WO1993003481A1

WO1993003481A1 - Optical recording apparatus

Info

Publication number: WO1993003481A1
Application number: PCT/GB1992/001405
Authority: WO
Inventors: Brian John Baker
Original assignee: Lasor Limited
Priority date: 1991-07-29
Filing date: 1992-07-29
Publication date: 1993-02-18
Also published as: GB9116308D0

Abstract

Apparatus for optical recording on a light-sensitive recording medium (4) includes an array (1) of discrete laser light sources for emitting a plurality of laser beams of a predetermined numerical aperture for forming an image of the array for focussing on the recording medium. The laser light sources of the array (1) are mounted at a predetermined pitch relative to each other and are arranged to be positioned at a predetermined distance from the recording medium. The apparatus also includes an optical arrangement (6, 7, 8), preferably including a microlens array (10), for splitting the image field into a plurality of lower numerical aperture light sources at an intermediate image space located between the laser light source array (1) and the recording medium (4) and for imaging the plurality of light sources of reduced numerical aperture through a final objective lens (11) in the focal plane of the recording medium (4) at a substantially reduced pitch relative to the pitch of the laser light source array (1).

Description

OPTICA RECORDING APPARATUS

The present invention relates to apparatus for optical recording on light-sensitive recording media, such as tapes or discs.

Known optical recording apparatus of this type comprises one or more laser diodes arranged to emit pulsed laser beams carrying data to be recorded. The pulsed beams are focussed on the light-sensitive surface of the record¬ ing medium to produces diffraction-limited spots represent¬ ing the data, the spots being of different reflectivity to the remaining areas of the recording surface. The recorded data can then be read from the recording surface by re¬ flecting a laser beam from the light-sensitive surface into a photodetector which generates an electrical output.

By using a plurality of laser diodes, each emitting a beam for recording data, a plurality of parallel tracks can be recorded simultaneously on a single recording medi¬ um, thereby increasing the data rate which is necessary in high-density data storage on optical recording media.

Such multi-channel optical systems have already been investigated for both tape and disc recording. The physical mounting, in side-by-side relationship, of the laser diodes which are required to produce the multi-chan¬ nel recording determines the minimum pitch of the laser diode array, i.e. the centre-to-centre spacing of adjacent diodes in the array. The pitch of the laser diode array is then required to be reduced by a large factor in order to record the separate channels on the recording medium. However, a simple de-magnification of the image of the laser diode array is not satisfactory, because a lens increases the numerical aperture of the beams by approxi¬ mately the same ratio as it reduces the image size. Howev¬ er, because of finite practical and theoretical limits to the increase in numerical aperture, reducing the image size results in reduced efficiency of light collection from the source, thereby negating the advantages of diode ar¬ rays, namely increased data rate.

The system disclosed in US Patent No. 4743091 (Gelbart) is designed to overcome the above-mentioned prob¬ lem when using laser diode arrays for optical tape recording. This system includes a separate collimating lens for each diode of the array and collects the laser beams together using an imaging lens. A separate diode collection lens reduces the numerical aperture of each light beam to a very low level, which enables the collec¬ tion lens in conjunction with the final imaging lens to achieve the required level of pitch de-magnification whilst still maintaining maximum optical throughput efficiency. Nevertheless, such a system is expensive, both in terms of piece parts and fabrication costs.

The use of laser diode arrays for optical recording on discs has additional problems due to the spiral ar¬ rangement of the tracks on the disc. Unless the pitch of the final multi-channel image is equal to the track pitch, then the array must be imaged at some angle less than perpendicular to the track direction. This presents a problem for use in an optical disc environment, because the incidence angle between the track and the array image is continuously varying from the outer extremities of the disc to the inner radius. Consequently, the image angle must be rotated in order to compensate for this. Such systems have been developed where a Dove prism is used to rotate the final image in order to vary the angle between the diode array and the track direction. If the pitch of the laser diodes is optically de-magnified, then the degree of angu¬ lar variation from inner to outer tracks is reduced, until in the final analysis, the pitch is reduced to the track pitch where no angular variation occurs. However, a fur¬ ther problem arises where low levels of de-magnification generate an unequal pitch in the written data. Consequent¬ ly, the minimum pitch achieved is that commensurate with achieving adequate signal to noise, and the larger pitches result in decreased areal density.

A system which was designed to enable a laser diode array to be used in disc recording is disclosed in a paper entitled "Multichannel optical recording using monolithic arrays of diode lasers : errata" by Carlin et al, APPLIED OPTICS, Vol. 23, No. 24, pages 4613-4619, 15 December 1984. Although by using modern laser diode arrays, this system could meet the requirements of equal recorded track pitch, it involves the use of lower numerical aperture collection systems than that required for efficient beam collection, and it also utilises a very high numerical aperture for the imaging objective. This is combined with the use of long optical paths to limit vignetting and cylindrical optics which are difficult to align in a production envi¬ ronment. Such a system would lead to an optical path length from source to image of nearly half a metre which would imply a not very compact optical system. Additional¬ ly, the system would still require the use of a Dove prism to compensate for the angular variation of the pitch axis from the outer disc diameter to the inner disc diameter. It is probably for these reasons that such a system has only been developed for the military market.

It is therefore an object of the present invention to provide apparatus for optical recording on light-sensi¬ tive recording medium, such as a tape or disc, which maxi¬ mises the optical throughout whilst also reducing the pitch to the dimensions of the order required for optical record¬ ing, and which is also cost-effective to manufacture and is of a compact design thereby rendering it suitable for mass-market applications.

Accordingly, one aspect of the present invention consists in apparatus for optical recording on a light- sensitive recording medium, the apparatus comprising an array of discrete laser light sources for emitting a plu¬ rality of laser beams of a predetermined numerical aperture for forming an image of the array for focussing on the re¬ cording medium, the laser light sources of the array being mounted at a predetermined pitch relative to each other and being arranged to be positioned at a predetermined distance from the recording medium, characterised in that the appa¬ ratus also comprises an optical arrangement for splitting the image field into a plurality of light sources of lower numerical aperture than said predetermined numerical aper¬ ture at an intermediate image space located between the laser light source array and the recording medium and for imaging the plurality of light sources of lower numerical aperture through a final objective lens in the focal plane of the recording medium at a substantially reduced pitch relative to the pitch of the laser light source array.

Thus the present invention effectively balances the numerical aperture of the laser light sources to be commen¬ surate with the reduced pitch of the image focussed on the recording medium.

The optical arrangement may preferably include an array of microlens elements. Advantageously, the pitch of the microlens elements of the array is substantially the same as the pitch of the laser light source array.

In one embodiment, the optical arrangement com¬ prises first lens means for creating an intermediate image and second lens means for collecting the laser beams from the intermediate image and for redirecting them through the final objective lens. In order to ensure that all the laser beams pass through the final objective lens, the magnification of the intermediate image is arranged such that the pitch of said intermediate image is slightly larger than the pitch of the laser light source array.

The laser light source array may comprise a plural- ity of laser diodes. In one example of the optical ar¬ rangement, the first lens means includes a relay arrange¬ ment, and preferably also anamorphic lens or prism means for transforming the elliptical cones of energy of the laser beams into circular cones of energy. The anamorphic lens or prism means may also be used to adjust the magnifi¬ cation of the intermediate image. The second lens means may include the array of microlens elements and a collimi- nated portion for sampling and/or summation of the laser beams passing therethrough and for ensuring that the image remains focussed in the focal plane of the recording medi¬ um.

Alternatively, the laser light source array may comprise a plurality of fibre optic illuminators coupled to one or more laser light-emitting means. In another example of the optical arrangement, first and second lens means may be provided to act as a telescope and for redirecting the laser beams through the final objective lens for focussing in the focal plane of the recording medium.

The laser light source array may comprise a plural¬ ity of discrete laser light sources arranged in at least two rows with the laser light sources of adjacent rows being staggered relative to each other in an interlacing configuration. In an embodiment including a microlens array, the microlens elements may be arranged in substan¬ tially the same interlacing configuration. In this way, the pitch of the laser light source array is reduced, which effectively reduces the separation distance between the microlens array and the final objective lens, which is required to image the intermediate image in the focal plane of the recording medium at the same reduced pitch which would have been obtained with a single row array of the laser light sources.

In an alternative embodiment, the optical arrange¬ ment may include holographic optical means. According to another aspect of the present inven¬ tion, there is provided a device for optical recording and/or reading of data on a light-sensitive recording medium, the device including optical recording apparatus constructed in accordance with the first aspect of the present invention, and including a further optical arrange¬ ment for reading data recorded on the light sensitive medium.

The invention will be further described by way of example with reference to the accompanying drawings, in which-

Figure 1 shows a schematic enlarged view of 'part of a laser diode array;

Figure 2 shows a schematic lay-out of optical recording apparatus in accordance with one embodiment of the first aspect of the invention;

Figure 3 shows a schematic lay-out of an optical tape recording device in accordance with one embodiment of the second aspect of the invention; and

Figure 4 shows a schematic lay-out of optical recording apparatus in accordance with another embodiment of the first aspect of the invention.

Referring firstly to Figures 1 and 2, an optical apparatus includes a laser diode array 1 comprising a row of laser diodes 2 mounted in side-by-side relationship with a predetermined pitch p, of for example lOOμm. As can be seen in Figure 1, each diode 2 emits a laser beam having an elliptical cone of energy 3 which, in the illustrated example, has a 30° angle of divergence along its major axis and a 10° angle of divergence along its minor axis, thereby resulting in a numerical aperture of approximately 0.5. As shown in Figure 2, the laser beams are directed through lens arrangements, to image the laser diode array 1 in the focal plane of a light-sensitive recording surface 4 of^" an optical recording medium 5, such as a tape or disc, the pitch of the image on the recording surface being much smaller than the pitch p of the laser diode array 1.

The apparatus shown in Figure 2 includes a first lens arrangement comprising a pair of relay lenses 6, 7 and a pair of anamorphic prisms 8 located between the relay lenses. This first arrangement collimates the ellip¬ tical cones of energy 3 emitted by the laser diodes 2 and then expands and reduces the cones. This has the effect of expanding the 10° divergence angle of the elliptical cones to produce a circular cone of 30° divergence angle and then reducing the circular cone to a divergence angle of 10°. The first lens arrangement then creates an image 9 of the laser diode array 1 at an intermediate image space between the laser diode array and the light-sensitive surface 4 of the recording medium 5. To create, for exam¬ ple, a circular image of the laser diode array, which image has a 3μm diameter with a cone angle of divergence of 10° and a pitch of lOOμm, the second lens 7 of the cor¬ rection is required to have a focal length of three times the focal length of the first lens 6 of the pair.

The image 9 of the laser diode array is then col¬ lected and redirected to focus on the light-sensitive recording surface 4 by a second lens arrangement. This second arrangement includes a microlens array 10 which collects the circular cones of energy from the image 9, splits the image into a plurality of images of a numerical aperture lower than that of the beams emitted by the laser diode array, and redirects them through a final objective lens 11 which images the image 9 in the focal plane of the recording surface 4 at a much reduced pitch, the reduced numerical aperture of the images being commensurate with that of the reduced pitch. To ensure all beams pass through the final objective lens 11, the intermediate image should have the correct magnification such that the pitch of the intermediate image is slightly larger than the pitch of the individual elements of the microlens array. Control of the image magnification is achieved by counter rotating the anamorphic prisms which expands the laser beams to achieve variation of the magnification whilst not altering the focus. The separation distance between the microlens array 10 and the final objective lens 11 and the focal length of the objective lens determine the de-magnifica¬ tion, and thus the final pitch, of the recorded image. For example, if the focal length of the objective lens 11 is 3 mm and the final required pitch of the recorded medium is 2.5μm then the separation between the microlens array and the objective lens is 120 mm. Consequently the ratio of the pitches of the intermediate to the microlens element is 1.0036:1 and the pitch of the intermediate image is 100.36μm.

In Figure 2, the final objective lens 11 is split in order to provide a collimated section including a sample prism 11a for beam sampling and/or summation and to ensure that the image remains focussed whilst scanning across the optical recording medium.

In a specific application of the apparatus in Figures 1 and 2, the medium is preferably an optical tape which is driven by a high speed tape transport system. By using such a system to record data at a given data rate, the number of laser-diodes in the array can be reduced. For example, a tape transport system operating at approxi¬ mately 6.1m/s transport rate requires twice the number of laser diodes (operating at half the power) than a transport system operating at approximately 12.2 m/s. Consequently, the image collection space is reduced if a lOOμm initial diode pitch is assumed.

Figure 3 shows a schematic lay-out of one example of a linear multichannel tape recording device including the apparatus of Figure 2 which is shown in less detail in Figure 3. In this arrangement, the cones of energy from the intermediate image 9 are collected by the microlens array 10 and directed through a lens 12 and a beam splitter cube 13 to a final objective lens 14, via a reflective prism 15, a quarter-wave plate 16 and a mirror 17, for focussing on the recording surface of the optical recording tape 18.

The read system of the device is generally conven¬ tional and includes a single read laser diode 19 which emits a beam which passes through a pulse-width modulated grating 20. This grating generates N+l beams from the single beam of laser diode 19 for an N beam recording system, i.e. from an array 1 of N laser diodes 2. The N+l beams pass through a beam splitter 21 and the beam split¬ ter 13 to be reflected from prism 15 and mirror 17, .to pass through the objective lens 14. The N+l beams are then reflected from the recording surface 18 and pass back along the same optical path until they reach the beam splitter 21 which then directs the beam through a focussing lens 22 to enable the reflected beams to be detected by an array of photodiodes 23. A sample of the N+l reflected beams is imaged via a semi-silvered mirror 24 onto a quadrant focus and tracking photo-detector 25. An electrical output from the photo-detector 25 is then passed to a control motor 26 of the tape transport to adjust the tracking and to a control 27 of the final objective lens 14 to adjust the focussing. Electronic data output 28 from the photodetec- tor array 23 is passed to electronic circuitry (not shown) for processing.

The enlarged schematic view shown ringed at 29 in Figure 3 illustrates the written data 30 from the N beams and the N+l read beams 31. As can be seen, the array of beams is slanted at an angle α to the track direction so that the pitch of the image on the recording surface is effectively greater than the pitch of the tracks. For example, if the recorded image pitch is 2.5μm and the slant angle o. of the array is approximately 40°, then the track pitch will be 1.6μm.

In the illustrated embodiment, the microlens array 10 comprises a linear array. However, it may alternatively comprise a two-dimensional array consisting of two .or more rows of microlens elements with the elements of adjacent rows being staggered with respect to each other in an interlaced configuration. This arrangement effectively reduces the pitch of the microlens array and enables the separation distance between the microlens array and the final objective lens to be proportionately reduced. For example, if the microlens array consists of a single row with a pitch of lOOμm then, in order to achieve a 40:1 reduction to an image pitch of 2.5μm on the recording surface, then a microlens to objective lens separation of 160 mm is required for an objective lens having a focal length of 4mm. If a two-row interlaced array of microlens elements is used, then the pitch of the microlens array is reduced to 50 μm and a microlens to objective lens separa¬ tion of only 80 mm is required. In the same manner, if a three-row interlaced array is used then the pitch of the microlens array is reduced to 33.3μm and a microlens to objective lens separation of only 60 mm is required.

The present invention is advantageous in that it can be made from cheap optical components in a very compact design, thereby enabling an effective and efficient record¬ ing system to be developed for the mass-market.

Whilst particular embodiments of the present inven¬ tion have been described, various modifications will be envisaged without departure from the scope of the invention as defined in the appended claims.

For example, the apparatus can be used for record¬ ing on disc, as well as tape, without incurring the above- mentioned problems of angular variation and unequal pitch from the inner to outer disc tracks. In the present inven- tion, a sample beam taken from the sample prism 12 can be focussed on a photodetector to monitor the angular varia¬ tion and track pitch and to adjust them accordingly.

In an alternative application, the optical record¬ ing apparatus may be used in optical printing techniques, such as laser excited dye diffusion thermal transfer.

Furthermore, holographic optical elements may be used to reduce the numerical aperture of the light sources and to re-direct the cones of energy through the final objective lens. In this alternative arrangement, the holographic optical elements would effectively replace both lens 7 of the pupil relay pair and the microlens array 10.

In another embodiment of the optical recording apparatus of the present invention shown in Figure 4, the discrete laser light sources of the array comprise a plu¬ rality of mono-mode fibre optic illuminators 33, each transmitting light from an individual laser diode (not shown) or from some other laser light-emitting means.

Limitations on the individual power available in laser diode arrays require the combining of a number of individual high power laser diodes. However, in this embodiment, individual high power laser diodes can be coupled, using discrete optics, into a mono-mode fibre arrangement with up to 70% coupling efficiency achieving higher array throughput power.

As shown in the enlarged view ringed in Figure 4, an array of the fibres 33 are brought together in an aligning block 34, which maintains the alignment of the axis of the fibre parallel with the main optical axis 35 and sets the pitch of the fibres. Practical limitations inhibit the placing of the pitches of the individual fibres, the individual diameters of which are of the order of 100 μ , closer than 250 μm. The laser beams emitted from the fibres 33 have a substantially circular or semi- elliptical cone, and pass through an array of microlenses 35 which creates a slightly diverging beam, that is_r having a reduced numerical aperture, onto the front face of lens 36. This lens 36 acts in conjunction with another lens 37 to form a telescope with the fibre array 33 being imaged between the lenses 36 and 37 at focal point 38. The light beams passing through the lens 37 are collected by a final objective lens 39 and focussed on a light-sensitive record¬ ing surface 40 at a much reduced pitch.

In one particular example, it can be assumed that the beams emitted by the individual fibres have a diver¬ gence of 0.11 NA. The microlens array, with a focal length of 1.3 mm, collects this to form an image of approximately 0.9 mm at the front focal plane 38 of lens 36 with a cone angle formed by the microlens array of 0.015. If lens 36 has a focal length of 5 mm and is separated from lens 37 by approximately 27 mm, then the beam will be expanded to a collimated beam approximately 4 mm in diameter. Using a final objective lens 39 with a 4 mm focal length and a numerical aperture of 0.5, will produce images on a 10 μm pitch in the focal plane of the surface 40.

When used in an optical tape recorder, the fibre array 33 can be interlaced into a two-dimensional array, and will produce a final pitch between the tracks of 10 μm/N, where N is the interlace factor.

Claims

1. Apparatus for optical recording on a light- sensitive recording medium (4), the apparatus comprising an array (1) of discrete laser light sources (2) for emitting a plurality of laser beams of a predetermined numerical aperture for forming an image of the array for focussing on the recording medium (4), the laser light sources (2) of the array (1) being mounted at a predetermined pitch rela¬ tive to each other and being arranged to be positioned at a predetermined distance from the recording medium (4), characterised in that the apparatus also comprises an optical arrangement (6,7,8,10) for splitting the image field into a plurality of light sources of lower numerical aperture than said predetermined numerical aperture at an intermediate image space located between the laser light source array (1) and the recording medium (4) and for imaging the plurality of light sources of lower numerical aperture through a final objective lens (11) in the focal plane of the recording medium (4) at a substantially re¬ duced pitch relative to the pitch of the laser light source array (1) .

2. Apparatus as claimed in claim 1, wherein said optical arrangement (6,7,8,10) includes an array (10) of microlens elements.

3. Apparatus as claimed in claim 2, wherein the pitch of the microlens elements of the array (10) is substantial¬ ly the same as the pitch of the laser light source array

(!)•

4. Apparatus as claimed in any preceding claim, where¬ in the lens arrangement (6,7,8,10) includes first lens means (6,7,8) for creating an intermediate image and second lens means (10) for collecting the laser beams from the intermediate image (9) and for redirecting them through the final objective lens (11).

5. Apparatus as claimed in claim 4, wherein, in order to ensure that all the laser beams pass through the final objective lens (11), the magnification of the intermediate image (9) is arranged such that the pitch of said interme¬ diate image is slightly larger than the pitch of the laser light source array (1) .

6. Apparatus as claimed in claim 5, wherein the laser light source array (1) comprises a plurality of laser diodes (2) .

7. Apparatus as claimed in claim 4, 5 or 6, wherein the first lens means includes a relay arrangement (6,7) and anamorphic lens or prism means (8) for transforming ellip¬ tical cones of energy of the laser beams into substantially circular cones of energy.

8. Apparatus as claimed in claim 7, wherein the anamorphic lens or prism means (8) is arranged to adjust the magnification of the intermediate image (9).

9. Apparatus as claimed in claim 7 or 8, wherein the second lens means includes an array (10) of microlens elements and a collimated portion (11a) for sampling and/or summation of the laser beams passing therethrough and for ensuring that the image remains focussed in the focal plane of the recording medium ( ) .

10. Apparatus as claimed in any one of claims 1 to 3, wherein the laser light source array (1) comprises a plu¬ rality of fibre optic illuminators (33) coupled to one or more laser light-emitting means.

11. Apparatus as claimed in claim 10, wherein the optical arrangement includes first and second lens means (36,37) acting as a telescope and for redirecting the laser beams through the final objective lens (39) for focussing in the focal plane of said recording medium (40).

12. Apparatus as claimed in any preceding claim, where¬ in the laser light source array (1) comprises a plurality of laser light sources (2) arranged in at least two rows with the laser light sources of adjacent rows being stag¬ gered relative to each other in an interlacing configura¬ tion.

13. Apparatus as claimed in claim 12, wherein the optical arrangement includes an array (10) of microlens elements and said elements are arranged in substantially the same interlacing configuration as said array (1) of laser light sources.

14. Apparatus as claimed in claim 1, wherein said optical arrangement includes holographic optical means.

15. Device for optical recording and/or reading of data on a light sensitive recording medium (4), the device including optical recording apparatus as claimed in any preceding claim, and including a further optical arrange¬ ment for reading data recorded on the light sensitive medium (4) .