Liquid-crystal planar-waveguide apparatus and method for fast control of polarization and other properties of light

A polarization controller includes a plurality of liquid crystal cells positioned as cladding on a waveguide that propagates a beam of light so that the evanescent field extends into the liquid crystal cells, and a ½-wave birefringent retarder for rotating the eigenstates of polarization between the liquid crystal cells. For fast response, the evanescent field preferably extends only into the surface effect region of the liquid crystal cells, where directors in the liquid crystal respond faster to changes in voltages applied across the liquid crystal cells.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is related generally to liquid crystal devices for controlling properties of light, and, more particularly, to use of fast response of liquid crystal molecules near boundary surfaces in combination with long interaction lengths of planar waveguide structures for fast control of polarization.

2. State of the Prior Art

It is often necessary for light beams, such as those used to transmit data, generate control signals, and many other applications to be polarized in a particular manner or in a particular state of polarization in a waveguide for purposes of routing, multiplexing, demultiplexing, signal processing, and other purposes. However, such desired polarization as well as phase relationships and other properties can become attenuated or degraded as the light propagates through various media or devices, or the polarization and/or phase may have to be changed or adjusted to correlate or combine with another beam of a different polarization state. Even after a particular desired polarization state is set, temperature changes, mechanical pressure or tension on optical fibers or other optic components, and many other variables can cause the polarization state of light to undergo changes.

Polarization controllers are used to set, recondition, or readjust polarization of light beams for these and other purposes. Practical considerations require that polarization controllers for such purposes be electrically controllable, and it is desirable that they respond fast to electric control signals. Liquid crystal materials have the largest electro-optic response of all currently known materials, and they are relatively easy to incorporate into optical devices. Therefore, they are used in many polarization controllers as well as in controllers of other light properties. However, the primary drawback of such ordinary liquid crystal polarization control devices is that their response times to changing electric fields, typically measured in tens of milliseconds, is slower than that required for many polarization controller applications.

Another class of crystalline materials, lithium niobate (LiNbO3), is much faster, with response times that can modulate light in gigahertz frequencies, and they can be used in applications that require faster response times than ordinary liquid crystals. However, LiNbO3is also very expensive to incorporate into devices, because it does not lend itself to high-volume manufacturing processes and requires tedious polishing. Further, while the tens of milliseconds speed of ordinary liquid crystal devices is too slow for many applications, the super-fast, gigahertz modulating frequency capabilities of LiNbO3are impressive, but unnecessary and do not justify the expense for many applications. Therefore, there is a need for light control devices, such as polarization controllers, that respond to electric control signals significantly faster than ordinary liquid crystal devices, but which are less expensive and easier to manufacture than the super-fast LiNbO3devices.

Additional objects, advantages, and novel features of the invention are set forth in part in the description that follows and others will become apparent to those skilled in the art upon examination of the following description and figures or may be learned by practicing the invention.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide fast-acting devices for controlling light properties, such as polarization, that respond quickly and reliably to electric control signals.

A more specific object of this invention is to provide a method and apparatus for controlling polarization of light that has significantly faster electro-optic response than ordinary liquid crystal polarization control devices, yet is easier to manufacture and less expensive than lithium niobate devices.

Additional objects, advantages, and novel features of this invention are set forth in the description and examples below, and others will become apparent to persons skilled in the art upon examination of the following specification or may be learned by practicing the invention. The objects and advantages of the invention may be realized and attained by the instrumentalities, combinations, compositions, or methods particularly included in the appended statements.

To achieve the foregoing and other objects and in accordance with the purposes of this invention, as embodied and described herein, a method of this invention may comprise using at least two, but preferably three or more, liquid crystal cells as variable birefringence cladding on a waveguide to control polarization of a beam propagating through the waveguide. The liquid crystal cells are prepared with directors oriented parallel to the propagation vector of light through the waveguide so that application of voltages across the individual liquid crystal cells varies retardance of extraordinary rays in the TM mode to alter polarization of light in the evanescent field of the light beam, which extends from the core of the waveguide into the cladding. Between liquid crystal cells, the eigenstates of polarization are rotated, preferably 45 degrees or multiples of 45 degrees, and each liquid crystal cell is preferably long enough to provide at least one wave of stroke, so that two liquid crystal cells can provide practical polarization control for many applications, but three or more liquid crystal cells in this configuration can change any polarization state to any other polarization state, i.e., can connect any point on the Poincare sphere to any other point on the Poincare sphere. Of course, other degrees of eigenstate rotation between other numbers of liquid crystal cells could be used to accomplish polarization control, once the principles of this invention are understood. Also, other numbers of liquid crystal cells, each of which has more or less than one wave of stroke could also be used. For example, but not for limitation, two liquid crystal cells, each having a half wave of stroke could be substituted for one liquid crystal cell with one wave of stroke. Other combinations can also be devised once the principles of the invention are understood. For fast polarization control, it is preferable that the evanescent field of the beam extend only into the surface effect region of the liquid crystal cells, where directors respond much faster to applied voltages and variations in applied voltages than other regions of the liquid crystal cells.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An example fast polarization controller10according to this invention is shown inFIG. 1in a single channel (i.e., single wavelength) embodiment for purposes of describing the principal features and operation of the invention. However, this fast polarization controller invention, which can operate as much as ten times or more faster than conventional liquid crystal polarizers, is also useable in multi-channel (i.e., multiple wavelength) embodiments, as will be described in more detail below.

Referring now to the isometric view of the example fast polarization controller10inFIG. 1in conjunction with the cross-sectional views inFIGS. 2 and 3, an input light beam13of any polarization condition can have its polarization controlled or tuned to emerge as an output beam14in any desired polarization state (e.g., horizontal linear, vertical linear, right-hand circular, left-hand circular, right-hand circular or elliptical) in any orientation (azimuth) by varying the magnitudes of the respective voltages Va, Vb, Vcacross the three birefringent liquid crystal cells20,30,40. The light beam13propagates through the fast polarization controller10via the waveguide core50, and the liquid crystal cells20,30,40are positioned sequentially adjacent the waveguide core50in such a manner that the evanescent field of the light beam13in the waveguide50extends into the liquid crystal cells20,30,40, as will be explained in more detail below. The eigenstates of polarization of the light beam13are rotated between the first and second liquid crystal cells20,30and between the second and third liquid crystal cells30,40by any convenient technique. For example, a first ½-waveplate60is positioned across the waveguide50between the first liquid crystal cell20and the second liquid crystal cell30, and a second ½-waveplate70is positioned between the second liquid crystal cell30and a third liquid crystal cell40in order to effectively rotate the polarization eigenstates of the light beam13between liquid crystal cells30,40, respectively. Polarization eigenstates are the directional distributions of the electric field components Exand Ey(seeFIG. 4), and they are rotated after liquid crystal cells20,30in order to expose different directional distributions of the electric field components Exand Eyto the successive retardation effects of the next following liquid crystal cells30,40, respectively, as will be discussed in more detail below.

Birefringent waveplates and other devices for controlling phase and polarization of light, including rotating planes or directions of polarization, are well-known to persons skilled in the art, thus do not have to be described in detail here for an understanding of this invention. Suffice it to say, in general, that birefringent materials have the characteristic of propagating light at different speeds in different directions, such that: (i) two sets of Huygens wavelets propagate from every wave surface in such a material, one spherical and the other ellipsoidal; (ii) the spherical and ellipsoidal wavelets are tangent to each other in a direction called the optic axis; (iii) the ray which corresponds to the wave surfaces tangent to the spherical wavelets is called the ordinary ray and the ray corresponding to the wave surfaces tangent to the ellipsoid wavelets is called the extraordinary ray; (iv) the velocity of the ellipsoidal waves is either greater or lesser than that of the spherical waves, depending on the birefringent material, except in the direction of the optic axis, where the two are equal; (v) a plate of such birefringent material retards an extraordinary ray in relation to an ordinary ray or vice versa, depending on the birefringent material, except in the direction of the optic axis; (vi) such a birefringent plate of a thickness which retards one of the rays with respect to the other by one-fourth of a wavelength is called a quarter-waveplate or λ/4 waveplate; and (vii) such a birefringent material of a thickness which retards one of the rays with respect to the other by one-half a wavelength is called a half-waveplate or λ/2 waveplate. A λ/2 waveplate has the effect of rotating a plane of polarization of a beam by two times the amount by which its optic axis is rotated in relation the incident plane of polarization. Therefore, the λ/2 waveplates60,70are effective to rotate eigenstates of polarization Exand Eyby any desired amount between the liquid crystal cells20,30and30,40, respectively.

It should be mentioned for clarity that the optic axis as defined above is distinct from the optical axis, which corresponds to the longitudinal axis95(FIG. 3) of the waveguide core50, i.e., the propagation vector94of the guided light beam13through the core50. Therefore, to avoid unnecessary confusion, the optical axis will not be mentioned again. Instead, the direction of propagation of the guided light beam13will be indicated and discussed by reference to the longitudinal axis95of the waveguide core50and/or the propagation vector94of the light beam13in the waveguide.

It is also well-known that three liquid crystal, tunable waveplates, each having one wave of stroke and having their respective optic axes oriented at 45 degrees to each other, will provide a full range of polarization control, i.e., be able to transform any incident state of polarization to any other state of polarization. See, for example, U.S. Pat. No. 5,005,952, issued to M. Clark et al. Only two of such liquid crystal, tunable waveplates are needed to get from any polarization state to linear polarization, but a third such liquid crystal, tunable waveplate is needed to then transform such linear polarization to any other polarization state, i.e., to move the polarization state off the equator of the Poincare sphere. Stroke is understood by persons skilled in the art to mean the phase difference, i.e., retardation, that can be achieved between one AC modulated voltage magnitude and another AC modulated voltage magnitude across a liquid crystal cell expressed in units of wavelength. For example, if a liquid crystal cell is effective to retard phase by one wavelength when a voltage across the cell is switched from a minimum voltage (for example, zero), to some maximum voltage, then the liquid crystal cell is said to have one wave of stroke.

In the preferred embodiment10of this invention shown inFIG. 1, however, where the light beam13is propagated in a direction parallel to the alignment layers24,26of the liquid crystal cells20,30,40, it is impractical, if not impossible, to orient the respective optic axes of different liquid crystal retarder cells20,30,40at different angles in relation to each other. Therefore, this invention departs from the convention of having three liquid crystal waveplates oriented with their respective optic axes rotated in relation to each other and propagating light through such three liquid crystal waveplates perpendicular to the alignment layers, as taught by U.S. Pat. No. 5,005,952 issued to Clark et al. Instead, substantially the same optical effect is provided in the preferred embodiment polarization controller10of this invention by having the respective optic axes of the liquid crystal retarder cells20,30,40all oriented in the same direction (e.g., parallel to the longitudinal axis95of the waveguide50) and using the λ/2 waveplates60,70or any other polarization rotation technique to rotate eigenstates of polarization of light in the waveguide50as it propagates between the liquid crystal retarder cells20,30,40, as will be explained in more detail below.

Essentially, the state of polarization of a lightwave signal, such as beam13, describes the magnitude and phase relationships of the electric field (E-field) component inside the signal. The electric fields (E-fields) of any light ray in the signal can be resolved into two arbitrary, orthogonal components Exand Ey, as illustrated in the X-Y coordinate system ofFIG. 4. The vector components Exand Eyare time dependent, so a more proper notation would be Ex(t) and Ey(t). However, it is common for persons skilled in this art to just assume such time dependency and simplify the notation form to merely Exand Ey. This convention will be used in this description.

All polarization states can be shown as unique elliptical displays with various degrees of ellipticity, such as the classic examples of horizontal polarization (0 degrees of ellipticity), right-hand circular polarization (45 degrees of ellipticity), and right-hand elliptical polarization, shown inFIG. 4. Other classic polarization states, such as, vertical linear polarization (90 degrees of ellipticity), left-hand circular polarization, left-hand elliptical polarization, as well as all states in-between those classic states mentioned above, are not shown inFIG. 4to avoid unnecessary complexity, but they are well-known to persons skilled in the art and can be handled or produced by the fast polarization controller10of this invention. Any particular state of polarization is determined by the relative magnitude and phase of the E-vector components (Exand Ey) of the light. Therefore, as is also well known to persons skilled in the art, any desired state of polarization can be created by adjusting the magnitude and phase relationships of the Exand Eycomponents of a lightwave signal. Such adjustment can be accomplished in a relatively fast manner with this invention, as explained below.

The waveguide comprising the core50, cladding15,17,19, and the liquid crystal cells20,30,40functioning as cladding with variable indices of refraction, has guided orthogonal transverse magnetic (TM) and transverse electric (TE) modes of light propagation. The alignment surfaces24′ of the liquid crystal cells20,30,40are preferably prepared in any manner known to persons skilled in the art (e.g., buffing and other methods or material compositions and structures) so that the liquid crystal directors28align parallel to the beam propagation vector94, as illustrated diagrammatically inFIG. 3. Therefore, the optic (slow) axis64(seeFIG. 5) of the liquid crystal cell20is parallel to the propagation vector94. Consequently, applying modulated voltages Va, Vb, Vcof various magnitudes across the liquid crystal cells20,30,40causes the directors to tilt upwardly in various amounts from the alignment surface24in a plane that is parallel to the propagation direction94of the light beam13in the waveguide50. Typically, the voltage is applied in a modulated AC (alternating current) square wave format, for example at 2 KHz, and the voltage Va(or Vbor Vc) is the magnitude of the modulated voltage. Such upward tilting of the directors28resulting from the application of such voltage changes the effective index of refraction Nefffor the TM mode of light, which, in effect, varies the birefringence of the liquid crystal cells20,30,40to cause varying degrees of birefringent phase retardance of Exvector components in relation to Eyvector components of the light. As mentioned above, the TM mode is perpendicular to the waveguide interface62between the core50and the respective liquid crystal cells20,30,40, so the TM mode is also perpendicular to the alignment layer24. Referring more specifically toFIG. 3, for example, essentially an increase in the applied voltage Vacauses the liquid crystal material22to become more birefringent by lowering the effective index of refraction for the TM mode of light propagation and thereby retarding the phase of the TM mode to create an optical path difference (OPD) between the TM and TE modes, which changes the polarization of the light beam13. Expressed in a different way, the optical phase delay in the TM mode changes the distribution of the E-field components (i.e., magnitude and phase relationships of the Exand Eyvector components) of the light beam13, which adjusts the polarization ellipticity between the various states described above, some of which are illustrated for example inFIG. 4and others of which are well-known to persons skilled in the art. The degree or extent of changes in the E-field components, i.e., the changes of relative magnitudes and/or phase relationship between the Exand Eycomponents, will depend on the change of magnitude of the modulated voltage Va. In the example illustrated inFIG. 5, such optical action of the first liquid crystal cell20is shown to change an arbitrary elliptical polarization state103having any azimuth α, and any elliptical angle ω, to another polarization state104in which the azimuth α2is zero degrees and the elliptical angle ω2is smaller than ω1. The azimuth α in this diagrammatic illustration inFIG. 5represents the tilt of the major elliptical axis in relation to a plane that includes the fast axis65and slow axis64of the cell20. Therefore, in an X-Y coordinate system, in which the X-axis is in said plane and the Y-axis is perpendicular to said plane, the azimuth α is the angle between the X-axis and the major elliptical axis of the polarization state of the beam13. The elliptical angle ω is the angle between the major elliptical axis and a straight line that extends through two points where the ellipse intersects its major and minor axes, as shown inFIG. 5.

Then, as illustrated diagrammatically inFIG. 5, the first ½-wave plate60, which is positioned between the first liquid crystal cell20and the second crystal cell30(seeFIGS. 1,2, and5) with its fast axis63oriented at 22½ degrees in relation to the fast axis65of the liquid crystal cell20, essentially rotates the eigenstates of polarization (i.e., the Exand Eycomponents) by 45 degrees before the beam13enters the second liquid crystal cell30. Therefore, as illustrated inFIG. 5, the polarization state104′ (i.e., elliptical angle ω2) of the light beam13emerging from the first ½-wave plate60is the same as the polarization state104(i.e., elliptical angle ω2) before the first ½-wave plate60, but its elliptical axes are rotated 45 degrees. Therefore, the azimuth α3in this example is 45 degrees.

The polarized beam13then encounters the second liquid crystal cell30, which has its slow axis64and fast axis65oriented the same as the first liquid crystal cell20. An applied modulated voltage Vbacross the second cell20, like the voltage Vaapplied across the first cell20, changes the effective index of refraction for the TM mode, thus changes the magnitude and phase relationships of the Exand Eyvector components to change the state of polarization of the beam13to some other state105with a different elliptical angle ω3, as illustrated inFIG. 5. The elliptical angle ω3in this arbitrary example is illustrated as being zero, which means that the state of polarization is “linear,” and the azimuth α4of zero degrees means it is “horizontal” linear polarization.

The second ½-wave plate70, with its fast axis73rotated 22½ degrees in relation to the fast axis65of the second cell30, then rotates the eigenstates of polarization by 45 degrees before the beam13encounters the third liquid crystal cell40. Therefore, the polarization state105′ of the beam13after the second ½-wave plate70is substantially the same ellipticity ω3as the polarization state105before the second ½-wave plate70, but the elliptical axis is rotated by 45 degrees. Then a modulated voltage Vcapplied to the third liquid crystal cell40, with its slow axis64and fast axis65oriented the same as the slow and fast axes64,65of the preceding cells20,30, can change the polarization of beam13to a desired polarization state106, as illustrated by the different ellipticity ω4of the outgoing beam14. The major axis of ellipticity of the example desired polarization state106is vertical, so the azimuth α6is 90 degrees.

Consequently, as can be understood from the explanation above, the ½-wave plates60,70enable the three liquid crystal cells20,30,40, with their slow axes aligned in the same direction, i.e., parallel to the beam propagation vector94, and each having at least one wave of stroke, to change any state of polarization to any other state of polarization, i.e., to connect any point on the Poincare sphere to any other point on the Poincare sphere, even though the respective slow axes64and fast axes65are oriented the same in all the cells20,30,40. This function of rotating the eigenstates of polarization between the liquid crystal cells20,30and between the liquid crystal cells could also be accomplished by any other means known to persons skilled in the art, for example, by twisting a polarization maintaining fiber or rotating the core of a waveguide, e.g., a circular core to an elliptical core. Also, rotating the eigenstates of polarization by 135 degrees or 270 degrees between the liquid crystal cells20,30and30,40will provide much the same enabling effect as the 45 degrees rotation described in the example above, so, to avoid unnecessary repetition, such rotation by those and other multiples of 45 degrees that provide the same benefit or function are considered to be included in any mention herein of rotating the eigenstates by 45 degrees between said liquid crystal cells.

Two of the cells, e.g., cells20,30with the first ½-wave plate60between them or cells30,40with the second ½-wave plate70between them, can be used to convert any polarization with an elliptical angle ω greater than zero, or with an elliptical angle ω of zero and an azimuth α greater than zero, to any other state of polarization, which may be sufficient for many applications. However, if the incoming beam13is linearly polarized and oriented horizontally, i.e., parallel to the plane than includes both the slow and fast axes64,65of a liquid crystal cell, for example, cell20, then that liquid crystal cell will be ineffective to change the linear polarization. Since the liquid crystal cells20,30,40only retard phase in the TM mode, as explained above, they have no effect on horizontal, linearly polarized light. Therefore, rotating the eigenstates of such polarization by 45 degrees with the ½-wave plate60or70or by any other means enables the next liquid crystal cell30or40to act on and alter the ellipticity, i.e., polarization state. Rotation of the eigenstates by more or less than 45 degrees (any amount greater than zero degrees) will also work for this invention, but 45 degrees is preferred.

Consequently, as mentioned above, the two ½-wave plates60,70between the liquid crystal cells20,30,40, as described above, enable the three cells20,30,40, each having their respective slow and fast axes64,65oriented the same as each other and each being individually controllable with at least one waive of stroke, to change any state of polarization of the input beam13to any other desired state of polarization in the output beam14. Therefore, this combination of elements is very conducive to structuring the fast polarization controller10on a common substrate, as illustrated inFIGS. 1-3, although it can also be used with any other convenient structure as well.

A significant feature of this invention, as will be described in more detail below, is that only liquid crystal directors28in regions23near alignment surfaces24′ (seeFIG. 3) of the liquid crystal cells20,30,40(FIGS. 1 and 2), which react much faster to voltage changes than middle region27liquid crystal directors (FIG. 3), are used to vary and control polarization by acting on only the evanescent portion92of the light beam13. Once these and other principles of this invention are understood, persons skilled in the art will be able to devise myriad ways to construct polarizer controllers with a waveguide core, at least two, and preferably three, liquid crystal cells positioned adjacent the core to function as waveguide cladding, and with the evanescent field of the light beam extending into the surface effect region of the liquid crystal cells. The polarization controller10shown inFIGS. 1-3illustrates one such embodiment for example, but not for limitation. The enlarged cross-section ofFIG. 3shows the structure of the polarization controller10in the region of the first liquid crystal retarder cell20, but it is representative of the regions of the second and third liquid crystal cells30,40as well. Therefore, referring now primarily toFIGS. 2 and 3in combination withFIG. 1, the polarization controller embodiment10has a bottom module11, which comprises the waveguide50, and a top module12, which, in this example, comprises the liquid crystal retarder cells20,30,40. When the top module12is affixed to the bottom module11, the liquid crystal cells20,30,40overlay the waveguide50and function as part of the cladding, but each with a variable and controllable index of refraction n2(V) that varies as a function of voltage Va, Vb, Vcacross the respective liquid crystal cells20,30,40as will be explained in more detail below. The variable index of refraction n2(V) is for the TM mode only, when the alignment surface24′ is prepared so that the directors28are parallel to the propagation vector94of the light beam13, as explained above.

In the fast polarization controller embodiment10inFIGS. 1-3, the waveguide core50is a transparent, preferably thin film, material, such as doped glass, polymer, or other material that is substantially transparent to the light beam13with a core index of refraction n1. The first or bottom electrodes or contacts21,31,41for each liquid crystal cell20,30,40, respectively, is deposited on the substrate18, followed by a bottom cladding layer15. The bottom cladding layer15has an index of refraction n2, which is less than the index of refraction n1of the core50to confine propagation of light13in the waveguide core50, as is well-known to persons skilled in the art. The core material50can be deposited on the bottom cladding layer15, for example, in a broad layer (not shown) of the desired core thickness, which can then be masked and etched away to leave the core50of a desired width and thickness, as illustrated diagrammatically inFIG. 1, or by any other deposition technique known to persons skilled in that art that works for creating the waveguide core50. Additional side cladding material16can be deposited on the bottom cladding layer15, as best seen inFIG. 1, to clad both lateral sides of the waveguide core50. The side cladding16can be the same material as the bottom cladding15with the same index of refraction n2or some other material with an index of refraction that is less than the index of refraction n1of the core50in order to guide light propagation in the core50. The top surfaces of the core50and side cladding16can be polished flat and then a bottom alignment layer24for the liquid crystal cells20,30,40can be deposited over the core50and side cladding16to complete the bottom module11of the fast polarization controller10. Again, the electric contacts21,31,41, bottom and side cladding15,16, and core50components of the bottom module11can be constructed and fabricated in any manner available to persons skilled in the art and not only in the manner described above. Recessed areas51,53,55in substrate18, as shown inFIG. 1, can be provide for access to outcropped portions21′,31′,41′ of electric contacts21,31,41for connection of electric leads52,54,56, or such electric connections can be made in any other manner known to persons skilled in the art.

The top module12of the fast polarization controller10comprises cavities81,82,83(FIG. 2) in a top substrate17to contain the liquid crystal material22,32,42of liquid crystal cells20,30,40, respectively. Before filling the cavities81,82,83with liquid crystal material, top electric contact layers29,39,49and top alignment layers26,36,46of the respective liquid crystal cells20,30,40are deposited into the cavities81,82,83. The top substrate17can serve as top cladding for the waveguide core50, or a separate top cladding layer19can be provided on top substrate17. In any event, the index of refraction of the top cladding17or19is less than the index of refraction n1of the core50in order to guide light beam13propagation in the core50and is preferably, but not necessarily, the same as the index of refraction n2of the bottom cladding15. The lateral side of the top substrate17can be recessed at57,58,59(FIG. 1) to expose outcropped portions29′,39′,49′ of the top electrodes29,39,49for connection of electric leads52′,54′,56′. Then, when the surfaces of the top alignment layers26,36,46and the surface of the bottom alignment layer24, which in this embodiment is illustrated for example to be common to all three liquid crystal cells20,30,40, are buffed in the desired alignment direction, the cavities81,82,83can be filled with the liquid crystal material22,32,42, respectively, and mounted on the bottom module11to form the liquid crystal cells20,30,40over the waveguide core50. The top module12and the bottom module11would be turned upside down during such filling and mounting to prevent spillage of the liquid crystal material22,32,42, and such brushing, filling, and mounting procedures are well-known to persons skilled in the art.

Of course, there are myriad other ways to structure and fabricate the bottom module11and/or the top module12and to accomplish the assemblage of the liquid crystal cells20,30,40adjacent the waveguide core50that would be well within the capabilities of persons skilled in the art, and this invention is not limited to the specific structures or assemblage described above. Suffice it to say that, once assembled, each liquid crystal cell has an index of refraction n2(V) for the TM mode that varies as a function of voltage Va, Vb, Vcacross the liquid crystal material22,32,42in the respective cells20,30,40, but which is less than the index of refraction n1of the waveguide core50, so that each liquid crystal cell20,30,40functions as a part of the waveguide cladding to confine propagation of light to the waveguide. Of course, as such voltage is increased, the index of refraction n2(V) increases to modify the phase and/or magnitude relationships between the Exand Eycomponents, as explained above, so lightwave guiding decreases. However, it is preferred that the index of refraction n2(V) should not increase enough to equal or exceed the index of refraction17of the core50so that the waveguide always has at least some lightwave guidance, unless it is desired to couple light out of the waveguide.

Referring now primarily to the enlarged cross-section view of the first liquid crystal cell inFIG. 3, essentially, it is known that liquid crystal molecules28in the edge regions, i.e., surface effect regions,23,25close to the alignment surfaces24′,26′ experience stronger restoration forces than the molecules in the midportion27of the liquid crystal cell20. See U.S. Pat. No. 4,385,806 to Ferguson. Therefore, those liquid crystal molecules28in the edge regions23,25respond much faster to changes in voltage Vathan the liquid crystal molecules in the middle region27, and switching times of 10 to 100 microseconds are possible. One way of interacting with the surface molecules is to apply a high bias voltage Va, for example, about 10 Vrms, at a frequency much higher than a frequency at which even the surface or edge region23,25molecules can respond and then modulating the molecules28, thus modulating index of refraction n2(V) in the edge regions23,25, by modulating the applied bias voltage Va.

It is also known that liquid crystal materials can be placed adjacent a guided wave in a planar waveguide to modulate light propagating in the optical path without coupling the light out of the waveguide, as long as the index of refraction n2(V) of the liquid crystal does not rise to equal or exceed the index of refraction n1of the core50, as mentioned above. See also U.S. Pat. No. 5,347,377 to Revelli. The fast polarization controller10of this invention utilizes those teachings of Revelli, i.e., modulation of a guided wave with a liquid crystal positioned adjacent a waveguide, and of U.S. Pat. No. 5,486,940 to Ferguson et al., i.e., fast response of liquid crystal molecules28in the edge region23(also called surface effect region23), to make a fast polarization controller10. Another significant feature of this invention, however, is that the waveguide core50and liquid crystal cells20,30,40are designed to limit the penetration of light energy92(FIG. 3), i.e., the evanescent field92, into the liquid crystal cladding material22to the surface effect or edge region23, where modulation response of the directors28is very fast. For purposes of this invention, the edge region23illustrated inFIG. 3(also called the surface effect region23) is considered to be the region of the liquid crystal material22where an applied voltage of eight (8) volts or less does not induce liquid crystal director (i.e., molecular) reorientation of more than five (5) degrees. There is, of course, a similar edge or surface effect region25adjacent the upper alignment layer26, but that surface effect region25is not used in this example of a fast polarization controller10embodiment to modulate the light beam13, as will become more clear from the description below.

As mentioned above, for the planar waveguide to always guide light and not allow the light to escape, the index of refraction n1of the core50must always be higher than the index of refraction n2of the passive cladding15,17, or19, and it must always be higher than the index of refraction n2(V) of the liquid crystal material22, which then also functions as cladding for the core50. The waveguide, therefore, comprises a combination of the core50, the passive cladding15,17,19, and the liquid crystal cells20,30,40. The side cladding16is not needed for one-dimensional wave guiding, but it can be included as part of the waveguide, if two-dimensional wave guiding is desired.

As shown by the intensity profile90of light13propagating through the waveguide (FIG. 3), some of the light energy92,92′ extends into the cladding, including the liquid crystal cell20on the top and the cladding layer15on the bottom, because there is not instant reflection of all light rays in the core50at the interfaces of the core50and cladding material15,17,19, or20. Instead, some light rays cross the interfaces from the core50into the cladding15,17,19, or20, where the lower index of refraction n2or n2(V) causes such light rays to diffract or bend and propagate back into the core50. The portion92,92′ of the light energy of the beam13that propagates in this manner in the cladding15,17,19,20is known as the evanescent field. The precise amount of the light beam13that propagates in the cladding15,16,19,20, i.e., that comprises the evanescent field92,92′, can be controlled for a particular wavelength of light by the design parameters and materials of the waveguide, such as the thickness of the core50and the differences in the indices of refraction of the core50and cladding15,16,19, or20, as is understood by persons skilled in the art. Typically, about 5% to 20% of the light beam propagates in the cladding15,17,19,20. Therefore, the portion92of this evanescent light energy that extends into the liquid crystal22is preferably confined to the fast-modulating, edge or surface effect region23of the liquid crystal material22, as explained above, for fast polarization modulation and control of the beam12.

The phase velocity of the guided beam13propagating through the waveguide in the region of a liquid crystal cell20,30,40, as illustrated in the enlarged longitudinal cross-section of cell20, inFIG. 3, is described by an effective index of refraction neff(V) of the waveguide, which is a function of n1, n2n2(V), λ and a, where λ is the wavelength of the light and a is the thickness of the waveguide core50. In general, a liquid crystal material22should be chosen so that the extraordinary index of refraction ne, which is the same as n2(V) in the example inFIG. 3, does not exceed the core index of refraction n1when maximum voltage Vais applied, so that the liquid crystal does not couple light out of the waveguide. Shutting off the applied voltage Vawill cause the liquid crystal index ne, i.e., n2(V), to drop toward the ordinary index of refraction noat the boundary surface24′. The alignment layer surface24′ in this application is buffed or otherwise prepared so that the directors28of the liquid crystal material22align in the direction of the light propagation vector94, i.e., parallel to the longitudinal axis95of the waveguide core50. Therefore, an increasing voltage Vahas the effect of changing the effective index of refraction neff(V) for the guided TM mode of light only and not for the TE mode. The change of the effective index of refraction Δneffof the waveguide will depend on the change of index of refraction Δn2(V) of the liquid crystal material22and the penetration depth of the optical wave92into the liquid crystal material22in the surface effect region23. In this regard, an estimate of the effective voltage dependent birefringence will be:

Δ⁢⁢neff≈βΔ⁢⁢ne2ne≈10-3,(1)
where β is a parameter that varies from 0 to 1 and depends on the mode number of the waveguide and penetration depth of the optical wave92into the liquid crystal material22, i.e., the surface effect region23. For a single mode fiber waveguide, β is typically on the order of 0.5.

As mentioned above, each liquid crystal cell20,30,40needs to have at least one wave of stroke in order for this example polarization controller inFIGS. 1-3to change any state of polarization of the input beam13to any other state of polarization in the output beam14. Using an example telecommunications wavelength λ of 1.5 μm, the required interaction length of the light beam with the liquid crystal cell20,30or40to produce one wave of stroke, i.e., to retard the extraordinary ray enough to produce an optical path difference (OPD) equal to one wavelength, can be estimated by the relationship:

OPD=Δ⁢⁢neff⁢Lλ.(2)
Setting OPD (optical path difference) to one wavelength results in an interaction length L≈1 mm, which is a very practical length for the liquid crystal cell20. Therefore, since the fast polarization controller10of this invention uses the surface mode, i.e., only liquid crystal material22in the edge region23, for polarization control of the light beam13, the liquid crystal cell20can be made practically as long as necessary to get one wave of stroke or even many waves of stroke, if desired, and still have the benefit of the fast modulation or control provided by the surface effect molecules according to this invention. Such surface-mode operation of this fast polarization controller10can be in a preferable range of 10 khz to 100 khz, although operation outside that preferred range is also possible.

The Δneffcan be altered as required by the design of a particular waveguide, i.e., selecting a liquid crystal with an index of refraction n2(V) that remains in the range described above. However, there will be a tradeoff between the magnitude of Δneffand temporal response. The closer n2(V) gets to n1, the weaker the guiding. Therefore, a larger Δneffmay provide a faster response, but it may also weaken the wave guiding, and vice versa.

The λ/2 waveplates60,70can be formed in any manner. One example of suitable ½-wave plates is a very thin polymer, which are well-known in the art, and they can be positioned in narrow saw kerfs61,71, respectively, extending into the top module12and bottom module13and orthogonally across the waveguide core50and cladding15,17,19(FIGS. 1 and 2). Such thin polymer ½-wave plates are preferred, because they keep light power losses to a minimum, although quartz and other materials can also be used. Various kinds of thin polymer ½-wave plates can be obtained, for example from Meadowlark Optics, of Frederick, Colo., Nippon Telegraph and Telephone Corporation, in Japan, or JDS Uniphase, of Ottawa, Canada. It is preferred that the saw kerfs61,71be on the order of about 25 μm or less so that free expansion of the unguided light wave across the kerfs61,71will not lead to excessive losses upon re-coupling to the waveguide core50on the other side of the kerfs61,71.

The liquid crystal material22can be any of a number of suitable varieties, including, but not limited to, nematic liquid crystals (NLC). Persons skilled in the art are familiar with the various varieties and characteristics of liquid crystals and are capable of selecting and implementing them for the purposes described herein, once they understand the principles of this invention.

As an example of an LC waveguide polarization controller that could be used in the present invention, one may use an inverted-ridge channel design on top of a p-doped silicon substrate that may serve as the lower electrode21. An inverted ridge design has the advantage that the top surface is smooth and does not interfere with subsequent layers used to align the liquid crystal molecules28.

It may be beneficial to keep the mean field diameter of the guided mode to be over 3 μm to improve coupling with optical fibers (not shown) and to enable the guided wave to traverse the narrow (20 μm) dicing saw cuts or kerfs61,71through the guiding channels into which thin polymer waveplates60,70are inserted to rotate the polarization vectors. In the following example design, a low index guide layer is chosen to give a low contrast waveguide with a large mode diameter.

For example, the silicon substrate could be covered with 4 μm of thermal oxide with an index approximately of 1.45 over which a 1 μm layer of silicon oxynitride SiOxNyis applied using plasma enhanced chemical vapor deposition (PECVD). It is known to those skilled in the art that the ratio of x:y can be controlled to yield a film with an index between 1.45 and 2. In this example, the silicon oxynitride layer may be n=1.54. The silicon-oxynitride layer can then be patterned into a ridge 1 μm thick by 3 μm wide using reactive ion etching to leave smooth sidewalls. Over this layer one can apply SiO2(n=1.45) and planarize back to the top of the channel using chemical mechanical polishing methods resulting in a 1 μm×3 μm ridge of index n=1.54 surrounded by SiO2on either side with an index of 1.45. Over this structure one can then apply a second film of silicon oxynitride 3 μm thick with an index of 1.54. The structure can then be annealed at high temperature to remove excess hydrogen and nitrogen giving a low loss waveguide at 1.55 μm.

The waveguide surface can then be coated with a standard polyimide as used in the manufacture of common liquid crystal displays (LCD) and rubbed with a velvet cloth in the direction of the k-vector of the light. A top coverglass whose underside can be coated with a conduction layer such as indium tin oxide (ITO) can then be spaced several to 10 microns from the waveguide surface using precision glass spacers.

To ensure guiding at all voltage values it is beneficial to use a liquid crystal material whose extraordinary index is below 1.5. For example, the liquid crystal ZLI2359 from EM Industries, Inc., has ordinary and extraordinary indices of 1.467 and 1.519 at a wavelength of 589 nm.

The voltage dependence of the TM0mode can be estimated by noting that the guided mode is similar to an asymmetric slab mode. Though the actual values of the propagation constants are slightly smaller than those computed for a slab waveguide with a 4 μm core, the difference between the effective indices of the TM modes for the LC index at n=n0and n=neis well approximated. It is well known to those skilled in the art to solve for and take the difference of the effective indices of a slab waveguide with a substrate index of 1.45, a 4 μm core with index of 1.54, and the top cladding index of 1.519 and 1.467, giving Δneff˜0.0015 (See Equation 2.29 ofFundamentals of Optical Waveguidesby Katsunari Okamoto, Academic Press, San Diego, (2000)). Typically, one realizes about half this value at a voltage of 50 Vrms. The electrode length required to give one wave of stroke at 1.55 μm is then

Between electrodes, as is known to those skilled in the art, the waveguide channel can be cut with a precision dicing saw to yield a trench approximately 20 μm wide. This waveguide is single mode with a mean-field diameter of approximately 4 μm in the out-of-plane dimension, which is sufficiently large to cross the trench with minimal losses. Into this channel a 15 μm optical half waveplate formed from high-birefringence polymers may be placed and fixed with index matching glue.

The fast polarization controller10described above is for a single channel, i.e., a single wavelength of light. A multi-channel, fast polarization controller100for controlling polarization of a plurality of wavelengths in a light beam113is illustrated inFIG. 6. Essentially, a plurality of single polarization controllers10′, each of which includes three liquid crystal cells20′,30′,40′ as described above, are combined together in an array on a single waveguide substrate150and/or superstrate (not shown inFIG. 6, but similar to the top module12described above except for having multiple channels). Two thin λ/2 waveplates160,170provide the function of rotating the eigenstates of polarization between all the respective first and second liquid crystal cells20′,30′ and between all the respective second and third liquid crystal cells30′,40′, as described above for the single channel embodiment10, but this function is for all the multiple channels in this multi-channel embodiment100.

As mentioned above, the incoming light beam113coupled to the fast, multi-channel, polarization controller100can comprise many individual wavelengths λ1, λ2, . . . , λN. For example, some telecommunications applications have multi-channel light beams comprising 40 to 100 independent wavelength channels separated by 50 GHZ to 100 GHZ centered around 1550 nm. In some applications, it is necessary to control the polarization of each channel or wavelength independently. For example, in certain situations, where the polarization states of multiple channels or wavelengths are uncorrelated, there is a need to convert all of them to a uniform polarization state. One such application is dispersion mode compensation. While the multi-channel, fast polarization controller100of this invention can accommodate these and other needs for such large numbers of channels or wavelengths λ1, λ2, . . . , λN, it is impractical to illustrate such a large number of channels in the drawings accompanying this description. Therefore, the illustrated multi-channel, fast polarization controller embodiment100illustrated inFIG. 6with only twelve channels, which is sufficient to describe the principles of the invention without unnecessary clutter. Once these principles are understood, persons skilled in the art can implement them with any desired numbers of channels or wavelengths λ1, λ2, . . . , λN.

In the multi-channel, fast polarization controller100illustrated inFIG. 6, the incoming light beam113is coupled to, enters, and passes through an arrayed waveguide grating (AWG)120, which separates each individual channel or wavelength λ1, λ2, . . . , λNand routes the light of each channel into a separate waveguide101,102, . . . , N. Such AWG's are well-known in the art and are described in detail, for example, in the article by J. Lam and L. Zhao, “Design Trade-offs For Arrayed Waveguide Grating DWDM (Dense Wave Division Multiplexing) MUX/DEMUX”, published on the website of Lightwave Microsystems, Inc. (now Neophotonics, Inc.), which is incorporated herein by reference. Essentially, the input waveguide115delivers the input beam113to a lens region116, which divides the light energy by wavelength λ1, λ2, . . . , λNamong the different individual channel waveguides101,102, . . . , N. Each grating channel waveguide101,102, . . . , N has a precise length difference with its neighbors so that the light in each channel waveguide emerges with a different phase delay at the output, where a second lens region117re-focuses the light from all the individual channel waveguides101,102, . . . , N onto the output waveguides101′,102′, . . . , N′ in the output array. Due to the precise differential phase tilt for different wavelengths, each wavelength λ1, λ2, . . . , λNis focused into a different output channel waveguide101′,102′, . . . , N′ in the output array.

However, because of the path lengths phase shifts, curvatures, and other characteristics of the channel waveguides101,102, . . . , N that occur or are encountered in the channel waveguides101,102, . . . , N, some polarization changes can occur. Therefore, a ½-wave plate122is positioned at the symmetry plane of the AWG120with its fast axis at 45 degrees with respect to the fast axes of the liquid crystal cells20′,30′,40′, which, as in the single channel polarization controller10described above, are in a plane parallel to the waveguide surface, thus perpendicular to the TM mode. This λ/2 waveplate renders the AWG120polarization insensitive by rotating the plane of polarization of the light in each of the channel waveguides101,102, . . . , N by 90 degrees. Essentially, polarization changes that occur in the first half of the AWG20approaching the symmetry plane, where the ½-wave plate122is positioned, will be undone as the light propagates through the second half of the AWG20.

The channel waveguides101′,102′ . . . , N′ then guide the respective individual wavelengths λ1, λ2, . . . , λNof light to the respective fast polarization controller components10′, which, with the three liquid crystal cells20′,30′,40′ and two ½-wave plates160,170, have the capability of changing any incoming polarization state to any other polarization state, as described above. Then, with the individual light channels λ1, λ2, . . . , λNadjusted by the fast polarization controller components10′ to whatever polarization state is desired, the output light is processed-out and recombined into a common output beam114in the output waveguide119by the output arrayed waveguide grating (AWG)130, which is essentially the reverse of the AWG120described above. Again, a ½-wave plate132is positioned in the symmetry plane of the AWG130to render the output AWG130polarization insensitive, as explained above for the input AWG120.

The above-described function of separating and routing the various wavelengths λ1, λ2, . . . , λNinto the various channel waveguides101,102, . . . , N can also be performed by other techniques, for example, other diffraction grating structures, prisms, etc., as will be understood by persons skilled in the art. Likewise, the recombination of the various wavelengths λ1, λ2, . . . , λNfrom the various channel waveguides101′,102′, . . . , N′ into the common output beam114can be accomplished with other diffraction grating structures, prisms, etc., as will be understood by persons skilled in the art.

The foregoing description is considered as illustrative of the principles of the invention. Furthermore, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and process shown and described above. Accordingly, resort may be made to all suitable modifications and equivalents that fall within the scope of the invention. The words “comprise,” “comprises,” “comprising,” “include,” “including,” and “includes” when used in this specification are intended to specify the presence of stated features, integers, components, or steps, but they do not preclude the presence or addition of one or more other features, integers, components, steps, or groups thereof.