Laser machining of materials

Details Laser machining of materials Laser machining of materials

2002-10-28

2004-09-07

Heinrich, Samuel M. (Department: 1725)

Electric heating

Metal heating

By arc

C219S121720

Reexamination Certificate

active

06787733

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATION
This application is a national stage of PCT/EP01/04629 filed Apr. 25, 2001 and based upon DE 100 20 559.3 filed Apr. 27, 2000 under the International Convention.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a device and a method for laser machining of materials such as, for example, metallic materials (metals, alloys), ceramics, glasses, plastics, cellulose materials (paper, board, etc.), biological tissues, fluids.
2. Description of the Related Art
The use of ultrashort laser pulses, i.e. pulses with a pulse duration in the range between approximately 10
−15
s and 5×10
−10
s for the purpose of high precision laser microstructuring is known.
Thus, C. Momma et al. describe experiments on laser ablation of metallic materials using pulse durations of 150 fs down to the nanosecond range in “Präzise Mikro-Bearbeitung mit Femtosekunden-Laserpulsen/Precise Micromachining with Femtosecond Laser Pulses”, Laser- und Opto-elektronik 29(3)/1997.
S. Nolte discusses aspects of material machining using femtosecond laser pulses in his thesis on “Mikromaterialbearbeitung mit ultrakurzen Laserpulsen, (Micromachining of materials with ultrashort laser pulses)”, Cuvillier Verlag Göttingen, 1999.
In an article on “Application of ultrashort laser pulses for intrastromal refractive surgery”, Graefe's Arch. Clin. Exp. Ophthalmol 238:33-39, 2000, H. Lubatschowski et al. describe the use of laser systems which generate ultrashort laser pulses with a duration of 100-200 femtoseconds in the field of intrastromal refractive surgery.
Kurtz et al. “Optimal Laser Parameters for Intrastromal Corneal Surgery”, SPIE, Vol. 3255, 56-66, January 1998, also use ultrashort laser pulses for tissue machining.
Loesel et al., “Non-thermal ablation of neutral tissue with femtosecond laser pulses”, Appl Phys. B, 66, 121-8, 1998, are also concerned with comparable objectives.
In WO 99/67048 Michael D. Perry and Brent C. Stuart describe methods for “Ultrashort Pulse Laser Machining of Metals and Alloys”. In this publication they explain the use of laser pulses with a repetition rate that is greater than 1 Hz and can even be more than 2 kHz, the laser beams having a wavelength in the range between 0.18 and 10 &mgr;m and the pulse duration being between 10 fs and 100 ps; values in excess of 10
12
W/cm
2
are quoted for the intensity (=power density).
Usually, femtosecond pulses with sufficient energy (in the &mgr;J to mJ range) for material machining are generated during so-called “chirped pulse” amplification. In this context see D. Strickland, G. Mourou, Opt. Commun. 56. 219 (1985) and WO 99/67048. With this amplification technique pulses with a duration of, for example, about 100 fs are first generated in a mode-coupled oscillator. The pulses are initially temporally stretched. This takes place in a structural unit termed a “stretcher”, which comprises a special arrangement of dispersion gratings and has the effect that the various wavelength fractions of the pulse (the bandwidth of a 100 fs pulse is approximately 10 mm for an average wavelength of 800 nm) travel optical paths of different lengths. The pulse is reversibly stretched by more than three orders of magnitude from 100 fs to a few 100 ps. Because of the reduction in intensity produced in this way, the subsequent amplification takes place with avoidance of non-linear effects, which would result in interference with the spatial pulse profile and could lead to destruction of the amplifier medium. The amplified pulse is then compressed again to virtually its original duration by a structural element termed a “compressor”, which likewise consists of dispersion gratings.
A typical mode-coupled oscillator for the generation of fs laser pulse (sic) is indicated by C. Momma et al., Laser- under Optoelektronik 29(3)/1997, p. 85 with reference to F. Salin et al., Opt. Lett. 16.1674 (1991).
For their experiments H. Lubatschowski et al., Graefe's Arch. Clin. Exp. Ophthalmol 238:33-39, 2000, use a “Kerr lens” mode-coupled titanium-sapphire laser system with subsequent “chirped pulse” amplification and to this extent refer to Morou G (1997) “The ultra high-peak power laser: present and future” Appl. Phys. B 65:205-211.
Further systems for defined generation of ultrashort laser pulses (fs laser pulses) are known to those skilled in the art from the pertinent literature.
The explanations on equipment and methods for the production of ultrashort laser pulses and on their use in material machining given in the literature references cited above are incorporated in the present text by reference. Laser systems of the type mentioned in the literature references cited above can be used in the context of the present invention.
In general, i.e. also in the context in the present invention, ultrashort laser pulses can be used for cutting material (for example metallic materials), for ablation and structuring of material (for example biological tissue) and for changing material properties (for example changing the refractive index in glass).
The particular advantages of material machining with ultrashort laser pulses (fs laser pulses) are evident in particular in extremely precise cutting and/or ablation of materials that gives rise to minimal damage, both thermally and mechanically. By focusing the ultrashort laser pulses, energy is deposited in the focus on a very restricted area by striking a microplasma and a cutting effect or material ablation is achieved by so-called photodisruption. Ablation rates in the sub-&mgr;m range with cut widths of less than 500 nm can be achieved. Because of a non-linear interaction mechanism during photodisruption, the material ablation with this technique is largely independent of the material properties. In particular, materials with high thermal conductivity (such as, for example, metals) and materials with low laser light absorption (such as, for example, polymers or certain biological tissues) can also be machined using fs laser pulses.
As an alternative to ablation on the surface of a material to be treated, it is also possible by focusing in materials that are transparent to laser radiation (such as, for example, the cornea of an eye) to achieve a cutting effect within the transparent material (tissue).
However, to date the use of the fs laser pulse technique for machining materials has been fairly inconvenient. For instance, up to now no suitable means are available for online monitoring of the machining result. As a general rule, therefore, in the case of fs material machining the machining result, for example a cut depth achieved, is currently determined, for example by optical or electron microscopy, only after machining. If post-machining of the material is required, the material sample must be re-positioned, which, however, as a general rule is no longer possible with the precision desired or even required. Therefore, frequently the ablation depth per pulse is initially determined by semi-empirical means and the cut depth in the sample to be machined is then estimated prospectively by counting the laser pulses. This procedure is increasingly being felt to be unsatisfactory.
SUMMARY OF THE INVENTION
The aim of the present invention was, therefore, to indicate a device and a method for laser machining of a material, for example in the solid or liquid aggregate state, with ultrashort laser pulses, which device and which method make it possible by simple means to monitor the result of machining with ultrashort laser pulses.
Preferably, in this context the device and the method should be so designed that online monitoring is possible.
According to a first aspect of the present invention, the stated aim is achieved by a device for machining a material with ultrashort laser pulses, comprising
(a) a device for generating a sequence of first laser pulses, where
the first laser pulses each have a duration of less than 300 picoseconds and
the repetition rate for the first laser pulses is in the range between 100 kHz and 1 GHz,
(b) a conv

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