Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2000-09-15
2003-07-15
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S253000, C359S199200, C359S199200, C349S119000
Reexamination Certificate
active
06594063
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to an optical device for controlling a light beam and in particular to an electrically controlled optical device that changes the path or intensity of a light beam.
Fiber optic communication network infrastructures are becoming more diverse and sophisticated. The demand for greater bandwidth has added complexity to their architecture. In these all-optical networks, channels are dynamically routed, switched, provisioned, restored, and protected in the optical layer. Switches and attenuators are critical elements in these architectures, and in support of these components, various technologies exist or are under development.
To date, numerous types of optical switches have been proposed or commercially made. Although the performance properties of these technologies are often very strong in one area, they tend to be deficient in others. For example, an electro-optical switch proposed by Soref suffers from huge insertion losses. In this switch, the incident beam is split by a birefringent crystal into two orthogonal polarizations. Only one of the polarizations is optically routed through the switch; the other portion of the incident light is lost. Optical switching technology based on lithium niobate crystals is extremely fast, but has high crosstalk and high insertion losses. Optomechanical devices, currently the most widely used switching components, have some very good switching characteristics, but are highly unreliable due to their moving parts.
An optical switch that uses electrically controllable rotators in combination with one or more birefringent crystals to route an optical signal is shown in U.S. Pat. No. 5,724,165. In a preferred embodiment, the rotators are electrically controllable liquid crystal elements. This patent uses two electrically controllable rotator elements in a 1×2 switch. The use of two electrically controllable rotator elements is inefficient and requires a more complex controller, making manufacturability difficult, increasing materials cost, and duplicating any electro-optical functional deficiency associated with the one electrical controller. Our invention simplifies the design by using only one electrically controllable electrical rotator and thus eliminates some of the aforementioned problems. It also incorporates other elements to overcome other deficiencies associated with the other technologies mentioned above.
SUMMARY OF THE INVENTION
In contrast to prior art, we propose an optical component with excellent switching properties, including low crosstalk, low insertion loss, low polarization loss, relatively fast dynamic response, relatively low switching voltage, good temperature stability, and high reliability. The optical component in accordance with the invention is also multifinctional, as opposed to typical single-function switches, attenuators, variable couplers, and polarization mode dispersion compensators. In particular, the optical component may operate in one or more different modes of operation wherein the particular mode of operation is determined by firmware without changing the optical hardware. In more detail, a microprocessor-based hybrid liquid crystal optical component is provided wherein the voltages applied to the liquid crystal rotator are varied in order to change the functionality of the optical component. For example, depending on the applied voltage, the functionality can be changed from a switch to an attenuator.
The optical component in accordance with the invention may include a set of fixed waveplate rotators located between the second and third birefringent crystals, which simplifies the control of the optical component. The optical component may also include a liquid crystal multicell rotator element that permits the optical component to be operated in one or more different modes. In addition, the optical component in accordance with the invention may also include a temperature control mechanism so that the optical component is not sensitive to temperature changes.
The present invention is thus a birefringent optical device that can employ a compound, zero-order, field-driven liquid crystal rotator in order to provide input-signal polarization independence. The invention provides an optical device that may include one or more variable-retardation, birefringent rotators, and one or more birefringent elements, such as crystals. Each rotator is a single retardation cell or a composite of two or more retardation cells with at least one compensator cell. The polarization-rotating properties of these rotators enable the optical device in accordance with the invention to be independent of the incident polarization. These rotators are broadband, with, high contrast ratio, relatively low insertion loss, and fast response time, resulting in an optical device that likewise has low crosstalk, low insertion loss, and fast response time relative to typical optical devices. The optical device may include birefringent beamsplitters alternating with active and passive rotation elements that guide the light signal. The birefringent beamsplitter separates an input signal into an ordinary ray (an s-polarization ray) and an extraordinary ray (a p-polarization ray). The ordinary ray travels in a forward direction, while the extraordinary ray travels forward and upward, or forward and sideways, depending on the orientation of the optical axis of the birefringent element. Both rays then pass through a rotator having dual rotating elements. Each rotating element may independently and controllably rotate the polarization of the particular incident beam. The rotating elements are a combination of field-addressed, near-zero-order, birefringent liquid crystal rotator cells and retardation waveplates.
In more detail, the optical device in accordance with the invention may include a means to generate and separate light into two orthogonal, linearly polarized beams. The optical device in accordance with the invention may further include the means to recombine the separated beams into a single beam or to keep both beams separated. Thus, the optical device includes the means for directing light very rapidly from one input port to any number of exit ports. The present invention also further minimizes the polarization dependence of the device by adjusting the voltages of the liquid crystal cells in the cell stack of the compound rotator at the appropriate input incident light polarization.
The present invention also solves the various deficiencies of typical optical switches and components with zero-order rotators by using an electrically variable retardation device. In particular an electrically tunable birefringent liquid crystal cell is used as the retarder to achieve close to zero-order retardation.
In accordance with the invention, an optical device is provided that comprises a birefringent element for separating incoming light into first and second signals having different polarizations, and an electrically controlled rotator element for independently rotating the polarization of the first and second polarization signals to generate third and fourth signals having polarizations similar to each other. The rotator element further comprises a stack of one or more rotator elements and a compensator element to provide low crosstalk and fast switching speed.
In accordance with another aspect of the invention, a polarization independent optical device is provided. The optical device comprises a first birefringent element for separating incoming light into first and second polarization signals and an electrically controlled rotator element for independently rotating the polarization of the first and second polarization signals to generate third and fourth signals having polarizations similar to each other, wherein the rotator element comprises a stack of one or more rotator elements and a compensator element. The optical device further comprises a second birefringent element for directing the third and fourth signals in predetermined directions based on the polarization of those sign
Albert Richard
Bergner Brent
Seekola D. L.
Epps Georgia
Gray Cary Ware & Freidenrich LLP
SpectraSwitch, Inc.
Thompson Tim
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