JP2003075622A - Diffraction grating, method for processing diffraction grating and optical element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a diffraction grating manufactured by mechanical processing with high accuracy, to provide a method for processing the diffraction grating and to provide an optical element which uses the diffraction grating. SOLUTION: A material of Ge single crystal or Si single crystal is used and grating grooves 2a, 2b to 2n are formed on the (111) plane of the crystal orientation by a fly cut method by using a diamond cutting tool.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a diffraction grating manufactured by cutting, a method of processing the same, and an optical element using the diffraction grating.

[0002]

2. Description of the Related Art Generally, a diffraction grating is divided into a transmission type diffraction grating and a reflection type diffraction grating depending on the property of diffracted light. Further, it can be divided into a plane diffraction grating and a concave diffraction grating depending on the shape of the substrate on which the gratings are arranged.

The transmission type diffraction grating utilizes diffraction of light transmitted through the surface of the grating groove, while the reflection type diffraction grating arranges the grating grooves on a flat or concave substrate to reflect the reflected diffraction light. This is the diffraction grating used.

The plane diffraction grating is a diffraction grating in which a grating structure is arranged on a plane substrate and has a spectrum spectral function, but usually does not have an image forming function. Therefore, in a spectroscopic optical system using a plane diffraction grating, it is essential to use an auxiliary optical element for spectrum imaging. On the other hand, the concave diffraction grating is usually a diffraction grating in which grating grooves are arranged on a concave spherical substrate, and has both the function of spectrum dispersion and the function of image formation.

Further, the phase type diffraction grating gives a phase difference to the incident light within one period of the grating by making the grating groove shape into a sawtooth shape (blaze type), a sine wave shape, a rectangular wave shape, etc. This is a diffraction grating whose intensity characteristics can be selected according to the application.

Since single crystal Ge and single crystal Si have excellent characteristics as an infrared optical material, they may be used as a phase type diffraction grating. Among them, the blazed type diffraction grating is a phase type diffraction grating having a grating groove of a triangular cross section, and usually, a carving line with a grating groove of a predetermined sectional shape is sequentially formed by grinding using a grindstone or shaving using a diamond cutting tool. Formed and manufactured.

[0007]

As described above, when a blazed diffraction grating made of a crystal material such as single crystal Ge or single crystal Si is manufactured by machining using a grindstone, the brittleness peculiar to the crystal material is used. The surface to be machined tends to be dominant, and in order to obtain the shape accuracy, the dimensional accuracy, and the surface roughness required for the optical element, it is an issue to realize the processing under the processing conditions in the ductility mode.

From the experiments conducted so far, it was found that there is a critical cut thickness at the boundary between the ductile mode and the brittle mode, and the critical cut thickness is the relationship between the crystal orientation of the sliding surface of the crystal and the processed surface, It is known to depend on the relationship between the crystal axis and the cutting direction and the relationship between the crystal axis and the cutting force direction (depending on the tool rake angle and the cutting direction).

For example, the machined surface is the (111) surface, the cutting direction is in the (111) surface, and the tool rake angle is -20 ° to-.
When the angle is 40 °, the critical cutting thickness in ductile mode is 0.1 μm, and for the diffraction grating machining method that becomes ductile mode in all cutting directions, ultra-precision cutting using a single crystal diamond tool as a tool Then, ultra-precision grinding using a diamond grindstone etc. as a tool can be considered.

In the former case, the shape is transferred by translating the tool in the dispersion orthogonal direction (groove direction) by using a tool of a single crystal diamond tool of a general shape having a cross-sectional shape of the diffraction grating groove surface in the dispersion direction (direction orthogonal to the groove). The shaving method is generally used. In the shaving process, the groove depth itself of several μm to several + μm is set as the maximum cutting thickness, or the maximum cutting thickness less than the critical cutting thickness is set, and the tens of times to several hundreds of times are set. It was necessary to repeat the processing, and ductile mode processing of the crystalline material was impossible.

In the latter case, the cross-sectional shape of the grindstone is transferred to the cross-sectional shape of the groove surface of the diffraction grating (direction orthogonal to the groove), so that the cross-sectional shape of the grindstone is cross-sectional direction of the groove surface of the diffraction grating (direction orthogonal to the groove). It is necessary to shape (truing) into a V-shape which is the shape. However, the diameter of the abrasive grains constituting the grindstone is 1 to 2 μm even for the smallest # 20,000. Since the bond fills the space between the abrasive grains, it was impossible to set the radius of curvature at the tip of the grindstone cross section to about 10 μm or less. Therefore, a shape error with a curvature radius of about 10 μm is transferred to the cross section of the diffraction grating groove surface produced by grinding in the dispersion direction (orthogonal direction), which becomes a phase error of the diffraction grating and the grating constant is several tens of μm. In the diffraction grating of 0-5
A diffraction grating having a loss of about 0% and a grating constant of 15 μm or less has an excessive loss, and thus it is difficult to apply the grinding process. In addition, the cross-sectional shape error of the grindstone due to wear or loss of the abrasive grains, and disturbances such as temperature changes that occur during long-time machining, which is peculiar to the grinding process, increase the shape error of the diffraction grating.

Further, since the cross-sectional curve passing through the rotation axis of the grindstone working surface is transferred to the surface roughness of the grinded diffraction grating groove surface in the dispersion direction (orthogonal direction), the abrasive grains are relatively small. 130 nm RMS with # 4,000 diameter
The roughness is about 45 nm RMS even when # 20,000 having the smallest grain size is used as the abrasive grain. The roughness of the surface of the diffraction grating groove causes a phase error of the diffraction grating, which causes a loss of 0 to 10% in the near infrared region of a wavelength of several μm and a wavelength of several to several +
In the infrared region of μm, a loss of about 0 to 40% was caused to reduce the diffraction efficiency.

The present invention has been made in view of these circumstances, and an object thereof is to provide a diffraction grating manufactured by high-precision machining, a processing method thereof, and an optical element using the diffraction grating.

[0014]

According to the first aspect of the present invention, a material of single crystal Ge or single crystal Si is used, and a lattice groove is formed on the (111) plane of the crystal orientation. And the diffraction grating.

Further, according to the means of the invention of claim 2,
The surface roughness of the lattice groove surface is 10 nm, which is the smoothest.
RMS, lattice constant error of ± 0.1 μm or less, dimensional error of each part in the dispersion direction cross section of ± 0.2 μm or less, and blaze angle error of ± 0.2 ° or less are satisfied, The diffraction grating is characterized in that

According to the third aspect of the invention,
It is a method of processing a diffraction grating, characterized in that a grating groove is formed in a work piece made of single crystal Ge or single crystal Si by a fly cutting method using a diamond cutting tool.

According to the means of the invention of claim 4,
In an optical element formed by combining a diffraction grating and an optical element, the diffraction grating is an optical element characterized by using the above diffraction grating.

According to the means of the invention of claim 5,
The optical element is an optical element characterized by being a grism.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a perspective view of a transmission or reflection type blazed diffraction grating made of single crystal Ge according to the present invention. The transmission type diffraction grating utilizes diffraction of light transmitted through the grating groove surface, and the reflection type diffraction grating has a structure in which the grating grooves are arranged on a flat or concave substrate to reflect the reflected diffraction light. Is a diffraction grating that utilizes

The blazed diffraction grating 1 has a grating groove surface 1a (grooved surface) of single crystal Ge and a triangular groove 2a, 2 having a triangular cross section.
2n are formed, and the crystal orientation of the lattice groove surface 1a is the (111) plane which is a cleavage plane. Ge is a brittle material and has a cleavage plane (111), and its surface properties differ depending on the cutting direction and have an orientation dependency. Therefore, if only the (110) plane, which is the cutting direction with good surface accuracy, is processed, brittle fracture is unlikely to occur, and uniform and good surface accuracy can be obtained.

FIG. 2 is an external perspective view of the precision cutting processing apparatus of the present invention, and FIG. 3 is a schematic perspective view of the processing section thereof. The precision cutting device 5 is a fly-cut type processing device using the diamond cutting tool 6 as a tool. On the upper surface of the bed 7, a tool spindle 8 and a triaxial table 9 which is movable in the triaxial directions of X axis, Y axis and Z axis are provided. The diamond cutting tool 6 is attached to the outer peripheral portion of a rotating cutting tool holder 12 fixed to a rotating shaft 11 that constitutes the tool spindle 8, and rotates along with the rotation of the tool spindle 8. The diamond cutting tool 6 is a general-purpose cutting tool having the same outer shape as the lattice grooves 2a, 2b ... 2n to be processed.
Further, a triaxial table 9 that holds the workpiece 13 and is movable in three axial directions of the X-axis, the Y-axis, and the Z-axis is arranged at a position facing the diamond cutting tool 6. The workpiece 13 is a work holder 14 provided on the triaxial table 9.
It is fixed with wax or a fixing jig.

With these configurations, the rotating diamond cutting tool 6 and the work piece 13 are relatively translated by the triaxial table 9 of the X-axis, Y-axis, and Z-axis to maintain a certain cutting depth. In this state, the lattice grooves 2a, 2b to 2n formed in the workpiece 13 are relatively translated in the dispersion orthogonal direction (groove direction).

Next, a method of setting the apex angle αo and the rake angle θ of the diamond cutting tool 6 will be described.

FIG. 4 shows the apex angle αo of the diamond bite 6.
It is explanatory drawing of the setting method of and rake angle (theta). The apex angle αo of the diamond bite 6 and the grating grooves 2a, 2 of the diffraction grating 1
b ... 2n in the dispersion direction (perpendicular to the groove) the vertical angle α of the bottom of the cross section, that is, the apparent vertical angle α of the rotating diamond bite 6, is αo = 2 * tan ⁻¹ {cos θ · tan (α / 2)}.

Here, θ is the rake angle along the maximum inclination direction of the rake face, and the direction is the diamond bite 6
The right rake angle and the left rake angle are set to the same value, so that they are set equal to the direction of the bisector of α. The relationship between the vertical rake angle θo of the cutting edge and the rake angle θ along the maximum inclination direction of the rake surface is θ = tan ⁻¹ [√2 * {cos ((α / 2)-(π /
4)) * sin ((α / 2)-(π / 4))} ⁻¹ * t
an θ ₀ ] The cutting edge vertical rake angle θo is appropriate in the range of −20 ° to −40 °. For example, when θ = 20 ° and α = 90 °, θ = 27.24 ° and αo = 83. It becomes 0.28 °.
The cutting edge angle of the diamond bite 6 is ± 0.
High accuracy of 1 ° or less.

FIG. 5 is a plan view of the schematic perspective view of the processing portion shown in FIG. 3, showing the maximum cutting thickness, the rake angle change amount, and the cutting direction change amount in the fly-cut method.

The maximum cutting thickness t is t = (F / S) * {2 (D / R) -1 (D / R) ² }
^0.5 where F is the feed rate, S is the rotational speed of the tool, D is the depth of cut, and R is the radius of rotation of the tool.

The cutting depth D is set to the grating groove 2 of the diffraction grating 1.
Even if the groove depths of a, 2b ... 2n are set equal to each other, t can be arbitrarily set by changing the feed rate F, the number of revolutions S, and the tool turning radius R. Therefore, the critical cutting of ductile mode cutting is possible. It is possible to achieve thickness. For example, when D = 10 μm, F = 10 mm / min,
By setting S = 3000 min and R = 60 mm,
It becomes t = 0.6 μm. The change amount Δφ in the cutting force direction is equal to the change amount Δφ in the cutting direction, and Δψ = Δφ = cos ⁻¹ [1- (D / R)] The cutting depth D is set to the grating groove 2a of the diffraction grating 1. 2b ... 2n
Even if the groove depth is set to be equal to the groove depth, the rake angle change amount Δθ and the cutting direction change amount Δφ can be made sufficiently small by setting R sufficiently large. For example, D =
When it is set to 10 μm, by setting R = 60 μm,
Δφ = Δφ = 1 °.

By performing the cutting process on the workpiece 13 by the above-described processing method, the ductile mode can be processed. Also. The wear amount of the diamond cutting tool 6 is negligible with respect to the processing accuracy required for the diffraction grating 1, and the processing accuracy of the diffraction grating 1 is the positioning accuracy and the dynamic motion accuracy of the precision cutting device 5, and the diamond cutting tool 6 It is determined only by the shape accuracy of.

By these, the grating grooves 2a, 2b ... 2n
RMS with the smoothest surface roughness is 10 nm RMS
Is. The error of the lattice constant is ± 0.1 μm or less. The dimensional error of each part of the cross section in the dispersion direction (orthogonal to the groove) is ±
It is 0.2 μm or less. In the case of the blazed diffraction grating 1, it was confirmed by actual processing that it is possible to obtain a highly accurate diffraction grating 1 in which the error of the blazed angle is ± 0.2 ° or less.

FIG. 6 is a perspective view showing an example of the diffraction grating 1 of the present invention. The lattice constant is 21 μm, the blaze angle is 5.7 °, the length in the dispersion orthogonal direction (groove direction) is 20 mm, and the length in the dispersion direction (groove orthogonal direction) is 18 mm.

FIG. 7 shows the grating grooves 2a, 2b ... Of the diffraction grating 1.
2D schematically shows a differential interference microscope (AMF) photograph of a 2n surface, and FIG. 8 is a schematic pattern of an AMF image of a cross section in the dispersion direction of the grating grooves 2a, 2b ... 2n. Lattice groove 2
The surface roughness of the surfaces a, 2b ... It was 0.2 ° or less.

Optical elements can also be formed using these diffraction gratings. FIG. 8 is an explanatory diagram of a grism which is an optical element. The grism 20 is a single dispersive optical element that utilizes both prisms and diffraction gratings.
The grism 20a is a prism 20a having a first surface 21 and a second surface 22, and the diffraction grating 23 is the prism 2a.
0a on the second surface 22.

The light incident on the prism 20a from the first surface 21 is further dispersed by the diffraction grating 23. As a result, the diffraction grating 23 and the prism 2 serve as dispersive optical elements.
The choice of both of the 0a characteristics results in dispersion characteristics with optimized resolution. That is, the parameters of the prism 20a can be determined by the selected shape and material, and the characteristics of the diffraction grating 23 can be specified by the number of diffraction gratings 23, the order of diffraction, and the type of diffraction grating 23 used.

In addition to the grism, an echelon or an immersion grating can be formed as the optical element.

[0037]

According to the present invention, it is possible to obtain a diffraction grating manufactured by high-precision machining and a processing method thereof.

Further, a highly accurate optical element using a diffraction grating is possible.

[Brief description of drawings]

FIG. 1 is a perspective view of a diffraction grating of the present invention.

FIG. 2 is an external perspective view of the precision cutting device of the present invention.

FIG. 3 is a schematic perspective view of a processing unit.

FIG. 4 is an explanatory view of a method for setting a vertex angle and a rake angle of a diamond bite.

FIG. 5 is a plan view of a processed portion.

FIG. 6 is a perspective view showing an example of a diffraction grating of the present invention.

FIG. 7 is a differential interference microscope photograph of the grating groove surface of the diffraction grating.

FIG. 8 is a schematic diagram of an AMF image of a cross section in the dispersion direction of a grating groove.

FIG. 9 is an explanatory diagram of a grism of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Diffraction grating, 1a ... Lattice groove surface, 2a, 2b-2n ... Lattice groove, 5 ... Precision cutting processing device, 6 ... Diamond tool, 8 ... Tool spindle, 9 ... Triaxial table, 13 ...
Workpiece

Claims (5)

[Claims] 1. A diffraction grating comprising a material of single crystal Ge or single crystal Si, wherein a grating groove is formed in the (111) plane of the crystal orientation. 2. The surface roughness of the grating groove surface is the smoothest, 10 nm RMS, and the error of the lattice constant is ± 0.1 μm.
The dimensional error of each portion of the cross section in the dispersion direction satisfies at least one of ± 0.2 μm or less and the blaze angle error of ± 0.2 ° or less. Diffraction grating. 3. A method of processing a diffraction grating, characterized in that a grating groove is formed in a workpiece made of single crystal Ge or single crystal Si by a fly cutting method using a diamond cutting tool. 4. An optical element formed by combining a diffraction grating and an optical element, wherein the diffraction grating uses the diffraction grating according to claim 1 or 2. . 5. The optical element of claim 4, wherein the optical element is a grism.