Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2000-02-15
2001-01-09
Mack, Ricky (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C385S015000, C385S047000
Reexamination Certificate
active
06172790
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to communications, and more specifically to conversion between communications in optical form and electrical or electromagnetic form.
BACKGROUND OF THE INVENTION
There has been increasing use of optical communications for terrestrial and vehicular use, partially because of the low cost and light weight of many optical components, particularly optical fiber transmission paths, and partially because an optical path can accommodate many light carrier wavelengths, each having a wide bandwidth capable of carrying multiple channels of information. Optical paths are also resistant to electromagnetic interference.
The wide bandwidth of optical channels allows carrying digital information, and also allows carrying many channels of analog information, each on an individual subcarrier. This use, in turn, results in a need for simple and effective ways to shift each electrical baseband frequency to a different carrier frequency. This frequency shifting task is normally accomplished by a heterodyne mixer or modulator.
Improved arrangements for frequency shifting baseband signals to subcarrier frequencies are desired for use in optical communication systems.
SUMMARY OF THE INVENTION
An optical modulator according to an aspect of the invention includes a splitter/combiner light coupler including first, second, and third ports. The splitter/combiner light coupler divides light applied to the third port into substantially equal first and second portions at the first and second ports, respectively. A first optical directional coupler includes first and second coupled transmission paths, the first coupled transmission path of the first optical directional coupler defines first and second ports, and the second coupled transmission path of the first optical directional coupler defines first and second ports. A first light transmission path includes first and second cascaded electrooptic portions. Each of the first and second electrooptic portions includes an electrical input port, by which the index of refraction of each electrooptic portion is responsive to electrical signals applied to its electrical input ports. The first light transmission path extends from the first port of the splitter/combiner light coupler to the first port of the first transmission path of the first optical directional coupler. A second light transmission path is provided, which extends from the second port of the splitter/combiner light coupler to the first port of the second transmission path of the first optical directional coupler. An optical reflector is coupled to the second port of one of the first and second transmission paths of the first optical directional coupler. An optical absorber is coupled to the second port of the other one of the first and second transmission paths of the first optical directional coupler. As a result of this arrangement, light applied to the third port of the splitter/combiner light coupler is modulated by the (multiplicative) product of the electrical signals applied to the electrical input ports of the first and second electrooptic portions. In a particularly advantageous version of this embodiment, a third electrooptic portion including an electrical input port is coupled in cascade with the second light transmission path.
In order to separate the modulated light from the unmodulated light, a reflective optical modulator as described above is further associated with a second optical directional coupler including a first port, a second port, and a third port, for coupling light applied to the first port to the second port, and for coupling light applied to the second port to the third port, the second port of the second optical directional coupler being coupled to the third port of the splitter/combiner light coupler, whereby signals applied to the first port of the second optical directional coupler are coupled to the reflective modulator, and modulated light from the reflective modulator is coupled to the third port of the second optical directional coupler by way of the second port of the second optical directional coupler.
Another hypostasis of the invention is an array antenna, which includes a plurality of antenna elements, each including a port at which received signals appear, and also includes an optical fiber onto which light signals representing the individual signals received by the plurality of antenna elements are to be coupled. A source of light is provided, as is a source of a plurality of electrical carriers, each at a frequency which is offset from the frequencies of the other carriers. A plurality of optical modulators is included, each including a light port, and first and second electrical ports. Each of the optical modulators modulates light applied to its light port in response to the product of the electrical signals applied to the first and second electrical ports. Each one of the plurality of optical modulators has its first electrical port coupled to the port of one of the plurality of antenna elements, its second electrical port coupled to the source of a plurality of electrical carriers, for receiving one of the carriers therefrom, and its light port coupled to the source of light. A coupling arrangement is coupled to the optical fiber and to the light ports of the plurality of optical modulators.
In a particular avatar of the array antenna according to the invention, each optical modulator comprises a splitter/combiner light coupler including first and second ports, and the light port. The splitter/combiner light coupler divides light applied to the light port into (preferably equal) first and second portions at the first and second ports, respectively. A first optical directional coupler includes first and second mutually coupled transmission paths. The first transmission path of the first optical directional coupler defines first and second ports, and the second transmission path of the first optical directional coupler defines first and second ports. A first light transmission path includes first and second cascaded electrooptic portions. The first and second electrooptic portions include the first and second electrical input ports, respectively. The index of refraction of the electrooptic portions is responsive to electrical signals applied to the electrical input ports. The first light transmission path extends from the first port of the splitter/combiner light coupler to the first port of the first transmission path of the first optical directional coupler. A second light transmission path extends from the second port of the splitter/combiner light coupler to the first port of the second transmission path of the first optical directional coupler. An optical reflector is coupled to the second port of one of the first and second transmission paths of the first optical directional coupler. An optical absorber is coupled to the second port of the other one of the first and second transmission paths of the first optical directional coupler. As a result, the light applied from the source of light to the light port of the splitter/combiner light coupler is modulated by the multiplicative product of the electrical signals applied to the electrical input ports of the first and second electrooptic portions. In a particular manifestation of this avatar, the coupling arrangement comprises a second light directional coupler associated with each of the splitter/combiner light couplers. Each of the second light directional couplers includes a first port, a second port, and a third port, for coupling light applied to the first port to the second port, and for coupling light applied to the second port to the third port. The second port of the second optical directional coupler is coupled to the third port of the associated one of the splitter/combiner light coupler. As a result, signals applied to the first port of the second optical directional coupler are coupled to the optical modulator, and modulated light from the optical modulator is coupled to the third port of the second optical directional coupler by way of th
Frey Richard Louis
Lotshaw William Taylor
Tiemann Jerome Johnson
Katz E. R.
Lockheed Martin Corp.
Mack Ricky
Meise W. H.
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