Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-09-02
2001-08-07
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06271953
ABSTRACT:
FIELD OF THE INVENTION
This invention is related to the field of optical communications, and more particularly, this invention is related to a method and apparatus of free space satellite communications using an optical carrier signal and a pointing and tracking beacon signal for non-mechanical steering.
BACKGROUND OF THE INVENTION
U.S. patent application Ser. No. 09/076,494, filed May 12, 1998, and entitled, “SYSTEM AND METHOD FOR FREE SPACE OPTICAL COMMUNICATIONS,” assigned to the present assignee by the same inventors, and hereby incorporated by reference in its entirety, describes a method by which next generation satellite communication systems could achieve extremely high data rates for direct intersatellite, satellite-to-ground, and ground-to-satellite communications over extremely large line-of-sight distances using optical technology. The application achieved a technological advance by circumventing various shortcomings, such as common with optical data communication technology that followed the development of terrestrial fiber optic networks and concentrated solely on the transmission of digital information, typically using a pulse-pulse modulation (PPM), on-off keying (OOK) format, or wavelength-division multiplexing (WDM) to increase the information rate in optical channels. Those techniques did not fully exploit the advantages of optical communication technology for high data rate space based applications.
The invention disclosed in the '494 application circumvented those shortcomings by electrically combining a number of data sources, digital or analog, using a frequency-division multiple access theme, and using this signal as a wide band modulating signal to alter the phase of a single optical carrier. The constant envelope of phase modulation was advantageous as compared to amplitude modulations (OOK), (PPM) for simplifying detection schemes as is well known in communications. At the receiving terminal, the carrier is coherently demodulated and the individual electrical signals recovered using filtering of amplification. The invention described in that application, unlike known prior art, allows digital and analog signals to simultaneously share a single optical carrier.
The invention of the '494 application allows both analog and digital data to be transmitted simultaneously on a phase modulated optical communication signal to a receiver, such as for intersatellite and satellite-to-ground communications. Because the optical carrier signal is phase modulated, the problems associated with moving targets and changes in distances between the targets, e.g., satellites and/or ground stations and satellites, are reduced. A constant envelope type of modulation, i.e., phase modulation, is used instead of the more conventional intensity modulation, which changes the amplitude of the signal. As a result, no auxiliary or pilot signal is necessary. Additionally, the phase modulated signal is readily adapted for non-mechanical steering, which decreases any payload weight for communications equipment, requires less fuel and decreases acquisition times, and increases reliability.
Typically, a mixture of analog, digital or RF signals are each passed through a mixer where respective signals are up converted into a unique signal slot or channel. The frequencies then are combined to form the broad band frequency division multiplexed signal. A laser generates an optical carrier signal. An electro-optic modulator phase modulates the optical carrier signal with the multiplexed signal to produce a phase modulated optical communication signal.
A receiver is positioned, such as in a satellite, to receive the phase modulated optical communications signal. The receiver comprises a demodulator for demodulating the phase modulated optical communications signal back into the original broad band frequency division multiplexed signal. A demultiplexer (e.g., filter) allows demultiplexing of the broad band frequency division multiplexed signal into the plurality of communication signals comprising the frequency division multiplexed signal.
A plurality of digital communication signals are generated and analog modulated onto an optical carrier using an electro-optic technique. The electro-optic modulator can preferably comprise a Mach-Zender electro-optic modulator. An antenna can receive communication signals to be multiplexed with a receiver, such as in a satellite, and can be connected through the frequency division multiplexer for receiving analog communication signals generated by a remote source. The electro-optic modulator preferably generates an optical carrier signal wavelength of about 1,550 nm. This wavelength is preferable because erbium-doped fiber amplifiers can be used at this wavelength for amplifying the phase modulated optical communication signals.
In many similar communication systems, a separate pointing and tracking (PAT) signal is used as a “beacon,” for determining the amount of mechanical, coarse steering in gimbaled telescopes and fine steering in fine steering mirrors. However, relative to optical steering systems, these types of mechanical steering systems increase the payload weight, which requires more fuel, and increases acquisition times.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a non-mechanical beam steering mechanism that allows efficient steering for optical free space communications with adequate beam steering control.
In accordance with the present invention, a method of communicating in free space now allows nonmechanical steering using a cascaded nematic liquid crystal and an acousto-optic Bragg cell. In one aspect of the present invention, the method comprises the steps of frequency division multiplexing a plurality of communication signals into a single broad band frequency division multiplexed signal. An optical carrier signal is generated with a laser. This optical carrier signal is phase modulated with the broad band frequency division multiplexed signal by mixing the multiplexed signal with the optical carrier signal in an electro-optic modulator. A pointing and tracking beacon is also generated.
The phase modulated optical communication signal and pointing and tracking beacon is then transmitted from a first communications terminal to a receiver unit of a second communications terminal. This phase modulated optical communications signal and pointing and tracking beacon is received within a cascaded Bragg cell and liquid crystal at the second communications terminal. The index of refraction through the receive Bragg cell and receive liquid crystal display is changed based on the detected pointing and tracking beacon. This operation continually updates the optical line-of-sight paths to maintain the communication channel. The phase modulated optical communications signal is then demodulated back into the broad band frequency division multiplexed signal and then demultiplexed into a plurality of analog communication signals.
A system for communicating in free space is also disclosed and includes a first communication terminal, having a multiplexer circuit for multiplexing a plurality of analog communication signals into a single broad band frequency division multiplexed signal. A laser generates an optical carrier signal and an electro-optic modulator receives the broad band frequency division multiplexed signal and the laser generated optical carrier signal to produce a phase modulated optical communications signal. A pointing and tracking beacon is also generated. These signals are passed through a transmit beam steering assembly that non-mechanically steers the optical communications signal and pointing and tracking beacon by the means of a cascaded Bragg cell and liquid crystal.
A receiver is positioned at a second communications terminal, such as on a second satellite or even ground station, and receives the transmitted phase modulated optical communications signal and pointing and tracking beacon. This receiver includes a receive beam steering assembly, which includes a cascaded Bragg cell an
Beadle Edward R.
Dishman John F.
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Harris Corporation
Pascal Leslie
Phan Hanh
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