A Multi-Pass Spectrometer

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Abstract
Optical aspects of a multi-pass spectrometer, suitable for the photoelectric measurement of low light energy, are described. In the optical systems of the multi-pass spectrometer presented here, light dispersed by a grating is not focused on a point by a collimating mirror but is made to incident on a plane mirror, arranged above the collimating mirror, and is reflected to the grating again as a second incidence. The dispersed light obtained after the second diffraction strikes another plane mirror under the collimating mirror and reflects to the grating as the third incidence, and so on. The dispersed light of the final diffraction is focused on an exit slit by the collimating mirror. With this optical system, spectral purity can be enhanced with a factor of 2N, where N is the number of plane mirrors, and the spectrometer has a better focusing, larger speed, and smaller stray components than other multi-pass systems. Employing this device to a photoelectric Raman spectrometer, it was possible to improve the resolution of the instrument and to use the sample tube of smaller volume than usual.

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