WO2000038183A2

WO2000038183A2 - Optical scanning head for data storage disks

Info

Publication number: WO2000038183A2
Application number: PCT/DE1999/004117
Authority: WO
Inventors: Edgar Pawlowski
Original assignee: HEINRICH-HERTZ-INSTITUT FüR NACHRICHTENTECHNIK BERLIN GMBH
Priority date: 1998-12-22
Filing date: 1999-12-20
Publication date: 2000-06-29
Also published as: WO2000038183A3; DE19860563C2; DE19860563A1

Abstract

The storage capacity of a DVD is already higher than that of a conventional CD by a factor of 20. The invention provides three alternatives for further increasing storage capacity and increasing the reading out speed. All of the inventive solutions are based on the main idea of developing the known optical scanning head into a 'multi-spot DVD pick-up' with which it is possible to produce a number of focusing points exceeding the minimum number, for synchronously scanning a large number of tracks and layers. To this end, the optical scanning head (1) may have a multiplex objective (7) with a superimposed grating structure (8) made up of a number of individual grating structures (J1, J2, J3,...Ji). The optical scanning head may also have additional beam splitters in front of or behind the collimator or a number of simultaneously operated laser diodes of adjacent wavelengths. Combinations are also possible. With the inventive scanning heads, DVDs can be used wherever large quantities of data have to be scanned quickly.

Description

Optical scanning head for data storage disks

description

The invention relates to an optical scanning head for multi-layer data storage disks with a focus-adjustable optical beam path, in which at least one collimator, an objective lens, a beam splitter and a focusing arrangement are contained, with at least one laser diode at the beginning and a detector arrangement at the end of the beam path and with a grating structure integrated into the beam path for the diffractive generation of a number of focus points corresponding at least to the number of storage layers for simultaneous detection of different data storage locations.

The optical data storage disk is the storage medium of the future with the advantage of high performance at a relatively low cost. Compared to the conventional "Compact Disk" (CD), the "Digital Versatile Disk" (DVD) has a much higher storage capacity. This is achieved on the one hand by a higher storage density and on the other hand by a lower storage layer thickness. For CDs, the substrate thickness is 1.2 mm, for DVDs only 0.6 mm. Up to four storage layers can be present on a double-sided DVD (double layer, double-side), the maximum storage capacity (17 GB) can thus be 26 times the storage capacity of a conventional CD. This means, for example, that complete feature films can be saved in digital quality on a DVD.

The data is read in and read out via an optical pickup to generate a focal point at the data storage location.

In order to achieve user compatibility between CD and DVD with their different storage layer thicknesses, the pickup must have two different focus points at different depths, ie have two different focal lengths. Such "dual focus pickups" initially had two different objective lenses, which could be inserted alternately into the beam path of a single laser diode via a rotary actuator ("DVD / CD-compatible pickup heads with dual lens rotating actuator", Takahashi et al., Jpn.J .Appl.Phys. Vol.36 (1997) pp.467-473) or two successive, axially displaceable refractive lenses ("0.8-Numerical-Aperture Two-Element Objective Lens for the Optical Disk", Yamamoto et al ., JpnJ.Appl.Phys.Vol.36 (1997) pp.456-459) The next step was to develop a single refractive lens with a circumferential correction ring to create two focus areas on one objective lens (“A Compact Disc Compatible Digital Versatile Disc Pickup Using Annular Mask ", Lee et al., Jpn.J.Appl.Phys. Vol.36 (1997) pp.486-490).

As early as 1988, researchers from the "NEC" company presented a small and light optical pickup with holographic optical elements (HOE) ("Compact optical head using a holographic optical element for CD players", Kimura et al., Appl. Opt. Vol .27 (1988), No. 4, pp.668-671). This consists of four optical components: photodiode, HOE, imaging lens and detector array. The HOE, consisting of four different grating sectors, fulfills the tasks of beam splitting and the generation of focus errors and readjustment signals. This showed for the first time that with the help of "diffractive optical elements" (DOE) compact, light and inexpensive pickups for CDs with the necessary high optical requirements can be realized.

In the classic explanation, a hologram is a storage of the phase and amplitude information of two beams of coherent light, which are mutually correlated, in the form of a microscopic interference pattern. The reconstruction of a hologram can be interpreted as an imaging process: a hologram transforms a wave field due to the diffraction in the hologram structure into another wave field. It therefore works like a complicated lens and can be referred to as a "phase imaging element". The amplitude (light / dark) of the light is usually not taken into account here. The basic advantages of HOEs are: optically ideal-looking element even with large apertures, low weight, Multifunctionality, complicated imaging geometries, application on curved surfaces. With "Computer-Generated Holograms" (CGH) complicated, computer-simulated and calculated interference patterns can be written directly into light-sensitive material with a laser scanner.

The Fresnel zone plate can be referred to as a "hologram of a point". If it is a segmented zone plate, several focus points can be generated holographically. Segmented Fresnel zone plates with two or four focus points with the key functions of beam splitter and focusing element are described in the article "Segmented Fresnel zone lens elements with several primary foci ", PCM Galloway et al., Opt. Communications 131 (1996) pp.371-379. These are single lattice structures in the form of symmetrical circular segments that are distributed over the full circle of a Fresnel lens. Such superimposed, computer-generated zone lenses of the Fresnel type for generating multiple focus points can also be used, for example, in optical pickup heads. In this circular segment structure with relatively large individual segments, however, the aperture is limited and the superposition of the individual light beams is not very finely coherent, so that disruptive superlattice images arise.

In 1989, Matsushita presented a compact focus sensor using an astigmatic Fresnel zone lens, integrated as a DOE structure on a beam splitter cube ("Reflection micro-Fresnel lenses and their use in an integrated focus sensor", Shiono et al., Appl Vol. 28 (1989), No.15, pp.3434-3442), which made it possible to shorten the overall length and save an optical adjustment step _

the pickup for the rewritable magneto-optical disk. At the same time, in particular by the Matsushita company in particular, the so-called "phase change process" was developed as a competing process for a rewritable disk. This process, in which the molecular state by reading and reading the storage layer is changed using a much simpler pickup, great future prospects are currently forecast.

For the video disk with a significantly increased storage density, pickups with integrated DOEs with blazed grids in front have also been presented, in which several beams are used for detection adjustment (“Optical Pickup Using Blazed Holography Optical Element for Video

Disk Players ", Yoshida et al., Jpn.J.Appl.Phys. Vol.33 (1994) pp. 3947-3951).

For the multi-layer disc, Matsushita developed the use of DOEs for the dual focus head ("Dual Focus Optical Head for 0.6 mm and

1, 2 mm disks ", Komma et al., Opt. Rev. Vol.1, No.1 (1994), pp.27-29). Here, the 0.6 mm focus with diffracted light O.Order (transmission ) and the

1, 2 mm focus set with diffracted light 1st order (main refraction).

By using the different diffraction orders, despite the use of only one DOE, reading from a 0.6 mm deep and a 1.2 mm deep storage layer is made possible.

In the described pickups with two focus points generated by different diffraction, a separate flat hologram is assigned to the objective lens. From the publication "Dual Focus Optical Head with a Hologram-Integrated Lens", Komma et al., Jpn.J.Appl.Phys. Vol.36 (1997) pp.474-480, the blazed HOE is directly in the first, This means that aberration errors due to different disc thicknesses can be corrected without mechanical circuitry. The pickup structure can be made even simpler and more compact. The scanning head known from the article "Lens Design for Optical Head Compatible with Compact Disk and Digital Versatile Disk" by Tanaka et al., Jpn.J.Appl.Phys. Vol.73 (1998) pp.2179-2183 is an optical scanning head with a dual-focus objective lens in the beam path, in the aspherical top of which a hologram is integrated. With the refracted light of the first order, CDs can be read, with the unbroken light (zeroth order) DVDs. Scattered light is caused by refractions of higher ones This can be reduced by the hologram, which is designed as a special grating profile. The spaces (pitches) in the hologram are determined with the aid of a computer. A distance of 0.5 mm between the two focal points creates sufficiently large spots for quadrant detection Pitches are so narrow, however, that they can no longer be produced, complicated replacement structures for lattice profiles with diffractive and refract Iive proportions for creating a hybrid lens are therefore calculated. However, these lattice structures only serve to avoid scattered focal points in one layer. This arrangement also aims to create exactly two focus points in two different layers. The consequence of this is that only storage disks with a limited storage capacity and with a limited reading speed can be scanned.

In addition to the dual focus pickups with two different optical diffraction ranges, the essay "Dual Wavelength Optical Head for 0.6 mm and 1.2 mm Substrate Thicknesses", Katayama et al., Jpn.J.Appl.Phys. Vol. 36 (1997) pp.460-466, a pickup known which has two laser diodes with widely differing wavelengths (635 nm and 785 nm), a first and a second one using a holographic or a variable magnetic optics Alternatively, a second focal point is generated by zero or first order diffraction, and simultaneous operation of the two laser diodes is not provided. A pickup is described in the publication "Digital Video Disc / Compact Disc Compatible Pick-up With Liquid Crystal Shutter", Tsuchiya et al., Jpn.J.Appl.Phys. Vol.36 (1997) pp.481-485 The diameter of the laser beam can be changed by means of a liquid crystal diaphragm. This creates two different apertures on one objective lens and thus two focal points. With liquid crystal diaphragms, the transmittance can be changed locally by changing the applied voltage.

German published patent application DE 19636595 A1 describes a device for reading optical storage media in several tracks in different layers. For this purpose, several laser diodes are used at the same time, the emitted radiation of which is passed through a refractive lens. However, this shows a strongly wavelength-dependent behavior in its refractive effect. The laser diodes must therefore have a very different emission behavior in terms of wavelength, which is relatively difficult to implement because the tuning range of a normal laser diode is relatively narrow. Furthermore, a simultaneous read / write operation in different tracks and layers is not possible with the apparatus described, since the information is always acquired by an oscillation of the scanning beam between the individual tracks in a single layer.

In addition to this overview of the technological background to the present invention, the closest prior art from which the invention is based is disclosed in US Pat. No. 5,526,336 A. An optical pickup of the type mentioned at the outset is described here, which has a Fresnel and an objective lens which can be moved separately from one another for exact control of the focus point position. The Fresnel lens serves for the diffractive splitting of the incident light beam into a minimum number of focal points corresponding to the number of existing layers of a multi-layer data storage disk. This is done by using the different diffraction orders, especially the zeroth and the first; but higher diffraction orders should also be able to be used - at least theoretically. However, it should be noted here that the intensity of the deflected rays decreases sharply from the second diffraction order, so that such rays are generally regarded as undesirable, unusable scattered light. An arbitrary increase in the number of deflected rays is therefore not possible with a simple Fresnel lens. As a result, only storage disks with a limited storage capacity and with a limited reading speed can be scanned.

The multifocal objective lens known from US 5838496 A is a biconvex converging lens which has a Fresnel zone lens integrated into its more curved surface. The geometric design of the individual zones is chosen so that the color shifts of the diffracted light beam that normally occur when different diffraction orders are used are corrected. The various diffraction orders are also used again in this lens - and the disadvantages that occur are at least partially attempted to be compensated for - but the disadvantages described in connection with US Pat. No. 5,526,336 A remain.

The problem with the invention is therefore to be seen in developing an optical scanning head of the known type for data storage disks in such a way that it enables a significantly increased storage capacity on the data storage disk to be scanned and a significant increase in the scanning speed. The construction of such a pickup should continue to be very compact. The aim should be that the measures for further training are simple and that the means are inexpensive. The manufacture, handling and maintenance of the pickup according to the invention should be problem-free and largely free of errors. With the present invention, different forms of design of a scanning head of the structure mentioned at the outset are implemented for this problem, all of which are realized in the form of a “multi-spot DVD pickup” according to the invention with a number of focus points that can be generated that exceeds the required minimum number (one focus point per layer), described as equal approaches.

First of all, to solve the problem, the optical scanning head can be designed such that in order to generate a number of focus points that exceeds the minimum number, the objective lens integrates the integrated grating structure in the form of a complex superimposed grating structure from a number of different, curved individual grating structures that corresponds to the number of focus points to be generated individual segments, which are arranged in an irregular, statistically distributed manner in the overlay grid structure.

The basic idea behind this proposed solution is that by increasing the number of focus points in a same time unit, correspondingly more data can be scanned than in the previously described conventional scanning heads with a maximum of two focus points for scanning two tracks in different layers. The result is a significant increase in the achievable storage capacity of a DVD and the reading speed. The superimposition of different curved grating structures in a complex diffraction grating creates the possibility of parallel execution of the functions of the underlying individual components, so that targeted distributions of the incident light beam in the individual diffraction directions can be carried out to generate different focus points. The occurrence of each individual element only as a fraction of the respective basic element occurring in accordance with the total number of individual elements is sufficient for the transmission of the incident light beam to achieve the same diffraction effect as the complete basic element. The number of individual elements and thus the number of the focal points that can be generated are theoretically increased arbitrarily and only finds a limitation in the production of the overlay lattice structure. The irregular statistical distribution of the individual segments can also minimize superlattice images.

As the individual segments become smaller, an increasingly fine, coherent superposition of the light beams is achieved. The individual segments can have any geometric shapes. According to an advantageous embodiment of the optical scanning head according to the invention described first, however, it is particularly useful if the segments of the individual grating structures are geometrically, in particular square, delimited. The arrangement of geometric segments, for example in a hexagonal shape, in a corresponding superposition structure is simple and versatile. In particular, an arrangement of square segments in a "checkered" overlay structure is particularly easy to produce and is always easy to understand. Consideration of the arrangement of the segment shape, for example a circular segment shape with a centered arrangement in a circle, need not be taken into account in the case of square segments in addition, a preferred direction of light refraction avoided.

Another advantage of the superposed single lattice structures is that their optical properties can be selected and implemented separately. The multi-spot pickup according to the invention can be used for simultaneous scanning (reading and writing) of different data tracks and / or different data layers. According to a further embodiment of the invention, it is useful for simultaneous scanning of different data tracks if the focal lengths of the individual grating structures are all the same and their diffraction angles to the optical axis of the objective lens are different in order to generate a number of focus points on different tracks in a storage layer that exceeds the minimum number. Conventional CDs with only one storage layer can thus be read out much higher speed can be scanned. With different diffraction angles in one plane one speaks of an "off-axis beam path".

According to a further development of the invention, it is useful for simultaneous scanning of different data tracks and data layers if the focal lengths of the individual grating structures and their diffraction angles to the optical axis of the objective lens are different in order to generate a number of focus points on different tracks in different storage layers which exceeds the minimum number. With each individual lattice structure, a different focus point can be generated from a corresponding number of segments at a different depth (layer) and at a different diffraction angle (track). With such a configuration, a modern storage disk (DVD) can be optimally used in its maximum storage capacity and the readout speed can be maximized. All beams can be imaged on different photodetectors. The regulation of the individual focus points can be done simultaneously via a main actuator. The fine tuning of the relative distances between the focus points can be done relatively easily by varying the central light wavelength (tunable laser diode). In the case of a laser diode, the change in wavelength can be generated, for example, by varying the operating temperature or by rotating a grating when using an external resonator.

The implementation of the complex superimposed grating structure described from a large number of different, curved single grating structures in an optical element must be carried out under the conditions of simplicity in terms of construction and production technology and cost-effectiveness. According to a next embodiment of the scanning head according to the invention, it is therefore advantageous if the objective lens is designed as a multiplex objective with a multiplex grating applied to a converging lens as a superimposed grating structure. Such a grating, which can be produced by etching, which is applied to a flat surface of a converging lens, for example by gluing has the advantage that the transmittance is particularly high since higher order diffraction orders can be reduced.

According to another embodiment of the invention, the objective lens can, however, also be designed as a multiplex objective with a computer-generated hologram designed as a segmented Fresnel zone lens as a superimposed grating structure. The computer-generated hologram (CGH) has several advantages over the grating-lens combination described above. In this way, the CGH can be made more compact, lighter and even cheaper. Furthermore, the imaging properties are of higher quality since no optical aberrations occur. With the CGH, the focal length is inversely proportional to the wavelength of the incident light, so the focus point can be adjusted by varying the light wavelength. If it is also necessary, for example due to increased layer tolerances in DVD production, to adjust all individual focal lengths separately, then it makes sense to add several laser diodes of different wavelengths to such a CGH multiplex lens with an off-axis beam path and as a light source use. By varying the individual frequencies, the different focal lengths can also be individually adjusted. The off-axis beam path ensures that the signals from the different layers can also be mapped independently of one another. Fresnel zone lenses are the simplest holographic imaging elements. They can be used, for example, to transform a plane wave into a converging and a diverging wave. The hologram thus behaves like a converging lens and a diverging lens at the same time.

The grating elements described can be produced using the modern microstructuring techniques of VLSI technology and have proven their strength in various optical applications in recent years. Decisive and of great advantage for binary optics are the aspects that the lattice structures can be calculated with the aid of a computer, the mask production with a high-precision electron beam Writer can be done and further structuring is possible with the proven manufacturing techniques of microelectronics. Such surface relief structures are particularly suitable for later mass production of inexpensive and high-precision optical components using replication technology.

As an alternative solution to the problem described above, an optical scanning head of the type mentioned at the outset can also be characterized according to the invention in that an additional beam splitter, the position of which is arranged in front of or behind the collimator in order to generate a number of focal points on different tracks that exceeds the minimum number is changeable in the direction of the beam path, the first beam splitter is designed as a polarization beam splitter with a downstream polarization plate, the objective lens as a diffractive optical element and the collimator with a grating structure.

With such an arrangement, the storage capacity and reading speed of a DVD can also be increased considerably. The advantage of this alternative embodiment is a simplification of the pickup structure with a separation of the scanning function "reading / writing of several tracks in one layer". Beam splitters are used for reading / writing different tracks in a layer. These can be positioned in front of or behind the collimator By changing the position of the beam splitter in the direction of the beam path, the relative focal distance of all partial beams generated on a DVD can be varied, so with such a pickup design, separate geometric regulation of the read / write function of the pickup in one layer is possible. In this variant, the collimator arranged in the beam path and the focusing arrangement are provided with grating structures which serve to forward the light beam which has now already been split at the beginning of the beam path into individual partial beams in order to generate a plurality of focal points ht from a grid and a cylindrical lens containing lens, also a version as Fresnel zone lens is possible. The polarization beam splitter is a diffractive grating, the λ / 4 plate is used for further beam polarization. The objective lens is also designed as a diffractive optical element (DOE), for example in the form of a simple Fresnel lens. It is also possible to use a segmented Fresnel zone lens.

After a continuation of this solution according to the invention, the additional beam splitter can be designed as a simple transmission grating, as a diffractive and / or refractive deflection element or as an acousto-optical modulator. The diffractive deflection elements (DAE) can be realized in the form of computer-generated holograms and the refractive deflection elements (RAE) in the form of prismatic plates. According to a next embodiment, the deflection element can be controllable in its optical effect by means of a liquid crystal cell. By combining a liquid crystal cell with DAEs, the partial beams can be changed so that only one beam is transmitted with high intensity. This then has enough power (approx. 5 mW with a laser diode power between 7 mW and 10 mW) to be able to write (burn) data to the DVD disc. This element is then referred to as a diffractive controllable deflection element (DSAE). By combining a liquid crystal cell with RAEs, the spot distance can be controlled on a DVD. This element is then referred to as a refractive controllable deflection element (RSAE). The combination of DAEs and RAEs - also in controlled form S - is also possible.

The inventive variant described above enables the read / write function to be separated from several tracks in one layer with separate geometric control. Simplification of the structure with separate geometrical control for separating the read / write function from one track in each case in several layers is also achieved if an alternative solution to the problem for an optical one is achieved Scanning head of the type described at the outset for generating a number of focus points in different storage layers which exceeds the minimum number, a number of different tunable laser diodes of closely adjacent wavelengths corresponding to the number of focus points, which can be operated simultaneously, and the first beam splitter as a polarization beam splitter with a downstream one Polarizing plate, the objective lens as a diffractive optical element and the collimator are formed with a lattice structure. For the first time, several laser diodes are used simultaneously in an optical scanning head. A diffractive optical element in the form of a Fresnel zone lens (FZL) has the property that its focal length is inversely proportional to the wavelength. By using several wavelengths, which can be adjacent and not at a large distance as in the prior art, several data layers can therefore be scanned at the same time. The individual spots can be readjusted by fine-tuning the wavelengths if the layers are not completely homogeneous. The separation of the individual partial beams on the detectors is determined by the geometry of the laser and by the polarization beam splitter. The pole beam splitter as a diffractive grating also acts as a function of frequency, so that all beams are mapped individually onto different photodetectors depending on the wavelength. The regulation of the individual focus points can additionally be carried out simultaneously via a focusing actuator.

The two scanning heads described last have, as special advantages, a simplified structure and a separate geometric regulation of the focus point setting. This is achieved by separating the read-write functions in different tracks and in different layers. According to a next embodiment of the invention, an additional beam splitter is arranged behind each laser diode for the scanning head with a plurality of laser diodes to generate a number of focal points on each of a plurality of parallel tracks in different storage layers, the position of which in the direction of the Beam path is changeable. This combines the measures of the two alternative solutions and enables optimal use of the DVD with simultaneous and therefore very fast scanning of a large number of tracks in different storage layers. Nevertheless, the simplified structure is still preserved and the separate geometric control functions can be carried out.

Even with the combination of the functions, it is possible according to further embodiments of the invention that the additional beam splitter is designed as a simple transmission grating, as a diffractive and / or refractive deflection element or as an acousto-optical modulator and the optical element of the deflection element can be controlled by means of a liquid crystal cell. The associated effects and advantages have already been discussed above.

Forms of embodiment of the invention in their alternative forms and in details are explained in more detail below with reference to the schematic figures for their further understanding. It shows:

Figure 1 shows the beam path in a multi-spot according to the invention

DVD pickup with a multiplex lens,

FIG. 2 shows a multiplex objective as a grating-lens combination,

FIG. 3 shows a multiplex objective as a CGH lens,

FIG. 4 shows a multi-spot image of a multiplex objective in different tracks in one layer

FIG. 5 shows a multi-spot image of a multiplex objective in different tracks in different layers,

6 shows the beam path in an alternative according to the invention

Multi-spot DVD pickup with an additional beam splitter and Figure 7 shows the beam path in another alternative multi-spot DVD pickup according to the invention with several laser diodes and additional beam splitters.

FIG. 1 shows a possible beam path of an optical scanning head 1 in a multi-spot design with a multiplex objective. At the beginning of the beam path, a laser diode 2 is arranged, which in the selected exemplary embodiment emits coherent light with a wavelength of 650 nm. By means of a downstream beam former 3, an initially diverging, oval light beam B is converted into a diverging, round light beam BD for intensity centering. This diverging light beam BD then falls into a collimator 4, which transforms it into a linearly polarized, parallel light beam BP. An objective lens 5 with an integrated grating structure 6, which is designed as a multiplex objective 7, is arranged as the next element in the beam path.

The multiplex lens 7 is the centerpiece of the scanning head 1 designed as a multi-spot DVD pickup and splits the incident light beam BP into a plurality of partial beams B _{1 (} B ₂ , B ₃ Bj), all of which are separate

Lead focus point Pi, P ₂ , P ₃ j. The number of partial beams Bi, B ₂ ,

B ₃ , Bj depends on a grating structure 6 integrated in the multiplex objective 7. This is designed as a complex superposition grid structure 8, not shown here, which is developed by superposition from a number of individual grid structures corresponding to the number of split individual beams B ₍ B ₂ , B ₃ Bj). Further details are given in the explanations for FIGS. 2 and 3 refer to.

The split light beams Bi, B ₂ , B ₃ Bj still pass through a beam splitter 9, which acts as a semitransparent mirror, before they reach the surface of a data storage disc at the focal points P-ι, P ₂ , P ₃ , P 10, for example a DVD. At the focal points Pi, P ₂ , P ₃ , Pi, data can then be read in and read out via the scanning head 1. In the illustrated embodiment, the focus points Pi, P ₂ , P ₃ , Pj lie both in different tracks Ti, T ₂ , T _3ι Tj and in different layers L |, L ₂ , L ₃ Lj, so that the DVD 10 with a maximum Amount of data memory occupied and can be processed with a maximum scanning speed. The partial beams BRi, BR ₂ , BR _3> BRj reflected by the data pixels are guided by the beam splitter 9 onto a mirror 11, which is used for beam deflection to save space and for focusing for detection with a detector arrangement 12. The focus at the detection site can be readjusted with a downstream cylindrical lens 13. This also provides the controlled variable for a focusing actuator for the detection (not shown further here). The detector arrangement 12 consists of individual detectors Di, D ₂ , D ₃ Dj, which are assigned to the individual tracks T-ι, T ₂ , T _3ι Tj and layers Li, L ₂ , L ₃ , .... Lj. The individual detectors Di, D ₂ , D ₃ , Dj here consist of four quadrants with equal rights, which, by evaluating the respective signal levels (in the range of approx. 10 μW), precisely adjust the focus points P _{1 (} P ₂ , P ₃ , Pj and thus allow a highly precise adjustment of the scanning head 1 to the DVD 10, which may have manufacturing tolerances in its tracks and layers.

The generation of a large number of different focal points Pi, P ₂ , P ₃ , Pj is made possible in the scanning head 1 by using the multiplex objective 7, the construction of which is shown in detail in FIGS. 2 and 3. The multiplex objective 7 has a complex superimposed grating structure 20, which is composed of individual segments S _n . This individual

Segments Si, S ₂ , S ₃ , S _{n come} from different individual grating structures J-ι, J ₂ , J ₃ , .... J,. The multiplex objective 7 shown in FIG. 2 is designed as a converging lens 21 with an applied multiplex grating 22 as an overlay grating structure 20. The Multiplex grid 22 is a composite grid of individual grids J-ι, J ₂ ,

Y ₃ yy The first parameter for these grids is the lattice constant Ki, K ₂ ,

K ₃ , ... Ki for the achievable diffraction. The second parameter k-ι, k ₂ , k ₃ , .... kj is the position of the individual segments Sι, S ₂ , S ₃ , S _n in the individual grids Jj.

If, for example, ten individual grids Jj are used to generate ten different focus points Pj, one tenth of the segments S _n of each individual grille Ji is included in the multiplex grating 22. If these are additionally randomly and randomly arranged in the superimposed grid structure 20 via a random generator R, optimal maintenance of the parallel grid functions with maximum suppression of superlattice structures is ensured. Increasing the number of segments S _n further improves these properties.

The same applies to the multiplex objective 7 in FIG. 3. However, this is a segmented Fresnel zone lens 23, the superimposed grating structure 20 of which is designed as a computer-generated hologram CGH. The first parameter of the individual Fresnel zone lenses FZL-i, FZL ₂ , FZL ₃ FZLj forming the CGH lens 23 is the focal length f |, f ₂ , f ₃ , ... fj. The second parameter F _{1 (} F ₂ , F ₃ ,... Fj is again the position of the individual segments S, S ₂ , S ₃ ,... S _n . Their random distribution in the superimposed grid structure 20 is also determined via a random generator R. Such a CGH lens 23 has particularly favorable focusing properties with the same aperture of the individual Fresnel zone lenses FZLi,

In Figures 4 and 5 are different positioning options

Focus points P _1f P ₂ , P ₃ , Pj are shown. In Figure 4, all focus points Pi are

P ₂ , P ₃ Pj in a layer Lι, but in different traces Ti, T ₂ , T ₃ Tj. For this purpose, the focal lengths fi, f ₂ , f ₃ , ... fj of the individual grating structures Ji, J ₂ ,

J ₃ , .... Jj or FZLi, FZL ₂ , FZL ₃ , FZLj all the same, their diffraction angles to optical axis of the objective lens 7, however, different (off-axis). In figure

5 all focus points Pi, P ₂ , P ₃ , Pj are both in different

Layers Lι, L ₂ , L ₃ , ... Lj as well as in different tracks T-ι, T ₂ , T ₃ , .... Tj. For this purpose, both the focal lengths f |, f ₂ , f ₃ , ... fj of the single grating structures J _1f J ₂ , J ₃ Jj or FZLi, FZL ₂ , FZL ₃ , FZLj and their diffraction angle to the optical axis of the objective lens 7 are different ( off-axis). In the case of an on-axis arrangement (not shown), the tracks Ti, T ₂ , T ₃ Tj all lie in different layers L-ι, L ₂ , L ₃ , ... Lj directly above one another.

A scanning head 30 for separate scanning of a single layer Lj of a DVD 31 is shown in FIG. The essential difference from the previously described scanning head lies in the arrangement of an additional beam splitter 32 in a position I in front of a collimator 33 or in a position II behind the collimator 33. The additional beam splitter 32 causes the wavelength of a laser diode 34 to be divided λj emerging light beam B into a number of partial beams Bi, B ₂ , B ₃ , ... Bj corresponding to the selected grating structure. The focal points Pi, P ₂ , P ₃ , -Pi then lie in the layer Lj of the DVD 31. For this purpose, they first pass through a polarization beam splitter 35 with a downstream λ / 4 plate 36 and a simple objective lens 37 as a diffractive optical element (DOE, for example Fresnel lens). By changing the positioning of the additional beam splitter 32 in the direction of the beam path, the focus points Pj can be precisely adjusted. Both the collimator 33 and a focusing device 38 for a detector device 39 for the wavelength λj are designed with additional grating structures Jj.

In the selected embodiment, the additional beam splitter 32 is designed as a simple transmission grating. However, this can also be a refractive or diffractive deflection element (RAE, DAE) or an acousto-optical modulator (AOM). When using a Deflection elements can also be designed to be controllable in their optical effect via a liquid crystal cell (RSAE, DSAE).

Analogous to the scanning head 30 with a scanning option in one layer, a scanning head can be seen which can scan in different layers in one track each. As a further development of this, a scanning head 40 is shown in FIG. 7, which allows the possibility of synchronously scanning several parallel tracks Tj in several layers Lj. For the function of layer scanning, as a significant difference to the two preceding scanning heads 20 and 30, this has a large number of laser diodes LDi, LD ₂ , LD ₃ , .... LDj, whose wavelengths λ-i, λ ₂ , λ ₃ , .... λj are closely adjacent. Each light beam B _{n is} split into partial beams B _n ι, B _n2 , B _n3 ,... B _n j via an objective lens 41 designed as a DOE lens, for example in the form of a simple (or segmented) Fresnel lens. This has the property that its focal length is inversely proportional to the wavelength of the incident light. By using several wavelengths λi, λ ₂ , λ ₃ λj, several layers Lj can therefore be read out simultaneously.

By varying (λj ± Δ λj) each wavelength λi, λ ₂ , λ ₃ λj everyone can

Beam B _n ι, B _n2 , B _n3 , ... B _n j on the respective layer Lj can be readjusted if necessary.

With the scanning head 40 shown, several tracks Tj can now be scanned simultaneously in the different layers Lj. For this purpose, an additional beam splitter BS-ι, BS ₂ , BS ₃ , ... BSj is arranged behind each laser diode LDj, which each beam B _n in partial beams B _n ι, B _n 2, B _n3 , ... B _ni the same Wavelength λj splits, which are then diffracted in groups by the objective lens 41 depending on their wavelengths λj. The further measures required in connection with the additional beam splitters BS-i, BS ₂ , BS ₃ ,... BSj correspond to those in FIG. 6. A detector arrangement 42 in this scanning head 40 consists of one row of detectors per wavelength λj. The number of individual detectors Di, D ₂ , D ₃ , Dj per row corresponds to that of the additional ones

Beam splitter number of partial beams of the same wavelength λj.

Reference list

I. optical scanning head 2nd laser diode

3. Beamformer

4. Collimator

5. Objective lens

6. integrated grating structure 7. multiplex lens

8. Overlay lattice structure

9. Beam splitter

10. Data storage disk

I I. Mirror 12. Detector arrangement

13. Cylinder lens

20. Overlay lattice structure

21. converging lens

22. Multiplex grating 23. Segmented Fresnel zone lens

30. optical scanning head

31. DVD

32. additional beam splitter

33. collimator 34. laser diode

35. Polarization beam splitter

36. λ / 4 plate 37. Objective lens

38. Focusing device

39. Detector device

40. optical scanning head 41. objective lens

42. Detector arrangement

B light beam

B _n light beam

B _{1 f} B ₂ , B ₃ , Bj partial beam B _n ι, B _n2 , B _n3 , ... B _n i partial beam

BD diverging light beam

BP linearly polarized light beam

BRL BR ₂ , BR ₃ , BRi reflected partial beam

BSi, BS ₂ , BS ₃ , ... BSi additional beam splitter DL D ₂ , D ₃ Dj single detector fi, f ₂ , f ₃ , ... fι focal length

Fi, F ₂ , F ₃ , ... Fj position parameters

FZLi, FZL ₂ , FZL ₃ FZLj single Fresnel zone lens

Ji, J ₂ , J ₃ , Jj single lattice structure ki, k ₂ , k ₃ , kj position parameters

K ₁ , K ₂ , K ₃ , ... Ki grating constant λi, λ ₂ , λ ₃ , .... λi wavelength i, L ₂ , L ₃ Li layer

LDi, LD ₂ , LD ₃ , .... LDi laser diodes Pi, P ₂ , P ₃ , Pi focus point

R random generator

S1. S2. S3 S _n segment

Tι, T ₂ , T ₃ , track

Claims

claims

1. Optical scanning head (1) for multilayer data storage disks (10) with a focus-adjustable optical beam path, in which at least one collimator (4), an objective lens (5, 7, 21, 23), a beam splitter (9) and a focusing arrangement (13) are included, with at least one laser diode (2) at the beginning and a detector arrangement (12) at the end of the beam path and with a grating structure (6, 22) integrated in the beam path for the diffractive generation of at least a number of storage layers (L) Focus points (Pi) for the simultaneous acquisition of different data storage locations, characterized in that, in order to generate a number of focus points (P _{1 (} P ₂ , P ₃ Pi) exceeding the minimum number), the objective lens (5, 7, 21, 23) has the integrated grating structure (6 , 22) in the form of a complex overlay lattice structure

(8, 20) from a number of different, curved individual lattice structures (Ji, J ₂ , J ₃ Ji) corresponding to the number of focus points (P _{1 (} P ₂ , P ₃ Pi)) in the form of individual segments (Si, S ₂ , S ₃ , S _n ), which are arranged in an irregular, statistically distributed (R) manner in the superposition grid structure (8, 20).

2. Optical scanning head according to claim 1, characterized in that the segments (S ₁ , S ₂ , S ₃ S ") of the individual grating structures (J ^ J ₂ , J ₃ , .... Ji) are geometrically, in particular square limited.

3. Optical scanning head according to claim 1 or 2, characterized in that for generating a number of focus points (Pi, P ₂ , P ₃ Pi) exceeding the minimum number on different tracks (T ^ T ₂ , T ₃ , Tj) in a storage layer (Lj) the focal lengths (fi, f ₂ , f ₃ , ... fi) of the Single grating structures (J _{1 (} J ₂ , J ₃ , .... Ji) all the same and their diffraction angles to the optical axis of the objective lens (5,7,21, 23) are different.

4. Optical scanning head according to claim 1 or 2, characterized in that to generate a number exceeding the minimum number of

Focal points (P _{1 (} P ₂ , P ₃ Pj) on different tracks (Ti, T ₂ , T ₃ , Tj) in different storage layers (Li, L ₂ , L ₃ Lj) the focal lengths (f _{1 (} f ₂ , f ₃ , ... fi) of the individual grating structures (Ji, J ₂ , J ₃ Jj) and their diffraction angles to the optical axis of the objective lens (5, 7, 21, 23) are different.

5. Optical scanning head according to at least one of the preceding claims 1 to 4, characterized in that the objective lens (6) is designed as a multiplex objective (7) with a multiplex grating (22) applied to a converging lens (21) as a superimposed grating structure (8).

6. Optical scanning head according to at least one of the preceding claims 1 to 4, characterized in that the objective lens (6) is designed as a multiplex objective (7) with a computer-generated hologram (CGH) designed as a segmented Fresnel zone lens (23) as a superimposed grating structure (8).

7. optical scanning head (30) for multilayer data storage disks (31) with a focus-adjustable optical beam path, in which at least one collimator (33), an objective lens (37), a beam splitter (35) and a focusing arrangement (38) are contained, With at least one laser diode (34) at the beginning and a detector arrangement (39) at the end of the beam path and with a grating structure integrated in the beam path for the diffractive generation of at least the number of storage layers (Lj) _

25

Corresponding number of focus points (Pj) for the simultaneous acquisition of different data storage locations, characterized in that in order to generate a number of focus points (Pi, P ₂ , P ₃ , Pi) exceeding the minimum number on different tracks (Ti, T ₂ , T _3, Tj ) an additional beam splitter (32) is arranged in a storage layer (L) in front of or behind (I; II) the collimator (33), the position of which can be changed in the direction (x) of the beam path, the first beam splitter (9) as a polarization Beam splitter (35) with a downstream polarizing plate (36), the objective lens (37) as a diffractive optical element (DOE) and the collimator (33) with a grating structure.

8. Optical scanning head according to claim 7, characterized in that the additional beam splitter (32) is designed as a simple transmission grating, as a diffractive and / or refractive deflection element (DAE, RAE) or as an acousto-optical modulator (AOM).

9. Optical scanning head according to claim 8, characterized in that the deflection element (DSAE, RSAE) can be controlled in its optical effect by means of a liquid crystal cell.

10. Optical scanning head (40) for multilayer data storage disks with a focus-adjustable optical beam path in which at least one

Collimator, an objective lens (41), a beam splitter and a focusing arrangement (13) are included, with at least one laser diode (2, LDj) at the beginning and a detector arrangement (12) at the end of the beam path and with a grating structure (6 ) for the diffractive generation of a number of focus points (Pi) corresponding at least to the number of storage layers (Li) for simultaneous detection of different data storage locations, characterized in that in order to generate a number exceeding the minimum number

Focal points (Pi, P ₂₎ P ₃ Pi) in different storage layers (Li, L ₂ ,

L ₃ L,) a number of different tunable laser diodes (LDi, LD ₂ , LD ₃ , .... LD,) corresponding to the number of focus points of closely adjacent wavelengths (λi, λ, λ ₃ , .... λ _:) are provided, which can be operated simultaneously, and the first beam splitter (9) as a polarization beam splitter (35) with a downstream polarizing plate (36), the objective lens (37) as a diffractive optical element (DOE) and the collimator (33) with a Grid structure are formed.

11. Optical scanning head according to claim 10, characterized in that in order to generate a number of focus points (P, P, P ₃ , P,) which exceeds the minimum number, in each case a plurality of parallel tracks (Ti, T ₂ ,

T ₃ , T,) in different storage layers (Li, L ₂ , L ₃ , .... L) behind each

Laser diode (LDi, LD ₂ , LD ₃ , .... LD,) an additional beam splitter (BSi, BS ₂ , BS ₃ , ... BS |) is arranged, the position of which can be changed in the direction (x) of the beam path.

12. Optical scanning head according to claim 11, characterized in that the additional beam splitter (BSi, BS ₂ , BS ₃ , ... BSj) as a simple transmission grating, as a diffractive and / or refractive deflection element (DAE, RAE) or as an acousto-optical Modulator (AOM) is formed.

13. Optical scanning head according to claim 12, characterized in that the deflection element (DSAE, RSAE) can be controlled in its optical effect by means of a liquid crystal cell.