Optical: systems and elements – Polarization without modulation – By relatively adjustable superimposed or in series polarizers
Reexamination Certificate
1999-11-29
2001-09-04
Schuberg, Darren (Department: 2872)
Optical: systems and elements
Polarization without modulation
By relatively adjustable superimposed or in series polarizers
C359S199200, C359S494010, C359S490020, C385S011000
Reexamination Certificate
active
06285500
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to optical switches, and particularly to wavelength selective switches using a polarization rotating device.
2. Technical Background
In the past two-decades fiber optics have transformed the telecommunications market place. Initially, network designs included relatively low-speed transceiver electronics at each end of the communications link. Light signals were switched by being converted into electrical signals, switched electronically, and reconverted into light signals. The bandwidth of electronic switching equipment is limited to about 10 GHz. On the other hand, the bandwidth of single mode optical fiber in the 1550 nm region of the electromagnetic spectrum is in the Terahertz range. As the demand for bandwidth increases exponentially, network designers have sought ways to exploit the available bandwidth in the 1550 nm region. Thus, a need exists for optically transparent cross-connects and switches.
One approach that has been considered involves a frequency-selective optical switch employing a polarization beam splitter, Wollaston prism and a liquid crystal switch element. However, this design has a major drawback. The polarizing beam splitter, which is used to recombine the beams, is always located between the focusing lens and the spatial light modulator. One effect of this is that the polarizing beamsplitter must be able to accept a large acceptance angle, which leads to poorly superimposed beams if birefringent crystals are used. If beamsplitting cubes are used contrast ratio is reduced and crosstalk is increased. This was addressed by using a Wollaston Prism. Wollaston Prisms are designed to convert a collimated beam of mixed polarization into two deflected collimated beams, which are separated by an angle that is roughly bisected by the optical axis of the original mixed polarization beam. This solves many of the problems associated with placing the polarizing beam separator between the focusing lens and the LC switch element, but there are substantial problems associated with using Wollaston Prisms. The most significant of these lies is the fact Wollaston Prisms cannot produce beams that are exactly symmetrically deflected. Because the effect of the Wollaston Prism is not symmetrical, the beams cannot be superimposed at the LC switch element. Thus, the positions of the beams at the LC switch element must be balanced with the differing angles of incidence at the LC switch element to minimize crosstalk and insertion loss variation for the different switched states. Due to this asymmetry, the optical system must grow to unattractively long lengths in order to achieve acceptable crosstalk with an acceptable channel bandwidth.
Thus, what is needed is a wavelength selective switch having an optical system that is symmetric about a polarization modulator and capable of delivering superimposed beams at the polarization modulator in order to reduce crosstalk, reduce insertion loss, and improve spectral resolution.
SUMMARY OF THE INVENTION
A wavelength selective switch is disclosed that includes an optical system that is symmetric about a polarization modulator and capable of delivering superimposed beams at the polarization modulator in order to reduce crosstalk, reduce insertion loss, and improve spectral resolution.
One aspect of the present invention is an optical device for selectively directing a first signal and a second signal to a selected output. The optical device includes: a birefringent optical system having a system input that receives the first signal and the second signal, and a system output to which the birefringent optical system transmits a superimposed signal formed by superimposing a first polarized signal and a second polarized signal, wherein the first polarized signal and the second polarized signal are polarized versions of the first signal and the second signal, respectively; and a polarization modulator coupled to the system output, whereby the polarization modulator selectively rotates a polarization state of the superimposed signal.
In another aspect, the present invention includes an optical device for selectively directing a first signal and a second signal to a selected output. The optical device includes a first polarization beam splitter for separating the first signal and second signal into first signal polarization components and second signal polarization components, respectively. A first half-wave retarder is coupled to the polarization beam splitter, the first half-wave retarder causes all of the first signal polarization components and the second signal polarization components to be uniformly polarized in a first polarization state. A first grating is coupled to the first half-wave retarder, for producing a plurality of first signal wavelength channels and a plurality of second signal wavelength channels. A second half-wave retarder is coupled to the first grating, for causing the plurality of second signal wavelength channels to be uniformly polarized in a second polarization state. A first optical compensator is coupled to the first grating, for causing an optical distance of the plurality of first signal wavelength channels to be substantially equal to an optical distance of the plurality of second signal wavelength channels. A first polarization beam combiner is coupled to the optical compensator and the second half-wave retarder, for combining the plurality of first signal wavelength channels and the plurality of second signal wavelength channels into a plurality of superimposed wavelength channels. A focusing lens is coupled to the polarization beam combiner; and an array of polarization modulators coupled to the focusing lens, each of the modulators has a switch state, wherein each superimposed wavelength channel is focused onto a predetermined modulator.
In another aspect, the present invention includes a method for selectively directing a first signal and a second signal to a selected output in an optical device. The method includes the following steps. Providing a polarization modulator. Converting the first signal into at least one first polarized component and the second signal into at least one second polarized component. Superimposing the at least one first polarized component with the at least one second polarized component to form a superimposed signal, wherein the at least one first polarized component and the at least one second polarized component are co-linear in at least one axis direction; and focusing the superimposed signal onto the polarization modulator.
In another aspect, the present invention includes a method for selectively directing a first signal and a second signal to a selected output in an optical device that includes a birefringent optical system. The method includes the following steps. Providing an array of liquid crystal pixels, wherein each of the liquid crystal pixels includes a switch state. Demultiplexing the first signal and the second signal to thereby form a plurality of first signal wavelength channels and a plurality of second signal wavelength channels, respectively. Superimposing each first signal wavelength channel over its corresponding second signal wavelength channel to thereby form a plurality of superimposed wavelength channels; and focusing each superimposed wavelength channel onto a predetermined liquid crystal pixel.
The features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that the following detailed description is merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and
Ranalli Eliseo R.
Scott Bradley A.
Corning Incorporated
Malley Daniel P.
Schuberg Darren
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