Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to -the optical xead-out
of information recorded in a track carried by a moving data
carrier in the form of disc o~ tape, and relates more
particularly to an optical focussing read-out device.
The optical read-out of information recorded at high
density, poses the problem of accurate focussing o~ the light
beam on the track which latter has to be illuminated by a
spot o very small dimensions.
It is well known to make the displacement of an
element of the optical read-out device, with which element
the vertical displacement of the focussing spot is associated,
` subject to an error signal characteristic of the vertical
deviation between the track and the focussing spots. Optical
; readers of this kind make it possible to control the displace-
ment of the focussing spot in accordance with slow variations
in said error; however, their speed is too slow to compensate
for spot de~ects when the error varies rapidly.
According to the invention, there is provided an
optical read-out device adapted ~or reading data recorded
along a track carried by a moving data carrier, said optical
read-out device comprising an optical reading head having an
optical axis and comprising a first and a second optical
element which can be displaced along said optical axis, said
reading head being adapted for projecting read-out radiation
onto said track and for focussing said radiation at a focussing
spot which is displaced by displacements of said first and
second optical elements, said read out device ~urther comprising
detection means adapted for detecting read out radiation
emexging from said data carrier and for delivering an error
signal representing distance between said focussing spot
and said track, filtering means receiving said error signal
and furnishing ~ first and a second control signal having
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frequencies respectively lower and higher than a limiting
frequency, and first and second displacement devices,adapted
for respectively receiving said first and said second control
signals, for respectively controlling said displacements of
said first and second optical elements.
For a better understanding of the present invention
and to show how the same may be carried into effect, reference
will be made to the following description and the attached
drawing among which:
Fig. 1 illustrate a first embodiment of an optical
read-out device in accordance with the invention;
Figs. 2, 3 and 4 illustrate variant embodiments of
the optical reading head included in optical Lead-out device
in accordance with the invention.
In Fig. 1, which represents a catGptric optical reader
designed to read-out a track carried by a data carrier, by
transmitting a read-out light beam, the beam 1 coming from a
coherent light source is received by a mirror 2, the center of
which, is located upon the optical axis of the beam 1. The mirror
2, which is of low weight, can be displaced by means of a
piezoelectric element 3 to which it is attached, the piezo-
electric element being capable of developing vibrations under
the control of an alternating electrical signal of high
frequency which causes the mirror 2 to perform rapid movements
of low amplitude, less than some tens of mlcrons. The light
rays received by the mirror 2 are reflected towards a mlrror
4 containing a circular orifice to pass the incident beam 1.
The mirror 4, also centered on the optical axis of the beam 1,
~ reflects the light rays which it receives~ The radii of curva-
- 30 ture of the mirrors 2 and 4 are such that in the rest sta~e,
the rays reflected by the mirror 4 converge at a point 0 upon
the optical axis of the incident beam 1. The data carrier 6,
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carrying the track 7, is arranged in such a fashion that at
~he time of origin, the point 0 is located on the track 7.
The mirror 4 is attached to a mounting 5. The radiation
diffracted by the track 7 and transmitted by the data carrier
6 is picked up ~y two photoelectric detectors 8 and 9 arranged
symmetrically in relation to the optical axis of the system,
one in front of said axis in the direction of motion of the
track, the other behind said axis~
Known devices make it possible, from electrical
signals furnished by these two detectors, to obtain a focussing
error signal which is characteristic of the interval between
the focussing spot 0 of the beam and the track being read out.
Such a device is for example a phase comparator 10.
The Eocussing error signal is applied to filters 11 and
12, the first being a low-pass filter whose cut-off frequency if
fc and the other being a high-pass filter whose cut-off frequency
is equal to that of the filter 11, namely fc~ The output
signal from the filter 11 therefore represents the slow variations
in the focussing error whilst the output signal from the filter
12 represents the rapid variations therein. The first is
applied to an electrodynamic coil 13 and the second to the
piezoelectric crystal 3. The common frequency fc to the bands
of the two filters 11 and 12, is imposed by the dynamic range
of the slow device and the phase-shift in said device at the
corresponding frequency. It may be of the order of 150 Hz
for example.
The range of the fast control is deliberately restricted
to a zone which does not include the saturation zone, so that
said control reacts rapidly when the amplitude of the control
signal comes into the pull in range of the system.
Thus, for displacements of the focussing spot which
take place at ~igh frequency, this corresponding to substantial
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acceleration, only a light-weight optical element is displaced.
This device ma~es it possible also to reduce the
acoustic noise generated by the displacement of a substantial
volume within the range of the audible frequencies spectrum.
This optical device, as also the devices described
hereinafter, is designed for a pair of well-defined object
and image points. The displacements of one or more elements
of the device introduce aberrations which impair the readout
spot. This device must therefore be a high-grade device, and
limited by diffraction. The introduction of small aberrations,
the displacements always being less than some few tenths of
a millimetre, has the effect of reducing the intensity of the
radiation at the center of the diffraction spot and of increasing
the intensity of the diffraction rings. It is reasonable to
assume that the ~iffraction spot is not too severely distorted
by the aberratlons if the intensity at the spot center experi-
-ences a drop of less than 20 % of the maximum intensity.
The elements of the optical device are therefore
designed so that the focussing spot is of "good" quality within a
~olume determined by the variations in the vertical positioning
of the data carrier and by the deformation of the data carrier
tra~ks.
In practice, the output signals from the filters ll
and 12 are amplified so that the voltages respectively applied
to the electrodynamic coil and to the piezoelectric element
give rise to a resultant displacement compensating for that
measured by means of the error signal. Thus, the voltages to be
applied to the piezoelectric element are of the order of several
-; ; h~ndreds of volts for a displacement of some few microns.
Fig. 2 illustrates the optical section of a dioptric
optical reader in accordance with the invention.
Similar elements are designated by similar references
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in all the figures.
The light beam 1 is focussed at a point 0 upon a
track 7 carried by a data carrier 6, using a read-out head
comprising a convergent lens 1~, a second convergent lens 15,
a convergent meniscus 16 and a second meniscus 17.
The elements 14, 16 and 17 are secured to a frame
18 capable oE being displaced a-t low frequencies parallel to
the optical axis o-~ the system, by rneans of
an electrodynamic device such as that shown in E`ig. 1.
10~lhe convergent lens 15 is also carried by the mounting
18 but it can displaced parallel to the optical axis of the
system relatively to said mounting. To do this, an annular
piezoelectric element 19 is secured to -the lens 15 in order
to cause -the latter to follow its own motion when an electrical
signal is applied to it.
When the frame and the lens 15 displace together, the
; resultant displacement as ar as the focussing spot is concerned,
is equal to the resultant of the displacements due respectively
to the displacement of the frame and that of the lens 15. It
is well k~own that for an optical device having a transverse
magnification of G - ~ , where p is the distance of the
object from the center of the lens and p' the image interval,
the corresponding axial magniEication for a pair of object and
image points is equal to G2. Since the radiation source is fixed~
if the optical device is displaced along its optical axis by
~x, the corresponding displacement of the image point will be
equal to ~x - ~x/G2.
~ he assembly o~ the optical device described herein-
before, can be split down into two elements, the first having
a magnification G' which is high, and being constituted by the
assembly of the read-out head with the exception of the low-
weight element 15, this giving an lmage point I, and the other,
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the lens 15, having a magnific~tion G' which is less than
unity. A displacement a x of the frame 18, gives rise to a
displacement d in the image point I by the first element, sub- -
stantially equal to ~ x. The low-weight element also being
displaced by ~ x since it is attached to the frame, the
relative distant between the image point I~ this bein~ the
object as far as the lens 15 is concerned, and the center of
said lens is constant in the absence of any displacement of the
lens, and the corresponding displacement of the focussiny spot
is substantially equal to Qx since ~ x/G2 is negligible. If,
in addition, the lens 15 is simulkaneously displaced by ~x', the
corresponding displacement of the focussing spot 0 will be sub-
stantially equal to ~X2 , ~x' being negligible. The displace-
ment which is the ~esultant of the two simultaneous displacements
is therefore substantially equal to d = ~ x ~ ~ 2 G' being
considered as constant within the range of variations. ~ x can
reach some tenths of a millimetre and ~ 2 is limited to some
G'
tens of microns as indicated earlier, so that the focussing
spot retains good quality
Fi~. 3 illustrates the optical section of a catadiop-
tric optical reader in accordance with the invention.
The read-out light beam 1 is focussed at the point 0
on the track 7 carried by the data carri.er 6, by means of an
optical device comprising a splitter plate 20, a convergent
lens 21 and a~light.-weight mirror 22 which reflects the incident
radiationl the reflected radiation which passes back through
the convergent lens 21 and the splitter plate 20, being focussed
.~ : at O by an objective lens 23.
The mirror 22 is secured to a piezoelectric element .
.
24 capable of performing low-amplitude displacements along the
. optical axis of the system.
The mounting for the piezoelectric element, the lens : .
. ~ _ 7 _ :
. . .
21, the spitter pla~e 20 and the objective lens 23 are assembled
in a frame 25 capable oE displacement at low frequenciec parallel
to the optical axis of the beam reflected by the mirror 22, this,
for example, with the help of an electrodynamic system such as
that shown in Fig. 1.
'rhe displacement ~x of this frame gives rise to a
displacement ~x on the part of the focussing spot 0. A
displacement ~x' on the part of the mirror 22, which may be
a flat mirror or a spherical mirror, gives rise to a displace-
ment of 0 which is a function of the focal lengths of the lens21 and the objective lens 23. In other words, the displacement
of this mirror corresponds to the displacement of an object,
which is the spot formed upon the mirror, relatively to said
two lenses. This displacement is equal to K ~ x' where K is a
constant which is a function of the local lengths. The resultant
displacement of the focal spot 0 is a x ~ K a x~. :
In Fig~ 4, the optical section of an optical reader
in accordance with the invention has been shown, in which the
high frequency displacement of the focussing spot is pxoduced
by varying the convergence of a dioptric element of the
optical device.
The light beam 1 is focussed at 0 on the track 7
carried by a data carrier 6, using two lenses 26 and 27 and a
read-out objecti~e lens 28 capable of being displaced at low
; frequencies. The two lenses 26 and 27 are arranged in a zone
in which the read-out beam has a small diameter, and the interval
between them e, is capable of variation by means of an annular
- piezoelectric element 29 attached to the lens 27.
This piezoelectric element is excited by the high
frequency electrical signal obtained from the focussing error
signal. The low frequency displacement of the objective lens
28 produ~es a substantially identical displacement of the focal
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spot since the magnification of the objective lens 28 i~ high.
A variation ~e in the interval between the two lenses 26 and
27 is equivalent to a displacement on the part of the virt,ual
object point in the case of the read-out objective lens 28, and
gives rise to a displacement ~ x' on the par-t oE the focussing
spot. The beam incident upon the ob]ective lens 28 has a narrow
angle so that the axial magnifi~ation ( _~_ ,)2 is high, and it
is -therefore necessary that a small variation in th,e interval e
between the two lenses should give rise to a large displacement
in the virtual object, the latter being the source of the incident
beam upon the objective lens 28, so that the displacement a x~
of the focussing spot is perceptibleO
The invention is not limited to the embodiments
described and illustrated. In particular, the high Erequency
movement in order to displace the light-weight element, can be
~' achieved by means of a small high-speed electrodynamic system
replacing the piezoelectric element.
Moreover, any optical read-out head capable of being
displaced slowly and comprising an optical element which can
be displaced rapidly, or to which it is possible to add an
; optical element capable of rapid displacement, can be used to
,~ form the optical device in the controlled variable focus optical
reader in accordance with the invention.
, Finally, the optical readers illustrated have been
applied to the read-out~oE data carriers which are read by ~
transmission. ~'
This illustration is by no means limitative of the
, scope of the invention and the various embodiments of the
optical~reader in accordance with the invention can be applied
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~ ~ 30 to the read-out of data carriers which are read by reflexion. '
