Free Space Optics (FSO)Naveen Krishnan H

Reg No: 00604561

S7 CSE, MG College of Engineeringnaveenkrishnanh@gmail.com

Abstract— Free Space Optics or Free Space Photonics is a line of sight transmission in which modulated visible or infrared beams are transmitted through the atmosphere to obtain optical communications. Here the transmission uses lasers to transmit data, but instead of enclosing the data stream within a glass fibre, it is transmitted through the atmosphere. The transmission can be done with invisible or visible beams of light. The data rates up to 1.25Gbps can be achieved by this. Optical wireless also eliminates the need to buy expensive spectrum for radio frequency solutions.

I. INTRODUCTION

Free Space Optics may sound like a new buzz-word-enabled technology that seems to have very outer-space connotations, but in reality it is an old technology that was originally developed by the military over 30 years ago. In technical terms, Free Space Optics is an optical wireless, point-to-point, line-of-sight high bandwidth broadband solution with data rates ranging from 1 Mbps to over 2.5 Gbps that provides the best solution to the “lastmile” needs of bandwidth hungry applications created due to the convergence of telecommunications and data communications. What makes this technology so special is the compelling economic advantages and the relative speed and ease of deployment when compared to typical fibre or copper connectivity.

Most people think that optical communication is only possible through a fiber. However, light can be made to travel through air as its medium for a lot less money than with typical fiber optic deployments.

Fig. 1 Example of a network employed using FSO

II. HISTORY

Optical communications, in various forms, have been used for thousands of years. The Ancient Greeks polished their shields to send signals during battle. Later on a wireless solar telegraph called heliograph was developed, that signals using Morse code flashes of sunlight.

The fundamental concept of FSO was experimented by Alexander Graham Bell himself, through his invention the

‘photophone’ in 1880s. In 1880, four years after having invented the telephone, Alexander Graham Bell made his first wireless optical communication in which the rays of the sun replaced electric wire.

The basic idea is to focus sunlight onto a flexible reflective membrane. The user speaks into this membrane. The spoken word is transmitted in the air by modulating the reflected sun rays. This modulated reflected light, after its displacement in the air, is collected by a photoconductive selenium cell connected to a pile and the ear-phones.

In 1960s it was taken up by the military with the help of lasers. But it was much later that this technology was tried as a method for implementing commercially.

Fig. 2 Photophone in action

Fig 3 FSO implemented to transmit voice

III. FEATURES

The main features of FSO are:-

A. Still a Relatively New Technology

The FSO even though uses the same basic idea devised by Graham bell in 1880s; implementation of FSO using the concept of lasers is still quite new. And the usage of FSO as a commercial network solution is very new.

B. Is a Line of Sight Technology

FSO is implemented where the sender can ‘see’ the receiver. That is there should be a direct visible path between the sender and receiver. Also FSO is for short range communications. The range of FSO networks are within 3 to 4km between each link. If we want to connect nodes farther than that, we would have to ‘bounce’ it off in an intermediate position.

C. Offers Full Duplex Gigabit Ethernet Through-put

Gigabit Ethernets are as the name suggests Ethernet connections in which the transfer rate is in the rate of Gigabits per second. In other words Gigabit Ethernet networks can transmit more than 1 Giga bit of data in one second. FSO can provide speeds even greater than 1.25 Gbps. That is it can be used to implement gigabit Ethernet networks. Even though Gigabit Ethernets can be implemented FSO are also used to implement cheaper Fast Ethernet connections with speeds of 166Mbps. This is done through LEDs and is thus far cheaper than the laser versions.

D. Can be Implemented With Cost-Efficient LEDs

As mentioned before we can use cheaper LEDs in place of laser as well. This is done if the required network contains only short-distant links. At which case we can implement FSO network using cost efficient LEDs instead of costly lasers.

The performance is similar between LEDs and LASERs when used for short-distance networks. But as the distance of the links increase, LASER becomes far superior.

E. Is Perfectly Eye-Safe

Infrared radiations of wavelength 850nm and above are used here. They tends not to reach retina but are absorbed by cornea. Thus FSO is perfectly eye safe. If more power is required, lasers of greater wavelength can be used (usually 1550nm) and still remain eye-safe. Thus FSO connections pose little physical harm. That is here class1 lasers are used and thus pose no physical harm.

TABLE ILASER CLASSIFICATIONS

IV. COMPONENTS

FSO does not require added wirings. It just requires the placement of the Transceivers. The transceivers can both send and receive messages and can thus function in full-duplex mode. Basic transceiver working is shown in Fig.3. The red beam is the transmitted beam, while yellow is the received beam. Received beam is recognized by the detector present in the transceiver.

Fig. 4 Basic diagram of a Light Pointe transceiver

V. APPLICATIONS

Free Space Optical Communications have many advantages. They include:-

A. To Implement Wireless Gigabit Ethernet

FSO is a cost effective and quick way of implementing gigabit Ethernet connections. Here there are a lot of restrictions like, there should be a visible link between nodes, and also the atmospheric constraints should be in an acceptable limit.

The main ways of implementing an FSO network (the different topologies) are:-

1) Point-to-Point:

Here a dedicated connection is provided between two computers or nodes. The connection speed between 155Mbps

and 2.5Gbps can be realized. Many companies like Air Fiber and Terabeam provide with even more speed. The range here is about 4km.

Fig. 5 Point-to-Point FSO network diagram.

2) Mesh:

Mesh architectures may offer redundancy and higher reliability with easy node addition but restrict distances more than the other options.

Fig. 6 Mesh FSO network diagram.

3) Point-to-Multipoint:

Point-to-multipoint architecture offers cheaper connections and facilitates node addition but at the expense of lower bandwidth than the point-to-point option. This provides the same speed as of point-to-point links.

Fig. 7 Point-to-multipoint FSO network diagram.

B. Communication Between Space Shuttles and Satellite Constellations

In the mid-1960's NASA initiated experiments to utilize the laser as a means of communication between the Goddard Space Flight Center and the Gemini-7 orbiting space capsule. Thus FSO was and still is a preferred method of implementing communications between Satellite constellations. FSO here is not degraded in performance due to the lack of atmosphere and can provide maximum efficiency.

Fig. 8 FSO link between two satellites

C. In Classified Sites

In military and other sites where there is a high demand of security. Wireless radio frequency networks are thus prone to intruder attacks. Also the wired links can also be tampered. But FSO links are narrow and are very hard if not impossible to ‘tap’ into. Thus FSO in itself without any need for any encryption technique is already very secure.

D. In Special Cases

When there are obstacles like motorways, canals etc between the network stations, we can implement FSO easily. In these cases laying optical fibers are either too costly or cannot be physically achieved.

Another Special Case is when there are stricter regulations in using Radio Frequencies like sites near an airport. Here we can implement FSO.

E. When There is a Saturation of Radio Operator Links

If we want to use Radio Frequencies to implement wireless networks, first we want to buy a particular band of frequencies in-order to avoid cross-talk and noise. But if there are already a lot of RF networks there, then it will be difficult to get a RF frequency. FSO can step-in such cases.

F. For Temporary Network Installations

FSO network installation is very simple and quick. It just requires setting the Transceivers . There is no need for laying the cable in the case of wired communication. In the case of wireless RF networks, there is no need for applying for the use a radio frequency.

Also once implemented, FSO networks can also be dismantled just as quickly as it is implemented. Thus FSO is the suited network solution when the work place tends to shift periodically.

G. For Quick Restoration of Networks

FSO networks are preferred networks in emergencies like earthquakes. In the event of earthquakes, there are usually smaller disturbances known as aftershocks. So using terrestrial cables is not an intelligent option as they may get damaged

due to these aftershocks. RF links are affected by radio-magnetic waves and thus they too are not preferred.

Another reason why FSO is chosen is when a new link needs to be setup. At which case FSO networks can be setup quickly.

VI. ADVANTAGES

A. Ease of Implementation

As mentioned before FSO links can be setup quickly and also dismantled just as quickly. There is no need for acquiring any legal license at present to implement an FSO network. Thus whole setup process is easy and fast.

Fig. 8 FSO Transceiver placed inside an office

B. Full-Duplex Support

FSO transmitter is equipped with a photo-detector as well. Thus they are capable of transmitting as well as receiving information. Thus by setting up an FSO network, we are implementing a potential Full-duplex network as we can send and receive messages at the same time.

C. High Bitrates and Low Error Rates

FSO provides gigabit Ethernet connections. Here the transmission is optic and thus it is not affected by crosstalk, electromagnetic radiations (not majorly) etc. It has been seen that FSO networks are about 99.9% accurate when used in link distances of 500 to 1000 metres. This incredible accuracy is under normal or recommended conditions. Under adverse atmospheric conditions it may degrade, but still the amount of accuracy is still high comparably.

D. High security

In other Ethernet systems we employ security features like cryptography to secure our data. But in FSO, the data path can actually be ‘seen’ and is very narrow. Thus it is very difficult to ‘tap into’. Thus FSO in itself is very secure and thus suited for transmission of sensitive data.

E. Protocol Transparency

FSO is a layer 1 device, thus they can be used with any network, both old and new. Also it can be used in all the Ethernet configurations, namely Ethernet, Fast Ethernet, and Gigabit Ethernet. It also works in SONET, FDDI, ATM etc. FSO can also integrate easily with other wireless communications as well namely 802.11 or Wi-Fi.

F. Cost

Air-fiber is a company produces FSO products. It has prepared a cost model based on deploying an FSO mesh in Boston. According to its analysis, deployment would cost about $20,000 per building, with an average link length of 55 meters and a maximum length of 200 meters. The mesh would also provide full redundancy.

Thus a comparable fiber network costs between $50,000 to $200,000 per building. Considering the cost of optical fibers this is very promising. It would take about $50,000 to $80,000 (per kilometer in an urban location) to connect the buildings alone few thousands more to connect the buildings!

Light Pointe another company proclaims only about $8000 per link!

VII. LIMITATIONS

There is no ‘perfect’ Ethernet solution. All the Ethernet techniques have their plus and minuses, so does FSO.

The main limitations of FSO are:-

A. Only for Line-of-Sight Communication

Free Space Optical Communication can be done only if the network stations are visible to each other. That is there should be possible for a beam of light to intersect the network stations. This always is not possible. Thus only LAN can be provided using FSO.

B. Fog, Snow and Rain

Atmospheric adversities are a significant restraint in Free Space Optical communications. Fog, smog, snow storm, sand storm, rain etc does degrade the network’s bandwidth and/or power of the beams.

Fog is the main problem. It is composed of extremely small moisture particles that act like prisms upon the light beam, scattering and breaking up the signal.

In the case of rain and snow the network may not be shut off completely, but far away network stations might degrade in connectivity (that is signal attenuation is effected).

Most vendors know they have to prove reliability in bad weather cities in order to gain carrier confidence, especially if those carriers want to carry voice. So these vendors try to distinguish themselves by running trials in foggy cities. Terabeam, for example, ran trials in Seattle, figuring if it could make it there, it could make it anywhere.

Sand storm and Smog are rare and occur scarcely at high altitudes. Most of these phenomenons affects Wi-Fi network also.

Fig. 9 first picture shows normal condition and second in fog condition and last one is under snow storm condition.

C. Scintillations

Scintillation is the result of solar energy heating small pockets of air to slightly different temperatures, thereby creating regions of varying refractive index along the propagation path.

Here the beam from the transceiver is spread when they pass through these air pockets as shown in Fig 10. This does not cut off the transmission but can induce errors in the transmission.

Fig. 10 Scintillation

D. Low Clouds

This is a case when we are connecting offices in 2 skyscrapers. Here also the beam is scattered due to the small water particles present in the clouds. Attenuation is effected here as well.

Fig. 11 shows a skyscraper being engulfed in a low cloud. In cases like this the beam strength of the FSO communication can be lost. But here also the connection only gets degraded not cot-off completely.

Fig. 11 Low clouds

E. Building Motion

There are mainly tree types of motions and are shown in the TABLE II. There are mainly two ways to cope with the building movements. They are:-

1) Automatic Pointing and Tracking

Here the narrow beam of light is used. The system always monitors the communication and aligns the transceivers automatically. But this technique requires additional components and added complexities. It is also costly to provide this feature.

But this is employed when there is added requirement of security. Here the beam is directed towards the transceivers alone and is thus highly directed.

2) Large Divergence

Here the beam is spread out over a larger margin, and there by nullifying the sway of the building. This is relatively cheap to employ but the security is compromised here. But we can over come this by employing added security features.

Even though it degrades security, this approach is often used due to the low implementation cost.

But the recent surveys conducted in Seattle show that only 15% percentage of building sways more than 4 degrees. Thus the beam need not be spread out to a large extend.

TABLE IIBUILDING MOTIONS

Type Cause(s) Magnitude Frequency

Tip/tilt Thermal expansion

High Once per day

Sway Wind Medium Once every several seconds

Vibration Equipment (e.g., HVAC), door slamming, etc.

Low Many times per second

VIII. COMPARISON

TABLE III shows the comparison of various Ethernet solutions there is. The various parameters are ranked, 10 being best and 1 being worst.

TABLE IIIETHERNET SOLTIONS

Network Speed Distance Latency Security CostFSO 10 4 10 10 8Telco 10 10 10 10 15.8 RF 2 9 5 1 1024GHz RF 5 6 6 2 560GHz RF 9 3 9 3 6

IX. PRACTICAL SOLUTIONS

There are several commercial products available in the market. Some of them are FSona, Sonabeam, Ronjan, Light Pointe etc

These provide with both internal and external FSO solutions. The basic devices in all the above mentioned products are similar and the entire cost of implementing an FSO network is only some thousands of dollars.

Fig 12 some practical implementations

X. CONCLUSIONS

Well designed FSO networks can deliver up to 99.9% efficient networks. The FSO networks provide added security and high data rates. FSO implementation is easy and is cost efficient. There are some short comings to FSO like any other Ethernet solutions, but most can be overcome or is usually tolerable.

Thus Free Space Optical communication has large future scope as it enable implementation of cheap Gigabit Ethernet connections

REFERENCES

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Ghani, Marek Hajduczenia, Paul Havala, “Delivering Carrier Ethernet: Extending Ethernet Beyond the LAN”

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Fiber” IEEE Spectrum August 2001[15] Allen Doug, “The Second Coming of Free Space Optics”, Network

Magazine, March 2001