Optical: systems and elements – Optical modulator – Having particular chemical composition or structure
Reexamination Certificate
2000-10-24
2002-06-11
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Having particular chemical composition or structure
C359S239000, C359S251000, C359S322000
Reexamination Certificate
active
06404538
ABSTRACT:
BACKGROUND OF THE INVENTION
Much progress has been made in the last thirty years in developing optical switches or modulators, but current devices are not very satisfactory for many applications. The majority of active fiberoptic devices used in present day systems, for example, fiberoptic intensity attenuators, are based on electromechanical operation. In one type, fibers are positioned end to end and mechanically moved in or out of line. In another type, mirrors are rotated to direct beams into or away from a receiving fiber. This can be accomplished mechanically or with piezoelectric or electrostatic drivers. Mechanical devices intrinsically lack speed and long term reliability. Solid-state light controlling devices (without moving parts) are needed for fiber communication systems. A key problem for these developing fiberoptic components is realizing speed and reliability, as well as the essential fiberoptic systems requirement of low insertion loss and polarization insensitivity. For devices used between regular fibers, low insertion loss and polarization insensitivity operation is the basic performance requirement.
Others have proposed an optical switch/attenuator using a liquid crystal cell as the modulation element situated between an input and an output birefringent element, each fed by optical fibers. When the liquid crystal cell is turned on, light emerging from the output birefringent element is deflected and not focused by the subsequent collimator onto the corresponding optical fiber. Although it has the desirable features of low insertion loss, and low required operating voltage, being liquid crystal-based, the long term reliability of organic materials and the relatively low switching speed are not suitable for many applications.
Others have also proposed a fast (less than one microsecond) optical switch using an electro-optic crystal in which birefringence can be induced by application of an electric field. Operation is based on rotating the plane of polarization of light with respect to the orientation of a subsequent passive polarizer that blocks or transmits light depending on the angle. The basic arrangement works efficiently with incoming light polarized with a particular orientation. Randomly polarized light suffers a loss. This is overcome by using additional elements that split incoming light into two orthogonal polarizations, passively rotates one to match the other, and combines the two into a single beam fed to the basic modulator. However, the suggested electro-optic crystals, require voltages of a kV or more for operation.
Still others have described a modulator having a tapered plate, a Faraday rotator or electro-optic crystal, and a second tapered plate. The Faraday rotator is controlled by varying the current in an external coil which varies a magnetic field. The suggested electro-optic crystals require high drive voltages of kilovolts. Electrode design also effects polarization dependence and modulation efficiency.
SUMMARY OF THE INVENTION
Accordingly, the main objects of the invention are to provide an electrically controllable solid state optical modulator, attenuator, or switch that is insensitive to the polarization of the incoming light, has low insertion loss and, has a fast (one hundred nanoseconds or less) response time. Another object of the present invention is to provide a system for compensating the solid state devices against environmental changes, for example, temperature. Additional objects are to provide a device using rugged oxide materials and using easy assembly and alignment processes.
These objectives and other features and advantages are realized in two basic modes. In the transmission mode, arbitrarily polarized light beam enters from one side (the input surface) and exits the other side (the output surface). In one embodiment, the modulator comprises, between the input and output, a polarization separator, e.g., a birefringent plate with an oriented c-axis, followed by an electro-optic phase retarder with electrodes to generate an internal electric field when a voltage is applied, followed by a polarization recombiner. The separator breaks the light beam into two polarization rays, an ordinary one having a polarization direction (angular orientation with respect to the separator c-axis) perpendicular to the c-axis and an extraordinary one with a polarization direction parallel to the c-axis. In addition, the extraordinary ray is deflected in a plane containing the c-axis while the ordinary ray travels straight through. These two paths define a separation plane. The recombiner doesn't effect ordinary rays either, but causes extraordinary rays to be deflected an equal amount but opposite the separator deflection back to be recombined with undeflected ordinary rays at the output. The modulator is normally-on. The phase retarder has an electric field that extends across the optical path at an angle, preferably at about 45° to the separation plane which is also at 45° to both the extraordinary and ordinary polarization directions. When a voltage is applied to the phase retarder, portions of the extraordinary ray become ordinary and are not deflected to the output. In addition, portions of the ordinary ray become extraordinary and, instead of traveling through the recombiner to the output are deflected away from it. With sufficient voltage, the two rays are completely interchanged so that none of their components reach the output.
A normally-off modulator can be obtained simply by orienting the deflection of the recombiner to be in the same direction as the separator. If the output is placed equidistant between the undeflected ordinary ray and the twice deflected extraordinary ray, none will normally reach the output. However, if a voltage is applied to the phase retarder, portions of the ordinary ray will be deflected once and portions of the extraordinary ray will be not be deflected and both will reach the output. With sufficient voltage, all light will reach the output. Addition of a 90° polarization direction rotator, i.e., a polarization direction interchanger, to the normally-off modulator produces a normally-on modulator with low polarization mode dispersion. Addition of two 45° polarization direction rotators allows the fields in the phase retarder to be at 90° to the separation plane which produces a modulator with the minimum spacing between phase retarder electrodes thereby reducing the control voltage.
In a reflection mode, the simplest version comprises a separator covering an input area and a transversely displaced recombiner covering an output area, both followed by an electro-optic phase retarder, in turn followed by a reflector which directs the rays which have traveled through the separator and retarder back through the retarder for a second pass and then through the recombiner to the output. Having the input and output on the same side is considered useful in certain applications. A further advantage is that having two passes through the phase retarder means that each pass adds to the phase so that less voltage is required for full modulation. In full modulation, linear polarized extraordinary and ordinary rays with polarization directions at 45° to the electric field become circularly polarized on one pass and rotated by 90°, i.e., interchanged, after two passes.
As in the transmission mode, the deflection of the recombiner can be arranged to provide normally-on or normally-off modulation. The control voltage can be reduced by adding a 45° polarization direction rotator, e.g., a half-wave plate with a c-axis at 22.5°+N×45° (N an integer), between the separator/recombiner and the phase retarder so that the electric field can be at 90° to the separation plane. Insertion of a circular polarizer, e.g., a quarter-wave plate with a c-axis at 22.5°+N×45° (N an integer) will convert any configuration from normally-on to normally-off and vice versa.
The described modulator/attenuator can be built advantageously to control power levels in, for example, fiberoptic communication systems. In
Chen Qiushui
Jiang Hua
Melman Paul
Tsang Dean
Wang Feiling
Corning Applied Technologies Corporation
Epps Georgia
Hamilton Brook Smith & Reynolds P.C.
Spector David N.
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