Optical communications – Optical communication over freee space – Transceivers
Reexamination Certificate
2000-11-08
2004-09-21
Chan, Jason (Department: 2633)
Optical communications
Optical communication over freee space
Transceivers
C398S118000
Reexamination Certificate
active
06795655
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to free-space optical communication systems and may be used for wireless/fiberless two-way information transfer between remote objects, including situations with many objects participating in point-to-point or point-to-multipoint information exchanges.
BACKGROUND
Through-out the developed and developing world, modem society continues to create exponentially increasing demands for digital information and the communication of such information between data devices. A variety of optical fiber and wired technologies now provide high bandwidth with attendant high data rates for communications to customer premises, but installation of such facilities is not practical in all locations. In many major population centers, installation of a new physical facility of this type requires underground installation with a high construction cost. The construction and the attendant requirement for local government approval impose considerable time delays. In many instances, the actual available capacity through long delayed deployments lags far behind the ever-increasing demand. Radio frequency (RF) wireless solutions reduce the time, complexity and cost of installation, but those solutions are inherently limited by their use of shared RF spectrum. As the number of users on a given piece of spectrum grows, the average capacity available to any one user declines.
Free-space optical communications systems offer two-way information transfer between remote objects without the use of wires and/or optical fibers. Because such systems can implement point-to-point links to the individual customer premises, such systems are not subject to the limits of shared capacity, as in the existing RF wireless technologies. Free-space optical communications systems may implement network (mesh) technologies for information transfer or point-to-multipoint technology for a two-way information exchange.
One known optical communication system uses two terminals, each which includes a transmitter in a form of a light source with modulation means, and a receiver (see U.S. Pat. No. 4,960,315). This known system is used as a backup, allowing restoration of communication upon disruption of a section of fiber optic connectivity. The disclosed system provides communication via an optical radiation beam propagating in free space. A drawback of the known system, however, is that its application is limited because it provides communication only via a single channel (one transmitter to one receiver) and hence has a low transmission capacity.
A known multi-channel optical communication system uses several transmitting devices implemented as lasers with modulation means, a transmitting medium implemented as an optical fiber transparent for the laser radiation, and several receiving devices (see U.S. Pat. No. 5,589,968). This system achieves an increase in transmission capacity due to spectral, time, and code multiplexing of channels. A drawback of this known system, however, is its limited application and high cost. This is because optical fiber is used as the optical radiation transmission medium. As noted, use of optical fiber requires expensive installation by laying optical cable, e.g. underground or on masts and typically requires government permissions and/or rights-of-way, which adds considerable further expenses.
A multi-channel free-space optical communication network is known based on use of a multi-address distributing device (router) comprising a scanner which can alternately point the light beam through the atmosphere at one or several receiving devices from a multitude of receiving devices, and a lens array where each lens provides collimation of the beam pointed at the corresponding to it receiving device or at several receiving devices (see U.S. Pat. No. 5,786,923). The known communication network may also use radiation of different wavelengths, wherein each wavelength is pointed at the corresponding receiving device by means of a dispersing optical device. Also, this known network may use spectral or time division of channels formed within the light beam dedicated to each of the receiving devices, in the same manner as when transmitting information via an optical fiber.
A drawback of the known free-space multi-channel system is that it does not provide for maximization of information transmission rate through the atmosphere. This drawback is critical, because the free-space channel providing information transmission through the atmosphere is, as a rule, the “bottleneck” in any communication network consisting of fiber optic and free-space (atmosphere) links.
The lower capacity of a free-space path in the atmosphere relative to a fiber optic path results from the presence of excessive noise arising from background light (primarily sunlight). Other factors that reduce capacity include the considerable losses of signal power. Substantial signal losses result from absorption and scattering in the atmosphere. Also, there are “geometrical losses” caused by widening of the light beam, during passage through the atmosphere, up to diameters exceeding the light receiving aperture size, i.e. so that substantial portions of the radiant energy are not directed on the intended receiver. To mitigate these drawbacks, the use of the free-space atmospheric channels requires additional methods of channel multiplexing (in comparison to fiber optic systems).
A free-space optical communication system, known from the Description to the Japanese patent application No. 06303198, Oct. 10, 1994, includes some multiplexing. The known system comprises several light-emitting elements and several optical receivers, operating simultaneously. The system is intended to achieve higher bit rate by information transfer through the atmosphere from the simultaneous use of several receivers and transmitters to create several spatially-divided information transmission channels (with number of channels equal to the number of receivers and corresponding number of transmitters interconnected with optical beams).
The known free-space optical communication system with spatial mutliplexing, however, suffers from several disadvantages. First, the system has limited application and complex design when the number of channels is large, because the maximum achievable number of channels in this system is equal to the number of transmitters or the number of corresponding receivers. Also, to separate the channels, it is necessary for each receiver to receive only the transmitter signal intended for or addressed to this receiver, which makes the system exceedingly complex. To provide division of channels, each transmitter should have a lens forming a narrow optical beam, and the receivers should be placed far enough from each other to avoid overlapping of optical beams from different transmitters, caused by the beam divergence. This, in turn, causes an increased size of each optical communication device, if the number of channels is large. This also limits the communication distance, because each lens is forming optical beams of non-zero divergence, and the transverse dimensions of each beam, starting from some particular distance, will increase proportionally to the distance from the transmitter.
Thus, the known free-space optical communication system is inconvenient for creating of a large number of independent communication channels, which in turn limits the ability to increase the system capacity for servicing a large number of subscribers with large data-rate demands.
The known system also suffers from the drawbacks common for prior art free-space optical systems: a) requirement for clear line-of-site between communicating terminals; b) lack of flexibility in transmission capacity available to terminals with changing demand for bandwidth. Hence, there is a continuing need for a free-space optical communication system with increased capacity, providing flexible connectivity between multiple terminals not necessarily having line-of-site with all other terminals,—without requiring overly complex optical elements or ot
Leshev Aleksei A.
Ragulsky Valery V.
Sadovnikov Mikhail A.
Sidorovich Vladimir G.
Vasiliev Mikhail V.
Chan Jason
Meklyn Enterprises Limited
Payne David
LandOfFree
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