Abstract:
An optical recording/reproducing apparatus featuring stable information signal detection ability and reduced number of components. The optical recording/reproducing apparatus includes: a plurality of light sources emitting lights with different wavelengths for use in recording/reproducing information onto/from various types of optical recording media of different recording densities, in which at least two lights emitted from the light sources have polarization components orthogonal to each other; an objective lens for focusing a light from each of the light sources to a corresponding optical recording medium; a collimating lens disposed between the light sources and the objective lens for collimating lights from the light sources; a hologram element installed between the collimating lens and the objective lens for refracting a light emitted from one of the light sources, in which the light to be refracted is selected by wavelength and polarization components; and a photodetector receiving a light that is reflected from the corresponding optical recording medium after being focused by the objective lens.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of Korean Patent Application No. 2005-48257, filed on Jun. 7, 2005 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     An aspect of the present invention relates in general to an optical recording/reproducing apparatus, and more specifically, to an optical recording/reproducing apparatus capable of recording/reproducing information onto/from various types of optical recording media of different recording densities and different thicknesses.  
         [0004]     2. Description of the Related Art  
         [0005]     In an optical recording and/or reproducing apparatus for recording and/or reproducing information on and/or from an optical information storage medium, such as an optical disc, the recording density (or recording capacity) of the optical information storage medium is determined by the size of a light spot, using a light beam focused by an objective lens. Equation 1 below shows the relation between the size of a light spot S and a wavelength (λ) of light and a numerical aperture (NA) of the objective lens. 
 
S∝λ/NA   [Equation 1]
 
         [0006]     In general, in order to increase the area recording density of the optical disc, the size of a light spot formed on an optical disc needs to be reduced. As is evident from Equation 1, to reduce the light spot size, the wavelength should be short and the NA should be large. For instance, light with a short wavelength such as a blue violet laser and an objective lens with an NA of 0.6 or higher are required.  
         [0007]     As well known already, a digital versatile disc (DVD) records and/or reproduces information using light with a wavelength of 650 nm (or 635 nm) and an objective lens with an NA of 0.6 (or 0.65 for a recording type DVD). Considering that a typical DVD is 120 mm in diameter and has a track pitch of 0.74 μm, its recording density on each side will be greater than 4.7 GB.  
         [0008]     Nevertheless, the DVD is not sufficient for recording high definition moving images. This is because at least 23 GB of recording density for a single side is needed to record 135-minute-long moving images in high definition.  
         [0009]     To meet the demand for high-density recording capacity, much research and studies are underway to develop and set standards of a high density optical disc, namely a next-generation DVD (hereinafter referred to as HD-DVD), or a Blu-ray, using light with a shorter wavelength than red light (i.e., 405-408 nm), such as blue light, and an objective lens with an NA of 0.6 or higher and having a narrow track.  
         [0010]     Meanwhile, in order to secure a tolerance due to the tilt of the optical disc, the thickness of the optical disc should be reduced if the NA of the objective lens is increased for high-density recording. Therefore, in consideration of an allowable tolerance due to the tilt of the optical disc, the thickness of a conventional CD, i.e., 1.2 mm is reduced to 0.6 mm for a DVD and an HD-DVD. Also, the NA of an objective lens is increased from 0.45 for a conventional CD to 0.65 for a DVD and an HD-DVD. In case of the HD-DVD, a blue light source will probably be used in consideration of its recording capacity as a light source. Thus, a problem in the development of an optical disc based on the new standards is the compatibility of the new optical disc with the existing optical disc.  
         [0011]      FIG. 1  is a schematic view of a related art optical recording/reproducing apparatus compatible with CD, DVD and HD-DVD using a single objective lens.  
         [0012]     Referring to  FIG. 1 , the optical recording/reproducing apparatus includes first to third light sources  1   a,    1   b,    1   c  for use with various types of optical recording media of different recording densities and disk thicknesses, respectively; first to third optical path converters  2   a,    2   b,    2   c  for converting the path of a light emitted from each of the light sources  1   a,    1   b,    1   c;  a collimating lens  3  for collimating lights that passed through the optical path converters  2   a,    2   b,    2   c;  a reflective mirror  4  for reflecting lights that passed through the collimating lens  3  to the optical recording medium; a λ/4 wavelength plate  5  for changing polarization components of an incident light; an objective lens  6  for focusing lights that passed through the λ/4 wavelength plate  5  onto the optical recording media; and first and second photodetectors  7   a,    7   b  for detecting information from a reflected light from the optical recording media. Here, the first light source  1   a  is for a CD (D 1 ), the second light source  1   b  is for a DVD (D 2 ), and the third light source  1   c  is for an HD-DVD (D 3 ). Reference numeral  8  in  FIG. 1  denotes a hologram element for diffracting and refracting light, and reference numeral  9  in  FIG. 1  denotes a sensor lens for magnifying a spot of a light received to the first photodetector  7   b.    
         [0013]     In the above-described optical recording/reproducing apparatus, the second and third light sources  1   b,    1   c  emit lights for recording or reproducing information onto or from the DVD (D 2 ) and the HD-DVD (D 3 ) that require the same thickness and the same numerical aperture (NA). However, the first light source  1   a  must emit a light for recording or reproducing information onto or from the CD (D 1 ) which has a different thickness and numerical aperture than the DVD (D 2 ) and the HD-DVD (D 3 ). Therefore, a different lens configuration having a different NA from the NA of the objective lens  6  is required, as shown in  FIG. 1 . As can be seen in the drawing, the first light source  1   a  is arranged closer to the collimating lens  3  than the second and third light sources  1   b,    1   c,  so a light from the first light source  1   a  enters the objective lens  6  at a certain incidence angle.  
         [0014]     Drawbacks of the optical system illustrated in  FIG. 1  are that aberration of the objective lens  6  increases and the aberration increases even more due to the motion of the objective lens  6  under servo control. The increase in aberration produces an error on an information signal and thus an error signal.  
         [0015]     Moreover, since the distances between the second and third light sources  1   b,    1   c  and the optical recording media D 1 , D 2 , D 3  are different from the distance between the first light source  1   a  and the optical recording media D 1 , D 2 , D 3 , two separate photodetectors  7   a,    7   b  are required. Therefore, a total number of components increases, resulting in increased material cost.  
       SUMMARY OF THE INVENTION  
       [0016]     It is, therefore, an aspect of the present invention to provide a compatible optical recording/reproducing apparatus, capable of detecting an information signal more stably and reducing a total number of components and material cost.  
         [0017]     According to another aspect of the present invention, there is provided an optical recording/reproducing apparatus, including: a plurality of light sources emitting lights with different wavelengths for use in recording/reproducing information onto/from various types of optical recording media of different recording densities, in which at least two lights emitted from the light sources have polarization components orthogonal to each other; an objective lens for focusing a light from each of the light sources to a corresponding optical recording medium; a collimating lens disposed between the light sources and the objective lens for collimating lights from the light sources; a hologram element installed between the collimating lens and the objective lens for refracting a light emitted from one of the light sources, in which the light to be refracted is selected by wavelength and polarization components; and a photodetector receiving a light that is reflected from the corresponding optical recording medium after being focused by the objective lens.  
         [0018]     According to another aspect of the present invention, distances between each of the light sources and the collimating lens are substantially the same, and the hologram element diffracts and refracts an incident light. Moreover, the plurality of light sources preferably consist of: a first light source for emitting a p-polarized light with a first wavelength; a second light source for emitting an s-polarized light with a second wavelength shorter than the first wavelength; and a third light source for emitting an s-polarized light with a third wavelength shorter than the second wavelength. The hologram element diffracts the p-polarized light with the first wavelength. Also, between the objective lens and the hologram element, a λ/4 wavelength plate having no effects on the p-polarized light is formed, and a diffraction grating is formed on a surface of the hologram element facing the λ/4 wavelength plate.  
         [0019]     According to another aspect of the present invention, the optical recording/reproducing apparatus further includes: a first and a second optical path converter for changing the optical path of a light emitted from each of the first and second light sources; a third optical path converter for guiding a reflected light from the corresponding optical recording medium to the photodetector; a reflective mirror disposed between the hologram element and the collimating lens; and a lens housing accommodating the objective lens, the λ/4 wavelength plate, and the hologram element. Here, servo control on the objective lens is achieved by controlling the movement of the lens housing.  
         [0020]     Additional aspects and/or 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. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:  
         [0022]      FIG. 1  is a schematic view of a related art optical recording/reproducing apparatus;  
         [0023]      FIG. 2  is a schematic view of an optical recording/reproducing apparatus according to an embodiment of the present invention;  
         [0024]      FIG. 3  is a cross-sectional view showing a λ/4 wavelength plate and a hologram element in  FIG. 2 ; and  
         [0025]      FIG. 4  is a front view of the hologram element in  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0026]     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 the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures.  
         [0027]     Referring to  FIG. 2 , an optical recording/reproducing apparatus according to an aspect of the present invention includes, a plurality of light sources  10   a,    10   b,    10   c,  a plurality of optical path converters  20   a,    20   b,    20   c,  a collimating lens  30 , a reflective mirror  40 , a hologram element  50 , a λ/4 wavelength plate  60 , an objective lens  70 , a lens housing  80 , and a photodetector  90 .  
         [0028]     The plurality of light sources  10   a,    10   b,    10   c  comprise a first CD-light source  10   a  for recording/reproducing information onto/from a CD D 1 , a second DVD-light source  10   b  for recording/reproducing information onto/from a DVD D 2 , a third HD-DVD light source  10   c  for recording/reproducing information onto/from an HD-DVD D 3 .  
         [0029]     The first light source  10   a  is formed of a laser diode for example, and emits a p-polarized light of approximately 785 nm. The second light source  10   b  emits an s-polarized light of approximately 655 nm. Lastly, the third light source  10   c  emits an s-polarized light of approximately 405 nm. The distances from each of the first to third light sources  10   a,    10   b,    10   c  to the collimating lens  30  are substantially the same.  
         [0030]     The optical path converters  20   a,    20   b,    20   c  comprise a first optical path converter  20   a  for reflecting a light emitted from the first light source  10   a  to the collimating lens  30 , a second optical path converter  20   b  for reflecting a light emitted from the second light source  10   b  to the collimating lens  30 , and a third optical path converter  20   c  for reflecting lights emitted from the corresponding optical recording media D 1 , D 2 , D 3  to the photodetector  90 .  
         [0031]     The collimating lens  30  collimates a light emitted from each light source  10   a,    10   b,    10   c.    
         [0032]     The reflective mirror  40  reflects a light that passed through the collimating lens  30  to the objective lens  70 , and reflects a light that was reflected from the optical recoding media D 1 , D 2 , D 3  to the collimating lens  30 .  
         [0033]     The hologram element  50 , as shown in  FIGS. 3 and 4 , is an element for diffracting and refracting a light emitted form the first light source  10   a,  and is disposed between the λ/4 wavelength plate  60  and the reflective mirror  40 . On the surface of the hologram element  50  facing the λ/4 wavelength plate  60 , a diffraction grating  51  for diffracting a light emitted from the first light source  10   a  is formed. The diffraction grating  51  includes a dome-shaped protrusion  52  formed at the central portion of the hologram element  50 , and two ring-shaped protrusions  53 ,  54  formed in a ring shape with respect to the dome-shaped protrusion  52 . Preferably, the cross section of each of the ring-shaped protrusions  53 ,  54  has a sawtooth shape, in order to increase the efficiency of light diffraction and refraction.  
         [0034]     The hologram element  50  having the above configuration is dependent on wavelength and polarization components of light. In other words, the diffraction efficiency is high only if a light is of a wavelength of a selected region or a light has selected polarization components, and the diffraction efficiency is very low otherwise, that is, if a light is not of a wavelength of a selected region or a light does not have selected polarization components. Therefore, the hologram element  50  operates only on the light with a wavelength of a selected region and with selected polarization components. In this embodiment of the invention, the hologram element  50  works only on a p-polarized light of a first wavelength. For instance, the hologram element  50  diffracts and refracts a p-polarized light of approximately 785 nm emitted from the first CD light source  10   a.  Hence, an s-polarized light of a second and a third wavelength emitted from the second and the third light sources  10   b,    10   c,  respectively, are not diffracted by the hologram element  50 . Moreover, p-polarized lights of the second and third wavelengths, which have reflected from the optical recording mediums D 2 , D 3  and passed through the λ/4 wavelength plate  60 , are not diffracted by the hologram element  50 , either.  
         [0035]     Although the exemplary embodiment of the invention introduced the diffraction grating  51  formed of a dome-shaped protrusion  52  and two ring-shaped protrusions  53 ,  54 , its configuration may change in diverse pattern, depending on the design of the optical system. By making a light from the first light source  10   a  refracted at a predetermined angle by the hologram element  50  before it enters the objective lens  70 , the light is changed adaptive to the thickness and numerical aperture required of a CD. Therefore, by setting the first to third light sources  10   a,    10   b,    10   c  to be the same distance from the collimating lens  30 , the light is changed adaptive to the thickness and numerical aperture required of a CD.  
         [0036]     The λ/4 wavelength plate  60  is disposed between the objective lens  70  and the hologram element  50 , and operates selectively according to wavelengths. For instance, in this embodiment of the invention, the λ/4 wavelength plate  60  does not operate on a light of the first wavelength, but operates on lights of the second and the third wavelengths. That is to say, a light emitted from the first light source  10   a  does not get changed to a circularly polarized light by the λ/4 wavelength plate  60 , but is scanned onto a recording medium in a form of p-polarized light. On the other hand, s-polarized lights emitted from the second and third light sources  10   b,    10   c  are changed into right-circularly polarized lights as they come in the λ/4 wavelength plate  60 , and the right-circularly polarized lights are reflected from the optical recording mediums D 2 , D 3  and changed into left-circularly polarized lights. These left-circularly polarized lights are changed into p-polarized lights as they pass through the λ/4 wavelength plate  60 .  
         [0037]     The lens housing  80  is a frame in which the object lens  70 , the λ/4 wavelength plate  60 , and the hologram element  50  are supported. The movement of the lens housing  80  is controlled under servo control of the objective lens  70 . In other words, the object lens  70 , the λ/4 wavelength plate  60 , and the hologram element  50  act as one unit through the lens housing  80 .  
         [0038]     The photodetector  90  receives lights reflected from the optical recording media D 1 , D 2 , D 3  and detects an information signal and an error signal. A good example of the photodetector  90  is a PDIC (Photo Diode Integrated Circuit). The error signal is used for focusing control, tilting control and tracking control of the objective lens  70 . Instead of placing the first to third light sources  10   a,    10   b,    10   c  the same optical distance from the collimating lens  30 , the optical distances from the light sources  10   a,    10   b,    10   c  to the corresponding optical recording media D 1 , D 2 , D 3  are equal. Therefore, a single photodetector  90  can detect all the lights emitted from each of the light sources  10   a,    10   b,  and  10   c.    
         [0039]     Reference numeral  91  denotes a sensor lens. The sensor lens  91  magnifies a spot of a light received by the photodetector  90 , or causes astigmatism to a light reflected from the third optical path converter  20   c  in case of performing a focusing control based on the differential astigmatism.  
         [0040]     With reference to FIGS.  2  to  4 , the following explains the operation of the optical recording/reproducing apparatus, according to one embodiment of the present invention.  
         [0041]     One of the first to third light sources  10   a,    10   b,    10   c  emits a light of a predetermined wavelength, and at least two of them emit two linearly polarized lights orthogonal to each other. Then optical path conversion is executed. Namely, a p-polarized light from the first light source is incident on the collimating lens  30  through the first optical path converter  20   a,  whereas an s-polarized light from the second light source  10   b  is incident on the collimating lens  30  through the second optical path converter  20   b.  Meanwhile, light emitted from the third light source  10   c  goes through the first to third optical path converters  20   a,    20   b,    20   c  and is incident on the collimating lens  30 . The incident light is changed to parallel light by the collimating lens  30 , and guided to the hologram element  50  by the reflective mirror  40 .  
         [0042]     The incident light on the hologram element  50  is selectively diffracted by wavelengths and polarization components thereof. For instance, a p-polarized light of the first wavelength emitted from the first light source  10   a  is diffracted and diverged by the hologram element  50 . At this time, among the light emitted from the first light source  10   a,  only a light received at a region within a dotted line in  FIG. 4  becomes an effective light to form a spot on the optical recording medium D 1 , and is detected by the photodetector  90 . However, among the light emitted from the first light source  10   a,  a light outside of the region within a dotted line in  FIG. 4  does not form a spot on the optical recording medium D 1  and on the photodetector  90 , so it cannot be used as an effective light for detecting an information signal and an error signal. Meanwhile, s-polarized light of the second and third wavelengths emitted from the second and third light sources  10   b,    10   c,  respectively is not diffracted by the hologram element  50  but is fully transmitted.  
         [0043]     When a light from the first light source  10   a  being diffracted and refracted by the hologram element  50  enters the λ/4 wavelength plate  60 , the light is fully transmitted therethrough without having any change in its polarization components and travels to the objective lens  70 . This light of the first wavelength is incident on the objective lens  70  as a non-parallel light and is scanned onto the optical recording medium D 1  according to a numerical aperture required of the CD D 1 . However, the s-polarized lights, which were emitted from the second and third light sources  10   b,    10   c  and transmitted through the hologram element  50 , are changed into right-circularly polarized lights by the λ/4 wavelength plate  60 , reflected from the optical recording media D 1 , D 2 , D 3 , and changed into left-circularly polarized lights. These left-circularly polarized lights are changed into p-polarized lights as they pass through the λ/4 wavelength plate  60  again.  
         [0044]     A light from the first light source  10   a,  which is reflected from the optical recording medium D 1  and transmitted through the λ/4 wavelength plate  60 , is emitted in a state where the light is not yet diffracted by the hologram element  50 . However, lights from the second and third light sources  10   b,    10   c,  which were reflected from the optical recording medium D 1 , are fully transmitted without being affected by the hologram element  50 .  
         [0045]     A light having passed through the hologram element  50  is incident on the first optical path converter  20   a  via the reflective mirror  40  and the collimating lens  30 . A part of the incident light on the first optical path converter  20   a  is reflected and the other part of the incident light enters the second optical path converter  20   b.  Again, a part of the incident light on the second optical path converter  20   b  is reflected and the other part of the incident light enters the third optical path converter  20   c.  A part of the incident light on the third optical path converter  20   c  is reflected to the photodetector  90 , and another part of the incident light passes through the third optical path converter  20   c,  thereby becoming a non-effective light. The photodetector  90  detects an intensity of the incident light thereon and calculates an information signal and an error signal from the detected signal. To perform servo control on the objective lens  70  by the error signal, the movement of the lens housing  80  is controlled.  
         [0046]     As explained so far, the optical recording/reproducing apparatus according to an aspect of the present invention, where the light sources are the same distance from the collimating lens, can be advantageously used for reducing aberration of the objective lens that is frequently observed in the existing limited optical system. In consequence, an information signal and an error signal are detected more stably.  
         [0047]     Moreover, since the distances from the objective lens to each of the light sources are the same, one single photodetector is sufficient to receive all the lights reflected from the optical recording mediums. Thus, a total number of components used in the apparatus can be reduced, and therefore the material cost of the apparatus can also be reduced.  
         [0048]     The foregoing embodiment and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. Also, the description of the embodiments of the present invention is intended to be illustrative, and not to limit the scope of the claims, and many alternatives, modifications, and variations will be apparent to those skilled in the art.