Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-10-19
2001-07-10
Negash, Kinfe-Michael (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200
Reexamination Certificate
active
06259544
ABSTRACT:
FIELD OF THE INVENTION
This patent application is related to the field of optical communications, and more particularly, this patent application is related to optical intersatellite and satellite-to-ground communication systems.
BACKGROUND OF THE INVENTION
This invention describes a method by which next generation satellite communication systems can achieve extremely high data rates for direct intersatellite, satellite-to-ground, and ground-to-satellite communication over extremely large line-of-sight distances using optical technology. Traditionally, intersatellite links have been implemented in the microwave and millimeter wave regions. However, these options have limitations imposed by wavelength, transmit power, and modulation bandwidth. Optical data transmission overcomes these limitations. The small wavelength provides extremely high gains for the required transmit power for reliable communication at very large distances. In addition, the modulation bandwidths achievable for optical based channel are on the order of 20 Ghz and still rapidly increasing, where radio-frequency technologies are experiencing only incremental improvements. Such wide bandwidth provides a suitable channel for data communications exceeding 20 Gbps. However, current optical data communication technology is following the development of terrestrial fiber optic networks and concentrating solely on the transmission of digital information, typically using a pulse-pulse modulation (PPM) or on-off keying (OOK) format. Also common is the use of multiple optical wavelengths, called wavelength-division multiplexing (WDM), to increase the information rate in an optical channel. These techniques do not fully exploit the advantages of optical communication technology for high data rate space-based applications. There are several weaknesses to those approaches. For example, the PPM technique is not suitable for high data communications due to difficulties in detection and low bandwidth efficiency, and OOK is typically limited to applications that can use direct modulation of the laser. Also, WDM is not desirable, especially for space as multiple lasers are required as size, weight, power, cost and reliability are all degraded when adding multiple active components such as lasers.
The present invention circumvents all of the above shortcomings by electrically combining a number of data sources, digital or analog, using a frequency-division multiple access scheme, and using this signal as a wideband modulating signal to alter the phase of a single optical carrier. The constant envelope of phase modulation is 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 recover using filtering and amplification. This invention, unlike known prior art, allows digital and analog signals to simultaneously share a single optical carrier.
Some prior art systems have used optical communication systems to an advantage. For example, U.S. Pat. No. 5,610,748 to Sakanaka et al. discloses a communications link having intensity modulation with a necessary pilot (e.g., auxiliary) signal. Intensity modulation is also more difficult in a system where the transmitter and receiver are moving relative to each other, such as with moving satellites. Also, the laser beam intensity changes with the distance between the transmitter and receiver, such as when satellites orbit, causing some data inconsistencies because the laser attenuation appears as a change in a data bit. Thus, intensity modulation is not as desirable as constant envelope modulation (i.e., phase modulation), for most free space communications. Although intensity modulation has been successfully used in some optical transmission systems, using a fiber system as disclosed in U.S. Pat. No. 5,351,148 to Maeda et al. However, it is desirable if another modulation besides intensity modulation were used for optical communications in free space. Additionally, because of the moving transmitter and receiver in intersatellite communication systems, more conventional mechanical steering elements are difficult to operate and it would be advantageous if a non-mechanical steering system could be used with such systems.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method and system of communicating in free space with intersatellite and satellite-to-ground communications that do not use intensity modulation or WDM of an optical carrier.
It is still another object of the present invention to provide a method and system of communicating in free space with an optical carrier signal produced by a laser that allows both analog and digital data to be sent simultaneously on the same optical carrier signal.
The present invention is advantageous because it now 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. The system and method of the present invention uses a constant envelope type of modulation, i.e., phase modulation, 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.
In accordance with the present invention, the system and method of the present invention allows communication in free space, such as in intersatellite communications, and includes a frequency division multiplexer for multiplexing a plurality of analog communication signals into a single broad band frequency division multiplexed signal.
Typically, a mixture of analog, digital or RF 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.
In still another aspect of the present invention, a plurality of digital communication signals are generated and analog modulated onto an optical carrier using 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.
The system further comprises a Bragg cell and a liquid crystal display that receives the phase modulated optical communication signal for non-mechanically steering the phase modulated optical communication signal. The
Beadle Edward R.
Dishman John F.
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Harris Corporation
Negash Kinfe-Michael
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