SYSTEMS FOR AND METHOD OF LASER MARKING WITH REDUCED MAXIMUM OPERATIONAL OUTPUT POWER

Abstract

A system for laser marking a substrate includes a multi-emitter array (16) for directing radiation onto a substrate. The multi-emitter array has a radiation guide (19) defining a number of discrete emission channels (20) with emitting ends (20a) of the emission channels (20) arranged in an array. Each emission channel (20) is coupled at its opposing end with two or more laser diodes (18a, 18b). The laser diodes (18a, 18b) are operated at a maximum operational output power (P.sub.op) sufficiently below their rated maximum power (P.sub.m) to provide acceptable levels of reliability whilst providing a combined radiation (24) emitted from each channel (20) having a power high enough to achieve increased operational speeds. The multi-emitter array (19) may comprise a number of optical fibres (26) whose emitter ends are arranged in an array. The system is particularly suited for inkless printing on substrates susceptible to colour change when irradiated.

Claims

1. A system for laser marking a substrate, the system comprising a multi-emitter array having a plurality of individually controllable discrete emission channels, each emission channel having an emitting end from which radiation is directed onto a selected area of the substrate in use, the emitting ends being arranged in an array, wherein each emission channel is coupled with at least two laser diodes and the system is configured, in use, to operate each laser diode at a maximum operating output power (P.sub.op) that is less than 63% of its rated maximum power (P.sub.m).

2. A system as claimed in claim 1, wherein the system is configured, in use, to operate each laser diode at a maximum operating output power (P.sub.op) that is less than 50% of its rated maximum power (P.sub.m).

3. A system as claimed in claim 1, wherein the system is such that, in use, the ratio of maximum operating output power P.sub.op to maximum rated output power P.sub.m of each laser diode satisfies the relationship: $\frac{P_{\mathrm{op}}}{P_m}\le {\left(\frac{1.58*10^{-6}}{n_c.n_p}{e}^{5.22x10^3/T_{jn}}\right)}^{1{/}_5}$ where n.sub.c is the number of emission channels, n.sub.p the number of laser diodes coupled to each emission channel and their product is >=32 and T.sub.jn is the laser diode junction temperature in Kelvin.

4. A system as claimed in claim 1, wherein the laser diodes coupled with each emission channel emit radiation at substantially the same wavelength.

5. A system as claimed in claim 4, wherein all the laser diodes emit radiation at substantially the same wavelength.

6. A system as claimed in claim 1, wherein the laser diodes emit radiation at a wavelength in the range of 900 nm to 1500 nm or at a wavelength in the range of 395 nm to 470 nm.

7. A system as claimed in claim 1, wherein the multi-emitter array is a multi-fibre array, the multi-fibre array comprising an array of emitting ends of optical fibres, each optical fibre defining one of said emission channels and being coupled with said at least two laser diodes at the opposing end.

8. A system as claimed in claim 7, wherein the optical fibres have a numerical aperture equal to or less than 0.24 and more preferably a numerical aperture in the range of 0.10 and 0.17.

9. (canceled)

10. (canceled)

11. (canceled)

12. A system as claimed in claim 1, wherein the system is configured for use with a substrate having a coating comprising a TAG leuco dye or AOM.

13. A system for marking a substrate susceptible to colour change upon irradiation, the system comprising a plurality of optical fibres, each optical fibre having an emitter end from which radiation is directed onto the substrate in use, the emitter ends of the optical fibres being arranged in an array, and at least two laser diodes coupled with each optical fibre at the opposing end, the system configured such that, in use, each laser diode is operated at a maximum operating output power (P.sub.op) that is less than 63% of its rated maximum power (P.sub.m).

14. A method of laser marking a substrate using a system comprising a multi-emitter array for directing radiation onto the substrate, wherein the multi-emitter array defines a plurality of emission channels, the emitting ends of which are arranged in an array with each emitter end being configured to independently direct radiation onto the substrate in use, and wherein each emission channel is coupled with at least two laser diodes, the method comprising operating each laser diode at a maximum operating output power (P.sub.op) that is less than 63% of its rated maximum power (P.sub.m).

15. A method as claimed in claim 14, the method comprising operating each laser diode at a maximum operating output power (P.sub.op) that is less than 50% of its rated maximum power (P.sub.m).

16. A method as claimed in claim 14, the method comprising operating the system such that the ratio of maximum operating output power P.sub.op to maximum rated output power P.sub.m of each laser diode satisfies the relationship: $\frac{P_{\mathrm{op}}}{P_m}\le {\left(\frac{1.58*10^{-6}}{n_c.n_p}{e}^{5.22x10^3/T_{jn}}\right)}^{1{/}_5}$ where n.sub.c is the number of emission channels, n.sub.p the number of laser diodes coupled to each emission channel and their product is >=32, and T.sub.jn is the laser diode junction temperature in Kelvin.

17. (canceled)

18. (canceled)

19. (canceled)

20. A method as claimed in claim 19, wherein the substrate has a coating comprising a TAG leuco dye or AOM.

21. A method as claimed in claim 19, wherein a colour change region of the substrate incorporates a NIR (near infrared) absorber which is effective in the radiation wavelength range 900 nm to 1500 nm and the laser diodes emit radiation at a wavelength falling within the range of the NIR absorber.

22. A method as claimed in claim 19, wherein a colour change region of the substrate responds to radiation in the range 395 nm to 470 nm and the laser diodes emit radiation at a wavelength falling within said range.

23. A method of marking a substrate susceptible to colour change when irradiated using a system comprising a plurality of optical fibres, emitter ends of the optical fibres being arranged in an array and each emitter end configured for independently directing radiation onto the substrate in use, and wherein at least two laser diodes are coupled with each optical fibre at the opposing end, the method comprising operating each laser diode at a maximum operating output power (P.sub.op) that is less than 50% of its rated maximum power (P.sub.m).

24. (canceled)

25. A system as claimed in claim 1, wherein the system has means for controlling emission of radiation from the laser diodes so as to controllably irradiate selected areas of the substrate with desired quantities of radiation so as to mark the substrate in a desired manner.

26. A method as claimed in any one of claim 14, wherein the substrate is susceptible to colour change when irradiated and the method comprises controlling the radiation emitted by the laser diodes such that the radiation emitted through each of the emission channels irradiates selected areas of the substrate with desired quantities of radiation so as to mark the substrate in a desired manner.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0041] In order that the invention may be more clearly understood an embodiment thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

[0042] FIG. 1 illustrates schematically an embodiment of a system for laser marking a substrate in accordance with an aspect of the invention; and

[0043] FIG. 2 is a schematic illustration of multi-emitter array forming part of an imaging head of the system of FIG. 1.

[0044] Turning now to FIG. 1, a system 10 for laser marking a substrate 12 is shown. The system 10 includes an imaging head 14 and is suitable for marking a substrate 12 which includes material susceptible to changing colour upon irradiation, so as to form an image.

[0045] The substrate 12 may be any suitable substrate which is susceptible to changing colour when irradiated. Such colour change technology is known in the art, for example from WO2016135468 A1, WO2016097667 A1, WO2015015200 A1, and WO2010026408 A2, to which the reader should refer for further details. The contents of WO2016135468 A1, WO2016097667 A1, WO2015015200 A1, and WO2010026408 A2 are hereby incorporated by reference. In accordance with one non-limiting embodiment, the substrate 12 is susceptible to colour change when irradiated by radiation in the NIR wavelength range 900 nm to 1500 nm and may comprise a NIR absorber. In an alternative non-limiting embodiment, the substrate is susceptible to colour change when irradiated by radiation having a wavelength in the range 395 nm to 470 nm

[0046] As illustrated schematically in FIG. 2, the imaging head 14 contains a multi-emitter array 16 comprising a number of laser diodes 18 and a radiation guide 19 for directing radiation from the laser diodes onto the substrate 12. The radiation guide 19 comprises a number of discrete emission channels 20, each of which has an emitter end 20a from which radiation from the laser diodes 18 is directed onto a selected area of the substrate in use. At least the emitter ends 20a of the emission channels 20 are arranged in an array. The laser diodes 18 are arranged in groups of two 18a, 18b, with each group of diodes being coupled with the opposing, inlet end of a respective one of the emission channels 20 by suitable coupling optics 22. The radiation emitted by all the diodes 18a, 18b in each group is combined and directed through the respective emission channel 20 to form a combined emission 24 which is directed onto the substrate 12 from the emitter end 20a. In the present embodiment, the multi-emitter array is a multi-fibre array having a number of optical fibres 26 which each define one of the emission channels 20. The emitting ends of the fibres 26 extend through a coupling block 28 which holds the emitting ends in an array. However, optical or radiation guide means other than optical fibres could be adopted provided they can be arranged to define discrete emission channels 20 in which the emitter ends 20a are arranged in array. In this regard, the term multi-emitter array should be understood as referring to an arrangement in which the emitting ends of multiple optical fibres or other optical or radiation guide means are arranged in an array. Whilst FIG. 2 illustrates the input ends of the optical fibres 26 (or other optical or radiation guide means) and the laser diodes 18 being aligned in an array, this is not essential and they can be configured in any suitable manner for incorporation in the imaging head 14 or indeed outside the head.

[0047] FIG. 2 is a schematic illustration which shows a simplified imaging head 14 with five discrete emission channels 20, in which the emitter ends 20a of the channels arranged in a one dimensional array and two laser diodes 18a, 18b are coupled to each channel. It should be understood though that the number of emission channels 20, the number of laser diodes 18, and the configuration of the emitter ends 20a can be varied as required to provide an array of the desired shape, size and resolution. For example, in a typical printing head there may be hundreds of emission channels 20 with a corresponding number of laser diodes and the emitter ends 20a of the emission channels 20 could be arranged in a two dimensional array or other configuration. It should also be understood that the number of laser diodes 18 coupled in a group with each emission channel/optical fibre 20 is not limited to two but can be three or more. Accordingly, the laser diodes 18 can be arranged in groups of three or more, with the laser diodes in each group being coupled with a respective emission channel/optical fibre 20.

[0048] The laser diodes 18 are selected and operated to emit radiation in a suitable wavelength to produce a colour change in the substrate. For example, for use with a substrate 12 susceptible to colour change when irradiated by radiation in the NIR wavelength, the laser diodes 18 emit radiation in the NIR wavelength, typically in the wavelength range 900 nm to 1500 nm. Alternatively, for use with other colour change substrates, the laser diodes could be selected and operated to emit radiation in the wavelength range 395 nm to 470 nm.

[0049] The laser diodes 18, or at least each group of laser diodes 18a, 18b, are individually addressable and are individually controlled by a microprocessor 30 via a drive amplifier 32.

[0050] The microprocessor 30 is operable to convert a digital image file to a set of emission instructions for the multi-emitter array 16. Typically, this involves mapping a particular pixel in the image file to a particular spot or area of the substrate 12; and determining the irradiation (duration and/or intensity) required from the individual emission channels 20 in the imaging head 14 to change the colour of each spot or area of the substrate to a colour matching that of each image pixel. Each of the optical fibre emission channels 20 directs the combined emission 24 of the respective diodes 18a, 18b coupled to it onto a spot on the surface of the substrate 12, such that a specific continuous (or discontinuous) pattern of irradiated spots is formed when the laser diodes are emitting. The system is arranged so that the combined emission 24 from the various emission channels 20 forms a pattern of irradiated spots on the substrate 12 which matches the pixels in an image file.

[0051] A further lens or other optical guidance arrangement may be provided between the emitter ends 20a of the optical fibres 26 or other emission channels 20 and the substrate.

[0052] The system has a conveyance mechanism (not shown) for moving the substrate 12 relative to the imaging head 14 and the microprocessor 30 is further operable to respond to the movement of substrate 12 relative to the imaging head 14. This movement may take place in a single direction as indicated by arrow 34 in FIG. 1 or in multiple directions. Typically, at faster operating speeds the substrate moves continuously in a single direction as indicated by the arrow 34.

[0053] The power of the combined emission 24 from each of the emission channels 20 is the sum of the output power from each of the laser diodes 18a, 18b coupled with it, subject to any losses in the optical system between the laser diodes and the substrate, including in this embodiment the coupling optics 22 and optical fibre 26. As discussed previously, imaging speed is dependent on the power of the radiation directed onto the substrate through each emission channel, with a power of >=7.5 W required to achieve imaging speeds of 3.2 m/s and above at a resolution of 200 dpi. By coupling two or more laser diodes 18a, 18b to each fibre optical channel 20, it is possible to obtain a combined emission 24 from each channel which has a high enough power to enable increased imaging speeds, say in excess of 3 m/s, to be achieved whilst operating each laser diode 18 at a maximum operating power P.sub.op which is sufficiently below its maximum rated power P.sub.m that the reliability of the system is acceptable for modern manufacturing processes. For example, if two laser diodes each with a maximum power rating P.sub.m of 8 W are coupled with each fibre optical emission channel 20, a combined emission power in the region of 8 W can be achieved whilst operating each diode at a maximum operating power P.sub.op which is around 50% of its maximum rated power P.sub.m. Using two laser diodes each with a P.sub.m of 12 W would enable a combined emission of >=8 W to be achieved whilst operating the each diode with a maximum operating power P.sub.op which is below 50% of its P.sub.m. By coupling two, three or more laser diodes to each emission channel, a combined emission 24 from each channel having a power sufficient for high imaging speeds (e.g. above 3 m/s) can be achieved whilst maintaining a P.sub.op/P.sub.max ratio suitable for acceptable reliability using currently available laser diodes. It has been found that operating each laser diode 18 at a P.sub.op which is below 50%, or below 63%, of its P.sub.m achieves satisfactory reliability levels for the system. In particular, it has been found that acceptable levels of reliability can be achieved by configuring the system so that the ratio of maximum operating power P.sub.op to maximum rated output power P.sub.m for each diode satisfies the relationship

[00005]$\begin{array}{cc}\frac{P_{\mathrm{op}}}{P_m}\le {\left(\frac{1.58*10^{-6}}{n_c.n_p}{e}^{5.22x10^3/T_{jn}}\right)}^{1{/}_5}& \left(3\right)\end{array}$

[0054] where n.sub.c is the number of emission channels, n.sub.p the number of laser diodes coupled to each emission channel and their product is >=32, and T.sub.jn is the laser diode junction temperature in Kelvin.

[0055] In an embodiment, the substrate 12 is susceptible to colour change when irradiated by radiation in the NIR wavelength range 900 nm to 1500 nm and may also contain a MR absorber to facilitate the use of NIR laser diodes. In particular, the colour change technology may comprise a metal oxyanion, a leuco dye, a diacetylene, and a charge transfer agent. The metal oxyanion may be a molybdate, which may be ammonium octamolybdate AOM. The colour change technology may further comprise an acid generating agent and leuco dye colour formers where the acid generators may be thermal acid generators (TAG) or photo-acid generators (PAG). The acid generating agent may be an amine salt of an organoboron or an organosilicon complex. In particular, the amine salt of an organoboron or an organosilicon complex may be tributylammonium borodisalicylate. The leuco dye colour former may be odb1 and odb2 and other colours. Suitable NIR absorbers include Indium Tin Oxide (ITO) and particularly non-stoichiometric reduced ITO, Copper Hydroxy Phosphate, Tungsten Oxides, doped Tungsten oxides and non-stochiometric doped tungsten oxides and organic NIR absorbing molecules such as copper pthalocyanines.

[0056] The above embodiment is described by way of example only. Many variations are possible without departing from the scope of the invention as defined in the appended claims. For example, whilst in the embodiment described the system is adapted for inkless printing of a substrate susceptible to colour change when irradiated, the system can be adapted for inkless printing of substrates which exhibit other visible changes in its physical properties when irradiated or indeed for other applications where a substrate is to be irradiated and where the speed of operation of the system is dependent on the power of radiation applied to the substrate.