Optical communications – Optical communication over freee space – Transmitter and receiver
Reexamination Certificate
2000-11-06
2004-12-07
Pascal, Leslie (Department: 2633)
Optical communications
Optical communication over freee space
Transmitter and receiver
C398S156000
Reexamination Certificate
active
06829439
ABSTRACT:
FIELD OF THE INVENTION
The invention is related to free-space optical communications systems and may be used for two-way information transfer between remote objects without use of wires and/or optical fibers, including the case when multiple users are taking part in the information exchange, e.g. if network (mesh) technologies are used for information transfer, or if point-to-multipoint technology is implemented for a two-way information exchange between a base station and several subscribers.
BACKGROUND
The modern information age has developed a pattern of continually increasing demand for communication capacity. 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 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.
U.S. Pat. No. 5,786,923, for example, discloses a point-to-multipoint bi-directional wide area telecommunications network employing atmospheric optical communication. The network comprises a primary transceiver unit, an optical router, and subscriber transceiver units. The primary transceiver unit generates a first light beam, which carries modulated data intended for end users. The optical router receives the first light beam and demodulates the data. The optical router then remodulates the data onto a second light beam and transmits the second light beam to the subscriber transceiver units. The optical router demodulates and retransmits to each of the subscriber transceiver units in a time-multiplexed fashion. In the patented system, each subscriber transceiver unit includes an optical receiver/transmitter. This optical “antenna” is coupled to an input/output device, such as a set-top box and television set or a computer or television, by an optical fiber or cable. The subscriber transceiver units receive the second light beam and demodulate the data. In the upstream direction, each subscriber transceiver unit atmospherically transmits a third light beam, which includes data from the user equipment. The subscriber transceiver sends this beam to the optical router. The optical router demodulates the upstream data, modulates it onto a fourth light beam, and transmits the fourth light beam to the primary transceiver unit.
Free-space optical communications involve multibeam communications. Accordingly, key components of free-space optical communication systems are multibeam optical transmitters with beams that may be targeted at prescribed objects arbitrarily positioned in space, as well as optical receivers for reception of signals arriving from such objects. These components enable wireless optical data communications and associated networking.
Realization of multibeam optical transmitters or receivers using the multichannel principle, i.e. as a set of separate channels of the same type operating point-to-point, multiplies the weight, dimensions, and cost of the entire system at least N-fold if N beams or directions should be implemented for communications.
The basic parameter of a multibeam communication system is a bit rate or bandwidth allocated to a communication network subscriber. It is evident that in a prior art multichannel system ratios between bandwidth dedicated to a single subscriber and total weight and cost of the entire system, decreases at least as fast as the number of beams. In other words, the efficiency of multibeam system implemented as a set of separate channels measured by the above mentioned ratios decreases, in comparison with a single-beam system, at least in proportion to a number of beams. This is the main technical and economical problem related to creation of multibeam free-space communication systems designed to provide information exchange for multiple objects (subscribers).
The optical space communication device is known, consisting of at least one receiver and one transmitter implemented as several sources of light located in the same plane around the receiver. The sources are installed with different inclinations to the setting plane so that the angles of deviation from the perpendicular to the plane increases with the distance to the receiver, providing the absence of intersection of beams from sources with different inclinations to the perpendicular (see the description of the Japanese patent application No. 63326970, publication No. H 04 B 10/10/1990 /1/). The disadvantages of the known device are: the difficulty of alignment of the sources relative to the receivers at which they shall be aimed, the difficulty of readjustment of the sources after the receivers have been moved, and large divergence of radiation illuminated by the sources, which requires either high power of the radiation, or high sensitivity of the receivers (both causing an increase of weight and size). The last circumstance (high divergence of the radiation) also requires narrowing the receiving bandwidth, as less power approaching a receiver provide for narrower telecommunication bandwidth for the same reliability of telecommunication.
A telemetry system is known, using an optical communication device (see the description of the Great Britain patent No. 2180116, H 04 B 9/00, 1987 /2/). The communication device is implemented in the form of several receivers and a transmitter consisting of several radiation sources installed on a curved surface, e.g. a hemisphere. The disadvantages of the known device are the following: difficulty of sources alignment relative to receivers corresponding to them and limited area of the system applications due to short ranges of communications caused by large energy losses (because of large divergence of optical beams radiated by the sources), and hence narrow bandwidth of the system.
U.S. Pat. No. 5,909,296 discloses an optical communication device having a transmitter with optical radiation modulation means, and one or more optical radiation receivers with light demodulation means. The transmitter is implemented as a set of radiation sources arranged on a convex surface and aimed at the receivers. To decrease irradiated beams divergence, each radiation source has its own microlens. The disadvantage of this device is its complicated design, which requires a separate lens for each transmitter radiation source. This complexity leads to a corresponding increase of the system weight and cost. Furthermore, the sources of optical radiation are fixed relative to the lenses, which prohibits any correction of the beamwidth and beam direction when the distance and/or angle between the receiver and the transmitter change. Thus, in spite of the irradiated beams divergence reduction due to the lenses, the system efficiency remains low because of excess energy losses as well as because of increased system weight and cost.
U.S. Pat. No. 3,713,163 discloses an antenna capable of delivering a plurality of beams centered about a single boresight axis t
Leshev Aleksei A.
Ragulsky Valery V.
Sadovnikov Mikhail A.
Sidorovich Vladimir G.
Vasiliev Mikhail V.
Li Shi K.
McDermott Will & Emery LLP
Meklyn Enterprises Limited
Pascal Leslie
LandOfFree
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