WO1998001851A1

WO1998001851A1 - Optical apparatus for scanning a tape-shaped record carrier

Info

Publication number: WO1998001851A1
Application number: PCT/IB1997/000712
Authority: WO
Inventors: Gerard Eduard Van Rosmalen
Original assignee: Philips Electronics N.V.; Philips Norden Ab
Priority date: 1996-07-04
Filing date: 1997-06-17
Publication date: 1998-01-15
Also published as: TW319854B

Abstract

An optical apparatus for scanning a tape-shaped record carrier (1) is described, in which a mirror polygon (20) is used for scanning tracks which extend perpendicularly to the tape travel direction (5), and in which the mirror polygon is tiltable so as to adapt the scanning direction to the track direction. By rendering the tilt axis (40) parallel to the chief ray of the scanning beam (b) incident on the polygon, it is ensured that the scanning track remains straight.

Description

Optical apparatus for scanning a tape-shaped record carrier.

The invention relates to an optical apparatus for scanning a tape-shaped record carrier for recording a structure of optically detectable information areas arranged in information tracks which extend in a direction transverse to the longitudinal direction of the tape, said apparatus being provided with a supply reel and a take-up reel for transporting the record carrier in its longitudinal direction, a radiation source detection unit for supplying a scanning beam and for converting the beam reflected by the record carrier into electric signals, a mirror polygon, which is rotatable about an axis of rotation, for realizing a scanning movement of the scanning beam in the track direction, and an objective system for focusing the scanning beam to a scanning spot on the record carrier, the mirror polygon being tiltable.

Since the introduction of the digital optical recording technique, there has been an increasing need of raising the storage capacity of the medium used so that, for example, a digital video signal can be stored on such a medium. In the known digital audio disc, or compact disc (CD) and media derived therefrom, such as CD-ROM, CD-I, etc. , the storage capacity is determined by the size of the scanning spot formed in the information plane of the record carrier, which scanning spot determines the resolving power of the scanning device and hence the minimum dimensions of the information details, for example information pits which can still be detected separately. It is true that the size of the scanning spot can be reduced by decreasing the wavelength of the read beam used and/or by increasing the numerical aperture of the objective system with which the scanning spot is formed, but this cannot lead to an increase of the storage capacity by a factor of ten or more.

As described in the article "A Compact Optical Tape Recording System" in^*. SPIE, Vol 2338 Optical Data Storage, 1994, pp. 8-14, the storage capacity and write speed may be increased by several orders of magnitude by making use of a tape-shaped optical record carrier which is moved in its longitudinal direction along an optical scanning device and in which the information is provided in information tracks extending in a direction perpendicular to the longitudinal direction of the tape. To write and/or read these information tracks, the scanning device comprises a mirror polygon having, for example six mirror faces, or facets. Upon rotation of the mirror polygon, each facet of the consecutive mirror facets ensures that an information track is scanned, hence is written or read.

It should then be ensured that the scanning spot accurately follows the instantaneously scanned track when the written information is being read, even if the track direction differs from the track described by the scanning spot on the record carrier at a nominal position of the mirror polygon. To this end, a tracking error signal is generated in known manner, so that the direction of the scanning track can be made equal to that of the tracks by means of this signal. Such a correction can be realized by adapting the angle between the axis of rotation of the mirror polygon and the chief ray of the scanning beam coming from the radiation source detection unit and being incident on the mirror polygon, as has been described in US Patent 4,901,297. In this Patent, which relates to magneto-optical writing and reading of information, it is proposed to tilt the axis of rotation of the mirror polygon about a first and/or second tilt axis, which are mutually perpendicular and are both perpendicular to the axis of rotation of the polygon in its nominal position. However, if tilting must take place through larger angles, the scanning spot on the record carrier will describe a curved scanning track which no longer matches the straight tracks.

It is an object of the invention to provide an optical tape scanning apparatus in which this problem does not occur. To this end, the tape scanning apparatus is characterized in that the axis about which the mirror polygon is tiltable is parallel to the chief ray of the scanning beam coming from the radiation source detection unit and being incident on the mirror polygon.

In this tape scanning apparatus, the track described by the scanning spot on the record carrier also remains straight in the case of larger tilt angles. A preferred embodiment of the apparatus is further characterized in that the axis about which the mirror polygon is tiltable coincides with said chief ray.

This embodiment has the advantage that the height, i.e. the dimension in the direction of the axis of rotation, of the facets and hence of the polygon does not have to be larger than the diameter of the scanning beam at the area of this polygon. Consequently, the mass of the polygon may be maintained small so that a rapid start-up of, and high rotation frequencies for, the polygon are possible.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings:

Fig. 1 shows the circuit diagram of the optical tape scanning apparatus, and

Fig. 2 shows an embodiment of a radiation source detection unit used in this apparatus.

In Fig. 1, the reference numeral 1 denotes a tape-shaped record carrier. This tape is directly transported from a supply reel 3 to a take-up reel 2 across a stationary guiding element 4. The apparatus does not have to comprise any further tape-guiding elements. Both reels are driven by separate motors. The motors may be driven in such a way that the tape tension remains constant. The tape travel direction is denoted by means of the arrow 5.

The scanning device of the apparatus comprises a radiation source detection unit 10 which supplies a scanning beam b, a rotating mirror polygon which reflects the, for example parallel, beam to an objective lens 30 focusing the beam to a radiation spot V on the tape. The mirror polygon comprises, for example ten mirror facets f,-f₁₀ and, during operation, this polygon rotates about the shaft 21 in the direction of the arrow 22. Each facet rotating in the radiation path of the beam b, facet f₂ in the drawing, will move the beam in the direction of the arrow 25, perpendicularly to the tape travel direction 5, across the entrance pupil of the objective lens. The radiation spot formed by this lens then scans a track extending in the direction perpendicular to the direction 5. A second, a third, etc. track are consecutively scanned by means of the facets f,, f_I0, etc.

The scanning beam b coming from the unit 10 and incident on a mirror facet is located in the plane defined by the scanning beam coming from the mirror polygon and extends at an angle of, for example 38° to the central position of the scanning beam which is moved, for example through an angle of 48° . The objective lens, in the form of an f-θ lens has, for example an effective focal length of 1.25 mm and a numerical aperture of 0.45. The scanning spot can then be moved, for example through a distance of 1 mm in the vertical direction. In this way, it is possible to write tracks having a length of 1 mm in the direction perpendicular to the tape travel direction.

A plurality of horizontal strips of vertical information tracks may be written on a tape. To this end, tracks with a length of 1 mm are first written from the beginning to the end of the tape. Then the travel direction of the tape is reversed, the tape and the optical system are displaced through a distance of slightly more than 1 mm with respect to each other and the next horizontal strip of vertical tracks is written. Thus, 12 strips with information tracks can be provided on a tape having a width of 12.7 mm. The apparatus is also suitable for recording tapes having a width of 8 mm. Reading a written tape is effected in a manner analogous to that for writing. Then the beam reflected by the tape traverses the same optical path in the reverse direction to the radiation source detection unit. The information signal, the focus error signal and the tracking error signal are obtained in this unit in a similar way as in an optical audio disc (CD) player.

The radiation source detection unit comprises a high-power diode laser having a wavelength of, for example 780 nm. If the objective lens has an NA of 0.45, a resolving power which is comparable to that of the Compact Disc system is obtained. Then an information density of 1 bit/μm can be achieved, and a tape having a width of 12.7 mm and a length of 42 m may comprise 50 Gbytes of information.

The information density in the track direction is, for example 0.6 μm/bit so that a track may comprise approximately 1600 bits. The nominal rotation frequency of the mirror polygon is, for example 2000 revolutions per sec. The scanning frequency of a mirror polygon with ten facets is then 20 kHz. At 1600 bits per track, a bitrate of 32 Mbits per second is achieved. The track period is, for example of the order of 1.6 μm. At a scanning frequency of 20 kHz, the tape speed is then 3.2 cm/ sec during reading and writing. This is a relatively low speed so that no complicated tape transport mechanism is required. Fig. 2 shows a possible embodiment of the radiation source detection unit

10. This unit comprises a radiation source 11 in the form of a diode laser which supplies a diverging beam b. This beam is converted by a collimator lens 13 into a parallel beam which is directed onto the mirror polygon. In order that the beam b'is separated from the on-going beam, which beam b'is reflected by the record carrier and enters the collimator lens 13 again after a second passage through the objective lens and a second reflection on the mirror polygon, the unit is provided in known manner with a diffraction grating 12 having such a blaze that a large part of the radiation of the returning beam is diffracted in the first diffraction order to a detector 14. An image V of the radiation spot V is formed on this detector. Blaze is understood to mean that the walls of the grating grooves have such a slope that the angle at which a ray is refracted by such a wall is equal to the angle at which a ray diffracted in the first diffraction order leaves the grating. As described in, inter alia, US Patent 4,665,310, the grating may be divided into two halves in which the directions of the grating strips are different. Then the beam b' is divided into two sub-beams and a focus error signal can be generated by means of a detector comprising a separate pair of detector elements for each sub-beam, i.e. a signal indicating whether the focal point of the objective lens is located or not located in the plane of the scanned track on the tape.

Such a signal may alternatively be obtained by means of an undivided grating having a grating period with a non-linear variation, in combination with a four- quadrant detector, as described in US Patent 4,358,200. This grating renders the beam astigmatic, and the four-quadrant detector allows determination of the shape of the radiation spot on the detector, which shape is dependent on the extent to which the beam b is focused on the tape.

Instead of using a grating, the ongoing and returning beam may alternatively be separated by using either a semi-transmissive mirror or the combination of a polarization-sensitive beam splitter and a λ/4 plate arranged between the beam splitter and the objective lens, in which λ is the wavelength of the scanning beam.

For obtaining a tracking error signal, two auxiliary beams may be used, which are split off the scanning beam by means of, for example, a diffraction grating. These auxiliary beams constitute radiation spots on the two edges of a track, which radiation spots are imaged on separate detectors. The difference between the output signals supplied by these detectors is the tracking error signal. This signal is independent of the nature of the information bits in the tracks. As the facets of the mirror polygon are parallel to the axis of rotation of this polygon, the auxiliary spots move along straight lines parallel to the path traversed by the scanning spot. Consequently, use can be made of the standard three-spot tracking system as used in the Compact Disc systems.

It is to be noted that the radiation source detection unit shown in Fig. 2 is only one of the possible embodiments. This unit may be modified in various ways known from the Compact Disc technology. For example, for the astigmatic focus error detection, a cylindrical lens instead of an astigmatic grating may be arranged in the path of the reflected scanning beam. Instead of the astigmatic focus error detection, the Foucault detection method may alternatively be used. In this method, use is made of a roof prism which splits the scanning beam reflected by the tape into two sub-beams each cooperating with a detector pair. The position of each sub-beam with respect to the associated detector pair is then a measure of focusing. The tracking error signal may not only be generated by using two auxiliary spots but may alternatively be generated from the scanning beam only, by splitting the detector for this beam into two parts. By subtracting the signals of these detector pans from each other, the tracking error signal is obtained. This method is known as the push-pull method. The objective, or scanning lens 30 may be arranged in an actuator (not shown) so that this lens can be moved in two directions, one movement serving for focus setting and the other for tracking. In order to define an average focus position, the tape is transported via the guiding element 4. However, a low-frequency change of the distance between the tape information surface and the lens 30 may then still occur, for example, due to variations in tape thickness. This may be compensated by said first movement, so that the distance between the lens and the tape surface can be varied in such a way that the focal plane of the lens always coincides with the information surface of the tape. This focus control may have a very small bandwidth. If the focal plane of the lens 30 is tilted with respect to the tape surface, a focus tilt correction must be performed. To this end, the guiding element 4 may have a tiltable implementation.

When writing an empty tape, the objective lens 30 is fixed in a stationary position. The system has a sufficiently high stability to write perfectly straight tracks with a constant track period. No vibrations at a frequency of the order of the scanning frequency occur in the system.

When reading a written tape, three unwanted relative movements of the scanning spot and a scanned track might occur. The first is a movement of the tape in a direction parallel to the track direction. This movement is not troublesome because the scanning path length is chosen to be larger than the track length. This ensures that the track is always within the scanning area. The only effect of this movement is that a small delay occurs in the signal which has been read. The second unwanted movement is caused by small variations of the tape travel speed. These variations may result in a mean tracking error. During reading and rewriting the tape, this error can be compensated by the second movement of the lens made possible by the lens actuator, namely a rotation of the lens about an axis which is parallel to, and preferably located in, the rear focal plane of this lens. This axis has a direction which is parallel to the track direction. In this way it is possible to shift the scanning path in a direction perpendicular to the track direction for the purpose of compensating said tracking error. The third unwanted movement is caused by a tilt of the tracks with respect to the scanning path. To compensate this, the scanning path may be tilted in a corresponding manner by tilting the mirror polygon.

In this way, all irregular tape movements can be compensated by active servocontrols so that a very accurate tape-guiding system with microprecision is not necessary. The tape transport mechanism is very simple: the tape is directly transported from one reel to the other via only one guiding element 4. In order to cause the direction of the track described by the scanning spot on the tape to coincide with the track direction, the mirror polygon is tilted in accordance with the invention in such a way that the tilt axis is parallel to the chief ray of the scanning beam b coming from the unit 10, as is shown in Fig. 1. In this Figure, the axis of rotation of the polygon is denoted by the reference numeral 35 and the tilt axis is denoted by the reference numeral 40. It is ensured by the choice of the tilt axis that the track described by the scanning spot V is always straight.

If, as shown in Fig. 1, the tilt axis 40 coincides with the chief ray of the beam b, the instantaneously used facet of the polygon will not shift with respect to the beam in the direction parallel to the axis of rotation when the polygon is tilted. This has the advantage that the dimensions of the facets and those of the mirror polygon in this direction do not have to be larger than the diameter of the beam b at the area of the facet. The polygon may then be thin so that its mass may remain small and a rapid start-up of, and high rotation frequencies for, the mirror polygon are possible. To be able to perform the above-mentioned movements, the mirror polygon is electromagnetically journaled and driven. The drive and journaling unit 45 may be implemented as described in US Patent 5,171,984. The polygon may move in six degrees of freedom. These movements may be detected so as to be corrected, if necessary. The position detection system as described in US Patent 5,245, 182 may be used for this purpose. With this system, displacements along the three axes of an orthogonal system of coordinates and the rotations about two of these axes can be measured. This system cooperates with the reflecting surface 21 of the mirror polygon and with a reflecting spherical element 23 which is centrally arranged on this surface. The position detection system may alternatively be implemented as described in the previously filed, non-prepublished European Patent Applications ... (PHN 15.885) and ... (PHN 15.891) in the name of the Applicant. If the polygon surface 21 is provided with at least an area 24 having a reflection which is different from its surroundings, the speed of rotation or the angle of the polygon about the axis 35 can also be measured with these position detection systems.

As already noted, the apparatus according to the invention may be used for reading a previously written tape-shaped record carrier and for writing and subsequent reading of a blank tape. The information layer of the last-mentioned tape may be a magneto- optical layer, while the apparatus should be provided in known manner with magnetic means for the local magnetization of the tape so that domains with a direction of magnetization opposite to that of their surroundings can be formed therein. Moreover, the unit 10 should then be provided in known manner with polarization-sensitive detectors. Instead of a magnetic layer, the tape may also have a phase-change layer in which information areas are formed by locally varying the structure from amorphous to crystalline, or conversely. In a tape which is only intended to be read, the information may alternatively be provided in the form of pits in the information layer, analogously as in the known CD audio disc.

Claims

Claims:

1. An optical apparatus for scanning a tape-shaped record carrier for recording a structure of optically detectable information areas arranged in information tracks which extend in a direction transverse to the longitudinal direction of the tape, said apparatus being provided with a supply reel and a take-up reel for transporting the record carrier in its longitudinal direction, a radiation source detection unit for supplying a scanning beam and for converting the beam reflected by the record carrier into electric signals, a mirror polygon, which is rotatable about an axis of rotation, for realizing a scanning movement of the scanning beam in the track direction, and an objective system for focusing the scanning beam to a scanning spot on the record carrier, the mirror polygon being tiltable, characterized in that the axis about which the mirror polygon is tiltable is parallel to the chief ray of the scanning beam coming from the radiation source detection unit and being incident on the mirror polygon.

2. An optical apparatus as claimed in Claim 1 , characterized in that the axis about which the mirror polygon is tiltable coincides with said chief ray.