JPS6318687B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a diffraction grating spectrometer having a double monochromator configuration.

In a monochromator that uses a concave diffraction grating and performs wavelength scanning only by rotating the grating, aberrations in the dispersion direction and the length direction of the slit (direction perpendicular to the dispersion direction) can be significantly reduced by using a holographic diffraction grating, etc. Although it is well removed, there is still astigmatism, and the wavelength resolution is usually on the order of several nm. This monochromator is of the meridional type, meaning that in the wavelength dispersion direction, the focal length is long near the center of the wavelength scanning range and becomes short at both ends, and conversely, in the sagittal direction, the focal length is short near the center of the wavelength scanning range and becomes long at both ends. ing.

The present invention was made for the purpose of improving the resolution of a spectroscopic device that performs wavelength scanning by rotating a diffraction grating. Conventionally, a double monochromator configuration has been employed as a means of measuring the reduction in the level of stray light. The present invention also employs a double monochromator configuration, but is intended to improve resolution with a particular emphasis on correction of astigmatism.

Astigmatism is a directional aberration, so if optical systems with the same astigmatism are arranged orthogonally with respect to the directionality of the astigmatism, the astigmatism of both will cancel each other out and correct the astigmatism. Ru. The present invention is based on this principle, and constitutes a double monochromator by combining two monochromators of the same structure using concave diffraction gratings so that their wavelength scanning directions are orthogonal. The present invention will be explained below with reference to Examples.

FIG. 1 shows a spectroscopic device according to an embodiment of the present invention. G
1 is the first diffraction grating, G2 is the second diffraction grating, and both have the same specifications, but as is clear from the figure, the direction of the grating lines is horizontal for the first diffraction grating, and vertical for the second diffraction grating. . These gratings are rotated in conjunction with each other about axes x1 and x2. FIG. 2 shows the interlocking mechanism, and the explanation will be given later. SI is the entrance slit, ST is the intermediate slit, and SO is the exit slit, and the positions of these three slits are fixed. The slits SI, ST, and SO all have the same length (vertical), the middle slit has the widest width, the input slit SI has a relatively narrow slit width, and the output slit SO has the narrowest width, which is the desired width. The slit width is variable depending on the resolution.

Regarding the first diffraction grating G1 in the above configuration, both the entrance slit SI and the intermediate slit ST are long in the wavelength scanning direction. Therefore, the diffraction image of the slit SI on the slit ST plane by the first diffraction grating G1 is a state in which the diffraction images of light of various wavelengths are arranged with slight deviations in the length direction, that is, in the vertical direction in the figure, as shown in Figure 3. It's getting old. intermediate slit
ST extracts the range ι from such a diffraction image and makes it incident on the second diffraction grating G2. Here, ι is the length of the intermediate slit ST. For this reason, a certain amount of light with wavelengths other than the target wavelength also enters the second diffraction grating, but this does not pose a problem because the second diffraction grating separates the unnecessary light. Considering the brightness, according to the above configuration, substantially all of the light of the target wavelength that passes through the input slit SI is sent to the second diffraction grating G2. If the input slit SI is made long in the direction perpendicular to the wavelength scanning direction of the first diffraction grating G1 (horizontal in the figure), the intermediate slit SI
Diffraction images of light of various wavelengths on ST are lined up in the image width direction as shown in Figure 4, and the intermediate slit ST extracts light in the dotted line range from such images. Same as the configuration, light of the target wavelength is only visible in the shaded area in a partial diagram of the diffraction image of the light of the wavelength, and the light transmittance of the first spectrometer is very poor. Therefore, if the intermediate slit ST is also lengthened in the horizontal direction, the wavelength selectivity and light transmittance of the first spectrometer will improve, but as for the second spectrometer, the length direction of the entrance slit will be parallel to the wavelength scanning direction, so A single wavelength diffraction image is formed long in the direction perpendicular to the exit slit SO,
The light transmittance of the second spectrometer becomes low, and in terms of wavelength resolution, the second spectrometer does not have wavelength resolution ability, so the double monochromator configuration becomes meaningless. Therefore, the most rational configuration of the present invention is to make all the slits long in the shape perpendicular to the wavelength dispersion direction of the second spectrometer (second diffraction grating G2).

Next, we will discuss astigmatism. 1st diffraction grating G
As mentioned earlier, the light beam diffracted at 1 is an astigmatic light beam whose focal length is short at both ends of the wavelength scanning range in the meridional direction, long at the center, and reversed in the sagittal direction. Since the dispersion direction of the second diffraction grating is perpendicular to that of the first diffraction grating,
The sagittal direction of the first diffraction grating is the meridional direction of the second diffraction grating, and in that direction, the incident light beam is emitted from a point closer to the center of the second grating G2 at both ends of the wavelength scanning range, and the meridional direction is In this direction, the tendency for the focal point to become short at both ends of the scanning wavelength range is canceled out. The same kind of relationship exists in the center of the scanning wavelength range and in the sagittal direction of the second grating, and astigmatism is significantly improved overall.

A holographic concave diffraction grating has a spectrometer configuration in which wavelength scanning is performed by rotating a grating with fixed input and output slits, which can minimize aberrations in the optical axis direction by evenly dividing them into positive and negative parts over the entire wavelength scanning range. However, the slit cannot be made longer because of astigmatism.
According to the present invention, since astigmatism is further corrected, the resolution does not decrease even though long input/exit slits are used, and the double monochromator configuration improves the resolution, making it possible to obtain a bright, high-resolution spectroscopic device.

The specifications of each part of the embodiment configuration shown in FIG. 1 will be described. 1st,
The second diffraction gratings G1 and G2 have the same specifications, are concave spherical surfaces, have a height and width of 50 mm, and a radius of curvature of 150 mm.The holographic grating formation conditions are as follows: As, Position of the first recording light source C γ = 0.445085° γc = 150.327640mm Position of the second recording light source D δ = 23.904170° γd = 149.697650mm Wavelength used 441.6 nm Incident slit relative to the center of the first diffraction grating G1
The angle between the SI and the intermediate slit ST is 21.6 degrees, which is the same as the angle between the intermediate slit ST and the output slit SO with respect to the center of the second diffraction grating G2. γ2 = γ2' = 149.84mm.

Each slit has a length of 8 mm.

FIG. 6 specifically shows the effect of the present invention.
Figure B shows the intermediate slits in Figure 1 except for SI and G1.
The performance of a single monochromator in which ST is an input slit, SO is an output slit, and the second diffraction grating G2 is a dispersion element is shown. Figure A shows the performance of the embodiment shown in Figure 1. 750nm
The intensity of the three types of monochromatic light in the dispersion direction on the output slit SO is shown, and it can be seen that the present invention improves the dispersion power by about twice as much as that of a conventional single monochromator.

Finally, as shown in Figure 2, the first and second diffraction gratings G
1. The interlocking mechanism of G2 will be explained. In this figure, 1 is a wavelength feed screw common to both the first and second spectrometers and is driven by a pulse motor. Nuts 21 and 22 are screwed onto the feed screw 1, and the front surface of each nut is a plane perpendicular to the axis of the feed screw 1, and the rotation axis 31 of the first and second diffraction gratings G1 and G2 is 3
Arms 41 and 42 fixed perpendicularly to the nuts 21 and 2 are brought into contact with each other, and serve as sine bars.
22, the gratings G1 and G2 are simultaneously rotated by the same amount.

[Brief explanation of the drawing]

Fig. 1 is a perspective view of a double monochromator according to an embodiment of the present invention, Fig. 2 is an interlocking mechanism of the first and second diffraction gratings in the same embodiment, and Fig. 3 is a perspective view of the first diffraction grating in the above embodiment. Diagram showing the diffraction image, No. 4
The figure shows a diffraction image when the length direction of the incident slit of the first diffraction grating is changed. Figure 6 is a graph showing the effects of the present invention. G1...First diffraction grating, G2...Second diffraction grating, SI...Incidence slit, ST...Intermediate slit,
SO...Output slit, 1...Wavelength feed screw.

Claims (1)

[Claims] 1 A double monochromator configuration in which the positions of the entrance slit, intermediate slit, and exit slit are fixed, and the first and second diffraction gratings are rotated in conjunction to perform wavelength scanning, and a concave diffraction grating, etc. A spectroscopic device that uses a first diffraction grating and a second diffraction grating whose light dispersion directions are orthogonal to each other.