Andrey G. Safronov, Boris S. Vinevitch, Valeriy M. Zharikov

Controllable curvature mirrors for laser techniques

The paper describes bimorph mirrors purposed for operation in industrial laser machines. A detailed study of the technical parameters of cooled single channel deformable mirrors has been carried out. Results are given of preliminary tests of the uncooled bimorph mirrors in CO₂ industrial laser machine with output radiation power exceeding 2 kW as well as in the laser machine using a pulsed copper vapour laser with mean radiation power of 35 W.

1. Introduction.

A wide and intensive application of various type lasers (CO₂-lasers, =10.6 m; Nd:YAG-lasers, =1.06 m etc.) in industrial production, say, in automotive industry1,2, makes laser processing technologies more and more complicated. Their operational capabilities are progressively widening with the concurrent increase both of the speed of technological operations and of the processing quality. The very next step on the way of improving laser technologies is the use of controllable (adaptive) optics, in particular of deformable mirrors, wherein at the initial stage it makes sense to use simplest elements and systems.

Present-day industrial laser machines propose a number of problems, which can be resolved by using these systems. On the one hand, these are traditional problems for adaptive optics, namely, improvement of the quality of laser systems due to the real-time compensation of optical radiation wavefront distortions, such as, for example, non-linear thermal lens, etc. On the other hand, laser processing systems demonstrate their specific problems; among them are, for example, real-time control (with the possibly high speed) over the spatial location of the focal spot of the laser head during the process of welding or cutting of complex-shaped parts [3].

As a practical matter of introduction into the existing laser machines, a bimorph adaptive optics is of special interest. A detailed analysis of its advantages has been carried out in paper [4]. The present paper describes results of experimental researches and testings of single channel deformable bimorph mirrors, which have been designed for use in industrial laser machines purposed for various applications. These mirrors are distinguished by their high and stable technical parameters, simplicity of the operation, high reliability and low cost.

2. Cooled deformable mirrors. Results of experimental reserches.

Fig.1. The molybdenum cooled one-channel adaptive mirrors for the middle-power (up to 10 kW) laser systems.

Uncooled single channel mirrors of the discussed type purposed for use in laser systems with the radiation power up to 1 kW were earlier studied in paper [5]. A bimorph technology [6,7] developed and used in those mirrors was used for production of molybdenum cooled deformable mirrors shown in fig. 1.

The outer diameter of the mirror is 70 mm and its height is 13.7 mm. A cooling system of the "oblique wafer" type is formed underneath of the reflective surface of the mirror. A coolant is supplied to the cooling system through metallic inlets, see fig. 1. Such cooling system allows to use the cooled bimorph mirrors in laser systems the integral radiation power of which may reach 10 kW. The detailed performance investigations of the cooling system of the type as described above for the case of use in cooled bimorph mirrors were earlier carried our in paper [8].

Light aperture diameter of cooled deformable mirrors is 42 mm. The initial shape of the optical surface is flat. The mirror is provided with the copper reflective coating and silicon dioxide protective coating. Regular reflectance is no less than 98.5% at =10.6 mm. Control voltage for cooled bimorph mirrors ranges from - 300 V up to + 200 V. Capacitance of the control electrode is of order 300 nF. The mirror weight is about 300 g.

2.1. Initial shape of the optical surface of cooled deformable mirrors.

Cooled deformable mirrors were studied using the interferometric complex Mark-II. Block diagram of the set is shown in fig. 2. The interferometer (1) realizes the known Fizeau scheme; the diameter of the output beam is 102 mm; the working wavelength is 632.8 nm; the power of the laser radiation is 1 mW. A reference flat was taken as an optical etalon (2) for measurements of deformable mirrors with a flat initial surface. The interferogram registration error on this complex is /20 for flat optical surfaces and /10 for spherical ones.

Fig.2. Block diagram of the measuring complex: 1 - Mark-II interferometer; 2 -optical etalon; 3 - attenuator filter; 4 - deformable mirror fixed in a 5-coordinate adjusting mount; 5 - control panel; 6 - VM-2 videomonitor; 7 - P50E videoprinter; 8 - electronic control unit of the deformable mirror.

Fig.3. The interferogram of initial shape for the cooled one-channel deformable mirror over full light diameter (42 mm).

Measurements of the initial shape of optical surface demonstrated good quality and high stability of the cooled deformable mirrors. This was verified by multiple interferometric control, which have been being carried out for few months including measurements at various air temperatures and humidities as well as before and after transportation of the mirrors under various conditions. A typical interferogram of the initial surface of the cooled deformable mirror is shown in fig. 3. As a result of analysis of a number of obtained interferograms the following has been determined:

1. the total deviation of the surface shape from the flat over the full light aperture is no more than 1 fringe (1F=/2) for cooled deformable mirrors;
2. local shape error of the surface over the full light aperture does not exceed 0.25F=/8.

2.2. Response functions and sensitivity of cooled adaptive mirrors.

Experimental measurements of response functions for the cooled bimorph mirrors were carried out using the above-described automatic complex, see fig. 2. Control voltage to the deformable mirrors was applied from the electronic unit (8). Fig. 4 shows a typical interferogram of the reflective surface of a cooled single channel mirror under the control voltage.

Fig.4. The interferogram of reflective surface for the cooled one-channel deformable mirror under U=80 V of control voltage over the full light diameter (42 mm).

An amplitude sensitivity of cooled adaptive mirrors was obtained as a result of experimental measurements and their further statistical processing. The mean value of the amplitude sensitivity over 96 measurements was 46±8 m/kV. Here the following should be noted. Value of the sensitivity of the bimorph mirrors generally and of the mirrors under consideration in particular depends on the sign of control voltage. This phenomenon is defined by the polarazation of piezoelectric ceramics. To be more concrete, if applied control voltage has the same sign as the polarization voltage of the piezoelectric ceramics, the sensitivity of the bimorph mirror will be higher than for the case when the above listed voltages have opposite signs. In particular for the case of the cooled mirrors under description, polarization voltage of the piezoelectric ceramics is negative with respect to the body of the mirror. Therefore for negative values of the voltage, the amplitude sensitivity is somewhat higher than the mean value and equals to 49±6 m/kV. Correspondingly for positive values of the voltage, the value of sensitivity is somewhat below the mean value and equals to 44±8 m/kV.

Influence of the polarization of piezoelectric ceramics is also the reason of asymmetry of the range of control voltage. In this particular case, it is shifted to the negative range. Taking this into account, the maximum controllable deformations of the cooled bimorph mirrors are +14.7 m (convex) for -300 V and -8.9 m (concave) for +200 V.

From the above discussed, an empirical formula for the response function given in [5] for the case of the cooled single channel bimorph mirrors described in the present paper can be written as follows (no hysteresis is taken into account):

where W(r) is the response function of the reflective surface of the mirror; r₁ is the radius of the reflective surface (here, r₁=21 mm); K is sensitivity; U is control voltage. For given deformable mirrors the value of K lies in the range 44 - 49 m/kV

2.3. Hysteresis of cooled deformable mirrors.

Piezoelectric adaptive mirrors are typified by existence of an electromechanical hysteresis, which reveals itself in the non-linear dependence of displacements of the reflective surface upon applied voltage. Hysteresis measurements for cooled mirror were carried out as follows. The shape of reflective surface of a deformable mirror was registered by means of the above-described method under various values of control voltage and afterwards sag of this surface was calculated. Control voltage during the procedure of measuring were being changed cyclically in the following sequence: 0 +U 0 -U 0 +U. A hysteresis curve was constructed using the obtained numerical data, wherein each point of the curve was averaged over 4 measurements in order to avoid random errors.

Fig.5. Deformation amplitude W vs control voltage U for a cooled single channel deformable mirror. Values of deformation amplitudes W are given in fringes F=/2.

Fig. 5 represents a shape of the hysteresis loop for cooled bimorph mirror. Square-shaped marks on this curve correspond to initial part of the dependence W(U). A numerical value of the hysteresis is defined as a ration of the loop width at zero control voltage to its total height and for the mirrors under consideration is 10.6 ± 0.3 %.

3. Non-cooled deformable mirrors. Results of preliminary tests.

Preliminary tests of non-cooled single-channel bimorph mirrors studied in detail in [5] were carried out with cw CO₂-laser and a pulsed copper-vapour laser.

3.1. Tests with CO₂-laser.

In spite of the fact that the non-cooled bimorph mirrors are calculated for use with lasers which radiation power does not exceed 1 kW and, moreover, under an outer cooling [5], the preliminary tests were carried out with a cw industrial CO₂-laser which radiating power exceeds 2 kW. That is explained by the similarity of geometrical parameters of deformable mirrors and intra-cavity optics of the laser. The given laser is a part of the "GARPUN-2000" technological set. Its base technical parameters are listed in the table 1:

Table 1.

Parameter	Value
1. Type of laser	cw, gas-discharge, axially fast-flowing
2. Radiation wavelength, m	10.6
3. Maximum radiation power, W: - in a multimodal regime - in a single-mode regime	2500 1000
4. Output beam diameter, mm: - in the multimodal regime - in the single-mode regime	max 45 max 20
5. Beam divergence in a single-mode regime, mrad	max 2
6. Range of adjustable radiation power, W	200 ... 2000
7. Type of the cavity	stable
8. End mirror of the cavity	concave, Rcurv = -30 m, dia = 60 mm
9. Output semi-reflecting mirror of the cavity: - for the multimodal regime  - for the single-mode regime	concave, dia = 60 mm Rcurv = -30 m Rcurv = -15 m
10. Total length of the cavity, m	6.5

It was revealed during the tests with CO₂-laser that, after the mounting of the uncooled bi-morph mirror into the evacuated cavity of the laser, its initially flat reflective surface is being deformed (buckled). The curvature radius of the deformed surface was ranging from 9.4 m (over the full light aperture of 42 mm) up to 7.4 m (over an aperture of 7-10 mm). A collimator comprising OSK-2 optical bench was used in order to estimate the curvature radius. The said deformations of the bimorph mirror are totally linear and elastic; after removal of the pressure the shape of the reflective surface cor-responds to the initial shape. Obviously, it is necessary to take into account such a phenomenon when using deformable mirrors in the evacuated optical systems, and the initial reflective surface must be formed introducing a correction for intra-cavity static deformations.

The used for the tests cw CO₂-laser has a stable cavity comprising a total reflecting end mirror, and an output semi-reflecting mirror, and 6 flat turning mirrors. All optical components of the cavity (excluding the output mirror) are made cooled. The uncooled bimorph mirror was mounted in the cav-ity of the laser instead of the end mirror, wherein the radiating power inside of the cavity was about 260 W. The outer cooling was not applied to the deformable mirror. In spite of a strongly non-optimized geometry of the cavity and the above-noted phenomena, a stable lasing was achieved. Under the control signal of a definite value applied to the deformable mirror, the lasing is suppressed due to the deformations of the reflective surface. Evidently, the optimization of the parameters of the deform-able mirrors directly for the given cavity as well as the use of a cooled bimorph optics, which was de-scribed in part 2 of the present paper, will give the possibility to achieve a regime of a modulated Q-factor for this cw CO₂-laser. Probably, the maximum modulation depth is achievable at the frequency of the control voltage, which is equal to the main resonant frequency of the deformable mirror because in this case the controllable displacements of the reflective surface will be the greatest. It is clear that un-der a quasi-static control voltage the application of the described deformable mirrors inside of the given laser will provide the regime of a controllable output radiation power at least within a limited range.

It should be noted that during the tests the multiple-layered bimorph mirror operated under a significant integrated load on its flexible reflective plate. That was the result of the following reasons as a minimum:

1. thermal deformations of the mirror under the action of the power of the intra-cavity radiation;
2. deformations of the reflective plate caused by the outer (with respect to the cavity) atmospheric pressure;
3. controllable deformations of the mirror.

3.2. Copper vapour laser tests.

Basic technical parameters of the ILGN-201 laser ("CARAVELLA" technological set), which was used in these tests, are listed in table 2. Testings were carried out with an uncooled single-channel deformable mirror, which has a 42 mm light aperture and a copper reflective surface. Regular reflec-tance at the given wavelengths is 44% for =0.51 m and ~60% for =0.58 m.

Table 2.

The base technical parameters of the Copper-vapour laser

Parameter	Value
1. Type of laser	pulsed
2. Radiation wavelength, m	0.51 and 0.58
3. Beam divergence, mrad	0.1...0.5
4. Output beam diameter, mm	20
5. Repetition frequency, kHz	8...12.5
6. Half-height laser pulse width, ns	15±5
3. Radiation power, W: - average - pulse	30...35 200 000...250 000

The deformable mirror under study was used outside of the cavity of a copper vapour laser for controlling the focus position of the laser beam, which is formed by the optical system with the focal length of 7.5 m. The bimorph mirror was located directly in front of the concave spherical mirror, see fig. 6. Under the changing from +100 V to -200 V control voltage, the focus position of the beam was travelling linearly (with regard to the electromechanical hysteresis) from its mean position 2.4 m from the cavity and 4.8 m to the cavity. Let us note that during the tests, the uncooled mirror under a quite strong heating by the laser radiation, up to about 80°C, without any changes in the operational parameters. In addition to that, uncooled mirrors demonstrated a stable performance under the laser radiation radiation while being heated up to 45° - 50°C. It is clear that such temperature conditions took place due to the very low index of reflection of the deformable mirrors at operational wavelengths of the laser. It seems likely that applying of suitable reflective coatings will provide a long and efficient operation of the uncooled molybdenum bimorph mirrors with the described laser at temperatures of about 20-25°C.

Fig.6. Block diagram of tests with copper-vapour laser. 1 - laser; 2 - non-cooled flat deformable mirror ; 3 - concave spherical mirror (f=7.5 m); 4 - electronic control unit.

4. Conclusions.

The carried out experimental studies of cooled and uncooled bimorph mirrors as well as their preliminary tests in technological lasers allow to consider the mirrors to be a convenient, reliable and inexpensive instrument which provides a widen range of functional capabilities of existing laser processing machines. Wherein, probably, application efficiency for the given deformable mirrors will increase with the shift to the region of shorter wavelengths of laser radiation.

5. References.

1. F.J. Gruber, "Audi places its bets on solid-state lasers. Three more laser-welding cells go online at Audi's Ingolstadt, Germany, body plant", Euro Laser, No. 2, pp. 20-25, 1996.
2. F.J. Gruber, "BMW leads the way. Laser welding in mass-production operations: 11 meter of laser-welded seams on BMW's new 5-series model", Euro Laser, No. 2, pp. 26-29, 1996.
3. K. Bar, B. Freisleben, C. Kozlik, R. Schmiedl, "Adaptive optics for industrial CO2 laser system", Lasers in Engineering, 4, pp. 233-242, 1995.
4. A.G. Safronov, "Adaptive bimorph optics: present state and principles of future development", Journal of Optical Technology, 65, No. 1, 1998.
5. A.G. Safronov, "Single-channel adaptive mirrors for laser optics", Quantum Electronics, 25, No. 11, pp. 1079-1083, 1995.
6. A.G. Safronov, "Multiple layer piezoelectric deformable bimorphic mirror", RU Patent, No. 2068191, 12 February 1996, Int. Cl. G02B 5/10.
7. A.G. Safronov, "Multiple layer piezoelectric deformable bimorphic mirror", International Patent Application, No. PCT/RU96/00053 filed 6 March 1996. International Publication: No. WO 96/18919, 20 June 1996, Gazette No. 1996/28.
8. A.G. Safronov, "Controllable bimorph optics based on piezoelectric structures", Ph. D. thesis's summary, General Physics Institute of Russian Academy of Sciences, 1995.

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