During the past several years, the telecommunication industry has enjoyed explosive growth. The industry has strained to meet the demand for increasing communication bandwidth. Global, national and regional telecommunication techniques primarily include: (1) telephone networks providing voice, data and FAX transmission using twisted pair wire, coaxial cable, fiber optics, microwave systems, and RF networks; and (2) television networks providing television through RF transmission, and coaxial cable, and (3) internet data utilizing some or all of the specified data transmission media. Television, telephone and data communication are also currently being provided through satellite-based systems. Growth in non-voice data communications including high-speed image and video data establishes the need for data communication at increasingly higher speeds than that required for voice communication alone.
Radio communication permits the user to be mobile. It does not require expensive wiring connecting the communication equipment. The problem with radio communication is that the available radio bandwidth is limited. A solution to limited bandwidth is to establish separate geographical cells which allows reuse of available bandwidth for each separate cell. By establishing a large number of independent cells, the number of users that can share the common bandwidth is increased. Nevertheless, since the RF data transmission is essentially broadcast, the permissible density of common-band cells, and likewise the maximum density of users that can be accommodated, is necessarily limited.
The need for alternative technologies for last mile and last few miles connectivity solutions is on the increase. In the past telephone and cable systems have generally operated on a regulated monopoly basis. Currently, however, the federal, state and local governments in the United States are encouraging competition in the provision of these services. Still, local telephone and cable companies are reluctant or charge heavily to share their installed infrastructure, and the installation of new cable or fiber connections can be prohibitively expensive, disruptive or otherwise not possible. In many developing countries there is no significant wired communication infrastructure in place and installing a wired infrastructure would be expensive and disruptive. Certain events such as the Olympic Games and the Super Bowl create temporary need for greatly expanded communication in a region. Disasters such as major ice storms or hurricanes can disrupt existing communications creating a need for temporary high-bandwidth communication equipment until the existing system can be repaired.
Techniques for providing free space optical communications are known. (See xe2x80x9cA Brief History of Free-Space Laser Communicationsxe2x80x9d by David L. Begley in Selected Papers on Free-Space Laser Communications, David L. Begley, ed., SPIE Optical Engineering Press, 1991.) For example free space laser communication has been proposed for satellite to satellite communication, where the laser beam can provide extremely high bandwidth over long distance point-to-point links. Satellite communications provide a nearly ideal venue for the application of laser communication technology, where the beam path is above the earth""s atmosphere. By contrast, ground-based free space laser communication has generally been constrained to relatively short distances because of the adverse effects of atmospheric conditions such as turbulence.
Laser sources are capable of providing extremely narrow beams, with footprints of tens of centimeters at kilometer distances easily achievable. These extremely narrow beams permit an almost unlimited number of local links to operate at the same carrier frequency without interfering with one another and yet requiring low transmission power levels to achieve good signal-to-noise. The difficulty with arbitrary reduction of transmitted beam size is in maintaining alignment between transmitters and receivers. Once the beam is reduced below a few hundred microradians, even the most minor vibration or other perturbation will be sufficient to cause the transmitted beam to entirely miss the remote receiver.
What is needed is a laser communication system with transceivers which can be installed and easily aligned and will remained aligned.
The present invention provides a free space laser communication system which dynamically maintains link alignment. The system comprises two laser transceivers positioned a distance apart. Each transceiver includes the following components. A beacon laser is provided for transmitting a beacon beam at a first wavelength in a beam having a first divergence. A signal laser is provided for transmitting a signal beam carrying information to be transmitted to the other transceiver. A special telescope system is provided for collecting both the incoming beacon beam and the incoming signal beam from the other transceiver and for transmitting an outgoing signal beam. A tracker is provided to maintain dynamic alignment. A high-speed detector is provided to receive the data transmitted from the remote terminal.
The optical system images the pupil of the telescope onto a tracking mirror, which pivots in tip and tilt. The received beacon beam is reflected from the tracking mirror and focused on a CCD array that monitors the location of the remote beacon source and provides feedback to the tracking mirror. Based on this feedback, the tracking mirror tips and tilts as necessary to maintain the received beacon beam on a predetermined position of the CCD array, while simultaneously directing the received signal beam through a small aperture to the high-speed signal detector. The small aperture isolates the incoming signal beam from spatially distinct background radiation. The outgoing laser beam from the signal laser is also reflected from the tracking mirror and out of the telescope system toward the remote transceiver. In a preferred embodiment the signal laser for both transceivers is a single mode diode laser operating at 850 nm. The divergence of the signal beam from each transceiver is approximately 200 microradians, providing a footprint at a two-mile transceiver separation of approximately 2 feet. The beacon laser is a diode laser operating 785 nm. The beacon beams are given much larger divergences on the order of one degree to provide for tracking over a wide dynamic range. Preferred embodiments permit communication at high data rates of up to 2.5 gigabits per second at ranges in excess of six miles in clear weather. The dynamic and automatic tracking features of the present invention permit the system to be installed and made operational in just a few minutes. In case alignment is disrupted for any reason the system automatically realigns itself as soon as the cause is removed.
The present invention provides a laser-based telecommunication system, which can be installed and aligned easily, efficiently and which will remain aligned for extended periods of operation. Additionally, this system can be operated over distances ranging from hundreds of meters to greater than ten kilometers, with communication speeds in excess of gigabits per second and with high link reliability in the face of local perturbations and path atmospheric turbulence.