Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-04-16
2002-04-30
Negash, Kinfe-Michael (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06381055
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to free-space optical telecommunication networks, and more particularly to the calibration and positioning of transceivers and transceiver assemblies used in such networks.
Free-space optical telecommunication offers an attractive alternative to hard-wired or radio communication in certain situations. For example, a telecommunications services provider who wants to enter a new geographical area may have little or no hard-wired plant in that area and may wish to avoid the cost and complexity of installing such plant to serve the new area. Similarly, radio communications resources are limited and regulated, and a new telecommunications services provider may not have sufficient rights to use those resources in a new geographical area.
Free-space optical telecommunication is therefore attractive because it avoids the need for hard-wired plant and because, unlike radio telecommunication, it is essentially unregulated. Optical telecommunication also has the advantage of very large information capacity. Thus optical telecommunications links can support a wide range of telecommunications services such as telephone, video, audio, and computer data transmission.
As described in commonly owned, co-pending U.S. application Ser. No. 08/847,196, filed May. 1, 1997, which is hereby incorporated by reference herein, free-space optical networks may employ substantially unguided, point-to-point, electromagnetic communication between area access points and end users. For example, an area may be served by one or more area access points, each having a transceiver for bi-directional, free-space, line-of-sight, electromagnetic communication with one or more nearby relay points. Relay points have at least two, and in some cases-more than two, free-space optical transceiver assemblies for line-of-sight optical communication with area access points, relay points, and/or end points. An end point is similar to a rely point except that an end point has only one transceiver. Relay point and end point transceivers may be located on the roofs of houses or other buildings in the neighborhood served by an above-mentioned area access point. Users of the network may be located at or near any relay point or end point.
Preferably, at least a fraction of the relay points are reachable via more than one path through the network of relay points for providing alternative communication between two points when direct service is undesired or temporarily unavailable.
A possible problem with providing alternative communication paths involves the calibration and alignment of transceiver assemblies during reconfiguration. For example, if communication between two points is temporarily interrupted, both points must normally reestablish communication with an alternative relay or access point. However, reestablishing communication may be problematic when information that indicates the alternative point's position is even slightly inaccurate, or when the positioning system itself has inaccuracies.
In fact, the problems associated with reestablishing communication between assemblies are compounded by the fact that both light beams must be aligned in order to establish two-way communication. For example, if the number of possible linking directions associated with a single assembly is represented by the variable N, then the time it takes to find one particular linking direction increases as N increases. It follows that the time it takes for two assemblies to independently find linking directions increases with the square of N. Of course, the number of possible linking directions scales with the solid angle in which the counterpart assembly is located. However, the number of possible linking directions (e.g., N
2
) associated with two assemblies could become so large that a search for a particular linking direction may become impractically slow.
Therefore, it would be desirable to provide a free-space optical network that can be reconfigured quickly. In particular, it would be desirable to provide a network that could be reconfigured in a time that does not scale as the square of N, even when a relay point or an end point has limited or no knowledge regarding the positions of the alternative points.
Another possible problem of such networks involves safety. Some free-space optical networks could, under certain circumstances, be a direct ocular viewing hazard. For example, in a reconfigurable network, the vertical height of the transceiver assembly and the direction of an electromagnetic beam emitted by the transceiver are adjustable for finding alternative relay, access, or end points. Normally, such alternatives are found using a scanning routine. However, a person may be located in the path of the beam during scanning, which may damage that person's eyes.
Therefore, it would be desirable to provide a free-space line-of-sight optical network that can be reconfigured safely.
Yet another possible problem associated with free-space, line-of-sight, optical communication involves misalignment. For example, if a transceiver assembly at a relay point were mounted to a roof of a home, the position of that assembly may move when the position of the home moves. Such movement is known to occur periodically with temperature changes that accompany the change of seasons and continuously over a period of many years because the foundation of the home may settle over time. When the position or direction of an assembly changes relative to its counterpart assembly, communication therebetween may degrade, or even be lost, especially when the distance between the assemblies is large and/or the diameter of the optical beam carrying the information is small.
Another problem associated with free-space optical communication involves inaccuracies in the positioning system due to intermittent mechanical slippage and loss of positional data. This problem is compounded by the fact that both beams must be aligned.
Therefore, it would be desirable to provide a reconfigurable, free-space, line-of-sight optical network that is immune to misalignment. In view of the foregoing, it is an object of this invention to provide improved free-space, point-to-point, optical telecommunication.
It is a more particular object of this invention to provide safe, reliable, and accurate telecommunication using a telecommunication network that employs substantially unguided, free-space, electromagnetic radiation between spatially distributed points, even when a particular telecommunications pathway of the network is nonfunctional.
It is another particular object of this invention to provide accurate calibration and rapid alignment and realignment of transceiver assemblies mounted at access, relay, and end points.
SUMMARY OF THE INVENTION
These and other objects of the invention are accomplished in accordance with the principles of the invention by providing free-space optical telecommunications methods and apparatus that employs substantially unguided, point-to-point, free-space, electromagnetic communication between transceiver assemblies located at end points, relay points, and access points.
For example, in one embodiment of this invention, a self-calibrating, reconfigurable free-space optical transceiver assembly is provided that includes a transceiver, a calibration retro-reflector, and a control unit. The transceiver should be at least rotatable about a vertical axis and include a transmitter and a receiver. The transceiver should face the vertical axis and the retro-reflector should be at a known rotary position with respect to the transceiver. The control unit moves the transmitter so that it emits a rotating electro-magnetic beam. In operation, a portion of the beam is reflected by the retro-reflector when the transceiver faces the retro-reflector and the receiver detects that portion. Other methods and apparatus for calibrating a transceiver, which do not use a retro-reflector, are also provided.
The self-calibrating assembly may be calibrated according to the following method. In a first step, the height of t
Javitt Joel I.
Rutledge Christopher Lee
AT&T Corp.
Negash Kinfe-Michael
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