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Description  |
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The present invention relates to a novel information record, to novel
optical recording apparatus and methods for use in formation of such an
information record, and to novel optical playback apparatus and methods
for use in recovery of information from such an information record.
In the prior art, optical recording methods have been proposed in which
light from a laser is focused upon the surface of a recording medium with
sufficient intensity to cause ablation of surface material. By suitably
controlling the intensity of the focused light in accordance with
information to be recorded while relative motion is established between
the recording medium and the focused light spot, an information
representative pattern of pits may be formed in the surface of the
recording medium.
In application of such an ablation process, for example, to the
high-density recording of video information at "real-time" recording
rates, the achievement of an adequate recording sensitivity requires
efficient coupling of energy from the recording light beam into the
material to be ablated. In accordance with the principles of the present
invention, such efficiency of energy coupling may be enhanced by
construction of the recording medium in the form of a substrate having a
surface which is highly reflective (at least at the frequency of the light
forming the recording beam), with a thin layer of material, highly
absorptive at the recording beam light frequency, overlying the reflective
surface. With both incident light and reflected light (reflected from the
substrate surface) passing through the thin absorptive layer, the ability
to rapidly elevate the temperature of the absorptive layer material to
that required for ablation to take place is enhanced.
Pursuant to a further aspect of the present invention, the efficiency of
energy coupling into the absorptive layer may be further enhanced by
choosing a thickness for the absorptive layer which establishes an
anti-reflection condition for the coated substrate at the recording beam
light frequency. With energy loss via reflection minimized due to the
anti-reflection condition establishment, and with energy loss via
transmission into the substrate minimized by the reflective surface
presence, a highly efficient heating of the absorptive layer to an
ablation temperature is realizable.
In accordance with an illustrative embodiment of the present invention, a
surface of a glass substrate of disc form is processed so as to form
polished flat surface, which is then coated with a thin layer of metal
(e.g., aluminum) to establish a highly reflective surface for the
substrate. This reflective surface is then coated with a layer of material
(e.g., an organic dye, such as fluorescein) which is highly absorptive at
the light frequency of a monochromatic light source available for
recording use (e.g., an argon laser, providing an output as a wavelength
of 4579 angstrom units). The thickness of the dye layer is chosen to
establish an anti-reflection condition for the coated substrate at the
recording wavelength.
The coated disc structure, formed in the above-described manner,
constitutes a record blank which may be utilized in conjunction with a
suitably controlled light beam source of the appropriate frequency to
efficiently effect coupling of energy from the light beam into the
absorptive layer.
In an illustrative recording system embodying the principles of the present
invention, a record blank of the above-described disc form is subject to
rotation at a constant rotational speed while a beam of light from a light
source (e.g., a laser, providing light at a frequency at which the
anti-reflection condition is obtained) is focused on the coated surface of
the disc. The intensity of the light beam is controlled in accordance with
information to be recorded. Illustratively, the control is effected in
accordance with carrier waves modulated in frequency by
picture-representative video signals, with the light beam intensity
varying as a result between a high level sufficient to effect ablation of
the absorptive material and a low level insufficient to effect such
ablation, the frequency of the level alternations varying as the video
signal amplitude changes.
An information track comprising a succession of spaced pits is formed in
the coated surface of the disc, the pits appearing in those surface
regions exposed to the high level beam, due to vaporization of the
absorptive layer material in response to the high level beam exposure.
Variations in the length and separation of the pits are representative of
the recorded information. Where a continuous sequence of pictures is to be
recorded, a spiral information track may be formed by providing relative
motion, in a radial direction and at a constant rate during the recording
between the recording beam and the rotating disc. Alternatively, in the
absence of such relative motion during the recording, a circular
information track may be formed; appropriate for "slide" recording
purposes.
The result of the above-described recording process is the formation of an
information record of a form which facilitates recovery of the recorded
information by optical playback processes. The information track of such
an information record comprises (1) undisturbed surface regions that
exhibit very low reflectance at an appropriate light frequency (due to the
anti-reflection thickness choice described previously), alternating with
(2) pit regions, formed by the ablation process, that exhibit appreciably
higher reflectance at the same light frequency (due to complete, or at
least partial, removal of the absorptive layer covering of the reflective
surface of the substrate, ensuring departure from the anti-reflection
condition). A high ratio between the reflectance of the pit regions and
the reflectance of the intervening (undisturbed surface) regions is
readily provided.
In playback operations pursuant to the principles of the present invention,
a light beam is focused upon the information track of a rotating
information record of the above-described type. The playback beam has a
constant intensity at a level insufficient to effect ablation of the disc
coatings, and is of a frequency substantially corresponding to that at
which the undisturbed surface regions exhibit an anti-reflection
condition. A photodetector, positioned to receive light reflected from the
successive regions of the information track as they pass through the path
of the focused light, develops a signal representative of the recorded
information. A high readout contrast ratio (due to the large differences
in a reflectance of the pit regions and the intervening track regions, at
the light frequency of the playback beam) is readily obtained, permitting
recovery of the recorded video signals with an excellent signal-to-noise
ratio.
In the accompanying drawings:
FIG. 1 provides a cross-sectional view of a portion of a recording medium,
exhibiting a construction in accordance with principles of the present
invention;
FIG. 2 illustrates a cross-sectional view of a portion of the information
track of an information record formed from a recording medium of the type
shown in FIG. 1 pursuant to principles of the present invention;
FIG. 3 provides a representation, partially in block diagram form, of
optical recording apparatus suitable for use in formation of an
information record of the type shown in FIG. 2 pursuant to principles of
the present invention, as well as playback apparatus suitable for use in
recovery of recorded information from an information record of the type
shown in FIG. 2 in accordance with principles of the present invention;
and
FIG. 4 provides a graph of the relationship between surface layer thickness
and reflectance for an illustrative form of the recording medium of FIGS.
1 and 2.
In FIG. 1, a cross-sectional view of a portion of a record blank 10, formed
for use in an optical recording system, shows the construction of a
recording medium pursuant to an illustrative embodiment of the present
invention. The record blank 10 includes a substrate 11, which is
illustratively formed in the shape of a disc, a major surface(s) of which
is processed to be polished and flat. Desirably, the substrate 11 is
formed of a material, such as glass, for example, which may be
conveniently processed to achieve such surface.
Overlying the surface "s" of the substrate 11 is a thin layer 13 of a
material exhibiting a high reflectivily (over at least a given portion of
the light spectrum). Illustratively, the reflecting layer 13 is formed of
a metal, such as aluminum, for example, deposited on the surface "s" by an
evaporation process.
Overlying the reflecting layer 13 is a layer 15 of material which is highly
light absorptive (at least over the aforementioned given portion of the
light spectrum). Illustratively, the absorptive layer 15 is formed of an
organic dye, such as fluorescein, for example, deposited on the reflecting
layer 13 by an evaporation process.
An advantage of the use, in the illustrated construction of the recording
medium, of an absorptive layer overlying a reflective surface will be
recognized by consideration of the effect achieved when a light beam L (of
a frequency in the aforementioned given portion of the spectrum) is
directed along an axis (x) normal to the surface "s," and focused at or
near the surface of the absorptive layer 15. Most of that portion of the
incident light which reaches the interior boundary of the absorptive layer
15 is not "lost" through transmission into the substrate 11, as would
occur in the absence of the reflecting layer 13, but rather is reflected
back into the absorptive layer 15. The consequence is exposure of the
absorptive layer 15 to both incident and reflected light. Where it is
desired to effect surface ablation in response to recording light
exposure, this avoidance of transmission losses into the interior of the
recording medium increases the coupling of energy from the recording light
beam into the surface material, enhancing recording sensitivity. While a
similar avoidance of transmission losses into the interior of the
recording medium would be achieved by forming the surface layer of a
reflecting material (not covered by an absorptive layer), the transmission
loss avoidance would be offset by high reflection losses.
For optimum efficiency of coupling of energy from the recording light beam
L into the absorptive layer 15, reflection losses are desirably reduced to
a low level by choosing the thickness (d.sub.1) of the absorptive layer
15, with relation to the thickness (d.sub.2) of the reflecting layer 13
and the optical constants of the elements of the system 15-13-11, to
establish a so-called anti-reflection condition for the system at the
recording beam frequency. The achievement of an anti-reflection effect by
use of thin films of appropriate thicknesses and optical properties is
well-known, per se, and applications of the effect with films of
transmissive materials are widespread in optical equipment. Formulae which
may be used in arriving at parameter combinations for the absorptive media
(15,13) of the FIG. 1 system that result in the desired anti-reflection
condition are presented herein subsequently, in connection with a
discussion of FIG. 4.
When the intensity of the focused light beam L is of sufficient magnitude,
material of the absorptive layer 15 is elevated to an ablation
temperature, and vaporization of the material occurs, forming a pit in the
surface of the record blank 10. With suitable modulation of the intensity
of the beam L in accordance with a recording signal, as successive regions
of the record blank 10 pass through the beam path, an information track
may be formed comprising spaced pits in regions of the absorptive layer
subject to the high intensity beam exposure, separated by undisturbed
regions of the absorptive layer (that were not subject to such high
intensity beam exposure).
FIG. 2 illustrates a portion of an information record formed when the
record blank 10 of FIG. 1 is subjected to such controlled beam exposure.
As shown in cross-section in FIG. 2, the information track comprises a
succession of spaced pits p.sub.1, p.sub.2, p.sub.3, p.sub.4, separated by
regions (u.sub.1, u.sub.2, u.sub.3, u.sub.4) in which the surface of the
absorptive layer 15 is undisturbed. For illustrative purposes, the depth
of each pit is shown as being equal to the thickness of the absorptive
layer 15, whereby the reflective layer 13 is wholly uncovered in the pit
regions. As will be discussed subsequently, while such a depth of ablation
may desirably result in maximization of the readout contrast ratio, it is
not essential for good playback results. Thus, in an acceptable
alternative to the illustrated form of information record, a residual
portion of the absorptive material (with a thickness, of course, less than
d.sub.1) may overlie the reflecting layer 13 in the pit bottoms.
FIG. 3 illustrates combined recording/playback apparatus useful for the
purposes of the present invention. For an initial explanation of operation
of the illustrated apparatus in the recording mode, it should be assumed
that the disc 10', supported on a turntable 20 which is subject to
rotation at a constant rotational rate (e.g., 1800 rpm) by the turntable
rotational drive mechanism 21, is a record blank of the form shown in FIG.
1.
The monochromatic light output of a laser 31 (illustratively, of an argon
type providing an output wavelength of 4579 angstrom units) is passed via
a polarizer 32 and an intensity modulator 33 to a polarized beam splitter.
The polarizer 32 effects a polarization of the laser output in a direction
that permits passage of the intensity modulated light through the beam
splitter 35. The intensity modulator 33 is driven by a modulator driver
55, responsive to a carrier wave source in the form of a frequency
modulated oscillator 53. The frequency of the carrier wave output of
oscillator 53 is subject to variation in accordance with the amplitude of
a modulating signal provided a source 51 of video signals to be recorded.
The light intensity of the output of modulator 33 is shifted between high
and low levels in consonance with the modulated carrier waves.
A lens 37 forms the light passed by beam splitter 35 into a beam which
passes through a quarter-wave plate 39 to a mirror 41 which reflects the
beam onto the entrance aperture of a lens 43. The lens 43 focuses the
light beam reflected by mirror 41 on the absorptive surface layer (15) of
the disc 10'. An absorptive layer region in the path of the focused light
beam when the light beam intensity level is high suffers ablation, while
an absorptive layer region in the path of the focused light beam when the
light beam intensity level is low remains undisturbed. The result is
formation of an information track of the general type shown in FIG. 2.
When the frequency of the carrier waves controlling the intensity
modulator 33 is high, the spacing between successive pits in the
information track is short (see e.g., spacing between pits p.sub.1 and
p.sub.2); when the carrier wave frequency is low, the pit spacing is long
(see, e.g., spacing between pits p.sub.3 and p.sub.4).
A lens translational drive mechanism 61 provides a constant rate radial
motion of an assembly 40 (incorporating lens 43 and mirror 41), when a
spiral track formation is desired, or a stepped radial motion when it is
desired to form a circular track, or a series thereof.
When the light frequency of the recording beam provided by laser 31 falls
in the given spectrum portion for which the disc's layer 13 is highly
reflective and for which the disc's layer 15 is highly absorptive, and at
or close to the frequency at which the system 15-13-11 exhibits the
anti-reflection effect, a high recording sensitivity is obtained.
For an explanation of operation of the apparatus of FIG. 3 in a playback
mode, it may be assumed that the rotating disc 10' is of a recorded form
as shown by the information record construction of FIG. 2. In the playback
mode, variation of the intensity of the laser output is discontinued by a
suitable disabling of the modulation control system 51-53-55. The mode of
operation of the translational drive mechanism 61 is chosen to be
appropriate for scanning of the form of information track desired to be
played back. The constant intensity of the laser output is set at a
playback level safely below the level causing ablation of the material of
the absorptive layer 15. The laser beam follows the previously described
path (via elements 32, 33, 35, 37, 39, 41) to lens 43 which focuses the
beam on the desired information track of disc 10'. Light reflected from
the information track is returned via elements 43, 41, 39 and 37 to the
beam splitter 35. As the returned light has made two passes through the
quarter-wave plate 39, its polarization has been altered to a direction
which results in reflection of the returned light by beam splitter 35 to a
photodetector 71.
The intensity of light falling upon the photodetector 71 alternates between
a minimum level and a maximum level as the successive regions (p.sub.1,
u.sub.1, p.sub.2, u.sub.2, etc.) of the information track pass through the
path of the focused beam. The minimum intensity level for light reaching
the photodetector 71 is obtained when an undisturbed region (u.sub.1,
u.sub.2, etc.) of the absorptive layer 15 is in the focused beam path,
while the maximum intensity level for light reaching the photodetector 71
is obtained when a pit (p.sub.1, p.sub.2, etc.) is in the focused beam
path.
The output of photodetector 71 comprises carrier waves with zero crossings
recurring at frequencies varying in consonance with the pit edge spacing
variations passing through the focused beam path. The photodetector output
is applied to a bandpass filter 73, which selectively passes signal
components falling within the deviation range employed for oscillator 53
and appropriate sidebands thereof. The output of bandpass filter 73 is
applied via a limiter 75 (removing spurious amplitude modulation of the
frequency modulated carrier waves) to the input of an FM demodulator 77,
which recovers the recorded video signal information.
When the light frequency of the playback beam provided by laser 31 falls in
the given spectrum portion for which the disc's layer 15 is highly
absorptive and for which the disc's layer 13 is highly reflective, and at
or close to the frequency at which the undisturbed regions of the system
15-13-11 exhibits the anti-reflection effect, a high readout contrast
ratio is realized, permitting the video signal recovery with an excellent
signal-to-noise ration. Illustratively, with use of a deviation range of
7-10 MHz., color television signals of an NTSC format have been recovered
with a video signal-to-noise ratio of 55-60 db (peak-to-peak video to rms
noise) for a video bandwidth of 5 MHz.
The reflectance of a system of two absorbing layers, such as the system of
FIG. 1 (incorporating a dye layer on top of a metallic layer on a glass
substrate), can be calculated using formulae well known in the theory of
thin film optics. The reflectance of such a system is given by the square
of the modulus of the amplitude reflection coefficient of the system. The
amplitude reflection coefficient of such a system is given by the
following expression:
##EQU1##
where r.sub.1, r.sub.2, r.sub.3 are the Fresnel reflection coefficients at
the air to dye, dye to metal, and metal to substrate interfaces
respectively, and are complex numbers, and where i = .sqroot.-1.
The Fresnel reflection coefficients are given in terms of the complex
indices of refraction, as follows:
##EQU2##
where .eta..sub.0, .eta..sub.1, .eta..sub.2, .eta..sub.3 are the complex
indices of refraction respectively of the media: air, dye, metal, and
glass substrate. The complex index of refraction is a complex number,
.eta. = .eta.-ik, which incorporates the real part .eta. and the imaginary
part k and characterizes the intrinsic optical properties of a medium. For
non-absorbing media the imaginary part of the index is zero.
The quantities .delta..sub.1 and .delta..sub.2 are given by
##EQU3##
where d.sub.1 and d.sub.2 are the thicknesses of the dye and metal layers
respectively and .lambda. is the wavelength of the light in air. Note that
.delta..sub.1 and .delta..sub.2 are complex quantities because of the
dependence on the complex indices.
The reflectance of the system of FIG. 1 can thus be readily evaluated with
a computer as a function of the film thicknesses and optical constants.
One such calculation for fluorescein on aluminum on glass is plotted in
FIG. 4. In this case the assumed parameters were: aluminum thickness = 100
A; wavelength of light = 4579 A; index of dye = (1.842 - i.362); and index
of aluminum = (0.47 - i4.84).
The graph of FIG. 4 shows that the minimum reflectance for the illustrative
system parameter choices for the system of FIG. 1 occurs at a dye layer
thickness of approximately 500 angstrom units. Thus, when this thickness
value is associated with the illustrative system parameter choices, the
record blank of FIG. 1 (and the non-depressed regions of the information
record of FIG. 2) exhibits an anti-reflection condition for the argon
laser output.
While the principles of the present invention have been demonstrated with
particular regard to the illustrative structures of FIGS. 1 and 2, it will
be recognized that various departures from such illustrative structures
may be undertaken in practice of the invention. For example, the substrate
itself may be formed of material having high reflectivity, eliminating the
need for employing a separate reflecting layer in forming a reflective
surface underlying the absorptive layer. For another example, since
wideband reflection is not required of the reflecting layer, the metal
coating may be supplanted by a multilayer (or even a single layer)
dielectric reflector. It should also be appreciated that other forms of
optical recording (such as pulsed holographic recording) may use the
advantageous record blank structure described herein.
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