Cheng-Hao Ko

Analytical modeling and tolerance analysis of a linear variable filter for spectral order sorting

This paper proposes an innovative method to overcome the low production rate of current linear variable filter (LVF) fabrication. During the fabrication process, a commercial coater is combined with a local mask on a substrate. The proposed analytical thin film thickness model, which is based on the geometry of the commercial coater, is developed to more effectively calculate the profiles of LVFs. Thickness tolerance, LVF zone width, thin film layer structure, transmission spectrum and the effects of variations in critical parameters of the coater are analyzed. Profile measurements demonstrate the efficacy of local mask theory in the prediction of evaporation profiles with a high degree of accuracy.

Theoretical modelling and experimental verification of gradient thickness function for thin film deposition of linear variable filter

This study proposes a theoretical model for a gradient thickness function for thin film deposition of a linear variable filter (LVF), which is used for order sorting for a diffraction grating. To achieve linear variable filtering, the thin film must be fabricated with a gradient of increasing thickness along the direction of wavelength dispersion. The thickness gradient is created using a local mask on top of the substrate during thin film deposition inside the evaporation chamber. A theoretical model and ray tracing simulation are developed to obtain the thin film thickness distribution and the parameters for the fabrication of LVFs. Within the 25%–75% thickness range, the profile distribution exhibits a high degree of linearity (coefficient of determinations are greater than 0.9939). This study shows that the proposed theoretical model and ray tracing predict a thin film profile for an LVF with high precision (root–mean–square deviations are less than 4.52%).

Two-dimensional analytical modeling of a linear variable filter for spectral order sorting.

A two-dimensional thin film thickness model based on the geometry of a commercial coater which can calculate more effectively the profiles of linear variable filters (LVFs) has been developed. This is done by isolating the substrate plane as an independent coordinate (local coordinate), while the rotation and translation matrices are used to establish the coordinate transformation and combine the characteristic vector with the step function to build a borderline which can conclude whether the local mask will block the deposition or not. The height of the local mask has been increased up to 40 mm in the proposed model, and two-dimensional simulations are developed to obtain a thin film profile deposition on the substrate inside the evaporation chamber to achieve the specific request of producing a LVF zone width in a more economical way than previously reported [Opt. Express23, 5102 (2015)OPEXFF1094-408710.1364/OE.23.005102].

Manufacturing of a Linear Variable Filter for Spectral Order Sorting

An analytical thin film thickness model based on the geometry of a commercial vacuum coating system is proposed. This model can calculate the profiles of linear variable filters (LVFs), which are used to eliminate overlapping orders of spectra due to the use of a diffraction grating and which are fabricated using a local mask, producing a linearly variable thickness. While the filter transmits the first-order wavelength and blocks the second-order wavelength. The 75% and 25% relative thicknesses deviation between the evaporated film and the theoretical model is less than 5%, indicating good suitability for LVF design and fabrication.

The hyperspectral imaging achieved by aberration-corrected freeform concave blaze grating with variable line spacing

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.

Design and verification of an aberration-corrected free-form concave blaze grating with variable line spacing for hyperspectral imaging (Conference Presentation)

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.

Design and verification of a flat-field aberration-corrected concave blaze grating for hyperspectral imaging

An optimization process to control the aberration of a concave grating is developed. The approach has a dramatic reduction in aberration and great improvement in spectral resolution.

Two-dimensional analytical modeling of a linear variable filter for spectral order sorting

A two-dimensional thin film thickness model based on the geometry of a commercial coater which can calculate more effectively the profiles of linear variable filters (LVFs) has been developed. This is done by isolating the substrate plane as an independent coordinate (local coordinate), while the rotation and translation matrices are used to establish the coordinate transformation and combine the characteristic vector with the step function to build a borderline which can conclude whether the local mask will block the deposition or not. The height of the local mask has been increased up to 40 mm in the proposed model, and two-dimensional simulations are developed to obtain a thin film profile deposition on the substrate inside the evaporation chamber to achieve the specific request of producing a LVF zone width in a more economical way than previously reported.

Analytical Modeling of a Linear Variable Filter for Computational Hyperspectral Imaging

An analytical thin film thickness model calculates the profiles of linear variable filters, which perform spectral filtering. Coupled with an image sensor and using a computational algorithm, this device becomes a LVF-hyperspectral imager.

Aberration Control of a Concave Grating for Hyperspectral Imager

An optimization process to control the aberration of a concave grating is developed. The approach has a dramatic improvement in aberration reduction and spectral resolution. Calculated result is in good agreement with measurement.