We have proposed an analytical 2D model [1] of a thickness gradient function for thin film deposition of a LVF for order sorting of a diffraction grating using an evaporation chamber. The LVF was fabricated and its thickness profile was measured using a probe-type surface analyzer. This study proposes an innovative method for overcoming the low production rates currently associated with LVF fabrication.
Within the 25% - 75% thickness range, the profile distribution exhibits a high degree of linearity, with R2 are greater than or equal to 0.9914 for both the cases. The LVF zone width appears to be a linear function of the mask height h, with R2 are greater than or equal to 0.9982 for all the cases. This indicates that the thickness gradient function is a more accurate model for obtaining the thickness profile of an LVF than any other modeling mentioned in previous published results. This study demonstrates that the developed theoretical 2D model can be used to predict accurately the thin film profile of an LVF.
The effective zone width of the LVF is defined as a thickness range of 25% - 75%, which appears to have high degree of linearity as a function of the mask height, h (mask-to-substrate gap). Thus, these results also confirms that the linear variable area increases as the mask-to-substrate gap increases. Thin film layer structures are constructed to demonstrate the efficacy of the proposed LVF design concept. Transmission spectrum result (wavebands 400nm to 1000nm) for varying mask heights at different positions shows maximum number of wavebands with transmittance (>99.9%). Comparison of both the theoretical and the evaporating results matches satisfactorily.
To design a concave grating for a hyperspectral imaging (HSI) system, it is critical to achieve flat field focusing in both the horizontal and vertical directions on the image sensor. We have developed a generalized automation aberration reduction procedure (ARP) that can be applied in any cases of a concave grating spectrometer.
The concave grating, which has a free-form profile with blaze grating pitch and variable line spacing [3], is fabricated using five-axis CNC machine with nanometer machining precision for hyperspectral imaging. In order to evaluate the performance, an optical system is designed and setup to measure the focused spot size, spectral resolution and diffraction efficiency.
A flat-field aberration corrected concave blaze grating for 400-1100nm is designed and fabricated. The concave grating, which has a free-form profile with blaze grating pitch and variable line spacing, is fabricated using five-axis CNC machine with nanometer machining precision. An optical system is setup to measure the focused spot size, spectral resolution and diffraction efficiency to evaluate the performance of aberration-corrected concave grating. The blaze grating reaches a diffraction efficiency of 70%. The focused vertical spot size is 50µm, which indicates a 50µm spatial resolution at the image plane. The focused horizontal spot size is 300µm, which converts to a spectral resolution of 6nm. The design methodology can be applied to an Offner type hyperspectral imager with a free-form convex grating and variable line spacing to achieve high efficiency and high spatial and high spectral resolving power.
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