Optical Switching Device

This application is related to one, or more, of the following U.S. patent applications: Ser. Nos. 795,156; 795,151; 595,150; 795,149; 795,155; 795,138; 795,148; 795,154; 795,152; 795,296 all filed on even date herewith. All of the above applications are assigned to the assignee hereof.

The present invention generally relates to an optical switching device and, in particular, relates to one such optical switching device having means for selectively changing the polarization of the separated components of an inputted light beam.

The use of optical fibers as a telecommunication and transmission medium has numerous advantages compared to existing telecommunication and transmission media. For example, optical fibers can sustain a broader bandwidth signal and hence can convey larger quantities of information over the same period of time than existing media. Further, light waves are shorter than the conventional microwaves commonly used in existing telecommunication systems and thus a substantial reduction in the physical size of components is readily achievable. This further results in a cost reduction for materials, packaging and manufacturing. Still further, current optical fibers exhibit little or no electromagnetic radiation or radio frequency radiation thus resulting in negligible impact on the surrounding environment. In addition, fiber optics are relatively insensitive to radio frequency interference from surrounding devices or systems.

To be viable, every telecommunication system must include some means for controllably redirecting a signal, or at least a portion thereof, to or from a transmission media or between one or more such medium. In the case of an optical telecommunication system the preferred means is an optical switch. Currently, optical switches are generally mechanical in nature.

However, mechanical switches require relatively high driving power and are subject to wear, abrasion and fatigue. As a result, mechanical switches are prone to failure after repeated use. In addition, since a rather small optical fiber is usually displaced from alignment with one port fiber into alignment with another port fiber, mechanical switches can easily become expensive. One particular reason for this expense is the very small tolerances required to ensure the proper alignments between the optical fiber moved and the optical fibers of the ports.

Recently, liquid crystal optical switching devices have been proposed as an alternative to mechanical switches. Typical of liquid crystal optical switches proposed to date are those described in U.S. Pat. No. 4,201,422 issued to McMahon et al. on May 6, 1980, U.S. Pat. No. 4,278,327 issued to McMahon et al. on July 14, 1981, and U.S. Pat. No. 4,385,799 issued to Soref on May 31, 1983. Therein, a plurality of liquid crystal switch designs are described wherein optical fibers are attached to the side angled surfaces of a pair of opposing trapezoidal prisms. The trapezoidal prisms are arranged with the bases thereof parallel and with liquid crystal material positioned therebetween.

Although the optical switches described in these references have some advantages over mechanical switches, specifically no moving parts, these liquid crystal switches are both difficult to manufacture and expensive since all the surfaces of the trapezoidal prisms must not only be optically flat, but be fixed at a precise angular relation to each other. As a result, the liquid crystal optical switches, as described in the above references, are presently impractical for optical communication systems.

Consequently, since a typical telecommunications network may include hundreds, it not thousands, of switching devices, a low cost, readily manufactured optical switching device is needed in order to more fully realize the advantages of an optical telecommunication system.

Accordingly, it is one object of the present invention to provide a relatively inexpensive liquid crystal optical switching device that is amenable to mass production techniques.

This object is accomplished, at least in part, by an optical switching device having means for selectively changing the polarization of the components of a separated incident light beam.

Other objects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description read in conjunction with the appended claims and the drawing attached hereto.

FIG. 1 is a cross sectional view, not drawn to scale, of an optical switching device embodying the principles of the present invention and showing a first optical path for an incident light beam;

FIG. 2 is a cross sectional view of the device shown in FIG. 1 showing a second optical path for the incident light beam; and

FIG. 3 is a cross sectional view of another embodiment of an optical switching device embodying the principles of the present invention.

An optical switching device, generally shown at 10 in FIGS. 1 and 2 of the drawing and embodying the principles of the present invention, includes means 12 for splitting a light beam into polarized components thereof, means 14 for recombining the polarized components, first means 16, positioned in the path of one of the polarized components, for selectively changing the polarization thereof and second means 18, positioned in the path of the other one of the polarized components, for selectively changing the polarization thereof, whereby the recombined components can be directed along a preselected optical path.

Preferably, the device 10 further includes at least a first input port 20 and first and second output ports, 22 and 24, respectively. In the embodiment shown in FIGS. 1 and 2, the device 10 further includes a second input port 26. In addition, each port, 20, 22, 24 and 26, is associated with a transparent member, 28, 30, 32 and 34, respectively. Preferably, two of members, 28 and 34, provide means for redirecting the polarized components toward the means 14 for recombining the polarized components.

In one implementation, the means 12 for splitting an incident light beam includes a liquid crystal beam splitter. As more fully discussed below the liquid crystal beam splitter is, preferably, disposed such that a light beam conveyed to the device 10 by the first input port 20 thereof is incident thereon at an angle greater than the critical angle, for example, between the first and fourth transparent members, 28 and 34, respectively. Further, the liquid crystal beam splitter can either be defined by a means 36 for retaining a liquid crystal material 38 between the members, 28 and 34, or alternatively, the liquid crystal beam splitter can be a discrete element, or cell, that is self-contained. Preferably, the liquid crystal material is a nematic type, although other types may also be used. A discrete cell beam splitter is preferred because the ease of fabrication of the device 10 is improved.

In one particular embodiment, the means 14 for recombining the polarized components is identical to the means 12 for splitting a light beam. In fact, the means 12 for splitting a light beam and the means 14 for recombining the components thereof can be a single discrete cell. One such discrete cell particularly adaptable for use in the device 10 is discussed and described in U.S. patent application Ser. No. 795,150 entitled LIQUID CRYSTAL CELL FOR USE IN AN OPTICAL SWITCH filed on even date herewith and assigned to the assignee hereof. This application is deemed incorporated herein by reference. Therein, a dicrete liquid crystal beam splitter is discussed that includes a pair of glass slides having a, for example, nematic, liquid crystal material retained therebetween. Since such cells are inherently bidirectional such a discrete cell is directly applicable for the means 12 for splitting a light beam and as the means 14 for recombining the polarized components thereof in the device 10.

In the embodiment shown in FIGS. 1 and 2 the first and second means, 16 and 18, respectively, for selectively changing the polarization of the light beam component passing therethrough are substantially identical. Each of the means, 16 and 18, include first and second boundary members, 40 and 42, respectively, having a twisted nematic liquid crystal material 44 disposed therebetween, means 46 for retaining the twisted nematic liquid crystal material 44 therebetween and means 48 for sustaining an electromagnetic field across the twisted nematic liquid crystal material 44. In the preferred embodiment, the twisted liquid crystal is nematic, although other types of twisted liquid crystal material can also be used. Preferably, each of the means 48 for sustaining an electromagnetic field includes first and second electrodes, 50 and 52, respectively, extending to an edge 54 of the first and second means, 16 and 18, respectively, whereat the electrodes, 50 and 52, can be connected to electrically conductive leads 56 for connecting to a source control voltage 58. As known in the liquid crystal art, a twisted nematic liquid crystal material is one wherein the liquid crystal molecules are oriented parallel to both of the glass slides but rotated 90.degree. with respect to each other. This produces a 90.degree. twist of the molecules from one side of the liquid crystal material to the other. Thus, when an electromagnetic field is applied thereacross the molecules are reoriented so that they are perpendicular to both slides of the liquid crystal cell. In the unpowered condition, i.e., no electromagnetic field applied, the light incident on the twisted liquid crystal material is rotated 90.degree., that is, perpendicularly polarized components are rotated so that they become parallel polarized and vice versa.

In the preferred embodiment, each port, 20, 22, 24 and 26, is associated with one of the transparent members, 28, 30, 32 and 34, respectively. Each port, 20, 22, 24 and 26, preferably includes a blind hole 60 extending into the associated transparent member, 28, 30, 32 and 34, and a means 62 disposed therein for collimating and a light beam traversing the port, 22, 24, 26 or 28. Each port, 20, 23, 24 and 26, is adapted to receive an optical fiber 64 for connecting the device 10 with an optical system. The use and parameter of recessed ports for connecting optical fiber to a liquid crystal optical switching device is discussed and described in U.S. patent application Ser. No. 795,156 entitled LIQUID CRYSTAL OPTICAL SWITCHING DEVICE filed on even date herewith and assigned to the assignee hereof. This application is deemed incorporated herein by reference.

In the device 10 the first and second transparent members, 28 and 30, respectively, include the first input and output ports, 20 and 22, respectively, and are disposed on opposing sides of the first means 16 for selectively changing the polarization of a light beam component passing therethrough. In addition, the first polarization changing means 16 and the first and second transparent members, 28 and 30, respectively, are all disposed on one side of the beam splitting means 12 and recombining means 14. In one specific arrangement, the first electrode 50 of the first polarization changing means 16 is oriented proximate the first transparent member 28 and the second electrode thereof is proximate the second transparent member 30. Hence, the first polarization means 16 and the first and second transparent members, 28 and 30, as discrete elements can be preassembled prior to assembly with the discrete beam splitting means 12 and recombining means 14.

Similarly, the third transparent member 32 having the second output port 24 associated therewith, the fourth transparent member 34 having the second input port 26 associated therewith and the second polarization changing means 18 can be preassembled prior to the final assembly of the device 10.

In the embodiment shown in FIGS. 1 and 2 the first and fourth transparent members, 28 and 34, respectively, are cooperatively sized with the first and second polarization changing means, 16 and 18, respectively, and the second and third transparent members, 30 and 32, respectively, such that the polarized components of an incident light beam are redirected toward the beam recombining means 14 by the air/member interface thereof prior to traversing the polarization changing means, 16 and 18.

In the specific implementation, the beam splitting means 12 and the beam recombining means 14 includes a nematic liquid crystal material 38 such as EM 1132 manufactured and marketed by E. Merck of Rahway, N.J. The boundary members, 40 and 42 are glass slides having a thickness of about 0.5 millimeters and spaced apart about 12 micrometers. The twisted nematic liquid crystal material 44 is, for example, EM 1132 marketed and manufactured by E. Merck of Rahway, N.J. The transparent members, 28, 30, 32 and 34, are, preferably, molded clear plastic.

In operation, the nematic liquid crystal material 38 of the beam splitting means 12 and the beam recombining means 14 characteristically completely reflects perpendicularly polarized light, represented by solid line paths in the drawings, and appears transparent to parallel polarized light, represented by the dashed line paths in the drawings. Hence, when a plane polarized light beam is incident thereupon at, or greater than, the critical angle, the parallel polarized component is substantially completely transmitted therethrough, i.e., the incident light beam is split into two components. Upon being split into the parallel and perpendicular components, the components are directed to the transparent member/air interfaces of the first and fourth transparent members, 28 and 34, respectively, whereat they are redirected, i.e., reflected through the first and second polarization changing means, 16 and 18, respectively, and recombine in the nematic liquid crystal recombining means 14. Whereafter, as shown in FIG. 1, the recombined light beam exits the device 10 via the first output port 22.

With regard to the functional operation of twisted nematic liquid crystal material, it is understood that in an unpowered mode, i.e., when no electromagnetic field is sustained across the twisted nematic liquid crystal, the polarization of the light beam component traversing that particular cell are rotated 90.degree.. That is, the parallel polarized component entering the one of the means, 16 or 18, exits as a perpendicular polarized component. Likewise, a perpendicularly polarized component entering one of the means, 16 or 18, in the powered mode exits as a parallel polarized component. However, in the powered mode, i.e., when an electromagnetic field is sustained across the twisted nematic liquid crystal material, the polarization components of the light traversing that particular cell is unchanged.

Referring now to FIG. 2, the device shown in FIG. 1 is shown with an incident light beam entering the first input port 20 in the powered mode. As shown therein, the incident beam is split into its perpendicular and parallel polarization components. In the unpowered mode, the perpendicularly polarized component is reflected from first transparent member/air interface through the first polarization changing means. 16. In this instance, however, this light component exits the means 16 as a parallel polarized light beam and is thus transmitted directly through the beam recombining means 14 to the second output port 24. In a similar fashion the initially transmitted parallel polarized component is reflected from the second transparent member/air interface through the second polarization changing means 18 exiting as a perpendicularly polarized component that is reflected by the beam recombining means 14 and thus directed to the second output port 24. Hence, the application of an electromagnetic field across the first and second polarization changing means, 16 and 18, effects the switching of the recombined light beam from the first output port 22 to the second output port 24.

A second embodiment, generally indicated at 66 in FIG. 3 and embodying the principles of the present invention, is configured such that the first and second polarization changing means, 16 and 18, are in the path of the split components prior to the reflection thereof by the transparent member/air interfaces. For convenience, reference numerals designating particular elements in FIGS. 1 and 2 are used to designate similar elements in FIG. 3. In addition, the embodiment shown in FIG. 3 includes a discrete beam splitting means 12 positioned between the first transparent member 28 and the second transparent member 34. The discrete beam splitting cell can, in one embodiment, be a liquid crystal cell, or alternatively, can be a cross polarized quarter wavelength plate. In addition, the means 14 for recombining the components of the split light beam is a discrete liquid crystal cell. Further, the first and second polarization changing means, 16 and 18, respectively, are formed in a single discrete twisted nematic liquid crystal cell. The operation of the device 66 shown in FIG. 3 is identical to that discussed above with respect to the device 10 shown in FIGS. 1 and 2.

The optical switching devices, 10 and 66, are advantageous in that cross talk between the output ports, 22 and 24, is considerably reduced compared to other known liquid crystal optical switches. In addition, the devices, 10 and 66, require considerably less voltage to effect the switching between the output ports, 22 and 24. For example, those devices shown in the above-referenced U.S. patent application entitled LIQUID CRYSTAL OPTICAL SWITCHING DEVICE and in U.S. patent applications Ser. Nos. 795,151 and 795,155 entitled LIQUID CRYSTAL OPTICAL SWITCHING DEVICE HAVING MINIMIZED INTERNAL LIGHT PATH and LIQUID CRYSTAL OPTICAL SWITCHING DEVICE HAVING REDUCED CROSSTALK, respectively, all filed on even date herewith and assigned to the assignee hereof. These above referenced patent applications are deemed incorporated herein by reference. Therein, a typical voltage for switching a light beam between the output ports thereof would be on the order of about 30 volts. The devices, 10 and 66, as described herein requires less than 5 volts applied to each of the polarization changing means, 16 and 18.

Further, the basic structures of the devices, 10 and 66, described herein are particularly applicable for use in other configurations, such as, a device for multiplexing/demultiplexing a particular frequency component with a multifrequency incident light beam and an optical interface to, for example, an analog system. Such configurations are discussed and described in U.S. patent applications Ser. Nos. 795,154 and 795,149 entitled LIQUID CRYSTAL WAVE DIVISION DEVICE and OPTICAL INTERFACE APPARATUS, respectively, both filed on even date herewith and assigned to the assignee hereof. These applications are deemed incorporated herein by reference.

Although the present invention has been described herein with respect to specific exemplary embodiments, it will be understood that other arrangements and configurations may be developed that nevertheless do not depart from the spirit and scope of the present invention. Hence, the present invention is deemed limited only by the appended claims and a reasonable interpretation thereof.