The preparation technology of high groove density gratings is generally complex and costly, while nanoimprint technology has the ability to prepare large quantities of high-precision micro-nano structures. Therefore, nanoimprint technology is used to study the replication process of grazing incidence high groove density gratings used in Littman-type external cavity semiconductor lasers (ECDL). A UV composite rolling technology is used to imprint the high groove density grating groove area on the ULE glass substrate, and the grating replication is completed after coating. Replicated grating performance was tested and characterized with high fidelity.
The immersion grating is one of the key components of the imaging spectrometer due to its high spectral resolution. The silicon immersion grating is conducive to the miniaturization and weight reduction of related satellite-borne instruments to reduce the launch cost of satellite payloads. This paper introduces the application of silicon immersion grating at home and abroad, its working principle and manufacturing methods. The symmetrical trapezoidal groove silicon immersion gratings with groove density of 200 lp/mm and 400 lp/mm were designed. The grating diffraction efficiency and polarization were analyzed by finite element analysis. The photoresist mask was fabricated by UV exposure and holographic exposure, and then the uniform arrays of V-shaped grating on a 2-inch Si substrate were achieved by reactive ion etching and wet etching. The measured results of groove parameters met the design requirements.
To obtain the fine characteristics of the atmospheric carbon dioxide absorption spectrum, the imaging spectrometer must have high spectral resolution, but the volume of the instrument will increase simultaneously which is not conducive to remote sensing detection on satellite platforms. To solve this problem, we used a silicon immersion grating as dispersion element that diffraction occurs inside the silicon grating medium, the dispersion and spectral resolution are increased by 3.4 times respect to the conventional grating, and the volume of the spectrometer can be reduced to 1/40 at most. Smile is introduced because of the dispersion difference of light between the non-principal section and the principal section, which affects the spectral fidelity, data retrieval and detection accuracy. We established a model to analyze the smile by the silicon immersion grating and used the distortion of off-axis lens to correct. To miniaturize the instrument, optical system adopted the Littrow structure that the dimension is 410 mm×130 mm×110 mm. The optical system works in the short-wave infrared carbon dioxide weak absorption band 1.594-1.624 μm with a spectral resolution of 0.08 nm, entrance slit length is 16 mm and F number is 2. The optimized MTF is greater than 0.87 at the Nyquist frequency of the detector, the RMS radius of the spot diagram is within the Airy disk, and the maximum smile is less than 0.1 m, which is less than 1% of a single detector pixel. The optical system has excellent imaging performance, meeting the requirements of atmospheric carbon dioxide detection.
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