DE202012102794U1 - Optics for beam measurement - Google Patents

Abstract

Optics for beam measurement of optical radiation, which spreads divergent behind a beam waist, comprising a first partially reflecting mirror, a second partially reflecting mirror, a collimation lens consisting of at least one optical lens, and a third partially reflecting mirror arranged one after the other in the beam direction.

Description

FIELD OF THE INVENTION

The invention relates to an optics for beam measurement with integrated beam attenuation.

BACKGROUND OF THE INVENTION

In the measurement of the properties of laser beams very different tasks can be followed, correspondingly large is the variety of known devices. On the one hand, there are devices for measuring the power of the beam and, on the other hand, devices for determining the geometric properties of the beam. The geometric properties such as beam profile, focus diameter, divergence angle and beam parameter product are quantities which, just like the beam power, have an influence on the laser processing process and are thus decisive for the quality of the processing result. The measurement of these parameters is therefore an essential part of quality assurance in laser material processing.

The devices for power measurement can be divided into those that capture the entire beam, for example in an absorber and determine the performance of the temperature change of the absorber. Devices of this kind are eg in the DE 42 43 902 and the DE 102 53 905 described. In another type of devices not the entire beam is collected, but it is coupled out a very small fraction of the beam, so that the main beam can continue to be used and can be done on the basis of the decoupled sub-beam so simultaneously measuring the power (eg DE 196 30 607 ).

To determine the geometric properties, a spatially resolved measurement of the beam intensity is necessary. The spatial resolution can be achieved by scanning the beam. That's how it shows DE 35 10 937 a device in which by rapidly moving a needle-shaped probe through the beam, a signal is generated, the time course of which reproduces the spatial distribution of the beam intensity. Another possibility for spatially resolved measurement of the beam profile shows the DE 101 29 274 in which several wire lattice planes pass through the beam and from the heating and the resulting change in resistance of the individual wires can be deduced to the local intensity. The DE 10 2008 022 015 describes an array of temperature-sensitive sensors for spatially resolved measurement of a beam profile, wherein the sensor array receives a fraction of the beam power by placing the array behind a semitransparent mirror that reflects the bulk of the radiation.

Particularly convenient for spatially resolved beam measurement is the use of a semiconductor sensor such as a CCD camera, as this allows the entire two-dimensional beam profile in a plane can be detected with a single measurement. The disadvantage is that the beam power for this purpose must be very low, otherwise the sensor is overdriven or even destroyed. The sensor can therefore not be positioned directly in the plane to be measured in the beam, but the beam must be imaged and at the same time greatly attenuated. For this purpose, a fraction of the beam must be coupled out, the decoupled fraction having to match its geometric properties with the original beam.

Although such beam splitters are known in principle, the characteristics of the beam extraction are generally dependent on the beam incidence angle and / or the polarization state of the beam. Usually, the reflection properties of a material interface are exploited (Fresnel reflection), or these properties are influenced by structuring of the surface, whereby in addition scattering and diffraction occurs. So will in the DE 103 55 866 a beam splitter in the form of inclined plane plates described, the non-effective surfaces are provided with an anti-reflection coating. The DE 10 2008 028 347 describes a measuring module for measuring the power of a laser beam, is initially coupled via a partially reflecting mirror, a portion of the radiation and a further attenuation of the decoupled beam takes place by reflection on uncoated glass plates.

In the in the DE 40 06 618 and in the DE 102 17 657 As shown, a holographic structure is applied for coupling the beam onto the plane plate designed as a beam splitter or as a reflector, the diffraction efficiency of which determines the fraction of the decoupled radiation.

The beam of interest to be measured, in particular in laser systems and beam guides for 1 μm laser radiation, is usually divergent, such as e.g. the beam emerging from a fiber tip or the beam focused by a processing optics.

However, the use of an inclined plane plate for coupling a sub-beam from a divergent beam is problematic because the angle of incidence within the beam varies on the plane plate and therefore the reflectance according to the Fresnel formulas is not constant and additionally from the polarization direction of the beam depends. Such a simple arrangement therefore changes the beam profile and is thus not suitable for exact beam measurement.

One possible way to reduce this angular dependence of the reflection is from the already mentioned above DE 10 2008 028 347 known. There, another mirror with the same deflection angle is used to reduce the angular dependence in the reflection at the inclined plate. The in the DE 10 2008 028 347 However, the device shown is not suitable for decoupling from a divergent beam, nor for direct extraction from a high power beam; this takes over there an additional upstream mirror. Another disadvantage of the device shown there is the difference in the reflection of P- and S-polarized beams. Furthermore, the beam there is deliberately scattered for homogenization with a small angular distribution, so that the device is not suitable for beam analysis, but only to determine a relative, proportional to the power of the beam size.

To circumvent the problem of the angular dependence of the reflection, it is also conceivable first to collimate the beam by means of a collimation objective or at least one lens and only then to decouple a partial beam by means of a plane plate. Although the reflectance is then constant within the beam, since the angle of incidence no longer varies, but there is still a dependence on the polarization direction.

To reduce the reflection differences in different polarization states, it is known from the prior art to provide the interfaces of a partially reflecting mirror with suitable coatings, for example with special antireflection coatings, which should be designed so that the residual reflectance over a small angle range varies as little as possible and at the same time the reflectivity for the two polarization directions should be as equal as possible. For this purpose, several thin dielectric layers must be applied to the substrate. This way, for example, pursues the DE 20 2004 007 511 , The layer system for coating a mirror disclosed in this document consists of 6 individual layers whose layer thickness must be precisely adhered to. It will be apparent to those skilled in the art that production tolerances occur and the desired consistency of reflection is difficult to ensure, especially under long-term conditions that can be known to cause changes in thin films due to aging, humidity, etc. Another The problem is that this coating must be applied on both sides of the plate and the reflected radiation components from the front and back of the plate can interfere with each other, which in turn can lead to fluctuations in the measured intensity.

If the beam is first collimated by means of an objective or a lens or imaged in some other form before a partial beam is decoupled, another problem arises. As the radiation passes through the lens, a minimal portion of the radiation is absorbed by the material of the lens, resulting in the generation of a radial temperature gradient in the lens, the magnitude of which depends on the radiated power. Because of the temperature coefficient of the refractive index of the lens material, this leads to an additional refractive power, the so-called "thermal lens". As a result, the position of the image shifts, a so-called "focus shift" arises, as well as additional aberrations, ie aberrations.

This focus shift is particularly disadvantageous when it comes to the measurement of high-brilliance radiation to achieve maximum imaging accuracy or to determine the size of the focus shift itself, for example. from an optic to be tested or from the end cap of an optical fiber tip.

Accordingly, there is a need for an apparatus for determining the intensity profile of a high power laser beam near the beam waist on a suitable sensor while attenuating the beam. The beam imaged on the sensor must therefore be an exact, unaltered image of the original beam. For this purpose, the optics must on the one hand highly accurate, i. on the other hand, the beam must be attenuated by many orders of magnitude, or a correspondingly smaller, precisely representative component must be coupled out of the beam so that the actual measuring sensor, e.g. a CCD camera, not destroyed by the beam.

Object of the present invention is thus to provide a device in which a small fraction of a divergent laser beam high power is coupled out without the decoupled beam part is changed in relevant properties.

BRIEF DESCRIPTION OF THE INVENTION

The object of the invention is achieved by a device according to the claims. The device according to the invention is suitable for eliminating the disadvantages of the devices known from the prior art, or the disadvantages are reduced to a practically no longer measurable size.

The invention provides an optical system for optical beam radiation measurement which diverges behind a beam waist, comprising in each case a first partially reflecting mirror, a second partially reflecting mirror, a collimating lens consisting of at least one optical lens, and a third one in the beam direction partially reflecting mirror.

It is provided for the three partially reflecting mirrors that they can each consist of a transparent optical material and can each have at least one plane surface. Furthermore, the three partially reflecting mirrors may each have an uncoated planar interface on which a small part of the radiation of the respective mirror is reflected by utilizing the Fresnel reflection.

In one embodiment, no optically imaging element should be arranged between the beam waist and the first partially reflecting mirror.

The reflection plane, which is spanned by a mirror through incident and reflected beam, may be parallel to the reflection plane at the first mirror for the second mirror. It is also contemplated that the beam may be collimated by the collimating lens prior to reflection at the third partially reflective mirror. In addition, the reflection plane at the third mirror can be arranged perpendicular to the reflection planes of the first and the second mirror.

The angle of incidence α ₂ at the second mirror with respect to the optical axis of the beam, may be in a range which is defined by the formula: α1 (n ₂ / n ₁₎ ^1/2 ≤ α ₂ ≤ α ₁ (n ₂ / n ₁ ), where α _{1 is} the angle of incidence at the first mirror with respect to the optical axis of the beam, and n ₁ and n _{2 are} the refractive indices of the first and second mirrors.

A relay lens may be disposed between the first partially reflecting mirror and the second partially reflecting mirror, the relay lens having a magnification of about -1 relative to the beam waist, and the opening angle of the convergent beam behind the relay lens being about the same can be large as the opening angle of the divergent beam in front of the relay lens.

Furthermore, a fourth mirror may be arranged downstream of the third partially reflecting mirror in the beam direction, and the fourth mirror may be arranged such that, after the reflection, the optical axis of the reflected beam is arranged parallel to the optical axis of the original beam.

Behind the third mirror or the fourth mirror, a focusing lens can be arranged, which produces an image of the original beam waist in its image plane. In the image plane of the focusing lens, a sensor may be arranged.

To attenuate the beam, means may be arranged behind the third mirror, these being selected from the group consisting of partially absorbing plane plates, for example gray glasses, partially transmitting plane plates, partially reflecting plane plates, a pair of polarizers arranged one behind the other, whose polarization angle can be adjusted to each other ,

The remaining beam portions transmitted by the second and the third mirror can each be captured by an element which has an absorbing surface and is provided with cooling fins for cooling by the ambient air or holes through which a coolant flows for cooling.

The rear surfaces of the first, second and third mirrors not used for the partial reflection of the radiation can be provided with an antireflection coating.

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:

1 : Schematic representation of a known from the prior art apparatus for beam measurement, in which first by a partially reflecting mirror, a fraction of the beam is coupled out, and then the beam is collimated or further imaged.

2 : Representation of the reflectance at a quartz glass-air interface according to the Fresnel formulas for a beam with S polarization direction (dotted curve) and a beam with P polarization direction (dashed curve). The solid portion of the curves corresponds to the angular range in the device according to 1 is used, with a deflection of the beam by 90 °.

3 : Schematic representation of an alternative known from the prior art apparatus for beam measurement, in which the beam is first collimated, and then a fractional part of the beam is coupled out by a partially reflecting mirror.

4 : Schematic representation of the device according to the invention for measuring the beam in a first embodiment.

5 : Representation of the reflection of the beam to be measured at the first two mirrors as part of the device according to the invention for explaining the principle of compensation of the angular dependence of the reflectance.

6 : Representation of the resulting intensity curve in the inventive device according to 4 for both polarization directions as a function of the angle of incidence. The solid portion of the curves is that in the device 4 used by the beam angle range.

7a : Representation of a portion of the device according to the invention with the reflection at the first two mirrors, wherein the beam is deflected at both mirrors by the same angle of 45 ° and the resulting working distance is very small.

7b : Representation of a portion of the device according to the invention with the reflection at the first two mirrors, wherein the beam is deflected at both mirrors by the same angle of 50 °. The resulting working distance is relatively small.

7c : Representation of a portion of the device according to the invention with the reflection at the first two mirrors, wherein the beam is deflected at both mirrors by different angles, first by 40 ° and subsequently by 60 °. The resulting working distance is significantly greater than when using the same deflection angle.

8th : Representation of the device according to the invention in a second embodiment, in which between the first and the second mirror, a relay lens is inserted and thereby a much larger working distance is achieved.

9 : Representation of the device according to the invention in an embodiment similar to the first embodiment according to 4 , By using a retrofocus collimating lens, a smaller collimated beam diameter is achieved, allowing a more compact overall design.

10 : Representation of the device according to the invention. Additionally shown here is the beam deflection by a fourth mirror and the focusing of the decoupled and attenuated sub-beam on a sensor.

11 : Representation of the device according to the invention in a second embodiment. Additionally shown here is the beam deflection by a fourth mirror and the focusing of the decoupled and attenuated sub-beam on a sensor.

DETAILED DESCRIPTION OF THE FIGURES

The basic principle of the invention is to almost completely compensate for the dependencies of the degree of reflection on the angle and the state of polarization by means of a special spatial arrangement of at least three partially reflecting mirrors and a collimation objective, so that the objectives required for collimation or imaging are not behind the power beam but behind at least one mirror can be arranged and thus the focus shift of the device is negligibly small overall.

For this purpose, a device is provided which comprises a first partially reflecting mirror, a second partially reflecting mirror, a collimating lens and a third partially reflecting mirror, the three partially reflecting mirrors each consisting of a transparent optical material and each having at least one plane surface and each of these uncoated plane surfaces a small portion of the radiation is reflected by utilizing the Fresnel reflection. The reflection planes, which are spanned by the incident and reflected beam of a mirror, are arranged parallel to one another in the first two mirrors. Between the second and the third mirror, the beam is collimated by the collimating lens. The reflection plane of the third mirror is perpendicular to the reflection planes of the first and second mirrors.

1 shows a typical schematic arrangement for decoupling a partial beam, as used in many of the prior art devices. partial beam 73 gets out of a beam 71 , which is behind a beam waist 70 spreads, by a tilted plan plate 10 with an uncoated surface 11 and an antireflective coated surface 12 decoupled. The main part 74 the radiation passes through the plane plate 10 therethrough. Because the angle of incidence 16 . 17 on the partially reflecting mirror 10 due to the divergence of the beam 71 varies, according to the Frensnel formulas, the reflectance within the beam changes. As a result, the beam profile is distorted, which is why this type of beam extraction is disadvantageous. It is favorable that a collimation by a lens 50 or another image takes place in the attenuated partial beam, as a result of which the thermo-optical influences of the objective, such as the focus shift, are small.

The in the usual devices after 1 occurring angle-dependent reflectance is in 2 shown. In the example shown, a beam with a numerical aperture of NA = 0.14 (corresponds to approx. +/- 8 °) is deflected by 90 ° from an uncoated quartz glass plate, ie the beam incidence angle is 45 ° on the optical axis and varies within of the beam in the range of about 37 ° to 53 ° (solid area of the curves). It can be seen that in this example, the reflectance for a P-polarized beam by a factor of about 30 varies, resulting in a corresponding variation in intensity over the beam aperture. As a result, the profile of the beam to be measured is strongly distorted, the arrangement is thus not suitable for exact beam measurement. For S-polarized beams, the reflectance varies by a factor of about 2, while the ratio between S- and P-polarized beams on the optical axis is 12.5.

In 3 an alternative way of extracting a partial beam is shown, as it is also used in devices of the prior art. Here is the first behind a beam waist 70 spreading beam 71 collimated by a lens 50 and only subsequently with a tilted plane plate 10 a partial beam 73 decoupled, with the main portion 74 the radiation transmits the plane plate. Although the falsification of the beam profile is avoided because the angle of incidence on the plane plate is constant, the reflectivity for the two polarization directions of the beam is generally different. Also disadvantageous are the thermo-optical effects in the collimation objective 50 , By the absorption of the radiation 71 in the lens 50 creates an additional thermal power 53 which produces a focus shift and thermo-optical aberrations (aberrations).

4 shows a schematic representation of the device according to the invention for beam measurement in a first embodiment. He is behind a beam waist 70 divergently spreading beam 71 is first, ie without prior collimation or other mapping, successively by two partially reflecting mirrors 10 . 20 reflected, so that the intensity of the thus coupled-out partial beam has dropped to a fraction. Below is the beam from a lens 50 collimated; the collimated beam 72 is then from a third partially reflecting mirror 30 reflected, wherein the plane which is spanned by incident and reflected beam, the so-called reflection plane, for the third mirror 30 is perpendicular to the reflection planes of the first and the second mirror. To illustrate this spatial arrangement, a perspective form of the representation is selected. For all three partially reflective mirrors 10 . 20 . 30 For reflection, the uncoated interface of a plane surface is utilized. The main part 74 the radiation transmitted through the mirror 10 , Through the mirror 20 and 30 each small residual shares occur 75 the radiation through, which can be absorbed by absorbers, not shown.

5 serves to explain the principle of compensation of the angular dependence of the reflectance in the device according to the invention. These are the reflections of the beam to be measured 71 at the first two mirrors 10 and 20 shown. At the same time the beam becomes 71 , related to the beam 76 on the optical axis, on the first mirror 10 around a first deflection angle 15 diverted and then at the second mirror 20 around a second deflection angle 25 , Looking at a marginal ray 77 of the beam 71 , it can be seen that the angle of incidence on the first mirror 10 for example, greater than the angle of incidence of the main beam 76 , on the second mirror 20 it is the other way around. Consequently, the reflectance for the marginal ray 77 once larger and once smaller than the reflectance for the main beam 76 , After both reflections, therefore, the differences in the resulting intensity of the beam have 72 almost balanced.

This compensation of the angle dependence is in 6 on the basis of the resulting intensity profile for the decoupled partial beam 73 in the inventive device according to 4 shown for both polarization directions as a function of the angle of incidence. The reflection on the third partially reflecting mirror 30 is also included in this illustration. The solid portion of the curves is that in the device 4 from the beam 71 used angle range.

In the 7a . 7b and 7c is shown as the free working distance 90 the device according to the invention, arising from the distance of the device to the beam waist 70 results, with the deflection angles 15 and 25 at first 10 and at the second mirror 20 related. In the arrangements after the 7a and 7b is the deflection angle 15 . 25 the same at both mirrors. In 7a the deflection angle is in each case 45 °, so that for the collimated beam 72 gives a favorable rectangular geometry, but the working distance 90 very small, the beam waist to be measured 70 is just outside the device. In 7b the deflection angle is 50 °, so that the collimation lens 50 can be arranged closer to the beam, the resulting working distance 90 is therefore a bit bigger, but still relatively small.

Only when using different deflection angle 15 and 25 at the first 10 and the second mirror 20 , as in 7c As shown, all the optical elements can cling well to the beam and you get a much larger one working distance 90 , Here is the deflection angle 15 at the first mirror 10 (For example, 40 °) smaller than the deflection angle 25 at the second mirror 20 (for example 60 °).

8th shows the device according to the invention in a second embodiment, in which between the first 10 and the second mirror 20 a relay lens 55 is inserted and thereby a much greater working distance is achieved, ie the beam waist 70 is much further away from the device. At the same time, a smaller diameter is obtained for the collimated beam 72 and 73 , which allows a more compact design, as with the smaller dimensions of the third mirror 30 is recognizable. Here, the beam only by a partially reflecting mirror 10 is attenuated before a lens 55 is arranged in the beam, the thermo-optical effects in this embodiment are not reduced as much as in the first embodiment.

In 9 the device according to the invention is shown in an embodiment similar to the first embodiment according to 4 , In this embodiment, a collimating lens 51 used in retro-focus construction. The retrofocus design causes the focal length of the lens, ie the distance from the focus, ie the beam waist 70 , to the first lens surface of the collimating lens 51 , greater than the effective focal length of the lens 51 , This will result in a smaller collimated beam diameter 72 such as 73 achieved, thereby enabling a more compact overall design.

10 shows the device according to the invention in the first embodiment by analogy 4 or 9 , with other components for complete imaging of the beam 71 with his beam waist 70 are shown. Additionally shown here is the beam deflection by a fourth mirror 40 , whereby the decoupled partial beam again parallel to the original beam 71 is aligned, and the focus 60 of the decoupled and attenuated sub-beam to a sensor 80 , The focusing lens 60 is designed in tele-construction, ie the focal distance from the last lens to the image plane 80 is smaller than the effective focal length of the lens, thus supporting a compact overall system design, especially at higher magnifications of the system.

In 11 the device according to the invention in a second embodiment is analogous to 8th shown, wherein other components for complete imaging of the beam are shown. In addition, the beam deflection is represented by a fourth mirror 40 , whereby the decoupled partial beam again parallel to the original beam 71 is aligned, and the focus of the decoupled and attenuated sub-beam on a sensor 80 , The focusing lens 60 is executed in tele-construction.

DETAILED DESCRIPTION OF THE INVENTION

The invention comprises in a basic embodiment three partially reflecting mirrors and a collimating lens in a special spatial arrangement. The beam to be measured has a beam waist 70 which may be formed by a beam exit, such as an optical fiber tip, or by the formation of a beam focus behind an optic. Behind the beam waist 70 the beam spreads and forms a divergent beam 71 , A first partially reflective mirror 10 is in the beam 71 arranged and coupled by reflection a small proportion of the radiation, thereby by a certain angle 15 is redirected, whereas the main part 74 of the beam 71 through the mirror 10 transmitted and subsequently to a beam trap 85 or a beam power meter can hit. The reflected portion of the beam hits the second partially reflecting mirror 20 which in turn reflects a portion and thereby at an angle 25 diverts while a residual share 75 the beam through the mirror 20 is transmitted and collected behind the mirror by an absorber, not shown. After reflection by the second mirror 20 The beam is from a collimation lens 50 collimated, the beam thus forms after collimation an almost parallel beam 72 , The collimated beam 72 is now through a third partially reflecting mirror 30 reflected. In this reflection, the plane of reflection, which is spanned by the incident and reflected beam of a mirror, is perpendicular to the reflection planes of the first and second mirrors. After the reflection at the third mirror 30 is now a collimated, attenuated beam 73 available, which is an accurate reflection of the original beam 71 represents. This partial beam 73 can now, for example, by means of a focusing lens 60 on a sensor 80 , eg a CCD camera.

The decoupled partial beam 73 intended to be an exact replica of the original ray 71 represent. This is ensured by the operation explained below.

Through the reflection on the first two mirrors 10 and 20 the angular dependence of the reflection is compensated, because eg a marginal ray 77 who is on the first mirror 10 has a larger angle of incidence than the main beam 76 , when reflecting on the second mirror 20 a smaller angle than the main beam 76 owns (cf 5 ). Due to the angular dependence of the reflection of the edge beam thus has, for example, at the first mirror a greater degree of reflection than the main beam, while at the second mirror, the reflectance is smaller. After the two-time reflection, the differences in the resulting intensity are almost balanced. The angular dependence is thereby almost eliminated, the very small residual error is now advantageously symmetrical to the optical axis and no longer disturbs.

To compensate for the remaining different beam intensities for the two polarization directions, the reflection on a third partially reflecting mirror 30 needed. In order to be able to compensate for the difference, instead of enlarging it further, the third mirror must be used 30 in a direction perpendicular to the other two mirrors 10 and 20 reflect. The re-occurrence of a variation of the reflection by different angles of incidence in the beam is prevented by the collimation by means of the collimating lens 50 which is therefore in front of the third mirror 30 is arranged.

6 shows the resulting intensity of the beam for both polarization directions in the collimated beam 73 for the in 4 or 9 embodiment shown; the residual variation in intensity is extremely small for a polarized beam in both directions of polarization, and for an unpolarized beam that can be thought of as being composed of both polarization components, and the resulting intensity therefore results from the average of the two curves, is practically immeasurable ,

• – Verwendung von mindestens 3 teilreflektierenden Spiegeln, die zur teilweisen Reflexion des Strahls die unbeschichtete Grenzfläche des Materials ausnutzen;
• – der erste Spiegel ist direkt im zu vermessenden, divergenten Strahl angeordnet, d.h. zwischen der zu abzubildenden Strahl-Ebene (die Strahl-Taille, der Strahl-Fokus oder die Strahl-Austrittsebene) und dem ersten Spiegel befindet sich kein abbildendes optisches Element;
• – die von einfallendem und reflektiertem Strahl eines Spiegels aufgespannte Ebene, die Reflexions-Ebene, ist bei den ersten beiden Spiegeln gleich;
• – nach der Reflexion an den ersten beiden Spiegeln wird der Strahl durch ein Kollimations-Objektiv kollimiert;
• – nach der Kollimation des Strahls wird dieser durch den dritten Spiegel reflektiert, hierbei steht die durch einfallenden und reflektierten Strahl gebildete Ebene senkrecht auf der entsprechenden Ebene der ersten beiden Spiegeln.

The device according to the invention can have various embodiments. Common to all embodiments are the following essential features:

• - using at least 3 partially reflective mirrors which utilize the uncoated interface of the material to partially reflect the beam;
• The first mirror is arranged directly in the divergent beam to be measured, ie there is no imaging optical element between the beam plane to be imaged (the beam waist, the beam focus or the beam exit plane) and the first mirror;
• The plane spanned by the incident and reflected beam of a mirror, the reflection plane, is the same in the first two mirrors;
• After reflection at the first two mirrors, the beam is collimated by a collimating lens;
• - After the collimation of the beam, this is reflected by the third mirror, in this case, the plane formed by incident and reflected beam is perpendicular to the corresponding plane of the first two mirrors.

• – Der Fokus-Shift (bzw. die Laserleistungs-induzierte Fokus-Verschiebung) ist äußerst gering und praktisch nicht mehr messbar.
• – Sehr genaue Übertragung des Strahlprofils vom ursprünglichen Strahl in den abgeschwächten Mess-Strahl.
• – Praktisch keine Abhängigkeit von der Polarisationsrichtung des Strahls.
• – Hohe exakte Abschwächung, typischerweise um mehr als drei Größenordnungen.

The features of the invention provide the following special advantages:

• - The focus shift (or the laser power-induced focus shift) is extremely low and virtually impossible to measure.
• - Very accurate transmission of the beam profile from the original beam into the attenuated measuring beam.
• - Virtually no dependence on the polarization direction of the beam.
• - High exact attenuation, typically more than three orders of magnitude.

The essential properties of the system are defined by the immediate physical relationships at the interfaces of the mirrors, so that there are no production tolerances and deviations.

A focusing lens is used to image the decoupled and attenuated partial beam onto a sensor.

Further embodiments of the invention may have the following features: The rear side of the partially reflecting mirrors may be antireflected to reduce stray light.

By means of a fourth mirror, preferably arranged between third mirror and focusing lens, the axis of the attenuated measuring beam can be aligned again parallel to the axis of the input beam.

Between the first and second mirror, a relay lens can be located, which has a magnification of about -1. In this way, the free working distance of the entire system to the measurement level (the beam waist to be measured) can be significantly increased.

In order to optimize the working distance in an embodiment with first and second mirrors arranged directly behind one another, it makes sense to design the reflection angle on the first and second mirrors differently, namely to select the reflection angle on the first mirror to be smaller. The refractive index of the first mirror is then also smaller to choose; the reflection angles and the refractive indices of the first and second mirrors are in a specific relationship to each other.

The refractive index of the first mirror can be chosen as small as possible, so that the reflectance is low.

The third mirror, which is in the collimated beam, may be wedge-shaped; uncoated front and anti-reflective coated back are not parallel to each other but include a small angle.

To obtain a particularly low focus shift, it makes sense to arrange a collimating or imaging optical system only after the second partially reflecting mirror. Unfortunately, this arrangement "consumes" extra working distance, i. the beam plane to be measured (the beam waist) runs the risk of running into the reflected beam. This could be acceptable for measuring the beam focus of a processing optics, the measurement of the beam from an optical fiber, in particular with a welded end cap, can be impossible.

To optimize the working distance, it is necessary to choose different angles of incidence on the two mirrors, relative to the optical axis, or the deflection angle of the beam at the mirrors, so that the optical elements as close to the beam "snuggle" can. 7 shows by way of example various arrangements. It can be seen that device variants with the same deflection angles on the first two mirrors ( 7a and 7b ) do not lead to a maximum working distance, this succeeds only with different deflection angles ( 7c ).

At different deflection angles on the first mirror and the second mirror, however, one loses the advantage of optimal compensation of the angle-dependent reflection, which again leads to a varying intensity over the beam aperture and thus to the falsification of the beam profile.

To get around this problem, choose materials with different refractive indices for the first and second mirrors. With a suitable ratio of the refractive indices, which depends on the ratio of the beam deflection angle, one obtains again an optimal compensation of the angular dependence.

α₁

Einfallswinkel auf erstem Spiegel, bezogen auf die optische Achse (= halber Umlenkwinkel am ersten Spiegel)

α₂

Einfallswinkel auf zweitem Spiegel, bezogen auf die optische Achse (= halber Umlenkwinkel am zweiten Spiegel)

n₁

Brechzahl des ersten Spiegels

n₂

Brechzahl des zweiten Spiegels

Optimal compensation of the angular dependence of the reflection, ie a minimal residual variation of the intensity, is obtained when the resulting intensity curve after the two reflections is symmetrical to the principal ray, that is symmetrical to the ray on the optical axis (cf. 6 ). The resulting intensity curve thus has an extremum (minimum or maximum) on the optical axis. However, different refractive indices for the first mirror and the second mirror give different optimal angles, depending on whether one considers an unpolarized, a P-polarized or an S-polarized beam. For an unpolarized beam, approximately the following relationship for the optimum angles of incidence arises: α ₂ / α ₁ ≈ (n ₂ / n ₁ ) ^1/2 [Formula 1] Explanation of the formula symbols:

α ₁

Angle of incidence on the first mirror, relative to the optical axis (= half deflection angle at the first mirror)

α ₂

Incident angle on the second mirror, relative to the optical axis (= half deflection angle at the second mirror)

n ₁

Refractive index of the first mirror

n ₂

Refractive index of the second mirror

For a polarized beam, there are other optimal angles, and slightly different angles depending on the polarization direction. For a polarized beam of arbitrary orientation, therefore, a mean optimum angle is to be used, for this applies approximately: α ₂ / α ₁ ≈ n ₂ / n ₁ [formula 2]

Thus, for given refractive indices and given angle of incidence for the first mirror, the optimum angle of incidence on the second mirror is in the following range: α ₁ (n ₂ / n ₁ ) ^1/2 ≦ α ₂ ≦ α ₁ (n ₂ / n ₁ ) [Formula 3]

If one has no information about the state of polarization, ie if it can be arbitrary as well as the beam can be unpolarized, then it makes sense to choose as compromise, for example, an angular ratio which results as mean value of formulas 1 and 2: α ₂ / α ₁ ≈ [(n ₂ / n ₁ ) ^1/2 + (n ₂ / n ₁ )] / 2 [Formula 4]

The 10 and 11 show variants of the device in an expanded form. The fourth mirror is used to align the beam axis again parallel to the input beam, and by means of a focusing lens, the beam is imaged onto a sensor (eg camera chip). Other components that are useful for operating the device but are not functionally necessary, such as jet absorber behind the second and the third mirror, or a further attenuation of the beam in front of the sensor, are not shown here.

Suitable materials for the mirrors are in principle the same materials that are otherwise used for optical elements such as lenses at highest Laser powers are suitable. These are in particular crystal glasses, which can be produced with a particularly high degree of purity and therefore have a very low absorption, for example quartz glass, calcium fluoride, sapphire or zinc sulfide.

The material for the mirror 1 is the use of quartz glass; on the one hand quartz glass has a relatively low refractive index, so that a particularly high proportion of the radiation is transmitted and only a small portion is coupled out, and secondly quartz glass has an extremely low temperature expansion coefficient, so that the disturbance of the wavefront of the coupled-out beam by thermo -optical effects is very low even at the highest beam power.

The in the 4 or in 9 or 10 embodiment of the invention shown can be used with the following numerical examples. An incident angle of 20 ° is used on the first mirror and an angle of incidence of 30 ° on the second mirror. This results in how in 7c shown, a favorable working distance. The material used for the first mirror is quartz glass (ie n ₁ = 1.45), since this material is very well suited for highest laser powers and has a low Fresnel reflection of about 3.5% on average due to the small refractive index. From the present angle ratio at the second mirror and the first mirror results according to formula 4, an optimal refractive index for the second mirror of n ₂ ≈ 2.46. A suitable material with a near refractive index is zinc sulphide (ZnS) with a refractive index of 2.29. The refractive index pairing 2.29 / 1.45 and an angle of incidence of 20 ° on the first mirror according to formula 3 gives a range for the optimum angle of incidence on the second mirror between 25.1 ° and 31.6 °. The selected angle of 30 ° in the embodiment is thus well in the optimum range.

Due to the reflections on the first two mirrors, the ratio between S- and P-polarized radiation is 2.4. This ratio must be compensated after the collimation. This is done by the reflection on the third mirror. It can be used for a mirror of zinc sulfide with an angle of incidence of 37.8 °, or alternatively a mirror made of sapphire with an angle of incidence of 33.2 °, or a mirror made of quartz glass with an angle of incidence of 30.2 °. If a mirror of zinc sulfide with the angle of 37.8 ° is selected as the third mirror, then the reflections at the three mirrors result in a weakening of the power of the coupled-out partial beam to approximately 0.07% of the power of the input beam.

The effective focal length of the collimating lens is preferably in a range of 50 mm to 500 mm, when the first and second mirrors are arranged directly one after the other and no relay lens is used.

When a relay lens is inserted between the first and second mirrors, the focal length of the collimating lens is preferably in a range of -50 mm to -200 mm.

The effective focal length of the focusing lens is preferably in a range of 100 mm to 1000 mm.

The magnification ratio defined by the focal length ratio of the focusing lens to the collimation objective is preferably in a range of 1 to 10.

LIST OF REFERENCE NUMBERS

10

first partially reflecting mirror

11

uncoated interface of the first mirror

12

Anti-reflection coating

15

Beam deflection angle of the first mirror

16

smaller angle of incidence than on the optical axis

17

greater angle of incidence than on the optical axis

20

second partially reflecting mirror

21

uncoated interface of the second mirror

22

Anti-reflection coating

25

Beam deflection angle of the second mirror

30

third partially reflecting mirror

31

uncoated interface of the third mirror

32

Anti-reflection coating

40

fourth mirror

50

Collimating lens

51

Collimation lens in retrofocus construction

53

Formation of a thermal lens by absorption of the laser radiation

55

Relay Lens

60

Focusing lens

70

Beam focus, beam waist or beam exit plane, as well as the plane of the beam to be measured

71

divergent beam

72

collimated beam

73

decoupled partial radiation for further imaging on a sensor

74

Main part of the original beam

75

transmitted residual portion of the beam

76

Main beam or beam on the optical axis of the beam

77

single marginal ray of the beam

80

Image plane with sensor, e.g. CCD camera

85

Jet trap, absorber or beam power meter

90

working distance

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 4243902 [0003]
• DE 10253905 [0003]
• DE 19630607 [0003]
• DE 3510937 [0004]
• DE 10129274 [0004]
• DE 102008022015 [0004]
• DE 10355866 [0006]
• DE 102008028347 [0006, 0010, 0010]
• DE 4006618 [0007]
• DE 10217657 [0007]
• DE 202004007511 [0012]

Claims (17)

Optics for beam measurement of optical radiation, which propagates divergently behind a beam waist, comprising in each case in the beam direction successively arranged a first partially reflecting mirror, a second partially reflecting mirror, a collimating lens consisting of at least one optical lens, and a third partially reflecting mirror. Apparatus according to claim 1, characterized in that the three partially reflecting mirror each consist of a transparent optical material and each having at least one planar surface. Apparatus according to claim 1 or 2, characterized in that the three partially reflecting mirrors each have an uncoated flat boundary surface on which a small part of the radiation of the respective mirror is reflected by utilizing the Fresnel reflection. Device according to one of the preceding claims, characterized in that no optically imaging element is arranged between the beam waist and the first partially reflecting mirror. Device according to one of the preceding claims, characterized in that it is designed such that the reflection plane, which is spanned at a mirror by incident and reflected beam, for the second mirror is parallel to the reflection plane at the first mirror. Device according to one of the preceding claims, characterized in that the beam is collimated by the collimating lens before reflection at the third partially reflecting mirror. Device according to one of the preceding claims, characterized in that it is designed such that the reflection plane at the third mirror is perpendicular to the reflection planes of the first and the second mirror. Device according to one of the preceding claims, characterized in that the angle of incidence α ₂ at the second mirror with respect to the optical axis of the beam, in a range defined by the following formula: α ₁ (n ₂ / n ₁ ) ^1/2 ≦ α ₂ ≦ α ₁ (n ₂ / n ₁ ), where α _{1 is} the incident angle at the first mirror with respect to the optical axis of the beam, and n ₁ and n _{2 are} the refractive indices of the first and second mirrors. Device according to one of the preceding claims, characterized in that a relay objective is arranged between the first partially reflecting mirror and the second partially reflecting mirror. Apparatus according to claim 9, characterized in that the relay lens relative to the beam waist has a magnification of about -1, and wherein the opening angle of the convergent beam behind the relay lens is about the same size as the opening angle of the divergent beam in front of the relay -Lens. Device according to one of the preceding claims, characterized in that in the beam direction of the third partially reflecting mirror is subsequently arranged a fourth mirror. Apparatus according to claim 11, characterized in that the fourth mirror is arranged such that after the reflection, the optical axis of the reflected beam is arranged parallel to the optical axis of the original beam. Device according to one of the preceding claims, characterized in that behind the third mirror or the fourth mirror, a focusing lens is arranged, which generates an image of the original beam waist in its image plane. Apparatus according to claim 13, characterized in that in the image plane of the focusing lens, a sensor is arranged. Device according to one of the preceding claims, characterized in that behind the third mirror means for attenuating the beam are arranged, which are selected from the group comprising partially absorbing plan plates such as gray glasses, partially transmitting plan plates, partially reflecting plan plates, a pair of successively arranged polarizers , whose polarization angle can be adjusted to each other. Device according to one of the preceding claims 1 to 10, characterized in that the second and the third mirror transmitted residual beam components are each collected by an element having an absorbent surface and with cooling fins for cooling by the ambient air or holes through which has a coolant flowing for cooling. Device according to one of the preceding claims, characterized in that not for the partial reflection of the radiation used rear surfaces of the first, second and third mirror are provided with an anti-reflection coating.

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