CN101797666A - Laser cutting head with extended depth of focus - Google Patents

Abstract

A laser cutting head for extending the focal depth of a laser cutting machine, which comprises a collimating beam expanding lens group, a reflector and an aberration correcting focusing lens in sequence along the incident direction of the laser, and is characterized in that: a binary phase element is added in front of the aberration correcting focusing lens, the binary phase element is coaxial with the aberration correcting lens, the aperture of the binary phase element is consistent with the incident pupil of the laser cutting head, the binary phase element is a multi-zone structure of concentric circular rings with central symmetry, and the phase of the binary phase element is distributed alternately from the inside to the outside according to zones 0 and π. The present invention is conducive to improving the cutting speed and the maximum cutting thickness of such a laser cutting machine, and at the same time improving the quality of the cut section. It has important practical value and application prospects for improving the cutting speed and energy utilization rate of the laser cutting machine and improving the quality of the cut section.

Description

Laser cutting head with extended depth of focus

technical field

The invention relates to a laser cutting machine, in particular to a laser cutting head for extending the focal depth of the laser cutting machine.

Background technique

Since the advent of lasers in 1960, laser technology has developed rapidly. Due to the unique high brightness, high directivity, high monochromaticity and high coherence of laser, it has been proposed to use laser for material processing since its invention. At present, laser processing technology has developed into an important laser application technology. Compared with traditional mechanical processing, the main characteristics of laser processing are: the laser beam energy is highly concentrated, the processing area is small, so the thermal deformation is small; the processing quality is high, the precision is high, and the speed is fast; the processed parts are not limited by size and shape; Cooling medium, and no pollution, low noise. After nearly fifty years of development, laser processing has been combined with multiple disciplines to form multiple application technology fields. The main processing technologies include: laser cutting, laser welding, laser marking, laser drilling, laser heat treatment, laser rapid prototyping, Laser coating etc. Among them, laser cutting has become the most widely used laser processing method in the current industrial processing field, accounting for more than 70% of the entire laser processing industry.

The main principle of laser cutting is to rely on the highly concentrated light field energy of the laser to directly vaporize or melt the part of the workpiece to be cut to achieve the purpose of cutting. That is to say, the high energy density of the laser is a crucial factor. It is precisely because of the extremely high energy density of the focused laser beam that the workpiece can be vaporized or melted instantly, and the thermal effect is not transmitted to the surroundings in time, so that thermal deformation will not occur. It is this extremely high energy density that makes fast cutting possible. What determines the energy density of the focused laser beam is its light intensity distribution near the focus. This focused light field distribution near the focus is the most essential factor affecting the quality and speed of laser cutting. In the field of laser processing, people usually use the focal spot radius R and focal depth DOF near the focal point to measure the performance of a laser cutting machine. Currently, longitudinal depth of focus is a bottleneck factor affecting laser cutting speed and cutting thickness. For relatively thin workpieces, defocusing often occurs during the rapid movement of the workpiece relative to the laser cutting head, especially for workpieces with uneven surfaces (such as curved surfaces). At this time, it is necessary to increase the working energy of the laser, which will reduce the utilization rate of laser energy and affect the quality of the cutting section of the workpiece. A possible solution is to add a focus servo system for autofocus, which will increase the cost again. At the same time, if the speed of the focus servo cannot keep up with the moving speed of the workpiece, defocusing will also occur. At this time, the speed of the focus servo has become a serious bottleneck for increasing the cutting speed. On the other hand, when cutting thicker workpieces, if the thickness of the workpiece exceeds the depth of focus, then it will be a great challenge. At this time, the problem cannot be solved simply by increasing the laser output energy. Because, on the one hand, the output energy of the laser is limited, when the laser is in high power and overload operation for a long time, its life will be significantly shortened. At the same time, if the laser output energy is only increased, the light intensity distribution is very uneven beyond the focal depth range, and the result of this is that the parallelism of the cutting section of the workpiece is very poor. That is to say, any laser cutting machine has a maximum cutable workpiece thickness, and we call this thickness its limit cutting thickness. Obviously, the larger the depth of focus, the larger the limit cutting thickness.

We know that the depth of focus and the numerical aperture of the focusing lens are closely related. In the low numerical aperture range (NA<0.5), the relationship between the depth of focus DOF and the numerical aperture NA can be written as DOF∝λ/NA ² , that is, the depth of focus is inversely proportional to the square of the numerical aperture of the focusing lens. To prolong the depth of focus, the numerical aperture must be reduced, and as the numerical aperture decreases, the focus spot will increase, and the energy density will decrease, which is not conducive to laser cutting. That is to say, simply reducing the numerical aperture is a method to extend the depth of focus at the expense of reducing the resolution of the focused spot and the energy density of the focused laser beam.

Based on the above reasons, extending the focal depth without reducing the optical field distribution of the lateral resolution of the focused spot, that is, extending the focal depth on the premise of ensuring that the energy density in the focal area of the focused laser beam is sufficiently large, will be very beneficial to improving the speed of laser cutting and improving Cut section quality.

Contents of the invention

The object of the present invention is to propose a laser cutting head for extending the focal depth of a laser cutting machine, the laser cutting head for extending the focal depth can significantly improve the focusing focal depth of the laser beam, while keeping the transverse focal spot basically unchanged , It is very beneficial to increase the cutting speed and limit cutting thickness of the laser cutting machine, and improve the quality of the cutting section.

Technical solution of the present invention is as follows:

A laser cutting head with an extended focal depth, which consists of a collimating beam expander lens group, a mirror and an aberration-eliminating focusing lens in sequence along the incident direction of the laser light, and is characterized in that: A binary phase element, the binary phase element is coaxial with the aberration-ablation lens, the clear aperture of the binary phase element is consistent with the entrance pupil of the laser cutting head, and the binary phase element is The multi-zone structure of the center-symmetrical concentric rings, the phases of the multi-zones are distributed sequentially according to zone 0 and π from the inside to the outside.

The binary phase element is a 3-zone binary phase element, and the normalized radii of the 3-zone binary phase element for a plane wave with uniform intensity distribution are: 0, 0.3029, 0.9167 and 1 in sequence.

The binary phase element is a 5-zone binary phase element, and the normalized radii of the 5-zone binary phase element for a plane wave with uniform intensity distribution are: 0, 0.0196, 0.3421, 0.5523, 0.9251 and 1 in sequence.

Technical effect of the present invention

The present invention adds a pure phase-modulated binary diffractive optical element to the focusing optical path of the laser cutting machine, thereby prolonging the focal depth of the focused beam without increasing the size of the transverse focusing spot and improving the uniformity of the longitudinal energy density distribution of the focused light field Effect. This focused light field with a more uniform energy density distribution in the longitudinal direction has important practical value and application prospects for improving the utilization rate of laser energy, improving the processing efficiency and processing quality of laser cutting machines.

Description of drawings

Fig. 1 is a schematic diagram of the optical path of a laser cutting machine with extended focal depth according to the present invention. The optical path according to the serial number is: 1-laser; 2 collimating beam expander lens group; 3 mirror; 4 binary phase element; 5 aberration-eliminating focusing lens;

Fig. 2 is a schematic diagram of a physical model of a focused laser beam in a laser cutting head.

Figure 3(a) Typical 3-zone binary phase element; (b) Typical 5-zone binary phase element.

Fig. 4 The variation of the depth of focus with the truncation ratio β of the Gaussian intensity distribution before and after adding the binary phase element.

Fig. 5. Longitudinal (a) and transverse (b) light intensity distributions before and after adding 3-zone phase elements in two cases (β→0 and β=1).

Detailed ways

1. Theoretical design

1. The establishment of the physical model of the laser cutting machine focusing optical path

A typical optical path of a laser cutting machine is shown in Figure 1. According to the number, it is: laser 1; collimating beam expander lens group 2; mirror 3; aberration-eliminating focusing lens 5; The focusing optical path of the laser beam after being collimated and expanded can be simplified to the physical model in Figure 2. As shown in FIG. 2 , a Cartesian coordinate system is established with the focal point of the focusing lens as the center of the circle, wherein the optical axis is along the z-axis direction. The laser beam after collimation and expansion can be regarded as a plane wave with a Gaussian intensity distribution. The numerical aperture of the focusing lens used in laser cutting is generally relatively low (NA<0.1), so the back field of the laser beam passing through the focusing lens can be calculated by classical scalar diffraction. Its normalized electric field distribution near the focus can be expressed as:

$\mathrm{<span\; class="notranslate">\; E.</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; v</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}{<span\; class="notranslate">\; \&Integral;</span>}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; 1</span>}}\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; J</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; vr</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; (</span><span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="iur"\; filenames="HSA00000063397600011.png,HSA00000063397600031.png"\; state="\{\{state\}\}">iur</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; rdr</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

Where: r=ρ/R is the normalized transverse coordinate, ρ is the actual coordinate of the plane where the focusing lens is located, and R is the clear aperture of the system. u and v are respectively

$\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="R"\; filenames="HSA00000063397600011.png,HSA00000063397600031.png"\; state="\{\{state\}\}">R</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; f</span>}}<span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; f</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; z</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; a</span>}<span\; class="notranslate">\; )</span>$

$\mathrm{<span\; class="notranslate">\; v</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;Rr</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; \&lambda;f</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}\sqrt{{\mathrm{<span\; class="notranslate">\; x</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; the\; <figure-callout\; id="2"\; label="y"\; filenames="HSA00000063397600011.png,HSA00000063397600031.png"\; state="\{\{state\}\}">y</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; \&lambda;\; f</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; b</span>}<span\; class="notranslate">\; )</span>$

And L(r) characterizes the transverse amplitude distribution of the beam. For a Gaussian beam whose spot radius on the focusing lens (the spot radius is defined as the transverse distance from the optical axis when the intensity decays to 1/e of the on-axis intensity) is ω, it can be Expressed as

L(r)＝exp(-β ² r ² ) (3)

where β=R/ω.

In order to characterize the change of the light field after adding the phase element, we defined the following parameters: Snell ratio (S), side lobe intensity ratio (M), longitudinal focal depth and lateral focal spot radius. Among them, the Snell ratio S is defined as the ratio of the central main lobe intensity to the central main lobe intensity of the Airy disk after adding the phase element; the side lobe intensity ratio M is defined as the maximum side lobe intensity and the central main lobe intensity in the transverse light intensity distribution ratio. The lateral spot radius is defined as the lateral distance when the light intensity drops to 1/e of the on-axis light intensity. In the field of laser processing, the longitudinal depth of focus is defined as the distance between two points in the longitudinal direction (on the optical axis) when the spot size increases by 5% relative to the smallest spot (on the focal plane). In the present invention, we add two other restrictions, that is, within the focal depth range, the Snell ratio S must not be lower than 0.1, and at the same time, the side lobe intensity ratio M must not be higher than 1/e. Therefore, the depth of focus can be defined as

DOF＝2|z _t | (4)

Among them, z _t = min(z _m , z _s , z _p ), and z _m , z _s , z _p respectively satisfy M(z _m )=1/e, S(z _s )=0.1 and R(z _p )=1.05R(z=0).

2. Optimal design of binary phase element for extended depth of focus

Generally, the optical system is circularly symmetric, so the general binary phase element also adopts circularly symmetric structure. At the same time, binary structures are easy to process and widely adopted. The present invention just adopts this binary pure phase element with simple structure and easy processing. Figures 3a and b are schematic diagrams of typical 3-zone and 5-zone binary phase elements, respectively. Adding this specially designed binary phase element before the focusing lens constitutes the optical path of the cutting head with extended focal depth of the present invention, as shown in FIG. 1 . After the binary phase element 4 is added to the optical path of the cutting head to focus the rear field of the lens, in the physical model of the optical field distribution near the focal spot, we can preliminarily calculate the distribution of the rear field intensity by formula (1). Then, according to this approximate physical model, by changing the normalized geometric radius value of each area of the binary phase element and optimizing the focal depth of the field behind the focusing lens (defined according to formula (4)), the depth of focal length can be obtained Binary phase element. It can be seen from the normalized field intensity distribution formula (1) of the backfield of the focusing optical path (the optical head focusing optical path after adding the binary phase element) that the normalized backfields of Gaussian beams with different truncation ratios are also different . That is to say, if the normalized radius of the binary phase element is optimized for a Gaussian beam with a small β value, it may not be optimal for a Gaussian beam with a large β value, and may not even have the effect of extending the depth of focus. Therefore, for different laser cutters (ie Gaussian beams with different β values), the normalized radius of the binary phase element needs to be re-optimized.

Table 1 shows the optimal solutions of the normalized radii of the 3 and 5 regions obtained through simulated annealing optimization under the assumption that the incident laser field has a uniform intensity distribution (which can be regarded as a Gaussian beam of β→0). At the same time, the table also lists performance parameters such as the corresponding Snell ratio, side lobe intensity ratio, lateral spot radius ratio, and corresponding focal depth ratio (the ratio of the focal depth before and after adding the phase element). It can be seen that the binary phase element designed by us can significantly prolong the focal depth of focus (2-4 times). At the same time, the size of the lateral light spot is basically unchanged (within 8%). At the same time, the performance parameters such as Snell ratio, side lobe intensity ratio, transverse spot radius ratio and corresponding focal depth ratio of the two optimized structures are given in the case of Gaussian intensity distribution with incident laser field β=1.

Figure 4 shows the depth of focus of the original focused spot (without binary phase element) and the binary phases of zone 3 and zone 5 given in Table 1 when the Gaussian intensity cutoff ratio β of the incident laser field is different. The variation of the depth of focus after the element. 100 represents the variation of focal depth with β before adding binary phase element; 300 represents the variation of focal depth with β after adding optimized 3-zone binary phase element; 500 represents the variation of focal depth with β after adding optimized 5-zone The variation of the depth of focus after the meta-phase element with β. We can clearly see that the depth of focus decreases gradually with the increase of the β value. Among them, after adding the binary phase element, the focal depth of focus drops faster, and the area 5 drops faster than the area 3, that is to say, it is more sensitive to the β value. From this figure, we can also see that in the case of β<1, the depth of focus extension of the binary phase elements in zones 3 and 5 is relatively obvious; when β>1, basically there is no effect of extending the depth of focus , when β=2, the depth of focus is compressed instead. It can be seen that for the incident laser field with a large β value, the normalized radius of the binary phase element needs to be re-optimized.

Figures 5a and b are the comparisons of the longitudinal focal depth and transverse focal spot size before and after adding binary phase elements in the two cases of uniform intensity distribution and Gaussian intensity distribution (when β=1), and 301 represents the Gaussian intensity distribution (β= 1) the light intensity before adding the phase element, 302 represents the light intensity after the Gaussian intensity distribution (when β=1) is added to the phase element; 303 represents the light intensity before the uniform intensity distribution (β→0) before adding the phase element, 304 Indicates the light intensity after adding a phase element with a uniform intensity distribution (when β→0).

This further illustrates the results in Table 1. At the same time, we can see from the figure that, compared with the uniform intensity distribution, the lateral focal spot of the Gaussian intensity distribution is larger, and the vertical focal depth is smaller. Therefore, in the actual optical path, we should collimate and expand the beam as much as possible into a plane wave with uniform intensity distribution, that is, a Gaussian intensity distribution with a small β value. Under the premise of such a Gaussian intensity distribution with a small β value, compared with the 3-zone phase element, the 5-zone has a better ability to extend the depth of focus. Theoretically speaking, we can further extend the depth of focus by adding more areas, but this will cause a large drop in the energy of the main spot, which means that we need to further increase the laser output power, and the actual The working power of the laser is limited, that is to say, there is a limit to extending the focal depth. The 3-zone and 5-zone binary phase elements proposed in the present invention are the results of comprehensively considering the extension of the focal depth and the limitation of laser power. Of course, if the laser power is high enough, more partitioned binary phase elements can be used.

Table 1 Optimized binary phase element parameters under different number of zones

Note: The lateral focal spot radius ratio and longitudinal focal depth ratio are relative to the corresponding parameters of the focused light field before adding the phase element.

Two, the embodiment

The laser cutting head with extended focal depth of the present invention includes: a collimating beam expander lens group 2, a reflector 3 and an aberration-absorbing focusing lens 5, and is characterized in that a two-dimensional lens is added in front of the aberration-absorbing focusing lens 5 A metaphase element 4, the binary phase element 4 is coaxial with the aberration-elimination lens 5, the clear aperture of the binary phase element 4 is consistent with the entrance pupil of the laser cutting head, and the binary phase element 4 is The multi-zone structure of the center-symmetrical concentric rings, the phases of the binary phase element 4 are distributed sequentially according to zone 0 and π from the inside to the outside.

Taking the solid Nd:YAG laser cutting machine as an example, a specific implementation plan is proposed for its working wavelength (1064nm). What the above-mentioned binary phase element 4 adopts is the fused silica glass of low refractive index (at 1064nm wavelength band, n=1.4496), and when the laser light is normal incident, the reflectivity of this binary optical element is 7.2% (assuming element The surface is an ideal plane), and the visible laser loss is relatively low. At the same time, the damage threshold of pure fused silica glass is very high, and its damage threshold for 1064nm pulsed laser is as high as 20-40J/cm ² . The refractive index of fused silica at a wavelength of 1064nm is 1.4496, so its

The phase depth is d ₀ =1.064/(2×(1.4496-1))=1.183 μm. Assume that the beam waist radius of the 1064nm laser emitted by the solid Nd:YAG laser cutting machine after collimation and expansion is 5mm, the clear aperture (diameter) of the lens is 10mm, and the focal length is 100mm (NA=0.05). Then the radii of each ring of the three-zone phase element we optimized are: 1.783mm, 4.655mm, 5mm. Considering the mechanical performance, optical performance and cost of the phase element comprehensively, its thickness is set to 1 mm. For fused silica substrate materials, we can use mature photolithography and plasma etching techniques to process this binary phase element.

In summary, the present invention proposes a laser cutting head that simply utilizes a binary phase element to effectively extend the focal depth, and a feasible technical route is proposed by taking a solid Nd:YAG laser cutting machine as an example. This binary phase element with extended focal depth has wide practical value and good application prospect in laser processing technology.

Claims (3)

1. laser cutting head that prolongs depth of focus, its formation comprises along laser incident direction collimator and extender set of lenses (2) successively, speculum (3) and anaberration condenser lens (5), it is characterized in that: at described anaberration condenser lens (a 5) binary phase element of preceding adding (4), this binary phase element (4) is coaxial with anaberration lens (5), the clear aperature of this binary phase element (4) is consistent with the entrance pupil of described laser cutting head, this binary phase element (4) is the multi-region structure of centrosymmetric donut, and the phase place of this binary phase element (4) is from inside to outside successively by district 0, π distributes alternately.

2. the laser cutting head of prolongation depth of focus according to claim 1, it is characterized in that described binary phase element (4) is 3 district's binary phase elements, the normalization radius of this 3 district binary phase element of plane wave that uniform strength is distributed is followed successively by: 0,0.3029,0.9167 and 1.

3. the laser cutting head of prolongation depth of focus according to claim 1, it is characterized in that described binary phase element (4) is 5 district's binary phase elements, the normalization radius of this 5 district binary phase element of plane wave that uniform strength is distributed is followed successively by: 0,0.0196,0.3421,0.5523,0.9251 and 1.

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