Reconfigurable laser communications terminal

Details Reconfigurable laser communications terminal Reconfigurable laser communications terminal

1998-10-02

2001-12-04

Pascal, Leslie (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C359S199200

Reexamination Certificate

active

06327063

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a reconfigurable laser communications terminal particularly suited for optical intersatellite links.
BACKGROUND ART
Optical intersatellite link terminals are utilized to communicate between two satellites. One of the principal technical challenges in designing communication networks involving these terminals is to isolate the receiver channel within any terminal from back-scatter or any other spurious radiation that may be produced by the transmitted beam originating within the same optical terminal. For typical intersatellite ranges and acceptable transmitter/receiver aperture sizes, the isolation requirement is often greater than 90 dB, i.e. any spurious signal generated by the transmitter beam which can possibly enter the receiver channel must be less than the transmitter beam itself by at least 90 dB.
In communications networks incorporating optical intersatellite links (OISLs), one common isolation approach which achieves acceptable size and weight while also producing the required isolation between transmitted and received beams is to operate the beams at different wavelengths. A dual wavelength OISL network requires (at least) two types of terminals which are characterized by their operating wavelengths. For example, terminals of type A transmit radiation at a first wavelength and receive radiation at a second wavelength, while terminals of type B transmit at the second wavelength and receive at the first wavelength. A successful communication link requires that an A terminal communicate with a B terminal. The operating wavelengths are typically separated by about 5 to 10 nanometers (nm) which is large enough that available receiver bandpass filters can adequately reject spurious transmitter radiation and meet the isolation requirement. This approach offers various advantages, including the fact that the transmit and receive functions can share a common telescope and pointing optics to reduce weight and cost. To separate the transmitted beam from the incoming received beam, a dichroic beamsplitter is used which reflects the first wavelength while transmitting the second wavelength.
A second known isolation method employs polarization-based switching techniques to isolate the transmitted and received beams which operate at the same wavelength but orthogonal polarizations. This approach provides isolation of only about 30 dB to 40 dB depending upon the particular implementation and is therefore not appropriate for many OISL applications which have more stringent isolation requirements.
While physical isolation of the optical paths provides acceptable isolation, this third approach requires separate telescopes and pointing optics and therefore results in greater weight and cost compared to the approaches described above. Likewise, temporal isolation, i.e. transmitting and receiving at different times, represents a fourth approach that may be used to provide sufficient isolation but imposes severe constraints on the communication format that can be used. Furthermore, temporal isolation requires the system to adapt to changes in intersatellite range and is therefore undesirable.
Designers of space-based communications networks recognize that some fraction of the OISL terminals will become inoperable during the desired system lifetime. To compensate, sufficient redundancy should be provided so that the overall system can function acceptably despite the loss of a considerable fraction of the terminals. A typical system design may specify 50% more terminals per satellite than is required to maintain minimum acceptable performance. An implicit assumption when this reliability approach is applied in conjunction with dual-wavelength isolation is that any given satellite will always have enough terminals of the proper type (A or B) to maintain its links with complementing terminals of the opposite type on one or more other satellites. To reduce cost while providing acceptable system redundancy, the terminals of the communication system should be reconfigurable such that an A terminal can convert to a B terminal (and vice versa) upon receipt of an appropriate command from the network controller.
One approach to providing this reconfigurability is to mechanically exchange a dichroic beamsplitter so that the appropriate wavelength for the A or B terminal is directed to the receiver channel and from the transmitter. However, this approach requires complex and costly mechanical and electronic componentry to achieve the precise alignment necessary for the repeated switches between type A and B terminals throughout the life of the satellite network.
SUMMARY OF THE INVENTION
Thus, it is an object of the present invention to provide a reconfigurable communication terminal having acceptable weight, cost, and isolation for use in optical intersatellite links.
Another object of the present invention is to provide a system and method for separating a transmit beam from a receive beam in a dual-wavelength optical communication system which exhibits similar performance characteristics for both types of terminals to facilitate reconfigurability.
A further object of the present invention is to provide a method for reconfiguring an optical communication terminal while reducing or eliminating mechanical motion of optical elements.
Yet another object of the present invention is to provide a polarization-based reconfigurable communication system and method which combine optical paths of the transmit and receive beams while improving isolation.
An additional object of the present invention is to provide a system and method for reconfiguring an optical communication terminal in response to a control command by exchanging transmit and receive signal types.
Another object of the invention is to provide a system and method for reconfiguring an optical communication terminal which reduce or eliminate repositioning of beam steering components.
A system and method for optical communication include at least one reconfigurable terminal using dual-wavelength operation for isolation between transmitted and received signals in combination with polarization switching to separate and steer the transmitted and received signals to and from corresponding receivers and transmitters, respectively. The polarization based switching provides wavelength independent beam steering to facilitate interchanging of wavelengths for transmitted and received signals. A controllable or passive polarization changer such as a wave plate or polarization rotator, in conjunction with selectable or tunable bandpass filters, allow the communication terminal to be reconfigured without also requiring the repositioning and associated precision alignment of beam steering optics.
In one embodiment, an optical communication terminal according to the present invention includes a transmitter for generating an optical signal having a first-polarization and first wavelength and a receiver for receiving an optical signal having a second polarization and second wavelength. A polarization beamsplitter directs optical signals having the first polarization and first wavelength from the transmitter while directing optical signals having the second polarization and second wavelength to the receiver. A reconfigurable polarization changer operable to change polarization of optical signals passing therethrough is responsive to a command signal to select one of a first state in which output signal polarization is changed from the first polarization to a third polarization (while also changing a fourth polarization to the second) and at least a second state in which output signal polarization is changed from the first polarization to the fourth polarization (while also changing the third polarization to the second polarization).
A method for optical communication according to the present invention includes transmitting optical communication signals at a first wavelength and first polarization, and receiving optical communication signals at a second wavelength and second polarization. The transmitted and received si

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