Optical waveguides – Polarization without modulation
Reexamination Certificate
1999-03-03
2001-02-13
Kim, Robert (Department: 2877)
Optical waveguides
Polarization without modulation
C385S015000
Reexamination Certificate
active
06188810
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to optical fiber based communication networks, and more particularly, to couplers for selectively transferring data between two such networks.
BACKGROUND OF THE INVENTION
Communication networks based on optical fibers for transferring data between terminals are attractive because of the high bandwidth of the optical fibers. Data is transmitted on these networks by modulating a light source, usually a laser. A plurality of users may share a single fiber either via time or wave division multiplexing (WDM). Wave division multiplexing is typically implemented by assigning a different wavelength to each user or channel.
A limited number of stations may be accommodated on any given fiber ring. Accordingly, a coupling device is used to selectively couple data from a first fiber to a second fiber. The data to be coupled is typically isolated to a sub-set of the channels on the first fiber. In some cases, it is advantageous to simultaneously remove the transferred channel from the first fiber after the channel is connected to the second fiber. Such coupling devices include optical circulators.
In a telecommunications network each subscriber communicates with a central office over a fiber that is arranged in a ring with the subscriber and central office stations disposed along the ring. If the fiber is broken, communication between one or more of the users and the central office will be interrupted. In principle, these users can still communicate with the central office by sending messages along the uninterrupted portion of the loop. However, this requires that the direction of propagation along the fiber be reversed over a portion of the fiber.
Unfortunately, the fiber ring typically includes components that are unidirectional in nature such as the optical circulators used to couple the fibers discussed above. To reverse the direction of propagation in response to a fiber break, duplicate optical circulators configured to propagate signals in the opposite direction are included in the network. These components are inserted into the fiber in place of the corresponding components by utilizing bypass switches. Such bypass arrangements substantially increase the cost and complexity of the optical network, and hence, it would be advantageous to avoid these bypass arrangements.
Broadly, it is the object of the present invention to provide an improved optical circulator.
It is another object of the present invention to provide an optical circulator whose direction of light transmission can be reversed by applying a control signal to the optical circulator without the need to utilize bypass switches and additional optical circulators.
It is yet another object of the present invention to provide an optical coupling arrangement in which the direction of propagation of the coupled signals may be switched without the need to utilize bypass switches and other circulators.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
SUMMARY OF THE INVENTION
The present invention is a reversible optical circulator and a coupling device constructed therefrom. The optical circulator has first, second, and third ports for receiving and transmitting light signals. A light separator resolves an incoming light signal on one of the ports into first and second light signals having orthogonal polarizations with respect to one another. A first non-reciprocal optical element that includes a first magnetic field generator for generating a first magnetic field operates on the first optical signal. The first magnetic field has a direction that is determined by a first control signal. The first non-reciprocal optical element rotates the polarization of the first optical signal by either 90 degrees or 0 degrees depending on the direction of the first magnetic field. A second non-reciprocal optical element that includes a second magnetic field generator for generating a second magnetic field operates on the second optical signal. The second magnetic field has a direction determined by a second control signal. The second non-reciprocal optical element rotates the polarization of the second optical signal by either 90 degrees or 0 degrees depending on the direction of the second magnetic field. A light collector combines the first and second optical signals after the first and second optical signals have traversed the first and second non-reciprocal optical elements, respectively, to create a combined light signal, the combined light signal leaving the optical circulator by another of the first and second ports. The non-reciprocal optical elements are preferably constructed from Faraday rotators and a half-wave plate.
The light coupling device adds a first light signal having a wavelength of &lgr;
1
to a second light signal of wavelength &lgr;
2
traveling in an optical channel. The coupling includes first and second optical channel ports, the second light signal entering one of the first and second optical channel ports and leaving by the other of the first and second optical channel ports and an optical channel input port for receiving the first light signal. The coupler includes an optical circulator and first and second reflectors. The optical circulator has first, second, and third ports and a circulation direction determined by the circulator input signal. Each of the reflectors has first and second ports, light entering one of the ports exits the other of the ports unless the light is reflected by the reflector. Each reflector has first and second reflection states, the reflector reflecting light of wavelength &lgr;
1
in the first reflection state, and passing light of wavelength &lgr; in the second reflection state. The reflection state of the reflector is set in response to a reflector input signal associated with that reflector. Only one of the first and second reflectors is set to the second reflection state at any one time, the identity of the reflector in the second reflection state depending on the circulator input signal which also determines the direction of travel of the first light signal in the optical channel. The optical circulator preferably includes a Faraday rotator having a magnetic field direction that is determined by the circulator input signal. The reflectors are preferably constructed from variable wavelength fiber Bragg reflectors.
REFERENCES:
patent: 5982539 (1999-11-01), Shidasaki
patent: 6088491 (2000-07-01), Sorin et al.
Agilent Technologie,s Inc.
Kim Robert
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