Apparatus and Method for Controlling Laser Processing of a Remote Material

Abstract

Apparatus for controlling laser piercing of a remote material (10), which apparatus comprises: at least one piercing laser (1) for emitting laser radiation (2) for piercing the remote material (10), which laser radiation (2) is characterized by a first wavelength (16); a probe laser (3) for emitting a probe signal (4) for monitoring the piercing of the remote material (10); beam delivery optics (5) configured to direct the laser radiation (2) and the probe signal (4) onto the material (10); at least one detector (6) for detecting optical radiation (7) that is emitted or reflected by the material (10) in response to the probe signal (4); and an electronic filter (8) for filtering an electronic signal (9) emitted by the detector (6) in response to the detector (6) detecting the optical radiation (7); and the apparatus being characterized in that the probe laser (3) is configured such that the probe signal (4) is able to be modulated by a modulation signal (13); and the electronic filter (8) comprises a phase sensitive detector (14) which is configured to receive the electronic signal (9) and the modulation signal (13) and to provide phase sensitive detection of the electronic signal (9), which phase sensitive detection is used to improve a signal to noise ratio of an amplitude of the electronic signal (9), thereby enabling detection of a reduction in the amplitude of the electronic signal (9), which reduction is indicative of the laser radiation (2) piercing the remote material (10).

Description

FIELD OF INVENTION

This invention relates to an apparatus and a method for controlling laser piercing of a remote material.

BACKGROUND TO THE INVENTION

High power lasers have important applications in the laser processing of industrial materials. Pulsed lasers, with peak powers exceeding 10 kW, are used in marking, engraving, cutting, welding, and drilling applications. Continuous wave lasers with powers exceeding 500 W are used in cutting and welding applications. These high power lasers advantageously have optical fibre beam delivery systems for delivering the laser radiation from the laser to a work piece, which work piece can be located tens or hundreds of metres from the laser.

Laser cutting of materials typically commences with piercing through the material in order to form at least one hole. There can be many holes required and so the piercing can contribute a significant amount of time to the cutting process. The ability to know accurately when the material is pierced allows the apparatus to start the cut or move on to another pierce, hence improving reliability and productivity as dwell times for piercing are kept to a minimum.

There are many industrial lasers that use a beam delivery cable between the laser and the work piece. These lasers include fibre lasers, disk lasers, and rod lasers. Fibre lasers are especially attractive owing to their excellent reliability, cost, and wall plug efficiency. Also, the fibre lasers provide power levels with good beam quality to multi kilowatts. The beam delivery cable contains optical fibre, which can be tens or even hundreds of metres long, allowing the laser to be located in a location remote from the work piece. There is thus a continuous optical path through the optical fibre from the laser to the work piece. However, there is also an optical path for optical radiation emitted from the work piece to propagate back to the laser itself. Backward travelling optical radiation is generally considered a nuisance and needs to be removed in a controlled manner to prevent damage to the laser.

In certain applications, the material being pierced is remote from the laser, and the inclusion of a beam delivery cable is not practical. Industrial applications would include remote cutting, for example at heights or in ship yards. Defence applications would include using lasers to cut holes in mines and bombs in order to make them safe, and the use of coherently or spectrally combined lasers that may be used to cut holes in drones or missiles in order to disable them.

The ability to detect that the material has been pierced, and to proceed promptly with the next task, provides operational advantages. However, the ability to detect the piercing of the material can be difficult because of back reflections of the laser signal emitted from the laser and/or the difficulty of capturing enough signal emitted from the piercing process to enable the piercing process to be monitored.

For piercing remote materials, the back reflected signals can be so small that the detection process is swamped by electronic noise, making control of the laser piercing process very difficult. This is highly undesirable in applications in which the laser radiation needs to be turned off once the material has been pierced.

It is an aim of this invention to improve the signal to noise ratio of pierce detection in the control of laser piercing of a remote material.

The Invention

According to a non-limiting embodiment of the present invention there is provided apparatus for controlling laser piercing of a remote material, which apparatus comprises:

• • at least one piercing laser for emitting laser radiation for piercing the remote material, which laser radiation is characterized by a first wavelength;
  • a probe laser for emitting a probe signal for monitoring the piercing of the remote material;
  • beam delivery optics configured to direct the laser radiation and the probe signal onto the material;
  • at least one detector for detecting optical radiation that is emitted or reflected by the material in response to the probe signal; and
  • an electronic filter for filtering an electronic signal emitted by the detector in response to the detector detecting the optical radiation;

and the apparatus being characterized in that

• • the probe laser is configured such that the probe signal is able to be modulated by a modulation signal; and
  • the electronic filter comprises a phase sensitive detector which is configured to receive the electronic signal and the modulation signal, and to provide phase sensitive detection of the electronic signal, which phase sensitive detection is used to improve a signal to noise ratio of an amplitude of the electronic signal, thereby enabling detection of a reduction in the amplitude of the electronic signal, which reduction is indicative of the laser radiation piercing the remote material.

Improving the signal to noise ratio of the amplitude of the electronic signal enables the control of laser piercing in cases where the laser radiation has been directed over free-space distances of tens, if not hundreds or thousands of metres. The amplitude of the electronic signal is proportional to the intensity of the optical radiation that is reflected or emitted by the material in response to the probe signal. Once the material is pierced, the probe signal passes through the pierced hole in the material, and the optical radiation that is reflected or emitted by the material falls. Improving the signal to noise ratio of the amplitude of the electronic signal enables the reduction in the intensity of the optical radiation to be monitored, enabling reliable and accurate pierce detection.

Phase sensitive detection involves the synchronous demodulation of a signal with a reference signal, followed by low pass filtering. Typically, both in-phase and quadrature demodulation are performed, with the outputs from each demodulator combined together to provide the amplitude of the signal component that is synchronous with the reference signal. Electronic noise components that are not synchronous with the modulation signal are attenuated in comparison to signals and electronic noise components that are synchronous with the reference signal. The electronic noise components that are not synchronous with the modulation signal are averaged nearly to zero. Phase sensitive detection enables detection of signals up to one million times smaller than electronic noise components.

The Invention takes advantage of the signal to noise advantages of the phase sensitive detection process by amplitude modulating the probe signal with the modulation signal. The optical radiation that is emitted or reflected by the material in response to the probe signal is detected by the detector. The electronic signal that is emitted by the detector is then synchronously demodulated within the phase sensitive detector to provide a filtered electronic signal whose amplitude is proportional to the intensity of the optical radiation received by the detector. Electronic noise components within the electronic signal that are not synchronous with the modulation signal are strongly attenuated by the phase sensitive detection process. This makes it possible to detect the reduction in the amplitude of the filtered electronic signal which occurs when the laser radiation pierces the material. The laser radiation can then be turned off, or directed elsewhere, in order to prevent materials beyond the pierced hole being irradiated.

The electronic filter may comprise a sampler for sampling the electronic signal in synchronism with the modulation signal.

The probe signal may have a second wavelength. The first wavelength may be different from the second wavelength.

The apparatus may include an optical filter configured to filter the optical radiation emitted or reflected by the material in response to the probe signal.

The probe laser may be a pulsed laser.

The modulation signal may be synchronized with a pulse repetition frequency of the pulsed laser.

The pulsed laser may be a nanosecond pulsed fibre laser.

The piercing laser may be characterized by a maximum power. The probe laser may be characterized by a peak power. The probe laser may be selected such that the peak power is greater than the maximum power.

The piercing laser may be a continuous wave laser.

The apparatus may comprise a plurality of the piercing lasers.

The piercing lasers may have the same first wavelength. The beam delivery optics may be configured to coherently combine the laser radiation emitted from the piercing lasers.

The piercing lasers may have different first wavelengths. The beam delivery optics may comprise a diffraction grating configured to combine the laser radiation emitted from the piercing lasers.

The detector may be configured to be adjacent to the material.

The detector may be configured to be adjacent to the probe laser.

The invention also describes a method for controlling laser piercing of a remote material, which method comprises:

• • providing a piercing laser for emitting laser radiation for piercing the remote material, which laser radiation is characterized by a first wavelength;
  • providing a probe laser for emitting a probe signal;
  • directing the laser radiation and the probe signal onto the material;
  • detecting optical radiation that is emitted or reflected by the material in response to the probe signal with at least one detector; and
  • filtering an electronic signal emitted by the detector in response to the optical radiation with an electronic filter;
    and the method being characterized in that it includes the following steps:
  • modulating the probe signal with a modulation signal;
  • inputting both the electronic signal and the modulation signal to the electronic filter;
  • performing phase sensitive detection in the electronic filter in order to improve a signal to noise ratio of an amplitude of the electronic signal; and
  • using the amplitude of the electronic signal to control the piercing of the remote material with the laser radiation.

The laser radiation may be directed over a free-space distance of at least ten metres. The free-space distance may be at least one hundred metres. The free-space distance may be at least one thousand metres.

The method may include the steps of providing a plurality of the piercing lasers end coherently combining the laser radiation emitted by the piercing lasers.

The method may include the steps of providing a plurality of the piercing lasers and spectrally combining the laser radiation emitted by the piercing lasers.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which:

FIG. 1 shows apparatus according to the present invention for controlling laser piercing of a remote material;

FIG. 2 shows a modulation signal that is synchronized to the pulse repetition frequency of a pulsed laser;

FIG. 3 shows a peak power of a pulsed laser that is greater than a maximum power of a continuous wave laser;

FIG. 4 shows a coherently combined laser system comprising a plurality of piercing lasers and a probe laser;

FIG. 5 shows a cross-sectional side view of a beam combiner comprising a bundle of optical fibres laid side by side;

FIG. 6 shows a cross-sectional end view of the bundle of optical fibres; and

FIG. 7 shows a spectrally combined laser system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows an apparatus for controlling laser piercing of a remote material 10, which apparatus comprises:

• • at least one piercing laser 1 for emitting laser radiation 2 for piercing the remote material 10, which laser radiation 2 is characterized by a first wavelength 16;
  • a probe laser 3 for emitting a probe signal 4 for monitoring the piercing of the remote material 10;
  • a beam delivery optics 5 configured to direct the laser radiation 2 and the probe signal 4 onto the material 10;
  • at least one detector 6 for detecting optical radiation 7 that is emitted or reflected by the material 10 in response to the probe signal 4; and
  • an electronic filter 8 for filtering an electronic signal 9 emitted by the detector 6 in response to the detector 6 detecting the optical radiation 7;

and the apparatus being characterized in that

• • the probe laser 3 is configured such that the probe signal 4 is able to be modulated by a modulation signal 13; and
  • the electronic filter 8 comprises a phase sensitive detector 14 which is configured to receive the electronic signal 9 and the modulation signal 13, and to provide phase sensitive detection of the electronic signal 9, which phase sensitive detection is used to improve a signal to noise ratio of an amplitude of the electronic signal 9, thereby enabling detection of a reduction in the amplitude of the electronic signal 9, which reduction is indicative of the laser radiation 2 piercing the remote material 10.

Improving the signal to noise ratio of pierce detection enables the control of laser piercing in cases where the laser radiation 2 has been directed over a free-space distance 29 of tens, if not hundreds or thousands of metres. In such cases, the intensity of the optical radiation 7 that is reflected or emitted by the material 10 in response to the probe signal 4 can be very small, and in general, not visible in the electronic signal 9 emitted by the detector 6. The electronic signal 9 shown in FIG. 1 is dominated by electronic noise, making it very difficult to discern the modulation signal component. The detection process is thus swamped by electronic noise, making control of the laser piercing process very difficult. This can be undesirable in applications in which the laser radiation 2 needs to be turned off once the material has been pierced.

The phase sensitive detector 8 shown in FIG. 1 comprises a first demodulator 91 and a second demodulator 92 for demodulating the electronic signal 9 with respect to the modulation signal 13. The first demodulator 91 performs demodulation in phase with the modulation signal 13, and the second demodulator 92 performs demodulation in quadrature with the modulation signal 13. The outputs from the first demodulator 91 and the second demodulator 92 are filtered by the low pass filters 93, 94, and combined together in the processor 95 to provide a filtered electronic signal 12 that is proportional to the amplitude of the electronic signal 9. The first and the second demodulators 91, 92 may be multipliers or mixers. Depending on the waveform of the modulation signal 13, the processor 95 may comprise taking the square root of the sums of the squares of the outputs of the two low pass filters 93, 94. The phase sensitive detector 14 may be configured in analogue electronics, a digital signal processor, or a combination thereof. In certain applications, the phase relationship between the modulation signal 13 and the electronic signal 9 is such that only the first demodulator 91 is needed. The relative phase of the modulation signal 13 at the input to the first demodulator 91 may be adjusted in order to maximize the signal to noise ratio of the filtered electronic signal 12.

Electronic noise components in the electronic signal 9 that are not synchronous with the modulation signal 13 are attenuated in comparison to signals and electronic noise components that are synchronous with the modulation signal 13. The electronic noise components that are not synchronous with the modulation signal 13 are averaged nearly to zero. Phase sensitive detection enables detection of signals up to one million times smaller than electronic noise components.

The invention takes advantage of the signal to noise advantages of the phase sensitive detection process by amplitude modulating the probe signal 4 with the modulation signal 13. The optical radiation 7 that is emitted or reflected by the material 10 in response to the probe signal 4 is detected by the detector 6. The electronic signal 9 that is emitted by the detector 6 is then synchronously demodulated within the phase sensitive detector 14 to provide a filtered electronic signal 12 whose amplitude is proportional to the intensity of the optical radiation 7 received by the detector 6. Electronic noise components within the electronic signal 9 that are not synchronous with the modulation signal 13 are strongly attenuated by the phase sensitive detection process. This makes it possible to detect the reduction in the amplitude of the filtered electronic signal 12 which occurs when the laser radiation 2 pierces the material 10. The laser radiation 2 can then be turned off, or directed elsewhere, in order to prevent materials beyond the pierced hole being irradiated.

The laser radiation 2 and the probe signal 4 are shown being combined by a coupler 22. The coupler 22 can be a beam splitter.

The phase sensitive detector 14 may be a lock-in amplifier.

The electronic filter 8 is shown further comprising a sampler 15 for sampling the electronic signal 9 in synchronism with the modulation signal 13. The sampler 15 may be a linear gate.

The modulation signal 13 may be supplied by a modulator 20 that may form part of the apparatus. Alternatively, the modulation signal 13 may be provided by an external modulator which may comprise a computer algorithm.

The apparatus may include a discriminator 11 for analyzing the filtered electronic signal 12 from the electronic filter 8. The discriminator 11 can be configured to detect a change in the characteristics of the optical radiation 7 emitted by the material 10 when the material 10 is pierced by the laser radiation 2.

The probe signal 4 may be characterized by a second wavelength 17. The first wavelength 16 may be different from the second wavelength 17.

The apparatus may include an optical filter 18 configured to filter the optical radiation 7 emitted by the material 10. Advantageously, the optical filter 18 may be selected to attenuate optical radiation at the first wavelength 16. This is advantageous if the optical radiation 7 emitted from the material 10 during the piercing process has signal wavelengths that would be masked by optical radiation at the first wavelength 16 and/or the second wavelength 17 that is not characteristic of the piercing process. Such optical radiation can occur from atmospheric scattering, or from reflections, and can reduce the ability of the apparatus to detect when the material 10 has been successfully pierced. Selecting and configuring the optical filler 18 to attenuate such masking optical radiation in preference to the optical radiation 7 that is characteristic of the piercing process can improve signal to noise ratio and thus increase the ability to monitor the piercing process.

The probe laser 3 is preferably a single mode laser.

The probe laser 3 may be a pulsed laser. FIG. 2 shows the modulation signal 13 synchronized with a pulse repetition frequency 19 of the pulsed laser. Six pulses 26 of the pulsed laser are shown for each ON state 25 of the modulation signal 13. However, the number of pulses 26 for each ON state 25 may be one. Alternatively, the number of pulses 26 may be greater than one.

The probe laser 3 may be a nanosecond pulsed fibre laser.

As shown in FIG. 3, the laser radiation 2 may be characterized by a maximum power 31, and the probe signal 4 may be characterized by a peak power 32. The probe laser 3 may be selected such that the peak power 32 is greater than the maximum power 31. The peak power 32 may be at least ten times greater than the maximum power 31.

FIG. 4 shows a coherently combined laser system 40. The piercing lasers 1 are optical amplifiers 44. The optical amplifiers 44 are preferably selected to provide a single mode output. A seed laser 41 is coupled to the optical amplifiers 44 via a demultiplexer 42 and phase modulators 43. The outputs from the optical amplifiers 44 are combined with the output from the probe laser 2 in the multiplexer 47. The multiplexer 47 emits the laser radiation 2 and the probe signal 4.

The multiplexer 47 can comprise optical fibres 45 and 46 laid in a bundle 61 as shown in cross-sectional side view in FIG. 5, and in cross-sectional end view in FIG. 5. The optical fibres 45 and 46 preferably have end caps 51 for reliable high-power operation. The end caps 51 can comprise fused silica. The end caps 51 can comprise graded index lenses in order to collimate the laser outputs. Alternatively, or additionally, separate collimating lenses 52 can be provided. Preferably the optical fibre 46 from the probe laser 3 is located near or at the centre of the bundle 61 as shown in FIG. 6 to ensure that the probe signal 4 can be imaged near the centre of the laser radiation 2 and thus overlay the area being pierced on the material 10.

The seed laser 41 is preferably a narrowband laser such as a distributed feedback semiconductor or distributed feedback fibre laser. Preferably it is characterized by an optical bandwidth less than 40 GHz, and preferably less than 20 GHz in order to reduce the effects of stimulated Brillouin scattering in the optical amplifiers 44. Use of the seed piercing laser 1 ensures that the optical amplifiers 43 have the same first wavelength 16.

The demultiplexer 42 can comprise at least one optical fibre coupler. The demultiplexer 42 can comprise a planar waveguide splitter.

The phase modulators 43 can comprise lithium niobate modulators. The phase modulators 43 are controlled in order to lock the relative phases of the laser radiation emitted from the optical amplifiers 44 such that the laser radiation 2 comprises a single-mode beam.

Further details of how to implement a coherently combined laser system such as shown in FIG. 4 (though without the probe laser 3) can be found in U.S. Pat. No. 8,520,306, which patent is hereby incorporated by reference herein.

FIG. 7 shows a spectrally combined laser system 70. The piercing lasers 1 are single mode lasers 71, 72, 73, 74 that have different first wavelengths 16. The probe laser 3 has a second wavelength 17 that is different from the first wavelengths 16. The beam delivery optics 5 comprises a first diffraction grating 75 configured to combine the laser radiation 2 emitted from the piercing lasers 1. The first diffraction grating 75 can also combine the probe signal 4 from the probe laser 3. A second diffraction grating 76 can be provided in order to compensate for the angular dispersion induced by the first diffraction grating 75. The first and the second diffraction gratings 75, 76 can be of identical or similar design. The first and the second diffraction gratings 75, 76 are preferably arranged parallel to each other. The piercing lasers 1, the probe laser 3, and the first and the second diffraction gratings 75, 76 are preferably selected and configured such that the laser radiation 2 and the probe signal 4 are collimated and are able to illuminate and pierce the material 10 shown with reference to FIG. 1.

The probe laser 3 shown with reference to FIGS. 1, 4 and 7 can be a continuous wave laser. The continuous wave laser may be a fibre laser, a disk laser, a rod laser, a solid state laser, or a carbon dioxide laser.

Referring again to FIG. 1, the detector 6 is shown located adjacent to the piercing laser 1 and the probe laser 3. A lens 21 focuses the optical radiation 7 onto the detector 6. In certain applications, the detector 6 may be located remotely from the piercing laser 1 and the probe laser 3, for example the detector 6 may be located adjacent to the material 10. The probe laser 3 may be located remotely from the piercing laser 1. The detector 6 may be located remotely from both the piercing laser 1 and the material 10. By “remote” or “remotely” it is meant separated by a free space distance 29 of at least 10 metres, preferably at least 50 m, and more preferably at least 1 km.

The invention described with reference to the Figures can be used in a variety of ways, including;

• • (i) determining when the material 10 has been pierced, so that the piercing laser 1 can be turned off;
  • (ii) to provide a monitor of cutting quality; and
  • (iii) to provide a weld quality monitor operating such that an increase in noise indicates weld quality diminishing.

It is to be appreciated that the embodiments of the invention described above with reference to the accompanying drawings have been given by way of example only and that modifications and additional steps and components may be provided to enhance performance. Individual components shown in the drawings are not limited to use in their drawings and may be used in other drawings and in all aspects of the invention. The present invention extends to the above mentioned features taken singly or in any combination.

Claims

1. Apparatus for controlling laser piercing of a remote material, which apparatus comprises: and the apparatus being characterized in that

at least one piercing laser for emitting laser radiation for piercing the remote material, which laser radiation is characterized by a first wavelength;

a probe laser for emitting a probe signal for monitoring the piercing of the remote material;

beam delivery optics configured to direct the laser radiation and the probe signal onto the material;

at least one detector for detecting optical radiation that is emitted or reflected by the material in response to the probe signal; and

an electronic filter for filtering an electronic signal emitted by the detector in response to the detector detecting the optical radiation;

the probe laser is configured such that the probe signal is able to be modulated by a modulation signal; and

the electronic filter comprises a phase sensitive detector which is configured to receive the electronic signal and the modulation signal, and to provide phase sensitive detection of the electronic signal, which phase sensitive detection is used to improve a signal to noise ratio of an amplitude of the electronic signal, thereby enabling detection of a reduction in the amplitude of the electronic signal, which reduction is indicative of the laser radiation piercing the remote material.

2. Apparatus according to claim 1 wherein the electronic filter comprises a sampler for sampling the electronic signal in synchronism with the modulation signal.

3. Apparatus according to claim 1 wherein the probe signal has a second wavelength, and wherein the first wavelength is different from the second wavelength.

4. Apparatus according to claim 3 and including an optical filter configured to filter the optical radiation emitted or reflected by the material in response to the probe signal.

5. Apparatus according to claim 1 wherein the probe laser is a pulsed laser.

6. Apparatus according to claim 5 wherein the modulation signal is synchronized with a pulse repetition frequency of the pulsed laser.

7. Apparatus according to claim 5 wherein the pulsed laser is a nanosecond pulsed fibre laser.

8. Apparatus according to claim 5 wherein the piercing laser is characterized by a maximum power and the probe laser is characterized by a peak power, and the probe laser is selected such that the peak power is greater than the maximum power.

9. Apparatus according to claim 1 wherein the piercing laser is a continuous wave laser.

10. Apparatus according to claim 9 and comprising a plurality of the piercing lasers.

11. Apparatus according to claim 10 wherein the piercing lasers have the same first wavelength.

12. Apparatus according to claim 11 wherein the beam delivery optics is configured to coherently combine the laser radiation emitted from the piercing lasers.

13. Apparatus according to claim 10 wherein the piercing lasers have different first wavelengths, and the beam delivery optics comprises a diffraction grating configured to combine the laser radiation emitted from the piercing lasers.

14. Apparatus according to claim 1 wherein the detector is configured to be adjacent to the probe laser.

15. A method for controlling laser piercing of a remote material, which method comprises: and the method being characterized in that it includes the following steps:

providing at least one piercing laser for emitting laser radiation for piercing the remote material, which laser radiation is characterized by a first wavelength;

providing a probe laser for emitting a probe signal;

directing the laser radiation and the probe signal onto the material;

detecting optical radiation that is emitted or reflected by the material in response to the probe signal with at least one detector; and

filtering an electronic signal emitted by the detector in response to the optical radiation with an electronic filter;

modulating the probe signal with a modulation signal;

inputting both the electronic signal and the modulation signal to the electronic filter;

performing phase sensitive detection in the electronic filter in order to improve a signal to noise ratio of an amplitude of the electronic signal; and

using the amplitude of the electronic signal to control the piercing of the remote material with the laser radiation.

16. A method according to claim 15 wherein the laser radiation is directed over a free-space distance of at least ten metres.

17. A method according to claim 15 wherein the free-space distance is at least one hundred metres.

18. A method according to claim 17 wherein the free-space distance is at least one thousand metres.

19. A method according to claim 15 wherein the method includes the steps of providing a plurality of the piercing lasers and coherently combining the laser radiation emitted by the piercing lasers.

20. A method according to claim 15 wherein the method includes the steps of providing a plurality of the piercing lasers and spectrally combining the laser radiation emitted by the piercing lasers.