Dual-channel Synchronous Calibration VIPA Spectrometer with Optical Waveguide Input

In the realm of observational astronomy, achieving high-precision spectral detection has become a crucial necessity, particularly for scientific endeavors such as studying terrestrial planets via radial velocity methods, probing cosmological variations in fundamental constants, and measuring the universe's expansion rate. This demand drives the advancement of spectrometers with high spectral resolution and high wavelength calibration accuracy. Over the past two decades, several high-resolution astronomical spectrometers tailored for high-precision radial velocity measurements have been developed worldwide. These spectrometers typically employ echelle gratings as the primary dispersion components, characterized by complex structures, large dimensions, stringent mechanical and thermal stability requirements, and considerable manufacturing and maintenance costs. Compared with the gratings, the Virtually Imaged Phased Array (VIPA), employing the side-entrance Fabry-Perot etalon geometry, features a simple and compact structure, ultra-high angular dispersion, minimal sensitivity to slit width variations, and ease of calibration when combined with laser frequency combs. These characteristics render it highly promising in astronomical spectrum detection. Consequently, research endeavors focused on VIPA spectrometers for astronomical applications have been initiated. A kind of VIPA spectrometer equipped with dual-channel optical fiber input and calibrated using a laser frequency comb was developed, and its long-term stability was investigated. However, significant relative shifts and poor synchronization of parallel optical fiber channels under environmental disturbances limit the calibration repeatability of this spectrometer. A substantial disparity persists between the dual-channel synchronous calibration accuracy and the photon noise limit. To mitigate the significant impact of relative shift between channels on synchronous calibration accuracy, this paper adopts a novel dual-channel optical waveguide input mode. Compared to the side-by-side optical fiber arrangement, the optical waveguide chip exhibits superior stability and reduced spatial position deviation attributed to the utilization of photolithography and reactive ion etching fabrication techniques. Additionally, the proximity between two optical waveguides is significantly closer than that of parallel optical fibers. When subjected to environmental disturbances, the relative displacement between waveguides is smaller compared to optical fibers, resulting in enhanced synchronicity. In principle, the VIPA spectrometer with optical waveguide input can achieve superior calibration synchronization. Hence, this study develops a VIPA spectrometer utilizing a dual-channel optical waveguide as the input port and examines the calibration shifts and dual-channel synchronous calibration accuracy of the VIPA spectrometer across diverse environmental conditions. Research findings demonstrate that under comparable experimental conditions, the VIPA spectrometer with optical waveguide input achieves superior dual-channel synchronous calibration accuracy compared to its counterpart with optical fiber input. This represents the highest dual-channel synchronous calibration accuracy attained by VIPA spectrometers to date. Furthermore, the stability performance of the VIPA spectrometer has not reached its optimal state under current experimental conditions. Employing an astronomical optical comb with higher repetition frequency and a flattened spectrum will improve the signal-to-noise ratio of the spectrum detected by the VIPA spectrometer, thereby leading to further improvements in the wavelength calibration accuracy. It is anticipated that the dual-channel synchronous calibration accuracy will surpass current levels significantly, thereby maximizing the advantages of optical waveguides as innovative input ports for spectrometers.