Optical pickup system

An optical pickup system for generating information signals recorded on a recording medium includes a semiconductor laser generating source for generating and emitting a laser beam, an object lens for condensing the generated laser beam for an optical disc and receiving the laser beam reflected by an optical disc, a beam separator for changing a forwarding direction of the laser beam reflected by an object lens reflected by the optical disc, a prism for directing a portion of the beam to a pair of photo detector units provided on the prism and for forwarding the remainder of said laser beam in the same direction it was travelling when first incident on the prism, a condensing lens for condensing the laser beam reflected by the prism, and a focusing error compensating means for compensating two signals to be zero by an actuator.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates in general to an optical pickup system for 
generating information signals recorded on a recording medium. 
2. Description of the Invention 
In a conventional optical recording and reproducing system, a three beam 
detecting system for an optical disc as an optical recording unit has been 
widely used. But, the three beam detecting system has problems in that it 
requires a precise grating as well as highly precise processing and 
assembling of a deck which supports a pickup system. Further, the three 
beam detecting system involves additional problems, even though the 
operational process is securely performed, in that it requires a 
diffraction grating which slightly reduces the light quantity of the main 
beam. In addition, in the case of mounting the diffraction grating on a 
player, it is difficult to achieve both the proper mounting position and 
mounting angle. 
SUMMARY OF THE INVENTION 
Accordingly, the primary object of the present invention is to eliminate 
the problems of the prior art described hereinbefore. 
Another object of the present invention is to provide an optical pickup 
system not requiring a precise grating. A further object of the present 
invention is to provide enhanced performance irrespective of the precision 
of assembly and manufacture of a deck which supports the pickup system. 
To achieve the above objects of the present invention, the optical pickup 
system of the invention includes a semiconductor laser generating source 
for generating and emitting a laser beam, an object lens for condensing 
the generated laser beam onto an optical disc and for receiving the laser 
beam reflected by the optical disc, a beam separator for changing a 
forwarding direction of the laser beam reflected by the object lens to a 
prism, a prism for directing a portion of the laser beam to photo 
detectors and for forwarding the remainder of the beam in the same 
direction it was travelling when first incident on the prism, a condensing 
lens for condensing the laser beam from the prism, and a photo detecting 
unit for detecting the laser beam condensed by the condensing lens.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention will be more readily understood by reference to the 
accompanying drawings which illustrate the optical pickup system according 
to the present invention. Referring to FIG. 1, a laser beam generated by a 
semiconductor laser generating source 1 is condensed through a beam 
separator 3 and an object lens 2 as a spot on an information recording 
medium such as an optical disc D. Further, the condensed position can be 
adjusted by controlling the object lens 2 by an actuator 9. 
Meanwhile, the laser beam reflected by the optical disc D is received onto 
the beam separator 3 through the object lens 2. The reflected laser beam 
passing the objects lens 2 has its direction changed by the beam separator 
3. There is provided a Pechan prism 4 having an interface, at least an 
upper and lower inclined surface, so as to direct a portion of the laser 
beam to photo detectors and for forwarding the remainder of the beam in 
the same direction it was travelling when first incident on the prism. In 
addition, there are also provided photo detecting units 7 and 8 on a 
central portion of the lower and upper inclined surfaces of the prism 4, 
respectively. On the right side of the prism 4, a condensing lens 5 is 
provided for condensing the laser beam coming from the prism 4 and, 
further, a photo detecting unit 6 is provided for detecting the laser beam 
condensed at a predetermined distance. The photo detecting unit 6 is 
divided into two parts, PD3 and PD4 (FIG. 4B). 
The optical pickup system according to the present invention, as described 
above, has the distance L between the semiconductor laser generating 
source 1 and the object lens 2 the same as the distance L1+L2 between the 
object lens 2 and the focus position P6 as illustrated in FIG. 2. 
Meanwhile, the prism has .theta..sub.1 =45.degree. where .theta..sub.1 is 
measured between an interface in the prism and an incidence side, and 
.theta..sub.1 '=45.degree. where .theta..sub.1 ' is measured between the 
interface and a reflection side. Also, .theta..sub.4 =67.degree.30' where 
.theta..sub.4 is measured between the upper inclined surface and the 
interface, .theta..sub.3 =22.degree.30' where .theta..sub.3 is measured 
between the lower inclined surface and the interface; and .theta..sub.2 
=112.degree.30' where .theta..sub.2 is measured and between the surface of 
incidence and a side of the prism, as illustrated in FIG. 3. 
As illustrated in FIGS. 3 and 4A, photo detecting units 7 and 8, having a 
diameter "d" of effective light receiving surface, are provided on the 
lower and upper inclined surfaces and of the prism, respectively. In 
addition, photo detecting unit 6 is provided at position P9 which is just 
in front of a position of which the laser beam is condensed by the 
condensing lens 5. 
In FIGS. 1 and 2, L denotes a distance between the semiconductor laser 
generating source 1 and the object lens 2, L1 a distance between the photo 
detecting unit 7 and the object lens 2, L2 a distance between the photo 
detecting unit 7 and the focus point P6, L3 a distance between focus point 
P6 and the photo detecting unit 8, L4 a distance between the photo 
detecting unit 8 and the condensing lens 5, L5 a distance between the 
condensing lens 5 and the photo detecting unit 6, and P1 denotes the 
position of the semiconductor laser light source. 
The operational effects of the optical pickup system will be described 
hereinbelow. 
As illustrated in FIG. 1, the laser beam generated by the semiconductor 
laser generating source 1 is condensed by the object lens 2 through the 
beam separator 3, and the condensed beam is reflected by the optical disc 
D. 
Thus, the reflected beam is condensed again by means of the object lens 2 
and is received on the prism 4 after having its direction changed by the 
beam separator 3 provided between the object lens 2 and the semiconductor 
laser generating source 1. 
The laser beam received on the prism 4 is divided by an interface so that a 
portion thereof proceeds along a predetermined path, as illustrated by 
arrows in FIG. 3, and is incident on photo detector units provided on the 
prism. The remainder of the beam is forwarded to the condensing lens 5 and 
is condensed on the photo detecting unit 6 by means of the condensing lens 
5. 
The photo detecting units 7 and 8 detect the moving state of the optical 
disc D, and the photo detector unit 6 detects the information recorded on 
the optical disc D. 
A more detailed description of a procedure for detecting the moving state 
and the recorded information will be given hereinbelow. 
After the laser beam generated by the semiconductor laser generating source 
1 is condensed on the optical disc D by means of the object lens 2, and 
after the laser beam, which has been reflected by the disc D is again 
condensed by means of the object lens 2, the position of the focus will be 
at a position which is a distance L from the object lens 2. Thus, the 
position of the focus, if beam separator 3 were not disposed in the beam 
path, would be at the generating point of the semiconductor laser 
generating source 1. 
With the beam separator 3 in place and disposed between the object lens 2 
and the semiconductor laser generating source 1, the beam which has been 
reflected by disc D is condensed to a focus at P6. 
As illustrated in FIG. 1, when the prism 4 is provided at a position such 
that L=L1+L2, the laser beam will be condensed at position P6. After being 
condensed at position P6, the laser beam will begin diverging again after 
the prism 4, as illustrated in FIG. 2. 
The laser beam coming from the prism 4 is condensed again by means of the 
condensing lens 5, and the condensed beam is detected by means of the 
photo detecting unit 6. 
When the optical disc D is present at the precise position the beam is to 
be condensed, the laser beam is condensed on P6, but if wobble occurs 
during the rotation of the optical disc D, or if a difference in thickness 
occurs during manufacturing the optical disc there will be a focusing 
error because of not having been exactly condensed on the optical disc D, 
so that the laser beam will be condensed before or behind P6. 
Thus, when the distance between the optical disc D and the object lens 2 
increases, as illustrated in FIG. 7A, the condensing point A moves to A' 
so that the laser beam is condensed to P located before P6, as illustrated 
in FIG. 7B. When the distance between the optical disc D and the object 
lens 2 decreases, the condensing point A moves to A", so that the laser 
beam is condensed to P" located behind P6. 
As described above, when the laser beam is condensed before or behind P6, 
the light quantities which the photo detecting units 7 and 8 (mounted on 
the prism 4) detect will be changed. If the condensing point is before P6, 
as illustrated in FIGS. 5A and 7A, the beam spot size becomes narrower 
than an effective receiving surface of the photo detecting unit 7, so that 
the photo detecting unit 7 is available to detect the intensity of the 
laser beam without any loss. But, as illustrated in FIGS. 5B and 7B, if 
the laser beam is condensed behind P6 the photo detecting unit 8 mounted 
at P7 can't detect a part of the laser beam since the size of the laser 
beam becomes larger than the effective light receiving surface of the 
detector. 
Thus, when the condensing point is at P6, the size of the laser beam which 
the photo detecting units 7 and 8 detect will be the same. Thus, since 
both the size of the laser beam and the effective light receiving surface 
will be the same, the intensity of the laser beams which are detected by 
the two photo detecting units 7 and 8 will be the same. 
Therefore, the focusing error can be detected by comparing the outputs of 
the two photo detecting units 7 and 8. 
For example assuming the detecting signal of the photo detecting unit 7 is 
S1, and the detecting signal of the photo detecting unit 8 is S2, the 
focusing error signal (FES) is S1-S2. If FES is zero, there will not occur 
any focusing error. In the case of FES&gt;0, the distance between the object 
lens 2 and the optical disc D is farther than desired. In case of FES&lt;0, 
the distance between the object lens 2 and the optical disc D is closer 
than desired. 
Thus, the focusing error can be adjusted by adjusting the object lens 2 
using the actuator 9 according to the FES. 
In addition, the optical information signals detecting the information 
recorded on the optical disc D can be obtained by adding the two signals 
S1 to S2. 
Thus, one formula may be obtained: "Optical information signals =S1+S2." 
Meanwhile, in the case of obtaining information recorded on the optical 
disc D, there will be a tracking error when the laser beam deviates from 
the track of the optical disc D in addition to the above described 
focusing error. 
The function of detecting the tracking error is achieved with the photo 
detecting unit 6 mounted on P9, as described above. 
The case of there being no tracking error is illustrated in FIGS. 8 and 9A, 
wherein the outputs of detectors PD3 and PD4, (which consist of the photo 
detecting units 6) are equal. As illustrated in FIGS. 8 and 9B, when the 
laser beam is incident off track-center toward the center of the optical 
disc D, the signal S4 grows larger than the signal S3. 
In addition as illustrated in FIGS. 8 and 9C, when the laser beam is 
incident off track-center outward from the center of the optical disc D, 
the signal S3 grows larger than the signal S4. 
Therefore, assuming the tracking error signal (TES) is S3-S4, if TES is 
0(zero), there will not have occurred any tracking error. However, if 
TES&gt;0, the laser beam is incident off track-center outwardly from the 
center of the optical disc D. And if TES&lt;0, the laser beam is incident off 
track-center inwardly from the center of the optical disc. Thus, the 
tracking error can be compensated by means of adjusting the actuator 9. 
Accordingly, in the optical pickup system according to the present 
invention, the optical information recorded on the optical disc D can be 
generated without error by compensating for the focusing error and 
tracking error when obtaining the optical information signals. 
As described above, the optical pickup system according to the present 
invention can be used for Compact Disc Player (CDP) , Video Disc Player 
(VDP), Optical Disc Driver(ODD), Mini Disc Player (MDP), and any kinds of 
Mini Disc which use the optical pickup system. 
Thus, the optical pickup system according to the present invention does not 
require the conventional precise grating as previously used in 3-beam 
detecting system. In addition, detecting the focusing error by means of 
the beam size by using the prism achieves higher precision than the 
conventional three beam system with its expected manufacturing effects.