Abstract:
An optical system capable of enhancing a specific polarization state of light beam comprises a polarization beam splitter and a polarization state converter. The polarization beam splitter separates an input light beam into a first light beam of first polarization state and a second light beam of second polarization state. The first polarization state is different from the second polarization state. The second light beam is input into the polarization state converter and converted to a third light beam having significantly much more components of first polarization state. The polarization state converter has a configuration providing total reflection or high reflection function. The configuration includes at least one anisotropic optical thin film that is disposed between an incident medium of high refractive index and a medium of low refractive index.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This utility application claims priority to Taiwan application serial number 099119780, filed Jun. 18, 2010, which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This present invention relates to an optical system which is capable of enhancing a specific polarization state of a light beam and a light source system including the same. 
         [0004]    2. Description of the Prior Art 
         [0005]    The demand for various types of display devices has increased in the past decade. For instance, the most popular at this stage is the LCD display device and projector. These display machines use polarized light beam propagating through the components within them to achieve desired functions. Therefore, how to get a specific polarized light efficiently has been an important topic. Instead of using a conventional polarizer to absorb the unwanted polarized component from an unpolarized light, a polarization conversion device can be utilized to recycle the unwanted polarization component to increase the efficiency of display and projector. A previously proposed way to convert the non-polarized light beams of a light source into linearly polarized light beams having a single polarization state, was taught in, for example, U.S. Pat. No. 5,122,895 (Polarization Converter for converting Randomly Polarized Light to Linearly Polarized Light). In this patent, a so-called PS converter is disclosed. The P-polarized light component refers to the electric field oscillation of P light component being parallel to the plane of the incident light beam. The S-polarized light component refers to the electric field oscillation of S light component being vertical to the plane of the incident light beam. 
         [0006]    Typically, in a system of LCD display or projector, the conventional polarization plate (polarizer) absorbs the unwanted polarized light component within the incident light beam, to obtain the required polarized light beam. The maximum output efficiency for the conventional polarizer is about 50%. In addition to lacking of efficiency as to the conventional approaches, the configuration of conventional PS converters is complex and typically involves relatively high costs of production. In addition, the display apparatus which implements the polarization state converter consumes more power than expected. 
         [0007]    To meet the needs which cure the drawbacks mentioned above, the present invention intends to provide a novel solution of optical system capable of enhancing a specific polarization state of a light beam. By this invention, a thin film approach is used to implement a simple anisotropic film system which induces a strong polarization conversion. Together with a polarization beam splitter, other than boosting to a higher conversion efficiency, the requirements for enhancing one polarization state from an unpolarized light can be met easily. 
       SUMMARY OF THE INVENTION 
       [0008]    In accordance with one aspect of the invention, the optical system is capable of enhancing a polarization state of the light beam. 
         [0009]    The aforesaid objective of the invention is achieved by combining a polarization beam splitter and a polarization conversion configuration. 
         [0010]    In specific, the present invention is an optical system capable of enhancing a polarization state of the light beam, comprising a polarization beam splitter, for splitting an inputted light beam into a first light beam and a second light beam, the first light beam having a first polarization state, the second light beam having a second polarization state different from the first polarization state; a polarization state converter, inputting the second light beam, for converting the polarization state and outputting a third light beam, the third light beam including significantly more of the first polarization state, wherein the polarization state converter is configured as such for providing total reflection or high reflection of light, the configuration including at least an anisotropic optical thin film, disposed between an incident medium of high refractive index and a medium of low refractive index. 
         [0011]    More details of the respective embodiments can be found in the respective iterations in the dependent claims hereinafter recited. 
         [0012]    According to one embodiment, the optical system includes a laminated beam splitter, the laminated beam splitter includes two triangular prisms and an optical thin film stack disposed between the two triangular prisms, and the optical thin film stack includes multiple layers of high refractive index thin film and low refractive index thin film that are interposed to each other. The optical thin film stack is highly reflective to the incident S polarized light beam, is highly transmissive to the incident P polarized light beam. 
         [0013]    Optionally, according to another embodiment, the polarization state converter includes a prism, and the light beam reflection surface of the prism is coated with a first isotropic film/an anisotropic film/a second isotropic film for performing partial conversion or entire conversion of polarization state of light beam. 
         [0014]    In accordance with another embodiment, the optical system further includes an element for performing partial or entire combination of the first light beam and the third light beam. 
         [0015]    The light source system of the embodiment includes an optical system which is capable of enhancing a specific polarization state of the light beam, which is output from, for example, an LED light source. One polarization state of the output light beam from this light source system is therefore enhanced. 
         [0016]    All aspects of the present invention will no doubt become apparent to those of ordinary skills in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the following figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE APPENDED DRAWINGS 
         [0017]      FIG. 1  shows the optical system  1  of first embodiment; 
           [0018]      FIG. 2   a  shows a multiple layers structure of the glass prism  104  of high refractive index; 
           [0019]      FIG. 2   b  discloses the relative relationship between the anisotropic thin film  210  and corresponding three principal axes; 
           [0020]      FIG. 3  shows the optical system  3  of second embodiment; 
           [0021]      FIG. 4  shows the embodiment of a light source system of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Some preferred embodiments and practical applications of this present invention would be explained in the following paragraph, describing the characteristics, spirit and advantages of the invention. 
         [0023]    As shown in  FIG. 1 , the first embodiment of the optical system of the invention for enhancing a specific polarization state of a light beam includes a polarization beam splitter  10 , a polarization state converter  12  and an optical (light combination) element  14 . 
         [0024]    The polarization beam splitter  10  can be a laminated beam splitter  10  which includes a triangular prism  100  and a triangular prism  102  forming a cube of the laminated beam splitter  10 . Between the two triangular prisms  100  and  102 , an optical thin film stack  110  is disposed, and the optical thin film stack  110  includes multiple layers of high refractive index thin film and low refractive index thin film that are interposed to each other. The optical thin film stack is highly reflective to the incident S polarized light beam, is highly transmissive to the incident P polarized light beam. Optionally, other surfaces of the cube formed by the triangular prism  100  and the triangular prism  102  are coated with multiple layers of anti-reflection films to reduce the possible loss of light reflection. 
         [0025]    In one embodiment, the polarization beam splitter  10  can be a broadband polarization beam splitter, for instance the model No. 05FC16BP.3, which is implemented by material of SF2, produced by Newport Corporation (http://www.newport.com/). 
         [0026]    By one embodiment, the polarization state converter  12  includes a glass prism  104  of high refractive index with the reflection surface  112  coated by at least an isotropic film and an anisotropic film. In addition, on this glass prism  104 , except the light beam reflection surface  112 , other surfaces can be coated with multiple layers of anti-reflection films to lower the loss of light reflection. 
         [0027]    As shown in  FIG. 1 , as a non-polarized light beam  120  enters the optical system  1  of the first embodiment, the non-polarized light beam  120 , while going through the polarization beam splitter  10 , is divided into, by the multiple layers of film  110 , a first P polarized light beam component  122  and an S polarized light beam component  124 . The first P polarized light beam component  122  passes through the multiple layers of film  110  and transmits from the polarization beam splitter  10 . The S polarized light beam component  124  is reflected by the multiple layers of film  110  and leaves the cube of polarization beam splitter  10 . In succession, the S polarized light beam component  124  enters the polarization state converter  12 , i.e. the glass prism  104  of high refractive index. The incident S polarized light beam component  124  is converted, at the reflection surface  112  of glass prism  104 , to a second P polarized light beam component  126  which leaves from the polarization state converter  12 . The second P polarized light beam component  126  has same polarization state and an identical (or not-identical) advancing direction, compared to those of the first P polarized light beam component  122 . Afterwards, using a light combination element  14 , the second P polarized light beam component  126  and the first P polarized light beam component  122  can be combined partially or entirely forming a P polarized light beam  128 . 
         [0028]    A multiple-layer structure (configuration) at the reflection surface  112  of glass prism  104  of high refractive index is shown in  FIG. 2   a . The multiple-layer structure includes a first isotropic film  202 /an anisotropic film  210 /a second isotropic film  204  sequentially disposed as shown (not to scale). That is, the light beam reflection surface  112  is formed by a first isotropic film  202 /an anisotropic film  210 /a second isotropic film  204 . In other words, the shown configuration includes at least an anisotropic thin film  210  disposed between an incident medium of higher refractive index and a medium of lower refractive index. For the embodiment of  FIG. 2   a , the incident medium of higher refractive index is the glass prism  104  and the second isotropic film  204  is the medium of lower refractive index. In accordance with one embodiment, the glass prism  104  has higher refractive index of 1.515 and the material is BK7. The first isotropic film  202  utilizes the material of MgF 2  with thickness of 200 nm, and as the wavelength of the light beam is about 632.8 nm, the refractive index of the first isotropic film  202  is about 1.397. Similarly, the second isotropic film  204  is in form of thin film material of MgF 2  with thickness of 30 nm, as the wavelength of the light beam is about 632.8 nm, the refractive index of the second isotropic film  204  is also about 1.397. Furthermore, the anisotropic film  210  is in form of thin film material of MgF 2  with thickness of 800 nm. Relative to the anisotropic three principal axes ( 21 ,  22 ,  23 ), as shown in  FIG. 2   b , the three principal refractive indices of the anisotropic film  210 , i.e. n 21 , n 22 , n 23 , are n 21 =1.215, n 22 =1.216, n 23 =1.260 respectively. It is noted that the first principal axis  21 , the second principal axis  22  and the third principal axis  23  are respectively perpendicular to each other. The third principal axis  23  and the film normal  20  intersects at an angle α, wherein α=33 degrees. The plane formed by third principal axis  23  and the film normal  20  is noted as the deposition plane  220 , the second principal axis  22  is on the deposition plane  220  and is vertical to the third principal axis  23 , the deposition plane  220  is angled with the incident surface  222  at about 90 degrees. 
         [0029]    In some embodiments, the multiple-layer film  110  is such that allows transmission of light in the visible regime from 420 nm to 680 nm. 
         [0030]    For the light beam advancing through polarization beam splitter  10 , i.e. unpolarized incident light beam  120 , the first P polarized light beam component  122 , the S polarized light beam component  124  and the second P polarized light beam component  126 , the portion of being absorbed by the polarization beam splitter  10  can be neglected during the optical design. The Extinction Ratio (Tp/Ts) of the prism ( 100 ,  102 ) is about 1000:1 for the visible light range, wherein Tp is the transmittance for P polarized light beam which is greater than 90% in average, and Ts=(1−Rs), wherein Rs is the reflectance for S polarized light beam which is greater than 99.5% in average. 
         [0031]    In some embodiments, the anisotropic film  210  is an inclined column array formed by a dielectric material. The direction of columnar growth is not parallel to the plane of incidence producing the polarization conversion effect. Furthermore, the dielectric material of the anisotropic film  210  is selected from a group consisting of MgF 2 , SiO 2 , or TiO 2 . 
         [0032]    As shown in  FIG. 3 , the second embodiment for the optical system  3  is disclosed. The polarization beam splitter  10 , the polarization state converter  12  and the optical element  14  shown are respectively similar in structure and function with those shown in  FIG. 1 . In  FIG. 3 , the first internal angle  330 , the second internal angle  332  and the third internal angle  334  of the glass prism  104  of high refractive index are respectively about 51.5 degrees, 74.09 degrees and 54.41 degrees. As the incident angle is about 51.5 degrees, within the range of visible light, there are more than 90% of the incident S polarized light component  124  that are converted into the second P polarized light beam component  126 . Afterwards, the second P polarized light beam component  126  is reflected to a prism interface  314  of the glass prism  104  and then refracted to outside of the prism  104 . After the second P polarized light beam component  126  is refracted at the prism interface  314 , it emits as a third P polarized light beam component  326 . The third P polarized light beam component  326  has identical polarization state with the first P polarized light beam component  122 , and identical or different advancing direction. A light combination element  14  can be used for partially or entirely combing the third P polarized light beam component  326  and the first P polarized light beam component  122  resulting in a P polarized light beam  328 . The elements in  FIG. 3  having same legend as elements shown in  FIG. 1  respectively have same or similar structure and function and will not be redundantly iterated herein again. 
         [0033]    As the optical system illustrated in  FIG. 1 ,  FIG. 2   a ,  FIG. 2   b  or  FIG. 3  is utilized at output section of various light source, e.g. LED light source, specific one polarization state of the outputted light beam passing through this system can be enhanced. As shown in  FIG. 4 , a light source element  40 , e.g. LED light source, emits an unpolarized light beam  120  entering the optical system  1 . At first, via the polarization beam splitter  10 , the first P polarized light beam component  122  and S polarized light beam component  124  are formed. Then, after the S polarized light beam component  124  enters the polarization state converter  12 , the second P polarized light beam component  126  is formed. At last, using the optical element  14 , the first P polarized light beam component  122  and the second P polarized light beam component  126  are combined to form P polarized light beam  128 . The elements in  FIG. 4  having same legend as elements shown in  FIG. 1  respectively have same or similar structure and function and will not be redundantly iterated herein again. 
         [0034]    With the example and explanations above, the features and spirits of the invention will be hopefully well described. It is understood that the invention is not only limited to those described embodiments and it is highly possible for persons skilled in the arts, without departing the spirit of the invention, might make various alteration, modification or equivalent transformation. 
         [0035]    For example, the polarization beam splitter  10  can be designed as such the outputted polarized light beam  122 ,  124  are respectively S polarized light beam component and P polarized light beam component. Under this situation, the function of polarization state converter  12  is to convert the P polarized light beam component into S polarized light beam component. 
         [0036]    For a possible alteration, the polarization beam splitter  10  may be such that the outputted polarized light beam  122 ,  124  are respectively left circular (or elliptical) polarized light beam component and right circular (or elliptical) polarized light beam component. Under this situation, the function of polarization state converter  12  is to convert right circular (or elliptical) polarized light beam component into left circular (or elliptical) polarized light beam component. 
         [0037]    Still for another possible alteration, the thin film  202 ,  204  and  210  in the polarization state converter  12  can use the anisotropic thin film of 800 nm thickness with principal refractive index of n 21 =1.751, n 22 =1.685, n 23 =1.897 respectively. This configuration can result in a broadband polarization conversion effect. 
         [0038]    For another possible modification, the thin film  202 ,  204  and  210  of polarization state converter  12  can be replaced by stack of the multiple anisotropic thin films resulting in a broadband and wide angle polarization conversion effect. 
         [0039]    With the example and explanations above, the features and spirits of the invention will be hopefully well described. Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teaching of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.