WO2009007721A2 - Apparatus and method for laser processing a material - Google Patents

Apparatus and method for laser processing a material Download PDF

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Publication number
WO2009007721A2
WO2009007721A2 PCT/GB2008/002357 GB2008002357W WO2009007721A2 WO 2009007721 A2 WO2009007721 A2 WO 2009007721A2 GB 2008002357 W GB2008002357 W GB 2008002357W WO 2009007721 A2 WO2009007721 A2 WO 2009007721A2
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WO
WIPO (PCT)
Prior art keywords
amplifier
laser
pump
power
seed
Prior art date
Application number
PCT/GB2008/002357
Other languages
French (fr)
Other versions
WO2009007721A3 (en
Inventor
Andrew Paul Appleyard
Duncan John William Walker
Roland Neal
Original Assignee
Spi Lasers Uk Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Spi Lasers Uk Limited filed Critical Spi Lasers Uk Limited
Priority to DE602008002842T priority Critical patent/DE602008002842D1/en
Priority to CA2693496A priority patent/CA2693496C/en
Priority to EP08775901A priority patent/EP2165394B1/en
Priority to US12/668,396 priority patent/US20100177797A1/en
Priority to KR20107002219A priority patent/KR101480932B1/en
Priority to JP2010515597A priority patent/JP2010533373A/en
Priority to CN2008800238864A priority patent/CN101689747B/en
Priority to AT08775901T priority patent/ATE483265T1/en
Priority to DK08775901.5T priority patent/DK2165394T3/en
Publication of WO2009007721A2 publication Critical patent/WO2009007721A2/en
Publication of WO2009007721A3 publication Critical patent/WO2009007721A3/en

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/094003—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light the pumped medium being a fibre
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/06—Shaping the laser beam, e.g. by masks or multi-focusing
    • B23K26/062—Shaping the laser beam, e.g. by masks or multi-focusing by direct control of the laser beam
    • B23K26/0622—Shaping the laser beam, e.g. by masks or multi-focusing by direct control of the laser beam by shaping pulses
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/08—Devices involving relative movement between laser beam and workpiece
    • B23K26/082—Scanning systems, i.e. devices involving movement of the laser beam relative to the laser head
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/10007—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers
    • H01S3/1001—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers by controlling the optical pumping
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/094003—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light the pumped medium being a fibre
    • H01S3/094007—Cladding pumping, i.e. pump light propagating in a clad surrounding the active core
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/094042—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light of a fibre laser
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/10007—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers
    • H01S3/10015—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers by monitoring or controlling, e.g. attenuating, the input signal
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/102—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the active medium, e.g. by controlling the processes or apparatus for excitation
    • H01S3/1022—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the active medium, e.g. by controlling the processes or apparatus for excitation by controlling the optical pumping
    • H01S3/1024—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the active medium, e.g. by controlling the processes or apparatus for excitation by controlling the optical pumping for pulse generation
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
    • H01S3/16—Solid materials
    • H01S3/1601—Solid materials characterised by an active (lasing) ion
    • H01S3/1603—Solid materials characterised by an active (lasing) ion rare earth
    • H01S3/1618—Solid materials characterised by an active (lasing) ion rare earth ytterbium
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/23—Arrangements of two or more lasers not provided for in groups H01S3/02 - H01S3/22, e.g. tandem arrangements of separate active media
    • H01S3/2308—Amplifier arrangements, e.g. MOPA

Definitions

  • This invention relates to an apparatus and method for laser processing a material.
  • pulsed lasers that require a very wide dynamic range together with consistent pulse shapes and pulse timing.
  • a good example is in processing rolls for printing machines. These require lasers with high average powers which are used to mark the print rolls. The power in the pulses needs to be varied accurately in order to achieve the required print resolution. The greater the dynamic range, the wider the range of print wheels that can serviced by one particular laser. Failure to provide consistent timing and predictable pulse shapes degrades the print quality.
  • Prior art solutions include use of continuous wave lasers (such as carbon dioxide lasers or fibre lasers) with an external modulator for attenuating the output power.
  • the modulator is typically an acousto-optic modulator which gates and attenuates the laser beam accordingly.
  • An aim of the present invention is to provide an apparatus and method for laser processing a material which reduces the above aforementioned problem.
  • apparatus for laser processing a material which apparatus comprises a seed laser pumped by a seed pump, an amplifier pumped by an amplifier pump, a controller. and a scanner, wherein the controller controls the seed pump, the amplifier pump and the scanner in synchronism such that optical pulses emitted by the seed laser are propagated through the amplifier and directed to the material by the scanner, the apparatus being characterized in that controller controls the amplifier pump to cause the amplifier to act as an optical attenuator when the optical radiation is in a first power range, and as an optical amplifier when the output power is in a second power range.
  • the invention enables an improved dynamic range. This is an important benefit in manufacturing environments and applications in " ⁇ nicro" materials processing where the laser would be used at the top end of its rated operating range (average power, peak power, pulse energy) in order to machine “coarse" featured items with acceptable productivity, whilst also being equally capable of processing "fine” featured items at the lower end of the rated range where very low average power / low peak power / very low pulse energy are processing requirements.
  • the present invention is able to provide a dynamic range of at least three orders of magnitude (30dB) compared with typically less than two orders of magnitude (20dB) for lasers and master oscillator power amplifiers not configured and operated in accordance with the present invention.
  • the scanner is preferably a mechanical unit that scans the print roll with respect to the laser beam.
  • the scanner may comprise a galvanometer and a reflector.
  • printing roll it is meant a roll for printing applications, such as a ceramic printing roll, an anilox printing roll, an ink- transfer roll, an engraved printing rolls, or an engraved sheet that is attached to a roll.
  • the controller can be a computer controller.
  • the controller may Include a switch (such as a relay or a transistor) for turning off the electrical drive power to the amplifier pump when in the first power range.
  • the seed laser may comprise a cladding-pumped fibre laser.
  • the cladding- pumped fibre laser may comprise a rare-earth dopant.
  • the rare-earth dopant may be ytterbium.
  • the seed laser may comprise erbium, thulium, holmium or neodymium.
  • the amplifier may comprise a cladding-pumped fibre amplifier.
  • the cladding- pumped fibre amplifier may comprise a rare-earth dopant
  • the rare-earth dopant may be Ytterbium.
  • the seed laser may comprise erbium, thulium, hoimium or neodymium.
  • the apparatus may include at least one additional amplifier stage.
  • the apparatus may include an absorber or a saturable absorber.
  • the saturable absorber may introduce a delay.
  • the apparatus may include a variable delay for compensating the delay.
  • the invention also provides a method for marking a material, the method comprising providing an apparatus according to the present invention, and applying the laser radiation to the material.
  • the material can be a printing roll.
  • Figure 1 shows apparatus for laser processing a material according to the present invention
  • Figure 2 shows pulse shapes according to apparatus not of the present invention.
  • Figure 3 shows examples of a pulse obtained according to the present invention
  • Figure 4 shows examples of missing pulses
  • Figure 5 shows examples of noisy pulses
  • Figure 6 shows examples of the increase in dynamic range afforded by the invention.
  • Figure 1 shows apparatus for laser processing a material 8 with optical radiation 7, which apparatus comprises a seed laser 1 pumped by a seed pump 2, an amplifier 3 pumped by an amplifier pump 4, a controller 5, and a scanner 6, wherein the controller 5 controls the seed pump 2, the amplifier pump 4 and the scanner 6 in synchronism such that optical pulses 10 emitted by the seed laser 1 are propagated through the amplifier 3 and directed to the material 8 by the scanner 6, the apparatus being characterized in that controller 5 controls the amplifier pump 4 to cause the amplifier 3 to act as an optical attenuator when the optical radiation 7 is in a first power range 61, and as an optical amplifier when the output power is in a second power range 62.
  • the first and second power ranges 61, 62 are described with reference to Figure 6, and can be overlapping.
  • the power amplifier 2 preferably comprises double clad optical fibre 19 that is doped with rare-earth dopant (not shown).
  • the ability to achieve an improved dynamic range is an important benefit in manufacturing environments and applications in "micro” materials processing where the laser would be used at the top end of its rated operating range (average power, peak power, pulse energy) in order to machine “coarse" featured items with acceptable productivity, whilst also being equally capable of processing "fine” featured items at the lower end of the rated range where very low average power / low peak power / very low pulse energy are processing requirements.
  • the apparatus shown in Figure 1 provides a dynamic range of three orders of magnitude or greater, compared with typically less than two orders of magnitude for lasers and master oscillator power amplifier not configured and operated in accordance with the present invention.
  • the material 8 can be a printing roll, for example, one used in the Anilox process.
  • the scanner 6 is preferably a mechanical unit that scans the print roll with respect to the laser beam.
  • the scanner 6 may comprise a galvanometer and a reflector.
  • the controller 5 can be a computer controller.
  • the controller 5 can include a switch 14, such as a relay or a transistor, for turning off the electrical drive power to the amplifier pump 4 when in the first power range 61.
  • the controller 5 can include a variable delay 15 for delaying a control signal 17 comprising pulses 11.
  • the delayed control signal would be passed to both the seed laser 1 and the amplifier 3 in order to avoid the amplifier 3 from self-Q-switehing.
  • the pulses 11 can have different amplitudes 12.
  • Figure 2 shows pulses 21, 22, 23, 24, 25, 26, 27, 28, 29 produced by modulation of the laser output with various set points corresponding to continuous-wave equivalent output powers of 200W, 180W, 160W, 140W, 120W, 100W, 80W, 60W and 40W respectively.
  • the seed laser 1 is a cladding pumped Ytterbium-doped fibre laser operating at around 1070nm, comprising double clad optical fibre (not shown) and two optical fibre Bragg gratings (not shown).
  • the amplifier 3 is a cladding pumped Ytterbium-doped fibre amplifier.
  • the output pulse shapes shown correspond to the amplified first relaxation oscillation from the seed laser 2.
  • the variations in pulse peak power and thus pulse energy were obtained without using the present invention by reducing the output power of both the seed pump 2 and the amplifier pump 4 for each successive measurement. As is clearly visible, as the power level falls, the pulse is delayed, and becomes smeared.
  • FIG 3 shows a result according to the invention.
  • Pulse shape 31 (see left hand axis 33) was obtained with the amplifier 3 in the second power range 62 in which the amplifier 3 is operated with approximately a 3dB large signal amplification.
  • Pulse shape 32 (see right hand axis 34) was obtained with the amplifier 3 in the first power range 61 in which the amplifier 3 is operated with an attenuation of approximately 1dB.
  • the pulse shapes 31, 32 are virtually identical.
  • the apparatus can be operated at lower output powers than in the conventional operating mode without there being missing or very noisy pulses. This is because the seed laser 1 is being driven harder for equivalent output power when operating in the first power range than when being operated in the second power range. Thus the dynamic range has been extended.
  • the pulse shapes in Figure 3 are nearly identical, it should be noted that the pulse shape 31 was delayed in order for the pulses to overlay in the Figure. This is because the pulse turn-on delay varies with drive power, as shown in Figure 2. Thus it is desirable for the controller 5 to include the variable delay 15 if pulse synchronization at different power levels is required.
  • the variable delay 15 can be controlled internally to the laser, or applied externally.
  • the dynamic range over which the apparatus provides useful output can be further extended by increasing the dynamic large signal amplification range of the amplifier 3. This can be accomplished by use of additional amplification stages to increase the maximum amplification in the second power range 62. Alternatively or additionally, by inserting an absorber or a saturable absorber to increase the effective attenuation in the first power range.
  • the saturable absorber will introduce an additional time delay (not shown) with respect to the pulse entering the saturable absorber corresponding to the time required to bleach the absorption.
  • the rare-earth doped optical fibre 19 will act as a saturable absorber when not pumped.
  • the power amplifier 3 will in practice operate as both an amplifier when pumped, and as an attenuator plus saturable absorber when not pumped.
  • the apparatus is a two-stage master oscillator power amplifier comprising the seed laser I and the power amplifier 3, in which the optical power partitioning between seed laser 1 and the power amplifier 3 is approximately 1:2.
  • the pumps 2, 4 are implemented as laser diodes.
  • the seed laser 1 would thus contribute approximately 66W to the overall output and the power amplifier 3 would add 133 W to achieve the nominal rated power,
  • both the seed laser 1 and the power amplifier 3 are pumped, and the pump diodes (the pumps 2 and 4) driving both the seed laser 1 and the amplifier 3 are operated in synchronism, the output power being controlled by varying the power supplied by the pumps 2, 4 to the two stages proportionately.
  • the pump diode current is typically modulated at high speed in order to produce a train of output pulses with the parametric properties required for the printing roll (or other work piece) being processed.
  • the seed laser 1 comprises a double clad fibre and fibre Bragg gratings to create the laser cavity, and the power amplifier 3 uses the double clad fibre 19. It is preferred that the double clad fibres are side-pumped via a side-pumping fibre (not shown) that couples pump power into the double clad fibre gradually along its length.
  • Optical amplifiers based on double clad fibres pumped by side pumping fibres are described in more detail in United States patent number 6,826,335 which is hereby incorporated herein by reference.
  • the diameter of the side-pumping fibre may be optimized according to the power rating of the laser.
  • the diameter of this fibre for the seed laser 1 is preferably smaller than for the power amplifier.
  • the seed laser 1 might contain double clad fibre in which the "active fibre” (Yb-doped core or similar) is approximately I25um in diameter and the "pump fibre” (pure silica fibre for injection of pump diode power into the cavity) is 125um.
  • active fibre Yb-doped core or similar
  • pump fibre pure silica fibre for injection of pump diode power into the cavity
  • the double clad fibre 19 contained therein might require larger pump fibre (e.g.200um) to enable the higher pump diode power to be efficiently coupled into the amplifier 3 without excessive optical loss;
  • the "active fibre” double clad fibre 19
  • the table below shows the application requirements of the laser for a range of anilox processing parameters.
  • the dynamic range is from a pulse energy of 0,08mJ to 40mJ, a range of approximately 500 times (or 27dB). This range cannot be achieved with the conventional laser.
  • a typical range might be 100 times (or 20dB).
  • the pulse energy can be varied from 40mJ to 0.03mJ, a range of 1 ,333 (or 31.2dB).
  • Figure 6 shows some measurements made with a SP200C-0004 laser made and operated in accordance with the invention.
  • the laser is a 200W continuous wave laser that can be modulated to provide pulses at repetition rates up to and exceeding 100kHz.
  • the measured average output power 63 is shown plotted against the set point average power 64 for both extended range operation, shown by the triangles 65, and standard range operation shown by the circles 66.
  • the output power is stable above approximately 0.9W to 200W when operated in standard mode (the second power range 62), but is unstable below 0.9W (shown by the dashed line 68),
  • the dynamic range can be extended to the dashed line 67 by switching the laser diodes pumping the power amplifier 2 off. This is the extended range operation referred to in Table 2, (the first power range 61).
  • the output is then stable down to 0.05 W.
  • the overall dynamic range is thus 0.05W to 200W, a range of 4,000, or 36dB.
  • the laser can operate over the full range of Anilox processes, with pulse energies from 30 ⁇ J. Note that the laser can be operated stably in the extended range at power levels in excess of 0.9 W, but the laser cannot be operated stably in the standard range below 0.9W without instabilities.
  • the apparatus of the invention thus increases the dynamic range of a pulsed laser system, especially one that utilizes relaxation oscillations from lasers. It can also be used to increase the repetition frequency from lasers before reaching the lower power levels at which there is pulse drop out and related instabilities.
  • the pulsed laser system comprises the seed laser 1 and at least one amplifier 3.
  • the seed laser 1 and the amplifier 3 are pumped with an aggregate of between 100W and 1000W of pump power. Approximately 10% to 80% of the pump power is provided to the seed laser 1.
  • the seed laser is pumped with approximately 20% to 50% of the pump power. More preferably the seed laser 1 is pumped with approximately 25% to 40% of the pump power.
  • the method of the invention comprises providing the apparatus according to the present invention, and applying the laser radiation to the material 8.
  • the method can include the step of increasing the dynamic range of the laser radiation applied to the material 8 by changing the ratio of pump power provided by the seed pump 2 and the amplifier pump 4.
  • the pump power provided by the seed pump 2 can be substantially more than the pump power provided by the amplifier pump 4.
  • the pump power provided by the amplifier pump 4 can be reduced to zero.
  • the material 8 can be a printing roll.
  • printing roll it is meant a roll for printing applications, such as a ceramic printing roll, an anilox printing roll, an ink-transfer roll, an engraved printing rolls, or an engraved sheet that is attached to a roll. This is especially advantageous for ceramic coated printing rolls such as used in the Anilox process.
  • Anilox printing machines are available from Aquaftex, a branch of F.L. Smithe Machine Company, Inc. of Duncansville, Pennsylvania, United States of America.

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  • Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • Electromagnetism (AREA)
  • Mechanical Engineering (AREA)
  • Lasers (AREA)
  • Laser Beam Processing (AREA)
  • Electrical Discharge Machining, Electrochemical Machining, And Combined Machining (AREA)
  • Removal Of Insulation Or Armoring From Wires Or Cables (AREA)

Abstract

Apparatus for laser processing a material (8) with optical radiation (T), which apparatus comprises a seed laser (1) pumped by a seed pump (2), an amplifier (3) pumped by an amplifier pump (4), a controller (5), and a scanner (6), wherein the controller (5) controls the seed pump (2), the amplifier pump (4) and the scanner (6) in synchronism such that optical pulses (10) emitted by the seed laser (1) are propagated through the amplifier (3) and directed to the material (8) by the scanner (6), the apparatus being characterized in that controller (5) controls the amplifier pump (4) to cause the amplifier (3) to act as an optical attenuator when the optical radiation (7) is in a first power range, and as an optical amplifier when the output power is in a second power range.

Description

Apparatus and Method for Laser Processing a Material
Field of Invention
This invention relates to an apparatus and method for laser processing a material.
Background to the Invention
There are many applications for pulsed lasers that require a very wide dynamic range together with consistent pulse shapes and pulse timing. A good example is in processing rolls for printing machines. These require lasers with high average powers which are used to mark the print rolls. The power in the pulses needs to be varied accurately in order to achieve the required print resolution. The greater the dynamic range, the wider the range of print wheels that can serviced by one particular laser. Failure to provide consistent timing and predictable pulse shapes degrades the print quality.
Prior art solutions include use of continuous wave lasers (such as carbon dioxide lasers or fibre lasers) with an external modulator for attenuating the output power. The modulator is typically an acousto-optic modulator which gates and attenuates the laser beam accordingly.
Unfortunately, as the pulse power is varied in prior art pulsed lasers, the pulse shape smears, and the pulse is also delayed. Alternatively, long pulsed fibre lasers have been used. But the long pulse places restrictions on dynamic range, thereby placing limits on the print resolution (specified in terms of lines per inch) and the quantity of ink transferred per unit area of printed material. The effect is a degradation to print quality and the need to have multiple lasers for marking at different resolutions, Prior ait lasers used for this application typically have output powers of between 100W and 500W, with dynamic ranges less than 20dB. It would be advantageous to increase this dynamic range to at least 30dB.
An aim of the present invention is to provide an apparatus and method for laser processing a material which reduces the above aforementioned problem. The Invention:
According to a non-limiting embodiment of the invention, there is provided apparatus for laser processing a material, which apparatus comprises a seed laser pumped by a seed pump, an amplifier pumped by an amplifier pump, a controller. and a scanner, wherein the controller controls the seed pump, the amplifier pump and the scanner in synchronism such that optical pulses emitted by the seed laser are propagated through the amplifier and directed to the material by the scanner, the apparatus being characterized in that controller controls the amplifier pump to cause the amplifier to act as an optical attenuator when the optical radiation is in a first power range, and as an optical amplifier when the output power is in a second power range.
Advantageously, the invention enables an improved dynamic range. This is an important benefit in manufacturing environments and applications in "τnicro" materials processing where the laser would be used at the top end of its rated operating range (average power, peak power, pulse energy) in order to machine "coarse" featured items with acceptable productivity, whilst also being equally capable of processing "fine" featured items at the lower end of the rated range where very low average power / low peak power / very low pulse energy are processing requirements. The present invention is able to provide a dynamic range of at least three orders of magnitude (30dB) compared with typically less than two orders of magnitude (20dB) for lasers and master oscillator power amplifiers not configured and operated in accordance with the present invention.
For marking printing rolls, the scanner is preferably a mechanical unit that scans the print roll with respect to the laser beam. Alternatively, or additionally, the scanner may comprise a galvanometer and a reflector. By printing roll, it is meant a roll for printing applications, such as a ceramic printing roll, an anilox printing roll, an ink- transfer roll, an engraved printing rolls, or an engraved sheet that is attached to a roll.
The controller can be a computer controller. The controller may Include a switch (such as a relay or a transistor) for turning off the electrical drive power to the amplifier pump when in the first power range.
The seed laser may comprise a cladding-pumped fibre laser. The cladding- pumped fibre laser may comprise a rare-earth dopant. The rare-earth dopant may be ytterbium. Alternatively or additionally, the seed laser may comprise erbium, thulium, holmium or neodymium.
The amplifier may comprise a cladding-pumped fibre amplifier. The cladding- pumped fibre amplifier may comprise a rare-earth dopant The rare-earth dopant may be Ytterbium. Alternatively or additionally, the seed laser may comprise erbium, thulium, hoimium or neodymium.
. The apparatus may include at least one additional amplifier stage. Alternatively or additionally, the apparatus may include an absorber or a saturable absorber. The saturable absorber may introduce a delay. The apparatus may include a variable delay for compensating the delay.
The invention also provides a method for marking a material, the method comprising providing an apparatus according to the present invention, and applying the laser radiation to the material. The material can be a printing roll.
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:
Figure 1 shows apparatus for laser processing a material according to the present invention;
Figure 2 shows pulse shapes according to apparatus not of the present invention; and
Figure 3 shows examples of a pulse obtained according to the present invention;
Figure 4 shows examples of missing pulses;
Figure 5 shows examples of noisy pulses; and
Figure 6 shows examples of the increase in dynamic range afforded by the invention. Preferred Embodiment
Figure 1 shows apparatus for laser processing a material 8 with optical radiation 7, which apparatus comprises a seed laser 1 pumped by a seed pump 2, an amplifier 3 pumped by an amplifier pump 4, a controller 5, and a scanner 6, wherein the controller 5 controls the seed pump 2, the amplifier pump 4 and the scanner 6 in synchronism such that optical pulses 10 emitted by the seed laser 1 are propagated through the amplifier 3 and directed to the material 8 by the scanner 6, the apparatus being characterized in that controller 5 controls the amplifier pump 4 to cause the amplifier 3 to act as an optical attenuator when the optical radiation 7 is in a first power range 61, and as an optical amplifier when the output power is in a second power range 62. The first and second power ranges 61, 62 are described with reference to Figure 6, and can be overlapping. The power amplifier 2 preferably comprises double clad optical fibre 19 that is doped with rare-earth dopant (not shown).
The ability to achieve an improved dynamic range is an important benefit in manufacturing environments and applications in "micro" materials processing where the laser would be used at the top end of its rated operating range (average power, peak power, pulse energy) in order to machine "coarse" featured items with acceptable productivity, whilst also being equally capable of processing "fine" featured items at the lower end of the rated range where very low average power / low peak power / very low pulse energy are processing requirements. The apparatus shown in Figure 1 provides a dynamic range of three orders of magnitude or greater, compared with typically less than two orders of magnitude for lasers and master oscillator power amplifier not configured and operated in accordance with the present invention.
The material 8 can be a printing roll, for example, one used in the Anilox process. For marking printing rolls, the scanner 6 is preferably a mechanical unit that scans the print roll with respect to the laser beam. Alternatively, or additionally, the scanner 6 may comprise a galvanometer and a reflector.
The controller 5 can be a computer controller. The controller 5 can include a switch 14, such as a relay or a transistor, for turning off the electrical drive power to the amplifier pump 4 when in the first power range 61. The controller 5 can include a variable delay 15 for delaying a control signal 17 comprising pulses 11. Preferably the delayed control signal would be passed to both the seed laser 1 and the amplifier 3 in order to avoid the amplifier 3 from self-Q-switehing. The pulses 11 can have different amplitudes 12.
Figure 2 shows pulses 21, 22, 23, 24, 25, 26, 27, 28, 29 produced by modulation of the laser output with various set points corresponding to continuous-wave equivalent output powers of 200W, 180W, 160W, 140W, 120W, 100W, 80W, 60W and 40W respectively. The seed laser 1 is a cladding pumped Ytterbium-doped fibre laser operating at around 1070nm, comprising double clad optical fibre (not shown) and two optical fibre Bragg gratings (not shown). The amplifier 3 is a cladding pumped Ytterbium-doped fibre amplifier. The output pulse shapes shown correspond to the amplified first relaxation oscillation from the seed laser 2. The variations in pulse peak power and thus pulse energy were obtained without using the present invention by reducing the output power of both the seed pump 2 and the amplifier pump 4 for each successive measurement. As is clearly visible, as the power level falls, the pulse is delayed, and becomes smeared.
Another problem that is illustrated in Figures 4 and 5 is that reducing the pulse power by such conventional operation not only smears the pulse shape, but also leads to "missing pulses" or noisy pulses. In Figure 4, the control signal 41 for the seed pump 2 and amplifier pump 4 should result in output pulses at the same repetition frequency as the control signal 41. However, as is clearly seen, there are missing pulses. This is because in pulse trains at low powers, there can be insufficient energy stored in the seed laser 1 to produce a relaxation oscillation. When a missing pulse situation occurs, the energy is stored until the next pulse, and can then be sufficient to trigger the laser above threshold. A related issue is one of noisy pulses, where the seed laser is operating just above threshold. This is shown in Figure 5 where the pulses 52 are clearly very much more noisy than those shown with reference to Figures 2 and 3. The output powers at which missing pulses or noisy pulses occurs provide effective lower limits to the dynamic range.
Figure 3 shows a result according to the invention. Pulse shape 31 (see left hand axis 33) was obtained with the amplifier 3 in the second power range 62 in which the amplifier 3 is operated with approximately a 3dB large signal amplification. Pulse shape 32 (see right hand axis 34) was obtained with the amplifier 3 in the first power range 61 in which the amplifier 3 is operated with an attenuation of approximately 1dB. The pulse shapes 31, 32 are virtually identical. Importantly, the apparatus can be operated at lower output powers than in the conventional operating mode without there being missing or very noisy pulses. This is because the seed laser 1 is being driven harder for equivalent output power when operating in the first power range than when being operated in the second power range. Thus the dynamic range has been extended.
Although the pulse shapes in Figure 3 are nearly identical, it should be noted that the pulse shape 31 was delayed in order for the pulses to overlay in the Figure. This is because the pulse turn-on delay varies with drive power, as shown in Figure 2. Thus it is desirable for the controller 5 to include the variable delay 15 if pulse synchronization at different power levels is required. The variable delay 15 can be controlled internally to the laser, or applied externally.
The dynamic range over which the apparatus provides useful output can be further extended by increasing the dynamic large signal amplification range of the amplifier 3. This can be accomplished by use of additional amplification stages to increase the maximum amplification in the second power range 62. Alternatively or additionally, by inserting an absorber or a saturable absorber to increase the effective attenuation in the first power range. The saturable absorber will introduce an additional time delay (not shown) with respect to the pulse entering the saturable absorber corresponding to the time required to bleach the absorption. The rare-earth doped optical fibre 19 will act as a saturable absorber when not pumped. Thus the power amplifier 3 will in practice operate as both an amplifier when pumped, and as an attenuator plus saturable absorber when not pumped.
EXAMPLE I
In this example of the invention, the apparatus is a two-stage master oscillator power amplifier comprising the seed laser I and the power amplifier 3, in which the optical power partitioning between seed laser 1 and the power amplifier 3 is approximately 1:2. The pumps 2, 4 are implemented as laser diodes. In a 200W laser system, the seed laser 1 would thus contribute approximately 66W to the overall output and the power amplifier 3 would add 133 W to achieve the nominal rated power,
In the "standard" mode of operation, both the seed laser 1 and the power amplifier 3 are pumped, and the pump diodes (the pumps 2 and 4) driving both the seed laser 1 and the amplifier 3 are operated in synchronism, the output power being controlled by varying the power supplied by the pumps 2, 4 to the two stages proportionately.
The pump diode current is typically modulated at high speed in order to produce a train of output pulses with the parametric properties required for the printing roll (or other work piece) being processed.
The seed laser 1 comprises a double clad fibre and fibre Bragg gratings to create the laser cavity, and the power amplifier 3 uses the double clad fibre 19. It is preferred that the double clad fibres are side-pumped via a side-pumping fibre (not shown) that couples pump power into the double clad fibre gradually along its length. Optical amplifiers based on double clad fibres pumped by side pumping fibres are described in more detail in United States patent number 6,826,335 which is hereby incorporated herein by reference.
The diameter of the side-pumping fibre may be optimized according to the power rating of the laser. In particular, the diameter of this fibre for the seed laser 1 is preferably smaller than for the power amplifier.
Preferably (but not essentially) the seed laser 1 might contain double clad fibre in which the "active fibre" (Yb-doped core or similar) is approximately I25um in diameter and the "pump fibre" (pure silica fibre for injection of pump diode power into the cavity) is 125um.
To attain the desired power level from the power amplifier stage 3, the double clad fibre 19 contained therein might require larger pump fibre (e.g.200um) to enable the higher pump diode power to be efficiently coupled into the amplifier 3 without excessive optical loss; the "active fibre" (double clad fibre 19) would usually be of the same design and dimensions as the "active fibre" in the seed source.
The table below shows the application requirements of the laser for a range of anilox processing parameters. The dynamic range is from a pulse energy of 0,08mJ to 40mJ, a range of approximately 500 times (or 27dB). This range cannot be achieved with the conventional laser. A typical range might be 100 times (or 20dB).
Figure imgf000010_0001
Table 1: Anilox Processing Requirements
Operating the laser in accordance with the invention permitted the operational parameters as shown in Table 2 to be achieved, The pulse energy can be varied from 40mJ to 0.03mJ, a range of 1 ,333 (or 31.2dB).
Figure imgf000011_0001
Table 2: Performance achieved using the invention
EXAMPLE II
Figure 6 shows some measurements made with a SP200C-0004 laser made and operated in accordance with the invention. The laser is a 200W continuous wave laser that can be modulated to provide pulses at repetition rates up to and exceeding 100kHz. The measured average output power 63 is shown plotted against the set point average power 64 for both extended range operation, shown by the triangles 65, and standard range operation shown by the circles 66. The output power is stable above approximately 0.9W to 200W when operated in standard mode (the second power range 62), but is unstable below 0.9W (shown by the dashed line 68), The dynamic range can be extended to the dashed line 67 by switching the laser diodes pumping the power amplifier 2 off. This is the extended range operation referred to in Table 2, (the first power range 61). The output is then stable down to 0.05 W. The overall dynamic range is thus 0.05W to 200W, a range of 4,000, or 36dB. The laser can operate over the full range of Anilox processes, with pulse energies from 30μJ. Note that the laser can be operated stably in the extended range at power levels in excess of 0.9 W, but the laser cannot be operated stably in the standard range below 0.9W without instabilities.
The apparatus of the invention thus increases the dynamic range of a pulsed laser system, especially one that utilizes relaxation oscillations from lasers. It can also be used to increase the repetition frequency from lasers before reaching the lower power levels at which there is pulse drop out and related instabilities.
Preferably the pulsed laser system comprises the seed laser 1 and at least one amplifier 3. Preferably the seed laser 1 and the amplifier 3 are pumped with an aggregate of between 100W and 1000W of pump power. Approximately 10% to 80% of the pump power is provided to the seed laser 1. Preferably, the seed laser is pumped with approximately 20% to 50% of the pump power. More preferably the seed laser 1 is pumped with approximately 25% to 40% of the pump power.
As indicated above, the method of the invention comprises providing the apparatus according to the present invention, and applying the laser radiation to the material 8. The method can include the step of increasing the dynamic range of the laser radiation applied to the material 8 by changing the ratio of pump power provided by the seed pump 2 and the amplifier pump 4. The pump power provided by the seed pump 2 can be substantially more than the pump power provided by the amplifier pump 4. The pump power provided by the amplifier pump 4 can be reduced to zero. The material 8 can be a printing roll. By printing roll, it is meant a roll for printing applications, such as a ceramic printing roll, an anilox printing roll, an ink-transfer roll, an engraved printing rolls, or an engraved sheet that is attached to a roll. This is especially advantageous for ceramic coated printing rolls such as used in the Anilox process. Anilox printing machines are available from Aquaftex, a branch of F.L. Smithe Machine Company, Inc. of Duncansville, Pennsylvania, United States of America.
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 components may be provided to enhance performance. The present invention extends to the above-mentioned features taken in isolation or in any combination.

Claims

Claims
1. Apparatus for laser processing a material, which apparatus comprises a seed laser pumped by a seed pump, an amplifier pumped by an amplifier pump, a controller, and a scanner, wherein the controller controls the seed pump, the amplifier pump and the scanner in synchronism such that optical pulses emitted by the seed laser are propagated through the amplifier and directed to the material by the scanner, the apparatus being characterized in that controller controls the amplifier pump to cause the amplifier to act as an optical attenuator when the optical radiation is in a first power range, and as an optical amplifier when the output power is in a second power range.
2. Apparatus according to claim 1 wherein the scanner is a mechanical unit.
3. Apparatus according to claim 1 or claim 2 wherein the scanner comprises a galvanometer and a reflector,
4. Apparatus according to any one of the preceding claims wherein the controller is a computer controller.
5. Apparatus according to any one of the preceding claims wherein the controller includes a switch for turning off electrical drive power to the amplifier pump when in the first power range.
6. Apparatus according to any one of the preceding claims wherein the seed laser comprises a cladding-pumped fibre laser.
7. Apparatus according to any one of the preceding claims wherein the cladding- pumped fibre laser comprises a rare-earth dopant.
8. Apparatus according to claim 7 wherein the rare-earth dopant comprises ytterbium.
9. Apparatus according to claim 7 or claim 8 wherein the rare-earth dopant comprises erbium, thulium, hohnium or neodymium.
10. Apparatus according to any one of the preceding claims wherein the amplifier comprises a cladding-pumped fibre amplifier.
11. Apparatus according to any one of the preceding claims wherein the cladding- pumped fibre laser comprises a rare-earth dopant.
12. Apparatus according to claim 11 wherein the rare-earth dopant comprises Ytterbium.
13. Apparatus according to claim 11 or claim 12 wherein the rare earth dopant comprises erbium, thulium, holmium or neodymiυm.
14. Apparatus according to any one of the preceding claims and including at least one additional amplifier stage.
15. Apparatus according to any one of the preceding claims wherein the apparatus includes an absorber
16. Apparatus according to any one of the preceding claims wherein the apparatus includes a saturable absorber.
17. Apparatus according to claim 16 in which the saturable absorber introduces a delay and wherein the apparatus includes a variable delay for compensating the delay.
18. A method for marking a material, the method comprising providing an apparatus according to the present invention, and applying the laser radiation to the material.
19. A method according to claim 18 wherein the material is a printing roll.
PCT/GB2008/002357 2007-07-09 2008-07-09 Apparatus and method for laser processing a material WO2009007721A2 (en)

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US12/668,396 US20100177797A1 (en) 2007-07-09 2008-07-09 Apparatus and Method for Laser Processing a Material
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AT08775901T ATE483265T1 (en) 2007-07-09 2008-07-09 DEVICE AND METHOD FOR MATERIAL PROCESSING USING LASER
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