WO2000032348A1

WO2000032348A1 - Laser image formation in multiple transparent samples

Info

Publication number: WO2000032348A1
Application number: PCT/IL1999/000654
Authority: WO
Inventors: Vladimir Dmitriev; Nikolay Guletsky; Sergey Oshemkov
Original assignee: Universal Crystal Ltd.
Priority date: 1998-12-03
Filing date: 1999-12-02
Publication date: 2000-06-08
Also published as: IL127387A0; AU1407900A

Abstract

A method and system for the formation of laser images in multiple transparent samples (140, 142, 144) by means of optical breakdown produced by focusing a the laser beam (112) within the samples. A pulsed laser beam is divided by beam splitters (116, 118) into separate marking beams latterly spaced from each other. The samples are stacked such that the separate beams must pass through upper samples before reaching lower samples in the stack. Each of the marking beams is separately focused by lenses (130, 132, 134) onto a separate sample, such that a beam marking a lower sample in the stack passes in an unfocused state through samples disposed above it in the stack, and thus does not effect the upper samples. All of the samples are moved (145) simultaneously relative to the laser beams to produce the desired images.

Description

LASER IMAGE FORMATION IN MULTIPLE TRANSPARENT SAMPLES

FIELD OF THE INVENTION

The present invention relates to the field of the formation of images by lasers in multiple samples of transparent samples by means of optical breakdown phenomena, especially using pulsed lasers.

BACKGROUND OF THE INVENTION

There is well known in the prior art methods whereby internal marking of transparent material is performed by means of the focusing of laser pulses inside the volume of the material, such that optical breakdown takes place, causing the disintegration and breakdown of the material at an atomic level. This process is caused by effects which occur when the power density of the focused beam far exceeds the threshold above which non-linear effects occur in the transmission properties of the otherwise transparent materials, and the material strongly absorbs the focused beam.

This process has been described under various names in the prior an. The effect was first described by V.V. Agadjanov et al. in USSR Inventor's Certificate No. 321422 entitled "Process for Making Decorative Articles", issued on January 18, 1972, which described the formation of three dimensional images within the volume of transparent materials by means of highly focused Q- switched solid state lasers. In that publication, the authors use the term "heat treatment" to describe what they thought was the physical basis of the effects which they obtained. Subsequent publications have shown, however, that in all probability, the effects obtained using the laser parameters and focusing means described therein, cannot be ascribed to simple local heat treatment, but are in fact only obtained when the volume optical breakdown threshold of the material is exceeded. η

Since that initial publication, additional prior art has appeared on the subject of the production of images or marks by means of optical breakdown, though this terminology has not been used in all of them. Some examples of such prior art are in U.S. Patent No. 5,206.496, to R.M. Clement et al, entitled "Subsurface Marking^" issued on April 27. 1993, USSR Patent No. SU 1 ,838, 163 to P.V. Λgrynsky et al. entitled "Process for Forming Images". Russian Patent No. RU 2.008,288 to S.V. Oshemkov, one of the current applicants, entitled "Process for Laser Forming of Images in Solid Media", U.S. Patent No. 4,092,518 to R.R. Merard entitled "Method of decorating a transparent plastics material article by- means of a laser beam", U.S. Patent No. 4.467.172. entitled "Method and apparatus for engraving diamonds with permanent identification markings ^', U.S. Patent No. 5,637,244 to A.I. Erokhin, entitled "Method and apparatus for creating an image by a pulsed laser beam inside a transparent material", as well as others.

All of the above prior art publications deal with different aspects of the physical process itself, and the technology by which the process can be applied on an industrial scale has been dealt with very little. In U.S. Patent No. 5,575,936 to B. Goldfarb entitled "Process and apparatus for etching an image within a solid article", there is described a method and apparatus for production of articles incorporating such images produced by the optical breakdown effect. In that patent, an automatic motion system is described for increasing the production rate of such images.

However, to the best of the applicant's knowledge, all of the prior art deals with the production of one image at a time, with low resultant productivity levels for the process. The method of dividing a laser beam up into several secondary- beams by means of multiple beam splitters, and the application of each secondary beam to a separate object is well known, both in surface marking technology, and in high power material processing applications, such as sheet cutting. In spite of the extra expense of this method, in that a separate focusing head is required for each marking or cutting station, this method has been commonly used for multiple image production on small pans.

For use on large parts, such as large panes of glass, which need to be marked with internally inscribed images or messages by means of the optical breakdown process, such a multiple head method becomes impractical. Firstly, since the panes are disposed one after the other along the line of beam splitters, the optical path lengths required become unnecessarily long, and vibrations present in this path length will result in distortions or lack of sharpness of the image. Secondly, because of the expense of multiple motion systems, a large sample is often mounted on one motion system and the multiple head method is used to focus the multiple beams onto separate areas of sample, such that the image formed by each of the heads is identical. In this way, only repetitive messages can be inscribed on a single pane, instead of one long message. Since the use of large panes of glass with side illuminated messages or images is becoming an important medium, both as decorative covering and as advertising medium, the currently available methods of producing such panes is not very cost effective.

There therefore exists a need for a method and a svstem of efficient multiple marking of images in transparent samples by means of optical breakdown marking, especially of large samples, for which no satisfactorily solution is currently available in the prior art.

The disclosures of all publications mentioned in this section and in the other sections of the specification, and the disclosures of all documents cited in the above publications, are hereby incorporated by reference.

SUMMARY OF THE INVENTION

The present invention seeks to provide a new method and apparatus for the laser marking of images in multiple samples of transparent materials, in such a way that the productivity of the process is significantly increased. The method enables a stack of transparent samples to be marked simultaneously by means of a plurality of beams split from the output beam of the marking laser, by means of a number of beam splitters. Each beam is focused to a different height so that it is correctly located in height with respect to the sample it is meant to mark. In the system according to the present invention, beams which are destined to mark the lower samples are allowed to pass through the upper samples. This they can do unhindered and without causing damage to the upper samples because the beams are only absorbed in the otherwise transparent material at their focal point. At the focal point, where the energy density is such that the linear region of transmission in the material is exceeded, optical breakdown ensues, causing marking of the samples. Alternatively, even at lower energy densities, slight linear absoφtion of laser radiation at the focal point may cause thermal effects and breakdown of the material, with consequent marking of the sample.

The definition of what constitutes a thermal effect and what constitutes optical breakdown is dependent on the laser pulses used, and on the nature of the material. The present invention is directed at any marking effect performed using the method or apparatus described herein, regardless of the exact physical mechanism by which the marking process is performed, or the nomenclature of that mechanism. It is therefore to be understood that the use of the term "optical breakdown" in this specification and as claimed, is meant to include any mechanism whereby a focused laser beam causes marking in a sample substantially transparent to the unfocused laser beam.

The focal points are arranged such that the threshold for optical breakdown is only achieved in the sample it is desired to mark. The stack of transparent samples is moved relative to the beams in a predetermined path by means of a single motion system, to produce the desired image in all of the samples simultaneously. Alternatively and preferably, the laser beams can be simultaneous scanned relative to the transparent samples by means of translation motion of the laser source and beam delivery system, or by means of angular scanning systems, optionally in combination with translation motof the laser source and beam delivery system. There is thus provided in accordance with a preferred embodiment of the present invention, a system for the formation of laser images in multiple transparent samples, consisting of a pulsed laser source, with a pulsed beam such that when focused onto one of the samples, the sample undergoes optical breakdown, at least one beam splitter operative to split the pulsed beam into at least two separate pulsed beams, each of the at least two separate pulsed beams also being capable of causing one of the samples to undergo optical breakdown when focused thereon, at least two focusing optics, one for each of the at least two separate pulsed beams, each focusing optic being operative to focus its beam onto a separate one of the multiple transparent samples, the samples being disposed relative to each other such that at least one of the at least two beams is directed through one of the samples before reaching a focus in its designated sample; and a mechanism for moving, the multiple transparent samples relative to the at least two separate pulsed beams, according to a predetermined path.

In accordance with yet another preferred embodiment of the present invention, there is provided a system as described hereinabove and wherein the predetermined path is a three dimensional path.

There is further provided in accordance with yet another preferred embodiment of the present invention a system as described hereinabove and wherein the pulsed laser source is a Nd:YAG laser, which is pulsed with a repetition rate in excess of 10 Hz, and whose pulses are of duration less than 1 millisecond.

There is further provided in accordance with still another preferred embodiment of the present invention a system as described hereinabove and wherein at least one of the transparent samples is made of glass or plastic.

In accordance with still another preferred embodiment of the present invention, there is provided a system as described hereinabove and wherein at least one of the transparent samples has its width and length considerably greater than its height.

In accordance with a further preferred embodiment of the present invention, there is also provided a system as described hereinabove and wherein at least two of the transparent samples disposed relative to each other have plane surfaces substantially parallel to each other.

There is provided in accordance with yet a further preferred embodiment of the present invention a system as described hereinabove and wherein the mechanism for moving the multiple transparent samples relative to the at least two separate pulsed beams, according to a predetermined path consists of a mechanical motion system or an angular scanning system, or a combination of both.

There is even further provided in accordance with a preferred embodiment of the present invention a method for the formation of laser images in multiple transparent samples, consisting of the steps of splitting the beam of a pulsed laser into at least two beams, each of which is capable of causing one of the samples _to undergo optical breakdown when focused thereupon, focusing each of the at least two beams onto a separate one of the multiple transparent samples, the samples being disposed relative to each other such that at least one of the at least two beams is directed through one of the samples before reaching a focus in its designated sample, and moving the multiple transparent samples relative to the at least two separate pulsed beams, according to a predetermined path.

Furthermore, in accordance with yet another preferred embodiment of the present invention, there is provided a method as described hereinabove and wherein the predetermined path is a three dimensional path.

There is also provided in accordance with a further preferred embodiment of the present invention a method as described hereinabove and wherein at least one of the transparent samples is made of glass or plastic.

In accordance with yet another preferred embodiment of the present invention, there is provided a method as described hereinabove and wherein at least one of the transparent samples has its width and length considerably greater than its height.

There is further provided in accordance with yet another preferred embodiment of the present invention a method as described hereinabove and wherein at least two of the transparent samples disposed relative to each other have plane surfaces substantially parallel to each other.

In accordance with still another preferred embodiment of the present invention, there is provided a method as described hereinabove and wherein the mechanism for moving the multiple transparent samples relative to the at least two separate pulsed beams, according to a predetermined path consists of a mechanical motion system or an angular scanning system, or a combination of both.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:

Fig. 1 shows a prior art system used for multiple marking of images within transparent articles.

Fig. 2 shows a multiple marking system according to the present invention, as used to mark images in multiple, large-dimensioned transparent samples, such as panes of glass.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is now made to Fig. 1 , which illustrates schematically a prior art system used for multiple marking of images within transparent articles. In the system shown, three marking stages are operated simultaneously. A marking laser 10 provides an output beam 12 which is pulsed 14 such that it can accomplish marking of images within transparent samples by means of optical breakdown. The threshold above which optical breakdown occurs is dependent on the pulse width, and is of the order of 1-10 J/cm for pulse widths of the order of picoseconds. The threshold increases with increase of pulse width and is of the order of 10'- 10 J/cπT for pulse widths in the nanosecond range. For longer pulse widths and in transparent materials, the threshold is typically higher, which can cause an increase in the extent of the damage zone. At the same time, for slightly absorbing materials where thermal breakdown occurs, the threshold is inversely proportional to the absoφtion coefficient of the material, and may be significantly less This mechanism may thus also be used for the marking of slightly absorbing materials using long pulse lasers, with pulse widths of up to about 1 msec. The beam passes through beam splitters 16, 18 and 20. each of which directs about one third of the beam power towards the triple focusing lenses 30. 32 and 34. which focus the beams respectively onto the three samples 40, 42 and 44. Each sample is mounted on its own motion stage. 45. 46 and 47. movable in the x- and y- directions. Z-axis motion is typically provided by mounting each lens on a vertical controlled slide.

It is evident from the system shown in Fig. 1 that if large samples are to be marked, the system becomes very large and cumbersome, since the samples are disposed serially. Also, such an arrangement requires very long optical path lengths, which are sensitive to vibrations. This is in addition to the expense of providing a separate motion stage for each sample to be marked simultaneously.

Reference is now made to Fig. 2 which shows a system for multiple marking of images within transparent articles, constructed and operative according to a preferred embodiment of the present invention. The system is shown with similar functions to that shown in Fig. 1. As previously, the output beam 1 12 from the marking laser 1 10 is pulsed 1 14, such that it can accomplish marking of images within transparent samples by means of optical breakdown. In order to achieve optical breakdown, the characteristics of the laser are preferably- such as to give ultra-short pulses of less than 100 psec. for high resolution marking, and up to 1 μsec for lower resolution marks, with pulse energies preferably as described above. The beam passes through beam splitters 1 16, 1 18 and 120, each which directs about one third of the beam power towards the triple focusing lenses 130, i 32 and 134, which focus the beams respectively onto the focal positions 150, 152 and 154 inside the three samples 140. 142 and 144. The samples in the preferred embodiment shown are large slabs, such as panes of glass in each of which a message or image is to be inscribed.

The slab samples are stacked, one on top of the other, such that the beam which produces the marking in the bottom-most slab passes through slabs stacked on top of it before reaching its focus inside the bottom slab. The beam passes through the upper slabs without any effect, since only when it is at its focus does it have sufficient power density to interact non-1 inearly with the glass, and to cause optical breakdowτi therein. Though in Fig. 2, the marking beams are shown passing through only one sample before reaching focus position on the target sample, the number can in practice be significantly greater. The complete stack of slabs is mounted on one motion system 145, which represents a significant COST saving compared with the prior art systems described in Fig. 1 , where a separate motion system is required for each sample. The motion system 145 provides x- and y-axis motion, while z-axis motion is provided by mounting each focusing lens on a vertical slide. The combined motion of these three axes enables any three dimensional image to be formed in the slab being treated.

According to further preferred embodiments of the present invention, angular beam scanning devices 160, 162, 164 may be used to produce the image, or a combination of angular scanning and translation motion 166, 168 of the laser source and beam delivery system, including angular scanning devices. All of these preferred embodiments enable the relative movement of the laser beams and the samples to be accomplished without moving the samples themselves, which could be large and cumbersome to move.

Furthermore, slight motion of the focusing lens in the z direction enables slightly diffused marking to be obtained, thus enabling a concept of depth to be obtained even with a two dimensional image. The individual slabs in the stack may preferably be offset laterally by the same amount that the focusing lenses are spaced apart, in order that each beam inscribe its image at the same relative location in each slab

The advantages of the system according to the present invention, as shown in Fig 2. when compared to previously available s\ stems, such as that shown in Fig 1. include savings in floor area, savings in capital cost of the svstem_. and savings in the processing time required to process a number of parts

In accordance with another preferred embodiment of the present invention, there is further provided a method for the formation of laser images in multiple transparent samples In this method, a pulsed laser output beam is split into a number of substantially similar separate beams. b\ means of a series of beam-splitters Each beam is then focuseα b means of its own focusing lens onto the samples, which have been previously stacked in a pile, such that in order to focus onto lower samples, the beam must pass through upper samples The focusing lenses are disposed at different heights, commensurate with the height at which is disposed the sample onto which each lens must focus the beam The stack of samples are located on a single motion system, which can move the samples relative to the beams in such a manner as to form a two dimensional image in each sample If the motion system includes vertical axis motion also, three dimensional images can be simultaneously formed in each sample

It will be appreciated by persons skilled m the art that the present invention is not limited by what has been particularly shown and described hereinabove Rather the scope of the present invention includes both combinations and subcombinations of various features described hereinabove as well as variations and modifications thereto which would occur to a person of skill in the an upon reading the above description and which are not in the prior art

Claims

CLAIMSWe claim:

1. A system for the formation of laser images in multiple transparent samples, comprising: a laser source emitting a pulsed beam; at least one beam splitter, operative to split said pulsed beam into at least two separate pulsed beams, each of said at least two separate pulsed beams being capable of causing one of said samples to undergo optical breakdown when focused thereon; at least two focusing optics, one for each of said at least two separate pulsed beams, each focusing optic being operative to focus its beam onto a separate one of said multiple transparent samples, said samples being disposed relative to each other such that at least one of said at least two beams is directed through at least one of said samples before reaching a focus in its designated sample; and a mechanism for simultaneously moving at least two of said multiple transparent samples relative to said at least two separate pulsed beams, according to a predetermined path.

2. A system according to claim 1 and wherein said predetennined path is a three dimensional path.

3. A system according to claim 1 and wherein said laser source is a Nd:YAG laser.

4. A system according to claim 1 and wherein said laser source is pulsed with a repetition rate in excess of 10 Hz.

5. A system according to claim 1 and wherein said laser source emits pulses of duration less than 1 millisecond.

6. A system according to claim 1 and wherein at least one of said transparent samples is made of glass.

7. A system according to claim 1 and wherein at least one of said transparent samples is made of plastic.

8. A system according to claim 1 and wherein at least one of said transparent samples has dimensions in the plane peφendicular to the direction of impingement of said laser beam on said sample considerably greater than its dimension in the direction of impingement of said beam.

9. A system according to claim 1 and wherein at least two of said transparent samples disposed relative to each other have plane surfaces substantially parallel to each other.

10. A system according to claim i and wherein said mechanism for moving said multiple transparent samples relative to said at least two separate pulsed beams according to a predetermined path comprises a mechanical motion system.

1 1. A system according to claim 1 and wherein said mechanism for moving said multiple transparent samples relative to said at least two separate pulsed beams according to a predetermined path comprises an angular scanning system.

12. A system according to claim 1 and wherein said mechanism for moving said multiple transparent samples relative to said at least two separate pulsed beams according to a predetermined path comprises an angular scanning system in combination with a mechanical motion system.

13. A method for the formation of laser images in multiple transparent samples, comprising the steps of: splitting the beam of a pulsed laser into at least two beams, each of which is capable of causing one of said samples to undergo optical breakdown when focused thereupon; focusing each of said at least two beams onto a separate one of said multiple transparent samples, said samples being disposed relative to each other such that at least one of said at least two beams is directed through at least one of said samples before reaching a focus in its designated sample; and simultaneously moving at least two of said multiple transparent samples relative to said at least two beams, according to a predetermined path.

14. The method of claim 13 and wherein said predetermined path is a three dimensional path.

15. The method of claim 13 and wherein at least one of said transparent samples is made of glass.

16. The method of claim 13 and wherein at least one of said transparent samples is made of plastic.

17. The method of claim 13 and wherein at least one of said transparent samples has dimensions in the plane peφendicular to the direction of impingement of said laser beam on said sample, considerably grthan its dimension in the direction of impingement of said beam.

18. The method of claim 13 and wherein at least two of said transparent samples disposed relative to each other have plane surfaces substantially parallel to each other. 19 The method of claim 13 and wherein said mechanism for moving said multiple transparent samples relative to said at least two separate pulsed beams according to a predetermined path comprises a mechanical motion svs_tem

20 The method of claim 13 and wherein said mechanism for moving said multiple transparent samples relative to said at least two separate pulsed beams according to a predetermined path comprises an angular scanning system

21 The method of claim 13 and w herein said mechanism for moving said multiple transparent samples relative to saiα at least tw o separate pulsed beams according to a predetermined path comprises an angular scanning svs_temin combination with a mechanical motion svstem