Study on the immersion grating with dielectric film

Abstract

Imaging spectrometer is an important optical instrument for monitoring the content of carbon dioxide (CO2) in the atmosphere. Using an immersion grating as the dispersive element in the spectrometer can significantly reduce the size of the grating while maintaining the same spectral resolution. The immersion grating with dielectric film is one of the key elements in greenhouse gases imaging spectrometer. In this paper, the quartz immersed grating was designed for weak CO₂ band. The rectangular groove structure with high refractive index dielectric film was designed in consideration of possible fabrication errors. In the 1.595 ~ 1.625 μm band, the duty cycle of the rectangular groove is 0.3 to 0.35, the thickness of TiO₂ is 110 to 125 nm, and the grating groove depth is 560 to 640 nm, the diffraction efficiency can reach more than 80%. When the duty cycle is 0.4~0.45, the TiO₂ thickness is 95~105 nm, and the grating groove depth is 570~700 nm, the diffraction efficiency can reach more than 80%. For the trapezoidal groove, when the duty cycle is 0.3~0.35, the bottom angle of the trapezoid is 82°~86°, the thickness of TiO₂ is 120~125 nm, and the grating groove depth is 565~620 nm, the diffraction efficiency can reach more than 80%. When the thickness of TiO₂ is 115~120 nm, the overall degree of polarization is less than 0.1.The immersed grating with the period of 697nm in a quartz substrate of 245 mm × 138mm will be fabricated by holographic lithography - ion beam etching in late 2021. The TiO₂ film will be deposited by atomic layer deposition.