JPH10260378A - Laser light converging and irradiating device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a spatial intensity distribution of laser light which has been converged weakly at its center part and intensely at its peripheral part by arranging a specific composite Fresnel zone plate right before or behind a condenser lens. SOLUTION: The composite Fresnel zone plate 1 has a unit cell obtained by providing a Fresnel zone plate, which is optically different in pitch, concentrically in a Fresnel zone plate together. Then this unit cell and a complementary unit cell which is opposite in phase difference of 0 and π radian from the unit cell are arranged in quantities at random on the same plate so that they are in half-and-half proportion. The obtained composite Fresnel zone plate 1 is arranged right before or behind the condenser lens. For example, the laser light L outputted from a large-output laser after passing through the composite Fresnel zone plate 1 is converged by the condenser lens 2. An object 3 is placed almost at a distance of the focal length (f) of the condenser lens 2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser beam condensing / irradiating apparatus for converging and irradiating a laser beam onto an object, and more particularly to a laser beam after condensing without depending on the spatial intensity distribution of the laser beam before condensing. The present invention relates to a technique for controlling the spatial intensity distribution of light.

[0002]

2. Description of the Related Art In a high-power laser device, it is not only difficult to obtain a complete Gaussian beam output due to the influence of a nonlinear refractive index and the like, but also a Gaussian beam has a low energy extraction efficiency in a laser device and is practical. is not. For this reason, in most current high-power laser devices, a rectangular beam output is extracted in most cases, and the output wavefront is considerably distorted due to the influence of the nonlinear refractive index and the like. When this light is condensed on an object using a lens or the like, an extremely non-uniform intensity distribution corresponding to the Fourier transform of the spatial frequency is obtained on the light condensing surface, and energy called a hot spot is concentrated due to wavefront distortion. The location occurred and was an obstacle to use.

In order to solve this problem, the present inventor has previously used a combination of a broadband laser device and a wavelength dispersive medium on a random phase plate (Japanese Patent Laid-Open No. Hei 4-1057).
No. 81) and a combination of multiple reflecting mirrors (Japanese Patent Application Laid-Open No. Hei 8-94961). These devices have the advantage that the spatial intensity distribution of the laser light after focusing can always be kept constant irrespective of the spatial intensity distribution of the laser light before focusing and the distortion of the wavefront.

[0004]

However, the above-mentioned conventional inventions (JP-A-4-105578 and JP-A-8-10781)
Japanese Patent Application Publication No. 94961) uses a random phase plate, so that the spatial intensity distribution of the condensed laser light is a sinc square function, that is, only one specific intensity distribution function having a strong center and a weak peripheral portion. I couldn't get it.

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and has as its object to provide a laser beam condensing / irradiating apparatus capable of obtaining a spatial intensity distribution of a condensed laser beam having a weak central portion and a strong peripheral portion. It is assumed that

[0006]

According to the present invention, there is provided a unit cell in which a plurality of Fresnel zone plates having optically different pitches coexist concentrically in place of the random phase plate used in the prior art. And a composite Fresnel zone plate created by randomly arranging the same number of unit cells having the same pattern in a complementary phase relationship with the unit cell on the same surface, and immediately before or after the condenser lens. It was done.

In addition, a multiple reflection mirror for forming a laser beam slightly different in angle from a laser beam is arranged on the optical axis in front of the composite Fresnel zone plate and the condenser lens.

Further, a wavelength dispersive medium is arranged on the optical axis in front of the composite Fresnel zone plate and the condenser lens using a laser beam having a wide bandwidth.

In addition, a laser beam having a wide bandwidth is used,
A wavelength dispersive medium and a multiple reflecting mirror are arranged on an optical axis before a composite Fresnel zone plate and a condenser lens.

As described above, according to the present invention, by installing a novel phase plate called a composite Fresnel zone plate, it is possible to obtain a spatial intensity distribution of a focused laser beam having a weak central portion and a strong peripheral portion. And

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, a composite Fresnel zone plate which is a main component of the present invention will be described. In general, a Fresnel zone plate is obtained by alternately filling a half-period zone of Fresnel with black or changing its phase. For high-power lasers, blacking out discards half of the output energy,
Usually, a phase difference of 0 and π radian is alternately applied to a half-period zone. In the composite Fresnel zone plate of the present invention, a unit cell in which another Fresnel zone plate having a different optical pitch coexists at the same center in the ordinary Fresnel zone plate. A large number of the unit cells are arranged at random on the same plate together with the complementary unit cells in which the phase difference between 0 and π radian is opposite to each other so that the ratio between the two becomes half. In the present invention, the composite Fresnel zone plate thus obtained is disposed immediately before or immediately after the condenser lens.

A normal Fresnel zone plate works like a lens, and if parallel laser light is incident, strong light concentration occurs at one point. When a normal Fresnel zone plate is placed immediately before or after the condenser lens, they act as if two lenses are superimposed, and the focal length moves to the shorter focal length side than when the condenser lens is used alone. .
As a result, at the original focal position of the condenser lens, the laser light is shifted from the focal point by an amount corresponding to the reduction of the focal length, and the spatial intensity distribution of the laser light has a geometric optical spread. At first glance, the unit cells of the composite Fresnel zone plate in the present invention tend to look like a double focal length lens having different focal lengths at the center and the periphery, but actually interfere with each other due to light interference effects. It is not a simple superposition. The situation can be examined by computer simulation considering the phase index. Then, from the combinations of the pitches, it is possible to select a combination in which the center of the spatial intensity distribution of the laser light is weak and the peripheral portion is strong near the focal position of the original condenser lens. However, when only this one unit cell is used, the same spatial intensity distribution as that of the calculation is actually obtained only when the intensity distribution and wavefront of the laser beam before focusing are spatially uniform. As described above, such a laser beam cannot be obtained with an actual high-power laser, so that this unit cell alone is not practical.

In the composite Fresnel zone plate of the present invention, a large number of this unit cell and a plurality of complementary unit cells having a phase difference opposite to each other are randomly arranged on the same plate so that the ratio between the two is evenly divided. doing. As a result, the incident laser light is divided into a number of minute beams each having the size of a unit cell. Since the divided minute beams are condensed by one and the same condensing lens, the micro beams overlap each other near the focal plane of the condensing lens while interfering with each other. The situation where this interference occurs and overlaps is a collection of many speckle patterns, similar to the conventional random phase plate, but unlike the conventional random phase plate, its envelope is the same as that of the random phase plate. It is not a sinc square function, but a spatial intensity distribution calculated by a unit cell. And the spatial intensity distribution does not depend on the laser light before focusing like the case of the random phase plate.

Further, it is clear that the speckle pattern and the vibration of the spatial intensity distribution after the fine focusing can be removed by a combination of a broadband laser beam and a wavelength dispersive medium, or a multiple reflecting mirror, or both. This is similar to the case of the random phase plate.

[0015]

FIG. 1 shows an optical arrangement of a basic configuration of a first embodiment of the present invention. The laser light L output from the high-power laser passes through the composite Fresnel zone plate 1 and is condensed by the condenser lens 2. The target object 3 is placed near a position f away from the focal length of the condenser lens 2. An example of a specific phase pattern of the composite Fresnel zone plate 1 is shown as a part in FIG.

The composite Fresnel zone plate 1 is shown in FIG.
Such a pattern is spread over an area that sufficiently satisfies the aperture of the output laser light L. In the figure, a black portion indicates a phase shift of π radian, and a white portion indicates that the phase shift is 0. 1
A and 1B are unit cells having a mutually complementary phase relationship, and the same number is randomly arranged. FIGS. 3 and 4 show the diameters d1 to d3 of the half-period zone of each Fresnel for one of the unit cells 1A and 1B.

FIGS. 3A and 4B are a plan view of the unit cells 1A and 1B and a schematic view of a cross section passing through the center, respectively, and all show only the diameters d1 to d3. The specific values of d1 to d14 are as follows.

In FIGS. 3 and 4, the black portions in each of the plan views (a) are portions having a phase shift of π radian.
For example, when this is realized by using a material having a refractive index different from that of air, such as optical glass, the thickness is increased by the amount of the phase shift. There is a step due to the difference in t2.

The n th half-period zone of Fresnel is generally In this embodiment, d1 to d3 are r0
= 2, whereas r0 = 3 outside of d4. In this way, a plurality of Fresnel zone plates having optically different pitches coexist concentrically to form the unit cell 1.
A and 1B are features of the present invention. FIG. 5 shows, by computer simulation, the spatial intensity distribution at a distance f when the laser beam L passing through the unit cells 1A and 1B is condensed by the condenser lens 2 having a focal length of 1 m.

In the calculation, since the result is a complete rotation target, only one side from the center is shown in FIG. That is, 0 is the center, and the area indicated by 500 is the area around the outer edge of the condensed laser light L. Although a vibration waveform due to interference is included, this vibration is actually mitigated by slightly shifting the object 3 from the position of the distance f. Actually prototyped this example,
FIG. 6 shows a measurement of the spatial intensity distribution of the laser light L at the position of the target 3.

FIG. 6 is different from FIG. 5 in that the whole is displayed instead of the half. Accordingly, the center where 0 is displayed is the center, and the places where -500 and 500 are displayed correspond to the outer peripheral edge of the laser beam L. 5 and 6, the vertical axis represents the light intensity.

Next, a second embodiment will be described with reference to FIG. The second embodiment is a composite Fresnel zone plate 1
And a multi-reflection mirror arranged on the optical axis before the condenser lens 2. In FIG. 7, reference numeral 4 denotes a multiple reflecting mirror, 5 denotes a laser amplifier, and 10 denotes a laser oscillator. Multiple reflection mirror 4
Is partially coated on both sides thereof, and a part of the laser beam L that has arrived is transmitted and the rest is reflected. Both surfaces of the multiple reflecting mirror 4 are not parallel to each other but are planes having a slight angle. Therefore, a part of the incident laser light L is transmitted through the multiple reflecting mirror 4 as it is, but the rest is reflected and a part is transmitted to the outside of the multiple reflecting mirror 4 while the rest passes through the inside of the multiple reflecting mirror 4 many times. Go back and forth. Multiple reflection mirror 4
Since both surfaces have angles, the lights emitted to the outside of the multiple reflection mirror 4 have angles to each other, and as a result, a beam bundle La composed of a large number of laser beams L having slightly different angles is obtained. In FIG. 7, a large number of beams generated here are indicated by a single line (see Japanese Patent Application Laid-Open No. 8-94961). As shown in FIG. 7, a laser beam L from a laser oscillator 10 is converted into a beam bundle La having a slightly different angle by a multiple reflecting mirror 4, which is amplified by a laser amplifier 5, and then the same composite Fresnel zone plate as in the first embodiment. The light is condensed by 1 and a condenser lens 2. As in the first embodiment, the spatial intensity distribution on the object 3 is strong at the outside and weak at the center as in the first embodiment. However, due to the effect of the multiple reflecting mirror 4, a vibration component finer than in the first embodiment is generated. Relaxed and smoother can be obtained.

Next, a third embodiment will be described. In the third embodiment, as shown in FIG. 8, a laser beam L having a wide bandwidth generated from a broadband laser device is used as a laser beam.
This is an example in which the wavelength dispersive medium 6 is disposed on the optical axis before the composite Fresnel zone plate 1 and the condenser lens 2 using the “b”. As shown in FIG. 8, a prism is used as the wavelength dispersive medium 6 in the third embodiment. When the broadband laser light Lb passes through the wavelength dispersive medium 6, an angular spread occurs corresponding to the wavelength band of the broadband laser light Lb which was originally parallel (see Japanese Patent Application Laid-Open No. 4-1055781). In this manner, the laser beam can be given a slight angular spread in the wavelength dispersive medium 6,
As in the second embodiment, the spatial intensity distribution on the object 3 is such that the outside is strong and the center is weak as in the first embodiment, but the vibration is finer than in the first embodiment. The components are relaxed and a smoother product can be obtained.

Next, a fourth embodiment will be described. In the fourth embodiment, as shown in FIG. 9, the multiple reflection mirror 4 used in the second embodiment and the wide band laser light Lb and the wavelength dispersive medium such as a prism used in the third embodiment are used. 6 shows a case where both of the combinations of 6 are simultaneously used. Both effects are arranged so that the multiple reflecting mirrors 4 are orthogonal to each other, such as in a direction parallel to the plane of the paper and the wavelength dispersive medium 6 in a direction perpendicular to the plane of the paper. As a result, the fine vibration components appearing in the first embodiment are two-dimensionally improved, and a smoother spatial intensity distribution with a stronger outer portion and a weaker central portion can be obtained.

[0025]

According to the present invention, a unit cell in which a plurality of Fresnel zone plates having optically different pitches coexist concentrically, and a unit cell having the same pattern and a complementary phase relationship with the unit cell And a composite Fresnel zone plate created by randomly arranging the same number on the same surface, and arranged immediately before or immediately after the condensing lens. It is possible to obtain a spatial intensity distribution of the condensed laser light, which is weak and the peripheral part is strong.

The composite Fresnel zone plate used in the present invention can be made to have a high proof strength capable of coping with a large output laser beam by a technique for forming a transmission type phase plate, and has a small transmission loss. When a fine vibration component in the spatial intensity distribution on the object is a problem, as in the present invention, a combination of a multi-reflecting mirror or a laser light having a wide bandwidth and a wavelength dispersive medium, or both. Can be removed by applying

In laser processing or laser processing, it is often the case that the intensity distribution according to the present invention, in which the peripheral portion is strong and the central portion is weak, is more preferable than the spatial intensity distribution in the ordinary condensing irradiation in which the central portion is strong. . Therefore, the present invention can be used to improve the quality and efficiency of such an industrially important laser application device.

Also, in the case of future energy development such as laser fusion, in order to uniformly irradiate a laser to a spherical fuel sphere, the focused irradiation spatial intensity distribution of each laser beam should be higher than the center. It is considered that it is preferable that the peripheral portion is strong, and the present invention is also useful for promoting the development.

[Brief description of the drawings]

FIG. 1 is a diagram showing an optical arrangement of a basic configuration according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a part of a phase pattern of a composite Fresnel zone plate.

3 is a diagram specifically illustrating the diameter of a half-period zone of each Fresnel for one unit cell in the phase pattern of the composite Fresnel zone plate of FIG. 2;

FIG. 4 is a diagram specifically illustrating the diameter of a half-period zone of each Fresnel for one unit cell in the phase pattern of the composite Fresnel zone plate of FIG. 2;

FIG. 5 is a diagram showing, by computer simulation, a spatial intensity distribution at a distance f when a laser beam passing through the unit cell of FIG. 3 is condensed by a condenser lens having a focal length of 1 m.

FIG. 6 is a diagram showing an actual measurement example of a spatial intensity distribution of condensed irradiation on a target obtained by actually producing a first embodiment of the present invention.

FIG. 7 is a diagram showing an optical arrangement according to a second embodiment of the present invention.

FIG. 8 is a diagram showing an optical arrangement according to a third embodiment of the present invention.

FIG. 9 is a diagram showing an optical arrangement according to a fourth embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Composite Fresnel zone plate 1A, 1B Unit cell 2 Condensing lens 3 Object 4 Multiple reflection mirror 5 Laser amplifier 6 Wavelength dispersive medium f Focal length of condensing lens L Laser beam Lb Laser beam with wide bandwidth

Claims (4)

[Claims] 1. A laser beam condensing and irradiating apparatus for converging and irradiating a laser beam onto an object through a converging lens, wherein a plurality of optically different pitch Fresnel zone plates coexist concentrically. A composite Fresnel zone plate created by randomly arranging the same number of unit cells and unit cells of the same pattern having a complementary phase relationship with the unit cell on the same surface, immediately before or immediately after the condenser lens A laser beam condensing and irradiating device, which is disposed in a laser beam. 2. A laser beam condensing and irradiating apparatus according to claim 1, wherein the multi-reflecting mirror converts the laser beam into a beam bundle slightly different in angle on an optical axis before the composite Fresnel zone plate and the converging lens. A laser beam condensing and irradiating device characterized by having an arrangement. 3. The laser beam condensing and irradiating apparatus according to claim 1, wherein a wide band laser beam generated from a broadband laser device is used as the laser beam, and the composite Fresnel zone plate and the converging lens are provided before the condensing lens. A laser beam focusing / irradiating apparatus comprising a wavelength dispersive medium disposed on an optical axis. 4. The laser beam condensing and irradiating apparatus according to claim 1, wherein a wide band laser beam generated from a broadband laser device is used as the laser beam, and the composite Fresnel zone plate and a converging lens are provided. A laser beam condensing / irradiating apparatus comprising a wavelength dispersive medium and a multiple reflecting mirror arranged on an optical axis.