Optical waveguides – Polarization without modulation
Reexamination Certificate
1999-08-03
2002-12-10
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Polarization without modulation
C385S002000, C385S008000, C359S246000
Reexamination Certificate
active
06493473
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to methods and apparatus for the control of the phase and magnitude of light, and more particularly, to an endless polarization transformer for transforming the state of polarization (SOP) of light.
BACKGROUND OF THE INVENTION
In a typical optical communication system, an optical transmitter generates an optical beam and modulates the beam with an electrical signal representative of the information to be transmitted by the communication system. An optical fiber propagates the modulated optical beam to a receiver that demodulates the optical beam to recover the electrical signal. Fiber amplifiers, disposed at appropriate intervals in the fiber optic link between the transmitter and the receiver, maintain the strength of the modulated optical beam. The low loss, light weight, small size, flexibility and high intrinsic bandwidth of optical fiber make optical communication systems highly desirable for the communication of both of digital and analog signals. Examples of optical communication systems include cable TV (CATV) systems, telephone and other cross-country or cross-continent communication systems, and other microwave and RF systems, such as phased array antenna systems used by the military. DWDM (Dense Wavelength Division Multiplexing) systems are also becoming increasingly popular, as they increase the bandwidth of existing optical fibers by transmitting multiple beams, each of a different wavelength and representing a different channel for the communication of information, over the optical fiber.
One important concern with optical communication systems is control of the polarization of the light beam (referred to herein as the state of polarization, or SOP) received by the various optical components in the optical communication system. Polarization is property of light relating to the direction in space of the vibrations of the electromagnetic fields of which light is composed. For example, the electric field of the light can vibrate along either or along both of two orthogonal axes of a coordinate system, where the orthogonal axes of the coordinate system define a plane that is perpendicular to a third axis along which the light propagates. For example, assume that a light beam is propagating along the z axis. The electric field can vibrate solely along the direction of the x axis, can vibrate solely along the direction of the y axis, or can vibrate along both axes, in which case the SOP of the light is determined by the superposition of the orthogonal polarization components, i.e., of the vibrations along the y axis and along the x axis. The superposition can result in the electric field vector tracing out particular shapes in the x-y plane, such as a circle or an ellipse. The shape and the direction in which it is traced (counterclockwise or clockwise) are determined by the relative phase and magnitude of the orthogonal polarization components of the light beam, and correpsond to a particular SOP.
Transmitters and receivers, as well as other components often present in an optical communication system, such as modulators and photonic switches, add-drop multiplexors, etc., are typically designed to operate with light having a particular SOP. However, as is well known to those of ordinary skill in the art, the SOP of a light beam often changes as the beam propagates in the optical communication system. These changes can be random over time. Thus although the transmitter may transmit the proper SOP that would result, if unchanged, in efficient operation of the other components in the optical communication system, the SOP changes as the beam propagates, and the performance of one or more of the components, and hence of the overall optical communication system, can suffer. The use of polarization maintaining optical fiber can reduce the problem of changes of the SOP, but such optical fiber is expensive, does not typically totally eliminate changes of the SOP, and hence is typically only used in short lengths to interconnect adjacent components.
Polarization transformers are known in the art. A polarization transformer can receive the light beam prior to delivery of the beam to the receiver or other optical component and transform the SOP of the beam to the SOP that the receiver or other component is designed to process. For example, it is known in the art that any input SOP can be transformed into any desired output SOP by a cascade of two quarter-wave plates and a half-wave plate. The half-wave plate is placed in between the quarter-wave plates, and each plate is disposed at a selected angle, where the angles can be, and often are, selected to be different. Varying the selected angles varies the SOP to which the input polarization of the beam is transformed.
Mechanical polarization transformers that use a cascade of quarter-wave and half-wave plates are available. However, such mechanical devices are inherently slow, bulky, complex, and not readily miniaturized. Polarization transformers are often combined with devices that track the input or output SOP and a controller for automatically controlling the transformer such that the varying input SOP is continuously transformed to the desired output SOP, which usually should not vary over time.
Another device known in the art is a “fiber squeezer” polarization transformer. However, this device is also mechanical and can require the use of complicated “reset” algorithms. “Reset” is a known phenomenon wherein a polarization transformer is controlled by a parameter that can only vary over a finite range. As the tracking and controlling described above progresses, the control parameter can approach one of its limits, and must be reset to a value within the control parameter range to continue to properly transform the SOP. During the reset the transformed polarization varies from the desired output SOP, and this can result in the loss of signal, and hence information received by the receiver or other component while the transformer is reset. Such loss is considered unacceptable in many applications. A polarization transformer that does not require a reset is known in the art as an “endless” polarization transformer, and is more desirable. The quarter-wave and half-wave transformer cascade discussed above is one example of an endless polarization transformer, because the waveplates can be endlessly rotated without reaching a limit.
Other optical apparatus useful for endless polarization transformation are disclosed in U.S. Pat. Nos. 5,212,743; 5,359,678; and 5,361,270, issued May 18, 1993, Oct. 25, 1994, and Nov. 1, 1994, respectively, and all of which include Heismann as an inventor. These patents are referred to herein as the “Heismann patents,” and all are herein incorporated by reference. A polarization transformer using the apparatus disclosed in the Heismann patents uses cascaded electrode sections and a titanium indiffused optical waveguide realized on z-propagating lithium niobate. The cascaded electrode sections apply electric fields to the optical waveguide for inducing a selected phase shift between, and a selected relative amplitude between, the orthogonal components of the polarization of the light propagated by the optical waveguide. (In such a waveguide, the orthogonal components of polarization of the light are often referred to as the TM mode and the TE mode. Transfer of power between the orthogonal polarization components is referred to as mode conversion, and changing the relative phase of the orthogonal polarization components referred to as mode phase shifting.) The voltages are placed on the electrodes to produce electric fields that vary, via the electrooptic effect of the electrooptic lithium niobate substrate, the above conversion and phase shift such that the overall SOP determined by the superposition of the TE and TM modes is the desired output SOP.
The devices disclosed in the Heismann patents are often constructed and operated as the electronic analogs of the quarter-wave plate, half-wave plate, quarter-wave plate mechanical device described above, without,
McCormick Paulding & Huber LLP
Sanghavi Hemang
Uniphase Telecommunciations Products, Inc.
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