The original idea came to me when I saw an old magazine photograph that showed a pyramid building with an intense laser beam coming out of its top to the sky. Then the beam was split on to the sky and formed a laser grid. This was a science fiction picture but it was the first time I thought about a network that would use open space laser beams to transfer data. Optical fibers are used to transfer huge amounts of data but they are still cables. The idea of data over a laser beam in the air is not new. There are already commercial solutions up to several Mbps and also good amateur solutions like the Ronja project for 10Mbps. Laser data links have some very important advantages over wire and radio communications, so they must be considered as great solutions for specific applications:
Low cost because of the absence of the carrier medium. The carrier medium is the laser light and it is "created" when the laser is switched on. For a greater distance link, no extra cost is needed (limited only by the laser power and focus capability) as there is no additional cable required. This causes zero link material cost, like in the case of radio.
Physically secure. No easy interception or eavesdropping can be made by intruders. Even if one has somehow access to the laser beam and installs a beam splitter device, the receiver will know that something has been wrong because the beam intensity will be smaller. Also, in this case the laser beam may not reach the receiver, because of the loss from the beam splitter or the misalignment. Furthermore, if low power visible or IR laser is used no one will know that there is a link exists. Security is also enhanced by the placement of the beam high enough in the sky for intruders to physically reach. Also, the carrier medium can be made to be temporary, if the laser is switched on only for the communications session. Furthermore, because of the resistance to interference described below, security against eavesdropping is enhanced. I would say that interception from intruders and eavesdropping in a carefully designed laser link, is at least as much impossible as in optical fibers. On the other hand, eavesdropping is a huge disadvantage of radio links.
Link flexibility. Unlike cables and like radio, the laser beam can be redirected to any location by realignment, without extra link material cost or cable re-routing. For a greater distance link, no additional cable is required (limited only by the laser power and focus capability).
Resistance to interference from and to other links. In cables there is no interference in a link from other links (unless the cable links are physically very close together), because each of them has its own physical medium. In radio, in order to avoid interference from adjacent links, each radio link must use its own frequency channel. This is a problem because this introduces a limit of the radio links that can simultaneously exist in a specific area, for a specific band. Radio links also must have a predefined output power level and low level harmonics. If a transmitter has a big power level and many harmonics it may interfere with the adjacent links, even if it operates in a different frequency from them. Laser links are wireless links, but because of the nature of the laser medium, there is no interference from other links. Even if two laser beams with the same frequency cross themselves, no interference will be introduced to the links. It is only the end points that matter and in the case of the laser, these points are only tiny spots, unlike radio, where the beam widths are wide. Furthermore, like radio, we can use multiple laser wavelengths (frequencies) to carry the data, but because laser beams are highly directive, we can also use the same frequency (e.g. red laser beam) for as many laser links as we want in the same area, without interference problems. Thousands of laser links all in the same frequency, may cross together in an area and they will be completely isolated as long as they point to different locations.
Health friendly. In the last decades many health issues have arised by the use of radio (especially microwaves) even at low RF powers. Low power coherent laser light, like ordinary non-coherent light is totally safe, as long as reasonable precautions are taken on the receiver, so that one does not stare into the laser beam.
Although laser links seem to be the solution to many of our problems, they have some disadvantages that is worth to be mentioned:
A clear line-of-sight path is always required in contrast with the radio links, where radio-line-of-sight may be acceptable.
Transceivers need stable mounting and careful alignment. Because Laser beams are highly directive it is more difficult to align them to point to the receiver. As the distance between the transmitter and the receiver becomes greater, this problem becomes also greater. In my opinion this is the greater disadvantage of the laser links. It is like trying to keep a laser pointer that points to a point 200 meters away from you stable.
The laser beam spreads at big distances. The little dot you see when you point a laser pointer at a wall, becomes huge when talking about big distances. This is mainly a problem of the cheap lenses used and not the laser itself. To overcome this problem, big lenses may be used at the receiver side, to collect as much of the received laser light in a single point back again.
Adverse weather can disrupt communication. Like radio, a very rainy or foggy day may cause huge attenuation or diffraction to the laser beam. This becomes worst as the distance between the transmitter and the receiver gets higher.
Flying birds may instantly interrupt the laser beam, so error correcting
protocols are necessary unless occasional errors are tolerable.
From the currently available amateur solutions, Ronja seems to be the best among them. The only problem with Ronja is that it is too complicated for the amateur to build and it costs too much. It is a link of high data rate and long distance though. In my approach for the SkyNet I thought that it might be more affordable and easier for many applications, to have more than two smaller and cheaper transceiver modules to cover the same distance (repeaters) rather than a big one. On the other hand, if talking about backbones Ronja may be better, but if talking about a crowded neighborhood where we want to interconnect the houses, then we may not need so advanced electronics, neither so precise alignment.
There is also another thing that must be considered when working with laser data links. All the commerce and amateur links that I have seen are point to point links. There are just two transceivers that communicate with each other. No third transceiver can join the link as the laser beam is highly directive. I did not mention it as a disadvantage of the laser links because I intend to propose a few solutions to this problem.
Solution 1: The passive way
Using special lenses, called beam splitters, a single laser beam can be split into two other. In an ideal beam splitter each produced beam is half the power of the original beam. Beam splitters can provide a way of making a laser link network, nevertheless they may be considered as impractical solution for the next reasons:
All the transceivers that exist in the network must be in the same altitude (X-axis). The beam splitter will cause beam misalignment otherwise.
Beam splitters introduce 50% loss because they half the power of the original beam. So they are not good for long distance links.
Beam splitters have to intercept the original beam. This means that they must be installed at some point onto the original link. They can not introduce an angle on it.
Not all transceivers can communicate with each other directly. In a hypothetical network with 4 nodes A, B, C, D, since the beam splitter splits the original beam into two, node A can only transmit to nodes B and C and not D. Node B will be able to transmit back to A and D but not to C. Node C will be able to transmit back to A and D but not to B. Node D will be able to transmit back to B and C but not to A. In order to have a network where each node will see all the others, the original data has to be retransmitted by the other nodes, but this has a lot of problems, like failures in case of a note being switched off, latency problems etc.
Solution 2: The active way
A more practical networked approach for laser links seems to be the use of active repeaters. Each repeater accepts a laser beam and transmits a copy (or more than one copies) of the original data. By the use of active repeaters many of the problems of passive repeaters can be overcame.
A line of sight between the receiver and the transmitter is not required. For example the repeater may be installed on a hill and connect the transceivers on both sides of the hill.
The repeater can be installed at any place, provided that both transceivers have line of sight with it.
The use of repeaters overcomes the problems of laser beam power loss, spreading and misalignment. The original laser beam is disrupted by the repeater and a fresh new laser beam comes out of it. So, the transmitted laser beam by the repeater has greater power than the received one, it is re-focused and re-aligned to the receiver.
Each repeater may be a back-to-back transceiver or a more simple and cost effective hardware solution that just repeats the received data, without further data processing.
I have talked so far about repeaters that connect a pair of transceivers, but how can multi node networks be created? Most of the home and company networks today use the star topology. Star topology is good because the network is up, even if one or some of the nodes are offline. It allows all nodes to communicate with each other through a central point (called hub in computer terminology). The critical point now is the hub and not the nodes. A user has to only take care about its link with the hub and not the links to the other users. If we need to connect two star networks together then we can use a backbone between the two hubs, or we can connect them using another central node. The last case introduces the tree network and it is used for greater scale networks.
For SkyNet, the star network topology seems to be satisfactory since we want the network to be always available, regardless of the availability of the nodes and since the links are relatively of small distance. Later on, if we want to connect two sub networks, we can make a higher data rate and reliability backbone between their two hubs. In order to implement this topology, there must be a central hub with as many transceivers as the clients. Each transceiver in the hub must point to a specific client.
(to be continued)
Current files for download:
Laser transceiver schematic (Express PCB file)
Laser transceiver schematic (jpg file)
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