Optical data recordal

The present invention relates to optical data recording equipment and methods, and is particularly, although not exclusively, concerned with dye thermal transfer printing.

In dye thermal transfer printing, heat is transferred to selected pixel areas of a dye layer to cause dye to be transferred from the heated areas to form printed pixels on an adjacent dye-receiving layer and thereby form a printed image.

Laser diodes are often selected as the heat sources since they are inexpensive, reliable and compact. In a typical printing device, the output beam is modulated in intensity by a single laser diode as it scans across the dye layer, and each scan prints one row of pixels on the recording layer. The beam can only be modulated on or off so that one pixel is either printed or not, or the beam intensity can be varied over a range of values ​​to vary the amount of dye transferred from a pixel area and provide pixels with different tones. to enable continuous tone printing.

However, laser diodes typically emit light from an elongate stripe region, and thus the beam quality is asymmetrical, with the output beam having a high divergence in the axis of the stripe width and a small divergence in the axis of the stripe length. A problem with laser diodes is that although the beam can be fairly easily focused in its axis with high divergence to a relatively high resolution, it is more difficult to do so in the low-divergence axis, and thus it is difficult to do so to get high overall resolution.

Another problem with the previous device is that the printing speed is low, since again every pixel has to be printed one at a time. Further, the optical structure for beam scanning is complex and increases the cost and size of the device while lowering the reliability.

One way to increase the printing speed and to simplify the scan setup is to place a plurality of laser diodes in a field, and to modulate each laser diode individually in intensity while the output beams collectively scan the dye layer. This then allows a plurality of pixel lines to be printed simultaneously. However, the use of a plurality of laser diodes itself adds to the cost and complexity of the device. US-4162121 discloses replacing a conventional scanner in a printing system with an array of nearby converters.

According to a first aspect, the present invention provides a data recording device, comprising a laser diode, which, in use, emits a beam of light from an elongated strip area, a modulator means, which is arranged in one dimension only parallel to the axis with low divergence of the beam and is operable to individually modulate different regions of the beam cross-section along the axis, to generate a plurality of individually modulated beam sections, and means for focusing the beam sections onto a recording medium, to effect the data recording.

In another aspect, the present invention provides a data recording method comprising modulating the light beam from a laser diode in one dimension which is dimension parallel to the axis with low divergence of the beam such that different cross-sectional areas of the beam are individually modulated along the axis to provide respective beam portions which are focused on a recording medium.

The invention is preferably for use in dye thermal transfer printing, in which case the recording medium may be a pair of adjacent dye delivery and receiving elements. However, other applications may be considered, such as etching wells or reorienting regions of a magnetic material as data in a suitable recording medium.

By spatially dividing the main beam into a field of separate beam sections along the low-divergence axis (ie along the axis which is parallel to the long dimension of the laser diode emission strip), the problems of resolution in this axis are reduced because each separate beam section can be focused on a separate pixel area. Thus, a better resolution can be achieved in the low-divergence axis which, if desired, can be adjusted to match the resolution in the high-divergence axis along which the beam is not split.

Further, the separate beam sections may print a plurality of pixels simultaneously, thereby increasing the printing speed and avoiding or reducing the requirement for large optical beam scanning devices without requiring the use of a plurality of separate laser diodes.

The modulator means may take any suitable form and may comprise, for example, an LCD array consisting of a single row of LCD lines extending across the axis of low divergence of the laser beam, each cell being individually switchable to the intensity of the area of the laser beam incident thereto, so that each cell generates a separate, individually modulated beam section.

In a preferred embodiment, however, the modulator means comprises an acousto-optic modulator, which extends over the axis with low divergence of the laser beam, where means are provided to generate an acoustic wave train in the modulator, wherein regions of the wave train act as a diffraction grating, each generating a deflected beam portion from a respective region of the laser diode beam cross section, when the beam passes through the modulator, wherein the intensity of a deflected beam portion depends on the amplitude of the wave train range generating the same.

The means may be arranged to focus the deflected beam portions on the recording medium, and the portions of the beam which are not deflected are blocked by suitable means. Alternatively, means may be provided for blocking the deflected beam sections and the device may be configured to focus the undeflected beam sections onto the recording medium. In this latter arrangement, however, it is difficult to linearly vary the intensity of the undeflected beam portions. Furthermore, it is not possible to diffract all the light into a deflected beam section. Therefore, it is not possible to completely switch off the non-deflected beam sections. This may be undesirable, especially in continuous tone printing, and thus focussing of the deflected rather than undeflected beam section is preferred.

An acousto-optic modulator has a number of advantages over an LCD panel because it is less expensive, less complex and more reliable. Furthermore, the resolution, ie the pixel size, determined by the lengths of the diffraction regions of the wavetrain (subject to any magnification / reduction by the imaging means), which are easily and accurately controllable to enable higher resolutions to be achieved than LCD panels. Further, the resolution can be modified quickly and easily by suitably changing the acoustic wave, in contrast to LCD panels which provide only a fixed resolution. Another advantage is that it is relatively easy to modulate the amplitude of an acoustic wave to produce beam portions of varying intensity, while it is more difficult and more expensive to produce an LCD cell capable of producing one Beam intensity over a range of values ​​to vary exactly. Acousto-optic modulators also provide better contrast than LCD panels.

The acoustic waves applied to the acousto-optic modulator may be of any suitable shape and may, for example, comprise a plurality of wave pulses, each pulse having the same amplitude or a different amplitude. Preferably, however, the acoustic wave is in the form of an amplitude modulated carrier wave, as this provides a particularly simple and responsive arrangement, and control means may be provided to apply a carrier wave to the acousto-optic modulator and around the carrier wave based on the data to be recorded in it Modulate amplitude. For example, if the deflected beams are focused on a dye layer, then a portion of the carrier wave may be modulated with a high amplitude to print a dark pixel, with a lower amplitude to produce a brighter pixel, and with a very low or zero Amplitude to prevent a pixel from being printed.

Regardless of the type of acoustic wave, this preferably has a frequency which generates an optimum diffraction of the laser light.

Since the acoustic wave is not stationary but propagates through the modulator, means are preferably provided to pulse-generate the laser diode beam for a time which is short compared to the propagation velocity of the wave. This allows the laser diode beam to interact with the propagating wave at a momentary timing of the wave. Further, a pulsed beam may be required if other modulators are used, for example, to give the cells of an LCD panel time to change their state; Of course, if the LCD cells are able to change state quickly enough, then, for example, a printed image is not adversely affected during the change.

A potential disadvantage of the acousto-optic modulator is that the repetition time of the laser pulses, and thus, for example, the image printing time, is limited by the time required for the acoustic wave to travel the length of the modulator so that a new wave train , which defines new data to be recorded, can fill the modulator. The LCD feid does not have these long acoustic propagation latencies since the state of the cells can be changed in parallel.

Another example of a suitable modulator means for use in the present invention is an array of micromirrors which extends in the low-divergence axis of the beam, which can be actuated by, for example, a field of charge-coupled devices. Each mirror in the array could reflect light in different directions depending on the mirror orientation, so that in one orientation the reflected light would be focused on the recording medium while in another orientation the reflected light would be blocked by suitable means. A disadvantage of micromirror fields is that it is difficult to modulate rays from the field to have variable intensities. However, these fields have greater contrast than LCD panels, and the orientation of the micromirrors can be changed in parallel so that there are no long acoustic propagation latencies, as in the acousto-optic modulator.

Preferably, means are provided for focusing the laser diode beam in its high divergence axis so that an image of the laser diode strip is formed at a selected position either a short distance in front of the light receiving surface of the modulator means or at the surface of or within the modulator means. As noted, the beam can be focused to high resolution in the high-divergence axis, however, changes in that resolution can be further desired, for example, to equalize the pixel size in both beam axes. By moving the focused image strip from the center (or area) of the modulator means to a position slightly in front of the surface, the width of the image formed on the modulator means in the high-divergence axis is increased and thus the resolution is reduced. Another advantage of focusing the beam slightly in front of the modulator means is that the modulator means is not exposed to the full intensity of the beam and the likelihood of damage or burn-out of the modulator means is reduced.

The beam is preferably not focused on the modulator means in the low-divergence axis. Instead, a means is preferably provided to expand the beam in this axis, so that it substantially completely illuminates the length of the modulator means and is adapted to it. Preferably, means are provided for collimating the low-divergence laser diode beam in the axis before the beam is expanded.

Given the different optical requirements in the two axes, the device may include equalization optics means for focusing the laser diode beam in the high divergence axis in, on, or just in front of the modulator means and for collimating the beam in the low divergence axis. The device may further comprise means for anamorphic expansion of the laser diode beam in the axis of low divergence with respect to, for example without affecting the, high divergence axis. This latter means may for example be an equalizing cylindrical telescope system.

Downstream of the modulator means, the imaging means focuses the individually modulated beam sections onto the recording medium, and may be further configured to enlarge or reduce the pixel images provided by the beam sections to a desired resolution as required. The modulator means may be arranged in a conjugate plane to that of the recording medium so that both are in a focal plane of the imaging means, and in this case the modulator means is activated according to a desired print image, for example. However, in an alternative embodiment, the modulator means is arranged to lie in a Fourier plane such that the desired image is formed on the recording means by activating the modulator means in accordance with a Fourier transform of, for example, a desired print image. This may be advantageous because image data arriving at a printer may already be in the form of a Fourier transform, since this is a standard method of data compression, and thus the need for means for converting the Fourier transform into image data may be avoided. A transformed image also allows various image manipulation operations, and the optical transformation of a pixelated pattern looks blurred rather than pixelated, thus producing a better quality image (ie less pixelated).

The array of beam sections may be scanned across the recording medium, such as a dye layer, in a suitable manner, eg, in a direction orthogonal to the axis of the modulator means, so that multiple lines of pixel data are simultaneously generated.

The invention provides a particularly advantageous printing arrangement, as the beam quality of the laser diode is effectively improved in the low-divergence axis, thereby enabling high resolution and fast printing through cost-effective, reliable means. At the same time, the invention advantageously uses the high beam speed in the other axis.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of a device for one-dimensionally dividing a laser diode beam along its axis of low divergence when viewed in this axis;

Figure 2 is a schematic diagram of the device of Figure 1 viewed in the high divergence axis of the beam; and

Figure 3 is a schematic diagram showing the control means for the device of Figures 1 and 2.

Turning to Figures 1 and 2, a laser diode 1 emits light from an elongated strip surface area (not shown). The emitted beam 2 is of asymmetrical quality and has a high divergence in the axis of the beam width and a small divergence in the axis of the beam length.

A lens 3 collimates the beam 2 in the low-divergence axis, and a cylindrical telescope 4 expands the beam 2 in that axis so that the beam fully illuminates the length of an acousto-optic modulator 5. In the axis with high divergence of the beam 2, the lens 3 focuses the beam 2 on the center of the acousto-optic modulator 5, whereas the cylindrical telescope 4 has no effect. The acoustooptic modulator 5 is operated in a so-called "Schophoney mode".

An amplitude modulated acousto-optic wave train propagates along the length of the modulator 5, portions of which act as diffraction gratings to produce a deflected beam portion 6 and a non-deflected beam portion 7, respectively, whose relative intensities are determined by the amplitude of the wavetrain at the various ranges are determined.

The deflected beam sections 6 are focused by a telecentric imaging lens 8 onto a dye layer 9 so that each beam section 6 heats a separate pixel region of the dye layer 9 to cause dye to be transferred from the heated regions to a receiving layer 10 to printed pixels form. The undeflected beam sections 7 are imaged onto a stopper 11 of zero order.

By the amplitude modulation of the carrier wave train, the intensity of the deflected beam 6 can be varied so that a desired line pattern of printed pixels can be generated.

The resolution of the pressure in the direction of the low-divergence beam axis depends on the size of the deflected beam sections 6 in this axis and on any magnification / reduction provided by the telecentric imaging lens 8. Therefore, a higher printing resolution can be obtained than if the laser diode beam 2 were imaged onto the ink layer 9 only without being split. Further, the resolution can be easily adjusted by modifying the acoustic wave train to vary the lengths of the diffraction regions.

The beam splitting has no effect on the print resolution in the high divergence axis of the beam 2, which is defined by the size of the beam 2 in this axis when it impinges on the modulator 5 and by the reduction of the telecentric imaging lens 8. Thus, by focusing the beam in this axis, the resolution in this axis can be changed slightly in front of the modulator 5 so that the beam width in the high-divergence axis diverge after focusing and is slightly wider upon reaching the modulator 5. It should be noted that although Figure 2 shows the beam 2 focused on the center of the modulator 5 in the high-divergence axis, this arrangement could cause the modulator 5 to be damaged by the intensity of the beam 2. In practice, therefore, the beam 2 is typically focused slightly in front of the modulator 5.

FIG. 3 shows the control of the printing device. A controller 12 includes a laser pulse controller 13, a line data generator 14, a frame data generator 15, and a print transport controller 16.

Data representing the desired print image is stored in the frame data generator 15, which transmits the data serially pixel line by pixel line to the line data generator 14, which then outputs a control signal to the acousto-optic modulator 5, to propagate an amplitude modulated carrier wave train along the modulator length to effect. This wave train is modulated in accordance with the pixel data to have regions of high, low, and different intermediate amplitudes. The regions of low amplitude correspond to a pixel-off state, and the modulated regions of intermediate or high amplitude correspond to different degrees of pixel-on-state, with the higher the amplitude, the darker the pixel.

When the wave train with this data completely fills the modulator 5, the laser pulse controller 13 pulses the laser diode 1 so that it outputs a short light pulse 2 whose duration is considerably shorter than the propagation time of the acoustic wave train in the modulator 5, so that the pulse interacts with the wave train only at a momentary shaft position. The deflected rays 6 generated by this interaction are then imaged onto the dye layer 9 to effect dye transfer to the receiving layer 10. This process creates a single pixel line of print, and to create another print line, the print transport controller 16 advances the dye layer 9 and the receiver layer 10 by one pixel line while a new wave train of new pixel data propagates through the modulator 5. *** " The laser diode 2 is again turned on in pulses when the carrier wave train fills the modulator 5 with the new pixel data. In this way, a desired print image can be built line by line.

A suitable modulator 5 is a tellurium-shearing mode cell, which is typically 30mm in the low divergence axis of the beam 2, 1 to 4mm in the high divergence axis of the beam 2, and about 10mm in depth in the optical axis (the beam propagation direction). A carrier wave train may propagate along such a cell at a speed of approximately 600mS and will typically have a frequency between 20MHz and 100MHz, depending on the wavelength of the laser diode; the carrier frequency is selected so as to ensure the maximum diffraction efficiency at the laser beam wavelength.

The pixel height is generally a minimum of 2 times the carrier wave train wavelengths, and thus at 20 MHz, for example, the pixel height can only be 60 microns small. This can yield 500 pixels across a 30mm cell, ie a line section which is 500 pixels long can be printed by a single laser pulse. Typically, however, fewer pixels of lower resolution are used, and in one embodiment 60 pixels are fitted into a 30 mm cell at 0.5 mm per pixel.

The telecentric imaging lens 8 can typically reduce the printing pixel size from 0.5 mm to between 10 and 100 microns so that the 60 pixels cover a line length of between 0.6 and 6 mm. These print lengths typically require 25 uJ or 2.5 mJ laser energy. The former value corresponds to a peak laser power of about 50 W and an average power of about 0.5 W (ie a duty cycle of 1%). This performance can be achieved, for example, by a Spectra Diode Labs SDL 3230 T diode array with emission dimensions of 10 mm x 1 micron. A laser pulse time of 0.5 microseconds would be acceptable and the repetition rate must be slow enough to allow the acoustic wave train to propagate along the entire length of the modulator 5 so that the pulse-to-pulse interval would be greater than 50 microseconds ie a repetition rate of less than 20 kHz would be required.

In the present device, adjacent pixels are written in parallel on the print medium, and so there is a certain amount of thermal crosstalk between them. The controller 12 must compensate for this by controlling the intensity of each beam 6 in response to the state of the pixels surrounding the pixel to which the beam 6 is imaged.

In the foregoing, only one embodiment of the invention has been described, and many other modifications may be considered. For example, the undeflected beams 7 could be imaged by the modulator 5 on the dye layer 9 instead of the deflected beams 6. Furthermore, other modulators, such as LCD fields or micromirror fields, and the modulator 5 could be positioned in a Fourier plane of the dye layer 9. In the latter case, the amplitude modulation of the carrier wave train would have to be performed in response to data representing a Fourier transform of the desired image.