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摘要
Objective An optical window with a bandpass frequency selective surface (FSS) can transmit microwave signals at the desired frequency band and shield out-of-band signals, while transmitting visible light. However, designing frequency selective surfaces often require parameter scanning and full-wave simulation to obtain structural parameters corresponding to the desired design goals. This process is complicated and computational, so it is necessary to simplify the design. In this paper, a method is proposed to quickly design narrow bandpass optical transparent frequency selective surfaces. The method combines the genetic algorithm and the equivalent circuit (EC) model, and can quickly and accurately output the structural parameters of optical transparent windows under the desired design goals, which is verified by full-wave simulation. The loss of transmission peaks is less than 1.5 dB at the desired frequency, the out-of-band suppression is larger than 20 dB, the quality factors of transmission peaks are larger than 26, the visible light transmittance is acceptable, and the structure is not sensitive to polarization angles. Methods In this paper, the structure of an optical transparent bandpass FSS is proposed to verify the effectiveness of the design method. The structure is composed of a glass layer with silver grids plated on its upper and lower surfaces, as shown in Fig. 1(a). The equivalent circuit models of the metal grid and the dielectric layer are given (Fig. 1(b)) to calculate the total scattering parameters. The design goals of the band-pass electromagnetic shielding optical window are: 1) the frequency of the transmission peak is close to the design goal; 2) the transmission peak is high; 3) the out-of-band suppression is big enough. Considering the above points, the fitness function is written as Eq. (15). The overall optimization process is shown in Fig. 2. It can be seen that the initial population is established according to the value range of the independent variable. And the adaptive function is determined by the equivalent circuit model and the design goal. Appropriate values are screened out through the adaptive function, and the structural parameters conforming to the convergence rule are output. Results and Discussions Through the optimization by genetic algorithm and the EC method, two sets of structural parameters corresponding to the goals of design I and design II are obtained (Table 1). Compare the curves under two sets of structural parameters calculated by CST Studio software and equivalent circuit model (Fig. 3 and Table 2), it can be seen that the transmission peaks of design-I and design-II are close to the design goals, achieving a narrow passband and high out-of-band suppression. The full-wave simulation results show that the frequencies of transmission peaks are located at 11. 86 GHz and 12. 95 GHz, and the transmissivity losses are 1. 38 dB and 1.42 dB, respectively. Besides, the out-of-band shielding is greater than 20 dB, and the quality factors of transmission peaks are 26.91 and 28.13, respectively. Moreover, the accuracy of the EC relative to the full-wave simulation when calculating the transmissivities of design- I and design- II at the oblique incidence angle of 0°-60° for TE or TM polarization, is calculated by formula (17), and the results are shown in Table 3. It can be seen from Table 3 that the accuracy of the EC model relative to the full-wave simulation is high. Finally, the transmissivities of design- I and design- II at 0°-45° polarization angle (Fig. 5) are simulated, and it can be seen that the structure is not sensitive to polarization angle. Conclusions In summary, this paper proposes an accurate and fast design method to design a narrow bandpass optical transparent FSS. Two structural parameters are optimized under the design goals by this method and are verified to be close to the design goals by full-wave simulation, with the losses of transmission peaks less than 1. 5 dB at the desired frequency, the out-of-band suppression larger than 20 dB, the quality factors of transmission peaks larger than 26, the visible light transmittance acceptable, and the structure not sensitive to polarization angles. In addition to optimizing the structural parameters of the bandpass optical transparent FSS at the desired transmission frequency point, this design method can also be used to optimize design goals of different optical transmittances and dielectric materials, or a multi-layer FSS that consists of media and metal grids. Therefore, this method can effectively optimize the structural parameters of the FSS composed of dielectric and grid structures under desired design goals, greatly reduces the design time of the metal grid type FSS, and provides convenience for the design of the FSS.
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