Side-spot focus apparatus for optical disc recording and/or playback system

Focus error due to drift from a specified value of the pointing angle of a focus laser light beam with respect to the reflective surface of an optical disc is self-compensated by employing two light beams derived from the same focus laser that have a substantially constant angular displacement therebetween. Any change due to drift in the specified value of the pointing angle of one light beam is accompanied by a substantially equal and opposite change due to drift in the specified value of the pointing angle of the other light beam.

This invention relates to an improved side-spot focus apparatus for an 
optical disc recording and/or playback system. 
As known in the art, an optical disc is capable of extremely high density 
storage of information. An example of an optical disc recording and 
playback system is disclosed in U.S. Pat. No. 4,142,209, which issued Feb. 
27, 1979 to Hedlund, et al. Such a system employs a relatively high-power 
laser beam for recording information on the optical disc and a relatively 
low-power beam for reading information from the optical disc. In both 
cases, it is essential that the laser beam be precisely focused on the 
reflective surface of the optical disc. Further, precise focusing must be 
maintained despite relative motion of the disc with respect to the record 
and/or read beams which occur in addressing different regions of the disc. 
In order to maintain this precise focusing, optical disc recording and/or 
playback systems incorporate some type of focus apparatus. One particular 
type of focus apparatus, known in the art, is side-spot focus apparatus. 
Side-spot focus apparatus employs an auxiliary laser light beam, which is 
directed toward the reflective surface of the optical disc at a specified 
pointing angle. Present-day side-spot focus beam apparatus ignores the 
fact that the pointing angle of the auxiliary laser beam drifts somewhat 
in response to such factors as aging of the laser, temperature of the 
laser, etc. This drift causes a small but still significant focus error 
with which present-day side-spot focus beam apparatus cannot deal. The 
advantage of the improved side-spot focus apparatus of the present 
invention is that it is self-compensating for focus error due to drift in 
the value of the pointing angle of the laser beam. 
More specifically, the improved side-spot focus apparatus of the present 
invention comprises first means including a focus laser for deriving first 
and second separate collimated light beams respectively traveling toward a 
light-reflective surface of the optical disc in a first beam direction and 
in a second beam direction. Each of the first and second beam directions 
have a projected component thereof oriented parallel to a given plane, 
which given plane is oriented perpendicular to the light-reflective 
surface. The projected component of the first beam direction in the given 
plane is oriented at a first oblique angle .alpha..sub.I with respect to 
and on a given side to the normal of a disc surface. The projected 
component of the second beam direction in the given plane is oriented at a 
second oblique angle .alpha..sub.II with respect to and on the side 
opposite from the given side of said normal to said disc surface. Further, 
.alpha..sub.I has a non-constant value equal to C plus or minus a pointing 
error .DELTA. and .alpha..sub.II has a non-constant value C minus or plus 
the pointing error .DELTA., but the sum of .alpha..sub.I +.alpha..sub.II 
has a substantially fixed constant value equal to 2C. 
The improved side-spot focus apparatus of the present invention further 
comprises second means including an imaging lens having its optical axis 
oriented substantially parallel to said given normal, the imaging lens 
being situated between the first means and the disc surface in the path of 
the first and second beams for illuminating the reflective surface of the 
disc with incident image first and second beams. Each of the incident 
image first and second beams is focused on the disc surface only when the 
distance between the surface and the imaging lens has a given value. 
The improved side-spot focus apparatus of the present invention still 
further comprises third means including a set of photodetectors, 
responsive to the reflective positions of each of the first and second 
beams reflected from the disc surface and passed back through the imaging 
lens, for deriving an error signal that is dependent on the difference 
between the actual value of the distance from the imaging lens to the disc 
surface and the given value thereof and is independent of the pointing 
error .DELTA.. 
Finally, the improved side-spot focus apparatus of the present invention 
comprises fourth means including a lens mover responsive to the error 
signal for moving the imaging lens in the direction to minimize the 
difference between the actual value and given value of the distance from 
the imaging lens to the disc surface.

Referring to FIG. 1, there is shown optical disc 100 having a 
light-reflective surface 102. Situated at a distance q from 
light-reflective surface 102 is imaging lens 104. (Note: For this 
description, lens 104 is a finite conjugate lens; a lens with real image 
and object planes.) Originally collimated beam of light 105, from focus 
laser 108, is preferably passed through lens 107 to derive incident narrow 
beam 106 having a crossover in object plane 109 of imaging lens 104. Beam 
106 is inclined, in the plane of the paper, at a relatively small oblique 
angle .alpha. with respect to the normal 110 to the reflective surface 102 
of optical disc 100. Focus laser 108 could be either further to the right 
or further to the left so long as the pointing angle .alpha. of beam 106 
has a certain specified value and the crossover of beam 106 remains in 
object plane 109. 
After refraction by imaging lens 104, beam 106 is incident on the 
light-reflective surface 102 at an angle of incidence .beta. with respect 
to normal 110. In FIG. 1, surface 102 is shown as coinciding with image 
plane 112 of imaging lens 104. This is the desired relative position of 
disc 100 with respect to imaging lens 104. That is, if the distance q has 
that given value shown in FIG. 1, image plane 112 will coincide with 
reflective surface 102 and incident beam 106 will be focused on surface 
102. 
Reflection from surface 102 forms reflected narrow beam 114 having an angle 
of reflection .beta. equal to the angle of incidence .beta. of beam 106 
incident on reflective surface 102. However, after being passed back 
through imaging lens 104, which transforms the value of the angle of 
reflection of beam 114 from .beta. back to .alpha., assuming that 
reflective surface 102 coincides with imaging plane 112 as indicated in 
FIG. 1, the reflected beam 114 is imaged to a small spot that impinges on 
a pair of photodetectors included in disc error detector 116, since the 
photodetectors are effectively positioned in object plane 109, as shown in 
FIG. 1. (In practice, a reflected-beam beam splitter, not shown, is 
situated before detector 116, so as to permit the physical location of 
detector 116 to be removed from the path of incident beam 106). The 
distance to the disc error detector is adjusted only by changes in the 
length of the long conjugate design of the lens 104. 
The structure of disc error detector 116, shown in FIG. 1a, includes a pair 
of contiguous photodetectors A and B, which derive respective signals A 
and B that correspond in value to the intensity of illumination of the 
respective photodetectors by reflective narrow focused beam 114 (which has 
a narrow beam width 118). Respective signals A and B are applied as inputs 
to an error circuit 120 that produces as an output error signal 122 having 
a value corresponding to A-B. 
Lens mover 124, which is mechanically coupled to imaging lens 104 by link 
126, has error signal 122 applied as a control input thereto. Lens mover 
124, which may comprise a loud speaker drive, is capable of moving imaging 
lens 104 with sub-micrometer precision to the left toward optical disc 100 
or to the right away from optical disc 100 in acccordance with the 
polarity of the applied A-B error signal 122. This causes the value of the 
distance q from the reflective surface 102 of optical disc 100 to imaging 
lens 104 to be substantially maintained at that given value at which image 
plane 112 coincides with surface 102, despite small changes in the 
absolute position of optical disc 100. 
Any sub-micrometer increase .delta. in the value of the imaging distance q 
results in reflective surface 102 of disc 100 moving to the left of image 
plane 112 (as indicated by 100L and 102L in FIG. 1b). Any sub-micrometer 
decrease -.delta. in the value of the imaging distance q causes reflective 
surface 102 of disc 100 to move to the right of image plane 112 (as 
indicated by 100R and 102R in FIG. 1b). Further, as indicated in FIG. 1b, 
the beam position of the reflected beams varies in accordance with the 
value of .delta., although the angle of incidence of incident beam 106 is 
independent of the value of .delta.. In particular, as shown in FIG. 1b, 
the position of reflected beam 114L (reflected from the reflective surface 
102L to the left of image plane 112) and the position of reflected beam 
114R (reflected from the reflective surface 102R to the right of image 
plane 112) can move respectively below and above the position of reflected 
beam 114 from image plane 112 (when the image distance Q has its proper 
given value shown in FIG. 1). This occurs because the image of the input 
spot near the detector moves longitudinally along the axis. The centioid 
of the beam at the detector, thus, is displaced by 
.perspectiveto.2.alpha.M.sup.2 .delta., where M is the magnification of 
the system. 
As discussed above in connection with FIGS. 1 and 1a, the reflected beam, 
after passing back through imaging lens 104, is incident on photodetectors 
A and B, shown in FIG. 1a. Beam width 118 of the reflected beam is 
symetrically situated with respect to photodetectors A and B only when the 
reflected beam originates from image plane 112. Because of the inverting 
effect of an imaging lens, lower positioned reflected beam 114L 
illuminates photodetector A with more intensity than it does photodetector 
B and upper positioned reflected beam 114R illuminates photodetector B 
with more intensity than it does photodetector A. Lens mover 124, in 
response to the A-B error signal on the output 122 applied thereto from 
disc error detector 116, mechanically moves imaging lens 104 through 
mechanical link 126 in a direction to minimize the value of .delta. (shown 
in FIG. 1b). Thus, the distance q between optical disc 100 and imaging 
lens 104 is restored to its given value at which reflective surface 102 
coincides with image plane 112. ln this manner, the spot of light incident 
on reflective surface 102 is retained in focus. 
The proper operation of the prior-art side-spot focus apparatus shown in 
FIGS. 1, 1a and 1b is predicated on the assumption that the pointing angle 
.alpha. has a specified fixed value that remains constant under all 
conditions. However, the fact is that the value of the pointing angle 
.alpha. drifts slightly (.+-..DELTA.) from its specified value in 
accordance with such factors as the temperature of the focus laser, aging 
of the focus laser, etc. The effect of this drift is to shift the position 
of the reflected beam impinging on photodetectors A and B, thereby 
resulting in a spurious error signal. The spurious error signal causes 
erroneous changes in the value of the distance q, thereby defocusing the 
light spot incident on the reflective surface of the optical disc. 
Modification of the structure and operation of the the prior art side-load 
apparatus shown in FIGS. 1, 1a and 1b in the manner shown in FIGS. 2, 2a, 
2b and 2c provides an improved side-spot focus apparatus that self 
compensates for the drift error .DELTA. in the value of the pointing 
angle. More particularly, the improved side-spot focus apparatus of the 
present invention includes optical disc 100, imaging lens 104, lens mover 
124 and mechanical link 126 arranged substantially as shown in FIG. 1. 
However, focus laser 208 differs from focus laser 108 of FIG. 1 by 
deriving from a single laser, two angularly-displaced incident narrow 
collimated beams 206.sub.I and 206.sub.II (only the center-lines of which 
are shown in FIG. 2), rather than a single incident narrow beam 106. The 
first beam direction 206.sub.I (which corresponds in function to single 
beam 106 of FIG. 1) has a projected component thereof oriented parallel to 
the plane of the paper. Similarly, the second beam direction of beam 
206.sub.II has a projected component thereof oriented parallel to the 
plane of the paper. As indicated in FIG. 2, the projected component of the 
first beam direction of beam 206.sub.I in the plane of the paper is 
oriented at a first oblique angle .alpha..sub.I with respect to and on a 
given side of normal 110 to the surface of optical disc 100. The optical 
axis of imaging lens 104 is preferably substantially coincident with 
normal 110. In any case, the optical axis of imaging lens 104 is oriented 
substantially parallel to normal 110. 
The projected component of the second beam direction of beam 206.sub.II in 
the plane of the paper is oriented at a second oblique angle 
.alpha..sub.II with respect to and on the opposite side from the given 
side of normal 110. 
Focus laser 208 includes such means as beam splitters, mirrors, etc. for 
deriving beams 206.sub.I and 206.sub.II with an angular displacement 
therebetween of substantially .alpha..sub.I +.alpha..sub.II having a fixed 
constant value equal to 2C. C is the specified angular value of each of 
the upper pointing angle .alpha..sub.I and the lower pointing angle 
.alpha..sub.II. However, in fact, each of pointing angles .alpha..sub.I 
and .alpha..sub.II is likely to have a pointing error .DELTA.. Therefore, 
.alpha..sub.I has a non-constant value equal to C.+-..DELTA. and 
.alpha..sub.II has a non-constant value C.+-. .DELTA. (since the sum of 
the two angles has a substantially fixed constant value equal to 2C). 
Incident beam 206.sub.I performs in the same manner as single incident beam 
106, discussed above, to provide reflected beam positions of the type 
shown in FIG. 1b. It is essential that the reflected beam derived from 
incident beam 206.sub.I be separated from incident beam 206.sub.II in 
order to remain distinct. Therefore, as shown in FIG. 2a, respective beams 
206.sub.I and 206.sub.II, emerging from focus laser 208, are displaced 
from one another in a direction perpendicular to the plane of the paper in 
the view shown in FIG. 2. Specifically, in the preferred embodiment shown 
in FIG. 2, each of beams 206.sub.I and 206.sub.II actually lies in a 
separate plane that is parallel to the plane of the paper. However, this 
particular arrangement is not essential in order to maintain beams 
206.sub.I and 206.sub.II distinct. Alternatively, for instance, each of 
beams 206.sub.I and 206.sub.II could lie in a separate plane that is 
angularly displaced from the plane of the paper so long as that beam 
travels in a direction toward disc 100 and has a projected component lying 
in the plane of the paper, in the view shown in FIG. 2. Otherwise, angular 
displacement of the beams with respect to the plane of the paper, in the 
view shown in FIG. 2, is immaterial to the operation of the present 
invention. 
FIG. 2b, which corresponds to FIG. 1b, shows the reflected beam position 
derived from beam 206.sub.II. As indicated in FIG. 2b, reflected beam 
position 214L, 214 and 214R, derived from incident beam 206.sub.II 
traveling in an opposite direction (up) from the direction (down) from the 
reflected beam position (shown in FIG. 1b) derived from incident beam 
206.sub.I. 
Disc error detector 216, shown in FIG. 2c, includes a set of photodetectors 
230 and error circuit 220. The set of photodetectors 230 includes a first 
pair of photodetectors A and B and a second pair of photodetectors C and 
D. The first pair of photodetectors A and B, which correspond with 
photodetectors A and B of FIG. 1a, is situated in the path of the 
reflected narrow beam derived from incident beam 206.sub.I. The second 
pair of photodetectors C and D is situated in the path of the reflected 
narrow beam derived from incident beam 206.sub.II. Photodetectors A, B, C 
and D derive respective signals A, B, C and D therefrom, each of which has 
a value in accordance with the intensity of light impinging on that one of 
the set of photodetectors 230 with which that signal corresponds. Each of 
signals A, B, C and D is applied as an individual input to error circuit 
220. As indicated in FIG. 2c, error circuit 220 derives an error signal 
222, which is applied to lens mover 124, having a value in correspondence 
with (A-B)+(C-D). 
Based on the foregoing discussion of FIG. 1b and FIG. 2, it is apparent 
that the relative intensities with which each of photodetectors A and B of 
the first pair are illuminated by the reflected beam derived from incident 
beam 206.sub.I depend both on the relative position of reflecting surface 
102 with respect to image plane 112 and the actual value to the pointing 
angle .alpha..sub.I in the following manner: 
Based on the foregoing discussion of FIGS. 1, 1a, 1b and 2, it is apparent 
that movement of disc 100 to the left increases the intensity of 
illumination of photodetector A with respect to that of photodetector B. 
In a similar manner, a decrease in the value of the pointing angle 
.alpha..sub.I results in a decrease in the intensity of illumination of 
photodetector A with respect to that of photodetector B. Based on the 
foregoing discussion of FIGS. 2, 2a, 2b and 2c, it is apparent that 
photodetector C of the second pair corresponds in function to 
photodetector A of the first pair and that photodetector D of the second 
pair corresponds in function to photodetector B of the first pair with 
respect to changes in the position of reflective surface 102 with respect 
to image plane 112. Furthermore, the sum of .alpha..sub.I and 
.alpha..sub.II remains substantially constant. Any change in the value of 
the pointing angle .alpha..sub.I must be accompanied by a substantially 
equal and opposite change in the value of pointing angle .alpha..sub.II. 
Therefore, any change in the relative intensity of illumination of 
photodetector A with respect to photodetector B is accompanied by a 
substantially equal and opposite change in the intensity of illumination 
of photodetector C with respect to photodetector D. For this reason, the 
value of the (A-B)+(C-D) error signal 222 derived from error circuit 220 
is substantially independent of any error .DELTA. in the pointing angle 
because any change in the value of (A-B) is self compensated for by an 
equal and opposite change in the value of (C-D). 
On the other hand, the respective values of both (A-B) and (C-D) are 
increased by displacement of reflective surface 102 to the left of image 
plane 112 and decreased by displacement to the right of image plane 112. 
Therefore, the double-beam disc error detector 216 of the improved 
side-spot focus apparatus of the present invention is substantially twice 
as sensitive as is the single beam disc error detector 116 of the prior 
art side-spot focus apparatus of FIGS. 1 and 1a.