Abstract:
In an optical head apparatus including a first light source for emitting a first light beam having a first wavelength, a second light source for emitting a second light beam having a second wavelength different from the first wavelength, an objective lens, a photodetector, and first and second optical combining/splitting elements, the first optical combining/splitting element receives the first light beam from the first light source to outgo most of the first light beam therefrom to the second optical combining/splitting element and receives the first and second light beams from the second optical combining/splitting element to outgo most of the first and second light beams therefrom to the photodetector. The second optical combining/splitting element receives the first light beam from the first optical combining/splitting element and the second light beam from the second light source to outgo most of the first and second light beams therefrom to the objective lens and receives the first and second light beams from the objective lens to outgo most of the first and second light beams therefrom to the first optical combining/splitting element.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical head apparatus for different types of disks such as a digital versatile disk (DVD) and a compact disk (CD). 
   2. Description of the Related Art 
   Recently, optical head apparatuses have been developed to be adapted to a DVD and a CD. In this case, a DVD requires a 650 nm wavelength light source, while a CD requires a 780 nm wavelength light source. Therefore, one of such optical head apparatuses is constructed by two light sources for emitting two kinds of light beams, i.e., a 650 nm wavelength light beam and a 780 nm wavelength light beam and transmitting them to a disk and one photodetector for receiving a light beam reflected from the disk. 
   In a first prior art optical head apparatus (see: JP-A-10-112050), first and second beam splitters for receiving a 650 nm wavelength light beam and a 780 nm wavelength light beam, respectively, are provided between a photodetector and an objective lens. The first beam splitter transmits half of the 650 nm wavelength light beam and most of the 780 nm wavelength light beam therethrough, while the first beam splitter reflects half of the 650 nm wavelength light beam and a small part of the 780 nm wavelength light beam. Also, a second beam splitter transmits most of the 650 nm wavelength light beam and half of the 780 nm wavelength light beam, while the second beam splitter reflects a small part of the 650 nm wavelength light beam and half of the 780 nm wavelength light beam. This will be explained later in detail. 
   In the above-described first prior art optical head apparatus, however, since losses of the intensity of the outgoing and incoming 650 nm wavelength light beams occur in the first beam splitter and also, losses of the intensity of the outgoing and incoming 780 nm wavelength light beams occur in the second beam splitter, the signal-to-noise (S/N) ratio would be decreased. 
   In a second prior art optical head apparatus (see: JP-A-10-228668), a wavelength selecting prism for selecting an S-polarized 650 nm wavelength light beam and an S-polarized 780 nm light beam is provided, and a beam splitter and a quarter-wave plate are provided between a photodetector and an objective lens. The beam splitter transmits most of the P-polarized components of the 650 nm wavelength light beam and the 780 nm wavelength light beam, while the beam splitter reflects most of the S-polarized components of the 650 nm wavelength light beam and the 780 nm wavelength light beam. This also will be explained later in detail. 
   In the above-described second prior art optical head apparatus, however, since losses of the intensity of the outgoing light beams occur in the wavelength selecting prism, the S/N ratio would be decreased. Additionally, since the difference in wavelength dependence characteristics between the P-polarized and S-polarized light beams cannot be large, it is difficult for the 650 nm wavelength light beam and the 780 nm wavelength light beam to be within a range of the polarizing beam splitter characteristics. Therefore, since the manufacturing margin of the beam splitter is small, the manufacturing cost would be increased. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an optical head apparatus capable of increasing the S/N ratio and decreasing the manufacturing cost. 
   According to the present invention, in an optical head apparatus including a first light source for emitting a first light beam having a first wavelength, a second light source for emitting a second light beam having a second wavelength different from the first wavelength, an objective lens, a photodetector, and first and second optical combining/splitting elements, the first optical combining/splitting element receives the first light beam from the first light source to outgo most of the first light beam therefrom to the second optical combining/splitting element and receives the first and second light beams from the second optical combining/splitting element to outgo most of the first and second light beams therefrom to the photodetector. The second optical combining/splitting element receives the first light beam from the first optical combining/splitting element and the second light beam from the second light source to outgo most of the first and second light beams therefrom to the objective lens and receives the first and second light beams from the objective lens to outgo most of the first and second light beams therefrom to the first optical combining/splitting element. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more clearly understood from the description set forth below, as compared with the prior art, with reference to the accompanying drawings, wherein: 
       FIG. 1  is a diagram illustrating a first prior art optical head apparatus; 
       FIG. 2  is a diagram illustrating a second prior art optical head apparatus; 
       FIG. 3  is a graph showing the transmittivity characteristics of the beam splitter of  FIG. 2 ; 
       FIG. 4  is a graph showing transmittivity characteristics of a typical beam splitter; 
       FIGS. 5A ,  5 B,  5 C and  5 D are graphs showing transmittivity characteristics of beam splitters according to the present invention; 
       FIG. 6  is a diagram illustrating a first embodiment of the optical head apparatus according to the present invention; 
       FIG. 7  is a diagram illustrating a modification of the optical head apparatus of  FIG. 6 ; 
       FIG. 8  is a diagram illustrating a second embodiment of the optical head apparatus according to the present invention; 
       FIG. 9  is a diagram illustrating a modification of the optical head apparatus of  FIG. 8 ; 
       FIG. 10  is a diagram illustrating a third embodiment of the optical head apparatus according to the present invention; 
       FIG. 11  is a diagram illustrating a modification of the optical head apparatus of  FIG. 10 ; 
       FIG. 12  is a diagram illustrating a fourth embodiment of the optical head apparatus according to the present invention; 
       FIG. 13  is a diagram illustrating a modification of the optical head apparatus of  FIG. 12 ; and 
       FIG. 14  is an enlarged plan view of the photodetector of  FIGS. 6 through 13 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Before the description of the preferred embodiments, prior art optical head apparatuses will be explained with reference to  FIGS. 1 ,  2  and  3 . 
   In  FIG. 1 , which illustrates a first prior art optical head apparatus (see: JP-A-10-112056), reference numeral  101  designates a disk which is either a DVD or a CD. That is, the DVD or the CD is mounted on the apparatus. A laser diode  102  generates a 650 nm wavelength light beam for the DVD, while a laser diode  103  generates a 780 nm wavelength light beam for the CD. A photodetector  104  receives a light beam reflected from the disk  101 . 
   A beam splitter  105  passes half of the 650 nm wavelength light beam and most of the 780 nm wavelength light beam therethrough, while the beam splitter  105  reflects half of the 650 nm wavelength light beam and a small part of the 780 nm wavelength light beam. 
   A beam splitter  106  passes most of the 650 nm wavelength light beam and half of the 780 nm wavelength light beam therethrough, while the beam splitter  106  reflects a small part of the 650 nm wavelength light beam and half of the 780 nm wavelength light beam. 
   The 650 nm wavelength light beam generated from the laser diode  102  is half-reflected by the beam splitter  105 , and passes through beam splitter  106  to reach a collimator lens  107  for converting the light beam passed therethrough into a collimated light beam. The collimated light beam is reflected by a mirror  108  and is focussed by an objective lens  109  at the disk  101  which is, in this case, a DVD. 
   A light beam reflected by the disk  101  is returned by the objective lens  109 , the mirror  108 , the collimator lens  107  and the beam splitters  106  and  105  to the photodetector  104 . In this case, only the half of the 650 nm wavelength light beam passes through beam splitter  105 . 
   On the other hand, the 780 nm wavelength light beam generated from the laser diode  103  is half-reflected by the beam splitter  106  to reach the collimator lens  107  for converting the light beam passed therethrough into a collimated light beam is reflected by the mirror  108  and is focussed by the objective lens  109  at the disk  101  which is, in this case, a CD. 
   A light beam reflected by the disk  101  is returned by the objective lens  109 , the mirror  108 , the collimator lens  107  and the beam splitters  106  and  105  to the photodetector  104 . In this case, only half of the 780 nm wavelength light beam passes through beam splitter  106 . 
   In the optical head apparatus of  FIG. 1 , however, since losses of the intensity of the outgoing and incoming 650 nm wavelength light beams occur in the beam splitter  105  and also, losses of the intensity of the outgoing and incoming 780 nm wavelength light beams occur in the beam splitter  106 , the S/N ratio would be decreased. 
   In  FIG. 2 , which illustrates a second prior art optical head apparatus (see: JP-A-10-228668), reference numeral  201  designates a disk which is either a DVD or a CD. That is, the DVD or the CD is mounted on the apparatus. A laser diode  202  generates a 650 nm wavelength light beam for the DVD, while a laser diode  203  generates a 780 nm wavelength light beam for the CD. A photodetector  204  receives a light beam reflected from the disk  201 . 
   A wavelength selecting prism  205  transmits most of the 650 nm light beam therethrough, and reflects most of the 780 nm light beam thereby. 
   A beam splitter  206  transmits most of P-polarized components of the 650 nm wavelength light beam and the 780 nm wavelength light beam, while the beam splitter  206  reflects most of S-polarized components of the 650 nm wavelength light beam and the 780 nm wavelength light beam. Transmittivity characteristics of the beam splitter  206  are shown in  FIG. 3 . 
   The S-polarized 650 nm wavelength light beam generated from the laser diode  202  is transmitted through the wavelength selecting prism  205  to reach a collimator lens  207  for converting the light beam passed therethrough into a collimated light beam. The collimated light beam is reflected by the beam splitter  206  due to the S-polarized light beam. Then, the light beam passes through a quarter-wave plate  208  so that the light beam is converted from a linearly-polarized (S-polarized) light mode into a circularly-polarized light mode, and then, is focussed by an objective lens  209  at the disk  201  which is, in this case, a DVD. 
   A light beam reflected by the disk  201  is returned by the objective lens  209  to the quarter-wave plate  208  and reaches the beam splitter  206 . In this case, the light beam is P-polarized due to the presence of the quarter-wave plate  208  for converting the mode from a circularly-polarized light mode into a linearly-polarized (P-polarized) light mode. Therefore, the light beam transmitted through the beam splitter  206  passes through a cylindrical lens  210  and a convex lens  211  to reach the photodetector  204 . 
   On the other hand, the S-polarized 780 nm wavelength light beam generated from the laser diode  203  is reflected by the wavelength selecting prism  205  to reach the collimator lens  207  for converting the light beam passed therethrough into a collimated light beam. The collimated light beam is reflected by the beam splitter  206  due to the S-polarized light beam. Then, the light beam passes through the quarter-wave plate  208  so that the light beam is converted from a linearly-polarized (S-polarized) light mode into a circularly-polarized light mode, and then, is focussed by an objective lens  209  at the disk  201  which is, in this case, a CD. 
   A light beam reflected by the disk  201  is returned by the objective lens  209  to the quarter-wave plate  208  and reaches the beam splitter  206 . In this case, the light beam is P-polarized due to the presence of the quarter-wave plate  208  for converting the mode from a circularly-polarized light mode into a linearly-polarized (P-polarized) light mode. Therefore, the light beam transmitted by the beam splitter  206  passes through cylindrical lens  210  and the convex lens  211  to reach the photodetector  204 . 
   In the optical head apparatus of  FIG. 2 , however, since losses of the intensity of the outgoing light beams occur in the wavelength selecting prism  205 , the S/N ratio would be decreased. Additionally, as shown in  FIG. 3 , since the difference Δλ in wavelength between the P-polarized and S-polarized characteristics cannot be large, for example, Δλ=150 nm, it is difficult for the 650 nm wavelength light beam and the 780 nm wavelength light beam to be within a range between the P-polarized and S-polarized characteristics. Therefore, since the manufacturing margin of the beam splitter  206  is small, the manufacturing cost would be increased. For example, if the P-polarized characteristics are shifted on the increase side, the P-polarized component of the 650 nm wavelength light beam is reflected by the beam splitter  206 , so that the S/N ratio of the DVD would be decreased. On the other hand, if the S-polarized characteristics are shifted on the decrease side, the S-polarized component of the 780 nm wavelength light beam is transmitted through the beam splitter  206 , so that the S/N ratio of the CD would be decreased. 
   Next, the principle of beam splitters according to the present invention will be explained with reference to  FIGS. 4 ,  5 A,  5 B,  5 C and  5 D. 
   In  FIG. 4 , which shows transmittivity characteristics of a beam splitter  5 , the beam splitter  5  transmits a part of an incident light beam I as a transmission light beam T and reflects a part of the incident light beam I as a reflected light beam R. Generally, the transmittivity characteristics of a P-polarized component of the incident light beam I are different from those of an S-polarized component of the incident light beam I. That is, there are generally five regions R 1 , R 2 , R 3 , R 4  and R 5 . In the regions R 1  and R 5 , the transmittivity of the P-polarized component is almost 100% (more than 90%) and also, the transmittivity of the S-polarized component is almost 100% (more than 90%). In the regions R 2  and R 4 , the transmittivity of the P-polarized component is almost 100% (more than 90%) and also, the transmittivity of the S-polarized component is almost 0% (less than 10%). In the region R 3 , the transmittivity of the P-polarized component is almost 0% (less than 10%) and also, the transmittivity of the S-polarized component is almost 0% (less than 10%). 
   Note that the transmittivity characteristics of the beam splitter  5  can be adjusted by changing the number and thickness of dielectric layers sandwiched by two transparent substrates. 
   According to the present invention, the 650 nm wavelength light beam falls into one of the regions R 1 , R 2 , . . . , R 5  and the 780 nm wavelength light beam falls into another of those regions. Therefore, since the manufacturing margin of beam splitters can be large, the manufacturing cost thereof would be decreased. 
   When the 650 nm wavelength light beam falls into the region R 1  of  FIG. 4  and the 780 nm wavelength light beam falls into the region R 2  of  FIG. 4 , a beam splitter  5 A as illustrated in  FIG. 5A  is obtained. That is, the beam splitter  5 A transmits most of the P-polarized and S-polarized components of the 650 nm wavelength light beam and the P-polarized component of the 780 nm wavelength light beam, and reflects most of the S-polarized component of the 780 nm wavelength light beam. 
   When the 650 nm wavelength light beam falls into the region R 2  of  FIG. 4  and the 780 nm wavelength light beam falls into the region R 3  of  FIG. 4 , a beam splitter  5 B as illustrated in  FIG. 5B  is obtained. That is, the beam splitter  5 B transmits most of the P-polarized component of the 650 nm wavelength light beam, and reflects most of the S-polarized component of the 650 nm wavelength light beam and the P-polarized and S-polarized components of the 780 nm wavelength light beam. 
   When the 650 nm wavelength light beam falls into the region R 3  of  FIG. 4  and the 780 nm wavelength light beam falls into the region R 4  of  FIG. 4 , a beam splitter  5 C as illustrated in  FIG. 5C  is obtained. That is, the beam splitter  5 C transmits most of the P-polarized component of the 780 nm wavelength light beam, and reflects most of the P-polarized and S-polarized components of the 650 nm wavelength light beam and the S-polarized component of the 780 nm wavelength light beam. 
   When the 650 nm wavelength light beam falls into the region R 4  of  FIG. 4  and the 780 nm wavelength light beam falls into the region R 5  of  FIG. 4 , a beam splitter  5 D as illustrated in  FIG. 5D  is obtained. That is, the beam splitter  5 D transmits most of the P-polarized component of the 650 nm wavelength light beam and the P-polarized and S-polarized components of the 780 nm wavelength light beam, and reflects most of the S-polarized component of the 650 nm wavelength light beam. 
   In  FIG. 6 , which illustrates a first embodiment of the optical head apparatus according to the present invention, reference numeral  1  designates a disk which is a DVD or a CD. That is, the DVD or the CD is mounted on the apparatus. A laser diode  2  generates an S-polarized 650 nm wavelength light beam for the DVD, while a laser diode  3  generates an S-polarized 780 nm wavelength light beam for the CD. A photodetector  4  receives a light beam reflected from the disk  1 . 
   Provided between the photodetector  4  and the disk  1  are the beam splitter  5 D having the transmittivity characteristics of  FIG. 5D  and the beam splitter  5 A having the transmittivity characteristics of  FIG. 5A . In this case, a light beam reflected from the disk  1  is transmitted through the beam splitters  5 A and  5 D to reach the photodetector  4 . 
   The S-polarized 650 nm wavelength light beam generated from the laser diode  2  is divided by a diffraction grating  6  into a zeroth order light component (transmission light component), a +1st order diffraction light component and a −1st order diffraction light component, which are converged by a coupling lens  7 . Most of the converged light beam of the coupling lens  7  is reflected by the beam splitter  5 D and is transmitted through the beam splitter  5 A. Then, the light beam transmitted through beam splitter  5 A is reflected by a mirror  8  to reach a collimator lens  9  for converting the light beam passed therethrough into a collimated light beam. The collimated light beam is converted by a quarter-wave plate  10  from a linearly-polarized (S-polarized) light mode into a circularly-polarized light mode, and then, is focussed by an objective lens  11  at the disk  1  which is, in this case, a DVD. 
   The light beam reflected by the disk  1  is returned by the objective lens  11  to the quarter-wave plate  10 . As a result, the mode of the light beam is converted from the circularly-polarized light mode into a linearly-polarized (P-polarized) light mode. Then, the P-polarized light beam is reflected by the mirror  8  and is transmitted through the beam splitters  5 A and  5 D, and further passes through the compound lens  12  to reach the photodetector  4 . 
   Note that the compound lens  12  has a cylindrical surface for incident light and a concave surface for outgoing light. Also, the photodetector  4  is positioned at an intermediate location between the two focal lines of the compound lens  12 . 
   On the other hand, the S-polarized 780 nm wavelength light beam generated from the laser diode  3  is divided by a diffraction grating  6 ′ into a zeroth order light component (transmission light component), a +1st order diffraction light component and a −1st order diffraction light component, which are converged by a coupling lens  7 ′. Most of the converged light beam of the coupling lens  7 ′ is reflected by the beam splitter  5 A. Then, the light beam reflected by the beam splitter  5 A is reflected by the mirror  8  to reach the collimator lens  9  for converting the light beam passed therethrough into a collimated light beam. The collimated light beam is converted by the quarter-wave plate  10  from a linearly-polarized (S-polarized) light mode into a circularly-polarized light mode, and then, is focussed by the objective lens  11  at the disk  1  which is, in this case, a CD. 
   The light beam reflected by the disk  1  is returned by the objective lens  11  to the quarter-wave plate  10 . As a result, the mode of the light beam is converted from the circularly-polarized light mode into a linearly-polarized (P-polarized) light mode. Then, the P-polarized light beam is reflected by the mirror  8  and is transmitted through the beam splitters  5 A and  5 D, and further passes through compound lens  12  to reach the photodetector  4 . 
   In the optical head apparatus of  FIG. 6 , since losses of the intensity of the outgoing and incoming light beams hardly occur in the beam splitters  5 D and  5 A, the S/N ratio would be increased. Additionally, as stated above, the manufacturing margin of the beam splitters  5 D and  5 A is large, so that the manufacturing cost thereof can be decreased. 
   In  FIG. 7 , which illustrates a modification of the optical head apparatus of  FIG. 6 , the locations of the laser diode  2 , the diffraction grating  6 , the coupling lens  7  and the beam splitter  5 D of  FIG. 6  are exchanged with the locations of the laser diode  3 , the diffraction grating  6 ′, the coupling lens  7 ′ and the beam splitter  5 A, respectively, of  FIG. 6 . The operation of the optical head apparatus of  FIG. 7  is similar to that of the optical head apparatus of  FIG. 6 . 
   In  FIG. 8 , which illustrates a second embodiment of the optical head apparatus according to the present invention, the laser diode  2  generates a P-polarized 650 nm wavelength light beam for the DVD, while the laser diode  3  generates an S-polarized 780 nm wavelength light beam for the CD. 
   The beam splitter  5 B having the transmittivity characteristics of  FIG. 5B  and the beam splitter  5 A having the transmittivity characteristics of  FIG. 5A  are provided between the photodetector  4  and the disk  1 . In this case, a light beam reflected from the disk  1  is transmitted through the beam splitter  5 A and is reflected by the beam splitter  5 B to reach the photodetector  4 . 
   The P-polarized 650 nm wavelength light beam generated from the laser diode  2  is divided by the diffraction grating  6  into a zeroth order light component (transmission light component), a +1st order diffraction light component and a −1st order diffraction light component, which are converged by the coupling lens  7 . Most of the converged light beam of the coupling lens  7  is transmitted through the beam splitters  5 B and  5 A. Then, the light beam transmitted through the beam splitter  5 A is reflected by the mirror  8  to reach the collimator lens  9  for converting the light beam passed therethrough into a collimated light beam. The collimated light beam is converted by the quarter-wave plate  10  from a linearly-polarized (P-polarized) light mode into a circularly-polarized light mode, and then, is focussed by the objective lens  11  at the disk  1  which is, in this case, a DVD. 
   The light beam reflected by the disk  1  is returned by the objective lens  11  to the quarter-wave plate  10 . As a result, the mode of the light beam is converted from the circularly-polarized light mode into a linearly-polarized (S-polarized) light mode. Then, the S-polarized light beam is reflected by the mirror  8  and is transmitted through the beam splitter  5 A. Then, the light beam transmitted through the beam splitter  5 A is reflected by the splitter  5 B and passes through the beam splitter  5 B, and further passes through the compound lens  12  to reach the photodetector  4 . 
   On the other hand, the S-polarized 780 nm wavelength light beam generated from the laser diode  3  is divided by the diffraction grating  6 ′ into a zeroth order light component (transmission light component), a +1st order diffraction light component and a −1st order diffraction light component, which are converged by the coupling lens  7 ′. Most of the converged light beam of the coupling lens  7 ′ is reflected by the beam splitter  5 A. Then, the light beam reflected by the beam splitter  5 A is reflected by the mirror  8  to reach the collimator lens  9  for converting the light beam passed therethrough into a collimated light beam. The collimated light beam is converted by the quarter-wave plate  10  from a linearly-polarized (S-polarized) light mode into a circularly-polarized light mode, and then, is focussed by the objective lens  11  at the disk  1  which is, in this case, a CD. 
   The light beam reflected by the disk  1  is returned by the objective lens  11  to the quarter-wave plate  10 . As a result, the mode of the light beam is converted from the circularly-polarized light mode into a linearly-polarized (P-polarized) light mode. Then, the P-polarized light beam is reflected by the mirror  8  and is transmitted through the beam splitter  5 A. Then, the light beam transmitted through the beam splitter  5 A is reflected by the beam splitter  5 B, and further passes through the compound lens  12  to reach the photodetector  4 . 
   In the optical head apparatus of  FIG. 8 , since losses of the intensity of the outgoing and incoming light beams hardly occur in the beam splitters  5 B and  5 A, the S/N ratio would be increased. Additionally, as stated above, the manufacturing margin of the beam splitters  5 B and  5 A is large, so that the manufacturing cost thereof can be decreased. 
   In  FIG. 9 , which illustrates a modification of the optical head apparatus of  FIG. 8 , the locations of the laser diode  2 , the diffraction grating  6  and the coupling lens  7  of  FIG. 8  are exchanged with the locations of the laser diode  3 , the diffraction grating  6 ′ and the coupling lens  7 ′, respectively, of  FIG. 8 . In this case, the laser diode  2  generates an S-polarized 650 nm wavelength light beam for the DVD, while the laser diode  3  generates a P-polarized 780 nm wavelength light beam for the CD. Also, the beam splitters  5 B and  5 A of  FIG. 8  are replaced by the beam splitters  5 C and  5 D having the transmittivity characteristics of  FIGS. 5C and 5D , respectively. The operation of the optical head apparatus of  FIG. 9  is similar to that of the optical head apparatus of  FIG. 8 . 
   In  FIG. 10 , which illustrates a third embodiment of the optical head apparatus according to the present invention, the laser diode  2  generates an S-polarized 650 nm wavelength light beam for the DVD, while the laser diode  3  generates a P-polarized 780 nm wavelength light beam for the CD. 
   The beam splitter  5 D having the transmittivity characteristics of  FIG. 5D  and the beam splitter  5 C having the transmittivity characteristics of  FIG. 5C  are provided between the photodetector  4  and the disk  1 . In this case, a light beam reflected from the disk  1  is reflected by the beam splitter  5 C and is transmitted through the beam splitter  5 D to reach the photodetector  4 . 
   The S-polarized 650 nm wavelength light beam generated from the laser diode  2  is divided by the diffraction grating  6  into a zeroth order light component (transmission light component), a +1st order diffraction light component and a −1st order diffraction light component, which are converged by the coupling lens  7 . Most of the converged light beam of the coupling lens  7  is reflected by the beam splitters  5 D and  5 C. Then, the light beam reflected by the beam splitter  5 C is reflected by the mirror  8  to reach the collimator lens  9  for converting the light beam passed therethrough into a collimated light beam. The collimated light beam is converted by the quarter-wave plate  10  from a linearly-polarized (S-polarized) light mode into a circularly-polarized light mode, and then, is focussed by the objective lens  11  at the disk  1  which is, in this case, a DVD. 
   The light beam reflected by the disk  1  is returned by the objective lens  11  to the quarter-wave plate  10 . As a result, the mode of the light beam is converted from the circularly-polarized light mode into a linearly-polarized (P-polarized) light mode. Then, the P-polarized light beam is reflected by the mirror  8  and is reflected by the beam splitter  5 C. Then, the light beam reflected by the beam splitter  5 C is transmitted through the beam splitter  5 D, and further passes through the compound lens  12  to reach the photodetector  4 . 
   On the other hand, the P-polarized 780 nm wavelength light beam generated from the laser diode  3  is divided by the diffraction grating  6 ′ into a zeroth order light component (transmission light component), a +1st order diffraction light component and a −1st order diffraction light component, which are converged by the coupling lens  7 ′. Most of the converged light beam of the coupling lens  7 ′ is transmitted through the beam splitter  5 C. Then, the light beam transmitted through the beam splitter  5 C is reflected by the mirror  8  to reach the collimator lens  9  for converting the light beam passed therethrough into a collimated light beam. The collimated light beam is converted by the quarter-wave plate  10  from a linearly-polarized (P-polarized) light mode into a circularly-polarized light mode, and then, is focussed by the objective lens  11  at the disk  1  which is, in this case, a CD. 
   The light beam reflected by the disk  1  is returned by the objective lens  11  to the quarter-wave plate  10 . As a result, the mode of the light beam is converted from the circularly-polarized light mode into a linearly-polarized (S-polarized) light mode. Then, the S-polarized light beam is reflected by the mirror  8  and is reflected by the beam splitter  5 C. Then, the light beam reflected by the beam splitter  5 C is transmitted through the beam splitter  5 D, and further passes through the compound lens  12  to reach the photodetector  4 . 
   In the optical head apparatus of  FIG. 10 , since losses of the intensity of the outgoing and incoming light beams hardly occur in the beam splitters  5 D and  5 C, the S/N ratio would be increased. Additionally, as stated above, the manufacturing margin of the beam splitters  5 D and  5 C is large, so that the manufacturing cost thereof can be decreased. 
   In  FIG. 11 , which illustrates a modification of the optical head apparatus of  FIG. 10 , the locations of the laser diode  2 , the diffraction grating  6  and the coupling lens  7  of  FIG. 10  are exchanged with the locations of the laser diode  3 , the diffraction grating  6 ′ and the coupling lens  7 ′, respectively, of  FIG. 10 . In this case, the laser diode  2  generates a P-polarized 650 nm wavelength beam for the DVD, while the laser diode  3  generates an S-polarized 780 nm wavelength light beam for the CD. Also, the beam splitters  5 D and  5 C of  FIG. 10  are replaced by the beam splitters  5 A and  5 B having the transmittivity characteristics of  FIGS. 5A and 5B , respectively. The operation of the optical head apparatus of  FIG. 11  is similar to that of the optical head apparatus of  FIG. 10 . 
   In  FIG. 12 , which illustrates a fourth embodiment of the optical head apparatus according to the present invention, the laser diode  2  generates a P-polarized 650 nm wavelength light beam for a DVD, while the laser diode  3  generates a P-polarized 780 nm wavelength light beam for a CD. 
   The beam splitter  5 B having the transmittivity characteristics of  FIG. 5B  and the beam splitter  5 C having the transmittivity characteristics of  FIG. 5C  are provided between the photodetector  4  and the disk  1 . In this case, a light beam reflected from the disk  1  is reflected by the beam splitters  5 C and  5 B to reach the photodetector  4 . 
   The P-polarized 650 nm wavelength light beam generated from the laser diode  2  is divided by the diffraction grating  6  into a zeroth order light component (transmission light component), a +1st order diffraction light component and a −1st order diffraction light component, which are converged by the coupling lens  7 . Most of the converged light beam of the coupling lens  7  is transmitted through the beam splitter  5 B and is reflected by the beam splitter  5 C. Then, the light beam reflected by the beam splitter  5 C is reflected by the mirror  8  to reach the collimator lens  9  for converting the light beam passed therethrough into a collimated light beam. The collimated light beam is converted by the quarter-wave plate  10  from a linearly-polarized (P-polarized) light mode into a circularly-polarized light mode, and then, is focussed by the objective lens  11  at the disk  1  which is, in this case, a DVD. 
   The light beam reflected by the disk  1  is returned by the objective lens  11  to the quarter-wave plate  10 . As a result, the mode of the light beam is converted from the circularly-polarized light mode into a linearly-polarized (S-polarized) light mode. Then, the S-polarized light beam is reflected by the mirror  8  and is reflected by the beam splitter  5 C. Then, the light beam reflected by the beam splitter  5 C is transmitted through the beam splitter  5 B, and further passes through the compound lens  12  to reach the photodetector  4 . 
   On the other hand, the P-polarized 780 nm wavelength light beam generated from the laser diode  3  is divided by the diffraction grating  6 ′ into a zeroth order light component (transmission light component), a +1st order diffraction light component and a −1st order diffraction light component, which are converged by the coupling lens  7 ′. Most of the converged light beam of the coupling lens  7 ′ is transmitted through the beam splitter  5 C. Then, the light beam transmitted through the beam splitter  5 C is reflected by the mirror  8  to reach the collimator lens  9  for converting the light beam passed therethrough into a collimated light beam. The collimated light beam is converted by the quarter-wave plate  10  from a linearly-polarized (P-polarized) light mode into a circularly-polarized light mode, and then, is focussed by the objective lens  11  at the disk  1  which is, in this case, a CD. 
   The light beam reflected by the disk  1  is returned by the objective lens  11  to the quarter-wave plate  10 . As a result, the mode of the light beam is converted from the circularly-polarized light mode into a linearly-polarized (S-polarized) light mode. Then, the S-polarized light beam is reflected by the mirror  8  and is reflected by the beam splitter  5 C. Then, the light beam reflected by the beam splitter  5 C is transmitted through the beam splitter  5 B, and further passes through the compound lens  12  to reach the photodetector  4 . 
   In the optical head apparatus of  FIG. 12 , since losses of the intensity of the outgoing and incoming light beams hardly occur in the beam splitters  5 B and  5 C, the S/N ratio would be increased. Additionally, as stated above, the manufacturing margin of the beam splitters  5 B and  5 C is large, so that the manufacturing cost thereof can be decreased. 
   In  FIG. 13 , which illustrates a modification of the optical head apparatus of  FIG. 12 , the locations of the laser diode  2 , the diffraction grating  6 , the coupling lens  7  and the beam splitter  5 B of  FIG. 12  are exchanged with the locations of the laser diode  3 , the diffraction grating  6 ′, the coupling lens  7 ′ and the beam splitter  5 C, respectively, of  FIG. 12 . The operation of the optical head apparatus of  FIG. 13  is similar to that of the optical head apparatus of  FIG. 12 . 
   In  FIG. 14 , which is an enlarged plan view of the photodetector  4  of  FIGS. 6 through 13 , reference numerals  4   a  through  4   h  designate photodetecting portions. The zeroth order light beam from the laser diode  2  or  3  forms a light beam spot PO on the photodetecting portions  4   a ,  4   b ,  4   c  and  4   d . Also, the +1st order diffraction light beam from the laser diode  2  or  3  forms a light beam spot P +1  on the photodetecting portions  4   e  and  4   f . Further, the −1st order diffraction light beam from the laser diode  2  or  3  forms a light beam spot P −1  on the photodetecting portions  4   g  and  4   h.    
   In the photodetector  4 , a focus error signal V f  is calculated by using a known astigmatism method:
 
 V   f   =V ( 4   a )+ V ( 4   d )− V ( 4   b )− V ( 4   c )
 
   Also, a track error signal V t  for a recordable DVD or a CD is calculated by using a known push-pull method:
 
 V (4 a )+ V (4 b )− V (4 c )− V (4 d )− K ( V (4 e )+ V (4 g )− V (4 f )− V (4 h ))
 
   where K is a constant. 
   On the other hand, a track error signal V t  for a read-only DVD is calculated by using a known differential phase detection method, i.e., a difference in phase between V( 4   a )+V( 4   d ) and V( 4   b )+V( 4   c ). 
   Further, an RF information V r  is calculated by
 
 V   r   =V ( 4   a )+ V ( 4   b )+ V ( 4   c )+ V ( 4   d )
 
   In the above-described embodiments, the quarter-wave plate  10  is a broadband quarter-wave plate for the 650 nm wavelength and the 780 nm wavelength, which is disclosed in JP-A-5-100114. Also, the coupling lenses  7  and  7 ′ are provided to converge the outgoing light beams, which would enhance the light output of the objective lens  11 . Also, the locations of the coupling lenses  7  and  7 ′ are adjusted in order to adjust the light beams to be collimated. In this case, although the location of the collimator lens  9  can be adjusted, it is better to adjust the locations of the coupling lenses  7  and  7 ′, since the diameter of the collimator lens  9  is much larger than that of the coupling lenses  7  and  7 ′. 
   As explained hereinabove, according to the present invention, since losses of the intensity of the outgoing and incoming light beams hardly occur in the beam splitters, the S/N ratio would be increased. Additionally, since the manufacturing margin of the beam splitters is large, the manufacturing cost thereof can be decreased.