Note: Descriptions are shown in the official language in which they were submitted.
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PHN 8736C
The invention relates to an apparatus for reading
an optical radiation-reflecting information carrier, which
apparatus comprises a radiation source which produces a read
beam, an objective system for focussing the read beam to a
read spot on the information structure of the information car~
rier and for imaging the read spot on a radiation-sensitive
information detector whose output signal represents the infor-
mation, and an opto-electronic focussing error detection
system for determining a deviation between the desired and the
actual position of the plane of focussing of the objective
system, which focussing error detection system comprises two
radiation-sensitive focussing detectors which co-operate with
a narrow focussing beam, the difference in the output signals
of the focussing detectors providing an indication about said
deviation.
In this respect "focussing beam" is to be under-
stood to mean an auxiliary beam with the aid of which the
focussing errors of the read beam are detected. The "focuss-
ing detectors" are radiation-sensitive detectors which co-
operate with said auxiliary beam.
Such apparatus is described in Canadian Patent1,017,858 - Xramer et al September 20, 1977 - PHN 6295C.
This apparatus is for example used for reading an information
carrier on which a (colour) television programme is stored.
The information structure then consists of a multitude of
areas alternating with intermediate areas which are arranged
in accordance with a spiral track, which areas and inter-
mediate areas have a different influence on a read beam. The
: 2
PHN 8736C
information is then for example contained in the lengths of
the areas and those of the intermediate areas. To obtain a
sufficiently long playing time the details of the information
s~ructure will be very small for limited dimensions of the
information carrier. For example, if a 30-minute television
programme is stoxed on one side of a disc-shaped round infor-
mation carrier in an annular area with an outer radius of
approx. 15 cm and an inner radius of approx. 6 cm, the width
of the tracks will be approx. 0.5/um and the average length
of the areas and of the intermediate areas will be approxi-
mately l/um.
- In order to enable such minute details to be read
an objective system with a fairly large numerical aperture
must be employed. However, the depth of focus of such an
objective system is small. As in the read apparatus varia-
tions in the distance between the plane of the information
structure and the objective system may occur which are greater
than the depth of focus, steps must be taken to enable these
variations to be detected and to enable the focussing to be
corrected.
In the apparatus in accordance with the said
Canadian Patent a narrow beam is therefore split from the
read beam before this beam enters the objective systemO The
narrow beam passes obliquely through the objective system.
After this beam has been reflected by the information carrier,
it traverses the objective system for a second time and then
forms a radiation spot, the focussing spot, in the plane of
the two focussing detectors. The symmetry of the focussing
spot relative the focussing detectors then provides
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an indication of the degree of focussing of the read beam
on the information structure.
In the known read apparatus a number of additional
e]ements are needed, such as a semitransparent mirror, a fully
reflecting mirror for the formation of the focussing beam,
and an additional lens for focussing the auxiliary beam in
the focal plane of the objective system. The positions of
the additional elements are very critical.
It is an object of the present invention to provide
an apparatus of the type mentioned in the preamble in which
a minimal number of additional elements is needed for
focussing detection. The apparatus in accordance with the
invention is characterized in that the radiation path of the
- rèad beam on one side of the optical axis of the objective
system includes a radiation-deflecting element whose surface
area is substantially smaller than the cross-sectional area
of the read beam.
Owing to the radiation-deflecting element a small
portion of the read beam is given an other direction than the
rest of the read beam. Thls portion is focussed on the
focussing d~tectors by the objective system, the position of
the radiation spot, whlch is formed in the plane of the
focussing detectors, relative to the detectors being governed
by the degree o`f focussing of the read beam on the in~ormation~
surface of the information carrier.
Preferably, the information detector and the
focussing detectors are disposed in the same plane perpendicular
to the optical axis.
The radiation~deflecting element may be constituted
by an optical wedge or by a diffraction grating.
A first cmbodiment of an apparatus in accordance
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27.7.1"7
with the invention is characterized in that the radiation path
of the radiation beam which is directed towards the informatlon
carrier includes a radiation deflecting element, in such a
way that the radiation which is incident on the radiation
deflecting element forms an additional radiation spot on the
information structure besides the read spot, which radiation
spot i5 imaged on the focussing detectors by the objective
system.
As a radiation source a gas laser may be used, such
as a helium-neon laser. In that case the distance between
the objective system and the plane of the detectors is
comparatively great. The focussing spot is then situated at
a comparatively great distance from the read spot image.
It is alternatively possible to employ a (semi-
conductor) diode laser as radia-tion source. Such a laser
may also be used as information detector. In that case the
radiation which is reflected by the information carrier need
not be separated from the radiation which is directed towards
the information carrier. The optical read unit can then be
kept simple and compact. Furthermore, the objective system
may then have a low magnification. If in such a read
apparatus a focussing beam is formed by means of a deflecting
element, the focussing spot may be situated so closely to the
image of the read spot that the focussing detectors cannot be
arranged within the required distance to the diode laser. If
- it were possible to arrange the focussing detectors in the
desired position, a part o~ the read beam would already be
incident on the focussing detectors in the case of a slight
,
focussing error of the read beam, resulting in an error in
the focussing control signal.
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In order to avoid these problems, in accordance
with a further characteris~ic feature of the first ernbodlment
of the in~ention, in which the radiation-deflecting element
is an optical wedge, a second optical we~ge may be included
in the radiation pa-th of the sub-beam which is formed by
the first optical wedge and which is reflected by the
information carrier.
Preferably, the second optical wedge is then disposed
- within the image of the first optical wedge which image is
formed with the aid of the information carrier and the
lens element of the objective system nearest the informa~on
carrier. Thi$ means that the area of the second optical wedge
- is smaller than or equal to the area of the first optical
- wedge.
The objecti~e system may comprise a plurality of
lens elements or one lens elcment. In the last mentioned
case thel~ens element of the objective system nearest the
information carrier" is the objective system itself.
The second optical wedge, whose angle of
refraction is preferably greater than that of the first
optical wedge, deflects the focussing beam reflected by the
information carrier additi.onally relative to the read beam,
so that the distance between the focusslng spot and the read
spot increases.
In order to ensur0 that the second wedge always
remains in the image of the first wedge independently of the
position of the information carrier relative to the obJective
system, the optical wedges, in accordance with a further
characteristic feature of the lnvention, are disposed in the
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bac~ focal plane of the lens element of the objective system
nearest the information carrier.
~ second embodiment of an apparatus in æccordance
with the invention is characterized in that a radiation-de-
flecting element is disposed in the path of the read beam
- which is reflected by the information carrier and which
originates from the read spot, in such a way that the
radiation which is incident on the radiation deflecting
element is deflected to the focussing detectors.
In accordance with a further characteristic feature
of an apparatus in accordance with the invention the dividing
line between the focussing detectors makes an acute angle
with the direction in which the focussing spot moves owing
to focussing errors. By means of this step it is avoided that
the position of the focussing detector is very critical.
The radiation-deflecting elements which are used
are substantially smaller than the cross-section of- the read
beam. As a result of this the si~e of the read spot and thus
- the actual information read-out is not affected significantlyO
The slight influence of the radiation-deflecting elements on.
the read-out can further be reduced by arranging for the line
of interconnection between the optical axis of the objective
system and the radiation-deflectlng element to make an angle
of 45 with the direction in which an information track of
` the information carrier is read.
The invention will now be described in more detail~
on the basis of an apparatus which employs a dlode laser as
radiati.on source and optical wedges as radiation~deflecting
element. In this description reference is made to the
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27.7.1~77
drawing, in which:
- Fig. 1 shows a firs-l; embodiment of an apparatus in
accordance with the'invention,
Figs. 2a,and 2b show different orientations of the
focussing detectors relative to the directions of movement
of the focussing spot, and
' ~igs. 3a and 3b show how,the focussing spot moves
rela*ive to the focussing detectors when the optical wedges
are rotated relative to the optical axis, and
Fig. 4 shows a second embodiment of an apparatus in
accordance with the invention.
Fig. 1 shows a part of a round disc-shaped informaticn
carrier 1 in radial cross-section. The information structure is
for example a phase structure and comprises a multitude of
concentric or quasi-concentric tracks 2, which tracks consist
of a sequence of areas and intermediate areas. The areas ma~
for example be situated at a different level in the information
carrier than the intermediate areas. The information may for
- example be a colour television programme, but it may alterna-
tively be other information? such as a multitude of different
images or digital information. Preferably, the information
structure is situated at the back of the information carrier 1.
The information carrier is illuminated by a read
beam 3 produced by a diode laser l~. An objective system, which
consists of a single lens, or as shown in Fig. 1, of` two
lenses ~1 and L2 focusses the read beam to a read spot ~i on
the information structure. The read beam 3 is then reflected
by the information s-tructure and upon rotation of the
information carrier it is modulated in accordance with the
information which is contained in a ~rack portion to be read.
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- Af`ter reflection the read beam traverses the objecti~e system for a second time, an image ~'i being formed of' the read
spot Vi. At the location of the radiation spot ~'i a'~detector
is situated which converts the modulated read beam into an
elec~rical signal Si.
If the radiation source is a diode laser it is
possible, as is described in the published German Patent
Application No. 2,2~4,119, to use this diode laser as a
detector. Depending on the intensity of the reflected read beam '
the electrical resistance across the diode ]aser or the
intensity of the radiation emit'ted from the rear of the diode
laser will vary. When a diode laser is used as radiation
source no beam-splitting element is necessary'to separate the ,
modulated read beam reflected by the information carrier
from the unmodulated read beam which is dirccted towards
i the information carrier.
' In accordance with the invention a small optical
wedge 5 is disposed in the path of'the read beam 3. This wedge
' splits-off a sub-beam 6 (represented by dashed lines in Fig. 1)
from the,read beam. This sub-beam is focussed to a radiation
spot ~f on the information structure by the lens,L1. After ~
reflection at the information structure and a second passage
through the objective system the focussing beam forms a
radiation spot V'f(focussing spot) on an assembly of two
focussing detec*ors 7 and 8. If the distance betwcen the plane
of the tracks 2 and the objective system is correct, the
focussing spot is symmetrical relative to the focussing
detectors, so that both detectors receive an equal amo1lnt of
radiatlon and -the output signals S7 and S~ are equal. If the
plane o~ the information structure moves downwards relative
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2~.7.1'377
to the objective system, the point where the principal ray
of the reflected beam 6 enters the lens L1 will be shifted
towards the optical axis 00~. The deflection of the beam 6
by the objective system is then slightly less and the
focussing spot Vlf moves to the left. The detector 7 then
receives more radiation than the detector ~. If the plane of
the tracks 2 moves upwards, the reverse takes place, and
the detector 7 receives less radiation than the detector 8.
The signals S7 and Sg from the detectors are
applied to an electronic circuit 9. In this circuit the
signals are subtracted from each other in a manner known
per se. ~t the output of the circuit 9 a focussing control
signal rf is then obtained with which the focussing of the
objective system can be corrected, for example by moving
this system along the optical axis 00'. If the radiation source
is a diode laser, the optical read unit may also be moved along
the optioal axis.
The optical wedge, or a diffraction grating, is
disposed in the path of~the read beam which is directed
towards the information carrier, and the focussing beam which
passes through the lens L1 is narrow. Thus it is ensured
that the spot Vf is appreciably larger than the spot Vi.
The details of the information structure then can not be
distinguished with the focussing beam, so that the signals S7
and Sg will not exhibit any high-frequency variations.
For the sake of clarity the reflected focussing
beam is shown to pass through the border of the lens L1 in
~ig. 1. In reality the point where the principal ray of this
beam enters the lens L1 uill be nearer the optical axis.
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In the apparatus in accordance with the invention
the focussing beam is formed with very simpLe means, namely
with a wedge-shaped element only or a small diffraction grating
only. The wedge of the diffraction grating may for ex~mple be
mounted on a transparent plate. This plate may be fixed
relative to the lens L1 in the direction of the optica]
-axis 00'.
The angle of refraction of the wedge 5 is subject
' to an upper limit, so that thisis also the case for the
deflection of the fooussing beam by said wedge. It is
desirable that the point of the information structure to which
the focussing is adjusted is nearest the point of the
information structure where read-out is effected. The distance
between Vi and Vf is for example 100/um. In cases that the
lnformation carrier is oblique relative to the optical axis or
that variations in thicl~ness of the information carrier occur
it is then also possible to main-tain a correct focussing of
the read beam.'
In order to have a sufficient distance between the
focussing spot V'f and the read spot V'i de~lection by the
wedge 5 alone suffices if'the magnification of the objeotive
system is sufficiently high, or if the-radiation source does
not at the same time constitute the information detec-tor, so
that the radiation reflected by the record carrier can be
mirror-di~erted and the detectors can be arranged at~a suitable
distance from the information carrier.
When a diode laser is used as radlation source
~see Fi~. 1)'and an objective system which images the diode
laser o~to the information structure with a ratio of 2-1,
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the distance between the objective system and the diode laser
beilig preferably small, the distance as a result of the
deflection by the wedge 5 between the spots V~i and V~f is too
small. In that case it is possible in accordance with the
invention to employ a second optical wedge 10. This~7edge is then
disposed in the path of the reflected focussing beam. The
wedge 10 may have a greater angle of refraction than the
wedge 5, because it does not affect the distance between the
spots Vi and Vf.
Also in the case that a satisfactory distance
between the spots V'i and Vlf can be obtained with a wedge 5,
a second wedge 10 may be used. By means of the second wedge
it is then possible to prevent radiation of the read beam
from being incident on the focussing detectors when the
information structure is out of focus, resulting in the
` read spot V'i being "blown up".
The wedge 10 should then be disposed in the shadow
of the wedge 5 or, in other words, thé wedges 5 and 10 must
be imaged onto each other by the lens L1 via the information
carrier. In Fig. 1 the marginal rays of said imaging are
represented by dash-dot lines.
If the plane of the wedges were situated at an
arbitrary height between the lenses L1 and I,2, the image of the
wedge 5 would depend on the distance between the plane of the
information s-tructure and the objective system. Therefore care
is taken, in accordance with the invention, that the plane Or
the wedges ooincide with tho focal plane F of th0 lens I,1.
In order to ensure -that all -the radiation which is
deflected by the first wedge (5) passes through the second
wedge (10), the second wedge wouLd ha~e to be slightly larg~er
than the first wedge. Howe~er, a small portion of the read beam3
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itseif would then pass -through the second wedge and result
in a separate racliation spot V on the surface of the
detectors; compare the sma:Ll beam 3~ indicated by the un-
interrupted lines in Fig. 1. In the sltuation of Fig. 1, in
which the read beam is correctly focussed on the information
structure the radiation spot Vn is situated closely to the
focussing detectors. If the plane of the tracks 2 should then
be moved upwards, the radiation spot V would even fall onto
the detector 7 in the case of a small focussing error, thus
giving rise to an erroneous signal rf.
~ Therefore, the area of the wedge lO should at the mos
be equal to that of the wedge 5 and the wedge 10 is dispoeed
in the shadow of the wedge 5. As a result of this, a part of
the focussing beam, the beam 6' represented by the dashed lines,
will not be incident on the detectors 7 and 8. However,
this merely results in the signals S7 and S8 being slightly
smaller. The sensitivity of the detection system for focussing
errors is not significantly affected thereby.
Furthermore, care is taken that the distance d
between the optical axis 00~ and the point where the focussing
beam enters the lens L1 is approximately 0.7 times the
radius r of the lens pupil. For the read method shown in Fig. 1,
where the read beam traverses the information carrier twice,
the influence of spherical aberration in the objective system
on the shape of the spot Vi in the case of variations in the
thickness of the informatlon carrier is then minimal for the
focussing control method described.
In Figs. 2a and 2b the two focussing de-tectors 7
and 8 are shown with the focussing spot V~ projected thereon.
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It is assumed that in the case of a variation o r the focussing
of the read beam the focussing spot V'f moves in the x direction.
- For an optimum sensitivity to focussing errors of the detection
system the line separating -the detectors 7 and 8 should be
perpendicular to the x-direction, as is shown in Fig. 2a.
However, the derived focussing control signa-L rf would then
greatly depend on the position of the focussing detectors
in the x-direction.
In accordance with the invention the cletectors 7
and 8 are arranged so that the line of separation ~ makes
an aeute angle, for example 1~5, with the x-d:irection, as is
shown in Fig. 2b. The zero passage of the signal rf can then
be adjusted by rotating the wedge 5 or the wedges 5 and 10
about the optical axis 00'. In Figs. 3a and 3b the path des-
cribed by the focussing spot Vlf if the wedges are rotated is
represented by the curve c. In the case of ~ig. 3a, in which
the detectors have the orientation of Fig. 2b, the radiation
distribution over the focussing detectors will
ehange when the focussing spot moves over the detectors in
aceordanoe with the curve c. During assembly of the read
apparatus, after the pl~e with the wedges has been mounted
between the lenses L1 and L2 and the focussing has been
adjusted correctly, the plates can then be rotated so that
the focussing spot is symmetrical relative to the detectors
7 and 8. This is not possible if the focussing detectors have
the orientation in aecordanee with Fig. 2a~ In that ease the
radiation distribution over the focussing detectors cannot
be inf~u~nced by rotating -the wedge plate through small
angles. Compare Figure 3b.
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2~.7.l977
If the focussing detectors have tlle orientation
of Fig. 2b, moving the focussing spot Vlf in -the x-direction,
i.e. a movement as a result of the focussing errors, will
result in a smaller variation of the signals S7 and S8 then
if these detectors were oriented in accordance with ~ig. 2a.
Consequently, the sensitivity of the detection system is
reduced. However, this presents no problems. The sensitivity
also remains adequate in the case of *he arrangement of ~igo 2b.
The advantage obtained in respect of the positional tolerance
of the focussing detectors is then more importa~t than the
loss of sensitivity.
` As a focussing beam is derived from the read beam,
this beam will no longer fill the pupil of the lens L1 in an
optimum manner. As a result, the radiation spot Vi will become
slightly ]arger in the direction of the line connecting the
optical axis 00' to the centre of the deflecting element
(a wedge or a grating). The resolution of the read beam in
this dirèction is then slightly reduced. The influence of this,
in itself minor effect, may further be reduced by arranging
the line which c~nnects the optical axis and the deflection
element at an angle of approximately ~5 with the direction
of a track portion to be read.
The two radiation-deflecting elemen~ 5 and 10 in
Figure 1, which are necessary to obtain an adequate distance
between the radiation spots V'i and V'f should correct]y bc
aligned relative to each other. Moreover, the elements 5 and 10
together should correctly be aligned relative to the objective
system. This is because the element 10 must be disposed in
the shadow of the element 5.
Figure 4 shows an embodiment of an apparatus in
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27.7.19i7
accordance with the inventin in which an adequate spacing is
obtained between the focussing spot Vlf and the re-imaged read
spot with the aid of only one radlation-deflecting element
whose position is not very critical. In ~igure 4 the elements
which correspond to those of Figure 1 bear the same reference
numeral s .
In the arrangement of Figure 4 a small optical wedge
10 is disposed so that a sub-beam, or focussing beam 6 is
deflect0d from the read beam which has been reflected by -the
information carrier. The dashed lines in Figure 4 indicate
which part of the read beam passes through the wedge. The lenses
Ll and L2 ensure that the focussing beam 6 lS concentrated on
the focussing detectors to a radiation spot, or focussing spot,
V~f.
1~ Now only one radiation spot on the information
structure is used for reading the information and for
generating a focussing error signal. The area of -the information
structure on which the read beam is focussed i.s thèn always
the area which is belng read.
The wedge 10 also deflects a part from the read
beam which is directed towards the information carrier. However,
this part is focussed on the information structure to an
additional radiation spot to the right of the read spot Vi.
The lens system Ll, L2 re-i1nages the additional radiation spot
in a position to the left of the optical axis 00~, i.e. not
on the focussing detectors.
The optical elements are allgned so that lf the
distance between the plane of the information tracks 2 and
the ob?ective system L1, L2 is correc-t, the radiation which is
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27.7.1'J7~
incident on the optical -wedge is directed as inclicated by the
dashed lines in Figure 4. The optical wedge then deflects the
focussing beam 6 so that the focussing spot is symmetrical
relative to the focussing detectors. These focussing c~tectors
then receive the same amount of radiation, and the output
signals S7 and S~ of the detectors 7 and 8 are then equal.
If the plane of the information structure moves
relative to the objective system L1L2, the convergence of the
- read beam which is reflected by the information carrier
changesO As a result of this, that part of the read beam which
is used as focussing beam will be incident on the wedge 10 at
an angle which differs from that indicated in Figure 4. As a
result of this the direction of the beam ~ which passes through
the wedge 10 and thus the position of the focussing spot V'~
relative to the focussing detectors also changes. If the
plane of the information structure moves towards the objective
system, the detector 7 will receive more radiation than the
detector 8. However, if the plane of the information structure
moves away from the objective system, the detector 7 will
receive less radiation than the detector 8~
The additional steps described with reference to
Figure 1 may also be applied to the arrangement of Figure 4.
Preferably, the dlstance a between the centre of
the wedge 10 and the optical axis 00~ is approximately 0.7 times
the radius of the read beam at the location of the wedge. In
the case of a variation in the thickness of the informatior
carrier the influence of the spherical aberrations in the
objective system on the shape of the spot V~i is then again
minimal.
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27.7.1'J/7
~urthermore, the lille which sep~rates the focussing
detectors preferably makes an acute angle, of for example
45, with the direction in which the radiation spot which is
formed in the plane of the focussing detéctors moves upon a
change in the position of the plane of the information
structure.
Finally, the line which interconrlects the optical
wedge 10 and the optical axis preferably makes an angle of
approximately 45 with the direction of a track portion to
be read.
Thefact that the invention has been described on
the basis of a wedge as a radiation-deflecting element does
not mean that the invention is limited to the use of such
a wedg~e. Instead of a wedg~e it is alternatively possible to use
any other radiation-deflecting element, such as a diffraction
grating.
Steps may also be taken to deflect the focussing
beam 6 in a direction opposite to that indicated in the
Figures, so tha-t the focussing detectors can be arranged on
the same side of the optical axis 00~ as the radiation~
deflecting element 10. ~or this purpose -the wedge 10 may for
example be rotated 1~0 about its own axis.
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