JPH0371024A - Monochromator - Google Patents

Abstract

PURPOSE:To compensate the spherical aberration of a diffraction grating and to increase the resolution more by making a wavelength scan through the rotation of a concave mirror, whose position and shape are so determined that monochromatic luminous flux diverged from a virtual image is converged on a projection slit, and the diffraction grating. CONSTITUTION:The diffraction grating 2 is rotatable around an axis of rotation containing a center grating groove at an optional angle and the relative position relation among an incidence slit 1, the grating center, the concave mirror 3, and the projection slit 4 is fixed. Then white luminous flux 6 diverged from the slit 1 forms the virtual image 8 of the slit 1 for optional monochromatic light at a fixed position on an arc 7 whose radius is equal to the distance be tween the center of the diffraction grating and slit 1. Then the monochromatic light 9 diverged from the virtual image 8 is converged on the slit by the concave mirror 3. Further, the wavelength scan is made through the rotation of the grating 2. Thus, the spherical aberration of the grating 2 is compensated to improve the resolution more.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a monochromator that extracts arbitrary monochromatic light from white light, and particularly in the soft X-ray region (wavelength of about 0.5 to 30 nm).
Regarding the planar diffraction grating monoc(3) m) suitable for m).

[Conventional technology]

In the soft X-ray region, all materials have a refractive index very close to the upper limit and absorb soft X-rays, so it is difficult to use hI-folding optical elements such as lenses or direct incidence (light is incident almost perpendicularly to the reflecting surface).
reflective optical elements cannot be used. Therefore, in soft X-ray monochromators, reflection-type diffraction gratings are used.
and reflectors are used with oblique incidence (light just barely enters the reflection). Even in this case, the reflectance is small, so in order to obtain high throughput, it is important to reduce the number of reflective surfaces as much as possible. Furthermore, in a soft X-ray monochromator, the optical system must be placed in a vacuum to avoid absorption of soft Xi by air, so it is desirable that the wavelength scanning mechanism be simple.

Traditionally, monochromators with a small number of reflective surfaces and relatively high resolution have been manufactured by Nuclear Instruments and
Metoz A246 (, 1986) 297-302
Page (Nucl, ], nstrum, and Metho
ds.

(4) A246 (1986) pp, 297-302). FIG. 2 shows the optical system of a conventional monochromator within a dispersion curve. The optical system includes an entrance slit 21, a plane diffraction grating (hereinafter referred to as a diffraction grating) 22
, a spheroidal mirror (an ellipsoidal surface is a surface obtained by rotating the interior of a tank around an axis passing through its bifocal points; hereinafter referred to as an ellipsoidal mirror) 23, and an exit slit 24. Ru. This diffraction grating has curved grating grooves arranged at irregular intervals using holography technology. The diffraction grating is rotatable around the rotation 4i11125 including the central grating groove, and the relative positional relationship among the entrance slit, the center of the diffraction grating, the ellipsoidal mirror, and the exit slit is fixed.

The white light beam 26 diverging from the entrance slit is reflected and diffracted by the diffraction grating, creating a virtual image of arbitrary monochromatic light (virtual image of the entrance slit)
27 is imaged at a fixed position. This virtual image has small aberrations in both the dispersion direction and the direction perpendicular thereto (stigmatic). On the other hand, one focal point and the other focal points of the ellipsoidal mirror are *images, respectively, and the eight scattered monochromatic light beams 28 are converged by the ellipsoidal mirror, and a stigmatic image is formed on the exit slit. Such a stigmatic imaging function is extremely useful in applications (such as soft X-ray microscopes and soft X-ray microprobes) in which monochromatic light beams diverging from an exit slit are focused on a minute spot.

Furthermore, this monochromator has the advantage of being capable of one-wavelength scanning using a simple mechanism that only rotates the diffraction grating.

Note that a French patent (patent number 2585468) has been registered regarding this type of monochromator.

[Problem to be solved by the invention]

Holographic diffraction gratings used in conventional monochromators have limited degrees of freedom in grating groove arrangement, so defocus and coma are corrected at the imaging position, but spherical aberration is not corrected at all. do not have. Therefore, aberrations in the dispersion direction remain in the image on the exit sleeve 1-, and this is one of the causes of limiting the sharpness resolution.

(6) The purpose of the misfire is to correct the spherical aberration of the diffraction grating, which is a problem with conventional monochromators, and to provide a monochromator with higher resolution.

[Means to solve the problem]

In order to achieve the above object, 1 the present invention arranges linear lattice grooves at irregular intervals, takes the central lattice groove with a groove interval σ0 as the origin, sets bz, ba, and ba as constants, and When the groove number at position W is n, n=(w+
b2w2+baw8+b4w')/σ0, and uses a flat diffraction grating with defocus, comatic aberration, and spherical aberration corrected at the imaging position. A mechanism is provided to rotate the mirror, the exit slit, and the diffraction grating at an arbitrary angle around a rotation axis including the central grating groove, and the relative positional relationship between the entrance slit, the center of the diffraction grating, the concave mirror, and the exit slit is fixed. (7) The diffraction grating is rotated while holding the position, and any monochromatic light component in the white light flux diverging from the incident slit is reflected and diffracted by the diffraction grating. A virtual image of the arbitrary monochromatic light is formed substantially without aberration at a fixed position on a circle whose radius is the distance from the slit, and the monochromatic light beam diverging from the virtual image is converged to the exit slit by the concave mirror, and the light is emitted from the slit. This is a monochromator configured to extract arbitrary monochromatic light from a slit.

[Effect]

FIG. 1 shows the monochromator optical system of the present invention in the dispersion plane, and the principle of the optical system is basically the same as that of the known example. The optical system is an SJI diagonal grating in which linear grating grooves are arranged at irregular intervals.
2, a concave mirror 3, and an exit slit 4. The diffraction grating can be rotated by any angle around the rotation axis 5 including the central grating groove, and the relative positional relationship among the entrance slit, the center of the diffraction grating, the concave mirror, and the exit slit is fixed. White light flux 6 diverging from the entrance slit
is reflected and diffracted by the diffraction grating, and an arbitrary monochromatic light beam (8) is placed at a fixed position on +34'7 with the center of the lpl grating as the center and the radius of the distance between the center of the diffraction grating and the incident sulin 1-. A virtual image (virtual image of the entrance slit) 8 is formed. Note that the aberration in the dispersion direction of this virtual image is even smaller than that of the known example. A monochromatic light beam 9 diverging from the virtual image is converged onto an exit slit by a concave mirror. Wavelength scanning is performed by rotating the diffraction grating.

Next, it will be explained that in the monochromator of the present invention, a virtual image with extremely small aberration is formed at a fixed position at any wavelength by the diffraction grating.

First, a general imaging theory regarding planar diffraction gratings with non-uniformly spaced straight grooves will be described. In FIG. 3, the center of the planar diffraction grating is the origin O, the normal to the diffraction grating is the X-axis, the y-axis is perpendicular to the grating grooves, and the X-axis is parallel to them. Below, we will consider the case where point light source A and image point B are on the xy plane. A ray of wavelength λ is located at an arbitrary point P (0, w, u) on the n-th grating groove from the origin.
When reflected and diffracted at
It is expressed as Here, (AP) and (II' B ) are the distances (including signs) from points A and B to point P, respectively (
9), and point A is an imaginary light source, A, P><O, and point B
When is a virtual image, (PB)<O. In addition, m is the diffraction order. If the ray and the incident ray are in the same region, let m, > O.

Furthermore, the clear number n is n=(w+b2w"+baw8+b4w')/σo
(2) is given by the function. Here, σ
0 is the groove spacing in the central lattice groove, and bz, b8° and b4 are coefficients that determine the lattice groove arrangement. Now, from the geometrical relationship, <A, P> and (P):3) become (A
P)”=r12+w2+u2-2wrzsinα-(3
)(PB)2=r2”+w2+u2-2wrzsinβ
−=(4). Here, rt and r2 are the distances (including signs) from points A and B to point O, respectively.

Also, α and β are the incident angle and till force i' angle of the chief ray, respectively, and α≧O, and the refracted ray is y>O.
When it is in the region, β>O. Expanding the optical path function into a grass series of W and U using equations (1) to (4), we get (]0
) F = r 1 + rx + w Fzo + w2F
xo+u'')'corresponding to the Houki index of o2) is expressed as follows.The specific formula is as follows.

Czo=-6, inα-5inβ −=
(7) Mxo = b2
...(J4)Mox=O...(15
) Mso=4tz+
...(16) Mxx=O...(■7) M40=b4
...(18) Now, the optical path function has the following physical meaning.

First, Fzo determines the relationship between the angle of incidence of the chief ray and the angle of diffraction, and can always be considered as O. In addition, Fzo is defocus, Foz is astigmatism, F2O is coma, and F1
a is related to spherical aberration. The aberration in the dispersion direction of the diffraction grating and in the direction perpendicular thereto becomes O when formulas (19) and (20) are satisfied, respectively, according to Fermat's principle.

W M10=E(11)...(13) U Therefore, as is clear from equations (5), (19), and (20), when F1□=O, the corresponding aberration is O. The theory of imaging one or more diffraction gratings is known.

(12) The present invention uses a planar diffraction grating with unevenly spaced linear grooves whose grating groove spacing is given by equation (2), and is 1 Fzol.

This takes advantage of the fact that l Faol and I Faol are sufficiently small and Fax=0. In the monochromator of the present invention, as mentioned above, the point light source and the image point (virtual image) are equidistant (constant) from the center of the diffraction grating, and the angle between the incident ray and the diffracted ray (called the declination angle) is -constant. be. Therefore, if the distance from the center of the diffraction grating to the point light source is r, and the angle of deviation is 2, then the following equation holds.

r engineering=r...(21)
rx=-r...(22
)α=θ1K...(23)
β=θ−K (24) Here, θ is the angle between the bisector OM of the declination angle and the X axis (the straight line O
rotation angle of the diffraction grating normal measured from M), and the straight line OM
When is in the region y>O, let 0>O. Formula (21
) to (24) into equations (8), (10), and (12), A=sinθ, B=coSθ, C=sinK,
If we set D=cos K, C2o=”--AB CD...(25)...(26) 2AB8C8D)...
(27). In particular, when 1θ1 is sufficiently small (for example, θ1<5°), equations (25) to (27) can approximately be expressed as C
xo''; --A CD ... (28) ... (30) On the other hand, from the basic formula of the diffraction grating (F'to=0) and formulas (23) and (24), =2AD ( 13) (14) Therefore, as is clear from equations (6) and especially 28) to (31), the coefficient that determines the lattice groove arrangement is expressed as bz=
- (32) If given by r2 r8, then at any wavelength within a sufficiently small range of 1 m,
lzo = 0, Fso'' = O, Fao'q O.

Also, since rl: C2, formulas (6), (9)
, (15), FO2=O always. These mean that the aberration of the virtual image is extremely small both in the dispersion direction and in the direction perpendicular thereto. Note that the magnification of the virtual image is co
It is represented by sα/cosβ and 1-.

As explained above, the monochromator of the present invention has 1
Defocus, coma aberration, and spherical aberration are corrected at any wavelength within a sufficiently small range θ1, and the image is formed by the diffraction grating. On the other hand, in the known monochromator, the spherical convergence k remains in the virtual image formed by the diffraction grating. Therefore, it is clear that the monochromator of the present invention provides higher resolution.

Now, the concave mirror that converges the monochromatic light beam diverging from the virtual image onto the exit slit may be selected from among a spherical mirror, a 1~loidal mirror, a 1' (TM) mirror, etc. depending on the purpose of use.In particular, If we use the 7th elliptical hemoscope - again and the other focal points coincide with the positions of the virtual image and the exit slit, we can create a stigmatic image on the exit slit. It is possible to form an image.

Note that when a single ellipsoidal mirror is used, the aberration of the ellipse ifllM increases as the virtual image moves away from the focal point of the shelf blood.

Therefore, the entrance slit is large and the exit slit is 1~
If a vertically oriented and consistent image is required, the number of reflection directions increases, but a concave mirror system with smaller aberrations than the elliptical 1'J rf+i mirror is used, and the monochromatic light beam diverging from the virtual image is sequentially reflected by multiple concave mirrors. The output light may be converged on the exit slit (16). A concave mirror system, for example of the Walter type, is suitable for this purpose.

By the way, in the monochromator of the present invention, since the declination angle is constant λ2, the upper limit of the wavelength λ is expressed by equation (23),
(24).

Applying the conditions α≦90° and 1β1≦90” to (31).

The wavelength range can be expanded simply by replacing the mirror. That is, if K is sufficiently large (for example, K>75°)
, Equations (32) to (34) are approximately expressed as b2 ma-・(36
) − is given. On the other hand, in order to obtain a crystalline reflectance with a soft X-ray blob, it is necessary to make the deflection angle sufficiently large.

In this case, as is clear from equation (35), the wavelength range becomes narrower. In addition, as mentioned above, the imaging function of the diffraction grating is
It is good only in a range where 1θ1 is sufficiently small.

For the above reasons, the ratio between the upper limit of the wavelength range and the r limit in the monochromator of the present invention is practically limited to about 3 to 4. This also applies to known monochromators.

However, if the polarization angle of the diffraction grating is sufficiently large, as shown in Figure 4 (in the figure, the same symbols as in Figure 1 represent the same parts), the polarization angle of the diffraction grating and 4 rfa (17) will be obtained. . In this way, the grating groove arrangement for aberration correction is
It is determined by the distance between the entrance slit and the center of the diffraction grating, and is almost independent of the declination angle. Therefore, at any sufficiently large deflection angle, a virtual image with extremely small aberrations is formed in an arc 7 of 1'' centered on the center of the diffraction grating and having a radius equal to the distance between the incident slit 1 and the center of the diffraction grating. Therefore, the diffraction grating is rotated from position 2 to position 2', and a virtual image of the desired monochromatic light (virtual image of the entrance slit) 8' is formed at another fixed position.

Further, another concave mirror 3' is arranged so as to converge the monochromatic light beam 9' diverging from this virtual image onto the exit slit (18). At this time, since the wavelength of the monochromatic light beam converged on the output slit is determined by equation (31), the wavelength range of the monochromator 2 is expanded by changing K.

The method of expanding the wavelength range by appropriately selecting the polarization angle of the diffraction grating is to use a monochromator (for example, M, R, Loweils:
Nucl, Instrument, and Methods
, 177 (1980) pp. 127-139). However, these monochromators do not utilize the imaging function of the aperture grating. (The parallel light beam is incident on the diffraction grating, and the parallel light beam is reflected and diffracted.) The present invention focuses on the fact that the imaging function of a flat curved diffraction grating with unevenly spaced straight grooves is almost independent of the polarization angle. , which is qualitatively different from the conventional method.

Although the above description has been directed to the soft X-ray region, it goes without saying that the present invention is basically applicable to vacuum ultraviolet, ultraviolet, and visible regions.

〔Example〕

As a first embodiment of the present invention, a monochromator suitable for laser plasma X-ray applications will be described (19). In this example, the light source (several μm in size) is treated as an entrance slit, and the distance r between the light source and the center of the diffraction grating is set to 400.
Let it be mn. Also, the grating groove spacing σ0 at the center of the diffraction grating is 1/600nn+, l! ! The argument angle 2K of the l-analytical lattice is 168
'', and the wavelength range 55-15n is scanned.

Note that in order to obtain a high reflectance, it is necessary to increase the incident angle on the short wavelength side, so the -1st order diffracted light is extracted from the output slit. In this case, the rotation angle θ of the diffraction grating varies from −0,822” to −2,467°.

Furthermore, the coefficients that determine the lattice groove arrangement are expressed by equations (32) to (34).
)% expression%ru. FIG. 5 shows the dimensionless lattice groove spacing σ/σ0 as a function of the lattice groove position W.

In this example, in order to form a stigmatic image on the exit slit, an ellipsoidal mirror is used, and the center of the elliptical mirror is The angle of incidence at the intersection point is 84°. Further, the distances from the center of the ellipsoidal mirror to the first focal point ((20) virtual image of the light source) and the second focal point (output slit) are both 50 On+m. Let the origin be the midpoint of the two foci of the ellipsoidal mirror, and let the axis of rotational symmetry be X 1llllll.

With the direction perpendicular to this as the Y axis, the equation of the ellipse is x”/
When expressed as a”+y”/b”=1, a=500.0n
yn, b = 52, 3 mm.

How the monochromator of the present invention reduces aberrations will be explained using the results of ray tracing.

In FIG. 6, (,) shows the image obtained on the exit slit in this example, and (b) shows b4=0 in this example.
The image is shown when (spherical aberration is not corrected). The Y-axis and two axes in the figure represent the dispersion direction and the direction perpendicular to this, respectively. Here, the divergence angle of the light beam diverging from a point light source is 14 mrad in the dispersion direction and 50 m in the direction perpendicular to the dispersion direction.
Calculations were performed for wavelengths of 5 and 10.15'nm as rad. As is clear from the figure, when the spherical aberration is corrected, the spread of the image in the dispersion direction is small and the resolution is improved. Furthermore, when spherical aberration is corrected, the spread of the image is small even in the direction perpendicular to the dispersion direction.

(21) The reason for this is that the aberrations of the ellipsoidal mirror are reduced.

As a second embodiment of the present invention, a monochromator suitable for synchro 1 to ron synchrotron radiation applications will be described. Generally, synchrotron radiation has a small light source, and a monochromator is installed far away from the light source. In this example, the light source is treated as an entrance slit, and the distance r between the light source and the center of the diffraction grating is 15 m. In addition, the grating groove spacing σ0 at the center of the diffraction grating is 1'/600 nu,
l! ! It is assumed that the polarization angle 2K of the I-fold grating is 174°, and the wavelength range of 2 to 6 nm is scanned. Note that the diffraction order used is the 11th order. In this case, the rotation angle θ of the diffraction grating is 0.
It varies from 657° to -1,971°.

Furthermore, the coefficients that determine the lattice groove arrangement are expressed by equations (32) to (34).
)% expression% becomes. FIG. 7 shows the dimensionless lattice groove spacing σ/σ0 as a function of the lattice groove position tOW. In this example, elliptical white rust is used, and the angle of incidence at the center of the ellipsoidal mirror (22) is set to 87°. Further, the distances from the center of the ellipsoidal mirror to the first focal point (virtual image of the light source) and the second focal point (output slit) are 5287nwn.

The length shall be 1000mm. Therefore, the magnification of the Hinoki IIJ curved mirror is 0.0654. If the equation of the ellipse is expressed as X2/a2+y2/b2-1- as mentioned above, a-81
4-3,5n+m, b=204.6an.

FIG. 8(a) shows that 1'? The image obtained on the exit slit was obtained by ray tracing.

Also, the 81st! ! ! 1(b) is an image obtained when the optical system is completely aberration-free in this example, based on the magnification of the diffraction grating and the elliptical 1'J plane mirror, and the size of the light source. The Y-axis and Z-axis in the figure represent the dispersion direction and the direction perpendicular thereto, respectively. Here, the size and divergence angle of the light source are 0.4 mm x 2 nm in the dispersion direction and the direction perpendicular to this.
, LmradX2mrad, and calculations were performed for wavelengths of 2 and 4.6 nm.

It can be seen that an image with small aberrations is formed at any wavelength, and high resolution can be obtained.

The monochromatic light beam diverging from the virtual image is divided into multiple concave If! An example in which the light is sequentially reflected by one mirror and converged on the exit slit will be explained.

This embodiment replaces the ellipsoidal mirror in the second embodiment with a Walter type concave mirror system. As shown in Figure 9, the Walter type concave mirror system consists of an onboard ellipsoidal mirror.
91 and a rotating hyperboloid mirror (hereinafter referred to as a hyperboloid mirror) 92, the first focal points of each of which are located at the same position 93.

Further, the second focal point 94 of the wiIJ-1 blood mirror and the second focal point 95 of the hyperboloid mirror are located above the virtual image of the entrance slit (light source) and the exit slit, respectively. Here, the incident angles at the centers of the ellipsoidal mirror and the hyperboloidal mirror are both 87°. Also,
Wt t'J if+i The distances from the mirror center to the virtual image of the entrance slit and the hyperboloid mirror center are respectively 15287no,
300nn+doshi, double 111'ff+i'Jlt The distance from the center to the exit slit is 865.5mm. The magnification of this concave 1rIi ME thread is 0.0654, the same as the ellipsoidal mirror of the second embodiment. Define the coordinate system of the elliptical mirror as described above, and define the equation of the ellipse as X2/a
2 +y 2 /299.5mm. Also, hyperbolic 1
Define the coordinate system for the in mirror in the same way, and express the hyperbolic equation as X2/a2-X2/b2;1, then a=4.
88.7m.

b=66.1 mm.

FIG. 1.0 shows the image obtained on the exit slit in this embodiment by ray tracing, and the symbols in the figure and the ray tracing conditions are the same as described above.

In this example, the aberration is even smaller than in the second example.
It can be seen that a substantially aberration-free image is formed even though the size of the light source is large.

As a fourth embodiment of the present invention, an example will be described in which the wavelength range is expanded simply by replacing the polarization angle of the diffraction grating and the four-pointed mirror.

Here, in the second example, the polarization angle of the diffraction grating is set to 1.
68° and scan the wavelength range 5 to i 5 n,m. In addition, to prevent the position of the exit slit and the direction of the exit light from changing, another ellipsoidal mirror was installed at an incident angle of 84°.
Use with. At this time, from the center of the elliptical mirror to the first focal point (
virtual image of the light source) and iI′l! to the second focal point (output slit 1~). The distance is 15144m and 1144mm respectively.
Therefore, (25) The magnification of the ellipsoidal mirror is 0.0755. If the equation of ellipse 11 is expressed as x''/a2+y2/b2-K as mentioned above, a,=8144.3in+, b=435.1+n
It is n.

111d(a) is an image obtained by ray tracing of the image obtained on the exit slit in this example. Further, Fig. 1 (I)) shows an image obtained when the optical system is completely aberration-free in this example, based on the magnification of the diffraction grating and the ellipsoidal mirror, and the size of the light source. The symbols in the figure and the ray tracing conditions are the same as above. Wavelength range 5~]-5
In the same way as in the second embodiment, an image with small aberration is formed at nm, and it can be seen that a wide wavelength range can be scanned using the same diffraction grating.6 [Effects of the Invention] Detailed above As described above, according to the present invention, it is possible to correct the spherical aberration of the diffraction grating, which is a problem with conventional monochromators, and to realize a monochromator with even higher resolution.

[Brief explanation of drawings]

(26) Fig. 1 is a block diagram explaining the optical system of the monochromator of the present invention, Fig. 2 is a block diagram explaining the optical system of a conventional monochromator, and Fig. 3 is a perspective view of the plane diffraction grating optical system. , 4th
The figure shows the configuration of the optical system when replacing the deflection angle of the diffraction grating and the concave mirror in the monochromator of the present invention. Figure 5 shows the change in the grating groove spacing of the diffraction grating in the first embodiment of the present invention. FIG. 6(a) is a characteristic diagram showing the image at the exit slit in the first embodiment of the present invention, FIG. 6(b) is a characteristic diagram shown in FIG.
is a characteristic diagram showing the image at the exit slit when the spherical aberration of the diffraction grating is not corrected in this embodiment, and FIG. 7 is a characteristic diagram showing changes in the grating groove spacing of the diffraction grating in the second embodiment of the present invention. , FIG. 8(a) is a characteristic diagram showing the image at the exit slit in the second embodiment of the present invention, FIG. 8(b)
is a characteristic diagram showing the image at the exit slit assuming that the optical system is completely aberration-free in this embodiment, FIG. 9 is a configuration diagram illustrating the Walter type concave mirror system, and FIG. 10 is the third embodiment of the present invention. Characteristic diagram showing the image at the exit slit in the fourth embodiment, FIG. 11(b)
is a characteristic diagram showing an image at the exit slit in this example, assuming that the optical system is completely aberration-free. 1...Incidence slit, 2,2'...Plane diffraction grating,
3゜3'...Concave mirror, 4...Output slit, 5...
・Rotation axis of plane diffraction grating, 6... White light flux, 7...
Arc formed by the virtual image of the entrance slit, 8, 8'...virtual image of the entrance slit 1-, 9, 9'...monochromatic light flux. (28) λ25nm (0-) (b) Input=-7ρnm Z(m...) 1(mrn) λ=15r) River

Claims (1)

[Claims] 1. An optical system comprising an entrance slit, a plane diffraction grating, a concave mirror, an exit slit, and a mechanism for rotating the diffraction grating at an arbitrary angle around a rotation axis including its central grating groove,
The relative positional relationship between the entrance slit, the center of the diffraction grating, the concave mirror, and the exit slit is fixed, and the diffraction grating has linear grating grooves arranged at unequal intervals, with a central grating groove having a groove interval of σ_0. As the origin, b_2, b_3, b
When _4 is a constant and the groove number at the position w perpendicular to the grating groove is n, then n=(w+b_2w^2+b_
There is a relationship of 3w^3+b_4w^4)/σ_0, and the diffraction grating reflects and diffracts an arbitrary monochromatic light component in the white light flux diverging from the incident slit, and the diffraction grating center is the center, and the diffraction grating center and the incident slit are A virtual image of the arbitrary monochromatic light is formed substantially without aberration at a fixed position on a circle whose radius is the distance from the slit, and the concave mirror is positioned so as to converge the monochromatic light beam diverging from the virtual image onto the exit slit. 1. A monochromator comprising: a concave mirror giving a shape; and wavelength scanning is performed by rotating the diffraction grating. 2. The monochromator according to claim 1, further comprising the optical system that sequentially reflects a monochromatic light beam diverging from the virtual image of the arbitrary monochromatic light by a plurality of concave mirrors and converges it on the output slit. monochromator. 3. The monochromator according to claim 1 or 2, wherein the single or plural concave mirrors are fixed while the relative positional relationship between the entrance slit, the center of the diffraction grating, and the exit slit is fixed. is replaced with another single or multiple concave mirrors, and the diffraction grating is located on a circle whose center is the center of the diffraction grating and whose radius is the distance between the center of the diffraction grating and the incident slit, and at a fixed position different from the fixed position. to form a virtual image of arbitrary monochromatic light substantially without aberration, and the single or plural concave mirrors have the above-mentioned optical system that converges a monochromatic light beam diverging from the virtual image of arbitrary monochromatic light onto the exit slit. monochromator. 4. In the monochromator according to claim 1, the concave mirror is a spheroidal mirror, and one focus and another focus of the spheroidal mirror are located above the virtual image and the exit slit, respectively. A monochromator featuring: 5. The monochromator according to claim 2, wherein the concave mirror is an ellipsoidal mirror of revolution and a hyperboloid of revolution,
One focal point of each of the spheroidal mirror and the hyperbolic mirror is coincident, and the other focal points of the ellipsoidal mirror and the hyperbolic mirror are located above the virtual image and the exit slit, respectively. Monochromator.