Optical design of a crossed double-grating spectrometer for extracting pure rotational Raman lines of N₂

In this paper, we introduce a spectrometer design with crossed double-grating for extracting pure rotational Raman (PRR) lines of N₂. The design applies a linear optical fibre array as the entrance slit and a linear array photomultiplier tube (LaPMT) as the detector. The linear dispersion and structural parameters of the proposed configuration were analysed theoretically, and the optical layout of the spectrometer was simulated using the Zemax software. A spectrometer with a 32-channel LaPMT was simulated to obtain the PRR spectrum of N₂ when the excitation wavelength was 532 nm. The Stokes and anti-Stokes Raman lines, with rotational quantum states from 0 (2) through 14 (16), were obtained on separate channels of the LaPMT, and the resolution was approximately 0.22 nm per channel.