JP5190421B2 - Compact thermal lens compensating head - Google Patents

Description

  The present invention relates to a machining head used in laser machining for cutting or welding a metal plate, and to a small thermal lens compensating machining head that compensates for a thermal lens effect.

In laser processing for cutting or welding a metal plate, high-power laser light is converged in a range of about several hundred μm. As a processing laser, a CO ₂ laser is mainly used.

  However, in recent years, a high-power fiber laser having a shorter wavelength and a wavelength of 1 μm has been used. The high power fiber laser has advantages such as higher brightness, higher absorption rate by metal, higher power density by shorter wavelength, higher electro-optical conversion efficiency, smaller processing marks and easy supply by fiber.

  A laser power of several kW is usually supplied by a multimode fiber. Recently, however, single-transverse mode high-power fiber lasers can be used, which can focus the light beam to a smaller spot and have a longer depth of focus. It can be applied to thick materials.

  An optical system composed of two lenses is used to converge the fiber laser output. The first lens is a collimator lens (CL), and has a focal length of 10 cm or more, for example. The role of the collimator lens is to collect the divergent light emitted from the fiber end up to a parallel beam of the order of cm. The second lens is a converging lens (FL), and is a lens that concentrates the light power to the processing point. A transparent protective window is provided on the surface of the converging lens in order to prevent scattered objects and dust from the object to be adsorbed. These optical systems are attached to a portion called a processing head (PH).

  A jet of high-pressure gas is blown to the machining area for the purpose of removing the metal that melts and scatters during workpiece machining.

  Patent Document 1 discloses a light beam between a collimator lens and a laser processing head in a collimator device for a laser beam in which a collimator lens is provided on an optical path between a laser oscillator and a laser processing head so as to be movable in the optical axis direction. On the road, a laser beam analyzer for detecting the beam diameter of the laser beam at a plurality of locations in the optical axis direction is provided so as to be movable in the optical axis direction, and the beam diameter at the plurality of locations detected by the laser beam analyzer is adjusted. A control means is provided for controlling the servo motor for calculating the contraction state or the diffusion state of the laser beam based on the calculation result and moving the collimator lens in the optical axis direction based on the calculation result. A collimator device for a laser beam having a configuration in which a collimator lens is moved and adjusted to be incident on a laser processing head with a laser beam diameter adjusted. It has been described.

  In Patent Document 2, the light diameter or light area of the laser light on the surface to be processed is obtained, and when the light diameter or light area deviates from a predetermined reference value, the focal position of the laser light is set as a reference. Assuming that the laser head is displaced from the position, the laser head and the workpiece are moved relative to each other according to the amount of deviation, and the moving direction of the laser head or workpiece at this time is the previous laser head or workpiece. It is possible to perform a highly accurate correction control of the focal position deviation with a simple structure by determining based on the movement direction and the change in the light diameter or light area obtained before and after this movement. A simple laser processing apparatus is described.

Japanese Patent No. 2970927 JP 2002-239768 A

Incidentally, in the lens, the lens material itself or the antireflection film contains a small amount of impurities, and these impurities absorb the laser light incident thereon and generate heat. The diffusion of heat forms a parabolic temperature distribution. This is because the effect of the thermal lens is added to the effect of the convergent lens due to the temperature dependence of the refractive index. Regarding the focal length f of the lens including such a thermal lens, the following relationship is established.
1 / f = 1 / f _L + 1 / f _TL

Here, f _L is a focal length specific to the lens, and f _TL is a focal length of the thermal lens.

  This phenomenon is known as the thermal lens effect. Immediately after the laser output is changed or laser oscillation is started, the thermal lens effect is formed taking several tens of seconds to nearly one minute, and the focal length changes during that time. During this time, the optical power density on the surface of the processing point changes, and the quality of cutting and welding changes. For this reason, it is necessary to wait until it reaches a steady state in which machining can be started.

  The thermal lens effect can be avoided by using an optical component that contains negligible impurities that absorb light. However, such an impurity-free material is difficult to manufacture and difficult to obtain at a realistic price. Therefore, it is desirable to design a machining head that minimizes the thermal lens effect.

  Regarding the time t required for the temperature to reach a steady state by thermal diffusion over the distance L, the following relationship is established.

t∝L ² / D
Here, D is the thermal diffusion coefficient of the material forming the lens, and L is the diameter of the lens. If the lens has a diameter of the order of mm, the heat diffusion time can be shortened. That is, the time until the lens temperature reaches a steady state can be shortened.

  However, such a design has the following problems. First, if the beam diameter is small at the position of the converging lens, the power density becomes excessive at the target, the distance to the focal plane becomes extremely short, and the distance between the lower end (end) of the processing head and the object to be processed becomes too narrow. Therefore, it is not suitable for a laser processing head.

Next, since the thermal lens effect is proportional to the optical power density I [W / cm ² ], if the design is made such that the incident beam diameter is small, the optical power density is increased and the thermal lens effect is increased. is there.

  It has been proposed to reduce the optical power density in the optical component by increasing the beam occupation area in the converging lens and the filling ratio of the optical system, that is, [beam occupation area / lens occupation area]. Yes. However, to date, there is no solution that can reduce the thermal lens effect to a negligible level in actual laser processing.

  As another solution, there is a method of compensating for the focal shift by dynamically adjusting the position of the collimator lens or the position of the processing head in accordance with the process in which the thermal lens effect is formed. However, this method requires a driving device that moves the lens along the optical axis, and measures the beam diameter at the focal plane of the workpiece and maintains the beam diameter at a predetermined size. Since a feedback circuit is required, the configuration of the machining head becomes complicated and large.

  It is an object of the present invention to provide a compact thermal lens compensation processing head capable of compensating a high brightness, that is, a single transverse (quasi-single transverse) mode laser beam without moving the focal point due to the thermal lens effect. And

  Furthermore, an object of the present invention is to provide a small thermal lens compensation processing head capable of reducing the transition time for forming the thermal lens effect from about 1 minute to about 1 second. In addition, the present invention can reduce the ratio of power absorbed by a single lens system, that is, a lens group and a window, by reducing the total thickness of the optical component to be transmitted and reducing the number of interfaces. It is another object of the present invention to provide a compact thermal lens compensation processing head that is reduced in size and weight.

  Furthermore, an object of the present invention is to provide a compact thermal lens compensation processing head that is designed to be small in size, enables faster processing, and can reduce power consumption.

  The small thermal lens compensating head according to the present invention has the following configuration.

[Configuration 1]
A high-intensity laser, that is, a single transverse mode or quasi-single transverse mode fiber laser, a lens comprising a collimator lens and a converging lens, which is irradiated with the laser beam from the fiber laser and irradiates the workpiece with the laser beam and a system, when the focal length of the collimator lens was set to f _1, the focal length f ₁ of the collimator lens with respect to a change in f ₁ of the distance z ₁ from the lens to the beam waist of the image (beam minimum radius location) The change is 0, that is, [d (z ₁ ) / d (f ₁ ) = 0], and the converging lens is installed so that the distance from the collimator lens is 2z _1. , the focal length of the converging lens is f _2, the beam radius and wavefront radius of curvature of the incident surface and the exit surface of the collimator _{lens, (ω 1, R 1)} , (ω 2, R ), And the beam radius and wavefront radius of curvature of the incident surface and the exit surface of the convergent lens, (omega _{3, R} 3), when the (omega _{4, R 4),}
[Dω ₃ / df ₁ = 0]
The change in f ₁ is Δ (1 / f ₁ ), the change in f ₂ is Δ (1 / f ₂ ), the change in R ₃ is Δ (1 / R ₃ ), and R ₄ When the change in Δ is Δ (1 / R ₄ ),
Δ (1 / f ₂ ) = Δ (1 / R ₃ ) + Δ (1 / R ₄ )
Δ (1 / R ₄ ) = Δ (1 / f ₂ ) −Δ (1 / R ₃ ) = 0
The change in wavefront curvature at the entrance surface of the converging lens due to the thermal lens effect in the collimator lens is compensated by the thermal lens effect generated in the converging lens. Therefore, the wavefront curvature at the exit surface of the convergent lens does not change, and the convergent lens is arranged at a position where the beam radius at the exit surface of the convergent lens is constant regardless of the change in the focal length of the collimator lens. It is characterized by being.

[Configuration 2]
Each of the collimator lens and the converging lens has a diameter of 1 mm.

  In the small thermal lens compensation processing head according to the present invention, since the thermal lens effect in the collimator lens is offset by the thermal lens effect in the converging lens, the focal position movement due to the thermal lens effect is suppressed. . Thereby, laser processing becomes possible immediately after turning on the power to the laser light source. That is, the present invention can contribute to productivity improvement, reproducibility improvement, and quality improvement in the cutting process.

  Further, in the small thermal lens compensating machining head according to the present invention, by having the configuration 2, the beam radius is reduced, the machining head is miniaturized, the lens diameter is reduced, and the machining head can be reduced in weight. As a result, the movement of the machining head is simplified and the machining head can be more easily accelerated and positioned. In addition, by shortening the thermal diffusion time based on the miniaturization of the lens, the time until the steady state is reached is shortened, and the time for forming the thermal lens effect is shortened. That is, the present invention can contribute to productivity improvement, reproducibility improvement, and quality improvement in the cutting process.

It is a side view which shows the structure of the conventional process head, Comprising: It is a figure explaining the change of the focal distance by the presence or absence of a thermal lens effect. It is a correlation diagram of the beam waist position and the beam waist at the image position with respect to the convergence force of the lens, the position is normalized by the focal length. It is a side view which shows the structure of the Example of the small thermal lens compensation processing head which concerns on this invention. FIG. 4 is a side view showing an example of a small thermal lens compensation processing head according to the present invention, which can realize a beam diameter of about 100 μm on the surface of an object to be processed placed at a position of about 5 cm from the convergent lens. is there.

  FIG. 1 is a side view showing a configuration of a conventional processing head, and is a diagram for explaining a change in focal length depending on the presence or absence of a thermal lens effect. In FIG. 1, a solid line indicates a case without a thermal lens effect, and a broken line indicates a case with a thermal lens effect. The amount of change in focus is generated by superimposing the thermal lens effects of the collimator lens and the converging lens.

  The small thermal lens compensation processing head according to the present invention is used in laser processing for cutting or welding a metal plate, and has a collimator lens 7 and a converging lens 8, and has high brightness, that is, Applied to a single transverse mode beam or a quasi-single transverse mode beam.

  First, high brightness, ie single transverse mode beam or quasi-single transverse mode beam has higher spatial coherence compared to multimode beam, It is possible to converge to a smaller spot size. In addition, as shown in FIG. 4, it is possible to design the incident beam having a diameter of sub millimeters to be reduced to a size of 100 μm at a position several cm away from the focusing lens.

  Next, in the present invention, the thermal lens effect is first-order approximated by optimizing the positions of the converging lens and the collimating lens for a Gaussian beam having a high luminance, that is, a single transverse (quasi-single transverse) mode. Can be canceled.

For a lens with a focal length f, the lens formula for the object position and the image beam waist position (the location of the minimum beam radius) is:
1 / [z ₀ + {z _R ² / (z ₀ −f)}] + (1 / z ₁ ) = (1 / f) (Equation 1)

Here, z ₀ represents the distance from the lens to the object, z ₁ represents the distance from the lens to the beam waist of the image, and z _R [= πω ₀ ² / M ² λ] represents the Rayleigh length (beam Parameter). λ is the wavelength, and ω ₀ is the beam waist diameter. M ² is a quality parameter, which is a parameter called beam propagation (diffusion). In the single transverse mode and the quasi-single transverse mode, M ² ≈1, but this does not limit the present invention. For short Rayleigh lengths, that is, [z _R << | z ₀ (z ₀ -f) | ^1/2 ], the lens formula described above is a well-known geometric optical lens as follows: The formula becomes valid.
(1 / z ₀ ) + (1 / z ₁ ) = (1 / f) (Equation 2)

  FIG. 2 is a correlation diagram of the beam waist position with respect to the convergence force of the lens and the beam waist at the image position, and the position is normalized by the focal length.

Unlike a multimode beam, geometrically, in the case of a Gaussian beam, as shown in FIG. 2, there is a limit maximum value for the focal length at which a beam with a beam waist diameter ω ₀ can be narrowed. . The lower diagram in FIG. 2 shows the position of the beam waist in question.

Here, as shown in the following (Formula 3), the focal length of the collimator lens L ₁ can be selected such that the waist position z ₁ is constant with respect to the change of f ₁ .
d (z ₁ ) / d (f ₁ ) = 0 (Expression 3)

This distance z ₁ is equal to the Rayleigh parameter as follows:
z ₁ = πω ₀₂ ² / M ² λ≈πω ₀₂ ² / λ

Here, ω ₀₂ = ω ₂ / √2, ω ₂ is the beam radius. However, ω ₀₂ (the beam waist diameter on the image side) is not constant and decreases monotonously as the lens convergence force (1 / f ₁ ) increases.

However, as long as the beam waist position and the beam size are constant with respect to the thermal lens effect, placing the second lens, i.e. the converging lens (8) in the proper position, can be kept constant as a result. it can. The collimator lens (7) and the converging lens (8) have a function of changing only the wavefront shape according to the following equation without changing the beam diameter at the incident position and the exit position.
(1 / f ₁ ) = (1 / R ₁ ) + (1 / R ₂ )
(1 / f ₂ ) = (1 / R ₃ ) + (1 / R ₄ ) (Equation 4)

Here, R ₁ (1) is the incident wavefront radius of curvature of the collimator lens, R ₂ (2) is the outgoing wavefront radius of curvature of the collimator lens, and R ₃ (3) is the incident wavefront curvature of the convergent lens. R ₄ (4) is the outgoing wavefront curvature radius of the converging lens.

  In the present invention, the converging lens is installed so that the distance from the collimator lens satisfies the following.

(1) The beam diameter at the entrance surface of the convergent lens does not change with respect to the change in the focal length of the collimator lens, that is, [dω ₃ / df ₁ = 0] (Equation 3) holds.

(2) The change in f ₁ , that is, Δ (1 / f ₁ ) causes an additional curvature change, Δ (1 / R ₃ ), on the wavefront at the convergent lens entrance surface. This is compensated by a 1 / f ₂ change, ie, Δ (1 / f ₂ ), so that a wavefront curvature change Δ (1 / R ₄ ) at the exit surface of the converging lens is cancelled. That is, the following is derived from (Equation 4).
Δ (1 / f ₂ ) = Δ (1 / R ₃ ) + Δ (1 / R ₄ )
Δ (1 / R ₄ ) = Δ (1 / f ₂ ) −Δ (1 / R ₃ ) = 0

(3) Since the beam diameter and the wavefront curvature are constant on the exit surface of the converging lens, the beam is as designed at any position after the converging lens. Thus, even if the convergence force changes due to the thermal lens effect or other causes, the focal length does not change as a primary approximation.

  FIG. 3 is a side view showing the configuration of an embodiment of the small thermal lens compensating head according to the present invention. In FIG. 3, the solid line represents the beam divergence shape having the steady beam waist position 9, and the broken line represents the beam divergence shape when the converging force of the collimator lens 7 is slightly increased.

  In this small thermal lens compensating head, as will be described below, the thermal lens effect in the converging lens 8 compensates for an increase in the wavefront curvature at the entrance surface 3 and has no effect on the beam divergence after the converging lens 8. Does not reach.

In this small thermal lens compensation processing head, a coherent Gaussian beam 5 is emitted from a single transverse mode fiber 6. A Gaussian beam is characterized by a beam radius ω and a wavefront curvature radius R in its cross section at any position. The collimator lens 7 has a focal length f ₁ of 2 mm to 3 mm, and its diameter (lens diameter) is about 1 mm. The converging lens 8 has a focal length of f ₂ and has a diameter of about 1 mm. The beam radius and the wavefront curvature radius at each of the positions (1), (2), (3), (4), that is, the incident surface and the exit surface of the collimator lens 7 and the converging lens 8, are (ω ₁ , R ₁ ), (Ω ₂ , R ₂ ), (ω ₃ , R ₃ ), (ω ₄ , R ₄ ).

The focal length f ₁ of the collimator lens 7 is such that the change in the beam waist position z ₁ with respect to the change in the distance is 0, that is, [d (z ₁ ) / d (f ₁ ) = 0] from (Equation 3). Designed to. The converging lens 8 is installed so as to be symmetrical with respect to the distance 2z ₁ from the collimator lens 7, that is, the beam waist position 9. Then, the beam radii at the positions (1), (2), (3), and (4) are all equal. That is, [ω ₁ = ω ₂ = ω ₃ = ω ₄ ], which is invariant to the change Δf _{1 of} f ₁ . Due to the thermal lens effect, the converging force of the collimator lens 7 changes as Δ (1 / f ₁ ). From (Equation 4), this leads to an increase Δ (1 / R ₂ ) in wavefront curvature after the collimator lens 7 (after position (2) in FIG. 3). R ₁ is not affected by the thermal lens effect. That is, Δ (1 / R ₁ ) = 0.
Δ (1 / R ₂ ) = Δ (1 / f ₁ ) (Formula 5)

In the first order approximation, the minimum beam waist position z ₁ does not change. For this reason, the change in the wavefront curvature at the position (3) is the same as the change at the position (2) due to symmetry, and is expressed as follows.
Δ (1 / R ₃ ) = Δ (1 / R ₂ ) (Expression 6)

Similarly, since R ₃ and R ₄ satisfy (Equation 4), the following holds.
Δ (1 / R ₄ ) = Δ (1 / f ₂ ) −Δ (1 / R ₃ )

Then, the following expression is obtained from (Expression 5) and (Expression 6).
Δ (1 / R ₄ ) = Δ (1 / f ₂ ) −Δ (1 / f ₁ ) (Expression 7)

The optical power density of both the lenses 7 and 8 is the same, and the lenses 7 and 8 are made of the same material and have substantially the same thickness. Therefore, the same thermal lens effect is generated. That is, the following holds.
Δ (1 / f ₁ ) = Δ (1 / f ₂ )

Then, from (Equation 7), [Δ (1 / R ₄ ) = 0]. Thus, it can be seen that the beam waist radius ω ₄ and the wavefront curvature R ₄ at the position (4) are constant and do not change. That is, the beam after the converging lens 8 is constant, and the design value is maintained.

In this design, it is not necessary to know the amount of light absorption in each lens 7, 8, and it is not necessary to know the magnitude of the thermal lens effect. Further, this design does not depend on the focal length f ₂ of the converging lens 8.

  FIG. 4 shows an embodiment of a small thermal lens compensation processing head according to the present invention, which has a configuration capable of realizing a beam diameter of about 100 μm on the surface of the processing object placed at a position of about 5 cm from the converging lens. FIG.

In this embodiment, a Gaussian beam 5 having a beam radius ω ₀₁ of 12 μm and a wavelength λ of 1.1 μm is supplied from a single transverse mode fiber 6. The distance between the collimator lens 7 and the converging lens 8 is z ₁ + z ₁ = 2z ₁ = 30 cm. The radius ω ₀₂ of the beam waist 9 is 229 μm, the respective lens positions, the beam radius ω = √ (2z ₁ λ / π) = 324 μm at each surface (1), (2), (3), (4). On the other hand, z ₁ is 15 cm. The beam radius ω ₁ of the Gaussian beam along the z-axis changes as follows.
ω ₁ = ω ₀ √ (1 + z ₀ ² / z _R ² )

From this, if the collimator lens 7 is placed at a position of z ₀ = 11 mm from the fiber end, the beam radius ω becomes 324 μm. The radius of curvature R ₁ at position (1) is as follows.
R ₁ = z ₀ {1+ (z _R ² / z ₀ ² )} ≈z ₀

From this, R ₁ = 11 mm. Thus, the value of f ₁ is obtained from (Equation 4), and f ₁ = 11 mm. Therefore, if it is desired to obtain a spot having a size of ω ₀₃ = 50 μm on the metal surface, tz ₂ = 4.6 mm and f ₂ = 41 mm are obtained by the same calculation.

  Note that the embodiments shown in FIGS. 3 and 4 do not limit the application of the present invention. In particular, even when the thermal lens effect of the collimator lens 7 and the thermal lens effect of the converging lens 8 are not the same, it is always possible to compensate for the thermal lens effect by setting each lens position and designing the focal length.

  The present invention is used in laser processing used for cutting or welding a metal plate.

5 Gaussian beam 6 Single transverse mode fiber 7 Collimator lens 8 Converging lens 9 Beam waist position

Claims (2)

High intensity, ie single transverse mode or quasi-single transverse mode fiber laser;
A lens system comprising a collimator lens and a converging lens, a laser beam from the fiber laser being incident, and irradiating the workpiece with the laser beam;
When the focal length of the collimator lens was set to f _1, the focal length f ₁ of the collimator lens is changed with respect to a change in f ₁ of the distance z ₁ to beam waist image (beam minimum radius location) from the lens is 0 That is, it is designed to be [d (z ₁ ) / d (f ₁ ) = 0],
The convergent lens is installed so that the distance from the collimator lens is 2z ₁ ;
Wherein the focal length of the converging lens is f _2, the beam radius and wavefront radius of curvature of the incident surface and the exit surface of the collimator lens, and _{(ω 1, R 1),} (ω 2, R 2), wherein the converging lens When the beam radius and the wavefront curvature radius at the entrance surface and the exit surface are (ω ₃ , R ₃ ) and (ω ₄ , R ₄ ),
[Dω ₃ / df ₁ = 0]
Is established,
The change in f ₁ is Δ (1 / f ₁ ), the change in f ₂ is Δ (1 / f ₂ ), the change in R ₃ is Δ (1 / R ₃ ), and the change in R ₄ is Δ (1 / R ₄ )
Δ (1 / f ₂ ) = Δ (1 / R ₃ ) + Δ (1 / R ₄ )
Δ (1 / R ₄ ) = Δ (1 / f ₂ ) −Δ (1 / R ₃ ) = 0
Is established,
The change of the wavefront curvature at the entrance surface of the convergent lens due to the thermal lens effect in the collimator lens is compensated by the thermal lens effect generated in the convergent lens, and converges with respect to the thermal lens effect occurring in both lenses. The wavefront curvature at the exit surface of the lens does not change,
The converging lens is disposed at a position where the beam radius on the exit surface of the converging lens is constant regardless of the change in the focal length of the collimator lens. The small thermal lens compensation processing head according to claim 1, wherein each of the collimator lens and the converging lens has a diameter of 1 mm.

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