RU2725685C1 - Tunable optical shaper of scalable flat homogeneous laser beam - Google Patents

Abstract

FIELD: optics.SUBSTANCE: tunable optical shaper includes a housing, an optical input and an output and a tunable beam diverger for inputting a divergent laser beam from a source with a Gaussian radiation intensity profile and outputting said beam to an optical intensity converter having a cylindrical lens and a Fresnel biprism, behind which a substantially flat beam-neck portion is formed in the caustic, elongated in a transverse direction to the optical axis due to presence of divergence beam moving in direction of its optical axis of positive lens. Cross-section of the beam in the neck is a segment of a straight line with a length which is a multiple of the radius of the Gaussian beam in the case of a collimated mode of operation of the divergence divider, and with a Gaussian width of the line which is a multiple of the output aperture of the laser source.EFFECT: technical result is a possibility of smooth adjustment of the focused laser beam size with preservation of uniformity of power density of laser radiation of kilowatt range within the image of the spot on the target in the form of a straight line segment.3 cl, 1 tbl, 7 dwg

Description

Technical field

The invention relates to an optical device from the field of laser optics for forming a laser beam, namely, the output of laser radiation in the direction of the processed body and the formation on it of a focused sharp image of a uniform flat laser beam.

State of the art

Optical shapers (configurators, converters) of laser radiation are used to convert an inhomogeneous laser beam into a uniform beam. In this case, various optical devices containing lenses, diffraction gratings, or matrix elements are used. The most convenient laser beam for conversion is a beam with a Gaussian power distribution in the cross section. After optical conversion, you can get a uniform power distribution on the target surface in the form of spots of various shapes: round, rectangular, square, flat. The uniformity within the stain allows to achieve a higher quality of material processing. So, for example, when laser surfacing of powder material over a large area, it is advantageous to use a rectangular beam of high power, and even better flat. Known configurators are static devices in the composition of the laser terminal devices (heads) of powerful lasers and are not able to change or rearrange the linear dimensions of a homogeneous laser spot without deteriorating the uniformity of the distribution of the radiation power density within it.

The present invention provides a tunable optical device that is capable of scaling an image of a configured laser spot without noticeable deterioration of the uniform distribution of power density within the spot in the form of a straight line segment of different lengths with sharp boundaries.

In the invention US 20090032511 A1, uniformity in the form of radiation homogenization in the beam caustic was realized by the beam configurator, which converts the radiation distribution of the Gaussian beam into radiation with another more uniform spatial distribution of the illumination field. In the invention, uniformity of the illumination field is achieved by using a plurality of micro-optical elements to homogenize the radiation. The system is designed for low laser radiation powers, it is not tunable and cannot be used at high radiation powers due to significant losses on the elements of microoptics.

A known non-tunable static optical achromatic system, EP 1998215 (A1), converts a laser beam with coaxial symmetry and a Gaussian intensity distribution over the beam section into a uniform distribution over the section through the use of two groups of lenses with different achromatic dispersion. Beam transformation is achieved by using aspherical lenses. It is clear that the cost of an optical system with such a configurator will be too high due to the presence of aspherical surfaces of the optical elements, which are much more laborious to manufacture compared to traditional spherical surfaces. In addition, for monochromatic laser radiation, the requirements for achromatic optics are also unnecessary.

A patent for the invention is known to US 7400457 B1, in which two aspherical astigmatic cylindrical lenses with orthogonally oriented axes are used to form a rectangular spot on the target. It is clear that in this case, the cost of an optical system with such a configurator will be too high due to the presence of astigmatic cylindrical surfaces of optical elements, which are much more laborious to manufacture compared to traditional cylindrical surfaces. In addition, the described system does not allow scaling the image size of the laser spot.

The closest in technical essence to the present invention is the invention WO 2012071050 A1, in which a combination of a cylindrical lens and a Fresnel lens is used to form a uniform beam profile of a high-power laser. However, due to the discrete set of static optical elements, there is no possibility of scaling the image of the spot.

The present invention is devoid of the above disadvantages of previously known optical laser power distribution configurators and allows solving the problem of scaling and outputting laser radiation in the direction of the body being processed with the formation of a focused sharp image of a uniform spot on it in the form of a tunable line of a given length and with a uniform distribution of laser power density in its limits.

Disclosure of invention

The present invention is based on the task of developing an optical device that does not contain aspherical optical elements, capable of performing the required scaling of the image of a uniform plane laser beam on the target. The image is obtained in the form of a line of the required length from a laser source with a Gaussian profile of the output beam of high power, sufficient for metal processing.

The technical result is the possibility of smoothly adjusting the size of the focused laser beam while maintaining the uniformity of the power density of the laser radiation in the kilowatt range within the image of the spot on the target in the form of a straight line segment, and, within the line, the radiation is not only uniform, but also has pronounced sharp outlines, therefore, the maximum possible radiation power density on the treated surface of the material is achieved.

To solve this problem, it is proposed to use an optical device - an optical beam former, which runs in a rigid case, has an optical input and output, and movable optical elements (lenses). Its optical input is connected to the output of the laser device. For high-power fiber lasers of the kilowatt power range, the radiation aperture of the outgoing Gaussian beam is determined by the size of the fiber core (more than 14 μm) and in the form of a diverging beam is fed to the input of the optical beam former and then to the optical beam divergence beam, which provides the required beam divergence in accordance with the required scaling images of the laser spot on the treated surface (screen, target) due to mobility inside the body of the optical shaper. The beam divergence shaper is capable of forming a convergent, collimated and diverging beam, depending on the size of the output beam scaling. The output beam itself undergoes a transformation of the intensity across the cross section (in the profile) of the original Gaussian beam passing through the beam intensity transducer, which is performed as a pair of optical elements - a cylindrical lens and Fresnel biprism. The presence of a movable beam divergence shaper provides smooth adjustment of the scaling of a plane beam without affecting its uniformity within the image spot on the target, which is presented in the form of a line as a projection of a plane laser beam.

Other advantages and features of the proposed invention will become apparent from the following description of preferred embodiments given with reference to the accompanying drawings.

Brief Description of the Drawings

FIG. 1 Known (traditional) optical scheme of a uniform linear profile former from a collimated Gaussian beam: 1 - Fresnel biprism, 2 - cylindrical lens, 3 - laser radiation source, 10 - optical beam collimator, 100 - optical beam intensity configurator, M - target (screen or part plane).

FIG. 2 Known (traditional) distribution of radiation intensity along the Z axis after the conversion of a collimated Gaussian beam of radius W ₀ . In the direction of the X axis, a distribution of size W ₀ . In the direction of the Y axis in the beam - Gaussian section, a profile whose size w (at the level of e ^-2 ) is equal to the size of a point radiation source multiplied by the magnification factor of the optical system (the ratio of the foci of the cylindrical lens 2 and the collimator 10 in the circuit in Fig. 1).

FIG. 3 Scheme of a tunable optical beam former for scaling a plane homogeneous diverging laser beam with a movable beam divergence former shifted by a distance x from its main focal plane F _c .

FIG. 4 Diagram of a tunable optical beam former for scaling a plane homogeneous diverging laser beam with a movable beam divergence former and mutually orthogonal menisci at its output, F _c coincides with f _c .

FIG. 5 Uneven beam intensity profile with the smallest possible parameter d = 5 mm (x = 60 mm). The output aperture of the laser source is accepted - 50 microns. Profile size L = 50 mm. Power loss ΔW = 0.5%, non-uniformity δ = 12.6%. The full scale along the X axis is equal to twice the length of the L profile. The full scale along the Y axis is 10 mm.

FIG. 6 The dependence of the size of a planar beam L on the displacement x in the circuit of FIG. 3 (d = 45 mm).

FIG. 7 Dependence of the empirical coefficient α _p (on the scaling factor m), which is necessary for the exact calculation of the radius of the correcting meniscus R _{p of the} Fresnel biprism to reduce the power loss and heterogeneity of the straight beam profile.

Legend:

W ₀ is the radius of the collimated Gaussian beam in the optical shaper;

w is the size of the Gaussian profile, determined by the size of the aperture (diameter of the light guide core of the optical fiber) of the laser radiation source, multiplied by the magnification factor of the optical system (the ratio of the foci of the cylindrical lens and collimator);

L ₀ is the size of the flat profile for the collimated Gaussian beam, equal to W ₀ ;

L is the size of the flat profile for a non-collimated Gaussian beam;

f _c is the focal length of the radiation divergence former in the beam collimation mode;

F _c is the position of the main plane of the divergence former;

f _l is the focal length of a cylindrical lens for a collimated beam;

f _p is the distance from the biprism to the waist of the caustic beam for a collimated beam;

F _p is the distance from the biprism to the caustic constriction for an uncollimated beam;

F _l is the distance from the cylindrical lens to the caustic constriction for an uncollimated beam;

d is the distance (distance) from the cylindrical lens to the main plane F _c of the divergence former;

X is the lens shift in the divergence shaper from its main plane F _c ;

d _l is the distance between the cylindrical lens and its corrective meniscus;

d _p is the distance between biprism and its correcting meniscus;

R _l is the estimated radius of curvature of the correction meniscus for a cylindrical lens;

R _p is the estimated radius of curvature of the correcting meniscus for Fresnel biprism;

α _p is the empirical coefficient for the proportional (α _p ⋅R _p ) correction of the calculated radius of curvature R _p ;

δ is the relative heterogeneity of the intensity of the uniform profile of the beam;

m is the scaling factor;

ΔW is the relative loss of radiation power.

The implementation of the invention

A typical known optical transducer (Fig. 1), a beam intensity configurator 100 for converting a collimated Gaussian laser beam into a flat and uniform cross section in the caustic constriction, consists of a Fresnel biprism 1 at the radiation output and a cylindrical lens 2 in front of it. A collimated radiation from a laser source 3 is supplied to its input. An optical collimator of a beam 10 forms a collimated beam, which traditionally contains one or more lenses and converts a diverging laser beam into a collimated one. On Target M (the screen or in the plane of the part), in the constriction of the caustics, a light spot is thus formed in the form of a straight line with a length equal to the radius of the collimated Gaussian beam W ₀ and width w, determined by the size of the point Gaussian laser source 3.

As is known, Fresnel biprism divides a collimated Gaussian beam into two half-beam converging towards each other. At a certain distance from the Fresnel biprism in the constriction of the beam caustic in a plane orthogonal to the optical axis, the image size of the light spot will be minimal. This distance will be called the “focus” of the Fresnel biprism. The intersection of wave fronts produced by the Fresnel biprism does not lead to interference and the corresponding spatial modulation of the radiation intensity, if the radiation coherence length is significantly less than its focal length. For example, for radiation from high-power fiber lasers, the coherence length is less than 0.05 cm with a radiation spectrum width of 2–4 nm.

A cylindrical lens compresses a collimated Gaussian beam in one plane. In the orthogonal plane of the Fresnel biprism, at its focus, which coincides in position with the focus of the cylindrical lens, forms a total homogeneous pattern of the intensity profile by superimposing two halves of a Gaussian profile with a width equal to the radius W ₀ (at the level of e ^-2 ) of the initial beam. As a result of this summation by combining, a uniform distribution of radiation intensity in the projection of the beam image onto a flat screen is obtained, in the form of a straight line segment of length L ₀ = W ₀ and width w of a Gaussian profile (Fig. 2), determined by the size of a point Gaussian radiation source multiplied by a coefficient increasing the optical system (the ratio of the foci of the cylindrical lens 2 and the collimator 10). To compare the options for forming a flat linear profile, we determine the parameters of the beam inhomogeneity and relative power loss. The relative heterogeneity of the radiation intensity of the beam profile δ is determined by the formula:

where I _max is equal to the maximum, and I _min is the minimum intensity within a uniform beam intensity profile. The relative power loss in percent ΔW in the beam is defined as the fraction of power in the wings of the intensity distribution outside the sharp edges of a uniform profile.

In the present invention (Fig. 3, 4) to control the image size of the caustic constriction in the form of a line L, without loss of uniformity (uniformity) in the range of the top of the flat power distribution (Fig. 2), it is proposed to use a tunable optical beam former 102 in the housing of which a beam divergence shaper 101 containing at least one movable lens 4, instead of the optical beam collimator 10 (Fig. 1). The divergence shaper at its output forms not only a collimated, but also diverging (converging) beam, which allows using the beam intensity converter 100 to achieve the goal. The divergence angle at the output of the beam divergence former can vary from converging to diverging (in particular, it can correspond to the collimated case). Accordingly, the converging beam forms a reduced image at the output of the Fresnel biprism 1 (compared to the size L ₀ with a collimated beam in rays A, B and A ', B'), and the diverging beam forms an increased image size of the line L (in rays A, D and A ', D'), which is the projection of a flat beam onto a screen or onto a laser-treated flat surface of a body.

It should be noted that the image of the line L is located between the rays A and A '(Fig. 3, 4). The angle between these rays is 2β (β = α⋅ (n-1), where α is the angle at the base of the Fresnel biprism, n is its refractive index) and determines the location of the sharp edges of the image of the line L. Thus, its size is strictly proportional to its distance from Fresnel biprism.

A prerequisite for obtaining a uniform intensity distribution in the beam profile when the optical intensity transformer 100 is operating with a beam divergence shaper 101 configured for a collimated beam operation is the coincidence of the caustic constrictions from the cylindrical lens 2 and the Fresnel biprism 1, and the divergence shaper 101 is removed from source 3 a distance equal to the focal length f _c , defined as for the case of a collimated beam at the output of the device 101.

If a parallel Gaussian beam is incident on the intensity transducer 100, then after it, in the constriction of the caustic of the beam, in focus from the cylindrical lens 2 and the Fresnel 1 biprism substantially confocal with it, we have a fixed non-tunable distribution of radiation intensity, shown in Fig. 2, which corresponds to the course of the rays B, A and B ', A' in FIG. 3. If an uncollimated, for example, diverging Gaussian beam is incident on the transducer 100, then the arrangement of the foci of the Fresnel biprism and the cylindrical lens F _p and F _l will not coincide (Fig. 3). In this case, the focus of the Fresnel biprism F _p is at the intersection of the rays A, D and A ', D', and the focus of the cylindrical lens F _{l is} noticeably larger. Since the sharpest image is in the focus of the cylindrical lens, and its location can differ markedly from the position of the focus of the Fresnel biprism, the condition for the strict superposition of the halves of the Gaussian beam on one another is not satisfied, a noticeable gap is formed in the distribution of the radiation intensity in the center of the image of the line L (Fig. five).

To reduce the magnitude of the dip (unevenness), it is necessary to reduce the divergence angles between the rays A and D, and, accordingly, between A 'and D', which determine the location of the focus of the Fresnel biprism (by changing the parameter x), and also increase the distance d (Fig. 3), allowing to reduce the focus of a cylindrical lens. Such a setup leads to a smaller discrepancy between the locations of the constrictions of the beams from the Fresnel biprism and the cylindrical lens. As a result, the unevenness in the plane beam will decrease as the image size L increases due to a further decrease in the divergence angles between the rays A, D and A ', D', respectively.

It is possible to approximate the divergence driver 101 to the source of powerful radiation 3, up to some reasonable limits. The limitation arises from the maximum allowable power density at its input. And if a further increase in image size is required, then use two meniscus cylindrical lenses 5 and 6 (Fig. 4) with mutually orthogonal optical axes corresponding to the position of the cylindrical lens 2 and the Fresnel biprism 1, installed between the original cylindrical lens 2, at the output of the divergence shaper 101 One lens changes the focus position of the cylindrical lens 2, the other focus position of the Fresnel biprism 1. These additional lenses ensure that the caustic constrictions from the cylindrical lens coincide with the Fresnel biprism in an uncollimated Gaussian beam when constructing an L image for a planar uniform beam (Fig. 4).

The use of two meniscus orthogonally oriented cylindrical lenses 5 and 6 as part of the optical shaper allows you to significantly increase the linear size of the flat beam in the focus area of the image of the cylindrical lens, without causing an undesirable increase in power density on the input lenses of the shaper. However, it should be borne in mind that an increase in the linear image size by a factor of m leads to a decrease in the radiation intensity in the spot by a factor of m ² .

Thus, further, the invention proposes the following device to achieve the above objectives.

A tunable optical shaper of a scalable flat homogeneous laser beam in its caustic constriction region, characterized by the presence of a housing 102, an optical input and output, and containing, along the beam, a tunable beam divergence shaper 101 at the input to input a diverging laser beam from a source 3 with a Gaussian profile the radiation intensity and the output of this beam to an optical intensity transducer 100 located at the output of the optical shaper, behind which an essentially flat beam waist is formed in the caustic, elongated in the transverse direction to the optical axis; wherein:

- the beam intensity converter 100 comprises a cylindrical lens 2 and a Fresnel biprism 1;

- divergence shaper 101 (Fig. 3) contains at least one positive lens 4, movable in the direction of its optical axis, by shifting which the dimensions of the elongated beam constriction in the caustic at the output of the optical shaper 102 are smoothly adjusted, and the beam cross section in the constriction represents essentially a segment of a straight line of length L, a multiple of the radius of a Gaussian beam in the case of a collimated mode of operation of the divergence driver, and with a Gaussian line width multiple of the output aperture of a point laser source;

- scaling of the essentially planar waist of the caustic beam is carried out due to the angular divergence of the beam at the output of the divergence driver 101 with a focal length f _c (determined in accordance with the collinear path of the rays in the beam), namely due to the displacement of the divergence driver lens to the laser source by a distance X from the main focal plane F _c of the divergence shaper, for a given distance (f _c + d) of the cylindrical lens from the laser source, and the value of L is calculated by the formula:

Where

d is the remoteness of the cylindrical lens from the main focal plane F _c of the beam divergence former;

f _l is the focal length of the cylindrical lens;

L ₀ is the value of L during the collinear path of the rays in the beam at the input of the intensity transducer.

The dependence L (x) for d = 45 mm, f _c = 120 mm, f _l = 200 mm, f _p = 185 mm is shown in FIG. 6.

The focal lengths of a cylindrical lens 2 and Fresnel biprism 1 F _l (x, d) and F _p (x, d), respectively, in a diverging beam are calculated by the formulas:

It is significant that at the output of the beam divergence shaper there are two interchangeable mutually orthogonally oriented cylindrical meniscus lenses 5 and 6 with a refractive index n and curvature radii R _l and R _p for correcting a cylindrical lens 2 and Fresnel biprism 1, respectively, providing the required scaling by m times the image size of the line on the target, compared with the collinear path of the rays in the beam at the input of the cylindrical lens in the absence of meniscus lenses, and when calculating the meniscus radii, the formulas are

where f _l is the focal length of the cylindrical lens;

f _p is the distance from the biprism to the constriction in the region of the caustic of the beam for the case of a collinear path of rays in the beam at its entrance;

d _l is the distance between the cylindrical lens and its corrective cylindrical meniscus;

d _p is the distance between the Fresnel biprism and its corrective cylindrical meniscus.

In this case, in the presence of menisci, it is possible to obtain almost any enlarged image size in a discrete way (in combination with smooth adjustment due to lens shift 3), using a meniscus cartridge with radii corresponding to the above formulas in the divergence 101 specifying the magnification factor m.

The table shows the results of calculating the inhomogeneity δ and the relative radiation power loss ΔW depending on the increase coefficient m of the size of the flat profile L.

It is also significant that when calculating the heterogeneity of power losses in the presence of correcting menisci, the dependence of the radius of the correcting meniscus for the Fresnel biprism R _{p was} empirically established to obtain better results. For this, the coefficient α _{p was used} , the dependence of which on the increase in m is shown in FIG. 7. This means that in order to reduce the inhomogeneity of the beam intensity profile and radiation losses when scaling a flat beam, the calculated radius of curvature of the meniscus R _p , which corrects the position of the caustic constriction from the Fresnel biprism, is multiplied by the empirical coefficient α _p from the range 1.1 ÷ 1, 25, thereby clarifying its curvature with respect to the previously calculated value according to claim 5, within the scale factor m of the plane beam from 20 to 2, respectively (Fig. 7).

Obviously, the composition of the tunable optical beam former is not limited to the set of elements described above, for example, instead of two menisci one can use one complex one, but the production of this requires considerable costs.

It should be noted that the radiation intensity of a plane beam in a radiation spot in the form of a uniform line is much higher than in a rectangle, which contributes to a more efficient use of cw laser energy in a power range above 1 kW in industrial technologies for processing materials over large areas with the possibility of real-time laser scale adjustment spots - the size of a flat uniform beam.

It will be apparent to those skilled in the art that the invention is not limited to the embodiments presented above, and that changes within the scope of the claims may be included. Distinctive features presented in the description together with other distinctive features, if necessary, can also be used separately from each other.

Claims (13)

1. A tunable optical shaper of a scalable plane homogeneous laser beam in the region of its waist in the caustic, characterized by the presence of a housing, an optical input and output, and containing, along the beam, a tunable beam divergence driver at the input to input a diverging laser beam from a source with a Gaussian radiation intensity profile and outputting this beam to an optical intensity transducer containing a cylindrical lens and Fresnel biprism and located at the output of the optical shaper, behind which an essentially flat section of the beam waist is formed in the caustic, elongated in the transverse direction to the optical axis due to the beam divergence in the shaper, at least one positive lens movable in the direction of its optical axis, characterized in that by shifting the positive lens, the dimensions of the elongated beam constriction in the caustic at the output of the optical shaper are continuously adjusted, the cross section of the beam in the constriction is essentially a segment of a straight line of length L that is a multiple of the radius of the Gaussian beam in the case of a collimated mode of operation of the divergence driver, and with a Gaussian line width that is a multiple of the output aperture of the laser source, and scaling of the essentially flat constriction of the caustic beam due to the angular divergence of the beam at the output of the divergence diverter with the focal length f _c (determined in accordance with the collinear path of the rays in the beam), namely, due to the displacement of the positive divergence lens of the divergence driver to the laser source by a distance X from the main focal plane F _c of the divergence driver, predetermined distance (f _c + d) of the cylindrical lens from the source of laser radiation, and the value of L is calculated by the formula:

where d is the distance of the cylindrical lens from the main focal plane F _c of the beam divergence former; f _l is the focal length of the cylindrical lens; L ₀ is the value of L during the collinear path of the rays in the beam at the input of the intensity transducer. 2. A tunable optical shaper according to claim 1, characterized in that at the output of the beam divergence shaper there are two interchangeable mutually orthogonally oriented cylindrical meniscus lenses with a refractive index n and curvature radii R _l and R _p for correction of a cylindrical lens and Fresnel biprism, respectively, providing the required scaling in m times the size of the image of the line on the target, compared with the collinear path of the rays in the beam at the input of the cylindrical lens in the absence of meniscus lenses and in calculating the meniscus radii are used the formulas: R _l = (m + d / f _p ) / (m - 1) ⋅ (f _l - d _l ) ⋅ (n - 1), R _p = m / (m - 1) ⋅ (f _p - d _p ) ⋅ (n - 1), where f _l is the focal length of the cylindrical lens; f _p is the distance from the biprism to the constriction in the region of the caustic of the beam for the case of a collinear path of rays in the beam at its entrance; d _l is the distance between the cylindrical lens and its corrective cylindrical meniscus; d _p is the distance between the Fresnel biprism and its corrective cylindrical meniscus. 3. The tunable optical shaper according to claim. 2, characterized in that for reducing the inhomogeneity of the beam intensity profile and the radiation loss when scaled planar beam, the calculated radius of meniscus curvature R _p, correction beam waist position from the caustic Fresnel biprism is multiplied by a factor α _p from the range 1.1 ± 1.25, thereby clarifying its curvature with respect to the previously calculated value according to claim 2, within the scaling factor m of the plane beam from 20 to 2, respectively.

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