APPARATUS FOR THERMALLY RECORDING DATA

Abstract: An optical information-bearing-signal recorder employs thermally-induced recording of ablative, material movement, material phase change, magneto-optic or similar technologies. Synchronous noise in the recording is reduced by employing a heat-inducing energy pulse (25), coherent light for example, shaped to have an initial recording-initiating intensity amplitude (44, 44A) followed by a recording-sustaining amplitude (55, 55A). The duration of the recording-initiating amplitude portion is constant for all lengths of signals being recorded. When a rotating optical disk is employed as a record-bearing medium, the recording-initiating and recording-sustaining amplitudes are increased with increasing radius of the recording on the disk.

Description: The present invention relates to apparatus for thermally recording data. The use of lasers for emitting heat-inducing coherent radiation and E-beams, which are heat-inducing beams commonly used with semiconductors, has been widely practiced in micromachining and signal recording applications. In the micromachining area, harmonic distortion that may be caused by the recording pattern of the thermally-induced recording is of no significance since information-bearing signals are not recorded. On the other hand, when lasers and the like are employed for recording information-bearing signals, the sensing of the recording is sensitive to the quality of the recording; that is, it is desired to maintain as high as possible signal-to-carrier and signal-to-noise ratios. The higher these ratios, the greater the probability of faithfully recording and reproducing information-bearing signals. The problems associated with such recording become acute as the areal densities increase which means that the energy levels in recording and sensing of information-bearing signals are reduced. As a result of the reduced energy levels involved in the recording and read-back, the signal-to-noise ratios are also reduced. Part of the noise is synchronous noise; that is, noise that is induced by either the readback or recording processes. At relatively modest areal densities, the induced synchronous noise may be tolerated; at higher areal densities such noise is more troublesome. In the micromachining area, US-A-4,475,028, McGuire, Jr. et al., shows a multimode constant-potential pulsed welding apparatus having a hot-start feature in which the number of pulses per second is doubled at the beginning of the welding operation. The purpose of the "hot-start" is to facilitate initiating an arc at a lower current level than that provided by a lower-frequency pulsing, i.e., the hot-start, or increased energy, at the beginning of the welding period, is for reliably establishing an arc for welding. There also has been substantial work in the laser machining of thin films and integrated circuits, as reported in the BELL SYSTEM TECHNICAL JOURNAL; for example, by the article by M. I. Cohen, et al., "Laser Machining of Thin Films and Integrated Circuits", BELL SYSTEM TECHNICAL JOURNAL, March 1968, pp. 385-405. This article teaches the use of solid-state lasers in fabricating tools which provide sharp definition and localised nature of working regions for allowing heating, melting or vaporising minute amounts of material with minimal effect on adjacent material or components. The article teaches that it is the optical power density in the focused spot at the piece being machined rather than the laser-power output itself that determines the suitability of a laser for removing or ablating material. Further, the size of the affected zone on the target material being machined depends upon the thermal properties of the material as well as the laser beam's spot size and the energy-intensity distribution across the laser-emitted beam. Edge definition of the affected zone depends primarily on thermal properties of the target and the duration of the exposure. Reflectance of the surface of the material being machined may decrease abruptly as the material melts or reacts with the atmosphere, with subsequent laser-beam absorption occurring at greater efficiencies. Initial laser-output energy has to be sufficient to initially break down the surface. The machining taught by this article consists of a sequence of pulses of equal duration, size and shape and of substantially constant amplitude. In another article, also in the BELL SYSTEM TECHNICAL JOURNAL, by D. Maydan, entitled "Micromachining and Image Recording on Thin Films by Laser Beams", August 1971, pp. 1761-1789, on page 1772, the relationship of the spot-size diameter to the size of the machining area is illustrated, see Fig. 6. An example of micromachining light pulses is shown in Fig. 7 on page 1773 as being light pulses of identical amplitude and of short equal durations. Fig. 8, on page 1774, shows that lines of differing widths may be provided using different laser-intensity outputs. Fig. 9, on page 1775, shows photographs of individual machining spots obtained from the pulsed-laser machining operation. The various pulse shapes used in the machining operations are shown in Fig. 13, page 1780. The use of laser machining using pulses of constant amplitude for video signal recording is shown in Fig. 17 on page 1784. An article by Cohen, et al., entitled "Application of Lasers to Microelectronic Fabrication", published by the New York Academic Press, 1968, in pp. 139-186, is an additional reference showing the effects of a laser beam on material being treated. At page 156, the article teaches that spot size increases with power- or light-intensity levels. Statements on page 164 compare laser welding with fusion welding. Page 167 discusses the effects of pulse duration. This article teaches that, in welding using a pulsed laser, one of the most important parameters to be considered is pulse duration. Fig. 17 on page 168 shows the effect of reducing the pulse duration while increasing the intensity to retain the same pulse energy and the resultant effect of the machining operation on the material. The authors state that it is possible to make satisfactory welds over a wide range of pulse durations so long as a minimum time required for adequate heating conduction is provided. While such a parameter is satisfactory for welding, it is certainly not an appropriate approach for high-areal-density information-bearing-signal recording. The authors also discuss the importance of maintaining proper energy control. On page 171, it is stated that the authors have previously suggested that a pulse shape that quickly rises to a peak value and then drops or decays to a lower level may be desirable for welding. This observation apparently is to prevent unintended splatter of the material as may be caused by overheating the area being subjected to a laser beam. US-A-3,962,558 to Kocher, et al., shows a pulsed- laser drilling machine having an initial high-energy pulse followed by a sequence of lower-energy pulses. Again, this arrangement is apparently selected to prevent undue splattering of the material being machined. An improvement over the Kocher, et al., patent is shown in US-A-4,114,018 to Von Allmen, et al., which cites the machining techniques shown in US-A-3,962,558 in Figure 2 as well as the amplitude-decaying technique of Cohen, et al., supra, in Figure 3. Von Allmen, et al., teach, as shown in Fig. 4 of the patent, that an optimum-current amplitude which is substantially constant is the preferable way of laser machining. This stated approach is confirmed by US-A-4,410,968 to Frohbach, et al., which teaches that, for ablative information-bearing signal recording, the energy level of the recording laser beam should be such as to move the material using constant recording power such as to deform the film of the record-bearing medium for causing local redistribution of the material without vaporising or splattering the material. Another form of pulsed-laser recording is shown in US-A-4,473,829 to Schouhamer Immink, et al., which uses overlapped circular-pulse-shaped beams for producing thermally-induced recordings of diverse lengths on a record-bearing medium. While rapidly pulsing a laser, or similar beam-emitting device, may be appropriate at relatively modest linear recording densities, at higher linear recording densities with a relatively rapidly-moving record-bearing medium, such pulsing can be difficult to achieve, i.e., it is more desirable to turn the laser or other beam-emitting source on and leave it on for the pulse duration. For variable-pulse-length recording operations, by not pulsing the laser, higher linear densities should be achievable. Yet another form of pulsed laser recording is shown in EP 109,130 (Philips) which uses a complex feedback mechanism analysing the reflected energy from a recording surface to detect the onset of pit formation, ie local melting of the recording material, and using this information to reduce the output energy of the laser thereby producing pulses of high initial intensity and lower final intensity in an attempt to produce pits in the recording medium of constant width. This approach may be acceptable at low or moderate recording rates but at high recording rates the response time of the feedback loop would be unacceptably long, causing excessive heating of the recording medium between the points at which melting begins and the laser output reduces. Additionally, this approach would be useless in certain types of pulsed-laser recording in which the surface reflectivity does not significantly change as a result of data being recorded onto the surface, eg: magneto-optic recording (see later). JP-A-55-139693 describes a write system for an optical recording medium wherein each of the write signal pulses consists of a first portion having a relatively high constant amplitude followed by a second portion having a relatively low constant amplitude. The stated purpose is to perform bit by bit writing of high precision whereby the influence of heat in writing is eliminated even when the write pulses have long durations. Pulsed lasers, in addition to ablative or material-redistribution recordings, have also been used for crystalline-to-amorphous phase-change recording. It is not readily apparent that the laser controls for ablative recording would apply to other forms of optical recording. For example, in the article in the IEEE TRANSACTIONS ON ELECTRON DEVICES by Ovshinsky, et al., entitled "Amorphous Semiconductors for Switching, Memory, and Imaging Applications", on pp. 91-105 of Vol. ED-20, No. 2, February 1973, page 97 in Fig. 10 shows electrical impedance characteristics of the amorphous-crystalline switchable material. In particular, there is shown an initial high electrical impedance value once the material has switched phase states; then the electrical impedance level drops as evidenced by the lowered voltage across the material. This article also shows the reversibility of optical effects of amorphous semiconductors that switch between amorphous and crystalline states. The term "semiconductors" is also to be applied to semi-metals, which are more commonly used in the phase-change optical recording. Apparently constant current pulses were used in the recording rather than constant power pulses. Another type of optical recording is the so-called magneto-optic disk such as described by Tsujiyama in ELECTRONIC AND COMMUNICATIONS IN JAPAN, Vol. 60-C, No.7, 1977, in an article entitled "Magneto-Optical Disk Memory Utilising Multilenses", on pp.89-97. This article teaches recording on a magneto-optic record-bearing medium using pulsed lasers having a constant amplitude and shape for generating recording in circular recording areas. In a magneto-optic recording system, an ancillary magnetic field steers the remnant magnetisation in one direction or the other while the laser is heating the material above the Curie point. Of interest here is that increases in energy density increase the size of the spot, hence the width and size of the recording. A series of pulses is used for recording short or long pulses, as shown in Figs. 12 and 13 of this article. Magneto-optic recorders are also described in the March 1983 issue of Japan Electronic Engineering (JEE) in an article by N Imamura entitled "Research Applies Magnetic Thin Films and the Magneto-optical Effect in Storage Devices." The use of E-beams in semiconductor manufacture is quite well known. For example, see the IBM TECHNICAL DISCLOSURE BULLETIN article, "E-beam Kinetic focus for High-Speed Pattern Generation" by Koste, et al., December 1978, Vol. 21, No. 7, pp.2768-2769. The use of E-beams for thermally recording identification indicia on a semiconductor chip is shown by P. M. Ryan in an article entitled "Automatic Serialisation of Chips for Identification and Traceability", IBM TECHNICAL DISCLOSURE BULLETIN, Vol. 22, No. 1, June 1979, pp. 108-111. The present invention provides data recording apparatus comprising a thermally responsive record medium having a predetermined thermal profile, means for thermally recording data on the record medium during motion thereof; said recording means comprising an energy-emitting source capable of locally heating the surface of such a record medium in response to an applied recording signal with an intensity related to the amplitude of the recording signal, and recording signal generating means responsive to an input data pulse train to produce a corresponding train of recording signal pulses, each recording signal pulse comprising a high amplitude portion followed by a lower amplitude portion, characterised in that the recording signal generating means is arranged to reduce the amplitude of each recording signal pulse so that the profile of said high amplitude portion matches the predetermined thermal profile of the record medium. Preferably the recording signal generating means includes means for producing for each input data pulse a first signal of predetermined amplitude and of duration proportional to that of the corresponding input pulses, means for producing a second signal, initially coincident with the first signal, having a profile matching the thermal profile of the record medium with gradually reducing amplitude and of fixed duration, and means for adding the first signal to the second signal to produce said recording signal pulse train. Preferably the thermally sensitive record-medium is in the form of a rotatably-mounted disk, said energy-emitting source is at a variable distance from the axis of rotation of said disk, and said recording signal generating means is arranged to vary the amplitude of the recording signal in a manner dependent linearly on said variable distance. The invention is of particular value when the thermally-sensitive record-medium is also magnetically-sensitive and said energy-emitting source is a laser. In a further aspect of the invention there is provided a method for thermally recording data on a thermally responsive record medium during motion thereof comprising the steps of: determining the thermal profile of the record medium, generating a train of recording signal pulses in response to a corresponding train of data pulses, said recording signal pulses comprising a high amplitude portion followed by a lower amplitude portion and locally heating the surface of the record medium in response to said recording signal pulses; the step of generating the recording signal pulses being characterised by reducing the amplitude of each recording signal pulse so that the profile of said high amplitude portion matches the predetermined thermal profile of the record medium. Figure 1 is a simplified drawing showing apparatus for thermally recording data according to the present invention; Figure 2 shows a pulse-length recording on a thermally-responsive record-medium illustrating the effect of the present invention; Figure 3 illustrates read-back signals from symmetrical test pattern recordings illustrating the effect of the present invention; Figure 4 is a schematic diagram of recording circuits employed in the apparatus of Figure 1; Figure 5 is a wave-form diagram showing various signals in the circuitry of Figure 4; and Figure 6 shows an alternative recording pulse waveform which may be produced by the circuitry of Figure 4. Referring now more particularly to the drawings, Figure 1 illustrates a portion of an optical information-bearing-signal recorder. The arrangements of such recorders are well known in the art and are not detailed for that reason. Data to be recorded is supplied over data line 10 to recording, or write, circuits WRT 12 for recording on rotatable optical-record disk 11, which is an optical thermally responsive record-bearing medium. In a preferred form, record-bearing medium 11 includes a plurality of radially-displaced circular tracks indicated on disk 11 by two circular dashed lines. The actual configuration of the tracks can be either a plurality of concentric circular tracks or a single spiral track having a plurality of circumvolutions, each circumvolution being termed a single track portion of the spiral track. Assuming that record-bearing medium 11 is rotating at a relatively constant rotational speed, the intensity of the heat-inducing laser beam impinging on record-bearing medium 11 is varied with the radial position of the record track. To achieve this variation in intensity, the track number, which doubles as a radial-position indicator, is supplied over cable 13 to recording circuits 12. Recording circuits 12 generate recording-controlling pulses by combining the radial indicator on cable 13 with the incoming data signals on line 10 to produce a series of control pulses on control line 14 for modulating solid-state laser 15 output-light intensities. Solid-state laser 15 can be any gallium arsenide or other solid-state laser which provides output-light intensities in accordance with diverse input-control amplitude signals, particularly in accordance with a constant-current amplitude. The laser 15 output-light beam follows light-path axis 16 through focusing objective lens 17, then proceeds over light path 18 to disk 11. Omitted from the drawing are the tracking circuits and the track seeking circuits which move beam 18 radially of disk 11 for tracking and seeking, as is well known. The usual focusing circuits and other control and data-transfer circuits also are not shown. Figure 2 illustrates a typical thermally-induced recording pattern for pulse-length recording as found in ablative recording, phase-change recording, magneto-optic recording, and the like. Pulse-length recording represents information by the length of a recorded pattern; one length is unity, a second length is two, etc. Therefore, a relatively elongated pulse-length-recorded spot 21 represents a plurality of informational units in accordance with the length of the spot 21 along a track on record-bearing medium 11. When a constant-energy heat-inducing pulse records information into spot 21, the width of the recording area 21 at the onset portion of the recording is at a desired radial width. As the pulse length gets longer, the constant-energy heat-inducing beam causes the recording area to radially enlarge, as at point 22. The radial enlargement of the recording area results in generation of synchronous noise in the read-back signals, as later explained with respect to Figure 3. If the shape of the recording area 21 can be controlled to that represented by dashed lines 23, the synchronous noise is either eliminated or greatly reduced. The present invention provides a recording area 21 of any desired arbitrary length which substantially eliminates the radial enlargement 22, thereby reducing synchronous noise from the recording induced by recording using a recording-initiating energy intensity. Figure 3 is a simplified showing of a test pattern of complementary recording patterns which result in a so-called "M W" read-back pattern and which show synchronous noise effects. Such synchronous noise effects are caused by even harmonics having substantial amplitude with respect to the base-frequency amplitude. The present invention avoids the synchronous-noise problem by supplying a control pulse over line 14 and a resultant light beam having varying intensities with the shape of wave form 25 (Figure 5). This wave form shows an initial high recording-initiating amplitude, followed by a lower recording-sustaining amplitude in the heat-inducing beam recording information-bearing signals on record-bearing medium 11. Such amplitudes are empirically determined for each type of record medium. The recording-initiating energy level enables high-speed recording--the level quickly heats the recording area to a recording-inducing temperature. The advantages of practising the present invention, as shown in Figure 3, can be seen by comparing solid read-back wave form 26, which is read back using constant-amplitude recording-initiating energy-level pulses, while dashed line 27 employs the recording wave form 25. The zero-axis crossing is caused by recording long pulses to create read-back wave form 26 dipping well below the zero axis 31, as at point 28, while the read-back signal 27 from the compensated recording using this invention crosses the zero axis 31 at point 30. The signal 27, at point 30, is substantially similar to the zero-axis crossing 29. This similarity indicates a low level or absence of synchronous noise. The wave form 26, having a substantial portion below the zero axis at point 28 and insubstantial zero-axis crossing at point 29, indicates second harmonic distortion or synchronous noise in the read-back signal. This read-back signal 27 inferentially indicates that the recording area 21 has a substantial elongated set of parallel sides, as indicated by dashed lines 23. Generating the compensated recording-light-beam pulse is best understood by referring next to Figures 4 and 5. The data input represented by idealised signal 40 is received in variable delay circuit VD 35. Variable-delay circuits are well known and not described for that reason. The radial-position information, i.e., track address, supplied over cable 13 is received by digital-to-analogue converter DA 36. The variation in signal delay is controlled by the analogue signal supplied over line 37 to VD 35. Generally, the recording signal is delayed with respect to a disk cell (not shown) on the record-bearing medium 11, which delay is varied with radius. The greater the radius, the longer the delay provided to the data signal by VD 35. The output signal 41 of VD 35 on line 38 is variably delayed by amount 42, as above described. The Figure 4 illustrated circuits have first and second current-control pulse-generating portions respectively indicated as current sources 50, 56. A first portion generates the recording-initiating amplitude 44 (Figure 5) by adding an output-current pulse from each of the pulse-generating portions, as later described. Single-shot circuit SS 43 (monostable multivibrator) responds to signal 41 to generate an output pulse 45 of constant duration. Pulse 45 travels over control line 46 to actuate electronic switch 47 for passing a constant-amplitude-current pulse received from current source I1 50 to control line 14, thence to laser 15. The output amplitude of current source 50 is controlled by DA 36 supplying a radial-related control signal over line 51 and is independent, in this implementation of the invention, of the optional pulse shaper 62. The greater the radial-position indication in cable 13, the greater the amplitude supplied by current source 50. The second pulse-generating portion supplies a base-current portion 55, generated from signal 41. Signal 41 travels through fixed-delay circuit FD 52, which has a delay equal to the signal-propagation delay of SS 43 such that the leading edges of the current outputs of the two pulse-generating portions are time-synchronous. The output signal 53 of FD 52 activates electronic switch 54 to pass a constant-current amplitude from current source I2 56 to line 14, then to laser 15. Signal 53, passed from FD 52, results in generation of base-current portion 55 of wave form 25. Line 14 ohmically interconnects the outputs of switches 47 and 54 for current summing the output of the two switches 47, 54 resulting in wave form 25. The laser 15 outputs a light beam having an energy content and intensity along path 16 with the same shape as the control wave form 25. Data signal 40 includes a short-duration pulse 58, resulting in an output-control pulse 59, a portion of wave form 25. The recording-initiating portion 44A has the same amplitude and duration as the recording-initiating portion 44 for the longer-duration data pulse. Base portion 55A has a duration representative of the duration of shorter data pulse 58, whereas base portion 55 has a duration representative of the duration of the longer data pulse 40A. The duration of the pulses in wave form 25 represents the information-bearing signals being recorded. In the magneto-optic recording, a first polarity of magnetic remanence represents an absence of a bit-indicating area 21, while the opposite or second polarity of magnetic remanence represents a bit-indicating area 21. In other optical recordings, two surface reflectance levels respectively represent medium background and record area 21. The shape of recording-initiating portion 44, 44A of wave form 25 can be modified for enhanced recording. While the constant current amplitude shape provides satisfactory noise reduction, further noise reduction is achievable by shaping the recording-initiating portion to the thermal profile of the record medium. Such a profile is empirically determined, as is well known. It is desired to raise the temperature of the recording area as quickly as possible; therefore, the leading edge 60 of pulse portion 44B of Figure 6 should be as steep as reasonably possible. Once the maximum current amplitude is reached, the shape of pulse portion 44B is the same as the thermal profile of the record medium receiving the recording. This profile typically has an exponential appearance. The trailing portion 61 of pulse portion 44B has a shape determined by the heating characteristic (thermal profile) of the record medium such that the temperature of the recording area is controlled to ensure constant width recording. Trailing pulse edge 61 leads into the recording-sustaining portion 55B of the laser control pulse. The time duration of trailing edge 61 may be longer than the duration of constant-current pulse portions 44 and 44A. The thermal profile of a record medium is a time measure of the temperature increase of the record medium for a constant-intensity laser beam. The profile of trailing edge 61 corresponds to the temperature increase profile preferably measured (as by infrared sensors) at the central portion of the laser beam image on the record medium. While such a profile can be determined by relatively stationary record medium and laser source, the profile is preferably obtained from a relatively moving record medium and laser source. Once the thermal profile is determined, the usual wave-shaping circuits can be designed to achieve the Figure 6-illustrated pulse 44B, 55B. Referring now to Figure 4, introducing the thermal profile shape to recording-initiating pulse portion 44B is achieved by including the optional pulse shaper 62; it is initiated by the leading edge of pulse 45 (Figure 5). Pulse shaper 62 supplies its thermal profile representative control signal 63 to current source 50. Current source 50 responds to the shaper 62 control signal 63 to vary the source 50 supplied current amplitude to switch 47 in accordance with the shaper 62 amplitude. Initially, the current amplitude indicated by the radial-indicating signal supplied by DA 36 over line 51 is supplied by I1 50, then decreasing current amplitudes are supplied following the thermal profile indicated by the pulser 62 control signal 63. The amplitude of control signal 63 is subtracted by I1 50 from the amplitude of the line 51 signal to activate the laser 15 to generate the trailing edge 61 of recording-initiating pulse portion 44B.

Claims: Data recording apparatus comprising a thermally responsive record medium having a predetermined thermal profile, means for thermally recording data on the record medium (11) during motion thereof; said recording means comprising an energy-emitting source (15) capable of locally heating the surface of such a record medium in response to an applied recording signal with an intensity related to the amplitude of the recording signal, and recording signal generating means (12) responsive to an input data pulse train (40) to produce a corresponding train (25) of recording signal pulses, each recording signal pulse (54) comprising a high amplitude portion (44) followed by a lower amplitude portion (55), characterised in that the recording signal generating means is arranged to reduce the amplitude of each recording signal pulse so that the profile of said high amplitude portion matches the predetermined thermal profile of the record medium. Apparatus as claimed in claim 1, in which said recording signal generating means includes means (52, 54, 56) for producing for each input data pulse a first signal (53) of predetermined amplitude and of duration proportional to that of the corresponding input pulse, means (43, 47, 50, 62) for producing a second signal, initially coincident with the first signal, having a profile matching the thermal profile of the record medium with gradually reducing amplitude and of fixed duration, and means (14) for adding the first signal to the second signal to produce said recording signal pulse train. Apparatus as claimed in any preceding claim, in which said thermally-sensitive record-medium is also magnetically-sensitive and said energy-emitting source is a laser. Apparatus as claimed in any preceding claim, in which said thermally-sensitive record-medium is in the form of a rotatably-mounted disk, said energy-emitting source is at a variable distance from the axis of rotation of said disk, and said recording signal generating means is arranged to vary the amplitude of the recording signal in a manner dependent linearly on said variable distance. A method for thermally recording data on a thermally responsive record medium during motion thereof comprising the steps of: determining the thermal profile of the record medium; generating a train of recording signal pulses in response to a corresponding train of input data pulses, said recording signal pulses comprising a high amplitude portion followed by a lower amplitude portion and locally heating the surface of the record medium in response to said recording signal pulses; the step of generating the recording signal pulses being characterised by reducing the amplitude of each recording pulse so that the profile of said high amplitude portion matches the predetermined thermal profile of the record medium.

Inventors: LEWKOWICZ, JULIAN
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: G11B7
IPC codes: G11B7, G11B11