Variable optical attenuator systems

Low loss, reliable variable optical attenuators and 1×2 switches and polarization insensitive low loss, reliable variable optical attenuators and 1×2 optical switches are described. In one embodiment, a system of the present invention includes a polarization separating sub-system a polarization recombining sub-system and one or more switchable volume diffraction gratings to provide polarization insensitive low loss, reliable variable optical attenuators and 1×2 optical switches.

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

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 may be 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. Furthermore, 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.

Optical attenuators perform numerous tasks associated with optical signal transmission systems. One function of an attenuator is to reduce the intensity of an optical signal which enters a photosensitive component. Photosensitive components are affected by variations in light intensity. Therefore, an attenuator causes the light intensity to be within the dynamic range of the photosensitive components. By using an attenuator, damage to the component is precluded. Additionally, the component does not become insensitive to small changes in the optical signal.

In other applications, attenuators serve as noise discriminators by reducing the intensity of spurious signals received by the optical device to a level below the device's response threshold. Moreover, optical attenuators are used to reduce the power of optical signals from an input fiber to an output fiber, and especially to balance optical power between several lines of an optical system. Many optical attenuators are also capable of actively attenuating an optical signal. Variable attenuators are required in some applications where different optical components require dissimilar incident optical signals, and hence variable sensitivities and saturation points. A fixed (i.e., passive) attenuation device is impractical for this purpose.

Attenuators serve to maintain the light level at a constant to compensate for component aging i.e., loss of efficiency in fiber amplifiers and reduced laser output from source, and changing absorption in optical waveguides. Variable attenuators serve to control feedback in optical amplifier control loops to maintain a constant output (e.g., as an automatic gain control element (AGC)).

Some variable attenuator designs require mechanical components or a number of optical components. Both of this type of attenuators exhibit a number of characteristics that are not desirable, such as high manufacturing and assembly costs, reduced reliability and extreme sensitivity to alignment.

There is a need for low loss, reliable variable optical attenuators.

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 sometimes 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.

A 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, scatter, or divert a significant part of the input signal.

A commonly utilized optical switch is a one input, two output switch, also referred to as a 1×2 switch. There is a need for low loss, reliable 1×2 switches.

It is one object of this invention to provide polarization insensitive variable optical attenuators and 1×2 switches.

It is another object of this invention to provide low loss, reliable 1×2 switches.

It is a further object of this invention to provide low loss, reliable variable optical attenuators.

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.

Low loss, reliable variable optical attenuators and 1×2 switches and polarization insensitive low loss, reliable variable optical attenuators and 1×2 optical switches are disclosed in the present invention.

In one embodiment, a system of the present invention includes a polarization separating sub-system a polarization recombining sub-system and one or more switchable volume diffraction gratings to provide polarization insensitive low loss, reliable variable optical attenuators and 1×2 optical switches. The polarization separating sub-system is optically disposed to receive an input optical beam of arbitrary polarization and is also capable of separating the input optical beam into a first optical beam of a first polarization and a second optical beam of a second distinct, orthogonal polarization. The polarization separating sub-system is also capable of emitting a first emitted optical beam of the first polarization and a second emitted optical beam of the first polarization, the emitted first and emitted second optical beams constituting an input channel of the first polarization. The one or more switchable volume diffraction gratings are optically disposed to receive the input channel and are also capable of providing one or more transmitted channels. The one or more transmitted channels include at least one transmitted optical beam of the first polarization and at least one other transmitted optical beam of the first polarization. The polarization recombining sub-system is optically disposed to receive the at least one transmitted optical beam of the first polarization and the at least one other transmitted optical beam of the first polarization and is capable of recombining the at least one transmitted optical beam of the first polarization and the at least one other transmitted optical beam of the first polarization into at least one final output beam of combined polarization.

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.

Low loss, reliable variable optical attenuators and 1×2 switches and polarization insensitive low loss, reliable variable optical attenuators and 1×2 optical switches are disclosed hereinbelow.

FIG. 9shows a pictorial, schematic representation of an embodiment of a variable optical attenuator (VOA) of this invention. The embodiment shown inFIG. 9operates in a configuration hereinafter called the normally-off configuration. In one embodiment the input optical beam420is derived from a single-mode (SM) fiber that is coupled to a collimating lens. Other embodiments derive the input beam from collimated free space beams or collimated sources. The input optical beam420is a beam of arbitrary polarization. The beam420is received by (enters) a polarization separating sub-system430(also referred to as a polarization diversity filter, or a compensator), which separates the input optical beam420into a first optical beam of a first polarization and a second optical beam of a second polarization, the second polarization being distinct from the first polarization (in one embodiment, the two polarizations are s- and p components). The polarization separating sub-system430is capable of emitting a first emitted optical beam of the first polarization and a second emitted optical beam of the first polarization (in one embodiment, the emitted beams are p-polarized). The emitted first and emitted second optical beams constitute an input channel of the first polarization. One or more switchable volume diffraction gratings440(one in the embodiment shown inFIG. 9) are optically disposed to receive the input channel and are capable of providing a transmitted channel. The transmitted channel includes a first transmitted optical beam of the first polarization and a second transmitted optical beam of the first polarization. A polarization recombining sub-system450is optically disposed to receive the first transmitted optical beam of the first polarization and the second transmitted optical beam of the first polarization and is capable of recombining the first transmitted optical beam of the first polarization and the second transmitted optical beam of the first polarization into a final output beam460of combined polarization.

One embodiment of the switchable volume diffraction grating element utilized in the variable optical attenuators/1×2 switches of this invention is the switchable diffraction element (grating) such as that described in U.S. Pat. No. 5,771,320, herein incorporated by reference.

The following paragraphs are excerpted from the detailed description of U.S. Pat. No. 5,771,320, herein incorporated by reference.

The embodiments of the optical switching and routing systems described herein 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-128 photopolymer (as described below) or, for example, polymer dispersed liquid crystals. So as to provide an example of a switching mechanism, one of the electrical switching techniques is described below. Further, switching to intermediate diffraction efficiency status permits switching of a given input signal to more than one output channel (“fan out” as opposed to “one to one” switching).

Recently it has been demonstrated in the literature that high efficiency volume diffraction gratings which are recorded in permeable media, such as the DMP-128 photopolymer manufactured by Polaroid Corporation, Cambridge, Mass., can be made to be rapidly switchable between high and low diffraction efficiency states under electric control by imbibing the structure with nematic liquid crystals. In this technique the crystals are rotated by the applied electric field and their refractive index is switched between ordinary and extraordinary values. By choosing the materials so that one of these switchable values matches that of the host grating material, the grating modulation is effectively switched “off” and “on,” thus switching the diffraction efficiency of the gratings and toggling the diffracted beam between the 0 and first diffracted order. It should also be appreciated that the switching systems described above use switched transmission diffractive gratings.

The embodiments of the optical switching and routing systems described in U.S. Pat. No. 5,771,320 utilize volume phase diffraction (holographic) 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, thermal 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 (for example, switching times in the microsecond regime). Electrical switching can be obtained in materials such as liquid crystal-imbibed Polaroid DMP-128 photopolymer (as described below) or, for example, polymer dispersed liquid crystals. So as to provide an example of a switching mechanism, one of the electrical switching techniques is described below. Further, switching to intermediate diffraction efficiency status permits switching of a given input signal to more than one output channel (“fan out” as opposed to “one to one” switching).

It has been previously demonstrated in the literature that high efficiency volume diffraction gratings which are recorded in permeable media, such as the DMP-128 photopolymer manufactured by Polaroid Corporation, Cambridge, Mass., can be made to be rapidly switchable between high and low diffraction efficiency states under electric control by imbibing the structure with nematic liquid crystals. In this technique the crystals are rotated by the applied electric field and their refractive index is switched between ordinary and extraordinary values. By choosing the materials so that one of these switchable values matches that of the host grating material, the grating modulation is effectively switched “off” and “on,” thus switching the diffraction efficiency of the gratings and toggling the diffracted beam between the 0 and first diffracted order.

In some embodiments of the switchable volume diffraction grating, input beams of electromagnetic radiation with polarization in a predetermined plane of polarization are diffracted by the enabled grating. A substantially polarization insensitive variable optical attenuator (VOA)/1×2 switch can be obtained from the systems of this invention, even with polarization sensitive embodiments of the switchable gratings.

Embodiments of polarization separating/recombining sub-systems are described in U.S. patent application Ser. No. 10/668,975, filed on Sep. 23, 2003, incorporated by reference herein.

The following paragraphs are excerpted from the detailed description of U.S. patent application Ser. No. 10/668,975, incorporated by reference herein.

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 polarization25to 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 polarization25are 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 polarization25into 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, λ 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 logPm.  (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=sin⁡[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.

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.

As described in U.S. patent application Ser. No. 10/668,975, one embodiment of the polarization separating/recombining sub-system includes a polarizing beam-splitter and a patterned polarization converter. Other embodiments can also be utilized in the present invention.

During operation of the normally-off configuration of the embodiment shown inFIG. 9, in the off-state (i.e. where the grating440is diffracting or “on”) the grating440diffracts light into the first-order with high diffraction efficiency, and very little light propagates to the VOA output460. In the on-state (i.e. where the grating440is non-diffracting or “off”), the optical beam propagates through the grating440with very little loss and eventually, exits the VOA to the output460. Since the diffraction efficiency of a switchable transmission volume grating varies continuously with applied voltage, the output optical power of the VOA of this invention is therefore continuously variable.

A common mode for switchable gratings fabricated using PDLC materials is to be diffracting when un-powered and non-diffracting when powered. For VOAs, it is often important to know the state of the devices in case of a power failure. If a VOA ofFIG. 9was made using such a grating, a lack of electrical power would leave the grating in a diffracting state, with little or no optical power transmitted to the output, and thus this configuration has been termed a “normally-off” configuration. It should also be noted that it is also possible to fabricate switched gratings, including PDLC switched gratings, so that they are non-diffracting without applied electrical power, and diffracting when powered. When this alternative type of switched grating is used with the configurations of this invention, the sense of “normally-off” and “normally-on”, as used herein, are reversed.

It should be noted the voltage is only one embodiment of the means for controlling the switching of the switchable grating440. Other embodiments exist for optical switching and for polarization switching.

It should also be noted that for the VOA configuration ofFIG. 9, additional switched gratings440may be cascaded and simultaneously switched to partial diffraction efficiencies in order to extend the depth of available attenuation, if desired.

Shown inFIGS. 10a-10bare pictorial, schematic representations of other embodiments of a variable optical attenuator of this invention.FIG. 10ashows an embodiment of a variable optical attenuator of this invention including a polarization separating sub-system430, a switchable volume diffraction grating510, a static grating520, and a polarization recombining sub-system450. The static grating520can be, in one embodiment, a volume diffraction grating that is non-switchable.

The configurations of the embodiments shown inFIGS. 10aand10btypically operate as “normally-on” configurations. Again, this nomenclature is based on the assumption of using common switched gratings510that are diffracting with no switching power applied, and non-diffracting when power is applied. The input optical beam420is a beam of arbitrary polarization. The beam420is received by (enters) a polarization separating sub-system430(also referred to as a polarization diversity filter, or a compensator), which separates the input optical beam420into a first optical beam of a first polarization and a second optical beam of a second polarization, the second polarization being distinct from the first polarization. The polarization separating sub-system430is capable of emitting a first emitted optical beam of the first polarization and a second emitted optical beam of the first polarization. The emitted first and emitted second optical beams constitute an input channel of the first polarization.

In the normally-on configuration ofFIG. 10a, the input channel encounters the switchable volume diffraction grating510and is diffracted into the first-order when no voltage is applied across the grating. The input channel is then diffracted to the polarization recombining sub-system450by the static grating520. The diffracted channel includes a first transmitted optical beam of the first polarization and a second transmitted optical beam of the first polarization. A polarization recombining sub-system450is optically disposed to receive the first transmitted optical beam of the first polarization and the second transmitted optical beam of the first polarization and is capable of recombining the first transmitted optical beam of the first polarization and the second transmitted optical beam of the first polarization into a final output beam460of combined polarization. When an appropriate voltage (in the electrical switching embodiment) is applied across the switchable volume diffraction grating510, the input channel is transmitted (not diffracted) through switchable grating510and is not coupled into the output at the location shown inFIG. 10a. Since the diffraction efficiency of a switchable transmission volume grating varies continuously with applied voltage, varying the voltage of the control signal on switchable grating510varies the percentage of the optical power of the input channel that is diffracted to the output460, and therefore the output optical power of the VOA of this invention is continuously variable.

It should be noted that embodiments operating in the normally-off configuration are also possible. For example, if a switchable grating that is transmitting with no applied power (and diffracting with applied power) is used as grating510inFIG. 10a, the configuration will operate as normally-off.

It should be noted that since the switchable gratings can be optically switched, electrically switched, polarization switched, or switched based on other mechanisms, the switching controls (voltage, or optical or polarization control) are means for varying the diffraction efficiency of the switchable gratings.

Shown inFIG. 10bis an embodiment of the VOA system of this invention in which the input channel encounters the static grating520first and then encounters the switchable transmission volume grating510. The operation of the VOA system of this invention shown inFIG. 10bis analogous to that of the VOA system of this invention inFIG. 10a.

It should also be noted that switchable gratings in addition to switchable gratings510could replace the static grating520inFIGS. 10aand10b. This allows, for example, for additional degrees of attenuation of the input signal if required. For example, consider the configuration ofFIG. 10a. If static grating520is replaced by a switchable grating, it can be set to a diffracting state and the operation of the VOA is as described above. However, if it is desired to heavily attenuate the output channel, both the switchable grating replacing static grating520and switchable grating510can be switched to non-diffracting states. In such a configuration, if the switchable gratings give a 25 dB contrast between diffracting and non-diffracting states, switching both gratings can provide a roughly 50 dB optical attenuation level to the output. This approximately doubles the dBs of attenuation available by switching only a single grating.

FIG. 11shows a pictorial, schematic representation of an embodiment600of a 1×2 optical switch of this invention. The embodiment600of the 1×2 optical switch of this invention includes a polarization separating sub-system430, a switchable volume diffraction grating510, a static grating610, and a polarization recombining sub-system450. The static grating610includes a transparent region620.

As in the embodiment ofFIG. 10a, the input optical beam420is a beam of arbitrary polarization. The input beam420is received by (enters) the polarization separating sub-system430(also referred to as a polarization diversity filter, or a compensator), which separates the input optical beam420into a first optical beam of a first polarization and a second optical beam of a second polarization, the second polarization being distinct from the first polarization. The polarization separating sub-system430is capable of emitting a first emitted optical beam of the first polarization and a second emitted optical beam of the first polarization. The emitted first and emitted second optical beams constitute an input channel of the first polarization.

During operation of the 1×2 switch600of this invention, the input channel is incident on the switchable volume diffraction grating510. The switchable grating510is set to either fully diffracting, fully transmitting, or some intermediate state of diffraction efficiency by the control515(voltage, in one embodiment). The fraction of the input channel that is undiffracted by switchable grating510is transmitted through switched grating510as transmitted beams615. These beams are then incident on the transparent region620of static grating610, where they are again transmitted as beams615.

Similarly the fraction of the input channel that is diffracted by switchable grating510propagates as diffracted beams625. These diffracted beams625are then incident on the static diffraction grating610and are diffracted by grating610.

A polarization recombining sub-system450is optically disposed to receive the transmitted beams615and the diffracted beams625. The transmitted beams615and the diffracted beams625each includes a first optical beam of the first polarization and a second optical beam of the first polarization. The polarization recombining sub-system450is capable of recombining the first optical beam of the first polarization and the second optical beam of the first polarization for each of beams615and625into two final output beams660and650, respectively, of combined polarization. In one embodiment the 1×2 switch600of this invention includes two output beam ports630,640(for example, two collimator/single mode fiber combinations). The two output beam ports630,640receive two final output beams650,660of combined polarization.

Since the diffraction efficiency of switchable diffraction grating510can be varied in a continuously varying manner using control515(electrical in one embodiment), variable amounts of the optical power in input20can be switched among outputs630and640. This also includes the cases where substantially all of the input power is switched to either output630or to output640. For the case where it is desired to switch all of the incident power to only one of the outputs at a time, it may be advantageous to replace static grating610and transparent region620by a single switched grating. In such a case, when it is desired to send all of the power to output630, both switchable gratings could be set to fully diffracting. This would not only route substantially all of the power to the desired output, but any input signal leaking through the first switchable grating as undiffracted light, would be additionally attenuated from being crosstalk in the output640by diffraction from the second switchable grating. Similarly, when setting the switch to direct substantially all of the power to the output640, both switched gratings would be set non-diffracting. This would not only direct substantially all of the input power to the output640, but any input signal leaking through the first grating as diffracted light will be further attenuated from being crosstalk in the output650by the non-diffracting second grating.

If the embodiment of the switchable diffraction grating is not polarization sensitive, i.e., if it diffracts with the same diffraction efficiency regardless of the state of the incident polarization, then the polarization separating sub-systems430and the polarization combining sub-systems450of the configurations ofFIGS. 9-11are not necessary. Similarly, if the embodiment of the switchable diffraction gratings switch a single polarization, but that polarization is the only one incident on the system, then the polarization separating sub-systems430and the polarization combining sub-systems450of the configurations ofFIGS. 9-11are not necessary.

For example, if the embodiment of the switchable volume diffraction grating is such that beams of electromagnetic radiation with polarization in a predetermined plane of polarization are diffracted by the enabled grating and if the input beam420has a polarization in that predetermined plane of polarization, the polarization separating sub-system430and the polarization recombining sub-system450are not necessary and can be omitted from the system of this invention. Systems of this invention in which the input beam420has a polarization in the predetermined plane of polarization in which the switchable volume diffraction grating510preferably operates are shown inFIGS. 12a,12band13.

During operation of the embodiment ofFIG. 12a, when the switchable volume diffraction grating720is enabled (either by its initial state or by a switching control such as a voltage), the input beam encounters the switchable volume diffraction grating720and is diffracted. The diffracted beam is then further diffracted by the static grating730resulting in output beam740. Since the diffraction efficiency of the switchable transmission volume grating varies continuously with applied voltage (switching control), the percentage of the optical power of the input channel that is diffracted to the output740, and therefore the output optical power of the VOA of this invention, is continuously variable.

During operation of the embodiment ofFIG. 12b, when the switchable volume diffraction grating720is enabled (either by its initial state or by a switching control such as a voltage), the input beam710encounters the static grating730and is diffracted. The diffracted beam then encounters the switchable volume diffraction grating720and is diffracted resulting in output beam740.

FIG. 13shows an embodiment700of the 1×2 switch of this invention in which the input beam710has a polarization in the predetermined plane of polarization in which the switchable volume diffraction grating720preferably operates. During operation of the embodiment800ofFIG. 13, when the switchable volume diffraction grating720is enabled (either by its initial state or by a switching control such as a voltage), the input beam encounters the switchable volume diffraction grating720and its optical power is divided among transmitted beam810and diffracted beam820. Diffracted beam820is further diffracted by the static grating760. Transmitted beam810is further diffracted by the static grating760. Transmitted beam810is transmitted through the transparent region770. The diffracted beam and the transmitted beam comprise the output beams840,830. The optical power in input beam710is variably divided into the two outputs830and840by setting the switchable grating720to a predetermined diffraction efficiency which is determined by the switching control715(voltage, in one embodiment). The switchable grating720is set to either substantially fully diffracting, substantially fully transmitting, or some intermediate state of diffraction efficiency by the control715(voltage, in one embodiment). It should be noted that by setting the switchable grating720to either substantially fully diffracting or substantially fully transmitting, the transmitted beam can be substantially absent or the diffracted beam can be substantially absent.

It should be noted that, in the embodiments ofFIGS. 12a,12b, and13, the static grating may be, but is not limited to, a volume diffraction grating or may be replaced by a switchable volume diffraction grating.

It should also be noted that the switchable gratings of this invention may be volume holographic gratings, but may also be switchable gratings of other types including switchable surface relief gratings.