JP5071868B2 - Laser processing method, laser processing apparatus, optical element manufacturing method, and optical element - Google Patents

Description

The present invention relates to a laser processing method, a laser processing apparatus, an optical element manufacturing method, and an optical element, and in particular, an ultrashort pulse laser beam (hereinafter referred to as a “femtosecond laser” having a pulse width of 10 ⁻¹⁵ seconds to 10 ⁻¹¹ seconds. Is also applied to the surface of the material to be processed or the inside thereof to process the material.

  In recent years, apart from methods of manufacturing optical elements by processes such as lithography, femtosecond laser light is irradiated inside an optical material such as glass or a polymer structure to induce a change in the refractive index of the irradiated part. For example, a method of processing an optical element such as a diffractive optical element or an optical waveguide into an optical material is known. This processing method has an advantage that the manufacturing process can be shortened because the optical element can be directly manufactured on the optical material. Further, since the optical element is formed inside the optical material, there is an advantage that the optical element can be integrated.

  By using this processing method, modification or processing may be performed in which the thickness or depth is large with respect to the diameter, that is, the so-called aspect ratio is large. Here, “thickness” or “depth” is the length of the modified region (or processed region) along the optical axis direction (incident direction) of the laser beam, and “diameter” is the thickness thereof. It is defined as the length of the modified region (or processed region) along the direction perpendicular to the vertical direction. For example, in a processing method in which a refractive index change is induced inside an optical material by femtosecond laser light to form a diffractive optical element, the thickness of the modified portion where the refractive index has changed is increased by performing a modification with a large aspect ratio. Become. Thereby, the advantage that the diffraction efficiency of a diffractive optical element becomes high is acquired.

  As a method for increasing the aspect ratio in the above processing method, in Patent Document 1 and Patent Document 2, a method of realizing a long focal depth by utilizing the wavelength spectrum of a femtosecond laser and the wavelength dispersion of a diffractive lens is disclosed. Has been proposed. The methods proposed in these documents specifically change the focal position of the diffractive lens according to the wavelength of the femtosecond laser, that is, increase the depth of focus using chromatic aberration. For example, according to Equation (38) described in Patent Document 2, the reference (center) wavelength of the femtosecond laser is Λ, the pulse width is Δt, the focal length of the diffractive lens with respect to the wavelength Λ is F, and the speed of light is In the case of c, a range ΔF (depth of focus) in which the focal length changes is derived as approximately ΔF = 2 × ln2 × F × Λ / π / c / Δt.

  From the above equation, in order to increase the range ΔF in which the focal length changes, it is necessary to increase the reference (center) wavelength Λ of the femtosecond laser, shorten the pulse width Δt, or increase the focal length F. . Since the former two methods relate to the laser oscillator, there is a possibility that the laser apparatus may be restricted when realizing these methods. The method of increasing the focal length F, which is the latter method, requires the lens diameter to be increased particularly when the numerical aperture is increased in order to make the modified diameter and the processed diameter fine. There is a problem that the optical system is difficult and an optical system is enlarged.

  Further, in the laser processing apparatus described in any one of Patent Documents 1 and 2, the design wavelength of the diffractive lens, that is, the wavelength when the annular zone of the diffractive lens is designed so that the light condensing performance is the best, The wavelength of the femtosecond laser is designed to be the same as that of the femtosecond laser, and is designed with the intention that no aberration other than chromatic aberration due to dispersion occurs. Patent Document 1 describes that the modified thickness is controlled by changing and adjusting the intensity of laser light for each wavelength. However, an apparatus for changing and adjusting the intensity of laser light is provided outside the laser apparatus. Therefore, there is a problem that the configuration of the processing apparatus becomes complicated.

  As another method for increasing the aspect ratio in the above processing method, in Non-Patent Document 1, spherical aberration is increased by condensing a femtosecond laser deep inside the material with a high-aperture objective lens. It is described that the light emitting region (filament length) is elongated in the optical axis direction of the femtosecond laser. Non-Patent Document 1 describes that the filament length is about 145 μm when the numerical aperture of the objective lens is 0.40 and the depth position where light is collected is 3.1 mm from the material surface, for example. . In Non-Patent Document 2, the aspect ratio of the waveguide formed changes according to the depth position where light is condensed inside the material. Specifically, the aspect ratio increases as the light collection position becomes deeper. It is stated that this is due to the influence of spherical aberration. In Non-Patent Document 2, for example, when the numerical aperture of the objective lens is 0.5 and the depth position where light is collected is 2.1 mm from the material surface, an ellipse having a minor axis of 3 μm and a major axis of 12 μm. It is described that a refractive index changing region is formed.

However, in order to increase the aspect ratio by the method described in Non-Patent Document 1 or Non-Patent Document 2, it is necessary to increase the depth position. Therefore, there is a problem that there is a restriction that a modified layer having a large aspect ratio cannot be formed at an arbitrary depth position, or that a thickness of an optical material itself to be processed becomes large. Further, the modified / processed thicknesses shown in Non-Patent Document 1 and Non-Patent Document 2 are as short as about a few tens of μm, and have not yet obtained a sufficient length for the demand to increase the modified thickness. . Note that the method described in Non-Patent Document 1 or Non-Patent Document 2 is caused by the depth position in the material so that the modified portion is formed into a substantially circular shape in order to utilize the modified portion mainly for an optical waveguide. Spherical aberration is corrected by an external optical system.
JP 2005-262290 A JP 2006-142342 A Q.Sun et al, “Effect of spherical aberration on the propagation of a tightly focused femtosecond laser pulse inside fused silica”, J. Opt. A: Pure Appl. Opt. 7, pp655-659 (2005) D.LIU et al, “Influence of focusing depth on the microfabrication of waveguides inside silica glass by femtosecond laser direct writing”, Appl. Phys. A84, pp257-260 (2006).

  In view of the above-described conventional problems, the present invention has a large aspect ratio without any restrictions on the femtosecond laser device or an increase in the size of the optical system, and without restriction on the depth position inside the material. It is an object of the present invention to provide a technique for processing a modified layer or a processed part into a material inside or a surface.

In summary, the present invention is a laser processing method, in which a femtosecond laser having a pulse width of 10 ⁻¹⁵ seconds to 10 ⁻¹¹ seconds is focused on either the surface or inside of a material to be processed by a diffractive lens. Thus, a process of processing the material to be processed and a process of setting the center wavelength Λ of the femtosecond laser and the design wavelength λ of the diffractive lens prior to the process of processing. In the setting step, the focal length of the femtosecond laser incident on the innermost ring of the diffractive lens is f _1, and the focal length of the femtosecond laser incident on the outermost ring of the diffractive lens is f _2. Then, the longer of the focal lengths f ₁ and f ₂ is equal to the focal length of the comparison target lens that is a diffractive lens having the center wavelength Λ of the femtosecond laser as a design wavelength, and the femtosecond laser The absolute value of the wavelength difference between the center wavelength Λ and the design wavelength λ is set so that the absolute value of the difference between the focal length f ₁ and the focal length f ₂ is larger than the focal depth of the comparison target lens determined according to the central wavelength Λ. Including a setting step.

  Preferably, in the step of setting the absolute value of the wavelength difference, the center wavelength Λ is set smaller than the design wavelength λ.

  Preferably, the laser processing method further includes a step of controlling the number of ring zones of the diffractive lens that contributes to the diffraction of the femtosecond laser.

  Preferably, the step of controlling the number of annular zones controls the incidence range of the femtosecond laser to the diffractive lens.

  Preferably, in the step of controlling the number of annular zones, the number of annular zones is controlled to a predetermined number by using an aperture configured so that the diameter of the opening through which the femtosecond laser passes is variable.

  Preferably, in the step of controlling the number of annular zones, the number of annular zones is controlled to a predetermined number using a beam expander that spatially changes the beam diameter of the femtosecond laser.

  Preferably, in the step of processing, the modified region is formed inside the material to be processed by focusing the femtosecond laser inside the material to be processed. In the laser processing method, the length of the modified region along the optical axis direction of the femtosecond laser is defined as the thickness L of the modified region, and the length of the modified region along the direction orthogonal to the optical axis direction is modified. When the region diameter φ is defined and L / φ is defined as the aspect ratio of the modified region, the method further includes a step of controlling the aspect ratio by controlling the energy of the femtosecond laser.

  Preferably, the step of controlling the aspect ratio controls the energy of the femtosecond laser so that the aspect ratio becomes 100 or more.

  Preferably, in the step of controlling the aspect ratio, the energy of the femtosecond laser is controlled so that the diameter φ is 2 μm or less and the aspect ratio is 100 or more.

Preferably, the material to be processed is an optical material including a polymer structure and glass.
Preferably, the processing step forms a diffractive optical element inside the optical material.

According to another aspect of the present invention, a laser processing apparatus that emits a femtosecond laser having a pulse width of 10 ⁻¹⁵ seconds to 10 ⁻¹¹ seconds, and a femtosecond laser on the surface or inside of a material to be processed And a diffractive lens including a diffractive surface for condensing the light. The absolute value of the wavelength difference between the center wavelength Λ of the femtosecond laser incident on the material to be processed and the design wavelength λ of the diffractive lens is the focal length of the femtosecond laser incident on the innermost ring of the diffractive lens. _1, and the focal length of the femtosecond laser that has entered the annular outermost diffractive lens is f _2, more long or any of the focal lengths f ₁ and f ₂ are the femtosecond laser center wavelength Λ The difference between the focal length f ₁ and the focal length f ₂ is equal to the focal length of the comparison target lens, which is a diffractive lens as a design wavelength, and is larger than the focal depth of the comparison target lens determined according to the center wavelength Λ of the femtosecond laser. The absolute value of is determined to be large.

  Preferably, the center wavelength Λ of the femtosecond laser is smaller than the design wavelength λ of the diffractive lens.

  Preferably, the apparatus further includes a ring number control device configured to be capable of adjusting the number of ring bands of the diffractive lens that contributes to the diffraction of the femtosecond laser.

  Preferably, the ring number control device controls the incident range of the femtosecond laser to the diffractive lens.

  Preferably, the ring number control device is an aperture configured such that the diameter of the opening through which the femtosecond laser passes is variable.

  Preferably, the ring number control device is a beam expander that spatially changes the beam diameter of the femtosecond laser.

  Preferably, the laser processing apparatus forms the modified region in the processing target material by condensing the femtosecond laser in the processing target material. The laser device defines the length of the modified region along the optical axis direction of the femtosecond laser as the thickness L of the modified region, and defines the length of the modified region along the direction orthogonal to the optical axis direction as the modified region. When the diameter φ is defined and L / φ is defined as the aspect ratio of the modified region, a femtosecond laser having energy adjusted so that the aspect ratio satisfies a desired condition is emitted.

  Preferably, the femtosecond laser has energy adjusted so that the aspect ratio is 100 or more.

  Preferably, the femtosecond laser has energy adjusted such that the diameter φ is 2 μm or less and the aspect ratio is 100 or more.

Preferably, the material to be processed is an optical material including a polymer structure and glass.
Preferably, the laser processing apparatus forms an optical element inside the optical material.

According to still another aspect of the present invention, there is provided a method for manufacturing an optical element, wherein a femtosecond laser having a pulse width of 10 ⁻¹⁵ seconds to 10 ⁻¹¹ seconds is collected on either the surface or inside of an optical material by a diffractive lens. The optical material includes a step of processing the optical material, and a step of setting the center wavelength Λ of the femtosecond laser and the design wavelength λ of the diffractive lens prior to the processing step. In the setting step, the focal length of the femtosecond laser incident on the innermost ring of the diffractive lens is f _1, and the focal length of the femtosecond laser incident on the outermost ring of the diffractive lens is f _2. Then, the longer of the focal lengths f ₁ and f ₂ is equal to the focal length of the comparison target lens that is a diffractive lens having the center wavelength Λ of the femtosecond laser as a design wavelength, and the femtosecond laser The absolute value of the wavelength difference between the center wavelength Λ and the design wavelength λ is set so that the absolute value of the difference between the focal length f ₁ and the focal length f ₂ is larger than the focal depth of the comparison target lens determined according to the central wavelength Λ. Including a setting step.

  According to still another aspect of the present invention, an optical element is manufactured by the method for manufacturing an optical element described above.

  Preferably, the optical material includes a modified region formed by focusing a femtosecond laser inside the optical material. The length of the modified region along the optical axis direction of the femtosecond laser is defined as the thickness L of the modified region, and the length of the modified region along the direction orthogonal to the optical axis direction is defined as the diameter φ of the modified region. When L / φ is defined as the aspect ratio of the modified region, the aspect ratio is 100 or more.

Preferably, the diameter φ is 2 μm or less.
Preferably, the optical element includes a diffractive optical element formed inside the optical material.

  According to the present invention, a diffractive lens is used as a lens for condensing femtosecond laser light, and the design wavelength of the diffractive lens is different from the wavelength of the incident femtosecond laser. By setting a large difference between the design wavelength of the diffractive lens and the incident wavelength of the femtosecond laser, spherical aberration can be increased with a simple and small optical system. By actively utilizing this spherical aberration, processing with a large modified thickness can be performed while the modified diameter is substantially constant. This makes it possible to increase the aspect ratio of the modified layer and the processed part without being restricted by the apparatus or the depth of the material.

  Embodiments of the present invention will be described below in detail with reference to the drawings. In the following, the same or corresponding parts in the drawings are denoted by the same reference numerals, and the description thereof will not be repeated in principle.

(Embodiment 1)
FIG. 1 is a diagram showing a configuration of a laser processing apparatus 50 according to the first embodiment. Referring to FIG. 1, a laser processing device 50 includes a laser device 1, an aperture 2, a diffractive lens 3, a stage 4, an ND filter (attenuating filter) 11, an attenuator 12, and an electromagnetic shutter 13. Is provided.

The laser device 1 includes, for example, a titanium / sapphire crystal as a laser medium, and emits laser light 5 having a pulse width of 10 picoseconds or less (specifically, 10 ⁻¹⁵ to 10 ⁻¹¹ seconds). Hereinafter, the laser beam 5 is also referred to as a “femtosecond laser”. Although the laser beam 5 is pulsed light, the locus of the laser beam 5 is continuously shown in FIG.

  The ND filter 11 and the attenuator 12 are for adjusting the energy (pulse energy) of the femtosecond laser. The electromagnetic shutter 13 is for obtaining a desired processing / modification pattern in the processing target material 8. In the present embodiment, the irradiation of the femtosecond laser and the driving of the stage 4 are synchronized by opening and closing the electromagnetic shutter 13.

The aperture 2 has an opening for allowing the laser beam 5 emitted from the laser device 1 to pass therethrough. When the laser beam 5 passes through the opening, the beam diameter of the laser beam 5 incident on the diffractive lens 3 is adjusted. In this embodiment, the beam diameter is a 1 / e ² (e is the base of natural logarithm) beam diameter generally used in the field of laser optics. The 1 / e ² beam diameter is a radial distance at which the light intensity decreases to 1 / e ² (about 0.135) times.

  The diffractive lens 3 condenses the laser light 5 that has passed through the aperture 2. A diffraction pattern for condensing the laser beam 5 is formed on at least one surface of the diffractive lens 3. That is, the diffraction pattern is formed on one or both of the incident surface 6 of the laser beam 5 and the exit surface 7 of the laser beam 5. The diffraction pattern formed on the diffractive lens 3 will be described in detail later.

  The laser beam 5 condensed by the diffractive lens 3 is applied to the material 8 to be processed. In the present embodiment, the inside of the material 8 to be processed is processed or modified by the laser beam 5. The irradiation unit 9 is an irradiation region of the laser beam 5 inside the processing target material 8, and is a region to be processed or modified.

  The material 8 to be processed is placed on the stage 4. Stage 4 is, for example, a movable stage. Although the movable direction of the movable stage is not particularly limited, it is preferable that the stage 4 is movable in a three-dimensional direction from the viewpoint of increasing the degree of freedom of the processing position and the shape of the processing region.

  The laser processing device 50 is a device that processes the material 8 to be processed by condensing the femtosecond laser on the surface of the material 8 to be processed or the inside thereof with the diffraction lens 3. The application of the laser processing apparatus 50 is not particularly limited, and can be used, for example, for fine processing of molds, drilling or cutting of various materials (for example, polymer, glass, metal, etc.).

  Also, the type of the material to be processed 8 is not particularly limited. For example, when the material to be processed 8 is an optical material, the laser processing apparatus 50 can be used as an optical element manufacturing apparatus. For example, by irradiating the inside of an optical material such as glass or a polymer structure with a femtosecond laser, the irradiation portion inside the optical material is modified such that the refractive index changes, for example. By changing the refractive index of the region irradiated with the femtosecond laser, for example, an optical element having a diffractive optical element or an optical waveguide can be manufactured.

  The optical element that can be manufactured by the laser processing apparatus 50 is not limited to the diffractive optical element or the optical waveguide, and may be a photonic crystal or an optical memory. However, below, the form which used the laser processing apparatus 50 as a manufacturing apparatus of a diffractive optical element is demonstrated.

  FIG. 2 is a diagram for more specifically explaining the production of the optical element by the laser processing apparatus 50 shown in FIG. Referring to FIG. 2, a laser beam 5 is irradiated inside the optical material as the processing target material 8. Thereby, the irradiation unit 9 is modified. When the stage 4 is stationary and the laser beam 5 is not scanned, the irradiation unit 9 (modified portion) is formed elongated (almost one-dimensionally) along the direction of the optical axis Z of the laser beam. The stage drive mechanism 10 moves the stage 4 in the Y direction, so that the modified portion can be expanded in a two-dimensional direction (YZ direction). Hereinafter, the modified portion having a two-dimensional region is referred to as a “modified layer”. The stage drive mechanism 10 moves the stage 4 in the X direction and repeats the irradiation of the laser beam 5 and the movement of the stage 4 in the Y direction, thereby forming a plurality of modified layers. Thereby, a diffractive optical element is manufactured inside the optical material.

  The scanning method of the laser beam 5 for forming the diffractive optical element is not limited to the method of moving the stage 4. The laser beam 5 may be scanned in the X direction and the Y direction using an optical system, or the stage movement 4 and the scanning of the laser beam 5 by the optical system may be combined.

  In the present embodiment, the length of the modified layer (modified region) in the direction perpendicular to the direction of the optical axis Z of the laser beam 5 (the incident direction of the laser beam 5) is defined as the modified diameter φ, and the laser beam 5, the length of the modified layer (modified region) in the direction of the optical axis Z is defined as a modified thickness L, and L / φ is defined as the aspect ratio of the modified layer. By performing the modification or processing with a large aspect ratio, the thickness of the modified portion where the refractive index has changed is increased. Thereby, the diffraction efficiency of the diffractive optical element can be increased.

  In the present embodiment, a diffractive lens is used to focus the laser beam 5. Although the diffractive lens is different from a normal plano-convex lens (refractive lens) in shape, it has a function equivalent to that of a refractive lens in terms of condensing. The diffractive lens will be described in detail below with reference to FIGS. FIG. 3 is a schematic diagram of the shape of the refractive lens and the shape of the diffractive lens. FIG. 4 is a diagram showing a wavelength spectrum of a femtosecond laser.

  Referring to FIG. 3, line a indicates the thickness of the refractive lens. The thickness of the refractive lens is the largest at the center of the lens and decreases as it goes from the center of the lens to the periphery. Let the coordinate in the lens radial direction be r, the thickness (physical length) of the lens at a portion away from the center of the refractive lens by h (r), and the refractive index of the lens be n. When the physical length is h (r), the optical path length of the light traveling through the medium having the refractive index n is longer than the optical path length of the light traveling through the vacuum by (n−1) × h (r), and the refractive index. The phase of light passing through the n medium is delayed by α (r). Since the phase is increased by 2π for one wavelength, the phase α (r) applied to the light after passing through the lens is expressed by the following equation (1).

Here, λ ₀ is the reference (center) wavelength of the femtosecond laser. As shown in FIG. 4, the femtosecond laser has a wavelength spectrum spread Δλ with λ ₀ as the center wavelength. Δλ increases as the center wavelength λ ₀ increases. For example, when the center wavelength λ ₀ is 800 nm, Δλ is approximately 10 nm.

Returning to FIG. 3, the phase α (r) shown in the equation (1) is the same even if it is shifted by 2πm (m is a natural number). A Fresnel lens that utilizes this fact and has a step of 2πm based on the shape of the refractive lens. That α (r) = - a circle of radius r _m of the 2.pi.m, separate the refractive lens surfaces concentrically, the thickness of the portion λ / (n-1) by reducing the lens is a Fresnel lens.

A line b indicates the radial thickness of the Fresnel lens. The Fresnel lens is a concentric serrated lens. The inclined part of the concentric stripes (corresponding to the hatched part in the line b) will be referred to as “ring zone”. In the present embodiment, the innermost ring zone b1 of the Fresnel lens is defined as the inclined surface next to the central convex portion. Assuming that the function indicating the line b is h _f (r), h _f (r) is expressed as shown in, for example, Expression (2).

Here, the function mod (a, b) represents the remainder when a is divided by b, m represents the order of the Fresnel lens, and h ₀ represents the thickness of the base lens. The Fresnel lens is one of diffractive lenses using a step of 2πm in phase, and a monochromatic laser with a wavelength λ ₀ behaves the same as a refractive lens.

Not only a Fresnel lens but also a lens (binary lens) designed so that a phase change when light passes takes only binary values of 0 and π is a kind of diffractive lens. Line c indicates the thickness of the binary lens. In the present embodiment, the innermost ring zone c1 of the binary lens is defined as the convex part next to the central convex part. Assuming that the function indicating the line c is h _b (r), h _b (r) is expressed, for example, as in Expression (3).

  Here, the function int (A) is a function for rounding down the decimal value of A to an integer value. As shown in the equation (4), in these diffractive lenses, when the wavelength λ changes, the focal length F changes in inverse proportion to the wavelength λ.

Here f ₀ is the focal length determined by the central wavelength lambda ₀ of the femtosecond laser, Equation (4) represents the wavelength dispersion of the diffraction lens. On the other hand, in the case of a refractive lens, the wavelength dependence of the focal length due to material dispersion appears. At this time, the focal length fluctuation rates Δ _r and Δ _F due to the wavelength dispersion of the refractive lens and the diffractive lens follow the following equations (5) and (6), respectively.

Here, n is the refractive index at the center wavelength, Δλ is the wavelength variation, and Δn is the refractive index variation associated with the wavelength variation. That is, the focal length variation rate delta _F is dependent on the center wavelength of the femtosecond laser lambda ₀ and the wavelength fluctuation amount [Delta] [lambda].

  As shown in FIG. 4, the femtosecond laser has a wavelength spectrum broadening. Assuming that the pulse width (full width at half maximum) is Δt, the minimum wavelength spectrum width (full width at half maximum) Δλ that can be taken by this pulse is approximately expressed by the equation (7).

  Here, c represents the speed of light. In the following, the wavelength spectrum width represents the full width at half maximum unless otherwise specified. From the equation (7), it can be seen that the wavelength spectrum becomes wider as the pulse width becomes shorter. Therefore, the shorter the pulse width of the femtosecond laser, the greater the influence of chromatic dispersion on the diffractive lens.

  Refractive lenses have little influence on chromatic dispersion, so the change in focal length is small even when a femtosecond laser is incident. However, with diffractive lenses, the focal length is affected by the wavelength, so various wavelengths in the wavelength spectrum are optical axes. Focus on a certain range above. That is, axial chromatic aberration occurs in the diffractive lens. Therefore, the diffractive lens can increase the depth of focus compared to the refractive lens. The range ΔF (that is, the depth of focus) in which the focal length of the diffractive lens changes is expressed according to the following equation (8) from equations (4) and (7).

Expression (8) is the design wavelength λ of the diffractive lens, that is, the wavelength when the annular zone of the diffractive lens is designed so that the light condensing performance is the best, and the center wavelength λ of the femtosecond laser incident on the diffractive lens. The range of change in focal length when ₀ is the same is shown.

  In a processing method in which a diffractive optical element is formed inside an optical material with femtosecond laser light, the aspect ratio of the modified layer is preferably as large as possible. For this purpose, it is preferable to make the focal length and the range in which the focal length changes (depth of focus) as long as possible.

  FIG. 5 is a diagram showing a method for obtaining a high aspect ratio by utilizing the wavelength spectrum of a femtosecond laser and the wavelength dispersion of a diffractive lens. This method is, for example, a method disclosed in Japanese Patent Application Laid-Open No. 2006-142342.

  In this method, the design wavelength of the diffractive lens, that is, the wavelength when the annular zone of the diffractive lens is designed so as to have the best light collecting property, and the reference (center) wavelength of the femtosecond laser are set to be approximately equal. Referring to FIG. 5, the design wavelength of diffractive lens 3A is equal to the reference (center) wavelength of the femtosecond laser. Since the laser beam 5 has a wavelength spectrum width according to the equation (7), the focal length changes due to the influence of wavelength dispersion received by the diffractive lens 3A. This realizes a long focal depth. The trajectory of the light beam indicated by the solid line and the broken line in the figure indicates that the focal length changes due to the difference in wavelength included in the wavelength spectrum width. The range in which the focal length changes is equal to ΔF obtained by the above equation (8). Hereinafter, the diffractive lens 3 will be described by appropriately comparing the diffractive lens 3A of FIG. 5 (that is, a diffractive lens having a design wavelength equal to the incident wavelength Λ of the femtosecond laser) and the diffractive lens 3.

  FIG. 6 is a diagram showing a method for obtaining a high aspect ratio according to the first embodiment. Referring to FIG. 6, in the present embodiment, laser light 5 (femtosecond laser) having a reference (center) wavelength different from the design wavelength of diffractive lens 3 is condensed by diffractive lens 3. In this embodiment, spherical aberration (incidence of light from the lens) is obtained by actively utilizing the high aberration property of the diffractive lens by making the design wavelength of the diffractive lens 3 different from the center wavelength of the femtosecond laser. Long focal depth is realized by changing the focal length according to the position). Thereby, the range Δf in which the focal length changes can be made larger than ΔF obtained by the above equation (8).

  FIG. 7 is a diagram showing the relationship between the design wavelength of the diffractive lens 3 and the center wavelength of the femtosecond laser in the first embodiment. In the following description, the center wavelength of the femtosecond laser is denoted by Λ so that the design wavelength of the diffractive lens and the center wavelength of the femtosecond laser can be easily distinguished.

  Referring to FIG. 7, in this embodiment, the absolute value | Λ−λ | of the difference between the center wavelength Λ of the femtosecond laser and the design wavelength λ of the diffractive lens 3 is Λ = λ. The wavelengths Λ and λ are selected so as to be larger than the wavelength spectrum spread Δλ (full width at half maximum) of the second laser. When the absolute value | Λ−λ | of the wavelength difference is equal to or less than Δλ, the range in which the focal length of the diffractive lens changes is limited to a range determined by chromatic aberration. In the present embodiment, by setting the absolute value | Λ−λ | of the wavelength difference to be larger than Δλ, the influence of chromatic aberration on the focal length change range is reduced, and a long focal depth by spherical aberration is realized.

FIG. 8 is a diagram for explaining the relationship between the center wavelength Λ of the femtosecond laser shown in FIG. 7 and the design wavelength λ of the diffractive lens 3 in more detail. Referring to FIG. 8, the laser light 5 incident on the diffractive lens 3 is condensed at the focal length f ₁ by the innermost ring 31 of the diffractive lens 3, and the outermost surface of the diffractive lens 3. Is condensed at the position of the focal length f ₂ .

When the design wavelength of the diffractive lens 3 is λ and the focal length of the diffractive lens 3 with respect to the design wavelength λ is f, the radius rm of the _m- th annular zone from the center O of the diffractive lens 3 is It is shown according to equation (9).

  The above equation (9) represents that the interval (period) of the annular zone is reduced as m is increased.

Δf shown in FIG. 8 is the absolute value (= | f ₁ −f ₂ |) of the difference between the focal lengths f ₁ and f ₂ . In the present embodiment, the longer value of the focal lengths f ₁ and f ₂ is a diffractive lens (in the diffractive lens 3A in FIG. 5) in which the center wavelength Λ of the incident femtosecond laser is the same as the design wavelength. When the design wavelength is equal to the focal length F of the femtosecond laser incident on the diffractive lens 3A (corresponding to the case where the design wavelength is equal to the center wavelength Λ), the wavelength difference is such that Δf is equal to or greater than ΔF expressed by the equation (8) The design wavelength λ of the diffractive lens and the center wavelength Λ of the femtosecond laser are selected so as to have | Λ−λ |. This makes it possible to increase the depth of focus using spherical aberration.

If the wavelength difference | Λ−λ | satisfies the above condition, the center wavelength Λ of the femtosecond laser may be shorter or longer than the design wavelength λ of the diffractive lens 3 as shown in FIG. However, as shown in the equation (4), the focal length becomes longer as the wavelength becomes shorter. Therefore, the center wavelength Λ of the femtosecond laser is preferably shorter than the design wavelength λ of the diffractive lens 3. As a result, the focal length can be made longer, so that the absolute value (= | f ₁ −f ₂ |) of the difference between the focal lengths f ₁ and f ₂ can also be increased. Therefore, the thickness of the modified layer formed on the optical material can be increased.

FIG. 9 is a diagram showing the beam behavior of the femtosecond laser due to spherical aberration when the center wavelength Λ of the femtosecond laser is shorter than the design wavelength λ of the diffractive lens 3. Referring to FIG. 9, the broken line shows the light beam behavior when the laser beam 5A having the center wavelength λ is incident on the diffractive lens 3 having the design wavelength λ. In this case, the focal length is f. The solid line shows the light beam behavior when the laser beam 5 having the center wavelength Λ (<λ) is incident on the diffractive lens 3 having the design wavelength λ. The focal length f ₂ indicates the focal length when the laser beam 5 is incident on the outermost ring of the diffractive lens, and the focal length f ₁ indicates that the laser beam 5 is on the innermost ring of the diffractive lens. Indicates the focal length when incident.

An angle between the optical axis Z of the laser beams 5 and 5A incident on the diffractive lens 3 and the light beam bent by the diffractive lens 3 is defined as θ. Assuming that the period of the annular zone is d, the light on the outer side of the lens with the smaller period of the annular zone becomes smaller from the relationship of dsin θ = λ, so that the light becomes smaller outside the optical axis Z (to the diffractive lens 3). Concentrate farther away). That is, the focal length f ₂ is larger than the focal length f ₁ . The focal length f ₁ of the laser beam 5 incident on the innermost (first) annular zone is the same as that when the laser beam 5 A having a center wavelength equal to the design wavelength λ of the diffractive lens 3 is incident on the diffractive lens 3. Slightly larger than the focal length f. Since the period d does not change in relation to dsin θ = λ and λ> Λ, the diffraction angle of the laser beam 5 incident on the innermost ring is the laser beam 5A incident on the innermost ring. Smaller than the diffraction angle. For this reason, f <f ₁ .

  FIG. 10 is a diagram showing the light beam behavior of the laser beam 5 condensed inside the optical material by the optical system shown in FIG. Referring to FIG. 10, the laser beam 5 incident on the optical material (processing target material 8) is refracted at the interface between the atmosphere and the optical material. The spherical aberration is caused by the difference between the refractive index of the atmosphere and the refractive index of the optical material. This spherical aberration further enlarges the spherical aberration caused by the diffractive lens 3. Therefore, although the change range of the focal length in the atmosphere is Δf, the change range Δf ′ of the focal length inside the optical material is equal to or greater than the product of the refractive index n and Δf of the optical material.

FIG. 11 is a diagram showing the beam behavior of the femtosecond laser due to spherical aberration when the center wavelength Λ of the femtosecond laser is longer than the design wavelength λ of the diffractive lens 3. Referring to FIG. 11, the broken line indicates the light beam behavior when the laser beam 5 </ b> A having the center wavelength λ is incident on the diffractive lens 3 having the design wavelength λ. In this case, the focal length is f. The solid line shows the light beam behavior when the laser beam 5 having the center wavelength Λ (> λ) is incident on the diffractive lens 3 having the design wavelength λ. The focal length f ₂ indicates the focal length when the laser beam 5 is incident on the outermost ring of the diffractive lens, and the focal length f ₁ indicates that the laser beam 5 is on the innermost ring of the diffractive lens. Indicates the focal length when incident.

From the relationship of dsin θ = λ described above, θ increases as the light on the outer side of the lens with a smaller period of the annular zone becomes larger. Condensate. That is, the focal length f ₂ is smaller than the focal length f ₁ . In this case, the laser beam 5A having a center wavelength equal to the design wavelength λ of the diffractive lens 3 is incident on the diffractive lens 3 at the focal length f ₁ of the laser beam 5 incident on the innermost (first) annular zone. This is slightly smaller than the focal length f. Since the period d does not change in relation to dsin θ = λ, and λ <Λ, the diffraction angle of the laser beam 5 incident on the innermost ring zone is the laser beam 5A incident on the innermost ring zone. It becomes larger than the diffraction angle. Therefore, f> f ₁ is satisfied.

  FIG. 12 is a view showing the light beam behavior of the laser beam 5 condensed inside the optical material by the optical system shown in FIG. Referring to FIG. 12, the spherical aberration corrects the spherical aberration due to the diffractive lens 3 by the difference between the refractive index of the atmosphere and the refractive index of the optical material. Therefore, although the change range of the focal length in the atmosphere is Δf, the change range Δf ′ of the focal length is equal to or less than the product of the refractive index n and Δf of the optical material inside the optical material. 10 and 12 that the focal length in the optical material can be made longer by making the center wavelength Λ of the femtosecond laser shorter than the design wavelength λ of the diffractive lens 3.

FIG. 13 is a flowchart for explaining the laser processing method according to the first embodiment. Referring to FIG. 9, first, in step S1, the center wavelength Λ of the femtosecond laser and the design wavelength λ of the diffractive lens are determined. In this step S1, the longer value of the focal lengths f ₁ and f ₂ is a diffractive lens having the same design wavelength as the center wavelength Λ of the incident femtosecond laser (in the diffractive lens 3A in FIG. When the design wavelength is equal to the focal length F of the femtosecond laser incident on the diffractive lens 3A (corresponding to the center wavelength Λ of the femtosecond laser), Δf (= | f ₁ −f ₂ |) The design wavelength λ of the diffractive lens and the center wavelength Λ of the femtosecond laser are selected so as to have a wavelength difference | Λ−λ | For example, when the center wavelength Λ of the femtosecond laser is fixed, a diffractive lens having a design wavelength λ corresponding to the wavelength Λ is selected. On the other hand, if the diffractive lens cannot be freely selected and the center wavelength Λ of the femtosecond laser is variable, the center wavelength Λ of the femtosecond laser is selected according to the design wavelength of the diffractive lens.

  In step S2, femtosecond laser irradiation conditions are set. Irradiation conditions include, but are not limited to, femtosecond laser pulse energy, repetition frequency, and the like. By controlling the pulse energy, the aspect ratio of the modified layer can be controlled. Thereby, the thickness of the modified layer can be increased or the diameter of the modified layer can be decreased.

  For example, the pulse energy is controlled so that the aspect ratio of the modified layer is 100 or more. Further, for example, the pulse energy is controlled so that the modified diameter is 2 μm or less and the aspect ratio of the modified layer is 100 or more. The reason why the aspect ratio of the modified layer is preferably 100 or more is that the diffraction efficiency of the diffractive optical element is increased, and thus the light utilization efficiency is increased. Further, the reason why the modified diameter is preferably 2 μm or less is that, as the period becomes finer, the diffraction angle becomes larger, the wavelength resolution is improved, and a lens with a high aperture can be formed. It is.

  In step S3, the material is processed by irradiation with a femtosecond laser. In the case of manufacturing an optical element, the optical element is manufactured by irradiating an optical material with a femtosecond laser to process or modify the irradiated portion.

  Note that the order of the processes in steps S1 and S2 is not limited as described in FIG. 9, and may be reversed. Further, further processing may be added prior to the processing in step S3. Further, the processing shown in FIG. 9 may be executed by causing a computer to execute a program. In this case, in order to cause the computer to execute the processing shown in FIG. 9, the user may input information to the computer as necessary.

  As described above, the present invention focuses on positively utilizing spherical aberration as a method for increasing the modified thickness. In the present embodiment, as a specific means for realizing this, the design wavelength of the diffractive lens and the reference (center) wavelength of the femtosecond laser are made different. Thereby, a large spherical aberration can be generated with a simple and small optical system. Normally, when spherical aberration occurs, the light condensing property is deteriorated, so that it is considered that modification or processing with a uniform diameter cannot be performed. According to the present embodiment, it is possible to actually process a modified layer having a long modified thickness while maintaining a substantially uniform fine diameter even when spherical aberration occurs. This point will be described in detail in the following examples.

  When the laser processing method according to the present embodiment is used as a method for manufacturing a diffractive optical element, an element with high diffraction efficiency can be obtained because the modified thickness can be increased. Moreover, it becomes possible to manufacture a diffractive optical element having any thickness at any position inside the material.

(Embodiment 2)
FIG. 14 is a diagram showing a configuration of a laser processing apparatus 50A according to the second embodiment. Referring to FIG. 14, the laser processing apparatus 50 </ b> A includes a beam diameter control apparatus 14 instead of the aperture 2. In this respect, the laser processing apparatus 50A is different from the laser processing apparatus 50, but the configuration of other parts of the laser processing apparatus 50A is the same as the configuration of the corresponding part of the laser processing apparatus 50.

  The beam diameter control device 14 controls the beam diameter of the laser light 5 to a desired size. For example, an optical aperture (aperture) that can adjust the diameter of the opening can be used as the beam diameter control device 14. Further, a beam expander may be used as the beam diameter control device 14. The beam diameter control device 14 spatially changes the beam diameter of the femtosecond laser incident on the diffractive lens 3 so that the incident range of the laser light of the diffractive lens 3 is controlled. That is, the number of ring zones of the diffractive lens 3 that contributes to the diffraction (condensation) of the femtosecond laser is controlled. As a result, when the modified layer is formed by irradiating the femtosecond laser inside the optical material, the modified thickness can be changed (controlled) without changing the modified diameter.

  The flowchart for explaining the laser processing method according to the second embodiment is the same as the flowchart of FIG. The process for controlling the number of ring zones of the diffractive lens 3 is included in the process of step S2 in FIG. 13, for example, but may be executed before the process of step S3 as a process different from the process of step S2. Further, as described above, the order of processing before step S3 is not particularly limited.

  In the configuration shown in FIG. 14, the number of annular zones contributing to diffraction in the diffractive lens is controlled by controlling the beam diameter of the femtosecond laser incident on the diffractive lens 3. However, the method for controlling the number of annular zones contributing to diffraction is not limited to the above method. For example, the intensity distribution of the femtosecond laser on the surface of the diffractive lens 3 may be formed in a ring shape by providing a shielding plate at the center of the lens or providing a plurality of axicon lenses (also called conical lenses). .

  According to the second embodiment, it is possible to easily control the aspect ratio with a simple optical system without complicating the laser processing apparatus and without needing to control the modified thickness depending on the depth into the material. become.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated in detail, this invention is not limited by a following example. In the following, the design wavelength of the diffractive lens and the center wavelength of the femtosecond laser incident on the diffractive lens may be referred to as “design wavelength” and “incident wavelength”, respectively.

  In Examples 1 to 5, the following conditions are applied to the laser processing apparatus 50 according to the first embodiment. In Examples 1 to 5, the conditions of the diffractive lens and the femtosecond laser were made common as shown below.

  (1) For the diffractive lens, a Fresnel zone plate made of quartz glass having a design wavelength of 635 nm, a diameter of 4 mm, a focal length of 4 mm was used.

(2) The femtosecond laser incident on the diffractive lens has a laser beam diameter of 7 mm (1 / e ^binary value). In addition, an aperture having a diameter of 5.5 mm was installed in front of the diffractive lens, and the femtosecond laser that passed through this aperture was condensed by the diffractive lens and irradiated onto the material.

Example 1
In this example, polymethacrylate (manufactured by Mitsubishi Rayon Co., Ltd., trade name Acrylite (registered trademark), model # 000) was used as the optical material to be processed. The size of the optical material was 10 mm on one side and 5 mm in thickness. The femtosecond laser incident on the diffractive lens has a center wavelength of 400 nm, a pulse width of 120 fs (fs; indicates femtosecond), a repetition frequency of 1 kHz, and an incident energy to the diffractive lens of 10 μJ. A condensing depth position inside the material was 1 mm from the material surface, and a femtosecond laser was scanned at a speed of 1 mm / s to produce a diffraction grating having a period of 2 μm and a side of 1 mm.

  FIG. 15 is a diagram showing an entire observation image by a transmission optical microscope of the modified layer formed in the optical material by performing femtosecond laser irradiation according to the conditions of Example 1. FIG. Referring to FIG. 15, the horizontal length of the modified layer (corresponding to the length in the Y direction in FIG. 2) is 1 mm, and the modified thickness of the modified layer (corresponding to the Z direction in FIG. 2) is 700 μm. there were. FIG. 16 is an enlarged observation image of a part A of the modified layer shown in FIG. Referring to FIG. 16, the length of one side in the partially enlarged image of the modified layer was 160 μm. The modified diameter was approximately 1 μm. From this result, the aspect ratio was determined to be 700/1 = 700.

  As shown in FIG. 15 and FIG. 16, the refractive index change modification processing with a fine and long modified thickness was achieved by irradiation with the femtosecond laser in accordance with the above-described conditions. By subjecting this sample to heat treatment at 70 ° C. for 1 day after irradiation with a femtosecond laser, the Bragg first-order diffraction efficiency was 82.5%. As a result, it was found that a highly efficient diffraction grating can be processed with a period of 2 μm.

  The result of considering the long modified thickness obtained under the conditions of this example will be described. When the annular zone of the diffractive lens is designed with a wavelength of 635 nm, a focal length of 4 mm, and a lens diameter of 4 mm (radius 2 mm), the radius of the innermost annular zone is 0.071 mm from the equation (9), and the period Was determined to be 71.27 μm. Similarly, from equation (9), the radius of the outermost ring zone was 2.000 mm, and the period was determined to be 1.42 μm. The radius of the outermost ring zone was assumed to be equal to the radius of the lens.

When a femtosecond laser having a central wavelength of 400 nm is incident on this diffractive lens, the focal length f ₁ due to the innermost annular zone is 6.350 mm, the focal length f _{2 due} to the outermost annular zone is 6.812 mm, and the difference Δf was | 6.812-6.350 | = 462 μm. Since this is Δf in the air (refractive index 1), Δf ′ (see FIG. 10) inside the material (refractive index 1.49) was considered. When the condensing position by the innermost ring zone (that is, the focal length) is 1 mm deep from the material surface, the light collecting position by the outermost ring zone is 1.726 mm deep from the material surface. Therefore, Δf ′ = 1.726-1.000 = 726 μm.

  The actual modified thickness was about 700 μm as shown in FIG. 15, but this value is greatly different from the calculated modified thickness due to chromatic aberration in Comparative Example 1 described later. It was considered that it was influenced by the long focal length due to the generated spherical aberration.

  The spherical aberration according to the present example was considered to be spherical aberration particularly when the difference between the design wavelength and the incident wavelength was 235 nm and the incident wavelength was reduced to 400 nm with respect to the design wavelength 635 nm. According to the equation (7), the wavelength spectrum spread of the femtosecond laser having the center wavelength of 400 nm and the pulse width of 120 fs is about 2 nm, but the difference between the design wavelength and the incident wavelength in this embodiment may be larger. I understood. In this embodiment, the incident wavelength is short with respect to the design wavelength, but in this case, it is effective in that the influence of the material refractive index by focusing the light inside the material works in the direction of further increasing the focal length. Was confirmed.

(Comparative Example 1)
In order to calculate the wavelength spectrum and the dispersion of the diffractive lens in Example 1, that is, the effect of chromatic aberration, the design wavelength is 400 nm, which is the same as the incident wavelength, the pulse width is 120 fs, and the focal length is longer in Example 1. The value was 6.812 mm. In this case, the range in which the focal length changes, that is, ΔF was calculated according to equation (8). ΔF was 33 μm as shown below.

ΔF = 2 × ln2 × (6.812 × 10 ⁻³ ) × (0.4 × 10 ⁻⁶ ) / π / (3 × 10 ⁸ ) / (120 × 10 ⁻¹⁵ ) = 33 μm
When the femtosecond laser is focused inside the material, the focused position changes depending on the material refractive index (1.49) (see FIG. 10). Considering this point, ΔF ′, which is ΔF inside the material, was calculated. ΔF ′ was 33 × 1.49 = 49 μm.

  Since ΔF ′ is slightly less than 50 μm due to the action of chromatic aberration, it has been found that ΔF ′ has deviated to such an extent that the modified thickness (700 μm) of the actual machining shape according to Example 1 cannot be explained. From the comparison results with Example 1 and Comparative Example 1, it was considered that the long modified thickness shown in Example 1 was due to the influence of spherical aberration. Furthermore, according to Example 1 and Comparative Example 1, it was found that a long modified thickness effect that cannot be obtained by chromatic aberration can be obtained even when lenses having the same focal length are used.

  In order to obtain a range in which the focal length changes in the same degree as in Example 1 (ie, 700 μm) from the wavelength spectrum of the femtosecond laser and the wavelength dispersion of the diffractive lens, ΔF ′ is increased by about 14 times. It was found that the focal length in this case is 97.3 mm. Further, it was found that when the numerical apertures were made the same, the outermost ring zone period was 1.42 μm, so the numerical aperture was about 0.28. From these, the lens diameter was determined to be 57 mm (radius 28.5 mm). From the above considerations, it was found that according to the comparative example 1, the diffractive lens becomes larger and the lens fabrication becomes difficult as compared with the first example.

(Example 2)
In this example, polymethacrylate (manufactured by Mitsubishi Rayon Co., Ltd., trade name Acrylite (registered trademark), model # 000) was used as the optical material to be processed. The size of the optical material was 10 mm on one side and 5 mm in thickness. The femtosecond laser incident on the diffractive lens has a center wavelength of 400 nm, a pulse width of 120 fs, a repetition frequency of 1 kHz, and an incident energy to the diffractive lens of 25 μJ. The depth position inside the material was set to 1 mm and 3 mm from the material surface, and a femtosecond laser was scanned at a speed of 1 mm / s to produce a diffraction grating having a period of 10 μm and a side of 1 mm.

  As a result, the modified thickness was about 850 μm regardless of whether the depth position was 1 mm or 3 mm, and almost no difference was observed. That is, according to the conditions of this example, it has been found that a large modified thickness can be obtained regardless of the depth position inside the material. It has also been found that the modified thickness does not increase as the depth position increases, and the modified thickness does not vary greatly. For these reasons, the action of increasing the modified thickness is not mainly due to the influence and action of spherical aberration caused by the focusing of the femtosecond laser inside the material, but to the influence and action of spherical aberration of the diffractive lens. I was able to confirm that.

(Example 3)
In this example, polymethacrylate (manufactured by Mitsubishi Rayon Co., Ltd., trade name Acrylite (registered trademark), model # 000) was used as the optical material to be processed. The size of the optical material was 10 mm on one side and 5 mm in thickness. The femtosecond laser incident on the diffractive lens has a center wavelength of 800 nm, a pulse width of 120 fs, a repetition frequency of 1 kHz, and an incident energy to the diffractive lens of 1.2 μJ. The depth position inside the material was set to 1 mm from the surface of the material, and a femtosecond laser was scanned at a speed of 1 mm / s to produce a diffraction grating having a period of 2 μm and a side of 1 mm.

  FIG. 17 is a diagram showing an entire observation image by a transmission optical microscope of the modified layer formed in the optical material by performing femtosecond laser irradiation according to the conditions of Example 3. Referring to FIG. 17, the modified diameter was about 1 μm and the modified thickness was 120 μm. From this result, the aspect ratio was determined to be 120. As shown in FIG. 17, fine and long modified thickness processing was possible by performing femtosecond laser irradiation according to the conditions of this example. By performing heat treatment at 70 ° C. for 1 day after irradiation of this sample, the Bragg first-order diffraction efficiency was 7.8%, and it was found that a diffraction grating with a period of 2 μm could be processed.

Next, the result of considering the modified thickness obtained according to the conditions of this example will be described. When a femtosecond laser with a central wavelength of 800 nm is incident on the diffractive lens, the focal length f ₁ due to the innermost ring is 3.174 mm, the focal length f _{2 due} to the outermost ring is 2.933 mm, and the difference Δf | 2.933−3.174 | = 241 μm. Since this is Δf in air (refractive index 1), Δf ′ (see FIG. 12) inside the material (refractive index 1.49) was considered. When the light collection position by the outermost ring is 1 mm deep from the material surface, the light collection position by the innermost ring is 1.252 mm deep from the material surface. Therefore, Δf ′ = It was 252 μm.

  As shown in FIG. 17, the actual modified thickness was about 120 μm, but this value was a modified thickness that could not be explained by the calculated modified thickness due to chromatic aberration in Comparative Example 2 described later. Therefore, it was considered that it was influenced by the long focal length due to the spherical aberration generated by the diffractive lens.

  The spherical aberration according to this example was considered to be spherical aberration particularly when the difference between the design wavelength and the incident wavelength was 165 nm and the incident wavelength was increased to 800 nm with respect to the design wavelength of 635 nm. According to the equation (7), the wavelength spectrum spread of the femtosecond laser having the center wavelength of 800 nm and the pulse width of 120 fs is about 8 nm, but the difference between the design wavelength and the incident wavelength in this embodiment may be larger. I understood. In this example, although the incident wavelength is longer than the design wavelength, in this case, it was confirmed that the influence of the material refractive index by condensing inside the material works in the direction of shortening the focal length.

(Comparative Example 2)
In order to calculate the wavelength spectrum and the dispersion of the diffractive lens in Example 3, that is, the effect due to chromatic aberration, the design wavelength is set to 800 nm which is the same as the incident wavelength, the pulse width is set to 120 fs, and the focal length is set to the longer one in Example 3. The value was 3.174 mm. In this case, the range in which the focal length changes, that is, ΔF was calculated according to equation (8). ΔF was 29 μm as shown below.

ΔF = 2 × ln2 × (3.174 × 10 ⁻³ ) × (0.8 × 10 ⁻⁶ ) / π / (3 × 10 ⁸ ) / (120 × 10 ⁻¹⁵ ) = 29 μm
When the femtosecond laser is focused inside the material, the focused position changes depending on the material refractive index (1.49) (see FIG. 12). Considering this point, ΔF ′, which is ΔF inside the material, was calculated. ΔF ′ was 29 × 1.49 = 43 μm.

  Under the action of chromatic aberration, ΔF ′ is a little over 40 μm. Therefore, it has been found that ΔF ′ is deviated to the extent that the modified thickness (120 μm) of the actual machining shape according to Example 3 cannot be explained. From the comparison results of Example 3 and Comparative Example 2, it was considered that the long modified thickness shown in Example 3 was due to the influence of spherical aberration. Furthermore, it was confirmed from Example 3 and Comparative Example 2 that a long modified thickness effect that cannot be obtained by chromatic aberration can be obtained even when lenses having the same focal length are used.

(Comparative Example 3)
In this comparative example, polymethacrylate (manufactured by Mitsubishi Rayon Co., Ltd., trade name Acrylite (registered trademark), model # 000) was used as the optical material to be processed. The size of the optical material was 10 mm on one side and 5 mm in thickness. The femtosecond laser incident on the lens had a center wavelength of 800 nm, a pulse width of 120 fs, and a repetition frequency of 1 kHz.

  In this comparative example, an objective lens having a numerical aperture of 0.4 (manufactured by Olympus, trade name LMPlanFL, magnification 20 times) was used instead of the diffractive lens. The incident energy of the femtosecond laser to the objective lens is set to 0.32 μJ, the depth position inside the material is set to 4.5 mm from the surface of the material, and the femtosecond laser is scanned at a speed of 1 mm / s to perform processing with a period of 10 μm. Conducted multiple times. As a result, a modified layer having a modified thickness of 50 μm could be obtained.

  In this comparative example, the depth position at which light is condensed is set to 4.5 mm from the material surface. Therefore, Δf generated by spherical aberration due to light condensing inside the material was calculated to be 230 μm. However, the modified thickness of actual processing was only 50 μm. In Example 3, the actual machining value was 120 μm with respect to the calculated value of Δf ′ of 252 μm, whereas in Comparative Example 3, the actual machining value was 50 μm with respect to the calculated value of Δf of 230 μm. From the comparison result between Example 3 and Comparative Example 3, even if the calculated value is the same degree of spherical aberration, it is more practical to apply spherical aberration with a diffractive lens than to apply spherical aberration with a refractive lens. It was found that the modified thickness can be brought close to the calculated value and the modified thickness can be increased.

Example 4
In Example 4, a cycloolefin polymer (manufactured by Nippon Zeon Co., Ltd., trade name ZEONEX (registered trademark), model 480) was used as an optical material to be processed instead of polymethacrylate. The size of the optical material was 10 mm on one side and 2 mm in thickness. The femtosecond laser incident on the diffractive lens has a center wavelength of 400 nm, a pulse width of 120 fs, a repetition frequency of 1 kHz, an incident energy to the diffractive lens of 15 μJ, a depth position inside the material of 1 mm from the material surface, and a femtosecond. A laser was scanned at a speed of 1 mm / s to produce a diffraction grating having a period of 2 μm and a side of 1 mm. As a result, a fine and long modified thickness with a modified diameter of approximately 1 μm and a modified thickness of 800 μm was obtained. From this result, the aspect ratio was determined to be 800. In addition, it was found that the Bragg first-order diffraction efficiency was 84.7% by performing heat treatment at 100 ° C. for 1 day after irradiation of this sample, and it was possible to process a highly efficient diffraction grating with a period of 2 μm.

  From Example 4, the effect that the modified thickness can be increased by causing the spherical aberration of the diffractive lens by changing the design wavelength and the incident wavelength is independent of the type of the polymer material. I was able to confirm.

(Example 5)
In Example 5, BK7 (borosilicate crown optical glass) which is a kind of optical glass was used as an optical material to be processed. The size of the optical material was 10 mm on one side and 3 mm in thickness. The femtosecond laser incident on the diffractive lens has a center wavelength of 800 nm, a pulse width of 120 fs, a repetition frequency of 1 kHz, and an incident energy to the diffractive lens of 4.5 μJ. The depth position inside the material was set to 1 mm from the material surface, and a femtosecond laser was scanned at a speed of 0.3 mm / s to produce a diffraction grating having a period of 2 μm and a side of 1 mm. As a result, a fine and long modified thickness with a modified diameter of approximately 1 μm and a modified thickness of 215 μm was achieved. From this result, the aspect ratio was determined to be 215. The Bragg first-order diffraction efficiency at this time was 76.5%, and it was found that a diffraction grating with a period of 2 μm could be processed.

  From Example 5, it can be seen that the effect that the modified thickness can be increased by causing the spherical aberration of the diffractive lens by changing the design wavelength and the incident wavelength is also effective for glass. It was. As a result, it turned out that the said effect is not based on a material kind.

(Example 6)
In Example 6, the following conditions were applied to the laser processing apparatus 50A according to Embodiment 2. In Example 6, a Fresnel zone plate made of quartz glass having a diameter of 8 mm, an annular zone design wavelength of 1000 nm, a focal length of 4 mm with respect to the design wavelength was used as the diffractive lens. The femtosecond laser incident on the diffractive lens has a center wavelength of 800 nm and a laser beam diameter of 14 mm (1 / e ² value). A femtosecond laser was focused by a diffraction lens and irradiated onto the material. Polymethacrylate (Mitsubishi Rayon Co., Acrylite (registered trademark), model # 000) was used as the optical material to be processed. The size of the optical material was 10 mm on one side and 5 mm in thickness. The incident pulse energy to the diffractive lens was 12 μJ, the depth position inside the material was 1 mm from the material surface, and the femtosecond laser was scanned at a speed of 1 mm / s. In front of the diffractive lens, an aperture is arranged as a beam diameter control device for controlling the beam diameter of the femtosecond laser. The aperture diameter was changed to 8 mm, 4 mm, 2 mm, and 1 mm, and processing with a period of 5 μm was performed several times for each aperture diameter.

  As a result, when the aperture diameter was 8 mm, the modified thickness was 330 μm. The modified thickness was 260 μm when the aperture diameter was 4 mm. When the aperture diameter was 2 mm, the modified thickness was 120 μm. When the aperture diameter was 1 mm, the modified thickness was 70 μm. In this way, it was possible to process the modified diameter to be constant (approximately 1 μm) at any aperture diameter.

  From Example 6, the diffractive lens that generates spherical aberration by making the design wavelength and the incident wavelength different from each other is the incident range of the femtosecond laser incident on the diffractive lens, in other words, the femtosecond laser in the diffractive lens. It was found that the modified thickness can be controlled while the modified diameter is kept substantially constant by controlling the number of annular zones that contribute to the diffraction (condensation) of light. Furthermore, in this embodiment, it was found that the modified thickness can be controlled even when a beam diameter control device having a simple configuration such as an aperture is used.

  It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

It is the figure which showed the structure of the laser processing apparatus 50 which concerns on Embodiment 1. FIG. It is a figure explaining manufacture of the optical element by the laser processing apparatus 50 shown in FIG. 1 more concretely. It is a schematic diagram of the shape of a refractive lens and the shape of a diffractive lens. It is the figure which showed the wavelength spectrum of the femtosecond laser. It is the figure which showed the method of obtaining a high aspect ratio by utilizing the wavelength spectrum of a femtosecond laser, and the wavelength dispersion of a diffraction type lens. 3 is a diagram illustrating a method for obtaining a high aspect ratio according to Embodiment 1. FIG. FIG. 3 is a diagram showing the relationship between the design wavelength of the diffractive lens 3 and the center wavelength of the femtosecond laser in the first embodiment. FIG. 8 is a diagram for explaining the relationship between the center wavelength Λ of the femtosecond laser shown in FIG. 7 and the design wavelength λ of the diffractive lens 3 in more detail. FIG. 4 is a diagram showing the beam behavior of the femtosecond laser due to spherical aberration when the center wavelength Λ of the femtosecond laser is shorter than the design wavelength λ of the diffractive lens 3. It is the figure which showed the light beam behavior of the laser beam 5 condensed inside an optical material by the optical system shown in FIG. FIG. 4 is a diagram showing the beam behavior of the femtosecond laser due to spherical aberration when the center wavelength Λ of the femtosecond laser is longer than the design wavelength λ of the diffractive lens 3. It is the figure which showed the light beam behavior of the laser beam 5 condensed inside an optical material by the optical system shown in FIG. 3 is a flowchart illustrating a laser processing method according to Embodiment 1. It is the figure which showed the structure of 50 A of laser processing apparatuses which concern on Embodiment 2. FIG. It is the figure which showed the whole observation image by the transmission optical microscope of the modified layer formed in the optical material by performing the femtosecond laser irradiation according to the conditions of Example 1. FIG. It is a figure which shows the enlarged observation image of the part A of the modified layer shown in FIG. It is the figure which showed the whole observation image with the transmission optical microscope of the modified layer formed in the optical material by performing the femtosecond laser irradiation according to the conditions of Example 3. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Laser apparatus, 2 Aperture, 3, 3A diffractive lens, 4 stage, 5, 5A laser beam, 6 entrance surface, 7 exit surface, 8 processing object material, 9 irradiation part, 10 stage drive mechanism, 11 ND filter, 12 attenuator, 13 an electromagnetic shutter, 14 beam diameter control device, 31 and 32, b1, c1 annular zone, 50 laser processing apparatus, 50A laser processing apparatus, a to c line, F, f _1, f ₂ the focal length, S1 to S3 Step, Z optical axis.

Claims (27)

A step of processing the material to be processed by condensing a femtosecond laser having a pulse width of 10 ⁻¹⁵ seconds to 10 ⁻¹¹ seconds on either the surface or inside of the material to be processed with a diffraction lens;
Prior to the processing step, comprising setting a center wavelength Λ of the femtosecond laser and a design wavelength λ of the diffractive lens,
The step of setting includes
The focal length of the femtosecond laser is incident on the annular zone top inner contour of the diffractive lens is f _1, a focal length of the femtosecond laser that has entered the annular outermost of the diffraction lens and f ₂ Then, the longer one of the focal lengths f ₁ and f ₂ is equal to the focal length of the comparison target lens which is a diffractive lens having the central wavelength Λ of the femtosecond laser as a design wavelength, and the femto The central wavelength Λ and the design wavelength are set such that the absolute value of the difference between the focal length f ₁ and the focal length f ₂ is larger than the focal depth of the comparison target lens determined according to the central wavelength Λ of the second laser. A laser processing method including a step of setting an absolute value of a wavelength difference from λ.   The laser processing method according to claim 1, wherein the step of setting the absolute value of the wavelength difference sets the center wavelength Λ smaller than the design wavelength λ. The laser processing method includes:
The laser processing method according to claim 1, further comprising a step of controlling the number of annular zones of the diffractive lens that contributes to diffraction of the femtosecond laser.   The laser processing method according to claim 3, wherein the step of controlling the number of ring zones controls an incident range of the femtosecond laser to the diffractive lens.   5. The step of controlling the number of ring zones is a method of controlling the number of ring zones to a predetermined number using an aperture configured to vary the diameter of an opening through which the femtosecond laser passes. Laser processing method.   The laser processing according to claim 3 or 4, wherein the step of controlling the number of annular zones controls the number of annular zones to a predetermined number using a beam expander that spatially changes a beam diameter of the femtosecond laser. Method. The processing step forms a modified region in the processing target material by focusing the femtosecond laser inside the processing target material,
The laser processing method includes:
The length of the modified region along the optical axis direction of the femtosecond laser is defined as the thickness L of the modified region, and the length of the modified region along the direction perpendicular to the optical axis direction is the modified region. The method further comprises the step of controlling the aspect ratio by controlling the energy of the femtosecond laser, where L is defined as the diameter φ of the mass region and L / φ is defined as the aspect ratio of the modified region. The laser processing method as described.   The laser processing method according to claim 7, wherein in the step of controlling the aspect ratio, energy of the femtosecond laser is controlled so that the aspect ratio becomes 100 or more.   The laser processing method according to claim 8, wherein the step of controlling the aspect ratio controls the energy of the femtosecond laser so that the diameter φ is 2 μm or less and the aspect ratio is 100 or more.   The laser processing method according to claim 1, wherein the material to be processed is an optical material including a polymer structure and glass. The laser processing method according to claim 1, wherein in the processing step, a diffractive optical element is formed inside an optical material as the processing target material . A laser device for emitting a femtosecond laser having a pulse width of 10 ⁻¹⁵ seconds to 10 ⁻¹¹ seconds;
A diffractive lens including a diffractive surface for condensing the femtosecond laser on the surface or inside of the material to be processed;
The femto the absolute value of the wavelength difference between the design wavelength λ heart wavelength Λ and the diffractive lens in the femtosecond laser is incident on the processing target material, which enters the annular zone top inner contour of the diffraction lens When the focal length of the second laser is f ₁ and the focal length of the femtosecond laser incident on the outermost ring of the diffractive lens is f ₂ , the longer one of the focal lengths f ₁ and f _2. Is equal to the focal length of the comparison target lens, which is a diffractive lens having the center wavelength Λ of the femtosecond laser as a design wavelength, and the focal point of the comparison target lens is determined according to the center wavelength Λ of the femtosecond laser. A laser processing apparatus, wherein an absolute value of a difference between the focal distance f ₁ and the focal distance f ₂ is determined to be larger than a depth. Wherein the said central wavelength Λ of the femtosecond laser, is smaller than the design wavelength λ of the diffraction lens, the laser processing apparatus according to claim 12.   The laser processing apparatus according to claim 12, further comprising an annular number control device configured to be capable of adjusting an annular number of the diffractive lens that contributes to diffraction of the femtosecond laser.   The laser processing apparatus according to claim 14, wherein the ring number control device controls an incident range of the femtosecond laser to the diffractive lens.   The laser processing apparatus according to claim 14 or 15, wherein the ring number control device is an aperture configured to have a variable diameter of an opening through which the femtosecond laser passes.   The laser processing apparatus according to claim 14 or 15, wherein the ring number control device is a beam expander that spatially changes a beam diameter of the femtosecond laser. The laser processing apparatus forms a modified region in the processing target material by condensing the femtosecond laser in the processing target material,
The laser device is
The length of the modified region along the optical axis direction of the femtosecond laser is defined as the thickness L of the modified region, and the length of the modified region along the direction perpendicular to the optical axis direction is the modified region. The femtosecond laser having an energy adjusted so that the aspect ratio satisfies a desired condition is defined by defining a diameter φ of a mass region and defining L / φ as an aspect ratio of the modified region. 12. The laser processing apparatus according to 12.   The laser processing apparatus according to claim 18, wherein the femtosecond laser has energy adjusted so that the aspect ratio is 100 or more.   The laser processing apparatus according to claim 18, wherein the femtosecond laser has energy adjusted such that the diameter φ is 2 μm or less and the aspect ratio is 100 or more.   The laser processing apparatus according to claim 12, wherein the material to be processed is an optical material including a polymer structure and glass. The said laser processing apparatus is a laser processing apparatus of any one of Claims 12-21 which forms a diffractive optical element inside the optical material as the said process target material . Processing the optical material by condensing a femtosecond laser having a pulse width of 10 ⁻¹⁵ seconds to 10 ⁻¹¹ seconds on either the surface or inside of the optical material with a diffractive lens;
Prior to the processing step, comprising setting a center wavelength Λ of the femtosecond laser and a design wavelength λ of the diffractive lens,
The step of setting includes
The focal length of the femtosecond laser is incident on the annular zone top inner contour of the diffractive lens is f _1, a focal length of the femtosecond laser that has entered the annular outermost of the diffraction lens and f ₂ Then, the longer one of the focal lengths f ₁ and f ₂ is equal to the focal length of the comparison target lens which is a diffractive lens having the central wavelength Λ of the femtosecond laser as a design wavelength, and the femto The central wavelength Λ and the design wavelength are set such that the absolute value of the difference between the focal length f ₁ and the focal length f ₂ is larger than the focal depth of the comparison target lens determined according to the central wavelength Λ of the second laser. A method for manufacturing an optical element, comprising a step of setting an absolute value of a wavelength difference from λ.   An optical element manufactured by the method for manufacturing an optical element according to claim 23. The optical material includes a modified region formed by focusing the femtosecond laser inside the optical material;
The length of the modified region along the optical axis direction of the femtosecond laser is defined as the thickness L of the modified region, and the length of the modified region along the direction perpendicular to the optical axis direction is the modified region. 25. The optical element according to claim 24, wherein the aspect ratio is 100 or more when the diameter φ is defined as a quality region and L / φ is defined as an aspect ratio of the modified region.   The optical element according to claim 25, wherein the diameter φ is 2 μm or less.   The optical element according to any one of claims 24 to 26, wherein the optical element includes a diffractive optical element formed inside the optical material.