U.S. patent application number 13/645758 was filed with the patent office on 2013-04-18 for optical pickup and optical read/write apparatus.
This patent application is currently assigned to PANASONIC CORPORATION. The applicant listed for this patent is PANASONIC CORPORATION. Invention is credited to Jun-ichi ASADA, Hideki HAYASHI, Kazuo MOMOO, Yohichi SAITOH.
Application Number | 20130094338 13/645758 |
Document ID | / |
Family ID | 47892336 |
Filed Date | 2013-04-18 |
United States Patent
Application |
20130094338 |
Kind Code |
A1 |
SAITOH; Yohichi ; et
al. |
April 18, 2013 |
OPTICAL PICKUP AND OPTICAL READ/WRITE APPARATUS
Abstract
An optical pickup includes: a light source; a first diffractive
element which diffracts light polarized in a particular direction;
an objective lens; a lens actuator which shifts the objective lens
so that the magnitude of shift from its initial position in a
tracking direction has an upper limit of 0.3 mm to 0.6 mm; a wave
plate; a second diffractive element which has two diffraction
regions configured to diffract light polarized in a direction that
intersects with the particular direction at right angles and which
splits the write beam reflected from the optical storage medium
through each diffraction region into a transmitted light beam and
at least one diffracted light beam; and a photodetector which
detects the transmitted light beam, the diffracted light beams that
have left the two diffraction regions, and the read beam reflected
from the optical storage medium.
Inventors: |
SAITOH; Yohichi; (Kyoto,
JP) ; ASADA; Jun-ichi; (Hyogo, JP) ; MOMOO;
Kazuo; (Osaka, JP) ; HAYASHI; Hideki; (Nara,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
PANASONIC CORPORATION; |
Osaka |
|
JP |
|
|
Assignee: |
PANASONIC CORPORATION
Osaka
JP
|
Family ID: |
47892336 |
Appl. No.: |
13/645758 |
Filed: |
October 5, 2012 |
Current U.S.
Class: |
369/47.15 ;
369/112.04; G9B/20; G9B/7.113 |
Current CPC
Class: |
G11B 7/003 20130101;
G11B 7/00458 20130101; G11B 7/14 20130101; G11B 7/1353
20130101 |
Class at
Publication: |
369/47.15 ;
369/112.04; G9B/20; G9B/7.113 |
International
Class: |
G11B 7/1353 20120101
G11B007/1353; G11B 20/00 20060101 G11B020/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 17, 2011 |
JP |
2011-227530 |
Claims
1. An optical pickup that writes data on a track on an optical
storage medium and reads the data being written on the track in
parallel, the optical pickup comprising: a light source configured
to emit a light beam; a first diffractive element configured to
diffract light that is polarized in a particular direction and
split the light beam that has been emitted from the light source
into a plurality of light beams including a write beam and a read
beam; an objective lens configured to converge the write beam and
the read beam onto the same track on the optical storage medium; a
lens actuator configured to shift the objective lens to perform a
focus control and a tracking control so that the magnitude of shift
of the objective lens from its initial position as measured in a
tracking direction has an upper limit of 0.3 mm to 0.6 mm; a wave
plate that is arranged between the first diffractive element and
the optical storage medium and that is designed so that the
polarization direction of light going from the first diffractive
element toward the wave plate is perpendicular to the polarization
direction of light going from the wave plate toward the first
diffractive element; a second diffractive element including two
diffraction regions with different diffraction properties that are
arranged in a direction corresponding to the tracking direction,
each said diffraction region being configured to diffract light
that is polarized in a direction that intersects with the
particular direction at right angles, the second diffractive
element configured to split the write beam that has been reflected
from the optical storage medium through each said diffraction
region into a transmitted light beam and at least one diffracted
light beam; and a photodetector including a plurality of
photosensitive elements that are configured to detect the
transmitted light beam, the diffracted light beams that have left
the two diffraction regions, and the read beam that has been
reflected from the optical storage medium.
2. The optical pickup of claim 1, wherein the first diffractive
element is a diffraction grating and the second diffractive element
is a polarization hologram element.
3. The optical pickup of claim 2, comprising a polarization
hologram plate in which the first and second diffractive elements
are combined with each other.
4. The optical pickup of claim 3, wherein the polarization hologram
plate, the wave plate and the objective lens are combined together
to form a single unit, and wherein the lens actuator is configured
to shift the unit.
5. The optical pickup of claim 1, wherein the photodetector is
configured to perform a differential operation between the output
signals of two of the photosensitive elements, which detect the
diffracted light beams that have left the two diffraction regions,
thereby generating a tracking error signal.
6. The optical pickup of claim 1, wherein if the track pitch of the
optical storage medium is a and the wavelength of the light emitted
from the light source is .lamda.,
a/0.85.ltoreq..lamda..ltoreq.a/0.75 is satisfied, and wherein the
objective lens has a numerical aperture of 0.81 to 0.89, and
wherein if the radial Rim intensity of the light entering the
objective lens is R and the track groove depth of the optical
storage medium is d, then
0.25.lamda.-(1.2R.sup.2-R+0.395).lamda./2.ltoreq.d.ltoreq.0.25.lamda.+(1.-
2R.sup.2-R+0.395).lamda./2 and
0.125.lamda.-(-0.2R.sup.2+0.45R-0.085).lamda./2.ltoreq.d.ltoreq.0.125.lam-
da.+(-0.2R.sup.2+0.45R-0.085).lamda./2 are both satisfied, or
0.25.lamda.-(1.2R.sup.2-R+0.395).lamda./2.ltoreq.d.ltoreq.0.25.lamda.+(1.-
2R.sup.2-R+0.395).lamda./2 and
0.375.lamda.-(-0.2R.sup.2+0.45R-0.085).lamda./2.ltoreq.d.ltoreq.0.375.lam-
da.+(-0.2R.sup.2+0.45R-0.085).lamda./2 are both satisfied.
7. An optical read/write apparatus comprising: at least one optical
pickup; a signal processing section configured to process the
output signal of the optical pickup; and a controller configured to
control the optical pickup based on the output of the signal
processing section, and wherein the optical pickup comprises: a
light source configured to emit a light beam; a first diffractive
element configured to diffract light that is polarized in a
particular direction and split the light beam that has been emitted
from the light source into a plurality of light beams including a
write beam and a read beam; an objective lens configured to
converge the write beam and the read beam onto the same track on
the optical storage medium; a lens actuator configured to shift the
objective lens to perform a focus control and a tracking control so
that the magnitude of shift of the objective lens from its initial
position as measured in a tracking direction has an upper limit of
0.3 mm to 0.6 mm; a wave plate that is arranged between the first
diffractive element and the optical storage medium and that is
designed so that the polarization direction of light going from the
first diffractive element toward the wave plate is perpendicular to
the polarization direction of light going from the wave plate
toward the first diffractive element; a second diffractive element
including two diffraction regions with different diffraction
properties that are arranged in a direction corresponding to the
tracking direction, each said diffraction region being configured
to diffract light that is polarized in a direction that intersects
with the particular direction at right angles, the second
diffractive element configured to split the write beam that has
been reflected from the optical storage medium through each said
diffraction region into a transmitted light beam and at least one
diffracted light beam; and a photodetector including a plurality of
photosensitive elements that are configured to detect the
transmitted light beam, the diffracted light beams that have left
the two diffraction regions, and the read beam that has been
reflected from the optical storage medium.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present disclosure relates to an optical pickup and
optical read/write apparatus which writes information on an optical
storage medium such as an optical tape and read the information
being written in parallel.
[0003] 2. Description of the Related Art
[0004] An optical tape drive system which performs read and write
operations on an optical tape medium using a plurality of optical
pickups at the same time by utilizing a high density optical
recording technique has been proposed as an optical read/write
apparatus that can be used effectively for the purpose of bulk data
archival and storage. Japanese Laid-Open Patent Publication No.
2006-286070 (which will be referred to herein as "Patent Document
No. 1" for convenience sake) discloses an example of such an
optical tape drive system.
[0005] Meanwhile, in a conventional magnetic tape drive system, a
write head and a read head are arranged separately with respect to
a track on which data is going to be written. And by writing data
and reading the data being written simultaneously, verification can
be made to see if the data has been written just as intended. In
this manner, high speed processing can get done with a sufficiently
high degree of reliability ensured.
[0006] It is known that such a verify technology is also applicable
to an optical read/write apparatus that is designed to read and
write data from/on a disc medium such as a magneto-optical (MO)
disc, a Blu-ray Disc (BD), a DVD or a CD using an optical pickup.
According to such a technology, a light beam that has been emitted
from a laser light source is split through a diffraction grating
into a zero-order light beam and .+-.first-order light beams, with
which a storage layer is irradiated. In this case, a write
operation can be performed by irradiating the storage layer with
the zero-order light beam and a verify operation can be performed
by detecting the .+-.first-order light beams. Such a technology is
called a DRAW (direct read after write) technology. According to
such a technology, an error check can be made right after data has
been written, and therefore, the processing can get done quickly
and the transfer rate can be increased. A read/write apparatus that
adopts the DRAW technology is disclosed in Japanese Laid-Open
Patent Publication No. 6-162532 (which will be referred to herein
as "Patent Document No. 2" for convenience sake), for example.
[0007] An optical read/write apparatus needs to perform a focus
control and a tracking control appropriately during read and write
operations. Particularly when an optical tape is used as an optical
storage medium, the tracking control needs to be performed in
accordance with the properties of the tracks of the optical tape,
which are different from those of a normal optical disc.
[0008] Examples of known tracking control methods for optical disc
drives include the push-pull (PP) method, the advanced push-pull
(APP) method, and the correct far field (CFF) method, which are
disclosed in Patent Document No. 2 and Japanese Laid-Open Patent
Publications No. 8-306057 and No. 2000-306262 (which will be
referred to herein as "Patent Documents Nos. 3 and 4",
respectively, for convenience sake), respectively.
[0009] However, none of these known optical read/write apparatuses
can obtain a tracking signal that is suitably used in an optical
storage medium such as an optical tape where the tracking position
may change significantly during reading or writing.
[0010] Thus, the present disclosure provides an optical read/write
apparatus that can stabilize the tracking performance even when
dealing with an optical storage medium such as an optical tape
where the track position may change significantly during the
operation.
SUMMARY OF THE INVENTION
[0011] The present disclosure provides an optical pickup which
writes data on a track on an optical storage medium and reads the
data being written on the track in parallel. The optical pickup
includes: a light source configured to emit a light beam; a first
diffractive element configured to diffract light that is polarized
in a particular direction and split the light beam that has been
emitted from the light source into a plurality of light beams
including a write beam and a read beam; an objective lens
configured to converge the write beam and the read beam onto the
same track on the optical storage medium; a lens actuator
configured to shift the objective lens to perform a focus control
and a tracking control so that the magnitude of shift of the
objective lens from its initial position as measured in a tracking
direction has an upper limit of 0.3 mm to 0.6 mm; a wave plate that
is arranged between the first diffractive element and the optical
storage medium and that is designed so that the polarization
direction of light going from the first diffractive element toward
the wave plate is perpendicular to the polarization direction of
light going from the wave plate toward the first diffractive
element; a second diffractive element including two diffraction
regions with different diffraction properties that are arranged in
a direction corresponding to the tracking direction, each said
diffraction region being configured to diffract light that is
polarized in a direction that intersects with the particular
direction at right angles, the second diffractive element
configured to split the write beam that has been reflected from the
optical storage medium through each said diffraction region into a
transmitted light beam and at least one diffracted light beam; and
a photodetector including a plurality of photosensitive elements
that are configured to detect the transmitted light beam, the
diffracted light beams that have left the two diffraction regions,
and the read beam that has been reflected from the optical storage
medium.
[0012] According to the present disclosure, a tracking control can
be performed with good stability even on an optical storage medium
such as an optical tape where the track position changes
significantly during the operation. It should be noted that the
idea of the present disclosure is applicable to not only an optical
tape but also an optical disc or any other optical storage medium
as well.
[0013] Other features, elements, processes, steps, characteristics
and advantages of the present invention will become more apparent
from the following detailed description of preferred embodiments of
the present invention with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 illustrates a configuration for an optical tape
machine as an embodiment of the present disclosure.
[0015] FIG. 2 is a perspective view schematically illustrating a
portion of an optical tape according to an embodiment on a larger
scale.
[0016] FIG. 3 is a block diagram illustrating a circuit
configuration for an optical tape machine according to an
embodiment.
[0017] FIG. 4 schematically illustrates an optical configuration
for an optical pickup according to an embodiment.
[0018] FIG. 5 is a schematic representation illustrating the
functions of respective members of the optical pickup shown in FIG.
4.
[0019] FIG. 6(a) is a perspective view illustrating a polarization
hologram plate 8 and FIG. 6(b) is a plan view illustrating one side
of the polarization hologram plate 8 with a polarization hologram
element 8b.
[0020] FIG. 7 is a schematic representation illustrating how a main
spot and two sub-spots are formed on a track on an optical
tape.
[0021] FIG. 8 schematically illustrates an exemplary configuration
for a photodetector and its photosensitive elements.
[0022] FIG. 9 is a schematic representation illustrating how
diffracted light beams that have been transmitted through a
polarization hologram element 8b are incident on photosensitive
elements 13 and 14.
[0023] Portion (a) of FIG. 10 shows an exemplary waveform of an
optical drive signal. Portion (b) of FIG. 10 illustrates the shapes
of marks to be recorded. Portions (c) and (d) of FIG. 10 show the
waveforms of signals representing the reflected light of sub-beams
that irradiate the optical tape 2 after and before the main beam
irradiates it, respectively. Portion (e) of FIG. 10 shows the
waveform of a signal obtained by calculating the difference between
those two read signals.
[0024] FIG. 11A shows a result of a simulation that was carried out
to see how the TE balance would change with the groove depth d and
the radial Rim intensity.
[0025] FIG. 11B is a graph showing how the radial Rim intensity R
changes with the groove depth range b in which the absolute value
of the TE balance can be reduced to 15% or less.
[0026] FIG. 12A shows a result of a simulation that was carried out
to see how the TE amplitude would change with the groove depth d
and the radial Rim intensity.
[0027] FIG. 12B is a graph showing how the radial Rim intensity R
changes with the groove depth range a in which the TE amplitude can
be reduced to 0.5 or less.
[0028] FIG. 13 illustrates a photodetector and its photosensitive
elements for use to obtain a tracking signal by the spot sized
detection method.
[0029] FIG. 14 shows a definition of a TE balance.
[0030] FIGS. 15(a) and 15(b) show how the TE balance and the TE
amplitude calculated change with the magnitude of lens shift
according to the PP method. FIGS. 15(c) and 15(d) show how the TE
balance and the TE amplitude calculated change with the magnitude
of lens shift according to the APP method. And FIGS. 15(e) and
15(f) show how the TE balance and the TE amplitude calculated
change with the magnitude of lens shift according to the CFF
method.
[0031] FIG. 16 illustrates an exemplary optical arrangement for an
optical pickup that performs a tracking control by the APP
method.
[0032] FIG. 17 illustrates an exemplary arrangement for a
photodetector that obtains a tracking signal by the APP method.
[0033] FIG. 18 illustrates an exemplary optical arrangement for an
optical pickup that performs a tracking control by the CFF
method.
[0034] FIG. 19 illustrates an exemplary arrangement for a
photodetector that obtains a tracking signal by the CFF method.
[0035] FIG. 20 shows how the read beam deviates with respect to the
center of the aperture of an objective lens.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0036] Hereinafter, embodiments of the present disclosure will be
described with reference to the accompanying drawings as needed. It
should be noted that the description thereof will be sometimes
omitted unless it is absolutely necessary to go into details. For
example, description of a matter that is already well known in the
related art will be sometimes omitted, so will be a redundant
description of substantially the same configuration. This is done
solely for the purpose of avoiding redundancies and making the
following description of embodiments as easily understandable for
those skilled in the art as possible.
[0037] It should be noted that the present inventors provide the
accompanying drawings and the following description to help those
skilled in the art understand the present disclosure fully. And it
is not intended that the subject matter defined by the appended
claims is limited by those drawings or the description.
1-1. Overall Configuration
[0038] FIG. 1 illustrates a configuration for an optical read/write
apparatus as an embodiment of the present disclosure. The optical
read/write apparatus of this embodiment is an optical tape machine
1 that can write data on an optical tape 2 and can read the data
from the optical tape 2. The optical tape machine 1 may be used to
back up a huge quantity of data, for example. In order to back up
such an enormous quantity of data in a short time with the transfer
rate increased, the optical tape machine 1 includes a lot of
optical pickups 4. By using those optical pickups 4 at the same
time, the DRAW operation of writing data on the optical tape 2 and
reading the data being written in parallel can be carried out. Any
number of optical pickups 4 may be arranged in any pattern. In this
embodiment, twelve optical pickups 4 (which are also identified by
"P. U. 1" through "P. U. 12") are arranged so as to cross the track
direction of the optical tape 2. As a result, such an enormous
quantity of data can be read and written from/on multiple tracks in
parallel. Also, this optical tape machine 1 is configured so that
the optical tape 2 can run both in the forward direction and in the
reverse direction. That is why when a read or write operation gets
done through the end of the optical tape 2, the read/write
operation can be continued just by reversing the tape running
direction without rewinding the optical tape 2.
[0039] On the optical tape 2, tracks 3 were transferred in advance
at a pitch of sub-microns (i.e., less than 1 .mu.m) onto a
tape-shaped film by nano-printing technology and a storage layer
and a protective layer are stacked thereon. The tracks 3 can be
formed substantially parallel to the running direction of the tape.
Although only some of those tracks 3 are illustrated in FIG. 1 to
make this drawing easily understandable, actually a huge number of
tracks 3 cover the entire recordable area of the optical tape 2.
The width W of the recordable area of the optical tape 2 may fall
within the range of a few millimeters through several centimeters.
Also, the optical tape 2 may have a thickness of a few .mu.m
through several ten .mu.m and its groove depth may be set to be 1
.mu.m or less, for example.
[0040] Those twelve optical pickups 4 are fixed and arranged so
that each of those optical pickups 4 is located in an associated
one of twelve recording zones, which are defined by evenly dividing
the recordable area of the optical tape 2 into twelve in the width
direction. That is why the tracks in each recording zone can be
accessed just by moving the objective lens 5. The objective lens 5
is driven by the lens actuator 20 so as to be able to shift
perpendicularly to the tracks (i.e., in the tracking direction).
Supposing the recordable area has a width W of 4.8 mm, for example,
each recording zone has a width T of 0.4 mm (=4.8 mm/12). In that
case, the optical tape machine 1 could be configured so that the
objective lens 5 can be shifted within the range of .+-.0.2 mm with
respect to the center of its associated recording zone as its
initial position. In this embodiment, however, with the error
involved with the manufacturing process of the optical tape 2 and
the influence of wobbling of the tape running taken into
consideration, the objective lens 5 is configured to be able to
shift as long a distance as .+-.0.3 to 0.6 mm at maximum. In the
following description, to make the objective lens 5 shift in the
tracking direction will be sometimes referred to herein as a "lens
shift".
[0041] FIG. 2 is a perspective view schematically illustrating a
portion of an optical tape 2 on a larger scale. The optical tape 2
may include a base film 2a, a back coating layer 2b that is adhered
to the back surface of the base film 2a, and an imprint layer 2c
that is supported by the base film 2a. On the upper surface of the
imprint layer 2c, lands 2d and grooves 2e have been formed.
Although not shown in FIG. 2, a reflective film and a recording
material film are deposited over the entire upper surface of the
imprint layer 2c. The optical tape 2 is extended in the
longitudinal direction L and may have a length of several hundred
meters, for example.
[0042] It should be noted that FIG. 2 illustrating the optical tape
2 is not to scale. Actually, the optical tape 2 may have several
hundreds, or an even greater number, of lands 2d and grooves 2e. In
one embodiment, data is written on either the lands 2d or the
grooves 2e. The lands 2d or the grooves 2e on which data is written
will be referred to herein as "tracks", which may have a pitch of
0.2 .mu.m to 0.4 .mu.m, for example. In the following description,
data is supposed to be written on the grooves 2e. That is why the
tracks will be sometimes referred to herein as "track grooves".
[0043] On the optical tape 2, a mark can be recorded optically by
irradiating the tape 2 with a light beam. More specifically, such a
mark is recorded on its recording material film. The light beam is
radiated by an optical pickup 4 that includes a light source and an
objective lens 5 that focuses the light beam emitted from the light
source on the optical tape 2. When the optical pickup 4 irradiates
the optical tape 2 with a light beam, the irradiated portion of the
optical tape 2 comes to have a different optical property such as a
reflectance or a refractive index from the rest (i.e., the
non-irradiated portion) of the optical tape 2. Such a portion, of
which the optical property has changed in this manner, is called a
"recorded mark".
[0044] In optical tape technologies, data can be read out from the
optical tape 2 by irradiating the tape 2 with a relatively weak
light beam with a constant intensity and detecting the light that
has been modulated by, and reflected from, the optical tape 2. In
writing data on the optical tape 2, data is written there by
irradiating the optical tape 2 with a pulsed light beam, of which
the optical power has been modulated according to the data to be
written, and locally changing the property of the recording
material film.
[0045] When data is going to be written on the recording material
film, the recording material film is irradiated with such a light
beam, of which the optical power has been modulated as described
above, thereby recording an amorphous mark on a crystalline
recording material film. Such an amorphous recorded mark is left
there by heating a portion of the recording material film that has
been irradiated with a write light beam to a temperature that is
equal to or higher than its melting point and then rapidly cooling
that portion. If the optical power of a light beam that irradiates
the recorded mark is set to be relatively low, the temperature of
the recorded mark being irradiated with the light beam does not
exceed its melting point and the recorded mark will turn
crystalline again after having been cooled rapidly (i.e., the
recorded mark will be erased). In this manner, the recorded mark
can be rewritten over and over again. However, if the power of the
light beam for writing data had an inappropriate level, then the
recorded mark would have a deformed shape and sometimes it could be
difficult to read the data as intended.
[0046] To read or write data from/on the optical tape 2, the light
beam always needs to maintain a predetermined converging state on a
target track. For that purpose, a "focus control" and a "tracking
control" are performed. In order to perform a focus control and a
tracking control, the focus error or the tracking error is detected
based on the light that has been reflected from the optical tape 2
and the position of the light beam spot is adjusted so as to reduce
the error as much as possible. The magnitudes of the focus error
and the tracking error are respectively represented by a "focus
error signal" and a "tracking error signal", both of which are
generated based on the light that has been reflected from the
optical tape 2. The focus error signal and the tracking error
signal are output from a photodetector that each optical pickup 4
has. The controller of the optical tape machine 1 performs a focus
control and a tracking control on each optical pickup 4 in response
to the focus error signal and tracking error signal supplied from
the photodetector of the optical pickup 4. In the following
description, the focus error signal and the tracking error signal
will be sometimes referred to herein as a "focus signal" and a
"tracking signal", respectively.
[0047] Hereinafter, an exemplary circuit configuration for the
optical tape machine 1 will be described with reference to FIG. 3,
which is a block diagram illustrating a circuit configuration for
the optical tape machine 1 of this embodiment. The optical tape
machine 1 illustrated in FIG. 3 includes an optical pickup assembly
40 which is a set of optical pickups 4, motors 506 and 507 which
make the optical tape 2 run, and circuit blocks that are
electrically connected to the optical pickup assembly 40 and the
motors 506 and 507 and that include a frontend signal processing
section 520, an encoder/decoder 530, a servo control section 550, a
driver amplifier 560, and a CPU (system controller) 540 to be
described below.
[0048] The output of each optical pickup 4 is supplied to the
encoder/decoder 530 by way of the frontend signal processing
section 520. In reading data, the encoder/decoder 530 decodes the
data that is stored on the optical tape 2 based on the signal that
has been generated by each optical pickup 4. The encoder/decoder
530 includes an optical power modulator 531. In writing data, the
encoder/decoder 530 encodes the data to generate a signal to be
written on the optical tape 2. In this description, this signal
will be referred to herein as an "optical drive signal". The
optical drive signal is supplied to each optical pickup 4 by way of
the optical power modulator 531. Using this signal, the intensity
of the light beam emitted from the light source of each optical
pickup 4 is modulated so as to record a mark as intended on a
target track on the optical tape 2.
[0049] The frontend signal processing section 520 generates a read
signal based on the output of each optical pickup and also
generates a focus error signal FE and a tracking error signal TE.
The read signal thus generated is then supplied to the
encoder/decoder 530. The focus error signal FE and the tracking
error signal TE are then supplied to the servo control section 550.
In response, the servo control section 550 gets the motors 506 and
507 controlled by a driver amplifier 560. The servo control section
550 also gets the position of an objective lens 5 controlled by a
lens actuator 20 in each optical pickup 4. The encoder/decoder 530,
the servo control section 550 and all the other components are
controlled by the CPU 540. The respective circuit blocks
illustrated in FIG. 3 can be implemented by assembling together
integrated circuit elements, memories and other electronic parts,
which form the respective sections, on a circuit board.
1-2. Problem
[0050] Next, it will be described what problem will arise when the
optical tape 2 is used as an optical storage medium and will also
be described what configuration the optical pickup 4 may use to
overcome that problem.
[0051] In the optical tape machine 1, each optical pickup 4 is
arranged at a fixed position in its associated recording zone
unlike a normal optical disc apparatus. That is why to have access
to a target track 3, the objective lens 5 needs to be moved, which
is not easy, however, due to an optical tape machine's own problem
to be described below.
[0052] To make the optical tape 2, it is necessary to perform the
processing step of forming track grooves continuously on a long
strip of a sheet. Specifically, the optical tape 2 may be formed by
performing the following manufacturing processing steps. First of
all, a sheet on which a pattern of track grooves has been formed by
an electron beam process is attached to a metallic roller and
provided as a master. Next, a UV curable resin is applied onto a
base material, which is a roll of a film that is the material of
the tape, and the groove transferring metallic roller that has been
provided as the master is pressed against the resin, thereby
transferring the track groove pattern continuously. Thereafter, the
UV curable resin gets cured, thereby forming track grooves on the
tape. Subsequently, a recording material film is deposited by
sputtering or any other process on the tape on which the track
grooves have been formed and then a protective film is stacked
thereon, thereby completing an original sheet of an optical tape.
Next, the roll of original optical tape sheet is continuously cut
to a width of 1/2 inches by splitter process, thereby making a lot
of optical tapes at the same time.
[0053] In such an optical tape manufacturing process, some
positioning error could be caused while the track groove pattern is
transferred from the master onto the original tape material and the
original tape material being turned could swing in the axial
direction. As a result, misalignment could occur between the tape
material and the positions on which the track groove pattern has
been transferred. Likewise, the original tape material could also
swing along the axis of rotation during the slitter process.
Consequently, in the optical tape completed, sometimes the track
grooves are not parallel to the edges of the tape that has been cut
and their positions may have an error in some cases. Those errors
are not a problem in the case of a magnetic tape that needs no
grooves. And those errors will never be caused in an optical disc,
which is manufactured integrally with a disklike stamper. That is
to say, these are problems unique to an optical tape.
[0054] In addition, while the tape is running, the positions of the
respective tracks are determined by a tape guide with respect to
the edges of the tape. As a result, the positions of the track
grooves will vary with respect to the position of the optical
pickups. Such a phenomenon will be referred to herein as a "run
out" of an optical tape. Although it depends on the environment,
this variation in position is approximately .+-.0.1 to 0.3 mm. On
top of that, since the respective optical pickups 4 are fixed in
the machine, a lens shift of .+-.0.2 mm is needed to have access to
a particular track in a recording zone. Therefore, even if the
error involved with the optical tape manufacturing process is
neglected, an objective lens shift of .+-.0.3 to 0.5 mm is needed
in order to follow the tracks.
[0055] In a known optical disc apparatus, the objective lens has
been allowed to shift approximately .+-.0.5 mm for a recordable
disc, and approximately .+-.0.1 mm for a read-only disc, in order
to follow the tracks, and it has been believed sufficient to allow
a shift range of approximately .+-.0.2 mm during the design
process. In an optical tape machine such as the one of this
embodiment, however, the objective lens needs to be shifted within
a very broad range that is 1.5 to twice as large as the movable
range of an optical disc apparatus. Thus, according to this
embodiment, the objective lens 5 is shifted by the actuator 20 so
that the upper limit of the magnitude of shift with respect to the
initial position of the objective lens 5 in the tracking direction
becomes 0.3 mm to 0.6 mm. It would be more beneficial to set the
upper limit of the magnitude of shift the range of 0.35 mm to 0.55
mm and even more beneficial to set the upper limit within the range
of 0.4 mm 0.5 mm.
[0056] On the other hand, the optical pickup 4 with the DRAW
function splits a light beam that has been emitted from a light
source into a plurality of light beams including a main beam (i.e.,
a write beam) and a sub-beam (i.e., a read beam) and forms two or
more light beam spots on the same track on the optical tape 2. For
that reason, in order to use the given light as effectively as
possible and to ensure mass-productivity by minimizing the spot
position adjustment problem, a one-beam method is adopted as the
tracking detecting method. In this description, the "one-beam
method" is a method for obtaining a tracking error signal using
only the main beam. Examples of known one-beam tracking detecting
methods include the push-pull (PP) method, the advanced push-pull
(APP) method, and the correct far field (CFF) method. If any of
these known tracking detecting methods were used as it is, however,
the following problem would arise.
[0057] FIG. 13 illustrates an exemplary configuration for a
photodetector for use when the PP method is adopted. The main beam
and the sub-beam reflected from an optical storage medium form
light beam spots 214 and 215 on photosensitive elements 212 and
213, respectively, on the photodetector. In this case, as the
objective lens shifts, the light beam spot 214 on the photodetector
shifts to the position indicated by the dashed circle in FIG. 13,
for example. The magnitude of such a shift of the light beam spot
on the photodetector is proportional to the magnitude of lens
shift. If the light beam spot shifts, the tracking error signal
loses its symmetry. And according to the degree of that asymmetry
(which will be referred to herein as a "TE balance"), the tracking
error signal comes to have a different value from original one. As
shown in FIG. 14, the TE balance (%) is defined to be a quantity
represented by (A-B)/2(A+B).times.100, where A is the positive
amplitude of the tracking error signal and B is the negative
amplitude thereof.
[0058] FIGS. 15(a) and 15(b) show how the TE balance and the
tracking signal's amplitude (which will be referred to herein as
"TE amplitude") calculated change with the magnitude of lens shift
according to the PP method. In FIG. 15, the TE amplitude is the
amplitude of the TE signal that is normalized by regarding its
value when the magnitude of lens shift is 0 mm to be one. In this
case, the TE balance and TE amplitude were calculated under the
condition including a wavelength of 0.405 m, an objective lens'
numerical aperture (NA) of 0.85, a track groove pitch of 0.32
.mu.m, a track groove depth of 0.04 .mu.m, and a radial Rim
intensity of 0.6. In this case, the radial Rim intensity means the
ratio of the intensity of the incoming light at the end of the
aperture of the objective lens to that of the incoming light at the
center of the aperture of the objective lens. That is to say, if
the respective intensities at the center and end of the aperture
are the same, then the radial Rim intensity becomes one. The radial
Rim intensity indicates the degree of the diaphragm and varies
according to the size of the aperture and the distance between the
collimator lens and the light source, for example.
[0059] Considering vibrations and other disturbances, in order to
stabilize the tracking control and minimize abnormal track jump,
the magnitude of off track may be up to 5% of the track pitch and
the TE balance needs to be reduced to 15% or less. According to the
PP method, when the lens shift is 0.05 mm, the TE balance becomes
15%. If the lens shift is further increased to .+-.0.5 mm, then the
TE balance becomes 200% or more. In such a state, the tracking
control cannot be performed at all or a track jump will occur at
once, and therefore, this method cannot be applied to an actual
apparatus. That is why the optical tape machine 1 such as the one
of this embodiment cannot adopt the PP method.
[0060] Next, it will be described what problem the APP method,
which should be improved as far as a decline in TE balance during
the lens shift is concerned, may have. FIG. 16 is a schematic
representation illustrating a simplified exemplary optical
arrangement for an optical pickup that adopts the APP method. In
FIG. 16, the light source, the collimator lens and other members of
the optical system are not illustrated. The light 304 reflected
from an optical storage medium 301 is transmitted through an
objective lens 302 and incident on a photodetector 303.
[0061] FIG. 17 illustrates how the photodetector 303 and its
detector will look when viewed in the direction indicated by the
arrow A in FIG. 16. The photodetector 303 is divided into two in a
direction corresponding to the tracking direction and is further
divided into three in a direction corresponding to the track
direction. That is to say, the photodetector 303 has six
photosensitive cells C1 through C6.
[0062] According to the APP method, in order to reduce the offset
of the tracking signal due to the lens shift, a differential
operation is performed with the signs of the respective outputs of
the photosensitive cells C1, C3, C4 and C6, which are located in
surrounding areas where there is little tracking signal component,
inverted to a situation where the PP method is adopted. That is to
say, if the output signals of the photosensitive cells C1 through
C6 are identified by c1 through c6, respectively, the tracking
signal is represented by (c2-c5)+k(c4+c6)-k(c1+c3) as shown in FIG.
17. By making this arithmetic operation, the DC component offset
during the lens shift can be canceled and an offset-free tracking
signal can be obtained.
[0063] FIGS. 15(c) and 15(d) show how the TE balance and TE
amplitude were calculated with respect to the magnitude of lens
shift according to the APP method under the same simulation
condition as in the PP method described above. It should be noted
that the calculations were made with the coefficient k in the
equation for calculating the tracking signal by the APP method set
to be a value that would minimize the TE balance. As shown in FIG.
15(c), the TE balance appeared to be stabilized until the lens
shift reached .+-.0.4 mm and could be said to fall within the
permissible range even at .+-.0.5 mm. However, as shown in FIG.
15(d), the TE amplitude decreased steeply with the magnitude of
lens shift. Specifically, the TE amplitude decreased by 30% when
the lens shift was 0.15 mm and decreased by as much as 90% or more
when the lens shift was 0.5 mm. This is because as the magnitude of
lens shift increases, the point of incidence of the light reflected
from the optical disc on a detecting hologram will shift to
increasing degrees and the light representing the tracking signal
component will be incident on the detector in decreasing
quantities. As can be seen, according to the APP method, the TE
amplitude changes so significantly that the loop gain of the
tracking control changes considerably and loses its stability. For
that reason, the APP method cannot be applied to an optical tape
machine that requires a lens shift of approximately 0.5,
either.
[0064] Next, a result of a simulation that was carried out with the
CFF method adopted will be described. FIG. 18 is a schematic
representation illustrating a simplified exemplary optical
arrangement for an optical pickup that adopts the CFF method. In
FIG. 18, the light source, the beam splitter and other members of
the optical system are not illustrated. And FIG. 19 shows how
photodetectors 405 and 406 and their detector look when viewed in
the direction indicated by the arrow A in FIG. 18.
[0065] The light reflected from an optical storage medium 401 is
transmitted through an objective lens 402 and incident on a
detecting hologram 404, which has two regions that are divided in
the tracking direction and that have mutually different diffraction
properties. That is why the light that has been incident on the
detecting hologram 404 gets diffracted by those two regions, and
the diffracted light beams are directed to photodetectors 405 and
406, respectively. Since the reflected light beams that have been
split by the detecting hologram 404 are directed to the
photodetectors 405 and 406, the intensity of the reflected light
does not increase or decrease due to the lens shift as
significantly as in the PP method, and a relatively stabilized
output can be obtained.
[0066] FIGS. 15(e) and 15(f) show how the TE balance and TE
amplitude were calculated with respect to the magnitude of lens
shift according to the CFF method under the same simulation
condition as in the methods described above. As shown in FIG.
15(f), when the lens shift was 0.5 mm, the variation in amplitude
was within -10% and relatively stabilized. As shown in FIG. 15(e),
until the lens shift reached the vicinity of 0.12 mm, the TE
balance was within 15%. However, once the lens shift exceeded 0.12
mm, the TE balance was more than 15%. And when the lens shift was
0.5 mm, the TE balance increased to about 62%. Thus, even when the
CFF method is adopted under this condition, as the magnitude of
lens shift increases, the offset of the tracking signal also
increases. For that reason, the CFF method cannot be applied to an
optical tape machine that requires a lens shift of approximately
0.5, either.
[0067] On the other hand, in a configuration for splitting the
light beam emitted from a light source into a main beam and a
sub-beam using a diffraction grating to perform a DRAW operation,
the sub-beam travels obliquely to the main beam, thus raising the
problem of a deteriorated read performance. The main beam, which a
zero-order light beam, goes straight from the light source and
enters the aperture of the objective lens. Meanwhile, the sub-beam,
which is a diffracted light beam of first or higher order, goes
obliquely with respect to the main beam with a tilt angle
corresponding to its angle of diffraction defined with respect to
the main beam, and then enters the aperture of the objective lens.
That is why although the main beam can enter the entire aperture of
the objective lens, the sub-beam 210 cannot enter the entire
aperture 207 (i.e., is not incident on some portions of the
aperture 207) as shown in FIG. 20. This situation is equivalent to
causing a substantial decrease in the NA of the objective lens. As
can be seen, the sub-beam cannot be condensed as narrowly as the
main beam and will raise the problem of a deteriorated read
performance.
[0068] The present inventors discovered such a problem with the
related art and perfected an optical pickup that can perform read
and write operation with good stability even on an optical tape 2
which is an optical storage medium that will cause a significant
variation in track position. Hereinafter, it will be described in
further detail how the optical pickup 4 of this embodiment operates
in the optical tape machine 1 described above and what
configuration the optical pickup 4 may have.
1-3. Optical Pickup's Configuration
[0069] FIG. 4 schematically illustrates an optical configuration
for the optical pickup 4, which includes a light source 6, a
polarization beam splitter 11, a collimator lens 7, an objective
lens unit 18, a lens actuator 20, and a photodetector 12. The
objective lens unit 18 includes a polarization hologram plate 8, a
quarter wave plate 9 and an objective lens 5, which are supported
by an objective lens holder 10.
[0070] The light source 6 is a semiconductor laser light source and
is configured to emit an intensity modulated light beam in response
to an optical drive signal supplied from the optical power
modulator 531 shown in FIG. 3. In this manner, a light beam that
has had its intensity modulated according to the data to be written
can be emitted from the light source 6.
[0071] The polarization beam splitter 11 is an optical element that
reflects only a light beam with a particular polarization direction
and transmits the other light beams. The polarization beam splitter
11 leads not only the light beam emitted from the light source 6 to
the optical tape 2 but also the light beam reflected from the
optical tape 2 to the photodetector 12. The collimator lens 7
transforms the light beam that has been reflected from the
polarization beam splitter 11 into parallel light beams.
[0072] In this embodiment, the objective lens 5, the polarization
hologram plate 8, and the wave plate 9 are combined together to
form an objective lens unit 18. The objective lens unit 18 is
configured so as to be moved by a lens actuator 20 not only
perpendicularly to the storage layer of the optical tape 2 (i.e.,
in the focusing direction) but also parallel to the storage layer
and perpendicularly to the tracks (i.e., in the tracking direction)
as well. More specifically, as a voltage is applied to the focus
coil or tracking coil of the lens actuator 20, the objective lens
unit is moved by the focus coil, the tracking coil and an elastic
member such as a spring or a wire. These focus and tracking
controls by the lens actuator 20 are regulated by the servo control
section 550 shown in FIG. 3.
[0073] FIG. 5 is a schematic representation illustrating only a
portion of the configuration shown in FIG. 4 with the rest omitted
in order to describe the functions of the respective members of the
optical pickup 4. In FIG. 5, among the members shown in FIG. 4,
illustration of the light source 6 and the polarization beam
splitter 11 is omitted. Also, the intervals between the objective
lens unit 18, the collimator lens 7 and the photodetector 12 shown
in FIG. 5 are shorter than in FIG. 4.
[0074] The polarization hologram plate 8 has one surface with a
diffraction grating 8a which diffracts only a light beam that is
polarized in a particular direction and the other surface with a
polarization hologram element 8b which has two diffraction regions
that diffract only a light beam, of which the polarization
direction is perpendicular to the former direction. In the
polarization hologram element 8b, those two diffraction regions are
arranged so as to split the polarization hologram element 8b into
two in the tracking direction.
[0075] The diffraction grating 8a diffracts the light beam emitted
from the light source 6, thereby producing a plurality of
diffracted light beams including a zero-order diffracted light beam
and .+-.first-order diffracted light beams. In this embodiment, the
zero-order diffracted light beam is used as a main beam for writing
and the .+-.first-order diffracted light beams are used as
sub-beams for reading. The main beam and the sub-beams are
converged by the objective lens 5 onto the same track on the
optical tape 2. The hologram element 8b diffracts the light beam
that has been reflected from the storage layer of the optical tape
2, thereby producing a plurality of light beams including a
zero-order diffracted light beam (transmitted light beam) and
.+-.first-order diffracted light beams.
[0076] FIGS. 6(a) and 6(b) schematically illustrate the structure
of the polarization hologram plate 8. Specifically, FIG. 6(a) is a
perspective view illustrating the polarization hologram plate 8 and
FIG. 6(b) is a plan view of the polarization hologram plate 8 as
viewed in the direction indicated by the arrow shown in FIG. 6(a).
The diffraction grating 8a is designed so as to function as a
diffraction grating with respect to only a light beam going toward
the optical tape from the light source and function as a
transparent medium with respect to the light beam returning from
the optical tape toward the photodetector. On the other hand, the
polarization hologram element 8b is designed so as to function as a
transparent medium with respect to the light beam on the way toward
the optical tape and to diffract the incoming light in each of the
diffraction regions A and B with respect to the light beam on the
way back from the optical tape.
[0077] As shown in FIG. 4, the light beam emitted from the
semiconductor laser diode of the light source 6 is reflected from
the polarization beam splitter 11, and transmitted through the
collimator lens 7 to turn into a parallel light beam. This parallel
light beam is split by the diffraction grating 8a on one surface of
the polarization hologram plate 8, which is fixed on the objective
lens holder 10, into a main beam and two sub-beams. Those main and
sub-beams that have left the diffraction grating 8a are plane
polarized light beams, but turn into either circularly or
elliptically polarized light beams by being transmitted through the
quarter wave plate 9. The main beam and the two sub-beams that have
been transmitted through the quarter wave plate 9 are condensed by
the objective lens 5, thereby forming a main spot and two sub-spots
on the same track on the optical tape 2. The reflected light beams
that have left those spots on the optical tape 2 are transmitted
through the objective lens 5 and then transformed by the quarter
wave plate 9 into plane polarized light beams, of which the plane
of polarization has rotated 90 degrees with respect to the light
beam on the way toward the optical tape. As a result, the reflected
light is not diffracted by, but just transmitted through, the
diffraction grating 8a of the polarization hologram plate 8 shown
in FIG. 5. On the other hand, the polarization hologram element 8b,
which is arranged to face the diffraction grating 8a, does
diffracts the incoming light beam, thereby producing a zero-order
diffracted light beam and .+-.first-order diffracted light beams
out of each of the main and sub-beams. Of these diffracted light
beams, one of the .+-.first-order diffracted light beams that have
been produced from the main beam incident on the regions A and B is
used to generate a tracking error signal, while the other of the
.+-.first-order diffracted light beams that have been produced from
the main beam incident on the regions A and B is used to generate a
focus error signal.
[0078] The zero-order and .+-.first-order diffracted light beams
which have been produced from the main beam by the hologram pattern
that has been split into two regions A and B in the tracking
direction and the two sub-beams which have been reflected from the
optical tape 2 are incident on their associated photosensitive
elements on the photodetector 12. And based on the electrical
signals output from those photosensitive elements, a tracking error
signal, a focus error signal, and a DRAW read signal are obtained.
In this manner, the polarization hologram plate 8 generates not
only DRAW sub-beams by using the pattern of the diffraction grating
8a that intersects with the track direction at right angles but
also diffracted sub-beams to detect a tracking error signal by the
CFF method. That is to say, the polarization hologram plate 8 has
two functions.
[0079] FIG. 7 is a schematic representation illustrating how light
beam spots are formed on the optical tape 2. The optical system of
the optical pickup 4 is adjusted so that the main beam and two
sub-beams that have left the diffraction grating 8a form a main
spot 110 and two sub-spots 120a, 120b, respectively, on the same
track on the optical tape 2. The diffraction efficiency of the
diffraction grating 8a of the polarization hologram plate 8 is
determined so that if the intensity of light at the main spot 110
is set to be write power, the intensity of light at the sub-spots
120a and 120b is appropriate for reading a signal. If a write
operation is performed on the optical tape 2 in the direction
indicated by the arrow a, a mark that has just been written is
scanned and read with the first sub-spot 120a. On the other hand,
if a write operation is performed in the opposite direction to the
one indicated by the arrow a, a mark that has just been written is
scanned and read with the second sub-spot 120b. In this manner, the
optical pickup 4 of this embodiment is configured to carry out a
DRAW operation, no matter in which direction the optical tape 2 is
running.
[0080] In this embodiment, as the diffraction grating 8a and the
objective lens 5 are fixed on the objective lens holder 10 so as to
be arranged close to each other, the two sub-beams pass through the
aperture without deviating so much from the center of the aperture.
For that reason, unlike the known arrangement, the aperture of the
objective lens 5 will not have any portion that the sub-beam does
not enter at all depending on the angle of diffraction and the
distance. Consequently, according to this embodiment, the two
sub-beams, as well as the main beam, can also be condensed narrow
enough to produce a beam that can be used effectively to perform a
DRAW with high read performance.
[0081] FIG. 8 schematically illustrates an exemplary configuration
for the photodetector 12 and the signals generated. Specifically,
FIG. 8 illustrates how the photodetector 12 receives the light
reflected from the optical tape 2 and detects respective kinds of
signals. The photodetector 12 includes photosensitive elements 13
through 17 and generates those signals based on their outputs. In
the example illustrated in FIG. 8, the CFF method is adopted as the
tracking detecting method and the spot sized detection method is
adopted as the focus detecting method.
[0082] Of the reflected light that has left the two sub-spots 120a
and 120b on the optical tape 2, the light that has been transmitted
through the polarization hologram element 8b (i.e., the zero-order
diffracted light) is incident on the photosensitive elements 15 and
16. One of the output signals of the photosensitive elements 15 and
16 is used as a DRAW read signal. It depends on the direction in
which data is going to be written on the optical tape 2 which of
the two outputs should be used as the DRAW read signal. In this
embodiment, the photodetector 12 is configured to switch the
outputs of the photosensitive elements 15 and 16 according to the
writing direction on the optical tape 2.
[0083] On the other hand, of the reflected light that has left the
main spot 110 on the track on the optical tape 2, the light that
has been transmitted through the polarization hologram element 8b
(i.e., the zero-order diffracted light) is incident on the
photosensitive element 17. Although not shown in FIG. 8, the output
of the photosensitive element 17 may be used as a read signal in a
read mode that does not perform any DRAW operation.
[0084] Of the reflected light that has left the main spot 110 on
the track on the optical tape 2, the +first-order diffracted light
that has been diffracted by the polarization hologram element 8b is
incident on the photosensitive element 14, which is split into a
central portion and a peripheral portion. The photosensitive
element 14 performs a differential operation between the respective
outputs of the central and peripheral portions so as to generate a
focus signal by the spot sized detection method.
[0085] Meanwhile, of the reflected light that has left the main
spot 110, the -first-order diffracted light that has been
diffracted by the regions A and B of the polarization hologram
element 8b is incident on the two divided areas 13a and 13b of the
photosensitive element 13. By performing a differential operation
on the output signals of those two areas 13a and 13b, a tracking
signal is generated by the CFF method.
[0086] FIG. 9 is a schematic representation illustrating more
specifically how the .+-.first-order diffracted light beams that
have gone through each of the diffraction regions A and B of the
polarization hologram element 8b are incident on the photosensitive
elements 13 and 14. In FIG. 9, illustration of the optical elements
other than the polarization hologram plate 8 and the photosensitive
elements 13 and 14 is omitted. As shown in FIG. 9, the +first-order
diffracted light beams that have gone through the regions A and B
of the polarization hologram element 8b form light beam spots on
the left- and right-hand sides of the photosensitive element 14. As
the sizes of these light beam spots change according to the focus
state, a focus signal can be obtained by making the arithmetic
operation described above. On the other hand, the -first-order
diffracted light beams that have gone through the regions A and B
of the polarization hologram element 8b form light beam spots on
the left- and right-hand sides 13a and 13b of the photosensitive
element 13. When an abnormal track jump occurs, the horizontal
balance between these light beam spots changes. And therefore, a
tracking signal can be obtained by making the arithmetic operation
described above. In this example, the hologram pattern of the
regions A and B is designed so that the .+-.first-order diffracted
light beams are incident on the photosensitive elements 14 and 13,
respectively. However, this is only an example of the present
disclosure. That is to say, the regions A and B may also be
designed so as to lead the .+-.first-order diffracted light beams
to the photosensitive elements 13 and 14, respectively. Also, the
layout of the respective photosensitive elements may be changed
appropriately according to the hologram pattern of the polarization
hologram element 8b. In any case, as long as a tracking signal can
be obtained by the CFF method, the polarization hologram element 8b
and the photosensitive element 13 may have any other
configuration.
[0087] Next, it will be described how the verify operation can get
done by using the read signals that are output from the
photosensitive elements 15 and 16. In writing data on an unrecorded
optical tape 2, the verify operation can get done by calculating
the difference between the respective outputs of the photosensitive
elements 15 and 16 and by comparing their differential signal to an
optical drive signal. Hereinafter, this respect will be described
with reference to FIG. 10.
[0088] Portion (a) of FIG. 10 shows an exemplary waveform of an
optical drive signal. Portion (b) of FIG. 10 schematically
illustrates the shapes of marks to be recorded on a track. Each of
these marks is recorded on an area where the main spot 110 is
located when the optical drive signal is high.
[0089] Portions (c) and (d) of FIG. 10 show the waveform of a
signal representing the reflected light of a sub-beam that
irradiates the optical tape 2 after the main beam has irradiated it
(and which will be referred to herein as a "following sub-beam")
and that of a signal representing the reflected light of a sub-beam
that irradiates the optical tape before the main beam irradiates it
(and which will be referred to herein as a "preceding sub-beam"),
respectively. As can be seen from portion (c) of FIG. 10, the
waveform of the reflected light of the following sub-beam is
affected by a recorded mark because the spot of a light beam that
has been modulated with the optical drive signal has moved on a
track with the recorded mark. On the other hand, as can be seen
from portion (d) of FIG. 10, the waveform of the reflected light of
the preceding sub-beam is not affected by any recorded mark because
the spot of a light beam that has been modulated with the optical
drive signal has moved on a track with no recorded marks.
[0090] Portion (e) of FIG. 10 shows the waveform of a differential
signal that is obtained by subtracting the signal representing the
reflected light of the following sub-beam from the signal
representing the reflected light of the preceding sub-beam. This
waveform includes information about the positions and shapes of the
recorded marks, i.e., information about the data written. By
comparing this signal to the optical drive signal, it can be
determined whether or not data has been written properly. This
decision may be made by the CPU 540 shown in FIG. 3, for
example.
[0091] As described above, according to this embodiment, a verify
read signal is generated by calculating the difference between two
detection signals representing the reflected light beams of the two
sub-beams. However, the verify read signal may also be generated by
a different method. For example, if a photodetector is provided to
detect a light beam that has been emitted from the light source 6
but has not been reflected from the optical tape 2 yet, then the
verify operation can also be performed based on the detection
signal generated by the photodetector. That is to say, the same
verify read signal can also be obtained by calculating the
difference between a signal representing the following sub-beam and
either the output signal of the photodetector or a signal obtained
by making a correction on that signal as needed. Such a
photodetector may be arranged opposite to the light source 6 with
respect to the beam splitter 11. In such a configuration, the
signal representing the preceding sub-beam is not used. That is why
if the apparatus is configured so that the optical tape 2 always
runs in the same direction without reversing its running direction,
the preceding sub-beam does not have to be converged on the track
on the optical tape 2.
1-4. Condition for Obtaining Stabilized Tracking Signal
[0092] The CFF method for use in this embodiment is regarded as
achieving a relatively stabilized TE balance and not producing an
offset easily among various one-beam tracking detecting methods.
According to the result of the simulation described above, however,
the condition that the TE balance should be within 15% at a lens
shift of .+-.0.3 to 0.6 mm that an optical tape needs to have could
not be satisfied.
[0093] Thus, the present inventors carried out a simulation to
calculate how the tracking signal would behave when a lens shift
occurred with the track groove parameters and the values of
parameters to affect the offset of the tracking signal varied. As a
result, we found a condition for obtaining a stabilized tracking
signal even when a lens shift occurs. The following is such a
condition:
[0094] The tracking signal was calculated under the condition
including an objective lens' NA of 0.85, a lens shift of 0.5 mm, a
light source wavelength of .lamda., and a track groove pitch of
0.8.lamda. and with the radial Rim intensity R and the track groove
depth d varied. The results of the calculations are shown in FIGS.
11A through 12B.
[0095] FIG. 11A shows how the TE balance (%) at a lens shift of
-0.5 mm changed with the groove depth d, which was varied within
the range of 0 to 0.5.lamda., when the radial Rim intensity was set
to be 0.6, 0.76, 0.85 and 0.89. As a result, the TE balance changed
symmetrically with respect to a groove depth of 0.25.lamda..
Although not shown in FIG. 11A, the TE balance at a lens shift of
+0.5 mm is represented by a curve that is symmetrical with respect
to the TE balance 0% axis shown in FIG. 11A. In this case, the
width b between the two intersections at which the TE balance curve
associated with any of the Rim intensities and a line representing
a TE balance of -15% cross each other is identified herein by b_1,
b_2, b_3, or b_4 (.lamda.). In FIG. 11A, shown is the range in
which the (absolute value of the) TE balance at a lens shift of 0.5
mm is equal to or smaller than 15% when b falls within any of those
ranges. FIG. 11B is a graph plotting the relation between the
radial Rim intensity and the width b. The following approximation
equation can be derived from this graph:
b=1.2R.sup.2-R+0.395
[0096] As the width b is the width of a region that is symmetrical
with respect to a groove depth of 0.25 .mu.m, the condition to be
satisfied by the groove depth d(.lamda.), the radial RIM intensity
R and the wavelength .lamda. is represented by the following
Inequality (1):
0.25.lamda.-(1.2R.sup.2-R+0.395).lamda./2.ltoreq.d.ltoreq.0.25.lamda.+(1-
.2R.sup.2-R+0.395).lamda./2 (1)
[0097] This equation specifies a condition imposed on the groove
depth d, the radial Rim intensity R and the wavelength .lamda. to
make the (absolute value of the) TE balance equal to or smaller
than 15% even when the lens shift is 0.5 mm.
[0098] Next, the results of the simulation that was carried out on
the TE amplitude will be described with reference to FIGS. 12A and
12B. FIG. 12A is a graph showing how the TE amplitude at a lens
shift of 0.5 mm changed with the groove depth d, which was varied
within the range of 0 to 0.5.lamda., when the radial Rim intensity
was set to be 0.6, 0.76, 0.85 and 0.89, under the same condition as
the calculations shown in FIGS. 11A and 11B. In this case, the TE
amplitude associated with a radial Rim intensity of 1 and a lens
shift of 0 mm is supposed to be one. The TE amplitude reached its
peaks when the groove depths were 0.125.lamda. and 0.375.lamda. but
decreased as the groove depth exceeded those values. Particularly
once the amplitude became smaller than 0.5, the amplitude decreased
steeply and became substantially equal to zero when the groove
depth was 0.25.lamda.. That is why according to this embodiment, TE
amplitude.gtoreq.0.5 is defined to be the condition to satisfy as a
range in which the TE amplitude is large enough and stabilized. In
this case, the width a between the two intersections at which the
TE amplitude curve associated with any of the Rim intensities and a
line representing a TE amplitude of 0.5 cross each other is
identified herein by a_1, a_2, a_3, or a_4. This width a is the
width of a range in which TE.gtoreq.0.5 can be maintained at a lens
shift of 0.5 mm.
[0099] FIG. 12B is a graph plotting the radial Rim intensity and
the width a. The following approximation equation can be obtained
from this graph:
a=-0.2R.sup.2+0.45R-0.085
[0100] Since the width a is the width of a range that is symmetric
with respect to a groove depth of 0.125.lamda., the condition to be
satisfied by the groove depth d(.lamda.), the radial Rim intensity
R and the wavelength .lamda. is represented by the following
Inequality (2):
0.125.lamda.-(-0.2R.sup.2+0.45R-0.085).lamda./2.ltoreq.d.ltoreq.0.125.la-
mda.+(-0.2R.sup.2+0.45R-0.085).lamda./2 (2)
[0101] Also, as TE amplitude.gtoreq.0.5 is also satisfied in a
range that is symmetric with respect to a groove depth of
0.375.lamda., the condition to be satisfied by the groove depth
d(.lamda.), the radial Rim intensity R and the wavelength .lamda.
is represented by the following Inequality (3):
0.375.lamda.-(-0.2R.sup.2+0.45R-0.085).lamda./2.ltoreq.d.ltoreq.0.375.la-
mda.+(-0.2R.sup.2+0.45R-0.085).lamda./2 (3)
[0102] Inequalities (2) and (3) specify the condition imposed on
the groove depth d, the radial Rim intensity R and the wavelength
.lamda. to satisfy TE amplitude.gtoreq.0.5 even when the lens shift
is 0.5 mm.
[0103] Consequently, if the radial RIM intensity R and the
wavelength .lamda. of each optical pickup 4 are set so as to
satisfy either Inequalities (1) and (2) or Inequalities (1) and
(3), then TE balance.ltoreq.15% and TE amplitude.gtoreq.0.5 are
satisfied even at a lens shift of 0.5 mm. That is to say, a
stabilized tracking signal with little offset can be obtained and
the tracking control can be stabilized.
[0104] In the simulations described above, the objective lens' NA
is supposed to be 0.85, the lens shift 0.5 mm and the track groove
pitch 0.8.lamda.. However, even if the actual values are somewhat
different from these values, the conditions represented by
Inequalities (1) to (3) are substantially satisfied. For example,
even if the objective lens' NA is set to be 0.81 to 0.89, the upper
limit of the lens shift 0.3 mm to 0.6 mm, and the track groove
pitch 0.75.lamda. to 0.85.lamda., the conditions specified by those
Inequalities (1) to (3) may also be applied.
[0105] As can be seen, according to this embodiment, even if the
upper limit of the lens shift is set to be 0.3 mm to 0.5 mm by
adopting the CFF method as a tracking detecting method, the offset
can still fall within a permissible range and a sufficient TE
amplitude can also be secured. As a result, a stabilized tracking
control is realized even in an optical tape machine with a
significant lens shift.
1-5. Effects
[0106] As described above, the optical pickup 4 of this embodiment
can write data on a track of an optical storage medium (such as the
optical tape 2) and read the data being written on that track in
parallel. The optical pickup 4 includes: a light source 6 which
emits a light beam; a first diffractive element (diffraction
grating 8a) which diffracts light that is polarized in a particular
direction; an objective lens 5 which converges a write beam and a
read beam onto the same track on the optical tape 2; a lens
actuator 20 which shifts the objective lens to perform a focus
control and tracking control; a wave plate 9 which is arranged
between the diffraction grating 8a and the optical tape 2; a second
diffractive element (polarization hologram element 8b) which has
two diffraction regions A and B with different diffraction
properties that are arranged in a direction corresponding to the
tracking direction; and a photodetector 12. The lens actuator 20
shifts the objective lens 5 so that the magnitude of shift of the
objective lens 5 from its initial position as measured in a
tracking direction has an upper limit of 0.3 mm to 0.6 mm. The
diffraction grating 8a splits the light beam that has been emitted
from the light source 6 into a plurality of light beams including a
write beam and a read beam. The wave plate 9 is designed so that
the polarization direction of light going from the diffraction
grating 8a toward the wave plate 9 is perpendicular to the
polarization direction of light going from the wave plate 9 toward
the diffraction grating 8a. In the polarization hologram element
8b, each of the diffraction regions A and B is configured to
diffract light that is polarized in a direction that intersects
with the particular direction at right angles. The polarization
hologram element 8b splits the write beam that has been reflected
from the optical tape 2 through each of the diffraction regions A
and B into a transmitted light beam and .+-.first-order diffracted
light beams. The photodetector 12 includes a plurality of
photosensitive elements that are configured to detect the
transmitted light beam, the .+-.first-order diffracted light beams
that have left the diffraction regions A and B, and the read beam
that has been reflected from the optical tape 2. By adopting such a
configuration, a DRAW operation can be performed on an optical tape
in which a track position may vary significantly during a read or
write operation and a stabilized tracking signal can be obtained.
As a result, the quality of the read/write operation can be
improved.
[0107] In the embodiment described above, a diffraction grating 8a
is used as the first diffractive element and a polarization
hologram element 8b is used as the second diffractive element. And
the diffraction grating 8a and the polarization hologram element 8b
are combined together to form a polarization hologram plate 8.
Thus, an element that generates a main beam and sub-beams for use
to perform a DRAW operation and an element that generates a
diffracted light beam to obtain a tracking signal by the CFF method
can be combined together. As a result, the overall size of the
apparatus can be reduced.
[0108] On top of that, the polarization hologram plate 8, the wave
plate 9 and the objective lens 5 are combined together to form a
single lens unit 18. And the lens actuator 20 shifts the objective
lens 5 by shifting the lens unit 18 in its entirety. Thus, it is
possible to prevent a DRAW sub-beam from deviating significantly
from the center of the aperture of the objective lens 5. As a
result, the DRAW read signal can have improved quality. In
addition, since the objective lens 5 and the polarization hologram
plate 8 always move together during a lens shift, deterioration in
the quality of a tracking signal can also be minimized.
[0109] Furthermore, the photodetector 12 generates a tracking
signal by calculating the difference between the output signals of
two photosensitive elements, which detect +first-order diffracted
light beams or -first-order diffracted light beams that have left
the two diffraction regions A and B of the polarization hologram
element 8b, among the plurality of photosensitive elements. As a
result, a relatively stabilized tracking signal can be
obtained.
[0110] Furthermore, if the track pitch of the optical tape 2 is a
and the wavelength of the light emitted from the light source 6 is
.lamda., a/0.85.ltoreq..lamda..ltoreq.a/0.75 is satisfied. The
objective lens 5 has a numerical aperture of 0.81 to 0.89. If the
radial Rim intensity of the light entering the objective lens is R
and the track groove depth of the optical storage medium is d, then
either Inequalities (1) and (2) or Inequalities (1) and (3) are
both satisfied. As a result, even if the lens shift is set to be as
large as 0.3 mm to 0.6 mm, a tracking signal, of which the TE
balance and TE amplitude are both stabilized, can be obtained.
Other Embodiments
[0111] Although an embodiment of an optical pickup has been
described herein as just an example of the present disclosure,
various modifications, replacements, additions or omissions can be
readily made on that embodiment as needed and the present
disclosure is intended to cover all of those variations. Also, a
new embodiment can also be created by combining respective elements
that have been described for that embodiment disclosed herein.
[0112] In the embodiment described above, the diffraction grating
8a is used as an example of the first diffractive element and the
polarization hologram element 8b is used as an example of the
second diffractive element. However, this is only an example of the
present disclosure. The first diffractive element may have any
other configuration as long as it is configured to split the light
beam emitted from the light source into a plurality of light beams
including a write beam and a read beam. Likewise, the second
diffractive element may also have any other configuration as long
as it has two diffraction regions that are arranged in a direction
corresponding to the tracking direction and that have mutually
different diffraction properties and as long as each of those
diffraction regions is configured to split the write beam that has
been reflected from the optical storage medium into a transmitted
light beam and at least one diffracted light beam. For example, the
polarization hologram plate 8 may be replaced with a hologram
pattern in which two layers that have the same properties as the
diffraction grating 8a and the polarization hologram element 8b are
stacked one upon the other.
[0113] Also, in the embodiment described above, the first and
second diffractive elements, the wave plate and the objective lens
are combined together to form a single lens unit. However, this is
only an example of the present disclosure. Alternatively, these
elements may be provided separately from each other. In that case,
a mechanism that shifts both the objective lens 5 and the second
diffractive element together in the tracking direction may be
provided.
[0114] Furthermore, the optical storage medium does not have to be
an optical tape. The optical pickup and optical read/write
apparatus with the configurations described above are also
applicable to any other kind of optical storage medium.
[0115] Furthermore, in the embodiment described above, the optical
read/write apparatus has twelve optical pickups. However, the
number of optical pickups provided may be determined arbitrarily.
The present disclosure is applicable to an optical read/write
apparatus that has at least one optical pickup.
[0116] Various embodiments of the present disclosure have been
described by providing the accompanying drawings and a detailed
description for that purpose.
[0117] That is why the elements illustrated on those drawings
and/or mentioned in the foregoing description include not only
indispensable elements that need to be used to overcome the
problems described above but also other inessential elements that
do not have to be used to overcome those problems but are just
mentioned or illustrated to give an example of the present
disclosure. Therefore, you should not make a superficial decision
that those inessential additional elements are indispensable ones
because they are illustrated or mentioned on the drawings or the
description.
[0118] Also, the embodiments disclosed herein are just an example
of the present disclosure, and therefore, can be subjected to
various modifications, replacements, additions or omissions as long
as those variations fall within the scope of the present disclosure
as defined by the appended claims and can be called
equivalents.
[0119] If an optical pickup according to an embodiment of the
present disclosure is used in a bulk data storage system that
includes a number of such pickups, data can be written accurately
either on multiple different areas of a given optical storage
medium or on multiple different optical storage media in parallel
with each other. Thus, the optical read/write apparatus of the
present disclosure can be used effectively as a cost-effective
read/write apparatus with a simplified configuration.
[0120] While the present invention has been described with respect
to preferred embodiments thereof, it will be apparent to those
skilled in the art that the disclosed invention may be modified in
numerous ways and may assume many embodiments other than those
specifically described above. Accordingly, it is intended by the
appended claims to cover all modifications of the invention that
fall within the true spirit and scope of the invention.
[0121] This application is based on Japanese Patent Applications
No. 2011-227530 filed Oct. 17, 2011 and No. 2012-171842 filed Aug.
2, 2012, the entire contents of which are hereby incorporated by
reference.
* * * * *