Focus error detector

A focus error detector for detecting a signal representing a degree of a focus error of an objective lens with respect to an optical disk includes two focusing lenses each for focusing the reflected light of the optical disk by splitting the reflected light into two beams with respect to the optical axis and two photo-detectors for receiving the respectively focused beams, which simplifies necessary components for a beam size method. The two focusing lenses whose focal lengths are different from each other does not necessitate use of a beam splitter, thereby contributing to reduction of necessary components and optimizing the arrangement of the components. Also, the operational stability is secured by removing the sensitivity to the tilt and shift of an optical axis.

BACKGROUND OF THE INVENTION 
The present invention relates to a focus error detector for detecting a 
signal representing a degree of a focus error of an objective lens with 
respect to an optical disk in an optical recording and reproducing pickup, 
and more particularly, to a focus error detector which is not sensitive to 
the tilt or shift of an optical axis. 
In general, an optical recording and reproducing pickup is constituted so 
that in order to place a recording medium such as an optical disk on a 
focal plane of an objective lens during record and reproduction 
operations, a signal representing a focus error ratio of an objective 
lens, caused by the vibration of the optical disk is detected and the 
objective lens is driven according to the signal. 
In detecting the focus error of the objective lens with respect to the 
optical disk, an astigmatism method disclosed in U.S. Pat. No. 4,841,507 
has a drawback in that, even if the optical disk is placed on the focal 
plane of the objective lens, the detected focus error signal is not zero 
when the optical axis is tilted or shifted by the vibration or a change of 
the optical disk over time. Thus, a high density record and reproduction 
with a small focus intensity is very unstable in operation, due to the 
frequent generation of servo errors. Also, desired data information is not 
recorded and reproduced cleanly. 
Meanwhile, there is a conventional focus error detecting method, a 
so-called "beam size" method, which is less sensitive to the tilt or shift 
of the optical axis by the vibration or optical disk changes over time, as 
described above. Such a conventional focus error detector is shown in FIG. 
1. 
Referring to FIG. 1, the light emitted from a light source 1 becomes 
parallel light by a collimating lens 2, to pass through a beam splitter 3, 
and is focused onto an optical disk 5 by means of an objective lens 4. The 
reflected light of optical disk 5 becomes parallel light again by means of 
objective lens 4, is reflected from beam splitter 3 and then is focused by 
a focusing lens 6. The focused reflected light is split into two beams by 
a beam splitter 7. One beam is received from one photo-detector 10 in a 
photo-detecting unit 9 and the other beam is received from the other 
photo-detector 11 in photo-detecting unit 9 via a reflective prism 8. 
Here, ternary detectors are each used for both photo-detectors 10 and 11 
as shown in FIG. 2 and are disposed before and after the focal point of 
focusing lens 6, so that same-sized beam spots are formed in the 
respective detectors when the optical disk is placed on the focal plane of 
the objective lens. That is to say, when the optical disk deviates from 
the focal plane of the objective lens, the size of beam spots formed on 
both photo-detectors 10 and 11 are changed with respect to each other. At 
this time, a signal whose magnitude and polarity are changed depending on 
the focus error is detected by a circuit shown in FIG. 2. 
According to the above-described conventional beam size method for 
detecting focus error, a beam splitter is used for splitting the reflected 
light into two beams. Also, a reflective prism is used for the purpose of 
installing two ternary photo-detectors on one plane. Accordingly, costs 
are increased due to the additional components and miniaturization is 
difficult due to the resulting large volume. 
Also, according to the prior art, since the distance from focusing lens 6 
to photo-detector 10 is not the same with that from focusing lens 6 to 
photo-detector 11, if the reflected light is tilted or shifted due to 
vibaration or change over time of the optical disk, the spot movement of 
photo-detector 10 is different from that of photo-detector 11. Thus, even 
if the optical disk is placed on the focal plane of the objective lens, 
the tilted or shifted reflected light often results in a focus error. 
SUMMARY OF THE INVENTION 
Therefore, it is an object of the present invention to provide a simplified 
and inexpensive focus error detector capable of being miniaturized, for 
detecting a focus error by adopting a beam size method which is not 
sensitive to the tilt and shift of the optical axis due to the vibration 
or change over time of the optical disk. 
To accomplish the above object, the focus error detector for an objective 
lens for focusing incident light onto an optical disk, according to the 
present invention, comprises: two focusing lenses having a different focal 
length from each other and each for splitting a reflected light from the 
optical disk into both sides with respect to the optical axis and focusing 
the split light; two ternary photo-detectors each for receiving split 
reflected light respectively focused by the two focusing lenses; and 
circuit means for detecting a signal representing the degree of the focus 
error of said objective lens with respect to the optical disk from the two 
ternary photo-detectors. 
As described above, according to the present invention, low cost and 
miniaturization can be realized by removing the conventional beam splitter 
and reflective prism and using two focusing lenses.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 3, an optical pick-up includes a light source 1 for 
emitting light, a collimating lens 2 for forming parallel light from the 
emitted light, a beam splitter 3 for directing the parallel light, and an 
objective lens 4 for focusing the incident light directed via the beam 
splitter 3 onto an optical disk 5, as conventionally. Similarly, the 
reflected light from the optical disk 5 is reflected from the beam 
splitter 3 via the objective lens 4. In the present invention, in order to 
split the reflected light reflecting from the beam splitter 3 into two 
beams, two focusing lenses 12 and 13 are installed in parallel on the 
emitting surface of the beam splitter 3. The two focusing lenses 12 and 13 
are designed so that the focal length is different from each other with 
respect to the same incident light. Also, in the present invention, as 
shown in FIG. 4, a photo-detecting unit 14 having two ternary 
photo-detectors 15 and 16 and each receiving two beams split and focused 
from the two focusing lenses 12 and 13 is disposed in the center of the 
respective focal points of the two focusing lenses 12 and 13. Meanwhile, 
circuit means for detecting a desirable focus error signal from the two 
photo-detectors 15 and 16 is shown in FIG. 4. 
In the focus error detector according to the present invention having the 
aforementioned configuration, if the optical disk 5 is placed on the focal 
plane of the objective lens 4, the reflected light reflected from the 
optical disk 5 and passing through the objective lens 4 becomes parallel 
light like the incident light. Therefore, the reflected light passes 
through the two focusing lenses 12 and 13 to then be focused onto the 
corresponding focal points. At this time, since the photo detecting unit 
14 is placed at the center of the respective focal points of the two 
focusing lenses 12 and 13, the beam spots landed on the two 
photo-detectors 15 and 16 are the same size, as shown in FIG. 5A. 
Therefore, the focus error signal detected by the circuit shown in FIG. 4 
becomes zero, which indicates that the optical disk 5 is placed exactly on 
the focal plane of the objective lens 4. 
However, if the optical disk 5 deviates from the focal plane of the 
objective lens 4, becoming more distant or closer, the reflected light 
passing through the objective lens 4 is converged or diverged, so that the 
respective focal lengths of the two focusing lenses 12 and 13 with respect 
to the reflected light are shortened or lengthened. At this time, as shown 
in FIGS. 5B and 5C, the size of the respective beam spots landed on 
photo-detectors 15 and 16 increase or decrease with respect to each other. 
Then, the focus error signals detected by the circuit shown in FIG. 4. are 
different in magnitude depending on the changed sizes of the beam spots, 
i.e., the degree of the optical disk 5 deviating from the focal plane of 
the objective lens 4, and in polarity depending on the deviated direction. 
Meanwhile, in an on-focus state, when the reflected light is tilted or 
shifted due to the vibration or change over time of the optical disk 5, 
since the distance from focusing lens 12 to photo-detector 15 is the same 
with that from focusing lens 12 to photo-detector 16, the spot movements 
of photo-detectors 15 and 16 is the same with each other. The difference 
between the signals detected depending on the change in light amount for 
the respective photo-detectors according to the spot movements are offset 
to then be removed. Therefore, in the on-focus state, the tilted or 
shifted reflected light does not cause a focus error. 
As described above, according to the present invention, the focus error 
detector adopting the beam size method is constituted only by two focusing 
lenses having the respective focal lengths designed differently, thereby 
simplifying the configuration to reduce the cost and to allow 
miniaturization. Specifically, since two focusing lenses focus beams onto 
the respective optical axes, the focus error signal detector becomes 
insensitive to the tilts and shifts of the optical axes for the reflected 
light incident thereto. That is to say, an optically excellent and stable 
operation of the present invention allows desired data information to be 
recorded and reproduced cleanly. Also, the focus error signal detector 
according to the present invention is advantageous for the optical 
recording and reproducing pickup of high density.