Patent Application
20240033851
The invention relates to the field of laser machining. In particular, the invention relates to an apparatus and a method for polarizing a laser beam, and a laser machining system including the apparatus.
When material is machined by using a laser beam, the polarization of the laser beam plays an important role. A defined polarization state of the laser beam allows certain interactions between laser beam and workpiece to be exploited in a targeted manner, for example in order to optimize the energy input into the workpiece by way of an adapted (e.g., increased) absorption of the laser radiation.
The majority of work currently undertaken within the scope of material machining by way of solid state lasers makes use of unpolarized radiation. This is due to the fact that, inter alia, the transportation fibers which guide the raw laser beam from the laser source to the machining optical unit and which typically have lengths of at least 20 m are unable to transfer a defined polarization state.
Although the polarization of a laser beam in the machining optical unit of a laser machining system may be unpolarized or randomly polarized, that is generally connected to a significant loss of energy. For example, a laser beam without a defined polarization state can be split into two polarized component beams by using a beam splitter. Since it is not readily possible to combine the two component beams to form a polarized machining beam (or output laser beam), only one of the component beams is used as a working beam. Thus, approximately 50% of the energy of the input laser beam is lost.
Chinese Publication CN 1484065 A describes an apparatus in which an incident unpolarized light beam is split into two differently polarized component beams, which are recombined by using optical elements. The incident light beam is split by using a birefringent element. The polarization state of one of the component beams is rotated by using a waveplate, with the result that both component beams have the same polarization state. The two component beams are then recombined via a lens and a K-shaped prism.
A substantial disadvantage of that apparatus lies in the fact that the resultant output beam does not have a defined, homogenous beam profile. Basically, the two component beams continue to exist in a spatially adjacent or partially overlapping manner, without “mixing” to form a common output beam.
It is accordingly an object of the invention to provide an apparatus and a method for polarizing a laser beam having an undefined polarization state and a laser machining system, which overcome the hereinafore-mentioned disadvantages of the heretofore-known apparatuses, methods and systems of this general type and which render it possible to modify an input laser beam having no defined polarization state, in such a way, without loss of power, that the input laser beam has a defined polarization state.
With the foregoing and other objects in view there is provided, in accordance with the invention, an apparatus for polarizing an input laser beam according to one aspect, with the input laser beam having an undefined polarization state. The apparatus includes a beam splitter device which is configured to split the input laser beam into a first component beam having a first defined polarization state and into a second component beam having a second defined polarization state. Further, the apparatus includes a polarization changing element for changing the polarization state of one of the polarized component beams, with the result that both component beams have the same defined polarization state. The apparatus includes a focusing element and a light-guiding element. The focusing element is configured to input couple both component beams into the light-guiding element in order to combine the component beams (by using the light-guiding element) to form an output laser beam while maintaining the defined polarization state (of the component beams).
The designations “input laser beam,” “first component beam,” “second component beam,” and “output laser beam” can preferably relate to different states of the same laser beam at different points in the beam path of the laser beam. The different designations in this respect merely serve to distinguish the various states or properties of the laser beam prior to or after the passage through the apparatus according to the invention.
Surprisingly, it was established that the component beams mix substantially without losses in the polarization state of the component beams to form a common laser beam (output laser beam) when passing through the light-guiding element, which is preferably configured as a fiber length. Consequently, a laser beam having an undefined polarization state can be polarized substantially without loss of power by using the apparatus disclosed hereinabove.
Examples of laser beams which have an undefined polarization state include unpolarized or randomly polarized laser beams, for which the polarization state is unknown.
It is understood that the components of the apparatus are preferably disposed successively in the described order in the propagation direction of the input laser beam.
It is also understood that the input laser beam or the component beams must be suitable for beam transmission by using optical waveguides (such as the light-guiding element). In particular, the input laser beam can be a solid-state laser beam. The laser beam (i.e., as input laser beam, and as the component beams, and as output laser beam) may have for example a wavelength of between 200 nm and 1300 nm, for example 515 nm or 1030 nm.
The input laser beam can preferably be directed in a collimated state at the beam splitting device. Alternatively, the input laser beam may also run slightly divergently or slightly convergently. For the purpose of providing a collimated input laser beam, it is possible to provide a collimation device (e.g., in the form of a lens or a mirror) which collimates the input laser beam prior to entry into the apparatus according to the invention.
Maintaining the polarization state of the component beams in the output laser beam should be understood to mean that the majority (i.e., at least 50%) of the output laser beam has the defined polarization state of the component beams, or a defined (e.g., slightly rotated) polarization state which deviates slightly from the aforementioned defined polarization state. According to a preferred variant, the proportion of the defined polarization state in the output laser beam can be at least 90%, preferably at least 95%, more preferably at least 98%. By way of example, the polarization state of a (fully) s-polarized component beam in the output laser beam should be considered maintained if the output laser beam still is 98% s-polarized.
For the purpose of the present invention, the correspondence of the polarization state of the output laser beam with the defined polarization state of the component beams can be determined in particular on the basis of the degree of polarization of the output laser beam. Here, the assumption can be made that the degree of polarization (P) of the component beams is P=1. The degree of polarization can be determined on the basis of the Stokes parameter, for example as described in Edward Collett (2005), Field Guide to Polarization, SPIE Press, p. 39 ff.
The light-guiding element preferably has a length long enough to combine the two component beams to form an output laser beam within the light-guiding element and/or short enough to maintain the defined polarization state of the two component beams in the output laser beam.
The combination of the component laser beams to form the output laser beam can be understood as a mixing of the component laser beams when passing through the light-guiding element, with the result that the output laser beam preferably has a uniform intensity distribution along its circumference. The output laser beam may have different cross-sectional shapes depending on the geometry of the light-guiding element. By way of example, the output laser beam may have a radially symmetric beam profile when an optical fiber with a circular cross section is used. The output laser beam may also have a polygonal, for example rectangular, or elliptical beam profile if a polygonal, for example rectangular, or elliptical fiber length is used. For example, an output laser beam with a defined polarization state can easily be rotated by using a waveplate, with the result that the polarization direction can be adapted to an advancement direction during the material machining.
Preferably, the beam splitting device can further be configured to deflect the first component beam and/or the second component beam in such a way that the two component beams run substantially parallel to one another. In this case, the phrase “substantially parallel” should be understood as including a deviation from the exact parallel position for as long as the two component beams are able to be input coupled into the light-guiding element by using the focusing element. By way of example, the phrase may include a deviation of up to 2°.
According to one variant, the length of the light-guiding element might be no more than 500 mm, preferably no more than 100 mm, more preferably no more than 50 mm.
In some fields of application, it may further be advantageous for the light-guiding element to have a length of one meter or a few meters. What needs to be taken into account in this context is that the defined polarization state of the component beams in the output laser beam is gradually lost with increasing length of the light-guiding element. The shorter the light-guiding element, the better the defined polarization state of the component beams is maintained in the output laser beam. By way of example, the polarization state of the output laser beam may still correspond to approximately 98% of the defined polarization state of the component beams when the light-guiding element has a length of 50 mm. This value may still be above 90% when the light-guiding element is a length of 1 m. It is therefore understood that the advantageous effects of the present invention—albeit in weakened form—still clearly come into effect when using a light-guiding element with a length of a few meters (e.g., up to 10 m).
Further, the length of the light-guiding element can be at least 15 mm, preferably at least 20 mm. Depending on the light-guiding element used, a specifiable minimum length of the light-guiding element is required to ensure a homogeneous beam profile of the output laser beam, in particular with a uniform intensity distribution along the circumference of the output laser beam.
Thus, it may overall be advantageous if the length of the light-guiding element is as short as possible in order to maintain the defined polarization state of the two component beams in the output laser beam to the greatest possible extent and is as long as necessary to ensure a sufficient homogeneity of the output laser beam.
The focusing element and the light-guiding element may be disposed symmetrically in the beam path of the component beams, to such an extent that the component beams are input coupled into the light-guiding element at the same angle.
In other words, the focusing element and the optical fiber can both be disposed on a central axis in the beam path which runs centrally between and parallel to the component beams. A ring-shaped beam profile arises in the far field of the output laser beam if both component beams are input coupled into the light-guiding element at the same angle.
According to an alternative variant, the focusing element and the light-guiding element may be disposed asymmetrically in the beam path of the component beams, to such an extent that the component beams are input coupled into the light-guiding element at different angles.
In this variant, the focusing element and the light-guiding element may have an axial offset from the central axis. In the case of a maximum offset, the focusing element and the light-guiding element can be located on the beam axis of one of the component beams, with the result that this component beam is input coupled into the light-guiding element at right angles. The other component beam is accordingly input coupled into the light-guiding element at a comparatively acute angle. Such maximal asymmetry of the input coupling of the two component beams into the light-guiding element yields a beam profile with a central spot and an outer ring in the far field of the focused output laser beam. It is understood that further asymmetric arrangements of the focusing element and the light-guiding element are possible between the symmetric arrangement and the maximally asymmetric arrangement, and cause corresponding beam profiles.
It is further understood that the angle of incidence of the component beams into the light-guiding element can additionally be regulated by the spacing between the component beams and the distance between the focusing element and optical fiber.
Thus, to regulate the beam profile of the output laser beam, provision can be made for the focusing element and the light-guiding element to be displaceably disposed along and/or across the beam propagation direction of the component beams.
According to a variant, the beam splitting device may include a thin-film polarizer and a mirror. The thin-film polarizer can be disposed at an angle in the beam path of the input laser beam such that a first component of the input laser beam, which has the first defined polarization state, is transmitted through the thin-film polarizer as first component beam and a second component of the input laser beam, which has the second defined polarization state, is reflected at the surface of the thin-film polarizer as second component beam. The mirror can be disposed at an angle in the beam path of one of the component beams in order to reflect the incident component beam in such a way that the latter is aligned substantially parallel to the other component beam.
The use of thin-film polarizers is particularly suitable for high laser powers.
It is understood that a separate mirror may also be disposed in each of the component beams, the respective mirror reflecting or deflecting the respective component beam in such a way that the component beams run substantially parallel to one another.
The use of a thin-film polarizer and a mirror for the beam splitting device has the advantage that the spatial distance between the component beams can be set as desired, independently of the laser power of the input laser beam.
According to an alternative variant, the beam splitting device can be a birefringent optical element which has different refractive indices in relation to the first defined polarization state and the second defined polarization state, with the result that the input laser beam is split into the first and the second component beam upon incidence in the birefringent element, with the component beams being aligned (substantially) parallel to one another by refraction effects upon the exit from the birefringent element.
The structure of this variant is particularly simple. Then again, the spatial offset of the component beams depends directly on the thickness of the birefringent element and cannot readily be set as desired.
The polarization changing element can be a waveplate, in particular a half-wave plate. In this way, the polarization state of the incident component beam can be rotated through 90°.
The purpose of changing the polarization of at least one of the component beams lies in aligning the (defined) polarization states of the component beams. It is understood that a multiplicity of possible combinations as to how the polarization states of the component beams can be aligned to one another arise in the process. A simple example would include the input laser beam being split into two linearly polarized component beams, with the first component beam having a p-polarization and the second component beam having an s-polarization. The polarization direction of one of the component beams can then be rotated, for example by using a half-wave plate, so that its polarization state is matched to the polarization state of the other component beam. By way of example, the polarization state of the second component beam can be rotated from an s-polarized component beam to a p-polarized component beam. However, it is understood that the component beams may also have a different defined polarization state. For example, the component beams can be or become elliptically polarized, more particularly circularly polarized.
The focusing element can preferably be an optical lens. According to one basic variant, a lens may be provided as a focusing element which focuses the first component beam and the second component beam at an end of the light-guiding element. However, it is understood that, according to an alternative variant, each of the component beams can also be focused into the light-guiding element by using a separate lens serving input coupling purposes. In such a case, it is not necessary for the two component beams to run parallel to one another. For input coupling into the light-guiding element, the component beams may also each be initially focused into a (short) connecting fiber, wherein the connecting fibers can be welded to the light-guiding element (e.g., likewise an optical fiber) by splicing. In such a case, the component beams reach the light-guiding element, where they are mixed to form the output laser beam, via the connecting fibers.
The light-guiding element may preferably have a circular cross section. Alternatively, the light-guiding element may also have a polygonal, for example rectangular, or elliptical cross section. By way of example, the light-guiding element can be an optical fiber, in particular a step-index fiber. However, other fiber types are also usable, for example a gradient-index fiber or a hollow-core fiber. The numerical aperture (NA) and the core diameter of the optical fiber play only a subordinate role for the effectiveness of the effects caused by the invention. For example, a light-guiding element according to the invention might be a step-index fiber with a core diameter of Ø=20 μm to Ø=400 μm, for example Ø=100 μm, and a numerical aperture of NA=0.065 to NA=0.22, for example NA=0.11. According to an alternative variant, the light-guiding element might also be a cylindrical or conical glass rod. The greater the numerical aperture of the fiber, the greater the angle with respect to the fiber longitudinal axis with which a component beam propagates through the fiber can be.
Further, the light-guiding element may have a tapering cross section. For example, the light-guiding element may be in the form of a conical optical fiber, specifically in the form of what is known as a tapered fiber. That is to say, the cross section of the fiber core reduces over the length of the fiber from the entrance end to the exit end (conical fiber). In comparison with an optical fiber which has an unchanging cross section (cylindrical fiber), the beam quality of the output laser beam can be improved using such a fiber. However, it should be observed that, in the case of the same length of the optical fiber, the retention of the polarization state may reduce with the reduction of the fiber diameter since the incident laser beams are reflected correspondingly more frequently within the optical fiber in the case of a reduced diameter.
With the objects of the invention in view, there is also provided a method for polarizing an input laser beam having an undefined polarization state according to a further aspect. In a first step, the method includes a splitting of the input laser beam into a first component beam having a first defined polarization state and into a second component beam having a second defined polarization state. In a second step, the method includes a changing of the polarization state of one of the polarized component beams, with the result that both component beams have the same defined polarization state. In a third step, the method includes an input coupling of both component beams into a light-guiding element in order to combine the component beams to form an output laser beam while maintaining the defined polarization state (of the component beams). It is understood that the method steps are performed in the sequence described.
For example, the method is able to be carried out by using an apparatus according to the invention in accordance with one of the above-described variants. In particular, the method may have one or more features and/or advantages of the above-described apparatus.
With the objects of the invention in view, there is concomitantly provided a laser machining system according to a further aspect. The laser machining system comprises a laser beam source for generating an input laser beam; a transportation optical fiber with a length of several meters, in particular more than 10 m, which is connected at a first of its ends to the laser beam source; and a machining optical unit which is connected to a second end of the transportation optical fiber. The machining optical unit includes: a collimation device for collimating the input laser beam incident in the machining optical unit from the transportation optical fiber; an apparatus according to any of the above-described variants for polarizing the input laser beam; and a focusing device for focusing the polarized output laser beam on an object to be machined.
For example, the laser machining system can be a laser cutting system for cutting preferably metallic workpieces.
The machining optical unit may further include a waveplate, in particular a half-wave plate, which is rotatably disposed in the beam path of the output laser beam, to be precise preferably between the polarization apparatus according to the invention and the focusing apparatus. By way of an appropriate rotation of the waveplate, the polarization direction of the output laser beam can be adapted to the cutting direction or advancement direction of the laser.
In summary, the above-described apparatus is based on the principle of a polarization-maintaining combination of at least two laser beams which, by using a focusing device, are input coupled into a light-guiding element which preferably has a length long enough to allow the at least two laser beams to be combined or mixed within the light-guiding element to form an output laser beam and/or short enough to maintain the polarization state of the at least two laser beams in the output laser beam.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an apparatus and a method for polarizing a laser beam having an undefined polarization state and a laser machining system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
Variants of an apparatus according to the invention for polarizing an input laser beam are described in detail hereinafter in conjunction with
Referring now to the figures of the drawings in detail and first, particularly, to
The apparatus 1 according to
A waveplate 20, for example a half-wave plate, is also disposed in the beam path of the second component beam 52b and transforms the second polarization state of the second component beam 52b (for example, s-polarization) such that it corresponds to the first polarization state of the first component beam 52a (for example, p-polarization). That is to say, once the second component beam 52b has passed through the waveplate 20, both component beams 52a, 52b are parallel to one another and have the same defined polarization state (specifically the first polarization state, for example p-polarization).
The apparatus 1 further includes a lens 30 which is disposed in the beam path of the two component beams 52a, 52b, in order to focus the latter and input couple these into a light-guiding element 40. The light-guiding element 40 has a length L (cf.
A variation of the apparatus 1 according to
In a manner analogous to
As an alternative to the symmetric arrangement (see
The illustration according to
In a manner analogous to
The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2021 108 759.7 | Apr 2021 | DE | national |
This application is a continuation, under 35 U.S.C. § 120, of copending International Patent Application PCT/EP2022/058324, filed Mar. 29, 2022, which designated the United States; this application also claims the priority, under 35 U.S.C. § 119, of German Patent Application DE 10 2021 108 759.7, filed Apr. 8, 2021; the prior applications are herewith incorporated by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2022/058324 | Mar 2022 | US |
| Child | 18483656 | US |
