Abstract:
An optical pickup head ( 100 ) for a high density recording and/or reproducing device compatible with first and second optical recording media. The pickup head includes a first light source ( 11 ) emitting first light beams with a first wavelength, a second light source ( 12 ) emitting second light beams with a second wavelength greater than the first wavelength, a prism unit ( 3 ), a collimating lens ( 4 ) located beside the prism unit for collimating incident first and second light beams, and an objective lens ( 7 ) for receiving the first and second light beams and transmitting the first and second laser beams to the first and second recording media respectively. The prism unit includes a first portion facing the first light source and receiving the first light beams, a second portion facing the second light source and receiving the second light beams, and an aspherical surface for the second light beams to pass therethrough.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to an optical pickup head and an information recording and/or reproducing device using the optical pickup head, the device being able to record information on and/or reproduce recorded information from plural types of optical recording media.  
       PRIOR ART  
       [0002]     Optical disks such as CDs (compact disks) and DVDs (digital versatile disks) have been used as information recording media for some time now. Recently, in order to satisfy ongoing requirements for recording and/or reproducing large quantities of information, optical disks with a memory capacity of more than 20 GB have been developed and utilized. The higher recording density of such optical disks requires that a focused spot of laser light generated by an information recording and/or reproducing device must be small and highly accurate. In general, the size of the focused spot (S) is proportional to the wavelength (λ) of the light, and inversely proportional to the numerical aperture (NA) of a lens that focuses the light, as expressed by the following formula (1): 
 
S∝λ/NA   (1) 
 
         [0003]     Therefore, there is a need to construct an optical pickup head for an information recording and/or reproducing device which utilizes a short wavelength light such as blue light, and which provides a large NA. An industry-wide standard for a next generation of high density optical disks has been proposed. The standard specifies that an objective lens have an NA of 0.85, and that light beams with a wavelength of about 405 nm be used.  
         [0004]     However, increasing the NA of an objective lens leads to sharp increases in coma aberration, a phenomenon which occurs when an optical disk is tilted. Coma aberration in turn leads to poor quality light convergence to the focused spot. Coma aberration caused by tilting of the optical disk is proportional to a thickness of an optical transmissive layer which is between a light entering plane and an information recording plane of the optical disk. Accordingly, increases in coma aberration caused by increasing the NA can be controlled by reducing the thickness of the optical transmissive layer. This approach forms the basis of a current proposal to reduce the thickness of the optical transmissive layer of next generation high density optical disks from 0.6 mm to 0.1 mm.  
         [0005]     In using next generation high density optical disks, the first consideration is the compatibility of corresponding equipment with existing optical disks. Stated differently, a recording and/or reproducing device for next generation high density optical disks should also be capable of recording and/or reproducing data on DVDs which are now in widespread use. However, as indicated above, there are many differences between the two types of disks. This makes it difficult to ensure compatibility of equipment with both types of disks.  
         [0006]     In a conventional solution to the above problem, an optical pickup head for a high density recording and/or reproducing device includes a first semiconductor laser emitting a first light beam at a short wavelength of about 405 nm, and a second semiconductor laser emitting a second light beam at a long wavelength of about 650 nm. The first and second light beams are rendered by two collimating lenses respectively. Then, a dichroic prism merges the first and second light beams and aligns the optical axes thereof. The merged light is transmitted to a common objective lens. The light then forms a small light spot on a recording plane of an optical disk.  
         [0007]     The major drawback of the optical pickup head is that it requires a large distance between the first and second semiconductor lasers and the corresponding collimating lens. This makes the overall size of the recording and/or reproducing device unduly large. Furthermore, because there is only the single common objective lens focusing light having the two different wavelengths, the focusing of the light of one of these wavelengths is subject to chromatic aberration. Moreover, the two kinds of disks have different thicknesses, including different thicknesses of light transmission layers thereof. Therefore the focusing of the light of either or both wavelengths is subject to spherical aberration. These problems in turn lead to poor quality light convergence to the focused light spot.  
       SUMMARY OF THE INVENTION  
       [0008]     Accordingly, an object of the present invention is to provide an optical pickup head for a high density recording and/or reproducing device compatible with at least two types of optical disks, in which optical aberrations are corrected and a size of the optical pickup head is reduced.  
         [0009]     Another object of the present invention is to provide an information recording and/or reproducing device using the above-described optical pickup head.  
         [0010]     To achieve the first above object, an optical pickup head for a high density recording and/or reproducing device compatible with first and second optical recording media is provided. The optical pickup head includes a first light source emitting first light beams with a first wavelength, a second light source emitting second light beams with a second wavelength greater than the first wavelength, a prism unit, a collimating lens located beside the prism unit for collimating incident first and second light beams, and an objective lens for receiving the first and second light beams and transmitting the first and second light beams to the first and second recording media respectively. The prism unit includes a first portion facing the first light source adapted to receive the first light beams emitted by the first light source, a second portion facing the second light source adapted to receive the second light beams emitted by the second light source, and an aspherical surface for the second light beams to pass therethrough.  
         [0011]     To achieve the second above object, an information recording and/or reproducing device includes an optical pickup head as described in the above paragraph, a drive mechanism for changing a relative position between an information storage medium and the optical pickup head, and an electrical signal processor for receiving signals output from the optical pickup head and performing calculations to obtain desired information.  
         [0012]     Other objects, advantages and novel features of the present invention will be drawn from the following detailed description of preferred embodiments thereof with the attached drawings, in which: 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is an exploded, isometric view of an arrangement of parts of an optical pickup head according to a preferred embodiment of the present invention, also showing essential optical paths thereof; and  
         [0014]      FIG. 2  is an exploded, top plan view of a prism unit of the optical pickup head of  FIG. 1 , also showing essential optical paths thereof. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     Referring to  FIG. 1 , an optical pickup head  100  according to the preferred embodiment of the present invention is illustrated. The optical pickup head  100  is used in an information recording and/or reproducing device (not shown) compatible with a first optical disk (not shown) having a higher recording density and a second optical disk (not shown) having a lower recording density. The optical pickup head  100  includes first and second semiconductor modules  11  and  12  juxtaposed with each other. Each of the first and second semiconductor modules  11  and  12  includes a semiconductor laser (not shown), and a photo detector (not shown) formed integrally with the semiconductor laser. The two semiconductor lasers generate laser beams with different wavelengths to be employed as irradiation light beams. First light beams from the first semiconductor module  11  have a short wavelength, such as 405 nm; and second light beams from the second semiconductor module  12  have a long wavelength, such as 650 mn.  
         [0016]     The optical pickup head  100  also includes first and second diffraction gratings  21  and  22 , a prism unit  3 , a collimating lens  4 , a mirror  5 , a wavelength selector  6 , and an objective lens  7 . The prism unit  3  comprises three prisms  31 ,  32  and  33 . The first and second prisms  31  and  32  are juxtaposed at a common side of the third prism  33 , and respectively face the first and second semiconductor modules  11  and  12 . The first diffraction grating  21  is located in a light path between the first semiconductor module  11  and the first prism  31 , and the second diffraction grating  22  is located in a light path between the second semiconductor module  12  and the second prism  32 . The collimating lens  4  is positioned at an opposite side of the third prism  33 , and accords with the wavelength of the first light beams so as to converge the first light beams into parallel light beams. The mirror  5  is aslant so as to reflect light beams from the collimating lens  4  to the wavelength selector  6 . The objective lens  7  has a numerical aperture specified by the first optical disk, which is larger then a numerical aperture specified by the second optical disk. The wavelength selector  6  is located beside the objective lens  7 , to selectively transmit light beams thereto.  
         [0017]     Referring to  FIG. 2 , the first prism  31  is parallelepiped, and includes a first incident surface  310 , a first emergent surface  311  parallel to the first incident surface  310 , and two parallel reflective surfaces  312  and  313  interconnecting the first incident surface  310  and first emergent surface  311 . The second prism  32  is formed with an aspherical surface, and includes a second incident surface  320  and a second emergent surface  321 . In the illustrated embodiment, the aspherical surface is provided at the second emergent surface  321 . In alternative embodiments, the aspherical surface can be provided at the second incident surface  320  or on the third prism  33 . The third prism  33  includes a third incident surface  330 . Part of the first emergent surface  311  and the second emergent surface  321  are juxtaposed beside two opposite ends of the third incident surface  330  respectively. The third prism  33  also includes a third emergent surface  331  parallel to the third incident surface  330 , a third reflective surface  332  interconnecting the third incident surface  330  and the third emergent surface  331  at corresponding ends thereof, and an optical path synthesizing/separating surface  333  parallel to the third reflective surface  332  at an opposite side of the third prism  33 .  
         [0018]     When recording an information signal on and/or reproducing an information signal from the first optical disk, the semiconductor laser of the first semiconductor module  11  emits first light beams with the short wavelength 405 nm. The first light beams propagate through the diffraction grating  21  along their original direction, and enter the first prism  31  through the first incident surface  310 . In the first prism  31 , the first light beams are reflected by the two opposite first reflective surfaces  312  and  313 , and are then output from the first emergent surface  311 . The first light beams transmit into the third prism  33  through the third incident surface  330 , and propagate to the optical path synthesizing/separating surface  333 . The first light beams pass through the optical path synthesizing/separating surface  333  along their original direction, because of their short wavelength. Subsequently, the first light beams transmit out from the third emergent surface  331 .  
         [0019]     After exiting the prism unit  3 , the first light beams are condensed by the collimating lens  4  and transformed into a first luminous flux of parallel light beams. Because the collimating lens  4  accords with the wavelength of the first light beams, it can enable beams of the first luminous flux to be fully parallel to each other. The first luminous flux transmits to the mirror  5 , which changes the transmitting direction toward the first optical disk. Accordingly, the first luminous flux illuminates the wavelength selector  6 . The wavelength selector  6  does not block any of the first luminous flux, so that the first luminous flux completely passes through the wavelength selector  6  and is incident on the objective lens  7 . The objective lens  7  converges the first luminous flux to form a focused light spot (not shown) on the first optical disk.  
         [0020]     After forming the light spot on the first optical disk, the first optical disk reflects the incident beams, so as to form first return beams (not labeled). The first return beams sequentially pass through/from the objective lens  7 , the wavelength selector  6 , the mirror  5 , the collimating lens  4 , and the prism unit  3 , and reach the first diffraction grating  21 . The first diffraction grating  21  diffracts the first return beams toward the photo detector of the first semiconductor module  11 . The photo detector translates the first light beams into electrical signals. An electrical signal processor of the information recording and/or reproducing device receives electrical signals output from the optical pickup head  100 , and performs calculations to obtain the desired information. Furthermore, a drive mechanism of the information recording and/or reproducing device changes a relative position between the first optical disk and the optical pickup head  100 , also based on the electrical signals output from the optical pickup head  100 .  
         [0021]     When recording an information signal on and/or reproducing an information signal from the second optical disk, the semiconductor laser of the second semiconductor module  12  emits second light beams (not labeled) with the long wavelength 650 nm. The second light beams propagate through the second diffraction grating  22  along their original direction, and enter the second prism  32  through the second incident surface  320 . The second light beams propagate to the second emergent surface  321  of the second prism  32 , and are converged first by the aspherical surface of the second emergent surface  321 . The converged second light beams transmit into the third prism  33  through the third incident surface  330 , are reflected by the third reflective surface  332 , and propagate to the optical path synthesizing/separating surface  333 . The optical path synthesizing/separating surface  333  reflects the second light beams because of their long wavelength. Subsequently, the second light beams transmit out from the third emergent surface  331 .  
         [0022]     After exiting the prism unit  3 , the second light beams are condensed by the collimating lens  4  and transformed into a second luminous flux of substantially parallel light beams. The second luminous flux transmits to the mirror  5 , and is reflected by the mirror  5  toward the second optical disk. Accordingly, the second luminous flux illuminates the wavelength selector  6 . The wavelength selector  6  transmits a center part of the second luminous flux, and blocks a peripheral part of the second luminous flux. Thus, only the center part of the second luminous flux can pass through the wavelength selector  6  and is incident on the objective lens  7 . The objective lens  7  converges the second luminous flux to form a focused light spot (not shown) on the second optical disk.  
         [0023]     After forming the light spot on the second optical disk, the second optical disk reflects the incident beams, so as to form second return beams (not labeled). The second return beams sequentially pass through/from the objective lens  7 , the wavelength selector  6 , the mirror  5 , the collimating lens  4 , and the prism unit  3 , and reach the second diffraction grating  22 . The second diffraction grating  22  diffracts the second return beams toward the photo detector of the second semiconductor module  12 . The photo detector translates the second return beams into electrical signals. The electrical signal processor of the information recording and/or reproducing device receives electrical signals output from the optical pickup head  100 , and performs calculations to obtain the desired information. Furthermore, the drive mechanism of the information recording and/or reproducing device changes a relative position between the second optical disk and the optical pickup head  100 , also based on the electrical signals output from the optical pickup head  100 .  
         [0024]     In the above-mentioned optical pickup head  100 , both (i) the working wavelength of optical elements, such as the first semiconductor module  11 , the collimating lens  4  and the objective lens  7 , and (ii) the numerical aperture of the objective lens  7 , are directly matched with requirements of the first optical disk. Therefore, when recording an information signal on and/or reproducing an information signal from the first optical disk, the optical pickup head  100  provides high quality light convergence to the focused light spot. Further, because the aspherical surface is formed on the second prism  32 , aberrations caused by non-matching between the second luminous flux and the collimating lens  4  and objective lens  7  are corrected. Moreover, the wavelength selector  6  selects a part of the light beams with long wavelength transmitting to the objective lens  7 , so that only central part of the objective lens  7  is illuminated by the second light beams. Thus the NA of the objective lens  7  is reduced when focusing the second light beams, and corresponds to the small NA required by the second optical disk. Hence, when recording an information signal on and/or reproducing an information signal from the second optical disk, the optical pickup head  100  provides high quality light convergence to the focused light spot.  
         [0025]     Furthermore, because the first and second light beams are reflected between the surfaces of the prism unit  3 , the distance between the collimating lens  4  and the first and second semiconductor modules  11  and  12  is reduced. This enables the optical pickup head  100  to be miniaturized. Moreover, the aspherical surface is directly formed on the second prism  32 , so that no extra optical element need be added to the optical pickup head  100 . This further facilitates miniaturization of the optical pickup head  100 , and improves the efficiency of mass production.  
         [0026]     Although the present invention has been described with reference to specific embodiments, it should be noted that these embodiments are not necessarily exclusive, and that various changes and modifications may be made to the described embodiments without departing from the scope of the invention as defined by the appended claims.