Abstract:
A holographic three-dimensional (3D) printing apparatus and a method of driving the same are provided. The holographic 3D printing apparatus includes a light source configured to emit a beam, a beam splitting and expanding unit configured to split the emitted beam into a reference beam and a signal beam and expand the signal beam, an illumination unit configured to extract the expanded signal beam and collimate the extracted signal beam, a spatial light modulator (SLM) configured to modulate the collimated signal beam, an objective lens unit configured to emit the modulated signal beam to a holographic recording medium, and a reference beam forming unit configured to emit the reference beam to the holographic recording medium.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit under 35 U.S.C. §119(a) of Korean Patent Application No. 10-2011-0084821, filed on Aug. 24, 2011, in the Korean Intellectual Property Office, the entire disclosure of which is incorporated herein by reference for all purposes. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    The following description relates to holographic three-dimensional (3D) printing apparatuses and methods of driving holographic 3D printing apparatuses. 
         [0004]    2. Description of Related Art 
         [0005]    As interest in three-dimensional (3D) stereoscopic images has increased, devices configured to display such stereoscopic images have been developed. Since high resolution stereoscopic images having a natural appearance may be made using holography, holographic 3D printing apparatuses have also been actively studied. 
         [0006]    Holographic 3D printing apparatuses record 3D image information on a holographic recording medium, which is a photosensitive storage medium, as an interference pattern. An interference pattern is formed when a reference beam emitted from a light source interferes with a signal beam emitted from the light source. The interference pattern is recorded on a holographic recording medium by chemically or physically changing a holographic recording medium. When a holographic 3D printing apparatus is used to emit a reference beam to the holographic recording medium on which the interference pattern is recorded, a 3D stereoscopic image is reproduced from the holographic recording medium. Such holographic 3D printing apparatuses may be applied to holographic displays and stereoscopic image output devices used in homes or offices. 
         [0007]      FIG. 1  is a plan view illustrating a general example of a holographic three-dimensional (3D) printing apparatus. Referring to the example illustrated in  FIG. 1 , a beam emitted from a light source  10  is split into a reference beam R and a signal beam S by a beam splitter  20 . In an example, the light source  10  is a laser source for emitting a pulse laser beam. In another example, the beam splitter  20  is a polarizing beam splitter. 
         [0008]    The signal beam S obtained by the beam splitter  20  is expanded while passing through a phase mask  31  and a predetermined lens  32 . The expanded signal beam S passes through a collimating lens  33  and an illumination unit  34  and, thereafter, becomes incident on a spatial light modulator (SLM)  35  on which predetermined color information is displayed. The signal beam S modulated by the SLM  35  passes through an objective lens unit  36  and is emitted to a holographic recording medium  50 . In an example, the illumination unit  34  is a polarizing beam splitter. In another example, the objective lens unit  36  is a Fourier objective lens. 
         [0009]    The reference beam R obtained by the beam splitter  20  passes through a first reflection mirror  41 , a lens  42 , and a second reflection mirror  43 , and is thereby emitted to the holographic recording medium  50 . Accordingly, the signal beam S interferes with the reference beam R on the holographic recording medium  50  to record an interference pattern. 
       SUMMARY 
       [0010]    In one general aspect, a holographic three-dimensional (3D) printing apparatus includes a light source configured to emit a beam, a beam splitting and expanding unit configured to split the emitted beam into a reference beam and a signal beam and expand the signal beam, an illumination unit configured to extract the expanded signal beam and collimate the extracted signal beam, a spatial light modulator (SLM) configured to modulate the collimated signal beam, an objective lens unit configured to emit the modulated signal beam to a holographic recording medium, and a reference beam forming unit configured to emit the reference beam to the holographic recording medium. 
         [0011]    The holographic 3D printing apparatus may further include that the beam splitting and expanding unit, the illumination unit, and the objective lens unit include first, second, and third holographic optical elements, respectively. 
         [0012]    The holographic 3D printing apparatus may further include that the first, second, and third holographic optical elements are on a first light guide member. 
         [0013]    The holographic 3D printing apparatus may further include that the first holographic optical element is configured to expand the signal beam through the first light guide member to be incident on the second holographic optical element. 
         [0014]    The holographic 3D printing apparatus may further include that the reference beam forming unit is further configured to adjust an optical delay of the reference beam to adjust a phase difference between the emitted reference beam and the emitted signal beam. 
         [0015]    The holographic 3D printing apparatus may further include that the reference beam forming unit includes a second light guide member through which the reference beam passes. 
         [0016]    The holographic 3D printing apparatus may further include that each of the first and second light guide members includes a transparent plate. 
         [0017]    The holographic 3D printing apparatus may further include that either the second light guide member is spaced apart from the first light guide member or the second light guide member is in contact with the first light guide member. 
         [0018]    The holographic 3D printing apparatus may further include that the holographic recording medium is between the first light guide member and the second light guide member. 
         [0019]    The holographic 3D printing apparatus may further include that the first and second holographic optical elements are on a first surface of the first light guide member, and the third holographic optical element is on a second surface of the first light guide member. 
         [0020]    The holographic 3D printing apparatus may further include that the light source and the SLM are adjacent to and spaced apart from the first holographic optical element and the second holographic optical element, respectively . 
         [0021]    The holographic 3D printing apparatus may further include that the first holographic optical element is on a first surface of the first light guide member, and the second and third holographic optical elements are on a second surface of the first light guide member. 
         [0022]    The holographic 3D printing apparatus may further include that the light source is disposed adjacent to and spaced apart from the first holographic optical element, and the SLM is on the first surface of the first light guide member. 
         [0023]    The holographic 3D printing apparatus may further include that the light source includes a continuous wave (CW) laser source or a quasi-CW laser source. 
         [0024]    In another general aspect, there is provided a method of driving a holographic three-dimensional (3D) printing apparatus, the holographic 3D printing apparatus including a light source, a beam splitting and expanding unit, an illumination unit, a spatial light modulator (SLM), an objective lens unit, and a reference beam forming unit, the method including emitting a beam from the light source, splitting the emitted beam into a reference beam and a signal beam via the beam splitting and expanding unit, expanding the signal beam via the beam splitting and expanding unit, extracting the expanded signal beam via the illumination unit, collimating the extracted signal beam via the illumination unit, modulating the collimated signal beam via the SLM, emitting the modulated signal beam from the objective lens unit to a holographic recording medium, and emitting the reference beam from the reference beam forming unit to the holographic recording medium. 
         [0025]    The method may further include that the signal beam is expanded via the first holographic optical element through the first light guide member to be incident on the second holographic optical element. 
         [0026]    The method may further include that emitting of the reference beam includes adjusting an optical delay of the reference beam via the reference beam forming unit to adjust a phase difference between the emitted reference beam and the emitted signal beam. 
         [0027]    The method may further include that the emitted beam is a continuous wave (CW) laser beam or a quasi-CW laser beam. 
         [0028]    Other features and aspects may be apparent from the following detailed description, the drawings, and the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]      FIG. 1  is a plan view illustrating a general example of a holographic three-dimensional (3D) printing apparatus. 
           [0030]      FIG. 2  is a perspective view illustrating an example of a holographic 3D printing apparatus. 
           [0031]      FIG. 3  is a cross-sectional view illustrating an example of the holographic 3D printing apparatus of  FIG. 2 . 
           [0032]      FIG. 4  is a cross-sectional view illustrating another example of a holographic 3D printing apparatus. 
           [0033]      FIG. 5  is a cross-sectional view illustrating yet another example of a holographic 3D printing apparatus. 
           [0034]      FIG. 6  is a cross-sectional view illustrating still another example of a holographic 3D printing apparatus. 
       
    
    
       [0035]    Throughout the drawings and the detailed description, unless otherwise described, the same drawing reference numerals will be understood to refer to the same elements, features, and structures. The relative size and depiction of these elements may be exaggerated for clarity, illustration, and convenience. 
       DETAILED DESCRIPTION 
       [0036]    The following detailed description is provided to assist the reader in gaining a comprehensive understanding of the methods, apparatuses, and/or systems described herein. Accordingly, various changes, modifications, and equivalents of the systems, apparatuses and/or methods described herein will be suggested to those of ordinary skill in the art. Also, descriptions of well-known functions and constructions may be omitted for increased clarity and conciseness. 
         [0037]      FIG. 2  is a perspective view illustrating an example of a holographic 3D printing apparatus.  FIG. 3  is a cross-sectional view illustrating an example of the holographic 3D printing apparatus of  FIG. 2 . Referring to examples illustrated in  FIGS. 2 and 3 , the holographic 3D printing apparatus includes a light source  110 , a beam splitting and expanding unit  120 , an illumination unit  130 , an SLM  135 , an objective lens unit  140 , and a reference beam forming unit  160 . The beam splitting and expanding unit  120  and the illumination unit  130  is disposed on a top surface of a first light guide member  170 . The objective lens unit  140  is disposed on a bottom surface of the first light guide member  170 . 
         [0038]    In an example, the first light guide member  170 , in which light is guided via total reflection, includes, for example, a transparent plate. In another example, the first light guide member  170  is formed of glass. In further examples, the first light guide member  170  is formed of any of various materials known to one of ordinary skill in the art to be used for guiding light by total reflection. 
         [0039]    The light source  110  and the SLM  135  are disposed adjacent to and spaced apart from the beam splitting and expanding unit  120  and the illumination unit  130 , respectively. That is, the light source  110  is disposed over the beam splitting and expanding unit  120 , and the SLM  135  is disposed over the illumination unit  130 . In an example, the light source  110  is a continuous wave (CW) laser source or a quasi-CW laser source. The beam splitting and expanding unit  120  splits a laser beam emitted from the light source  110  into a reference beam R and a signal beam S and expands the signal beam S. 
         [0040]    In an example, the beam splitting and expanding unit  120  includes a first holographic optical element. In general, a holographic optical element is a diffractive optical element having a fine grating pattern manufactured using holography. In an example, the holographic optical element performs various optical functions according to the grating pattern. The first holographic optical element is configured to perform a beam splitting function and a beam expanding function. That is, in an example, when a laser beam emitted from the light source  110  is incident on the first holographic optical element, the laser beam is diffracted at a predetermined angle by the first holographic optical element, and is split into a signal beam S passing through the first light guide member  170  and a reference beam R transmitted through the first holographic optical element. In this example, the diffraction of the signal beam S results in the expansion of the signal beam S. Although a laser beam emitted from the light source  110  is perpendicularly incident on the first holographic optical element in  FIG. 1 , in yet another example, the laser beam is incident on the first holographic optical element obliquely at a predetermined angle. 
         [0041]    The signal beam S is continuously expanded and totally reflected through the first light guide member  170  to have a desired size. The signal beam S having the desired size reaches the illumination unit  130  disposed on the top surface of the first light guide member  170 . The illumination unit  130  extracts and collimates the signal beam S, and causes the signal beam S to be incident on the SLM  135 . 
         [0042]    In an example, the illumination unit  130  includes a second holographic optical element. A fine grating pattern configured to extract and collimate the signal beam S is formed on the second holographic optical element. Accordingly, when the signal beam S passing through the first light guide member  170  reaches the second holographic optical element, the second holographic optical element extracts and collimates the signal beam S from the first light guide member  170 . 
         [0043]    The signal beam S emitted through the illumination unit  130  is incident on the SLM  135  on which predetermined color information is displayed. The SLM  135  modulates the signal beam S incident thereon. The modulated signal beam S emitted from the SLM  135  passes through the first light guide member  170  from the SLM  135 , and becomes incident on the objective lens unit  140 . The objective lens unit  140  focuses the signal beam S emitted from the SLM  135  and emits the focused signal beam S to a desired position on the holographic recording medium  150 . 
         [0044]    In an example, the objective lens unit  140  includes a third holographic optical element. A fine grating pattern configured to perform a Fourier objective lens function is formed on the third holographic optical element. Accordingly, the signal beam S emitted from the SLM  135  through the first light guide member  170  is focused by the third holographic optical element and emitted to a desired position on the holographic recording medium  150 . The holographic recording medium  150  is disposed, for example, under the first light guide member  170 . 
         [0045]    The reference beam forming unit  160  is disposed under the first light guide member  170  to be spaced apart from the first light guide member  170 . While not being limited thereto, the holographic recording medium  150  is disposed between the first light guide member  170  and the reference beam forming unit  160 . In an example, the hologram recording medium  150  is disposed at another position. 
         [0046]    The reference beam forming unit  160  guides the reference beam R obtained by the beam splitting and expanding unit  120  and transmitted through the first light guide member  170  and emits the reference beam R to a desired position on the holographic recording medium  150 . In addition, the reference beam forming unit  160  adjusts an optical delay of the reference beam R to adjust a phase difference between the reference beam R and the signal beam S. 
         [0047]    The reference beam forming unit  160  includes a second light guide member  161 . In an example, like the first light guide member  170 , the second light guide member  161 , in which light is guided via total reflection, includes, for example, a transparent plate. In another example, the second light guide member  161  is formed of glass. In further examples, the second light guide member  161  is formed of any of various materials known to one of ordinary skill in the art to be used for guiding light by total reflection. 
         [0048]    In an example, a first side surface  161   a  of the second light guide member  161  is inclined. Accordingly, in this example, the reference beam R transmitted through the first light guide member  170  and incident on the second light guide member  161  is reflected by the first side surface  161   a  of the second light guide member  161  to pass through the second light guide member  161 . 
         [0049]    In another example, as the reference beam R passes through the second light guide member  161 , an optical delay of the reference beam R is adjusted. Thus, in this example, a phase difference between the reference beam R and the signal beam S is adjusted. 
         [0050]    In an example, a material layer  162  is disposed on a second side surface  161   b  of the second light guide member  161 . The material layer  162  refracts the reference beam R passing through the second light guide member  161  at a predetermined angle, to cause the reference beam R to be emitted to a desired position on the holographic recording medium  150 . In another example, while not being limited thereto, the second side surface  161   b  of the second light guide member  161  is inclined at a predetermined angle. Accordingly, the signal beam S interferes with the reference beam R on the holographic recording medium  150  to record an interference pattern. 
         [0051]    According to the examples illustrated in  FIGS. 2 and 3 , each of the beam splitting and expanding unit  120 , the illumination unit  130 , and the reference beam forming unit  160  of the holographic 3D printing apparatus performs various functions to realize a holographic 3D printing apparatus having a compact structure. Further, in examples, each of the beam splitting and expanding unit  120 , the illumination unit  130 , and the objective lens unit  140  uses a holographic optical element. 
         [0052]      FIG. 4  is a cross-sectional view illustrating another example of a holographic 3D printing apparatus. Referring to the example illustrated in  FIG. 4 , a beam splitting and expanding unit  220  and an illumination unit  230  are spaced apart from each other on a top surface of a first light guide member  270 . An objective lens unit  240  is disposed on a bottom surface of the first light guide member  270 . In an example, the first light guide member  270  includes a transparent plate. A light source  210  is disposed over the beam splitting and expanding unit  220 . An SLM  235  is disposed over the illumination unit  230 . 
         [0053]    In an example, the light source  210  is a laser source configured to emit a CW laser beam or a quasi-CW laser beam. In another example, the beam splitting and expanding unit  220  includes a first holographic optical element configured to split a laser beam emitted from the light source  210  into a reference beam R and a signal beam S and expand the signal beam S. The first holographic optical element is an optical element configured to perform both a beam splitting function and a beam expanding function. That is, in an example, when a laser beam emitted from the light source  210  is incident on the first holographic optical element, the laser beam is split into the signal beam S and the reference beam R and the signal beam S is expanded. In another example, the laser beam emitted from the light source  210  is incident on the first holographic optical element obliquely at a predetermined angle. 
         [0054]    The signal beam S is continuously expanded and totally reflected through the first light guide member  270  to have a desired size. The signal beam S having the desired size reaches the illumination unit  230  disposed on the top surface of the first light guide member  270 . The illumination unit  230  extracts and collimates the signal beam S, and then causes the signal beam S to be incident on the SLM  235 . 
         [0055]    In an example, the illumination unit  230  includes a second holographic optical element. A fine grating pattern configured to extract and collimate the signal beam S is formed on the second holographic optical element. Accordingly, when the signal beam S passing through the first light guide member  270  reaches the second holographic optical element, the second holographic optical element extracts and collimates the signal beam S from the first light guide member  270 . 
         [0056]    The signal beam S emitted through the illumination unit  230  is incident on the SLM  235  on which predetermined color information is displayed. The SLM  235  modulates the signal beam S incident thereon. The modulated signal beam S emitted from the SLM  235  passes through the first light guide member  270 , and becomes incident on the objective lens unit  240 . 
         [0057]    In an example, the objective lens unit  240  includes a third holographic optical element configured to focus the modulated signal beam S emitted from the SLM  235  and emit the focused signal beam S to a desired position on a holographic recording medium  250 . A fine grating pattern configured to perform a Fourier objective lens function is formed on the third holographic optical element. Accordingly, the modulated signal beam S emitted by the SLM  235  to pass through the first light guide member  270  is focused by the third holographic optical element and emitted to a desired position on the holographic recording medium  250 . While not being limited thereto, the holographic recording medium  250  is disposed under the first light guide member  270 . 
         [0058]    A reference beam forming unit  260  is disposed under the first light guide member  270 . While not being limited thereto, the holographic recording medium  250  is disposed between the first light guide member  270  and the reference beam forming unit  260 . The reference beam forming unit  260  guides the reference beam R obtained by the beam splitting and expanding unit  220  and transmitted through the first light guide member  270  and emits the reference beam R to a desired position on the holographic recording medium  250 . In addition, the reference beam forming unit  260  adjusts an optical delay of the reference beam R to adjust a phase difference between the reference beam R and the signal beam S. The reference beam forming unit  260  includes a second light guide member  261 . In an example, the second light guide member  261  is formed of a transparent material, like the first light guide member  270 . 
         [0059]    A top surface  261   a  of a first side of the second light guide member  261  contacts the bottom surface of the first light guide member  270 . In this example, the reference beam R obtained by the beam splitting and expanding unit  220  and transmitted through the first light guide member  270  is incident on the second light guide member  261 , is reflected by a first side surface  261   b  of the second light guide member  261 , and passes through the second light guide member  261 . In an example, due to the second light guide member  261 , an optical delay of the reference beam R and a phase difference between the reference beam R and the signal beam S are adjusted. 
         [0060]    A material layer  262  is disposed on the second light guide member  261 . In an example, the material layer  262  refracts the reference beam R passing through the second light guide member  261  at a predetermined angle and causes the reference beam R to be emitted to a desired position on the holographic recording medium  250 . Accordingly, the signal beam S and the reference beam R interfere with each other on the holographic recording medium  250  to record an interference pattern. 
         [0061]      FIG. 5  is a cross-sectional view illustrating yet another example of a holographic 3D printing apparatus. Referring to the example illustrated in  FIG. 5 , a beam splitting and expanding unit  320  is disposed on a top surface of a first light guide member  370 , and an illumination unit  330  and an objective lens unit  340  is disposed on a bottom surface of the first light guide member  370  to be spaced apart from each other. In an example, the first light guide member  370  includes a transparent plate. A light source  310  is disposed over the beam splitting and expanding unit  320 . In another example, the light source  310  is a laser source configured to emit a CW laser beam or a quasi-CW laser beam. An SLM  335  is disposed on the top surface of the first light guide member  370  between the illumination unit  330  and the objective lens unit  340 . 
         [0062]    In an example, the beam splitting and expanding unit  320  includes a first holographic optical element configured to split a laser beam emitted from the light source  310  into a reference beam R and a signal beam S and expand the signal beam S. The first holographic optical element is an optical element configured to perform both a beam splitting function and a beam expanding function. That is, when a laser beam emitted from the light source  310  is incident on the first holographic optical element, the laser beam is split into the signal beam S and the reference beam R and the signal beam S is expanded by being diffracted. In another example, the laser beam emitted from the light source  310  is incident on the first holographic optical element perpendicularly or obliquely at a predetermined angle. 
         [0063]    The signal beam S is continuously expanded while passing through the first light guide member  370  to have a desired size, and the signal beam S having the desired size reaches the illumination unit  330  disposed on the bottom surface of the first light guide member  370 . The illumination unit  330  extracts and collimates the signal beam S passing through the first light guide member  370  and causes the signal beam S to be incident on the SLM  335 . 
         [0064]    In an example, the illumination unit  330  includes a second holographic optical element. When the signal beam S passing through the first light guide member  370  reaches the second holographic optical element, the second holographic optical element extracts and collimates the signal beam S from the first light guide member  370 . 
         [0065]    The signal beam S emitted through the illumination unit  330  passes through the first light guide member  370  and is incident on the SLM  335  disposed on the top surface of the first light guide member  370 . The signal beam S is modulated by the SLM  335 , passes through the first light guide member  370 , and is incident on the objective lens unit  340  disposed on the bottom surface of the first light guide member  370 . 
         [0066]    In an example, the objective lens unit  340  includes a third holographic optical element configured to focus the signal beam S emitted from the SLM  335  and emit the signal beam S to a desired position on the holographic recording medium  350 . The signal beam S modulated by the SLM  335  and passing through the first light guide member  370  is focused by the third holographic optical element and emitted to a desired position on the holographic recording medium  350 . While not being limited thereto, the holographic recording medium  350  is disposed under the first light guide member  370 . 
         [0067]    While not being limited thereto, a reference beam forming unit  360  is disposed under the first light guide member  370  to be spaced apart from the first light guide member  370 . In addition, while not being limited thereto, the holographic recording medium  350  is disposed between the first light guide member  370  and the reference beam forming unit  360 . 
         [0068]    The reference beam forming unit  360  guides the reference beam R obtained by the beam splitting and expanding unit  320  and passing through the first light guide member  370  and emits the reference beam R to a desired position on the holographic recording medium  350 . In addition, the reference beam forming unit  360  adjusts an optical delay of the reference beam R to adjust a phase difference between the reference beam R and the signal beam S. While not being limited thereto, the reference beam forming unit  360  includes a second light guide member  361 . In an example, the second light guide member  361  includes a transparent plate, like the first light guide member  370 . 
         [0069]    In an example, a first side surface  361   a  of the second light guide member  361  is inclined. Accordingly, in another example, the reference beam R passing through the first light guide member  370  and incident on the second light guide member  361  is reflected by the first side surface  361  a of the second light guide member  361  and passes through the second light guide member  361 . Due to the second light guide member  361 , in yet another example, an optical delay of the reference beam R is adjusted to adjust a phase difference between the reference beam R and the signal beam S. 
         [0070]    A material layer  362  is disposed on a second side surface  361   b  of the second light guide member  361 . In an example, the material layer  362  refracts the reference beam R passing through the second light guide member  361  at a predetermined angle and causes the reference beam R to be emitted to a desired position on the holographic recording medium  350 . While not being limited thereto, in another example, the second side surface  361   b  of the second light guide member  361  is inclined. Accordingly, the signal beam S and the reference beam R interfere with each other on the holographic recording medium  350  to record an interference pattern. 
         [0071]      FIG. 6  is a cross-sectional view illustrating still another example of a holographic 3D printing apparatus. Referring to the example illustrated in  FIG. 6 , a beam splitting and expanding unit  420  is disposed on a top surface of a first light guide member  470 , and an illumination unit  430  and an objective lens unit  440  is disposed on a bottom surface of the first light guide member  470  to be spaced apart from each other. A light source  410  is disposed over the beam splitting and expanding unit  420 . In an example, the light source  410  is a laser source configured to emit a CW laser beam or a quasi-CW laser beam. An SLM  435  is disposed on the top surface of the first light guide member  470  between the illumination unit  430  and the objective lens unit  440 . 
         [0072]    In an example, the beam splitting and expanding unit  420  includes a first holographic optical element configured to split a laser beam emitted from the light source  410  into a reference beam R and a signal beam S and expand the signal beam S. The first holographic optical element is an optical element configured to perform both a beam splitting function and a beam expanding function. That is, when a laser beam emitted from the light source  410  is incident on the first holographic optical element, the laser beam is split into the signal beam S and the reference beam R by the first holographic optical element, and the signal beam S is expanded by being diffracted. In another example, the laser beam emitted from the light source  410  is incident on the first holographic optical element obliquely at a predetermined angle. 
         [0073]    The signal beam S is continuously expanded while passing through the first light guide member  470  to have a desired size, and the signal beam S having the desired size reaches the illumination unit  430  disposed on the bottom surface of the first light guide member  470 . In an example, the illumination unit  430  includes a second holographic optical element configured to extract and collimate the signal beam S passing through the first light guide member  470 , thereby causing the signal beam S to be incident on the SLM  435 . When the signal beam S passing through the first light guide member  470  reaches the second holographic optical element, the second holographic optical element extracts and collimates the signal beam S from the first light guide member  470 . 
         [0074]    The signal beam S emitted through the illumination unit  430  passes through the first light guide member  470 , and is incident on the SLM  435  disposed on the top surface of the first light guide member  470 . The signal beam S is modulated by the SLM  435 , passes through the first light guide member  470 , and is incident on the objective lens unit  440  disposed on the bottom surface of the first light guide member  470 . In an example, the objective lens unit  440  includes a third holographic optical element configured to focus the signal beam S emitted from the SLM  435  and emit the signal beam S to a desired position on a holographic recording medium  450 . The signal beam S modulated by the SLM  435  and passing through the first light guide member  470  is focused by the third holographic optical element and is emitted to a desired position on the holographic recording medium  450 . While not being limited thereto, the holographic recording medium  450  is disposed under the first light guide member  470 . 
         [0075]    While not being limited thereto, a reference beam forming unit  460  is disposed under the first light guide member  470 , and the holographic recording medium  450  is disposed between the first light guide member  470  and the reference beam forming unit  460 . The reference beam forming unit  460  guides the reference beam R obtained by the beam splitting and expanding unit  420  and passing through the first light guide member  470  and emits the reference beam R to a desired position on the holographic recording medium  450 . In addition, the reference beam forming unit  460  adjusts an optical delay of the reference beam R to adjust a phase difference between the reference beam R and the signal beam S. While not being limited thereto, the reference beam forming unit  460  includes a second light guide member  461  that, in an example, is formed of a transparent material, like the first light guide member  470 . 
         [0076]    While not being limited thereto, a top surface  461   a  of a first side of the second light guide member  461  contacts the bottom surface of the first light guide member  470 . In this case, in an example, the reference beam R obtained by the beam splitting and expanding unit  420  and passing through the first light guide member  470  is incident on the second light guide member  461 , is reflected by a first side surface  461  b of the second light guide member  461 , and passes through the second light guide member  461 . Due to the second light guide member  461 , in another example, an optical delay of the reference beam R is adjusted to adjust a phase difference between the reference beam R and the signal beam S. While not being limited thereto, a material layer  462  is disposed on the second light guide member  461 . In an example, the material layer  462  refracts the reference beam R passing through the second light guide member  461  at a predetermined angle, and causes the reference beam R to be emitted to a desired position on the holographic recording medium  450 . Accordingly, the signal beam S and the reference beam R interfere with each other on the holographic recording medium  450  to record an interference pattern. 
         [0077]    A number of examples have been described above. Nevertheless, it will be understood that various modifications may be made. For example, suitable results may be achieved if the described techniques are performed in a different order and/or if components in a described system, architecture, device, or circuit are combined in a different manner and/or replaced or supplemented by other components or their equivalents. Accordingly, other implementations are within the scope of the following claims.