JPH01191801A - Condenser lens - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Technical Field The present invention relates to a condensing lens that condenses laser light concentrically.

Co2 laser light is powerful and has high processing efficiency, so it is used for purposes such as cutting and welding.

In addition to machining equipment, C02 lasers are also widely used in medical equipment.

Since CO2 laser light has a wavelength of 10.6 μm, it cannot be guided by a silica glass fiber. However, it has become possible to freely guide C02 laser light using crystalline optical fibers.

High-power CO2 laser light can be guided using a mirror.

The focusing optical system uses polycrystals such as Zn5e%GaAs as lenses.

Using CO2 laser light, a board can be cut into any desired shape. In this case, the laser beam must be scanned three-dimensionally along the cutting line.

(B) Prior art The beam intensity distribution of laser light used in laser processing machines is usually Gaussian.

This beam is focused onto a minute spot using a plano-convex lens or a meniscus lens.

The purpose of focusing is to increase the energy density of the beam and make the cutting point as narrow as possible.

When processing (cutting, welding, or heat treating) a workpiece into a ring shape, it is carried out by moving the work table along a circular path or by scanning the processing head along a circular path.

However, providing a scanning mechanism poses problems in terms of cost, weight, space, etc. It also takes time to scan.

If there is a scanning mechanism, it is possible to process any shape, but this increases the cost.

If the object to be processed is determined to be circular, the C02 laser beam can be shaped into a circular shape and irradiated.

(b) Axicon An axicon is different from a lens.
Rather than focusing light on one point, it focuses light on an area with a certain length. This is an optical component that combines a flat surface and a conical surface.

Axicons were invented by Mal, sod.

J, H, McLeod: J, Opt, Soc.
, Am, 44 (1954) 592J, H,M
cLeod: J, Opt, Soc, Am, 5
0 (1960) 166 Use a conical surface instead of a spherical surface. Therefore, all the emitted light has a conical shape. Cones are similar shapes with the same apex angle.

In other words, when viewed on the radius, the emitted light is parallel.

Outgoing light close to the axis cuts the optical axis quickly. Outgoing light that is far from the axis cuts the optical axis slowly.

For this reason, light does not gather at the negative point. In other words, there is no focus. Instead, a condensed line segment is generated.

When parallel light is incident, the emitted light cuts the optical axis, and the range that cuts the optical axis becomes a line segment. This is called focal 1ine. It should be called a focal line segment rather than a focal point.

Figure 4 shows an example of an axicon. This is a convex axicon, consisting of a flat surface and a conical convex surface. In addition to this, there is also a concave axicon.

The axicon alone has its uses.

However, by combining a convex lens and an axicon, the emitted light can be shaped into a ring.

If the axicon is used alone, the emitted light will become parallel in the radial direction and will not converge no matter how far it goes. Therefore, by placing a convex lens in front of or behind the axicon, this can be converged. Since the convergence point is not on the optical axis, the convergent light forms a ring around the optical axis.

Figure 5 shows such a combination. A lens U and a convex axicon W are combined. The parallel light is condensed by the convex lens U, and becomes like a ring C by the axicon W.

The axicon has no imaging effect. The distance between the surface where the torus C is formed and the lens U is equal to the focal length f of this lens.

Let the base angle of the axicon be α. Let n be the refractive index.

Due to this axicon, the outgoing light is (
n-1) Bend by α.

Therefore, when α is small and the lens U and axicon W are close to each other, the radius Rc of the ring C is Rc=(n-1)αF (1).

However, in this case, the radius of the ring is uniquely determined.

There is also a desire to freely change the size of the ring.

On the other hand, a combination of axicons as shown in FIG. 6 was proposed by Rioux.

M, Rioux, R, Tremblay & P,
A, Bdlanger: APPl.

0pt, Bureau (1978) 1532 In FIG. 6, a convex lens U and a concave axicon v1 and a convex axicon W are combined. The cone base angles α of the convex axicon and concave axicon are equal. When these are combined, they become a simple flat plate, so only the light focusing effect of the lens U remains. However, when the concave axicon is moved away from the convex axicon, a ring C is formed on the focal plane, and its radius Rc becomes. By increasing the axicon distance d, Rc can be increased. In other words, Rc can be made variable.

Equation (2) can be easily understood from the fact that when light passing through the center of the concave axicon V is emitted, it forms an angle of (n-1)α with respect to the optical axis.

B) Problems to be Solved by the Invention The conventional ring focusing system using an axicon lens can only obtain a single ring.

If two rings of different diameters are to be obtained, the lens axicon must be exchanged.

Therefore, if you want to heat-treat the workpiece by cutting it into a donut shape along two concentric circles, first use a ring condensing system to irradiate light in an annular shape, and then use another ring condensing system to irradiate the object with light. Then, the light must be irradiated again in an annular manner.

For example, as shown in FIG. 3, consider a case where it is desired to irradiate a laser beam along a small circular ring C with a radius r1 and a circular ring B with a radius r2.

Using the ring focusing system shown in FIGS. 5 and 6, the small ring C can be irradiated with a laser beam. The laser beam can also be applied to the great torus B. However, the laser beam cannot be applied to the ring B%C at the same time. One of the rings should be replaced first, and the optical system should be replaced before applying the laser beam to the other ring.

When applying laser light concentrically, it takes time to replace the ring condensing system midway through the process. In this case, it is easier to scan the beam in an annular manner using a highly versatile beam scanning system. The benefits of using an axicon are lost.

An object of the present invention is to facilitate the processing of such concentric rings B and C.

(3) Configuration FIG. 1 shows a cross section of the condensing lens of the present invention.

This is an optical component that is a combination of a lens and an axicon. This is a sectional view including the optical axis.

It is rotationally symmetrical around the optical axis.

The front surface of the condenser lens A is a convex lens surface S1 with a radius of curvature H. The rear surface is an axicon surface S2. However, it is not a simple axicon, but an FQ up to the diameter D1.
It has a convex axicon with a protruding center like G.

The diameter from D1 to Do is a concave axicon such as Fl: and GH.

A feature of the condensing lens of the present invention is that the axicon surface S2 is an axicon that is convex on the inside and concave on the outside. The conical base angle of the convex axicon FQG is α, and the concave axicon E
Let β be the cone base angle of F and GH.

FIG. 2 shows the distribution of light beams when parallel light is incident on the condenser lens A of the present invention.

The incident of parallel light means that the C02 laser light enters the lens directly or via a mirror waveguide or optical fiber. Since the light is not shaped by a lens, it is almost parallel light. The beam intensity distribution often has a Gaussian distribution in the radial direction.

Since there is a convex lens surface S1, this lens has a focal plane. The focal length f is determined by the radius of curvature R of the convex lens surface S1 and the refractive index n.

Using the paraxial ray approximation, f' = -(31) This can be easily proven.Also, the focal length of a biconvex lens whose radius of curvature is R at both the front and rear is R/
This is clear from the fact that 2(n-1).

In this way, the existence of a focal length f means that the axicon category is closer to that of FIG. 5, rather than that of FIG. 6.

Now, the inward rays that have passed through the inner convex axicons FQ and G pass through points L and M on the focal plane. Since this is a cross section, a ring C with radius r1 is actually drawn on the focal plane.

The axicon FQG has a conical surface, but the conical surface does not have the effect of concentrating light on one point. The function of collecting light is on the convex lens surface S1. Then, the focal plane is determined by the radius of curvature H of Sl. The light flux passing through the FQ crosses the optical axis and converges at point M on the opposite side.

On the other hand, the light flux that passes through QG crosses the optical axis and reaches L of the focal plane.
converge to a point. The radius r1 of the circular ring C is determined as follows.

Pay attention to the rays passing through the optical axis. This is point Q, which forms an angle of (n-1)α with the optical axis. This is because FQ and GQ are tilted by α with respect to a plane perpendicular to the optical axis.

The angle between the ray and the normal to the FQ%GQ plane is α
It is. Outside, the angle between this normal and the outgoing ray is nα
It is. The normal is tilted by α with respect to the optical axis. Therefore, the emitted light forms an angle of (n-1)α with respect to the optical axis.

Furthermore, QO is approximately equal to the focal length f. This is an approximation that ignores the thickness of the condenser lens A.

Therefore, rl = (n −1) cXf' (4
). Here, jan (n-1)α is (n-1)α
It is similar to

The diameter of the convex axicon FQG is Dl. This means that a portion of the incident laser beam having a diameter D1 is converged on the inner ring C.

Now, the light flux passing through the outer concave axicon EF%GH is
It is refracted further outward, and similarly draws a large ring B on the focal plane.

The light flux passing through EF converges on point K. The light flux passing through GH converges on point N. These converge to the %N point without crossing the optical axis. Similar convergence of the light beam occurs in any cross section other than this one. Therefore, the great torus B is drawn by the concave axicon.

The radius r2 of this ring can also be determined from the inclination of the luminous flux at points F and G, but this is somewhat complicated. Even if we assume that the concave axicon extends to the central axis, the point that it converges on the ring B remains the same. Based on this assumption, it is sufficient to consider how the light emitted from point P diverges. P point is EF%GH
is the intersection of the extension lines of

Since the cone base angle of the concave axicon is β, the light exiting the center point P makes an angle of (n-1)β with the optical axis.

Since the distance PO is approximately equal to f, r2 = (n-1)βf(5). This proof is intuitive, simple, and easy to understand.

The same thing can be proven because a ray passing through F or E passes through point K without using point P, which actually does not exist.

For example, consider a ray passing through point F. This is D1/from the optical axis.
Only 2 minutes apart. The angle between the normal to the lens surface S1 and the ray is . When it enters the lens, it is refracted and becomes a ray that makes an angle R with the optical axis. This is the effect of the convex lens S1.

The axicon plane EF' is tilted forward by β with respect to the plane perpendicular to the optical axis. Therefore, the angle of incidence from the lens toward space at point F is as follows.

Here, the incident angle and the outgoing angle are the angles formed by the normal line to the boundary surface and the incident light and the outgoing light. It follows the usual definition of geometric optics.

Then, the exit angle is (8) multiplied by n, and becomes. However, since the angle is small, an approximation of sin θ-θ is used.

However, since the exit surface EF is tilted by β, the angle that the output light makes with the optical axis is (9) minus β. Multiplying this by the focal length f and further adding D1/2, rl
becomes.

It is. Inserting the definition (3) of f, we get rln(n-1)βf Q2+. This is the same as (5).

In FIGS. 1 and 2, a convex axicon with a conical base angle α is formed on the inside, and a concave axicon with a conical base angle β is formed on the outside.

However, as can be seen from the equations of rl and r2, if α and β are the same, it is clear that both convex and concave axicons draw the same ring.

The only difference is that in the case of a concave axicon, it does not cross the optical axis, and in the case of a convex axicon, it does cross the optical axis.

Therefore, in the present invention, the inner side is a convex or concave axicon, and the outer side is a convex or concave axicon.

The base angles of the cone are then α and β, respectively.

The inner axicon shall be written first, and the outer axicon later. The axicons in FIGS. 1 and 2 are concave and convex axicons, but in addition to these, convex curved axicons, concave and convex axicons, and day-to-day axicons are possible. To make it a double ring, it is only necessary that α\β 03). Each axicon is shown in FIGS. 7 to 9.

However, when a convex axicon is used, the light crosses the optical axis once, so the surface and light become oblique at the focal plane. It is not possible for the light to hit vertically.

If it is desired that the laser beam be directed vertically, a portal axicon structure is preferable.

Among the gate axicon structures, DI: 2rt (+4) DI
+ D□: 2r2 (15)
It follows that the best option is the one where .

b) Operation The parallel beam of the CO2 laser is made incident on the condenser lens A. The light that passes through the part whose center diameter is Dl has a radius r1
The light is focused along the small ring C.

The light that has passed through the portion whose peripheral diameter is D1 to D2 has a radius r
The light is focused along the small ring B of No. 2.

Therefore, as shown in Fig. 3, the workpiece is circularly B%
It is possible to simultaneously heat along C. It can be processed into donut shapes by cutting, welding, and heat treatment.

In FIG. 2, a parallel beam is applied to the convex lens surface S1, and the light emitted from the axicon surface S2 is focused on the focal plane.

This may be reversed. That is, the lens is reversed from front to back, a parallel beam is made incident on the axicon surface, and the light emitted from the convex lens surface S1 is focused on the focal plane. Even in this case, the light can be focused on the double ring of rl and r2.

However, the ratio of light amounts is appropriately determined by the ratio of Dl and Do.

In reality, the laser beam is often a Gaussian beam, so the area ratio and light amount ratio are not equal. However, if the intensity distribution P (r) of the laser beam is known, the power Pc of the light passing through the inner ring C is given by Pc, and the power Pc of the light passing through the outer ring B is given by. To change the ratio of Pc and PB, the conical FQG is made narrower or wider while keeping the angle α constant. In other words, the inflection points F and G are
Move inward or outward.

Even though it is moved, the axicon surface is ground with a diamond cutting tool, so once this condensing lens is made, the ratio of light intensity cannot be changed.

Once the lens is made, f%r1%r2 becomes a constant. In other words, it can only be used when processing into the same shape. This is a dedicated optical component.

However, when the same processing is repeated many times, the condensing lens of the present invention is extremely effective.

As long as the light from the C02 laser is sufficiently powerful, it is possible to process a double ring shape without using a beam scanning mechanism.

From the purpose of processing, rl and r2 are first determined. This is related to the focal length f, but since there is a relationship between f and R, the refractive index can be eliminated and it can also be written as rl = αR (19 r2 = βR (21). From this, α, β, Decide R appropriately. Since the number of unknowns is one more than the number of equations, α, β
, R has one degree of freedom in determining.

This is convenient.

The refractive index n is arbitrary. However, in reality, since the material is determined, n cannot be freely selected. C
02 Zn5e% can pass the laser light well.
These include GaAs and alkali halide crystals.

However, there is no problem even if the refractive index n is limited. Since α, β, and R can be selected freely, once these are determined and n is determined, the focal length f is determined. f
Since is the distance between the lens and the workpiece, there is no problem even if it is determined by these constants.

(→Example A condensing lens of the present invention was made using Zn5e polycrystal as a material. The constants are as follows.

Lens outer diameter Do 63.5mm Lens convex axicon diameter D13' 7.9mm Lens center wall thickness tc 15. Own convex axicon base angle α 5゜concave axicon base angle
β 10° focal length f' 127
mm Inner ring radius rl 15.5mm Outer ring radius
r2 31. Oram400WC02 laser light was irradiated, the acrylic plate was placed on the focal plane, and the pan pattern was examined. As a result, it was found that the laser beam could be focused into a ring shape with an accuracy of ±1.0 IMl.

(i) Effects By using the axicon lens of the present invention, parallel light beams can be focused into a double concentric ring shape at the focal plane.

When used as a condensing lens for CO2 laser light, double concentric circles can be processed (cutting, welding, heat treatment) at the same time.

For example, when cutting a steel plate into a donut shape,
This is particularly useful when the donut shape (rl, r2) is constant and must be mass-produced.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an example of the condensing lens of the present invention. FIG. 2 is a diagram showing the convergence of a luminous flux when parallel light is incident on the condenser lens of FIG. 1. FIG. 3 is a diagram showing a double ring. Figure 4 shows the principle of an axicon. Figure 5 is a diagram showing that parallel light can be condensed into an annular shape using an axicon or a convex lens. FIG. 6 is a diagram showing that parallel light can be focused into an annular shape using a convex lens, a concave axicon, and a convex axicon. FIG. 7 is a ray distribution diagram of a haloaxicon which is another example of the present invention. FIG. 8 is a ray distribution diagram of an uneven axicon which is another example of the present invention. FIG. 9 is a ray distribution diagram of a sun-day axicon, which is another example of the present invention. Inventor Miki Kusao Patent applicant Sumitomo Electric Industries Co., Ltd. Application agent Patent attorney Shigeru Kawase 2nd-No.
7 Figure 8 Figure 9

Claims (1)

[Claims] It is rotationally symmetric around the axis and one surface is a convex spherical surface S_1
The other surface is an axicon surface S_2, and the axicon surface S_2 is divided into two inner and outer axicon surfaces by a circle with a diameter D_1, the inner axicon is a concave or convex axicon, and the outer axicon is a concave or convex axicon. A condensing lens that is a convex axicon and has different cone base angles for the inner and outer axicons.