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Recent Progresses in VIPA Spectroscopy
Update time: 2023-07-09
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Recently, the Laboratory of Astrophotonics in NIAOT has made some new progresses in VIPA (Virtually Imaged Phased Array) spectroscopy.
1: The team studied the dispersion characteristics of VIPA with multimode fiber input and found that the spectral resolution of the VIPA spectrograph is nearly not affected by the input fiber core diameter. 
2: A VIPA spectrograph with a high spectral resolution and a broad spectral range is designed and built. And a spectroscopic pipeline, extracting one-dimensional spectra from the raw exposures produced by such a spectrograph and calibrating them in wavelength and flux, is accomplished. 
3: The team developed a miniaturized, high-resolution, high-precision and broad-band VIPA spectrograph for the actual needs of the aerospace sector. 
Due to the influence of atmospheric turbulence, ground-based spectral observation generally requires the use of multimode optical fiber as input. In order to expand the application of VIPA spectroscopy in astronomical obervations, the team studied the dispersion characteristics of VIPA under multimode fiber input. The experimental results verify that the spectral resolution of the VIPA spectrograph is much less affected by the core diameters of the input fibers when compared with that of échelle spectrographs (as shown in Figure 1). This property gives VIPA an advantage over gratings in ground-based high-resolution spectral observations. The research results were published in the academic journal The Astronomical Journal on May 4, 2023. (https://iopscience.iop.org/article/10.3847/1538-3881/accc30/meta)
Fig. 1 The spectral resolution and the total transmission efficiency of the VIPA with different kinds of input fibers.
To obtain broad and high-resolution spectra, the aberration due to the large field of view in the spectral imaging system has been effectively compensated. Based on this, a VIPA spectrograph with the spectral resolution greater than 500,000, and the spectral range covering 1013~1125 nm is designed and built. The spectral information is shown in Figure 2. The team also completed the spectroscopic pipeline, including extraction of one-dimensional spectra from the raw exposures and calibration of the wavelength and flux.
Fig. 2 The linewidth accuracy of the VIPA spectrograph with spectral range of 112 nm.
Based on the above research, the team has developed a set of VIPA spectrograph (images shown in Figure 3) for the demand of the aerospace sector, with the following typical parameters: (1) the spectral resolution of 60k~87k; (2) the spectral range of 110 nm; (3) wavelength accuracy <1 pm (in laboratory environment, temperature fluctuation < 1℃); (4) dimensions of 35 cmx35 cmx15 cm. The instrument will be delivered shortly.
Fig 3 Images of the VIPA spectrograph
The team will carry out further research on VIPA spectroscopy, focusing on broad-band, high-resolution, high-precision and muti-objective spectral measurements. 
The above research is supported by the National Natural Science Foundation of China under Grant Nos. 11773045 and 11973009 and Key Project No. 11933005, and also supported by funds from CAS.
Nanjing Institute of Astronomical Optics & Technology ,National Astronomical Observatories ,CAS