1. Field of the Invention
The present invention relates to a double pass monoch-omator having an improved wavelength resolution.
This application is based on patent number Hei 10-197881 and Hei 11-051944 filed in Japan, the contents of which are incorporated herein by reference.
2. Description of the Related Art
FIG. 11 shows an example of the structure of a conventional double pass monochromator. The double pass monochromator shown in FIG. 11 comprises a light source 1, an entrance slit 2A, an exit slit 2B, middle slits 2C, 2D, concave mirrors 3, a diffraction grating 4, lenses 60, a return reflection means 7, and a photo detector 8. Moreover, in this conventional example, a rectangular prism 50 is used as the return reflection means 7. In the following explanation, FIG. 11 shows two concave mirrors 3, but this is for convenience of explanation, and in fact one concave mirror is satisfactory.
In this double pass monochromator, the light emitted from the light source 1 is diffracted by the same diffraction grating 4 two times, and is incident on the photo detector 8 as output light.
First Diffraction
In this double pass monochromator, a wide wavelength band source incident light 1a emitted from light source 1 transits the entrance slit 2A to be incident on the concave mirror 3. After being reflected by the concave mirror 3, this light is incident on the diffraction grating 4 as first pass incident light 1b, and is diffracted by the diffraction grating 4. At this time, the first pass incident light 1b is reflected at different angles with respect to the direction perpendicular (x-axis) to the grating on the diffraction grating surface depending on wavelength. In addition, among this first pass incident light 1b, the particular wavelength component determined by the angle of rotation of the diffraction grating 4, having an axis of rotation on the axis (y-axis) parallel to the grating, is incident on the concave mirror 3 as first pass emitted light 1c. Thereby, the first pass emitted light 1c is reflected by the concave mirror 3, and the wavelength component that transits the middle slit 2C among the reflected light transits the lens 60, and then is incident on the return reflection means 7 as return reflection means incident light 1d.
Second Diffraction
The return reflection means emitted light 1e reflected by this return reflection means 7 transits the lens 60, then the light that transits the middle slit 2D is incident on the concave mirror 3. The light reflected by the concave mirror 3 is incident again on the diffraction grating 4 as second pass incident light 1f, and is diffracted. At this time, the second pass incident light 1f is reflected at a different angle with respect to the x-axis depending on wavelength, and the particular wavelength component determined by the angle of rotation of the diffraction grating 4 is incident on the concave mirror 3 as second pass emitted light 1g. Thereby, the second pass emitted light 1g is reflected by the concave mirror 3, and the wavelength component that transits the exit slit 2B among this reflected light is incident on the photo detector 8 as output light 1h. Thereby, light of a narrow wavelength band can be obtained.
In the conventional double pass monochromator, the return reflection means 7 sometimes is structured with a combination of planar mirrors in place of a rectangular prism. One example of this is shown in FIG. 15. Moreover, the double pass monochromator shown in FIG. 15 uses one concave mirror. The double pass monochromator shown in FIG. 15 comprises a light source 1, ar entrance slit 2A, an exit slit 2B, a concave mirror 3, a diffraction grating 4, a planar mirror 6B, a return reflection means 7 which comprises a middle slit 2C and planar mirrors 6C, 6D, and a photo detector 8.
In this double pass monochromator, a wide wavelength band source incident light 1a emitted from light source 1 transits the entrance slit 2A to be incident on the concave mirror 3. After being reflected by the concave mirror 3, this light is incident on the diffraction grating 4 as first pass incident light 1b and is diffracted by the diffraction grating 4. At this time, the first pass incident light 1b is reflected at different angles with respect to the direction perpendicular (x-axis) to the grating on the diffraction grating surface 4 depending on wavelength. In addition, among this first pass incident light 1b, the particular wavelength component determined by the angle of rotation of the diffraction grating 4, having an axis of rotation on the axis (y-axis) parallel to the grating, is incident on the concave mirror 3 as first pass emitted light 1c, is reflected by the concave mirror 3, and is incident on the return reflection means 7 as return reflection means incident light 1d. In this conventional example, the return reflection means 7 comprises two planar mirrors 6C, 6D and the middle slit 2C.
The return reflection means incident light 1d reflected by this return reflection means 7 is reflected by the planar mirror 6C in the return reflection means 7, then the light that transits the middle slit 2C, and is reflected again by the planar mirror 6D. In addition, this reflected light is incident on the concave mirror 3 as return reflection means emitted light 1e, the light reflected by the concave mirror 3 is incident again on the diffraction grating 4 as second pass incident light 1f, and is diffracted. At this time, the second pass incident light 1f is reflected at a different angle with respect to the x-axis depending on wavelength, and the particular wavelength component determined by the angle of rotation of the diffraction grating 4 is incident on the concave mirror 3 as second pass emitted light 1g. In addition, after being reflected by the concave mirror 3, it is reflected by the planar mirror 6B, and among this reflected light, only the wavelength component that transits the exit slit 2B is incident on the photo detector 8 as output light 1h.
In the above-described double pass monochromator, in the case of the double pass monochromator having the structure shown, for example, in FIG. 11, in the return reflection means incident light 1d, the dispersion direction X of the wavelength of light, which is the direction of the spreading of the wavelength dispersed by the diffraction grating 4, in FIG. 12, as indicated by a broken arrow, is perpendicular to the y direction, which is the longitudinal direction of the slit hole 2c of the middle slit 2C. That is, in the width direction S of the slit 2c, the wavelength band is dispersed from short wavelength light P (e.g. purple light) to long wavelength light R (e.g. red light). Moreover, in FIG. 12, the arrow showing the dispersion direction X is pointing in the direction from the short wavelength to the long wavelength. This return reflection means incident light 1d, as is shown in FIG. 13, is incident on the return reflection means 7, and after being reflected two times by the rectangular prism 50, is emitted as return reflection means emitted light 1e. At this time, the dispersion direction X of the wavelength of light is not changed by the process of transiting the rectangular prism 50.
In addition, in the double pass monochromator shown in FIG. 15 as well, the return reflection means incident light 1d is incident on this return reflection means 7, and after being reflected two times by planar mirrors 6C, 6D, the dispersion direction X of the light is not changed by the process of being emitted as return reflection means emitted light 1e. Therefore, in the case of separating the special components of light by the conventional double pass monochromator, the directions of dispersion X of the light of the first pass emitted light 1c and the second pass incident light 1f are the same. That is, the diffraction grating 4 diffracts the light in the same dispersion direction X two times, and due to this point, there is a limit on wavelength resolution.
In addition, in these conventional double pass monochromators, because of the distance between the two reflection points in the rectangular prism 50 or the two planar mirrors 6C, 6D, which comprise the return reflection means 7, when separating the spectral components of the light on the diffraction grating 4, the angle changes between the first pass incident and emitted light and the second pass incident and emitted light with respect to the grating surface of the diffraction grating. In this situation, because the structure of the light path from the light source 1 to the light receiver 8 is complicated, it is necessary to make the angle of these two passes of incident and emitted light with respect to the grating surface substantially approximately identical by sufficiently extending the focal distance of the optical system in the monochromator, in particular the focal distance of the concave mirror 3. Because of this, there is the problem that the body of the double pass monochromator becomes long.
In addition, in the double pass monochromator, in order to obtain a sufficient dynamic range by eliminating stray light, precise adjustment of the slit widths of a plurality of slits introduced into the light path and the setting position is necessary. However, in the conventional double pass monochromator, when the diffraction grating is rotated in order to select the diffraction wavelength, it is necessary to adjust the slit system introduced in each course of the light path at this time, and there is the problem that the adjustment is complicated.