Optical waveguides – With optical coupler – Switch
Reexamination Certificate
2001-10-24
2004-02-24
Sanghavi, Hemang (Department: 2874)
Optical waveguides
With optical coupler
Switch
C385S011000, C385S016000, C359S245000, C359S246000, C359S315000, C359S320000, C359S483010, C359S494010
Reexamination Certificate
active
06697550
ABSTRACT:
BACKGROUND
Recently, fiber optics networks have been used instead of electrical systems to provide significantly higher bandwidth. However, some of these fiber optics networks employ electrical switches to direct communication signals from various input ports to one or more output ports. In these systems, the optical signal is converted to an electrical signal prior to the switching process. The electrical switch redirects the transmission of the electrical signal which is then converted back to an optical signal before being transmitted to the output port.
In all-optical networks, optical switches are used to direct a signal from one port or channel to another port or channel. For example, some have proposed using mechanical optical switches, such as micromachined mechanically actuated mirrors, or liquid crystal based switches, to redirect the light signal. As all-optical networks increase in complexity, the speed at which the optical switching process occurs becomes more important. But because the switching times of mechanical optical switches are in the order of milliseconds, and liquid crystal based switches offer only slightly higher speeds, their use is limited.
All-optical switches based on electro-optic (E-O) effects of certain solid-state materials offer significantly higher switching times, for example, in the order of microseconds to nanoseconds. Some proposed systems use E-O switches based on polarization conversion, while others use phase modulation.
It has also been proposed to use optical switches based on E-O beam deflection effects to provide high-speed, multi-channel switching. Typically, however, the light being transmitted through an ordinary optical fiber is randomly polarized while the E-O effect is polarization dependent. To make the fiber optic switching process polarization independent, some systems include an optical device that converts the incoming light signal into two linearly polarized light beams having the same polarization direction. An E-O based beam deflector is used to deflect these two beams which are then combined by a second optical device into a single output beam. In these system, each of the optical devices is typically formed from a pair of prisms which are expensive and difficult to fabricate. Further, these prisms typically cannot be made in a compact size.
SUMMARY
Birefringent crystals can be used as compact devices for separating a randomly polarized beam into two beams with orthogonal polarization directions, or combining two such beams into a single beam based on “walk-off” effects. These devices alone, however, are unsuitable for use as beam deflectors. Moreover, the walk-off distance between the two beams that have been separated or that are to be combined varies with the angle of incidence with the walk-off device. Accordingly, these variations cause two incoming beams to recombine imperfectly, and hence induce a polarization dependent loss. Therefore, there is a need to use walk-off devices with a compensator that eliminates these variations to provide an optical device with low polarization dependent loss.
The present invention implements a high-speed 1×N optical switch with an Electro-Optic (E-O) deflector. The optical switch redirects or steers an incoming light signal to a number of output light ports in a polarization independent manner.
In one aspect of the invention, the optical switch includes a walk-off device, and a compensator which compensates for walk-off distance variations of two polarized beams, with orthogonal polarization directions, associated with the walk-off device.
In some embodiments, the walk-off device is a crystal with birefringence such as, for example, yttrium vanadate or rutile. The compensator can be an isotropic plate such as glass. In certain embodiments, the walk-off device is arranged to separate an incoming beam into the two polarized beams, while in other embodiments, the walk-off device is arranged to combine the two polarized beams into a single beam.
In another aspect of the invention, the optical switch includes a separator which separates the incoming light signal into a first polarized beam and a second polarized beam so that the polarization direction of the first beam is orthogonal to that of the second beam. A first converter rotates the polarization direction of one of the two beams to align the polarization direction of that beam with the polarization direction of the other beam. An electro-optic (E-O) deflector deflects the two beams and a compensator displaces one of the two deflected beams relative to the other beam to minimize polarization dependent losses. A second converter rotates the polarization direction of the displaced beam so that it is orthogonal to the polarization direction of the other beam. A combiner then combines the two beams into a single output light signal which is directed to the output port.
Embodiments of this aspect can include one or more of the following features. In some embodiments, the separator and the combiner are walk-off devices. The walk-off devices can be crystals with birefringence properties such as yttrium or rutile. The first and the second rotators can be Faraday rotators, quartz rotators, or half-wave plates, and the compensator can be an isotropic plate, such as glass.
In certain embodiments, the E-O deflector includes a prism having an index of refraction that changes as an electric field is applied across the prism. For example, the prism can be a crystal made from lithium tantalate, lithium niobate, or KTN. In other embodiments, the prism can be made from a ferroelectric ceramics. Alternatively, the prism can be a liquid crystal dispersed polymer.
Some switches have an E-O deflector having two or more prisms. In some embodiments, the prisms have linear E-O properties such that the deflection of the two beams produced by the prisms is additive when the electric field applied to each prism has the same polarity. In other embodiments, the prisms are configured in a domain inversion arrangement. In this arrangement, the deflection of the two beams produced by the prisms is additive when the electric field applied to a respective prism has a polarity that is opposite to that applied to an adjacent prism. In yet other embodiments, the prisms are made from materials with quadratic E-O properties.
The optical switch can include or can be coupled with a collimator which directs the incoming light signal to the separator. The collimator can include, for example, an input lens and an input port. The optical switch can also include an output collimating lens and an array of output ports. The input port and the output ports are typically optical fibers. The output ports can be coupled to a fiber pigtailed channel wave-guide array which directs the light signal from the output lens to the array of output ports.
Related aspects of the invention include a method of using the device as an ultrafast 1×N switch. The method includes separating the input light signal into a first polarized beam and a second polarized beam so that the polarization direction of the first beam is orthogonal to that of the second beam. The polarization direction of one of the two beams is rotated to align with that of the other beam. The two beams are then deflected, and subsequently one of the two deflected beams is displaced an appropriate distance to minimize polarization dependent losses. The polarization direction of the displaced beam is rotated so that its polarization direction is again orthogonal to that of the other beam. The two beams are then combined into a single output light signal.
Embodiments of the invention may have one or more of the following advantages. The optical switch of the present invention is able to quickly redirect a light signal from one output fiber to another in time periods in the order of nanoseconds. The optical switch is compact in size, reliable, and requires low power to operate. A particular advantage of the optical switch is that it can switch the direction of a light signal in a polarization independent manner.
R
Renka Corporation
Rojas Omar
Sanghavi Hemang
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