GB2167202A - Data storage systems - Google Patents

Abstract

Data is stored on an optical tape (16) housed in a cassette (19). An acousto-optic deflector (12) is employed to scan the tape (16) for both reading and writing. The data is recorded rasterwise, in one or more tracks, generally across the width of the track but at an angle thereto. By use of passive scanning techniques in association with tape, a simple system with high volumetric storage efficiency is achieved. <IMAGE>

Description

SPECIFICATION Data storage systems This invention relates to data storage systems and in particular, but not exclusively, to optical systems for archival storage.

According to one aspect of the present invention there is provided a data storage system comprising an optical tape on which data can be stored and acousto-optic means for reading and/or writing the data.

According to another aspect of the present invention there is provided a method of storing data comprising the steps of modulating an optical beam in accordance with the data to be stored and scanning the beam over an optical tape by acousto-optic means such as to write a data pattern thereon.

Optical tape is typically, a tape of photosensitive material into which data can be written and subsequently read by optical means. Local changes in the transmissive or reflective properties of the tape caused by the recording illumination describe the data pattern.

Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 illustrates a possible track format; Figures 2a and 2b illustrate two possible deflector formats; Figure 3 illustrates the principle of Bragg cell deflection; Figure 4 illustrates acousto-optic deflector response; Figure 5 illustrates schematically an embodiment of optical tape storage system.

Optical storage techniques are of interest for many applications involving low, medium or high capacity storage means. In the archival storage market simple optical disc systems are proving of limited adequacy and there is a growing emphasis on complex Juke-Box arrangements to achieve sufficient data capacity.

By resorting to a tape system, however, such capacities can be directly obtained and correspondingly simpler access means adopted.

The present invention proposes a scheme comprising an optical tape store addressed by means of an acousto-optic deflector to achieve a high speed write/read data facility.

The tape may comprise any one of a number of media being developed for optical storage use, reference for example, SPIE Proceedings, Vol. 420, on "Optical Storage Media". The tape 16 may hold data is one or more tracks each N data points wide, where typically 102tut103. The data is written rasterwise into each track as indicated in Fig. 1, that is generally across the width of the track but at an angle thereto. Taking N=1000, and assuming a 5,am diameter spot on a iOjtm pitch, both along and across the tape, a capacity of 108 bits per meter length of track results.

Tape capacities of 1012 bits or greater are therefore readily envisaged, particularly if multi-tracking is employed. By way of example, for the format illustrated and with the dimensions quoted a 100,us scan at 10cm sec-1 gives a write/read speed of up to 107 bits per second.

The data is written into, and read from, the tape by the same acousto-optic deflector, which may be of the bulk form illustrated in Fig. 2a or the surface wave form illustrated in Fig. 2b, although the same general principles apply in the two cases. The technique is to launch an acoustic wave into an acousto-optic crystal medium from a suitable located transducer, or transducers to form a diffraction grating and hence by varying the drive frequency of the acoustic signal cause the diffracted (deflected) optical output beam to scan.

A general example of this is shown in Fig. 3 which indicates the deflection principles of a Bragg cell. Light from a laser source 1 is collimated into a beam by a lens 2 and coupled to an acousto-optic medium 3 at an angle of incidence 68, the Bragg angle. Transducers (not shown) generate an acoustic wave in the acousto-optic medium substantially at right angles to the light beam, causing the refractive index of the acousto-optic medium to be modulated and so produce a diffraction grating. The diffracted beam output from the medium 3 is focussed by a lens 4, into a spot on, for example, a tape (not shown). Variation of the drive frequency applied to the transducers causes a change in the grating period with corresponding deflection of the spot.The efficiency of the diffraction process is dependent on a number of parameters but approaches 100% at Bragg incidence if the relationship: HCos26B PL= 2M2 is satisfied. In this expression P is the power in the acoustic beam.

L and H are the width and depth respectively of this beam, and M2 is the figure of merit for the device material, and, A is the optical wavelength.

The achievable resolution 2N of such a device, given N data points across the track, can be written as, rBATr N= T Where AB is the acoustic bandwidth of the device, X is the transit time of the acoustic wave across the optical beam and T is the scan time, AB+I relate respectively to the deflection and aperture of the device and are uniquely dependent on the material concerned.

In the particular case of lithium niobate, however, which is typical for a device of surface wave construction, performance along the lines of Fig. 4 is indicated. Wide bandwidth is possible in this case. Thus with AB=250MHz resolutions of several thousand spots are apparent for scan times as low as a few tens of microseconds, enabling data rates in the order of 107 bits per second as mentioned above with respect to Fig. 1. in practice rates may vary from this figure dependent on the sensitivities of the detection device, and recording material, the system optical throughput characteristics and the scan power level. The approach, therefore, is essentially one of low to medium bit rate capability.

The full optical tape storage system is illustrated schematically in Fig. 5. A laser 10 produces a beam of light which is collimated by lens 11 and input to an A-O deflector 12 which in reality may be of either bulk or surface wave type as per Fig. 2. Under control of the R.F. drive 13 the light beam is diffracted, focussed by lens 14 and tape medium 15 scanned thereby, as described above. The light 15 scanning the tape medium 16 either "marks" (writes) the tape, being suitably modulated with data for this purpose, or produces (reads) a detectable signal corresponding to the data pattern scanned, which is detected by photodiode 17 following focussing thereon by lens 18. Detection is illustrated in the reflection mode but depending on the construction of the tape 16 may also be considered in transmission terms, and may be amplitude or phase encoded.The tape is preferably housed in a sealed dust-free cassette to avoid environmentally induced reading errors.

Fig. 5 illustrates a tape with an integral drive/spooling mechanism 19 (cassette housing omitted) although other tape transport arrangements may be used, for example, a conventional cassette type of arrangement. The input/output optics required to shape the light for transmission through the deflector and subsequently achieve a planar scanning focus may be based on those described in our co-pending Application No. 8227750 (Serial No.

2128355) (J.S. Heeks-R.E. Cooke 37-5) although a finer focus is required for the tape storage application and a correspondingly shorter linear scan. The RF drive 13 may be comprised by a voltage controlled oscillator (VCO) using either analogue or digital control, the circuit being optimised for high speed linear scan and minimum focus distortion. The data processing circuit 20 provides modulation/timing instruction for the laser drive 21, synchronisation information for the RF drive 13 and bit-by-bit interpretation of the data signal detected by the photodiode 17, and has a two-way interface 22 with a data network indicated at 23.

In its simplest form, with a single data track, scanning is by straight forward acoustooptic means across the tape. With parallel tracks, however, incremental mechanical sideways movement and/or multiply stackedheads are required for track selection scan.

The laser source 10 is typically a low power CW He.Ne laser combined with an acoustooptic modulator for data modulation, or a semiconductor laser, which can be directly modulated. With the bulk deflector approach, however, a high power argon laser may also be used.

Thus there is provided a simple optical tape storage system in which data scanning is achieved principally by passive acousto-optic deflection means. The one deflector may perform both write and read functions and, subject to the tape medium employed, both permanent and erasable storage s achievable.

The tape storage system is suited particularly to bulk archival storage, Where storage volume is at a premium.

Claims (14)

1. A data storage system comprising an optical tape on which data can be stored and acousto-optic means for reading and/or writing the data.

2. A system as claimed in claim 1 wherein the acousto-optic means comprises an acousto-optic deflector employed for both reading and writing of data on the optical tape.

3. A system as claimed in claim 1 or claim 2, wherein the tape has one or more tracks for the writing of data.

4. A system as claimed in claim 3, and including, for multi-track optical tape, incremental mechanical scanning means for use between tracks.

5. A system as claimed in claim 3, and including, for multi-track optical tape, multiplystacked acousto-optic heads for use with the multi tracks.

6. A system as claimed in any one of the preceding claims, wherein the data is written rasterwise generally across the width of the track but at an angle thereto.

7. A system as claimed in claim 2, wherein for reading of data on the tape the tape is scanned by a light beam as deflected by the acousto-optic deflector and a signal corresponding to a scanned pattern is detected by a photodiode.

8. A system as claimed in any one of the preceding claims, wherein the acousto-optic means is a surface wave acousto-optic deflector.

9. A system as claimed in any one of claims 1 to 7, wherein the acousto-optic means is a bulk acousto-optic deflector.

10. A system as claimed in any one of the preceding claims, wherein the optical tape is housed in a sealed dust free cassette.

11. A data storage system substantially as herein described with reference to the accompany drawings.

12. A method of storing data comprising the steps of modulating an optical beam in accordance with the data to be stored and scanning the beam over an optical tape by acousto-optic means such as to write a data pattern thereon.

13. A method as claimed in claim 12, and wherein for reading said data pattern the optical tape is scanned with a reading optical beam, whereby to produce an optical signal corresponding to the data pattern, and the op tical signal is detected.

14. A method of storing data substantially as herein described with reference to the ac companying drawings.