Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PATEN~
The present invention relates to optical
heads for use in data recording and retrieval systems.
Optical heads produce a focused beam of light
on a medium containing information and detect the light
lS reflected from the medium to determine the information
content of the medium. Mechanisms for maintaining the
focus and tracking of the optical head are required.
with the recent advances in semiconductor lasers, there
has been an increasing use of these lasers in data
retrieval and recording systems. The compact audio
disc player is a significant example of how lasers are
used in playing back prerecorded music, which is a form
of information. The concept of the compact audio disc
player or the long play video disc player can be
appiied to the storage of data for a large computer
network, mini computers or even personal computers.
When lasers are employed in these devices,
the light emitted by the lasers must be controlled by
appropriate optical component~ to produce a very small
spot of light on the medium surface. Light reflected
off of the medium i8 projected back to a detector from
which recorded information and other signals relating
to the status of the focus and tracking can be derived.
Some examples of patents covering optical systems for
such applications are U.S. Patent 4,486,791, U.S.
Patent 4,193,091, U.S. Patent 4,135,083, U.S. Patent
4,034,403, U.S. Patent 3,969,573, U.S. Patent
. ~.
r
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4,057,833, U.S. Patent 3,962,720, West German Patent
2501124 and U.S. Patent 4,198,657.
In some compact disc players the optical unit
is separated into two parts. The first part contains a
5 laser and a collimating lens to produce a nearly ;
collimated (parallel3 laser beam. It also contains a
beam-splitter to direct the return light reflected off
the medium to a detector reading the recorded informa-
tion. The second part contains a focusing objective
lens and a mechanism for moving it up and down so that
the focus spot is maintained on the medium surface.
In another version of these devices, a laser
beam from the laser diode is directly imaged onto the
medium by an objective lens without the use of a
collimater. In the return path, the light is imaged on ~
a detector by a beam-splitter.
Figure 1 shows one embodiment of a prior art
optical read head for a compact disc player. The head
consists of a laser pen 10 and a focusing and tracking
actuator 12. The laser beam is focused on an informa-
tion medium 14 at a spot 16. A laser beam 18 in the
shape of an elliptical cone is emitted from the semi-
conductor laser diode 20. Laser beam 18 passes un-
changed throu~h a beam-splitter 22 to a collimating
lens 24. Collimating lens 24 produces a substantially
parallel beam of light 26 which impinges upon an
objective lens 28. Objective lens 28 focuses beam 26
onto medium 14 at spot 16. The focusing of lens 28 is
accomplished through the use of a magnetic coil 30
which moves objective lens 28 up and down with respect
to medium 14. In addition, a tracking actuator 12 may
move objective lens 28 radially along the direction of
medium 14, which is typically a disc.
When the laser beam is returned or reflected
off of medium 14, part of the beam is reflected by
beam-splitter 22 and passes through a biprism or
cylindrical lens 32 to a photodetector 34. Lens 32
- ( ~
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produces a pattern on photodetector 34 which varies
according to the focus of spot 16. Thus, when detec-
tor 34 detects a variation f,om the ideal focus,
: appropriate electrical signals can be supplied to
; 5 coil 30 to move objective lens 28 to the correct
pos~tion. This mechanism is somewhat complicated and
requires a large number of elements which must be
precisely aligned relative to each other.
Figure 2 illustrates another embodiment of a
prior art optical head. A laser diode 36 emits a
, diverging laser beam 3B which passes through a
beam-splitter 40 directly onto an objective lens 42.
Again, lens 42 is mounted in a focusing and tracking
: actuator 44 which includes a coil 46. The beam im-
pinges upon a medium 48 and a portion of the reflected '~
beam is directed by beam-splitter 40 through biprism 50
onto photodetector 52. This embodiment eliminates the
need for collimating lens 24 of Figure 1, but requires
that the laser pen and focus and tracking actuator of
Figure 1 be combined in one unit because of the need to
precisely align objective lens 42 and beam-splitter 40.
Thus, the embodiment of Figure 2 cannot be produced
modularly as can the embodiment of Figure 1. In the
embodiment of Figure 1, the use of collimater lens 24
obviates the need for precise alignment of focusing and
tracking actuator 12 and laser pen 10. Thus, the
disadvantage of the embodiment of Figure 2 is that in
the event of a m,alfunction the entire unit must be
repaired or replaced.
A third prior art optical head utilizing a
pair of hologram lenses is shown ln Figure 3. A
hologram lens is a diffraction qrating which was
produced using holographic methods. A diffraction
grating is a grating having a series of slits so that
it diffracts light shined upon it. Light impinging on
a diffraction grating will produce a series of
diffracted beams at different angles from the central
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axis of the impinging light beam. The value of the
angles of diffraction depend upon the wavelength of the
light and the spacing of the grating. Diffraction
gratings can be created mechanically, but there is a
limit to the size of the spacing that can be achieved.
A hologram lens is a diffraction grating created by the
use of two interfering coherent laser beams in such a
manner that the beams form a suitable angle relative to
each other and a diffractiun grating corresponding to
the resulting interference pattern is formed. This
interference pattern i5 projected onto a substrate,
such as glass coated with photoresist. Upon develop-
ment of the photoresist, unexposed areas (negative
photoresist) or exposed areas (positive photoresist)
are removed, leaving a number of parallel grooves.
Vacuum deposition of a suitable metal on the grooves
provides diffraction grating of the reflection type,
comprising a number of equidistant parallel lines. A
discussion of the formation of a hologram lens accord-
ing to various techniques is set forth in U.S. Patent4,560,249.
The optical head of Figure 3 uses a laser
diode 54 to emit a laser beam 56. Laser beam 56
impinges upon a hologram lens 58, and one of the
diffracted beams from hologram lens 58 impinges upon
hologram lens 60. The diffracted beam from lens 58 to
lens 60 is a parallel beam of light, and thus hologram
lens 58 replaces the collimating lens of Figure 1.
This beam hits hologram lens 60 at an angle, causing
hologram lens 60 to emit a focused beam onto a medi-
um 62. Thus, hologram lens 60 replaces objective
lens of Pigures 1 and 2. On the return path, the
undiffracted, collimated beam o~ light passing through
hologram lens 58 impinges upon a photodetector 64 after
passing through a biprism or wedge 66. The collimation
of the return beam allows a photodetector 64 to be
placed a different distance from medium 62 than
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laser 54. Photodetector 64 includes four separate
; photodetectors 68. Biprism 66 splits the laser beam to
create two focused beams of light which fall on differ-
ent ones of detectors 68. A change in focus will cause
these beams of light to move from one of detectors 68
to another, thereby enabling the detection of an
out-of-focus condition. The apparat~s of Figure 3 is
disclosed in U.S. Patent 4,458,980.
An alternative to a biprism lens is a cylin-
drical lens which is polished with two separate curva-
tures to produce an as~igmatic beam. The astigmatic
beam is focused on the center of a four quadxant
photodetector and will be a circle when in focus. When
out of focus in one direction, it will be an elliptical
15 beam at a first angle and thus two of the ~
photodetectors will detect more light than the other
two! indicating a focus error. When out of focus in
the other direction, an elliptical beam at a different
angle is produced, which can also be detected.
In addition to correcting for focus error, or
the distance from the objective lens to the medium, the
optical head must also track the data. The data is
typically written onto a series of concentric or
spiraling grooves on a disc. The grooves are very
narrow and are spaced by approximately 1.6 microns to
allow the placement of pits having a size on the order
of 1 micron. Data is typically stored in the form of a
combination of pits and "lands, n where lands are the
area between the recessed pits. The pits serve to
scatter the laser beam while the lands reflect it. A
change in the amount of reflected light indicates a
transition from a pit to a land. Often, it is these
transitions which are used to represent bits of data
rather than the pits and lands themselves.
~ecause the thin groove which the pits and
lands are centered on is separated from other grooves
by a distance of the same order of magnitude as the
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laser beam diameter, a diffraction effect is produced
on the beam reflected back to the detector. This
diffrac~ion effect produces three beams which partially
overlap. If the beam moves off the groove to the area
between grooves, interferences of the overlapping beams
causes the right and left half of the pattern to
alternate in brightness. By using multiple
photodetectors, this change in brightness can be
monitored to detect tracking errors and produce a
feedback signal to put the beam back on track.
Another method for tracking is the use of a
diffraction grating in front of the laser to split the
laser beam into three beams before it hits the medium.
The center tracking beam is focused on the track with
the left and right sides being on the left and right ~
sides of the track. These three beams are reflected
back and split off by a beam-splitter to a separate set
of photodiodes which detect the intensity of the two
weaker beams. When they are of different intensities,
the error signal activates a servo mechanism that moves
the optical head to correct for the error.
Another type of medium uses thermal magnetic
recording to provide an erasing and rewriting capabil-
ity. The principle of thermal magnetic recording is
based on a characteristic of certain ferromagnetic
material. When the temperature of the material is
raised above the Currie temperature, the magnetization
of the material can be affected by a small magnetic
field. This principle is used for thermal
magneto-optics data storage where a laser beam is
focused on the recording medium to raise the tempera-
ture of the medium above the Currie temperature. A
small electro-magnet is placed on the other side of the
medium to create a magnetic field near the medium to
change the magnetizatiorl of the medium. To retrieve
information from the medium a laser beam is again
focused on the medium but at lower power. Depending on
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the magnetization of the medium, the polarization of
the beam reflected off the medium is eîther unchanged
or rotated by about 0.4 degree. A polarizer inserted
before a photodetector allows the detector to sense
these two different states of polarization of the
returned beam. One method of erasing the recorded
information is to first reverse the direction of
magnetization of the electro-magnet and then apply a
focused laser beam to raise the temperature of the
medium to above the Currie temperature to uniformly
magnetize the medi~m in one direction.
To use the above principle in optical data
storage systems an optical head is needed to produce a
focused laser beam on the thermal magnetic medium.
Moreover, a polarizer is needed to permit the detector
to read the information recorded on the medium.
Polarizers needed for the thermal magnetic
optical heads are available commercially in two forms.
One is a sheet type polarizer based on dichroism, which
is the selective absorption of one plane of
polarization in preference to the other orthogonal
polarization during transmission through a materia~.
Sheet polarizers are manufactured from organic mate-
rials which have been imbedded into a plastic sheet.
The sheet is stretched, thereby aligning the molecules
and causing them to be birefringent, and then dyed with
a pigment. The dye molecules selectively attach
themselves to the aligr.ed polymer molecules, with the
result that absorption is very high in one plane and
relatively weak in the other. The transmitted light is
then linearly polarized. The optical quality of the
sheet type polarizers is rather low. They are used
mostly for low power and visual applications.
Another type of polarizer is based on the use
~5 of wire grid structures to separate the two orthogonal
polarizations. When light radiation is incident on an
array o parallel reflective stripes whose spacing is
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on the order of or less than the wavelength of the
radiation, the radiation whose electric vector is
perpendicular to the direction of the array is reflect-
ed. The result is that the transmitted radiation is
largely linearly polarized. The disadvantage of both
types of polarizers is that their light efficiencies
are typically less than 30%.
There is thus a need for a simpler optical
head with a more efficient polarizer.
.
The present invention is an improved optical
head which uses a single diffraction grating to elimi-
nate the need for both a beam-splitter and a biprism
lens. This inYention allows the placement of all the
elements except the photodetector along a single
optical axis with the photodetector being placed
immediately adjacent to this optical axis. This
arrangement reduces alignment problems and vibrational
errors. A semiconductor laser is provided which
produces a laser beam which impin~es upon a movable
objective lens. The objective lens focuses the laser
beam onto an information medium. A diffraction grat-
ing, which may be holographic, is placed between the
laser and the objective lens. The diffracted beams on
the forwaxd path from the laser to the objective lens
are not used, but on the return path, one of the
diffracted beams is focused onto a photodetector.
In a first embodiment, the photodetector is
3C adjacent the semiconduCtor laser in 5ubstantially the
same plane ~within 2 mm). ln a second embodiment, an
additional lens is used to produce an additional focus
point intermediate the laser and the medium, with the
photodetector being ln substantially the same plane as
this additional focal point. By using an appropriate
pattern for the hologram lens, the focus point for the
return laser beam can be positioned in front of the
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laser or the additional focus point. This simplifies
construction since the photodetector cannot easily be
fabricated in the same plane as the laser face.
PrPferablyr the photodetector is a plurality
~5 of photodetectors having a center within five millime-
ters of the semiconductor laser. The semiconductor
laser and the photodetector may be mounted on the same
heat sink, thus ensuring that the photodetector will
move in the same amount and direction as the laser as a
result of any vibration, thus improving the accuracy of
the optical head.
A collimating lens may be included, or a
mirror may be used to allow the laser and photodetector
to be mounted at an angle to the axis of the objective
15 lens and the hologram lens. Focusing may be accom- ~
plished by variations in the grating pattern of the
hologram lens. For instance, an astigmatic image may
be produced by having the spacing increase from one
side to the other of the hologram lens. A
four-quadrant photodetector can then determine whether
the beam is in focus and tracking by comparing the
amount of light impinging on the separate quadrant
detectors. Alternately, the hologram lens may have a
grating with a first spacial frequency (spacing dis-
tance) on a first half of the lens and second spacialfrequency on a second half of the lens. This will
produce two diffraction beams, one from each side,
which are both imaged on four parallel photodetectors.
By monitoring the movement of the two beams among the
four photodetectors, the focus and tracking of the beam
can be monitored.
For applications utilizing a thermal magnetic
medium, a polarizer is placed in front of each
photodetector to enable detection of changes in
polarization on the medium. The polarizer is pref-
erably formed from a holographic grating with a line
spacing less than the wavelength of the laser beam used
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for reading and writing. A single holographic grating can be
used as the polarizer, with the grating having a central
portion without lines for allowing the passage of the source
beam and having lines on respective sides which are
orthogonal to each other to present orthogonal polarizations
to a pair of photodetectors. A method for making such a
polarizer is also disclosed.
The present invention thus provides a simple
optical head with less parts than the prior art which is less
susceptible to error due to vibration than the prior art
optical heads. The beam-splitter and biprism lens of the
prior art are both replaced by the single hologram lens,
allowing the placement of all of the elements of the optical
lS head except the photodetector along a single optical axis.
For a fuller understanding of the nature and
advantages of the invention, reference should be made to the
ensuing detailed description taken in conjunction with the
accompanying drawings, in the drawings:
Figure 1 is a diagram of a prior art optical
head using a collimating lens;
Figure 2 is a diagram of a prior art optical head
without a collimating lens;
Figure 3 is a diagram of a prior art optical head
using a pair of hologram lenses;
Figure 4 is a diagram of a preferred embodiment of
an optical head according to the present i.nvention having a
laser diode and a photocletector .immediately next to each
other;
Figure 5 is a front plan view of the semiconductor
laser and photodetector of Figure 4;
Figure 6 is a diagram of the grating of a hologram
lens according to the present invention having different
spacing on each half;
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Figure 7 and 7A are diagrams of an image from
the hologram lens of Figure 5 projected on four paral-
lel photodetectors according to the present invention;
Figure 8 is a diagram of an astigmatic
-5 hologram lens having parallel, straight frinyes accord-
ing to the present invention;
Figures 9A, 9B and 9C are diagrams of three
focus conditions of a laser beam from the hologram lens
of Figure 8 imposed upon a four-quadrant photodetector
according to the present invention;
Figures 10 and 11 are diagrams of alternate
configurations of astigmatic hologram lenses having
parallel, curved fringes and nonparallel, curved
fringes, respectively;
Figures 12A-l~C show the pattern produced by ~
the lens of Figure 11 in three different focal planes;
Figure 13 is a diagram of a variation of the
embodiment of the optical head of Figure 4 without a
collimating lens;
Figure 14 is a diagram of an optical head
according to the present invention utilizing an angled
mirror;
Figure 15 i5 a diagram of a second preferred
embodiment of the present invention using a
photodetector at a intermediate focal point of the
source laser beam;
Figure 16 is a front plan view of the
photodetector and polarizer of Figure 15;
Figure 17 is a diagram of the embodiment of
Figure 15 without the collimating lens;
Figure 18 is a diagram of an apparatus for
producing the polarizer of Figure 15; and
Figures l9A-D are diagrams of the polarizer
of Figure 15 during various stages of its manufacture
with the apparatus of Figure lB.
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A first embodiment of an optical head accord-
ing to the present invention is shown in Figure 4.
This optical head arrangement can be used for different
recording mediums, such as a thermal-magnetic medium or
a medium usin~ lands and pits. A semiconductor la~er
and detector 68 radiates a laser beam 70 to a
collimating lens 72. The collimated beam passes
through a hologram lens 74 to an objective lens 76.
~ologram lens 74 can also be put between semiconductor
laser and detector 68 and the collimating lens 72.
Objective lens 76 focuses the beam onto a medium 78.
Objective lens 76 can be moved by a coil 80 in a
focusing and tracking actuator 82. Semiconductor laser
and detector 68, collimating lens 72 and hologram
lens 74 form a laser pen 84 portion of the optical
head.
Figure 5 shows a front view of the semicon-
ductor laser and photodetector 68. A semiconductor
laser 86 is mounted on a heat sink 88. A four-quadrant
photodetector 90 is mounted on the face of heat
sink 88. A photodetector 92 is located behind semicon-
ductor laser 86 to measure the light emitted from the
semiconductor laser. Photodetector 92 is at an angle
so that it does not reflect light back into semiconduc-
tor laser 86. Four-quadrant detector 90 is preferably
within 5 millimeters of semi~onductor laser 86 and is
preferably within 2 mm of the same plane as laser 86.
A typical manufacturing process results in A
photodetector with a thickness of 0.25 to 0.5 mm, and
thus a separation of this amount hetween the
photodetector surface and the laser diode face. This
separation can be compensated for with a hologram lens
combining an astigmatic wavefront for focus error
generation and a spherical wavefront similar to a
conventional lens. This should pl~ce the detector
within the focus error range of the optical head. A
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more detailed description of a preferred embodiment of
semiconductor laser and photodetector 68 is presented in
United States Patent 4,757,197 issued July 12, 1988~
In operation, laser beam 70 from semiconductor
laser 86 is collimated or made parallel by collimating lens
72. This collimated beam passes through hologram lens 74 to
produce a zero order diffracted beam and a number of higher
order diffracted beams. The zero order diffracted beam is the
one which continues on the same path, and not at an angle,
and is the only beam used in the forward light path of the
optical head. This beam is focused on medium 78 by objective
lens 76 which can be mo~ed with coil 80.
On the return path, the beam again hits hologram
lens 74 producing zero and higher order diffracted beams~
The zero order beam is returned to the laser and is not used
for detection. ~Some prior art systems utilize the change in
power of the laser due to the reflected beam to measure the
intensity of the reflected beam. These systems, however,
cannot do focusing and tracking in this manner.) The
reflected beam on the return path also produces higher order
diffracted beams from holographic lens 74. One of these
higher order diffracted beams is imaged onto photodetector
90. Preferably, this is the first order diffracted beam
which is diffracted by an angle of approximately 10 degrees.
This structure has the advantage of having the semiconductor
laser and photodetector mounted on the same mechanical
structure. Thus, motion of this mechanical unit has little
effect on the signals received by the optical head because
the detector will move in the same amount and direction as
the beam will move due to movement of the semiconductor
laser.
The embodiment of Figure 4 is modular in that laser
pen 84 can be replaced separately from focusing
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14
and tracking actuator 82. In fact, the focusing and
tracking actuator of Figure 4 is the same as the
focusing and tracking actuator of the prior ar~.
In addition to diffracting the beam of light
to move it slightly so that it will impinge upon the
photodetector, hologram lens 74 can also be constructed
to perform a tracking and focusing function in conjunc-
tion with the photodetectar. Figure 6 illustrates one
construction of holographic lens 74 which can be used
in conjunction with four parallel photodetectors as
shown in Figure 7. Figure 6 shows a portion of a
hologram lens 94 having a first half 96 and a second
half 98 divided by a centerline 100. The spacing
between lines or stripes 102 on side 98 of hologram
lens 94 is less than the spacing between lines 104 on
side 96. Side 98 thus has a grating with a spacial
frequency fA = f+~f while side 96 has a grating with
spacial frequency fB = f-~f- When a laser beam is
directed at centerline 100, the light pattern on
photodetector 106 is shown in Figure 7. The light
pattern consists of a first spot 108 and a second
spot 110. Spots 108 and 110 represent the first order
- diffracted beam from sides 96 and 98, respectively,
with the zero order beam being to the right of
photodetector 106 of Figure 7.
This diffraction pattern can be seen more
clearly in Figure 7A. The zero order beam from both
the left side of the grating 98 having the spacial
frequency fA and the right side of the grating 96
having the spacial frequency fB shows up as a spot 112
on laser 114. Because ~ is a lower spacial frequency
(larger spacings, or period) ~an fA, its first order
diffraction beams will show up on either side of the
zero order ~eam at a closer distance to the center as
spots 116, 118, respectively. Spot 116 is focused on
photodetector 106. Similarly, the first order
diffraction beam for side 98 at frequency fA shows up
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as spots 120 and 122. The second order diffraction
beams produce spots 124 and 126, respectively, from the
right and left sides of hologram lens 94. The zero
order beam, the right side first order beam and all the
high order beams are not used.
Optical detector 106 of Figure 7 contains
four separate photodetectors A, B, C and D. ~he focus
and tra~king can be monitored by comparing the signals
detected by photodetectors A, B, C and D. When the
distance between medium 78 and the objective len~
increases beyond the focus distance, spot 108 will move
from detector B to detector A, and spot 110 will move
from detector C to detector D. Similarly, when medi-
um 78 moves closer to the objective lens than the focal
distance, spot 108 will move from detector A to detec-
tor B and spot 110 will move from detector D to detec-
tor C. Accordingly, the focus error signal is generat-
ed by (A - B) + (D - C).
The sideways movement of the objective lens
relative to the medium away from the tracking groove
will be indicated by the modulation of the reflected
beams. This modulation is a variation in brightness
which will affect one of spots 108 or 110 before it
affects the other of spots 108 or 110. Accordingly,
the tracking error signal is generated by ~A + B) - (C
+ D). This signal will show any difference between the
brightness of the two spots.
The detection of data is done by comparing
the total brightness of the two beams for spots 108 and
110 or A + ~ + C + D, to a referenced threshold in-
dicating the diference between a pit and a land.
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16
Figure 8 shows a linear spacial frequency
grating in which the spacial frequency increases as a
function of X. The position of the fringes is given
by:
x = A*n~
Where n = Nl, Nl + 1, Nl , 2
A = constant
The nonuniform spacing of the fringes creates
an astigmatic aberration in the diffracted beams. The
beam produced by this grating is shown in Fig-
ures 9A-9C, with Figure gB showing the beam in its best
focus, Figure 9A showing the beam out of focus when the
medium is too close to the lens and Figure 9C showing
lS the beam out of focus when the medium is too far from
the lens. The best focus of Fiqure 9B is also called
the circle of least confusion. Figure 9B also shows
overlapping beams 132 and 134 which are produced by the
grating effect of the grooved structure of the medium.
The beam will be on track when beams 132 and 134 are o
equal brightness, thus the tracking error signal is
given by A - C. The focus error signal can be de-
termined by noting that detectors B and D receive more
light than detectors A and C in the out-of-focus
condition of Figure 9A, while detectors A and C recei~e
more light in the out-of-focus condition of Figure 9C.
Accordingly, the focus error signal is given by (A + C~
- (B + D).
Figure 10 shows another astigmatic grating
which has an identical effect on a beam of light as the
grating of Figure 8. The position of the fringes of
Figure 10 is given by:
x = n*T ~ B*y2
Where n = 0, 1, 2, ..., N;
T is the spacing ~etween the lines;
B is a constant; and
y is a coordinate perpendicular to x.
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Figure 11 is another version of an astigmatic
grating having astigmatic focal lines oriented at 45
with respect to the axis of the grating. The position
of the fringes i~ given by the following equation:
-5 x = n*T~ C*y)
Where n = -N, -N~1, ... , N-l, N;
C is a constant; and
T is the grating period (line spacing).
The light pattern produced by the diffraction
grating of Figure 11 shown in Figures 12A-12C, with the
best focus being shown for Figure 12B. As can be seen,
the astigmatic focal lines have been rotated by 45
relative to the pattern shown in Figures 9A-9C. This
orientation puts the tracking signal pattern of
beams 136 and 138 in a better position with respect to
the photodetectors. As can be seen from Figure 9B,
beam 132 was partially in detectors A, B and D with
beam 134 bring partially within detectors ~, B and C.
In Figure 12B, beam 136 is wholly contained within
detectors B and C and beam 138 is wholly contained
within detectors A and B, thereby eliminating overlap
on the detectors. The tracking signal is thus given by
(A ~ D) - (B + C). The focus error siqnal is given by
(A + C) - (~ + D).
The functions of the hologram lens are not
restricted to those listed above. A hologram lens can
generally be made to perform the functions of lenses of
any kind. In particular, a hologram lens can be used
for (a) beam-splitting, (b) focus error signal gen-
eration, (c) changing the focal point of a beam and (d)
compensating for ~or creating) off-axis aberrations
such as astigmatism or coma ~which may be caused by a
collimating lens). ~y changing the focal point of the
return beam, it can be made to focus on a photodetector
which is in front of the laser source.
These functions of the hologram are
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18
determined by the lines or fringes placed on the
hologram lens. The general formula for these fringes
is given by the following equation:
2TrX/T + ~ ~x,y) -- 2NT!
where
N = a series of negative and positive
integers (typically -800 to +800);
T = the period of the grating; and
~(x,y) = a phase function of x,y.
The period T is chosen to cause the 1st order
diffracted beam to hit the photodetector according to
the equation:
T = A FC
ds
where
~ = wavelength of the laser beam;
Fc = distance from the hologram lens to the
laser or the focal length of the
collimating lens when the hologram is
placed after the collimating lens; and
ds = distance between the laser and the
photodetector~
The series of integers N, when plugged into
the equation, will give values of the x and y coordi-
nates which define the fringes of the hologram lens.
The phase function can be any number of
functions or combinations of functions. For example, a
phase function according to the following equation
~275731
19
could be used: -
~(x,y) = ~(x~+y~3/AF ~ ~xa/AFa + 2~xy/~f
where -
Fa = astigmatism of collimating lens - i'
F is defined by the equation
l/(Fc-d) -1/Fc = l~F
where
Fc = focal length of the collimating
lens or, where no collimating lens is
used, the physical distance between
the laser source and the h~logram
lens; and
d = distance between the laser source and
the photodetector along the optical
axis.
f = Fol/S, where Fo is the focal length
of the focusing ~objective) lens and
S is the focal error range.
The first term of the phase function example
above is used to focus the return laser beam on the
photodetector in front of the laser source. The second
term corrects for the astigmatism of the collimating
lens and the third term generates the focus error
signal.
Figure 13 shows another embodiment of an
optical head according to the present invention in
which a collimating lens has been eliminated. A laser
diode and detector 140 produces a laser beam 142 which
passes through a hologram lens 144 and an ob~ective
lens 146. The beam is imaged by objective lens 146
onto a medium 148. On the return path, the reflected
beam has one of its first order diffraction beams
imaged on the detector portion of the laser diode and
detector 140. An actuator 150 is used to move objec-
tive lens 146 in response to focus and tracking error
signals. This embodiment represents a trade-off
between the modularity of the embodiment of Figure 4
and the elimination of the need for a collimating lens.
~ ` ~275731
Figure 14 shows another embodiment of an optical
head according to the present invention. A laser diode and
detector 152 is mounted on a moving mechanism 154. A laser
- 5 beam 156 is reflected off of a mirror 158 which is at a 45
angle relative to laser diode and detector 152. The beam
passes through a hologram lens 160 to an objective lens 162
which focuses the beam on a medium 164. On the return path,
hologram lens 160 produces a first order diffracted beam
which is reflected off mirror 158 and impinges upon the
detector portion of laser diode and detector 152. This
embodiment allows focusing to be done by moving objective
lens 162 closer to or farther from medium 164. Tracking
errors can be corrected by moving laser diode and detector
152 sideways. Alternately, mechanism 154 can move laser
diode and detector 152 towards or away from mirror 158 to
perform focus correction, while objective lens 162 can be
moved sideways to perform tracking error corrections.
The embodiments shown in Fig. 4 and Fig. 13 can be
used as magnetic optical heads when they are constructed with
a semiconductor laser/detector device containing polarizers
(see Fig. 4 of the above United States Patent 4,757,197
issued July 12, 1988).
Figure 15 shows another thermal magnetic optical
head for use with a thermal magnetic medium. A semiconductor
laser 170 produces a laser beam 172 which is imaged by a lens
174 to a focal point 176. After converging on focal point
176, the laser beam again diverges and passes through a
hologram lens 178 to a collimating lens 180. Collimating
lens 180 produces a parallel beam of light which is imaged by
an objective lens 182 onto a medium 184. All of the elements
of the optical head are aligned along a single optical axis
186.
Upon a return path, a reflected beam off of medium
184 passes through objective lens 182 and
27S73~ ~
collimating lens 180 to diffraction grating 178,
preferably a hologram lens. A pair of 0th order
diffracted beams 188, 190 are focused upon a pair of
photodetectors 192, 194, respectively. Diffraction
grating 178 and photodetectors 192, 194 are constructed
in any of the ways previously mentioned with respect to
the preceding embodiments to enable focus and tracking
of the laser beam. A polarizing lens 196 is mounted
immediately in front of photodetectors 192, 194.
Lens 196 i~ constructed, as discussed below, so that
different polarizations of the beam are allowed to
impinge upon photodetectors 192 and 194, respec~ively.
Photodetectors 192 and 194 are mounted on a
substrate 198 as shown in more detail in Figure 16.
Figure 16 shows an embodiment of the
photodetector package of Figure 15. Two four-quadrant
photodetectors 200 and 202 are shown mounted on a
supporting substrate 198 which is preferably made of
either ceramic material or printed circuit board
material. The photodetectors are wire bonded to
conducting paths 204 which provide an external con-
nection to the photodetectors. The centers of the two
four-quadrant detectors 200, 202 are preferably less
than Smm from the center of the detector package. A
hole 206 is provided in the center of the detector
package to allow the passage of the source laser beam
at focus point 176 as shown in Figure 15. Since the
image of the laser beam at this ~OCU8 po$nt, or lasing
junction, i5 less than 3 micrometers in diameter, this
defines the minimum diameter of hole 206.
A polarizer 208 is shown partially ~roken
away in front of the photodetector package. A first
side 210 of polarizer 208 in front of photodetector 200
has a series of lines which are orthogonal to a series
of lines in a second portion 212 of the polarizer in
front of photodetector 202. A central portion 214 of
polarizer 208 has no lines at all to allow the passage
~275731
~2
of the source laser beam without polarization. The
lasing junction of laser diode 170 of Figure 15 is
arranged at a 4S angle with respect to both
polarization directions of portions 210 and 212. The
45 orientation improves the ability to detect the
small (0.4 degree) change in polarization of the beam
by putting this change in the linear region of the
cosine function. This results in a detectable differ-
ence in the intensity of the beams hitting the two
photodetectors.
Figure 17 shows an alternate embodiment to
the embodiment of Figure 15 for a thermal magnetic
optical head in which collimating lens 180 has been
removed. In this embodiment, all of the elements must
be precisely aligned without the margin for error
allowed in Figure 15 between collimating lens 180 and
objective lens 182O
Figure 18 shows an apparatus for producing
polarizer 208 of Figure 16 as a hologram lens. It is
well known in the art that light diffraction efficiency
is sensitive to the polarization of the incident light.
Herwig Kogelnik discussed this effect in his paper
"Coupled Wave Theory for Thick Hologram Gratings,"
published in The Bell System Technical Journal, volume
48, number 9, pp. 2909-2946, November 196~. A more
recent work on a similar topic is a paper entitle~
"Grating Efficiency Theory as it Applies to Blazed and
Holographic Grating" by E. G. Loewen, M. Neviere and D.
Maystre, published in APplied oPtiCs~ volume 16,
number 10, pp. 2711-2721, October 1977. This
polarization effect becomes very strong when the
grating period is less than the wavelength of light.
Figure 18 shows an arrangement for producing
polarizer 208 of Figure 16. A laser source 216 pref-
erably has a wavelength below 520nm. A laser with this
wavelength is used to produce a polarizer for an
optical head laser with a wavelength between 0.78
2~5~3~ ~
micrometers and 0.85 micrometers. The wavelength is
selected so that a diffraction grating can be recorded
on commercially available photoresist materials. With
a shorter wavelength the grating period formed on the
diffraction grating can be made smaller than the near
infrared wavelength of semiconductor lasers.
The laser beam from laser source 216 is
passed through a collimater 218 to produce a collimated
beam 220 which passes through a beam-splitter 222 to
produce a pair of collimated beams 224 and 226, respec-
tively. The expanded, collimated beams preferably have
a diameter of approximately 15mm. Beams 224 and 226
are then reflected off of mirrors 228 and 230, respec-
tively, towards a recording material 232. Each of the
expanded laser beams hits medium 232 at an angle ~ to a
vector normal to the recording surface of medium 232.
The interference pattern of the two intersecting laser
beams will produce a series of lines which form the
fringes of the hologram lens. The preferred process
differs from the process discussed in the bac~ground
section because no metal is deposited. Instead, the
glass is etched to produce a relief pattern of a series
of grooves on the glass. Since all portions of the
hologram lens are thus transparent, its efficiency is
much greater than a lens using a series of metal lines
which block a portion of the light.
The distance between the lines, or the
grating period, is d = ~/2 sin ~, where A is the
wavelength of a laser source 216. For a laser source
wavelength of 488nm and an angle ~ of 20, the grating
period will be approximately 0.71 micrometer.
Figure 19A shows the grating lines which can
be formed by such a process. The parallel lines of
Figure l9A are the fringes formed by the interference
of the laser beams as shown in Figure 18. To manufac-
ture a polarizer 208 as shown in Figure 16, a little
more than one-half of the polarizer medium is covered
" ~ ~I Z7573~ ~-
24
by a mask 234 as shown in Figure l9B. This mask is
arranged so that its boundary 236 is at a 45 angle to
the fringes 238 which are produced by the interference
pattern of the laser beams. After this first set of
~5 fringes 238 are formed, mask 234 is removed and a
mask 240 is placed over the fringes 238 as shown in
Figure 19C. This mask extends beyond the fringes 238
so that it covers more than half of the recording
medium. The recording medium is rotated by 90 and a
separate set of fringes 242 are formed on the second
. half of the medium. The resultant polarizer is shown
in Figure l9D as having a series of fringes 238 on a
first side which are orthogonal to a second set of
fringes 242 on a second side, with a central por-
tion 244 of the polarizer having no fringes at all.
This central portîon of the polarizer allows the
passage of the source laser beam without polarization.
The relief pattern formed on photoresist on
the recording medium can be replicated by an embossing
process. This makes mass production of the -
polarization elements possible at low cost for use in
the thermal magnetic optical head according to the
present invention. Such a phase diffraction grating
has an advantage in terms of light diffraction effi-
ciency over wire grid polarizers because the entirepolarizer is transparent as discussed above.
As will be understood by those familiar with
the art, the present invention may be embodied in other
specific forms without departing from the spirit or
essential characteristics thereof. For example, a
different pattern could be embodied on the hologram
lens to produce an image for focus and tracking error
detection. In the embodiment of Figure 15, the
polarizer could be placed on the other side of
diffraction grating 178 or collimating lens 180 and
diffraction grating 178 could be interchanged. Accord-
ingly, the disclosure of the preferred embodiments of
~27S731
the invention is intended to be illustrative, but not
limiting, of the scope of the invention which is set
forth in the following claims.
