WO2017080540A1

WO2017080540A1 - Beam analysis device and method using a variable optical element

Info

Publication number: WO2017080540A1
Application number: PCT/DE2016/000384
Authority: WO
Inventors: Reinhard Kramer; Otto MÄRTEN; Stefan Wolf; Roman Niedrig
Original assignee: Primes Gmbh
Priority date: 2015-11-09
Filing date: 2016-10-31
Publication date: 2017-05-18
Also published as: DE102015014387B3

Abstract

The invention relates to a device for analyzing light beams, said device allowing a quick and precise detection of geometric parameters of a light beam. For example, the beam parameter product or the beam propagation factor of a laser beam can be determined. The device comprises a variable optical element, an objective, and a spatially resolving detector. The variable optical element has an adjustable focal length and an image-side main surface, and the objective has a constant focal length and an object-side main surface. The distance between the image-side main surface of the variable optical element and the object-side main surface of the objective equals the constant focal width of the objective with a deviation of maximally +/- 5%. The objective is arranged downstream of the variable optical element in the beam direction, and the spatially resolving detector is arranged downstream of the objective in the beam direction. By changing the adjustable focal length of the variable optical element and by subsequently focusing the light beam through the objective, a focal point of the focused light beam can be variably adjusted in the axial direction with respect to the spatially resolving detector. The invention also relates to a method for the quick and precise detection of geometric parameters of a light beam.

Description

Title: Apparatus and method for beam analysis with a variable optical element

Applicant: PRIMES GmbH, Max-Planck-Str. 2, 64319 Pfungstadt

Our sign: PRI-2016-W-3

DESCRIPTION

FIELD OF THE INVENTION

The invention relates to an apparatus and a method for beam analysis of light rays. The invention is suitable for the rapid and accurate determination of geometric parameters such as the beam diameter, the beam parameter product or the beam propagation factor. The apparatus and method can be used for beam analysis of laser beams. BACKGROUND OF THE INVENTION

Geometric parameters of a light beam or a laser beam are important parameters for characterizing the beam. Such parameters may be, for example, the beam diameter, the beam profile, or the beam parameter product. The beam parameter product describes the product of radius of the beam waist and opening angle of the beam and is therefore a measure of the focusability of a light beam or laser beam. Other measures or terms for the same thing are the beam quality, the beam quality index, the beam propagation factor, the mode factor, or the diffraction metric. Beam parameters must be measured at regular time intervals for quality control in many production processes using light beams. The definitions and mathematical relationships for the determination of geometric parameters of a light beam are described in ISO 11146. For a complete determination of a beam which also includes the propagation properties, scanning of the beam in multiple planes along the beam is required. The most accurate results are expected when the beam is scanned over a distance of several Rayleigh lengths in the region of its beam waist.

For sampling the intensity distribution in a cross-sectional plane of the light beam, various methods are known. A basic possibility for measurement exists for example, to direct the beam directly or indirectly at a spatially resolving sensor or detector, for example a CCD camera, and in this way to determine the intensity distribution in the cross section of the beam. From this data, further information such as the beam diameter, the beam profile or the position of the beam can be derived or calculated. Another possibility is to scan the intensity distribution in a plane with an approximately punctiform detector in a raster motion, eg line by line.

The use of a spatially resolving sensor has the advantage over the raster scanning that the recording of the intensity distribution in a plane only requires a very short measuring time. A short measurement time is important if the intensity distribution in several different planes is to be scanned in order to determine the total beam parameter or the beam propagation factor. For scanning in several levels, in turn, various methods and devices are known. For example, the spatially resolving sensor can be placed on a linear guide, so that the scanning unit can be moved in the axial direction along the beam. In many cases, the beam waist of the light or laser beam is not directly accessible, or the beam is collimated, so that the Rayleigh length of the beam is very large and scanning across several Rayleigh lengths around the beam waist does not possible or impractical. In these cases, it is customary to first focus the beam by means of a lens or by means of a lens and to position the linear guide with the sensor in the focus area behind the lens. Although the scanning of the beam and subsequent determination of the geometric parameters of the beam then delivers the image-side beam parameters, these can be converted into the object-side parameters via the imaging equations of the lens. The conversion of the beam parameters can be simplified in that not only the sensor is placed on a linear guide, but also the focusing lens is arranged together with the sensor on the same Linearfuhrung, so that the distance between the lens and the sensor is constant, and for scanning the beam, the lens and the sensor are displaced axially relative to the light beam. In this case will in a sense, the object-side plane conjugate to the image-side sensor plane is virtually driven through the original beam.

A disadvantage of all of the aforementioned systems and methods is that a precise and mechanically complex linear guide is needed and relatively large masses must be moved. Thus, the positioning time for setting different measurement levels can not be arbitrarily reduced.

To solve the problem, a device is proposed in WO 2011/127400 A2, in which the light beam to be measured is divided by means of partially transmissive mirror into a plurality of parallel partial beams, each with different path lengths laterally offset hit the same sensor, so that a simultaneous recording multiple beam cross sections is possible. A disadvantage of this device is that the individual beam cross sections share the same sensor and therefore the area available for a single beam cross section and thus the pixel resolution is reduced by a considerable factor.

Another solution to the problem is disclosed in US 8,736,827 B2. The device shown there consists of a variable lens and a sensor. For the variable lens, for example, an electro-optical lens or a pressure-controllable fluid lens is proposed, whose focal length can be set variably. The moving mass when changing the focal length of the variable lens is very small. In this way, the image-side beam waist can be adjusted very quickly axially and beam cross sections in different planes can be quickly taken in a row. The disadvantage is that due to the variable focal length, the image-side parameters of the beam depend on the focal length and thus also be changed. An evaluation of the data according to the formulas defined in ISO 11146 is therefore not directly possible. Rather, a modified formula must be used for the evaluation (compare second formula in column 4 of US Pat. No. 8,736,827 B2). Because of the variable focal length, a relatively complex calibration of the device is required. This limits the accuracy of the method.

For variably adjustable lenses, the adjustment range of the focal length is limited by design and set to a specific range. The range required to pass through multiple Rayleigh lengths may vary depending on the parameters of the one being measured Beam, be greater than the focal length adjustment range. Conversely, it is also possible that the Rayleigh length of the focused beam is very short, and therefore the required adjustment range is very short, but a high accuracy and reproducibility of the focal length adjustment in a very small area is required. In both cases, the usability of the device disclosed in US Pat. No. 8,736,827 B2 can be clearly restricted.

The known from the prior art devices and methods therefore have significant disadvantages in terms of resolution, accuracy, speed or with respect to the usability for a large parameter range.

BRIEF DESCRIPTION OF THE INVENTION

The invention is therefore an object of the invention to provide a method and apparatus for beam analysis, in which several different cross-sectional planes of a light beam can be measured in a very short time, and allow a determination of beam parameters of the light beam with high accuracy ,

To achieve the object, an apparatus for determining geometric parameters of a light beam is proposed, which comprises a variable optical element, a lens, and a spatially resolving detector. At this time, the variable optical element has an adjustable focal length and an image-side major surface, and the objective has a constant focal length and an object-side major surface. The distance between the image-side major surface of the variable optical element and the object-side main surface of the objective is equal to the constant focal length of the objective with a deviation of at most +/- 5%. The objective is connected downstream of the variable optical element in the beam direction. The spatially resolving detector is connected downstream of the objective in the beam direction. By changing the adjustable focal length of the variable optical element and by subsequently focusing the light beam through the objective, a focus position of the focused light beam relative to the spatially resolving detector is variably adjustable in the axial direction.

There is provided an embodiment of the device in which the relative positions of the light beam, the variable optical element, the lens and the spatially resolving detector are stationary relative to each other. In one possible embodiment of the invention, the total focal length of the system consisting of the variable optical element and the lens is equal to the constant focal length of the lens with a maximum deviation of +/- 5%.

There is also provided an embodiment of the device in which the variable optical element comprises a fluid lens.

In a further embodiment of the invention, the variable optical element comprises an adaptive lens.

In yet another embodiment of the invention, the variable optical element comprises an adaptive mirror. In a possible embodiment of the device, a lens for divergence adjustment in the beam direction is arranged in front of the variable optical element.

An embodiment of the device according to the invention is provided in which the objective comprises a first lens group and a second lens group.

In a possible embodiment of the invention with an objective comprising a first lens group and a second lens group, the first lens group of the objective may have a negative refractive power, and the second lens group of the objective may have a positive refractive power, and the constant focal length of the objective Lens has a positive overall value.

To solve the problem, a method for the determination of geometric parameters of a light beam is proposed, which comprises the method steps listed below. An opening angle of the light beam is changed by means of a variable optical element having an adjustable focal length and an image-side major surface. The light beam changed by the variable optical element with respect to the opening angle is focused by means of a lens having a constant focal length and an object-side major surface. The adjustable focal length of the variable optical element is changed. At least three are successively set different focal lengths. Intensity distributions of the light beam focused by the lens are registered by means of a spatially resolving detector, which is arranged in the beam direction behind the objective. At each of the at least three different set focal lengths, an intensity distribution is registered in each case. Finally, a geometric parameter of the light beam is determined from the registered intensity distributions. Here, the distance between the image-side major surface of the variable optical element and the object-side major surface of the objective is equal to the constant focal length of the objective with a deviation of at most +/- 5%.

There is also provided a method in which the relative positions of the light beam, the variable optical element, the objective and the spatially resolving detector are mutually stationary.

In another possible method, determining a geometric parameter of the light beam from the registered intensity distributions comprises determining a beam propagation factor of the light beam.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be described in more detail with reference to the following figures, without being limited to the embodiments shown. It shows:

Figure 1: A known from the prior art device for

Beam analysis of a light beam with a variable lens with adjustable focal length, by means of which a beam is focused and an axially adjustable focus position is generated, and with a sensor which is arranged in the region of the adjustable focus position.

Figure 2: A schematic representation of a first embodiment of

Invention with a variable optical element, with a lens, which is arranged behind the variable optical element, and with a spatially resolving detector. In this embodiment, the lens has a relatively short focal length. Figure 3: A schematic representation of an embodiment of the invention similar to the embodiment shown in Figure 2. In this embodiment, the lens has a relatively long focal length. The focus adjustment range is only partially in the real image area of the lens.

Figure 4: A schematic representation of another embodiment of the

Invention similar to the embodiment shown in Figure 3. The light beam is not collimated in this example, but has a focus or beam waist in front of the device and then spreads divergently. The focus adjustment range can be completely in the real image area of the lens.

Figure 5: A schematic representation of a second embodiment of the

Invention in which a lens for divergence adjustment in the beam direction is arranged in front of the variable optical element. The lens for divergence adjustment has a negative refractive power in this embodiment. This moves the focus adjustment range backwards.

Figure 6: A schematic representation of another embodiment of the

Invention. In this embodiment, the lens comprises a first one

Lens group and a second lens group, which together form a lens in retrofocus construction, which has a rearwardly displaced main surface and thus the focus adjustment range is shifted to the rear. Figure 7: A schematic representation of an embodiment of the second

Embodiment of the invention, wherein a lens for divergence adjustment in the beam direction in front of the variable optical element is arranged. In the embodiment shown here, the lens for divergence adjustment has a positive refractive power and allows the analysis of rays that are not collimated or have a focus or a beam waist relatively close to the device.

Figure 8: A schematic representation of a third embodiment of

Invention in which the original light beam is transmitted by means of a beam Attenuator device is attenuated. By way of example, two beam splitters are shown for this purpose, each of which reflects a small part of the power of the light beam and in each case transmits a larger part of the power of the light beam. The transmitted portion of the power of the light beam is absorbed by absorbers or beam traps.

Figure 9: representation for determining the position of the main surfaces of a composite of two lens groups lens in retrofocus construction. DETAILED DESCRIPTION OF THE FIGURES

Figure 1 shows a schematic representation of a known from the prior art apparatus for beam analysis with a variable optical element 20, for example a variable lens LV, and with a spatially resolving detector 50. The light beam 10 to be measured by the variable lens LV focused. The focal length of the variable lens LV is adjustable in a range between a minimum focal length fLv, A and a maximum focal length fLv.c- In the example shown in Figure 1, the light beam 10 to be measured is collimated, ie the lines representing the light beam 10 are parallel to the optical axis 15. In this case, the focus 18 of the light beam 17 focused by the variable lens LV is just at the focal point of the variable lens LV. With the variation of the focal length of the variable lens LV, therefore, the focus 18 of the focused light beam 17 is axially displaced along an adjustment range As along the optical axis 15. In the example shown here, in which the light beam 10 is collimated, the focus adjustment range As is equal to the focal length adjustment range f _L v, A to f _L v _, c of the variable lens LV. Within this adjustment range As, the position-resolving detector 50 is positioned, preferably approximately in the middle of the adjustment range As. Thus, the focus or the beam waist 18 of the focused beam 17 can be positioned in an axial region on both sides of the detector 50, ie, before and virtually behind the spatially resolving detector 50. Thus, a series of intensity distributions in different cross-sectional planes of the focused beam 17 can be recorded by the detector 50. Among other things, the beam diameter for each cross-sectional plane can be calculated from the intensity distributions. From the application of the beam diameter above the axial position in the beam, the propagation factor of the beam can be determined. The focused beam 17 has a beam waist 18, ie an axial position at which the beam diameter is minimal. This axial position may, as in the example shown in FIG. 1, be the focus position of the focused beam 17. The diameter of the beam waist of the light beam 17 focused by the variable lens LV depends on the respectively set focal length of the variable lens LV, that is, the magnification of the device changes over the adjustment range As. This manifests itself also at different opening angles of the beam cone of the focused light beam 17, as can clearly be seen in the three focal length settings shown in FIG. The beam parameters of the focused light beam 17, in particular the beam waist diameter and the opening angle, are therefore not constant, but change with the set focal length. Because of this varying magnification of the measuring device, the evaluation of the measured beam diameters in different cross-sectional planes for determining a propagation factor can not take place according to the usual formulas described in ISO 114614. Furthermore, a conversion of the measured image-side beam parameters to the searched object-side parameters of the original beam is required. Not only the variable focal length of the variable lens LV, but also the distance of the spatial resolution detector 50 to the variable lens LV affect the measurement results. The exact determination of the beam parameters therefore requires a complex calibration of the device. In many variable lenses is not the focal length, but the refractive power of the lens, so the reciprocal value of the focal length, approximately linearly dependent on a manipulated variable, such as an electric current. This has the consequence that the axial focus position depends non-linearly on the manipulated variable. In Figure 1, this is recognizable by the fact that the focus position B 46 for the average adjustable power 26 is not in the middle of the adjustment range As, but much closer to the focus position A, 45, at the largest adjustable power 25. The achievable axial resolution thus not constant within the setting range As.

The varying magnification, the complex calibration, the required conversion to the beam parameters of the original beam 10 to be measured, as well as the fluctuating axial resolution make the device shown in Figure 1, known from the prior art and the associated methods error-prone and limited the achievable accuracy. FIG. 2 shows a schematic representation of a first possible embodiment of the invention. The light beam 10 to be measured strikes a variable optical element (VOE) 20 with an adjustable focal length. The variable optical element 20 does not image the light beam 10 onto a spatially resolving detector 50, but adjustably varies the aperture angle, ie, the divergence angle or the convergence angle of the light beam. Between the variable optical element 20 and the detector 50, an objective 30 is arranged. The objective 30 focuses the light beam so that a beam waist 18 of the focused light beam 17 is generated in an adjustment range As. The spatial resolution detector 50 is disposed within the adjustment range As so that, as the focal length of the variable optical element 20 changes, the beam waist 18 can be displaced around the detector 50 in an axial range. In this way, successively several different cross sections of the beam 17 can be scanned quickly in succession. The distance between the variable optical element 20 and the objective 30 is approximately the focal length of the objective 30. More specifically, the distance between the image-side major surface H ' _VOE 22 of the variable optical element 20 and the object-side major surface H ₀ bj 31 of the objective 30 approximately equal to the focal length fb of the lens _j 30. the magnification and the magnification of the beam waist 18 is not dependent on the focal length of the variable optical element twentieth This can be seen from the identical opening angles of the beam cone of the focused light beam 17 in the three beams exemplarily shown in FIG. 2 for different focal lengths of the variable optical element 20. In the exemplary embodiment of a first embodiment of the invention shown in FIG. 2, the focal length of the objective 30 is relative short, so that the adjustment range As of the beam waist 18 with the positions A, B and C of the focal positions 45, 46, 47 shown by way of example lies completely in the image space behind the objective 30.

FIG. 3 shows a further embodiment of the first embodiment of the invention. In contrast to the example shown in FIG. 2, the focal length of the objective 30 is chosen to be relatively long. As a result, on the one hand the adjustment range As is significantly greater than in the case of a short focal length of the objective 30, on the other hand it may happen that the adjustment range As of the beam waist 18 no longer lies completely in the image space behind the objective 30. As a result, the scanning range of the focused beam 17 accessible to the detector 50 can be restricted. In Figure 4, an embodiment of the first embodiment of the invention is similar to that shown in Figure 3. In contrast to FIG. 3, however, the light beam 10 to be measured is not collimated here, but rather has a focus or a beam waist 11 relatively close in front of the device and subsequently spreads divergently. Such an application is typical for the measurement of a laser beam 10, which was previously focused by a processing optics. The processing optics is not shown here, as this is not part of the measuring device. In the example shown in FIG. 4, due to the divergence of the beam 10, the adjustment range As of the beam waist 18 lies completely in the image space behind the objective 30.

In the second embodiment of the invention shown in Figure 5, a divergence adjustment lens LD 60 is disposed in front of the variable optical element 20. In the embodiment shown here, the lens for divergence adjustment 60 has a negative refractive power, whereby the previously collimated light beam 10 is divergent. As a result, the adjustment range As of the beam waist 18 is displaced to the rear and lies completely in the real image space behind the objective 30.

FIG. 6 shows a further embodiment of the first embodiment of the invention. In this embodiment, the objective 30 is composed of a first lens group 35 and a second lens group 36. In the illustrated embodiment, the first lens group 35 has a negative refractive power and the second lens group 36 has a positive refractive power. The first lens group 35 and the second lens group 36 together form a retrofocus-type objective 30 having a rearwardly displaced main surface. As a result, the adjustment range As of the beam waist 18 is also displaced to the rear and can lie completely in the real image space behind the objective 30.

In Figure 7, a further embodiment of the second embodiment of the invention is shown, in which a lens 60 for divergence adjustment in the beam direction in front of the variable optical element 20 is arranged. When the light beam 10 to be measured has a relatively high divergence and a beam waist 11 near the measuring apparatus, the focus adjustment area As of the beam waist 18 may be displaced relatively far backward. With the divergence adjustment lens 60, a refractive power offset is added to the adjustable refractive power of the variable optical element 20, thereby adjusting the adjustment range As by one certain amount is shifted axially. In the exemplary embodiment illustrated here, the divergence adjustment lens 60 has a positive refractive power, so that the adjustment range As is moved closer to the objective. The analysis of beams 10 that are not collimated and have a focus or beam waist 11 relatively close to the device can thus be made with a compact device whose size is not unnecessarily long.

A third embodiment of the invention is shown schematically in FIG. When measuring light beams 10 with high light intensity, for example laser beams, the spatially resolving detector 50 may possibly be overdriven. In such situations, it is beneficial to attenuate the beam. FIG. 8 shows an exemplary embodiment with a device for beam attenuation. The device for beam attenuation consists in this embodiment of two beam splitters 70, which are arranged in front of the variable optical element. Each beam splitter 70 reflects a small portion of the light beam 10, so that after two reflections, the light beam 10 has a substantially reduced intensity. If the two beam splitters 70 are arranged spatially so that the reflection planes are rotated by 90 ° to each other, a very precise polarization-independent attenuation can be achieved by means of this device. The superfluous beam portions transmitted by the beam splitters can be picked up by beam traps or absorbers 74.

Figure 9 illustrates the location of the major surfaces and the focal points in a composite of two lens groups 35, 36 retrofocus lens. Such a lens is exemplified in the embodiments of the invention in Figures 6, 7 and 8 as a lens 30. The object-side main surface 31 is where the rays emanating from the object-side focal point 33 would meet with the rays parallel to the axis after imaging by the objective 30, if the rays are virtually extended (shown in the upper part of FIG. 9). The image-side major surface 32 is located where the paraxial rays would meet with the rays passing through the image-side focal point 34 after imaging through the objective 30, if the rays are virtually extended (lower part of FIG. 9). The main surfaces 31, 32 are thus the surfaces on which one can imagine the refractive effect of all elements of the objective 30 reduced to one surface. In the example shown, the main surfaces 31, 32 are moved very far to the rear. This is advantageous in the embodiments of the invention in Figures 6, 7 and 8, in spite of a very large focal length of the lens 30 has a compact To realize design. Because of the rearwardly displaced main surface 31, the objective must be arranged very close behind the variable optical element 20 in order to fulfill the distance condition between the variable optical element 20 and the objective 30 (see FIGS. 6, 7, 8). , This also contributes to a compact construction.

DETAILED DESCRIPTION OF THE INVENTION

It is intended to provide a solution to the problem that known from the prior art devices for beam analysis require relatively complex devices for scanning multiple levels, require a long measurement time, have low accuracy, or are prone to systematic error sources , In contrast, an apparatus and a method for beam analysis are to be created, which allow a measurement of several different cross-sectional planes of a light beam in a short time and allow determination of beam parameters of the light beam with high accuracy.

To solve the task, a device is proposed which comprises a variable optical element (VOE) 20, an objective 30, and a spatially resolving detector 50, which are arranged one behind the other in the beam path of a light beam 10 to be measured. The variable optical element 20 has an adjustable focal length, which can be varied by means of a manipulated variable between a minimum focal length and a maximum focal length. The refractive power of an optical element is equal to the reciprocal focal length of the optical element, that is, the minimum adjustable focal length corresponds to a maximum refractive power, and the maximum adjustable focal length corresponds to a minimum refractive power.

The variable optical element 20 further has an image-side major surface H ^' _VOE 22. It is known from the field of technical optics that the image-side main surface of a lens is a virtual surface on which the refraction of axially parallel rays would take place, if one considers the refraction usually taking place at several optical interfaces of a lens reduced to one surface. The distance of the intersection between the image-side major surface of a lens and the optical axis to the image-side focal point of the lens is therefore equal to the focal length of the subject lens. The objective 30 has a constant focal length fo _bj and an object-side main surface H ₀ bj 31. According to the invention, the objective 30 is arranged in the beam direction behind the variable optical element 20 at a defined distance from the variable optical element 20. The distance H'VOE Ho _b j from the image-side major surface H ^' VOE 22 of the variable optical element (VOE) 20 to objektsei main surface Ho _bj 31 of the lens 30 should be approximately equal to the focal length fo _{bj of} the lens 30.

The position of the image-side major surface H ^' VOE 22 of the variable optical element 20 can, for example, when using a fluid lens with variable center thickness, vary in a small range. This variation can typically be from a few 1/10 mm to a few mm. Therefore, it is intended to allow a small deviation from the ideal distance condition between the variable optical element VOE 20 and the objective 30. The distance H ^' VOE Ho _b j between the image-side main plane H ^' VOE 22 of the variable optical element 20 and the object-side main surface Ho _bj 31 of the objective 30 should be in a range of 0.95 * fobj <H'VOE Hobj <1.05 * fobj lie. In other words, the distance between the major surfaces H ^' VOE Ho _bj should be equal to the focal length fo _b j of the objective 30 with a maximum deviation of + 1-5%.

In the beam direction behind the lens 30 of the spatially resolving detector 50 is arranged. The spatial resolution detector 50 is a photosensitive sensor that can register the local light intensity. In this context, the term "spatially resolving" means that the sensor does not register a single measured value integrally over its entire surface but has a plurality of cells distributed over the surface and can thus register a two-dimensional lateral light distribution The spatial resolution detector 50 records the intensity distribution of the focused light beam 17 in a cross-sectional plane of the light beam 17. The position-sensitive detector 50 is at a distance from the objective 30 arranged, in which by varying the focal length of the variable optical element 20, the beam waist 18 of the focused light beam 17 both before (eg front end position A, 45, the focus adjustment range As) and virtually behind the position-resolving detector 50 can be positioned (eg rear Final Stor e C, 47, the focus adjustment range As). Due to the features of the invention results in a special operation of the device. The light beam 10 to be measured is incident on the variable optical element (VOE) 20. The variable optical element 20 influences the light beam 10 such that the aperture angle of the beam cone of the light beam 10 is changed due to the refraction by the variable optical element 20. Depending on the set focal length of the variable optical element, the opening angle of the beam cone is influenced more or less. In the example shown in FIG. 2, the beam 10 to be measured is collimated and, after being influenced by the variable optical element 20, converges, the convergence angle (or angle of oscillation) depending on the set focal length or refractive power of the variable optical element 20. By way of example, FIG. 2 shows three beams which represent the refracted beam at minimum, average and maximum refractive power. Subsequently, the light beam 10 influenced by the variable optical element 20 is focused by the objective 30. Due to the adjustable opening angle or convergent angle of the light beam 10 after the variable optical element 20 results after focusing by the lens 30 an adjustable axial position (for example, A, B or C) of the beam waist 18 of the focused light beam 17. After focusing by the Lens 30 is the Öfmungswinkel or convergence angle of the focused light beam constant. This is equivalent to that the total focal length of the system composed of the variable optical element 20 and the objective 30 is constant, although the focal length and the refractive power of the variable optical element VOE 20 may have different values.

The total focal length fb of a system composed of two lenses 1 and 2 can be determined according to the following formula known from the technical view: f _G = f, f ₂ / (f, + f ₂ -e)

Here, fi is the focal length of the first lens 1, f _{2 is} the focal length of the second lens 2, and e is the distance of the two lenses. Applied to the device according to the invention, corresponds to the variable optical element (VOE) 20 with the focal length f _V 0E of the first lens 1, the lens 30 with the focal length fobj corresponds to the second lens 2, and the distance e of the lenses is optically accurately formulated , the distance of the image-side main surface of the first lens 1 to the object-side main surface of the second lens 2. This distance e should be according to the invention approximately equal to the focal length fo _{bj of} the lens 30. It follows: fG - fvOE fobj / (fvOE + fobj ~ ^β )

With e ~ f ₀ bj follows: f _G "f obj [0056] The overall focal length fo is thus approximately equal to the focal length of the lens 30 and thus independent of the focal length of the variable optical element (BOE) 20. Due to the constant total focal length the diameter of the beam waist 18 is constant over the entire adjustment range As. The beam parameters of the focused light beam 17 are therefore constant and do not change when the axial position of the beam waist 18 is adjusted. The propagation factor or the beam parameter product can therefore be determined from the beam radii in the different cross-sectional planes of the light beam in accordance with the procedure according to ISO 1146 ,

It is envisaged that the distance H ^' VOE Hobj between the image-side major surface of the variable optical element 20 and the object-side major surface of the objective 30 should be equal to the focal length fobj of the objective 30 with a maximum deviation of + 1-5%. This maximum deviation has only a small effect on the constancy of the total focal length or on the magnification of the system; their fluctuations then typically amount to at most a few percent.

Not only the constancy of the focal length of the system composed of the variable optical element 20 and the objective 30 is a favorable property. A further advantageous feature results from the fact that the constant focal length of the composite system is equal to the focal length of the objective 30. Thus, via the proper choice of the focal length of the objective 30, the size of the beam waist 18 of the focused beam 17 can be adjusted to a desired value irrespective of the fixed focal length adjustment range of the variable optical element 20 to provide the metrological resolution in recording the intensity distributions in the cross-sectional planes of the light beam 17 to optimize.

Another favorable feature of the device according to the invention is evident when considering the adjustment range As for the beam waist position. In simplified terms, a light beam 10 to be measured is considered, which is collimated. In a known from the prior art device as in Figure 1 then the adjustment range As is immediate As the difference between the maximum focal length and the minimum focal length of the variable optical element or the variable lens LV: As = f _L v, c - ίίν, Α · In the prior art, therefore, the adjustment range As by the properties of the variable optical element established.

In the case of the device according to the invention, on the other hand, the setting range As results from the following formula:

AS = (fobj) ² [(1 / fvOE, min) ^~ (1 / fvOE, max)]

Thus, by selecting a suitable focal length fobj of the objective 30, the adjustment range As can be adjusted to the desired or required range without being limited by the fixed focal length adjustment range fvoE.min to fvoE.max of a variable optical element 20 ,

For most accurate determination of the beam parameter product or the beam propagation factor, it is convenient to scan a beam in a range of several Rayleigh lengths around its beam waist. The Rayleigh length is the distance from the beam waist where the beam diameter has increased to V2 times the beam diameter. At a distance of a Rayleigh length from the beam waist, the intensity of the beam has fallen by half for Gaussian beams. With a large diameter of the beam waist and a small aperture angle or divergence angle of the beam, the Rayleigh length can be very large. Conversely, the Rayleigh length is very small with a small diameter of the beam waist and a large aperture angle. It is therefore desirable for the most accurate possible measurement of the beam parameters to be able to adapt the adjustment range As of the device to the radiation to be measured. This is possible in the device according to the invention by selecting the focal length of the objective 30 in a wide range. In FIG. 2, for example, the focal length fobj of the objective 30 is selected to be relatively short, and consequently the focus adjustment range As is relatively short. In contrast, in FIGS. 3 and 4 embodiments are shown in which the focal length fobj of the objective 30 is chosen to be significantly longer. As a result, the setting range As is also much larger, although the focal length setting range of the variable optical element 20 is the same as in FIG. 2. It is further found that the adjustment of the axial position of the beam waist 18 of the focused light beam is proportional to the change in the refractive power of the variable optical element 20. This is advantageous because with many optical elements or lenses with adjustable focal length not the focal length, but the refractive power is proportionally dependent on a manipulated variable such as an electric current or a pressure. This results in the device according to the invention also a proportional adjustment of the axial position of the beam waist when changing the manipulated variable of the variable optical element 20th

As a variable optical element 20, for example, a fluid lens may be used. Such a lens is offered, for example, by the company Optotune AG under the product name EL 10-30 as "Fast Electrically Tunable Lens." In this lens, an electrically controllable actuator presses on a container which is filled with an optical fluid and with a Depending on the pressure in the container, the elastic membrane is more or less curved, so the focal length of the lens is controlled by the electrical current applied to the actuator, and the manufacturer gives a roughly linear relationship between the electric current and the refractive power of the lens.

Any other lenses with adjustable focal length can also be used as the variable optical element 20. For example, adaptive lenses are possible in which the refractive power can be set by means of actuators; Fluid lenses in which an interface can be electrostatically adjusted; or adaptive mirrors in which the curvature can be adjusted by a pressure chamber or by actuators. The list is to be understood as an example; the invention is not limited to the said types of variable optical elements.

To determine the beam parameters or the propagation factor of a light beam, it is provided to scan at least three different cross-sectional planes of the light beam. For this purpose, the beam waist 18 of the focused light beam 17 is displaced by adjusting the focal length of the variable optical element 20 at three different axial positions, so that the spatially resolving detector 50 three different cross-sectional planes of the beam 17 come to rest. The three adjusted positions of the beam waist 18 can However, any other three positions may be within the adjustment range As of the beam waist 18, and more than three positions may be selected. In order to obtain the most accurate results, it is favorable, on the one hand, to select the setting positions of the beam waist 18 equidistantly and, on the other hand, to select an approximately equal number of positions in front of and behind the spatially resolving detector 50. At each approached position of the beam waist 18, the intensity distribution of the focused light beam 17 is registered by the spatially resolving detector 50. From the registered intensity distributions, the respective beam radii or beam diameters can be determined. Plotting the ray radii over the axial position gives the envelope or caustics of the beam 17, and the propagation factor or beam parameter product can be calculated therefrom.

The objective 30 may consist of a single lens. For example, to minimize aberrations, the single lens may be an aspherical lens. Reduction of aberrations can also be achieved by using multiple lenses. It is therefore also envisaged that the objective 30 is composed of several lenses.

In order to be able to scan the beam in a range of several Rayleigh lengths around the beam waist 18, it is not necessary that the entire adjustment range As lies behind the objective 30 in the real image space. However, at least about half of the setting range As should be accessible in real terms so that the spatially resolving detector 50 can be arranged approximately in the vicinity of the central focus position B, 46. From the requirement of a real image position at the middle focus position B, 46, the beam waist 18, a restriction in the choice of the focal length fobj of the lens 30 may result if the lens 30 consists of only a single thin lens, in which the object-side Main surface is typically within the lens (eg in a biconvex lens). Consequently, the focal length of the objective 30 in this case should not be greater than the focal length of the variable optical element 20 at the average adjustable refractive power of the variable optical element 20. The following numerical example illustrates the relationship: The focal length of the variable optical element 20 is adjustable, for example from fvoE.min = 50 mm to fvoE.max = 250 mm. This corresponds to a refractive power of 20 dpt to 4 dpt (dpt: diopter, refractive power in 1 / m). The average refractive power in this example is about 12 dpt, corresponding to a focal length of 83 mm. When the lens 30 with a single Lens then has a focal length of for example 100 mm and therefore would have to be placed about 100 mm behind the variable optical element, then the image position at the middle focus position (B) 46 of the beam waist 18 is not in the real area behind the lens 30. Das The exemplary embodiment shown in FIG. 3 corresponds approximately to the mentioned numerical example. In the present invention, this limitation on the focal length of the objective 30 is overcome because not only single lenses can be used as the objective 30 according to the present invention, but also other types of the objective 30 are provided in which the major surfaces are not within the objective or lenses of the objective must lie.

It is also contemplated that the objective 30 comprises a first lens group 35 and a second lens group 36. Both lens groups 35, 36 together form the objective 30. The first lens group 35 may consist of a single lens or may comprise a plurality of lenses. The second lens group 36 may also consist of a single lens or comprise a plurality of lenses. The first lens group 35 may have a negative focal length, and the second lens group 36 may have a positive focal length, and the lens 30 composed of both lens groups 35, 36 has a positive focal length. This creates a lens in so-called retrofocus construction. A retrofocus lens has an image section that is larger than the focal length of the lens. The image-side main surface H 'is therefore shifted backwards into the image space. For the device according to the invention, however, the position of the object-side main surface H is essential. The position of the object-side main surface is also shifted to the rear with a retrofocus lens. FIG. 9 shows the position of the main surfaces in a retro-focus-type objective 30 with a first lens group 35 and a second lens group 36. FIGS. 6, 7 and 8 show exemplary embodiments of the invention with an objective 30 designed as a retrofocus objective. The retrofocus Construction of the objective 30 may be advantageous in several respects. On the one hand, the position of the adjustment range As can be shifted backwards. Furthermore, this significantly shortens the distance between the objective 30 and the variable optical element 20, thereby reducing the size of the device. Finally, despite a small size, a very large focal length of the objective 30 can be realized in order to achieve a large adjustment range As. In the following, further embodiments and further developments of the invention are shown.

When measuring a laser beam with high brilliance, so with a small beam parameter product or mode factor, it may be desired, for example, to choose a very large focal length for the lens 30 to the beam waist 18 of the focused beam 17 as large as possible to image and achieve a high lateral resolution. In this case, the situation may occur that part of the focus adjustment range As or even the entire focus adjustment range is not located in the real image space behind the objective 30 and thus is not available for the measurement with the spatially resolving detector. A situation in which only part of the adjustment range lies in the real image space is shown by way of example in FIG. 3. In order to be able to completely scan the beam caustic in such situations, further embodiments of the invention are proposed. In a second embodiment of the invention is provided to arrange in the beam direction in front of the variable optical element 20, a lens (LD) 60 for divergence adjustment. With this lens 60, a refractive power offset is generated and thereby the entire focus adjustment range As is displaced axially by a certain amount. If the divergence adjustment lens 60 has a negative refractive power, the focus adjustment area As is shifted rearward in the beam direction. A corresponding embodiment is shown in FIG.

The refractive power of divergence adjustment lens (LD) 60 may also be positive. As a result, the adjustment range As of the beam waist 18 is shifted toward the front, closer to the objective 30. This may be advantageous in the measurement of light rays 10 having a relatively high divergence and a beam waist 11 near the measuring device. FIG. 7 shows an exemplary embodiment with a positive lens 60 for divergence adaptation.

The lens (LD) 60 for divergence adjustment may also be a variable lens whose focal length or power can be varied. Thus, a device can be provided which can be flexibly adjusted to many geometric configurations of the beam 10 to be measured. Thereby, the measurement of convergent, divergent and collimated beams 10 and beams with different layers of the beam waist 11 is possible without having to make a conversion of the device. It will only the focal length of the divergence adjustment lens 60 is properly adjusted to the respective beam 10, eg, such that the beam 10 is collimated to the divergence adjustment lens 60. During the measurement of the beam 10, the focal length or refractive power of the divergence adjustment variable lens 60 is constant; In order to vary the position of the beam waist 18 during the beam measurement, the focal length or refractive power of the variable optical element 20 is changed.

In a third embodiment, the invention further comprises a device for beam attenuation. The device for beam attenuation may comprise, for example, a filter glass or a neutral density glass, also called gray glass. The device for attenuation may also consist of a pair of polarizing filters with mutually adjustable angle. The attenuator may also include one or more beam splitters 70 which split the beam into a reflected and a transmitted beam, each having low intensities than the original beam. The beam splitter 70 may be formed by an interface of an optical device such as a plane plate, a wedge plate or a prism. The interface of the optical device may be uncoated or coated, for example provided with a dielectric coating. FIG. 8 shows by way of example the device according to the invention with a device for beam attenuation, which consists of two beam splitters 70, which are arranged in front of the variable optical element 20. Each beam splitter 70 reflects a small portion of the light beam 10, so that after two reflections, the light beam 10 has a substantially reduced intensity. The beam portions transmitted by the beam splitters with the majority of the beam power are captured by beam traps or absorbers 74. The two beam splitters 70 may be arranged spatially so that the reflection planes are rotated by 90 ° to each other. Thus, a very precise polarization-independent attenuation can be achieved, which is also suitable for high beam intensities and high beam powers. The invention is thus also applicable to the measurement of high power laser beams. The invention is not limited to the illustrated and described embodiments. Rather, the features of individual embodiments can also be combined with each other. For example, a device including both a divergence adjustment lens (LD) and a beam attenuation means is within the scope of the present invention. The invention offers numerous advantages over the prior art, which are summarized below:

• The device allows the rapid and precise adjustment of the axial position of the image-side beam waist 18 of a light beam 10 to be measured relative to a spatially resolving detector 50 without elements of the device must be stored axially movable.

The focal length of the overall optical system consisting of the variable optical element 20 and the objective 30 is constant and independent of the focal length of the variable optical element 20.

Due to the constant focal length or the constant imaging properties of the overall system consisting of variable optical element 20 and objective 30, the evaluation for the determination of propagation factors or beam parameter products can be carried out according to the formulas of ISO 11146.

• By means of a construction of the objective 30 consisting of two lens groups 35, 36, a compact design with a long focal length of the overall system and thus a large axial focus adjustment range As can be achieved.

The calibration of the device required for the correct determination of the beam parameters and for the conversion of the parameters to the subject beam parameters is simpler and less error-prone than with devices with varying total focal length.

The beam parameter product, which is determined from the beam diameters measured at different cross-sectional planes, is independent of the exact axial positioning of the spatially resolving detector 50, thus reducing the number of potential systematic sources of error.

The focal length of the overall optical system can be chosen independently of the limitations of the variable optical element 20 in order to adapt the magnification or the diameter of the image-side beam waist 18 to the conditions of the light beams 10 or laser beams to be measured.

The focal length of the objective 30 can be suitably selected to realize a sufficiently large or small adjustment range As for the axial position of the image-side beam waist 18 irrespective of the predetermined focal length variation range of the variable optical element 20. The change in the axial position of the image-side beam waist 18 is proportional to the refractive power change of the variable optical element 20 and is therefore approximately linearly dependent on the manipulated variable in the case of most of the variable optical elements 20.

Determining a geometric parameter of the light beam from the registered intensity distributions may also include determining the axial position of a beam waist or focus position 18 of the focused light beam 17. From the position of the image-side focus position 18, the axial position of the beam waist 11 of the original beam to be measured can be determined via the imaging equation of the overall system. The invention is therefore also intended to control or monitor a focus position of a light beam or laser beam. The monitoring of a focus position may be advantageous, for example, in a laser processing system to diagnose changes in the desired focus position of a processing laser beam. Such changes can be caused for example by thermal effects. Changes in the target focus position may also be caused by the laser beam itself due to absorption of the radiation by the processing optics.

The invention can be used for example for measuring laser beams. It can be measured laser beams emitted by beam sources. It is also possible to measure laser beams which are emitted by a beam guidance system, such as an optical fiber, or which are imaged or focused by laser processing optics, or which are imaged by beam shaping optics for shaping a desired beam geometry.

The invention can also be used for on-line beam diagnosis, for example on a laser processing optical system which decouples a fraction of the laser beam by means of a beam splitter and makes it available at a diagnostic beam output. The device according to the invention can be coupled to the diagnostic beam output of the laser processing optics. The invention can also be integrated as a fixed component in the laser processing optics. The laser processing optics may be, for example, a scanner optics. LIST OF REFERENCE NUMBERS

10 Light beam to be measured

11 beam waist of the light beam to be measured

15 Optical axis

17 Focused light beam

18 beam waist or focal position of the focused light beam

20 Variable optical element with adjustable focal length

22 image-side main surface of the variable optical element

25 Image-side focal point of the variable optical element at maximum power

26 Image-side focal point of the variable optical element at average power

27 Image-side focal point of the variable optical element with the smallest refractive power

30 lens

31 Object-side main surface of the lens

32 Image-side main surface of the lens

33 Object-side focal point of the objective

34 Image-side focal point of the lens

35 First lens group of the lens

36 Second lens group of the lens

45 Front end position A of the focus adjustment range

46 Middle position B of the focus adjustment range

47 Rear end position C of the focus adjustment range

50 Spatial detector

60 divergence adjustment lens

70 beam splitter

74 jet trap (absorber)

Claims

A device for determining geometric parameters of a light beam (10), comprising a variable optical element (20), a lens (30), and a spatially resolving

A detector (50), wherein the variable optical element (20) has an adjustable focal length and an image-side major surface (22), the objective (30) having a constant focal length and an object-side major surface (31), the distance between the image-side major surface (22) of the variable optical element (20) and the object - side main surface (31) of the objective (30) is equal to the constant focal length of the objective (30) with a deviation of at most +/- 5%, the objective (30) variable optical element (20) is connected downstream in the beam direction, wherein the spatially resolving detector (50) is downstream of the lens (30) in the beam direction, and wherein by changing the adjustable focal length of the variable optical element (20) and by subsequent focusing of the light beam (10 ) by the objective (30) a focal position (18) of the focused light beam (17) relative to the spatially resolving detector (50) in the axial direction variable is ellable.

2. Apparatus according to claim 1, wherein the relative positions of the light beam (10), the variable optical element (20), the lens (30) and the spatially resolving detector

(50) are stationary relative to each other.

3. Apparatus according to claim 1 or 2, wherein the total focal length of a system consisting of the variable optical element (20) and the lens (30) is equal to the constant focal length of the lens (30) with a maximum deviation of +/- 5%.

4. Device according to one of claims 1 to 3, wherein the variable optical element (20) comprises a fluid lens.

A device according to any one of claims 1 to 3, wherein the variable optical element (20) comprises an adaptive lens.

6. Device according to one of claims 1 to 3, wherein the variable optical element (20) comprises an adaptive mirror.

7. Device according to one of claims 1 to 6, wherein a lens (60) for divergence adjustment in the beam direction in front of the variable optical element (20) is arranged.

8. Device according to one of claims 1 to 7, wherein the lens (30) comprises a first lens group (35) and a second lens group (36).

9. Apparatus according to claim 8, wherein the first lens group (35) of the objective (30) has a negative refractive power, wherein the second lens group (36) of the objective (30) has a positive refractive power, and wherein the constant focal length of the objective (30 ) has a positive value overall.

10. A method for determining geometric parameters of a light beam (10), comprising the method steps:

Changing an opening angle of the light beam (10) by means of a variable optical element (20) having an adjustable focal length and an image-side main surface (22),

Focusing the light beam changed by the variable optical element (20) with respect to the opening angle by means of an objective (30) having a constant focal length and an object-side main surface (31),

Changing the adjustable focal length of the variable optical element (20), wherein successively at least three different focal lengths are set,

Registering intensity distributions of the light beam (17) focused by the objective (30) by means of a spatially resolving detector (50) arranged in the beam direction after the objective (30), wherein an intensity distribution is registered at each of the at least three different set focal lengths, and

Determining a geometric parameter of the light beam (10) from the registered intensity distributions,

wherein the distance between the image side major surface (22) of the variable optical element (20) and the object side major surface (31) of the objective (30) is equal to the constant focal length of the objective (30) with a maximum deviation of +/- 5%.

11. The method of claim 10, wherein the relative positions of the light beam (10), the variable optical element (20), the lens (30) and the spatially resolving detector (50) are fixed to each other.

The method of claim 11, wherein determining a geometric parameter of the light beam (10) from the registered intensity distributions comprises determining a beam propagation factor of the light beam (10).