1.  Field of the Invention
The present invention relates to an optical pickup system applied to compact disc players (CDPs), video disc players (VDPs), optical disc players (ODPs), multi-disc players (MDPs), etc.,  and more particularly to an optical pickup system capable of facilitating the arrangement and assembly of components to improve productivity thereof, and preventing malfunction caused by wavelength variation of lights emitted from a laser diode. 
2.  Description of the Prior Art
FIG.  1 illustrates a construction of one example of a conventional optical pickup system. 
Referring to FIG.  1, the optical pickup system includes a laser diode 11 being a light source, a grating 12, a hologram 13, an objective lens 14, a photodetector 15, and an optical disc 16 having optical information thereon. 
The grating 12 is arranged between the laser diode 11 and hologram 13 to make lights from the laser diode 11 be three diffracted lights of zero, +first and -first orders. 
At this time, the diffracted light of zero order is utilized to detect a focus error of the optical pickup system and read out the optical information recorded on the optical disc 16, and the diffracted lights of +first and -first orders are to detect a tracking error of the optical pickup system. 
The hologram 13 is placed between the grating 12 and objective lens 14 for transmitting the three diffracted lights having passed through the grating 12 toward the objective lens 14, or diffracting reflected lights incident from the optical disc 16 via the objective lens 14 to the photodetector 15 divided-by-six. 
The objective lens 14 between the hologram 13 and optical disc 16 allows the three diffracted lights transmitting the hologram 13 to focus onto the optical disc 16, or the light reflected from the optical disc 16 to be incident to the hologram 13. 
The photodetector 15 divided-by-six is partitioned into six split areas PD11 to PD16 as shown in FIG.  2, so that the lights diffracted by the hologram 13 focus onto respective split areas.  Thus, the diffracted light of zero order diffracted by the hologram 13 focuses onto the central first to fourth split areas PD11 to PD14 to be used for detecting the focus error of the optical pickup system and reading out the optical information recorded on the optical disc 16.  Also, the diffracted lights of +first and -first orders diffracted by the hologram 13 focus onto the fifth split area PD15 and sixth split area PD16 to be used for detecting the tracking error of the optical pickup system. 
The operation of the conventional optical pickup system having the above-mentioned construction will be described as below. 
The light from the laser diode 11 is classified into the three diffracted lights of zero order for the focus error detection and information reading, and of +first and -first orders for the tracking error detection via the grating 12. 
The three diffracted lights classified by the grating 12 focus on the optical disc 16 by means of the objective lens 14. 
The lights focusing onto the optical disc 16 are incident to the hologram 13 via the objective lens 14, in which the lights reflected from the optical disc 16 have the information recorded on the optical disc 16 and information required for detecting the focus error and tracking error. 
The lights reflected from the optical disc 16 are diffracted through the hologram 13 to focus onto the photodetector 15 divided-by-six.  The diffracted light of zero order focuses onto the first to fourth split areas PD11, PD12, PD13 and PD14, and the diffracted lights of +first and -first orders focus onto the fifth and sixth split areas PD15 and PD16, respectively. 
Therefore, in accordance with the pattern of the light focusing on respective split areas PD11 to PD16 of the photodetector 15 divided-by-six as shown in FIG.  3, the tracking error and focus error are detected while reading out the information recorded on the optical disc 16. 
More specifically, a tracking error signal TES is detected by the difference between the diffracted lights of +first and -first orders focusing on the fifth split area PD15 and sixth split area PD16 of the photodetector 15 divided-by-six, which is given by the following equation (1).  EQU TES=S15-S16 (1)
where reference numerals S15 and S16 respectively denote electrical signals of the lights focusing on the fifth split area PD15 and sixth split area PD16 of the photodetector 15 divided-by-six. 
It can be appreciated whether the diffracted light of zero order (main beam) correctly traces tracks of the optical disc or not in view of the relation that TES&gt;0 or TES&lt;0. 
A focus error signal FES is detected by the difference between the light focusing on the first and third split areas PD11 and PD13 and the light focusing on the second and fourth split areas PD12 and PD14, which is expressed as: EQU FES=(S11+S13)-(S12+S14) (2)
where reference numerals S11 to S14 respectively denote electrical signals of the lights focusing on the split areas PD11 to PD14 of the photodetector 15 divided-by-six. 
As shown in FIG.  3, the light focusing on the split areas PD11 to PD14 of the photodetector 15 divided-by-six varies in accordance with changing the distance between the optical disc 16 and objective lens 14.  When the objective lens 14 is normally spaced from the optical disc 16 (i.e., when the focus error signal FES equals zero), the light circularly focuses on the split areas PD11 to PD14 as shown in FIG.  3A. Meanwhile, as shown in FIGS.  3B and 3C, if the focus error appears due to a remote distance between the objective lens 14 and optical disc 16 (i.e., if the focus error signal FES is larger than zero), otherwise the focus error appears due to a close distance between the objective lens 14 and optical disc 16 (i.e., if the focus error signal FES is smaller than zero); the light focusing on the split areas PD11 to PD14 is transformed to have the elliptical shape from the circular shape. 
As can be noted in the above equations, the tracking error signal TES becomes zero and the focus error signal FES equals zero when neither the tracking error nor the focus error occur.  Consequently, the tracking error and focus error are corrected in conformity with the signals, thereby accurately reading out the information recorded on the optical disc. 
The information recorded on the optical disc 16 can be read out by means of the main beam, i.e., the diffracted light of zero order, focusing on the first to fourth split areas PD11 to PD14 of the photodetector 15 divided-by-six.  Wherefore, the variation of the amount of light focusing on the first to fourth split areas PD11 to PD14 of the photodetector 15 divided-by-six is checked for reading out the information.  Here, the optical information signal is defined by: EQU Optical Information Signal=S11+S12+S13+S14 (3)
In the conventional optical pickup system as described above, however, the wavelength of the laser beams is changed owing to an ambient such as surrounding temperature while the laser diode employed as the light source is operated, which in turn alters the diffraction angle of the diffraction elements such as the grating and hologram.  For this reason, a fault of wrongly inferring the presence of the focus error and tracking error is induced although neither focus error nor tracking error occurs. 
In other words, when the wavelength of the laser beam from the laser diode is 780 nm, the diffracted light normally focuses on a point "a" of the first to fourth split areas PD11 to PD14 as shown in FIG.  2.  On the other hand, the diffracted light focuses on a point "b" of the first split area PD11 in case of the wavelength of 770 nm, while focusing on a point "c" of the third split area PD13 in case of the wavelength of 790 nm.  As described above, the position to focus on the photodetector divided-by-six is relocated by the wavelength variation of the laser beam from the laser diode. 
Therefore, since the conventional optical pickup system shown in FIG.  1 which uses astigmatism cannot compensate for the wavelength variation of the laser beam from the laser diode, the optical pickup system possibly results in malfunction regardless of nonexistence of the focus error or tracking error. 
Furthermore, the wavelength varies for each laser diode fabricated, so that precise assembling of the hologram and photodetector divided-by-six involves a demanding operation originated from the predetermined wavelength of the laser beam from the laser diode to degrade productivity. 
FIG.  4 illustrates a construction of another example of the conventional optical pickup system proposed for solving the malfunction resulting from the wavelength variation of the laser beam of the optical pickup system shown in FIG.  1. 
Referring to FIG.  4, the optical pickup system includes a laser diode 41 used as a light source, a grating 42, a hologram element 43 divided-by-two, a collimator 44, an objective lens 45, a photodetector 46 divided-by-five, and an optical disc 47. 
The grating 42 is arranged between the laser diode 41 and collimator 44 to make light from the laser diode 41 be three diffracted lights of zero, +first and -first orders, and the three diffracted lights incident to the collimator 44 via the hologram element 43 divided-by-two. 
The collimator 44 disposed between the hologram element 43 divided-by-two and objective lens 45 allows the three diffracted lights passed through the grating 42 to be three parallel lights.  The diffracted light of zero order is utilized to detect a focus error of the optical disc 47 and read out optical information recorded on the optical disc 47, and the diffracted lights of +first and -first orders are to detect a tracking error of the optical disc 47. 
The objective lens 45 arranged between the collimator 44 and optical disc 47 focuses the three parallel lights passed through the collimator 44 onto the optical disc 47, or allows the lights reflected from the optical disc 47 to be parallel lights again. 
The hologram element 43 consisting of two holograms H41 and H42 is placed between the grating 42 and collimator 44 for transmitting the three diffracted lights having passed through the grating 42 toward the collimator 44 or for diffracting three parallel lights reflected from the optical disc 47 prior to passing through the collimator 44 at different angles to form six diffracted lights which then focus onto to the photodetector 46 divided-by-five. 
The photodetector 46 divided-by-five is partitioned into five split areas PD41 to PD45 as shown in FIG.  5, so that the diffracted light of zero order diffracted by the first hologram H41 of the hologram element 43 focuses onto the fourth split area PD44 to be used for reading out the optical information.  Also, the diffracted light of zero order diffracted by the second hologram H42 of the hologram element 43 focuses on the boundary plane of the second and third split areas PD42 and PD43 to be used for detecting the focus error.  The diffracted lights of +first and -first orders diffracted by the first and second holograms H41 and H42 of the hologram element 43 focus onto the first and fifth split areas PD41 and PD45 to be used for detecting the tracking error by the difference between the lights focusing on the split areas PD41 and PD45. 
The operation of the another example of the conventional optical pickup system having the above-mentioned construction will be described as below. 
The light from the laser diode 41 is classified into the three diffracted lights of zero order for the focus error detection and information reading, and of +first and -first orders for the tracking error detection via the grating 42. 
The three diffracted lights through the grating 42 are incident to the collimator 44 via the hologram element 43 to be three parallel lights which then focus onto the optical disc 47 by means of the objective lens 45. 
Meantime, the lights incident to the optical disc 47 are reflected to be parallel lights again via the objective lens 45 and collimator 44, and incident to the hologram element 43 divided-by-two. 
At this time, the lights reflected from the optical disc 47 have the information recorded on the optical disc 47 and information required for detecting the focus error and tracking error. 
The three parallel lights are incident to the hologram element 43 divided-by-two to be separated to have different angles one another by means of the first and second holograms H41 and H42.  Accordingly, the photodetector 46 divided-by-five receives six parallel lights.  Among the separated six parallel lights, the diffracted light of zero order separated by the hologram H41 focuses onto the fourth split area PD44 of the photodetector 46 divided-by-five, and the diffracted light of zero order separated by the second hologram H42 focuses on the boundary plane of the second and third split areas PD42 and PD43.  The diffracted lights of +first order separated by the first and second holograms H41 and H42 respectively focus on the upper and lower sides of the first split area PD41.  The diffracted lights of -first order separated by the first and second holograms H41 and H42 focus onto the fifth split area PD45. 
Therefore, in accordance with the pattern of the lights focusing on respective split areas PD41 to PD45 of the photodetector 46 divided-by-five as shown in FIG.  5, the tracking error and focus error are detected while reading out the information recorded on the optical disc 47. 
More specifically, a tracking error signal TES is detected by the difference between the diffracted lights focusing on the first split area PD41 and fifth split area PD45 of the photodetector 46 divided-by-five, which is expressed as: EQU TES=S41-S45 (4)
where reference numerals S41 and S45 respectively denote electrical signals of the lights focusing on the first split area PD41 and fifth split area PD45 of the photodetector 46 divided-by-five. 
It can be appreciated whether the diffracted light of zero order (main beam) correctly traces tracks of the optical disc or not in view of the relation that TES&gt;0 or TES&lt;0. 
A focus error signal FES is detected by the difference between the lights focusing on the second and third split areas PD42 and PD43, which is given by the following equation (5).  EQU FES=S42-S43 (5)
where reference numerals S42 and S43 respectively denote electrical signals of the lights focusing on the second and third split areas PD42 to PD43 of the photodetector 46 divided-by-five. 
As shown in FIG.  6, the amount of lights focusing on the second and third split areas PD42 to PD43 of the photodetector 46 divided-by-five vary in accordance with changing the distance between the optical disc 47 and objective lens 44.  Here, FIG.  6A shows the focusing pattern of the light on the second and third split areas PD42 and PD43 when the objective lens 44 is distant from the optical disc 47 to cause the focus error (i.e., when FES&gt;0).  Meanwhile, FIG.  6B shows the focusing pattern of the light on the second and third split areas PD42 and PD43 when the objective lens 44 is normally spaced from the optical disc 47 to have no focus error (i.e., when FES=0).  FIG.  6C shows the focusing pattern of the light on the second and third split areas PD42 and PD43 when the objective lens 44 nears to the optical disc 47 to cause the focus error (i.e., when FES&lt;0). 
As can be noted in the above equations, the tracking error signal TES becomes zero and the focus error signal FES equals zero when neither the tracking error nor the focus error occur.  Consequently, the tracking error and focus error are corrected in conformity with the signals, thereby accurately reading out the information recorded on the optical disc. 
The information recorded on the optical disc 47 can be read out by means of variation of the amount of the diffracted light of zero order (main beam) focusing on the second to fourth split areas PD42 to PD44 of the photodetector 46 divided-by-five.  Here, the optical information signal is defined by: EQU Optical Information Signal=S42+S43+S44 (6)
where reference numerals S42, S43 and S44 respectively denote electrical signals of the lights focusing on the second to fourth split areas PD42, PD43 and PD44 of the photodetector 46 divided-by-five. 
In the another example of the conventional optical pickup system illustrated in FIG.  4, however, the wavelength of the laser beam from the laser diode is changed owing to an ambient such as surrounding temperature while the laser diode employed as the light source is operated, which in turn alters the diffraction angle of the diffraction elements such as the grating and hologram element. 
Under the state that the boundary plane of the second and third split areas PD42 and PD43 of the photodetector 46 divided-by-five is provided to form a right angle as indicated by L1 of FIG.  5, when the laser beam of a predetermined wavelength is emitted from the laser diode 41 as denoted by a reference symbol "a" of FIG.  5, the light focuses on the boundary plane of the second and third split areas PD42 and PD43 of the photodetector 46 divided-by-five to normally operate the optical pickup system. 
However, as denoted by reference symbols "b" and "c" of FIG.  5, if the wavelength of the laser beam varies, the light does not focus on the boundary plane of the second and third split areas PD42 and PD43 of the photodetector 46 divided-by-five, but deviates from the boundary plane to place on the second or third split area PD42 or PD43.  For this reason, the optical pickup system results in malfunction regardless of nonexistence of the focus error or tracking error. 
In order to solve the error created by the wavelength variation of the laser beam from the laser diode, another example of the conventional optical pickup system shown in FIG.  4 has the photodetector divided-by-five provided in such a manner that the second and third split areas PD42 and PD43 of the photodetector 46 contact to form the boundary plane tilted by a minute angle .theta.  as indicated by L2 of FIG.  5. 
In other words, when the wavelength of the laser beam from the laser diode 41 is 780 nm, the light focuses on the point "a" at the boundary plane of the second and third split areas PD42 to PD43 of the photodetector 46 divided-by-five.  On the other hand, the light focuses on the point "b" at the boundary plane of the second and third split areas PD42 and PD43 in case of the wavelength of 770 nm, while focusing on the point "c" at the boundary plane of the second and third split areas PD42 and PD43 in case of the wavelength of 790 nm. 
Therefore, the boundary plane of the second and third split areas of the photodetector divided-by-five is obliquely provided by a predetermined angle in consideration of the wavelength variation of the laser beam from the laser diode, so that the focusing position of the light is relocated to the boundary plane of the second and third split areas when the wavelength of the laser beam is varied. 
By this structure, the focus error signal FES maintains the zero state in case of involving no focus error even though the wavelength of the laser beam from the laser diode is varied to thereby solve the problem caused by the wavelength variation. 
In the another example of the conventional optical pickup system shown in FIG.  4, the second and third split areas PD42 and PD43 of the photodetector 46 divided-by-five contact to provide the boundary plane tilted by the predetermined angle for the purpose of correcting the wavelength variation of the laser beam from the laser diode as illustrated in FIG.  5.  However, the angle at the boundary plane is so minutely provided that the focus error occurs because of the wavelength variation even with a slight error in the angle to result in malfunction of the optical pickup system. 
Furthermore, not only the fabrication of the photodetector divided-by-five for precisely adjusting the minute angle of the boundary plane, but also the arrangement of the hologram element and photodetector divided-by-five are very fastidious to significantly lower the productivity.