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Description  |
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DESCRIPTION
1. Technical Field
The invention relates to a read/write head for optical disks and to a
method of manufacture for such heads.
2. Background Art
Optical disks are storage media where information is recorded by a
modulated light beam on a movable surface in the form of indicia (or
spots) that are arranged along tracks and whose optical characteristics
can be detected by a readout light beam. The high storage density
achievable with these optical disks is interesting both for digital and
non-digital recording; the latter has already found widespread use in the
form of video disks.
The individual recording indicia have a typical diameter of about 1 .mu.m.
An example of such indicia are small pits that are burnt by the writing
laser into the surface of the disk to change its local reflectance.
Generation and readout of such pits (or other optical discontinuities)
requires focussing and track control of the laser beams with micrometer
accuracy or even better. As optical systems for focussing a beam to a
micrometer spot have very small depth of field, the height (or the working
distance) of the optical system above the information area on the disk
surface has also to be controlled very closely.
In known optical disk systems focussing, height and position control of the
laser beams are achieved by optical systems that consist of discrete
elements (lenses, mirrors, etc.) in combination with elaborate electronic
control systems and mechanical actuators for vertical and lateral
displacements. Such optical beam guidance systems tend to be rather bulky,
expensive and difficult to adjust. Their great inertia further forbids
rapid displacements and requires strong actuators.
A typical example of such known optical heads can be found in DE-A-2918931
U.S. Pat. No. 4,298,974, filed May 10, 1979, and issued Nov. 3, 1981,
where beam focussing is achieved by displacing an objective lens mounted
in an electromagnetic coil.
The complexity and the mass of these optical heads are in contrast to the
read/write heads used in magnetic disks, which are of simple design, can
be mass-fabricated and keep a predetermined distance above the disk by
flying on an air cushion. An optical disk whose design is based on the
same principle has been proposed by E. G. Lean in IBM Technical Disclosure
Bulletin, Vol. 23, No. 7A, December 1980, pp. 2994-2995, wherein the
optical read/write disk is aero-dynamically shaped to fly over the disk
surface. Input and output is achieved by optical fibers and wave guides
within the slider to direct the light to and back from the surface of the
optical disk. The use of optical fibers limits the flexibility of this
head and complicates its manufacture. Additionally optical imaging is only
of moderate quality. If additional fibers are required, e.g., for
auxiliary beams to control the position of the head, the complexity of
this head is further increased.
Another drawback of this proposal includes the small working distance of a
fibre optical system which requires a very small distance between the disk
and the head and, thus, leads to increased sensitivity against dust.
DISCLOSURE OF THE INVENTION
It is, therefore, an object of the present invention to provide an optical
read/write head of the aforementioned kind with excellent optical imaging
qualities, small dimensions, little weight, low cost and generalized
applicability in optical imaging systems. In addition, a method of
manufacturing such heads is provided as well as a method of reading out
magneto-optic media that is particularly suited for such heads.
The read/write head in accordance with the teachings of this invention is a
compact, flat, one-piece arrangement that can be easily fixed to an
actuator arm and used with one or several stacked optical disks. The
optical beams are guided exclusively in the interior of the head so that
no readjustment of the beam path is necessary and the sensitivity to dust
reduced. The optical elements to shape and to control the beams are
provided at the surface of the optical head and can be manufactured with
known techniques, e.g., photolithographic methods or molding, in batch
mode where the individual heads are separated only in the last processing
step. The low inertia of an individual head allows rapid displacements,
e.g., during track search, and the low manufacturing cost permits the use
of such high quality heads even in entertainment devices such as video
disks.
Preferred embodiments of the invention will now be described in detail with
reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B illustrate a first embodiment of the present invention in
cross sectional and top views, respectively, wherein optical beam guidance
elements are applied on a flat surface of a transparent substrate,
FIG. 2 illustrates a second embodiment of the present invention wherein the
beam guidance elements are realized in a plastic form part that may be
attached to a flat transparent substrate,
FIGS. 3A and 3B illustrate an enlarged cross section of the beam focussing
element in the form part of FIG. 2 and an isometric view of calculated ray
paths in the focussing element, respectively,
FIGS. 4A and 4B illustrate the principal beam path and the polarization
vector diagram, respectively, with an optical arrangement to read-out
magnetically coded information from an optical disk,
FIG. 5A illustrates a third embodiment of the present invention wherein the
read-out scheme of FIG. 4A is incorporated in the beam guidance elements
of FIG. 2,
FIGS. 5B-5E illustrate cross-sections of the embodiment of FIG. 5A along
various axes, and
FIG. 6 is a schematic representation of the manufacturing steps for making
the read/write head of FIG. 2.
BEST MODE FOR CARRYING OUT THE INVENTION
In the following discussion of the figures of the drawings like reference
numerals refer to corresponding elements of the invention. Referring to
the drawings in more detail, FIGS. 1A and 1B illustrate an embodiment of
the invention in cross-sectional and top views, respectfully, wherein the
optical read/write head includes simply a slab-like transparent substrate
with the beam guidance elements applied to the plane top. An information
bearing surface 1 of the optical disk 2 carries optical discontinuities 3,
e.g., pits, along tracks 4 which are to be scanned by a focussed light beam
5, preferably, a monochromatic laser beam. The surface 1 is preferably,
arranged at the bottom surface of the transparent optical disk 2 to reduce
the sensitivity of the system to dust particles.
Beam 5 exits as a convergent beam from one end of transparent substrate 6.
It is generated at the opposite end of the substrate 6 by a laser 10 which
may be obliquely oriented in a kerf cut into the upper surface of the
substrate 6. The entire light path of the beam from the laser 10 to its
exit over the optical disk 2 is located within the transparent substrate
6, which guides the oblique beam by multiple internal reflections. Optical
elements to influence and to shape the beam are applied on the external
surface of the substrate 6 at locations where the internal reflections
occur. These elements can be used on both external surfaces 6a and 6b of
the substrate 6, or, for ease of manufacturing, only at one surface 6a. In
particular the following elements may be provided in the light path
starting at laser 10:
A zone plate or hologram lens 12 that acts as a cylindrical mirror to
compensate for the generally elongated shape of the beam leaving the laser
10;
A polarizing beam splitter 14a on top of which a grating 14b is arranged to
split beam 5 into a main beam and two auxiliary beams which are
symmetrically offset with respect to the main beam and are focussed in F'
and F", respectively, on track 4 in front of and behind the focus F of the
main beam. Grating 14b is blazed such that the diffracted beams travel
parallel to each other in the direction corresponding to the regular
reflection of the beams at the upper surface 6a;
A quarter wavelength layer 15 to polarize the beams circularly; such a
layer can be obtained by isotropic sputtering; and
A zone plate 16 (or a hologram lens) which reflects the beam 5 in the
direction normal to substrate surface 6a and focuses it on information
bearing surface 1 of the optical disk 2.
The focussed exit beam 5 is reflected at the information bearing surface 1
and travels back via the zone plate 16 to the quarter wavelength layer 15
which retransforms the circular polarization to a linear polarization that
is perpendicular to the polarization of the original laser beam. The
polarizing beam splitter 14a will thus divert the reflected beam from the
axis of laser beam 5a into beam 5b which passes a zone plate 13 (acting as
a cylinder lens) to be focussed on a four-quadrant detector 11, arranged in
the kerf together with the laser 10. The auxiliary beams for track servo
control are directed to further photodiodes 11b and 11c. Servo signals are
generate from the outputs of the four-quadrant photodiode detector 11 and
the auxiliary photodiodes 11b and 11c and processed in the conventional
way to derive control signals for a head actuator symbolically shown at
reference numeral 7.
In this embodiment the optical elements 12-16 are applied directly to the
surface 6a of the substrate 6 by conventional techniques like evaporating,
sputtering and etching and the zone lenses or holographic elements are
defined with conventional methods, e.g., by directing beams that
correspond to the desired entry and exit beams to a photosensitive plate
as object or reference beams or by using synthetic hologram lenses. Laser
10 and photodiodes 11 could also be manufactured directly on the substrate
or be discrete elements that are fixed to the substrate, e.g., with resins
of matched indices. The back surface of substrate 6b can be covered with a
reflecting layer and both surfaces can be covered with an additional
protecting layer.
The height of the substrate 6 over the optical disk 2 can be either
controlled via the actuator 7 or in the manner of magnetic read/write
heads by shaping the lower surface 6b (or part of that surface) as an air
foil to control the flight height automatically. In that case the arm of
the actuator 7 must be flexible to allow for automatic height adjustments.
The substrate 6 itself is rigid and may include glass, quartz or plastic
material. Typical dimensions of the substrate are 11.times.3.times.1.5 mm.
Aerodynamic height control is, however, only advisable in dust free
environments, as the distance between the head and the disk is very small.
Greater clearance (circa 1 mm) is obtained by known optical servo control
methods which can be easily incorporated in the heads proposed here.
In an alternative embodiment of the subject invention the particular
elements 12-16 and the kerf for laser 10 and photodiode assembly 11 are
not directly formed on a surface of a substrate but in a separate part,
e.g., a transparent plastic body which is then fixed to the upper surface
of a plane substrate. The optical elements like gratings, zone plates,
catadioptric lenses, etc., can be made in the form part by molding or
etching, with appropriate dies, etc.
FIG. 2 shows an example of this preferred embodiment of the invention with
a form part 20, e.g., molded plastic, whose flat bottom surface is fixed
without optical discontinuity to the top surface of transparent substrate
6. The illuminating light beam 5a that is to be focussed on information
bearing surface 1 exits from laser 10 in a horizontal direction and is
deflected by an optical grating 21 at the surface of form part 20 that
simultaneously deflects the laser beam into the substrate 6 and corrects
for its original elongated cross section. Optical gratings with these
features are known in the art. After a first reflection on the bottom
surface of substrate 6, the illuminating beam 5a reaches a dome-shaped
part 23 of form part 20 with a further grating at its top that is blazed
to reflect the illuminating beam back to the substrate and to generate the
two auxiliary beams for track servo purposes. The reflected beams pass a
polarizing beam splitter 24 which is located on top of a
quarterwave-length plate 25 at the interface between substrate 6 and form
part 20. The beam is then reflected at a first and a second aspherical
reflector 26a and 26b which form an imaging system that focuses the beam
on the information bearing layer 1 of optical disk 2.
The beam reflected at information bearing layer 1 reaches again the first
and the second aspherical reflectors 26a and 26b from which it exits as a
parallel beam whose plane of polarization will be perpendicular to the
plane of polarization of the incident beam when it has passed again the
quarterwave-length coating or plate 25 before being reflected at
polarization beam splitter 24. The reflected beam then reaches reflector
22 (e.g., in the form of a toroid) which compensates for the different
optical path length from the laser (in the incident beam) and to the
photodetector element (in the reflected beam). In addition, reflector 22
introduces an artificial astigmatism that allows it to generate a servo
signal from the four-quadrant photodiodes 11 to be used for automatic
focus control.
For increased reflection efficiency the form part 20 and the bottom surface
of substrate 6 may be covered with a reflective coating 27 and a protective
coating 28.
For high optical quality the substrate and the form part should include
optically matched materials showing no birefringence and having the same
thermal expansion. Appropriate material combinations could be glass-glass,
plastic-plastic or glass-plastic.
The use of a separate form part 20 has several advantages with respect to
the embodiment of FIG. 1 where all of the beam shaping elements are
directly applied to the upper surface of substrate 6. The most important
aspect is that the two aspherical reflectors 26a and 26b provide an
aberration free aplanatic imaging element for optimal transmission between
the laser light source and the photo detector assembly. A second advantage
of the beam path in FIG. 2 is that the laser and the photo detector
assembly can be arranged in a common housing on the substrate. The
reflector/beam shaper 21 and the reflector 22 can be formed easily in the
form part 20 so that no kerf is required in substrate 6.
The embodiment of FIG. 2 can also be realized in a homogeneous body without
a separate substrate. In this case optical elements shown in FIG. 2 at the
interface between the substrate 6 and form part 20 can be placed into the
relief surface or at the plane lower surface.
The exact shape of the aspherical reflectors 26a and 26b can be calculated
from geometrical optics by tracing individual rays in the embodiment of
FIG. 2 and observing the following conditions: constancy of the light path
between two conjugate planes, laws of reflection and refraction, and sine
condition.
The surfaces of the two reflectors can then be expressed analytically by
polynomials of the form
##EQU1##
The x, y-plane is the symmetry plane and contains the optical axis so that
a.sub.ij =0 for odd j. Sufficient optical quality is obtained for
i+j.ltoreq.9, i.e., for polynomials of up to the 9.sup.th degree.
It has been shown that an aplanatic imaging system can be obtained with two
reflectors 26a and 26b. FIGS. 3A and 3B show a cross-section and an
isometric view, respectively, of the image forming part in the optical
read/write head with calculated ray paths for a telecentric entry beam 30.
Such a beam can be made available in the embodiment of FIG. 2 by
appropriate elements in the remaining parts of the optical path. If white
light is used instead of a monochromatic laser, an achromat compensator
plate 32 has to be added at the exit plane of the imaging element in FIG.
3.
A very simple optical system with reduced optical quality uses an ellipsoid
for surface 26b and a plane mirror for surface 26a.
The optical read/write held described above can be used for all storage
media where information is recorded by small spots or indicia on the
information bearing surface that differ in their reflectivity from their
environment. For magneto-optical storage media, however, the polarization
state of the readout beam is affected rather than its intensity by indicia
having their magnetization reversed with respect to their neighborhood. In
this case the design of the proposed optical read/write head must ensure
that the polarization directions of the optical beams are not changed by
oblique reflections at non-plane surfaces. This condition is satisfied by
the above-described imaging system with two aspheric reflecting surfaces.
Known magneto-optical materials yield, however, only rather small changes
of the polarization direction and render it difficult to obtain acceptable
signal/noise ratios in the readout signal. It is, therefore, proposed to
modify conventional readout schemes for magnetic optical media by an
arrangement that is schematically shown in FIGS. 4A and 4B and whose
implementation in an integrated optical read/write head is illustrated in
FIGS. 5A-5E.
In FIG. 4A a magneto-optical information bearing surface 1 (the
magnetization direction is perpendicular to the surface) on an optical
disk 2 is illuminated by a light beam 40 from laser 10 through lens L1,
beam splitter 41 and lens L2. Beam splitter 41 is a non-perfect polarizing
beam splitter such that only a fraction of the beam with intensity I1 is
transmitted, which was reflected at surface 1. This fraction in the order
of 10% is designated by I2 and passes a plate 42 that rotates the
polarization direction by 45.degree. to impinge on (perfect) polarizing
beam splitter 43 and mirror 44 which deflect the beam intensities I3 and
I4 to lenses L3 and L4, respectively, for focussing on photodiodes 46 and
45, respectively. The outputs of these photodetectors are connected to a
differential amplifier 47.
The non-perfect polarizing beam splitter 41 can be obtained by reducing the
number of layers in a conventional multilayer polarizing beam splitter.
FIG. 4B shows the polarization diagram with the intensities Ii and the
polarization directions of the various partial beams of FIG. 4A. The
nonperfect polarizing beam splitter 41 operates to increase the
polarization angle of I2 with respect to the conventional polarization
angle of the total intensity I1 reflected at layer 1. Intensity I2 is then
decomposed by polarizing beam splitter 43 and mirror 44 into components I3,
I4 along axes oriented under 45.degree. to I.sub.1. This decomposition
makes the output signal of differential amplifier 47 symmetrical to zero
and insensitive to all disturbances that do not rotate the polarization
plane. The signal/noise ratio of this signal corresponds to a conventional
read-out setup for magneto-optical media using at least an eight fold laser
power.
FIGS. 5A-5E show an integrated optical read/write head for magneto-optical
materials where the optical elements of FIG. 4A are integrated in a form
part 20 fixed to a plane parallel substrate 6. The elements in FIGS. 5A-5E
corresponding to the optical components in FIG. 4A carry the same reference
numerals, but have a dash added. FIGS. 5B-5E show cross sections along the
axes denoted by Roman numerals in FIG. 5A.
The exit beam of laser 10 in a kerf of form part 20 is deflected by a
concave reflector 50, passes nonperfect polarizing beam splitter 41 and is
imaged by aspherical reflectors 26a and 26b to recording layer 1. The
reflected light travels back along the same optical path and is partially
reflected by non-perfect polarizing beam splitter 41 to a plane mirror 51,
which is arranged above the nonperfect polarizing beam splitter 41 and
reflects the beam back to the beam splitter 41 in the direction of axis
II--II.
The orientation of mirror 51 is such that it simulates, together with beam
splitter 41, the 45.degree. rotator in FIG. 4A. The two intensities I3 and
I4 are generated by reflecting the partial beams generated by beam splitter
41 at a further plane mirror 55 and the back plane of the substrate,
respectively, and by recombining the beams under a slight angle before
they are deflected by a mirror 54 to photodetectors 11. The mirror 54 can
be either concave to focus the beams on the detectors or, if plane, be
combined with image forming elements in the optical path.
The form part 20 can easily be mass produced by precision molding with a
mold form that incorporates several of these heads. The mold form itself
can be derived from an original mold that is manufactured with high
precision, e.g., by electro erosion. After the molding process the
individual heads need only be separated, glued to the substrate and
equipped with the laser/photodetector assembly. The latter step is the
only one requiring exact adjustment before the electronic assembly is
rigidly fixed on the head. For efficient cooling the housing of the
electronic assembly can be thermally connected with an actuator arm.
FIG. 6 indicates schematically the manufacturing steps for the read/write
head of the present invention. In a first step the substrate 6 is prepared
with plane and parallel surfaces. Each optical element at the interface
between the substrate and form part is generated by depositing an
appropriate layer with the desired optical characteristics (quarter
wavelength layer or beam splitter layer), followed by a photolithographic
etch process to obtain the desired shape at the appropriate locations of
the surface (steps 63 and 64, as indicated in FIG. 6).
A number of form parts are cast (step 61) in a common negative mold 60 and
polished (step 62) to improve the optical quality of the surfaces before
the molded part is bonded to the substrate (step 65) in a way that no
optical discontinuity develops at the interface. This can be achieved by
using adhesives with indices matched to those of the substrate and the
form part. After the bonding process the laser diodes and photodetector
assemblies 10, 11 are added to the form part and adjusted (step 66). A
function test of the complete assembly can be performed either before or
after the individual heads are separated (step 67) from each other by
cutting.
Embodiments of the invention that do not use a substrate can be
manufactured in one simple molding process, followed by insertion of the
laser/photodetector assembly.
The imaging part of the read/write head of the present invention can also
be used in isolation for other optical applications. The parallel beam
leaving the plane exit surface of the imaging element shown in FIGS. 3A
and 3B easily allows attaching other optical components so that an imaging
system can be realized that has the following advantages: small size, long
working distance, free working space, overhead clearance, insensitivity to
high accelerations, and simple mass fabrication.
This imaging system is therefore suited in particular for manipulator and
inspection systems.
While the invention has been particularly shown and described with
reference to preferred embodiments thereof, it will be understood by those
skilled in the art that various changes in form and details may be made
therein without departing from the spirit and scope of the invention.
* * * * *
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