Abstract
This paper establishes the modulation transfer function (MTF) and signal-to-noise ratio (SNR) characterization models of the infrared array-aperture diffractive optical system based on the diffraction imaging mechanism. Subsequently, the imaging system diffraction efficiency is calculated based on the three-dimensional Finite Difference Time Domain (FDTD) method and the imaging characteristics are represented by combining the MTF and SNR. Finally, the effects of different working wavelengths, field of views and filling factors of the primary lens imaging characteristics are analyzed. The analysis results show that the diffraction efficiency, the MTF and SNR of infrared array-aperture diffractive optical system all have spectral and spatial variation characteristics, which reduce with the decrease of the primary lens filling factor. When the filling factor is 0.6, the integral area of MTF decreases by 45.42% and the SNR decreases by 4.92 dB compared with the ideal full aperture system. The established model can be used to characterize the imaging quality of infrared array-aperture diffractive optical system and provide reference to the imaging system design.
The high-resolution GEO satellites realize continuous observation of specific areas, which are of great significance in disaster relief, resource exploration and military reconnaissance. GEO satellites put forward new requirements for space optical imaging technology, such as lightweight and foldable. Some new optical imaging technologies arise accordingl
Some researches have been conducted on the array-aperture diffractive optical system. Lawrence Livermore National Laboratory(LLNL) launched the Eyeglass project in 199
However, compared with the traditional refractive/reflective full-aperture system, the array-aperture primary lens decreases the modulation transfer function (MTF) and the focal plane energ
In this paper, we build the MTF and signal-to-noise ratio (SNR) models to express the spatial frequency modulation and energy transmission characteristics separately by considering the comprehensive influence of array-aperture structure and diffraction imaging. Furthermore, we present a fast method to solve the diffraction efficiency based on Finite Difference Time Domain (FDTD) according to the subwavelength micro structure of the array-aperture primary lens surface. This work can be applied to analyze the imaging quality and provide reference to the design of the infrared array-aperture diffractive imaging system.
The imaging mechanism of infrared array-aperture diffractive optical system is shown in

Fig. 1 Mechanism of degradation
图1 成像机理
The primary lens of the system is constituted with array-aperture Fresnel diffractive lenses, which modulate incident light through surface microstructure to achieve imaging. We analyze the light field distribution in the focal plane by calculating the diffraction efficiency, which is the important index to characterize the imaging quality of the diffraction system. The diffraction efficiency is defined as the ratio of the energy within the airy spot zero-order to the total incident energ
, | (1) |
where and are the energy distribution in the focal plane and incident plane respectively.
Nevertheless, due to the diffractive element microstructure and the total reflection, a part of incident light cannot converge to the focus limited by the deflection angle, resulting in the decline of diffraction efficiency. This phenomenon is the shadow effect in continuous phase structure, whose mechanism is shown in

Fig. 2 Shadow effects in continuous phase structure,(a) normal incidence, (b) oblique incidence
图2 连续相位结构中阴影效应,(a)正入射,(b)斜入射
The Shadow effect on the Fresnel lens multi-level micro structure with four steps is shown as

Fig. 3 Shadow effects in multi-level structure
图3 多台阶结构阴影效应
Based on the above analysis, the infrared array-aperture diffractive optical system has lower energy on image plane compared to the full aperture system. Simultaneously, the limit of diffractive microstructure for incident light makes the diffraction efficiency decreases gradually at the outer rings of the primary lens. In addition, the intermediate frequency information reduces due to the sparseness of array-aperture structure. These factors makes the imaging degradation characteristics more complex. From the perspective of spatial frequency modulation and energy transmission, we choose MTF and SNR to characterize the quality degradation of images, and establish the coupled degradation model of the infrared array-aperture diffractive optical system.
The entrance pupil of primary lens contains several identical circular apertures arranged in certain structure instead of whole connected domain. Typical infrared array-aperture structures include Golay3, Ring6 and Tri-arm7. The generalized pupil function of primary lens can be expressed as:
, | (2) |
where is circular function, d is the diameter of the sub-aperture, , N are the center coordinate and number of sub-apertures separately.
According to the scalar diffraction theory, the Point Spread Function (PSF) and the MTF of the primary lens can be obtained with
, | (3) |
, | (4) |
where represents the Fourier transform function, and is the modulus operator. By combining
, | (5) |
where and represent the MTF of the primary lens and array apertures respectively, is the relative coordinates of n-th pair of array apertures, is the distance from exit pupil to image plane, are spatial frequency, is the radial coordinate, , and is the cut-off frequency of array apertures.
The decreases when the array apertures get sparse. Meanwhile, the relative coordinates of array apertures increase, which makes the secondary peak of move laterally and the decrease. We define the filling factor to represent the sparsity of array apertures, which can be expressed a
, | (6) |
where D is the equivalent aperture of the infrared array-aperture primary lens, which is defined as the maximum circumscribed circle diameter of each aperture.
, | (7) |
, | (8) |
where and is the pupil function and transmittance function of designated light respectively, is the integral diffraction efficiency, which can be calculated by
The SNR is an important index in the imaging performance evaluation of the optical system, which can represent the energy utilization efficiency. It can be defined a
, | (9) |
where var(g) and are variance of signal and noise respectively. The
, | (10) |
where G is the gain, ,,, and represent the noise of dark current, the readout noise, the mode noise, the amplifier noise and the quantization noise separately. Particularly, is the grainy noise, which is related to the total energy of incident light.
Subsequently, we analyze the energy in the focal plane of the infrared array-aperture diffractive optical system. The illuminance E of the radiation source on the image plane ds' can be expressed as:
, | (11) |
where Φ, Φ’ are the luminous flux on the object and image plane separately, K is the transmittance, L is the radiation brightness, ds is the radiation source on object plane, Ω is the solid angle of entrance pupil. , which is defined as the lateral magnification of optical system, , S is the area of the entrance pupil and r is the object distance.
As for the infrared array-aperture diffractive optical system, the energy of non-designated order cannot be used in imaging.
, | (12) |
where is the integral diffraction efficiency, Si is the area of i-th sub-aperture, .
The effective energy Q1 received by detector on the focal plane of the infrared array-aperture diffractive optical system can be expressed as:
, | (13) |
where A is the pixel area, t is the integration time.
And the energy Q2 received by detector on the focal plane of the equivalent full-aperture optical system can be calculated as:
. | (14) |
When r1=r2:
. | (15) |
Assuming the detector output is a linear response to the incident light energy, the SNR of the infrared array-aperture diffractive optical system can be written as:
, | (16) |
where g0 is the output energy of the full-aperture optical system and SNRβ is the SNR of infrared array-aperture primary lens under the condition of 100% diffraction efficiency.
It can be seen from Sects. 1.1 and 1.2 that calculating the diffraction efficiency is the significant process of solving the MTF and SNR model. For the infrared array-aperture diffractive optical system, the size of the primary lens diffraction microstructure is in subwavelength level. The coupling effect of electromagnetic field components on the primary lens discontinuous microstructure surface enhances. In the circumstances, the vector diffraction theory should be applied for analysis instead of the scalar diffraction theory. For the non-rotational symmetry structure of the infrared array-aperture diffractive primary lens, we apply the Finite Difference Time Domain (FDTD) method to solve the diffraction efficiency based on the vector diffraction theory.
As shown in

Fig. 4 Schematic of FDTD analysis, (a) 3D schematic, (b) side view of schematic
图4 FDTD原理图, (a) 3D原理图,(b)侧视图
Limited by computing resource, the FDTD method can only acquire the light field distribution on the output plane near the primary lens. When the light field in computational space reaches the steady state, we apply the vector-based plane-wave spectrum (VPWS) method to obtain the light field distribution in the focal plane and compute the diffraction efficiency of primary len

Fig. 5 Flow to solve diffraction efficiency
图5 衍射效率计算流程
In this section, the proposed model and method are verified and analyzed through experiments. We consider a certain infrared array-aperture diffractive optical system to analyze the influences of wavelength, FOV and filling factor on diffraction efficiency. Further, we take the diffractive optical system design parameters as the input to analyze the variation of MTF and SNR at different wavelengths, FOV and filling factor based on the established analysis model. Finally, we verify the validity of model by comparing with the theoretical analysis results in Sect. 1, and provide suggestions for infrared array-aperture diffractive optical system design.
We select the Golay3 structure to design the primary lens in

Fig. 6 Infrared array-aperture structure and Airy spot profile with different filling factors, (a) infrared array-aperture structure, (b) focal plane energy distribution of different filling factors, (c) diffraction efficiency curve
图6 不同填充因子红外合成孔径结构及艾里光斑剖面, (a)红外合成孔径结构,(b)不同填充因子焦平面能量分布,(c)衍射效率变化曲线
Wavelength | Refractive index n | Focal length f | F number | Number of steps | Stitching structure | Fill factor | Base thick |
---|---|---|---|---|---|---|---|
900~980 nm | 1.5 | 10 mm | 1.43 | 4 | Golay3 | 0.4~0.6 | 0.45 µm |
In addition, in order to calculate the imaging system diffraction efficiency accurately and efficiently, we adopt the time harmonic plane wave with amplitude of 1 as the incident wave source, and set the space step and the thickness of PML layer as λ/20 and 8 grids respectively.
First, we carry out experiments to explore the influence of filling factor on the imaging characteristics in the infrared array-aperture diffractive optical system. The primary lens array-aperture structure and the central section of Airy spots with different filling factor are shown in
According to the MTF analysis model established in Sect. 1.1,

Fig. 7 MTF in the central FOV with different filling factors
图7 不同填充因子中心视场MTF
Meanwhile, according to the SNR characteristic analysis model established in Section 1.2, when β are 0.6, 0.5 and 0.4 respectively, the SNR of the system decreases by 4.92 dB, 6.85 dB and 9.63 dB compared with the ideal full aperture system by considering the particle noise simply. Ulteriorly, due to the existence of other noises such as dark current noise, the SNR will decrease more violently with the filling factor decreasing.
In order to further verify the correctness of the proposed model, we analyze the effects of different wavelength and FOV on the imaging characteristics.

Fig. 8 Focal plane energy distribution, (a) different incident wavelengths, (b) different FOVs
图8 焦平面能量分布, (a)不同入射波长,(b)不同视场
Meanwhile, we analyze the degradation characteristic of MTF at different incident wavelengths and FOVs in

Fig. 9 The degradation of MTF, (a) different incident wavelengths, (b) different Angle of incidence
图9 MTF变化曲线, (a)不同入射波长,(b)不同入射角
Wavelength/nm | MTF/(%) | SNR/dB | FOV/(°) | MTF/(%) | SNR/dB |
---|---|---|---|---|---|
900 | 45.42 | 4.92 | 0 | 45.42 | 4.92 |
950 | 50.45 | 6.02 | 3 | 48.70 | 5.14 |
980 | 60.64 | 8.18 | 5 | 53.17 | 5.76 |
Summarily, for the infrared array-aperture diffractive system, the center FOV has the optimal imaging quality. While with the deviations of wavelength and FOV, the diffraction efficiency, the MTF and SNR decrease gradually, which reflect the spectral and spatial variation characteristics in the image quality degradation. These results are consistent with the theoretical analysis in Section 1, which prove the correctness of the proposed analytical method. In addition, the infrared array-aperture diffractive optical system with the Ring6 and Tri-arm7 structure also have similar imaging characteristics. And the proposed method is applicable to the visible band similarly, while the medium-long wave infrared imaging is not applied to the diffractive imaging system generally for its wide band. Currently, the infrared array-aperture diffractive sensing imaging technology is still in the demonstration stage. The analytical method in this paper can support the optimization design and indicator demonstration of the imaging system, which needs to be further verified in future applications.
In this paper, in order to analyze the complex imaging characteristics of the infrared array-aperture diffractive optical system, we establish the MTF and SNR analysis models based on the imaging mechanism of primary lens array-aperture structure and diffraction imaging. Subsequently, considering the primary lens surface microstructure on the subwavelength order, we propose a fast method based on FDTD to solve the diffraction efficiency, and represent the imaging characteristics with the MTF and SNR. The experimental results show that the MTF, the SNR and the diffraction efficiency of the infrared array-aperture diffraction optical system have the characteristics of spectral and spatial variation. When the primary lens filling factor is 0.6, the SNR of the system decreases by 4.92 dB and the MTF area reduces by 45.42%. The correctness and effectiveness of the analytical model are verified by experiments, which can be used as significant references for the application of infrared array-aperture diffractive imaging technology.
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