BRPI0605596B1 - DEVICE AND METHOD FOR CHARACTERIZATION OF LOW AND HIGH POWER LASER BEAMS BASED ON LIGHT SPREADING - Google Patents

Abstract

A device and method for the characterization of low and high power light scattering laser beams The present invention relates to a device and method for the characterization of high and low power light scattering laser beams which allows you to measure laser beam quality with a single image of the scattered light from the laser beam spread. More specifically, the present invention discloses a novel way of measuring the spatial parameters of laser beams in real time, and which can also be used to characterize a single pulse of a laser beam. With the data provided in real time by this device and the laser beam characterization method, laser applications become more efficient, as it allows to know the quality of the beam at the moment of laser use.

Description

(54) Title: DEVICE AND METHOD FOR THE CHARACTERIZATION OF LOW AND HIGH POWER LASER BEAMS BASED ON LIGHT SPREADING (51) Int.CI .: G01J 1/40 (73) Holder (s): ADVANCED STUDY INSTITUTE - IEAV (72) Inventor (s): KELLY CRISTINA JORGE; RUDIMAR RIVA; NICOLAU ANDRÉ SILVEIRA RODRIGUES ····· · · · · ··· • · ··· · · ··· ··· · · ··· ·· · · · · · · ··· · ·· · • · · · · · · · · · * · ·

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DEVICE AND METHOD FOR CHARACTERIZATION OF LOW AND HIGH POWER LASER BEAMS BASED ON LIGHT SPREADING

Field of the Invention ------ - __ '5 The present invention relates to a device and method for characterizing low and high power laser beams based on light scattering, which allows measuring the quality of the laser beam with a single image of the scattered light from the laser beam, in one or more transverse directions.

The present invention reveals a new way to obtain the spatial parameters of low and high power laser beams in real time, using the light scattering effect.

The present invention also makes it possible to characterize a single laser pulse.

Fundamentals of the Invention

Characterization of low and high power laser beams means measuring the spatial parameters of the beam: the minimum diameter and the divergence of the laser beam. To obtain the value of these parameters, it is necessary to measure the diameter of the laser beam in at least ten different positions of its propagation. In laser applications, it is essential to know these laser beam propagation parameters for the goals of these applications to be successfully achieved.

Devices and methods for characterizing laser beams comprising a digital sensor, or a slit, or even a blade, positioned in front of the laser beam, which scan the beam's frontal profile in each of the several propagation positions. Based on these measurements of the beam diameter, the quality of the laser beam is determined according to the pre-standard (ISO / 11146). The quality of a real laser beam is estimated in comparison to an ideal laser beam, which has a pure Gaussian profile and with the quality factor (M ² ) equal to one. Laser beams that have a quality factor greater than one (M ² = n> 1), have a divergence n times greater than the divergence of an ideal beam.

The sectors that involve laser applications, such as industrial, medicinal and scientific, require a device that makes it possible to know the characteristics of the propagation of laser beams at the time of laser processing. In recent years some devices have appeared that determine the quality of the beam in real time

2/6 · * ··· · »·· ·· ··« ** ··· ·· * · »· · ♦ · · · · · ·» · _ · * * ··· · ···· ··· · ···· ·· · · · »* · · · · · · · · · · · r ·· · · · · · · ·« »······ · ·« · « «T« „« (pulse to pulse) using multiple slits or a lens with a diffraction grating. In Brazil, the only filing of a patent application (PI0203571-5) refers to a method and apparatus for characterizing Gaussian laser beams using a thermal lens.

- Traditional devices (knife scanning technique or '5 beam frontal scan by a digital sensor) do not indicate the quality of a single pulse and, moreover, are time consuming, as it is necessary to make several measurements, but do not fail to be efficient. The devices and methods that have emerged in recent years are still not entirely simple. The device with multiple slits does not allow the measurement of a single pulse, as there is a small interval, but longer than the time of a pulse, so that all slits scan a certain area of the laser beam. The other device, which emerged recently, consists of a lens with a diffraction grid, which makes it possible to obtain nine images of the beam at once and, thus, the propagation of a single pulse and its quality. This device is complex, since it is necessary to make a very efficient calibration due to the variation of beam intensity in each propagation position.

With the present invention, the quality of laser beams is determined by measuring the diameters of the laser beam at a propagation distance of three Zr (Rayleigh length), whereas, for existing methods, at least one propagation distance of ten regions of Zr.

In view of the problem of characterizing a single laser pulse, the present invention aims to make possible, in an easy and efficient way, the characterization of low and high power laser beams in real time.

Brief Description of the Invention

The present invention has as its first object a light scattering medium. The scattering of light occurs when the laser beam (light) strikes an environment that contains particles randomly distributed. This laser beam propagates in the medium and meets the particles in the medium (or its photons collide with the particles in the medium) and, thus, occurs what we call light scattering, that is, a part of this beam of light is spread in several directions. In order to obtain the scattering of light in a medium, it is necessary that that medium has a sufficient amount of particles that cause a homogeneous scattering of the light in all directions. The light scattering effect of the laser beam makes it possible to visualize the spread of the profile

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AAA AA AA A • AAAAAA ♦ · AAAA • AA AA · · of beam intensity. This spreading medium is stored in a transparent cell, which does not deform the characteristics of the spread profile of the laser beam. -Focuses the laser beam spreader cell and this is observed on the side of ^_ cell, the propagation of the laser beam scattered. - - - _ - _ ___

The present invention also has as its object an optical image system to register the image of the spread of the laser beam that was spread when passing through the spreading cell. This image shows the diameter of the beam in several different positions of its propagation, in one or more transversal directions. In this way, the measurement of several beam diameters in different propagation positions is obtained simultaneously. With these measurements, the laser beam quality factor is calculated. The optical image system must be orthogonal to the direction of propagation of the laser beam in the spreader cell and present the image of the spread of the scattered laser beam clearly. For the optical image system to present an image with good visibility, it is necessary that the signal intensity of the scattered light of the laser beam is greater than the electronic background noise and less than the saturation limit of the digital sensor; you must use a / -number that does not cause an aberration effect; use a depth of field greater than the diameters of the laser beam and use a resolution ten times less than the minimum diameter of the laser beam.

More objects, features, aspects and advantages of the present invention will appear more clearly in the Detailed Description of the Invention.

Detailed Description of the Invention

Various beam quality measuring devices are already known. However, none of them use the light scattering effect.

In addition, there are very few devices (three) that claim to measure the quality of a single laser pulse. However, these devices are complicated and require skilled people to handle the technique. In fact, none of these existing devices provides, in a single image, all the information to determine the quality of the laser beam as the present invention.

The attached drawings show the disposition of the device for the characterization of low and high power laser beams based on the scattering of light, object of the present patent, and which will help to understand the invention.

Figure 1 shows the device in an exploded perspective;

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Figure 2 shows the image of the spread of the laser beam spread in a transverse direction (10) collected by the optical image system (6, 7, and 8).

Figure 3 shows an anamorphic optical image system mounted orthogonal to the

--direction direction of the laser beam in the spreader cell (4), to obtain the digital image of the spread of the spread laser beam (10).

In accordance with what the above figures illustrate, the device works as follows:

- the laser beam (1) must have a minimum diameter at least ten times the size of the image resolution (one pixel} of the optical image system, that is, the minimum beam diameter must be measured with, at least ten pixels, so that the error of the method is not greater than 10% in the final result. For that, a spherical lens (2) of focal length / in front of the laser (1) is used to focus the beam to inside the spreader cell (4), with the following conditions;

- the spreader cell (4) must be a transparent container so as not to deform the characteristics of the beam propagation profile and not to impair the visualization of the scattered light from the laser beam propagation (3). The spreader cell (4) must have the face of the beam entrance parallel to the laser exit cavity (1) together with the spherical lens (2). The side of the scattering cell (4), in which the scattered light from the beam propagation (3) is observed, must be orthogonal to the optical image system (6, 7 and 8);

- the spreader cell (4) must have a length of at least three Z _R (Rayleigh length). The _{Z R} indicates a distance in which the diameter of the laser beam increases 72 in relation to the minimum diameter, that is, each time the diameter of the

beam increases an amount of 72 of the minimum diameter, the laser beam propagated over a distance equal to Z _R. At least three Ζ _Λ must be measured;

- the spreading cell (3) must have a width and height greater than the diameter of the laser beam (1) and its faces must be thick enough to avoid damaging the optical path of image formation and, even less, the characteristics of propagation of the laser beam (1);

- the spreading medium (5) of the spreading cell (4) must be homogeneous, spread in all directions evenly and can be liquid, gaseous or solid. The concentration of the spreading medium must be sufficient to cause the scattering of light from the

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5/6 laser, with enough intensity to sensitize the digital sensor (8) of the optical image system without causing the signal to saturate;

- the optical image system must be formed by lenses (6) that do not influence the formation of the image; Use-openings (diaphragms) (7) that do not cause an aberration effect and are sufficient to obtain the intensity signal of the scattered light from the spread of the laser beam (3). The scattered light from the laser beam propagation (3) must be greater than the background noise signal caused by the electronic equipment in the digital sensor (8) and it must also not be too intense to avoid signal saturation in the digital sensor ( 8);

- in obtaining more than one transverse direction of propagation of the laser beam, for example, collecting the image of the propagation of the laser beam spread over two axes (x and y), simultaneously, one must use diffraction grating and appropriate lenses.

- another important characteristic of the optical imaging system is that the depth of field must be greater than the diameters of the scattered laser beam (3) in the spreader cell (4);

- the / -number factor is inversely proportional to the numerical opening of the optical system (F # = f / D). In the system, the / -number must always indicate a depth of field sufficiently greater than the diameter of the scattered laser beam (3);

- the simple relationship between depth of field and the / -number factor is as follows:

P = 8. λ. (F #) ² where P indicates the total depth of field, λ indicates the wavelength of the laser beam and F # is the symbol for the / -number factor;

- the image resolution of the optical system (pixel) must have a dimension of at least 10% of the beam diameter, so that its influence is, at most, 10% in the measurement of the beam diameter;

- a simple optical image system is used, composed of a spherical lens or an anamorphic optical system, formed by two cylindrical lenses, is used.

- an anamorphic optical image system is used (Figure 3), when the conditions for image resolution and minimum diameter are limited. The anamorphic imaging system is determined by two cylindrical lenses (6), with the same focal length and

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6/6 aperture (7), that is, ο same factor / -number. In these conditions, each cylindrical lens (6) acts individually in one plane of the image and the positions of the object and the image in relation to the lenses are identical in both planes, applying an inverse magnification between the vertical and horizontal planes;

- the cylindrical lens (6), which acts in the vertical plane, is aligned in relation to the z axis, and the cylindrical lens (6), which acts in the horizontal plane, is aligned in relation to the y axis;

- the electronic file of the image of the scattered light from the spread of the laser beam (10) must be recorded in a format that allows access to the data in any mathematical software;

- with the data in this image file, the beam diameters are determined in each propagation position (adding the pixels occupied by the beam diameters in each propagation position);

- finally, based on the spatial measurement of each diameter of the spread of the spread beam, a curve is determined, by which the minimum diameter is located in the focusing region and the beam divergence is calculated in relation to the region of the smaller diameter (beam waist). With the minimum radius value and the beam divergence, the beam quality factor (M ² ) is calculated.

Claims (15)

Claims 1) “DEVICE FOR THE CHARACTERIZATION OF LOW AND HIGH POWER LASER BEAMS BASED ON LIGHT SPREADING”, characterized by the fact that it is formed by a transparent spreading cell (4) with faces parallel to the cavity of a laser beam (1) and an optical image system (6, 7 and 8), said optical image system (6, 7 and 8), comprising cylindrical lenses (6), an aperture (7) and a digital sensor (8) , and the spreading cell (4) contains, inside, a spreading medium (5) gaseous, liquid or solid, whose homogeneous spreading particles are randomly distributed, with the laser beam (1) being focused by means of a spherical lens (2) into the spreader cell (4), with a minimum diameter at least ten times greater than the resolution (pixel) of the optical image system (6, 7 and 8); the light from the laser beam (3) being spread evenly across the spreading medium (5), in all directions and, on the side of the spreading cell (4), the scattered light from the spread of the laser beam (3) is observed ), and, orthogonal to the spreader cell (4), there must be an optical image system (6, 7 and 8) to collect (9) the image (10) of the scattered light from the spread of the laser beam (3) in one or more transversal propagation directions. 2) Device, according to claims 1, characterized by the fact that, in order to obtain the image of more than one transverse direction of propagation of the laser beam, simultaneously, diffraction grating with lenses must be used. 3) Device, according to claims 1 and 2, characterized by the fact that, to obtain the image of the scattered light of the spread of the laser beam, an anamorphic image system is used, formed by two cylindrical lenses (6) and reverse magnifications of the horizontal and vertical planes of the image or use a simple optical image system, consisting of a spherical lens and unit magnification. 4) Device, according to claims 1, 2 and 3, characterized by the fact that, to measure a greater distance of scattered light from the spread of the laser beam (3) in the image, it is necessary to use, in the horizontal plane, a magnification (M <1) and, in the vertical plane, use an inverse magnification of the horizontal plane, that is, use an “anamorphic” optical image system. 5) Device according to claims 1, 2, 3 and 4, characterized by the fact that, in the optical system of anamorphic image, it is necessary to use two cylindrical lenses Petition 870180012158, of 02/15/2018, p. 20/22 (6), with the same focal length f and aperture D (7) (same factor / -number), one of which is aligned in relation to the vertical plane and the other, to the horizontal plane. 6) Device according to claims 1, 2, 3, 4 and 5, characterized by the fact that the distance from the object (scattered light from the laser beam (3)) to the lens, in the horizontal plane, is equal to distance from the lens to the image (digital sensor (8)) in the vertical plane. 7) Device according to claims 1, 2, 3, 4, 5 and 6, characterized by the fact that the relative positions of the object (scattered light from the laser beam (3)) and the image (digital sensor (8 )) are identical for the two cylindrical lenses (6). 8) “METHOD FOR THE CHARACTERIZATION OF LOW AND HIGH POWER LASER BEAMS BASED ON LIGHT SPREADING”, according to the device described in claims 1 to 7, characterized by the fact that from the image (10) collected by the system optical image (6, 7 and 8), containing the beam diameter in several different propagation positions, in one or more transverse directions, the minimum diameter and divergence of the laser beam, which are the characterization parameters, are calculated of the laser beam, with the / -number factor (F #) of the optical image system (6, 7 and 8) given by the relationship between the focal / lens distance and the D aperture and said / -number factor (F #) it must always indicate a depth of field greater than the diameter of the laser beam in any of the propagation positions. 9) Method, according to claims 8, characterized by the fact that the / number (F #) of the optical image system (6, 7 and 8) must be such that the image system is not impaired with spherical aberration effects . 10) Method, according to claims 8 and 9, characterized by the fact that the o / -number factor (F #) must be correlated with the sensitivity of the digital sensor (8) and the intensity of the light scattered from the spread of the beam laser (3) in the spreading medium (5). 11) Method, according to claims 8, 9 and 10, characterized by the fact that the concentration and / or the density of particles of the scattering medium (5) must be sufficient to cause a signal of intensity of the scattered light of the propagation of the laser beam (3) on the digital sensor (8) (detector) of the camera, without exceeding the saturation limit of the optical digital sensor (8). 12) Method, according to claims 8, 9, 10 and 11, characterized by the fact that the scattered light signal from the laser beam propagation (3) must be greater than the electronic background noise signal. Petition 870180012158, of 02/15/2018, p. 21/22 13) Method, according to claims 8, 9, 10, 11 and 12, characterized by the fact that the image resolution of the optical system must have a dimension of at least 10% of the beam diameter, that is, each pixel must be at least ten times smaller than the diameter of the laser beam. 14) Method according to claims 8, 9, 10, 11, 12, and 13, characterized by the fact that it is necessary to measure a propagation distance of at least three Z _R (Rayleigh length) to determine the quality of the laser beam. 15) Method according to claims 8, 9, 10, 11, 12, 13 and 14, characterized by the fact that the image of the scattered light from the spread of the laser beam (10) presents the value of the diameter of the beam in each different propagation position, in one or more transverse directions. Petition 870180012158, of 02/15/2018, p. 22/22 1/1 Graphics * · · · ··· · ·"·· ··· · ···· ··" · · ·· ·· · · · ··· · · " ·· · · · · · · ·· · · · ♦