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BACKGROUND OF THE INVENTION
This invention relates to optical read-out apparatus and, more
particularly, to such apparatus which is capable of reading optically
detectable indicia by transmitting and focussing a light beam on the
surface of an optically encoded medium and by sensing modulations which
are imparted to the light beam by the optically encoded information.
Optical read-out apparatus is used to recover information which has been
recorded in an optically detectable format on a record carrier. This
information may be represented by analog or digital data which is recorded
as optically encoded signals. As examples, this information may be video
signal information, audio signal information, binary or digital signal
information, and the like. The record carrier may be a rotatable disc, a
film strip, a movable card or other record carrier capable of having
encoded information recorded thereon. The encoded information is in the
form of optically detectable indicia, such as markings which diffract,
distort, modulate or otherwise modify some parameter of a light beam which
is incident thereon. As a typical example, video signal information may be
recorded on a video disc as pits which are provided in substantially
circular, concentric tracks, or in a single spiral track, whereby the pits
modulate the intensity of a light beam which is transmitted to the record
disc.
In optical read-out apparatus for a video disc, a source of light, such as
a laser, emits coherent light which is directed and focussed by an optical
head to a spot on the surface of the disc. The head generally includes
various lenses, mirrors and/or prisms to properly control and shape the
beam emitted by the laser. If the video information which is recorded on
the disc is in the form of pits, the focussed beam is modulated by such
pits and then is reflected to photo-detecting devices which convert the
intensity modulations of the reflected beam into corresponding electrical
signals. These electrical signals then are demodulated so as to recover
the video information which had been recorded on the disc. So-called
tracking and time-base errors can be sensed by detecting the reflected
beam in order to drive suitable servo control circuits for correcting such
errors. In some systems, a separate light beam additionally is directed to
the video disc, and reflections of this separate beam are used to
determine whether the optical head is in a proper focussing condition.
Additional servo control circuitry is provided to maintain a correct
focussing condition.
Although the optical head which is comprised of one or more optical lenses
and mirrors functions in a generally satisfactory manner, the individual
optical elements which constitute this head usually are precise optical
instruments and are very expensive to manufacture. In addition, the
overall structure of the optical head is relatively complex and expensive
to assemble. In some applications, the optical head includes an objective
lens which serves the dual function of focussing the laser-emitted light
beam onto the surface of the video disc and also collecting the light
which is reflected from the disc to transmit the reflected light to the
photo-detecting devices. The aperture of this objective lens is small to
enhance its focussing of the laser-emitted beam and, consequently, this
aperture limits the amount of reflected light which can pass through the
lens to the photo-detecting devices. As a result, the sensitivity of the
read-out apparatus may be limited.
OBJECTS OF THE INVENTION
Therefore, it is an object of the present invention to provide improved
optical read-out apparatus which overcomes the aforenoted defects of the
prior art.
Another object of this invention is to provide optical read-out apparatus
having an improved optical head which does not require highly precise
optical mirrors and lenses as heretofore used.
A further object of this invention is to provide apparatus for reading
optically detectable indicia from a record carrier having improved
sensitivity.
An additional object of this invention is to provide apparatus for reading
optical markings on a record carrier wherein a focussing lens system used
by the prior art is replaced by a hologram.
Yet another object of this invention is to provide improved apparatus
having one or more holograms for use in an optical head to read optically
detectable indicia from a record carrier.
Various other objects, advantages and features of this invention will
become readily apparent from the ensuing detailed description, and the
novel features will be particularly pointed out in the appended claims.
SUMMARY OF THE INVENTION
In accordance with the present invention, apparatus is provided for reading
optically detectable indicia which are recorded on a record carrier. A
source of coherent light emits a light beam which is transmitted to the
record carrier whereat the beam is modulated by the recorded indicia. A
hologram receives the transmitted beam and focusses same on the record
carrier. One or more photo-detectors are disposed to receive the modulated
beam so as to detect the intensity of the coherent light which is
transmitted from the record carrier. In a preferred embodiment, the record
carrier is a disc which reflects the modulated beam to the photo-detector.
The hologram, as used in this invention, may be formed by recording the
interference pattern which is established between a reference beam and a
subject beam, the latter diverging from a convergence point to interfere
with the reference beam.
BRIEF DESCRIPTION OF THE DRAWINGS
The following detailed description, given by way of example, will best be
understood in conjunction with the accompanying drawings in which:
FIG. 1 is a schematic diagram of prior art optical readout apparatus;
FIG. 2A is a schematic diagram of an improved optical head for use with
optical read-out apparatus in accordance with the present invention;
FIG. 2B is a plan view of one embodiment of the hologram which can be used
with the optical head shown in FIG. 2A;
FIG. 3 is a schematic illustration of the manner in which the hologram can
be formed;
FIG. 4 is a schematic illustration of the optical effects achieved by the
hologram;
FIGS. 5A and 5B are schematic diagrams of still further embodiments of the
present invention;
FIGS. 5C and 5D are schematic diagrams of various embodiments of the
optical head which can be used with this invention;
FIG. 6A is a schematic diagram of a still further embodiment of the present
invention;
FIG. 6B is a plan view of a portion of the optical head which can be used
with the apparatus shown in FIG. 6A;
FIG. 7 is a graphical representation of the manner in which the embodiment
shown in FIG. 6A can be used;
FIG. 8 is a schematic diagram of tracking control circuitry which can be
used with the embodiment shown in FIG. 6A;
FIGS. 9A-9C depict the manner in which the embodiment shown in FIG. 6A
functions to correct tracking errors;
FIG. 10 is a graphical representation of the distribution of light which is
reflected to the optical head shown in FIG. 6A;
FIG. 11 is a perspective sketch showing a still further embodiment of the
present invention;
FIG. 12 is a schematic diagram of an additional embodiment of the present
invention;
FIG. 13 is a schematic diagram of yet a further embodiment of the present
invention;
FIG. 14 is a schematic diagram of another embodiment of this invention;
FIG. 15A is a schematic diagram of an additional embodiment of this
invention;
FIG. 15B is a plan view of the optical head used in FIG. 15A; and
FIG. 16 is a perspective diagram of yet another embodiment of this
invention.
DETAILED DESCRIPTION OF CERTAIN OF THE PREFERRED EMBODIMENTS
The present invention is applicable to optical read-out apparatus in
general. In such apparatus, information is recorded as optically
detectable indicia on a record carrier. This indicia represents analog or
digitally recorded information signals. Various uses of such optically
recorded indicia are known to those of ordinary skill in the art, such as
the storage of data. For the purpose of the present description, this
stored data represents video signal information; although the following
description is not intended to be limited solely to a video information
storage and read-out device. Furthermore, the record carrier upon which
the optically detectable indicia are recorded may be either of the
selectively transmissive type or selectively reflective type. In the
former, the indicia modulates a light beam which is transmitted through
the record carrier, and a suitable photo-detector is disposed on the
opposite side of the carrier so as to receive the modulated, transmitted
beam after is passes through the carrier. When a reflective carrier is
used, both the source of light and the photo-detector are disposed on the
same side of the carrier. Hence, light which is incident on the carrier is
modulated by the indicia and this modulated light is reflected from the
carrier to the photo-detector. For the purpose of the present description,
it will be assumed that the record carrier upon which the optically
detectable indicia are recorded is of the reflective type. The indicia
itself may take any one of well-known forms. However, for the purpose of
the present description, it will be assumed that the video information
recorded on the record carrier is in the form of pits which are disposed
in at least one track. The record carrier itself may be a rotatable disc,
a movable card or a movable film strip. For the purpose of the present
description, it will be assumed that the record carrier is a rotatable
disc having substantially concentric record tracks on its surface, the
pits being recorded in such tracks. In a preferred embodiment, the record
tracks are constituted by a spiral track. Accordingly, the following
description will refer to a video disc formed of one or more tracks of
pits representing video information, which pits selectively modulate the
intensity of light incident thereon, and this modulated light is reflected
from the surface of the video disc. Nevertheless, it should be readily
understood that other types of record carriers can be used, the optically
detectable indicia need not be limited solely to pits, and the information
represented by such indicia need not be limited solely to video
information.
The advantages achieved by the present invention will best be understood by
first considering a typical prior art optical read-out device which has
been used to read out and recover video information from a video disc.
Turning now to FIG. 1, a video disc 3 has its surface 3a provided with
optically detectable indicia represented as pits shown by indentations in
the surface. Surface 3a is provided with a reflective layer, such as by
vaporization of aluminum, so that the geometric pattern of pits
representing the video information serves to selectively reflect light
which impinges on surface 3a. A turntable 2 secured to a rotary shaft 1
supports disc 3 and serves to rotate the disc so as to permit the optical
scanning of the tracks within which the pits are recorded.
Optical scanning of the tracks on disc 3 is achieved by projecting a laser
beam and focussing same to be incident on the tracks. To this effect, a
laser device 4 emits a beam of coherent light which is polarized in a
predetermined direction. The light beam emitted by laser device 4 passes
through a lens 5 and is transmitted over a folded optical path established
by a reflecting surface 6, such as a mirror, to pass through a polarizing
prism, a quarter-wavelength plate 8 and an objective lens 9 to disc 3.
Lens 5 functions to focus the beam emitted by laser device 4 to a point of
convergence. This converging point is disposed between mirror 6 and
polarizing prism 7. Polarizing prism 7 is a conventional device adapted to
transmit light therethrough which is polarized in a predetermined
direction but to reflect light which is polarized in a direction
perpendicular to that predetermined direction. As shown in FIG. 1, the
beam reflected from mirror 6 passes through the aforementioned converging
point and then diverges therefrom toward polarizing prism 7. The
polarization of the beam transmitted to the polarizing prism from mirror 6
is the predetermined direction such that this beam passes through prism 7
and thence through quarter-wavelength plate 8. A quarter-wavelength plate
is a conventional optical component which is adapted to rotate the
direction of polarization of a beam passing therethrough. If a given beam
passes through this quarter-wavelength plate in a first direction and then
is reflected to pass through the very same quarter-wavelength plate in an
opposite direction, the resultant beam which passes through this plate the
second time has its direction of polarization rotated by 90.degree. with
respect to the original beam.
After passing through quarter-wavelength plate 8, the beam emitted from
laser device 4 is focussed by objective lens 9 to a spot of predetermined
diameter on surface 3a of disc 3. As one example, the beam is focussed to
a spot whose diameter is approximately 1 .mu.m (micrometer). The focussed
spot is modulated by the pits provided in the track being scanned while
disc 3 rotates about shaft 1. Consequently, the intensity of the focussed
beam is modulated in accordance with such pits. The modulated beam is
reflected from surface 3a of disc 3 back through objective lens 9 and
quarter-wavelength plate 8. Thus, by passing through the
quarter-wavelength plate in opposite directions, that is, in the
transmitting and reflecting directions, the direction of polarization of
the reflected beam is rotated by 90.degree. relative to the direction of
polarization of the transmitted beam. Hence, the modulated beam which is
reflected to polarizing prism 7 has its direction of polarization
perpendicular to the predetermined direction associated with this prism
and, therefore, is reflected thereby. As shown, this modulated, reflected
beam is received by a photo-detector 10, preferably having a predetermined
stop, or aperture, to produce an electrical signal whose amplitude varies
as a function of the intensity modulations of the beam received by the
photo-detector. This electrical signal is amplified by an amplifier 11 and
supplied to further apparatus (not shown) for demodulation. This further
apparatus recovers the video signal information which had been recorded in
the form of pits on surface 3a of disc 3.
As may be appreciated, lens 5, mirror 6, prism 7, quarter-wavelength plate
8 and objective lens 9 comprise an optical head of relatively complex
construction and exhibiting substantial mass or weight. If tracking,
time-base or focussing errors are present, portions or all of the optical
head must be physically moved in order to compensate for such errors.
Because of the mass of this optical head, it may be recognized that a
significant time delay may be present before such errors can be corrected.
Furthermore, the moving or adjustment mechanisms are expensive. Also,
because of the usual diaphragm associated with objective lens 9, the
reflected light which returns through this objective lens and is
transmitted to photo-detector 10 will have its intensity limited by the
aperture of that diaphragm. This means that the intensity of the light
received by photodetector 10 may be less than desired, thus decreasing the
sensitivity of the optical read-out apparatus. Furthermore, and as
mentioned above, the overall cost of manufacture and assembly of the
illustrated prior art optical head is high.
Turning now to FIG. 2A, one embodiment of improved optical read-out
apparatus is shown wherein the problems associated with the prior art
optical head shown in FIG. 1 are avoided. In particular, the optical head
of FIG. 1 is replaced by a support structure 12, hereinafter referred to
as a holder, a hologram 13 supported on holder 12 and a photo-detector 14
also supported on holder 12. Holder 12 may be a plate that is
substantially transparent throughout or, alternatively, is transparent in
the vicinity juxtaposed with hologram 13 and photo-detector 14. As an
example, holder 12 may be formed of glass, and is relatively thin so as to
exhibit small mass as compared to the mass of the optical head shown in
FIG. 1.
Hologram 13, described in greater detail below, is adapted to receive a
coherent beam of light, such as a laser beam emitted from a laser device
(not shown) and functions to focus this received beam to a spot on the
surface of the record carrier. As shown in FIG. 2A, the record carrier is
a disc 3', similar to disc 3 of FIG. 1, and the reference numerals used to
identify the video disc and rotary support structure in FIG. 2A are the
same as the reference numerals used in FIG. 1 with the addition of a
prime. Thus, the beam focussed on the surface 3a' of disc 3' by hologram
13 is reflected from the disc to impinge upon photo-detector 14.
In the embodiment shown, hologram 13 is ring-shaped, such as illustrated in
FIG. 2B, and photo-detector 14 is juxtaposed to the center portion of this
ring. One of ordinary skill in the art will appreciate that a hologram is
the recording of an interference pattern established between a reference
wave and a subject wave. In general, a common source of coherent light is
used to transmit the reference wave and to irradiate a subject so as to
transmit from that subject the subject wave. The reference and subject
waves intersect at a location to form an interference pattern or fringe.
If a photo-recording medium is disposed in the plane of intersection, a
latent image of the interference pattern is produced thereon and may be
subsequently developed. When the developed image of the interference
pattern subsequently is irradiated with a beam of coherent light, the
incident beam is diffracted, generally to form a zero order diffraction
beam and higher order diffraction beams. One of these higher order
diffraction beams forms a virtual image of the original subject and
another of the higher order diffraction beams forms a real image of the
original subject. This real image is formed by a converging beam, while
the virtual image appears because of a diverging beam whose convergence
location defines the virtual image.
The recorded hologram generally is formed on a relatively thin medium, such
as a film, and as is conventional, the hologram may be a so-called plane
hologram or volume hologram, as desired.
When hologram 13 is used in the embodiment shown in FIG. 2A, the coherent
light beam transmitted thereto may be a plane wave. Hologram 13 converts
this plane wave beam to a spherical wave beam which is focussed to a spot
on pits P.sub.1 which are recorded on the surface 3a' of disc 3'. This
spherical wave, which is converging, is modulated by pits P.sub.1 and is
reflected through the central opening of hologram 13 to impinge upon
photodetector 14.
A technique for forming hologram 13 is schematically illustrated in FIG. 3.
A laser device 15 emits a beam of coherent light which passes through
half-mirror 16 to impinge upon a photo-sensitive plate 17. Although
referred to as a plate, the photo-sensitive medium may be a film, a
thermoplastic layer, a silver haloid coating or the like.
A portion of the laser beam emitted by laser device 15 is reflected by
half-mirror 16 and further reflected by a mirror 18 to a lens 19. Thus,
half-mirror 16 serves as a beam-splitter to divide the amplitude of the
emitted beam as shown. Lens 19 serves to focus the beam reflected by
mirrors 16 and 18 to a location P. The reflected beam b.sub.2 then
diverges from location P to impinge upon photo-sensitive plate 17 whereat
it interferes with the beam which is transmitted through mirror 16. The
beam transmitted through mirror 16 may be considered as the reference beam
and beam b.sub.2 may be considered as the subject beam. Thus, the
reference and subject beams form an interference pattern at
photo-sensitive plate 17, and it is this interference pattern which is
recorded. If lens 19 forms a focussed spot at location P the subject image
represented by subject beam b.sub.2 may be considered to be the focussed
spot. Thus, the hologram recorded on plate 17 is the hologram of a
focussed spot, or convergence point, P.
Referring to FIG. 4, the manner in which the hologram formed by the
technique shown in FIG. 3 affects particular light beams is shown. Let it
be assumed that the interference pattern formed on photo-sensitive plate
17 is developed so as to form hologram 17'. Now, if the apparatus shown in
FIG. 3 is modified to the extent that half-mirror 16 is replaced by a
"full" mirror 20, but the optical path length between laser device 15 and
hologram 17' remains the same, then the beam emitted by laser device 15 is
focussed to convergence point P. The beam then diverges from point P and
is directed as beam b.sub.1 to hologram 17'. Beam b.sub.1 which is an
off-axis beam as shown, is a spherical wave which is converted by the
hologram to a plane wave. This plane wave is transmitted from hologram 17'
as beam b.sub.2. Thus, one function of hologram 17' is to convert a
diverging spherical wave, represented as beam b.sub.1, to a plane wave
represented as beam b.sub.2. The propagation of beam b.sub.2 is similar to
that of the beam emitted by laser device 15.
Now, let it be assumed that beam b.sub.2, which is similar to the beam
emitted by laser device 15, is transmitted in the opposite direction, as
shown by beam b.sub.2 '. Beam b.sub.2 ' is a plane wave and is incident on
hologram 17'. This hologram now functions to convert the plane wave to a
spherical wave, represented as beam b.sub.1 ', this spherical wave
converging to convergence point P. Convergence point P of converging wave
b.sub.1 ' is coincident with convergence point P of diverging beam
b.sub.1. If hologram 17' is used as hologram 13 in FIG. 2A, it is
appreciated that the beam transmitted thereto by the laser device (not
shown in FIG. 2A) corresponds to beam b.sub.2 ' and is converged by the
hologram to point P on the surface 3a' of disc 3'. This focussed spot is
modulated by pits P.sub.1 and is reflected by disc 3' to photodetector 14.
As may be appreciated, the optical head formed of holder 12 and hologram 13
is quite simplified when compared to the prior art optical head
illustrated in FIG. 1. Furthermore, this simplified head exhibits
relatively low mass and can be easily adjusted by a simple mechanism to
correct for various errors. Also, because of the low mass of this head,
the response time for error correction is reduced. Still further, since
the improved head avoids the necessity of an objective lens, the usual
stop element associated with that lens is not needed. As a result, the
intensity of the reflected beam impinging upon photo-detector 14 is not
unnecessarily limited, and the sensitivity of the improved optical
read-out apparatus is increased. The amount of light flux received by the
photo-detector can, therefore, be determined merely by the aperture of the
photo-detector.
Since hologram 13 converts the incident plane wave laser beam to a
converging spherical wave beam which is focussed on surface 3a' of disc 3'
to be modulated by pits P.sub.1 and reflected back along the axis of the
ring-shaped hologram, the embodiment of FIG. 2A can be modified as shown
in FIGS. 5A and 5B. Thus, photo-detector 14 need not be supported on
holder 12 but, rather, may be located at a position spaced from the
holder. In FIG. 5A, the reflected beam may pass through the opening 15a of
a light shield 15b to be focussed by a lens 16 on the photo-detector.
Although light shield 15b is used, this does not reduce the intensity of
light reflected to the photo-detector by an undesirable amount. In FIG.
5B, a reflecting prism 17a reflects the returning, modulated beam to a
lens 18a which focusses the modulated beam upon photo-detector 14. In the
embodiments of FIGS. 5A and 5B, the amount of light flux received by
photodetector 14 may be selected as a function of light shield 15b and/or
the diameter of the central opening of hologram 13. Hence, reflected light
flux and photo-detector response can be matched relatively easily.
If photo-detector 14 is supported by holder 12, as in the FIG. 2A
embodiment, the amount of light flux received by the photo-detector can be
selected, as desired, by providing suitable light blocking members. In
FIG. 5C, the light blocking member is a stop member 28 having a selected
aperture. This may be similar to light shield 15b shown in FIG. 5A. In
FIG. 5D, the light blocking member is a mask 29 which may be considered to
be the inverse of stop 28. Another function of the illustrated light
blocking members is to selectively block some orders of the diffraction
beams while passing others to the light detector. Hence, the zero order
diffraction beam, also known as the simple component of reflected light,
or selected ones of the higher order diffraction beams can be transmitted
to the photo-detector.
Another embodiment of an improved optical head in accordance with the
present invention is shown in FIGS. 6A and 6B. In FIG. 6A, a holder 21
supports a hologram 22 and individual photo-detectors 23 and 24. Hologram
22 functions in the manner described hereinabove to convert the plane wave
laser beam to a converging spherical wave beam for focussing on surface
3a' of disc 3'. Photo-detectors 23 and 24 are supported upon holder 21 and
are particularly disposed so as to intercept the higher order diffraction
beams which are reflected from surface 3a'. FIG. 6B represents the
relative locations on holder 21 whereat hologram 22 and photo-detectors 23
and 24 are positioned. If desired, holder 21 may be opaque except for
those portions juxtaposed with the hologram and photo-detectors, which
portions are substantially transparent.
The embodiment of FIG. 6A is readily adapted to detect tracking errors
wherein the spot focussed on the surface 3a' of disc 3' deviates from the
track being scanned. Let it be assumed that a track is represented by the
cross-sectional depression, or groove, and that the focussed spot may
deviate in a radial direction from the scanned track. Since different
optical paths are traversed by the higher order diffraction beams
reflected from surface 3a' to photo-detectors 23 and 24, the
aforementioned tracking errors will produce corresponding changes in the
intensities of these respective reflected beams. For example, if the
focussed spot incident on surface 3a' deviates from its predetermined
incidence location relative to the scanned track, the electrical signal
produced by one of the photo-detectors will increase while the electrical
signal produced by the other photo-detector will decrease. This is
graphically depicted in FIG. 7 wherein the ordinate represents the output
signal level and the abscissa represents incident beam deviation. The
solid curve of FIG. 7 corresponds to, for example, the signal produced by
photo-detector 23 and the broken curve corresponds to the signal produced
by photo-detector 24. When the focussed beam is displaced in the positive
direction, that is, from left to right, the signal produced by
photo-detector 23 increases while that produced by photo-detector 24
decreases. When the focussed beam is displaced in the opposite, negative
direction, the signal produced by photo-detector 24 increases while that
produced by photo-detector 23 decreases. This beam displacement is
relative to a scanned track and, generally, is caused by the actual
movement of disc 3' with respect to the beam incident thereon.
In FIG. 8, an example of a tracking-error detecting circuit is
schematically illustrated. As shown, photo-detectors 23 and 24 may be
connected electrically through low-pass filters 25 and 26, respectively,
to separate inputs of a comparator 27. The output of comparator 27 is of a
magnitude and polarity representing the direction and amount of relative
displacement of the beam with respect to the scanned track. This output is
the tracking error signal and may be used to control a servo control
device whereby the tracking error is compensated. As an alternative, and
as appreciated from the graphical illustration of FIG. 7, tracking errors
can be detected and compensated merely by using only one of
photo-detectors 23 and 24. That is, the signal level produced in
accordance with, for example, the solid curve of FIG. 7 (or the broken
curve) can be used to represent the direction and magnitude of the
tracking error.
As a further explanation of this tracking error detecting capability,
reference is made to FIGS. 9A-9C and FIG. 10. Let it be assumed that the
beam focussed onto surface 3a' of disc 3' by hologram 22 has a diameter of
10 .mu.m, as shown in FIGS. 9A-9C. This focussed beam is reflected from
surface 3a' with respective intensities for the zero order diffraction
beam and higher order diffraction beams. Let it assumed that the reflected
zero order diffraction beam is transmitted along a center line O, the +1
order diffraction beam is transmitted along the line "+" and the -1 order
diffraction beam is transmitted along the line "-" as shown in FIGS.
9A-9C. FIG. 10 is a graphical depiction of the intensities of the
reflected beams along the axis shown in FIGS. 9A-9C, this axis being
substantially perpendicular to the optical path traversed by the zero
order diffraction beam. If the beam incident on surface 3a' is as shown in
FIG. 9A, the intensity distribution measured along the axis is as shown by
the solid curve in FIG. 10. That is, when the incident light beam is as
shown in FIG. 9A, the reflected beam is comprised substantially of the
zero order diffraction beam. Thus, maximum intensity will be detected
along the zero order path while minimum intensities will be detected along
the +1 and -1 order paths. If the beam incident on surface 3a' is as shown
in FIG. 9B, the intensity distribution of the reflected light appears as
shown by the one-dot chain curve of FIG. 10. Thus, the intensities of the
higher order diffraction beams are greater than the intensity of the zero
order diffraction beam. If photo-detectors are disposed at locations
corresponding to locations b and c in FIG. 10, these photo-detectors will
produce output signals of greater amplitude when the incident beam is as
shown in FIG. 9B than when the incident beam is as shown in FIG. 9A. Now,
if the beam incident on disc 3' is as shown in FIG. 9C, the intensity
distribution will appear as represented by the broken curve of FIG. 10
wherein maximum intensities are included in the higher order diffraction
beams. Thus, if photo-detectors are located at positions corresponding to
positions b and c of FIG. 10, the output signals produced thereby when the
incident beam is as shown in FIG. 9C are of greater amplitude than when
the incident beam is as shown in FIG. 9A. Furthermore, the particular
signal levels produced for the FIG. 9C beam geometry differ from the
amplitudes of the signals produced in response to the FIG. 9B beam
geometry. Consequently, by detecting these signals, the amount and
direction of beam deviation with respect to pit P.sub.1 can be detected.
Thus, photo-detectors 23 and 24 may be positioned on holder 21 at
locations corresponding to positions b and c of FIG. 10.
While the foregoing description has referred to the +1 order diffraction
beam and -1 order diffraction beam, it is appreciated that other orders
can be detected, as desired. The intensity distributions shown in FIG. 10
by the one-dot chain curve and by the broken curve thus may correspond to
these other higher order diffraction beams. While the locations of
photo-detectors 23 and 24 at positions corresponding to positions b and c
will result in the detection of these higher order diffraction beams, it
is appreciated that if the photo-detectors are located at positions
corresponding to position a in FIG. 10, the output signal produced by
these positioned photo-detectors will correspond to the intensity of the
zero order diffraction beam shown by the solid curve in FIG. 10.
If the higher order diffraction beams are detected, and if the diameter of
the focussed spot incident on surface 3a' | | |
