V. D. Popov, S. Yu. Soukhorossov

Application of deformable mirrors
in laser material processing systems

Laser method of material sizing first appeared more than twenty years ago, but it still is quite an exotic technology. The first reason for this is the high cost of equipment, materials and other resources. This is why the question of profitability of the application of laser material processing systems exists. Even industrial giants resolve this problem using their laser systems with full load and attracting external commercial orders. Nevertheless, laser technologies are becoming more and more popular. According to the expert estimation, several hundreds of laser machines work only in Russia, the number rises up to thousands worldwide.

Among those who is acquainted with laser technologies the common point is that laser cutting has a number of advantages compared with standard cutting methods in the productive capacity (more than 12 meters per minute) while cutting metals with a comparatively small thickness (below 10 mm). Maximum thickness of metal that can be cut by use of a standard laser cutting machine is limited by 20 mm. Unexcelled is accuracy and quality of surface of the cut made by the laser. Reports have been published announcing a micron-level accuracy and corresponding surface smoothness, that means that the cutting which traditionally is regarded as a blanking operation, becomes a finishing one at the same time. This opens up a possibility to cast the engineering elements directly of blanks.

However the main purpose of the present report is not aimed on the discussion of pros and cons of laser material processing technology. The authors believe that the majority of the audience shares their conviction that laser technology has its particular field of application and to widen it is a common goal.

The principal fact for the present consideration is that the laser radiation is transmitted in a relatively wide beam that needs to be focused to making a useful technological effect. This is the laser beam focusing that limits the thickness of materials that can be processed by use of laser machines.

Area of active interaction of the laser beam and the material under processing can be represented by two truncated cones conjugated by their small bases. Small bases are the area of the focal spot. Two physical phenomena have influence on the focus parameters in the laser machine. First, the diameter of focus depends on and is proportional to power of radiation. Second, the position of focus on the optical axis and angle at the vertex of the cones depend on the wavefront curvature of the laser beam (convergent or divergent) at the focusing optics.

Task of material processing requires that at least the position of the area of active interaction is controlled. With respect to the method of how the position of the area of active interaction is controlled, laser cutting and welding machines can be separated into two classes.

First class includes the machines where the laser is fixed on and moves together with a portal. The example of such machines is laser machines manufactured by Messer. Advantage of such a configuration is a relatively constant distance between the laser head and the material processed. A disadvantage is that the laser with significant weight and dimensions is located on the moving part of the machine and thus imposes certain requirements on the carrying ability of the portal as well as on its weight, dimensions, power consumption, and range of available speeds, etc.

The second class includes machines where the laser is fixed and the moving part is the optics. These are so called machines with flying optics. Flying optics is 1-2 order of magnitude lighter and smaller, but due to the significant distances to be serviced it cannot maintain the constant position of the focus and size of the beam. It results in a decreased accuracy and worse quality of the cut, but these two factors are the most attractive aspects of laser material processing technology.

The authors propose to solve the above problems using mirrors with a controllable shape of surface in the optical system of laser material processing system.

In any case the application of a practically inertialess focusing optics gives advantages due to the increased productive capacity and reduced cost. For example, such an optics reduces significantly the risk of a possible direct contact between the laser head and the material under processing.

To achieve an optimum control over the parameters of focus, its radius and position must be controlled independently (or almost independently) of one another. The best way to provide this is to use two deformable mirrors, placing one of them near the source of laser radiation and the another near the focusing head.

First mirror, placed near the laser, serves to maintain constant illumination and beam size on the focusing optics. Since the distance between the laser and the focusing head is, as a rule, big enough and reaches several meters, influence of this mirror on the laser beam divergence is insignificant and therefore the influence on the focus position itself is weak.

The second mirror placed near the focusing optics defines the focus position in greater degree practically not influencing on the beam size.

Since 1993 Turn Ltd. has been working on the development of deformable mirrors basing on the bimorph technology. At the present moment the company produces a line of single-channel deformable mirrors for application in laser material processing systems.

External appearance of the deformable mirrors is shown on fig. 1 and their specifications are given in Table 1.

Fig.1. External appearance of AT22 (left) and AT24 deformable mirrors for laser material processing systems.

Table 1.

Specifications of deformable mirrors for industrial laser machines

Turn Ltd. works in the active competition with companies who use traditional technologies of deformable mirrors with actuators. The most active on the market is Diehl Stiftung & Co., Rothenbach (Germany).

Alike mirrors produced by Turn Ltd. a mirror with actuators has a protective reflecting coating with the regular reflectance of 99.8 % at the wavelength of 10.6 m and is provided with a cooling system. Deformable mirrors produced by different companies have very close and comparable parameters. But since Turn's mirrors are 5-6 times lighter and about 10 times shorter, they can be built into the laser material processing system machines without making any changes in its design and moreover can be built into the laser itself. Additionally, Turn's mirrors have frequency parameters several orders of magnitude better due to the absence of any massive moving parts.

Recently the company has carried out a number of testings in cooperation with various companies including Germany-based.

AT24 deformable mirror was built into the optical system of an operating laser material processing system LFP-Laserbeam Cutting and Welding System. The machine comprises a CO₂-laser with the power of 6 kW which moves along one axis while the flying optics serves another axis. Optical path length from the laser to the piece under processing reaches 6 meters. Focal length of the optical head is about 30 cm. AT24 deformable mirror was placed near the optical head and was capable to displace the focus position within the range of 15 mm. The mirror was controlled by means of AT32 control unit interfaced with a computer using a special software. It should be noted than due to the mismatch of light aperture of the mirror to the beam size of the laser system some vignetting occurred that influenced negatively on the system efficiency.

Testing was carried out while operating in the modes of cutting and perforating.

In the mode of perforating a doubled efficiency was achieved while processing a specimen of stainless steel with a thickness of 8 mm, all other parameters of processing being unchanged.

In the mode of cutting a significant improvement of the cut smoothness occurred, all other parameters of processing being unchanged (and in spite of the above noted vignetting). On fig 2 a photo of typical elements processed during the testing are shown, fig. 3 represents a view of the cut.

Fig.2. Typical view of the elements produced by laser cutting.	Fig.3. View of the cut after activating the deformable mirror. Specimen is a piace of stainless steel with the thickness of 8 mm. Cutting speed is 0.46 m/min.

Turn Ltd. carried out a number of experiments on the use of its deformable mirrors inside of the laser cavity.

First experiment was carried out together with Lasertech joint venture using a laser of "Hebbr-1A" type (co-production of Russia and Bulgaria) with output power up to 1 kW. During the experiment a pulsed-periodic lasing has been easily achieved with time and frequency parameters of the pulses were defined by the same parameters of the control voltage. It was found during the testing that while operating in pulsed-periodic mode the distribution of power over the spot is better compared to the continuous operation, all other factors being equal.

During the similar experiment carried out in cooperation with specialised scientific and production enterprise "Istok-Laser" using CO₂-laser "Harpoon-2000" (Russia) with the output power up to 2 kW not only the pulsed-periodic mode was realised but a stable modulated Q-factor mode was achieved. While operating in this mode with time and frequency parameters of the pulses defined by the same parameters of the control voltage a specific overshoot of power 2-3 times exceeding the mean power of radiation was registered.

So, the pulsed-periodic lasing and the mode of modulated Q-factor were realised in the originally continuously operating lasers "Hebbr-1A" and "Harpoon-2000", that widens their operating parameters and potential applications.

At the present time Turn Ltd. is concentrated both on extension and cost reduction of its deformable mirror production. A manufacturing is being finished of an earlier developed 19-channel deformable mirror purposed for compensating thermal deformations of the working medium of a solid state laser. Among the company's plans for the near future are the release of a cheap mirror with dimensions matching the geometry of the commercial industrial lasers with the power of up to 1 kW (of the "Hebbr-1A" type) and launch of a mirror with the light aperture of 70 mm.

It should be stressed that all operating capabilities of industrial lasers using controllable mirrors in their configuration has not been studied yet by now because the worldwide exploitation of such intelligent machines only begins.

Additionally to the above advantages, the application of deformable mirrors promises to become a solution for technical difficulties which arises while drilling long holes, where the cylindricity of the hole along all its length is a critical factor, or cutting thick materials when the typical X-shape of the cut is undesirable.

It cannot be doubted that the use of deformable mirrors inside or outside of the cavity of laser material processing systems offers improved operating parameters and wider processing capabilities, for example, greater processing capacity of cutting and welding, better smoothness of the cut or weld, wider range of thicknesses and expanded range of materials that can be processed.

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