WO2008061516A1

WO2008061516A1 - Method and device for adjusting the beam diameter of a laser beam passing through an optical element by means of temperature changes

Info

Publication number: WO2008061516A1
Application number: PCT/DE2007/002106
Authority: WO
Inventors: Michael Von Borstel; Mark Geschwandner
Original assignee: Trumpf Laser- Und Systemtechnik Gmbh
Priority date: 2006-11-25
Filing date: 2007-11-21
Publication date: 2008-05-29
Also published as: DE102006055738B4; DE102006055738A1

Abstract

In a method for adjusting the beam diameter of a laser beam (10) passing through an optical element (9), wherein the optical element (9) is transmissive or partially transmissive for the wavelength of the laser beam (10) and has a temperature-dependent refractive index, according to the invention the temperature of the optical element (9) is adjusted according to the desired beam diameter.

Description

Method and device for changing the beam diameter of a laser beam passing through an optical element by means of temperature change

The present invention relates to a method and apparatus for varying the beam diameter of a laser beam passing through an optical element, wherein the optical element is transmissive or partially transmissive to the wavelength of the laser beam and has a temperature-dependent refractive index. Particular application of the invention in Iπfrarot gas lasers, especially in CCVGaslasern with a wavelength of 10.6 microns. In the case of CO ₂ gas lasers, it is known that the beam diameter of the laser beam changes with increasing service life or service life of the laser. The main reason for this effect is considered to be increasing aging over time and soiling of the optical elements, in particular of the coupling-out mirror. In stable laser resonators, the laser beam is coupled out of the laser resonator via a partially transmissive coupling-out mirror. The Auskoppelspiegel is made of infrared-permeable materials, usually made of zinc selenide, in some cases of gallium arsenide, and is cooled as well as possible.

The beam guidance and shaping of CO ₂ laser beams takes place predominantly in free beam propagation via reflective, transmissive and partially transmissive optical elements. Each optical element absorbs a small part of the laser beam power of an incident laser beam and heats up by the absorbed laser beam power. Dust particles or other contaminants, such as abrasion, which are present in a beam guiding space, deposit on the surface of the optical elements and lead to an increased absorption of the incident laser beam and thus to an additional heating of the optical elements. The absorbed laser beam power leads to a thermal load on the optical elements, which reduces the lifetime, and also changes the optical properties. By cooling the optical elements to reduce the thermal load and improve the optical properties. Transmissive and partially transmissive optical elements have the disadvantage that cooling can take place only over the edge of the optical elements, since the laser beam passes through the optical elements.

In the case of transmissive and partially transmissive optical elements, the laser beam at least partially traverses the optical element. For example, a power density distribution of the transmitted laser beam with a maximum in the center and a decrease in the power density outwards in the optical element in the radial direction leads to a corresponding distribution of temperature and thermal expansion. The temperature gradient in the radial direction causes according to the thermal conductivity and the specific heat capacity of the optical element, a heat flow in the direction of the cooler areas. Zinc selenide and Gallium arsenide has a thermal conductivity that decreases with increasing temperature. This leads to steeper temperature gradients in the optical element at higher temperatures, since the heat is transported away worse.

The refractive index n is a temperature-dependent property of an optical element. Due to this temperature dependence, a spatial temperature distribution in an optical element causes an incident laser beam to be refracted differently. Materials having a positive refractive index gradient + dn / dT or a negative refractive index gradient -dn / dT have different effects on an incident laser beam. Zinc selenide and gallium arsenide have a positive refractive index gradient + dn / dT.

In the case of an optical element without curvature, a temperature distribution with a maximum in the center of the optical element and a temperature drop to the edges (eg Gaussian temperature distribution) and a positive refractive index gradient + dn / dT lead to a focusing of the incident laser beam, a negative refractive index gradient -dn / By contrast, dT generates a widening of the laser beam. A focusing optical element generates a laser beam path with a smallest beam diameter (beam waist) at the focal point of the optical element, behind the beam waist increases the beam diameter. Heating the optical element at a positive refractive index gradient + dn / dT causes the beam diameter in the region from the optical element to the beam waist and just behind it to be reduced with respect to a cold optical element, whereas the beam diameter at distances that are large in relation to the focal length of the optical element are enlarged with respect to a cold optical element.

In order to keep the beam diameter of a laser beam constant or to adjust it to different values, adaptive mirrors, in which the

Curvature of the mirror surface is selectively changed, or adaptive telescopes, in which the curvature of the mirror surface and / or the distance of the telescope mirror are selectively changed, used. In known adaptive mirrors, the deformation of the mirror surface takes place, for example, by means of piezoelectric actuators (For example, DE 42 36 355 A1), which can be produced depending on the number used almost any mirror surfaces, or by changing the cooling water pressure of a cooled mirror (eg DE 39 00 467 A1), whereby a spherical surface change is generated, which is usually sufficient is to change the laser beam diameter. However, these known adaptive optics require additional optical components.

It is therefore an object of the present invention to provide a method and a device suitable for carrying out this method, with which the beam diameter of a laser beam can be changed without additional optical components.

This object is achieved according to the invention in the method mentioned that the temperature of the optical element is changed according to the desired beam diameter of the laser beam.

The idea of the present invention is to change the refractive index of an optical element or the spatial distribution of the refractive index and consequently the beam diameter of an incident laser beam in a targeted manner via the temperature of the optical element. The particular advantage of the invention is that no additional optical components (adaptive mirrors, adaptive telescopes) are required for beam diameter adjustment.

On the one hand, the invention can be used to specifically change the beam diameter of a laser beam as a function of a processing task. Another application is to counteract a focusing or expanding action of one or more optical elements to set a constant beam diameter of the laser beam.

The temperature change of the optical element can take place via a direct cooling or heating of the optical element or indirectly via a cooling or heating of a holder (for example optical support or frame) of the optical element. For example, the temperature of a liquid cooling medium can be changed. Also on the speed of the cooling medium can its cooling effect to be influenced. Other options are the cooling of the optical element or its support by eg Peltier elements or heating by eg electrical heating elements, such as a heating wire. Combinations of these methods are also possible.

In a preferred variant of the method, the temperature of the optical element is controlled manually or automatically, in particular as a function of the total switch-on duration of the laser beam. For example, a machine operator or service technician can perform a temperature decrease for a positive refractive index gradient optical element + dn / dT when the beam diameter of the laser beam has decreased due to aging or fouling of the outcoupling mirror, for optical elements having a negative refractive index gradient -dn / dT an increase in temperature occurs , Another simple option is an automatic temperature change depending on the total duty cycle of the

Laser beam. A new coupling-out mirror with a positive refractive index gradient + dn / dT, eg of zinc selenide, could therefore be operated with eg 60 ^° C. Over time, the temperature is lowered and thus the change in diameter of the laser beam compensated by aging / pollution of the coupling-out mirror. These two solutions, manual and automatic temperature change as a function of the total switch-on duration of the laser beam, offer the advantage of achieving a lifetime extension of the optical element, in particular of the coupling-out mirror, without additional sensors.

In another preferred variant of the method, the temperature of the optical element is regulated as a function of a desired value, in particular by the beam diameter of the laser beam at a specific location in the beam path of the laser beam or the intensity of the thermal radiation radiated by the optical element. The control can also take place as a function of certain process parameters of a laser processing, such as plasma temperature, or depending on the materials to be processed or material thicknesses. Advantageously, the required for temperature change heating or cooling power can be used to first an early warning and then an error message for the To provide cleaning or replacement of the optical element, in particular the Auskoppelspiegels.

According to the invention, the temperature change can be made over a large area or via a spatially resolved cooling or heating of one or more specific areas of the optical element. Preferably, the temperature of the optical element in the range between about 60 ⁰ C and about 10 ⁰ C is changed.

The invention also relates to a device suitable for carrying out the method according to the invention for varying the beam diameter of a laser beam with an optical element with temperature-dependent refractive index arranged in the beam path of the laser beam and transmissive or partially transmissive for the laser wavelength, wherein the optical element is in thermal contact with a cooling or heating element (Eg Peltier element or electric heating element) is, whose cooling or heating temperature is adjustable.

Preferably, the cooling or heating element is in thermal contact with a holder of the optical element, ie in the case of a Auskoppelspiegel with a peripheral mirror mount or a peripheral mirror support.

In a preferred embodiment, the device according to the invention has a control unit which adjusts the temperature of the optical element to different temperature values, in particular as a function of the total switch-on duration of the laser beam. In another preferred embodiment, a control unit is provided, which changes the temperature of the optical element as a function of a desired value, in particular depending on the beam diameter of the laser beam at a specific location in the beam path of the laser beam, the intensity of the heat radiation of the optical element or from a process or material parameter of a laser processing.

The transmissive or partially transmissive optical element is particularly preferably the output mirror of a laser resonator. Preferred coupling-out mirror material is zinc selenide; however, all other infrared transmissive materials (IR- Materials) possible, such as gallium arsenide.

In particularly preferred embodiments of the invention, the laser beam is an infrared laser beam, in particular a CO ₂ laser beam with a wavelength of 10.6 μm, and its power is greater than 1000 W.

Further advantages of the invention will become apparent from the description and the drawings. Likewise, the features mentioned above and the features listed further can be used individually or in combination in any combination. The embodiments shown and described are not to be understood as exhaustive enumeration, but rather have exemplary character for the description of the invention.

Show it:

1 shows a CCV gas laser with a folded laser resonator.

Rg. 2 a CCVGasIaser with an external beam guide and a

Processing head;

3 shows a first embodiment of the device according to the invention with a heated mirror socket of the coupling-out mirror;

Fig. 4 shows a second embodiment of the device according to the invention with a cooled by means of a Peltier element mirror frame of

output mirror;

5 shows the second embodiment of the device according to the invention with a device for temperature monitoring of the coupling-out mirror; and FIG. 6 shows a third embodiment of the device according to the invention with a mirror frame of the mirror cooled by a mirror support

Output mirror.

The CCVGasIaser 1 shown in Fig. 1 comprises a square-folded laser resonator 2 with four adjoining laser discharge tubes 3, which are connected to each other via corner housing 4, 5. One in the direction of Axes of the laser discharge tubes 3 extending laser beam 6 is shown in phantom. Deflection mirror 7 in the corner housings 4 are used to deflect the laser beam 6 by 90 °. In the corner housing 5, a rearview mirror 8 and a Auskoppelspiegel 9 partially transmissive for the laser wavelength are arranged. The rearview mirror 8 is designed to be highly reflective for the laser wavelength and reflects the laser beam 6 by 180 °, so that the laser discharge tubes 3 are traversed again in the opposite direction. A part of the laser beam 6 is coupled out of the laser resonator 2 at the partially transmissive output mirror 9, the other reflected part remains in the laser resonator 2 and passes through the laser discharge tubes 3 again. The decoupled from the laser resonator 2 via the Auskoppelspiegel 9 laser beam is denoted by 10. In the center of the folded laser resonator 2 is arranged as a pressure source for laser gas, a radial fan 11, which is connected via supply lines 12 for laser gas with the corner housings 4, 5 in combination. Suction lines 13 extend between suction boxes 14 and the radial fan 11. The direction of flow of the laser gas inside the laser discharge tubes 3 and in the supply and suction lines 12, 13 is illustrated by arrows. The excitation of the laser gas via electrodes 15, which are arranged adjacent to the laser discharge tubes 3.

Thus, the decoupled from the laser resonator 2 laser beam 10 as

Machining tool can be used, the laser beam 10 is guided as shown in Fig. 2 in an external beam guide 16 via reflective and transmissive optical elements, such as mirrors and lenses, from the laser resonator 2 to a processing head 17. The decoupled laser beam 10 is expanded by means of two lenses 18, 19, which form a beam telescope 20, to a desired beam diameter and deflected by a deflection mirror 21 to the processing head 17. The processing head 17 includes a focusing lens 22 which focuses the laser beam to a required beam diameter for processing.

Fig. 3 shows a device 30 for changing the Strahidurchmessers a decoupled from the Laεerresonator 2 via the Auskoppelspiegel 9 laser beam 10. The Auskoppelspiegel 9 is mounted in a mirror mount (holder) 23, which in turn is mounted on a cooling water cooled mirror carrier 24 so that at Auskoppelspiegel 9 forms a temperature gradient from the inside out. The device 30 has an electrical heating element 31 provided on the mirror mount 23 and a control unit 32 which changes the heating power of the heating element 31 as a function of the beam diameter of the coupled-out laser beam 10 measured with a sensor 33. In the exemplary embodiment shown, mirror mount 23 and mirror mount 24 are thermally insulated from one another by an insulator 34 provided therebetween.

Zinc selenide, which is the preferred material for partially transmissive outcoupling mirrors, and gallium arsenide have temperature-dependent thermal conductivities and refractive indices that cause the refractive power of the outcoupling mirror 9 and thus the focusing properties to change with temperature. Specifically, at high temperatures, the thermal conductivity decreases, resulting in steeper temperature gradients with increasing temperatures because the heat dissipates less. The steeper temperature gradient leads due to the positive refractive index gradient + dn / dT for zinc selenide and gallium arsenide to increased refractive power and thus to a different propagation of the laser beam behind the Auskoppelspiegβl compared to a cold Auskoppelspiegel. A focusing outcoupling mirror generates a laser beam path with a beam waist (smallest beam diameter) in the focus of the outcoupling mirror, behind the beam waist the beam diameter increases. Heating at the focusing outcoupling mirror causes the beam diameter in the near field (region from the outcoupling mirror to the waist of the beam and shortly after it) to decrease, in the far field, i. at intervals which are large against the focal length of the coupling-out mirror, the beam diameter increases in relation to a cold coupling-out mirror.

With the aid of the device 30, the refractive index of the coupling-out mirror 9 or the spatial distribution of the refractive index and consequently the beam diameter of the laser beam 10 are varied in a targeted manner via the temperature or the temperature gradient of the coupling-out mirror 9. Based on the measured with the sensor 33 beam diameter, the control unit 32, the temperature of the Auskoppelspiegels 9 according to the desired beam diameter. In experiments on a 5 kW CO ₂ laser with a Coupling mirror of zinc selenide, a change in diameter of 4% per 15 ⁰ C was determined. A 25 mm large laser beam 10 is thus 1 mm smaller, when the temperature of the mirror mount 23 increases by 15 ⁰ C.

FIG. 4 shows another device 40 for changing the beam diameter of the laser beam 10 coupled out via the outcoupling mirror 9. The device 40 has a cooling element (eg Peltier element) 41 arranged between mirror mount 23 and mirror support 24, the cooling power of which is determined by a control unit 42, for example Total switch-on of the laser beam 10 is changed automatically. The Peltier element 41 requires on the one hand a reference heat sink that keeps this page at a fixed temperature, for example, the mirror support 24, which is flowed through with laser cooling water (eg to 25 ⁰ C). The temperature difference of the Peltier element 41 to this reference is determined by the current flow through the Peltier element 41. In principle, temperatures above and below the reference temperature can be generated. In the simplest case, there is a linear change (decrease in + dn / dT and increase in -dn / dT) of the temperature over time, in order to counteract aging and / or contamination and the associated heating of the coupling mirror 9. Thus, a new Auskoppelspiegel 9 could be operated from zinc selenide with eg 60 ⁰ C and then the temperature in the course of a year to eg 25 ⁰ C are lowered.

FIG. 5 shows a device 50 for changing the beam diameter of the coupled-out laser beam 10 analogously to FIG. 4 with a cooling element 51 arranged between mirror mount 23 and mirror mount 24, the device 50 being combined with a device 52 for temperature monitoring of the device

Auskoppelspiegels 9 is operated. The temperature monitoring device 52 comprises a e.g. formed as a photodiode or pyrometer temperature sensor 53, which is integrated on the laser resonator 2 side facing away from the Auskoppelspiegels 9 in the mirror frame 23. The temperature sensor 53 detects the radiated heat from the coupling-out mirror 9 whose intensity I of the

Temperature T of the Auskoppelspiegel 9 (I ~ T ⁴ ), so that the temperature of the Auskoppelspiegel 9 is measured on the intensity of the heat radiation. The measured intensity is applied to a device 54, eg a microprocessor, for measuring the measured intensity with a stored reference intensity to compare. As soon as the measured intensity of the heat radiation deviates from the reference intensity by a previously set value, the device 54, which is designed as a control unit, changes the cooling capacity of the cooling element 51 in order to change the temperature of the outcoupling mirror 9. The temperature change of the Auskoppelspiegels 9 can be made gradually until a previously set limit is exceeded, in which the laser 1 is turned off and an exchange of Auskoppelspiegels 9 must be made.

FIG. 6 shows a further device 60 for changing the beam diameter of the laser beam 10 coupled out via the outcoupling mirror 9

Mirror mount 23 directly on the mirror support 24, so is in direct thermal contact with the mirror support 24. About a change in temperature of the mirror carrier 24 by flowing cooling water (flow arrows 61) in the temperature range between eg 60 ⁰ C and 10 ⁰ C, the temperature of the Auskoppelspiegel 9 changed and thus the beam diameter of the decoupled laser beam 10 can be adjusted.

The control unit 42 is disclosed in FIG. 4 for the device 40 together with a cooling element 41 which is arranged between the mirror mount 23 and the mirror mount 24. Of course, the control unit 42 can also be operated with the further devices 30, 50 and 60 according to the invention. The same applies to the control unit 32, 54, which is disclosed in FIGS. 3 and 5 for the devices 30 and 50 and is also applicable together with the devices 40 and 60.

The devices 30, 40, 50, 60 according to the invention for changing the beam diameter of a laser beam are shown in FIGS. 3 to 6 for the partially transmissive coupling-out mirror 9 of a stable laser resonator 2. Alternatively, the beam diameter of a laser beam may be varied over the temperature of other transmissive or partially transmissive optical elements in the external beam guide 16 or in the processing head 17 for the wavelength of the laser beam, e.g. via the lenses 18, 19 of the beam telescope 20 or the focusing lens 22 in the machining head 17.

Claims

Patent claims

A method of varying the beam diameter of a laser beam (10) passing through an optical element (9; 18, 19, 22), wherein the optical element (9; 18, 19, 22) is transmissive to the wavelength of the laser beam (10) is partially transmissive and has a temperature-dependent refractive index, characterized in that the temperature of the optical element (9; 18, 19, 22) is changed according to the desired beam diameter of the laser beam (10).

2. The method according to claim 1, characterized in that the temperature change of the optical element (9; 18, 19, 22) directly via a cooling or heating of the optical element (9; 18, 19, 22) or indirectly via a cooling or heating a holder (23) of the optical element (9; 18, 19, 22) takes place.

3. The method according to claim 1 or 2, characterized in that the temperature of the optical element (9; 18, 19, 22), in particular as a function of the total duty cycle of the laser beam (10), is controlled manually or automatically.

4. The method according to claim 1 or 2, characterized in that the temperature of the optical element (9; 18, 19, 22) in dependence on a desired value, in particular the beam diameter of the laser beam (10) at a certain location in the beam path of the laser beam ( 10), from the

Intensity of the thermal radiation of the optical element (9; 18, 19, 22) or by a process or material parameter of a laser processing, is regulated.

5. The method according to any one of the preceding claims, characterized in that the temperature change of the optical element (9; 18, 19, 22) via a spatially resolved cooling or heating of one or more portions of the optical element (9; 18, 19, 22) ,

6. The method according to any one of the preceding claims, characterized in that the temperature of the optical element (9; 18, 19, 22) in the range between about 60 ⁰ C and about 10 ⁰ C is changed.

7. Device (30; 40; 50; 60) for changing the beam diameter of a

A laser beam (10) having an optical element (9, 18, 19, 22) arranged in the beam path of the laser beam (10) is transmissive or partially transmissive for the wavelength of the laser beam (10) and has a temperature-dependent refractive index, the optical beam Element (9; 18, 19, 22) in thermal contact with a cooling or heating element

(24; 31; 41; 51) whose cooling or heating temperature is adjustable.

8. Device (30; 40; 50; 60) according to claim 7, characterized in that the cooling or heating element (24; 31; 41; 51) in thermal contact with a particular edge-side holder (23) of the optical element

(9, 18, 19, 22).

9. Device (30; 40; 50; 60) according to claim 8, characterized by a mirror carrier (24), which is cooled in particular with cooling water, wherein the Hatterung (23) of the optical element (9; 18, 19, 22) to the mirror carrier

(24) is attached.

10. Device (30) according to claim 9, characterized by a on the holder (23) provided heating element (31) and an insulator (34), the holder (23) of the mirror carrier (24) thermally insulated.

1 1. Device (40; 50) according to claim 9, characterized by a cooling element (41; 51) arranged between the holder (23) and the mirror carrier (24).

12. Device (60) according to claim 9, characterized in that the mirror carrier (24) is designed as a cooling or heating element.

13. Device (30; 40; 50; 60) according to claim 7 to 12, characterized by a control unit (42), which determines the temperature of the optical element (9; 18, 19, 22), in particular as a function of the total switch-on duration of the laser beam (10), to different temperature values.

14. Device (30; 40; 50; 60) according to claim 7 to 12, characterized by a control unit (32; 54), which determines the temperature of the optical element (9; 18, 19, 22) in dependence on a desired value, in particular the beam diameter of the laser beam (10) at a specific location in the beam path of the laser beam (10), the intensity of the thermal radiation of the optical element (9; 18, 19, 22) or of a process or

Material parameters of a laser processing, changed.

15. Device (30; 40; 50; 60) according to one of claims 7 to 14, characterized in that the transmissive or partially transmissive optical element (9) is the outcoupling mirror of a laser resonator (2).

16. Device (30; 40; 50; 60) according to one of claims 7 to 15, characterized in that the laser beam (10) is an infrared laser beam, in particular a CCV laser beam with a wavelength of 10.6 μm.

17. Device (30; 40; 50; 60) according to one of claims 7 to 16, characterized in that the power of the laser beam (10) is greater than 1000 W.