Broadband and high-resolution spectroscopy plays a significant role in many research fields such as atmospheric trace gas detection, industrial monitoring, precision measurement, and basic physics and chemistry. Large spectral bandwidth allows for the simultaneous detection of multiple species, which enables a single instrument to have many functions. However, detection techniques that can provide a pm-level spectral resolution over a wide bandwidth still need to be further studied. The virtually imaged phased array (VIPA) is a plane-parallel etalon, where the input beam is injected at an angle through an entrance window on the front face. The multiple reflections occur within the VIPA etalon. The emerging light interferes to make different frequencies exit at different angles. VIPA spectrometer is an orthogonal dispersion system composed of VIPA and grating and can achieve spectral coverage of tens of nm in a single frame and spectral resolution of pm. In the past years, the VIPA spectrometer has been widely applied in high-precision broadband spectral measurement. However, practical applications of VIPA spectrometer face the following problems. First, some algorithms that employ gas absorption to calibrate the VIPA spectrometer ignore the instrument lineshape function (ILS), and second, these algorithms are difficult to calibrate when weakly absorbed. Additionally, the adjustment structure of the VIPA spectrometer can still be improved. Our paper reports an improved near-infrared spectrometer based on the VIPA and presents the experimental details and performance evaluation. The broadband and high-resolution measurement technology of CO₂ in 1.43-1.45 μm is carried out by combining the supercontinuum source and multi-pass cell. The results verify the reliability of the system and the accuracy of the improved data processing algorithm.

The experimental system mainly consists of a supercontinuum laser, a Chernin muti-pass cell, and a VIPA spectrometer. The broadband light is collimated by the aspheric collimator. Then the emergent light is reflected eight times inside the gas cell and finally connects to the interface of the VIPA spectrometer by a single-mode fiber to acquire the CO₂ absorption spectrum. The experimental source is a supercontinuum laser with a spectral coverage of 0.47-2.4 μm. The Chernin cell is composed of five pieces of plano-concave mirrors with a radius of 0.5 m. To obtain CO₂ absorption of appropriate intensity and avoid absorption saturation, the mirror angle of the Chernin cell is adjusted to realize the reflection number of 8 and the optical path of 4 m. The VIPA spectrometer is made of high-strength hard aluminum alloy with dimensions of 400 mm×280 mm×120 mm. The main improvements of the spectrometer structure are as follows. The adjusting structure of the cylindrical lens and the collimator is combined to change the incident optical axis, and the off-axis aberrations of the VIPA spectrometer are reduced. The adjusting structure of the imaging lens is improved and the CCD leads to more compact spectrometer. Meanwhile, the grating rotation structure is added and the spectral coverage of the VIPA spectrometer is extended. The system employs pure N₂absorption as the background image (I0) and pure CO₂absorption as the signal image (I). The algorithm subtracts the dark image from each of the signal and background images and then adopts Eq. (10) to subtract the baseline to get the absorption image. Finally, the algorithm extracts the one-dimensional spectra according to the rules shown in Fig. 2 and realizes the absorption spectral inversion.

The fitting residual of the CO₂ absorption spectrum at 6971.0021 cm^-1 is 3×10^-3[Fig. 4(c)], which verifies the correctness of the improved algorithm with the spectral resolution of the VIPA spectrometer being 4.5 pm [Fig. 4(d)]. By generalizing unimodal fitting to multimodal fitting, the broadband theoretical absorption spectrum can be obtained by line-by-line integration [Fig. 5(a)]. The minimum fitting residual of the whole spectrum (1.43-1.45 μm) is 5.31×10^-1, proving that the developed VIPA spectrometer can be utilized for broadband and high-resolution spectral measurement of gases. The standard deviation (SD) of the baseline is 2.68×10^-1[Fig. 5(a)], and the detection limit of CO₂ molecules corresponding to the highest absorption peak of line intensity is 1.85×10^-1, which can be improved by increasing the optical path.

A high-resolution near-infrared VIPA spectrometer with a relatively simple structure, a spectral resolution of 4.5 pm, and a spectral coverage of 25 nm in a single frame is developed. Improving the adjustment structure of the VIPA spectrometer makes the spectrometer more compact, reduces the off-axis aberrations, and extends the actual spectral coverage of the VIPA spectrometer. In terms of the data processing algorithm, the extraction accuracy of weak signals is improved by adding image enhancement algorithms, and the accuracy of gas parameter inversion is improved by considering the ILS. Finally, the broadband and high-resolution measurement technology of CO₂ in 1.43-1.45 μm is carried out by combining the supercontinuum source and multi-pass cell. The fitting results of the single absorption peak at 6971.0021 cm^-1 verify the spectral resolution of the VIPA spectrometer. The accuracy and reliability of the VIPA spectrometer applied to the measurement of broadband and high-resolution gas absorption spectrum are verified by comparing the measured absorption spectrum with the theoretical absorption spectrum. In the future, the VIPA spectrometer combined with optical cavity can realize broadband and high-resolution spectral measurement of trace gases.