Micro-ablation with high power pulsed copper vapor lasers

doi:10.1364/OE.7.000050

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Micro-ablation with high power pulsed copper vapor lasers

Martyn Knowles »View Author Affiliations

Optics Express, Vol. 7, Issue 2, pp. 50-55 (2000)
http://dx.doi.org/10.1364/OE.7.000050

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Abstract

Visible and UV lasers with nanosecond pulse durations, diffraction-limited beam quality and high pulse repetition rates have demonstrated micro-ablation in a wide variety of materials with sub-micron precision and sub-micron-sized heat-affected zones. The copper vapour laser (CVL) is one of the important industrial lasers for micro-ablation applications. Manufacturing applications for the CVL include orifice drilling in fuel injection components and inkjet printers, micro-milling of micromoulds, via hole drilling in printed circuit boards and silicon machining. Recent advances in higher power (100W visible, 5W UV), diffraction-limited, compact CVLs are opening new possibilities for manufacturing with this class of nanosecond laser.

[Optical Society of America ]

1. Introduction

Laser micro-machining is an increasingly important production method and is used in the automotive, aerospace, electronics, telecommunications and medical device industries. A variety of laser types are used in laser micro-machining and micro-ablation is the dominant mechanism for material removal. It is clear the laser type must be matched to the application and that no single laser type will be optimum for all applications. The laser types that are most commonly used are the copper vapour laser, excimer laser and Nd solid state laser. In recent years there has been considerable research interest in micro-machining with ultra-short pulses from titanium sapphire lasers. This has sparked a lively debate as to what is the optimum pulse width for micro-machining [1

X. Chen and X. Liu, “Short pulsed laser machining: how short is short enough?,” J. Laser Applications 11, 268–272 (1999). [CrossRef]

–4

M.R.H. Knowles, G. Rutterford, A.I. Bell, A.J. Andrews, G. Foster-Turner, and A.J. Kearsley, “Sub-micron and high precision micro-machining using nanosecond,” Proceedings of ICALEO 98 85e, 112–120 (1998).

]. It is generally accepted that short pulses (<100ns) generate considerably less recast and thinner heat affected zones (HAZ) than long pulse lasers (>100ms). There is also evidence that the intrinsic machining characteristics with ultra-short pulses (<1ps) are different to short pulses (<100ns) and theoretical models which aim to describe the photon-material interaction in the ultra-short pulse [2

M.D. Shirk and P.A. Molian, “A review of ultrashort pulsed laser ablation of materials,” J. Laser Applications 10, 18–28 (1998). [CrossRef]

], short pulse [15

C. Körner, R. Mayerhofer, M. Hartmann, and H.W. Bergmann, “Physical and material aspects in using visible laser pulses of nanosecond duration for ablation,” Appl. Phys. A 63, 123–131, (1996). [CrossRef]

, 16

J.J. Chang and B.E. Warner, “Laser-plasma interaction during visible laser ablation of metals,” Appl. Phys. Lett. 69, 473–475, 1996. [CrossRef]

] and their comparison [3

D. Kapitan, D. W. Coutts, and C. E. Webb, “On pulsed laser ablation of metals: comparing the relative importance of thermal diffusion in the nanosecond-femtosecond regime,” Conference on Lasers and Electro-Optics - Europe paper CThH76 (1998).

, 4

] have been proposed. The reader is encouraged to consult these references [2

M.D. Shirk and P.A. Molian, “A review of ultrashort pulsed laser ablation of materials,” J. Laser Applications 10, 18–28 (1998). [CrossRef]

, 3

, 4

, 15

, 16

J.J. Chang and B.E. Warner, “Laser-plasma interaction during visible laser ablation of metals,” Appl. Phys. Lett. 69, 473–475, 1996. [CrossRef]

] However, in practice, it is possible to produce both excellent and relatively poor results with both short and ultra-short pulses. The difference is often due to the exact application under consideration and the degree of process optimization. Whilst the initial results demonstrated by ultra-short pulse lasers were interesting, their quality was too poor to be of practical interest. However recent results using ultra-short pulses [5

H.K. Tönshoff, C. Momma, A. Ostendorf, S. Nolte, and G. Kamlage, “Microdrilling of metals with ultrashort laser pulses,” J. Laser Applications 12, 23–27 (2000). [CrossRef]

, 17

H.K. Tonshoff, F. von Alvensleben, A. Ostendorf, G. Kamlage, and S. Nolte, “Micromachining of metals using ultrashort laser pulses,” IJEM Review International Journal of Electrical Machining 4, 1–6, 1999.

] show a considerable improvement. However, for certain applications the quality of the holes is still insufficient for demanding applications. An example of this is micro-drilling of orifices in conventional fuel injectors where both roundness, zero taper and wall smoothness criteria must all be met simultaneously.

The CVL is a high power, pulsed visible (511nm and 578nm) laser. It emits intense (50–500kW), short (25ns) pulses at high repetition frequencies (2–50 kHz). Typical average powers are 10–100W although commercial systems up to 1.5kW have been supplied. The beam quality from these devices is typically diffraction limited [6

M. Knowles, R. Benfield, A. Andrews, and A. Kearsley, “Development of high power compact copper vapour lasers,” Advanced High-Power Lasers and Applications 99 M. Osinski, H.T. Power, and K. Toyoda, eds., Proc SPIE 3889, paper 3889-63, (1999).

, 7

D. K Kapitan, D. W. Coutts, and C. E. Webb, “Efficient Generation of near diffraction-limited beam-quality output from medium-scale copper vapour laser oscillators,” IEEE J. Qu. Electronics 34, 419–426 (1998). [CrossRef]

]. The excellent beam quality, short pulses, high peak power and high pulse repetition frequency is an ideal combination for precision micro-ablation. One of the key advantages of the CVL is its ability to generate high beam quality at very high average powers. Recently there has been an improvement in terms of beam quality and output power in the latest generation of diode-pumped solid state lasers for micro-machining applications and that good beam quality is now available at low to medium powers. It is interesting to note that their specifications are largely based on that of the CVL. That is, they have a high pulse repetition rate, short pulses, and visible or UV output. However, they still differ in certain intrinsic features. The solid state lasers can only produce diffraction limited beam quality with gaussian profiles whereas the CVL can produce diffraction limited beam quality with top hat or gaussian-like profiles. In certain applications the top hat beam profile gives a significant processing advantage since the wings of the gaussian profile can cause undesirable sub-threshold machining effects such as recast, heat affected zones and poor edge definition. The gas laser properties of the CVL enable it to easily generate diffraction limited beam quality at very high powers. The thermal lensing and birefringence issues in solid state lasers combined with the need to use non-linear process to produce shorter wavelengths means that power scaling of solid state lasers whilst maintaining diffraction limited beam quality is much more challenging. This is the principal reason whilst the output power of solid state lasers of this type has lagged behind the output power of the CVL and more conventional (low beam quality) solid state lasers. The modern CVL is easy to use, reliable and has low running costs. In addition to this, new features such as active power stabilization, frequency doubling (to produce high power UV) [12

R.I. Trickett, M.J. Withford, and D.J. Brown, “4.7W, 255nm source based on second-harmonic generation of a copper vapour laser in cesium lithium borate,” Optics Letters 23, 189–191, 1998. [CrossRef]

] and fibre beam delivery through small core fibres (100µm diameter) have been added. The modular concept has been retained thereby permitting simple power scaling of the laser system.

The copper vapour laser (CVL) has been at the forefront of laser micro-machining technology for some years. It has been used with the visible fundamental output [8

M.R.H. Knowles, A.I. Bell, G. Rutterford, G. Foster-Turner, and A.J. Kearsley, “Advances in copper lasers for micromachining,” Proceedings of ICALEO 97 82, (1997).

–11

J.J. Chang, B.E. Warner, E.P. Dragon, and M.W. Martinez, “Precision micromachining with pulsed green lasers,” J. Laser Applications 10, 285–291 (1998). [CrossRef]

] and frequency doubled UV [13

A.C. Glover, D.W. Coutts, D.J. Ramsay, and J.A. Piper, “Progress in high-speed UV micro-machining with high repetition rate frequency doubled copper vapour lasers,” Proceedings of ICALEO 94 79, 343–351, (1994).

–14

A.C. Glover, M.J. Withford, E.K. Illy, and J.A. Piper “Ablation threshold and etch rate measurements in high-speed ultra-violent micro-machining of polymers with uv-copper vapour lasers,” Proceedings of ICALEO 95 80, 361–370, (1995).

]. Principal applications of these lasers are in micro-hole drilling and precision cutting. In a number of applications it has been demonstrated that the copper laser is the only viable tool because of the excellent results that are achieved with its combination of high power, short pulses, visible radiation and diffraction-limited beam quality, high reliability and low cost of ownership. Holes as small as 1.5µm have been drilled and the laser is capable of achieving tolerances of better than ±0.25µm in holes with diameters in the range 5–200µm. The high peak power and excellent beam quality enable high aspect ratio holes and cuts with smooth walls and controlled or zero taper to be produced in metals, ceramics, diamond and other materials.

Recent developments in kinetic enhancement of CVLs have enabled very high powers with excellent beam quality to be produced from a compact industrial package [6

]. This technology increases the output power from a given device by a factor 3. Output powers of over 40W with diffraction-limited beam quality from stand-alone oscillators of 1m tube length. The same tube when used as an amplifier can produce 60W. MOPA systems based around two tubes of this size will produce over 100W at close to the diffraction-limit. This power level, high repetition frequency, short pulse width and beam quality is not possible from any other laser source at visible wavelengths. Lasers based around this technology will be smaller and more cost-effective than previous generations

2. Micro-hole drilling

Micro-hole drilling is an enabling technology for several important manufacturing processes. One good example of this is the legislative drive to reduce emissions from diesel and gasoline engines. One part of the solution to achieving these lower emissions requires fuel injectors with smaller injection orifices. In some designs under test the orifices are no longer round but some other shape. The current production process for drilling the orifices in diesel fuel injectors is wire electric discharge machining (WEDM). Whilst there have been significant improvements in the WEDM capability, it is unlikely that WEDM will be able to meet the very small hole requirements or produce shaped micro-holes. The CVL has demonstrated the ability to drill these holes at process speeds that are economically attractive. Other manufacturing processes that require precision micro-hole drilling are gasoline fuel injectors, industrial inkjet printers and medical devices. Examples of these components are shown in figures 1 and 3. In these and the subsequent figures the following abbreviations will be used. P for average power, PRF for pulse repetition frequency, F for lens focal length, w for focused spot size, t for drilling time (when appropriate) and v for cutting speed (when appropriate). Figure 2 shows the set-up for laser drilling a diesel fuel injector nozzle.

Fig. 1. (a) A high quality CVL drilled hole in 1mm thick steel for diesel fuel injectors (P=35W, PRF=10kHz, F=200mm, w=10 microns, t=20s trepanned) and (b) a high performance gasoline swirl injector manufactured using a CVL (P=45W, PRF=10kHz, F=250mm, w=13 microns, t=60s trepanned). These holes demonstrate the exceptional hole quality (roundness, smooth walls, sharp edges and sub-micron recast layers).

Fig. 2. Photographs of CVL drilling of a diesel fuel injector. Photograph (a) on the left shows an overview. See reference [18

http:/www.oxfordlasers.demon.co.uk/movies/ddi_drilling_large.mov

] for the link for the movie of this drilling (5.26MB). Photograph (b) on the right shows a close up of the drilling process and jig. See reference [19

http:/www.oxfordlasers.demon.co.uk/movies/ddi_drilling_close.mov

] for the link for the movie of this drilling (6.12MB). [Media 3]

Fig. 3. An example of a CVL drilled industrial inkjet printer nozzle orifice. In fig 3(a) the hole diameter is 50mm through 100mm stainless steel and was trepanned. The recast laser is sub-micron and the diameter reproducibility is +/-0.2mm. In fig 3(b) the hole is 5mm diameter (part of a 22,000 hole array) in a medical device and was percussion drilled (20 pulses at 10kHz, w=4 microns).

3. Micro-cutting

The high beam quality of the CVL enables it to be focused to very small spot sizes which not only enables micro-hole drilling but also precision cutting and milling. The high peak power of the laser coupled with the small spot sizes produces very high focal intensities (1–1000 GW/cm²) which are ideal for ablating a wide variety of materials including diamond and ceramics. Figures 4a below shows the intersection of 4 laser diode heat sinks which were cut by a CVL. These heat sinks are used to conduct heat away from the laser diode and the laser-cut edge is used to align a fibre optic to the laser diode. Hence the taper of the cut must be less than 2° full angle for correct fibre alignment. With the CVL the taper was less than 1° full angle. Typical dimensions of the diamond heat sinks are 0.75×0.5×0.25 mm and several thousand devices are cut from a single diamond wafer. One important feature of the CVL cutting of diamond compared to other laser types and one of the main reasons that it was chosen for this process was the very low graphite content on the cut wall. Figure 4b shows an example of precision cutting of complex micro-parts. The device in the figure is a vacuum chuck for a pick-and-place tool head. The material was 1.6 mm thick diamond on tungsten carbide and was cut by a CVL.

Fig. 4. (a) Low taper cutting of CVD diamond (250 micron thickness) to form heat sinks for laser diodes, reproduced with the permission of Marconi Materials Technology by a CVL (P=40W, PRF=10kHz, F=300mm, w=50 microns, v=0.5 mm/s) and (b) a vacuum chuck made from 1.6mm thick diamond/tungsten carbide for a pick-and-place tool (P=40W, F=250mm, w=13 microns, PRF=10kHz, v=0.01 mm/s).

4. Laser surface machining

The figures below show two very different surface machining applications. The CVL is used to precisely ablate material from the surface and through the use of a CNC or scanning optics complex patterns can be formed. Figure 5a shows an example of micro-circuit fabrication using a CVL. The laser cuts are 25 µm wide through a 5 µm nickel layer on ceramic. Typical write speeds are 50–250 mm/s. Laser patterning of micro-circuits is used when the track/gap dimensions are beyond the capability of chemical etching or when the material to be removed is not easy to etch by chemicals. Lasers also have an advantage when the volumes are low and the direct-write capability of the laser from a computer file reduces set-up times and costs. Figure 5b is an example of sub-micron patterning using a CVL where the beam is split and then recombined to produce an interference pattern on the material surface [4

Fig. 5. (a) The photograph on the left shows direct writing of micro-circuits using a CVL. This example has 25mm wide tracks on a 50 mm pitch through 5mm thick nickel on ceramic. Note the lack of burr around the laser cuts. (b) Sub-micron surface patterning of polished steel (P=6W, PRF=10kHz, intensity=1J/cm², 60 pulses). The ablated squares are 2mm wide with corner radii of less than 0.25mm [4

5. Blind via drilling using frequency doubled CVL

Frequency doubling and frequency mixing of the 511 and 578nm output of a CVL produces high peak power, high repetition rate pulses in the UV (255nm, 271nm and 289nm). Output powers in excess of 4W have been achieved [12

]. Figure 6 shows a blind via drilled through a polyimide layer down to the copper layer below as required for PCB vias. The high power available from the CVL in the visible and UV spectrum and with appropriate wavelength switching, enables it to machine both the copper and insulating layers in a PCB.

Fig. 6. Blind via drilled through polyimide down to the copper layer below using the frequency mixed 271nm line from the copper laser (P=500mW, PRF=6kHz, w=40 microns, t=20ms trepanned). Note the excellent roundness, smooth walls and flat bottom to the via.

6. Conclusions

High power CVLs are being used increasingly in manufacturing processes of micro-components or components with micro-features. Typical applications are in micro-hole drilling and other micro-machining applications where the excellent beam quality and high power of the CVL enable processes that are not readily achieved with other lasers or processes. We believe that the results in this paper conclusively demonstrate that excellent micro-ablation results (sub-micron precision and sub-micron-sized heat affected zones) can be achieved with CVLs. The recent introduction of kinetic enhancement to CVLs has produced a dramatic improvement in output power from a given package size. This should further increase the uptake of CVLs in manufacturing by offering users a more compact package and a higher power per dollar ratio than previously available. Frequency doubling of the CVL to produce UV wavelengths (255nm, 271nm and 289nm) at high power (several watts) and high pulse repetition frequencies will enable new applications in polymers and other materials.

References and Links

1.	X. Chen and X. Liu, “Short pulsed laser machining: how short is short enough?,” J. Laser Applications 11, 268–272 (1999). [CrossRef]
2.	M.D. Shirk and P.A. Molian, “A review of ultrashort pulsed laser ablation of materials,” J. Laser Applications 10, 18–28 (1998). [CrossRef]
3.	D. Kapitan, D. W. Coutts, and C. E. Webb, “On pulsed laser ablation of metals: comparing the relative importance of thermal diffusion in the nanosecond-femtosecond regime,” Conference on Lasers and Electro-Optics - Europe paper CThH76 (1998).
4.	M.R.H. Knowles, G. Rutterford, A.I. Bell, A.J. Andrews, G. Foster-Turner, and A.J. Kearsley, “Sub-micron and high precision micro-machining using nanosecond,” Proceedings of ICALEO 98 85e, 112–120 (1998).
5.	H.K. Tönshoff, C. Momma, A. Ostendorf, S. Nolte, and G. Kamlage, “Microdrilling of metals with ultrashort laser pulses,” J. Laser Applications 12, 23–27 (2000). [CrossRef]
6.	M. Knowles, R. Benfield, A. Andrews, and A. Kearsley, “Development of high power compact copper vapour lasers,” Advanced High-Power Lasers and Applications 99 M. Osinski, H.T. Power, and K. Toyoda, eds., Proc SPIE 3889, paper 3889-63, (1999).
7.	D. K Kapitan, D. W. Coutts, and C. E. Webb, “Efficient Generation of near diffraction-limited beam-quality output from medium-scale copper vapour laser oscillators,” IEEE J. Qu. Electronics 34, 419–426 (1998). [CrossRef]
8.	M.R.H. Knowles, A.I. Bell, G. Rutterford, G. Foster-Turner, and A.J. Kearsley, “Advances in copper lasers for micromachining,” Proceedings of ICALEO 97 82, (1997).
9.	M.R.H. Knowles, R. Foster-Turner, A.I. Bell, A.J. Kearsley, A.P. Hoult, S.W. Lim, and H. Bisset, “Drilling of shallow angled holes in aerospace alloys using a copper laser,” Proceedings of ICALEO 95 80, 321–330, (1995)..
10.	M.R.H. Knowles, A.J. Kearsley, R. Foster-Turner, J.E. Abbott, J.M. Boaler, and K.H. Errey, “Visualization of small hole drilling using a copper laser,” Proceedings of ICALEO 94 79, 352–361, (1994).
11.	J.J. Chang, B.E. Warner, E.P. Dragon, and M.W. Martinez, “Precision micromachining with pulsed green lasers,” J. Laser Applications 10, 285–291 (1998). [CrossRef]
12.	R.I. Trickett, M.J. Withford, and D.J. Brown, “4.7W, 255nm source based on second-harmonic generation of a copper vapour laser in cesium lithium borate,” Optics Letters 23, 189–191, 1998. [CrossRef]
13.	A.C. Glover, D.W. Coutts, D.J. Ramsay, and J.A. Piper, “Progress in high-speed UV micro-machining with high repetition rate frequency doubled copper vapour lasers,” Proceedings of ICALEO 94 79, 343–351, (1994).
14.	A.C. Glover, M.J. Withford, E.K. Illy, and J.A. Piper “Ablation threshold and etch rate measurements in high-speed ultra-violent micro-machining of polymers with uv-copper vapour lasers,” Proceedings of ICALEO 95 80, 361–370, (1995).
15.	C. Körner, R. Mayerhofer, M. Hartmann, and H.W. Bergmann, “Physical and material aspects in using visible laser pulses of nanosecond duration for ablation,” Appl. Phys. A 63, 123–131, (1996). [CrossRef]
16.	J.J. Chang and B.E. Warner, “Laser-plasma interaction during visible laser ablation of metals,” Appl. Phys. Lett. 69, 473–475, 1996. [CrossRef]
17.	H.K. Tonshoff, F. von Alvensleben, A. Ostendorf, G. Kamlage, and S. Nolte, “Micromachining of metals using ultrashort laser pulses,” IJEM Review International Journal of Electrical Machining 4, 1–6, 1999.
18.	http:/www.oxfordlasers.demon.co.uk/movies/ddi_drilling_large.mov
19.	http:/www.oxfordlasers.demon.co.uk/movies/ddi_drilling_close.mov

OCIS Codes
(140.1340) Lasers and laser optics : Atomic gas lasers
(140.3390) Lasers and laser optics : Laser materials processing
(140.7300) Lasers and laser optics : Visible lasers
(160.3900) Materials : Metals

ToC Category:
Focus Issue: Laser ablation

History
Original Manuscript: May 19, 2000
Published: July 17, 2000

Citation
Martyn Knowles, "Micro-ablation with high power pulsed copper vapor lasers," Opt. Express 7, 50-55 (2000)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-7-2-50

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References

X. Chen and X. Liu, "Short pulsed laser machining: how short is short enough?," J. Laser Applications 11, 268-272 (1999). [CrossRef]
M.D. Shirk and P.A. Molian, "A review of ultrashort pulsed laser ablation of materials," J. Laser Applications 10, 18-28 (1998). [CrossRef]
D. Kapitan, D. W. Coutts and C. E. Webb, "On pulsed laser ablation of metals: comparing the relative importance of thermal diffusion in the nanosecond-femtosecond regime," Conference on Lasers and Electro-Optics - Europe paper CThH76 (1998).
M.R.H. Knowles, G. Rutterford , A.I. Bell, A.J. Andrews, G. Foster-Turner and A.J. Kearsley, "Sub-micron and high precision micro-machining using nanosecond," Proceedings of ICALEO 98 85e, 112-120 (1998).
H.K. Tönshoff, C. Momma, A. Ostendorf, S. Nolte and G. Kamlage, "Microdrilling of metals with ultrashort laser pulses," J. Laser Applications 12, 23-27 (2000). [CrossRef]
M. Knowles, R. Benfield, A. Andrews and A. Kearsley, "Development of high power compact copper vapour lasers," Advanced High-Power Lasers and Applications 99 M. Osinski, H.T. Power, K.Toyoda, eds., Proc SPIE 3889, paper 3889-63, (1999).
D. K Kapitan, D. W. Coutts and C. E. Webb, "Efficient Generation of near diffraction-limited beam-quality output from medium-scale copper vapour laser oscillators," IEEE J. Qu. Electronics 34, 419-426 (1998). [CrossRef]
M.R.H. Knowles, A.I. Bell,.G. Rutterford, G. Foster-Turner and A.J. Kearsley, "Advances in copper lasers for micromachining," Proceedings of ICALEO 97 82, (1997).
M.R.H. Knowles, R. Foster-Turner, A.I. Bell, A.J. Kearsley, A.P. Hoult, S.W. Lim, and H. Bisset, "Drilling of shallow angled holes in aerospace alloys using a copper laser," Proceedings of ICALEO 95 80, 321-330, (1995).
M.R.H. Knowles, A.J. Kearsley., R. Foster-Turner., J.E. Abbott, J.M. Boaler., K.H. Errey, "Visualization of small hole drilling using a copper laser," Proceedings of ICALEO 94 79, 352-361, (1994).
J.J. Chang, B.E. Warner, E.P. Dragon and M.W. Martinez, "Precision micromachining with pulsed green lasers," J. Laser Applications 10, 285-291 (1998). [CrossRef]
R.I. Trickett, M.J. Withford and D.J. Brown, "4.7W, 255nm source based on second-harmonic generation of a copper vapour laser in cesium lithium borate," Opt. Lett. 23, 189-191, 1998. [CrossRef]
A.C. Glover., D.W. Coutts, D.J. Ramsay and J.A. Piper, "Progress in high-speed UV micro-machining with high repetition rate frequency doubled copper vapour lasers," Proceedings of ICALEO 94 79, 343-351, (1994).
A.C. Glover,.M.J. Withford, E.K. Illy and J.A. Piper "Ablation threshold and etch rate measurements in high-speed ultra-violent micro-machining of polymers with uv-copper vapour lasers," Proceedings of ICALEO 95 80, 361-370, (1995).
C. Körner, R. Mayerhofer, M. Hartmann and H.W. Bergmann, "Physical and material aspects in using visible laser pulses of nanosecond duration for ablation," Appl. Phys. A 63, 123-131, (1996). [CrossRef]
J.J. Chang and B.E. Warner, "Laser-plasma interaction during visible laser ablation of metals," Appl. Phys. Lett. 69, 473-475, (1996). [CrossRef]
H.K. Tonshoff, F. von Alvensleben, A. Ostendorf, G. Kamlage, S. Nolte, "Micromachining of metals using ultrashort laser pulses," IJEM Review International Journal of Electrical Machining 4, 1-6, 1999.
http:/www.oxfordlasers.demon.co.uk/movies/ddi_drilling_large.mov
http:/www.oxfordlasers.demon.co.uk/movies/ddi_drilling_close.mov

Other

X. Chen and X. Liu, "Short pulsed laser machining: how short is short enough?," J. Laser Applications 11, 268-272 (1999). [CrossRef]
M.D. Shirk and P.A. Molian, "A review of ultrashort pulsed laser ablation of materials," J. Laser Applications 10, 18-28 (1998). [CrossRef]
D. Kapitan, D. W. Coutts and C. E. Webb, "On pulsed laser ablation of metals: comparing the relative importance of thermal diffusion in the nanosecond-femtosecond regime," Conference on Lasers and Electro-Optics - Europe paper CThH76 (1998).
M.R.H. Knowles, G. Rutterford , A.I. Bell, A.J. Andrews, G. Foster-Turner and A.J. Kearsley, "Sub-micron and high precision micro-machining using nanosecond," Proceedings of ICALEO 98 85e, 112-120 (1998).
H.K. Tönshoff, C. Momma, A. Ostendorf, S. Nolte and G. Kamlage, "Microdrilling of metals with ultrashort laser pulses," J. Laser Applications 12, 23-27 (2000). [CrossRef]
M. Knowles, R. Benfield, A. Andrews and A. Kearsley, "Development of high power compact copper vapour lasers," Advanced High-Power Lasers and Applications 99 M. Osinski, H.T. Power, K.Toyoda, eds., Proc SPIE 3889, paper 3889-63, (1999).
D. K Kapitan, D. W. Coutts and C. E. Webb, "Efficient Generation of near diffraction-limited beam-quality output from medium-scale copper vapour laser oscillators," IEEE J. Qu. Electronics 34, 419-426 (1998). [CrossRef]
M.R.H. Knowles, A.I. Bell,.G. Rutterford, G. Foster-Turner and A.J. Kearsley, "Advances in copper lasers for micromachining," Proceedings of ICALEO 97 82, (1997).
M.R.H. Knowles, R. Foster-Turner, A.I. Bell, A.J. Kearsley, A.P. Hoult, S.W. Lim, and H. Bisset, "Drilling of shallow angled holes in aerospace alloys using a copper laser," Proceedings of ICALEO 95 80, 321-330, (1995).
M.R.H. Knowles, A.J. Kearsley., R. Foster-Turner., J.E. Abbott, J.M. Boaler., K.H. Errey, "Visualization of small hole drilling using a copper laser," Proceedings of ICALEO 94 79, 352-361, (1994).
J.J. Chang, B.E. Warner, E.P. Dragon and M.W. Martinez, "Precision micromachining with pulsed green lasers," J. Laser Applications 10, 285-291 (1998). [CrossRef]
R.I. Trickett, M.J. Withford and D.J. Brown, "4.7W, 255nm source based on second-harmonic generation of a copper vapour laser in cesium lithium borate," Opt. Lett. 23, 189-191, 1998. [CrossRef]
A.C. Glover., D.W. Coutts, D.J. Ramsay and J.A. Piper, "Progress in high-speed UV micro-machining with high repetition rate frequency doubled copper vapour lasers," Proceedings of ICALEO 94 79, 343-351, (1994).
A.C. Glover,.M.J. Withford, E.K. Illy and J.A. Piper "Ablation threshold and etch rate measurements in high-speed ultra-violent micro-machining of polymers with uv-copper vapour lasers," Proceedings of ICALEO 95 80, 361-370, (1995).
C. Körner, R. Mayerhofer, M. Hartmann and H.W. Bergmann, "Physical and material aspects in using visible laser pulses of nanosecond duration for ablation," Appl. Phys. A 63, 123-131, (1996). [CrossRef]
J.J. Chang and B.E. Warner, "Laser-plasma interaction during visible laser ablation of metals," Appl. Phys. Lett. 69, 473-475, (1996). [CrossRef]
H.K. Tonshoff, F. von Alvensleben, A. Ostendorf, G. Kamlage, S. Nolte, "Micromachining of metals using ultrashort laser pulses," IJEM Review International Journal of Electrical Machining 4, 1-6, 1999.
http:/www.oxfordlasers.demon.co.uk/movies/ddi_drilling_large.mov
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