Polarization insensitive optical switching and routing systems and methods of manufacturing and operation

Polarization converter assemblies are utilized to provide switching and routing systems with effective coupling between a first and second router assemblies, and to provide polarization insensitive switching and routing systems. An improved optical switching/routing system of this invention includes a first router assembly, a second router assembly and a polarization converter assembly, each assembly optically aligned with one another, the polarization converter assembly being optically interposed between the first router assembly and the second router assembly. Embodiments of the polarization converter include, but are not limited to, liquid crystal spatial light modulators or half-wave retarders. A polarization insensitive switching and routing system of this invention includes a polarization separating sub-system, a selectable switching/routing sub-system, and a polarization combining sub-system. In one embodiment, the polarization separating sub-system includes a polarization splitter and a patterned polarization converter and the polarization combining sub-system includes a patterned polarization converter and a polarization combiner.

FIELD OF THE INVENTION

The present invention relates generally to interconnection and switching systems, and, more particularly, to optical switching/routing (interconnecting) systems which incorporate the use of selectable switching and routing components.

BACKGROUND OF THE INVENTION

With the advent of substantial new uses for high bandwidth digital and analog optical systems, there exists a greater need to effectively control the route of optical beams from among many possible paths. This is especially true in digital computing systems where signals must be routed among processors, in analog systems such as phased array radar, and in the switching of high bandwidth optical carriers in communication systems. However, it should be realized that these are just several of numerous systems that require the use of an optical switching or routing mechanism.

In many current and future systems light beams are modulated in a digital and/or analog fashion and are used as “optical carriers” of information. There are many reasons why light beams or optical carriers are preferred in these applications. For example, as the data rate required of such channels increases, the high optical frequencies provide a tremendous improvement in available bandwidth over conventional electrical channels such as formed by wires and coaxial cables. In addition, the energy required to drive and carry high bandwidth signals can be reduced at optical frequencies. Further, optical channels, even those propagating in free space (without waveguides such as optical fibers) can be packed closely and even intersect in space with greatly reduced crosstalk between channels. Finally, operations that are difficult to perform in the lower (e.g., radio) frequencies such as time shifting for phased array applications can often be performed more efficiently and compactly using optical carriers.

A common problem encountered in applications in which high data rate information is modulated on optical carrier beams is the switching of the optical carriers from among an array of channels. These differing optical channels may represent, for example, routes to different processors, receiver locations, or antenna element modules. One approach to accomplish this switching is to extract the information from the optical carrier, use conventional electronic switches, and then re-modulate an optical carrier in the desired channel. However, from noise, space, and cost perspectives it is more desirable to directly switch the route of the optical carrier from the input channel to the desired channel, without converting to and from the electronic (or microwave) regimes.

Another common problem arises in applications where there is a need to arbitrarily interconnect any of n electronic input channels to any of n output channels. This “crossbar switch” type of function is difficult to implement electronically. In such a case better performance may be obtained by modulating the electronic information on optical carriers, and switching the optical carriers to the desired channel where they may be reconverted to electronic information if desired. This conversion to optical carriers permits the use of optical switching techniques as in the present invention, as well as providing a ready interface to other optical interconnect schemes.

Another problem that is typical in optical switching systems is the insertion loss they impose. Some switching systems divide the input signal power into many parts, and block (absorb) the ones that are not desired. Others use switches that are inefficient and absorb or divert a significant part of the input signal.

The optical switching and routing systems of U.S. Pat. No. 5,771,320 (issued to T. W. Stone on Jun. 23, 1998), incorporated by reference herein, overcame some of the problems associated with complexity and performance, including number of required switching devices and control signals, switch isolation, noise and crosstalk suppression, insertion loss, spurious reflections, data skew, and compactness that were present in preceding optical switching systems.

One optical switching and routing system described in U.S. Pat. No. 5,771,320 utilizes a pair of router assemblies made up of a series of switchable diffraction gratings. The second router assembly is crossed in orientation with respect to the first router assembly. In one embodiment, the switchable diffraction gratings are polarization selective gratings. Such switchable polarization selective gratings typically transmit light of a first polarization in both the switched states and diffract light of a second distinct polarization in one switched state, and transmit light of the second distinct polarization in the other switched state.

In the type of switching and routing systems utilizing switchable polarization selective gratings, there is a need for improved coupling between the first and second router assemblies.

Further, in the type of switching and routing systems utilizing switchable polarization selective gratings, there is also a need for polarization insensitive switching and routing systems.

It is one object of this invention to provide polarization selective switching and routing systems with effective coupling between the first and second router assemblies.

It is another object of this invention to provide systems and methods for polarization insensitive switching and routing.

It is yet another object of this invention to provide optical systems and methods that can be utilized in polarization insensitive switching and routing systems.

BRIEF SUMMARY OF THE INVENTION

The objects set forth above as well as further and other objects and advantages of the present invention are achieved by the embodiments of the invention described hereinbelow and set out in the claims appended hereto.

The systems of present invention utilize polarization converter assemblies to provide switching and routing systems with effective coupling between a first and second router assemblies, and to provide polarization insensitive switching and routing systems.

An improved optical switching/routing system of this invention includes a first router assembly, a second router assembly and a polarization converter assembly, each assembly optically aligned with one another, the polarization converter assembly being optically interposed between the first router assembly and the second router assembly. Embodiments of the polarization converter include, but are not limited to, liquid crystal spatial light modulators or half-wave retarders.

The polarization insensitive switching and routing system of this invention includes a polarization separating sub-system, a selectable switching/routing sub-system, and a polarization combining sub-system. In one embodiment, the polarization separating sub-system includes a polarization splitter and a patterned polarization converter and the polarization combining sub-system includes a patterned polarization converter and a polarization combiner.

An embodiment of the polarization separating sub-system of this invention includes a polarizing beam-splitter and a patterned polarization converter. The polarization converter has an isotropic region and a second region. A substantially collimated optical beam with arbitrary polarization state incident on the beam-splitter will exit as two beams with parallel polarization vectors. Anisotropic crystalline materials, such the “walk-off polarizer” offered by Optics for Research, Inc. of Caldwell, N.J., can be utilized for the polarizing beam-splitter. Possible embodiments of the second region of the polarization converter are, but not limited thereto, a half-wave retarder and a twisted nematic configuration.

Another embodiment of the polarization separating sub-system of this invention includes a pair of polarization splitting gratings and a patterned polarization converter.

An embodiment of the polarization combining sub-system of this invention includes a patterned polarization converter and an anisotropic crystalline material acting as a combiner. Another embodiment of the polarization combining sub-system of this invention includes a patterned polarization converter and a pair of polarization combining gratings.

A method for fabricating an embodiment of the polarization converter is also disclosed.

For a better understanding of the present invention, together with other and further objects thereof, reference is made to the accompanying drawings and detailed description and its scope will be pointed out in the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to better understand the present invention described below, it should be noted that certain terms used in the description of the invention have been used interchangeably.

In the following descriptions of the present invention, the terms such as “light” and “optical radiation” may be used interchangeably, and these terms both include electromagnetic radiation over the entire spectrum of wavelengths such as, for example, ultraviolet, visible, and infrared. Also, the term “optical”, for example, as applied to components and systems, refers not only to optical components and systems, but also to electro-optical components and systems.

Furthermore, terms such as “beams” and “channels” may also be interchanged, in certain instances, based upon their usage as recognized in the art.

The optical switching/routing systems of this invention utilize polarization converter assemblies to provide switching and routing systems with effective coupling between a first and second router assemblies, and to provide polarization insensitive switching and routing systems.

FIG. 1depicts a schematic representation of an embodiment of an optical switching/routing system10of this invention with effective coupling between first and second router assemblies (front half and back half)15,40. Referring toFIG. 1, the first router assembly15is capable of receiving one or more individual beams5of electromagnetic radiation with polarization in a predetermined plane of polarization25. The first router assembly15has a predetermined orientation and includes grating means20defining several independently controlled segments for directing the one or more individual beams5of electromagnetic radiation from preselected locations35along the segments for input into a polarization converter assembly30. The polarization converter assembly30is capable of receiving the one or more individual beams5of electromagnetic radiation from preselected locations35along the segments20of the first router assembly15, and of rotating the predetermined plane of ploarization25to produce an output plane of polarization45. The second router assembly40being has an orientation different from the predetermined orientation of the first router assembly15. The second router assembly40includes grating means20defining several independently controlled segments for receiving each of the individual beams5from the polarization converter assembly30and directing the individual beams5for output50.

Embodiments of the router assemblies are described in U.S. Pat. No. 5,771,320, incorporated by reference herein. The gratings20are switchable gratings and the switching is controlled by control signals12(only two of which are shown). The gratings are separately switchable in segments22for each of the channels in the input array5. This independent switching of each of the gratings20for each input channel can be accomplished by pixellating each of the gratings20into m stripe segments22. In the embodiment shown inFIG. 1, the second router assembly40, which is nearly identical in structure the first router assembly15, is crossed in orientation with respect to the first router assembly15. The segments22of the second router assembly40are rotated 90 degrees with respect to the segments22of the first router assembly15.

During operation of the switching and routing system10ofFIG. 1, control signals12effect the “on-off” operation of the gratings20, thereby directing the input beams5of each channel to the desired output channels of output array50. The first router assembly15contains n cascaded gratings20, each of which is pixilated into m separately controllable segments22. Thus there are n*m control signals12required to independently route each of the input beams5to its selected column in the central plane37. The second router assembly40also needs m*n control signals12to route the selected beam from each column to the desired output channel. The total control line count for a general m channel to m channel switch for this embodiment is thus 2*m*n.

The embodiments of the optical switching and routing systems described in U.S. Pat. No. 5,771,320 utilize volume phase diffraction gratings that permit switching of the incident energy between two or more orders. The primary mechanisms considered which permit this diffracted-order switching are electrical switching, optical switching, and polarization switching. The switched gratings can be optically switched, electrically switched, polarization switched, or switched based on other mechanisms. Currently it is preferred that electrical and polarization switching techniques are used with the present invention since they are extremely fast (switching times in the microsecond regime or faster). Electrical switching can be obtained in materials such as Polaroid DMP-1 28 photopolymer imbibed with nematic liquid crystals or, for example, polymer dispersed liquid crystals. The gratings formed utilizing polymer dispersed liquid crystals or photopolymer imbibed with nematic liquid crystals are polarization sensitive gratings.

Referring again toFIG. 1, during operation of the optical switching and routing system10of this invention utilizing polarization sensitive gratings, control signals19effect the “on-off” operation of the gratings20. Input beams5of electromagnetic radiation with polarization in a predetermined plane of ploarization25are steered by the enabled segments22of gratings20to preselected locations on the output plane37of the first router assembly15. When a particular grating segment22is “on,” the beam incident on that segment is completely switched by diffraction with little or no loss from the incident beam to a diffracted beam traveling in a new direction. The steered beams5from the preselected locations on the output plane37of the first router assembly15are inputs to the polarization converter assembly30. The polarization converter assembly30rotates the predetermined plane of ploarization25into an output plane of polarization45. The output plane of polarization45is chosen so that the beams5are effectively transmitted by the second router assembly40. The beams5of electromagnetic radiation with polarization in an output plane of polarization45are steered by the enabled segments22of gratings20in the second router assembly40to an output location in output array50.

FIG. 2is a schematic representation of an embodiment of the optical switching/routing system ofFIG. 1in which the polarization converter30includes a liquid crystal spatial light modulator (SLM). In this embodiment, the SLM has two states. In one state, an SLM pixel rotates the polarization plane by 90°; in the other state, the polarization plane is not rotated. Exemplary embodiments are 2-D SLMs based on ferroelectric liquid crystals (such as those available from Displaytech), or SLMs based on nematic liquid crystals (such as those available from Meadowlark Optics). Other embodiments include an SLM based on a twisted nematic configuration. The SLM polarization converter60also includes steering gratings directly before and directly after the central plane SLM. In one embodiment, a first steering grating, disposed between the output plane37and the SLM, would steer beams5normal to the output plane37of the first router assembly15. The second steering grating, disposed after the SLM, would steer the beams5in the input vertical plane of the second router assembly40. The first steering grating ensures normal (perpendicular) incidence of the beams5onto the SLM. The steering gratings may be pixilated static gratings or switchable gratings.

FIG. 3is a schematic representation of an embodiment of the optical switching/routing system ofFIG. 1in which the polarization converter30includes a half-wave retarder. In this embodiment, the polarization converter70includes a zero-order half-wave retarder that has its optic axis in a plane parallel to the output plane37of the first router assembly15. The optic axis is oriented at 45° with respect to the polarization plane25of the incident beams. The polarization converter70also includes steering gratings directly before and directly after the central plane half-wave retarder. In one embodiment, a first steering grating, disposed between the output plane37and the half-wave retarder, would steer beams5normal to the output plane37of the first router assembly15. The second steering grating, disposed after the half-wave retarder, would steer the beams5in the input vertical plane of the second router assembly40. The first steering grating ensures normal (perpendicular) incidence of the beams5onto the half-wave retarder.

In one embodiment, half-wave retarders are comprised of anisotropic materials. In another embodiment, the half-wave retarder utilizes a solid twisted nematic film in the central plane. Such a solid twisted nematic film could include, but are not limited to, polymerizable nematic, or chiral nematic, liquid crystals. (Examples of half-wave retarders can be found in the products offered by Meadowlark Optics and Newport Research Corporation.) Other embodiments of half-wave retarders are within the scope of this invention.

A schematic representation of an embodiment of a polarization insensitive optical switching/routing system100of this invention is shown inFIG. 4. Referring toFIGS. 4 and 5, the polarization components are, as is usually done, defined with respect to the local interface. In an embodiment of the grating based switching/routing system shown inFIGS. 1,2, and3, if a pair of beams with “p” polarization constitute the input channel to the grating based switching/routing system, where the gratings diffract “p” polarized light when the gratings are “on”, the polarization of the output channel of the first router assembly15is rotated ninety (90) degrees by the polarization converter assembly30and provided as input to the second router assembly40. In this embodiment, the gratings (segments) of the second router assembly40are rotated 90 degrees with respect to the segments of the first router assembly15. Since the gratings in the second router assembly40diffract “p” polarized light when the gratings are “on” and the polarization components are defined locally with respect to the grating, the polarization component of the output channels of the second router assembly40will be labeled as a “p” component although the polarization component of the output channels is rotated by 90 degrees with respect to the polarization component of the input channels to the first router assembly15.

Referring toFIG. 4, the polarization insensitive optical switching/routing system100includes a polarization separating sub-system110, a selectable switching/routing sub-system120, and a polarization recombining sub-system130. The polarization separating sub-system110includes a polarization splitter140and a patterned polarization converter150. (“Patterned” as used herein includes a “tiled” polarization converter. A “tiled” polarization converter is one that has assembled from sub-units or components.) The patterned polarization converter150has an isotropic region152and a second region155such that an optical beam105with arbitrary polarization state incident on the polarization splitter140will exit the patterned polarization converter150as two beams with parallel polarization vectors125.

The polarization recombining sub-system130includes a patterned polarization converter160and a polarization combiner170. The patterned polarization converter160has an isotropic region162and a second region165such that two beams with parallel polarization vectors135incident on the patterned polarization converter160will exit the polarization combiner170as an optical beam175with arbitrary polarization state.

If selectable switching/routing sub-system120includes polarization sensitive gratings, the gratings operate on one component of polarization (labeled “p” inFIG. 4). In order to make the switching/routing system120function with the other component of polarization (labeled “s” inFIG. 4) or with light containing both components of polarization, the switching systems are placed between symmetric polarization splitter110and combiner130assemblies as shown inFIG. 4. Although “p” and “s” are used herein as polarization labels, it should be noted that “p” and “s”, and “ordinary” and “extraordinary”, as used herein refer to exemplary polarization labels and the methods of this invention are not limited to these exemplary cases. It should also be noted that the methods of this invention can be applied to, but are not limited to, orthogonal polarization components.

During operation of the system ofFIG. 4, an optical beam105with arbitrary polarization state is incident on the polarization splitter140. In one embodiment, the polarization splitter140includes a uniaxial crystal such as calcite, quartz, etc. The thickness of the splitter140is selected so that the s and p components are spatially separated into a pair of twin beams. The twin beams then encounter the patterned polarization converter150that rotates the s component beam into the p-polarized state. In one embodiment, the pattern is selected so as to leave the p-polarized beam in the p state. The two p-polarized twin beams125corresponding to each input beam105then propagate through the switching/routing system120, and are routed accordingly. In one embodiment, the patterned polarization converter150,160includes a polymerized twisted nematic rotator.

At the output of the switching/routing system120, the exiting twin beams135are then symmetrically recombined. In order to balance path lengths of the two component beams, the patterned polarization converter160is now aligned so that the beam that was transmitted through the splitter (undeviated) at the front of the system is now deviated symmetrically as shown inFIG. 4. The thickness of the combiner170is chosen so that the two polarization component beams162,165are brought back together again and are spatially combined in an output optical beam175with arbitrary polarization state.

In another embodiment of the a polarization insensitive optical switching/routing system100, shown inFIG. 5, polarization sensitive gratings175,180,185,190are used to accomplish the split and combine functions. The polarization sensitive gratings175,180are used to split and separate the s and p polarization components into twin, spatially separated beams152,155as inFIG. 4. And as inFIG. 4, a patterned polarization converter150produces two p-polarized twin beams125corresponding to each input beam105. The two p-polarized twin beams125corresponding to each input beam105then propagate through the switching/routing system120, and are routed accordingly. At the output of the switching/routing system120, the exiting twin beams135are then symmetrically recombined. The patterned polarization converter160operates as inFIG. 4. The polarization sensitive gratings185,190spatially combine the two polarization component beams162,165into the output optical beam175with combined (arbitrary) polarization state. Also as inFIG. 4, the recombination is symmetric so as to balance the path lengths of the two twin polarization component beams.

In one embodiment of the polarization insensitive optical switching/routing system100, the optical switching/routing system ofFIG. 1is utilized as the selectable switching/routing sub-system120. In the embodiment of this invention in which the switching/routing system120includes a pixilated switchable grating, such as that shown inFIG. 1, the two p-polarized twin beams125are typically switched/routed together (in tandem) in the same manner a single beam (channel) is switched through the switching/routing system120.

It should be noted that the polarization separating/recombining sub-systems could be considered as a separate optical systems (also referred to as a polarization diversity filters). The polarization sensitive grating based polarization diversity filters (PDF) have cost advantages (in particular at large aperture). Multi channel capabilities (for example, a single large aperture grating can accept many parallel input channels) absent in prior art splitters, such as anisotropic and micro-optic polarization beam splitters, can be achieved in polarization sensitive grating based PDFs. Since only two components are required and these two components are readily alignable, the polarization sensitive grating based PDFs have alignment advantages over multi-element PDFs such as micro-optic polarization beam splitters.

In one embodiment, shown inFIG. 5, a pair of identical polarization sensitive volume holographic diffraction gratings175,180, such as described in U.S. Pat. Nos. 5,771,320, and 5,692,077, or made with PDLC, with a photo-polymer such as Polaroid DMP-128, or with dichromated gelatin, which diffract only “p” polarized light and transmits “s” polarized light are used as polarization splitting gratings. The first grating diffracts the “p” polarized light while the “s” polarized light is transmitted undiffracted.

The second grating subsequently diffracts the “p” component in order to render it parallel to the undiffracted “s” polarization component. The separation between the two gratings is sufficient to spatially separate the “s” and “p” component beams. It should be noted that the diffraction angle (and accordingly the spatial frequency) of the two grating can be chosen to optimize the contrast in the polarization splitting.

For the polarization sensitive grating shown inFIG. 5, the diffraction efficiency of the “p” polarization is maximized and the diffraction efficiency for “s” polarization is simultaneously minimized (thus, the “s” polarized beam is transmitted). It should be noted that in another embodiment (not shown), the “s” polarization is maximized and the diffraction efficiency for “p” polarization is simultaneously minimized. The polarization combining performed by the polarization combining gratings185,190is symmetrical to the operation of the polarization splitting gratings175,180. The embodiment shown inFIG. 5results in an optical path balanced system and substantially reduces the temporal chromatic dispersion effects.

A schematic representation of an embodiment of a polarization converting system200(patterned polarization converter) of this invention, which can be utilized as the patterned polarization converter150,160ofFIGS. 4,5, is shown inFIGS. 6a,6b.A detailed description of the polarization converting system200and methods for fabricating one embodiment are given herein below. Referring toFIGS. 6aand6b,the polarization converting system200of this invention includes a polarizing beam-splitter220and a patterned polarization converter230, both of which are more fully described below. During use, as seen inFIGS. 6aand6b,a substantially collimated optical beam215with arbitrary polarization state is incident on the system200through the beam-splitter light receiving surface240and exits as two beams225,235with parallel polarization vectors, as shown inFIGS. 6aand6b.The beam-splitter light emitting surface255has two areas—a first area260and a second area265. The polarization beam-splitter separates the received beam of light215into a beam of light245of a first polarization (also called the ordinary polarization) emitted from the first area260and another beam of light250of a second polarization (also called the extraordinary polarization) emitted from the second area265.

In one embodiment, the polarization converter230of this invention has a first isotropic region270and a second region275. When a substantially collimated optical beam215with arbitrary polarization state is used as input to the polarizing beam-splitter220, the beam of light of the first (the ordinary polarization) polarization245enters the isotropic region270, at normal incidence, through the first region light receiving surface280and exits, as a beam225of the same first polarization, through the first region light emitting surface285. Thus, transport through the isotropic region leaves the polarization unchanged. The output beam225has the same polarization as input beam245.

The beam of light of the second (the extraordinary polarization) polarization50enters the second region275, at normal incidence, through the second region light receiving surface290and exits, as a beam235of the first polarization, through the second region light emitting surface295. Transport through the second region rotates the polarization of the incoming beam250producing an output beam235of the same polarization as the beam225emitted from the isotropic region. Both beams225and235exit the polarization converter230normal to the surface.

The first region light receiving surface280is substantially disposed on the first area260of the beam-splitter light emitting surface255by being in contact with or secured on area260by means of any conventional optically appropriate adhesive. The second region light receiving surface290is substantially disposed on the second area265of the beam-splitter light emitting surface255by also being in contact with or secured on area265by means of any conventional optically appropriate adhesive.

While the above embodiment is described in terms of a substantially collimated optical beam with arbitrary polarization state, containing both ordinary and extraordinary polarization components, incident on the beam-splitter light receiving surface, the embodiment could be also utilized for the case where the incident substantially collimated optical beam contains only ordinary or extraordinary polarization. In this case, one of the two beams entering the polarization converter has null amplitude and the same beam also has null amplitude upon exiting the polarization converter.

Although not limited thereto, anisotropic crystalline materials, such the “walk-off polarizer” offered by Optics for Research, Inc. of Caldwell, N.J., can be utilized for the polarizing beam-splitter. It should be noted that other configurations are possible utilizing one or more sub-elements. For example, micro-optic polarizing beam splitters (including polarizing cube beam splitters) can also be utilized.

In another embodiment of the polarization converting system of this invention, a pair of polarization sensitive gratings is used as the beam splitter.

Possible, but not limited to, embodiments of the second region275of polarization converter230are a half-wave retarder and a twisted nematic polarization converter. As shown inFIGS. 6aand6b,the polarization converter230is utilized at normal incidence.

For a better understanding of the present invention, reference is now made of the following analysis. More specifically, bandwidth considerations can be used to compare the half-wave retarder and a twisted nematic embodiment. For linearly polarized light incident on a half-wave retarder with its plane of polarization at 45° with respect to the optic axis (of the retarder), the optical power Pmstill remaining polarized parallel with the incident light is given by

Pm=cos2⁡(m⁢⁢π2⁢λcλ),(1)
where, λcis the center wavelength of the incident light, 1 is the wavelength of incident light and m is the order of the retarder, with m=1,3,5 . . . for zero-, first, second-order waveplates etc. Note that λc=2Δnd, where Δn is the retarder birefringence and d is the retarder thickness. It is apparent from Eq. (1) that the zero-order half-wave retarder (i.e. m=1) has the broadest bandwidth. In addition, it is the least sensitive to angle of incidence variations. The extinction ratio, or contrast, of the retarder may be defined as follows:
γm=10 log Pm.  (2)

Next, consider the 90° twisted nematic (TN) polarization converter. The optical power at the output of a 90° TN with polarization plane parallel to that of the incident light is given by

Pq=sin2⁢⌈π2⁢1+(λcλ⁢4⁢q2-1)2⌉1+(λcλ⁢4⁢q2-1)2,(3)
where λ is the design wavelength and q is referred to as the order of the TN; q=1, 2, 3 . . . refer to first-, second-, third-minimum TNs etc (as shown by C. H. Gooch and H. A. Tarry, J. Appl. Phys. D 8, 1575 (1975)). The center wavelength of the TN rotator is given by λc=2(Δnd)/(4q2−1), where Δn is the nematic birefringence and d is the TN film thickness. In the case of the TN, the first-minimum TN has the broadest spectral bandwidth. The extinction ratio, or contrast, of the TN is written analogously with Eq. (2).

FIG. 7is a graphical representation of the contrast of the zero-order retarder and the first-minimum TN as a function of wavelength for λc=1550 nm. As can be seen fromFIG. 7, the bandwidth of the first-minimum TN is broader than that of the zero-order retarder.

UV-curable nematic (N) or chiral nematic (N*), such as the RM (reactive mesogens) line of UV-curable nematics from EM Industries of Hawthorne, N.Y., could be used to construct the patterned polarization converter230. The N material could be used to construct retarder-based rotators, and the N or N* material could be used to make the TN rotators.

FIG. 8depicts a flowchart of an embodiment of the method for fabricating an embodiment of the polarization converter230. Referring toFIG. 8, first, a cell (also referred to as a receptacle) is constructed to contain and align the UV-curable nematic (step310,FIG. 8). The cell will generally consist of two substrates separated by appropriately sized spacers. The inner substrate surfaces will be coated with an alignment layer that aligns the nematic along a desired direction. In the case of the retarder, the alignment direction of the top and bottom substrates is the same; for the TN, the alignment directions of the top and bottom substrates are perpendicular. A suitable alignment layer that could be used is a polyimide film provided by Brewer Science (Rolla, Mo.) that contains mechanically-sculpted furrows to align the nematic directors. For example, polyimide SE812 (sold by Brewer Science) is spin-coated onto clean glass substrates to about 1-μm thickness, baked, then mechanically rubbed with a soft cloth. Nematic molecules align on such a polyimide layer, parallel to the rubbing direction.

Next, the cell is filled with the UV-curable nematic (step320,FIG. 8). Filling may take place via capillary action; heating the cell may be necessary if the nematic materials are viscous. Alternatively, the nematic material may be heated on a single substrate that has spacers dispersed on it. A second substrate may be placed on top of this to create a nematic sandwich when the nematic is in the liquid state. Alternatively, the nematic may be solvent-cast onto a single substrate, as described, for example, in U.S. Pat. No. 5,926,241, issued to William J. Gunning, III on Jul. 20, 1999 (see, specifically, col. 6, lines 6–13).

The nematic-filled cell is annealed (step330,FIG. 8) until the liquid crystal achieves the desired configuration dictated by the alignment layers on the substrates: e.g. planar or TN.

A mask is placed in contact with the filled, annealed cell so that the open areas define where the polarization rotation regions of the film shall be (step340,FIG. 8). The film temperature is adjusted to achieve the desired layer anisotropy (step345,FIG. 8), as determined using an optical measurement. In this step, the nematic birefringence Δn is thermally tuned after it is introduced into a cell with fixed thickness d. Note that the polarization state of an optical beam exiting the polarization converter depends on the quantity Δn·d/λ for both the half-wave retarder and twisted nematic configurations where λ is the wavelength of the optical beam.

The mask is, then, exposed with UV light that is effective for curing the nematic (step350,FIG. 8). After the nematic is cured, the mask is removed (step360,FIG. 8) and the cell is heated above the clearing temperature of the un-cured nematic (step370,FIG. 8). The unexposed areas will then become isotropic; when this state has been achieved, the entire cell is flooded with UV light to cure the isotropic regions (step380,FIG. 8). After exposure, the nematic film is allowed to return to room temperature (step390,FIG. 8).

It should be noted that the although the above described embodiments have been described in terms of polarization rotation, other polarization conversion mechanisms are also within the scope of this invention. It should also be noted that although the embodiments of the polarization converter of this invention described above include a first isotropic region and a second polarization converting region, polarization converters including two polarization converting regions are also within the scope of this invention.

It should be further noticed that although the embodiment of the polarization insensitive switching/routing system of this invention described above includes a patterned polarization converter having an isotropic region and a polarization converting region, polarization insensitive switching/routing system including other polarization converters having two polarization converting regions are also within the scope of this invention.

An embodiment of a polarization insensitive switching/routing system of this invention including a polarization separating sub-system being capable of separating an input optical beam into a first optical beam of a first polarization and a second optical beam of a second polarization and emitting a first emitted optical beam of a third polarization and a second emitted optical beam of the third polarization, wherein the selectable switching/routing sub-system is capable of switching/routing the first emitted optical beam and the second emitted optical beam to an output channel of a fourth polarization, the output channel constituting a pair of output beams of said fourth polarization, and wherein the polarization recombining sub-system is capable of recombining the pair of output beams of the fourth polarization into a final output beam of combined polarization, is also within the scope of this invention. In such an embodiment, the polarization converters in either the polarization separating sub-system or the polarization recombining sub-system (or both) could include two polarization converting regions.

Although the invention has been described with respect to various embodiments, it should be realized this invention is also capable of a wide variety of further and other embodiments within the spirit and scope of the appended claims. For example, it should be noted that, although the invention is described above in terms of an embodiment where the two beams with parallel polarization vectors exiting the polarization converter have ordinary polarization, other embodiments are possible. For example, an embodiment in which the two beams with parallel polarization vectors exiting the polarization converter have extraordinary polarization is also possible.