Abstract:
A method of aligning an optical system and an optical system incorporating the method. A hologram element is installed in the optical system so that test beams diffracted by the hologram element travel along a same optical path as test beams incident on the optical system. Alignment errors in the optical system are measured using interference patterns formed on an image surface by test beams returned from the optical system and reference beams. At least one optical element of the system is aligned using measurements calculated from the interference patterns. The optical system may provide for temporarily installing the hologram element to perform the measurements or the optical system may be constructed with a hologram formed on any optical element which may require alignment. A direction and a magnitude of any misalignment are determinable based on an appearance of the pattern and a number of circles in the pattern, respectively.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application claims the benefit of Korean Application No. 2001-61641 filed Oct. 6, 2001, in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a method of aligning an optical system and an apparatus therefor, and more particularly, to a method of and an apparatus for easily aligning an optical system using a computer-generated hologram (CGH) and aligning elements.  
           [0004]    2. Description of the Related Art  
           [0005]    In general, an alignment of an optical system is achieved by aligning optical elements of the optical system and mechanically adjusting positions of the optical elements so that the optical system has an image quality within a prescribed tolerance. The conventional technique for aligning the optical system does not directly measure errors of the optical system, but measures light intensity representing the image quality to realign the optical elements where the image quality deviates from a tolerance limit.  
           [0006]    In the conventional method of aligning the optical system, an additional unit is necessary to measure the image quality and the optical system is realigned by measuring the image quality and indirectly calculating the alignment errors of the optical system. Thus, it is difficult to accurately calculate the alignment errors and errors may occur in realigning the optical system.  
           [0007]    In the conventional technique, optical elements are aligned through mechanical manipulation, which results in a slack alignment of the optical elements, and an additional alignment unit is necessary to perform the alignment. Thus, the configuration of the conventional apparatus becomes complicated and the manufacturing cost increases.  
         SUMMARY OF THE INVENTION  
         [0008]    To solve the above-described problems, it is an object of the present invention to provide a method of precisely aligning an optical system in real time and in an accurate manner by removing errors occurring through mechanical manipulation of optical elements of the optical system, and associated aligning elements.  
           [0009]    Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.  
           [0010]    Accordingly, to achieve the above and other objects, there is provided a method of aligning an optical system. An optical element and a hologram element are installed in a barrel. Alignment errors of the optical element are measured from interference patterns formed on an image surface by reference beams and test beams. The optical element is aligned to remove the alignment errors.  
           [0011]    The hologram element is a computer-generated hologram element which is manufactured according to the tolerance of the optical system so that test beams travel in a same optical path as an incident path.  
           [0012]    The barrel may further include a unit for installing the computer-generated hologram element.  
           [0013]    The optical element may be installed in front of the computer-generated hologram element, wherein the test beams transmitted through the optical element are reflected on the computer-generated hologram element and travel on the same optical path.  
           [0014]    Alternatively, the optical element may be installed after the computer-generated hologram element, wherein the test beams transmitted through the computer-generated hologram element are perpendicularly reflected on an incident surface of the optical element and travel on the same optical path.  
           [0015]    The alignment errors of the optical element are measured from errors of the interference patterns with respect to null interference patterns.  
           [0016]    To achieve the above and other objects, there is provided an optical system comprising at least one optical element with a hologram for diffracting test beams on the same optical path.  
           [0017]    The hologram may be a computer-generated hologram.  
           [0018]    To achieve the above and other objects, there is provided a method of aligning an optical system. An optical element with a hologram is aligned in a barrel so that test beams travel on the same optical path as an incident path. Alignment errors of the optical element are measured from interference patterns formed by reference beams and the test beams. The optical element is aligned to remove the alignment errors.  
           [0019]    The hologram may be a computer-generated hologram.  
           [0020]    The alignment errors of the optical element are measured from errors of the interference patterns with respect to null interference patterns.  
           [0021]    To achieve the above and other objects, there is provided a method of testing a barrel. A hologram element is placed in a predetermined position of the barrel and a predetermined lens is installed in a designated position of an optical element. A manufacturing condition of the barrel is measurable from interference patterns formed by reference beams and test beams.  
           [0022]    The hologram element is manufactured according to the tolerance of an optical element so that the test beams travel in the same optical path as an incident path.  
           [0023]    The hologram element may be a computer-generated hologram element.  
           [0024]    The barrel may include a unit for installing the computer-generated hologram element.  
           [0025]    The predetermined lens may be a spherical lens for measuring the manufacturing condition of the barrel.  
           [0026]    Where the predetermined lens is installed in front of the computer-generated hologram element, the test beams transmitted through the predetermined lens are reflected along the same optical path as an incident path.  
           [0027]    Where the predetermined lens is installed after the computer-generated hologram element, the test beams transmitted through the computer-generated hologram are perpendicularly reflected on an incident surface of the predetermined lens and travel on the same optical path as an incident path.  
           [0028]    The manufacturing condition of the barrel is measurable from errors of the interference patterns with respect to null interference patterns.  
           [0029]    An optical system according to the present invention includes at least one optical element with a hologram element placed in a predetermined position of a barrel so that test beams travel on the same optical path as an incident path.  
           [0030]    The hologram element may be a computer-generated hologram element.  
           [0031]    To achieve the above and other objects, there is provided an optical system comprising at least one optical element with a unit for placing a hologram element in a predetermined position of a barrel.  
           [0032]    The hologram element measures a manufacturing condition of the barrel or aligns the at least one optical element.  
           [0033]    The hologram element may be a computer-generated hologram element.  
           [0034]    The present invention provides a method of aligning an optical system using a computer-generated hologram in real time and in an accurate manner and a measured manufacturing condition of a barrel. Also, the present invention provides an optical element which is directly aligned and simultaneously used by writing a hologram directly on the optical element, and a method of aligning the optical system.  
           [0035]    Alignment errors of the optical system include a defocus error, a decenter error, a tip error, and a tilt error. The defocus error occurs where light rays are bent toward an optical axis by more or less than a correct amount as in focusing or defocusing the system. The decenter error occurs where the center of the optical element is not aligned with the optical axis of the system. The tip error occurs where the optical element is inclined on an X-axis. The tilting error occurs when the optical element is inclined on a Y-axis.  
           [0036]    The alignment of the optical system represents the arrangement of optical elements constituting the optical system without the defocus, decenter, tip and tilt errors. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0037]    The above object and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which:  
         [0038]    [0038]FIG. 1 is a schematic view of an optical projection system using a method of aligning an optical system according to a first embodiment of the present invention;  
         [0039]    [0039]FIG. 2 is another schematic view of the optical projection system using a method of aligning the optical system according to a second embodiment of the present invention;  
         [0040]    [0040]FIG. 3 is a schematic view of an interferometer useable in the method of aligning the optical system according to the embodiments of the present invention;  
         [0041]    [0041]FIG. 4 is a schematic view illustrating a method of testing a barrel according to the present invention;  
         [0042]    [0042]FIG. 5 is a schematic view illustrating another method of testing a barrel according to the present invention;  
         [0043]    [0043]FIG. 6 is a schematic view illustrating an optical system and a method of aligning the optical system according to a first embodiment of the present invention;  
         [0044]    [0044]FIG. 7 is schematic view illustrating another optical system and a method of aligning the optical system according to a second embodiment of the present invention;  
         [0045]    [0045]FIG. 8 is a view showing interference patterns resulting from alignment or manufacturing errors;  
         [0046]    [0046]FIG. 9 is view showing interference patterns resulting from decentering errors in a first direction; and  
         [0047]    [0047]FIG. 10 is a view showing interference patterns resulting from decentering errors in a second direction. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0048]    Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. Embodiments of a method of aligning an optical system, an optical system having aligned elements, and a method of testing a barrel according to the present invention will be described in detail.  
         [0049]    [0049]FIG. 1 is a schematic view of an optical projection system using a method of aligning an optical system having a hologram element, i.e., a computer-generated hologram (CGH) element, according to a first embodiment of the present invention. FIG. 3 shows a more detailed view of a Fizeau interferometer.  
         [0050]    Referring to FIGS. 1 and 3, in an embodiment of an optical system, a method of aligning the optical system, and a method of testing a barrel according to the present invention, a Fizeau interferometer  31  forms test beams  1  and reference beams  3  from a light source  41 . An optical projection system  35  has optical elements on which the test beams  1  transmitted through the Fizeau interferometer  31  are incident.  
         [0051]    Alternatively, another type of interferometer may be used as the interferometer  31 . Also, the present invention is described with reference to a method of aligning an optical projection system but the principles thereof may be applied to an alignment of another type of optical system.  
         [0052]    The Fizeau interferometer  31  is positioned in front of the optical projection system as shown in FIG. 1 and generates the test beams  1  and the reference beams  3  necessary for the method of aligning the optical system and the method of testing a barrel.  
         [0053]    The test beams  1  emitted from the Fizeau interferometer  31  are incident on the optical projection system  35  which is positioned rearward of the Fizeau interferometer  31 . Here, the optical elements which are components of the optical system are lenses or mirrors, but not a CGH.  
         [0054]    Referring to FIG. 3, the Fizeau interferometer  31  includes a light source  41 , a condensing lens  42 , a filter  43 , a beam splitter  45 , and a collimating lens  47 . The condensing lens  42  condenses beams generated from the light source  41 . The filter  43  transmits only ones of the specific wavelengths of the incident beams. The beam splitter  45  splits an optical path into two optical paths so that some of the incident beams, the reference beams  3  travel toward an image surface  49  and the test beams  1  travel toward the optical projection system  35 . The collimating lens  47  collimates the test beams  1 . The light source  41  may generate laser beams, some of which are the test beams  1  and some of which are the reference beams  3 .  
         [0055]    The condensing lens  42  condenses the laser beams generated by the light source  41  and transmits the condensed laser beams to the beam splitter  45 . The filter  43  is positioned on an optical path between the condensing lens  42  and the beam splitter  45  and is adapted to transmit only incident beams having optimum light intensity within a predetermined frequency range.  
         [0056]    The beam splitter  45  changes the optical path of the reference beams  3  of the incident beams to an angle of 90° so that the reference beams  3  travel toward the image surface  49  and transmits the test beams  1 , without alteration of the optical path, toward the optical projection system  35 .  
         [0057]    The collimating lens  47  collimates the test beams  1  transmitted through the beam splitter  45 . Also, a lens (not shown) is positioned in front of the collimating lens  47  to assist in making the test beams  1  transmitted through the collimating lens  47  into convergent light beams.  
         [0058]    The test beams  1  are transmitted to the optical projection system  35  through the Fizeau interferometer  31 , are reflected by an optical element or a CGH element of the optical projection system  35 , and are incident again on the Fizeau interferometer  31 . The beam splitter  45  changes the optical path of the reflected test beams  1  incident again on the Fizeau interferometer  31  to an angle of 90° so that the reflected test beams  1  travel toward the image surface  49  and form interference patterns with the reference beams  3  on the image surface  49 .  
         [0059]    Referring to FIG. 2, in the optical projection system using the optical system, the method of aligning the optical system, and the method of testing the barrel according to the present invention, convergent light beams may be used rather than collimated light beams as the test beams transmitted through the Fizeau interferometer  31 . The convergent light beams may be formed by positioning an optical lens in front of the collimating lens  47  of the Fizeau interferometer  31 , as described above. The structure and function of the optical system  35  shown in FIG. 2 are the same as the structure and function of the optical system  35  described with reference to FIG. 1.  
         [0060]    [0060]FIGS. 4 and 5 are schematic views illustrating a method of testing a barrel to measure the manufacturing condition of the barrel in which an optical element is installed. Referring to FIGS. 4 and 5, in the method of testing the barrel according to embodiments of the present invention, a hologram element, i.e., a CGH element  33 , is manufactured to reflect test beams  1  along an optical path which is the same as an optical path of incident test beams  1 . The CGH element  33  is placed in a predetermined position of a barrel  37 . In the embodiment shown in FIG. 4, a predetermined lens  32  is installed in a designated position of an optical element, and the manufacturing condition of the barrel  37  is measured from interference patterns formed by the reference beams  3  and the reflected test beams  1  emitted from the predetermined lens  32 . In the embodiment shown in FIG. 5, a predetermined lens  34  is installed in a designated position of another optical element, and the manufacturing condition of the barrel  37  is measured from the interference patterns formed by the reference beams  3  and the test beams  1  reflected by the predetermined lens  34  and emitted from the CGH element  33 . The CGH element  33  is manufactured in consideration of a tolerance limit of image quality.  
         [0061]    As shown in FIG. 4, the predetermined lens  32  is manufactured to the same size as a diameter and a thickness of an edge of an optical element which will be installed in the barrel  37  and is designed to have an optimum curvature radius so that beams are incident on an accurate predetermined position of the CGH element  33 . The predetermined lens  32  is manufactured in consideration of each optical element and is installed in a designated position where optical elements are placed to measure the manufacturing condition of the barrel  37 .  
         [0062]    The CGH element  33  is manufactured so as to reflect the test beams  1  transmitted from the predetermined lens  32  so that the test beams  1  travel inversely with respect to optical path where the predetermined lens  32  is positioned in front of the CGH element  33 .  
         [0063]    Where the predetermined lens  34  is positioned after the CGH element  33  as shown in FIG. 5, the CGH element  33  is manufactured so that the test beams  1  are transmitted to the CGH  33 , reflected on a mirror  36 , are perpendicularly incident and reflected on an incident surface of the predetermined lens  34 , and travel inversely with respect to the optical path, and transmitted to the interferometer  31  through the CGH element  33 .  
         [0064]    The CGH element  33  may be ring-shaped or fan-shaped depending on the optical path of the test beams  1  transmitted through the predetermined lens  32 . The barrel  37  may include an additional unit so as to install the CGH element  33 .  
         [0065]    Referring to FIG. 4, the predetermined lens  32  is specially manufactured to measure the manufacturing condition of the barrel  37 . Also, the predetermined lens  32  is placed in a designated position of each optical element that will be positioned in front of a hologram element, i.e., the CGH element  33 , to retransmit the test beams  1  reflected by the CGH element  33  to the Fizeau interferometer  31 .  
         [0066]    Referring to FIG. 5, the predetermined lens  34  is placed in a designated position of each optical element after the CGH element  33 . The test beams  1  transmitted through the CGH element  33  are perpendicularly incident, are reflected on an incident surface of the predetermined lens  34  according to the reflection law, change direction to an inverse direction, and are thus transmitted to the Fizeau interferometer  31 .  
         [0067]    As described above, the test beams  1  which are incident on the Fizeau interferometer  31  from the optical projection system  35  form interference patterns with the reference beams  3  on the image surface  49 . If these interference patterns are null interference patterns, the barrel  37  is determined to be manufactured to a design parameter. If the interference patterns are not null interference patterns, the barrel  37  is determined not to be manufactured to the design parameter. Manufacturing errors of the barrel  37  are correctable based on shapes of the interference patterns.  
         [0068]    [0068]FIG. 6 is a schematic view illustrating an optical system  35  in which a hologram element, i.e., a CGH element, is additionally installed and a method of aligning the optical system according to an embodiment of the present invention. Referring to FIG. 6, the optical system according to the embodiment of the present invention includes at least one optical element  32   a  on which test beams  1  are incident from an interferometer  31  and a barrel  37  having a hologram element. The hologram element is a CGH element  33 . To install the CGH element  33 , an additional unit may be prepared in the barrel  37 .  
         [0069]    Referring to FIG. 6, in the method of aligning the optical system, a hologram element, i.e., the CGH element  33 , is manufactured to reflect test beams  1  along a same optical path as an incident path of the test beams  1 . Optical elements  32   a  and  34   a  are installed in the barrel  37  and alignment errors of the optical elements  32   a  and  34   a  are measurable from interference patterns formed by reference beams  3  (FIG. 3) and test beams  1  transmitted through the optical element  32   a  and reflected by the CGH element  33  or transmitted through the CGH element  33  and reflected by the optical elements  34   a . Finally, the optical elements  32   a  and  34   a  are aligned to remove the alignment errors. The CGH element  33  is manufactured in consideration of a tolerance limit of image quality.  
         [0070]    The CGH element  33  used in the method of aligning the optical system according to the embodiment of the present invention is manufactured by the same method as the CGH element  33  in the method of testing the barrel. In other words, the CGH element  33  is manufactured so that the CGH element  33  is written in a position where test beams  1  transmitted through the optical elements  32   a  and  34   a  are incident. Light beams transmitted through the optical element  32   a  positioned in front of the CGH element  33  are reflected along the same optical path, and light beams transmitted through the optical element  34   a  positioned behind the CGH element  33  are reflected along the same optical path.  
         [0071]    Alignment errors of the optical system are measurable after the optical elements  32   a  and  34   a  are both arranged in the barrel  37  or after the optical elements  32   a  and  34   a  are separately installed.  
         [0072]    Similar to the method of testing the barrel, in the method of aligning the optical system where the CGH element  33  is additionally installed, alignment errors of the optical system are measurable from errors of the reference patterns with respect to null interference patterns formed by the reference beams  3  generated from the interferometer  31  and the test beams  1  returned from the optical system  35 .  
         [0073]    [0073]FIG. 7 is a schematic view illustrating an optical system using optical elements on which a CGH is written, according to the embodiment of the present invention, and a method of aligning the optical system. Referring to FIG. 7, holograms  30   a  and  30   b  are written directly on optical elements  32   b  and  34   b , respectively. The holograms may be computer-generated holograms.  
         [0074]    The CGH  30   a  and the CGH  30   b  which are designed through computer simulation are written on the optical elements  32   b  and  34   b , respectively, so that the test beams  1  are incident on the optical elements  32   b  and  34   b  and are reflected in the same optical path as the incident path. The CGH  30   a  and the CGH  30   b  may be written around the respective peripheral areas of the optical elements  32   b  and  34   b  so as not to degrade the quality of an image formed by beams transmitted through the optical elements  32   b  and  34   b.    
         [0075]    Referring to FIG. 7, in the method of aligning the optical system according to the embodiment of the present invention, a hologram is formed on the optical elements  32   b  and  34   b  so that the test beams  1  travel in the same optical path as the incident path. The optical elements  32   b  and  34   b  are arranged in the barrel  37 , and alignment errors of the optical elements  32   b  and  34   b  are measured from interference patterns which are formed on the image surface  49  by the reference beams  3  and the test beams  1  transmitted through the optical elements  32   b  and  34   b . Finally, the optical elements  32   b  and  34   b  are aligned to remove the alignment errors. The hologram formed on each optical element may be a CGH.  
         [0076]    The test beams  1  are incident on and reflected by the respective CGH formed on the optical elements  32   b  and  34   b , change optical paths to an inverse direction of the incident direction, and interfere with the reference beams  3  of the interferometer  31  shown in FIG. 3 to form an interference pattern on the image surface  49 .  
         [0077]    As described above, where the test beams  1  and the reference beams  3  form a null interference pattern in the method of aligning the optical system in which the CGH is additionally installed or in the method of testing the barrel, the optical system is determined to be correctly aligned. However, where another type of interference pattern is formed, alignment errors are determined to occur in the optical system.  
         [0078]    Manufacturing errors of the barrel or alignment errors of the optical system contribute to the defocus, tip, tilt, or decenter errors of the optical system. FIGS. 8 through 10 illustrate interference patterns formed on the image surface  49  by the errors. Errors may be calculated by equation  1  which is applicable to all of the above-described errors.  
         Δ E= λ/2×(number of interference patterns)   (1)  
         [0079]    Referring to FIG. 8, it can be seen that a number of interference patterns increases with an increase in the error. The −errors shown, g −2  and g −1 , indicate that the optical elements are close to the X-axis direction of the interferometer, while the +errors shown, g +1  and g +2 , indicate the opposite.  
         [0080]    A null interference pattern go shown in FIG. 8 is formed where optical system alignment errors or barrel test errors do not occur. The shape of the circle of the null interference pattern go represents the shape of the image surface, which means that an interference pattern is not formed.  
         [0081]    Interference patterns g +1  and g +2  are created where the optical system alignment errors or the barrel test errors are in a +error direction. Where errors occur, interference patterns increase by one or by two, and circles of the interference patterns become larger and spread out in a form of waves.  
         [0082]    If two circles occur as in the interference pattern g +1  of FIG. 8, an error AE determined according to equation 1 equals λ, where λ is the wavelength of the beam. In the same manner, if three circles occur as in the interference pattern g −2  shown in FIG. 8, the error ΔE determined according to equation 1 is 1.5 λ. The black and white colors of interference patterns corresponding to the −error are interchanged with the black and white colors of the interference patterns corresponding to the +error direction.  
         [0083]    [0083]FIGS. 9 and 10 show interference patterns created where decentering error occurs in the method of aligning the optical system or the method of testing the barrel. Referring to FIG. 9, where the center of the optical axis of the optical element moves upward on the Z-axis, as shown in FIG. 9, an interference pattern g +1  is created in the +error direction. Where the center of the optical axis of the optical element moves downward on the Z-axis, as shown in FIG. 9, an interference pattern g −2  is created in the −error direction.  
         [0084]    Referring to FIG. 10, where the center of the optical axis of the optical element moves along the Y-axis in a positive direction, as shown in FIG. 10, an interference pattern g+ 1  is created in the +error direction. Where the center of the optical axis of the optical element moves along the Y-axis in a negative direction, as shown in FIG. 10, an interference pattern g −2  is created in the −error direction.  
         [0085]    If tip or tilting error occurs in the method of aligning the optical system or the method of testing the barrel, the interference patterns shown in FIG. 9 are created. The errors can be calculated from equation  1  according to the number of interference patterns, as described above.  
         [0086]    In the method of aligning the optical system and the method of testing the barrel according to the embodiment of the present invention, the manufacturing condition of the barrel or the alignment errors of the optical system are measurable in real time and in an accurate manner using a CGH.  
         [0087]    In particular, in the optical system and the method of aligning the optical system according to the present invention, the CGH is not installed as an additional element, but the CGH is directly written on the optical element to simplify the configuration of the optical system. Also, since the CGH which is written directly on one or more elements of the optical system becomes a part of the optical system as sold, errors are prevented from initially occurring and the CGH may be used by a customer to aid in any realignment of the optical system without adding another element to the system.  
         [0088]    As described above, the method of aligning the optical system or the method of testing the barrel may be applied to an optical system easily and accurately measure alignment or manufacturing errors of the optical system in real time.  
         [0089]    Also, the aligned optical elements and the method of aligning the optical system allow measuring the alignment errors in optical systems in real time and in an accurate manner. Further, since an additional unit is unnecessary for the CGH, the configuration of the optical system is simplified and errors are remarkably reduced.  
         [0090]    Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.