GB2261296A

GB2261296A - Dual wavelength laser beam delivery system

Info

Publication number: GB2261296A
Application number: GB9123674A
Authority: GB
Inventors: Stewart Wynne Williams; Graeme Scott
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1991-11-07
Filing date: 1991-11-07
Publication date: 1993-05-12
Anticipated expiration: 2011-11-07
Also published as: GB2261296B; GB9123674D0

Abstract

A dual wavelength laser beam delivery system 40, for near co-linear delivery of two laser beams of different wavelength, comprises first and second lasers 42, 46 for producing first so and second laser beams 44, 48 respectively, beam delivery means 50 e.g. mirrors, conversion means 52, 54 for converting at least one of the beams into a diffraction free beam thereby to enable a reduction in the separation angle between the beams to be achieved without said beams interfering with each other prior to arrival at the working point on material 55. The conversion means 52, 54 may comprise shaping optics 59, 60 and may comprise axicon, waxicon or holographic elements. <IMAGE>

Description

DUAL WAVELENGTH LASER BEAM DELIVERY SYSTEM The present invention relates to an apparatus for co-linear or near co-linear delivery of two laser beams of different wavelengths and relates particularly, but not exclusively, to such apparatus suitable for use in cutting, drilling, welding or marking of materials.

Conventionally, materials are processed by using just one laser beam which is usually Infra-Red (IR). Recent developments have, however, illustrated that there are some considerable advantages to be gained by carrying out some processes using two lasers of different wavelengths, such as IR and Ultra-Violet (UV). Unfortunately, some practical difficulties present themselves when combining two lasers and these can best be illustrated by reference to figure 1 which shows a known two laser processing apparatus. The known apparatus comprises a first laser 12 of the Infra-Red CO2 type for delivering a first laser beam 14 onto the material to be processed 16, a second laser 18, of the Ultra-Violet excimer type, for delivering second laser beam 20 onto the material 16 and beam directing and shaping optics 22, 24.The beam directing optics 22 usually consist of one or more mirrors whilst the beam shaping optics 24 generally comprise one or more lenses. The two beams are arranged to impinge on the same point on the material to be processed. However, because the two beams have substantially different wavelengths it is not possible for the beams to be co-linear as the final beam shaping optic will not transmit both wavelengths. This means that one or other of the beams has to be incident on the workpiece at some large separation or delivery angle 0 as shown in figure 1. This is very undesirable for the following three reasons: (a) The large angle of incidence means that the area illuminated by the UV laser on the material is increased by an amount proportional to 1/Cos 0.As a result of this, the energy density of the beam or the material is reduced by the same proportion, so effectively reducing the power density of the laser. It also makes it difficult to produce small illuminated areas.

(b) It is difficult to produce aberration free images on the surface and so to provide uniform sharply defined beam areas.

(c) The reflectivity of the material varies with the angle of incidence for most materials and hence the amount of energy coupled in the workpiece also varies. In many cases the reflectivity significantly increases with increasing angle of incidence.

The angle of incidence 0 will be determined by the distance d between the final optical element and the workpiece and this will be determined by the input beam parameters, the required spotsize and depth of field at the workpiece. For typical cutting applications this distance will be in the range of 50-lOOmm. The diameter of the element is usually in the range of 25-40mm so that, allowing for the mounting of the components the typical value of 0 may be as much as 450. If the process uses co-axial gas assist then the interaction point will be surrounded by the gas nozzle and it may well be impossible to carry out dual wavelength processing under these conditions and with this type of setup.

Schemes have been devised to overcome the above mentioned problems but they are frequently complicated and expensive.

Figure 2 illustrates a typical arrangement. The Infra-Red beam 32 is converted to an annulus by a device such as a waxicon 34 and is then directed by means of a mirror 36 with a hole in it to an off axis parabolic reflector 37 which also has a hole in it.

The excimer laser beam 38 is brought to the interaction point via an imaging lens 39 and through the hole in the off axis parabolic mirror 37. This whole system would be prohibitively expensive to manufacture and difficult to set up.

There is therefore a need for an improved apparatus for near co-linear delivery of two laser beams of different wavelengths onto a common working spot having a reduced beam separation delivery angle F and a higher beam energy concentration.

Accordingly, the present invention provides an apparatus for near co-linear delivery of two laser beams of different wavelengths, the apparatus comprising: a first laser, for producing a first laser beam having a first wavelength; a second laser, for producing a second laser beam having a second wavelength; beam delivery means for delivering said first and said second laser beams along respective first and second paths co-incident at a working point but angled relative to each other by a separation angle; and first conversion means for converting at least one of said laser beams into a diffraction free beam or a substantially diffraction free beam thereby to enable a reduction in said separation angle to be achieved without said beams interfering with each other prior to arrival at said working point.

Preferably, the apparatus is further provided with a second conversion means for converting the second laser beam into a diffraction free or substantially diffraction free beam thereby to enable a still further reduction in the separation angle.

In a particularly advantageous arrangement the apparatus first and/or the second conversion means is operable to convert its associated laser beam into a diffraction free beam having the form of a Bessel-Gauss beam.

In an alternative, but still advantageous arrangement the apparatus first and/or second conversion means is operable to convert its associated laser beam into a diffraction free beam having the form of a zero order Bessel function of the first kind.

The first and/or second conversion means may comprise an axicon element, a waxicon element or a holographic element.

Preferably, the first and/or the second laser is/are operable to produce a Gaussian shaped laser beam for impingement on its associated conversion means.

Advantageously, the apparatus may include a nozzle means comprising a beam receiving end for receiving said first and/or said second laser beams, a beam exit end for exiting of said first and/or said second laser beams from said nozzle towards an object to be processed thereby, and a gas inlet for receiving gas into said nozzle.

Conveniently, the nozzle means may further comprise a first window at said receiving end for receiving said first laser beam and a second window at said receiving end for receiving said second laser beam.

Advantageously, the first window comprises Zinc Selenide or Germanium for receiving an Infra Red laser beam and the second window comprises Quartz for receiving an Ultra Violet laser beam.

The present invention will now be more particularly described by way of example only with reference to the accompanying drawings in which: Figure 1 is a schematic representation of a first conventional dual laser beam non colinear material processing apparatus not according to the present invention, Figure 2 is a schematic representation of a conventional dual laser beam co-linear material processing apparatus not according to the present invention, Figure 3 is a schematic representation of a dual laser beam material processing apparatus according to the present invention, Figure 4 is a schematic representation dual laser beam material processing apparatus when combined with a gas nozzle in accordance with a further feature of the present invention and Figure 5 is a graph illustrating the power or energy density of. a Bessel function diffraction free beam.

Referring briefly to figure 1 a known two laser processing apparatus 10 comprises a first laser 12, of the Infra Red C02 type, for delivering a first laser beam 14 onto the material to be processed 16, a second laser 18, of the Ultra Violet excimer type, for delivering a second laser beam 20 onto the material 16 and beam directing and shaping optics 22, 24. The beam directing optics 22 usually consist of one or more mirrors 22, whilst the beam shaping optics 24 generally comprise one or more lenses. The two beams 14, 20 are arranged to impinge on the same point on the material to be processed however, because the two beams have substantially different wavelengths it is not possible for the beams to be co-spatial as the final beam shaping optic will not transmit both wavelengths.This means that one or other of the beams 14, 20 have to be incident on the workpiece at some large separation, or delivery angle 0 as shown. This can result in a reduction in the beam energy density at the point of incidence and a non-uniform combined beam shape which results in the problems detailed above.

Figure 2 illustrates schematically one arrangement which can reduce the above mentioned problems. The apparatus comprises an Infra Red beam 32 produced by a suitable laser (not shown) and which is converted into an annulus by a waxicon element shown at 34 prior to being directed by mirror 36 having a hole therein to an off axis parabolic reflector 37 which also has a hole therein. UV excimer laser produced beam 38 is brought to the interaction point X via an imaging lens 39 and through the hole in the off axis parabolic mirror 37.

This whole system would be prohibitively expensive to manufacture and is difficult to set up.

The present invention attempts to overcome the problems associated with the above mentioned apparatus by providing an apparatus 40 for or near co-linear delivery of two laser beams of different wavelengths. The apparatus 40, as shown in figure 3, comprises a first laser 42 of the C02 type for providing an Infra Red laser beam 44 having a first wavelength, a second laser 46 of the excimer type for providing an Ultra Violet laser beam 48 having a second wavelength, a beam delivery means 50 and first and second conversion means shown at 52 and 54 respectively.

The beam delivery means 50 comprises first and second mirrors 56, 57 for reflecting said first and second laser beams 44, 48 towards a co-spatial processing point on the material to be processed 55 together with beam shaping optics 59, 60. One or both laser beams 44, 48 are passed through beam converting means 52, 54 for converting said beam into diffraction free beams or substantially diffraction free beams. In practice, the beam converting means 52, 54 may comprise the shaping optics 59, 60.

A diffraction free beam or substantially diffraction free beam has the advantage of having a very long depth of field over which the laser beam spot size is substantially constant.

The diffraction free beam may take the form of a zero order Bessel function of the first kind, in which case its transverse energy distribution will be substantially as shown in figure 5.

Alternatively, it may take the form of a Bessel Gauss beam in which case its transverse energy distribution will be similar to that shown in figure 5. For either shape beam the highest energy concentration is provided in the very middle portion of the beam as shown.

It has been found that a diffraction free range of 100mm for a central spot size of 0.5 mm is achievable. In addition to this, the diffraction free zone can be generated at a distance of at least lm from the optical element or converter producing the diffraction free beam.

Referring now once again to figure 3, it will be appreciated that by converting one or both laser beams 44, 48 into diffraction free beams and, if necessary, generating said beams more than lm from the optical elements it will be possible to significantly reduce the separation delivery angle 0 without compromising the performance of the apparatus. The high centrally biased concentration of energy distribution in diffraction free beams allows the two beams to be positioned more co-linear to each other than has been possible before and helps to increase the final laser spot energy concentration.The ability to generate the diffraction free beam more than lm from the optical elements allows the separation delivery angle 0 to be reduced as the mirrors and optical elements may be positioned more closely to each other than was previously possible without the beams interfering with each other.

The converting means themselves 52, 54 may comprise axicon elements, waxicon elements or holographic elements, all of which are well known in the art and are therefore not described further herein.

Referring now particularly to figure 4, the above mentioned apparatus may be combined with a gas assist nozzle shown generally at 70 in order to provide a more efficient processing apparatus. In addition to these elements previously described, the combined apparatus comprises a nozzle 72 having a beam receiving end 74 a beam exit end 76 and a gas inlet 78 for receiving gas into said nozzle 72. The receiving end 74 includes first and second windows 80, 82 made from Zinc Selenide or Germanium and Quartz respectively for receiving the Infra Red and Ultra Violet laser beams 44, 48 respectively. These materials being particularly good at transmitting said beams. It will however be appreciated that other window materials may be used. Mirrors 56 and 58 may be replaced by a suitably coated prism 84 if desired.

In operation, the two lasers 42, 46 are operated to produce laser beams 44, 48 which are directed into the gas nozzle by mirrors 56, 58 or prism 84. Gas is passed under pressure into the nozzle 72 through inlet 78 and is expelled therefrom via outlet 76. The gas may be Argon, C02 or Oxygen for example and has the effect of increasing the processing rate or preventing oxidisation. In some cases the gas may be supplied at high pressure and used to blow molten metal away from the point of processing.

Claims

1. An apparatus for near co-linear delivery of two laser beams of different wavelengths, the apparatus comprising: a first laser, for producing a first laser beam having a first wavelength; a second laser, for producing a second laser beam having a second wavelength; beam delivery means for delivering said first and said second laser beams along respective first and second paths co-incident at a working point but angled relative to each other by a separation angle; and first conversion means for converting at least one of said laser beams into a diffraction free beam or a substantially diffraction free beam thereby to enable a reduction in said separation angle to be achieved without said beams interfering with each other prior to arrival at said working point.

2. An apparatus as claimed in claim 1 including second conversion means for converting the second laser beam into a diffraction free or substantially diffraction free beam thereby to enable a still further reduction in the separation angle.

3. An apparatus as claimed in claim 1 or claim 2 in which the first and/or the second conversion means is operable to convert its associated laser beam into a diffraction free beam having the form of a Bessel-Gauss beam.

4. An apparatus as claimed in claim 1 or claim 2 in which the first and/or second conversion means is operable to convert its associated laser beam into a diffraction free beam having the form of a zero order Bessel function of the first kind.

5. An apparatus as claimed in any one of the preceding claims in which the first and/or the second conversion means comprises an axicon element.

6. An apparatus as claimed in any one of claims 1 to 4 in which the first and/or the second conversion means comprises a waxicon element.

7. An apparatus as claimed in any one of claims 1 to 4 in which the first and/or the second conversion means comprises a holographic element.

8. An apparatus as claimed in any one of the preceding claims in which the first and/or the second laser is/are operable to produce a Gaussian shaped laser beam for impingement on its associated conversion means.

9. An apparatus as claimed in any one of the preceding claims including a nozzle means comprising a beam receiving end for receiving said first and/or said second laser beams, a beam exit end for exiting of said first and/or said second laser beams from said nozzle towards an object to be processed thereby and a gas inlet for receiving gas into said nozzle.

10. An apparatus as claimed in claim 9 including a first window at said receiving end for receiving said first laser beam and a second window at said receiving end for receiving said second laser beam.

11. An apparatus as claimed in claim 10 in which the first window comprises Zinc Selenide or Germanium for receiving an Infra Red laser beam and the second window comprises Quartz for receiving an Ultra Violet laser beam.

12. An apparatus for near co-linear delivery of two lasers of different wavelengths substantially as hereinbefore described with reference to and as illustrated in figures 3 and 4 of the accompanying drawings.