Free Space Optical Communication System

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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, 1 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 2 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. 3 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 4 Proceedings of the 2011 ASEE North Central & Illinois-Indiana Section Conference Copyright © 2011, American Society for Engineering Education 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 Reciever 10mW Beam

BS1 (50/50%) M1

BS2 5mW M2 BS1

M2 2.5mW BS2 (50/50%)

2.5mW 2.5mW 5 mW

Laser 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 5 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

6 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.

Z C

X θ Y

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.

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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 8 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 9 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 Weather Visibility (km) Range (m)

Dense Fog 0.05 21

Moderate Fog 0.2 82

Light Fog 0.5 206 Rain 1 415 Haze 2 833 Haze 4 1676

Fair 10 4214

Clear 23 9692

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 10 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 Vrms Beam Diameter (m) (mV) (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

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.

Bibliography

[1] "Alexander Graham Bell Inventor of the Telephone." SOLAR NAVIGATOR. Web. 10 Nov. 2010. .

[2] Sinking., Delaying. "Habitat." Oracle ThinkQuest Library. Web. 14 Oct. 2010. .

[3] Majumdar, Arun K., and Jennifer Crider Ricklin. Free-space Laser Communications: Principles a Advances. New York, NY: Springer, 2008. Print.

[4] "Understanding the Importance of Free Space Optics." Journal of Optical Networking 6th ser. 2 (2003). Lightpointe White Paper Series. June 2003. Web. 10 Nov. 2010 .

[5] University of Brussels’ Free Space Communication Project .

[6] Georgia Institute of Technology Free Space Optical Communication Project .

[7] Kim, Issac I., Bruce McArthur, and Eric Korevaar. "Comparison of a Laser Beam Propogating 785 Nm and 1550 Nm in Fog and Haze for Optical Wireless Communications." Optical Access Incorporated. . Web. 10 Nov. 2010.