To keep the thermal expansion of mirrors for high power laser within a specified value, a cooling system has to be applied. The idea presented in this paper is, to heat up the mirrors to keep them at high operating temperature. Therefore several simulations were run and verified at specially designed and manufactured test parts. The test parts are fabricated with a type of an economic process chain

Börret, Rainer

Hänle, Manuel

Kelm, Andreas

Knopf, Marcel

Sinzinger, Stefan

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  Manuel Hänle, Andreas Kelm, Marcel Knopf, Rainer Börret,  Stefan Sinzinger : Optical Performance of heated laser mirrors for high power lasers Zuerst erschienen in: DGaO-Proceedings. - Erlangen-Nürnberg : Dt. Gesellschaft für angewandte Optik, ISSN 1614-8436. - Bd. 110 (2009), P36,  insg. 2 S.  URL: http://www.dgao-proceedings.de/download/110/110_p36.pdf     DGaO-Proceedings 2009 – http://www.dgao-proceedings.de – ISSN: 1614-8436  Optical Performance of heated laser mirrors for high power lasers Manuel Haenle*, Andreas Kelm*, Marcel Knopf*, Rainer Boerret*, Stefan Sinzinger** * Zentrum für optische Technologien, Aalen University, rainer.boerret@htw-aalen.de; ** TU Ilmenau To keep the thermal expansion of mirrors for high power laser within a specified value, a cooling system has to be applied. The idea presented in this paper is, to heat up the mirrors to keep them at high operating temperature.  Therefore several simulations were run and verified at specially designed and manufactured test parts. The test parts are fabricated with a type of an economic process chain. 1 Introduction and background When it comes to industrial applications like weld-ing, cutting or drilling, high-power CO2-lasers have become the state-of-the-art tool. In order to achieve a high quality beam, the mirrors guiding and form-ing the beam have to be cooled down during op-eration. In order to simplify the laser head, the idea is to use silicon carbide instead of copper mirrors. Ceramics can be used at much higher tempera-tures than cooper. The mirror has to be designed in a way, that it achieves its desired form at operating temperature. Then one can leave the cooling sys-tem making the head lighter and less complex. In a first step, test-parts are simulated under different conditions and the proof of concept is made by verifying the simulated results in an experimental set-up. The test parts were manufactured by using a new polishing tool for fast processing. 2 Simulation In a first step, a simple simulation using the finite element method calculated the behavior of different mirrors over time with a constant heating power of 20W simulating the thermal load of a CO2 laser. The equilibrium temperature was 120°C. The same amount of heat was put onto a copper mirror and an already preheated mirror with a temperature of 120°C.  Mirror Copper SiC Standard  SiC preheated to 120°C Simulated devia-tion 42µm 4µm 1,4µm Simulated tem-perature change 100K 100K 15K Tab. 1 Overview over simulated surface deviation and temperature change of a copper, a SiC and a preheated SiC mirror with a heating power of 20W. Table 1 shows the simulation results. The tempera-ture in equilibrium is independent of the material. Copper shows the largest surface deviation, while a preheated SiC mirror shows almost no deviation.         Figure 1 Simulated surface deviation (max. 1,4µm)and temperature distribution (max. 7K) of the preheated SiC mirror with an additional heat flow of 20 W Figure 1 shows the simulated surface deviation and temperature distribution of a preheated SiC mirror with an additional heat flow of 20W. Both suggest a deformation that leads to a change of focal length. Surface deviation is not rotational symmetrical be-cause of clamping. 3 Verification and measurements To verify the simulation results a test-part was manufactured and measured during heating with a thermo foil. The mirror was heated from the back with a heat flow density of 3W/cm². The tempera-ture was measured with a PTC and a thermo cam-era. The mirror was heated inside a Fizeau-interferomter, so in situ measurements were possi-ble. Maximum temperature over time, comparison0204060801001200 1000 2000 3000 4000Time [s]Temperature [°C]simulated PTC measured Thermal Camera measured Figure 2 Comparison of simulated and measured mirror temperature over time. The deviation is a result of the simple simulation model. 10mm  DGaO-Proceedings 2009 – http://www.dgao-proceedings.de – ISSN: 1614-8436  Figure 2 shows the simulated and measured tem-perature over time. With a load simulating the hold-ing and the interferometer, the two curves show good agreement. The measurement was aborted after one hour. Figure 3 shows the surface deviation of the mirror when it is heated to 100 °C. The maximum devia-tion is about 2µm, which is in good concordance with the simulation. The discrepancy between simulation and measurement may be caused by the simple simulation model. As the simulation results suggested, focus is the dominating image error. Higher orders of rotational image errors were negligible. Tilt resulting from deformation of the bearing was removed before analysing.  Figure 3 Measured deformation of the SiC mirror at a temperature of 100°C. The maximum deformation is about 2µm, the main surface error is the focal term. Tilt was removed. Since focus is the prime image error, the character-istics of the focus term was surveyed. Figure 4 shows the change of the focus term which is almost linear with temperature. Measured Defocus over temperatureR2 = 0,9687-1,8-1,6-1,4-1,2-1-0,8-0,6-0,4-0,200 20 40 60 80 100 120Temperature [°C]4th Zernike Figure 4 Typical development of focus (4th Zernike) over temperature. The focus term shows the largest change over temperature. 4 Manufacturing of the test parts For manufacturing the test parts a newly developed bond abrasive tool was used. The tool has a con-cave shape to avoid effects caused by the tool’s edge. Figure 4 left shows the tool in action on a mirror surface. The fine grinded surface was pol-ished with diamond slurry to achieve good rough-ness and high reflectivity. The total processing time for grinding, fine grinding and polishing was about 3 hours. Figure 5 right shows an interferometric measure-ment of the test part without heating    Figure 5 Newly developed bond abrasive tool used to manufacture the mirror face (left), interferometric meas-urement of the mirror without heating (right). 5 Summary and Outlook In this study a laser mirror made out of SiC under temperature load was simulated and tested. It is shown that focus term shows the largest change. In further studies a mirror will be manufactured which is designed to achieve the ideal form at operating temperature. The surface deviation of this mirror will be examined under an additional thermal load. Furthermore a more sophisticated simulation model will be developed. Acknowledgement The work is supported by the federal ministry of education and research BMBF/AIF under the con-tract “Keraform” and by the Landesstiftung Baden-Württemberg Foundation under the contract “F3 - Laseroptik”. Literature [1] Werkzeug Laser – Ein Lichtstrahl erobert die indus-trielle Fertigung; Leibinger-Kammüller, Nicola, Buch-fink, Gabriela; Vogel Verlag; ISBN: 978-38343330529 [2] Gundlagen und Anwendungen der Finite-Element-Methode im Maschinen- und Fahrzeugbau; 7. Aufla-ge; Klein, Bernd; Springer Fachbuchverlag; ISBN: 978-3834891495 [3] Untersuchung und Einführung eines neuen Feinstschleifverfahrens mit Diamantwerkzeug zur Bearbeitung von Planflächen; Schleitzer, Yvonne; Masterthesis Hochschule Aalen, 2008   

Optical Performance of heated laser mirrors for high power lasers

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Optical Performance of heated laser mirrors for high power lasers

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