Patent Publication Number: US-2007115552-A1

Title: Polarization beam splitter and combiner

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
      This application claims priority to provisional patent application No. 60/596,157, entitled “A New Hybrid PBS for LCOS Projection,” filed Sep. 2, 2005. This application also claims priority to provisional patent application No. 60/717,134, entitled “Hybrid PBS for LCOS Projection,” filed Sep. 14, 2005. 
    
    
     TECHNICAL FIELD  
      This application relates to a polarization beam splitter, and more in particular to a polarization beam splitter and combiner for projection systems.  
     BACKGROUND  
      Polarization beam splitters (PBS) are optical components used in front and rear projection systems to split input light into two beams with opposite polarization states. PBSs have also been used to combine light from two ports for output through a single port. In some projection applications, PBSs have been used for both splitting and recombining light, for instance as provided in the ColorQuad™ and CQ3® architectures, by ColorLink, Inc., Boulder, Colo.; also described in commonly-assigned U.S. Pat. Nos. 6,183,091 and 6,961,179, which are herein incorporated by reference. Various PBSs are known, including multilayer birefringent cube PBSs, MacNeille-type PBSs, and wire-grid polarizer-type PBSs. More detail on these PBSs and related projection system architectures can be found at M. Robinson, J. Chen and G. Sharp, P OLARIZATION  E NGINEERING  FOR LCD P ROJECTION  97-98 (Wiley &amp; Sons 2005) [hereinafter P OLARIZATION  E NGINEERING ], which is hereby incorporated by reference for all purposes.  
       FIG. 1  illustrates an exemplary Multilayer Birefringent Cube (MBC) PBS  100 . An MBC PBS  100  is typically made of multilayer birefringent stack  200  with alternating high/low refractive indices (i.e., from layers  208 ,  210 ). The multilayer birefringent stack  200  is sandwiched by two bulk glass prisms  102 ,  104 . Glass prisms have typically been made from high-index glass, with high lead content, for example, PBH56 and SF57.  
       FIG. 2  shows the structure of a typical multilayer birefringent stack  200  which is made from a polarizing film. The multilayer birefringent stack  200  comprises alternate layers of different uniaxial birefringent materials,  208  and  210 , with a common optic axis. In one direction, the refractive indices of the materials are matched, having refractive index n. In an orthogonal direction, in the plane of birefringent material, the refractive indices of birefringent materials  208  and  210  are n 2  and n 1  respectively. Because the structure of multilayer birefringent stack  200  appears homogeneous, light polarized along this direction (e.g., s-polarized light) is transmitted without loss. In the orthogonal direction, the structure is inhomogeneous, taking the form of a thin-film multilayer mirror (e.g., for the p-polarized light).  
      The polarizing film material that may be used in multilayer birefringent stack  200  may typically have between 100 and 800 layers of alternate polymers  208 ,  210 . In a color specific embodiment discussed herein, the number of layers may be closer to 300. Made from thermally processed extruded multilayer polymer, the entire layered structure is stretched to form the necessary birefringence within the layers  208 ,  210 . In this way the extraordinary index is controlled to achieve high reflectivity for one polarization, whereas the ordinary axis is matched yielding high transmission for the orthogonal polarization. The optic axes of all layers are substantially parallel to the stretch direction and can therefore be considered the optic axis of the composite film. Since the unaffected transmitted wave is orthogonal to this optic axis, the polarizer is effectively an o-type polarizer in transmission, but e-type in reflection.  
      Incorporating the multilayer birefringent reflecting polarizing material (multilayer birefringent stack)  200  of  FIG. 2  between glass prisms produces a multilayer birefringent cube PBS  100 , as shown in  FIG. 1 . With regard to the multilayer birefringent stack  200 , the orientation of the optic axis defines the transmitting and reflecting polarizations, as for a conventional sheet polarizer. This in principle allows transmission of either s- or p-polarizations.  
       FIGS. 3A and 3B  show two configurations and their respective optic axes.  FIG. 3A  is a schematic block diagram of an MBC PBS  300  that transmits p-polarized light  302  and reflects s-polarized light  304 . In an alternative configuration, in which multilayer birefringent stack  200  is orthogonally arranged,  FIG. 3B  provides an MBC PBS  350  that transmits s-polarized light  304  and reflects p-polarized light  302 . The configuration for MBC PBS  300  shown in  FIG. 3A  has been commercially developed by 3M, Inc. under the Vikuiti™ name.  
      There are various advantages to using an MBC PBS  100  over a MacNeille-type PBS. Such an MBC PBS  100  delivers good contrast and high transmission without the drawback of geometrical polarization axis rotation associated with conventional MacNeille cube PBSs.  
      A disadvantage, however, is its poor front wave distortion in the reflected channel due to poor flatness of the multilayer birefringent stack  200 . This limitation is unfortunate, since it precludes the reflected image from being adequately conveyed in a projection system, thus preventing four ports of MBC PBS  100  from being used in an optical imaging system, for example, such as one using a ColorQuad™ or CQ3® architecture.  
      The schematic block diagram of modulation subsystem  400  in  FIG. 4  illustrates the poor wave front distortion encountered with a conventional MBC PBS  100  and LC modulators  120 ,  122  for two different wavelength ranges. In this illustration, s-polarized light  130  reflects off multilayer birefringent stack  200  toward first LC panel  120 . The first LC panel  120  modulates the light  130  and transmits p-polarized light  132  through multilayer birefringent stack  200  toward an output port. Input p-polarized light  134  is transmitted through the multilayer birefringent stack  200  toward a second LC panel  122 . Light modulated by the second LC panel  122  is modulated and reflected back as s-polarized light  136  toward the multilayer birefringent stack  200 . Due to the poor front wave distortion in the reflected channel, s-polarized light  138  provides an inferior quality reflected image. Accordingly, MBC PBS  100  is unsuitable for use in a four port modulation subsystem  400  that uses two LC modulating panels  120 ,  122 .  
      Another performance concern with MBC PBS  100  relates to stress-induced birefringence in both the polymer layers  200  and the surrounding glass  102 ,  104 . It is a concern because any intrinsic or induced birefringence alters the polarization state of light, causing non-uniform system performance characteristics, such as poor system contrast, and a non-uniform picture, among others.  
      Induced birefringence in the PBS can result from several conditions. The first is internal stress due to the forming of glass. Second, bonding and mounting glass components should be done carefully to avoid stress. Finally, thermally induced birefringence should be controlled through careful system thermal management. Induced birefringence derives from non-uniform expansion of glass by thermal gradients and mismatched material thermal coefficients. The extent to which these thermal effects are seen is related not only to the glass photoelastic constant, but also to absorption, thermal expansion coefficient, and Young&#39;s modulus.  
      MBC PBS can avoid severe thermal stress issues using low index though highly transmissive glass such as SK8. This solution is however not so suitable to broad band, high incident angle MacNeille cubes.  
     SUMMARY  
      The high transmission performance of the birefringent multilayer cube PBS has always been attractive for the shared PBS LCoS projection systems based on ColorSelect® wavelength filters. The stumbling block has always been the severe reflected wavefront distortion. By incorporating a conventional MacNeille-type coating adjacent to a multilayer birefringent stack as part of a hybrid cube approach this challenge can be overcome and allow for high performing, compact projector architectures.  
      In an embodiment, a polarization beam splitter includes a multilayer birefringent stack adjacent to a dichroic coating, and sandwiched between a first prism and a second prism. The multilayer birefringent stack may include alternate layers of uniaxial polymeric birefringent material, the layers having a common optical axis. The dichroic coating may include alternate layers of high- and low-refractive index materials coated onto the hypotenuse of the second prism. In such an embodiment, the multilayer birefringent stack may be operable to reflect s-polarized light of a first wavelength range, and the dichroic coating may be operable to reflect s-polarized light of a second wavelength range.  
      According to another aspect, a polarization beam splitter has an optical interface for selectively reflecting and transmitting polarized light. The optical interface includes a multilayer birefringent stack and a dichroic coating adjacent to the multilayer birefringent stack. The multilayer birefringent stack is operable to reflect incident s-polarized light toward a first direction and transmit incident p-polarized light toward a second direction. The dichroic coating is operable to reflect s-polarized light from a fourth direction (parallel but opposite to the second direction) toward a third direction.  
      According to yet another aspect, a projection system includes a first, second and third PBS. The first PBS has an optical interface including a multilayer birefringent stack and a dichroic coating adjacent to the multilayer birefringent stack. The third PBS is operable to combine light from the first PBS and the second PBS, and operable to output the combined light through an output port.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a more complete understanding of the principles disclosed herein, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings in which:  
       FIG. 1  is a schematic block diagram of a conventional multilayer birefringent cube (MBC) polarization beam splitter (PBS);  
       FIG. 2  is a schematic block diagram of a stacked birefringent film in accordance with the present disclosure;  
       FIG. 3A  is a schematic block diagram of a conventional MBC PBS that transmits p-polarized light and reflects s-polarized light;  
       FIG. 3B  is a schematic block diagram of another conventional MBC PBS that transmits s-polarized light and reflects p-polarized light;  
       FIG. 4  is schematic block diagram of a conventional multilayer birefringent cube PBS and Liquid Crystal (LC) modulators for two different wavelength ranges;  
       FIG. 5  is a schematic block diagram of a PBS in accordance with the present disclosure;  
       FIG. 6  is schematic block diagram of a modulating subsystem that includes a hybrid PBS and LC modulators for two different wavelength ranges in accordance with the present disclosure; and  
       FIG. 7  is a schematic block diagram of an exemplary projection system architecture that includes a PBS in accordance with the present disclosure. 
    
    
     DETAILED DESCRIPTION  
       FIG. 5  is a schematic block diagram of a PBS  500  that addresses the above concerns and others. PBS  500  includes a first prism  502  and a second prism  504 , a multilayer birefringent stack  506  and a dichroic coating  512 , arranged as shown.  
      The multilayer birefringent stack  506  includes alternating layers of high/low refractive index uniaxial birefringent material,  508  and  510 , with a common optic axis. The structure of the multilayer birefringent stack  506  is similar to that described with reference to the multilayer birefringent stack  200  of  FIG. 2 . The multilayer birefringent stack  506  is sandwiched between first prism  502  and second prism  504 , and adjacent to the dichroic coating  512 .  
      The dichroic coating  512  is similar to the dichroic coating used in a MacNeille-type polarizer, in that it comprises alternate layers of high- and low-index materials. The dichroic coating  512  is coated on to the hypotenuse surface of the second prism  504 . The dichroic coating material is chosen such that Brewster&#39;s angle condition is met at all interfaces, preferably substantially satisfying the Banning relation between refractive indices in accordance with the following equation:  
               n   sub   2     =       2   ⁢           ⁢     n   L   2     ⁢     n   H   2           n   L   2     +     n   H   2                 (     equation   ⁢           ⁢   1     )             
 
 where n sub  is the refractive index of the glass prisms  502 ,  504  and adhesive layers (not shown), where n L  is the refractive index of the low-index coated material, and where n H  is the refractive index of the high-index coated material. For conventional coating materials, this favors high index glass cubes. Furthermore, high index glass helps with maintaining polarization performance over a range of input angles since the incident angles in air are squeezed inside the glass medium. 
 
       FIG. 6  illustrates a schematic diagram of an exemplary modulation subsystem  600 , including PBS  500 , a first modulating panel  620 , and a second modulating panel  622 . Modulating panels  620  and  622  may be of the LCoS variety, although other modulation panels that modulate the state of polarization may alternatively be used. The architecture of modulating subsystem  600  separates two orthogonally polarized wavelength ranges between the two modulator ports  619  and  621 . A quarter wave plate  624  may be disposed between modulator port  621  and LC modulation panel  622  to compensate for polarization axis rotation effects from the MacNeille polarizer  512 . With reference to embodiments described herein, it is assumed that the LC modulation panels include any further compensation necessary to create a mirror like off-state; and since these compensators are not germane to the present disclosure, they are not illustrated herein.  
      Both the multilayer birefringent stack  506  and the dichroic coating  512  should be designed to highly transmit p-polarized light (i.e., light  632 ,  634 ) of both wavelength ranges, which is desirable for high contrast and transmission. However, the reflection of s-polarized light (i.e., light  630 ,  636 ) can be wavelength-range specific, with the multilayer birefringent stack  506  optimized to reflect a first wavelength range (e.g., green); with the dichroic coating  512  optimized to reflect a second wavelength range (e.g., red). In some embodiments, the multilayer birefringent stack  506  and the dichroic coating  512  may act together to suppress s-transmission of the overlapping wavelength range (e.g., yellow), since any multiple reflections between the surfaces of  506  and  512  would, in general, be scattered out of the collection of a projection lens (not shown) and be lost.  
      Generally, in conventional PBSs, high-index glass such as PBH56 or SF57 has been preferred to achieve good angular performance. In the case of PBS  500 , however, the multilayer birefringent stack  506  has a very large field-of-view which would offset any roll-off in the angular performance of a dichroic coating  512 . By optimizing the wavelength ranges of the multilayer birefringent stack  506  for a first wavelength range, and the dichroic coating  512  for a second wavelength range, the high-index requirement of the glass prisms  502 ,  504  may be reduced. This potentially allows for lead-free or low-lead, environmentally friendly glass (e.g., SF6, SF1, SF2, N-BK7, et cetera) to be used for the first prism  502  and the second prism  504 , while retaining high performance optical characteristics. Such lower-lead glass is also typically cheaper to produce, leading to mass-production benefits.  
      In operation, s-polarized light  630  and p-polarized light  634  enters first prism  502 . S-polarized light  630  is reflected by the multilayer birefringent stack  506  toward first modulator port  619 , at which the first compensated LC modulating panel  620  is disposed. First LC modulating panel  620  modulates the first wavelength range light, which is reflected as p-polarized light  632  back toward the multilayer birefringent stack  506 . As p-polarized light  632 , both the multilayer birefringent stack  506  and the dichroic coating  512  transmit the first wavelength light toward the output port  638 .  
      Referring now to the path of the p-polarized light  634  entering first prism  502 , the light  634  is transmitted by multilayer birefringent stack  506  and dichroic coating  512  toward a second modulator port  621 . Located at the second modulator port  621  is a second compensated LCoS panel  622 , which may have a quarter wave plate compensator  624  disposed therebetween. In a similar fashion, second LCoS panel  622  modulates a second wavelength range of light  634  and reflects the modulated light  636 , which is s-polarized toward the dichroic coating  512 . At the dichroic coating  512 , the s-polarized light  636  is reflected toward output port  638 .  
      In an exemplary embodiment, the first wavelength range is representative of a green portion of the visible spectrum and the second wavelength range is representative of a red portion of the visible spectrum. In another embodiment, the first wavelength range may be a different color, for example, a blue portion of the visible spectrum and the second wavelength range may be a red portion of the visible spectrum. It should be apparent to a person of ordinary skill that various combinations of wavelength ranges are feasible for other embodiments.  
       FIG. 7  illustrates an exemplary projection system  700  utilizing projection subsystem  600  of  FIG. 6 . Projection system  700  includes a first polarization beam splitter  500  that has an optical interface with a multilayer birefringent stack  506  and a dichroic coating  512  adjacent to the multilayer birefringent stack. The projection system  700  also includes a second PBS  720  and a third PBS  730  that is operable to combine light from the first PBS  500  and the second PBS  720 , thus, directing light through an output port  732 . Generally, projection system  700  shares similarities to the CQ3 Architecture by ColorLink, Inc., of Boulder, Colo.; which is also described in commonly-assigned U.S. Pat. No. 6,961,179, and which is hereby incorporated by reference. In an embodiment, the second PBS  720  may be provided by an MBC PBS, or a MacNeille PBS, although an MBC PBS will likely offer superior performance. Third PBS  730  may be provided by a MacNeille-type PBS, and serves the purpose of combining the first, second, and third wavelength range modulated light. It should be noted that if a MacNeille type polarizer is used for second PBS  720 , then a quarter wave plate compensator (not shown) may be disposed between the PBS  720  and third LC modulating panel  715 , to compensate for the geometric effects of the MacNeille PBS.  
      The CQ3 architecture utilizes polarization-based, wavelength-selective, passive spectral filters  706 ,  708 ,  725 . Such filters may be ColorSelect® filters, which selectively rotate the polarization of one color relative to its complementary, and are available from ColorLink, Inc. in Boulder, Colo. The operation and technical description for ColorSelect® filters is provided in commonly-assigned U.S. Pat. Nos. 5,751,384 and 5,953,083, both of which are hereby incorporated by reference. The projection system  700  also includes a dichroic mirror  702  and wire-grid polarizers  704 ,  710  as well as third LC modulator  715  for a third wavelength range (e.g., blue). The “90 degree” configuration (shown here), where input light  701  is perpendicular to the output light  740 , can be easily modified into a “180 degree” configuration where input light  701  is parallel to the output light  740 .  
      In operation, s-polarized white light  701  enters the projection system  700 . At the dichroic mirror  702 , light  703 , comprising in one embodiment of the red and green color bands, is transmitted toward first PBS  500 , while blue light  705  is reflected toward second PBS  720 . Referring first to the operation of the first PBS  500 , the wire-grid polarizer  704  cleans up the s-polarized light  703 . Wavelength-selective filter  706 , which may be a red-cyan (RC) ColorSelect® filter, transmits cyan light (which includes green) as s-polarized light, and rotates the state of polarization of red light to an orthogonal state of polarization (p-). Thus, in an embodiment, a first wavelength range is green (falling within the cyan range), which is transmitted as s-polarized light, and a second wavelength range is red, which is transformed to p-polarized light. Accordingly, the operation of modulation subsystem  600  operates in accordance with the description in  FIG. 6  such that light at the output port  638  is directed toward a third PBS  730  as well as wavelength-selective filter  708  disposed therebetween. The wavelength-selective filter  708 , in this embodiment, may be a magenta/green (MG) wavelength-selective filter that serves to allow a first wavelength range light (i.e., green) to pass as p-polarized light, while rotating the state of polarization of the second wavelength range (i.e., red).  
      For s-polarized light  705  directed toward the second PBS  720 , the wire-grid polarizer cleans up the light  705  such that the s-polarized light reflects off boundary  722  toward a third LC modulating panel  715 . Third LC modulating panel  715  modulates a third wavelength range (i.e., blue) and modulated light  724  is p-polarized and directed toward third PBS  730 . Wavelength-selective filter  725  may be disposed between second PBS  720  and third PBS  730  to rotate the state of polarization from p- to s-polarized light. Accordingly, third wavelength range (i.e., blue) modulated light reflects at the boundary  731  toward output port  732 . The combined modulated light  740  may then be directed toward a projection lens (not shown).  
      Other embodiments may include variations from the above-described embodiment. For instance, an alternative embodiment may place red and blue LC modulation panels on the output ports of PBS  500 , while isolating green modulation port on the second PBS  720 . Another embodiment may utilize four LC modulating panels (e.g., for red, green, blue, and yellow wavelength ranges) in accordance with the teachings of commonly-assigned U.S. patent application Ser. No. 11/367,956, entitled “Four Panel Projection System”, which is hereby incorporated by reference. In such an embodiment, modulation subsystems, such as modulation subsystem  600  will be located where the modulation subsystem  600  and the PBS  720  are located. Furthermore, such an embodiment will provide reflective boundaries that are optimized for the respective wavelength ranges of the respective ports. It will be appreciated by those of ordinary skill in the art that the teachings herein can be embodied in other specific forms without departing from the spirit or essential character thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than the foregoing description, and all changes that come within the meaning and ranges of equivalents thereof are intended to be embraced therein.  
      Additionally, the section headings herein are provided for consistency with the suggestions under 37 C.F.R. § 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings refer to a “Technical Field,” the claims should not be limited by the language chosen under this heading to describe the so-called technical field. Further, a description of a technology in the “Background” is not to be construed as an admission that technology is prior art to any invention(s) in this disclosure. Neither is the “Summary” to be considered as a characterization of the invention(s) set forth in the claims found herein. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty claimed in this disclosure. Multiple inventions may be set forth according to the limitations of the multiple claims associated with this disclosure, and the claims accordingly define the invention(s), and their equivalents, that are protected thereby. In all instances, the scope of the claims shall be considered on their own merits in light of the specification, but should not be constrained by the headings set forth herein.