Abstract:
A polarization controller includes: a liquid crystal cell; a mechanism for determining a compensation voltage; and a voltage driving circuit for applying the compensation voltage to the liquid crystal cell, where the application of the compensation voltage optimizes an extinction ratio of the liquid crystal cell. The controller controls the extinction ratio using an applied voltage to compensate for temperature changes, liquid crystal cell gap variations, and other errors of the liquid crystal cell. This controller is aimed at liquid crystal devices used in a wide ambient temperature range and is also useful to compensate for the fabrication errors of the liquid crystal switches.

Description:
FIELD OF THE INVENTION 
   The present invention relates to optical switches utilized in optical communications systems. More particularly, the present invention relates to such switches that utilize liquid crystal cells to control the polarization of light. 
   BACKGROUND OF THE INVENTION 
   Liquid Crystal (LC) cells are used along with other optical elements in optical communication apparatuses to switch light along alternative optical pathways. In these apparatuses, LC cells usually serve as polarization controllers. The extinction ratio (R ex ) of polarization is the most important parameter for these controllers. 
     FIG. 1  shows the diagram of a typical LC cell  102  as may be used as a polarization controller within an optical switch. In this diagram, double-barbed arrows inscribed within circles represent p-polarized light that is polarized vertically within the plane of the page. Crosses inscribed within circles in  FIG. 1  represent s-polarized light polarized perpendicularly to the plane of the page. An input light  104  having an initial linear polarization, which is assumed herein to be p-polarization, is incident upon the LC cell  102 . 
   Upon passing through the cell  102 , the light becomes output light  106 . Without an applied voltage (that is, in the OFF state), the polarization of the light is switched (rotated by 90°) such that, nominally, all the output light becomes s-polarized output light  106   s . However, due to imperfection of the cell, a small proportion of the light is the p-polarized output light  106   p.    
   When voltage is applied to the cell  102  (the ON state) shown in  FIG. 1 , the polarization of the output light  106  nominally remains the same as that of the input light  104 . Thus, most of the light is the p-polarized light  106   p . Once again, however, due to imperfection of the cell, a small proportion of the light is the s-polarized output light  106   s.    
   For clarity of presentation, the two output lights  106   s  and  106   p  shown in  FIG. 1  are shown separated from one another. In practice, however, these two output lights overlap. The output light represented by a dashed line is the one of lesser intensity. If the liquid crystal cell  102  comprises a component within an optical switch, other components of the switch may be utilized to route the s-polarized output light  106   s  to a first destination and the p-polarized output light  106   p  to a second destination. Thus, unless the light of lesser intensity is minimized, either in the ON state or the OFF state, an undesirable portion of the light may be directed to an incorrect destination. 
   The extinction ratio, R ex , of the cell  102  shown in  FIG. 1  is defined by 
                   R   ex     =     10   ⁢           ⁢       log   10     ⁡     (       I   s       I   p       )                 Eq   .           ⁢   1               
where I s  is the intensity of s-polarized output light  106   s  and I p  is the intensity of p-polarized output light  106   p . If the input light  104  is s-polarized instead of p-polarized, then the quantities I s  and I p  must be interchanged with one another in Eq. 1.
 
   As the ambient temperature increases, the birefringence of the LC material generally decreases, leading to a different R ex  in the OFF state.  FIGS. 2A–2B  show simulated curves of R ex  versus applied voltage for a Twisted Nematic LC cell with a 6.8 μm thick of LC layer. The different curves represent R ex  values at different temperatures. For instance, “T20”, “T30”, etc. indicate curves calculated for respective temperatures of 20° C., 30° C., etc. Assume that the OFF state of the cell  102  corresponds to an applied voltage of 0 V. Then, when the ambient temperature increases from 20° C. to 70° C., the R ex  in the OFF state increases from 22.8 dB at 20° C. to 30 dB at 50° C., and then drops to 15.5 dB at 70° C. (see  FIG. 2A ).  FIG. 2B  shows the same curves exhibited in  FIG. 2A , but at a wider range of voltages. If the ON state has an applied voltage of 4 V, then, as the temperature changes, the R ex  in the ON state remains constant at around −50 dB (see  FIG. 2B ). The minus sign indicates that the p-polarized output light  106   p  is dominant in ON state. 
   During fabrication of an LC cell, the control of parameters cannot be ideal, so the parameters of each individual cell, such as cell gap, twist angle, etc., may vary.  FIGS. 3A–3B  show that the cell gap variation leads to different curves of R ex  versus applied voltage.  FIGS. 3A–3B  show graphs of the simulated curves of R ex  versus applied voltage for various Twisted Nematic liquid crystal cells with different LC thickness and twist angle. The different curves in each of  FIG. 3A  and  FIG. 3B  represent R ex  values for cells having different cell gaps. For instance, “d65”, “d68, “d70” and “d72” represent curves for cells having gaps of 6.5° μm, 6.8° μm, 7.0° μm and 7.2° μm, respectively. Comparison between  FIGS. 3A and 3B  shows that twist angle variation also leads to different curves of R ex  versus applied voltage. 
   Because, in general, a liquid crystal cell will output some light that is polarized orthogonal to the desired dominant polarization, an undesirable portion of the output light may be directed to an incorrect destination. Further, since the extinction ratio can vary with temperature within a single cell and can vary between cells, depending upon fabrication parameters, the amount of such mis-directed light is difficult to predict. This presents a problem in reproducibly fabricating optical switches based upon liquid crystal devices and in ensuring stable operation of the switches. 
   Accordingly, there exists a need for an improved liquid crystal cell apparatus. The improved apparatus should be made to operate at an optimum extinction ratio, regardless of temperature variations and cell-to-cell variations. The present invention addresses such a need. 
   SUMMARY OF THE INVENTION 
   A polarization controller includes: a liquid crystal cell; a mechanism for determining a compensation voltage; and a voltage driving circuit for applying the compensation voltage to the liquid crystal cell, where the application of the compensation voltage optimizes an extinction ratio of the liquid crystal cell. The controller controls the extinction ratio using an applied voltage to compensate for temperature changes, liquid crystal cell gap variations, and other errors of the liquid crystal cell. This controller is aimed at liquid crystal devices used in a wide ambient temperature range and is also useful to compensate for the fabrication errors of the liquid crystal switches. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  shows the diagram of a typical optical switch using LC cells. 
       FIGS. 2A–2B  show graphs of the simulated curves of R ex  versus applied voltage at various temperatures for a Twisted Nematic liquid crystal cell with a 6.8 μm thick LC layer. 
       FIGS. 3A–3B  show graphs of the simulated curves of R ex  versus applied voltage for various Twisted Nematic liquid crystal cells with different LC thickness and twist angle. 
       FIG. 4  shows graphs of the simulated curves of R ex  versus applied voltage at various temperatures for a Twisted Nematic liquid crystal cell configured in accordance with the present invention. 
       FIGS. 5A–5B  illustrate two different liquid crystal polarization controller apparatuses, in accordance with the present invention, that maintain optimum extinction ratio through application of compensation voltages. 
       FIGS. 6A–6B  illustrate two different methods, in accordance with the present invention, for maintaining optimum extinction ratios of LC polarization controller devices. 
   

   DETAILED DESCRIPTION 
   The present invention provides an improved liquid crystal cell apparatus. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein. 
   The present invention discloses apparatuses and methods that enable compensation for extinction ratio (R ex ) variations in optical switches, wherein such variations might be caused by ambient temperature changes or by variations in the fabrication process of LC cells. To more fully appreciate the features and advantages of the present invention, the reader is referred to  FIGS. 4–6B  of the accompanying drawings in conjunction with the following discussion. 
     FIGS. 5A–5B  illustrate two different preferred embodiments of LC polarization controller apparatuses, in accordance with the present invention, that maintain high extinction ratio through application of compensation voltages to the LC. The apparatus  500  ( FIG. 5A ) comprises an LC cell  102  that receives a polarized input light  104  and outputs a polarized output light  106 , a temperature sensor  502  that is either physically coupled to or in close proximity to the LC cell  102 , a microprocessor  504  electronically coupled to the temperature sensor  502 , an electronic voltage driving circuit  508  electronically coupled to the microprocessor  504  and to the LC cell  102 , and a machine readable compensation voltage table  506  that is electronically read by the microprocessor  504 . The microprocessor  504  may be replaced by any other suitable electronic analog or digital logic processor or computer. 
   Preferably, the LC cell  102  within the LC polarization controller  500  ( FIG. 5A ) and the polarization controller  550  ( FIG. 5B ) is constructed such that its highest R ex  is achieved at the upper limit of the operating temperature and without any applied voltage. This property can be achieved by choosing an appropriate LC cell gap and appropriate refractive indices of the LC material during fabrication of the cell.  FIG. 4  shows a typical electro-optic (EO) response of a cell with such a design. In this design, the operating temperature range is chosen, for example, to be 20° C.–70° C. When no voltage is applied, an LC cell  102  fabricated with this design achieves the highest R ex  (50 dB) at the upper limit of the operating temperature (i.e., 70° C.). When the temperature decreases, the R ex  drops. Advantageously, if the operating temperature is lower than the upper limit, then there always exists a particular applied voltage for that temperature that enables the LC polarization controller to reach a maximum R ex . Frequently, the maximum R ex  at any temperature in question will comprise an optimum R ex  to be utilized at that temperature. 
   The compensation voltage table  506  of the apparatus  500  may contain a predetermined list of compensation voltages, tabulated against temperature, that should be applied to the LC cell to optimize the extinction ratio of the output light at any respective temperature. These compensation voltages are determined by the measured electro-optic (EO) response of the cell at various temperatures, an example of which is given in  FIG. 4 . The voltage compensation table  506  may also or alternatively be provided as a mathematical formula that is stored in either software or electronic firmware and calculated by the microprocessor  504 , such that the formula receives a temperature and provides a compensation voltage in response. 
   The temperature sensor  502  of the apparatus  500  measures the temperature of the LC cell or of its environs (i.e., the ambient temperature). This temperature is read by the microprocessor  504  and is compared to entries in the compensation voltage table  506  or else is entered into a formula stored in that table. As a result of this table entry retrieval or formula calculation, the microprocessor  504  determines a best compensation voltage to apply across the LC cell  102  so as to optimize the extinction ratio at that temperature. The voltage driving circuit  508  applies the determined voltage to the LC cell  102 . The compensation voltage may be applied in addition to any other operating voltage applied to the LC cell, such as the voltage required to place the cell in its “ON” state. The optimization may be such as to either maximize or minimize the extinction ratio, depending upon whether the cell  102  is in its ON state or OFF state and upon how the extinction ratio is defined. 
   The polarization controller apparatus  550  ( FIG. 5B ) comprises an LC cell  102  that receives a polarized input light  104  and outputs a polarized output light  106 , a beam splitter  510   a  disposed within the path of the output light  106  so as to divert a small sampled proportion  516   a  of the output light, a detector system  512  optically coupled to the beam splitter  510   a  and receiving the sampled proportion  516   a , a servo system  514  electronically coupled to the detector system  512  and a voltage driving circuit  508  electronically coupled to the servo system  514  and to the LC cell  102 . The apparatus  550  may also comprise an optional second beam splitter  510   b  disposed within the path of the input light  104  so as to divert a small sampled proportion  516   b  of the input light to the detector system  512 . 
   The detector system  512  measures the extinction ratio of the sampled proportion  516   a  of the output light  106  and, optionally, the intensity of the sampled proportion  516   b  of the input light. The extinction ratio of the sampled proportion  516   a  or the ratio of the two sampled proportions  516   a – 516   b  is related to the extinction ratio of the output light  106 . Preferably, the detector system  512  comprises a means—such as a polarization beam splitter or birefringent crystal—to separate the s-polarized and the p-polarized light and comprises a separate photo-detector for each such light. The detector system  512  may also comprise electronic logic (not shown) to correct the detected intensities of the s-polarized and p-polarized light output for any intensity variations in the input light  104 . The latter variations may determined through optional monitoring of the sampled proportion  516   b  of the input light. 
   Based upon the measurements and corrections described above, the detector system  512  provides a correction signal to the servo system  514 . The servo system  514  then controls the voltage driving circuit  508  so as to provide a voltage to the LC cell  102  that optimizes the measured extinction ratio, wherein the extinction ratio is determined from the sampled proportion  516   a  and, optionally, from the sampled proportion  516   b.    
     FIGS. 6A–6B  illustrate two different preferred embodiment of methods, in accordance with the present invention, for maintaining optimum extinction ratios of LC polarization controller devices through the application of compensation voltages to the devices. The method  600  illustrated in  FIG. 6A  and the alternative method  650  illustrated in  FIG. 6B  both include the following actions: 
   Action 1: Design the LC polarization controller  500  or  550  such that its highest R ex  is achieved at the upper limit of the operating temperature and without any applied voltage, such as shown in  FIG. 4 . This corresponds to step  601  in both method  600  and method  650 . 
   Action 2: Apply a compensation voltage to reach optimum R ex . The methods  600  and  650  differ in their respective approaches to achieving this action. Approach A corresponds to the steps  602 – 610  in the method  600  ( FIG. 6A ) whereas approach B corresponds to the steps  652 – 654  in the method  650  ( FIG. 6B ). 
   Approach A ( FIG. 6A ), utilized in apparatus  500  ( FIG. 5A ), comprises the following steps: Measure the EO curves of the LC polarization controller  500  for different temperatures in Step  602 ; Tabulate the compensation voltages which enable the LC polarization controller  500  to reach the optimum R ex  at different temperatures in Step  604 ; Use a temperature sensor  502  to measure the ambient temperature or the LC temperature in Step  606 ; Use a microprocessor  504  or other logic processor or computer to check the tabulated compensation voltages according to the measured temperature in Step  608 ; and Apply the appropriate compensation voltage to the LC cell  102  to reach optimum R ex  for any temperature in the operating range in Step  610 . Steps  601 ,  602  and  604  of method  600  are preparatory in nature and occur during fabrication of the LC cell  102  and prior to putting an LC cell  102  into operation. Steps  606 – 610  comprise a repeating loop that occurs when an LC cell  102  is in operation. The compensation voltage may be applied in addition to any other operating voltage applied to the LC cell  102 , such as the voltage required to place the cell in its “ON” state. 
   Approach B ( FIG. 6B ), utilized by the apparatus  550  ( FIG. 5B ), comprises the following steps: Use a photo-detection system  512  to measure the R ex  of the LC cell  102  in Step  652 ; and Use a servo system  514  to control applied voltage to the LC cell  102  according to the monitoring signal  516   a  in step  654  to maintain optimum R ex  automatically. Step  601  of method  650  is preparatory in nature and occurs during fabrication of the LC cell  102 . Steps  652 – 654  comprise a repeating loop that occurs when an LC cell  102  is in operation. 
   Fabrication errors, such as cell gap error and twist angle errors, are compensated automatically by Approach B utilized in method  650 . If Approach A of method  600  is used, the EO response of each individual device needs to be measured for different temperatures in order to compensate for all fabrication errors. 
   An improved apparatus and a method for controlling the extinction ratio of a liquid crystal cell have been disclosed. Although the embodiments shown here include a transmissive Twisted Nematic LC switch, the concept of controlling R ex  via compensation voltages can also be applied to other transmissive and reflective LC switches without departing from the spirit and scope of the present invention. Although the present invention has been described in accordance with the embodiments shown and discussed, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.