Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-11-03
2002-02-12
Mullen, Thomas (Department: 2632)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06347001
ABSTRACT:
BACKGROUND
1. Technical Field
This invention relates to free-space laser communication systems, and more particularly to a free-space laser communication system having at least two communicating transceivers.
2. Background Information
Free-space laser communication systems transmit and receive information by means of a light beam that propagates through space or the atmosphere. Such laser systems gain their principle advantages over radio frequency based broadcast systems by being highly-directional and more difficult to jam. Compared to microwave-based systems, such laser systems usually have greater bandwidth, lower input power, smaller size and less weight. One reason a laser communication source requires less radiated power is because the angular radiated laser beam divergence is much smaller than a microwave beam.
When used for space-based, air-to-air or air-to-ground communications, free-space laser communication systems pose a number of challenging problems. One such problem is the fact that having a smaller beam divergence requires greater accuracy in pointing the laser beam. Microwave beam divergence is typically on the order of milliradians whereas laser beam divergence is generally less than 0.1 milliradians. This characteristic requires pointing accuracies for laser beams on the order of 1 to 10 microradians.
Accordingly, the first step in establishing communication for two laser communication terminals is for each terminal to acquire and track the other terminal. Typically such laser communication terminals will include a tracking beacon, a beacon receiver, and a communication transceiver, which are referred to as optical apertures. The transceiver optical aperture generally is mounted on a gimbaled platform having at least two orthogonal axes of freedom and which has been stabilized against base motion. The optical apertures are optically aligned with each other to what is called the system boresight.
The beacon laser of each terminal radiates a signal with much larger beam divergence than the signal of a separate communication laser, thus providing a source to acquire and track. This source must be viewable with good signal strength in the presence of other light, such as sky light, sunlight, starlight, moon light, or light reflected from the earth and other objects.
The acquisition and tracking system of each of a pair of laser communication terminals must be able to initially point the transmitting and receiving apertures of each terminal as close as possible to the direction of the other terminal.
FIG. 1
is a stylized diagram showing two vehicles communicating by means of a free-space laser communication system. A first vehicle
1
(e.g., an airplane) has a gimbal-mounted host laser communication terminal
2
mounted so as to be able to “see” a similarly mounted target laser communication terminal
3
on a second vehicle
4
(e.g., an airplane). The host terminal
2
includes a pointing system, a coarse acquisition and tracking system for generating a large field of view (FOV) “footprint”
5
to illuminate the target terminal
3
, and a fine acquisition and tracking system for generating a smaller “footprint”
6
to more precisely illuminate the target.
Certain aspects of the system shown in
FIG. 1
are disclosed in U.S. Pat. No. 5,710,652, entitled “Laser Communication Transceiver and System” and U.S. Pat. No. 5,801,866, entitled “Laser Communication Device” and U.S. patent application Ser. No. 08/221,527, filed Apr. 1, 1994, entitled “Point-to-Point Laser Communication Device” (now U.S. Pat. No. 5,754,323); [Ser. No. 08/199,115, filed Feb. 22, 1994, entitled “Laser Communication Transceiver and System”;] and Ser. No. 07/935,899, filed Aug. 27, 1992, entitled “Voigt Filter” (now U.S. Pat. No. 5,731,585). Each of the above references is incorporated herein by reference.
FIG. 2
is a stylized diagram showing the angular field of view (FOV)
7
for the optical apertures of the target laser communication terminal
3
on the second vehicle
4
and the angular FOV
5
of the beacon beam from the transmitting host laser communication terminal
2
. Each laser communication terminal must be able to initially point its transmitting and receiving optical apertures as close as possible to the direction of the opposite, target terminal. The beacon beam
5
from the host terminal
2
of the first vehicle
1
must provide a large footprint
5
at the receiving target terminal
3
to give the greatest probability of illuminating the target terminal
3
. The target terminal sensor should have a large angular field of view
7
to improve the probability of seeing the host terminal's beacon beam on the first “look”. This will reduce the amount of searching time and the uncertainty in establishing the communication link. However, if the beacon beam divergence is made too large, the intensity of the received beacon signal may be so low that the tracking signal caused by the received beacon signal is obscured by system electronic noise and other illuminating light sources.
The platform on which each terminal is mounted must provide a means to stabilize the pointing of the transmitting and receiving optical apertures against angular disturbances of the base mount. The base mount could include a vehicle, such as an aircraft or space platform, which has a significant amount of angular motion which would cause pointing errors for the optical apertures. The ideal method of stabilization would be a totally frictionless mount which has freedom to rotate in two orthogonal directions. If a frictionless mount were possible, then system inertia would cause each terminal to stay pointed in the same direction in the presence of angular disturbances to the base mount. In practice, friction couples base motion to the optical apertures of a terminal, causing angular motion. Such angular motion should be removed with a servo system which both senses and provides opposing torques to stabilize against the base motion.
The frequency of base motion disturbances can vary widely. Aircraft often have base motion disturbances at propulsion system frequencies or some multiple of these frequencies. A military tank would have disturbances at the frequencies of the engine rotation. These base motion disturbances can cause large pointing errors in a laser communication system. The terminal servo system must sense these disturbances and stabilize the base mount of the terminal. In general, the servo system must have a frequency response sufficiently high to compensate for the highest frequency components of base motion that contribute to producing pointing error.
The tracking system also must be able to sense angular motion of a terminal and provide pointing correction. The transmitter should be pointed with greater precision than the receiver of a laser terminal, since the beam divergence angle of the transmitter is much smaller than the receiver's acceptance angle. The precision of pointing the communication laser beam is preferably a fraction of its beam divergence angle that is sufficiently small so that the received power will vary at the receiver as the beam jitters due to the exponential decay in the beam intensity from its central maximum to its edge. The system is usually configured to maintain the beam power density equal to or above one half of its maximum power density. This ensures that the received communication signal is at least one half of the maximum transmitted power density that could be received during normal motion of the transceivers and in the presence of base angular vibration. In general, the angular tracking rates are small when terminals are separated by large distances. However, satellite to satellite tracking rates can be quite large for some systems, and ground to air tracking rates can be large if smaller distances are involved.
To provide tracking, a beacon receiver of terminal will image an incoming beacon beam onto a pixel-imaging device of what is commonly referred to as a “centroider”. The centroider provides an error signal which is directly proporti
Arnold Robert
Bloom Scott
Trissel Richard G.
Fish & Richardson P.C.
Mullen Thomas
Trex Communications Corporation
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