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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to digital recording media and more particularly to
high capacity recording media for optical recording and read-out.
2. Description of the Related Art
Digital information has been recorded on many kinds of materials by a wide
variety of processes. One widely used system magnetizes selected areas of
a disk of magnetic material to represent the information to be preserved.
Other approcahes, used primarily when greater recording density is
desired, include the use of a substrate coated with a material capable of
being changed by selective treatment with a laser beam. For example, the
substrate may have a reflective surface that is caused to have lower
reflectivity in those areas where it is heated by a focused laser beam.
Conversely, non-reflecting absorbent surfaces have been rendered
reflective by the application of a lser beam as described in Optical
Memory News, September-October 1984, page 14. The reflectivity of the
surface may be altered by melting or deforming the surface or by actual
evaporation of material from the surface. Most often, the medium is in the
form of a disk and the information is recorded along a spiral track. The
timing and tracking information may also be recorded by a modulated laser
beam following the spiral track.
U.S. Pat. Nos. 4,214,249 to Kasai; 4,270,916 to Dil; 4,379,299 to
Fitzpatrick et al.; 4,314,262 to Reilly; and 4,334,299 to Komurasaki et
al. disclose media of the kinds referred to above.
The Dil Patent discloses recording on a disk having a grooved spiral and in
which timing marks are recorded on the sloping walls of the grooves.
The Kasai patent discloses the recording of digital information by the
selective exposure to a laser beam that causes deformation or evaporation
of a layer composed of S, Se, Te, or chalcogenide compounds thereof. The
rate of recording is limited by the head conductivity of the medium.
The Fitzpatrick patent describes a digital writing process in which a film
of semiconductor material, such as cadmium telluride, on a substrate of
plastic such as methylmethacrylate or polycarbonate, is exposed to a
recording laser beam that heats the plastic substrate to produce a
pressurized gas bubble that bursts the overlying semiconductor leaving a
pit or hole in the reflective surface that represents one bit of
information. The rate at which information can be recorded is limited by
the amount of heat required to cause the eruption and the heat
conductivity of the recording medium.
The Reilly patent describes a recording medium formed by a thin continuous
layer of metal in which bits of data are recorded by alterations produced
with focused spots of laser light. A transparent dielectric coating is
provided to increase the light absorption of the metal layer.
The Komurasaki patent describes a real-time monitor for use with a
recording medium comprising a continuous film of metal such as bismuth,
gold or chromium which is selectively melted or vaporized by a focused
light beam to record one bit of information.
U.S. Pat. No. 4,380,769 to Thomas et al. describes the recording of
information by the thermal deformation of a continuous thin film of
amorphous material carried by a plastic substrate. Individual depressions
surrounded by sharply defined ridges are produced in the amorphous film.
U.S. Pat. No. 4,334,233 to Murakami describes a dustprotecting shield over
the substrate that minimizes information distortion that might otherwise
occur because of dust particles on the recording medium.
U.S. Pat. No. 4,428,075 to Hazel describes a preformatted disk in which
synchronization marks are recorded in areas separate from the data
recording areas. These tracking and timing marks are distinct from the
alterations that represent bits of data and, to the extent they occupy
space that could otherwise be used for digital storage, reduce the
capacity of the disk.
The formation of arrays having microscopic relief patterns is known in the
photographic field where such techniques are used to reduce variations in
image density. U.S. Pat. Nos. 4,366,299 to Land and 4,402,571 to Cowan et.
al. discuss the formation of spaced discrete holes using as photoresist
that is exposed twice to the interference patterns of two laser beams, one
exposure being below the threshold for the development of the photoresist.
Land also describes for photographic purposes the formation of peaks
coated with silver as one step in formation of a silver halide coating.
The structure proposed by Land does not lend itself to the recording of
digital information.
U.S. Pat. No. 3,019,124 to Rogers discloses a method of manufacturing
photographic elements by applying a first light sensitive layer in a
uniform thickness to a support, embossing the coated layer to form a
relief impression having systematically arranged spaced elevated sections
joined by depressed sections interspersed between them, and applying a
second light sensitive layer having a different spectral sensitivity to
fill the depressions remaining in the surface to the level of raised
sections.
U.S. Pat. No. 4,362,806 to Whitmore describes a photographic substrate
comprising an array of microvessels that are filled with various
photographic materials. The object is to reduce lateral image spreading by
providing a discontinuous recording substrate. The microvessels are
separated only by minute distances that play no part in the recapture of
information. Any appreciable thickness of the walls separating the
microvessels detracts from the continuous image that is the object of the
Whitmore disclosure. The recording is done over mass areas and the
microvessel walls are used to prevent undesired lateral spreading of the
photographic image. Whitmore suggests electronically scanning the
photographic elements to read information in digital format. Whitmore also
discloses modifying the microvessels by scanning with a laser beam to
alter the character of selected microvessels by melting, sublimation or
change in viscosity. The microvessels of Whitmore require subsequent
photographic processing to provide optically readable information.
SUMMARY OF THE INVENTION
A digital recording medium has discrete spaced individually-alterable
storage elements which, in one embodiment, in the unaltered state, are
tiny mirror surfaces, sometimes calles here "micromirrors", arranged in a
substantialy regular array in a plane spaced from a reference plane of a
supporting substrate. Each micromirror is supported by a mesa extending
from the substrate so that the micromirrors are separated by valleys or
indentations between the mesas. Each micromirror is individually optically
alterable to store one or more bits of information. The substrate may be
protected from contaminates by a layer of transparent material of
substantial thickness that minimizes the effect of dust particles. The
array of micromirrors is arranged to be scanned by a recording device and
subsequently, without further processing, by a reading device.
Information is recorded by causing a change in the reflectivity of the
selected micromirrors, for example, by subjecting the surface of each
selected micromirror to an infrared light beam of sufficient intensity to
materially reduce the reflectivity of the mirror. Each micromirror, by
having one of two or more levels of reflectivity, becomes a depository for
one or more bits of digital information.
The regular spacing of the array of micromirrors enables them to serve both
as tracking and timing markers prior to and during recording and read-out.
In effect, the medium itself acts as an optical encoder for the scanning
device. This arrangement permits variations in the scanning velocity, a
particular advantage when reciprocating scanning procedures are used. The
micromirrors may be used to control the scanning path, both for recording
and read-out, by centering the beam along the path of maximum reflection.
The use of an array fo regularly spaced discrete reflective micromirrors
makes it possible to test the recording medium for defects prior to
recording and authenticate its quality. In a practical way, this
eliminates the need for monitoring the recording process because the
chance of failing to record on a mesa having the required level of
reflectivity is small. The reduction in reflectivity of exposed
micromirrors results from the absorption of sufficient energy to change
the mirror coating itself or to distort the thermoplastic mesa supporting
the mirror. The reflectivity of the recorded micromirrors preferably is
not reduced to zero, but rather only enough that it can be readily
distinguished as a recorded micromirror, the reflectivity of the recorded
micromirror remaining greater than that of the valleys separating the
micromirrors.
In another embodiment, the micromirrors are formed on the bottom surfaces
of spaced indentations in a first surface of a sheet of clear substrate.
Viewed from the second surface of the substrate, the indentations become
mesas supporting the micromirrors. The recording and reading light beams
are focused on the micromirrors through the second surface of the
substrate.
In a preferred embodiment, the recorded medium is moved linearly along a
full row of micromirros, while a small transverse oscillation of the
read-out light beam maintains the lateral position of the recorded medium
such that the read-out beam on the average, tracks the approximate center
of the row of mirrors being scanned. At the end of each linear scan, from
end to end of the row of micromirrors, the recorded medium is moved
laterally one row and then moved linearly in the opposite direction. The
arrangement of the micromirrors permits a wide choice in the selection of
a particular read-out procedure.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is an enlarged diagrammatic top view of a small section of a
recording medium embodying the invention;
FIG. 2 is a sectional view along line 2--2 of Figure 1;
FIG. 3 is a partial perspective view of the recording medium shown in FIGS.
1 and 2;
FIG. 4 is a reproduction of a scanning electron micrograph of the recording
medium of FIGS. 1-3 at a magnification of 10,000X;
FIG. 5 is a reproduction of a scanning electron micrograph of an embossed
layer of PVC for use in fabricating the recording medium;
FIG. 6 is a reproduction of a photograph of a video image of the medium
taken from a video screen at a magnification of 1,750X;
FIG. 7 shows a section of the medium of FIGS. 1-3 including a transparent
protective shield and in which a filler is placed in the valleys between
the mirrors;
FIG. 8 is a partial sectional view of another embodiment of the recording
medium in which the protective shield and the substrate are formed
integrally from a single sheet of plastic;
FIG. 9 shows diagrammatically a source of laser light and the associated
optics and detectors for recording on and reading from the medium;
FIG. 10 illustrates diagrammatically a preferred scanning sequence for both
recording and read-out; and
FIG. 11 illustrates the use of the micromirrors for tracking during
recording and read-out.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The drawings are not to scale and various elements have been exaggerated
for purposes of illustration. In the various figures, similar elements are
indicated by the same numerals or by the same numerals followed by an
identifying letter suffix.
As illustrated by FIGS. 1-3, a recording medium, generally indicated at 2,
in this example, is in the form of a rectangular plastic card about 2 by
3.5 inches which is capable of recording more than 800 megabits of digital
information. The medium comprises a substrate or base element 4 having an
array of uniformly spaced micromirrors 6, each supported by a minute
projection or mesa 8 extending from one surface of the substrate 4. The
projections 8 are integrally formed as part of the substrate 4, which may
be formed of thermoplastic material, and are separated by valleys,
generally indicated at 10.
The projections or mesas 8 which support the micromirrors 6 serve two
functions: to provide thermal isolation between adjacent mirrors and to
provide a light sink between the mirrors in the form of the valleys 10.
The height of the projections 8 above the substrate 4 is not critical and
is typically between 0.5 and 2.5 micrometers. The projections 8, in this
example, are arranged in the array to provide one storage element for each
two micrometers along each row of mirrors.
The thickness of the substrate 4 is not critical but may be of the order of
100 or more times the height of the mesas 8. The mirrors 6, which lie in a
common plane, are of material capable of reflecting laser light. Each
micromirror is of sufficient size and flatness to function as an effective
mirror at the frequency of light being used to read the data from the
medium. The mirrors may be formed by coating the tops of each mesa 8 with
a layer of reflective material capable of absorbing sufficient energy to
permit a low power laser beam to reduce significantly the reflectivity of
the micromirror with an exposure of less than about one microsecond. The
preferred mirror coating is a composite formed of gold and silicon
dioxide. The mirrors 6 should be as flat as possible and the surface
variations should be limited to a fraction of a wavelength of the incident
light, for example, from one-fifth to one-tenth of a wavelength. For the
present purposes, a mirror capable of reflecting 20-25% or more of the
incident light to be used for readout is defined as a "flat mirror". Each
micromirror is capable of immediately detectable alteration, for example,
by exposure to a source of focused energy, such as a laser beam, by which
is meant the alteration takes place substantially immediately upon such
exposure and may be detected without further processing such as is
required in photographic and other indirect processes.
Preferably, the micromirrors 6 form a regular array as illustrated by FIG.
1. By a regular array is meant an array in which the storage elements are
equally spaced in parallel rows that are separated by a distance equal to
the distance between adjacent storage elements in the rows. With this
arrangement, the medium can be tested for defects prior to recording by
scanning the surface of the medium with a non-destructive laser beam and
measuring the reflectivity from each micromirror. The reflective and
absorptive capacity of the micromirror is a function of the amount of
coated material on the mesa, therefore, if each micromirror is confirmed
for reflectivity, the medium can be certified for recording with a high
degree of assurance that the recording will be accurate. During this
pre-test, the physical position, as well as the reflectivity, of each
micromirror is verified. This may be done by any desired mode of scanning
in which the distance between micromirrors is verified, as by a counting
device related to the speed of the beam scan.
Each micromirror 6 represents one bit of information. Note that the size of
each bit of information is determined prior to recording: it is not the
recording device that determines either the position or size of the
information bits. With this arrangement, the micromirrors 6 themselves
provide the tracking guides for pre-testing, recording, and read-out. The
data can be packed with maximum density because no allowance is required
for variations in laser spot size during recording. The tolerances
permitted in the area of the focus of the laser beam at the plane of the
mirrors are thus greater than in those arrangements where the position and
size of each recorded bit is determined by the action of the laser beam.
The maximum surface dimension of each micromirror is preferably between 1
and 2.5 micrometers and the minimum dimension should not be less than the
wavelength of the light being used for reading. For special applications,
the mirror size may be less than one micrometer or substantially greater
than 2.5 micrometers. For most applications, where density of recording is
important, the area of the micromirror preferably is between 0.7 and about
5 square micrometers. It is preferred that the distance between the mesas
6 be not significantly less than the maximum surface dimension of the
micromirrors. The reflecting area of the micromirrors 6 may be round,
square, rectangular or any other desired shape. It is convenient, however,
to provide a regular array of generally round reflecting surfaces equally
spaced in parallel rows, such as result from the example set forth below.
Each micromirror preferably has an original reflectivity of at least 20%
of the particular laser light being used. After exposure to the laser beam
to destroy the reflectivity, the reflection preferably is significantly
less than 20% or at least significantly less than the reflectivity of the
orignial mirror surface.
In one system, preferred for many typical applications, the round
micromirrors of one micrometer diameter are spaced one micrometer apart
and a recording laser beam is arranged to scan the mirrors at a speed of
about two meters per second to record data at a one megabit/second rate.
If a higher data rate is desired, a faster scanning speed can be used. The
micromirrors preferably are spaced as closely as possible in the array,
for maximum storage capacity, but the dimensions of each mirror must be
large enough to permit it to function as a mirror at the wavelength of
light being used and the micromirrors must be separated by a distance
great enough that neither the recording or read-out laser beam can overlap
two micromirrors.
EXAMPLE
The following is an example of the steps in the preparation of the
recording medium embodying the present invention: A photoresist relief
pattern comprising a square array of flat bottoms with tapered peaks, with
center-to-center spacing of about 2 micrometers (see FIGS. 4, 5 and 6) was
prepared as follows. Positive photoresist (Shipley AZ-1450J, manufactured
by Shipley Company, Inc. Newton, Mass. was spin coated on a glass plate to
a thickness of several micrometers. The plate was then exposed to an argon
laser interference pattern using a glass prism to split the beam and to
recombine the two halves, thus forming a series of spaced parallel
interference lines at the photoresist target. The exposure was through the
glass plate so the greatest exposure was at the bottom of the layer of the
photoresist.
After a three-minute exposure, the plate was rotated 90 degrees and exposed
a second time, as described by M. T. Gale in Optics Communications, Volume
18, No. 3, August 1976, page 295. The plate was then developed for twenty
seconds in Shipley developer. FIG. 4 is a reproduction of a scanning
electron micrograph, at a magnification of 10,000X, of the photoresist
pattern, tilted at an angle of about 45.degree.. This micrograph shows
partly etched saddle points between adjacent peaks, indicating that each
exposure was above the threshold for development of the photoresist. It
shows also that at the intersection of the lines, etching of the
photoresist extends to the surface of the glass plate. The flat surfaces
thus created are important because they will define the flat substrate of
the reflective micromirrors of the optical recording medium.
A nickel mold was made from the photoresist plate, prepared as above. This
process is described in National Geographic, March 1984, page 373. A
second generation nickel electroform was made from the original nickel
master. The second generation nickel had contours corresponding to those
of the photoresist plate and served as a stamper to reproduce the pattern
by embossing sheets of plastic.
An array of flat-topped plastic mesas was produced by embossing a sheet of
PVC plastic with the nickel stamper, described above, in a Carver
Laboratory press, Model C. manufactured by Fred C. Carver, Inc., Menomonee
Falls, Wis. The nickel stamper was placed, contoured side up, on a sheet
of lead on the lower stage of the press. A sheet of 10 mil thick glossy
black PVC plastic, obtained from Ridout Plastics, San Diego, Calif., was
placed over the nickel stamper. The press was pumped to a pressure of
20,000 pounds and the lower heating unit was raised to a temperature of
250 degrees Fahrenheit. The heater in the upper platen was not energized
while the heat from the lower unit penetrated the lead, nickel and
plastic. When the thermometer in the upper platen read 200.degree. F., the
lower heater was turned off and the 20,000 pounds pressure was maintained
during cooling. When the temperature in the upper unit had dropped to
150.degree. F., the pressure was released and the PVC was peeled from the
niclel stamper. A bright diffraction pattern was visible on the embossed
PVC. FIG. 5 is a reproduction of a scanning electron micrograph of the
embossed PVC at a magnification of 10,000X tilted at an angle of about
45.degree..
A reflective material was then coated on the embossed surface of the PVC.
This material was chosen to be both reflective enough to permit
identification as a micromirror by an optical reading device and also
capable of absorbing sufficient laser energy to melt or cause distortion
of the plastic substrate during data recording. The preferred material is
a metal and ceramic composite of gold and silicon dioxide. Such materials,
known as cermets, have been used for thin film resistors and in light
absorbing applications such as solar collectors. The Au-SiO.sub.2 system
is described in the Handbook of Thin Film Technology, McGraw-Hill, 1983,
chapter 18, page 21.
The cermet layer, coated on glossy clear polyester, has about four times
greater absorbancy at 830 nm than a pure gold layer on the same substrate.
The cermet is also significantly more sensitive to alteration of
reflectivity by laser light. A pure gold layer showed no resonse to pulses
of several microseconds, at a power level of about 5 milliwatts. Under the
same conditions, the cermet coating showed significant changes in
reflectivity in response to pulses of less than one microsecond.
Finally, cermet was sputter coated on the embossed PVC described earlier.
This storage medium showed visible changes in reflectivity at pulse
durations of less than 0.3 microseconds at the same 5 milliwatt power
level.
FIG. 6 is a reproduction of a photograph of a sample of the recording
medium comprising an array of individually alterable micromirrors of
Au-SiO.sub.2 on embossed PVC plastic. The photograph was from a TV monitor
attached to an optical system providing a magnification of about 1750X on
the screen. Some of the micromirrors in a row near the bottom have been
exposed to a 0.5 microsecond pulse from an 830 nm diode laser, at a power
level of about 5 milliwatts. The darkened spots are clearly visible as
areas of significantly lower reflectivity in response to the laser pulses.
In this test, the response to the recording laser beam was only along the
rows of micromirrors, not between them. If only a portion of a micromirror
is exposed to the laser beam, the entire micromirror will still melt or be
distorted, although somewhat more slowly. These properties are especially
advantageous in optical data recording because the recorded spot size and
location is less sensitive to variations in the laser spot size and
alignment.
In reading a previously recorded area of the medium, it is desirable to be
able to distinguish between three levels of illumination: the level
represented by an untreated micromirror retaining its original
reflectivity, the level represented by a micromirror that has been treated
by the laser beam to destroy its reflectivity, and the reflectivity
represented by the valleys 10 between the mesas 8. The areas between the
mesas have no reflecting surface in the plane of the micromirrors 6, so
that the reflection is reduced by dispersion of the beam at positions
beyond the focal point of the beam. The untreated areas of the medium 2
between the mesas will also have an inherently lower reflectivity than the
micromirrors 6 even though some of the sputtered mirror coating material
is deposited in the valley areas. Generally, therefore, it is not
desirable to eliminate all reflectivity of the micromirrors 6, but to
reduce it only enough to make it readily distinguishable from the
untreated micromirrors. This allows recorded micromirrors to continue to
serve as tracking and timing markers. In this example, the unaltered
micromirrors have a reflectivity greater than 20% at 830 nm and a laser
power of about 3.2 nanojoules per square micrometer is sufficient to
reduce the reflectivity of the mirror coating by the desired amount. Other
kinds or quantities of mirror coatings can be used that require higher
recording energy, but it is preferable that the micromirror be destroyed
by exposure to focused energy no greater than 200 nanojoules per square
micrometer.
It is important to provide a mirror surface that is relatively immune to
oxidation or discoloration or dulling from other causes. It is important
also, to provide a surface that is affected only minimally by dust or
other contaminates. For those reasons, a layer of transparent material,
generally indicated at 12 in FIG. 7, is positioned over the surface of the
mesas 8. This layer, which may be formed of polyester, polycarbonate or
other transparent plastic, is in contact with the micromirrors 6 and is of
substantial thickness (100 or more times the height of the mesas 6) so
that, during recording and read-out by a laser beam focused on the
micromirrors 6, the converging laser beam covers a significant area at the
point where it enters the layer 12 and so minimizes the effect of a dust
particle on the surface of the layer 12.
A filler 14, which may be a liquid such as oil, fills the valleys 10 and
displaces any air that would otherwise be trapped between the micromirror
surfaces and the layer 12. The liquid is preferably selected with an index
of refraction near that of the plastic from which the layer 12 is formed
to avoid any undesirable reflection of the laser beam. The filler 14 may
remain as a liquid or it may be composed of a liquid plastic accompanied
by a catalyst so that after the filler is in position the plastic
solidifies. Alternatively the filler may be a UV curable polymer. With any
of the filler compositions, it is desirable to add an infrared absorbing
dye to the filler to further reduce any reflection from the valleys 10.
Such dyes are well known in the prior art.
In general, the recording of data by altering the reflectivity of selected
micromirrors results in each micromirror having only one of two possible
states, that is, the micromirror reflects the incident light as a mirror
or has a reflectivity below some predetermined level and is not considered
to be a mirror. The density of information storage can be increased by
providing for additional levels of reflectivity. For example, each
original micromirror 6 can be constructed to have a reflectivity of 40-45%
at the frequency of the laser beam used for read-out. A first level of
intensity of the laser recording beam may be adjusted so that during the
time of recording on one micromirror, the reflectivity is reduced to
between 25% and 35%. To record another state, the intensity of the
recording laser beam is increased sufficiently that during the time of
exposure the reflectivity is reduced to between 10% and 20%. A single
micromirror can thus be used to store any one of three information indicia
(1) the mirror retains full reflectivity, that is, between 45% and 55%;
(2) the reflectivity is between 25% and 35%; and (3) the reflectivity is
between 10% and 20%. The valleys 10 should have a reflectivity
substantially less than 10% to permit even the micromirrors with minimum
reflectivity to be used as timing and tracking guides.
In an alternative embodiment, the recording medium and the overlying
plastic protective sheet are fabricated as an integral structure. As
illustrated by FIG. 8, the stamper used to form the medium 2a is the
reverse of the one used to form the medium of FIGS. 1-3. In this instance,
the mesas 8a are formed as depressions in a first surface 16 of a
substrate 4a formed of clear thermoplastic. The micromirrors 6a are formed
by exposing the surface containing the indentations to the sputtering
action of the mirror coating. The micromirrors 6a are therefore formed on
the flat surfaces at the bottoms of the indentations. However, viewed from
the opposite side, in the direction of the arrow 18, the indentations
appear as mesas with the mirror coating on the flat tops.
The micromirrors 6a are exposed to the recording and reading laser beams,
in the direction of the arrow 18, through the plastic substrate 4a. With
this arrangement, the micromirrors 6a are in intimate contact with
substrate material providing superior protection of the reflecting
surfaces from contamination. One additional advantage of this construction
is that, in the process of fabrication, reflective material that is
inevitably sputtered onto the exposed surface 16 of the substrate 4a,
which forms the bottoms of the valleys 10a, may be completely removed by
abrading. The bottom surface of the substrate 4a between the indentations
may be provided with a layer of light absorbent material thereby rendering
the valleys 10a between the micromirrors substantially non-reflective. The
plastic material of the substrate 4a now replaces the layer 12 that is a
separate entity in the earlier embodiment. The plastic is continuous from
the surface exposed to the laser beam to the bottom of the valleys 10a at
the surface 16 with no disruptive reflections resulting from a change in
the index of refraction.
One scanning procedure for recording on and reading from either of the
embodiments of the medium 2 and 2a is illustrated by FIGS. 1 and 9. A
source of coherent light, such as a diode laser 22, produces a beam 24,
that is first made more uniform by a collimating lens and an anamorphic
prism, both indicated diagrammatically at 16, and then is focused through
an objective lens 32 onto the micromirrors 6. The maximum dimension of the
beam of the plane of the mirrors, indicated by the broken line 28, is
preferably no greater than the cross-sectional area of each individual
mirror, and in any event small enough to distinguish one micromirror from
any adjacent micromirror. The same optical system is used for both
recording and reading. The laser light reflected from the micromirrors is
directed by a beam splitter 20 to an optical detector, generally indicated
at 36.
One method for scanning the medium 2 is to cause the laser beam to traverse
one row of micromirrors from one end of the medium 2 to the other. At the
end of each row, the laser beam is caused to move to the next row of
micromirrors and to scan that row in the opposite direction. As
illustrated in FIG. 1, a first row of micromirrors is scanned along line
"a" from one end of the medium to the other. The scanning beam is then
moved sideways to the next row and scans along line "b" in the reverse
direction. The beam is then again moved sideways and the micromirrors
scanned along line "c". A preferred procedure, however, is to scan the
micromirrors diagonally as illustrated by FIG. 10 which provides an
improved signal to noise ratio by increasing the distance between
successive micromirrors. The equally spaced rows of micromirrors that make
up the regular array are represented by the broken lines "d" and "e". The
scanning track of the laser beam 24, however, is successively along lines
"f", "g" and "h" which are diagonal with respect to the parallel rows of
micromirros, such as "d" and "e", forming the regular array. At the end of
row "f", the scanning motion is interrupted and moved sideways in the
direction of the arrow "j" to place the row "g" in scanning position. The
laser beam then scans that row in the reverse direction along the line
"g". This process is repeated to scan the entire series of rows over the
entire surface of the medium. An end-of-row code is pre-recorded on each
row and is read by the recording and reading systems to cause the scan to
move to the next row of micromirrors at the appropriate point.
The scanning movement along the rows may be accomplished by moving either
the entire laser and optics assembly or by moving the medium 2. In this
example, because the mass of the medium 2 is only a fraction of that of
the laser-optics assembly, there is substantial advantage in moving the
medium. The reciprocating scanning action, which results in variations in
the scanning velocity, is made practical by the regular arrangement of the
micromirrors which can be used to control the timing both during recording
and read-out.
The transverse movement, to move the beam from one row of micromirrors to
the next, is preferably accomplished by a sideways movement of the laser
22 and the associated optics at the end of the scanning of each row of
micromirrors. The sideways movement may also be accomplished by movement
of the medium, by deflection of the laser beam, or by a combination of the
two. For example, the laser beam may be deflected, by means well known in
the art, to accommodate the scanning of a preselected number of rows and
then the medium moved sideways by a similar number of rows while the beam
deflection is returned to its original position.
During the scanning, the position of either the laser beam or the medium 2,
or both, are controlled by the use of the micromirrors as tracking guides.
During the scanning of each row of micromirrors, the beam 14 is caused to
oscillate transversely, at a frequency much lower than the data rate, by a
galvanometer-actuated mirror, or other means well known in the art, for a
distance at the point of focus somewhat less than the distance across one
micromirror. The transverse sweep of the scanning action is indicated by
the broken lines 38 and 42 in FIG. 11 as the scan proceeds along the
centerline "k". The magnitude of the transverse scan depends upon the size
of the micromirrors, the size of the scanning spot, and the distance
between adjacent micromirrors. The intensity of the reflected light is
averaged by a tracking control mechanism, which forms part of the optical
detector 36, over a substantial number of micromirrors before changing the
direction of oscillation, in order to improve the tracking precision. The
tracking control mechanism maintains the beam 24 centered on the row of
micromirrors being scanned. If the average intensity of the reflected beam
when it is deflected, say, to the right, as diagrammatically illustrated
at 44, is less than the average intensity when deflected an equal distance
in the opposite direction, the beam 24 is adjusted toward the left to move
it nearer the center line of the micromirrors. If desired, the area of the
beam 24 in the focus plane may be made slightly larger than the reflecting
area of one micromirror, so long as it is small enough that it cannot
encompass any substantial fraction of two mirrors at the same time, so
that it can detect the reflectivity of each micromirror despite small
misalignment of the read-out beam relative to the centerline of the row of
micromirrors being scanned.
Prior to recording, the medium 2 is scanned by the laser beam 24 at low
intensity to determine whether all or substantially all of the
micromirrors have requisite reflectivity. After the medium has been
certified as free from defects, or the defects "fenced off" as described
below, the permanent recording is made by the laser beam 24 which has a
first level of intensity sufficient to enable the optical detector 36 to
determine the presence of a reflecting micromirror 6 on the surface of a
mesa 8, and a second level of intensity great enough to destroy the
reflectivity of the micromirror at which it is directed. The intensity of
the laser beam 24 is modulated as the recording is made to destroy the
reflectivity of the mirrors in accordance with the information to be
recorded.
The laser beam 24 operates at its low or reading intensity until the
detector 36 indicates the beam is focused on a micromirror. If the digital
information to be recorded indicates that particular micromirror is to be
destroyed, the laser beam is pulsed to its higher recording intensity for
a period of one microsecond or less, but long enough to destroy the
micromirror. If that particular micromirror is not to be destroyed, the
laser beam passes over it at the lower non-destructive intensity leaving
the reflectivity of the micromirror unchanged.
By destruction of the reflectivity is meant a lowering of the reflectivity
by an amount sufficient that the optical detector 36 can determine the
difference between a micromirror that has been exposed to the laser beam
24 at recording intensity from one that retains its original reflectivity.
If the examination of the medium prior to recording indicates relatively
few defects, the rows of micromirrors containing defects can be "fenced
off", that is, the particular rows containing defects are marked with a
special code that causes the scan | | |
