Optical arrangement for attenuating laser beams

Abstract

Optical arrangement (1) for attenuating laser beams (2), comprising at least two optical elements (3, 4), each arranged such that it reflects a portion of the incident radiation power of a laser beam (2), characterized in that the attenuation of the laser beam (2) takes place by Fresnel reflection at the optical elements (3, 4) and between at least two of the optical elements (3, 4) a polarization-influencing optical element (5) is arranged, which rotates the polarization of the incident laser beam (2) by 90°, wherein the folding of the reflected beams (6) takes place in at least one plane.

Description

The present invention relates to an optical arrangement for attenuating laser beams, comprising at least two optical elements each arranged to reflect a portion of the incident radiation power of a laser beam.For applications which require the measurement and analysis of high-power laser beams, reliable attenuation is often necessary, which, however, influences the essential beam properties-such as polarization state, intensity distribution in location and angle-as little as possible. The challenge here is in particular to reduce laser powers in the kilowatts range to a lower power range in the milliwatt to watts range in order to make the beam manageable for measuring instruments without significantly distorting the original beam parameters in the process.In the prior art, as described in U.S. Patent Application US20112555088A1, a polarization-compensated beam splitter system is used for beam splitting, which is based on two uncoated optical wedges and a polarization rotator between them. Although this arrangement allows a certain stability with respect to polarization influences and environmental influences such as temperature and humidity, some fundamental challenges arise in this case. For example, such systems often rely on laser beams of highly stable polarization. Even minor changes in the polarization state can cause significant measurement inaccuracies, since the reflectivity of the wedges is not constant at different polarization states and small changes in the polarization lead to considerable variations in the beam splitting. This can impair the reliability and consistency of the beam measurement, which is particularly problematic if precise, repeatable results are required.In addition, the prior art often uses large angles of incidence (typically 45°) to achieve adequate reflection for attenuation.However, this geometry leads to a beam offset, which makes integration into compact measurement systems more difficult and can impair the optical stability at high powers.Alternative methods for attenuating laser beams, such as glass plates with HR (high reflection) coatings, variable polarization beam splitters or absorbing filters, are also known. However, these methods likewise exhibit restrictions, in particular due to thermal effects and an angle dependence of the attenuation. Coated elements tend to be subject to thermal distortions or can be damaged in intense lasers, which affects the precision and reliability of the attenuation. Absorbing filters also prove to be insufficiently loadable at high beam intensities and under certain circumstances influence the intensity distribution of the beam.Overall, there is thus the disadvantage in the prior art that most optical attenuation systems either alter the polarization or angular characteristics of the laser beam or offer insufficient thermal stability. This makes it difficult to measure high-power laser beams in the kilowatts in a precise and repeatable manner. There is therefore a need for a solution which provides constant attenuation independently of the polarization state, remains thermally stable and, in addition, leaves the intensity distribution unchanged in location and angle in order to capture the beam in the measurement process as unaffected as possible.It is therefore the object of the invention to avoid or at least reduce the disadvantages known from the prior art and to provide an optical arrangement for attenuating high-power lasers, which enables a compact, efficient solution for precise attenuation, which is developed especially for applications for measuring high-power lasers.This object is achieved by an optical arrangement for attenuating laser beams, comprising at least two optical elements which are each arranged such that they reflect part of the incident radiation power of a laser beam, wherein the attenuation of the laser beam is effected by Fresnel reflection at the optical elements, and a polarization-influencing optical element is arranged between at least two of the optical elements, which polarization-influencing optical element rotates the polarization of the incident laser beam by 90°, wherein the folding of the reflected beams is effected in at least one plane.This optical arrangement offers the advantage that the laser power is reduced precisely and efficiently by Fresnel reflection without significantly influencing the beam properties. The arrangement enables reliable attenuation by the reflecting optical elements and ensures with the polarization rotating element that the polarization remains stable. The configuration allows a compact, thermally stable and easily integratable solution for high power lasers, thus reducing maintenance and reducing overall costs.According to an advantageous embodiment of the invention, it can be provided that the polarization-influencing optical element is arranged between the optical elements in such a way that it enables uniform attenuation of the laser beam independently of the polarization state of the incident beam. The invention is advantageously further developed in such a way that the polarization behavior of the laser beam is changed and controlled in a targeted manner in order to enable uniform attenuation independently of the polarization state of the incident laser beam. This results in stable and reproducible performance in laser measurements and facilitates integration into systems operating with different polarization states, thereby reducing installation costs.According to a further preferred development of the invention, it can also be provided that the folding of the reflected beams takes place approximately in a single plane, which enables a compact construction. This facilitates the integration of the optical assembly into existing systems and reduces the space requirement, which is advantageous in industrial environments with limited space. The simple folding plane also reduces the adjustment effort during installation and improves the stability of the beam profile.Furthermore, according to a likewise advantageous embodiment of the invention, it can be provided that the optical elements consist of a material having a coefficient of thermal expansion of at most 5×10 -6 K -1 and an absorption of at most 0.1% at the relevant laser wavelength, preferably quartz glass, which minimizes thermal distortions. This choice of material ensures stable beam properties even at high power and extends the service life of the arrangement by low thermal loading. The thermally stable construction is particularly suitable for continuous operation and reduces the operating costs by means of less maintenance outlay.According to a further particularly preferred embodiment of the invention, it can be provided that the polarization-influencing optical element is designed such that it influences both the horizontal and the vertical polarization components of the laser beam. This combination of features provides the advantage of achieving uniform attenuation regardless of the polarization state of the incoming beam. The manipulation of the horizontal and vertical polarization components ensures consistent attenuation, which enables precise laser measurement and control. The arrangement is thus versatile and requires fewer adaptations, which saves time and effort.Furthermore, the invention can also be further developed to the effect that the polarization-influencing optical element is designed to be rotatable and is preferably designed as a λ / wave plate, which enables variable adaptation of the attenuation. A λ / 2 waveplate as variable component allows the attenuation to be set flexibly without the need for mechanical changes to the entire arrangement. This increases versatility and user-friendliness and enables rapid adaptation to various requirements.In a likewise preferred embodiment variant of the invention, it can also be provided that at least one optical element has a coating which adjusts the reflection properties in order to influence the overall attenuation and / or the polarization of the reflected beam. This combination of features offers the advantage that the reflected beams can be influenced in a targeted manner by a coating on the optical elements. The adjusted reflection allows exact control of the attenuation and the polarization of the beam. The optimized reflection improves the energy efficiency of the arrangement and minimizes unwanted light losses, which is particularly decisive in high-precision applications.It may also be advantageous to further develop the invention in that the rear sides of the optical elements are at an angle to the front side and are provided with an antireflection coating in order to avoid undesired reflections in the direction of the attenuated beam, whereby undesired reflections in the direction of the attenuated beam are avoided. This provides constant and clear beam guidance and increases the efficiency and precision of the attenuation. The reduction of undesired reflections also increases the safety of the system and protects adjacent optical or sensory components from disturbing deflections.According to a further preferred embodiment of the subject matter of the invention, it can be provided that the optical elements are designed as quartz glass elements with a nominal angle of incidence of 22.5° and the reflected beams are folded such that the attenuated beam exits at an angle of 0° or 90° with respect to the incident laser beam, wherein a polarization-influencing optical element, in particular a λ / waveplate, is arranged between the optical elements, said polarization-influencing optical element rotating the polarization of the incident laser beam by 90°. This combination of features offers the advantage that the attenuation of the laser beam takes place particularly precisely for applications with a small angle of incidence. A nominal angle of incidence of 22.5° stabilizes the beam parameters even at high intensity. The λ / 2 waveplate rotates the polarization 90° which ensures constant polarization and increases the effectiveness of attenuation. This arrangement is optimally suitable for compact measurement systems in which the precision and stability of the beam properties are of primary importance.Finally, the invention can also be advantageously embodied such that the optical elements are embodied as quartz glass elements with a nominal angle of incidence of 45° and the reflected beams are folded such that the attenuated beam exits at an angle of 0° or 90° with respect to the incident laser beam, wherein a polarization-influencing optical element, in particular a λ / waveplate, is arranged between the optical elements, said polarization-influencing optical element rotating the polarization of the incident laser beam by 90°. Advantageously, the 45° angle of incidence configuration provides a flexible option allowing 0° or 180° deflection, depending on the application. The λ / 2 waveplate rotates the polarization 90° to ensure uniform attenuation regardless of the polarization state. This configuration allows use in applications with larger deflection angles and ensures reliable attenuation of the beam at high output power.The invention provides a significant improvement in the attenuation of laser beams that have high variability in angle of incidence, as is often the case with highly divergent or focused beams after processing optics or at the output of a fiber. These particular beam conditions place particular requirements on the optical arrangement, as conventional attenuation systems often provide inconsistent results at varying angles of incidence. The optical system according to the invention, on the other hand, enables uniform and reliable attenuation over a broad spectrum of angles of incidence without significantly influencing the beam properties.In summary, an important advantage of the invention is therefore its ability to homogeneously attenuate divergent and convergent beams, which ensures excellent stability and reproducibility in comparison with the prior art. The attenuation is adapted by the specific configuration of the optical elements such that constant attenuation is achieved even at large angles of incidence and strong divergence or convergence. Thus, the arrangement of the invention can process beams which typically occur after focusing in processing optics and have substantial angular deviations without substantially altering the polarization characteristics or the intensity distribution of the beam.Another advantage of the invention is the suitability for high-precision attenuation directly after processing optics or at the output of fibers, where beams often have very large divergence or convergence angles. Due to the optimized design of the attenuation elements and the use of a polarization-rotating element, a high uniformity over the entire angle of incidence range is achieved, whereby the beam quality is maintained. This is particularly useful for high power applications where precise and stable beam parameters are critical to achieving reproducible measurement results or to accurately control laser power.The invention thus significantly reduces the effort for the adjustment and calibration of the attenuation systems and enables simpler integration into compact measurement and machining systems, which further increases the overall efficiency and reliability of these systems.The invention will be explained in more detail below with reference to figures without limiting the general concept of the invention.It shows: FIG. 1 shows a first embodiment of an optical arrangement according to the invention in a schematic illustration, FIG. 2 shows a diagram of the angle-dependent attenuation as a function of the vertical divergence angle and horizontal divergence angle with respect to the embodiment shown in FIG. 1, FIG. 3 shows a second embodiment of an optical arrangement according to the invention in a schematic illustration, FIG. 4 shows a diagram of the angle-dependent attenuation as a function of the vertical divergence angle and horizontal divergence angle with respect to the embodiment shown in FIG. 3, FIG. 5 shows a third embodiment of an optical arrangement according to the invention in a schematic illustration.FIGS. 1, 3, 5 show an optical arrangement 1 for attenuating laser beams 2, comprising at least two optical elements 3, 4, which are each arranged such that they reflect a part of the incident radiation power of a laser beam 2.The attenuation of the laser beam 2 by Fresnel reflection takes place at the optical elements 3, 4, wherein a polarization-influencing optical element 5 is arranged between at least two of the optical elements 3, 4, which element rotates the polarization of the incident laser beam 2 by 90°, wherein the folding of the reflected beams 6 takes place in at least one plane.In this case, the polarization-influencing optical element 5 is arranged between the optical elements 3, 4 in such a way that it enables uniform attenuation of the laser beam 2 independently of the polarization state of the incident beam. The folding of the reflected beams 6 takes place approximately in a single plane.The optical elements 3, 4 consist of a material having a coefficient of thermal expansion of at most 5×10-6K-1and an absorption of at most 0.1% at the relevant laser wavelength, preferably quartz glass.The polarization-influencing optical element 5 is designed such that it influences both the horizontal and the vertical polarization components of the laser beam 2. The polarization-influencing optical element 5 can also be designed to be rotatable in particular and preferably as a λ / 2 wave plate.At least one optical element 3, 4 may further comprise a coating that adjusts the reflection properties to influence the overall attenuation and / or the polarization of the reflected beam 6.The rear sides 7 of the optical elements 3, 4 are at an angle to the front side 8 and are provided with an antireflection coating in order to avoid undesired reflections in the direction of the attenuated beam.The optical elements 3, 4 are designed as quartz glass elements with a nominal angle of incidence of 22.5° and the reflected beams 6 are folded such that the attenuated beam exits at an angle of 0° or 90° with respect to the incident laser beam 2, wherein a polarization-influencing optical element 5, in particular a λ / 2 waveplate, is arranged between the optical elements 3, 4, said polarization-influencing optical element rotating the polarization of the incident laser beam 2 by 90°.Referring to the figures in detail: FIG. 1 shows a preferred embodiment of the optical arrangement 1 according to the invention for attenuating laser beams 2 which resorts to two glass transitions and a λ / 2 waveplate 5 (half-wave plate) positioned therebetween. The arrangement 1 is designed such that the incident laser beam 2 impinges on the first glass transition at a nominal angle of incidence of 22.5°. This angle of incidence was chosen to ensure as constant a Fresnel reflection as possible, which leads to uniform attenuation of the beam 2.Between the two glass transitions, a λ / 2 waveplate 5 is arranged with its fast axis (fast axis) in such a way that it is at an angle of 45° to the folding plane. This arrangement causes the polarization of the beam 2 to be rotated 90° after the first glass transition, whereby horizontal polarization components are converted into vertical polarization components and vice versa. This allows the total reflections .The beam 2 then passes through the second glass transition, in which reflection likewise takes place. By the specific arrangement of the λ / 2 waveplate 5 between the glass transitions, the polarization portion of the reflected light is rotated through 90° a further time, such that the original polarization of the beam 2 is restored after the second transition. As a result, the polarization state of the attenuated beam remains unchanged in the overall system.This configuration ensures that the polarization of the laser beam 2 is not affected in the attenuation. The symmetrical configuration of the arrangement also enables a compact and simple integration into systems for beam measurement, since no beam offsets arise due to the arrangement and the incident and the attenuated beam run in the same convolution plane.FIG. 2 shows an embodiment of the optical arrangement according to the invention for attenuating laser beams, which comprises two optical elements with a nominal angle of incidence of \(\theta_{i nominal}=22.5° b) and a λ / 2 wave plate arranged between the elements, as is also known from FIG. 1. This configuration enables polarization-independent attenuation by the targeted adaptation of the beam reflection, so that constant attenuation of the incident laser beam is achieved.In FIG. 2, the angle-dependent attenuation of this arrangement is graphically depicted, showing the change in total reflection \(\alpha_V\ )) and horizontal divergence angles. The graph shows the percent change in total reflectance belonging to R_H wax) at a vertical divergence angle of \(\alpha_V = waxpm7° B), where wax (R_H wax) varies from +32% to -27%. Despite these divergences, the total reflection of the arrangement according to the invention remains more constant over the entire angle of incidence range than in the prior art, which leads to a significantly more stable and reliable beam attenuation.The advantage of this configuration is that the folding of the beam guide takes place exclusively in one plane, which significantly facilitates the integration of the arrangement according to the invention into a measurement system. The one-sided folding does not produce any offset between the incoming and attenuated beam, which simplifies the installation and the adjustment into compact systems. The invention thus provides a homogeneous attenuation of the beam over the angle of incidence range and at the same time reduces the adjustment outlay, which increases the overall efficiency and stability in the measurement and attenuation of laser beams.FIGS. 3 and 4 show a variant of the embodiment known from FIGS. 1-2. The same applies to FIG. 5.The invention is not limited to the embodiments shown in the figures. The foregoing description is, therefore, not to be considered as limiting, but illustrative. The following claims should be understood to mean that a said feature is present in at least one embodiment of the invention. This does not exclude the presence of further features. If the patent claims and the above description define "first" and "second" features, this designation serves to distinguish two features of the same type without specifying a ranking.List of reference characters1 Optical arrangement 2 Laser beam 3 Optical element 4 Optical element 5 Polarization-influencing optical element 6 Beam 7 Rear side 8 Front sideReferences included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedUS 201110255088A1

[0003]

Claims

An optical arrangement (1) for attenuating laser beams (2), comprising at least two optical elements (3, 4) which are each arranged such that they reflect a part of the incident radiation power of a laser beam (2), characterized in that the attenuation of the laser beam (2) takes place by Fresnel reflection at the optical elements (3, 4) and a polarization-influencing optical element (5) is arranged between at least two of the optical elements (3, 4) which rotates the polarization of the incident laser beam (2) by 90°, wherein the folding of the reflected beams (6) takes place in at least one plane.Optical arrangement according to Claim 1, characterized in that the polarization-influencing optical element (5) is arranged between the optical elements (3, 4) in such a way that it enables uniform attenuation of the laser beam (2) independently of the polarization state of the incident beam.Optical arrangement according to Claim 1 or 2, characterized in that the folding of the reflected beams (6) takes place approximately in a single plane.Optical arrangement according to one of the preceding claims, characterized in that the optical elements (3, 4) consist of a material having a coefficient of thermal expansion of at most 5×10 -6 K -1 and an absorption of at most 0.1% at the relevant laser wavelength, preferably quartz glass.Optical arrangement according to one of the preceding claims, characterized in that the polarization-influencing optical element (5) is designed such that it influences both the horizontal and the vertical polarization components of the laser beam (2).Optical arrangement according to one of the preceding claims, characterized in that the polarization-influencing optical element (5) is designed to be rotatable and is preferably designed as a λ / 2 wave plate.Optical arrangement according to one of the preceding claims, characterized in that at least one optical element (3, 4) has a coating which adjusts the reflection properties in order to influence the overall attenuation and / or the polarization of the reflected beam (6).Optical arrangement according to one of the preceding claims, characterized in that the rear sides (7) of the optical elements (3, 4) have an angle with respect to the front side (8) and are provided with an antireflection coating in order to avoid undesired reflections in the direction of the attenuated beam.Optical arrangement according to one of the preceding claims, characterized in that the optical elements (3, 4) are designed as quartz glass elements with a nominal angle of incidence of 22.5° and the reflected beams (6) are folded such that the attenuated beam exits at an angle of 0° or 90° with respect to the incident laser beam (2), wherein a polarization-influencing optical element (5), in particular a λ / 2 waveplate, is arranged between the optical elements (3, 4), said polarization-influencing optical element rotating the polarization of the incident laser beam (2) by 90°.Optical arrangement according to one of the preceding claims 1 - 8, characterized in that the optical elements (3, 4) are designed as quartz glass elements with a nominal angle of incidence of 45° and the reflected beams (6) are folded such that the attenuated beam exits at an angle of 0° or 180° to the incident laser beam (2), wherein a polarization-influencing optical element (5), in particular a λ / 2 waveplate, is arranged between the optical elements (3, 4), said polarization-influencing optical element rotating the polarization of the incident laser beam (2) by 90°.

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