WO2009083164A1

WO2009083164A1 - Grating mirror for the online monitoring of a laser beam, and monitoring device comprising the same

Info

Publication number: WO2009083164A1
Application number: PCT/EP2008/010814
Authority: WO
Inventors: Joachim Schulz
Original assignee: Trumpf Laser- Und Systemtechnik Gmbh
Priority date: 2007-12-21
Filing date: 2008-12-18
Publication date: 2009-07-09
Also published as: DE102007062825A1

Abstract

The invention relates to a grating mirror (61) for the online monitoring of a laser beam (34). The grating mirror (61) has a variable local grating period and a variable local angle of grating lines (62) that are selected such that the grating mirror (61) focuses a laser beam (39) diffracted at a higher diffraction order (m=-1) onto at least one focal point (64). The grating mirror (61) is preferably utilized as part of a monitoring device (60) for the online monitoring of the laser beam (34).

Description

Grid for online monitoring of a laser beam and monitoring device with it

The present invention relates to a grating mirror for on-line monitoring of a laser beam, a monitoring device for on-line monitoring of a laser beam with such a grating mirror, and a laser resonator and a beam guide with such a monitoring device. Monitoring devices for the online monitoring of laser beams are variously known. For example, Spiricon offers monitoring devices for CO ₂ and solid-state lasers for analyzing and monitoring laser beams, in which a portion of the laser beam is coupled out of the processing laser beam via a partially transmissive beam splitter and fed to a detector device. The monitoring device comprises a focusing lens, deflecting mirror, filter and a camera.

The problem with the use of (semi-) transmissive materials such as ZnSe, GaAs, Ge, ZnS or Si as substrates for decoupling a portion of the laser beam is that the decoupled laser radiation passes through the substrate material, whereby a cooling of the optical element not can be done over the entire surface of the back and therefore must be along the circumference. In addition, most of the above materials have poor thermal conductivity, which, together with the altered cooling, results in greater susceptibility to destruction by fouling of the optical element used for decoupling. Furthermore, the focusing or imaging of the decoupled laser beam, if this is required, usually only be achieved by a further optical element.

German Patent DE 101 58 859 of the University of Applied Sciences Hildesheim / Holzminden / Göttingen discloses a method and a device for analyzing and monitoring the intensity distribution of a laser beam over the laser beam cross section. The monitoring method is based on the idea that sets in a laser mirror, which deflects a laser beam, due to the different intensities a corresponding temperature and heat distribution and, consequently, a local thermal expansion in the laser mirror, which can be detected interferometrically. The monitoring device comprises a laser mirror, a measuring light source, a beam splitter, a reference plate and a camera. The measuring light beam passes without deflection through the beam splitter and passes through the reference plate on the mirror surface. The measuring light beam is reflected in itself and forms an interference pattern between the mirror surface and a reference surface, which is observed with the camera. German patent application DE 101 49 823 describes a laser beam analyzer for detecting the intensity distribution and / or the position of a laser beam reflected by a deflection mirror. In this case, a detector arrangement intercepts scattered light coming from scattering centers in the mirror surface outside the direction of reflection in order to analyze its distribution.

Object of the invention

It is the object of the present invention to provide a grating mirror, a monitoring device, a laser resonator and a beam guide, with which the decoupling and focusing of a laser beam for on-line monitoring by means of a single optical element can be carried out.

Subject of the invention

This object is achieved by a grating mirror having a local grating period and a local orientation of the grating lines, which are each selected such that the grating mirror focuses a deflected into a higher diffraction order laser beam to at least one focal point. The diffraction in the higher diffraction order is carried out at a different diffraction angle than the zeroth order of diffraction, which corresponds to the reflected laser beam at deflecting mirrors without grating lines and is also referred to as a reflected laser beam, so that the diffracted into the higher diffraction order laser beam to a detector shown reduced or (Partially) can be focused without disturbing the beam path of the reflected laser beam. Focussing the diffracted laser beam of a higher diffraction order also enables the use of detectors with a small detector area, even for laser beams with large beam cross-sections, whereby costs can be saved. Furthermore, the same optical element can be used to decouple a measurement beam and to focus the measurement beam, so that additional focusing or imaging optics can be dispensed with. The intensity of the decoupled partial beam or the ratio between the intensities of the reflected and the diffracted into the higher diffraction order laser beam can in this case on the choice of the shape of the grid lines (flank angle, Duty cycle, grid depth) and the polarization of the laser beam can be adjusted.

If the orientation of the grid lines is not parallel, but varies locally over the surface of the grid mirror, the direction of the coupled-out sub-beam at each point of the grid mirror can be chosen differently. With a concentric grid arrangement (with center in the deflection plane), the laser beam can be focused (or defocused) perpendicular to the deflection plane. With additional choice of a suitably adapted, locally different grating period, the laser beam can also be focused (or defocused) in the deflection plane. In particular, these two foci are initially independent, so that a combination can be found in which the diffracted into the higher diffraction order laser beam is directed to a common focus point. Alternatively, it is also possible to focus the laser beam astigmatically on two different focal points in such a way that the elliptically diffracted laser beam is imaged or (partially) focused circularly onto a predetermined plane.

In a preferred embodiment, the wavelength and the angle of incidence of the laser beam and the local grating period of the grating mirror are selected such that only a diffracted into a higher diffraction order laser beam, in particular the diffracted in the minus first diffraction order laser beam occurs in addition to the reflected laser beam of zeroth diffraction order. This higher diffraction order is usually the (positive or negative) first diffraction order. Laser power that does not occur in the zeroth or first diffraction order reduces the power of the laser beam reflected in the zeroth diffraction order, without it being useful to use. Therefore, it is convenient to set the above parameters according to the diffraction equation such that only the zeroth and (minus) first diffraction orders exist.

In an advantageous embodiment, the local grating period and the local angle of the grating lines are each chosen such that the diffracted into the minus first diffraction order laser beam is focused on the at least one focal point. Since, due to physical reasons, the intensity of the diffracted laser radiation decreases rapidly with increasing diffraction order, it is advantageous to select the lowest possible order of diffraction for decoupling. For the extraction of the minus First diffraction order can be found here a suitable local orientation of the grid lines in a particularly simple manner.

In a preferred embodiment, the grid lines are formed as circles, which are arranged concentrically around a center. In this case, by suitable variation of the grating period in the radial direction, the focusing of the diffracted into a higher diffraction order laser beam to a focal point. The use of a radially symmetric grating mirror is particularly advantageous because it is easy to manufacture. In this case, the grid lines can be made in a conventional mirror by etching into the substrate or into the dielectric layers of a reflection-enhancing multilayer coating. In particular, the grating mirror can be made by off-axis diamond turning, which automatically creates a concentric structure. Since only a small portion of the laser beam is decoupled for online monitoring, the required structure depth is easy to produce.

In a preferred embodiment, the center of the concentrically arranged grid lines is arranged at a distance from the intersection of the optical axis of the diffracted into the higher diffraction order laser beam with the grating mirror. As a rule, in the case of a grating mirror with concentric grating lines, the laser beam is aligned with its optical axis to the center of the grating lines. In the present case, the focusing of the laser beam diffracted into the higher diffraction order onto a focal point can be realized by the lateral offset of the optical axis.

In an advantageous development, the distance between the center of the concentrically arranged grid lines and the intersection of the optical axis deviates by a maximum of ± 10% from a distance given by: Y _c = m λ f / P ₀ , where P _{0 is} the grating period at the intersection, f the distance between the intersection and the focal point, λ the wavelength of the laser beam and m the diffraction order of the laser beam denote. By the above equation, the optimum distance between the center point and the intersection point for focusing the laser beam diffracted into the higher diffraction order onto the focal point is given. Small deviations from this optimal distance in the range of approx. ± 10% but can be tolerated.

In an alternative embodiment, the grid lines run along a family of curves fj (x) = Vj + A (yi) x ² + B (Vj) x ⁴ . For the determination of the optimal set of curves along which the grid lines should run, solutions of a nonlinear equation system must be approximated, which is done numerically by a development around the point x = 0. It has been shown that for larger deviations from this point above approach to determine the family of curves provides better results than the use of an approach concentric circular grid lines. It is understood that other approaches for determining the family of curves of the grid lines can be used, wherein the most suitable approach depends on parameters such as the aperture of the grating mirror. If the deviations that occur are too large for a selected approach, a different approach is chosen until a set of curves with acceptable deviations has been determined.

A further aspect of the invention is implemented in a monitoring device for online monitoring of a laser beam, comprising: a grating mirror as described above, a detector unit for detecting the diffracted into the higher diffraction order, focused on the at least one focal point laser beam, and an evaluation unit for evaluating the detected laser beam. Such a monitoring device can be realized in a compact design, since the grating mirror takes over the focusing or imaging of the diffracted into the higher diffraction order laser beam to the detector, so that can be dispensed with an additional focusing or imaging optics.

In an advantageous embodiment, the detector unit is arranged at a distance of less than 20% of the focal length of the grating mirror from the focal point. In this case, a detector unit with a small detector surface can be selected, which on the one hand reduces costs and on the other hand, a further reduction of the size of the monitoring device can be realized.

Preferably, the detector unit is designed as a spatially resolving detector, as a quadrant detector or as a power detector. The use of a local Detaching detector, such as an array detector or an IR camera, provides the ability to monitor the intensity distribution and thus the beam shape of the laser beam online. Also, it is alternatively possible to monitor by means of a quadrant detector only the pointing of the laser beam. The use of a power detector is possible, which does not provide spatially resolved information, but only detects the total intensity of the laser beam. From the measured intensity of the diffracted into the higher diffraction order laser beam, the evaluation unit can determine the total power of the laser beam, since the division of the intensity components to the zeroth and the higher diffraction order is known.

Another aspect of the invention is realized in a laser resonator with a monitoring device as described above. The monitoring device is typically integrated in the laser resonator.

One aspect of the invention is embodied in a beam guide for delivering a laser beam from a laser cavity to a laser processing head with a monitoring device as described above. In this case, a deflecting mirror of the beam guide can be exchanged for the grating mirror. It is understood that the use of the monitoring device is not limited to the beam guidance described above, but can be used advantageously in any beam guidance for a laser beam having at least one deflection mirror.

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 CO.sub.2 gas laser with a folded laser resonator, 2 is a CG ^ gas laser with an external beam guide and a processing head,

3 shows the diffraction behavior of a grating mirror with parallel grating lines,

FIGS. 4a-c the design of a grid mirror in several steps,

Fig. 5 shows a grating mirror according to the invention with concentric circular grid lines and

Fig. 6 shows a monitoring device according to the invention with a grating mirror, which is designed as a highly reflective deflection mirror.

1 shows a square folded laser resonator 1 of a CCV gas laser with eight discharge paths. A partially transmitting outcoupling mirror 2, for example, having a reflectance of 70% and a transmittance of 30%, and a highly reflective end mirror 3 define the laser resonator 1. A generated in the laser resonator 1 laser beam 4 is eight deflecting mirror 5 to 12 in two superimposed , parallel planes 13, 14 folded square, with four deflecting mirrors 5 to 8 in the upper level 13 and four further deflecting mirrors 9 to 12 are arranged in the lower level 14. The laser beam 4 is deflected by the end mirror 3 at the first three deflecting mirrors 5, 6 and 7 in the upper plane 13 by 90 °. The laser beam 4, which is deflected from the third deflection mirror 7 to the fourth deflection mirror 8 in the upper plane 13, is also deflected by this and then by the first deflection mirror 9 in the lower plane 14 by 90 °, but in one of the planes 13, 14, and passes from the upper level 13 into the lower level 14. The other three deflection mirrors 10, 11 and 12 deflect the laser beam 4 by 90 ° in the lower level 14. Coming from the fourth deflecting mirror 12 of the lower level 14, the laser beam 4 impinges on the partially transmitting outcoupling mirror 2, at which a part of the laser beam is coupled out of the laser resonator 1 as a coupled-out laser beam 15 and another part is reflected as a reflected laser beam 16 into the laser resonator 1. The reflected laser beam 16 hits in the reverse direction to the laser beam 4 on the eight deflecting mirrors 5 to 12 and passes through all the discharge paths between the output mirror 2 and the end mirror. 3

In order for the laser beam 15 coupled out from the laser resonator 1 to be used as a machining tool, the laser beam 15 is irradiated in an external beam guide 20 via reflecting, partially transmitting and transmitting optical elements, such as those shown in FIG. Mirror and lenses, led by the laser resonator 1 to a processing head 21, in which a focusing of the laser beam 15 takes place. The decoupled laser beam 15 is spread over a beam telescope 22, which is formed in Fig. 2 as a mirror telescope with two mirrors 23, 24, to a desired beam diameter and deflection mirrors 25a, 25b, which are formed in Fig. 2 as 90 ° -Umlenkspiegel , deflected to the processing head 21. The machining head 21 can be designed as a mirror head as shown in FIG. 2 and, in this case, comprises a focusing mirror 26 which focuses the laser beam 15 on a beam diameter required for machining. If necessary, further deflecting mirrors can be arranged in the external beam guide 20, which deflect the laser beam 15.

In order to monitor the laser beam 4 in the laser resonator 1 or the laser beam 15 coupled out of the laser resonator 1 with the aid of a resonator-internal deflection mirror 5 to 12 or a resonator mirror 25a, 25b external to the resonator, the deflection mirror is provided with grid lines which split the laser beam into a plurality of diffraction orders. A provided with grid lines mirror, which is formed, for example, as a deflection mirror 5-12, 25a, 25b, Auskoppelspiegel 2 or end mirror 3 is referred to as a grating mirror. Preferably, the grating mirror is formed as a highly reflective deflection mirror. For the purposes of this application, a highly reflective grating mirror is understood to be a grating mirror in which the reflected laser beam has a proportion of 95% or more of the intensity of the incident laser beam. Such a deflection mirror can replace a conventional deflection mirror in the laser resonator or in the beam guide in order to carry out an online monitoring of the laser beam. As a rule, the deflecting mirror causes a deflection of the laser beam by 90 °. Fig. 3 shows the diffraction behavior of a grating mirror 30 with parallel grating lines 31 and a constant grating period P, which is defined as the sum of the grating line width and the distance between two grating lines. The diffraction behavior of the grating mirror 30 is determined by the generally valid diffraction equation

P [sin (β _m ) - sin (Θ)] = m λ

described, which states that at a selected grating period P and given wavelength λ of the laser beam only a limited number of diffraction orders m occurs (m = 0, ± 1, ± 2, ...), which in space at different diffraction angles ß _m spread. The angle between the surface normal 32 of the grating mirror 30 and the optical axis 33 of the incident laser beam 34 as the angle of incidence Θ and the angle between the surface 35 of the grating mirror 30 and the optical axis 33 of the incident laser beam 34 are defined as the angle γ.

The optical axis 36 of the zero-order diffracted laser beam (m = 0), hereinafter referred to as the reflected laser beam 37, is at a diffraction angle βo corresponding to the incident angle θ. The optical axis 38 of the diffracted laser beam of the minus first diffraction order (m = -1) 39 extends at a diffraction angle β.i = aresin [-λ / P + sin (Θ)], which differs from the diffraction angle β _{0 of} the reflected laser beam 37 ,

In the following, with reference to FIGS. 4a-c shows step by step how to modify a grating mirror 30 according to FIG. 3, so that a higher diffraction order can be focused onto a focal point and the laser beam can be monitored with the aid of the grating mirror.

4a shows a deflection mirror 40, which initially has no grating lines, and an incident laser beam 34, which impinges on the deflection mirror 40 at an angle γ relative to the surface of the deflection mirror 40 and is reflected therefrom as a reflecting laser beam 37 at the angle γ. The surface of the deflection mirror 40 is defined as a mirror plane 41. As a deflection plane 42, the plane defined by the optical axis 33 of the incident laser beam 34th and the optical axis 36 of the reflected laser beam 37 is clamped. The coordinate system XYZ is oriented such that the mirror plane 41 represents the XY plane and the deflection plane 42 represents the YZ plane of the coordinate system XYZ. The optical axes 33, 36 of the incident laser beam 34 and the reflected laser beam 37 intersect at the coordinate origin (0, 0, 0).

The incident laser beam 34 has a spatial extent and consists of parallel partial beams which are arranged about the optical axis 33. All partial beams strike the deflection mirror 40 at the same angle γ, but each partial beam has different coordinates (X _s , Ys, 0) of an intersection point 43 with the mirror plane 41. So that the parallel partial beams of the incident laser beam 34 are focused on a common focal point 44 with the coordinates (XF, YF, ZF), the partial beams must be deflected differently. In FIG. 4a, the focal point 44 is arranged in the deflection plane 42, which forms the YZ plane, ie for the coordinates of the focus point 44 (0, Y _F , Z _F ). The equations which are derived below apply to the case where the focal point 44 lies in the deflection plane 42. If the focal point is outside the deflection plane 42, the X coordinate XF of the focal point 44 must additionally be taken into account in the calculations.

The direction of a respective deflected sub-beam 45 is defined by a straight line between the coordinates (X _s , Ys, 0) of the respective intersection point 43 with the mirror plane 41 and the coordinates (0, YF, ZF) of the common focus point 44. Starting from the intersection 43, the direction of the deflected sub-beam 45 can be described by two angles ω and τ, where the first angle ω is the angle between the deflected sub-beam 45 and a parallel to the Y-axis and the second angle τ is the angle between the deflected partial beam 45 and a parallel to the X-axis is defined. For the angles ω and τ, the following equations result as a function of the coordinates (Xs, Ys, 0) of the intersection point 43 and the coordinates (0, YF, ZF) of the focal point 44:

cos (180 ° - ω) = - cos (ω) = | Y _F - Y _s | / V [Xs ² + (YF - Ys) ² + Z _F ² ]

cos (180 ° - τ) = - cos (τ) = X _s / V [Xs ² + (YF - Ys) ² + Z _F ^2]. 4b shows the deflection mirror 40 according to FIG. 4a, in which three different intersection points 43a-c of parallel partial beams of the incident laser beam 34 with the mirror plane 41 are drawn. The lower intersection 43a has the coordinates (0, 0, 0), the middle intersection 43b the coordinates (X _s -ι, Ysi, 0) and the upper intersection 43c the coordinates (Xs ₂ , Ys2, 0). In order for the diffracted laser beam of the minus first diffraction order 39 to be focused on the focal point 44, the different partial beams 45a-c must be deflected in different directions. The directions of the partial beams 45a-c emanating from the intersections 43a-c and intersecting at the focal point 44 are defined by different angles ω and τ according to the above equations.

A grating mirror according to FIG. 3 with linear grating lines and constant grating period splits an incident laser beam 34 into a plurality of diffraction orders, which are diffracted in different directions. However, such a grating mirror can not focus a diffracted laser beam of a higher diffraction order, for example the diffracted laser beam of the minus first diffraction order 39, onto a focal point.

A grating mirror with linear grating lines and variable grating period produces a focus line which is perpendicular to the deflection plane 42. A grating mirror with curved grating lines, for example circular grating lines, and a constant grating period focuses a laser beam linearly in a plane parallel to the deflecting plane 42. Focusing the laser beam on a focal point 44 results from a combination of these two grating types, ie by curved grating lines with a variable grating period. The task is now, the deflection mirror 40 of Fign. 4a, b to be provided with curved grid lines such that the diffracted laser beam of the minus first diffraction order 39 is focused on the focal point 44.

A grid is local, ie at one point, defined by an angle α of the grid lines and the grating period P. The angle α is defined as the angle between the tangent to the respective grid line and a parallel to the Y-axis. Wanted are angles α and grating periods P, which are the partial beams 45a-c, starting from the Distract intersections 43a-c to the focus point 44. This yields the following equation of determination for the angle α and the grating period P:

cos (ω) = cos (γ) - m λ sin (α) / P cos (τ) = m λ cos (α) / P

For the three intersections 43a-c, local grid lines are shown in FIG. 4b, which are shown as linear grid lines with a constant grid period P. This approximation (linear grid lines, constant grid period) applies to a small area around the intersections 43a-c. In practice, these locally linear grating areas may result in too much rasterization and the focus on the focal point may be too coarse. In order to increase the resolution of the grating mirror, the object now is to find a family of curves of closed grating lines, which generates the local angles α and grating periods P shown at the intersections 43a-c. In addition, a grating mirror with closed grating lines can be produced more easily when manufacturing the grating mirror by means of lithography or diamond turning than grating mirrors with locally linear grating regions.

4c shows the deflection mirror 40 according to FIGS. 4a, b with closed grid lines 46. The grid lines 46 are represented mathematically as a family of curves fj (x), the index i indicating the individual functions of the family of curves as an integer value. For the family of curves fj (x) the condition f, (x) = tan (90 ° -α) holds. By solving the above equations cos (ω) = ... and cos (τ) = ... according to the angle α and the grating period P, we obtain the following system of equations for f -, (x) as a function of the angles γ, ω and τ:

f, (x) = tan (90 ° -α) sin (α) = V [1-P ² cos (τ) ² / (m ² λ ² )]

P = - m λ / V [cos (τ) ² + (cos (ω) - cos (γ)) ² ]

This system of equations can generally not be solved analytically, but must be solved by numerical calculation methods. For this, the solutions of the system of equations are developed around the point x = 0. As a solution to the grid lines 46 concentric circular grid lines can be approximated become. However, it turns out that the deviations increase with increasing distance from the point x = 0. If the deviations are too large for concentric circular grid lines, for example, the approach fj (x) = y-, + Ax ² + Bx ⁴ can be selected, the parameters

and

depend on the parameter V ₁ , which represents the intersection of fj (x = 0) with the y-axis. It is understood that the chosen approach for the set of curves f, (x) depends on various parameters, such as the aperture of the grating mirror. If necessary, ie if the deviations occurring are too large for a chosen approach, a different approach is chosen. It is an iterative process in which the approach to the family of curves fj (x) varies and the occurring deviations are calculated until a set of curves with acceptable deviations has been determined.

FIG. 5 shows a grating mirror 50, which is designed as a deflection mirror with circular grating lines 51, which are arranged concentrically around a common center 52. The center 52 of the concentric grid lines 51 has the coordinates (0, Yc, 0), where the Y coordinate Yc is given by:

Yc = m λ f / P _0,

where P ₀ = P (X = O, Y = 0, 0) the grating period at the origin of coordinates and f = V [YF ² + ZF ² ] the distance between the origin of coordinates (0, 0, 0) and the focus point 44 with the coordinates (0, YF, ZF).

For the grating period P of the concentric circular grating lines 51, the following applies due to the diffraction equation for the minus first diffraction order (m = -1):

P = -λ / [sin (β.i) -cos (γ)],

with ß.-i = arctan [tan (δ ₀ ) - Yi / (f cos (δ ₀ ))] and δ ₀ = arctan (-Y _F / Z _F ).

δ ₀ is hereby defined as the angle between the Z-axis and a straight line 53 that passes through the coordinate orientation (0, 0, 0) and the focal point 44. Y is ₁ defined as the intersection of the i-th grid line with the y-axis, ie as the distance of the ith grid line from the coordinate origin at the position x = 0.

For example, by off-axis diamond turning, the grid lines 51 can be generated on the surface of the grid mirror 50 so that the grid mirror 50 produces the desired focusing effect. Such a grating mirror 50 with grating lines 51 can be used in monitoring devices for the on-line monitoring of a laser beam, of which an embodiment in FIG. 6 is shown by way of example.

6 shows a monitoring device 60 according to the invention for the online monitoring of a laser beam with a highly reflecting deflecting mirror 61, which is provided with grid lines 62 and is referred to below as grid mirror 61. The grating mirror 61 can be arranged both in the laser resonator 1 of FIG. 1 for monitoring the intracavity laser beam 4 and in the external beam guide 20 of FIG. 2 for monitoring the decoupled laser beam 15 and replaces one of the deflecting mirrors 5 to 12 or 25a, 25b. The incident laser beam 34 falls at an angle of incidence Θ relative to the surface normal 63 of the grating mirror 61 or at an angle γ relative to the mirror plane 41 on the grating mirror 61, at which the laser beam 34 is decomposed into a plurality of diffraction orders _m diffracted in different directions ß _m become.

In Fig. 6, the laser beam 37 reflected in the zeroth diffraction order as the processing laser beam and the laser beam 39 diffracted in the minus first diffraction order appear as a measuring beam for on-line monitoring of the laser beam. With respect to the surface normal 63 of the grating mirror 61, the optical axis 36 of the laser beam 37 reflected in the zeroth diffraction order has a diffraction angle β ₀ which corresponds to the angle of incidence Θ between the optical axis 33 of the incident laser beam 34 and the surface normal 63. The diffracted in the minus first diffraction order laser beam is reflected at a diffraction angle ß.i, which is different from the angle of reflection Θ of the reflected laser beam 37. The diffraction angle β-i is also measured between the optical axis 38 of the diffracted laser beam of the minus first diffraction order 39 and the surface normal 63 of the grating mirror 61. For monitoring the laser beam, the diffracted laser beam 39 is focused onto a focal point 64. In the region of the focal point 64, ie at a distance of less than 20% of the distance between the focal point 64 and the intersection 65 of the optical axes 33, 36 of the incident laser beam 34 and the reflected laser beam 37, a detector unit 66 is arranged, on which the focused laser beam the minus first diffraction order 39 is displayed. The distance between the intersection point 65 of the optical axes 33, 36 and the focal point 64 defines the focal length f of the grating mirror 61. In an evaluation unit 67, the detected focused laser beam 39 is analyzed. Depending on the design of the detector unit 66, for example as a power meter, as a quadrant detector or as a spatially resolving detector, the laser power, pointing and / or the power density distribution of the laser beam 34 can be monitored.

In order to limit diffraction losses of the laser beam 34, the grating mirrors 40, 50, 61 according to FIGS. 4a-c, 5, 6 designed so that next to the reflected laser beam of zeroth diffraction order 37, which represents the processing laser beam, only a further diffraction order occurs, which by suitable choice of parameters in the diffraction equation (angle of incidence, grating period, wavelength of the laser beam) becomes. Laser power, which occurs in further higher diffraction orders, is the processing laser beam is not available. Rather, higher diffraction orders can lead to unwanted heating of the mirror mount and surrounding components. The grating mirrors 40, 50, 61 are therefore designed so that only the part which has to be decoupled from the laser beam 34 for on-line monitoring occurs in a higher diffraction order.

In summary, focusing the laser beam 39 diffracted into a higher diffraction order onto a focal point allows a compact realization of a monitoring device for online monitoring, the scope of use of which is of course not limited to the monitoring device described in FIG. It is understood that the monitoring device 60 shown in FIG. 6 can be connected to units for driving the laser resonator 1 or the beam guide 20 in order to regulate the laser power of the laser beam 34 or its distribution to a desired value.

Claims

claims

A grating mirror (40, 50, 61) for on-line monitoring of a laser beam (34), characterized in that the grating mirror (40, 50, 61) has a local grating period (P) and a local angle (α) of the grating lines (46 , 51, 62) which are each selected such that the grating mirror (40, 50, 61) focuses a laser beam (39) diffracted into a higher diffraction order onto at least one focal point (44, 64).

2. grating mirror according to claim 1, wherein the wavelength (λ) and the angle of incidence (Θ) of the laser beam (34) and the local grating period (P) of the grating mirror (40, 50, 61) are selected such that in addition to the reflected laser beam the zero order of diffraction (37) only a laser beam diffracted into a higher diffraction order, in particular of the laser beam (39) diffracted in the minus first diffraction order (m = -1) occurs.

3. grating mirror according to claim 1 or 2, wherein the local grating period (P) and the local angle (α) of the grid lines (46, 51, 62) are each selected such that the diffracted into the minus first diffraction order laser beam (39) is focused on the at least one focal point (44, 64).

4. Grid mirror according to one of the preceding claims, wherein the grid lines (51) are arranged as concentric circles about a center (52).

5. grating mirror according to claim 4, wherein the center (52) of the concentrically arranged grid lines (51) at a distance from the intersection (65) of the optical axis (38) of the diffracted into a higher diffraction order laser beam (39) with the grating mirror (50 ) is arranged.

A grating mirror according to claim 5, wherein the distance between the center (52) of the concentrically arranged grating lines (51) and the intersection (65) deviates by at most ± 10% from a distance (Yc) given by: Y _c = m λ f / Po, where P _{0 is} the grating period at the intersection (65), f is the distance between the intersection (65) and the focal point (44), λ is the wavelength of the laser beam (39) and m is the diffraction order of the laser beam (39 ) describe.

7. grating mirror according to one of claims 1 to 3, wherein the grid lines (46) along a family of curves fj (x) = yi + A (Vj) x ² + B (yι) x ⁴ run.

8. A monitoring device (60) for online monitoring of a laser beam (34), comprising: a grating mirror (40, 50, 61) according to any one of the preceding claims, a detector unit (66) for detecting the diffracted in the higher diffraction order, the at least a focal point (44, 64) focused laser beam (39), and an evaluation unit (67) for evaluating the detected, focused laser beam (39).

9. Monitoring device according to claim 8, wherein the detector unit (66) at a distance of less than 20% of the focal length (f) of the grating mirror (40, 50, 61) from the focal point (44, 64) is arranged.

10. Monitoring device according to one of claims 8 or 9, wherein the detector unit (66) is designed as a spatially resolving detector, as a quadrant detector or as a power detector.

11. Laser resonator (1) with a monitoring device (60) according to one of claims 8 to 10.

12. beam guide (20) for supplying a laser beam (15) from a laser resonator (1) to a machining head (21) with a monitoring device (60) according to any one of claims 8 to 10.