JPS6212846B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a spectrometer using two stages of diffraction gratings, a so-called double monochromator, and particularly to a zero-dispersion type spectrometer.

Two types of double monochromators are known: additive-dispersion type and zero-dispersion type.The former doubles the dispersion, while the latter returns the once-dispersed light to a state without dispersion. It is also called inverse dispersion. FIG. 1 shows a schematic configuration of a conventional zero-dispersion double monochromator using a concave diffraction grating. entrance slit
The light incident from S ₁ is reflected and dispersed by the first concave diffraction grating G ₁ to form a spectral image I at the intermediate slit S ₂ . This light then reaches the second concave diffraction grating _G2 , is reflected there, is subjected to the effect of inverse dispersion, and exits from the exit slit _S3 . Here, in order to achieve zero dispersion for all wavelengths, it is necessary that the linear dispersion by the two concave diffraction gratings G ₁ and G ₂ be equal, and the lattice constant and convergence effect of each must be equal. . In addition, by simultaneously rotating each of the biconcave diffraction gratings in the direction of the arrow in the figure while maintaining symmetry with respect to the intermediate slit _S2 , the wavelength range of the spectral image formed within the intermediate slit _S2 can be changed. Furthermore, it is well known that by providing various apertures at the intermediate slit positions, it is possible to extract or exclude only light of specific wavelengths.

However, since the long wavelength light λ ₁ and the short wavelength light λ ₂ that form a spectral image off the optical axis at the position of the intermediate slit S ₂ have different diffraction angles, the second concave diffraction grating
By the time it reaches G ₂ , it has widened considerably. Therefore, in order to condense all of these diverging lights onto the exit slit _S3 , the second concave diffraction grating _G2 must be extremely large as shown by the dotted line in the figure. Otherwise, a large amount of vignetting would occur and, as a result, it would be impossible to achieve zero dispersion over a wide wavelength range.

Furthermore, since the short wavelength light and the long wavelength light are emitted from the exit slit _S3 in different directions, vignetting occurs in various measuring devices (not shown) following the exit slit _S3 , which is extremely inconvenient.

SUMMARY OF THE INVENTION An object of the present invention is to provide an excellent zero-dispersion spectrometer which is effective over a wide wavelength range, is compact, has no vignetting, and has no wavelength-based directivity.

Hereinafter, the present invention will be explained based on examples. FIG. 2 is a schematic diagram of an embodiment of the present invention, and the same numbers as in FIG. 1 represent the same members.

As shown in FIG. 2, in this embodiment, first and second plano-convex positive lenses L _{1 and} L ₂ , which are equal to each other, are arranged symmetrically with respect to the intermediate slit S ₂ with their flat surfaces facing each other, and the combination of both lenses is Depending on the system, the two concave gratings are conjugate to each other. Further, the composite imaging magnification γ of the spectral line I by the composite system of the two positive lenses L ₁ and L ₂ is γ=1. This point will be explained in detail using FIG. First, the spectral image I produced by the first concave diffraction grating G ₁ becomes a β-times spectral image I' by the first positive lens L ₁ without changing the imaging position in the optical axis direction. Here, the conditions for not changing the imaging position in the optical axis direction are as follows for the first plano-convex positive lens _L1 . As shown in Fig. 4, the focal length of this plano-convex lens is: d is the center thickness, n is the refractive index, and l is the distance to the spectral image i.
From Newton's formula, (+d+l)×(-d/n-l)= ² , that is, ^d2 +{(n+1)l-(n-1)}d+ ^nl2 =0. The magnification β at this time is β=/+d+l.

The spectral image I′ formed in this way is
Further, the second plano-convex positive lens _L2 produces a 1/β-times spectral image I without changing the imaging position in the optical axis direction. Therefore, for the composite system of both lenses L ₁ and L _{2 ,} the spectrum composite magnification γ becomes γ = 1, and the initial spectral image by the first concave diffraction grating G ₁ is affected by the effects of both lenses L ₁ and L ₂ . It matches the spectral image after

With this configuration, as shown in Fig. 2, the two concave diffraction gratings G ₁ and G ₂ , the input slit S ₁ , the exit slit S ₃ , and the double plano-convex positive lenses L ₁ and L ₂ are connected to the intermediate slit S ₂ , that is, the spectral image. It is completely symmetrical with respect to. Therefore, the light beam diffracted by the first concave diffraction grating _G1 is transmitted to the second concave diffraction grating having the same size.
All are accepted by G ₂ and are affected by inverse dispersion,
Completely zero dispersion without vignetting is achieved. Moreover, due to the above symmetry, the exit slit
The light beam emerging from S ₃ has the same directivity for all wavelengths as the light beam entering the entrance slit S ₁ .

In this embodiment, plano-convex positive lenses are arranged on both sides of the intermediate slit, but the invention is not limited to this as long as it is a positive lens. It is sufficient to have a configuration that satisfies that the two diffraction gratings are conjugate with each other and that the composite magnification γ of the spectral images is γ=1, and both positive lenses do not necessarily have to be equal.

Furthermore, although a concave diffraction grating is used in the above embodiment, the present invention is equally effective in a configuration consisting of a combination of a plane diffraction grating and a concave mirror, which have been known for a long time. In this case as well, two positive lenses are placed on both sides of the intermediate slit, and the spectral image is made equal in size by the synthesis system as in the above embodiment. In this case, two concave mirrors and two positive lenses are interposed in the optical path between the two plane diffraction gratings, but the two plane diffraction gratings are conjugated with respect to the composite system of these four. By doing so, the optical system becomes symmetrical with respect to the intermediate slit _S2 , as in the above embodiment.

As described above, according to the present invention, an excellent zero-dispersion spectrometer without vignetting and without wavelength-based directivity despite its compact configuration has been achieved. Furthermore, this zero-dispersion spectrometer can be used as a bandpass filter that can be arbitrarily selected over a wide wavelength range, and its rise and fall are extremely sharp and have flat characteristics.

[Brief explanation of the drawing]

Fig. 1 is a schematic configuration of a conventional zero-dispersion double monochromator using a concave diffraction grating, Fig. 2 is a schematic configuration of an embodiment according to the present invention, Fig. 3 is a partially enlarged view of Fig. 2, and Fig. 4 is an explanatory diagram of FIG. 3. Explanation of symbols of main parts, S ₁ ... Entrance slit,
_L1 ...First plano-convex positive lens, _S2 ...Intermediate slit, _L2 ...Second plano-convex positive lens, _S3 ...Exit slit, _G1 ...First concave diffraction grating, G ₂ ...Second concave diffraction grating.

Claims (1)

[Claims] 1. A first diffraction grating for forming a spectral image of an incident light beam, a second diffraction grating for inversely dispersing the light beam of the spectral image, and a second diffraction grating provided at the position of the spectral image. In a zero-dispersion spectrometer having an intermediate slit, a first positive lens and a second positive lens are respectively disposed near both sides of the intermediate slit, and a first positive lens and a second positive lens are respectively arranged between the first diffraction grating and the second diffraction grating. Both the diffraction gratings are made conjugate with respect to a synthetic system including the intervening positive lenses, and the composite magnification γ of the spectral image by the positive lenses is set to γ=1, so that the spectral image by the first diffraction grating and the A zero-dispersion spectrometer characterized by having a configuration that matches a spectral image after being subjected to the action of a synthesis system including a bipositive lens.