Abstract:
An optical head for a hologram optical apparatus and a method of operating the same are provided. The optical head for the hologram optical apparatus includes a reference light unit for guiding reference light, a signal light unit for guiding signal light, and a light source unit for providing 1 the reference light and the signal light to the reference light unit and the signal light unit, wherein the reference light unit and the signal light unit are stacked. The signal light unit includes: a plurality of optical waveguides stacked sequentially; composite hologram optical elements and lighting hologram optical elements disposed on the plurality of optical waveguides; an optical modulator for modulating light output from the plurality of the optical waveguides; and a lens for condensing light output from the optical modulator onto a recording layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Korean Patent Application No. 10-2012-0129091, filed on Nov. 14, 2012, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     1. Field 
     Apparatuses and methods consistent with exemplary embodiments relate to an optical apparatus, and more particularly, to an optical head for a hologram optical apparatus and a method of operating the same. 
     2. Description of the Related Art 
     A reference light and a signal light are used for hologram recording. Hologram technology is used for various applications such as printing, image displays, information recording, and advertising. 
     Since a reference light and a signal light are used for hologram recording, a head of an optical system for hologram recording includes optical elements for generating, processing and coupling of the reference light and the signal light. As in other fields, miniaturization and integration are also becoming main issues in the optical apparatus field. Since an optical head is the core of the hologram recording apparatus, miniaturization of the optical head may precede the miniaturization of the hologram recording apparatus as a whole. For color hologram recording, red light R, green light G and blue light B are used. The optical head includes optical elements for receiving and processing the R, G and B light simultaneously, and separating, enlarging and beam-shaping the processed light. Therefore the optical elements are necessary to be integrated in order to miniaturize the optical apparatus for color holograms. In this process, however, factors such as optical interference or optical crosstalk may arise, which may result in deterioration of hologram quality. 
     SUMMARY 
     One or more exemplary embodiments may provide an optical head for a hologram apparatus, which is capable of achieving miniaturization or integration and reducing a crosstalk that occurs in optical mixing. 
     One or more exemplary embodiments may provide methods of operating the optical head. 
     Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the presented embodiments. 
     According to an aspect of an exemplary embodiment, an optical head for a hologram optical apparatus includes a reference light unit for guiding reference light for hologram recording; a signal light unit for guiding signal light for hologram recording; and a light source unit for providing the reference light and the signal light, wherein the reference light unit and the signal light unit are sequentially stacked. 
     The signal light unit may include a plurality of optical waveguides stacked sequentially; a composite hologram optical element and lighting hologram optical element disposed on each of the plurality of optical waveguides; an optical modulator for modulating light emitted from the plurality of optical waveguides; and a lens for condensing light output from the optical modulator on a recording layer, wherein the composite hologram optical element and lighting hologram optical element each form a single layer hologram. 
     The signal light unit may include a single optical waveguide into which the light from the light source unit is incident; a composite hologram optical element and a lighting hologram optical element disposed on the single optical waveguide; a single optical modulator for modulating light output from the single optical waveguides; and a lens for condensing light emitted from the single optical modulator on a recording layer, wherein the composite hologram optical element and the lighting hologram optical element each form a three layer hologram having layers corresponding to red, greed, and blue light. 
     The plurality of optical waveguides may include three optical waveguides corresponding respectively to red, green, and blue light and being sequentially stacked. 
     The composite hologram optical element and the lighting hologram optical element may both be disposed on a top surface or a bottom surface of each of the plurality of optical waveguides. 
     The lens may be a Fourier lens. 
     The composite hologram optical elements disposed on each of the optical waveguides may not overlap each other. 
     Thicknesses of the composite hologram optical elements provided to each of the optical waveguides may be the same as or different from each other. 
     Thicknesses of the lighting hologram optical elements provided to each of the optical waveguides may be the same as or different from each other. 
     Thicknesses of the composite hologram optical element and the lighting hologram optical element for each of the optical waveguides may be the same as or different from each other. 
     Refractive index modulations of the composite hologram optical elements for each of the optical waveguides may be the same as or different from each other. 
     The lens may be a holographic Fourier lens. 
     The reference light unit may include: an optical waveguide; an upper hologram optical element disposed on a top surface of the optical waveguide; and a lower composite hologram optical element disposed on a bottom surface of the optical waveguide. 
     The upper and lower composite hologram optical elements respectively may each include a three layer hologram. 
     The light source unit may include: first, second, and third light sources; a reflection unit reflecting light emitted from the first, second, and third light sources; and a mirror reflecting light incident from the reflection unit to the signal light generating unit. 
     The first, second, and o third light sources may each be lasers emitting different colors, respectively. 
     The reflection unit may include a mirror and a beam splitter. 
     The lasers of the first, second, and third light sources may each be a continuous wave (CW) laser or a quasi CW laser. 
     The first, second, and third light sources may directly scan light onto the composite hologram optical elements. 
     According to an aspect of another exemplary embodiment, a method is provided for operating the optical head for a hologram optical apparatus according to the above description. The method includes adjusting at least one of a thickness and a refractive index modulation of the composite hologram optical elements, wherein the signal light generating unit includes at least: an optical waveguide; and a composite hologram optical element and a lighting hologram optical element each disposed on the optical waveguide. 
     The optical waveguide may include a plurality of optical waveguides which are sequentially stacked; and the composite hologram optical element and the lighting hologram optical element disposed on each of the optical waveguides may each respectively comprise a single layer hologram. 
     The optical waveguide may be a single optical waveguide; and the composite hologram optical element and the lighting hologram optical element may each include a three layer hologram. 
     The composite hologram optical elements may be disposed so as not to overlap each other. 
     The method further includes scanning light to each of the composite hologram optical elements at a certain time intervals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other exemplary aspects and advantages will become apparent and more readily appreciated from the following description of exemplary embodiments, taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a cross-sectional view illustrating an optical head for a color hologram optical apparatus according to an exemplary embodiment; 
         FIG. 2  is a cross-sectional view illustrating a modified example of a signal light generating unit in the optical head of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view illustrating an optical head for a color hologram optical apparatus according to another exemplary embodiment; 
         FIG. 4  is a cross-sectional view illustrating a case where red light is incident to a first composite hologram optical element and light is not incident to second and third composite hologram optical elements in  FIG. 3 ; 
         FIG. 5  is a cross-sectional view illustrating an optical head for a color hologram optical apparatus according to another exemplary embodiment; and 
         FIGS. 6A and 6B  are comparison graphs illustrating a case where crosstalk is minimized in a mixed light by adjustment of thicknesses and refractive index modulations of hologram optical elements in an optical head for a hologram optical apparatus according to exemplary embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. In this regard, the present embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the embodiments are merely described below, by referring to the figures, to explain aspects of the present description. Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. In the drawings, the thicknesses of layers and regions are exaggerated for clarity. It will also be understood that when a layer is referred to as being “on” another layer or substrate, it can be directly on the other layer or substrate, or intervening layers may also be present Like reference numerals in the drawings denote like elements, and thus their description will be omitted. 
       FIG. 1  is a cross-sectional view illustrating an optical head (hereinafter a first optical head) for a color hologram optical apparatus according to an exemplary embodiment. 
     Referring to  FIG. 1 , the first optical head includes first to fourth optical waveguides  20 ,  30 ,  40  and  50 , which are stacked in order. The first to fourth optical waveguides  20 ,  30 ,  40  and  50  may be formed to be spaced from each other or in contact with one another. The first optical waveguide  20  is used for generating a reference beam. The second to fourth optical waveguides  30 ,  40  and  50  are used for generating a signal light. The first optical waveguide  20  may have a length shorter than the second to fourth optical waveguides  30 ,  40  and  50 . The second optical waveguide  30  is for a blue light B. The third optical waveguide  40  is for a green light G. The fourth optical waveguide  50  is for a red light R. An optical modulator  72  is disposed adjacent to the first optical waveguide  20  under the second optical waveguide  30 . The optical modulator  72  may be a spatial light modulator (SLM). A condensing lens  74  is disposed under the optical modulator  72 . The condensing lens  74  may be a Fourier lens. The condensing lens  74  condenses light onto a predetermined region on a hologram recording layer  10 . The recording layer  10  may be positioned at a focal length of the condensing lens  74 . The optical modulator  72  and the condensing lens  74  are aligned on a same optical axis. 
     A first composite hologram optical element CH 1  and a first lighting hologram optical element LH 1  are formed on a top surface of the fourth optical waveguide  50 . The first composite hologram optical element CH 1  and the first lighting hologram optical element LH 1  are separated from each other. The first lighting hologram optical element LH 1  may have a greater surface area than the first composite hologram optical element CH 1 . The first composite hologram optical element CH 1  and the first lighting hologram optical element LH 1  may be formed in the same layer. The first composite hologram optical element CH 1  diffracts red light (indicated as an alternated long and short dash line) of the incident light (mixed light of R+G+B) and guides the red light into the fourth optical waveguide  50  and transmits the remainder of the incident light. The red light diffracted by the first composite hologram optical element CH 1  travels along the fourth optical waveguide  50  and is incident on the first lighting hologram optical element LH 1 . The first lighting hologram optical element LH 1  diffracts the incident red light into collimated light and directs the collimated light towards the third optical waveguide  40 . A second composite hologram optical element CH 2  and a second light hologram optical element LH 2  are separately disposed on a top surface of the third optical waveguide  40 . The second composite hologram optical element CH 2  may be disposed immediately under the first composite hologram optical element CH 1 . The second lighting hologram optical element LH 2  may be disposed immediately under the first lighting hologram optical element LH 1 . The second composite hologram optical element CH 2  diffracts green light (indicated as an alternated long and two short dashes line) of the incident light transmitted through the first composite hologram optical element CH 1  onto a predetermined position in the third optical waveguide  40  and transmits the remainder of the incident light. The diffracted green light travels along the third optical waveguide  40  and is incident on the second lighting hologram optical element LH 2 . The second lighting hologram optical element LH 2  diffracts the incident green light into collimated light and directs the collimated light toward the second optical waveguide  30 . The second composite hologram optical element CH 2  and the second lighting hologram optical element LH 2  may be formed in a single layer. 
     A third composite hologram optical element CH 3  and a third lighting hologram optical element LH 3  are separately disposed on a top surface of the second optical waveguide  30 . The third composite hologram optical element CH 3  may be formed immediately under the second composite hologram optical element CH 2 . The third lighting hologram optical element LH 3  may be formed immediately under the second lighting hologram optical element LH 2 . The second and third lighting hologram optical elements LH 2  and LH 3  respectively may have a same area as that of the first lighting hologram optical element LH 1 . The third composite hologram optical element CH 3  diffracts blue light (indicated as a dotted line) of the incident light transmitted through the second composite hologram optical element CH 2  in a predetermined direction in the second optical waveguide  30  and transmits the remainder of the incident light. The diffracted blue light travels along the second optical waveguide  30  and is incident on the third lighting hologram optical element LH 3 . The third lighting hologram optical element LH 3  diffracts the incident blue light into collimated light and directs the collimated light toward the optical modulator  72 . 
     The collimated light (indicated as an alternated long and short dash line) output from the first lighting hologram optical element LH 1  is transmitted through the second light hologram optical element LH 2 , the third optical waveguide  40 , the third lighting hologram optical element LH 3  and the second optical waveguide  30 , and is incident on the optical modulator  72 . The collimated light (indicated as an alternate long and two short dashes line) output from the second lighting hologram optical element LH 2  is transmitted through the third optical waveguide  40 , the third lighting hologram optical element LH 3  and the second optical waveguide  30 , and is incident on the optical modulator  72 . Accordingly the red collimated light output from the first lighting hologram optical element LH 1 , the green collimated light output from the second lighting hologram optical element LH 2 , and the blue collimated light output from the third lighting hologram optical element LH 3  are incident on the optical modulator  72  together. Namely, the optical modulator  72  receives white light. The modulated light transmitted through the optical modulator  72  is used as a signal light and is focused on a predetermined position on the recording layer  10  by the condensing lens  74 . 
     A fourth composite hologram optical element CH 4  is formed on a top surface of the first optical waveguide  20 . The fourth composite hologram optical element CH 4  may be formed under the third composite hologram optical element CH 3 . The first to fourth composite hologram optical elements CH 1  to CH 4  may be aligned on a same vertical line, one above the other. The fourth composite hologram optical element CH 4  diffracts white light, which passes through the first to third composite hologram optical elements CH 1  to CH 3  and the second to fourth optical waveguides  30 ,  40  and  50  and is incident on the fourth composite hologram optical element CH 4 , in a predetermined direction in the first optical waveguide  20 . The fourth composite hologram optical element CH 4  may include three laminated layers. At this time, the three layers may be hologram layers for diffracting the incident red light, green light and blue light in a predetermined direction in the first optical waveguide  20 . Light (indicated by a solid line) diffracted from the fourth composite hologram optical element CH 4  into the first optical waveguide  20  travels along the first optical waveguide  20  and is output through a bottom surface of the first optical waveguide  20  at the end of the first optical waveguide  20  in the light traveling direction. A fifth composite hologram optical element CH 5  is formed on the bottom surface of the first optical waveguide  20  through which the light is output. The fifth composite hologram optical element CH 5  diffracts the white light output through the bottom surface of the first optical waveguide  20  and focuses the light onto a predetermined position on the hologram recoding layer  10 . The light output from the fifth composite hologram optical element CH 5  is used as a reference light. On the hologram recording layer  10 , the predetermined position on which the reference light is focused may be identical to a position on which the signal light, condensed by the condensing lens  74 , is focused. 
     First to third light sources  60 ,  62  and  64  are disposed above an upper side of the fourth optical waveguide  50 , and spaced from the fourth optical waveguide  50 . The first to third light sources  60 ,  62  and  64  may be respectively a light source emitting red light, a light source emitting green light and a light source emitting blue light. Each of the first to third light sources  60 ,  62  and  64  may be, for example, a continuous wave (CW) laser or a quasi CW laser. 
     A first mirror M 1  is disposed immediately above the first composite optical element CH 1 . A second mirror M 2  is disposed under the third light source  64 . The first and second mirrors M 1  and M 2  face each other and are aligned along the same optical axis. First and second beam splitters BS 1  and BS 2  are disposed on the same optical axis between the first and second mirrors M 1  and M 2 . The first and second beam splitters BS 1  and BS 2  are disposed respectively under the first and second light sources  60  and  62 . The first mirror M 1  reflects a white light emitted and mixed from the first to third light sources  60 ,  62  and  64  and directs the white light onto the first composite hologram optical element CH 1 . Light emitted from the first light source  60  is reflected by the first beam splitter BS 1  and is incident on the first mirror M 1 . Light emitted from the second light source  62  is reflected by the second beam splitter BS 2 , is transmitted through the first beam splitter BS 1  and is incident on the first mirror M 1 . Light emitted from the third light source  64  is reflected by the second mirror M 2 , is transmitted through the second and first beam splitters BS 2  and BS 1 , and is incident on the first mirror M 1 . The first and second mirrors M 1  and M 2 , and the first and second beam splitters BS 1  and BS 2 , together may be a reflecting unit. 
     The second to fourth optical waveguides  30 ,  40  and  50 , the first to third composite hologram optical elements CH 1 , CH 2  and CH 3 , and the first to third lighting hologram optical elements LH 1 , LH 2  and LH 3  in  FIG. 1  may, together, be a signal light generating unit. The first optical waveguide  20 , and the fourth and fifth composite hologram optical element CH 4  and CH 5  may, together, be a reference light generating unit. 
       FIG. 2  illustrates a modified example of the signal light generating unit of  FIG. 1 . 
     Referring to  FIG. 2 , the first composite hologram optical element CH 1  and the first lighting hologram optical element LH 1  are attached to a bottom surface of the fourth optical waveguide  50 , and operations thereof are the same as in  FIG. 1 . The first composite hologram optical element CH 1  and the first lighting hologram optical element LH 1  are spaced from the third optical waveguide  40 . The second composite hologram optical element CH 2  and the second lighting hologram optical element LH 2  are attached to a bottom surface of the third optical waveguide  40 , and operations thereof are the same as in  FIG. 1 . The second composite hologram optical element CH 2  and the second lighting hologram optical element LH 2  are spaced from the second optical waveguide  30 . The third composite hologram optical element CH 3  and the third lighting hologram optical element LH 3  are attached to a bottom surface of the second optical waveguide  30 , and operations thereof are the same as in  FIG. 1 . The light output from the third composite hologram optical element CH 3  is incident on the fourth composite hologram optical element CH 4  and the path the light travels thereafter is the same as in  FIG. 1 . White light output from the third lighting hologram optical element LH 3  is incident on the optical modulator  72  and the path the light travels thereafter is the same as in  FIG. 1 . Diffraction superposition of the mixed light may be adjusted according to a multiplication (d*Δn) of thicknesses d 1 , d 2  and d 3  of the first to third lighting hologram optical elements LH 1 , LH 2  and LH 3 , and refractive index modulation for the light incident thereto, when the light is transmitted through the first to third lighting hologram optical elements LH 1 , LH 2  and LH 3  and is mixed together. Accordingly the diffraction superposition portions of the mixed light may be minimized by adjusting the thicknesses d 1 , d 2  and d 3 , and the refractive index modulation Δn. Therefore crosstalk of the light may be minimized while the light is transmitted through the first to third lighting hologram optical elements LH 1 , LH 2  and LH 3  and is mixed together. Consequently, efficiency may be maximized. For similar reasons, crosstalk may be minimized by properly adjusting thicknesses of the first to third composite hologram optical elements CH 1 , CH 2  and CH 3  and the refractive index modulation thereof. 
       FIG. 3  illustrates a main part of an optical head for a hologram optical apparatus according to another exemplary embodiment. Other parts may be the same as the optical head in  FIG. 1 . 
     Referring to  FIG. 3 , the first to third composite hologram optical elements CH 1 , CH 2  and CH 3  are not vertically aligned as shown in  FIGS. 1 and 2 . Namely, the first to third composite hologram optical elements CH 1 , CH 2  and CH 3  do not overlap. There are various other alignments other than the one shown in  FIG. 3  in which the first to third composite hologram optical elements CH 1 , CH 2  and CH 3  do not overlap. For example, positions of the second and third composite hologram optical elements CH 2  and CH 3  may be exchanged. An alignment of the first to third lighting hologram optical elements LH 1 , LH 2  and LH 3  may be the same as shown in  FIG. 1  or  FIG. 2 . The first to third composite hologram optical elements CH 1 , CH 2  and CH 3  are aligned so as not to overlap each other. Therefore the red light R, the green light G and the blue light B may be incident on the fourth composite hologram optical element CH 4  without any interference with each other. Other features of this embodiment may be the same as the description in relation to  FIG. 1 . In  FIG. 3 , the red light R, the green light G and the blue light B may be directly incident on the first to third lighting hologram optical elements LH 1 , LH 2  and LH 3  from the first to third light sources  60 ,  62  and  64  without being transmitted through a mirror, a beam splitter or the like. The red light R, the green light G and the blue light B may be incident on the first to third lighting hologram optical elements LH 1 , LH 2  and LH 3  simultaneously or sequentially at time intervals. 
       FIG. 4  illustrates a case in which the red light R is incident on the first composite hologram optical element CH 1  and light is not incident on the second and third composite hologram optical elements CH 2  and CH 3 . Light incident on the second or the third composite hologram optical elements CH 2  or CH 3  may be performed after the incidence of the red light R. 
       FIG. 5  illustrates an optical head for a color hologram optical apparatus according to another exemplary embodiment. Here, a description of portions that are different from the optical head as shown in  FIG. 1  will be given. 
     Referring to  FIG. 5 , a fifth optical waveguide  80  is disposed between a light source unit P 1  and the first optical waveguide  20 . The fifth optical waveguide  80  replaces the three optical waveguides  30 ,  40  and  50  in  FIG. 1 , a sixth composite hologram optical element CH 6  and a fourth lighting hologram optical element LH 4  are formed on a top surface thereof. The sixth composite hologram optical element CH 6  and the fourth lighting hologram optical element LH 4  may be formed on a bottom surface of the fifth optical waveguide  80  instead of the top surface. The sixth composite optical waveguide CH 6  diffracts a portion of white light incident from the light source unit P 1  in a predetermined direction of the fifth optical wave guide  80  and transmits the rest of the incident light and transfers it to the fourth composite hologram optical element CH 4 . Light diffracted into the fifth optical waveguide  80  travels along the fifth optical waveguide  80 , is incident on the fourth lighting hologram optical element LH 4 , and is diffracted to be output as collimated light. The light output from the fourth lighting hologram optical element LH 4  is incident on the optical modulator  72 . The light incident on the optical modulator  72  is modulated and incident to a holographic Fourier lens  90 . The holographic Fourier lens  90  diffracts the incident light to form an image on the recording layer  10 . The holographic Fourier lens  90  replaces the condensing lens  74  in  FIG. 1  and is disposed under the optical modulator  72 . The holographic Fourier lens  90  may be disposed to contact with or be spaced from the optical modulator  72  on a same optical axis. In  FIG. 5 , the sixth composite hologram optical element CH 6  and the fourth lighting hologram optical element LH 4  respectively may include a three layer hologram. The three layer hologram may respectively correspond to R, G and B. 
       FIGS. 6A and 6B  are comparison graphs illustrating a case in which crosstalk is minimized in the mixed light by adjustment of the thicknesses and refractive index modulations of hologram optical elements in an optical head for a hologram optical apparatus according to exemplary embodiments. 
       FIG. 6A  illustrates diffraction superposition which may occur when the light is mixed. A first graph G 1  shows a diffraction pattern for the green light G. A second graph R 1  shows a diffraction pattern for the red light R. Referring to the first and second graphs G 1  and R 1 , it may be seen that a first diffraction pattern GP 1  of the first graph G 1  is superposed on a 0th diffraction pattern of the second graph R 1 , and a first diffraction pattern RP 1  of the second graph R 1  is superposed on the 0th diffraction pattern of the first graph G 1 . 
     From  FIG. 6B , it can be seen that superposition of diffraction patterns disappears by adjustment of thicknesses d and refractive index modulations Δn of the lighting hologram optical element and the composite hologram optical elements. 
     Referring to  FIG. 6B , it can be seen that the 0 th  diffraction pattern of the first graph G 1  is not superposed on the diffraction pattern of the second graph R 1 , and the 0 th  diffraction pattern of the second graph R 1  is not superposed on the diffraction pattern of the first graph G 1 . This result may be caused by a fact that centers of the 0 th  diffraction patterns of the mixed light, for example, the red and green light, are separated further than before the adjustment of the thicknesses d and the refractive index modulation Δn. 
     As shown in  FIG. 6B , superposition between the mixed light is minimized by the adjustment of thicknesses d and refractive index modulations Δn of the lighting hologram optical element and the composite hologram optical element. Therefore crosstalk may be minimized. 
     As described above, according to the one or more of the above exemplary embodiments, a signal light generating unit and a reference light generating unit are laminated in an optical head for a hologram optical apparatus and the signal light generating unit may have a laminated structure on which optical waveguides for the red light R, the green light G and the blue light B are overlapped. 
     Therefore an optical head according to an exemplary embodiment may be smaller in volume that an optical head according to related art. 
     Crosstalk according to light mixing that occurs in a signal light generating process can be minimized by properly adjusting thicknesses and refractive index modulations Δn of hologram optical elements used in the signal light generating unit. 
     Efficiency can be maximized by separating R, G and B respectively and matching one wavelength with one hologram optical element. 
     It should be understood that the exemplary embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each embodiment should typically be considered as available for other similar features or aspects in other embodiments.