Apparatus for detecting a focus error and a tracking error of an optical head

An optical composite unit is provided to receive a light focused by a focusing lens for receiving a light reflected from an optical disc. The optical composite unit comprises a cylindrical lens and side-wedge prisms or side-cylindrical lenses which are provided on both sides of the cylindrical lens. The cylindrical lens has a height including a diameter of the focused light and a width narrower than the diameter, and an axis which is rotated by 45.degree. relative to a long axis of a cylindrical beam formed by the cylindrical lens. The side-wedge prisms or side-cylindrical lenses form side beams separated from the cylindrical beam. The cylindrical beam is for detecting a focus error, and the side beams are for detecting a tracking error.

FIELD OF THE INVENTION 
The invention relates to an apparatus for detecting a servo signal of an 
optical head, and more particularly to, an apparatus for detecting focus 
and tracking errors in accordance with one beam reflected from an optical 
disc. 
BACKGROUND OF THE INVENTION 
An optical disc apparatus has been developed to provide high density 
storage of information and high speed transfer rate of information. For 
instance, an optical disc apparatus to be adapted to a Hi-vision 
television system having an information amount which is five times greater 
than that of a conventional NTSC television system, and an external 
storage apparatus for computer peripheral having high speed and access 
properties of a magnetic disc used for a computer and a large capacity 
memory property of an optical disc have been intensively researched and 
developed. Disc apparatus for the next generation is expected for an 
optical disc apparatus in consideration of the necessity of high speed 
transfer rate and large capacity memory. Especially, it is required that 
the weight of an optical head itself is reduced to provide high speed 
access. 
The basic structure of an optical head is mainly contructed in one-beam 
system or three-beam system. It is said that each of them is advantageous 
in one aspect, and disadvantageous in the aspect. However, the one-beam 
system is considered to be advantageous in that no dispersion light is 
observed in recording information, although it depends on a sensitivity of 
a recording medium. In a recording and reproducing type of an optical 
head, the maximum output power of a laser which is available at the 
present time is approximately 35 mW. 
Conventional optical heads using one-beam system is described in the 
Japanese Patent Kokai Nos. 60-129943 and 62-88145. 
In the conventional optical heads, a first conventional apparatus for 
detecting a servo signal of an optical head comprises first and second 
four-divided optical sensors for focus and tracking error detections, and 
a second conventional apparatus for detecting a servo signal of an optical 
head comprises a six-divided optical sensor of focus and tracking error 
detections. The detail of the first and second conventional apparatus will 
be explained later. 
In the first and second conventional apparatus for detecting a servo signal 
of an optical head, however, there are disadvantages in that a focus error 
signal of high precision is difficult to be obtained, because a tracking 
error signal is leaked into the focus error signal, and stability and 
reliability of providing a six-divided optical sensor are low, because the 
divided photodetector regions are required to be adjusted by the order of 
.mu.m. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to provide an apparatus for 
detecting a servo signal of an optical head in which a focus error signal 
of high precision is obtained without influence of a tracking error 
signal. 
It is a further object of the invention to provide an apparatus for 
detecting a servo signal of an optical head in which a six-divided optical 
sensor having high stability and reliability is obtained with easy 
adjustment and small size. 
According to the invention, an apparatus for detecting a servo signal of an 
optical head, comprises: 
a focusing lens for focusing a light reflected from an optical disc storing 
information to provide a focused light; 
an optical composite unit for providing a cylindrical beam and side beams 
by receiving the focused light, the cylindrical beam having a long axis 
including a diameter of the focused light thus received and a short axis 
being narrower than the diameter, and the side beams being separated from 
the cylindrical beam; and 
an optical sensor for detecting focus and tracking error by separately 
receiving light beams based on the cylindrical beam and the side beams.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before explaining a method and an apparatus for detecting a servo signal of 
an optical head in the preferred embodiments according to the invention, 
the aforementioned conventional apparatus fore detecting a servo signal of 
an optical head will be explained. 
In FIG. 1, the first conventional apparatus for detecting a servo signal of 
an optical head comprises a laser 601, a collimator lens 602, beam 
splitters 603 and 604, an optical disc 605, an objective lens 606, a 
focusing lens 607, a beam splitter 608, a four-divided optical sensor 611, 
a cylindrical lens 612, a reflecting mirror 613, a four-divided optical 
sensor 614, and a magneto-optical detecting unit 615. 
In operation, a light emitted from the laser 601 is collimated by the 
collimator lens 602, and a P-polarization of the collimated light is 
transmitted through the beam splitters 603 and 604, and is focused on the 
optical disc 605 by the objective lens 606. A light reflected from the 
optical disc 605 is deflected to be supplied to the magneto-optical 
detecting unit 615 by the beam splitter 604 having a reflection efficiency 
of 100% for a S-polarization of the reflected light. The P-polarization of 
the reflected light is transmitted through the beam splitter 604 and is 
deflected to be supplied to the focusing lens 607 by the beam splitter 
603. The deflected light is focused by the focusing lens 607, and the 
focused light is divided to be supplied to the four-divided optical sensor 
611 as a beam 609 and to be deflected in the direction of the reflecting 
mirror 613 as a beam 610. The beam 609 is received by the four-divided 
optical sensor 611, and the beam 610 is transmitted through the 
cylindrical lens 621 and is reflected to be received in the four-divided 
optical sensor 614 by the reflecting mirror 613. 
FIG. 2 shows the four-divided optical sensor 611 having regions E and F 
connected to an adder 611A and regions G and H connected to an adder 611B, 
whose summation outputs of (E+F) and (G+H) are supplied to a differential 
amplifier 611C. 
In the differential amplifier 611C, a calculation of (E+F)-(G+H) is carried 
out to provide na output signal which is a tracking error signal. 
FIG. 3 shows the four-divided optical sensor 614 having regions A and B 
connected to an adder 614A and regions C and D connected to an adder 614B, 
whose summation outputs of (A+B) and (C+D) are supplied to a differential 
amplifier 614C. 
In the differential amplifier 614C, a calculation of (A+B)-(C+D) is carried 
out to provide an output signal which is a focus error signal. 
FIG. 4 shows the second conventional apparatus for detecting a servo signal 
of an optical head which comprises a laser 901, a collimator lens 902, a 
polarizing beam splitter 903, a quarter-wavelength plate 904, an objective 
lens 905, an optical lens 906, a convex lens 907, a double-wedge prism 
908, a cylindrical lens 911, and a six-divided optical sensor 912. 
In operation, a light emitted from the laser 901 is collimated by the 
collimator lens 902, and the collimated light is transmitted through the 
polarizing beam splitter 903 to be supplied to the quarter-wavelength 
plate 904, in which an input polarization is converted to an output 
circular polarization. The polarization-converted light is focused on the 
optical disc 906 by the objective lens 905. A light reflected from the 
optical disc 906 is deflected by the polarizing beam splitter 903, and the 
deflected light is focused by the convex lens 907. The focused light is 
divided into two beams 909 and 910 in the left and right directions by the 
double-wedge prism 908, and the divided beams 909 and 910 are diffused in 
an angle i the upper and lower directions by the cylindrical lens 911. 
Then, the two beams 909 and 910 are received in the six-divided optical 
sensor 912. 
As shown in FIGS. 5A to 5C, the six-divided optical sensor 912 comprises 
six regions P, Q, R, S, T and U, wherein the four regions P, Q, R and S 
are for a focus error, and the two regions T and U are for a tracking 
error. FIG. 5B shows the state in which the optical disc 906 is precisely 
focused by the objective lens 905, while FIG. 5A shows the state in which 
a distance between the objective lens 905 and the optical disc 906 is 
shorter than a precisely focused distance, and FIG. 5C shows the state in 
which a distance therebetween is longer tan the focused distance. 
In the error detection, a focus error signal FE is obtained as a result of 
calculating an equation (1) of output signals from the regions P, Q, R and 
S, and a tracking error signal TE is obtained as a result of calculating 
an equation (2) of output signals from the regions T and U, as set out 
below. 
EQU FE=(P+R)-(Q+S) (1) 
EQU TE=T-U (2) 
Next, an apparatus for detecting a servo signal of an optical head in the 
first preferred embodiment will be explained in FIGS. 6A and 6B. 
FIG. 6A shows the apparatus for detecting a servo signal of an optical head 
which comprises a focusing lens 104 for focusing a collimated beam 101 
which is a light reflected from an optical disc (not shown) storing 
information, a cylindrical lens 106 having an axis C which is rotated by 
45.degree. relative to a long axis of an elliptical beam 1061 including a 
diameter of a circular beam 105 formed by the focusing lens 104 (a cross 
section of the cylindrical lens 106 being shown in FIG. 6B), symmetrically 
arranged prisms 107 and 108 which are fixed on both sides of the 
cylindrical lens 106 in parallel to the long axis of the elliptical beam 
1061, a six-divided optical sensor 109 comprising a four-divided sensor 
110 having regions A, B, C and D for detecting a focus error, and two 
sensors 111 and 112 having regions E and F for detecting a tracking error 
signal. In this apparatus, the cylindrical lens 106 and the prisms 107 and 
108 constitute a composite optical member defined hereinafter "an iris 
prism 113" for providing the cylindrical beam 1061 for a focus error, and 
beams 1071 and 1081 for a tracking error. 
In the apparatus as described above, the collimated light 101 reflected 
from the optical disc (not shown) is represented by a light amount 
contour-line-pattern 102 and a light intensity indicating line 103 having 
two peaks which are generated by diffraction on the track groove of the 
optical disc. The diffracted light caused by the track groove will 
intermodulate into a focus error signal when the focus servo control is 
carried out. Therefore, this will cause a disturbance for the focus servo 
control, and it is desired that a light beam which is not affected by a 
track groove of the optical disc is obtained to overcome the disturbance. 
For this purpose the iris prism 113 is provided with the cylindrical lens 
106 which does not cover the side regions of the beam 105, and the prisms 
107 and 108 covering the side regions of the beam 105 which are not 
covered by the cylindrical lens 106, so that the cylindrical beam 1061 for 
a focus error signal which is not affected by a tracking error signal is 
obtained to be separated from the beams 1071 and 1081 for the tracking 
error signal. 
In operation, the elliptical beam 1061 is positioned in the vicinity of a 
focal point of the focusing lens 104 to be rotated by 90.degree., when it 
is in the vicinity of the photodetector 109, and it will be an 
approximately circular beam on the four-divided sensor 110 in accordance 
with astigmatism of the cylindrical lens 106, because the cylindrical lens 
106 has different focal lengths for x- and y-cordinate-axes. The regions A 
and B of the four-divided sensor are connected to the adders 110A, and C 
and D of the four-divided sensor 110 are connected to the adders 110B, 
and, outputs of the adders 110A and 110B are connected to a differential 
amplifier 110C, so that a focus error signal of (A+C)-(B+D) is obtained at 
an output of the differential amplifier 110C. 
The regions E and F of the optical sensors 111 and 112 are connected to a 
differential amplifier 115, so that a tracking error signal of (E-F) is 
obtained at an output of the differential amplifier 115. 
In FIG. 7C, the iris effect of the invention will be explained, wherein it 
is assumed that an iris member 114 having an iris 114a is provide between 
the focusing lens 104 and a cylindrical 106A is provided. Thus, a 
cylindrical beam 1061A is obtained in accordance with the provision of the 
iris 114a, and the cylindrical beam 1061A is supplied to the cylindrical 
lens 106A having an axis C which is rotated by 45.degree. relative to the 
cylindrical beam 1061A. Then, the focused beam 106a is obtained at the 
position for the six-divided optical sensor 109. 
This means that the side portions of the cylindrical lens 106A which are 
outside the dotted lines 106B and 106C are not necessary for the 
cylindrical beam 1061A, and, therefore, these side portions may be cut 
away. 
In FIG. 7B and 7C, the iris prism 113 will be illustrated more clearly. As 
explained before, the wedge prisms 107 and 108 are fixed on the both sides 
of the cylindrical lens 106, that is, the cut-away portions in FIG. 7A, in 
parallel to the long axis of the cylindrical beam 1061A. FIGS. 7D and 7E 
shows a top and a bottom of the iris prism 113 thus fabricated as seen in 
the directions indicated in FIG. 7C by the arrows 7D and 7E. 
In the first preferred embodiment, the wedge prisms 107 and 108 may be 
replaced by cylindrical lenses symmetrically divided on the left and right 
sides. 
An apparatus for detecting a servo signal of an optical head in the second 
preferred embodiment will be explained in FIG. 8A, wherein the reference 
numeral 301 indicates a cylindrical lens, 302 and 303 prisms, 304 an iris 
prism, 305 a focusing lens, 306 and 307, and 306a and 307a beams, and 308 
a six-divided optical sensor. 
In the second preferred embodiment, the iris prism 304 comprises the 
cylindrical lens 301 for detecting a focus error, and the wedge prisms 303 
and 304 for detecting a tracking error, and a cross-section of the iris 
prism 304 is shown in FIG. 8B. The light beams 306 and 307 are crossed 
after being transmitted through the wedge prisms 302 and 303 to be 
supplied to the regions E and F of the six-divided sensor 308, so that a 
tracking error signal is obtained by the calculation of (E-F). A focus 
error signal is obtained by the calculation of (A+C)-(B+D) in the regions 
A, B, C and D of the six-divided sensor 308. 
In the first and second preferred embodiments, the iris prisms 113 and 304 
may be molded by, for instance, polyolefine resin, poly methyle meta 
acrylete (PMMA) resin, glass mold, etc. 
An apparatus for detecting a servo signal of an optical head in the third 
preferred embodiment will be explained in FIG. 9, wherein the reference 
numeral indicates a focusing lens, 402 a cylindrical lens, 403 and 404 
cylindrical lenses or wedge prisms, and 405 a six-divided optical sensor. 
In the third preferred embodiment, a cylindrical axis of the cylindrical 
lens 402 is slat to Y-axis by 45.degree., and the split-type cylindrical 
lenses having shorter focal lengths than that of the cylindrical lens 402 
or wedge prisms 403 and 404 are provided on the both sides thereof. Focus 
and tracking error signals are obtained in the six-divided sensor 405 in 
the same manner as in the first and second preferred embodiments. 
An apparatus for detecting a servo signal of an optical head in the fourth 
preferred embodiment will be explained in FIG. 10, wherein the reference 
numeral 501 indicates a focusing lens, 502 a cylindrical lens, 503 and 504 
cylindrical lenses or wedge prisms, and 505 a lens, 506 to 509 beams, and 
510 a six-divided optical sensor. 
In the fourth preferred embodiment, the six-divided sensor 510 is arranged 
to be rotated to X-coordinate-axis by 45.degree., so that the beams 508 
and 509 are partially received by the regions E and F of the optical 
sensor 510, respectively, as shown in FIG. 10. 
In the first to the fourth preferred embodiments, the cylindrical lens may 
be in the form of a square having the same dimension in height and width 
or a rectangle having a height larger in dimension than a width. One 
example of the square cylindrical lens is shown in FIGS. 11A and 11B, 
wherein like reference numerals indicate like parts as used in the first 
to fourth preferred embodiments. 
Although the invention has been described with respect to specific 
embodiment for complete and clear disclosure, the appended claims are not 
to be thus limited but are to be constructed as embodying all modification 
and alternative constructions that may be occur to one skilled in the art 
which fairly fall within the basic teaching here is set forth.