the beam was less concentrated on the receiver which led to loss in signal information.
Even with the loss factors associated with an increasing beam diameter and low laser power, the
sound quality of the receiver was constant over the whole range. A future test will be conducted
with the ideal 100mW green laser in order to obtain the maximum transmission range, which is
sure to cover a much larger distance. The final distance characterization will include the
system‟s performance over various weather conditions.
Table 2: Communication Range Characterization
Distance
(m)
Vrms
(mV)
Beam Diameter
(mm)
1 250 6.35
10 250 11.43
20 250 25.40
25 240 25.40
30 200 31.75
35 170 31.75
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Proceedings of the 2011 ASEE North Central & Illinois-Indiana Section Conference Copyright © 2011, American Society for Engineering Education
Figure 8: Distance vs Received Signal‟s Vrms
Conclusion
The overall FSOC system is composed of the transmitter, receiver, tracking system, and
path redundancy components. The signal quality of the output will constantly be characterized,
debugged, and fine tuned. The interfacing between the microcontroller and image processing
software may be the most complex system within the FSOC. After communication system‟s
tracking abilities have been completed, it will then be possible to add on to the design.
Specifically, the next phase of the project is to implement frequency division multiplexing
circuitry to allow for multiple signals to be sent at the same time. This would allow for audio,
video, and data signals to be sent with the same laser beam at the same time. Complexity can
also be added to the tracking and path redundancy systems. With more microcontrollers it would
be possible to combine the path redundancy system and the tracking system. Controlling the
angles of the mirrors and beam splitters with servomotors would allow for a tracking system with
multiple transmission paths. Additional modifications could also be added to the tracking system
to increase the security of the communication channel. An array of additional lasers could be
added around the data-carrying-laser-beam in order to act as a detection halo. If any of the lasers
surrounding the communication laser are blocked, then the system could shut off to avoid any
security breaches of the data. As fiber optic cable revolutionized the communication system in
the past and present, free space optical communication systems will continue to do so in the
future. Society‟s desire for an enlarged bandwidth will surely increase; FSOC systems will be
there to help satisfy the demand. Overall, this project has many possible customers, significant
applications, and a promising future to help advance society‟s communication possibilities.
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Proceedings of the 2011 ASEE North Central & Illinois-Indiana Section Conference Copyright © 2011, American Society for Engineering Education
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