Spectroscope

Abstract

To improve the degree of freedom in arranging optical components.SOLUTION: An optical axis La and an optical axis Lb of first and second collimators 3 and 7 are arranged separately in a ruling direction C of diffraction grating 4. A first position a1, on which incident light is incident, and a second position a2, at which diffracted light is condensed on a first slit 6b, are present on a first circumference Ca centered on the optical axis La of the first collimator 3. A third position b1, at which the diffracted light passes through the first slit 6b, and a fourth position b2, at which the second collimator 7 condenses the diffracted light, are present on a second circumference Cb having the same radius as the first circumference Ca centered on the optical axis Lb in a plane perpendicular to the optical axis Lb of the second collimator 7. The first position a1 and the second position a2 are arranged symmetrically with respect to the optical axis La of the first collimator 3, the third position b1 and the fourth position B2 are arranged symmetrically with respect to the optical axis Lb of the second collimator 7, and the second position a2 and the third position b1 are arranged substantially parallel to the ruling direction C of diffraction grating 4.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a spectrometer that separates light to be measured into wavelengths. [Background technology]

[0002] A known example of a multi-path spectroscope that separates light to be measured into wavelengths is that disclosed in Patent Document 1. The spectroscope disclosed in Patent Document 1 collimates incident light and irradiates the light onto a diffraction grating, and emits return light from the diffraction grating through a slit provided in the spectroscope itself. Each of the two spectroscopic paths includes a collimator that collimates the incident light, and the collimators of each spectroscopic path are arranged so that the irradiation areas on the diffraction grating of the light emitted from the collimators of each spectroscopic path are shifted at least in the direction in which the ruling lines extend. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 7195189 Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present invention is to provide a spectrometer that allows greater freedom in arranging optical components than the spectrometer of Patent Document 1 described above. [Means for solving the problem]

[0005] In order to achieve the above object, a spectrometer according to claim 1 of the present invention comprises a diffraction grating 4, a first collimator 3 that converts incident light into collimated light, inputs the collimated light to the diffraction grating, and collects diffracted light from the diffraction grating; a folding optical system 6 including a first mirror 6a that reflects the diffracted light from the diffraction grating that is collected by the first collimator, a first slit 6b that is arranged at a position where the diffracted light reflected by the first mirror is collected, and a second mirror 6c that reflects the diffracted light that has passed through the first slit back to the diffraction grating, and that folds the diffracted light that is collected by the first collimator back to the diffraction grating; a second collimator 7 that inputs the diffracted light reflected by the reflecting optical system back into the diffraction grating and collects the diffracted light from the diffraction grating; a light receiving unit 8 having a second slit 8b disposed at a position where the diffracted light is collected by the second collimator, and receiving the diffracted light that has passed through the second slit to extract light of an arbitrary wavelength component, an optical axis La of the first collimator and an optical axis Lb of the second collimator are arranged to be spaced apart in a ruling direction of the diffraction grating so that irradiation areas on the diffraction grating of the first collimator and the second collimator are shifted in a ruling direction C of the diffraction grating; a first position a1, which is a position where incident light toward the first collimator is incident, and a second position a2, which is a position where diffracted light from the diffraction grating is focused on the first slit of the folding optical system, exist on a first circumference Ca centered on the optical axis of the first collimator; a third position b1, which is a position where diffracted light from the diffraction grating passes through the first slit of the folding optical system, and a fourth position b2, which is a position where diffracted light from the diffraction grating is focused by the second collimator, exist on a second circumference Cb of the same radius as the first circumference, which is centered on the optical axis of the second collimator in a plane perpendicular to the optical axis of the second collimator; the first position and the second position are arranged at positions symmetrical with respect to the optical axis of the first collimator; the third position and the fourth position are arranged at positions symmetrical with respect to the optical axis of the second collimator; and the second position and the third position are arranged so as to be approximately parallel to the ruling direction of the diffraction grating. [Effects of the Invention]

[0006] According to the present invention, by shifting the incident position and the exit position in the height direction, the degree of freedom in arranging optical components is improved, and stray light passing through the slit can be suppressed. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram illustrating the configuration of a spectrometer according to the present invention. [Figure 2] FIG. 2 is a diagram showing the internal configuration of a folding optical system in the spectrometer according to the present invention. [Figure 3] FIG. 2 is a diagram showing the internal configuration of a light receiving unit in the spectrometer according to the present invention. [Figure 4] FIG. 3 is an explanatory diagram showing the positional relationship between a first collimator, a diffraction grating, and a plane mirror in the spectrometer according to the present invention. [Figure 5] 3 is a conceptual diagram showing an example of the relationship between the incident position and the exit position of light on each of the first and second spectral paths in the spectrometer according to the present invention. FIG. [Figure 6] FIG. 10 is a conceptual diagram showing another example of the relationship between the incident position and the exit position of light on each of the first and second spectral paths in the spectrometer according to the present invention. [Figure 7] FIG. 10 is a conceptual diagram showing another example of the relationship between the incident position and the exit position of light on each of the first and second spectral paths in the spectrometer according to the present invention. [Figure 8] FIG. 10 is a conceptual diagram showing another example of the relationship between the incident position and the exit position of light on each of the first and second spectral paths in the spectrometer according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0008] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0009] As shown in FIG. 1, the multipath spectrometer 1 of this embodiment is a spectrometer with a differential dispersion configuration that includes an input connector 2, a first collimator 3, a diffraction grating 4, a plane mirror 5, a folding optical system 6, a second collimator 7, and a light receiving unit 8. The spectrometer passes incident light as light to be measured through a first spectroscopic path A and a second spectroscopic path B to extract light of any wavelength component, and obtains information (spectroscopic spectrum data) that resolves and arranges the light by wavelength.

[0010] The first spectral path A is a path through which light passes in the order of input connector 2 → first collimator 3 → diffraction grating 4 → plane mirror 5 → diffraction grating 4 → first collimator 3 → folding optical system 6. The second spectral path B is a path through which light passes in the order of folding optical system 6 → second collimator 7 → diffraction grating 4 → plane mirror 5 → diffraction grating 4 → second collimator 7 → light receiving unit 8.

[0011] In the spectrometer 1 of this embodiment, in the first spectroscopic path A, incident light (light to be measured) incident from the input connector 2 is collimated by the first collimator 3 to be irradiated onto the diffraction grating 4, the diffracted light from the diffraction grating 4 is reflected between the plane mirror 5 and the first collimator 3, and the diffracted light collected by the first collimator 3 is reflected back onto the diffraction grating 4 by the folding optical system 6, and in the second spectroscopic path B, the diffracted light returned by the folding optical system 6 is collimated by the second collimator 7 to be irradiated onto the diffraction grating 4, the diffracted light from the diffraction grating 4 is reflected between the plane mirror 5 and the second collimator 7 to be collected and received by the light receiving unit 8.

[0012] In Figures 1 and 4, the incident light and the outgoing light are shown separated vertically in order to clearly distinguish the light paths in the first light-splitting path A and the second light-splitting path B, but in reality the incident light and the outgoing light almost overlap.

[0013] In addition, in FIG. 1, a polarization scrambler can be disposed between the input connector 2 and the first collimator 3 to eliminate polarization dependency.

[0014] The following describes the configuration of each part of the spectrometer 1. The input connector 2 is a connector that takes in the light to be measured, which is the object of measurement, as input light.

[0015] The first collimator 3 is composed of a lens with an outer diameter of 35 mm and a focal length of 150 mm, for example, that can be moved and adjusted in the direction of the optical axis La (a direction perpendicular to the ruling direction C of the diffraction grating 4, which will be described later), and converts the incident light (measured light) that enters from the input connector 2 into collimated light (parallel light) and makes it enter the diffraction grating 4.

[0016] The first collimator 3 also collects the diffracted light from the diffraction grating 4 on the first light separation path A onto a first slit 6b via a first mirror 6a of the folding optical system 6, which will be described later.

[0017] The diffraction grating 4 is an optical element that separates and disperses the collimated light (parallel light) from the first collimator 3 and the collimated light (parallel light) from the second collimator 7 by wavelength. It is made up of a reflector of predetermined dimensions (for example, length (vertical) L: 70 mm, width (horizontal) W: 60 mm) and has on its surface ruling lines consisting of numerous grooves (sawtooth grooves, sinusoidal grooves, rectangular grooves) that extend parallel to each other at regular intervals in the direction of arrow C in Figure 1, and is rotatable around a central rotation axis 4a that is parallel to the ruling direction C along which the ruling lines extend.

[0018] As shown in Fig. 4, the plane mirror 5 is set at a predetermined deflection angle θ (for example, θ = 15°), and reflects the diffracted light from the diffraction grating 4 and returns it to the diffraction grating 4. Note that the configuration may be simplified by omitting the plane mirror 5. That is, in Figs. 1 and 4, the diffracted light from the diffraction grating 4 is reflected by the plane mirror 5 and returned to the diffraction grating 4, thereby dispersing the light twice in each of the first spectral path A and the second spectral path B. However, it is also possible to omit the plane mirror 5 and disperse the light once in each of the first spectral path A and the second spectral path B.

[0019] The folding optical system 6 is an optical system that folds the light (diffracted light) from the first spectral path A toward the second spectral path B, and as shown in Figures 1 and 2, is composed of a first mirror 6a, a first slit 6b, and a second mirror 6c.

[0020] As shown in Figures 1 and 2, the first mirror 6a reflects light from the first collimator 3 so that the diffracted light from the diffraction grating 4 on the first spectral path A is focused onto the first slit 6b via the first collimator 3.

[0021] As shown in Figures 1 and 2, the first slit 6b is positioned at a position where the diffracted light from the diffraction grating 4 of the first spectral path A is collected via the first collimator 3, and allows the light from the first collimator 3 to pass through an opening not shown.

[0022] As shown in FIGS. 1 and 2, the second mirror 6c reflects the light that has been reflected by the first mirror 6a and passed through an opening (not shown) of the first slit 6b to the second collimator .

[0023] In this way, the folding optical system 6 changes the optical path of the light on the first spectral path A using the first mirror 6a to pass through the first slit 6b, and then changes the optical path of the light that has passed through the first slit 6b using the second mirror 6c to fold it back toward the second spectral path B.

[0024] The second collimator 7, like the first collimator 3, is composed of a lens with an outer diameter of 35 mm and a focal length of 150 mm, for example, that can be moved and adjusted in the direction of the optical axis Lb (a direction perpendicular to the ruling direction C of the diffraction grating 4), and converts the incident light reflected from the first spectral path A by the reflection optical system 6 into collimated light (parallel light) that is incident on the diffraction grating 4.

[0025] The second collimator 7 also collects the diffracted light from the diffraction grating 4 on the second light-split path B onto the opening of the second slit 8b of the light-receiving unit 8 via a third mirror 8a (described later).

[0026] The light receiving unit 8 receives the diffracted light from the diffraction grating 4 on the second spectral path B and extracts light of any wavelength component, and as shown in Figures 1 and 3, is composed of a third mirror 8a, a second slit 8b, a first lens 8c, and a light receiver 8d.

[0027] As shown in Figures 1 and 3, the third mirror 8a reflects light from the second collimator 7 so that the diffracted light from the diffraction grating 4 on the second spectral path B is focused onto the second slit 8b via the second collimator 7.

[0028] As shown in Figures 1 and 3, the second slit 8b is positioned at a position where the diffracted light from the diffraction grating 4 on the second spectral path B is collected via the second collimator 7, and allows the light from the second collimator 7 to pass through an opening not shown.

[0029] As shown in Figure 3, the first lens 8c receives the light reflected by the third mirror 8a from the opening of the second slit 8b, which is located at a position twice the focal length f, and focuses the light onto the light-receiving surface of the photodetector 8d, which is located at a position twice the focal length f.

[0030] 1 and 3, the photoreceiver 8d receives the light collected by the first lens 8c. The light received by the photoreceiver 8d is resolved into wavelengths and converted into arranged information (spectral data) to be acquired.

[0031] Here, the first collimator 3 and the second collimator 7 in the above-mentioned spectrometer 1 are arranged so that the optical axis La and the optical axis Lb are spaced a predetermined distance apart in the ruling direction C of the diffraction grating 4, as shown in Figures 5 to 8, so that the irradiation area on the diffraction grating 4 of the first collimator 3 does not overlap with the irradiation area on the diffraction grating 4 of the second collimator 7.

[0032] In the example of FIG. 5 , the line connecting the optical axis La and the optical axis Lb is parallel to the ruling direction C of the diffraction grating 4, the first position a1 and the second position a2 are located on the first circumference Ca, the third position b1 and the fourth position b2 are located on the second circumference Cb, and the first collimator 3 and the second collimator 7 (not shown) are arranged a predetermined distance apart so that the return from the second position a2 to the third position b1 is parallel to the ruling direction C of the diffraction grating 4.

[0033] In the example of FIG. 6 , the line connecting the optical axis La and the optical axis Lb forms a predetermined angle with respect to the ruling direction C of the diffraction grating 4, the first position a1 and the second position a2 are located on the first circumference Ca, the third position b1 and the fourth position b2 are located on the second circumference Cb, and the first collimator 3 and the second collimator 7 (not shown) are arranged a predetermined distance apart so that the return from the second position a2 to the third position b1 is parallel to the ruling direction C of the diffraction grating 4.

[0034] In the example of Figure 7, the line connecting the optical axis La of the first collimator 3 and the optical axis Lb of the second collimator 7 is parallel to the ruling direction C of the diffraction grating 4, the first position a1 and the second position a2 are on the first circumference Ca, the third position b1 and the fourth position b2 are on the second circumference Cb, and the turning back from the second position a2 to the third position b1 is parallel to the ruling direction C of the diffraction grating 4 and are arranged at a predetermined distance apart so that the turning back distance is longer than in Figures 5 and 6.

[0035] In the example of Figure 8, the optical axis La of the first collimator 3 and the optical axis Lb of the second collimator 7 are arranged at a predetermined distance apart so that the line connecting them is parallel to the ruling direction C of the diffraction grating 4, the first position a1 and the second position a2 are on the first circumference Ca, the third position b1 and the fourth position b2 are on the second circumference Cb, and the turning back from the second position a2 to the third position b1 forms a predetermined angle with the ruling direction C of the diffraction grating 4.

[0036] In the above-described spectrometer 1, the position where the incident light toward the first collimator 3 is incident on the first spectral path A (the incident position on the end face of the input connector 2) is defined as the first position a1. Also, in the first spectral path A, the position where the diffracted light from the diffraction grating 4 is collected on the first slit 6b of the folding optical system 6 (the position of the opening of the first slit 6b, not shown, which is a virtual exit position on the optical axis of the folding optical system 6 that is parallel to the ruling direction C of the diffraction grating 4) is defined as the second position a2.

[0037] In the second spectral path B, the position where the diffracted light from the diffraction grating 4 passes through the first slit 6b of the folding optical system 6 (the position of an opening (not shown) of the first slit 6b, which is a virtual incident position on the optical axis of the folding optical system 6 that is parallel to the ruling direction C of the diffraction grating 4) is defined as a third position b1. In addition, in the second spectral path B, the position where the diffracted light is collected by the second collimator 7 (the position of an opening (not shown) of the second slit 8b) is defined as a fourth position b2.

[0038] The first position a1 and the second position a2 exist on a first circumference Ca centered on the optical axis La of the first collimator 3 in a plane perpendicular to the optical axis La. The third position b1 and the fourth position b2 exist on a second circumference Cb of the same radius as the first circumference Ca centered on the optical axis Lb of the second collimator 7 in a plane perpendicular to the optical axis Lb. The first position a1 and the second position a2 are arranged symmetrically with respect to the optical axis La of the first collimator 3, and the third position b1 and the fourth position b2 are arranged symmetrically with respect to the optical axis Lb of the second collimator 7. The third position b1 and the fourth position b2 are arranged so as to be approximately parallel to the ruling direction C of the diffraction grating 4.

[0039] Furthermore, the first position a1 is located at a position where the point where the first circumference Ca intersects with a plane including the optical axis La of the first collimator 3 and the optical axis Lb of the second collimator 7 is different. The second position a2 is located symmetrically to the first position a1 with respect to the optical axis La of the first collimator 3. The second position a2 and the third position b1 are located so as to be parallel to the ruling direction C of the diffraction grating 4. The fourth position b2 is located symmetrically to the third position b1 with respect to the optical axis of the second collimator 7.

[0040] Here, the diffraction by the diffraction grating 4 is expressed by the following formula. m·λ=2·d·cosα·cosγ·sinθ…Equation (1), γ=(β-i) / 2, θ=i+γ where m is the diffraction order, λ is the wavelength, d is the grating constant, α is the angle with respect to the main surface (the plane perpendicular to the ruling of the diffraction grating 4), i is the angle of incidence on the diffraction grating 4, and β is the diffraction angle at the diffraction grating 4.

[0041] The diffraction of the first light path A can be expressed by m·λ=2·d·cosα1·cosγ1·sinθ1...Equation (2), where the diffraction conditions are: incident angle i1, diffraction angle β1, angle with respect to the principal surface α1, β1-i1=2γ1, θ1=i1+γ1.

[0042] The diffraction of the second light path B can be expressed by m·λ=2·d·cosα2·cosγ2·sinθ2... (3) where the diffraction conditions are: angle of incidence i2, angle of diffraction β2, angle with respect to the principal surface α2, β2-i2=2γ2, θ2=i2+γ2.

[0043] In order for the rotation angle of the diffraction grating 4 to be the same for the wavelengths of the first and second spectral paths A and B, and for the output light wavelengths of the first and second spectral paths A and B to be the same, the arrangement needs to satisfy the following expressions (4) and (5) based on expressions (2) and (3). θ1=θ2 [i.e., i1+γ1=i2+γ2]...Equation (4) cosα1·cosγ1=cosα2·cosγ2…Equation (5)

[0044] Here, the ruling direction C of the diffraction grating 4 is the x-coordinate, the wavelength dispersion direction by the diffraction grating 4 is the y-coordinate, the optical axes (foci) of the first collimator 3 and the second collimator 7 are the origin, the focal length of both collimators 3 and 7 is f, the first position (incident position of the first spectroscopic path A) is (x, y) = (x1, y1), the second position (virtual exit position of the first spectroscopic path A) is (x, y) = (x2, y2), the third position (virtual incident position of the second spectroscopic path B) is (x, y) = (x3, y3), and the fourth position (exit position of the second spectroscopic path B) is (x, y) = (x4, y4).

[0045] Considering equation (4), if the first and second positions are symmetrical with respect to the optical axis La of the first collimator 3 and the third and fourth positions are symmetrical with respect to the optical axis Lb of the second collimator 7, the bisectors of the angles of incidence and diffraction of the diffraction grating 4 on the first and second spectral paths A and B will coincide, and θ1 = θ2 will be established, thereby satisfying equation (4). When θ1 = θ2, this means that there is no dependency on wavelength.

[0046] Furthermore, considering equation (5), if the first position and the second position are symmetrical with respect to the optical axis La of the first collimator 3, and the third position and the fourth position are symmetrical with respect to the optical axis Lb of the second collimator 7, then γ1, γ2, α1, and α2 can be expressed by the following equations (6) to (13) in any xy coordinate system with the optical axis La of the first collimator 3 and the optical axis Lb of the second collimator 7 as the origin. γ1=tan-1(|y1| / f)≒|y1| / f…Equation (6) =tan-1(|y2| / f)≒|y2| / f…Equation (7) γ2=tan-1(|y3| / f)≒|y3| / f…Equation (8) =tan-1(|y4| / f)≒|y4| / f…Equation (9) α1=tan-1(|x1| / f)≒|x1| / f…Equation (10) =tan-1(|x2| / f)≒|x2| / f…Equation (11) α2=tan-1(|x3| / f)≒|x3| / f…Equation (12) =tan-1(|x4| / f)≒|x4| / f…Equation (13)

[0047] Here, using equations (6), (8), (10), and (12) and the relationship [cosA ≒ 1-(A2 / 2)], equation (5): cosα1·cosγ1 = cosα2·cosγ2 is {1-(x1 / f)2 / 2}{1-(y1 / f)2 / 2}≒{1-(x3 / f)2 / 2}{1-(y3 / f)2 / 2}, x12 + y12 ≒ x32 + y32 + (x12·y12-x32·y32) / (2·f2), where x1, y1, x3, and y3 are f, so (x12·y12-x32·y32) / (2·f2) ≒ 0, and x12 + y12 ≒ x32 + y32...equation (14).

[0048] Furthermore, since |x1|=|x2|, |y1|=|y2|, |x3|=|x4|, and |y3|=|y4|, the following formula (15) can be obtained from the above formula (14). x12+y12≒x22+y22≒x32+y32≒x42+y42…Equation (15)

[0049] Therefore, the first position a1 and the second position a2 exist on approximately the same circumference in any plane perpendicular to the optical axis La of the first collimator 3, and the third position b1 and the fourth position b2 exist on approximately the same circumference in any plane perpendicular to the optical axis Lb of the second collimator 7, thereby satisfying equation (5).

[0050] From these, in order to satisfy equations (4) and (5), the first position a1 and the second position a2 should be located on the same circumference in any plane perpendicular to the optical axis La of the first collimator 3, the third position b1 and the fourth position b2 should be located on the same circumference in any plane perpendicular to the optical axis Lb of the second collimator 7, and the first position a1 and the second position a2 should be located in positions symmetrical with respect to the optical axis La of the first collimator 3, and the third position b1 and the fourth position b2 should be located in positions symmetrical with respect to the optical axis Lb of the second collimator 7.

[0051] As described above, according to this embodiment, the first position a1 and the second position a2 are located on a first circumference Ca centered on the optical axis La of the first collimator 3 in a plane perpendicular to the optical axis La, the third position b1 and the fourth position b2 are located on a second circumference Cb of the same radius as the first circumference Ca centered on the optical axis Lb of the second collimator 7 in a plane perpendicular to the optical axis Lb, the first position a1 and the second position a2 are located symmetrically with respect to the optical axis La of the first collimator 3, and the third position b1 and the fourth position b2 are located symmetrically with respect to the optical axis Lb of the second collimator 7, and the second position a2 and the third position b1 are located approximately parallel to the ruling direction C of the diffraction grating 4.

[0052] Furthermore, the optical axis La of the first collimator 3 and the optical axis Lb of the second collimator 7 are arranged a predetermined distance apart in the ruling direction C of the diffraction grating 4 so that the irradiation area on the diffraction grating 4 of the first collimator 3 and the irradiation area on the diffraction grating 4 of the second collimator 7 do not overlap. This allows for an arrangement that takes into account the characteristics of the optical components, improving the degree of freedom in arranging the optical components. Furthermore, stray light passing through the slits can be suppressed.

[0053] While the best mode for carrying out the spectrometer according to the present invention has been described above, the present invention is not limited to the description and drawings of this mode. In other words, all other modes, embodiments, and operational techniques that can be achieved by those skilled in the art based on this mode are naturally included in the scope of the present invention. [Explanation of symbols]

[0054] 1 spectrometer 2 Input connector 3 First collimator 4 Diffraction Grating 4a Rotation axis 5 plane mirror 6. Folded optical system 6a First mirror 6b First slit 6c Second mirror 7 Second collimator 8 Light receiving section 8a Third Mirror 8b Second slit 8c First Lens 8d receiver A. First spectroscopic path B. Second Spectroscopic Path C Score direction a1 1st position a2 second position b1 3rd position b2 4th position Ca 1st Circumference Cb Second Circumference La Optical axis of the first collimator Lb Optical axis of the second collimator θ declination

Claims

[Claim 1] A diffraction grating (4), a first collimator (3) that converts incident light into collimated light, inputs it to the diffraction grating, and collects diffracted light from the diffraction grating; a folding optical system (6) including a first mirror (6a) that reflects the diffracted light from the diffraction grating that is focused by the first collimator, a first slit (6b) that is arranged at a position where the diffracted light reflected by the first mirror is focused, and a second mirror (6c) that reflects the diffracted light that has passed through the first slit back to the diffraction grating, and that folds the diffracted light focused by the first collimator back to the diffraction grating; a second collimator (7) that inputs the diffracted light reflected by the reflection optical system back into the diffraction grating and collects the diffracted light from the diffraction grating; a second slit (8b) disposed at a position where the diffracted light is focused by the second collimator, and a light receiving unit (8) that receives the diffracted light that has passed through the second slit and extracts light of an arbitrary wavelength component, The optical axis (La) of the first collimator and the optical axis (Lb) of the second collimator are arranged to be spaced apart in the ruling direction of the diffraction grating so that irradiation areas on the diffraction grating of the first collimator and the second collimator are shifted in the ruling direction (C) of the diffraction grating; a first position (a1) at which incident light toward the first collimator is incident, and a second position (a2) at which diffracted light from the diffraction grating is focused on the first slit of the folding optical system, both of which are located on a first circumference (Ca) centered on the optical axis of the first collimator; a third position (b1) at which diffracted light from the diffraction grating passes through the first slit of the folding optical system, and a fourth position (b2) at which diffracted light from the diffraction grating is focused by the second collimator, both of which are located on a second circumference (Cb) of the same radius as the first circumference and centered on the optical axis of the second collimator in a plane perpendicular to the optical axis of the second collimator; the first position and the second position are located symmetrically with respect to the optical axis of the first collimator; the third position and the fourth position are located symmetrically with respect to the optical axis of the second collimator; and the second position and the third position are located approximately parallel to the ruling direction of the diffraction grating.

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