Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
__ PHN. 7016.
~;)394LQ2
The invention relates to a record carrier
on which information is recorded in an optical structure
of trackwise arranged areas and intermediate areas,
which areas have a different influence on a read beam of
radiation than the in~ermediate areas and the lands between
the tracks, the lengths of the individual areas and inter-
mediate areas defining the information, for which record
carrier the average spatial frequency of the areas in the
read apparatus varies. The invention also relates to an
apparatus for writing information on such a record carrier.
In this respect tracks, in the case of a
round record carrier, is to be understood to mean a
multitude of concentric tracks or one continuous spiral
track, which spiral track may be divided into a number
of quasi-concentric tracks. The average spatial fre-
quency of the areas is to be understood to mean the
average number of areas per unit of length.
For reading a record carrier provided with
an optical information structure it has been proposed (in
applicants Canadian Patent 1,005,907 - February 22, 1977
(PHN. 6518)) to project a radiation spot of dimensions
greater than
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the width of a track onto the information structure.
The radiation emerging from the record carrier is
concentrated onto a radiation sensitive detector by
a lens of ~imited numerical aperture. At the areas
in the information structure diffraction occurs, the
dlffracted beams of radiation for the most part falling
outside the lens aperture. Consequently, if an area
is located in front of the lens, less radiation will
reach the detector than if there is no area in front
of the lens.
In order to obtain an optimum optical signal
the geometry of the areas on the record carrier and
the dimensions of the radiation spot, as well as the
intsnsity distribution over the radiation spot should
~atch each other. The optimum aperture of the lens which
forms the radiation spot is then defined for a specific
geometry of the areas. The average spatial frequency
and thus the average length of the areas, howe~er, may
vary over the various tracks. Therefore, all the tracks
cannot be read in an optimum manner by means of a single
radiation spot formed by one specific lens.
The conversion of the information recorded
in the optical structure of the record carrier into
an el~ctrical signal need not always be linear. It may
happen that tha detector receives a certain threshold
intensity, whilst the edge of the area i~5 not yet
correctly positioned relative to the radiation spot~
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This results in a distortion of the detector signal,,
especially at low spatial frequencies, i.e. at'grea*
a~erage lengths of the areas and the intermediate
areas in the tracks.
It is an object of the invention to provide
a record carrier which mitigates one of the said
problems or both problems together. For this purpose,
according to a general feature of a record carrier
according to the invention, the average dimensions of
the areas in the tracks are adapted to the average
'' spatial frequency in the tracks in such a way that,
when reading the record carrier by means of a read
,beam of radiation which is focussed onto the information
structure to a radiation spot larger than the track
width and, in the case of several tracks, smaller
than the sum of the track width and twice the distance
betwesn two consecutive tracks, a signal of sufficient
modulation depth and minimal distortion is obtained.
According to a further feature of a record
carrier according to the invention, the track width
,increases as the average spatial frequency of the
block in the tracks increases. This allows the record
carrier according to the invention -to be read in an
optimum manner by means of a single radiation spot of
one specific geometry and ono specific intensity
distribution over the radiation spot., Adaptation of
the track width furthermore ensures that those
positions of the radiation spot and areas relative
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to each other, where the intensity of the radiation
to the detector is half the difference between the
maximum and minimum intensities of the radiation to
the detector, sufficiently approximate the desired
positionS.
According to a further feature of a record
carrier according to the invention this may also be
achieved for a low average spatial frequency of the
areas by making the average length of the areas smaller
lo than the average length which would correspond to the
-- intended spatial frequency.
According to the invention yet another
possibility at a low average spatial frequency is
to taper the ends of the areas off to a point in
the longitudinal direction of the tracks.
The invention will now be described in
more detail wlth referenc0 to -the drawing, in which:
Fig. 1 shows a part of the optical structure
of a record carrier to be read,
Fig. 2 represents a previously proposed
apparatus for reading such a record carrier,
Fig. 3 represents the amplitud~ distribution
of a radiation spot before and after interaction with an
area of the record carrier,
Fig. 4 shows parts of two information tracks
of different spatial frequencies,
Fig. 5 represents the signals produced by
interaction of` a radiation spot with the tracks of Fig. 4,
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Fig. 6 shows a part of a track of high
spatial frequency adapted according to the invention,
Fig. 7 shows the intensity variation of a
radiation beam emerging from the record carrier as a
function of the position of the edge of an area relative
to the radiation spot,
Figo 8 represents a deformation of the
detector signal which occurs at low spatial frequencies,
Figs. 9 and 10 show parts of optical information
structures which has been corrected for said deformation
- in accordance with the invention, and
Fig. 11 shows an apparatus for writing
information on a record carrier according to the invention.
Fig. 1 represents a part of an optical structure
of a record carrier to be read, specifically a round
record carrier 1. On the record carrier 1 a numbér of
- areas ~ are arranged in tracks 2. The blocks have a
different effect on~a rediation beam which is incident
on the record carrier than the rest of the record carrier,
i.e. than the intermediate areas t and the information-free
lands 3. The tracks may be arranged parallel to each
other, i.e. concentric relative to the centre of the
record carrier. The record carrier may also be provided
with a continuous spiral track. The lengths of the areas
and the intermediate areas are determined by the infor-
mation stored in the track.
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A radiation beam, not shown, produces a
radiation spot V on the optical structure. By moving
the record carrier in the dlrection indicated by the
arrow 4, the radiation beam is time-modulated in
accordance wi-th the sequence of areas and intermediate
areas in a track. The diameter d of the radiation
-spot V is greater than the width of the tracks 2,
but smaller than the sum of the track width and
twice the width of the information-free lands 3.
The numerical aperture of a lens L (see Fig.2~,
-- which serves to image the record carrier onto a detector
- D, is selected so that said lens cannot image the
narrow areas ~. Then, as long as a radiation spot is
projected outside an-ar0a ~, the detector D will
receive a ma~imum amount of radiation. When a radiation
spot is projec~ed onto an area g of a finer structure
than d much radiation is diffracted outside the en-
trance pupil of the lens L, so that substantially no
radiation impinges on the detector.
The information may be stored in the record
carrier as an amplitude structure, the areas being co
planar with the surface of the record carrier. The areas
may then be radiation-absorbing, whilst the record carrier
itself may be radiation-transmitting or radiation-
reflecting, or conversely, the areas may be radiation-
transmitting or radiation-reflecting, whilst the record
carrier is radiation-absorbing. The plane of the areas
may also be disposed at a small distance from the surface
.
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of the record carrier. An example of such a phase
structure is a reflecting record carrier, in which
pits are pressed at the location of the areas.
For a maximum modulatlon of the detector
current by said pattern of pits on the record carrier
the radiation beams reflected at the bottom of a pit
and at the record carrier surface adjacent thereto
must have a difference in phase of 1800 and have the
same intensity. The 1800 phase difference is obtained
by giving the pit a depth equal to a quarter of the
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~avelength of the radiation used, so that the difference
in path length between the two reflections will be half
a wa~elength. Equality of the intensities is obtained
by ensuring that an equal amount of radiation is incident
beside and in a pit. Thus, every track geometry is
associated with an optimum intensit~ distribution over
the radiation spot and, because that distribution
depénds on the diffraction at the lens aperture, also
with an optimum value of the aperture of the lens which
forms the radiation spot.
This is illustrated in Fig. 3. The curve a
represents the amplitude A as a function of the radius r,
in a round radiation spot. If said radiation spot is
projected onto a pit k which is si`tuated in a record
carrier at a depth of a quarter wavelength an amplitude
2~ distribution is obtained in accordance with the curvec.
The radiation reflected by the bottom of the pit is 180
phase shifted relatlve to the radiation reflected by the
PHN . 7 0 1 6
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area surrounding the pit. The detector is located on
the optical axis in a point which is situated compara-
tively far from the pit. The total radiation in this
point can be determined by addition of the complex
amplitudes of all components e and f. It i5 evident
that by an appropriate choice of the amplitude dis-
tribution and of the dimensions of the pit the sum
of the complex amplitudes and thus the intensity of
the radiation which is incident on the detector, can
be made negligibly small.
~s stated previously, the length of the pits
is determined by the information which is recorded in
the record carrier. For agreat number of consecuti~e
pits, however, an average length can be given. Said
average length may now vary over the record carrier.
This is for example the case with a round disc-shaped
record carrier on which a television programme is
written.and of which each concentric or quasi-concentric
track contains the information of one frarne. It is
obvious that for such a record carrier the average
length of the pits in the outer track is greater than
that of the pits in the inner track, for example by a
factor 3 for an inner diamter of 10 cm and an outer
diameter of 30 cm. This means that when the aperture
of the lens L is selected so that an outer track is
read in an optimum rnanner, an inner track cannot be
read in an optimum way by means o-f said lens.
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Fig. 4 represents parts of an information
structure with a great average pit length (1~ and
an information structure with small average pit length
(1'). In reality the lengths of the pits in a track
may still be modulated in accordance with the infor-
mation stored in ,the track.
It is assumed that the amplitude distribution
over the radiation spot and the dimensions of the
radiation spot are such that the track of an average
pit length 1 allows optimum reading. This means that
'~- when the radiation spot is projected on a pit the sum
of the oomplex amplitudes of the reflected radiation
components i and h is substantially zero, so that the
detector receives a ~inimum amount of radiation~
If the radiation spot is projected midway between two
- , pits, no diffraction will occur, so that the amount
of radiation on the detector is maximum. The variation
of the radiation inténsity as a function of~he travel
x of the read spot ,in the direction of the track is
then as represented by the curve u in Fig. 5.
' If the 'same radiation spot is used for
reading the information track of short average pit
length 1', the variation of the intensity of the
radiation intercepted by the detector will be far
more unfavourable as is represented by the curve u'
in Fig. 5. The spacing between the pits in the
longitudinal direction of the track is smaller than
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the diameter of the radiation spot, so that the
radiation spot is always projected on a pit or on
parts of adjacent pits. As a result, the maximum
radiation intensity at the detector will be sub-
stantially smaller than when reading a track of great
average pit length. When the radiation spot is projected
o~to one pit the complex amplitudes of the reflected
radiation components i and h will b0 anything but equal,
~ that the minimum of the radiation intensity which is
intercepted by the detector i9 higher than when a track
of great a~erag0 pit length 1 is read. The modulation
depth of the detector signal when reading a track of
small average pit length is smaller than when reading
a track of great average pit length.
According to the invention the situatio~ can
be improved upon for a track of small average pit length,
allowing for the amplitude distri`bution over the
radiation spot, by adapting the widths of the pits
in such a way that when the radiatiDn spot is projected
o~to a plt, the intensities of the radiation reflected
by the bottom of the pit and of the radiation rePlected
by the area surrounding the pit approximate each other
closely enough.
Fig. 6 shows a part of an information track
of an average length pit 1', according to the invention.
The width of the pits is substantially greater than in
Fig. ~. This ensures that the level of the minimurn
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radiation intensity at the detector is reduced, so that
the modulation depth of the detector s;gnal becomes
greater than that according to the curve u' in Fig. 5.
However, widening the tracks also has an
influence on the d.c. component in the detector signal.
It is obvious that the d.c. component varies with a
variation of the average spatial frequency i.e. such
that at increasing average spatial frequency of the
pits the d.c. component decreases; compare levels
n and n' in Fig. 5. The variation of the d.c.
component increases over a certain range of spatial
frequencies according as the pits become wider.
The variation of the d.c. component during reading of
a record carrier may present difficulties when elec-
tronically processing the electrical signal from the
detector. If a minimal variation of the d.c. component
is desirable, for example for a certain coding of a
television signal on the record carrier, the track
width will be the result of a compromise. In that case
some of the improvement in modulation depth attainable
according to the invention will have to be sacrificed
in favour of an as constant as possible d.c. component
in the detector signal.
Besides in a round record carrier with an
equal amount of time information per track, the invention
may also be employed in any record carrier in which
for some reason the spatial information density in the
optical structure changes in a specific direction.
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It is assumed that when the centre of the
radiation spot passes the beginning or end of a pit,
whilst the record carrier is being read, the radiation
intensity intercepted by the detector amounts to half
the difference between the possible maximum and
minimum radiation intensity at the detector. However,
in practice this requirement is not always met and
non-linearities result, especially at low average
spatial frequencies.
Fig. 7 shows how, in the case of a pit
structure of low spatial frequency, the intensity
of the radiation to the detector varies as a function
of the position of the radiation spo~ relative to the
pit. In this case it is assumed that a radiation spots
formed by Gaussian intenslty distribution has a
diameter of 1.3 /um. The pit has a length of 3 /um
and is 0.8 /um wide. The consecutive positions of the
end of the pit relative to the radiation spot are shown
dotted and are designated by 10, 11 and 12. The intensity
Io is equal to Imax Im;n~ Imax represe 9
intensity of the radiation to the detector if the
radiation spot is projected outside a pit Imjn is the
minimum intensity of the radiation directed towards
the detector if the radiation spot is projected onto
a pit. The "hump" in the curve which represents the
intensity in Fig. 7 is caused by the fact that the
pit and the radiation spot are not adapted to each other.
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For smaller pits the "hump" will disappear. The Figure
shows that already in situation 11, i.e. when the end
of the pit is still at a comparatively great distance
from the centre m of the radiation spot, the ;ntensity
of the radiation reflected to the detector has already
dropped to about 1/2 Io~ As a result of this, the
electrical signal supplied by the detector will have
a shape as represented by the uninterrupted line in
Fig. 8.
The signal is strongly distorted relative to
the signal which is represented by a dotted line, which
would be obtained if the value 1/2 Io were attained if the
end of the pit should coincide with the centre m of the
radiation spot s.
According to the invention the said non-
linearity can be substantially avoided by making the
tracks of low spatial frequency, for example the
outer tracks of the said round record carrier, narrow.
This ensures that the point in the radiation spot
where the end of a pit must be located in order that
the radiation intensity to the detector attains the
value 1/2 Io is shifted towards centre of the radiation
spot. At higher spatial frequencies, when the radiation
spot is continually projected onto a pit or parts of
26 adjacent pits, the said non-linearity is no longer
significant. The tracks with averagely shorter pits
may be wider, so that the requirement for optimum reading
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is met: as many as possible equal intensities of
the radiation emerging from a pit and of that
from the area surrounding a pit. Thus, the influence
of two errors can largely be eliminated by a single
step, namely by making the tracks with a low spatial
frequency of the optical information narrower than the
tracks with a high spatial frequency.
According to the invention it is equally
possible in the case of a record carrier with constant
track width to take a step so as to prevent non-linear
reading. The pits themselves can be made slightly
shorter. This is illustrated in Fig. 9 by dotted lines,
the uninterrupted lines represent a previously proposed
pit structure. By making the average lengths of the pits
smaller than half the average period it can be achieved
that the spacings between the positions at which the
reflected radiation intensity attains the level 1/2 Io
become equal to the desired spacing of half a period.
The same effect is obtained by tapering the
pits to a point as is shown in Fig. 10.
The invention is described with reference to
a record carrier having a reflecting pattern of pits.
However, because the invention in general yields an
improvement for a record carrier which is destined to
be read in accordance wlth the principle: radiation
spot greater than the track width, it may be employed
in many other record carriers. For example, a fully
transparent record carrier is conceivable with areas
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disposed at half a wavelength from the record carrier surface
(phase structure) or a transparent or reflecting record
carrier provided with trackwise arranged radiation-absorbing
areas (amplitude structure). The advantages of the read
principle: radiation spot greater than the track width,
however, are particularly predominant for a phase structure,
because a small phase variation results in a substantial
variation of the detector signal.
For writing inforDation on a record carrier
according to the invention use can be made of a special
write apparatus. Said apparatus is based on a previously
proposed device (described in applicants Canadian Patent
998,173 - October 5, 1976 (PHN. 6507)) and is provided with
special means according to the invention for influencing
the write beam of radiation, lndependently of the informa-
tion to be recorded, ln such a way that in the eventual
record carrier the average dimensions of the areas match
the average spatial frequencies on the record carrier.
The eventual record carrier may be the record carrier on
which information is written with the aid of the said
apparatus, called the master record carrier, or a record
carrier which via a pressing method is obtained froD the
master record carrier.
As is shown in Fig. 11, the apparatus includes
a radiation source 18, which supplies a radiation beam of
sufficient power. Said beaD is directed to the record
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carrier 1 on which information is to be written via
the prisms 19, 20 and 21 and concentrated to a small
radiation spot by an objective lens 27. The record
- carrier is provided with a layer which is sensitive
to the radiation used. The radiation path between the
source 18 and the record carrier 1 furthermore includes
a diaphragm 22 and an electro-optic radiation modulator
23. Said radiation modulator in conjunction with the
electronic control device 24 constitutes a modulation
unit. By means of said modulation unit the information,
for example a television programme, which is applied
to the terminals 30, 31 in the form of an electrical
signal, can be converted into radiation pulses of the
laser source. At specific instants, given by the
information at the terminals 30, 31 radiation spots
are projected on the record carrier.
The record carrier has a round circumference
and is rotated about its axis with the aid of a motor 15,
which by means of a carriage 16 is radially movable,
thus allowing for example a spiral track to be written
on the record carrier 1.
In order to enable the width of the tracks
to be written to be varied in accordance with the
average spatial frequency tCompare Figs. 4 and 6),
care is taken that according to the invention the
size of the radiation spot projected onto the record
carr;er is adjustable in one direction. For this purpose,
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for example diaphragm 22 may be adjustable in one
direction. However, preferably a diaphragm which is
adjustable in one direction is disposed in the imme-
diate vicinity of the objective lens 27, as is designated
by 32.
According to the invention, the areas with
tapered ends as shown in Fig. 10 can be written by
adapting the rise times of the radiation pulses to be
formed in the modulation device (23, 24). This can be
achieved, for example, by including a variable capa-
citance 33 in parallel with the capacitance of the
electro-optic modulator, so that the RC-time, which
determines the rise time, of the modulation device
can be adjusted to different ualues for different
spatial frequencies on the record carrier.
The modulation device (23, 24) also enables
a pattern of pits as shown dotted in Fig. 9 to be
obtained by electronic means. This is for example
possible, independent of the information to be written,
by inserting a variable delay between the leading and
trailing edges of the square-wave information signals
applied to the terminals 30 and 31.
