JP7360159B2 - Laser light diffraction focusing method and diffraction focusing optical element device - Google Patents

Description

The present invention relates to a laser beam diffraction focusing method and a diffraction focusing optical element device.

Research using high-intensity lasers, such as nuclear fusion, particle acceleration, laboratory astronomy, and the study of materials in extreme conditions at high temperatures and pressures, has developed as the peak intensity of lasers has increased. Increases in the focused intensity of these lasers have been achieved through the invention of chirped pulse amplification, larger and more precise optical elements, and advances in wavefront and phase correction technology. However, in order to further increase the intensity and output of these laser systems, there are many issues that need to be solved with respect to the optical elements in the systems, and there are major problems in realizing the next generation of high-intensity lasers.

In processing using pulsed lasers, the laser is focused by a lens or condensing mirror just in front of the object to be processed, creating a high-intensity optical field to process the material. At this time, fine particles (debris) that melt in the laser irradiation area during processing adhere to the laser condenser lens and upstream optical elements, causing problems such as a decrease in laser light transmittance and damage to the elements. ing.

To address this problem, for example, Patent Document 1 discloses an objective lens that focuses a laser beam on a workpiece, and a system that is arranged between the objective lens and the workpiece to prevent debris flying from the workpiece. It is described that the objective lens is provided with a lens protection cover that blocks out contamination of the objective lens.

Further, Patent Document 2 discloses that a passage hole is formed through which a laser beam condensed by a condensing lens and focused on a workpiece passes, and a suction path extends symmetrically with respect to the passage hole; One end and the other end of the suction path each have a suction source selectively connected to collect debris generated when the workpiece is irradiated with a laser beam focused by the condenser lens. It is described that the device is equipped with a means for collecting dust.

In addition, there are other methods to prevent debris from adhering by flowing low-pressure gas near the material (Non-patent Document 1), and a method to absorb debris by inserting a liquid between the lens and the workpiece (Non-patent Document 1). 2) A method of controlling the trajectory of charged debris by applying a magnetic field (Non-Patent Document 3) has been devised.

Japanese Patent Application Publication No. 2012-179642 Japanese Patent Application Publication No. 2014-205159

L. Malmqvist, L. Rymell, and H. M. Hertz, Appl. Phys. Lett. 68, 2627 (1996) D.J. Hwang, T.Y. Choi, C.P. Grigoropoulos, Applied Physics A, Vol. 79, Issue 3, pp 605-612 (2004) I. Prencipe, et al. High Power Laser Science and Engineering, Vol.5 e17, p.1-31 (2017)

However, these methods are not effective against debris with a size of several hundred micrometers, the intensity of laser irradiation on the workpiece decreases due to the introduction of extra substances such as liquid, and they cannot control neutral particles that are not charged. Complete debris control has not yet been achieved. The incompleteness of these debris protections determines the maintenance time for laser processing machines and requires periodic cleaning, which is a major impediment to the use of lasers for higher repetition rates and higher output than currently available. The current situation is that

SUMMARY OF THE INVENTION In view of the above-mentioned conventional circumstances, an object of the present invention is to provide a method for diffraction and focusing of a laser beam, which is capable of diffracting and focusing even a high-intensity laser beam, and is not affected by debris, and a diffraction focusing optical system. An object of the present invention is to provide an element device.

That is, in one embodiment of the present invention, a gas containing molecules that absorb light resonantly is supplied to form a certain region, and excitation laser light having a wavelength in the absorption band of the molecule is intersected within the region. In addition to optically exciting the gas by irradiating it in a manner similar to It is characterized by generating interference fringes within the region and using the interference fringes in the region as a transient diffraction and focusing element to diffract and focus laser light having a wavelength in the non-absorption band of the gas that is incident on the interference fringes. This is a method of diffraction and focusing of laser light.

According to one aspect of the present invention, a gas containing molecules that absorb light resonantly is irradiated with an excitation laser in a spatial periodic manner to generate a large-amplitude density modulation structure inside the gas. It can be made into a transmissive volume diffraction condensing element that diffracts and condenses light. In addition, since the medium of this element is gas, it is not affected by debris adhering to it like normal solid-state optical elements, and at the same time, the angle is deflected by diffraction, so the optical elements upstream from this element are It can also be protected.

At this time, in one aspect of the present invention, the gas containing molecules that absorb light resonantly may be a gas containing ozone, and the excitation laser may be an ultraviolet pulsed laser with a wavelength of 230 to 270 nm. Furthermore, the coherence of the laser beam is preferably high enough to generate interference fringes with clear contrast. It is desirable that the intensities of the two intersecting beams be adjusted to be equivalent within the gas.

As described above, a gas containing ozone is preferably used as the gas containing molecules that absorb light resonantly.

Further, in one aspect of the present invention, it is preferable that a certain region of gas be in a laminar flow state.

By doing so, density modulation with a curvature having a high contrast ratio can be generated by the excitation laser in a certain region caused by the gas.

Further, in one aspect of the present invention, the focal length of the laser beam to be diffracted and focused may be adjusted by adjusting the distance between the condensing optical system and the diverging optical system.

Since the focal length is determined by the curvature of interference fringes caused by the excitation laser beam, the focal length can be easily changed by adjusting the distance between the condensing optical system and the diverging optical system inside the interference optical system.

Further, in one embodiment of the present invention, the focal position of the diffracted and focused laser beam may be scanned by making the emission angle of the excitation laser beam variable.

The focal position of the laser beam can be scanned by adjusting the reflection mirror or the like inside the interference optical system to make the emission angle of the excitation laser beam variable.

Further, in one embodiment of the present invention, a shielding plate having an opening through which the laser beam passes may be provided near a certain region of gas.

By providing the shielding plate, it is possible to prevent, for example, debris generated from the object to be processed during laser processing from entering the optical system of the laser for light excitation and focusing.

Another aspect of the present invention provides a gas generating means for forming a certain area by supplying a gas containing molecules that absorb light resonantly, and an excitation laser beam having a wavelength in the absorption band of the molecule within the area. In addition to optically exciting the gas by irradiating the gas in such a manner that they cross each other, at least one of the optical paths of the two intersecting excitation laser beams is provided with one or both of a condensing optical system and a diverging optical system. and an optical excitation means for generating interference fringes having a curvature in the area, and using the interference fringes in the region as a transient diffraction and focusing element, a laser having a wavelength in the non-absorption band of the gas incident on the interference fringes. This is a diffractive and condensing optical element device characterized by diffracting and condensing light.

The above-described laser beam diffraction and focusing method can be realized as a diffraction and focusing optical element device having such a configuration.

As explained above, according to the present invention, there is provided a laser beam diffraction and focusing method and a diffractive focusing optical element device that can diffract and focus even high-intensity laser beams and are not affected by debris. I can do it.

1 is a schematic diagram showing an example of the configuration of a diffraction focusing optical element device according to an embodiment of the present invention. FIG. 2 is a conceptual diagram illustrating generation of interference fringes by an excitation laser. (A) is a diagram showing the light focusing characteristics in the vertical direction when implementing the laser beam diffraction focusing method according to an embodiment of the present invention, and (B) is a diagram showing the light focusing characteristics in the horizontal direction. FIG.

Hereinafter, preferred embodiments of the present invention will be described in detail. Note that this embodiment described below does not unduly limit the content of the present invention described in the claims, and all of the configurations described in this embodiment are essential as a solution to the present invention. Not necessarily.

FIG. 1 is a schematic diagram showing an example of the configuration of a diffraction focusing optical element device according to an embodiment of the present invention. A diffraction focusing optical element device 100 according to an embodiment of the present invention includes a gas generating means 10 that supplies a gas containing molecules that absorb light resonantly to form a certain region, and a gas generating means 10 that forms a certain area by supplying a gas containing molecules that absorb light resonantly, and an absorption band of the molecules. The gas is optically excited by irradiating excitation laser beams of different wavelengths so as to intersect in the region, and one of a condensing optical system and a diverging optical system is attached to at least one of the optical paths of the two intersecting excitation laser beams. Alternatively, by having both, the optical excitation means 20 is provided which generates interference fringes with curvature within the region.

In the diffraction focusing optical element device 100 according to one aspect of the present invention, the gas generation means 10 has a function of supplying a gas containing molecules that absorb light resonantly to form a certain region, for example, As shown in FIG. 1, it is composed of a gas generation section 11, a gas flow path 12, a gas discharge section 13, and the like.

The gas to be supplied is not particularly limited as long as it contains molecules that absorb light resonantly, but a gas containing ozone is preferably used. It is desirable to prepare ozone at a concentration (approximately 1 to 3%) sufficient for the ultraviolet laser (50 mJ/cm ² ) to be absorbed by a gas layer with a thickness of 3 to 10 mm. As a method for generating ozone, for example, dielectric barrier discharge using oxygen gas as a raw material is used.

When the gas generation unit 11 generates a gas containing ozone, for example, a source gas containing oxygen O ₂ is supplied into a space gap where dielectric barrier discharge is generated, and gas containing ozone O ₃ is generated by discharge. More specifically, it includes a pair of metal electrodes arranged in a parallel plate shape and a pair of dielectrics installed between the metal electrodes, and a raw material containing oxygen O ₂ is supplied from a raw material gas source such as an oxygen cylinder or an air tank. Gas (highly concentrated oxygen, dehumidified air, etc.) is pumped into the space gap between the pair of dielectrics by a diaphragm pump, and a high frequency high voltage is applied to the pair of metal electrodes by a high frequency power source. For example, a 13 MHz/100 W power source is used as the high frequency power source.

Note that the generation of the ozone-containing gas is not limited to the above embodiment, and the ozone-containing gas may be generated by an electrolytic method or an ultraviolet lamp method. Alternatively, the ozone-containing gas may be, for example, highly concentrated ozone gas obtained by vaporizing liquid ozone.

The gas flow path 12 is a flow path for supplying the gas generated by the gas generation unit 11, and has a function of forming a certain region for exciting the gas with an excitation laser. Although the shape of the gas flow path 12 is not particularly limited, for example, it has a rectangular cross-sectional shape with a sufficient length, and an opening 14 is provided in the center thereof. Excitation laser light and controlled laser light (processing laser light) are input into the gas flow path 12 through the opening 14 and exit through the opening 14 on the opposite side. As long as it has such a function, the number and shape of the openings 14 are not particularly limited, and the openings 14 through which the excitation laser beam and the controlled laser beam (processing laser beam) pass are the same. It may be different or it may be different. As an example, the gas flow path 12 is a gas flow tube or the like with a length of 250 mm and an inner diameter of 3 mm in depth (in the direction of propagation of the laser beam) x 10 mm on the sides, and the opening 14 is 6 mm in length x 10 mm in width.

The gas exhaust section 13 exhausts gas from the gas flow path 12. Since the laser irradiated portion of the gas flow path 12 is an opening as described above, the gas discharge speed is adjusted so that the gas does not leak from the opening. In one embodiment, the gas recovered from the gas outlet 13 may be returned to the gas generator 11 via a recirculation line.

In one aspect of the present invention, it is preferable that a certain region of gas be in a laminar flow state. By creating a laminar flow state, it is possible to generate density modulation with a curvature with a high contrast ratio when a certain region of gas is irradiated with an excitation laser. Therefore, it is desirable to control the gas discharge speed on the gas discharge section 13 side and the gas supply speed on the gas generation section 11 side so that laminar flow is maintained also in the opening section 14.

In the diffractive focusing optical element device 100 according to one aspect of the present invention, the optical excitation means 20 is configured spatially to generate a large-amplitude density modulation structure in a windowless region in the gas flow path 12 that serves as a lens. Used to create periodic intensity distribution. More specifically, an excitation laser light source 21 generates a laser light that excites the gas supplied from the gas generation means 10, and the laser light L emitted from this laser light source 21 is converted into two laser lights L1 and L2. a beam splitter 22 that splits the laser beams into two, reflecting mirrors 23A to 23F that reflect the optical path so that the two split laser beams L1 and L2 intersect in a certain area formed by the gas generation means 10; A condensing optical system 24 (for example, a convex lens), a diverging optical system 25 (for example, a concave lens), etc. are provided on at least one of the optical paths of the excitation laser beams L1 and L2 (on the laser beam L2 side in FIG. 1). In order to obtain an effective spatial periodic intensity distribution due to interference of the excitation laser beam at the opening 14 of the gas flow path 12, the distance from the beam splitter 22 to the opening 14 of the gas flow path 12 is set such that the distance between the beam splitter 22 and the opening 14 of the gas flow path 12 is It is desirable that they almost match. The spacing of interference fringes due to such a combination of spherical waves and plane waves gradually changes, but by generating several thousand or more high-order interference fringes, the difference in the fringe spacing within the transiently generated lens can be reduced. It is desirable to keep it below 1%.

FIG. 2 is a conceptual diagram illustrating generation of interference fringes by an excitation laser. In this way, one optical path (L1) of the excitation laser divided into two is left as a plane wave, and the other optical path (L2) is a combination of condensing and diverging optical systems such as a convex lens and a concave lens. By arranging it, the desired spherical wave is generated, and the two excitation laser beams are propagated with the same intensity into the windowless area of the gas flow path at an appropriate incident angle, resulting in a high contrast ratio. It is possible to generate interference fringes with a certain curvature. Due to the interference fringes, the controlled laser Lin irradiated from the controlled laser light source (for example, a processing laser light source) 40 can be turned into diffracted and focused laser light Lo. The interval between the interference fringes is, for example, several μm, but this interval can be made variable by an interference optical system 30, which will be described later.

When the gas containing molecules that absorb light resonantly is a gas containing ozone, the excitation laser is preferably an ultraviolet pulsed laser with a wavelength of 230 to 270 nm. At this time, the pulse width determines the lateral aperture of the ozone mixed gas lens, so it is set to, for example, several nanoseconds or less.

Several tens of nanoseconds after ultraviolet laser irradiation, when the refractive index modulation inside the gas is at its maximum, if a controlled laser is made incident at an appropriate angle of incidence, the first-order diffracted light will be diffracted with almost 100% diffraction efficiency and will be focused. It is possible to create light that is The controlled laser is, for example, a processing laser for laser processing the processing object 50.

The focal length of the diffractive focusing optical element device 100 according to an embodiment of the present invention is determined by the curvature of the excitation laser interference fringes, and the curvature is determined by the curvature of the two lenses inside the interference optical system 30 (the focusing optical system 24 and the divergent It can be easily changed by changing the distance of the optical system 25). Furthermore, the focal point can also be changed by changing the interval between the stripes.

That is, in one aspect of the present invention, by adjusting the distance between the condensing optical system 24 and the diverging optical system 25, the focal length of the diffracted and condensed laser beam Lo may be adjusted. Since the focal length is determined by the curvature of the interference fringes caused by the excitation laser beam, the focal length can be easily changed by adjusting the distance between the condensing optical system 24 and the diverging optical system 25 inside the interference optical system. can.

Further, in one embodiment of the present invention, the focal position of the diffracted and focused laser beam may be scanned by making the emission angle of the excitation laser beam variable. For example, by adjusting the reflecting mirrors (reflecting mirrors 23D, 23E, etc.) inside the interference optical system 30 to make the emission angles of the excitation laser beams L1 and L2 variable, it is possible to scan the focal position of the laser beam Lo. Therefore, the degree of freedom in laser processing of the processing object 50 can be increased.

Furthermore, in one aspect of the present invention, a shielding plate 60 having an opening through which the laser beams Lin and Lo pass may be provided near a certain region of gas in the gas flow path 12. By providing the shielding plate 60, it is possible to prevent debris 51 generated during laser processing of the processing object 50 from entering the interference optical system 30 and the like. As the shielding plate 60, for example, one having an opening of 2 mm can be used.

When a gas containing ozone is used as the gas containing molecules that absorb light resonantly, the principle of excitation of the refractive index modulation structure in the gas medium is considered to be as follows. That is, first, ozone is generated from raw material oxygen gas by dielectric barrier discharge or the like, and an ultraviolet laser having resonance absorption is made to interfere with the ozone and enter the medium into this medium. Here, the ozone in the area irradiated with the ultraviolet laser is photodecomposed (O ₂ +O) by the energy of the ultraviolet laser and then recombined to become ozone in a high energy state. After that, over several nanoseconds to tens of nanoseconds, the excited ozone with high kinetic energy collides with surrounding oxygen molecules, expands and compresses, and becomes dense and dense between the non-irradiated area and the irradiated area. form. This density difference becomes the refractive index necessary for diffraction.

This spatial density modulation structure is approximately determined by the interference irradiation pattern of the ultraviolet laser in the case of circular or straight interference fringes with a difference in fringe spacing of 1% or less. Therefore, by creating an ultraviolet laser interference pattern with a slight curvature in the gas, it is theoretically possible to provide it with the function of a focusing optical element in addition to the diffraction function, like a Fresnel zone plate. .

FIG. 3 shows typical vertical and horizontal focusing characteristics at a focal length of 260 mm when the laser beam diffraction focusing method according to an embodiment of the present invention is implemented. The horizontal axis represents the distance from the gas lens region, and the vertical axis represents the condensing diameter, showing excellent condensing characteristics with M ² =1.1 in both the vertical and horizontal directions.

In this way, in one embodiment of the present invention, a gas containing several percent ozone is irradiated with an ultraviolet laser in a spatial periodic manner to generate a large-amplitude density modulation structure inside the gas, thereby diffracting the laser light. It is a transmission type volume diffraction grating. Furthermore, in one embodiment of the present invention, by irradiating the gas with an ultraviolet laser under slightly curved interference conditions, the diffracted light can be focused in the same way as a lens.

In addition, in the present invention, a large-amplitude density modulation structure is generated in the optically excited gas, and a reproducible transient diffraction focusing optical element is created based on this, so even if the laser beam passes through the diffraction focusing optical element, Even if an optical optical element is destroyed, it can be regenerated each time, making it possible to create a diffractive focusing optical system that can handle high-intensity lasers. Therefore, even if light exceeding the destructive intensity is incident, it is regenerated every time, so it can be used with the intensity at the limit of use without a safety factor.

The gas containing ozone molecules used in one embodiment of the present invention, such as oxygen gas, has an absorption cross section of 10 ^-20 to 10 ^-23 cm ² for the laser wavelength used in general laser processing. Since it is small, the loss of laser energy due to gas can be ignored, and since it is a volume diffraction grating, almost 100% diffraction efficiency can be obtained by forming compression waves using a small amount of ultraviolet laser.

Furthermore, since the diffractive condensing optical element device according to one embodiment of the present invention uses gas as a medium, debris does not adhere to it unlike a normal solid optical element. In addition, this element has a damage threshold three orders of magnitude higher than that of solid-state optical elements even when exposed to high-intensity lasers such as processing lasers, so the element itself is resistant to optical damage caused by lasers, and even if it is destroyed, it can be immediately rebuilt. It is a transient element.

Although one embodiment of the present invention has been described in detail as above, those skilled in the art will easily understand that many modifications can be made without substantively departing from the novelty and effects of the present invention. It will be possible. Therefore, all such modifications are included within the scope of the present invention.

For example, a term that appears at least once in the specification or drawings together with a different term with a broader or synonymous meaning may be replaced by that different term anywhere in the specification or drawings. Furthermore, the structure and operation of the laser beam diffraction and focusing method and the diffraction and focusing optical element device are not limited to those described in the embodiment of the present invention, and various modifications are possible.

An embodiment of the present invention can be used as a final focusing lens for material processing using a pulsed laser, which is free of debris and has no problem of semi-permanent damage. Currently, laser processing requires an optical element that focuses a high-intensity laser and a debris shield element that protects the optical elements that make up the upstream laser system. Now, there is no need to clean or replace them, making it possible to create a nearly maintenance-free system.

In addition, the ozone mixed gas lens described in one embodiment of the present invention has a damage resistance of kJ/cm ² against nanosecond laser, so it can be used with high-intensity laser even with a small cross-sectional area, and it can be used for laser processing. It can also be applied to high-intensity laser plasma technology.

As one aspect of the present invention, the energy of the ultraviolet laser required to generate the ozone mixed gas lens is 50 mJ/cm ² , and the maximum energy of the controlled laser is 1.6 kJ/cm ² , and the control requires only a small amount of light. This makes it possible to efficiently generate lenses and control the focal position and focal length of the laser beam.

10 gas generation means, 11 gas generation section, 12 gas flow path, 13 gas discharge section, 14 opening, 20 optical excitation means, 21 excitation laser light source, 22 beam splitter, 23A to 23F reflecting mirror, 24 condensing optical system, 25 diverging optical system, 30 interference optical system, 40 controlled laser light source, 50 processing object, 51 debris, 60 shielding plate, 100 diffraction focusing optical element device, L, L1, L2 excitation laser light, Lin controlled laser light , Lo diffraction and focused laser light

Claims (12)

A certain area is formed by supplying a gas containing molecules that absorb light resonantly,
Excitation laser beams having a wavelength in the absorption band of the molecules are irradiated so as to intersect in the region to optically excite the gas, and condensing optics on at least one of the optical paths of the two intersecting excitation laser beams. generating interference fringes with curvature in the region by having one or both of a system and a diverging optical system,
A laser characterized in that the interference fringes in the region are used as a transient diffraction and focusing element to diffract and focus laser light having a wavelength in a non-absorption band of the gas that is incident on the interference fringes. Diffraction and focusing method of light. The gas containing molecules that absorb light resonantly is a gas containing ozone,
2. The laser beam diffraction and focusing method according to claim 1, wherein the excitation laser is an ultraviolet pulse laser with a wavelength of 230 to 270 nm. 3. The laser beam diffraction and focusing method according to claim 1, wherein the certain region of the gas is in a laminar flow state. 4. A focal length of the diffracted and focused laser beam is adjusted by adjusting a distance between the condensing optical system and the diverging optical system. A laser beam diffraction focusing method described in . 5. A focal position of the diffracted and focused laser beam is scanned by making the emission angle of the excitation laser beam variable. Diffraction and focusing method of laser light. 6. The laser beam diffraction and focusing method according to claim 5, further comprising providing a shielding plate having an aperture through which the laser beam passes in the vicinity of the region. a gas generating means for forming a certain area by supplying a gas containing molecules that absorb light resonantly;
Excitation laser beams having a wavelength in the absorption band of the molecules are irradiated so as to intersect in the region to optically excite the gas, and condensing optics on at least one of the optical paths of the two intersecting excitation laser beams. and an optical excitation means that generates interference fringes with curvature in the region by having one or both of a system and a diverging optical system,
Diffraction characterized by using the interference fringes in the region as a transient diffraction and condensing element to diffract and condense laser light having a wavelength in a non-absorption band of the gas that is incident on the interference fringes. Concentrating optical element device. The gas containing molecules that absorb light resonantly is a gas containing ozone,
8. The diffraction focusing optical element device according to claim 7, wherein the excitation laser is an ultraviolet pulsed laser having a wavelength of 230 to 270 nm. 9. The diffraction focusing optical element device according to claim 7, wherein the gas generating means controls the gas so that the certain region is in a laminar flow state. 10. The diffraction focusing optical element device according to claim 7, wherein the distance between the focusing optical system and the diverging optical system in the light excitation means can be adjusted. The diffraction focusing optical element device according to any one of claims 7 to 10, characterized in that the emission angle of at least one of the two intersecting excitation laser beams in the optical excitation means is variable. . 12. The diffraction focusing optical element device according to claim 7, further comprising a shielding plate having an opening through which the laser beam passes in the vicinity of the region.