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Dun Liu, Wei Yang, Shibin Wu, et al. Effect of the number of primary lens level on the MTF of diffractive imaging system[J]. Opto-Electronic Engineering, 2017, 44(8): 786-790. doi: 10.3969/j.issn.1003-501X.2017.08.004
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Effect of the number of primary lens level on the MTF of diffractive imaging system
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Abstract
In order to study the effect of diffractive stray light on the modulation transfer function (MTF) caused by the binary phase-type Fresnel lens, the wave propagation method was used to simulate the propagation of light waves. The point spread function (PSF) of the system was obtained by coherent superposition of infinite diffractive orders, and the MTF was obtained by the Fourier transform of the PSF. The differences between the modified values and the theoretical design values were analyzed when the number of level was 2, 4 and 8 at the diffraction imaging system with an 80 mm Fresnel lens as primary lens. The results show that the effect of diffractive stray light on the MTF of the system decreases with the increase of the number of level. However, the deviation from the design value is less than 0.5% when the level is 4. Finally, we put forward the idea that the central region of the lens is processed into several levels and the edge part is 2-level to reduce the effect of diffractive stray light. The results show that the idea can achieve the purpose of suppressing effect of diffractive stray light. -
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References
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Overview
There is a contradiction between high resolution and light weight of large aperture spaced imaging system. So many research institutions have begun to explore new imaging methods. Diffractive lens through microstructure to modulate light waves can be fabricated on thin films with very low surface mass density. With the benefits of small size, light weight and loose surface tolerance, diffraction imaging system has become a great potential technical solution. Fresnel zone plate (FZP) and photon sieves are the most commonly used microstructures, while the low diffraction efficiency limits the applications of photon sieves. FZP fabricated by binary optics technology can achieve 40.5% diffraction efficiency when the level is 2, and 81% for 4 levels. On the other hand, the diffraction efficiency is a function of the ratio of the design wavelength to the illumination wavelength. Therefore, the non design orders diffractive light may affect the performance of diffractive imaging system and can't be ignored. In order to study the effect of diffractive stray light caused by non design orders on the modulation transfer function of diffractive imaging system, the wave propagation method was used to simulate the propagation of diffractive light waves. By coherent superposition of finite diffractive orders, we calculated the PSF at 17 signal wavelength which evenly covers the spectral range. The sum of these results is the polychromatic PSF. After the point spread function (PSF) of the system was obtained, the Fourier transform of the PSF was done to calculate the modulation transfer function (MTF). The differences between the modified values and the theoretical design values were analyzed when the number of level was 2, 4 and 8 at the diffraction imaging system with an 80 mm Fresnel lens as primary lens. The MTF decreased at low frequency with 2-level Fresnel primary lens and the biggest decrease was 6.6%. The deviation from the design value is less than 0.5% when the level is 4 and 8. The results show that the effect of diffractive stray light on the MTF of the system decreases with the increase of the number of level. Finally, we found that only the incident light illuminating the primary's central area can directly attach the image plane by non design diffractive orders. So, we put forward the idea that the central area of the diffractive primary lens is processed into 4 or 8 levels and the edge part is 2 levels to reduce the effect of diffractive stray light. The MTF increased apparently after optimized and was close to the design value. It shows that the idea can achieve the goal of suppressing the diffractive stray light.
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Dun Liu, Wei Yang, Shibin Wu, et al. Effect of the number of primary lens level on the MTF of diffractive imaging system[J]. Opto-Electronic Engineering, 2017, 44(8): 786-790. doi: 10.3969/j.issn.1003-501X.2017.08.004
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Figure 1.
The layout of the prototype.
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Figure 2.
The design MTF of the prototype.
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Figure 3.
The on-axis MTF of the prototype. (a) 2-level primary diffractive lens. (b) 4-level primary diffractive lens. (c) 8-level primary diffractive lens. (d) Optimized primary diffractive lens.
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Figure 4.
The -1 order diffraction of the primary lens.
