Abstract:
The present invention provides a method and apparatus for utilizing light rays of differing wavelengths to read optical discs of different respective capacities. The present invention serves to prevent aberration of the minute optical spot used to read the respective optical discs and also serves to eliminate an offset component with respect to the signals received by the photo detector, as reflected from the respective optical discs.

Description:
The present invention is based upon and claims priority from Japanese Patent Application No. 10-102827 filed on Apr. 14, 1998, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical pick-up apparatus with two light sources. More specifically, the present invention relates to an optical pick-up apparatus capable of utilizing two respective light sources, each having a different wavelength, for reading two respective types of optical discs, and wherein, no photo detector adjustment is required in order to read each type of disk. 
     2. Description of the Related Art 
     FIG. 5 shows the general structure of a conventional optical pick-up apparatus. A light ray radiating from a laser diode  11  is collimated by a collimator lens  12  and its beam form is shaped by a beam shaping prism  13 . It passes through a beam splitter  14  and is deflected 90 degrees by a deflective prism  15 . It is then focused by an objective lens  16  and radiated on an optical disc  17  as a minute optical spot. Recording , reproducing and erasing information on the optical disc  17  are carried out by this optical spot. 
     A light ray reflected off of the optical disc  17  is collimated by the objective lens  16  again, and its path is deflected 90 degrees by the deflective prism  15 . It is reflected by the beam splitter  14  and is focused by a focusing lens  18 . Cylindrical lens  19  provides astigmatism and the light ray is received on photo detector  20 . It is photo detector  20  that detects the information signal and the servo signal as used within the optical pick-up apparatus. 
     Recent attempts to increase optical disc capacity have led to the practice of shortening the wavelength of the light source used to read these higher capacity discs. In general, the illuminated spot radial on an optical disc is proportional to the wavelength λ of the light source used to read the disc, and the capacity of the optical disc is inversely proportional to the square of the wavelength λ. Notwithstanding the trend toward using shorter wavelengths, there do exist optical disc drives that depend on the longer wavelengths. For example, the disc drive might depend on a reflective rate of the optical disc and also on the recording power. In such instances, it is impossible to reproduce and record information on a conventional disc by using a light source with a shortened wavelength. 
     Accordingly, for the purpose of establishing compatibility between conventional discs and optical discs has larger capacities, an optical disc drive might have two different kinds of light sources. One light source has a short wavelength (e.g., 650 nm), the other light source has a conventional wavelength (e.g., 785 nm). The simplest way to realize such a combined disc drive is to employ two separate pick-up apparatuses, each of which employs a light source having a different wavelength. However, in this case, such a drive would become too large and too expensive to be practical. 
     On the other hand, both the size and cost of such a combined arrangement could be reduced if two separate light sources, each having a different wavelength, could be processed using one common optical arrangement. 
     FIG. 6 shows such a conventional optical pick-up apparatus. The optical parts depicted in FIG. 6 are common to those optical parts depicted within the optical pick-up device of FIG. 5, the only difference being the light source  21 . For example, a first light source comprising a laser diode (LD) chip having a wavelength of 650 nm and a second light source comprising a laserdiode (LD) chip having a wavelength of 785 nm are separated by a very small distance which ranges from scores to hundreds of nanometers (nm). One light source, in the light source portion  21 , is located on the optical axis of the collimator lens  12 , a ray from this light source travels as a solid line of FIG.  6 . The other light source, in the light source portion  21 , is located such that it is slightly departed from the optical axis of the collimator lens  12 , a ray from this light source travels as a dotted line of FIG.  6 . These two light sources are used selectively. 
     For example, an optical information recording and reproducing apparatus, as disclosed in Japanese unexamined patent (KOKAI) No. 06-259804, comprises a first light source, a second light source, an optical beam composing means for composing rays from respective light sources on the same optical path, an optical arrangement which makes a beam from the first light source focused on the first optical disc and makes a beam from the second light source focused on the second optical disc, and photo detectors for receiving reflective rays from both the first optical disc and the second optical disc. 
     In the FIG. 6 optical pick-up apparatus, one light source, in the light source portion  21 , is located on the optical axis of the collimator lens  12 , the other light source in the light source portion  21 , is located such that it is slightly departed from the optical axis of the collimator lens  12 . A light ray radiating from the collimator lens  12 , which is departed from its optical axis, is incident to the inclining of that light ray from the objective lens. Furthermore, since aberration occurs and it is difficult to form a good optical illuminated spot on the optical disc  17 , and also, since two rays having different wavelengths reflected on the optical disc are incident to the converging lens  18  with different angles, the incident respective positions of the reflected light rays on the photo detector  20  are different. Therefore, when adjustments are made to the photo detector  20  for a light ray having one wavelength, a servo signal detected by the photo detector for a ray having a different wavelength necessarily has an offset component. 
     Thus, there exists a need for an apparatus and method which allows for the use of a single optical arrangement for utilizing light rays of differing wavelengths to read optical discs of different respective capacities, and wherein optical signals received by the photo detector as reflected from the respective optical discs do not contain an offset component. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the problems associated with the prior art and provides a method and apparatus for utilizing light rays of differing wavelengths to read optical discs of different respective capacities. The present invention serves to prevent aberration of the minute optical spot used to read the respective optical discs and also serves to eliminate an offset component with respect to the signals received by the photo detector, as reflected from the respective optical discs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other advantages and features of the invention will be more clearly understood from the following detailed description of the invention which is provided in connection with the accompanying drawings in which: 
     FIG. 1 depicts a first optical arrangement of the invention; 
     FIG. 2 depicts a portion of a second optical arrangement of the invention; 
     FIG. 3 depicts a portion of the FIG. 1 arrangement in more detail; 
     FIG. 4 depicts a portion of a third optical arrangement of the invention; 
     FIG. 5 depicts a conventional optical pick-up apparatus; and 
     FIG. 6 depicts a conventional optical pick-up apparatus. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will now be described with reference to FIGS. 1-4. Other embodiments may be realized and structural or logical changes may be made to the disclosed embodiments without departing from the spirit or scope of the present invention. 
     FIG. 1 depicts a first optical arrangement of the invention. The refractive index of optical glass, for a given light ray passing through it, varies with the wavelength of the light ray, and in general, if the light&#39;s wavelength is greater, the refractive index of the optical glass is lower. 
     As shown in FIG. 3, two light sources  32 ,  33 , having respective wavelengths λ1, and λ2, are located such that θ1&gt;θ2, where an angle in which the light of wavelength λ1, from the first light source  32 , is incident to the beam shaping prism  31  is θ1, and an angle in which the light of wavelength λ2, from the second light source  33 , is incident to the beam shaping prism  31  is θ2. In accordance with the present invention, a light with wavelength λ1 and a light with wavelength λ2 (λ1&lt;λ2) can be shaped by the beam shaping prism  31 , wherein the angles with which the lights are radiated from the beam shaping prism  31  are nearly equal. 
     As depicted in FIG. 1, light rays from two separate light sources  32 ,  33  are composed by beam composing means  34  (e.g., a dichroic prism). Light from the prism  34  is then collimated by the collimator lens  35  and its beam form is shaped by the beam shaping prism  31 . Upon leaving beam shaping prism  31 , the light rays are radiated in nearly equal angles. The light rays then pass through the beam splitter  36  and then their path is deflected 90 degrees by the deflective prism  37  where the light is then radiated on the optical disc  39  as a minute optical spot that has been focused by the object lens  38 . Recording, reproducing and erasing of information on the optical disc is carried out by the optical spot. 
     A reflective light from the optical disc  39  is collimated by the objective lens  38  and is again deflected 90 degrees by the deflective prism  37 . The light is then reflected by the beam splitter  36  and is converged by the converging lens  40 . Cylindrical lens  41  provides astigmatism and the light is then received by photo detector  42 . An information signal and a servo signal are detected by the photo detector  42  for use within the optical pick-up apparatus. 
     In accordance with the present invention, the particular optical disc which is being read determines which light source ( 32  or  33 ) is active. For example, for the first optical disc, a first light source  32  is active and light source  33  is inactive. Similarly, for the second optical disc, a second light source  33  is active and light source  32  is inactive. 
     Still referring to FIG. 1, a pick-up apparatus is depicted in accordance with a first embodiment of the present invention. The pick-up apparatus has a first light source  32  which radiates a light of wavelength λ1, and a second light source  33  which radiates a light of wavelength λ2, where λ2 is greater than λ1. Light from each source  32 ,  33  then passes through composing means  34  (e.g., a dichroic prism) where the light rays are composed. Next, the light rays pass through a collimate lens  35  which collimates the lights from each respective source  32 ,  33 . The lights then pass through a beam shaping prism  31  which shapes an optical beam form from the collimator lens  35 . An objective lens  38  is employed for focussing light from the beam shaping prism  31  onto the optical disc  39 . A photo detector  42  detects an information signal and servo signals by receiving light reflected from the optical disc  39 . 
     The FIG. 1 pick-up apparatus focuses light from the first light source  32  on a first optical disc, and focuses light from the second light source  32  on a second optical disc, wherein the substrate thickness of the second optical disc differs from that of the first optical disc. As an angle in which light from the second light source  33  is incident to the beam shaping prism  31  is smaller than an angle in which light from the first light source  32  is incident to the beam shaping prism  31 , the first light source  32  and the second light source  33  are located. Therefore, it is possible to make radiant angles of the beam shaping prism  31  for lights of the two different wavelengths λ1, λ2 nearly equal. Furthermore, an incident angle of the objective lens  38  for lights of the two different wavelengths λ1, λ2 is small, thereby reducing aberration of the optical spot for lights of the two different wavelengths λ1, λ2. 
     Turning now to FIG. 2, the portion of a second optical arrangement of the invention is depicted. In FIG. 2 embodiment, a single light source portion  43 , having two laser diode chips in the same package is used, thereby eliminating a need for composing means  34 . The two LD chips in this light source portion  43  comprise a first LD chip radiating a light of wavelength λ1 and a second LD chip radiating a light of wavelength λ2. 
     Lights of wavelength λ1, λ2 radiated from the two LD chips in the light source portion  43  arc collimated by the collimate lens  35 , and are shaped by the beam shaping prism  31 . As can be seen in FIG. 3, an angle in which the light of wavelength λ1, from the first light source  32 , is incident to the beam shaping prism  31  is θ1. Similarly, an angle in which the light of wavelength λ2, from the second light source  33 , is incident to the beam shaping prism  31  is θ2. As long as θ1&gt;θ2, the two LD chips, located within the same light source portion  43 , can respectively produce a light with wavelength λ1 and a light with wavelength λ2 (where λ&lt;λ2), wherein the two separate lights can be shaped by the beam shaping prism  31  such that respective angles radiated from the beam shaping prism  31  for both lights are nearly equal. 
     Defining an incident angle of the beam shaping prism  31  as θ0, a refractive index of material of the beam shaping prism  31  as n1 for wavelength λ1 light, and n2 for wavelength λ2 light, a focus distance of the collimator lens  35  as fc1, the distance between two LD chips in the light source portion  43  is L. Utilizing the above definitions, an optical arrangement of an embodiment of the invention is satisfied with the following expression: 
     
       
         L=fc1×tan (arcsin (n1×sin θ0))−(arcsin (n2×sin θ0)). 
       
     
     Therefore, radiating angles of the lights of two wavelengths λ1, λ2 are equal. 
     For example, where θ0=32 degrees, fc1=8, material of the beam shaping prism  31  is SF 11 , λ1=650 nm, and λ2=785 nm, n1 is 1.776653 and n2 is 1.765743, L=0.13 mm. That is, if the two LD chips in the light source portion  43  are located 0.13 mm apart, incident angles of the light of the two wavelengths λ1, λ2 for the objective lens  38  are nearly equal. 
     Still referring to FIG. 2, a second embodiment puts the first light source and the second light source in the same package. The first light source  32  and the second light source  33  are located in the light source portion  43 . As long as an angle in which a light from the second light source  33  is incident to the beam shaping prism  31  is smaller than an angle in which a light from the first light source  32  is incident to the beam shaping prism  31 , it is possible to make a radiant angle of the beam shaping prism for lights of the two wavelengths λ1, λ2 equal. Furthermore, incident angles of the objective lens for lights of the two wavelengths λ1, λ2 can be small and aberration of the minute optical spot for lights of the two wavelengths λ1, λ2 is reduced. 
     Turning now to FIG. 4, a portion of a third optical arrangement of the invention is depicted. The FIG. 4 embodiment puts two light sources  44 ,  45  and a photo detector  46  in the same package  47 . A hologram laser unit  49  comprises hologram  48  as a diffraction grating, wherein the hologram  48  is coupled to the package  47 . As depicted in FIG. 4, the light source  43 , the beam splitter  36 , the focusing lens  40 , the cylindorical lens  41  and the photo detector  42  are omitted. 
     A light radiated from one of the two light sources  44 ,  45  in the hologram laser unit  49  passes through the hologram  48  and is collimated by the collimator lens  35 . The beam form is shaped by the beam shaping prism  31  and its path is deflected 90 degrees by the deflective prism  37 . The light is then focused by the object lens  38  and is radiated on the optical disc  39  as a minute optical spot. Recording, reproducing and erasing are carried out by the optical spot. 
     A reflective light from the optical disc  39  is the collimated by the objective lens  38  at which point, the optical path is deflected 90 degrees by the deflective prism  37 . The light then passes through the beam shaping prism  31  and the collimator lens  35 , and is diffracted by the hologram  48 . Therefore, such optical path is separated from the radiating light path and is incident to the photo detector  46 . An information signal and a servo signal are detected by photo detector  46 . Therefore, it is possible to form a good optical spot on an optical disc while miniaturizing a drive by reducing the number of parts used in the disc drive. A cost savings is also realized through the incorporation of the resent invention. 
     As is apparent from FIGS. 1 and 4, incident angles of the lights of wavelengths λ1, λ2 for the objective lens  38  can be zero degrees, however, incident angles of the lights of wavelengths λ1, λ2 for the collimator lens  35  cannot be zero degrees. Therefore, in each embodiment, one of the two light sources which radiates a short wavelength λ1 light for reproducing, recording and erasing information on a high density optical disc is located on the optical axis of the collimator lens  35 . Therefore, it is possible to make the short wavelength λ1 light be incident to the collimator lens  35  and the objective lens  38  in an ideal condition, thereby allowing for the forming of a good optical spot on the optical disc. 
     On the other hand, the long wavelength λ2 light is incident to the collimator lens  35  with inclination. In accordance with the present invention, the inclination of the incident angle for the long wavelength λ2 light to the collimator lens does not pose a problem because a permitted level of incident angle error of the long wavelength λ2 light for the collimator lens is greater than that of the incident angle error of the short wavelength λ1 light for the object lens. Therefore, the margin for aberration of the optical spot for purposes of reproducing, recording and erasing information is greater. 
     While preferred embodiments of the invention have been described and illustrated, it should be apparent that many modifications can be made to the invention without departing from its spirit or scope. For example, while specific exemplary wavelengths have been discussed in connection with preferred embodiments of the present invention, the invention may be employed for use with light rays having wavelengths different than those depicted herein. Accordingly, the invention is not limited by the foregoing description or drawings, but is only limited by the scope of the appended claims.