Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
2002-02-20
2004-03-09
Porta, David (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C250S201200, C369S044140, C369S044230, C369S044320, C369S044410, C369S053280
Reexamination Certificate
active
06703595
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical data-processing apparatus of the type which is provided with an objective lens facing an optical data storage medium for making a light spot on the storage medium. In particular, the present invention relates to a technique applicable for such a data-processing apparatus whereby spherical aberration due to the thickness error of the substrate of the storage medium is properly detected. In this specification, an “optical data storage disk” may refer to any type of data storage medium with which desired information is written or read out optically. For instance, the optical storage medium may be a read-only disk (such as CD-ROMs), magneto-optical disk or phase change optical disk.
2. Description of the Related Art
For detection of a focus error in an optical data-processing apparatus, Foucault method is often employed. This method can be implemented in a conventional optical data-processing apparatus as shown in
FIG. 11
of the accompanying drawings.
Specifically, in the conventional apparatus, a laser beam emitted from a laser diode
90
passes through a collimating lens
91
, a first beam splitter
92
a
and an objective lens
93
, to strike upon an optical data storage disk D. The laser beam, after reflected on the disk D, passes through the objective lens
93
again and enters the first beam splitter
92
a
. This time, the laser beam is reflected in the beam splitter
92
a
, to be directed toward a second and a third beam splitters
92
b
,
92
c
. In the splitters
92
b
and
92
c
, as shown in
FIG. 11
, the laser beam is partly reflected (upward in the figure) and partly allowed to pass through. The reflected light in the second beam splitter
92
b
is led to a magneto-optical signal detector, while the reflected light in the third beam splitter
92
c
is led to a tracking error detector.
The laser beam having passed through the splitters
92
b
and
92
c
is led to a compound prism
94
and to a focus error detector which incorporates a light detecting device
95
. Then, a focus error signal is generated by the Foucault method in the manner described below.
Referring to
FIG. 12
, as passing through the compound prism
94
, the laser beam splits into an upper ray and a lower ray both of which have a semicircular cross section. These two rays are detected by the light detecting device
95
. In the illustrated situation, when a focus error occurs, the two semicircular light spots on the detecting device
95
shift in position. The detecting device
95
has a light-receiving surface quartered into first~fourth sections a~d by two division lines Lx and Ly perpendicular to each other. Each of the four sections a~d receives light, to generate a detection signal corresponding to the amount of the received light. The signals outputted from the detecting device
95
are supplied to a focus error signal generator (FESG)
96
to produce a focus error signal (FES). The focus error signal has a level LFES equal to {(La−Lb)+(Lc−Ld)}, where La~Ld are the levels of the detection signals outputted from the sections a~d, respectively.
The Foucault method will now be described with reference to FIGS.
13
A~
13
B,
14
A~
14
B and
15
A~
15
B.
When the focusing of the objective lens
93
is proper (FIG.
14
A), each of the two beam spots on the detecting device
95
has an oval form that is symmetrical with respect to the horizontal division line Lx. In this case, the L
FES
becomes zero. However, when the objective lens
93
is too close to the disk D (FIG.
13
A), the two beam spots will take a form and a position as shown in FIG.
13
B. In this case, the L
FES
becomes greater than zero. On the other hand, when the lens
93
is too distant from the disk D (FIG.
15
A), the two beam spots will take a form and a position as shown in FIG.
15
B. In this instance, the L
FES
becomes smaller than zero.
As seen from the above, the focus error signal can be used for detection of the defocusing of the objective lens
93
. More precisely, it is possible to detect the extent and direction of the defocusing of the lens
93
based on the focus error signal (FES). Thus, the focus control for the lens
93
can be performed based on the FES, whereby the lens
93
is moved toward or away from the disk D (i.e., in the focus direction) for focus adjustment.
A typical optical disk may include a transparent substrate and a recording layer formed on this substrate. In using such an optical disk, the laser beam is first led through the transparent substrate and then shone on the recording layer. Unfavorably, the substrate of an optical disk may lack uniformity in thickness (i.e., the substrate has a thickness error), which causes spherical aberration. Spherical aberration makes it difficult to bring the objective lens to the right focus position in performing the focus control. Accordingly, it is impossible to make a sufficiently small light spot on the storage disk, and therefore the required data-recording or data-reading cannot be performed. Recently, a high NA objective lens (NA stands for “numerical aperture”) is preferred for increasing the data storage density of the storage disk. However, since the spherical aberration is proportional to the fourth power of the NA, the apparatus incorporating a high NA objective lens may suffer unacceptably large spherical aberration. In the past, no easy but accurate technique has been proposed for detecting spherical aberration caused by the substrate thickness error.
SUMMARY OF THE INVENTION
The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to provide an optical data-processing apparatus whereby the occurrence of spherical aberration can be detected easily and accurately.
According to a first aspect of the present invention, there is provided an optical data-processing apparatus that includes: an objective lens for convergence of light beams emitted from a light source to make a beam spot on a recording layer of an optical data storage medium; a first light splitter for splitting reflected light from the storage medium into two semicircular rays; a second light splitter for splitting the two semicircular rays into non-biased light and biased light which has a different optical path length than the non-biased light; an optical detector that receives the non-biased light and the biased light, thereby producing a first signal corresponding to the received non-biased light and a second signal corresponding to the received biased light; a first signal processing unit for generating a focus error signal based on the first signal; and a second signal processing unit for generating a spherical aberration signal based on the second signal.
In the above data-processing apparatus, the focus error signal, which is obtained on the basis of the above-mentioned non-biased light, may be produced by the Foucault method as in the prior art discussed above. The spherical aberration signal, on the other hand, is obtained on the basis of the above-mentioned biased light. Since the biased light has an optical path length different from the counterpart of the non-biased light, the profile of the biased light will change when spherical aberration occurs. Based on this profile change, the spherical aberration signal is obtained. According to the present invention, both a focus error signal and a spherical aberration signal are obtained simultaneously. Thus, while the focus control is being performed, spherical aberration control can also be performed. As a result, an appropriately small beam spot can be formed on the recording layer of the storage medium, which is advantageous to performing proper data writing or data reading with respect to the storage medium.
Preferably, the biased light split by the second light splitter may include plus 1-order diffracted light and minus 1-order diffracted light. In this case, the second signal processing unit may generate the spherical aberration
Fujitsu Limited
Greer Burns & Crain Ltd.
Meyer David C.
Porta David
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