Imaging spectrometers are widely used to obtain spectrally-resolved images during spectroscopy or hyperspectral imaging. Offner systems are widely used to improve imaging spectrometers’ performance due to their compact size and tiny aberration. Typically, Offner systems comprise two spherical mirrors with overlapping centers. When used in imaging spectrometers, the first mirror is replaced by a concave grating or the second mirror by a convex grating. However, most Offner systems use holographic concave gratings, which have recently become the main focus in manufacturing and research. Considering the importance of determining the primary aberration induced by concave gratings, convex-blazed gratings have been recently proposed. Unlike the holographic concave gratings, convex-blazed gratings produced by swing ion beam etching or single-point diamond turning technology exhibit several advantages including, improved signal-to-noise ratio and improved energy efficiency of the Offner imaging spectrometers.
So far, only two methods for the design and optimization of convex-blazed gratings have been reported. One method is based on scalar differentiation theory, while the other method involves calculating the diffraction efficiency by assuming the convex grating as a plane grating. Unfortunately, these two methods treat the process of designing a convex-blazed grating and the optical design of the Offner imaging spectrometer as two separate entities. Other methods such as rigorous coupled-wave analysis also have several limitations. Therefore, the available methods for the design and optimization of convex-blazed gratings are inadequate. Being an essential element in Offner systems, developing effective methods for manufacturing high-performance Offner imaging spectrometers is highly desirable.
To address the challenges mentioned above, a group of researchers from Changchun University of Science and Technology: Mr. Zhencong Xiong, Professor Wenjun He, Dr. Qi Wang, Professor Zhiying Liu, Professor Yuegang Fu and Mr. Dejie Kong developed a new design and optimization method for convex blazed grating in Offner imaging spectrometer. They aimed at promoting the application of the convex-blazed gratings in manufacturing high-performance Offner systems used in imaging spectrometers. Their work is currently published in the research journal, Applied Optics.
In their approach, the proposed method integrated both the micro- and macro-design of the optical system. It combined multiple algorithms such as the traditional image quality evaluation and geometric optics theory methods used to design the structure parameters of the Offner system and the three-dimensional polarization ray-tracing algorithm used to determine the polarization states and the incident angles. Additionally, microstructure parameter optimization via a combination of particle swarm optimization and coupled-wave analysis was used to improve the diffraction efficiency. Finally, the feasibility of the proposed design approach was validated through the design of a convex-blazed grating in a mid-wave infrared Offner imaging spectrometer.
Results demonstrated the high imaging quality and high diffraction efficiency of the designed Offner imaging spectrometer. In the spectral range 3 – 5 µm, the first diffraction order of the convex blazed grating recorded an average diffraction efficiency of 82.24%. Compared to the traditional design method, an improvement in the lowest spectral diffraction efficiency from 59.88% to 69.24%, highest spectral diffraction efficiency from 90.45% to 91.84% and a reduction o the standard deviation from 7.82 to 6.62 were reported.
In summary, a robust and feasible method for designing, simulating and optimizing a convex blazed grating used in the Offner imaging spectrometer was reported. The methods creatively integrated different optical design processes and algorithms, thereby resulting in the design of the Offner imaging spectrometer with improved imaging quality and diffraction efficiency than the traditional design methods. The proposed method also satisfied the technical requirement of the imaging spectrometer design as it enabled automatic design optimization of the nanoconfiguration. In a statement to Advances in Engineering, Professor Wenjun He explained the new design approach was effective and would provide more opportunities for manufacturing high-performance imaging spectrometers based on convex reflective diffraction gratings.
Xiong, Z., He, W., Wang, Q., Liu, Z., Fu, Y., & Kong, D. (2021). Design and optimization method of a convex blazed grating in the Offner imaging spectrometer. Applied Optics, 60(2), 383.