Optical communications – Duplex
Reexamination Certificate
2000-12-15
2004-08-17
Tran, Dzung (Department: 2633)
Optical communications
Duplex
C398S120000, C398S123000, C398S128000
Reexamination Certificate
active
06778779
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention (Technical Field)
The present invention relates to passive optical communications, in particular to full-duplex communications on a single electro-magnetic beam and to the use of circular polarization keying as a modulation format.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Issued patents that have addressed the need for full-duplex communications on a single electro-magnetic beam include U.S. Pat. No. 4,882,770 to Miyahira, et al. entitled, “Wireless Optical Communication System Utilizing a Single Optical Carrier Frequency,” which discloses multiplexing for a full-duplex wireless optical short-range communications system by using two oscillators, one to modulate the audio data on a subcarrier frequency, and the other to modulate the subcarrier frequency on an optical infrared carrier frequency. Time division multiplexing is used as the modulation format. U.S. Pat. No. 5,121,243 also to Miyahira, et al. entitled, “Wireless Optical Communication System,” also discloses a wireless optical short-range communications system using time division multiplexing. U.S. Pat. No. 5,146,358 to Brooks entitled, “Optical Communications System and Method for Transmitting Information Through a Single Optical Wave Guide,” discloses simultaneous bi-directional, multi-channel communication. However, the data is either amplitude or frequency modulated. U.S. Pat. No. 5,272,555 to Suzuki entitled, “Bi-Directional Optional Transmission Method and Apparatus Therefore,” discloses bi-directional communication through a single optical path using either frequency modulation, amplitude modulation, or pulse modulation. U.S. Pat. No. 3,920,983 to Schlafer, et al. entitled, “Multichannel Optical Communications System Utilizing Multi-Wavelength Die Laser,” discloses a communications system having frequency multiplexed modulated signals in a single beam.
U.S. Pat. No. 5,966,224 to Hughes, et al. entitled, “Secure Communications With Low-Orbit Spacecraft Using Quantum Cryptography,” discloses using polarization as well as delay to accomplish secure communications. Vertical polarization and right-hand circular polarization is disclosed to distinguish between zeros and ones, or in the alternative, left-hand circular polarization and horizontal polarization to represent zeros and ones. U.S. Pat. No. 5,850,441 to Townsend, et al., entitled, “System and Method for Key Distribution Using Quantum Cryptography,” discloses a cryptograph communication system using polarization modulation. U.S. Pat. No. 5,978,121 to Fischer, et al., entitled, “Method and Device for the Optical Transmission of Data Over Distances in Space,” discloses the use of phase modulation to accomplish optical transmission. The light signal containing the information is circularly polarized, but does not use circular polarization as the modulation scheme. U.S. Pat. No. 4,689,625 to Barmat entitled, “Satellite Communications System and Method Therefore,” discloses transmitted and received signals operating in the same frequency band but with opposite linear or opposite handed circular polarizations, therefore requiring low power. U.S. Pat. No. 4,888,816 to Fica, Jr., entitled, “Two-Way Optical Communication System for Atmospheric Use,” discloses a two-way communication link with reduced light linkage and uses circular polarization, but with an amplitude modulation scheme.
None of these patents implement the unique differential circular polarization keying format and novel architecture for full-duplex communications on a single electro-magnetic beam as in the present invention. Additionally, conventional modulation formats for open air optical communications use amplitude shift keying (ASK), on-off keying (OOK), phase shift keying (PSK), or frequency shift keying (FSK). These modulation formats either only register signal for one state or they require a complex coherent detection system, the advantage of which falls away rapidly if there are any wave front distortions in the atmosphere. In contrast, circular polarization keying (CPK) uses simple paths of optical elements to separate the two circular polarization states and can direct each polarization state to a separate detector. Thus, the signal-to-noise ratio is increased by between 3 and 6 decibels simply by subtracting the output from those two channels. Each channel can be operated in a simple direct detection mode, thus avoiding the complexities of a coherent detection system. Passing circularly polarized light through a quarter-wave plate converts circularly polarized photons into linearly polarized photons. Furthermore, each left-hand circularly polarized photon is converted into a linearly polarized photon with the same polarization direction, while each right-hand circularly polarized photon is converted into a linearly polarized photons whose direction is orthogonal to the direction of the converted left-hand circularly polarized photons. Thus, CPK eliminates the difficulties associated with aligning linear polarization sensitive elements. This is particularly advantageous when certain polarization sensitive elements are used, such as Faraday Anomalous Dispersion Optical Filters, that provide ultra-high background rejection.
The present invention is a system allowing for full-duplex communications on a single electromagnetic beam in a communications system. This is accomplished by choosing data formats such that the forward data format (or “uplink” in the embodiment presented herein) is invisible to the return data format (or “downlink” in the embodiment presented herein). The present invention offers the potential of doubling the data rate of these systems while keeping the power consumption at the remote end very low. The present invention is useful for a passive retro-modulated communications system where a carrier beam is transmitted to a reflector that modulates the infinite carrier and returns it back to the source. In such a system full-duplex communications is often desired, while in conventional systems, time multiplexing is required between the transmit and receive phases. Time multiplexing reduces the data rate in each direction. The present invention overcomes this drawback. No time multiplexing is necessary because the data format in the forward direction is invisible to the data format in the return direction.
The present invention further implements a unique binary modulation format for optical communications. Using CPK modulation the zeros and ones are each represented by one of the two orthogonal circular polarization states. For example, a zero can be represented by right-hand circularly polarized light, and a one represented by left-hand circularly polarized light, or vice-versa. Thus, the presence of a zero or one can be determined by measuring the circular polarization of the incident light.
SUMMARY OF THE INVENTION (DISCLOSURE OF THE INVENTION)
The present invention is a method of full-duplex electromagnetic communication wherein a pair of data modulation formats are selected for the forward and return data links respectively such that the forward data electromagnetic beam also serves as a carrier for the return data. The present invention is further a method of encoding optical information wherein right-hand and left-hand circular polarizations are used to represent binary states.
One embodiment which implements the present invention is a full-duplex optical communication system wherein a laser diode transmitter and downlink receiver are located on the ground and a retro-modulator and uplink receiver are located on an object in space. The transmitter transmits a linearly polarized electro-magnetic beam to the object in space and a first quarter-wave plate converts the linearly polarized beam into a circularly polarized beam. A telescope di
Hazzard David A.
Horan Stephen
Payne Jason A.
Shay Thomas M.
Mays Andrea L.
New Mexico State University Technology Transfer Corporation
Peacock Deborah A.
Peacock Myers & Adams
Tran Dzung
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