A Research on Filed-of-View (FOV) Widening and Thermal-Phase-Drift (TPD) Compensating Technology Applied in a Polarized Interference Imaging Spectrometer (PIIS)
1. National Astronomical Observatories/Nanjing Institute of Astronomical Optics & Technology, Chinese Academy of Sciences, Nanjing 210042, China 2. Key Laboratory of Astronomical Optics & Technology, Nanjing Institute of Astronomical Optics & Technology, Chinese Academy of Sciences, Nanjing 210042, China 3. School of Electronic Engineering and Photoelectric Technology, Nanjing University of Science & Technology, Nanjing 210094, China
Abstract：The Polarized Interference Imaging Spectrometer (PIIS), which is based on the theory of Fourier Transform Spectroscopy, is consisted of a series of birefringent crystals such as polarizers, a beam splitter as well as various lengths of birefringent crystals required to achieve large delays. The PIIS, compared with a traditional grating-based dispersion spectrometer, has various advantages of multiple-channel measurements, simultaneous information acquisition of both original images and fringes containing spectral details, large light flux, better light signal-to-noise ratio (SNR) as well as anti-vibration etc. Therefore, the PIIS has also been developed in a range of astronomy and astrophysics areas such as remote sensing, extrasolar planet radial velocity measurements, spacecraft design, lunar exploration etc. However, by reviewing of former works and references, two major drawbacks still remain in PIIS and need to be fixed. For one thing, the classic PIIS has a very limited field-of-view (FOV) around ±2°, which means the acquired fringes on the image plane will show quite strong non-linear distortion and hence degrade the accuracy of spectral reconstruction via Fourier transform. For another, the random thermal-phase-drift (TPD), given rise from both thermal expansion and birefringence variation caused by the environmental temperature fluctuation, has barely been studied before and will inevitably result in extra radial velocity error based on Doppler Spectroscopy. In this paper, a noble polarization interference imaging spectrometer with the emphasis on the FOV widening technology is introduced. This technology, using a compensated Savart plate containing a half-wave plate sandwiched between two orthogonally placed displacer plates as a compensated Savart plate, produces an angle-dependent phase shear to create parallel spatial interference fringes with a FOV around ±10°. This improvement not only enhances the accuracy of Fourier Transform algorithm but also increases input luminous flux and therefore even weak input spectrum detection and calibration results with high SNR can be fully accomplished. Also, a secondary set of birefringent plates (α-BBO and LiNbO3) with opposite thermal properties is proposed to passively diminish TPD caused by temperature fluctuation. The experiment shows that thermal-drift-phase error is perfectly restricted within 0.02 rad in the laboratory environment. As a consequence, this advanced PIIS is eligible to realize the fast and accurate measurement and calibration application in the field of large astronomical spectral instruments with ultra-high spectral resolution occasions such as Astronomical Frequency Comb.
