reference will now be made in detail to the embodiments , examples of which are illustrated in the accompanying drawings , wherein like reference numerals refer to the like elements throughout . the embodiments are described below to explain the present invention by referring to the figures . fig3 is a diagram schematically illustrating an optical system 20 having an objective lens used in a general optical pickup apparatus according to an embodiment of the present invention . as shown in fig3 , the optical system 20 comprises a light source 21 , a diffraction grating 22 , a polarization beam splitter 23 , a collimating lens 24 , a reflective mirror 25 , a ¼ wave plate 26 , an objective lens 27 , an astigmatic lens 28 , and a photodetector unit 29 . here , according to an embodiment of the present invention , the light source 21 is an infrared laser light source having a wavelength of approximately 780 nm , red laser light source having a wavelength of approximately 650 nm , or a blue laser light source having a wavelength of approximately 405 nm when used for cd , dvd , and bd , respectively . however , the light source 21 of the present invention is not limited hereto and may vary as necessary . the diffraction grating 22 comprises a fine grating pattern formed on the surface thereof and diffracts light emitted from the light source 21 to form a zero - order diffracted main beam and ± 1st or more order diffracted sub - beams , the sub - beams having smaller amount of light than the main beam . the polarization beam splitter 23 allows light to pass through or to reflect depending on a polarization direction of the incident light . the collimating lens 24 collimates light , progressing from the light source 21 . the reflective mirror 25 bends a progressive direction of light from the light source 21 in order for the light to be incident on a disk d through the objective lens 27 . the ¼ wave plate 26 changes a polarization direction of the light reflected from the disk d after incident on the disk d . the objective lens 27 focuses the main beam and the sub - beam to form a main spot and sub spot on the track of the disk d . the astigmatic lens 28 introduces astigmatism of approximately 45 ° direction into light reflected by the polarization beam splitter 23 toward the photodetector unit 29 . the photodetector unit 29 comprises a number of photodetectors to measure a respective amount of light of the main beam and the sub - beam which is reflected from the disk d . for example , each photodetector of the photodetector unit 29 is a quad - detector . however , the present invention is not limited hereto , therefore , the photodetector unit 29 may be other photodetectors such as six divided or eight divided photodetector , for example . in an optical pickup apparatus in which all cd , dvd , hd - dvd , and bd are compatible , two optical systems having the above structure are installed respectively for cd / dvd / hd - dvd and bd . in this case , two objective lenses 27 can be installed in one actuator . however , when two objective lenses 27 are installed in a tangential direction of the disk d , problems described above may arise . in order to solve these problems , in the optical system comprising objective lenses 27 which are not arranged at the center line of the disk d , the diffraction grating 22 is simply rotated in the conventional optical pickup apparatus . however , while the diffraction grating 22 was rotated , an additional process to make the optical axis of the light source and the axis of diffraction grating 22 coincide was not performed , and thus , a difference in the amount of light generated among the sub - beams separated from the diffraction grating 22 occurs . fig4 is a diagram of the diffraction grating 22 according to an embodiment of the present invention . in fig4 , in the diffraction grating 22 , two diffraction regions 22 a and 22 b having grating patterns inclined in an opposite direction to each other are alternately arranged in parallel . as shown in fig4 , the two diffraction regions 22 a and 22 b are also inclined at a predetermined angle . the inclined angle of the diffraction regions 22 a and 22 b can be determined to correspond with a track direction , for example , at a center track t 2 . according to an embodiment of the present invention , a width of the diffraction regions 22 a and 22 b , a pitch of the grating patterns in the diffraction regions 22 a and 22 b , and a number of the grating patterns are determined so that the amount of light of the sub - beams are same when the centers of the light source 21 and the diffraction grating 22 are arranged to coincide with each other . for example , according to an embodiment of the present invention , when a blue laser diode having a wavelength of 405 nm is used as the light source 21 , the pitch of the grating patterns in the diffraction regions 22 a and 22 b is approximately 18 μm and the number of the grating patterns is approximately 6 . however , the present invention is not limited hereto and may vary , as necessary . fig5 a is a diagram illustrating positions of a main beam and a sub - beam generated from a conventional diffraction grating 22 ′ and fig5 b is a diagram illustrating positions of main beam and sub - beam generated from the diffraction grating 22 according to an embodiment of the present invention . fig5 a and 5b are illustrated to explain the principle that the amount of light of the sub - beams becomes the same by the diffraction grating 22 according to an embodiment of the present invention . the diffraction regions in fig5 a and 5b are horizontal for convenience but are substantially inclined along a track direction . as shown in fig5 a , position relation between a main beam m , sub - beams s 1 and s 2 , both generating from the conventional diffraction grating 22 ′, and diffraction regions 22 a ′ and 22 b ′ are illustrated . although the center of the main beam coincides with a boundary of the diffraction regions 22 a ′ and 22 b ′ in the diffraction grating 22 ′, the center of the first sub - beam s 1 and the center of the second sub - beam s 2 are disposed in relative positions in different diffraction regions of each other on the diffraction grating 22 ′. accordingly , the area a of the first diffraction region 22 a ′ in the first sub - beam s 1 and the area b of the second diffraction region 22 b ′ in the first sub - beam s 1 are different from each other and the area a of the first diffraction region 22 a ′ in the second sub - beam s 2 and the area b of the second diffraction region 22 b ′ in the second sub - beam s 2 are different from each other . subsequently , the amounts of light of the two sub - beams s 1 and s 2 are different from each other . therefore , an ac is generated in each sub push - pull signal for the first sub - beam s 1 and the second sub - beam s 2 , and thus , an ac is generated in the whole spp signal . if the main beam m in fig5 a does not coincide with the boundary of the diffraction regions 22 a ′ and 22 b ′ of the diffraction grating 22 ′, the area a of the first diffraction region 22 a ′ in the first sub - beam s 1 and the second sub - beam s 2 cannot be the same as the area b of the second diffraction region 22 b ′ in the first sub - beam s 1 and the second sub - beam s 2 , and thus , an ac is generated in the spp signal . as shown in fig5 b , the center of the first sub - beam s 1 and the center of the second sub - beam s 2 are both disposed at the boundary of a diffraction region 22 a and a second diffraction region 22 b on the surface of the diffraction grating 22 according to an embodiment of the present invention . therefore , the area a of the first diffraction region 22 a and the area b of the second diffraction region 22 b in the first sub - beam s 1 are same and the area a of the first diffraction region 22 a and the area b of the second diffraction region 22 b in the second sub - beam s 2 are same . therefore , when the center of the diffraction grating 22 according to an embodiment of the present invention accurately coincides with the optical axis of the light source 21 , the areas a and b of the first and second diffraction regions 22 a and 22 b in the first and second sub - beams s 1 and s 2 divided by the diffraction grating 22 are exactly same as each other . consequently , the amounts of light of the two sub - beams s 1 and s 2 are the same . according to the principle described above , when the center of the diffraction grating 22 and the optical axis of the light source 21 are arranged to accurately coincide with each other , an ac in the spp signal can be reduced by rotating the diffraction grating 22 and then accurately arranging the main beam and the sub - beams along a track direction . accordingly , since a direction of the track is changed according to a radial direction of the disk d , a rotational angle of the diffraction grating 22 may be determined corresponding to a track direction at the center track t 2 of the disk d in order to minimize an error . on the other hand , in the conventional optical system in which the objective lens is not arranged on the center line of the disk d described above , light emitted from the light source 21 which is incident on the disk d is only considered . that is , light reflected from the disk d and incident on the photodetector unit 29 is not considered . therefore , as illustrated in fig6 a , the main beam m and the sub - beams s 1 and s 2 which are received by three quad - photodetectors 29 a , 29 b , and 29 c are inclined as much as a direction of the track at which the objective lens 27 is positioned . consequently , a baseball pattern of each beam , an astigmatism direction , and division angles of the photodetectors 29 a , 29 b , and 29 c are off each other and thus it is hard to accurately determine a tracking error signal . in order to improve such problems , both an astigmatic lens 28 and a photodetector unit 29 which are indicated by a box a in fig3 , are rotated at a predetermined angle so that the quad - photodetectors 29 a , 29 b , and 29 c and beams received by the quad - photodetectors 29 a , 29 b , and 29 c coincide with each other . here , rotation of the astigmatic lens 28 is to introduce a 45 ° astigmatism into baseball pattern of the main beam m and the sub - beams s 1 and s 2 generated from the disk d . here , the direction of the tracks varies according to the position of the objective lens 27 based on the radial direction of the disk d as described above . therefore , in order to minimize an error , the astigmatic lens 28 and the photodetector unit 29 are rotated based on the center track t 2 of the disk d . consequently , spp signals not having an ac can be obtained . then , as illustrated in fig7 , a tracking error signal without deviation according to a radial direction can be obtained . for example , in the conventional optical pickup apparatus , the deviation of the tracking error signal according to a radial direction is approximately 40 % as illustrated in fig2 . however , in the present invention , the deviation of the tracking error signal according to a radial direction is approximately 5 % as illustrated in fig7 . as described above , in the optical recording / reproducing system according to the present invention which is compatible with respect to an optical recording medium having various kinds such as cd / dvd / bd / hd - dvd , even two objective lenses can be installed along a tangential direction in one actuator , and accurate tracking error signals can be obtained using either two optical systems . therefore , accurate tracking servo is possible for various kinds of optical recording medium . although a few embodiments have been shown and described , it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention , the scope of which is defined in the claims and their equivalents .