SYSTEM FOR SCANNING AN INFORMATION CARRIER WITH A PLURALITY OF

LIGHT SPOTS

FIELD OF THE INVENTION

The invention relates a system for scanning an information carrier with a plurality of light spots.

The invention has applications in the field of optical storage and microscopy.

BACKGROUND OF THE INVENTION

The use of optical storage solutions is nowadays widespread for content distribution, for example in storage systems based on the DVD (Digital Versatile Disc) standards. Optical storage has a big advantage over hard-disc and solid-state storage in that the information carriers are easy and cheap to replicate.

However, due to the large amount of moving elements in the drives, known applications using optical storage solutions are not robust to shocks when performing read/write operations, considering the required stability of said moving elements during such operations. As a consequence, optical storage solutions cannot easily and efficiently be used in applications which are subject to shocks, such as portable devices.

New optical storage solutions have thus been developed. These solutions combine the advantages of optical storage in that a cheap and removable information carrier is used, and the advantages of solid-state storage in that the information carrier is not moving and that its reading requires a limited number of moving elements.

A system according to such a new optical storage solution aims at reading data stored on an information carrier. The information carrier is intended to store binary data organized according to an array, as in a data matrix. If the information carrier is intended to be read in transmission, the states of binary data stored on the information carrier are represented by transparent areas and non-transparent areas (i.e. light-absorbing). Alternatively, if the

information carrier is intended to be read in reflection, the states of binary data stored on the information carrier are represented by non-reflective areas (i.e. light-absorbing) and reflective areas. The areas are marked in a material such as glass, plastic or a material having magnetic properties. This known comprises: - an optical element for generating an array of light spots from an input light beam said array of light spots being intended to scan said information carrier, a detector for detecting said array of output light beams generated by said information carrier.

In a first embodiment depicted in Fig.l, a known system for reading data stored on an information carrier 101 comprises an optical element 102 for generating an array of light spots 103 from an input light beam 104, said array of light spots 103 being intended to scan the information carrier 101.

The optical element 102 corresponds to a two-dimensional array of micro-lenses to the input of which the coherent input light beam 104 is applied. The array of micro-lenses 102 is placed parallel and distant from the information carrier 101 so that light spots are focussed on the information carrier. The numerical aperture and quality of the micro-lenses determines the size of the light spots. For example, a two-dimensional array of micro- lenses 102 having a numerical aperture which equals 0.3 can be used. The input light beam

104 can be realized by a waveguide (not represented) for expanding an input laser beam, or by a two-dimensional array of coupled micro lasers.

The light spots are applied on transparent or non-transparent areas of the information carrier 101. If a light spot is applied on a non-transparent area, no output light beam is generated in response by the information carrier. If a light spot is applied on a transparent area, an output light beam is generated in response by the information carrier, said output light beam being detected by the detector 105. The detector 105 is thus used for detecting the binary value of the data of the area to which the optical spot is applied.

The detector 105 is advantageously made of an array of CMOS or CCD pixels. For example, one pixel of the detector is placed opposite an elementary data area containing

one data (i.e. one bit) of the information carrier. In that case, one pixel of the detector is intended to detect one data of the information carrier.

Advantageously, an array of micro-lenses (not represented) is placed between the information carrier 101 and the detector 105 for focusing the output light beams generated by the information carrier on the detector, for improving the detection of the data.

There are, however, more efficient ways of generating a two-dimensional array of light spots, for example, aperture arrays and binary phase structures, and the present invention is not intended to be limited in this regard.

In a second embodiment depicted in Fig.2, a known system for reading data stored on an information carrier 201 comprises an optical element 202 for generating an array of light spots 203 from an input light beam 204, said array of light spots 203 being intended to scan the information carrier 201.

The optical element 202 corresponds to a two-dimensional array of apertures to the input of which the coherent input light beam 204 is applied. The apertures correspond for example to circular holes having a diameter of lμm or much smaller. The input light beam 204 can be realized by a waveguide (not represented) for expanding an input laser beam, or by a two-dimensional array of coupled micro lasers.

The light spots are applied to transparent or non-transparent areas of the information carrier 201. If a light spot is applied to a non-transparent area, no output light beam is generated in response by the information carrier. If a light spot is applied to a transparent area, an output light beam is generated in response by the information carrier, said output light beam being detected by the detector 205. Similarly to the first embodiment depicted in Fig.l, the detector 205 is thus used for detecting the binary value of the data of the area on which the optical spot is applied.

The detector 205 is advantageously made of an array of CMOS or CCD pixels. For example, one pixel of the detector is placed opposite an elementary data area containing a

data of the information carrier. In that case, one pixel of the detector is intended to detect one data of the information carrier.

Advantageously, an array of micro-lenses (not represented) is placed between the information carrier 201 and the detector 205 for focusing the output light beams generated by the information carrier on the detector, improving the detection of the data.

The array of light spots 203 is generated by the array of apertures 202 in exploiting the Talbot effect which is a diffraction phenomenon working as follows. When a coherent light beam, such as the input light beam 204, is applied to an object having a periodic diffractive structure (thus forming light emitters), such as the array of apertures 202, the diffracted lights recombine into identical images of the emitters at a plane located at a predictable distance zθ from the diffracting structure. This distance zθ is known as the Talbot distance. The Talbot distance zθ is given by the relation zθ = 2.n.d ² /λ, where d is the periodic spacing of the light emitters, λ is the wavelength of the input light beam, and n is the refractive index of the propagation space. More generally, re-imaging takes place at other distances z(m) spaced further from the emitters and which are a multiple of the Talbot distance z such that z(m) = 2.n.m.d ² /λ, where m is an integer. Such a re-imaging also takes place for m = ¹ A + an integer, but here the image is shifted over half a period. The re- imaging also takes place for m = ¹ A + an integer and for m = ³ A + an integer, but the image has a doubled frequency which means that the period of the light spots is halved with respect to that of the array of apertures.

Exploiting the Talbot effect allows to generate an array of light spots of high quality at a relatively large distance from the array of apertures 202 (a few hundreds of μm, expressed by z(m)), without the need for optical lenses. This allows to insert for example a cover layer between the array of aperture 202 and the information carrier 201 to prevent the latter from contamination (e.g. dust, finger prints....). Moreover, this facilitates the implementation and allows to increase in a cost-effective manner, compared to the use of an array of micro-lenses, the density of light spots which are applied to the information carrier.

Fig.3 depicts a detailed view of the known system. It depicts a detector 305 which is intended to detect data from output light beams generated by the information carrier 301. The detector comprises pixels referred to as 302-303-304, the number of pixels shown being limited to facilitate the understanding. In particular, pixel 302 is intended to detect data stored on the data area 306 of the information carrier, pixel 303 is intended to detect data stored on the data area 307, and pixel 304 is intended to detect data stored on the data area 308. Each data area (also called macro-cell) comprises a set of elementary data. For example, data area 306 comprises binary data referred to as 306a-306b-306c-306d.

In this embodiment, one pixel of the detector is intended to detect a set of data, each elementary data among this set of data being successively read by a single light spot generated either by the array of micro-lenses 102 depicted in Fig.l, or by the array of apertures depicted in Fig.2, or by a binary phase structure. This way of reading data on the information carrier is called macro-cell scanning in the following.

Fig.4 which is based on Fig.3, illustrates by a non-limitative example the macro-cell scanning of an information carrier 401.

Data stored on the information carrier 401 have two states indicated either by a black area

(i.e. non transparent) or white area (i.e. transparent). For example, a black area corresponds to a "0" binary state while a white area corresponds to a "1" binary state.

When a pixel of the detector 405 is illuminated by an output light beam generated by the information carrier 401, the pixel is represented by a white area. In that case, the pixel delivers an electric output signal (not represented) having a first state. On the contrary, when a pixel of the detector 405 does not receive any output light beam from the information carrier, the pixel is represented by a cross-hatched area. In that case, the pixel delivers an electric output signal (not represented) having a second state.

In this example, each set of data comprises four elementary data, and a single light spot is applied simultaneously to each set of data. The scanning of the information carrier 401 by

the light spots 403 is performed for example from left to right, with an incremental lateral displacement which equals the distance between two elementary data.

In position A, all the light spots are applied to non-transparent areas so that all pixels of the detector are in the second state.

In position B, after displacement of the light spots to the right, the light spot to the left is applied to a transparent area so that the corresponding pixel is in the first state, while the two other light spots are applied to non-transparent areas so that the two corresponding pixels of the detector are in the second state.

In position C, after displacement of the light spots to the right, the light spot to the left is applied to a non-transparent area so that the corresponding pixel is in the second state, while the two other light spots are applied to transparent areas so that the two corresponding pixels of the detector are in the first state.

In position D, after displacement of the light spots to the right, the central light spot is applied to a non-transparent area so that the corresponding pixel is in the second state, while the two other light spots are applied to transparent areas so that the two corresponding pixels of the detector are in the first state.

The scanning of the information carrier 401 is complete when the light spots have been applied to all data of a set of data facing a pixel of the detector. It implies a two- dimensional scanning of the information carrier. Elementary data which compose a set of data opposite a pixel of the detector are read successively by a single light spot.

Fig.5 depicts a three-dimensional view of the system as depicted in Fig.2. It comprises an array of apertures 502 for generating an array of light spots applied to the information carrier 501. Each light spot is applied and scanned over a two-dimensional set of data of the information carrier 501 (represented by bold squares). In response to this light spot, the information carrier generates (or not, if the light spot is applied to a non-transparent area) an output light beam in response, which is detected by the pixel of the detector 503

opposite the set of data which is scanned. The scanning of the information carrier 501 is performed in displacing the array of apertures 502 along the x and y axes. The array of apertures 502, the information carrier 501 and the detector 503 are stacked in parallel planes.

It is noted that the three-dimensional view of the system as depicted in Fig.l would be the same as the one depicted in Fig.5 in replacing the array of apertures 502 by the array of micro-lenses 102.

The scanning of the information carrier by the array of light spots is done in a plane parallel to the information carrier. A scanning device provides translational movement of the light spots in the two directions x and y for scanning all the surface of the information carrier.

Fig.6 represents a top-view of an information carrier as depicted in Fig.5. This information carrier comprises a plurality of square adjacent macro-cells (MCl, MC2, MC3...), each macro-cell comprising a set of elementary data areas (EDAl, ED A2,....). In this example, each macro-cell comprises 16 elementary data areas and is intended to be read by a single circular light spot (represented by black circles).

The data recovery is done in a massively parallel manner concurrently with the scanning of the probe array. Presently, it is envisioned that the bit recovery is done in a massively parallel fashion concurrently with the scanning of the probe array. When the probe array is accurately aligned with the information carrier, a "page" of data, corresponding to a certain position within a macro-cell, is visible on the detector. One page of data contains in the order of a million bits with a detector comprising an array of 1000*1000 pixels. This page- by-page detection method resembles the way in which bit detection takes place in traditional optical storage, the difference being that a multitude of light probes are operating in parallel instead of a single light probe.

Page-by-page detection has the benefit that a very large data rate is possible even if the speed with which the probes can be scanned is low. For instance, supposing that the probes

can be moved from one page position to the next in 0.1 seconds, the system would still have a potential bit rate in the order of 10 Mega bits per second.

Fig.7 represents a detector comprising an array of pixels (adjacent black squares xl to xl6, and yl to ylO) illuminated by an array of output light beams (black circles). This example illustrates the ideal case where the information carrier (from which output light beams are generated) and detector are perfectly aligned since each output light beam illuminates a single pixel of the detector. The addressing of individual pixels is done in a straightforward basis : left to right, then line-by-line.

However, in most of cases, a misalignment will occur between the output light beams generated by the information carrier and the detector. For example, the position accuracy of the set (information carrier + array of light spots generating means) is impaired by removability and deformation (stretch/shrink) of the information carrier.

Fig.8 represents a detector comprising an array of pixels (adjacent black squares xl to xl6, and yl to ylO) illuminated by an array of output light beams (black circles). This example illustrates the case where the information carrier (from which output light beams are generated) and detector are not perfectly aligned (e.g. x-y offset and rotation). The misalignment in this case corresponds to a rotation having an angle AO. From this figure, it is apparent that some individual projected output light beams may be situated on the edge of two consecutive pixels of the detector (e.g. output light beam OLBl) or even on the crossing of four pixels (e.g. output light beam OLB2), resulting in some errors during the reading step.

In order to partially solve the misalignment problem outlined above, a method of recovering data from an information carrier having data stored according to data pages is known, wherein each data page is formed by a set of data being spatially distant from each other, the method comprising the steps of: reading at least one data page so as to generate at least one read data page,

storing said at least one read data page in a storage memory, recovering data from said at least one read data page stored in said storage memory.

This solution allows in a first step to shift the problem caused by optical and mechanical constraints to the digital processing domain, then in a second step to recover data stored on the information carrier by performing dedicated data recovering algorithms. Since the data pages are stored in a memory, such data recovering algorithms can exploit the redundancy and/or correlation between adjacent data. Although no more optical and mechanical constraints are thus required (it may be envisaged that bit detection can be facilitated by non-mechanical realignment, or that no mechanical alignment be performed at all), bit recovery is achieved by means of too expensive signal processing means not suitable for consumer products.

OBJECT AND SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a cost-effective system for scanning an information carrier with a plurality of light spots.

To this end, there is provided a system for scanning an information carrier ,said system comprising: means for generating an array of light spots being intended to be applied to said information carrier, a detector comprising a plurality of pixels for receiving output light beams generated by said information carrier in response of said light spots, said detector having a pixel density higher than that of said array of light spots, an address decoder to address said plurality of pixels from a misalignment measure between said detector and said output light beams.

This system not only allows to compensate for the misalignment between said detector and said output light beams, but also implies cost-effective means since this compensation is done by address-shifting in the pixel addressing.

These and other aspects of the present invention will be apparent from, and elucidated with reference to the embodiments described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way of examples only and with reference to the accompanying drawings, in which:

Fig.1 depicts a first information carrier reading system;

Fig.2 depicts a second information carrier reading system;

Fig.3 depicts a detailed view of components dedicated to macro-cell scanning used in the systems of Figures 1 and 2;

Fig.4 illustrates the principle of macro-cell scanning;

Fig.5 depicts a three dimensional view of the system of Fig.1;

Fig.6 depicts an information carrier intended to be read by a plurality of light spots;

Fig.7 represents a detector comprising an array of pixels illuminated by an array of output light beams, without misalignment;

Fig.8 represents a detector comprising an array of pixels illuminated by an array of output light beams, with misalignment;

Fig.9 depicts a three-dimensional view of the system according to the invention with a detector comprising a high pixel density;

Fig.10 represents a detector according to the invention comprising an array of pixels illuminated by an array of output light beams, with misalignment;

Fig.11 illustrates various devices implementing the system according to the invention.

DETAILED DESCRIPTION OF THE INVENTION Fig.9 depicts a three-dimensional view of a system according to the invention. This system comprises an array of apertures 902 for generating an array of light spots applied to the information carrier 901. Each light spot is applied and scanned over a two-dimensional set of data of the information carrier 901 (represented by bold squares). In response to this light spot, the information carrier generates (or not, if the light spot is applied to a non- transparent area) an output light beam in response, which is detected by the pixel of the detector 903 opposite the set of data which is scanned. Contrary to the known system described in Fig.5, the detector in the system according to Fig.9 has a higher pixel density. Indeed, the density of pixels is higher than the density of light spots. The scanning of the information carrier 901 is performed in displacing the array of apertures 902 along the x and y axes. The array of apertures 902, the information carrier 901 and the detector 903 are stacked in parallel planes.

Fig.10 illustrates by an example a top view of a detector according to the invention having a high pixel density (adjacent black squares xl to x32, and yl to y20) by a factor of two in the x and y directions relative to the known system described with reference to Fig.5, resulting in the fact that a given output light beam is detected via the addition of four pixels of the detector.

Note that this increase in pixel density relative to the prior art is not restricted to two in x and y directions, neither is it necessary to use a square pixel topology, as will be apparent to a person skilled in the art. More accurate misalignment compensation can be achieved by using detector with higher density.

This higher pixel rate enables the use of misalignment information to compensate for this misalignment by changing the pixel-addressing scheme of the detector in order to resolve output light beams that otherwise are projected on pixel boundaries (see Fig.8), as explained in the following.

To read information on the information carrier, addressing of pixels and lines of the detector is done by means of x and y address decoders, adjacent pixels in the detector line being addressed by incrementing the x address, and adjacent lines being addressed by incrementing the y address.

The pixel addressing according to the invention is such that each pixel address is the sum of a base address that would be used in case of no misalignment between the output light beams generated by the information carrier and the detector, and an offset address determined from said misalignment.

Calculation of the base address : the x base address of the current addressed pixels is incremented (by two units in this example) for addressing an adjacent pixel. After a complete line is read, the y base address of the current addressed pixel is incremented (by two units in this example) for addressing a subsequent pixel.

The offset address is determined from the misalignment between the output light beams generated by the information carrier and the detector.

For example, if the misalignment corresponds to a rotation having a rotation angle AO as illustrated in Fig.10, the horizontal offset address is expressed as: Int [(I - cos AO) * D ] where cos is the cosine operator,

Int in the integer operator D is the distance between the pixel and the rotation center

(distance between x21 and x2 in this example) If the rotation angle is small, than the horizontal offset address is null.

Similarly, the vertical offset address is expressed as: Int [(sin AO) * D ] where sin is the sine operator,

Int in the integer operator

D is the distance between the pixel and the rotation center

(distance between x21 and x2 in this example)

For example, to read data carried out by output light beam OLB 1 , the base address of the four pixels are (x21, y3), (x22, y3), (x21, y4), (x22, y4). The horizontal offset address is null and the vertical offset address is one unit. As a result, the address of the four pixels to be addressed are (x21, y4), (x22, y4), (x21, y5), (x22, y5), as illustrated by bold square in Fig.10. It is apparent that all the pixels on which the output light beam OLBl falls, are now used to retrieve the corresponding data. The data is then retrieved from the value addition generated by the four addressed pixels of the detector.

For example, to read data carried out by output light beam OLB2, the base address of the four pixels are (x23, y7), (x24, y7), (x23, y8), (x24, y8). The horizontal offset address is one unit and the vertical offset address is one unit. As a result, the address of the four pixels to be addressed are (x22, y8), (x23, y8), (x22, y9), (x23, y9), as illustrated by bold square in Fig.10. It is apparent that all the pixels on which the output light beam OLB2 falls, are now used to retrieve the corresponding data. The data is then retrieved from the value addition generated by the four addressed pixels of the detector.

It is noted that the calculation of the offset address may also take into account various and additional misalignments, such as lateral shifts, or a local misalignment.

The misalignment can be calculated according to various methods. For example, the misalignment can be calculated via searching the positions of known data patterns on the detector, these data patterns having been previously recorded on the information carrier, and to compare these calculated positions with the theoretical known positions to derive the misalignment value.

As illustrated in Fig.11, the method according to the invention may advantageously be implemented in a reading apparatus RA (e.g. home player apparatus...), a portable device PD (e.g. portable digital assistant, portable computer, a game player unit...), or a mobile telephone MT. These apparatus and devices comprise an opening (OP) intended to receive an information carrier IC as previously described, and a system according to the invention for shifting light spots over said information carrier IC in view of data recovering.

The system in accordance with the invention may be used in a microscope. Microscopes with reasonable resolution are expensive, since an aberration-free objective lens with a reasonably large field of view and high enough numerical aperture is costly. Scanning microscopes solve this cost issue partly by having an objective lens with a very small field of view, and scanning the objective lens with respect to the sample to be measured (or vice-versa). The disadvantage of this single-spot scanning microscope is the fact that the whole sample has to be scanned, resulting in cumbersome mechanics. Multi-spot scanning microscopes solve this mechanical problem, since the sample does not have to be scanned over its full dimensions, the scanning range is limited to the pitch between two spots.

In a microscope in accordance with the invention, a sample is illuminated with the spots that are created by the probe array generating means, and a camera takes a picture of the illuminated sample. By scanning the spots over the sample and taking pictures at several positions, high-resolution data are gathered. A computer may combine all the measured data to a single high-resolution picture of the sample.

The focus distance can be controlled manually, by looking at a detail of the picture of the sample. It can also be performed automatically, as is done in a digital camera (finding the position in which the picture has the maximum contrast). Note that the focusing of the imaging system is not critical, only the position of the sample with respect to the probes is important and should be optimized.

A microscope in accordance with the invention consists of an illumination device, a probe array generator, a sample stage, optionally an imaging device (e.g. lens, fiber optic face plate, mirror), and a camera (e.g. CMOS, CCD). This system corresponds to the system of Fig. 9, wherein the information carrier (901) is a microscope slide on which a sample to be imaged may be placed, the microscope slide being deposited on a sample stage.

Light is generated in the illumination device, is focused into an array of foci by means of the probe array generator, it is transmitted (partly) through the sample to be measured, and the transmitted light is imaged onto the camera by the imaging system. The sample is

positioned in a sample stage, which can reproducibly move the sample in the focal plane of the foci and perpendicular to the sample. A position measurement system can be implemented into the stage, or it can be implemented in the system. In order to image the whole sample, the information carrier is scanned so that all areas of the sample are imaged by an individual probe.

Instead of a transmissive microscope as described above, a reflective microscope may be designed. In a reflective microscope in accordance with the invention, light that has passed through the sample is reflected by a reflecting surface of the microscope slide and then redirected to the camera by means of a beam splitter.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims. In the claims, any reference signs placed in parentheses shall not be construed as limiting the claims. The word "comprising" and "comprises", and the like, does not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The singular reference of an element does not exclude the plural reference of such elements and vice-versa. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In a device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.