CA1322791C - Method of optically scanning an information plane and optical record carriers and scanning apparatuses suitable for use of said method - Google Patents

Abstract

ABSTRACT:
Method of optically scanning an information plane and optical record carriers and scanning apparatuses suitable for use of said method.

When scanning an optical record carrier with a diffraction-limited scanning spot (S), the size of the effective scanning spot (S') can be considerably reduced by utilising the energy distribution within the scanning spot (S) in combination with a non-linear optical layer (22) in the record carrier (1) so that the resolution is increased both during reading and during writing.
Fig. 6.

Description

~L 3 2 2 7 9 Method of optically scanning an information plane and optical record carriers and scanning apparatuses suitahle for use of said method.

The invention relates to a method of optically scanning an information plane by means of a scanning beam which is focused to a scanning spot on the information plane, said scanning spot and the carrier of the information plane being moved relative to each other so that the scanning spot scans the information plane. The invention also relates to a record carrier, t-o a record carrier master which is used in the manufacture of such a record carrier and to an optical scannin~
apparatus, all of which are suitable for use of the method.
This ~ethod can be used for examining objects by ~eans of a scanning optical microscope, bu~ it is particularly suitahle for scanning an optical record carrier having an audio or vicleo program or digital data.
The informat.ion plane of a record carrier may be a plane recorded with optically readable information and a plane in which informat.ion is to be inscribed. In the former case the scanning beam is a read beam and in the latter case the scanning beam is a write beam modulated with the information to be inscrihed. The recor~ carxier may be a round disc-shaped record carrier, but also a tape-shaped record carrier.
Nowadays these record carriers and the associated read apparatuses are used in large numbers. In addition to the robustness of the disc~shaped record carriers, the large storage capacity is also an important factor wl1ich has contributecl to the success of the uptical recording syste~s. The optical disc having a diameter of 12 cm, which is known under the name of "Compact Disc" or "CD", may comprise, for example a digitised music program of one hour, whilst the larger optical disc having a diameter of 30 cm, which is known under the name of "Laservision ~isc" may comprise a video program of one hour.
Howeverj there is an ever increasing need of recording still larger quantities of informa~.ion on the said record carriers.
while maintaining the diameter of the present-day optical discs WlliCIl are easy to handle, this means that. the information areas such as the `

' ' ' known pits in the "CD" and ULa~ervision" discs and in the opti.cal discs for digital data storage, or the magnetic domains in the magneto-optical discs which are inscribahle and erasable must be smaller. The current techniques provide the possibility of inscrihing such smaller information areas. Hvwever, reading these smaller areas is a prohlem.
In the present.-day read apparatuses read objectives having a numer.ical aperture, NA, of the order of 0.45 are used.
Semiconductor diode lasers such as an AlGaAs laser emitting radiat.i~n at a wavelength of the order of 800 nm are used as radiation sources. This radiation can be focused by the read ohjective tn a diffraction-limited radiat.ion spot whose half-value width (FWHM) is of the order of 1 micron. The half-value width is understoo~1 to mean the distance between the points in the radiation spot where the intensity is half the maxlmum intensity in the centre of the radiation spot. Information areas having a width of the order of 0.5 micron and an average length of the order of 1 micron can be well-separated read by means of this scanning spot.
In this reading operation use is made of diffraction of the read beam by the informat.ion areas. At the stated values of the wavelength, the numerical aperture and the size of the information areas, the infnrmation structure can be considered to be a two-dimensional diffraction grating which spllts the incident radiation beam into a non-diffracted zero-order sub-heam, a plurality of diffracted first-order sub-beams and a plurality of sub-beams diffracted in higher orders. The zero-order suh-beam has a constant phase and amplitude anfl is not influenced by a movement of the ini.ormation structure with respect tn the scanning spot. However, ~he phase or amplitude of the first-order sub-beams is dependent on the movement of the information struct.ure.
An objective concentrating the zero-order sub-beam and a part of the first-order sub-beams from the record carrier on a detector is arranged between the record carrier and a radiation-sensitive detector. When mo~ing the information structure and the scanning spot with respect to each nther, the phase of a first-order sub-beam varies with respect to that of the zero-order sub-beam. ~y interference of a portion of a first-order suh-beam with the zero-order sub-beam, the intensity of the radiation received by the detector varies. This intensity variation represents the informat.ion which has been read, .. . . . .

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the succession of information areas in the scanning direction.
The angles at which the first-order sub-beams and the higher-order suh-beams are diffracted depend on the local period of the information structure, i.e. the distance hetween the beginning of a first information area and that of a subsequent information area. In tl1e case of a decreasing period, or an increasing spatial frequency, the diffraction angles become laxger until (,at a given spatial frequency referred to as the "conventional" cut.-off frequency,) the first-order sub-beams entirely fall outside the pupil of the objective. Information areas whose spat.ial frequency lies ahove this cut.-off frequency can therefore no longer be detected.
To be able to read an information structure at a spatial frequency above the said conventional cut-off frequency, United States Patent No. 4,242,57~ proposes to arrange the said objective, referred to as ohservation objective in this Patent, asymmetrically in the zero-order sub-heam coming from the record carrier. Since the ohservation objective is displaced in the direction in which one of the ~irst-order suh-beams is di~fracted, this sub-beam is captured also at higher spatial frequencies. This heam interferes in the detector plane with a part of the zero-order sub-beam, resulting in an interference pattern.
The variat.ion in this pattern representiny the information which has been read is detected by means of a detector whose width in the scanning direction is smaller than the period of the interference pattern. In the apparatus according to United States Patent No. 4,242,579 stringent requirements are imposed on the alignment of the optical elements, notably if a reflecting information structure is read and a transparent prot~ctive coating is present on this structure. In practice the cut-off frequency can be increased to approximately 1.5 times the conventional cut-off frequency with this apparatus.
The present invention has for its object to provi~e novel possibilities of reading optical information structures at spatial frequencies which are considerably higher than the conventional cut-off frequency, whilst the read apparatus in principle requires little or no adaptation .
The read method according to the invention is characterized in that use is made of a principally diffraction-lilnited scanning spot in combination with a layer of non-linear optical material :~322791 --in which the radiation of the scanning beam establlshes an optical effect which result.s in a ~iscrimination within the scanning spot so that the effective scanning spot is considerably smaller than the original scanning spot.
A diffractinn-limited radiation spot is understood to mean a scanning spot whose size is determined by the diffraction at the ap~rture of the lens wit.h which this spot is formed. Such a spot has a so-called Airy distribution, that is to say it comprises a central circular portion having an intensity decreasing from the centre to the edge and a plurality of rinys around the circle having a smaller, also outwardly decreasing intensit.y. The invention notably utilises the fact that only in a part of the central circle the intensity is high enough to pruduce a usable non-linear opt.ical effect so that scanning is substant.ially performed only with that portion of the radiation spot.
The said poItion of the scanning spot is referred to as the effective scanning ~pot.
A non-linear optical material is understood to mean a material whose optical properties vary under the influence of incident radiation. The said variation may be particularly a variation of the transmission or reflection coefficient or of the refractive index of the material, or a variation of the shape of the layer of non-linear optical material. As will hereinafter be described in greater detail, the following materials are generally suitahle as non-linear optical materials:
1. materials an optical property of which varies under the influence of a thermal effect, for exampla, thermochromic materials;

2. so-called optical bleaching mat.erials, for example, photochromic materials;

3. materials which are known as bistable opt.ical materials.

4. materials having a large third-order non-linear susceptibility, i.e.
a large X(3) (chi(3)number. The X(3) number is a known standard for non-linear optical materials, and for its definition reference may he made to the book by David J. Williams: "Non-linear Optical Prospectus of Organic and Polymeric Materials", American Chemical Society, Washing DC, 1983.
For a general introduction to non-linear optical materials reference is made to the book by H. ~loemberqen: "Non-linear ~1 3 2 ~ 7 ~

Optics~, ~ejamin, New York, 1965.
Similarly as for a direct nptlcal object-to-image transformation, that is to say optical imaging in the classical sense of the word, a modulation transfer function can be allocated to an opt.ical scanning, that is to say a point-by-point transformation. The modulation or contrast transfer function (MT~) is defined as the ratio hetween the contrast in the image and that in the object. This modulation transfer function decreases with an increasing spat.ial frequency in the ohject.
At a cut-off frequency, referred to as the conventional optical cut-off frequency, this function is equal to zero. In the conventional method of scanning optical information structures via linear media whose optical properties do not change, the modulation transfer function is entirely determined by the numerical aperture and the wavelength of the scanning beam if the det.ect.ion aperture is larger than or equal to the aperture of the beam incident on the information structure. The numerical aperture of the scanning beam i5 equal to the numerical aperture of the objective with which the scanning heam is focused to a scanning spot.
The detection aperture is the aperture of the beam which can be received by the radiation-sensitive detector. This aperture may be equal to the numerical apertuxe of a collecting lens between the record carrier and the detector, but it may be alternatively determined by the aperture of the detector itself. Under the above-ment:ione~ condition the cut-off frequency fc is given by: fc = 2.NA/A so that the highest spatial frequency in the information structure wh:ich can still be read is smaller than 2.NA/A.
The present in~ention is based on the recognition that the use of a non-linear material, i.e. a material whose optical propert.ies change under the influence of the scanning beam, in combination with the energy distribution across the cross-section of this beam leads to a variation of, for example the transmission or reflection coefficient within a small region of the radiation spot. On the original linear transmission or reflection coefficient a non-linear component is superimposed at the area of t11e scanning spot and movin~
along with this spot. The last.-mentioned c~mponent represents a su~-spot or effective scanning spot whicl1 is considerably smaller than the original scanning spot and whose side lohes, in contrast to those of the original radiation spot, are negligible. The effective scanning spot ~3227~ `

provides the possibility of reading considerably smaller information areas of a record carrier, or more generally smaller details of an object can be detected than with the orlginal scanning spot. The resolution of the optical scanning system is therefore impxoved.
In terms of the modulation transfer function the measure according to the invention results in a non-linear suh-function being superimposed on the original modulation transfer function determined by NA and A. It is true that this non-linear sub-function has a smaller maximum but it has also a considerahly smaller scope and a higher cut-off frequency than the original function. The difference in cut-off : frequency may be a factor of two or even more.
The cut-off frequency can even be further increased by using materials having higher-order non-linearities, either or not in . comhination with a larger detection aperture. The detection limit will be ultimately determined hy the signal-to-noise ratio, in contrast to t.he conventional optical scanning in which there is a sharp limit at the cut-off frequency fc = 2.NAI~.
It is to be nnted that the Article "Superresolution in Microscopy and the Abbe Resolution Limit" in "Journal of the Optical Societ.y of America", Vol. 57, No. 10 (October '67) pages 1190-1192 gives a theoretical disçourse on how superresolution, even to infinitely high . spatial frequencies, could be achieved by providing a so-called stop or diaphragm against the object. However, this Article also observes that .- it is virtually impossible in practice to manufacture such a diaphragm 25 with~the desired small opening and to place it in the desired position.
The great advantage of the present invention is that. the scanning spot.
itself creates a kind of stop function in the non-linear layer so that it is possible for the first time to realise the desired superresolutic)n , ~ in practice.
: 30 A very important application of the invention relates to reading optical record carriers. Thereby use is made of a novel type of record carrier which is characterized by a layer of non-linear optical : material which is located within the depth of focus of the information plane.
The layer of non-linear optical material may be constituted by the layer in which the information i~ or can he inscrihed. However, there are also embodiments in which a separate layer .

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of non-linear optical material is present in addition to the information layer. In that case it must be ensured that t.he first-mentioned layer is located so close to the information plane that the size of the scanning spot in this layer does not deviate very much from that of the diffraction-limited scanning spot in the information plane. Since the size of the scanning spot is adapted to the infc-rmation structure and since this size depends on the numerical aperture of the objective system with which the scanning spot is formed and since furthermore the depth of focus of the objective system is inversely proportional to the square value of t.he numerical aperture, it. is permitted to speak of the depth of focus of the information plane, which term is preferred in view of the wish for a concise formulation of the above-mentioned requirement.
The record carrier according to the invention has many emhodiment.s which are ~istinguishable in accordance with two criteria.
The cirst criterion relates to the facilities of the record carrier.
A first emhodiment in accordance with the first criterion is characterized in that the information plane is prov.ided with a permanent info:rmation structure which is constituted by information areas being arranged in information tracks, said areas alternating in the track direction with intermediate areas and being optically distinguished therefrom.
A permanent information st.ructure is understood to mean an information structure provided by the manufacturer of the record carriex in which the information areas are~ preferably constituted by p.its in, or hills on the record.ing surface. A record carrier having such a structure can only be read by the user, and it cannot be erased and inscribed again.
~ sec~nd embo~iment in accordance with the first criterion relates to a record carrier which can be inscribed once and is characterized in t.hat. the information plane has a permanent servo-track structure for positioning the scanning spot in the information plane.
Such a record carrier is often of the so-called ablative type in which, for éxample pits can he melted in the information surface by Means of the energy of the write beam. Another possibility is to inscribP by forming non-erasable crystalline aceas in an amorphous layer, or conversely. The permanent servo-track structure is provided by the manufacturer of the record carrier. With this structure it can be ~ 32~7~

ensured tllat the scannin~3 spot follows an accurately defined path in the information plane, but also that the scanning is in the correct posi.tion along the track direction at a given moment.
A third emhodiment. in accordance with the first cri~erion relates to a record carrier which can be inscribed and erased several times and is characterized in that the information plane has a permanent.
servo-track structure for positioning the scanning spot in the information plane. The servo-track structure has the same function as that in the second embodiment. Examples of an information layer for the third elnbodiment may be a magneto-opt.ical layer, or a so-called phase change layer in which a transition from the amorphous phase to the crystalline phase and cnnversely can he locally realised by means of optical radiation. A third example of an information layer which can be inscrihed and erased several times is a double layer of a first synthetic material layer having a relatively high thermal coefficient of expansion and a glass transition temperature which is below room temperature and a second synthetic material layer which ls connected to the first synt.hetic material layer and which has a relatively low coefficient of expansi-)n and a glass transition temperature which is above room temperature. Such a double layer whose first layer is reierred to as the expansion layer and whose second layer is referred to as the retention layer is descrihed in European Patent Application No. 0,136,070.
A fourth embodiment in accordanc:e with the first criterion relates to a record carrier intended as a master for .initially and optically inscribing a track-shaped structure which can be replicated in a record carrier according to the first, second or third embodiment, which record carrier has a suhstrate and a radiation-sensitive layer.
The track-shaped structure may be a track-shaped informat.ion structure or a servo-track structure which is converted into a relief pattern suitahle to be transferred on further record carriers by means of ~nown-pressing or xeplica techniques. ~he radiation-sensitive layer may be photo-sensitive layerl but alternatively an expansion-retention double layer referred to in the above in connection with the record carriers which can be erased and inscribed several times.
The above-mentioned first to fourth embodiments of the . :

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record carrier are distinguished from the known record carriers in thdt they have a layer of non-linear optical material which is located within the depth of focus of the information plane.
The second criterion by which the record carriers acc()rding to the invention are distinguishable relates to the type of non~linear ldyer with which the superresolution effect is realised.
A first embodilnent in accordance Witl1 the second criterion, is characterized in that the nvn-linear optical material is a material whose transmission changes with a changing intensity of the incident radiation. ~n example of such a layer is known in literature under the name of "bleaching layer" whose transmission increases witl1 an increasing int.ensity of the incident radiation. Suitable materials are, for example, gallium arsenide, gallium antimonide, indium arsenide and indium antimonide.
It is to he noted that the use of bleaching layers is known per se in photolithographic techniques for manufacturing integrated circuits. However, in these techniques the bleaching layer is used to sharpen the edges of the separate elements in the circuit patterns which must be provided in the semiconductor, and it is not used to limit the total imaging beam as in the present invention.
The first emh~diment of the record carrier may be further chardcterized in that the material having a transmission which changes under the influence of the intensity is a satura~ing dye. For example, with an increasing intensity this dye becomes more and more transparent to radiation o~ a given wavelength.
~ However, the first embodiment is preferably further characterized in that the mate~ial having a transmission which changes under the influence of t.he intensity is a bistable optical material.
This material is described in "Optics Communications", Vol. 59 (1984) Nn. 5, pages 359-361 for use in optical switches and optical co~puters. At a given intensity level the transmission of this material increases stepwise and the transmission decreases stepwise when the intensity is decreased to below this level. The bistable or optical material i5 elDinent~ly suitable for realising the superresolution effect because it acts substantially without delay.
~ layer whose transmission changes with a changing intens~ty can be used in a record carrier having a permanent information ': ', : . :, ' ...

:~3227~1 structure, in a record carrier which is inscribable once and in a record carrier which is inscrihahle and erasahle several times, and it is eminently suitable for use in a record carrier serving as a master in the manufacture of a large number of identical record carriers having an information track structure or a servo-track structure.
A second embodiment in accordance with the second criterion, of the record carrier is characterized in that the non~linear optical material is a material whose refractive index changes when chanqing the intensity of the inciden~ radiation.
Both the real part of the refractive index and the imaginary part of a complex refractive index may change, in other words the non-linear optical effect. may be both an amplitude effect and a phase effect. As an example of such a layer which may be provided both in front. of and behind the recording layer, a layer may be mention~d which comprises a phase change material. It has been found that the complex refract.ive index of this type of mat.erials, both in the amorphous and in the crystalline phase, is temperature dependent to such an extent that the ra~iation whose int.ensity is too small to realise a phase change can still estahlish a sufficiently laxge change of the refractive index in hoth phases to enahle these materials to be used as non-linear materials in the sense o~ the present invention.
A suh-embodiment of a record carrier having a phase change material as a non-linear optical material is further characterized in that the layer of non-linear optical material is also 2S the information layer. When inscribing this record carrier with a first high-intensity level, the amorphous phase is locally converted into a crystalline phase, or conversely, whilst during reading with a second low-intensity level the refractive index is temporarily changed without a phase change taking place.
a thir~ emho~iment in accordance with the second criterion, of the record carrier is characterized in that the non-linear optical material is a synthetic material having a relatively high thermal coefficient of expan~ion.
In such an expansion layer a scanning beam having an intensity which is lower than the write intensity can locally establish a surface deformation so that a discrimination witbin the scanning beam takes place again and effectively a smaller scanning spot is formed in ~32279~L

the information plane.
An expansion layer and a layer having a variable refractive index can he used in record carriers baving a permanent information structure or in record carriers which are inscribable once or several times. For example, in the embodiment having an expansion layer as a non-linear layer, the information layer may also be an expansion layer in comhination with a retention layer.
~ fourth emhodiment in accordance with t.he second cxiterion, of the record carrier in which the information layer comprises a magneto-optical material in which magnetic domains can be written by means of a magnetic field and a radiation beam, which domains can be read on the hasis of a change of polarisation of a scanning beam, is characterized in that the non-linear layer is constituted by a magneto-optical information layer whose polarisation change depends on the temperature.
This embndiment of the record car.rier utilises for the first time the fact that the polarisation change caused by a magnetic domain in the magneto-optical information layer, which change has the form of a rotation of the polarisation direction or the form of a change of the polarisation ellipticity, is dependent un the temperature of this layer.
The magneto-optical record carrier may be further characterized in that a layer of non-linear ~ptical material is provided within the depth of focus of the magnet~-opt.ical layer.
The non-linear effect can be increased considerably by means of the extra layer. It is also possihle to use substantially only the non-linear effect of the extra layer.
A preferred emhodiment of a record carrier having one of the above-lnentioned types of non-linear optical layers is characterized in that the non-linear optical layer forms part of a pack of layers constituting a resonance structure for khe scanning radiation.
~ y creat.ing a resonance st.ructure in the record carrier, the non-linear effect can be increased considerably. Tl1e non-linear layer itself has a transmission (T) versus optical thickness (nd: wit.h n being the refractice index and d being the geometrical thickness of the layer) characteristic which has a sinusoidal variation. The value nd is chosen to be such that around the corresponding point on the ~32279~

characteristic curve, the working point, the curve has a maximum slope.
The optical thickness nd of the layer will mostly deviate from N.~ in which A i5 the wavelength of the scanning radiation and N is an integer because for n.d = N.~ mostly an extremum is achieved in the characteristic curve. The non-linear effect of the layer thus optimised can be increased by adapt.ing the optical thicknesses ni.di of other layers which are already present in the record carrier so that these layers, combined with the non-linear layer, constitute a resonance structure. It holds for this pack of layers that x ni di i=1 in which x is the number of layers and ni and di are the refractive index and the geometrical thickness of the layer i, is chosen to be such that. around the corresponAing point on the characteristic curve the slope of this curve is maximum.
The embodiment. of the recor~ carrier having a resonance structure may be further characterized in that a plurality of extra layers is provided which, cnmhine~ with the non-linear layer, constitute the resonance structure.
The layers which are alxeady present in the recor~
carrier then need not be adapted. The same condition of the total opt.ical thickness a~ indicated above applies to the pack of layers with the extra layers.
When using non-linear optical materials an optimum superresolution signal is ohtained if the local non-linear optical effect is only present in the time interval during which the scanning spot remains in the area of this effect, in other words if the local effect has disappeared immediately after the central portion of the scanning spo~ has disappeared from this local position. The scanning spot is then reduced both on the front side and on the rear side, viewed in the direction of movement of tl1e spot. The superresolution signal then has a maximum sign~l-to-noise ratio. Materials which have the short recovery time reguired for this purpose, or which are quicXly xeversible are, for example, bleaching mat.erials or histable optical materials.
When us.ing non-linear optical materials havin~ a longer recovery time in which the non-linear effect is mostly established via a .: '.
. . , /

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thermal effect, the signal-to-noise rativ of the superresolution signal is smaller but it is still sufficiently large to utilise the effect.
When scanning a record carrier provided with such a material, the non-linear effect occurs on the front and rear sides of the scanning spot, but since this effect relnains present for some time and trails the scanning spot like a kind of comet tail, only the reduction of the scanning spot on its front side and the increase of the steepness at that side contrihutes to the superresolution effect.
For reading the record carriers according to the invention it is possihle to use the known scanning apparatuses comprising a radiation source, an objective system for focusing a radiation beam from the radiation source to a diffraction-limited ~canning spot on the information plane and a radiation-sensitive ~letect.or for converting radiation from the information plan~ into an electric s.ignal. In the known scanning apparatuses the objective system between the radiation source and the information plane has the same numerical aperture as the ohjective system between the information plane and the det.ector; in the case of a reflecting record carrier there is only one objective system.
According to the invention the scanning apparatus can be optimised for the superresolution effect Sucl1 a novel scanning apparatus is characterize~ in that the det:ection aperture is larger than the aperture of the scanning beam incident on the record carrier.
The detection aperture is the aE~erture of the beam which can be received by the radiation-sensitive detector. In most cases this aperture will be defined by the opening of an optical element between tl1e record carrier and the dete~tor.
A preferred emhodiment of the scanning appara~.us according to the invention, intended for scanning a reflecting record carrier is further characterized in that t.he object.ive system comprises round central portion which is sufficiently corrected and which has a sufficient optical quality for forming the diffraction-limited radiation spot, and a contiguous annular pvrtion whicl1 can recPive radiation from the record carrier.
The annular lens portion should only collect light and need not to image and it may therefore have a considerably lower opti.cdl quality than th~ cent.ral lens portion. With the annular lens portion : . ' 1322 ~91 radiat.ion from the xecord carrier, which radiation falls outside the central portion of the objective system can be captured and determined.
The information comprised in the information areas which are considerahly smaller than the scanning spot formed by the objective system and whicll can be read by means of the superresolution effect .is particularly present in t.his radiation so that the last-mentioned scanning apparatus is better suitable for the superresolution effect than the conventional apparatuses.
The objective system maY be foroed in known manner by means of a single lens element having one or two aspherical refractive surfaces, by a holographic lens or by a lens element having a radially varying refractive index.
The radiation through the annular lens portion can be received by the detector which also receives the radiation through the central lens portion. However, the scanning apparatus according to the invention may be further characterized in that an extra detector is provided for receiving the radiation passing through the annular lens portion .
~he signal which is due to t.he superresolution effect can then be separately detected and treated, whilst moreover convent.ional discs can also be read with this apparatus.
The invention will now be described in greater detail, by way of example, wit.h reference to the accompanying drawings in which Figs. 1a to 1k show the principle of the invention, Fig. 2 shows examples of modulation transfer functions which can be obtained when using the invention, Fig. 3 is a plan view of an emhodiment of an optical record carrier, Fig. 4 shows an embodiment of an optical scanning apparatus, Figs. 5a, 5h and 5c are sectional views of diffexent-order sub-heams in the fax field of the information structure in those cases where there is no non-linear effect or where there is a non-linear effect, Figs. 6, 7, 9, 10, 11, 13, 1~, 16, 17, 18, 19, 20 and 23 show embodiments of a record carrier hdving a non-linear layer, Fig. 8 shows the transmission as a function of the .: . ' ....................................... ,/ ~ , ... .

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intensity of a bistable optical material, Fig. 12 shows the variation of a thermally generated non-linear optical effect, Fig. 15 shows the transmission as a function of the aptical thickness of a non-linear layer and of a pack of layers, Fig. 21 shows a magnet.o-optical write and read apparatus, Fig. 22 shows a method of inscribing magnetic domains in a magneto-optical record carrier, and Fig. 24 shows an emhodiment of a read apparatus according to the invent.ion.
Fig. 1a shows diagrammatically how a radiation bea~ b with a plane wavefront. is focused by an objective L1 on a transmissic~n object 0 and how the radiation through the object is concentrated by a second objective L2 on a radiation-sensitive detector D. The variation vf the far field E(~) which is passed by the objective L1 as a function of the radius p is shown in Fig. lh. This field has a constant value E1 within the numerical aperture NA of the ~bjective and is equal to zero outside it. The objective L1 constitutes a diffraction-limited radiation spot S on the ohject 0, the field E~r) (Fig. 1c) having the known Airy distribution which is given by the function J1(r)/r in which J1tr) is the first-order Bessel function. This radiation spot consists of a central bright circle and several rings surrounding it and having a weaker, outwardly decreasing field strength. Fig. 1c alsn shows the half-value width FWHM of the radiation spot, i.e. the distance between the points where the field strength is equal to hal~ that in the centre of the spot. This FWH~ is, for exa~ple 0.71 ~/N~ in which A is the wavelength of tl1e radiation used.
The ohject 0 is assumed to he a homogeneous layer so that the transmission t is independent of the position of the spot S on the object (Fig. 1d). If this layer is a linear layer, i.e. the transmission is not influenced by the incident radiation, the field tO.E~r) behind the layer has the same shape and the same half-value width as the field E(r) incident on the layer as is shown in Fig. 1e. ~he field at the location of the detec~.ion aperture of the objective L2, also referred to as the far field of the object, is the Fourier-transform E~ ) af E(r), the inverse of the transformation which is brought abaut by the ~bjective L1. The said far field i5 shown in ~ig. 1f. For the sake of 1 3227~1 1fi simplicity it is assumed that the numerical aperture of the objective L2 is equal to t.he numerical apert.ure NA of the objective L1. This situation occurs inter alia when reading a reflecting record carrier.
The modulation tr~nsfer function MTF is o}ltained by convolution of the S far field with the detection aperture. This function, which is shown in Fig. 19, has a cut-off frequency fc which is equal to 2.NA/X for the case considered.
If the object 0 is a layer of non-linear material, its transmission changes under the influence of the energy of the scanning beam b. This energy is given by E(r).E~(r) in which E*(r) is the complex conjugate of E(r). For an object of non-linear material the transmission t(r) is given in a first approximation by:
t(r) = to + ~t, with ~t = ~ E(r).E(r)* in whi.ch ~ is a factor which is dependent on the type of material and its thickness and on the surrounding media. The term to represents the linear portion of the transmission and ~t is the non-linear contrihutor. The non-linear transm.iss.ion term is shown in Fig. 1h. To tl1is term also a half-value width ~tFWAM can be allocated which is for example 0.S1 ~/NA. Due to the non-linear term ~t. the field behind the object 0 not only has acquired the linear field term E(r) si1own in Fig. 1c but also a non-linear field term ~E~r). This term is given hy ~E(r) = ~t.E(r), hence by ~E(r) = ~.E(r).E(r).E(r)% and is shown in Fig. li. The term ~ E(r) represents a rad.iation spot witl1 negligible side lobes whose half-value width FWHM is considerably smaller, 0.42 ~/NA in the given model than that of the radiation spot which is ~epresented by the fiel~ t.erm E(r) in Fig. 1c. The resolving power of the first-mentioned radiatlon spot is larger than that of the last-mentioned spot. The Fnurier-transformed ~E(p ) of the field term ~E(r), or t.he non-linear contribution to thé far field at the location of the detection aperture, is given by the convolution:
~E(p ) = ~.E(p ) X E(~ )%X E(~ ) in which E(p ) is equal to the far field of the homogeneously filled Airy circle.
As is shown in Fig. 1j ~E(~ ) is more extensive than E~ ~ ). In the example considered E(p ) ranges from NA to +NA, whilst ' . " ', ~:

13227~

~E(p ) ranges from -3 NA to +3 NA. By convolution uf the far field term ~E(p) with the detect.ion aperture, the aperture of the objective L2,the non-linear portion of the modulation transfer function ~MTF is obtained which is shown in Fig. 1k. For the given model the cut-off S frequency fc of ~TF is twice as high as the cut-off frequency fc of a conventional diffraction-limited optical scanning system.
If the radiation from the ohject 0 is received by one detector D, the functions according to Figs. 1g and 1k are added so that a resultant modulation transfer function MTFs is obtained which has a cut-off frequency of fc~ The optical scanning apparatus thus has acquired an increased resolving power although the size of the scanning spot ~ is not reduced. This phenomenon is referred to as superresolution.
The nature of the non-linear effect is irrelevant for the superresolution effect itself. ~t may be large or small, positive or negative or even complex and anisot.ropic, as will hereinafter be explained. Anisotropic is understood to mean that the effect depends on t.he polarisation direction, or the direction of the E ~ector of the radiation. The nature of ~t is important only for spatial frequencies in the object which are smaller t.han fc~ If the non-linear portion in the frequency range from 0 to fc is added to the linear portion and if the phase of the non-linear effect is opposed to that of the linear effect, a local minimum in the resultant transfer function MTFs is produced, as is shown in Fig. 2.
The object which is scanned is of course not holnogeneous, as l!as been assumed in the foregoing, but it has a given structure. ~n the case of an optical record carrier this structure comprises a pattern of information areas. Fig. .3 shows a pvrtion of such a record carrier in a plan view. A large number of incormation tracks 3 which are separated from one another by informationless intermedia~e lands 4 is present in the information plane 2 of the record carrier 1. The information tracks may be concentrical tracks or quasi-concentrical contiguous tracks constituting one spiral track. The information tracks 3 are composed of informdtion areas 5 alternating in the track direction with intermediate areas 6. The information structure may be a so-called phase structure and may comprise pits sunk in the information surface or h.ills located on this surface. Such a st.ructure influences the phase of the radiation incident thereon. The information areas may also be disting~ished from , ~227~

the rest of the information surface in that they have a different reflection or tIansmission coefficient. When reading such infor~ation areas, the amplitude of the scanning beam is varied. Furthermore tl1e information aIeas may comprise magnetic domains whose magnetisation direction is opposed to that of the surroundings. In that case 2 magneto-optical record carrier is concerned which can be read by using the polarisation rotation "7r th~ Kerr effect, in the scanning beam caused by the areas. In all these cases the information is encoded in the succession of information areas in the track direction.
Fig. 4 shows a portion of the record carrier 1 in a radial cross-section and it shows diagrammatically a read apparatus for this caxrier. ~he information surface 2 is reflective and the su~strate 7 of the record carrier is transparent so that the information can be read with a scanning heam b which passes this substrate twice. The beam b is supplied by a radiation source 10, for example a semiconductor diode laser and is focused by an ohjective 11 to a scanning spot S on the information plane 2. The objective 11 may be preceded by a collimator lens 12 which converts the divergin~ beam from the source into a paralllel beam having such a cross-section that the pupil of the objective 11 is well filled and the scanning spot S is a diffraction-limited radiation spot of minimum dimension.
One track revolution can be scanned by rotating the record carrier witl1 the aid of a shaft 13 projecting through the central opening 14. All tracks or the entire spir,al track can be scanned by radially moving the scanning spot and the record carrier with respect to each other. The scanning beam is modulated in accordance with the succession of infvrmation areas and intermediate areas in the track direction. The modulated scanning heam is reflected by the information plane in the clirection of the source 10. A beam splitter 15, for example a prism having a semi-transparent face 16 is arranged between the source and the objective 11, whicl1 prism passes a part of the Ieflected beam t~
a radiation-sensitive det.ector 17. This detector then supplies an electric signal Si which is modulated in accordance with the information which has been read.
The struct.ure consisting of information areas can be considered as a two-d.imensional diffraction grating splitting the incident scanning heam into a nc)n-diffracted zero-order sub-beam, a ~322719~

plurality of first-order sub-beams and a plurality of higher-order ~ub-beams. Only the first-order sub-beams are important for the present invention because the energy of the higher-order sub-beams is only small.
Fig. 5a is a cross-sectional view of a linear grating g.
This grating is exposed with a beam b from an objective only the pupil p of wl1ich is shown. The grating g reflects the beam and splits it into a zero-order sub-beam b(O), a +1-order sub-beam b(+1), a -1-order sub-beam h(-1) and a plurality of higher-order sub-beams not shown. The sub-beams h(+1) and b(-1) are diffracted at angles +a and -~, respecti.vely. Fig.
5b shows the cross-sections of the beams at the location of the pupil.
The sub-beam b(O) which in the absence of a non-linear effect has the same angular aperture ~ and the same direction as the incident beam b entirely fails within the pupil and is passed on to the detector (17 in Fig. g) in the case of an optical scanning apparatus with which the information grating g is read. The zero-order sub-beam does not comprise information ahout the succession of the information areas and intermediate areas. This information is especially present in the first-order sub-beams b(~1), b(-1). Of these sub-beams only the portions denoted hy the shaded areas V1 and OV2 fall within the pupil. When reading the information, the phase variations in the sub-beams b(+1) and b(-1) with respect to the zero-order sub-beam are used.
In the areas V1 and OV2 in Fig. 5b the fi.rst-order sub-beams overlap with the zero-order suh-beam and i.nterferences occur. The phases of the first-order sub-beams change when moving the scanning spot o~er an infvrmation trach. Consequently, the intensity of the total radiation passing througll the objective and received by the detector changes.
If the cent.re of the scanning spot coincides with the centre of an information area, foI example a pit, there is a given phase difference ~ between a first-order sub-beam and the zero-order sub-beam. This phase difference is also referred to as phase depth of the information structure. If the scanning spot moves from a first information area to a second area, the phase of the +1-order suh-beam increases and this phase i5 increased by 2~ at the moment when the centre of the scanning ~pot has arrived in tl1e centre of the second information area. The phase of the -1-order beam simultaneously decreases. The phases of the first-order sub-beams with respect to those ~. ' ' ' ' . ' ' ' , ' ' .

~ 3227~

of the 2ero-ord~r sub-beam can thus be represen~ed by ~(+1) - ~ + 2~ xp ~ 2~ p in which x is the position in the tangenti.al direction of the scanning spot and Pt is the local tangential period of the information structure. The electric output signals of two det.ectors arranged behind the transition areas V1 and OV2 can then he represented hy S1 = cos (~+ 2~ p 52 = cos ( ~ - 2~ p ) The information signal ~i is obtained by adding these detector signals:
Si = S1 ~ S2 = 2~cos~Jcos 2~ ~ ) The above applies only as long as there is still an overlap between the suh beams h(+1) and h(-1) on the one hand and b(0) on the other hand. The angle ~ at which the f.irst-order sub-beam is diffracted is given by sin a ~ A/Pt = ~-f in which f is the spatial frequency of the grating g or the local spatial frequency of the information structure. There is no longer any overlap if ~ = 2~. Since sin ~ = NA, the classical cut-off fre~uency fc is given by:
fc = 2. NA/A
This applies if the medium which is in front of the info~mat.ion structure is a linear medium such as air or glass and if the angular aperture of the first-order sub-beam is equal ~o that of t.he zero-order sub-beam. If, as propo~ed in the present invention, a non-linear medium is arranged in the immediate vicinity of the information structure, and if the radiation beanl has the energy distribution shown : in Fig. 1c so that a temporary non-linear ef~ect is created in the path traversed by the scanning beam, the energy distribution in the far field of the information structure will be more extensive, as is shown in Fig. 1j. This means that the zero-order and the first-order sub-beams have become wider, as is shown in Fig. 5c by means of the cixcles ~b(0), ~b(+1) and ~h~-1). Wli.ilst in the situation of Fig. 5b the cut-off : frequency is reached if the centres B and C of the sub-beams b(+1) and b(-i) are shifted to D and E, respectively, the cut-off frequency in the situation of Fig. 5c is not achieved until B and C have moved to D and E , respectively, in the case of an equal size of the detection pupil P
because only then there is no overlap within the pupil p of the first.-order sub-beam Qb(~1) and ~b(-l) with the zero-order sub-beam Qb(0). The .

: . , .', .

. ' ` ~3227~

distances ~D and AE are twice as large as the distances AD and AE, respectively, so th~t the cut-off frequency is increased by a factor of two in the given model when a non-linear effect occurs.
It follows als~ from Fig. 5c that when using a larger detection aperture, hence a larger pupil p , there the situation of no overlap within this pupil between the first-order sub-beams and the zero-order sub-beam occurs only at even larger outward movements of the centres B and C ~f the first-order suh-beams ~b(+11 and ~b(-1), so that the cut-off frequency becomes even larger.
Due to the larger width of the suh-beams ~b(O), ~h(+1) and ~b(-1) the energy of these heams in the overlap areas is smaller than that. of the sub-beams b(O), b(+1) and b(-1) in the overlap areas oV1 and OV2 in ~ig. 5b. The amplitude of the superresolu$ic)n signal is therefore smaller than that of the information signal Si which is obtained in the conventional manner but is still sufficiently large to enahle reading. Only at. non-linear effects with very small dimensions, which would be required for reading very small information areas (for example smaller than 0.2 ~m), the sub-heams could "blow up~ to such an extent that the information signal would acquire a too small signal-to-noise ratio at. an acceptable energy of the scanning beam.
It .is important for the present invention that the non-linear opt.ical effect. is present. in th~ immediate vicinity of the informat.ion structure or in the information layer itself .in order that the heam is effectively narrowed in the pl,ane of the information structure. In other words: the non- l.inear layer must be present within the depth of focus of the information struct.ure, whilst the depth of focus is related in a first approximation to that of the ohjective witl which the scanning spot is formed on the information structure. Since the effective scanning spot is smaller than the diffraction-limited spot, the desired tolerance for the position of the non-linear layer is smaller than the depth of focus which is related to the diffraction-limited radiation spot. The non-linear layer may be present in front of the information structure but also behind it if the scanning beam passes through the information layer~ In the latter case the beam modulated with the information can be either passed to a detector on the rear side of the record carrier or it can be reflected alongside itself. The reflection layer required for this purpose may be a non-linear layer.

~3~2'~

The desired non-linear ef~ect can be realised in various manners, using various types of non-linear optical layers. The present invent.ion therefore also relates to a number of novel optical record carriers provided with such layers.
S A first emhodiment of such a record carrier is shown in a tangential cross-section in Fig. 6. The information structure in the information plane 2 comprises pits 5 having, for example a variahle length and alternating itl the track direction t Witi1 intermediate areas ~. A reflecting layer 20 is provided on this structure. This layer may be coated with a protective layer 21. A non-linear layer 22 whose transmission is increased if the incident radiation has a given intensity level is provided between the transparent suhstrate 7 and tlle reflecting informat.ion structure. The material of the layer 22 and the energy distribution within the scanning spot S are chosen to be such that the transmission is increased mainly in the central area 5' of the radidtion spot S. A sub-radiation spot S' is then produced with whicl information pits S which are considerably smaller than the radiation spot ~ can be read. It is also pos~lble for the transmission to be aecreased in the cent.ral area S' of the radiation spot. Then the non-linear effect has a different s.ign.
~ s is shown in Fig. 7 a layer 22 with variabletransmission may be alternatively provided behind the information plane 2. The reflectin~ layer 20 is then providled on the layer 22~ The layer 22 in Fig. 7 may be alternatively a reflecting layer whose reflection coefficient i5 locally changed by the incident radiation. In that case the reflecting layer 20 can be dispensed with.
The layer 22 may be a "~leaching layer" or a layer having a saturatin~ dye whose transmission for a given wavelength increases with an increasing intensity. The layer 22 may be alterna~ively a layer of a so-called bistable optical material. In Fig. a the transmission ~T~ of such a material is shown as a function of the intensity (I).
Below a given intensity I1 the layer has a relatively small transmission T1. As soon as the intensity exeeeds the threshold I, the transmission stepwise increases to T2 and further remains constant, even with an increasing intensity. If the intensity is decreased again t.o below I1, the transmission stepwise decreases to T1. The T-I
characterist.ic curve shows a hysteresis.

. ,: . .
:' '. , ' ' .

-~ ~ 2 2 ~

The advantage of tl)e bistahle optical materials and other materials which are not based on a thermal effect is that they are quickly reversible so that the non-linear optical effect has disappeared substantially immediatelY after t.he radiation spot has disappeared, resulting in an optimum superresolution effect.
Flgs. 6 and 7 show that. the layer 22, for example a bleaching layer has one flat side and therefore does not follow the profile of the information structure. When using a thinner layer 22 which is prov.ided, for example by vapour deposition or sputtering, ~his layer can fnllow the sai~ profile The two possibilities occur not only in the case of layers whose transmission is increased, but also in other non-linear layers which will be described hereinafter Fi~. 9 shows an embodiment of the record carrier in which a variation of the refractive index is utilised for creating a temporary non-linear effect For example, a layer .30 of a so-called phase change material is present in front. of the information structure, fnr which material it has previously been proposPd to use it as an optically inscrihahle material, hence in an information layer These materials, examples of which are GaSb and InSb, have the property that they change from the amorphous to the crystalline state, or conversely when they are irradiated w.ith light of a suffic.iently high intensity The complex refractive index then changes so that a written area has a different reflection or transmission coefficient than its surroundings. It has been founfl that the complex refract.ive index of this type of materials is temperature dependent to such an extent that, even in the case of irradiation with an intensity remaining below the level at which the conver~ion from amorphous to crystalline ur conversely occurs, there is : a sufficiently large variation of ~he complex refractive index to enahle layers of these materials to be used as non-linear layers in the sense of the present: invention. In the embodiment of Fig. 9 such a phase change layer 30 is provided in front of the information layer 2. The layer 30 ensures again that the scanning ~pot S is effectively limited to a sub-scanning spot S Witl1 which the sub-micron areas 5 can be read.
: Such a phase change layer may also be provided behind the information layer 2 as is shown in Fiq. 10. The layer 31 may then function as a reflecting layer whose reflection coefficient is locally, within an area S which is smaller than the ~canning spot S, increased, :: .,: , .

~32279~
`
2~ 2010~-8539 for example, by the scanning rad.tation.
The so-called expansion la~ers are also suitable as non-llnear layers ~or forming a temporary non-linear optlcal effect.
European Patent Application No. 0,136,070, lald open to public inspectlon on ~pril 3, 1985, descrlbes the use of such a layer as an inscrlbable layer ln an optical record carrier. Then the expanslon layer ls flxedly adherecl to a second layer, the so-called retention layer. By irradlatlon wlth laser pulses ln accordance wlth the in~ormatlon to be written informatlon areas ln the form of surface unevennesses can be written ln the expansion layer because thls layer has a large thermal coefficlent of expan-slon. Furthermore, this layer has a low glass transltion tempera~
ture. The unevennesses are passed on to the retention layer which has a relatlvely low coeff:Lcient of expansion and a glass transl-tion temperature above room temperature~ After the laser pulse has dlsappeared, the surface unevennesses contlnue to exlst be~
cause the retentlon layer prevents the expansion layer from assum-ing its original shape.
To obtain superresolutlon, only the expanslon layer 32 in Fig. 11 can be used as a non-linear layer. The energy distri-bution within the scanning spot S and the material parameters of the layer 32, notably the thermal coefflcient of expansion, are a~apted to each other in such a way that the unevenness 33 is smaller than the scannlng spot so that there is again a dlscrimi-nation withln thls spot and thus a smaller scannlng spot S' ls effectlvely formed.

.

11 3227!~1 ~ 24a 20104-853 The unevenness 33 will not lmmediately disappear after the radiation spot S has leEt the relevant posltion, but lt will gradually decrease wlth respect to time because heat must be dis-sipated. This means that in the record carrier an elongate de-formation becoming smaller towards the rear moves along with the scanning spot over the surface of the layer 32 as 1s illustrated in Fig. 12. Now this spot narrows and the steepness of the edge increases only on the Eront side as viewed ln the direction of movement VSl of the scannlng spot, compare S and S' ln Flg. 12.
The slgnal-to-nolse ratlo of the superresolution slgnal is there-fore smaller than when uslng non-llnear optical materlals in whlch the non-linear effect disappears substantlally simultaneously with the scanning spot.
The non-llnear optlcal effect which is realised by means of the layers 22, 30, 31 and 32 in the Figs. 6, 7, 9, 10 and 11 can be ` ~3~279~

increased hy having each of these layers form part of a pack of layers in the record carrier which pack functions as a resonance structure. The other layers of the packet may be constituted by layers which are already present in the record carrier, such as an information layer, a reflecting layer, separation layers and the like, dependent on the structure of the record carrier.
However, it is alternat.ively possible to add a resonance layer pack as a whole to the record carrier. Fig. 13 shows an example of such a pack of layers 24 which serves as a compnsite reflecting layer.
The pack of layers 24 comprises a suh-layer 24m of a non-linear optical material. The local reflection coefficient of the pack of layers is relatively low as long as it is not impinged by radiation on the relevant position. If a scanning spot is incident on this position, the reractive index of the sub-layer 24m changes so that the reflection of the composite layer 24, mainly where the scanning spot has a high energyl i.e. in the centre of t.he spot, is increased. Consequently t~he reflected radiation appears to be coming from a radiation spot S which is considerahly smaller than t.he scanning spot ~.
It is alternatively possible to reduce the reflection in the cent.re of the sc~nning spot. so that the energy in the centre S is smaller than outside it. As already noted, it is only important for the superresolution that there is a discont.inuity in the scanning spot and that the nature of the discontinuity is not so important.
A resondnce layer packet may also be provided in front of the information structure as is shown in Fig. 14. The pack 25 is now constituted in such that at a sufficiently high energy of the .incident radiation, the transmission is increased or decreased by varying the refractive indéx of one or more of t.he sub-layers 251 25n For a transparent layer the transmission as a function of the optical thickness n.d, in which n is the refractive index and d is the geometrical thickness, has for example a variation as is shown in Fig. 15. To obtain an optimally combined non-linear and resonance effect, it must be ensured that both for the non-linear layer and for the pack of layers the product n.d~ and the sum of the products I nidi of all layers is such that the corresponding working point Wp on the curve in Fig. 15 is located at the area where this curve has a maximum slope. In practice this means that n.d. is unequal tc) N.A/4, . . .
.~ ,, - .... .

~ 32~7~1 in which N is an integer, becal~s~ for N.~/4 tl1e curve will exhibit ext.re~a E. For a reflecting pack of layers the reflection as a function of n.d. and ~ nidi has a similar variation as the transmission of a transparent pack of layers and simildr considerations are valid.
In the embodiments described the information structure is a phase structure, which is read on the basis of the phase difference introduced in the read beam by the information areas which may comprise not only pits but also hills on the information surface. However, the information structure may alternatively be an amplitude structure, which likewise as the phase structure, can be read both in reflection and in transmission. The information areas then have a different reflection or transmission coefficient than their ambience~
In the record carriers described the information structure is a permanent structure which can be read only. The information areas are smaller than those of the known record carriers.
The manufacture of such record carriers is readily possible by means of known techniques. It is true that d radiation source, for example a laser with a shorter wavelength must be used then and the writing objective must then have a larger numerical aperture, but this is no drawback because the information need only be inscribed once in a so-called Master disc. A very large number of copies of the Master disc can be manufactured ~y means of knnwn pressing or replica techniques. The write apparatus may therefore be more expensive and it may be. equipped with a gas laser having a short wavelength and a write objective having a large numerica1 aperture and a small depth of focus. However, the invention may also be used when inscribing the master disc 50 as to be -able to write the sub-micron information areas with simpler apparatuses or to write even smaller information areas with the same apparatuses.
Fig. 16 shows a part of a master record carrier in a tangential cross-section. In principle, this master comprises a substrate 40 and a radiation-sensi.tive layer 41. According to the invention a non-linear optical l.ayer, for example a bleaching layer 23 is provided on the layer 41. A write beam b , which is intensity-modulated in accordance with the information to be written and which is focused to a diffraction-limited radiation spot S by means of an objective system not shown, increases the transmission of the layer 23 within an area which is smaller than the spot S. Consequently the photo-~32279~
: 27 sensitive laye} is exposed witl1 a radiation spot S which is smaller : than S and a~so the information areas are correspondingly smaller. A~ter the entire master record carrier has been scanned by the write bedm, the layer 23 is removed and the exposure pattern is converted by means of d known developing and etchin~ process into a relief pattern which issuitahle for replication.
In addition to the embo~iments shown in Figs. 6, 7,~ 9, 10, 11, 13 and 14 all of which relate to record carriers having a permanent information structure, the invention can be alternatively used in record carriers which can be written by a user himself. The inscribable record carriers are pre~erably provided by their manufacturers with a servo-track structuLe in the fQrm of a spiral track or a plurality of concentrical tracks. These tracks may be completely blank. ~hen writing information the position of the write spot with respect to the servo-tracks can be detected and re-adjusted so that. this spot accurately ~ollows the servo-tracks. This has the advantage that the write/read apparatus of the user need not be provided with very accurate and expensive mechanicdl positioning and guiding mechanisms.
The servo-tracks may also comprise address information to enable writing and reading at arbitrary pOsitiolls on the record carrier. If the inscribable record carriers are intended :Eor recording a user program a part of which is the same for all users, this part, or the standard information, may be prerecorded so that a user only needs to complete it with his own specific information.
The servo-track structure, or generally the prerecorded - permanent information can be provided via a mastering and replication process on a carrier which is subsequently provided with an inscribable layer. The master record carrier may be provided with a non-linear optical layer such as a bleaching layer, as described with reference to Fig. 16, so that also the servo-tracks can be narrower and the possible address and information areas can be smaller than has hitherto been ; conventional.
A record carrier with a ser~o-track structure is provided with a layer of inscribable material and, according to the invention also with a non-linear layer. Fig.17 shows a portion of such a record carrier in a ra~ial cross-section. The servo~tracks are denoted by 3 and the intermediate lands are denoted by 4. The reference numeral 42 . . .

~322'791 denotes the inscribable layer and 43 denotes the non-linear layer. This layer is not only used for reading with superresolution but also for forming information areas during writing, which areas may be considerably sma~ler than t.he write spot S. These information areas can be written in the servo-trac~s 3 as is shown in Fig. 17, but also in the intermediate lands 4. Moreover, the servo-tracks may be both grooves in the substrate surface and ridges on this surface.
The once inscribable and non-erasable layer 42 may be an ablative layer, for example a reflecting layer in which holes whicl1 do not reflect or reflect to a lesser extent can be formed by means of a write beam.
The layer 42 may alternatively comprise a phase change material in which a write beam locally establishes a transition from the amorphous to the crystalline phase, or conversely and hence a change of the refractive index (or reflection coefficient) so that the written information areas are distinguished from their ambience by a different transmission or reflection.
The phase r.hange materia~ is also suitable for writing and erasing information several times. The information which is written with a xadiation beam of high intensity is then read with a radiation beam of d lower intensity than that of the write beam, whilst erasing is effected at an intensity between the intensity of the w.rite beam and that of the read bean~. In practice the write, read and erasing functions can be performed by one beam which can be switched between three ~5 different intensity levels.
The inscribable infnrmation layer comprising a phase change material can be combined witl1 a non-linear layer also comprising a phase change material. However, it is alternatively possible that the record carrier comprises only one phase change layer whose properties are used both for writing the information and for generating a non-linear optical effect during reading.
Fig. 18 shows another emhodiment of a record carrier which can be inscribed and erased several times. The inscribable layer 44 com~rises a double layer of an expansion layer 45 and the retention layer 46 in which an information area in the form of an unevenness 47 can be written by means of a laser beam having a first wavelength. The information can be read or erased by means of a beam having a low ..... . .
.. .. .. .

- ~3227~

intensity and erased by means of d beam having a second wavelength.
With a suitable choice of the parameters of the expansion layer this layer may not only be used as an information layer but also as a layer for creatinq a non-linear optical effect. When reading the information areas in the form Or unevennesses in the expansion layer, d temporary small variation is provided in such an unevenness by means of the read beam, which variation disappears at a later stage so that the uneveness regains its origina]. shape and the information is maintained.
The mateLial of the said expansion layer which is a rubber-like material may not only be used as an inscribable and erasable material but also as a material for the information layer in a record carrier having a permanent information structure. A first possibility, as shown in Fig. 19, is for the information to be laid down in information pits 5 which are filled with the said rubber-like material ~5 48. When reading this information structure, the material in the pits is temporarily deformed (49) so that a discontinuity within the radiation spot S is created and a non-linear effect occurs.
It is also possible to provide a permanent information struc~ure in a rubber-like layer as is shown in Fig. 20. ~n this Figure the rubber-~.ike information layer is denoted by the reference numeral 50 and the information areas are denoted by the reference numeral 5. ~
second rubber-like layer 51 which deforms more easily than the layer 50 may be provided on the information layer. When reading the information, the read radiation temporarily deforms the layer ~1 at the location (52) of the scanning spot ~ so that a non-linear optical effect is obtained again.
Each of the inscribable information layers described so far: ti1e ablative layer, the phase chan~e layer and the expansion layer can be combined with each of the said non-linear layers: a layer having a changing transmission, a phase change layer or an expansion layer. The non-linear layers may be provided both in front of and behind the information layer and may form part of a pack of layers in the record carrier, ~hich pack forms a resonance structure.
Another embodiment of an inscribab].e and erasable record carriex is the magneto-optical record carrier comprising a substrate, a previously provided servo-track structure and a magneto-optical layer.
Such a record carrier and a write and a read apparatus for this carrier, .

- 132279~

described in Philips Technical Review, Vol. 42, No. 2, August 1985, pages 37-47 are shown diagrammatically in Fi~. 21.
The servo-tracks of tlle record carrier 1 are denoted by the reference numeral 3 . The reference numeral 7 denotes the substrate and 55 denotes the magneto-optical layer. A radiation beam b coming from a source 10, for example a semiconductor diode laser is converted by a collimdtor lens 12 into a parallel beam and focused by the objective system 11 to a diffraction-limited scanning spot S in the magneto-optical layer 55. The irradidted area of this layer is present in d magnetic field which is generated by a magnet coil 60. The magneto-optical layer is premagnetised in a direction perpendicular to the surface of the Iecord carrier.
During writing the radiation beam is intensity modulated in accordance with the information to be written. When moving the record carrier and the radiation spot S relati~e to each other, the magneto-optical layer is heated at certain positions in such a way that the direction of magnetisation is reversed in the direction of the external magnetic field which is opposed to the direction of premagnetisation.
When reading these written information areas in the form of magnetic domains, the record carrier is exposed wlth a continuous beam having a lower intensity than the write beam. The magnetic domains cause a small rotation of the polarisation direction of the read beam.
This rotation, ~ ~ dependent on the sign of the local magnetisation, is referred to as the Kerr rotation. The beam reflected by the record carrier and having a modulated polarisation direction, is directed towards the detection brdnch, for example by means of a beam-splitting prism 61. This branch comprises a ~/2 plate 62 whose principal axis extends at an angle of 22.5 to the original polaxisation direction of this beam. Consequently the polarisation plane is rotated through 45 with a modulation of + ~k A polarisation-sensitive beam splitter 63 splits the beam into two mutually perpendicularly polarised sub-beams which are received via the lenses 64 and 65 by the detectors 66 and 67. The angular modulations ~ ~k are then converted int.o amplitude modulations on the detectors. Since their phases are opposed, the difference signal is an accurate read signal.
The portion of the beam reflected by the record carrier and passed by the prism 61 may be directed by a further beam splitter :: .
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68, such as a semi-transparent mirror, to a further detection branch 69 in which tracking errors and focusing errors can be detected both during writing and during reading.
To erase the record carrier, the e~ternal magnetic field S is reversed over so thdt i.t acquires the same direction as the premagnetisation of the magneto-optica~ layer 55 and the record carrier is exposed with a continuous beam. Consequently the written domai.ns regain their original direction of magnetisation.
For further particuldrs of the record carrier and the detection methods and apparatuses therefor reference is made to ''PhiliE~sl Technical Review", Vol. 42, No. 2, pages 37-47.
The invention advantageously utilises the fact that the for said Rerr rotation is greatly dependent on the temperature for certain magneto-optical materials such as a gadolini~lm terbium-iron-cohalt compound. Due to the energy distribution within the diffraction-limited scanni.ng spot a portion of the scanning spot will experience a different Kerr rotation than the rest of the spot. The result is that a discrimination witl1in the scanning spot is established again.
Magneto-optical reading may not only he based on Kerr rotation but a]so on a variation of tl1e ellipticity of the read beam polarisation. Thi.s variation is also temperature dependent for certain magneto-optical materials so that the inv~ention can also be used in magneto-optical record carriers using the variations in ellipticity of the polarisation caused by the magnetic domains.
Use of the invention enables magnetic domains to ~e read which are smaller than has hitherto heen conventional. ~hese small magnetic domains can be obtained by means of a special way of writing, wh.ich is illustrated in Fig. 22. It is assumed that the write spot S
moves to the riqht at a speed V relative to the track portion 3 to be 30 written of the record carrier and that at the instant to the centre :5m vf the spot is present at point A. The magnetic field is then directed, for example, forwards, perp~ndicular to the plane of the dra~ y and the entire circular surface under the spot S is magnetised in that dir~ction. Aft.er the centre of the radia~ion spot has moved to ~, the magnetic field is reversed. Then the circular area present under the spot S acquires an opposite direction of ~agnetisation. As it were, a portion of the wxitten domain with centre A is erased thereby. The :
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result is that the remainin~ domain only covers the shaded area D. The length of this domain is determined by V ~ t in whi.ch.t is the period during which a ma~3netic field has been present in d given direction. The written domains are thus considerably smaller t.han the diameter of the . 5 write spot.
~ ne of the other above-mentioned non-linear layers can also be provided in a magneto-optical record carrier so that the non-linear effect of the magneto-optical layer is increased, resulting in both writiny and reading of this record carrier being improved. Fig. 23 shows an example of such a record carrier. In this Figure the reference numeral 7 denotes a transparent substrate on which a servo-track structure denoted by the tracks 3 is provided. This structure is coated with a dielectric layer 70 of, for example Aluminium Nitride. The magneto-optical layer 55 is present on this layer. The layer 55 is followed by a second dielectric layer 71 also comprising, for example Aluminium Nitride. This layer carries a reflecting non-linear optical layer, for examp]e a phase change layer 31.
It is of course alternatively possible to use only the non-linear effect of an extra layer such as the layer 31 in Fig. 23 in a magneto-optical record carrier in which the magneto-optical material is not so much dependent of temperature.
The signal-to-noise ratio of the signal read from a magneto-optical record carrier can be increased by increasing the intensity of the read beam. Such an intensity increase is also favourable when writing the magnetic domains.
The record carriers according to the invention can be read and written with known scanning apparatuses in which the detection aperture is in principle equal to the aperture of the objective with which the scanning spot is foxmed. According to a further aspect of the invention the detection aperture is considerably larger than the aperture of the objective. The superresolution effect can tllen be utilised to an optimum extent.
For an apparatus intended for scanning a record carrier in transmission the detection aperture can be increased without any problem hecause the part of the optical system which is present behind the record carrier on the side of the detect.or need not have any imaging quality and only needs to collect radiation.

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When scanning a refle~ting record carrier the reflected radiation can only reach the detector via the objective which must form the diff~action-limitPd rddiation spot and wl1ich must have a high optical quality~ Accordint3 to the present invention a larger objective 5 75 can be used whose central portion 76 is well corrected and whose peripheral portion has a lesser quality, as is shown in Fig. 24. The extra peripheral portion 77 ca~ collect the radiation diffracted undex larger angles and in which the non-linear effect is prominent. The beam b supplied by the source 10 and collimated by the lens 12 only fills the central portion 76 of the objective 75 so that a high-quality scanning spot S is formed on the information plane 2.
The objective 75 in Fig. 24 may be a sing]e aspherical lens element or a holograpllic lens or a lens having a radially varying refractive index.
It is of course alternatively possible to arrange an extra annu].ar lens around a well-corrected conventional objective.
The part of the radiation reflected by the record carrier and passing through the central portion 76 of the objective 75 and the part of this radiation which is collected by the annular portion 77 can be received by one detector 17.
Under circumstances it may be advantageous to detect these radiation parts separately. To this end a second beam splitter 78, for examp]e in the form of a plate having a r.adiation transmitting central portion 79 surrounded by a reflecting portion 80 may be arranged in the radiation path behind the first beam splitter 16. Now only the radiation passi.ng through the central portion 76 of the objective 75 reaches the original detector 17, whilst the radiation passing through the annular lens portion 77 is detected by a separate, for example annul~r detector 81. If the non-linear ef.fect introduces d phase which is opposed to that of the linear e~fect so that, as shown in Fig. 2, the modulation transfer function woul.d have a local minimum, this arrangement provides the possibility of compensdting for this minimum by subtracting the detector signals from each other instead of adding them together. Moreover, the sup~rresolution signal can then be detected separately and the apparatus is also suitab].e for scanning conventional, linear record carriers.

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Claims (19)

1. A method of optically scanning an information plane by means of a scanning beam which is focused to a scanning spot on the information plane, said scanning spot and the carrier of the information plane being moved relative to each other so that the scanning spot scans the information plane, characterized in that use is made of a principally diffraction-limited scanning spot in combination with a layer of non-linear optical material in which the radiation of the scanning beam establishes an optical effect which results in a discrimination within the scanning spot so that the effective scanning spot is considerably smaller than the original scanning spot.

2. An optical record carrier suitable to be scanned by means of the method as claimed in Claim 1, comprising an information plane, characterized by a layer of non-linear optical material which is located within the depth of focus of the information plane.

3. An optical record carrier as claimed in Claim 2, characterized in that the information plane is provided with a permanent information structure which is constituted by information areas being arranged in information, said areas alternating in the track direction with intermediate areas and being optically distinguished therefrom.

4. An optical record carrier as claimed in Claim 2, suitable to be read and being inscribable once, characterized in that the information plane has a permanent servo-track structure for positioning the scanning spot in the information plane.

5. An optical record carrier as claimed in Claim 2, suitable to be read and being inscribable and erasable several times, characterized in that the information plane has a permanent servo-track structure for positioning the scanning spot in the information plane.

6. A record carrier as claimed in Claim 2 intended as a master for initially and optically inscribing a track-shaped structure and comprising a substrate and an information layer, characterized in that the information layer is an inscribable layer which is free from information.

7. A record carrier as claimed in Claim 2, 3, 4, 5 or 6, characterized in that the non-linear optical material is a material whose transmission changes with a changing intensity of the incident radiation.

8. A record carrier as claimed in Claim 7, characterized in that the material having a transmission which changes under the influence of the intensity is a saturating dye.

9. A record carrier as claimed in Claim 7, characterized in that the material having a transmission which changes under the influence of the intensity is a bistable optical material.

10. A record carrier as claimed in Claim 3, 4 or 5, characterized in that the non-linear optical material is a material whose refractive index changes when changing the intensity of the incident radiation.

11. A record carrier as claimed in Claim 10 in which the non-linear optical material is a phase change material, characterized in that the layer of non-linear optical material is also the information layer.

12. A record carrier as claimed in Claim 3, 4 or 5, characterized in that the non-linear optical material is a synthetic material having a relatively high thermal coefficient of expansion.

13. A record carrier as claimed in Claim 5 having an information layer comprising a magneto-optical material in which magnetic domains can be written by means of a magnetic field and a radiation beam, which domains can be read on the basis of a change of polarisation of a scanning beam, characterized in that the non-linear layer is constituted by a magneto-optical information layer whose polarisation change depends on the intensity of the incident radiation.

14. A record carrier as claimed in Claim 13, characterized in that a layer of non-linear optical material is provided within the depth of focus of the magneto-optical layer.

15. A record carrier as claimed in Claim 2,3,4,5 or 6, characterized in that the non-linear optical layer forms part of a pack of layers constituting a resonance structure for the scanning radiation.

16. A record carrier as claimed in Claim 15, characterized in that a plurality of extra layers is provided which, combined with the non-linear layer, constitute the resonance structure.

17. A scanning apparatus suitable for performing the method as claimed in Claim 1 and comprising a radiation source, an objective system for focusing a radiation beam from the radiation source to a diffraction-limited scanning spot on the information plane of a record carrier and a radiation-sensitive detector for converting radiation from the information plane into an electric signal, characterized in that the detection aperture is larger than the aperture of the scanning beam incident on the record carrier.

18. A scanning apparatus as claimed in Claim 17, intended for scanning d reflecting record carrier, characterized in that the objective system comprises a round central portion which is sufficiently corrected and which has a sufficient optical quality for forming the diffraction-limited radiation spot, and a contiguous annular portion which can receive radiation from the record carrier.

19. A scanning apparatus as claimed in Claim 18, characterized in that an extra detector is provided for receiving the radiation passing through the annular lens portion.