DK145868B - DEVICE FOR READING A RADIATION REFLECTING REGISTRAR AND INCLUDING A FOCUS ERROR DETECTOR - Google Patents

Description

(19) DENMARK (J)

© (η) PUBLICATION MANUAL od 145868 B

DIRECTORATE OF THE PATENT AND TRADEMARKET SYSTEM

(21) Application No. 1205/78 (51) | nt.CI.3 β 11 B 7/08 (22) Filing day 17 · tnar. 1978 // G 02 B 7/11 (24) Race day 17 · tnar · 1973 (41) Aim. available 25 · s ep. 1978 (44) Presented 21. Raar. 1985 (86) International application no.

(86) International filing day (85) Continuation day - (62) Master application no. -

(30) Priority Mar 22 1977, 7703076, NL

(71) Applicant N.V. PHILIPS * LAMP LAMP FACTORIES, Eindhoven, NL.

(72) Inventor Hendrik * T Lam, NL.

(74) International Patent Bureau.

Apparatus for reading a beam = reflector recording = carrier and comprising a focusing = error detector.

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to an apparatus for reading a radiation-reflecting recording carrier on which information is recorded in an optically readable, traceably arranged information structure, which comprises a radiation source providing which system converts the read beam bundle modulated by the information structure into an electrical signal, and an optoelectronic in focusing error detector system to determine a deviation between the desired and actual position of the objective plane focusing plane, which focusing error detector system comprises an astig In a mathematical element forming two astigmatic firing lines and a radiation-sensitive detector located in a plane between the two astigmatic firing lines, which detector comprises four sub-detectors arranged in four different quadrants. of an imagined XY coordinate system with the xY plane parallel to the two astigmatic firing lines, and where the X and Y axes form an angle of 45 ° with the astigmatic firing lines of the astigmatic element, the sub-detectors being properly illuminated by the radiation from the the registration carrier wherein a focusing error signal is provided by diagonal addition of the output signals from the sub-detectors and subtraction of the sum signals.

Such an apparatus is disclosed in German Patent Application No. 2,501,124. This apparatus is used, for example, for reading a recording carrier on which a (color) television program is registered. The information structure then consists of a plurality of regions alternating with intermediate regions in a spiral track, which regions and intermediate regions affect a reading beam bundle in various ways. For example, the information is contained in the lengths of the regions and of the intermediate regions. To achieve sufficiently long playing time with a limited size registration carrier, the details of the information structure must be very small. For example, if a 30 minute television program is recorded on one side of a plate-shaped round record carrier in an annular field with an outer radius of approx. 15 cm and an inner radius of approx. 6 cm, the width of the grooves is approx. 0.5 µm and the average length of the regions and of the intermediate regions will be approx. 1 µm.

To enable these small details to be read, an objective system with a relatively large numerical aperture must be used. However, the focus depth of such a lens system is small.

Since the distance between the plane of the information structure and the lens system in the scanner can vary by an amount greater than the depth of focus, measures must be taken to enable these variations to be detected and the focus to be corrected.

According to the aforementioned German patent application, for this purpose a beam of beam reflected by the registration carrier is made astigmatic by the fact that a cylindrical lens is placed in the path of this beam of beam behind the objective system. Between the firing lines of the astigmatic system constituted by the objective system and the cylindrical lens, a radiation-sensitive detector is arranged which consists of four sub-detectors. As the position of the plane of the information structure changes relative to the objective system, the shape of the image formed on the sub-detectors will also change. This change in shape can be detected by combining the output signals from the sub-detectors appropriately.

Practical problems may arise when using the known scanner. The radiation distribution over the composite detector also depends on the diffraction of the beam from the details of the information structure. First of all, longitudinal diffraction is obtained by a track to be read, due to the sequence of regions and intermediate regions. The variations in the radiation distribution over the composite detector due to this diffraction have a high frequency relative to the focusing errors, so that the influence of this diffraction on the focusing control signal can be eliminated electronically.

Furthermore, diffraction occurs in a transverse direction. This diffraction depends on the position of the reading spot formed on the record carrier relative to the center of the track to be read. During assembly of the scanner, the focusing error j The detection system must be set such that when the center of the reading spot coincides with the center of a track to be read and the focus is correct, the focusing error signal is zero. However, if the reading spot while reading the record carrier is moved from the center of the track to be read toward its edge, the radiation distribution across the composite detector will change independently of the focus. This gives rise to an incorrect focus error signal, so that the focus of the lens system is set incorrectly.

The reading apparatus may also include a position error detector system for determining the deviation between the center of a beam spot on the record carrier and the center of a track to be read, as well as a servo system for correcting the position of the center of the reading spot. The position error signal also depends on the lens system's focus. If the focus is not correct, the position of the beam spot will also be adjusted incorrectly. As a result, the focusing error will grow again. In this case, both the focusing servo system and the servo system for correcting the position of the beam spot relative to a track to be read may be rendered unable to function properly.

As the optical read unit on its way to a specific section of the recorded program moves radially across the high speed record carrier, the edges of the individual tracks that the unit passes / cause a change in the focusing error signal. The focusing servo system will then step in unnecessarily and at a high frequency. When the focus of the lens system is corrected to move this system, the high-frequency control signals applied to the control system's controls will produce a nuisance noise. In addition, the controls will then use more power.

Since the width of the track is less than the distance between the tracks, the beam spot will, on average, be shorter at the tracks than between the tracks when the reading unit is moving rapidly across the track direction.

As a result, the average focusing error signal will also not be zero in the case of proper focusing. Consequently, the focus will generally be adjusted incorrectly.

The influence of the diffraction of the beam on the transverse track direction is particularly significant when a track section to be read and projected onto the surface of the sub-detectors - hereinafter referred to as the "effective track direction" - forms an angle of 45 ° with X and Y axis of the composite detector.

The invention is directed to an embodiment of the known reading apparatus in which the diffraction in a direction transverse to the track direction has no influence on the focusing error signal. To this end, the apparatus according to the invention is characterized in that one of the axes (X, Y). in the coordinate system in which quadrants the sub-detectors (A, B, C, D1 are arranged) are parallel to the effective track direction (7).

The invention is explained in more detail below with reference to the schematic drawing, in which fig. 1 shows the reading apparatus according to the invention, and fig. 2a, 2b and 2c show how the shape of the spot on the detector varies as a function of focusing.

In the apparatus of FIG. 1, a circular disk-shaped registration bazaar is denoted by 1. The information structure is e.g. a phase structure and comprises a large number of concentric or quasi-concentric tracks 7, which tracks consist of a sequence of regions g and intermediate regions t. The regions may, for example, be located at a different level on the registration carrier than the intermediate regions. The information can, e.g. be a (color) television program, but there may also be other information such as e.g. a large number of different images, or digital information.

5 145868

The recording carrier is illuminated by means of a reading beam bundle 3 which originates from a beam source 4, e.g. and a laser. An lens system, for convenience only represented by a single lens 5, focuses the beam of a reading spot V on the plane of the tracks 2. The focal length of the auxiliary lens 6 is selected such that the lens system pupil is appropriately filled. The beam reading beam is reflected from the record carrier and is then modulated in accordance with the information contained in a track section to be read. In order to separate the incident unmodulated beam from the reflected modulated beam, there is a beam beam splitter 8, e.g. a partially transparent mirror. The beam splitter directs the modulated read beam to a radiation sensitive detector 9. This detector is connected to an electronic circuit 10 which emits a high frequency information signal and, as will be explained below, a low frequency focusing error signal.

In order to detect focusing errors, the radiation path behind the beam bundle splitter 8 contains an astigmatic element 11. As shown in FIG. 1, this element may be a cylindrical lens. Astigmatism can also be obtained in other ways, e.g. by means of a flat transparent plate which is positioned obliquely to the bundle, or by means of a lens in an oblique position to the bundle. Instead of one focal point, an astigmatic system has two astigmatic focal lines which - in axial direction - occupy different positions and are perpendicular to each other. Therefore, the lens system and the cylindrical lens will provide the reading spot V with two further firing lines 12 and 13. The radiation-sensitive detector 9 is placed in a plane which, longitudinally along the optical axis, is located between lines 12 and 13, preferably at a location. , where the dimensions in two mutually perpendicular directions of the image spot plus the reading spot V are as similar as possible to each other in the case of proper focusing.

In order to be able to detect the shape of the spot V 'and thus the extent of focusing, the detector 9 includes four sub-detectors which are located in the four quadrants of an X-Y coordinate system. FIG. 2a, 2b and 2c show the four sub-detectors A, B, C and D with respect to line 2, 2 'in FIG. 1, with the different forms of the spot V 'on these detectors at different values of the distance between the objective system and the plane of the tracks. The X and Y axis 145868 6 forms an angle of 45 ° with the axis 15 through the cylindrical lens, i. with the astigmatic firing lines 12 and 13, while the X axis is now parallel to the effective trace direction. .

Pig. 2a illustrates the situation where the distance between the objective system and the plane of the tracks is correct. If this distance is too large, the firing lines 12 and 13 will be located closer to the cylindrical lens 11. In this case, the detector 9 is located closer to the firing line a 13 than the firing line a 12. The spot V then has the one shown in FIG.

2b. If the distance between the objective system and the plane of the tracks is too small, the firing lines 12 and 13 are further away from the cylindrical lens and the firing line 12 is closer to the detector 9 than the firing line 13. The spot V * then has the one shown in FIG. 2c.

If the signals from the sub-detectors A, B, C and D are represented by S ^, SB, Sc and SD, the focusing error signal is expressed by:

Sf = <SA + Sc) - <SB + SD).

It will be seen that in the case of FIG. 2a is Sa + S, = S ,, + S, -, so-

Oh yeah

is led to = 0. For those in FIG. 2b and 2c, the signal is negative and positive respectively. By adding the signals SA and Sc to each other and the signals Sg and Sg to each other and by subtracting the sum signals from each other, you obtain an unambiguous focusing error signal. This signal can be processed electronically in a manner known per se to produce a focusing control signal, with which one can correct the focusing of the objective system, for example by moving the objective system relative to the plane of the tracks by means of an electromagnetic coil.

As the radiation spot v is moved across the track direction, the amount of radiation of e.g. detectors A and B could grow relative to the amount of radiation for detectors C and D, regardless of the focus. However, since the focusing error signal is elicited by subtracting the signals and SB, one can disregard the influence of this movement on the signal S ^.

It should also be noted that the same beam can be used to read the information on the registration carrier and to generate the focusing error signal. The information signal may be provided by e.g. adding the signals from the four sub-detectors to each other. The variations in the radiation distribution over the sub-detectors should follow