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Claims  |
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What is claimed is:
1. Apparatus for reading a record carrier on which information is stored in
an optically readable track-shaped information structure, said apparatus
comprising a radiation source; a radiation-sensitive information signal
detector; objective system means for directing a radiation beam from said
radiation source to a spot on the information structure of the record
carrier and for thereafter directing said radiation from said record
carrier to said radiation-sensitive information signal detector; means for
relatively moving said record carrier and said radiation spot; and a focus
error detection system comprising two radiation-sensitive focus signal
detectors disposed in the far field of the information structure at one
side of a plane defined by the optical axis of the objective system means
and a line normal to the center line of the track portion on which said
radiation is directed, said detectors being disposed symmetrically
relative to a line effectively extending transversely to the track
direction, a subtraction circuit having inputs connected to said two focus
signal detectors, an adder circuit connected to said two focus signal
detectors, a multiplier circuit, shift circuit means for connecting
outputs of said adder circuit and said subtraction circuit to said
multiplier circuit and for relatively phase shifting the outputs of said
adder circuit and said subtraction circuit, and filter circuit means
connected to the output of said multiplier circuit for transmitting only
frequencies lower than a frequency corresponding to twice the average
spatial frequency of the information structure in the track direction
multiplied by the relative velocity of the radiation spot and the record
carrier in the track direction, a focus signal being thereby provided from
an output of said filter circuit means.
2. An apparatus as recited in claim 1, wherein each of the two focus signal
detectors is self-divided by lines extending substantially transversely to
the track direction into two sub-detectors to form two outer sub-detectors
and two inner sub-detectors, said shift circuit means comprising signal
operated swtiches responsive to said focus error signal for connecting
said two outer sub-detectors to said inputs of said outer circuit and said
subtraction circuit, and means connecting said inner sub-detectors
directly to said adder circuit and said subtraction circuit.
3. Apparatus as claimed in claim 1, wherein the dimensions of the detectors
in the effective track direction are substantially smaller than the
diameter of the effective exit pupil of the ojective system.
4. An apparatus as claimed in claim 3, wherein the detectors are disposed
at the periphery of the effective exit pupil.
5. An apparatus as claimed in claim 1, wherein the detectors have the shape
of isosceles triangles whose base sides effectively extend transversely to
the track direction.
6. An apparatus as claimed in claim 5 , wherein the height of the
triangular detectors is substantially smaller than the diameter of the
effective exit pupil.
7. An apparatus as claimed in claim 6, wherein the detectors are disposed
at the periphery of the effective exit pupil.
8. An apparatus as claimed in claim 5, wherein each of the triangular
detectors is subdivided into two sub-detectors shaped as isosceles
triangles, the outputs of the outer sub-detectors being connected via
switches which are actuated by the focus error signal, and the outputs of
the inner sub-detectors are connected directly to inputs of the adder
circuit and subtractor circuit.
9. Apparatus for reading a record carrier on which information is stored in
an optically readable track-shaped information structure, said apparatus
comprising a radiation source; a radiation-sensitive information signal
detector; objective system means for directing radiation from said
radiation source to a spot on the information structure of said record
carrier and for thereafter directing said radiation from said record
carrier to said radiation-sensitive information signal detector; means for
relatively moving said record carrier and said radiation spot; and a focus
and centering error detection system comprising four radiation-sensitive
focus and centering error detectors disposed in the far field of the
information structure at one side of a plane defined by the optical axis
of the objective system means and a line normal to the center line of the
track portion on which said radiation spot is directed, said focus and
centering error detectors being each disposed symmetrically in one of four
quadrants of an imaginary X-Y co-ordinate system whose X-axis extends in
the track direction and whose X-axis extends transversely to the X-axis,
said focus and centering error detectors thereby forming a four quadrant
detector, a first adder circuit having inputs connected to a first and
fourth quadrant of said four quadrant detector, a second adder circuit
having inputs connected to a second and third quadrant of said four
quadrant detector, a third adder circuit connected to the third and fourth
quadrants of said four quadrant detector, a fourth adder circuit connected
to said first and second quadrants of said four quadrant detector, a first
subtraction circuit having inputs connected to outputs of said first and
second adder circuits, a second subtraction circuit having inputs
connected to outputs of said third and fourth adder circuits, a fifth
adder circuit having inputs connected to outputs of said first and second
adder circuits, a first multiplier circuit providing a radial error
signal, a second multiplier circuit providing a focus error signal, and
coupling means for connecting outputs of said first subtraction circuit
and said fifth adder circuit to inputs of said first multiplier and for
connecting outputs of said second subtraction circuit and said fifth adder
circuit to said second multiplier circuit and for introducing a phase
shift between the output of said fifth adder and each of the outputs of
said first and second subtraction circuits.
10. An apparatus as claimed in claim 9, wherein in the far field of the
information structure one integrated radiation-sensitive detector with an
area at least equal to the cross-section of the undiffracted sub-beam is
disposed, which detector comprises areas which are separated from the
remaining major area of the detector, which areas constitute detectors for
the focussing detection system and centering detection system, and that a
sixth adder circuit is provided to which the sum signal from the said
areas and the signal from the major area of the detector are applied, the
read-out information signal being available at the output of this adder
circuit.
11. An apparatus as claimed in claim 10, wherein said major area of the
detector consists of two sub-areas, the boundary line being effectively
transverse to the track direction and intersecting the optical axis of the
objective system, and that a third subtractor circuit is provided to which
the output signal of the sixth adder circuit is applied together with the
signal obtained from the sub-area which is disposed at an other side of
the boundary line than the said areas, the read-out information signal
appearing at the output of the second subtractor circuit. |
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Claims |
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Description |
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The invention relates to apparatus for reading a record carrier on which
information, for example video and/or audio information, is stored in an
optically readable track-shaped information structure. The apparatus
comprises a radiation source, and an objective system for passing
radiation obtained from the radiation source to a radiation-sensitive
information detection system via the record carrier, which detection
system converts the read beam supplied by the radiation source and
modulated by the information structure into an electrical signal. The
apparatus furthermore comprises a focussing detection system which is
connected to an electronic circuit for deriving a control signal for
correcting the focussing of the objective system relative to the plane of
a track portion to be read.
A focussing detection system is to be understood to mean a
radiation-sensitive detection system and supplies an electrical signal
which provides an indication of a deviation between the plane of focussing
of the objective system and the plane of a track portion to be read.
Such an apparatus is inter alia known from the applicant's U.S. Pat. No.
3,876,841, issued Apr. 8, 1975. The record carrier described therein for
example stores a color television program. The information structure
consists of a spiral track which comprises a multitude of pits which are
pressed into the record carrier, the luminance information being contained
in the frequency of the pits, while the chrominance and audio information
is contained in a variation of the lengths of the pits. A read beam is
focussed at the information structure to a radiation spot whose dimensions
are the order of magnitude of those of the pits. By moving the record
carrier relative to the read beam, said beam is modulated in accordance
with the stored information. A radiation-sensitive information detector
converts the modulation of the read beam into an electrical signal. This
signal is processed in an electronic circuit, so that it becomes suitable
to be applied to color television receiving apparatus.
The objective system used in the read apparatus has a large numerical
aperture and a small depth of focus. Consequently, it is always necessary
to sharply focus at the information structure. Deviations between the
desired position of the plane of focussing and the actual position of this
plane, which deviations may be caused by errors in the position of the
record carrier or warping of the record carrier, or vibrations of the
elements in the read apparatus, must be detected constantly, and the
focussing must be corrected with the basis thereof.
In the apparatus in accordance with the U.S. Pat. No. 3,876,841 focussing
errors are detected with the aid of a separate focussing beam. This beam
which is derived from the read beam, traverses the objective system
obliquely and is comparatively narrow. By means of the objective system
the focussing beam which is reflected by the record carrier, is focussed
to a radiation spot in the plane of two radiation-sensitive detectors. The
degree of symmetry of the radiation spot relative to the detectors
provides an indication of the degree of focussing of the read beam on the
information structure. In addition to the optical elements required for
the actual read-out, the known apparatus requires optical auxiliary
elements for detecting focussing errors.
It is an object of the present invention to provide a read apparatus in
which focussing errors can be detected with the aid of a minimal number of
additional optical elements. The apparatus in accordance with the
invention is therefore characterized in that the focussing detection
system is constituted by two radiation sensitive detectors which are
disposed in the far field of the information structure at one side of a
plane defined by the optical axis of the objective system and a line
normal to the center line of a track portion to be read, the detectors
being disposed symmetrically relative to a line which effectively extends
transversely to the track direction. The outputs of the detectors are
connected both to a subtractor circuit and to an adder circuit, while the
outputs of the adder circuit and the subtractor circuit are connected to a
first and a second input of a multiplier circuit. One of the connections
between the adder circuits and the multiplier circuit and between the
subtractor circuit and the multiplier circuit includes a phase-shifting
circuit, while the multiplier circuit is connected to a filter circuit
which only transmits frequencies which are lower than the frequency which
corresponds to twice the average spatial frequency of the information
structure in the track direction. The control signal for focussing
correction is available at the output of the filter circuit.
The phrase "the detectors are disposed in the far field of the information
structure" is to be understood to mean that these detectors are located in
a plane where the various diffraction orders of the read beam formed by
the information structure are sufficiently separated; i.e. in a plane
which is disposed sufficiently far from the image of the information
structure formed by the objective system.
The phrase "a line effectively extends in the track direction or
effectively extends transversely dto the track direction" is to be
understood to mean that the imaginary projection of this line on the
information structure is parallel to or transverse to the track direction
respectively.
The invention is based on the recognition that during reading of the
information structure, which behaves as a two-dimensional diffraction
grating, focussing errors cause additional phase shifts between a
zero-order sub-beam and higher-order sub-beams. In the far field these
phase shifts are visible as a pattern of interference lines whose spatial
period is determined by the degree of focussing. Focussing errors can then
be detected solely with the aid of suitably disposed detectors and without
additional optical elements or an auxiliary beam. In accordance with the
invention, the sum of the detector signals is then used as a reference
signal for deriving the control signal for focussing correction.
The signal which provides an indication of focussing errors and the
reference signal are derived with the aid of the same elements. The
advantage of this is that these signals are affected in substantially the
same way by possible disturbances in the read system, such as optical
noise or vibrations of the elements. Owing to the manner in which said
signals are processed, namely via a so-called synchronous detection, the
resulting control signal for focussing correction is independent of said
disturbances. Another advantage is that the applicability of the invention
is not limited to one specific phase depth of the information structure.
Phase depth is to be understood to mean the difference in phase between
the zero-order sub-beam and the first-order sub-beams caused by the
information areas (or pits) of the information structure. The invention
may also be used for reading so-called black-white structures or amplitude
structures, whose phase depth may be assumed to be radians.
It is to be noted that it has been proposed previously, in the Applicant's
U.S. Pat. No. 4,006,293, issued Feb. 1, 1977, to detect focussing errors
with the aid of two detectors disposed in the far field of the information
structure. However, in that case the sum of the detector signals is not
used as a reference for deriving the focussing control signal. In the
previously proposed apparatus a d.c. control signal is obtained with the
aid of two detectors. For a dynamic detection of the focussing errors in
the latter apparatus the track portion to be read and the read spot should
be moved relative to each other periodically and transversely to the track
direction. For this purpose either the record carrier or the read
apparatus has to be adapted. The signals which provides an indication of
focussing errors and the reference signal are then derived in different
manners.
In the previously proposed read apparatus a first-order sub-beam which is
diffracted in the direction transverse to the track direction instead of
in the track direction is employed for detecting focussing errors.
A further advantage of the apparatus in accordance with the invention is
that the location of the detectors within the left-hand or right-hand part
of the effective exit pupil is not too critical. The detectors need not be
disposed substantially symmetrically relative to the so-called "neutral
line," as the detectors in the apparatus in accordance with the U.S. Pat.
No. 4,006,293.
The concept: The effective exit pupil covers both the actual exit pupil of
the objective system and an image of this exit pupil. Such an image can be
formed if the exit pupil itself is not readily accessible. The concept
"neutral line" will be explained hereinafter.
In accordance with a further feature of an apparatus in accordance with the
invention the dimension of the detectors in the effective track direction
is substantially smaller than the diameter of the effective exit pupil of
the objective system. This embodiment enables comparatively large
focussing errors to be detected.
An apparatus in accordance with the invention by means of which both large
and small focussing errors can be detected with high accuracy, is
characterized in that each of the detectors is subdivided into two
sub-detectors and that the outputs of the outer sub-detectors, are
connected via switches which are actuated by the derived control signal,
and the outputs of the inner sub-detectors are connected directly to
inputs of the adder circuit and the subtractor circuit.
An apparatus in accordance with the invention may also be characterized in
that the detectors have the shape of isosceles triangles whose base sides
effectivelfy extend transversely to the track direction. This enables an
unambiguous control signal to be derived for a wide range of focussing
errors.
In accordance with a further feature each of the triangular detectors may
be subdivided into two isosceles triangular detectors.
A preferred embodiment of an apparatus in accordance with the invention
with narrow detectors is characterized in that the detectors are disposed
at the periphery of the effective exit pupil. The focussing detection
system is then suitable for reading a record carrier in which the spatial
frequency of the information areas is subject to substantial variations.
The invention will now be described in more detail with reference to the
drawing, in which:
FIG. 1 shows an embodiment of an apparatus in accordance with the
invention;
FIGS. 2, 2a, 6a, 6b, 7, 8a, 8b, 9, 10a and 10b show possible forms of the
radiation-sensitive detection system used in this apparatus, and also
illustrate how the signals supplied by this system are processed,
and FIGS. 3, 4 and 5 clarify the principle of the invention.
FIG. 1 shows a round disc-shaped record carrier 1 in radial cross-section.
The information structure is assumed to be reflecting. The information
tracks are designated 3. A radiation source 6, for example a helium-neon
laser, emits a read beam b. This beam is reflected by the mirror 9 towards
an objective system 11, which is schematically represented by a single
lens. The path of the read beam b includes an auxiliary lens 7 which
ensures that the read beam fills the pupil of the objective system. Thus,
a radiation spot of minimal dimensions is projected on the plane 2 of the
information structure.
The read beam is reflected by the information structure and, when the
record carrier is rotated about a spindle 5 which extends through a
central opening 4, it is time-modulated in accordance with the information
stored in the track to be read. The modulated read beam traverses the
objective system again and is reflected by the mirror 9 in the direction
of the beam which is emitted by the source. The radiation path of the read
beam includes elements for separating the paths of the modulated and the
unmodulated read beam. These elements may for example comprise an assembly
of a polarization-sensitive dividing prism and a .lambda./4 plate. In FIG.
1 it has been assumed for the sake of simplicity that said means are
constituted by a semi-transparent mirror 8. This mirror reflects a part of
the modulated read beam to a radiation-sensitive information detector 12.
At the output of this detector a signal Si is available. The signal Si may
be decoded in known manner and subsequently, if a television program is
stored on the record carrier, it can be rendered visible and audible with
a conventional television receiving apparatus.
The optical details of the information structure are very small. For
example, the width of a track is 0.5 .mu.m, the track distance 1.2 .mu.m,
and the average spatial period of the pits 3 .mu.m for a disc-shaped round
record carrier on which a 30 -minute television program is stored within a
ring with an inner diameter of 12 cm and an outer diameter of 27 cm.
In order to enable such small details to be read an objective system with a
comparatively large numerical aperture (for example 0.45) is to be used.
Such an objective system, however, has a small depth of focus, which is
the reason why this beam should always remain sharply focussed at the
information structure.
In order to enable focussing errors to be detected two additional detectors
13 and 14 are provided in addition to the detector 12. In FIG. 2 these
detectors are shown in plan view. The origin O of the co-ordinate system
OXY is located on the optical axis of the objective system. The X-axis and
the Y-axis extend parallel to the longitudinal direction and transversely
to the longitudinal direction of a track portion to be read respectively.
The detectors 13 and 14 are for example disposed in the plane U in which an
image of the exit pupil of the objective system is formed by means of an
auxiliary lens 23. For the sake of simplicity, only the image (a') of a
point a of this exit pupil is represented in FIG. 1 by dashed lines. The
detectors 13 and 14 may also be arranged in an other plane, provided that
the sub-beams which are diffracted in different orders by the information
structure are sufficiently separated in this plane.
As is further indicated in FIG. 2, the output signals of the detectors 13
and 14 are applied to a subtractor circuit 15. The output of this circuit
is connected to a first input terminal of a multiplier circuit 18. By
means of the adder circuit 16 the output signals of the detectors 13 and
14 are added and via a phase-shifting circuit 17, which shifts the phase
of this signal by 90.degree., the resulting signal is applied to a second
input of the multiplier circuit 18. The output signal of this circuit is
applied to a low-pass filter 19. At the output of this filter, the desired
control signal S.sub.f is obtained as will be explained hereinafter.
Now the physical backgrounds of the invention will be explained. The
formation structure of the record carrier, which information structure
consists of tracks which in their turn comprise a multitude of areas and
intermediate areas, the areas for example being pits, may be regarded as a
two-dimensional diffraction grating. This grating divides the read beam b
into a zero-order sub-beam, a number of first-order sub-beams and a number
of higher-order sub-beams. A part of the radiation of the sub-beams passes
through the pupil of the objective system 11 and could be concentrated in
the image plane of the information structure. In this image plane the
individual diffraction orders are not separated. However, in the plane of
the exit pupil of the objective system, or in a plane in which an image of
this exit pupil is formed, the diffraction orders are more or less
separated. FIG. 3 shows the situation in the plane of the exit pupil.
The circle 20 with the center 23 in FIG. 3 represents the cross-section of
the zero-order sub-beam b (0, 0) in the plane of the exit pupil. The
circles 21 and 22 respectively represent the cross-sections of the
sub-beams b(+ 1,0) and b(- 1,O) which are diffracted in the longitudinal
direction of a track portion to be read. The X-axis and the Y-axis of FIG.
3 correspond to the X-axis and the Y-axis of FIG. 2. The distance d from
the centers 24 and 25 to the Y-axis is determined by .lambda./ p, p being
the local period of the pits in the track direction and .lambda. being the
wave length of the read beam b.
For deriving a focussing error use is made of the phase variations between
the first-order sub-beams which are diffracted in the track direction and
the zero-order sub-beam.
In the areas, shown hatched in FIG. 3, the first-order sub-beams b(+1,0)
and b(-1,0) partly overlap the zero-order sub-beam b(0,0) and interference
occurs. The phase difference of the sub-beams b(+1,0) and b(-1,0) relative
to the sub-beams b(0,0) varies with high frequency owing to the movement
of the read spot in the track direction, and with low frequency owing to
focussing errors. This results in intensity variations in the areas of
overlap, which variations can be detected with the detectors 13 and 14.
When the center of the read spot coincides with the center of a pit, a
specific phase difference .psi. is obtained between a first-order sub-beam
and the zero-order sub-beam. The value of .psi. depends on the shape of
the information structure, mainly on the phase depth of the pits. As the
read spot passes from a first pit to a second pit the phase of for example
the first-order sub-beam b(+1,0) relative to the zero-order sub-beam
increases continuously by 2 .pi.. Therefore, it may be assumed that as the
read spot moves in the track direction the phase of a first-order sub-beam
relative to the zero-order sub-beam varies by .omega. t. Here .omega. is a
temporal frequency which is determined th spatial frequency of the pits in
a track portion to be read and by the speed with which the read spot
passes over this track portion.
The phase difference between the beam b(0,0) and the beams b(+1,0) b(-1,0)
in the overlapping areas of FIG. 3 is determined by the nature of the
information structure and also by the degree of focussing of the read beam
at the plane of the information structure. This will be explained with
reference to FIG. 4.
In this Figure a portion of a track 3 is shown in longitudinal section. By
way of example it is assumed that the read beam is focussed in a plane
which is located at a distance .DELTA.z from the plane of the track. Owing
to this focusing error an additional pathlength difference is obtained
between the beam b(0,0) and the beams b(+1,0) and b(-1,0). Of these beams
only the chief rays are shown. For the direction at an arbitrary angle
.alpha. with the chief ray of the beam b (0,0), the pathlength difference
between the beam b (0,0) and the beam b(+1,0) is given by:
.DELTA. .omega. = .DELTA.z. cos .alpha. - .DELTA.z.cos(.beta. - .alpha.)
For a small angle .alpha. and for a small angular difference (.beta. -
.alpha.) the pathlength difference in good approximation, i.e. with an
accuracy up to the third order, equals:
##EQU1##
The phase shift caused by the defocussing in a direction at an angle
.alpha. with the optical axis of the objective system is then:
##EQU2##
The phase shifts .phi..DELTA.z 2 is a function of the angle 60 for a
specific value of the focussing error .DELTA.z. For each position in the
exit pupil the phase difference .phi..DELTA.z is determined by the
distance from this position to the Y-axis. For the positions disposed on
the two lines whose angular distance to the Y-axis is .beta./2, the phase
difference between a first-order sub-beam and the zero-order sub-beam is
.psi.(.phi..DELTA.z = 0) and is independent of a focussing error. These
two lines may be denoted as "netural lines" . In FIG. 3 one of these lines
is designated 1.sub.n.
FIG. 5 shows the total phase difference .phi. between the sub-beam b(+0,0)
and the sub beam b(+1,0) as a function of the position, in the angular
dimension .alpha., in the exit pupil for a specific focussing error
.DELTA.z. The position of the line which is parallel to the Y-axis and
which extends midway between the detectors 13 and 14 is denoted by
.alpha..sub.o. The centers of the detectors 13 and 14 are then located at
the positions .alpha..sub.o - .DELTA..alpha. and .alpha..sub.o +
.DELTA..alpha.. If the phase difference .phi..DELTA.z for the position
.DELTA..sub.o is represented by .phi..sub.o, the phase difference for the
position .alpha..sub.o - .DELTA..alpha. will be
(.phi..DELTA.z) .alpha..sup.. - .DELTA..alpha.= .phi..sub.o - .DELTA..phi.
and for the position
(.phi..DELTA.z) .alpha..sub.o + .DELTA..alpha. = .phi..sub.o + .DELTA..phi.
where .DELTA..phi. is given by .DELTA..phi. = 2.pi. (.DELTA.z/.lambda.)
.beta..DELTA..alpha.. Over the overlapping areas of FIG. 3 patterns of
interference lines extend. The spatial period of a pattern of interference
lines is determined by the magnitude of a focussing error, i.e. for a
large .DELTA.z the spatial period is small. Owing to the rapid scanning of
the pits in a track portion to be read by the read spot a pattern of
interference lines moves with high frequency. The sign of the displacement
of the pattern of interference lines is then determined by the sign of the
focussing error .DELTA.z.
The phase differences between the sub-beams which interfere at the location
of the detectors 13 and 14 are given by:
.phi.'.sub.13 = .psi. + .omega.t + .omega..sub.o - .DELTA..phi.
.phi.'.sub.14 = .psi. + .omega.t + .phi..sub.o + .DELTA..phi.
The time-dependent output signals of the detectors 13 and 14 may be
represented by:
S.sub.13 = A cos (.psi. + .omega.t + .phi..sub.o - .DELTA..phi.)
S.sub.14 = A cos (.psi. +.omega.t + .phi..sub.o + .DELTA..phi.)
The output signal of the subtractor circuit 15 (see FIG. 2) is then:
S.sub.15 = B sin (.psi. + .omega.t + .phi..sub.o).sup.. sin .DELTA..phi..
As is shown in FIG. 2 the output signals of the detectors 13 and 14 are
also added to each other in the circuit 16. In the signals S.sub.13 and
S.sub.14 the terms .omega.t have the same sign, while the sign of the
terms .DELTA..phi. in the signals S.sub.13 is opposite to that of this
term in the signal S.sub.14. As a result the variation in the sum of the
signals S.sub.13 and S.sub.14 owing to focussing errors will be
substantially smaller than this variation in the signal S.sub.15. The sum
signal may be represented by:
S.sub.16 = C cos (.psi. + .omega.t + .phi..sub.o) [1 + m (cos .DELTA..rho.)
]
Here m, for focussing errors which are not too large, is a constant smaller
than 1, so that if .DELTA.z is not too large the sign of S.sub.16 cannot
change. The signal S.sub.16 is applied to a phase-shifting circuit 17
which shifts the phase through 90.degree., yielding:
S.sub.17 = D sin (.psi. + .omega.t + .phi..sub.o) [1 + m cos (.DELTA..phi.)
]
In the multiplier circuit 18 the signals S.sub.15 and S.sub.17 are
multiplied by each other, yielding:
S.sub.18 = E sin.sup.2 (.psi. + .omega.t + .phi..sub.o) sin (.DELTA..phi.)
[1 + m cos (.DELTA..phi.) ]
This may be written as:
S.sub.18 = E [1 + m cos (.DELTA..phi.) ] .sup.. sin (.DELTA..phi.) [1/2 =
1/2 cos 2 (.psi. + .omega.t + .phi..sub.o) ]
After passing through the filter circuit, which transmits only frequencies
lower than 2 .omega., this yields a signal
S.sub.f = K (.DELTA..phi.) sin (.DELTA..phi.)
where
K(.DELTA..phi.) = 1/2E [1 + m cos (.DELTA..phi.) ]
and remains positive for focussing errors which are not too large.
Consequently, the signal S.sub.f is an odd function of .DELTA..phi. and
consequently also an odd function of the focussing error .DELTA.z, so that
with the described detector arrangement and with the described signal
processing the magnitude and the direction of the focussing error can be
detected. The signal S.sub.f may be used for correcting the focussing, in
a manner known per se, for example by moving the objective system in an
axial direction.
In FIG. 2 the reference numeral 17 denotes a phase-shifting circuit. This
circuit may be a differentiating network. However, preferably the
phase-shifting circuit takes the form of a so-called phase-locked loop.
FIG. 2aillustrates the principle of such a loop. An oscillator 26 supplies
a cosine function at its output 27 and a sine function at its output 28.
The output 27 is connected to a first input of a frequency comparator 29
in which the frequency of the oscillator 26 is compared with the frequency
of the signal cos (.omega. t), whose phase is to be shifted through
90.degree.The output signal of the frequency comparator is fed back to the
oscillator, so that the frequency of this oscillator becomes equal to that
of the signal cos (.omega. t). A sine function with the desired frequency
.omega. is then obtained at the output 28 of the oscillator.
Besides being diffracted in the longitudinal direction of a track portion
to be read, the radiation of the read beam is also diffracted in
directions transverse to this longitudinal direction and also in diagonal
directions. Thus, sub-beams of the orders (0, +1) and (0, -1) are also
obtained owing to the grating structure transverse to the track direction,
and sub-beams of the order: (+1, +1), (-1, +1), (-1, -1), and (+1, -1). In
FIG. 3, the directions of the sub-beams are indicated by arrows. As the
detectors 13 and 14 are disposed at either side of the X-axis, their
output signals will not be influenced by the sub-beams b(0, +1) and b(0,
-1). The directions of the lines of the interference patterns, which are
caused by the sub-beams diffracted in diagonal directions are oblique
relative to the detectors. The influence of the last-mentioned
interference patterns on the signals S.sub.13 and S.sub.14 will therefore
be averaged out.
The information structure also diffracts radiation of the read beam in
orders high than the first orders. However, the radiation energy in the
higher diffraction orders is comparatively low and the higher-order
diffraction angles are such that only a small part of the higher order
sub-beams falls within the pupil of the objective system. Therefore, the
influence of the higher-order sub-beams is negligible.
As previously stated, the spatial period of the pattern of interference
lines is determined by the focussing error .DELTA. z. The greater this
error, the smaller said spatial period will be. It has been assumed
hereinbefore that B and C in the expression of S.sub.15, S.sub.16 are
constants. However, in reality B and C vary in accordance with (sin x/x)
for rectangular detectors, x being given by .pi. (1/q), in which 1 is the
width of the rectangular detector and q the spatial period of the pattern
of interference lines. If the focussing error .DELTA. z becomes so large
that the period q of the interference pattern becomes equal to the width
1, the sign of B is reversed. The phase of the derived control signal then
changes 180.degree. and there is a risk that the focussing servo-control
will act in the wrong sense.
For deriving the signal S.sub.16 the output signals of the detectors 13 and
14 are added, so that a detector is used which is twice as wide as that
used in deriving the signal S.sub.15. Said sign reversal will consequently
occur for the first time for the signal S.sub.16 .
It is therefore proposed to make the detectors as narrow as possible. In
that case it is also possible to obtain a correct focussing control signal
for larger focussing errors, which may occur as the objective system
initially moves towards the record carrier, or in the event of a shock
against the read apparatus.
The use of narrow detectors has another advantage, namely that the two
detectors can be arranged close at the periphery of the effective exit
pupil. This is of importance if record carriers in which high spatial
frequencies of the information areas occur in the information structure
are to be read in a satisfactory manner.
The degree in which the sub-beams b(+1,0) and b(-1,0) and the sub-beam
b(0,0) overlap each other is determined by the spatial frequency of the
information areas in the track direction. In FIG. 3 the centers 24 and 25
of the circles 21 and 22 are nearly located on the edge of the circle 20
which represents the effective exit pupil.
Consequently, this Figure represents the situation in which the spatial
frequency in the track which is read is approximately equal to half the
cut-off frequency. When the spatial frequency increases, the first-order
sub-beams b(+1,0) and b(-1,0) will be diffracted through a larger angle
.beta.. At a specific spatial frequency of the information areas, which
correspond to the cut-of frequency of the optical read system, there will
no longer be any overlap of the first-order sub-beams with the zero-order
sub-beam. The information can be no longer be detected.
As for detecting focussing errors two detectors are used which are disposed
at one side of the Y-axis, the cut-off frequency for the focussing error
detection will be smaller than the cut-off frequency for the actual
information read-out. The cut-off frequency for the focussing detection is
already attained if the detector 13 is disposed partly outside the
overlapping area of | | |
