Optical pickup apparatus with single light source module

An optical pickup apparatus includes a light source module which emits a light beam, a beam splitter which reflects or transmits the light beam emitted from the light source module, an objective lens which focuses the light beam passing through the beam splitter onto a disc, and a photodetector which receives and detects the light beam reflected by the disc, wherein the light source module includes a first light source and a second light source that emit first and second light beams having different wavelengths and are formed into a single module. The optical pickup apparatus further includes a first grating which divides the first light beam emitted from the first light source into three beams of the first light beam and transmits the second light beam emitted from the second light source, and a second grating which transmits the first light beam emitted from the first light source and divides the second light beam emitted from the second light source into three beams of the second light beam. Since the first and second light sources are formed into a single light source module, the number of parts of the optical pickup apparatus can be reduced, and the efficiency of light is improved.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority of Korean Patent Application No. 2002-14708 filed Mar. 19, 2002 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical pickup apparatus, and more particularly, to an optical pickup apparatus in which the productivity and efficiency of light are improved by reducing the number of parts thereof.

2. Description of the Related Art

FIG. 1shows a conventional compatible optical pickup apparatus which records/reproduces data with respect to a disc. The optical pickup apparatus includes a first light holder105and a second light holder115. The first light holder105includes a first light source100which emits a light beam having a wavelength of about 650 nm and a first grating103which divides the light beam from the first light source100into three light beams to facilitate a servo tracking or a servo focusing. The first light source100and the first grating103are integrally formed. The second holder115includes a second light source110which emits a light beam having a wavelength of about 780 nm and a second grating113which divides the light beam from the second light source110into three light beams to facilitate a servo tracking or a servo focusing. The second light source115and the second grating113are integrally formed.

The first light holder105and the second light holder115are independently arranged at different positions. The first light source100is used for a disc107such as a DVD having a relatively thin thickness while the second light source100is used for a disc117such as a CD having a relatively thick thickness.

The light beam emitted from the first light source100is reflected by a first beam splitter120, passes through a second beam splitter123, and proceeds toward the relatively thin disc107. Then, the light beam is reflected by the relatively thin disc107, passes through the first and second beam splitters120and123, and is received by a photodetector130.

A reflection mirror125which changes paths of the light beams emitted from the first and second light sources100and110, a collimating lens127which makes an incident light beam parallel, and an objective lens129which focuses the incident light beam onto the disk107/117are provided on an optical path between the second beam splitter123and the disc107/117.

The light beam emitted from the second light source110passes through the second grating113, is sequentially reflected by the second beam splitter123and the reflection mirror125, and passes through the collimating lens127and the objective lens129, thus forming a light spot on the relatively thick disc117. Then, the light reflected by the relatively thick disc117passes through the objective lens129and the collimating lens127, is reflected by the reflection mirror125, passes through the second and first beam splitters123and120, and is received by the photodetector130.

Here, the first and second beam splitters120and123respectively split the light beams emitted from the first and second light sources100and110into approximately 50:50 and use only 50% of the received light, so that the efficiency of light is very low.

An astigmatism lens132is provided between the first beam splitter120and the photodetector130. The astigmatism lens132does not have a uniform curvature, but has different curvatures in vertical and horizontal directions to generate an astigmatism. The astigmatism lens132is arranged at an angle in a direction opposite to a direction in which the first beam splitter120is inclined, such that the size of the light beam focused on the photodetector130is increased by the interaction with the collimating lens129, and a coma aberration generated with respect to the light beam that passes through the first beam splitter120, is increased. Also, the curvatures of a lens surface in the vertical and horizontal directions are formed to be different from each other to generate the astigmatism. Here, a focusing error is detected in an astigmatism method by using the astigmatism generated as described above.

The light beams emitted from the first and second light sources100and110are respectively divided into three light beams by the first and second gratings103and113. A focusing error is detected by using the three light beams in a differential push-pull method with respect to the relatively thin disc107and in a three-beam method or push-pull method with respect to the relatively thick disc117. Since the differential push-pull method, the three-beam method, and the push-pull method are well-known techniques in the art, detailed descriptions thereof will be omitted herein.

Accordingly, CDs and DVDs can be compatibly recorded/reproduced by a single pickup apparatus having the above structure. However, since the conventional optical pickup apparatus has a separate light source and a separate grating for a CD and for a DVD, as well as two beam splitters, the number of parts increases. Therefore, the cost is raised, and portions of the pickup apparatus require numerous adjustments for an optical alignment. Accordingly, the productivity is lowered and a fraction defective is relatively high compared to a case having less number of parts. In other words, parts adopted in an optical pickup apparatus are designed according to a focal distance and optical length of a lens, and positions of the respective optical parts are determined accordingly thereafter. Here, an allowance in design or manufacture is unavoidably generated at the respective parts. Also, as the number of parts increases, allowance of each part increases. Thus, a light beam emitted from a light source in an above optical pickup apparatus does not accurately focus on a disc, thereby deteriorating a sensitivity of a signal thereof. Furthermore, where the light beam focuses on the disc, asymmetrically, a difference in the quantity of light is generated according to the position of the light beam so that the light beam focused on a photodetector becomes asymmetric and jitter increases.

Additionally, a motor which rotates a disc and chips may be presented in a layer where the optical pickup apparatus is installed. Thus, heat is generated from the motor or chips during a reproduction of a disc. In some cases, the internal temperature rises up to 60° C. even though a fan to cool the heat is installed. However, since parts inside the conventional pickup apparatus are attached by a UV bond, a portion attached by the UV bond is twisted or bulged by the high temperature. Accordingly, positions of the optical parts change and they are deviated from an optical axis so that the signal reproduction performance is lowered. Thus, it is necessary to reduce the portions, where the parts are attached by the UV bond, by reducing the number of parts in the optical pickup apparatus.

Also, a reflectance ratio of a light beam of a recordable disc is low due to its material, compared to a read-only disc. Accordingly, since the quantity of light that is reflected is small, it is disadvantageous in terms of detecting a signal, and the signal detection is affected more by noise. Thus, a light source used for a recordable disc must have a higher power than that of a light source used for a read-only disc. To increase optical power, as shown inFIG. 2B, a single mode laser light source is used. However, while the single mode laser light source may be effective in increasing the optical power, it is disadvantageous in terms of removing the effect by noise.

To reduce the noise, as shown inFIG. 2A, a multi-mode laser light source is used. A multi-mode laser light source having a high optical power for use as a laser light source having a wavelength of 780 nm has been developed. However, presently, it is difficult to manufacture a multi-mode laser light source having a high power for use as a laser light source having a wavelength of 650 nm. To solve the above problem, an HFM (high-frequency modulation), that is, a high frequency apparatus, is used for a multi-mode. However, where the high frequency apparatus is used, since a high frequency is dangerous to a human body, an electromagnetic shielding apparatus is necessarily added to protect the human body from the high frequency. This in turn increases the number of parts, and the dimension of an apparatus having the same is increased. Furthermore, where care is not taken to completely shield the high frequency, a user may be exposed to a very dangerous situation.

In addition, where a disc having a large birefringence is reproduced, since the conventional optical pickup apparatus does not have an apparatus to reduce a change in polarization according to a birefringence, a reproduction performance thereof is significantly deteriorated.

SUMMARY OF THE INVENTION

Accordingly, it is an aspect of the present invention to provide an optical pickup apparatus in which first and second light sources that emit light beams having different wavelengths are formed into a single module.

Another aspect of the present invention is to provide an optical pickup apparatus having a multi-purpose grating which is commonly used by first and second light sources.

Yet another aspect of the present invention is to provide an optical pickup apparatus having a polarizing beam splitter and a ¼ wave plate which improves the efficiency of light and provides an adaptability to a birefringence disc.

To achieve the above and/or other aspects of the present invention, there is provided an optical pickup apparatus for recording/reproducing with respect to a disc, comprising a light source module which emits a light beam, a beam splitter which reflects or transmits the light beam emitted from the light source module, an objective lens which focuses the light beam passing through the beam splitter onto the disc, and a photodetector which receives and detects the light beam reflected by the disc, wherein the light source module includes a first light source and a second light source that emit first and second light beams having different wavelengths and are formed into a single module. The optical pickup apparatus further comprises a first grating which divides the first light beam emitted from the first light source into three beams of the first light beam and transmits the second light beam emitted from the second light source, and a second grating which transmits the first light beam emitted from the first light source and divides the second light beam emitted from the second light source into three beams of the second light beam.

The first and second gratings may be integrally formed.

The optical pickup apparatus may further comprise a collimating lens which makes the light beam parallel, and is provided between the beam splitter and the objective lens.

The photodetector may comprise a first photodetector which detects the first light beam and a second photodetector which detects the second light beam.

The first photodetector may comprise a main photodetector having a main four-division structure and sub-photodetectors, each having a sub four-division structure, arranged at both sides of the main photodetector.

A depth of a pattern of the first grating may be 1.51 μm and a depth of a pattern of the second grating may be 1.23 μm.

To achieve the above and/or other asepcts of the present invention, there is provided another optical pickup apparatus for recording/reproducing with respect to a disc, comprising a light source module which emits a light beam, a polarizing beam splitter which selectively reflects or transmits the light beam emitted from the light source module according to a direction in which the light beam is polarized, a ¼ wave plate which converts a polarization of the light beam passing through the polarizing beam splitter, an objective lens which focuses the light beam passing through the ¼ wave plate onto the disc, and a photodetector which receives and detects the light beam reflected by the disc, wherein the light source module includes a first light source and a second light source that emits first and second light beams having different wavelengths and are formed into a single module. The optical pickup apparatus further comprises a first grating which divides the first light beam emitted from the first light source into three beams of the first light beam and transmits the second light beam emitted from the second light source, and a second grating which transmits the first light beam emitted from the first light source and divides the second light beam emitted from the second light source into three beams of the second light beam.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3shows an optical pickup apparatus according to an embodiment of the present invention. The optical pickup apparatus includes a light source module1having first and second light sources3and5, for light beams having different wavelengths, a beam splitter10which reflects or transmits a light beam emitted from the light source module1to change an optical path thereof, an objective lens15which focuses the light beam reflected by the beam splitter10on a relatively thin disc17or a relatively thick disc18, and a photodetector23which receives and detects the light beam reflected by the discs17and18.

In the light source module1, the first light source3and the second light source5are mounted on a mount2. The first light source3is, for example, a laser diode which emits a light beam having a 650 nm wavelength, and is used for the relatively thin disc17, for example, a DVD. The second light source5is, for example, a laser diode which emits a light beam having a 780 nm wavelength, and is used for the relatively thick disc18, for example, a CD. The first and second light sources3and5are arranged to be separated by, for example, about (110±2)μm.

A first grating7which divides the light beam emitted from the first light source3into three beams and a second grating8which divides the light beam emitted from the second light source5into three beams are provided on an optical path between the light source module1and the beam splitter10. The first grating7is manufactured to transmit over 90% of the light beam emitted from the second light source5. The second grating8is manufactured to transmit over 90% of the light beam emitted from the first light source3.

Where a pattern of a grating as shown inFIG. 4is formed, the following transmission equation can be obtained:

Here, m denotes the order of diffraction, T denotes the period of a diffraction pattern25, n0denotes the refractive index of air, n denotes the refractive index of a grating, d denotes the depth of the diffraction pattern25, and λ denotes the wavelength of a light source. The efficiency of diffraction of the conventional grating and the efficiency of diffraction of the first and second gratings7and8according to the present invention are calculated by using the Equation 1. Constant values used in the calculation of the efficiency of diffraction are shown in Table 1.

In the present invention, the efficiency of diffraction is changed by, for example, changing the depth d of the pattern of a grating. Here, the Equation 1 is calculated by substituting ½ for a, andFIG. 5shows a change in the quantity of light according to a change in the depth d of the pattern of the grating. An optimal depth d of the first and second gratings7and8adopted in the optical pickup apparatus according to the present invention can be determined from the graph ofFIG. 5.

Since the first and second gratings7and8are arranged on the optical path between the light source module1and the beam splitter10, a first light beam I and a second light beam11emitted from the first and second light sources3and5, respectively, all pass through the first and second gratings7and8. Here, the first and second gratings7and8selectively divide light corresponding thereto into three beams and transmit 90% or more with respect to the other light.

In detail, where the first light source3, for example, a laser diode for a DVD, is used, the first light beam I passing through the first grating7is divided into three beams, that is, a −1storder light beam, a 0thorder light beam, and a +1storder light beam in a ratio of 1:5:1. Where the divided first light beam I passes through the second grating8, 90% or more, for example, 95%, of the light passes through the second grating8.

Where the second light source5, for example, a laser diode for a CD, is used, 90% or more, for example, 95%, of the second light beam11passes through the first grating7. The second light beam11passing through the second grating8is divided into three beams, that is, a −1storder light, a 0thorder light, and a +1storder light in a ratio of 1:5:1.

The depth d of the pattern of a grating satisfying the above conditions is obtained with reference to the graph ofFIG. 5.

According to Table 2, the depth d of the pattern of the first grating7and the depth d of the pattern of the second grating8may be set to 1.51 μm and 1.23 μm, respectively.

The diffraction efficiency ratio according to the depth d of a pattern of a grating, where conventional gratings for a CD and a DVD are arranged along different optical paths, is shown in Table 3 to compare with the grating of the present invention.

In the present invention, a structure that is different from a conventional optical pickup is presented, as the first and second light sources3and5are formed into a single module, and the corresponding first and second gratings7and8are arranged along the same optical path. Here, although the first and second gratings7and8are independently formed, it is understood that the first and second gratings7and8may be integrally formed (as shown inFIG. 6). Thus, the number of parts in the optical pickup apparatus of the present invention can be reduced by forming light sources for light beams having different wavelengths into a single module and integrating gratings into a single multi-purpose grating.

Each of the light beams corresponding to the first and second light sources3and5is divided into three beams, reflected by the disc17or18, and detected by the photodetector23. By using the detected three beams, a tracking error detection and a focus error detection are performed, which will be described below.

The light beams reflected by the beam splitter10proceed to the objective lens15. A collimating lens13may be provided, as shown inFIG. 3, before the objective lens15to make the light beams parallel. The light beams are focused by the objective lens15onto the disc17or18. Accordingly, the optical pickup apparatus according to the present invention can be compatibly used for both the relatively thin disc17, for example, a DVD, and the relatively thick disc18, for example, a CD.

The light beams reflected by the disc17or18, passing through the objective lens15and the collimating lens13, and transmitted by the beam splitter10are converted into electrical signals by the photodetector23.

Here, a concave lens or an astigmatism lens20can be provided, as shown inFIG. 3, between the beam splitter10and the photodetector23. Where the concave lens or the astigmatism lens20is not provided, the size of a light spot formed on the photodetector23can be adjusted by controlling the thickness of the beam splitter10. Where the concave lens or the astigmatism lens20is provided, the size of a light spot can be adjusted by changing the focal distance of the concave lens or the astigmatism lens20.

FIG. 6shows an optical pickup apparatus according to another embodiment of the present invention. The optical pickup apparatus includes an optical module31having a first light source33and a second light source35, which emit light beams having different wavelengths and are integrally formed into a single module, a polarizing beam splitter40which transmits or reflect a light beam emitted from the light source module31according to a polarization direction, a ¼ wave plate41which changes a polarization state of the light beam whose optical path is changed by the polarizing beam splitter40, an objective lens45which focuses the light beam passing through the ¼ wave plate41onto the disc17or18, and a photodetector43which receives the light beam reflected by the disc17or18and passing through the objective lens45, the ¼ wave plate41, and the polarizing beam splitter40.

The optical pickup apparatus according to the present invention can be compatibly used, as a compatible optical pickup apparatus, for discs having different thickness. In other words, the present optical pickup apparatus can be used for both discs17and18formed of a relatively thin disc, for example, a DVD, and a relatively thick disc, for example, a CD.

The optical module31is manufactured by incorporating the first light source33which emits a first light beam I and the second light source35which emits a second light beam11having a different wavelength from that of the first light beam I into a single package. The first light source33is, for example, a laser diode which emits a light beam having a wavelength of, for example, 650 nm to read the relatively thin disc17. The second light source35is, for example, a laser diode which emits a light beam having a wavelength of, for example, 780 nm to read the relatively thick disc18.

A multi-purpose grating36is provided on an optical path between the light source module31and the polarizing beam splitter40. The multi-purpose grating36can be used for both of the first and second light sources33and35.

FIG. 7shows an enlarged view of the multi-purpose grating36. A first pattern37which divides the light beam emitted from the first light source33into three beams and simultaneously transmits 90% or more of the light beam emitted from the second light source35is formed on a surface36aof the multi-purpose grating36. A second pattern38which transmits 90% or more of the light beam emitted from the first light source33and simultaneously divides the light beam emitted from the second light source35into three beams is formed on another surface36bof the multi-purpose grating36. The above conditions are similar to those adopted in the first and second gratings7and8ofFIG. 3. Here, it is a characteristic that the multi-purpose grating36is integrally formed of the first grating7and the second grating8. By forming an integral multi-purpose grating, the number of parts used in the optical pickup apparatus can be further reduced.

Referring back to Table 2, the depth d of the first pattern37is 1.51 μm and the depth d of the second pattern38is 1.23 μm. Alternatively, the opposite formation is possible. Thus, the gratings for a CD and a DVD can be formed integrally.

Referring back toFIGS. 6 and 7, where the light beam emitted from the first light source33passes through the surface36aof the multi-purpose grating36, the light beam is divided into three beams, that is, into −1storder, 0thorder, and +1storder light beams in a ratio of 1:5:1. Then, 90% or more, for example, 95% or more, of the above divided light beams is transmitted while passing through the another surface36bof the grating36.

Where the light beam emitted from the second light source35passes through the surface36aof the multi-purpose grating36, 90% or more, for example, 95% or more, of the above light beam is transmitted. Then, where the above light beam passes through the another surface36bof the grating36, the light beam is divided into three beams, that is, into −1storder, 0thorder, and +1storder light beams in a ratio of 1:5:1.

A light beam divided into three beams by the multi-purpose grating36is incident upon the polarizing beam splitter40. The polarizing beam splitter40reflects or transmits the incident light according to a direction in which the incident light is polarized. For example, the polarizing beam splitter40transmits a P polarized beam and reflects an S polarized beam and vice versa. By using the above feature, most of the light beam emitted from the light source module31can be reflected to proceed toward the ¼ wave plate41. Here, the polarizing beam splitter40can be formed to reflect 95% or more of the light beam emitted from the light source.

Where the light beam passes through the ¼ wave plate41, the state of polarization of the incident light changes. For example, where it is assumed that a light beam of an S polarization is emitted from the light source module31, as the light beam passes through the ¼ wave plate41, the polarization of the light beam changes to a circular polarization. The light beam having the changed polarization passes through the objective lens45and is focused on the disc17or18. As the light beam reflected by the disc17or18passes through the ¼ wave plate41, the polarization of the light beam changes to a P polarization. Thus, 95% or more of the S polarization beam is first reflected by the polarizing beam splitter40. Then, 95% or more of the P polarization beam reflected by the disc17or18, and proceeding backward, passes through the polarizing beam splitter40and proceeds toward the photodetector43. Where the P polarization beam is emitted from the light source module31, the same method can be adopted. Thus, by adopting the polarizing beam splitter40and the ¼ wave plate41, most of the light beam emitted from the light source module31can be used as an effective light so as to improve the efficiency of light.

The light beam emitted from the light source module31is incident upon the disc17or18as a circular polarization beam by the ¼ wave plate41, regardless of whether the light beam is a P polarization or S polarization. Since the circular polarization beam has a stronger overcoming force to a birefringence than a linearly polarized beam, the ¼ wave plate41assists in increasing a corresponding force to a disk having a large birefringence.

A reflection mirror42, and a collimating lens43which makes the light beam parallel can be provided between the ¼ wave plate41and the disc17or18, as shown inFIG. 6. Here, by using the reflection mirror42to change an optical path of the light beam perpendicular, the overall thickness of the optical pickup apparatus can be reduced, so as to produce a slim optical pickup apparatus. Also, a lens47such as a concave lens or astigmatism lens can be provided between the polarizing beam splitter40and the photodetector50, as shown inFIG. 6.

As described above, since the light source module31and the multi-purpose grating36are provided in the optical pickup apparatus, the overall number of parts of the optical pickup apparatus can be reduced. Also, the efficiency of light can be maximized by using the polarizing beam splitter40and the ¼ wave plate41. Thus, while an additional high frequency apparatus is adopted by a conventional single mode laser diode, for a DVD, to increase the quantity of light, such a high frequency apparatus is not needed in the present invention. Since the efficiency of light of a light source is maximized in the present invention, where a multi-mode laser diode is used, a sufficient quantity of light can be secured.

The light beam reflected by the disc17or18passes through the objective lens45, the ¼ wave plate41and the polarizing beam splitter40, and is received by a photodetector50.

The following description about the photodetector50can be commonly applied to photodetectors23ofFIG. 3.

In the light source modules1and31, since the first light sources3and23and the second light sources5and35are separated by about (110±2)μm from each other, respectively, the optical axes are matched. Thus, each of the photodetectors23and50includes a first photodetector corresponding to the first light source3/23and a second photodetector corresponding to the second light source5/35.

FIGS. 8A through 8Eshow examples of the photodetector23/50adopted in the present invention.

As shown inFIG. 8A, a first photodetector51is formed of a main detector51ahaving a main four-division structure and a pair of sub-photodetectors51band51c, each having a sub four-division structure, arranged at both sides of the main photodetector51a. Three beams diffracted by the gratings7and8, or the grating36are detected by the first photodetector51so as to perform a servo tracking and a servo focusing. The first photodetector51can be used for a DVD.

Here, a differential push-pull (DPP) method can be used to detect a tracking error. A tracking error detection signal (TES) is obtained as follows:
TES=((A+D)−(B+C))−G(((J1+J4)(I1+I4))−((J2+J3)+(I2+I3)))  (Equation 2)

In the above equation, G is a gain applied to a detection signal of the sub-photodetectors51band51cto detect an optimal tracking error signal, since the quantity of light of the sub-photodetectors51band51cis smaller than that of the main photodetector51a. The gain G can be determined according to a ratio of the quantity of light between the 0thorder light beam and the 1storder light beam which are diffracted. A signal is amplified by the differential push-pull method.

On the other hand, a differential astigmatism method can be used to detect a focusing error, and a focusing error detection signal (FES) is obtained as follows:
FES=((A+C)−(B+D))+G(((J1+J3)(I1+I3))−((J2+J4)+(I2+I4)))  (Equation 3)

Here, for example, where the first photodetector51is applied to a DVD-RAM disc, since pits are formed in a groove and land portion, during a focusing, noise is generated not only to a main beam received by the main photodetector51a, but also to a side beam received by the sub-photodetectors51band51c. It is desired that the focusing is not influenced by the pits. A phase difference of 180° is formed between the main beam and the side beam, and the effect by the pits can be reduced by adding the main beam to the side beam.

Referring back toFIG. 8A, a second photodetector52is formed of a second main photodetector52ahaving a second four-division structure and second sub-photodetectors52band52carranged at both sides of the second main photodetector52a. The second photodetector52can be used for a CD.

A tracking error detection signal (TES) and a focusing error detection signal (FES) are obtained as follows in a three beam method and n astigmatism method, respectively:
TES=L−K(Equation 4a)
FES=((E+H)−(F+G))  (Equation 4b)

InFIG. 8B, a first photodetector53is formed of a main photodetector53ahaving a main four-division structure and sub-photodetectors53band53c, each having a sub four-division structure, arranged at both sides of the main photodetector53a. The configuration of the first photodetector53is the same as that of the first photodetector51ofFIG. 8A, and the tracking error detection signal and the focusing error detection signal are detected in the same manner using the equations 2 and 3.

A second photodetector54is formed of a second main photodetector54ahaving a second four-division structure and second sub-photodetectors54band54c, each having two-division structure, arranged at both sides of the second main photodetector54a. A focusing error detection signal (FES) and a tracking error detection signal (TES) are obtained by the following equations:
FES=(E+G)−(F+H)  (Equation 5a)
TES=((E+H)−(F+G))−G((L1+K1)−(L2+K2))  (Equation 5b)

InFIG. 8C, a first photodetector55is formed of a main photodetector55ahaving a four-division structure and sub-photodetectors55band55c, each having a two-division structure, arranged at both sides of the main photodetector55a. A second photodetector56is formed of a second main photodetector56ahaving a second four-division structure and second sub-photodetectors56band56carranged at both sides of the second main photodetector55a. The second photodetector56has the same configuration as that of the second photodetector52ofFIG. 8A. For the first photodetector55, a focusing error detection signal (FES) and a tracking error detection signal (TES) are obtained by the following equations:
TES=((A+D)−(B+C))−G((J1+I1)−(J2+I2))  (Equation 6a)
FES=(A+C)−(B+D)  (Equation 6b)

InFIG. 8D, a first photodetector57is formed of a main photodetector57ahaving a four-division structure and sub-photodetectors57band57c, each having a two-division structure, arranged at both sides of the main photodetector57a. A second photodetector58is formed of a second main photodetector58ahaving a second four-division structure and second sub-photodetectors58band58c, each having a second two-division structure, arranged at both sides of the second main photodetector58a. Here, the first photodetector57can obtain a focusing error detection signal and a tracking error detection signal by using the Equations 6a and 6b, while the second photodetector58can obtain a focusing error detection signal and a tracking error detection signal by using the Equations 5a and 5b.

InFIG. 8E, a first photodetector59is formed of a main photodetector59ahaving a main four-division structure and sub-photodetectors59band59c, each having a sub four-division structure, arranged at both sides of the main photodetector59a. A second photodetector60is formed of a second main photodetector60ahaving a second main four-division structure and second sub-photodetectors60band60c, each having a second sub four-division structure, arranged at both sides of the second main photodetector60a. The first and second photodetectors59and60have substantially the same structure. A tracking error detection signal can be obtained in a differential push-pull method, and a focusing error detection signal can be obtained in a differential astigmatism method. The first photodetector59can perform a tracking error detection and a focusing error detection by using the Equations 2 and 3.

The second photodetector60can obtain a tracking error detection signal (TES) and a focusing error detection signal FES as follows:
TES=((E+H)−(F+G))−G(((L1+L4)+(K1+K4))−((L2+L3)+(K2+K3)))  (Equation 7a)
FES=((E+G)−(F+H))+G(((L1+L3)+(K1+K3))−((L2+L4)+(K2+K4)))  (Equation 7b)

The present invention includes a first photodetector corresponding to a first light source and a second photodetector corresponding to a second light source. The first photodetectors51,53,55,57, and59shown inFIGS. 8A through 8Ecan be used for, for example, a DVD, and the second photodetectors52,54,56,58, and60can be used for, for example, a CD.

An optical pickup apparatus according to the present invention includes a light source module in which first and second light sources that emits light beams having different wavelengths are formed into a single module. Accordingly, the number of parts of the optical pickup apparatus can be drastically reduced. By reducing the number of parts, the productivity is improved, and the reliability is improved as the deterioration of performance, due to a deterioration of a bonding of the parts during a high temperature operation, is reduced.

Additionally, the present invention includes gratings suitable for the light source module of two wavelengths, or an integral grating for both the first and second light beams. This further reduces the number of parts of the optical pickup apparatus. Thus, the structure of the present optical pickup apparatus is simplified, the assembly thereof is made easy and the production cost thereof is reduced.

Also, by providing first and second photodetectors which correspond to the first and second light sources, and using a differential push-pull method and a differential astigmatism method, a signal for a recordable disc having a low reflectance is amplified, thereby allowing a servo tracking and a servo focusing to be efficiently performed.

Furthermore, by maximizing the efficiency of light using a polarizing beam splitter and a ¼ wave plate, where a multi-mode laser diode is used as a light source, recording/reproduction is possible with respect to a disc such as a DVD-RW, DVD-RAM, DVD-R and DVD+RW which are recordable discs having a low reflectance. Also, since a light beam reflected back to the light source module from the polarizing beam splitter hardly exists, a laser light source is stable. The laser light source can be driven with a particular quantity of light. Either a single mode laser diode or a multi-mode laser diode can be used as a light source.

In addition, since a circular polarization light beam is input to a disc by the ¼ wave plate, a corresponding force to a disc having a large birefringence can be improved.