With the development of modern communication technology, hybrid optics/microwave communication systems have become an attractive research area, resulting in new requirements for installed beam splitters^[1,2]. A beam splitter divides electromagnetic waves into multiple beams according to different properties, such as amplitude^[3], polarization^[4], and frequency^[5,6]. Frequency beam splitters are suitable options for the hybrid optics/microwave communication system to separate optical signals and microwave signals. To ensure efficiency of the modern communication system, the beam splitter should not cause excessive insertion loss. Owing to some constraints in structural strength and surface flatness, in some practical applications, the beam splitter usually has a certain physical thickness, and its corresponding electrical thickness is of the order of several tens of millimeters. For the first beam splitting case, the metal mesh and transparent conductive films are usually adopted to transmit the optical signals and simultaneously reflect microwave signals based on their conductive characteristics on the premise of ensuring the sufficient optical transparency^[7–10]. However, for these materials, the relationship between the optical transmission and the conductivity is contradictory. For the metal mesh, the diffraction pattern generated by the periodic structure, which affects optical imaging or optical communication, tends to be more obvious when the shielding effectiveness is improved. For transparent conductive films, an improvement in conductivity often results in decreasing optical transmittance, and the optical passband cannot easily cover the long-wavelength infrared^[9,10]. For the other case, when reflecting optical signals while simultaneously transmitting microwave signals, the interference effect, which results from the optical thickness of the beam splitter having the same order of magnitude as the operating wavelength, may lead to a high insertion loss for the microwave transmission, particularly at oblique incidence under TE polarization. Therefore, some novel strategies must be explored to achieve a high-efficiency splitting effect without breaking up constraints such as physical thickness.

Metamaterials and metasurfaces have attracted much attention because of their flexible regulation of electromagnetic properties^[11–14]. Among them, frequency selective surface (FSS) has been extensively studied for decades because of their unique spatial filter characteristics^[15–20], and many excellent results have been reported, such as hybrid radomes^[20], band-stop filters^[21], circuit analog absorbers^[22–24], and polarization converters^[25–27]. Among the widespread applications, hybrid radomes are widely known because of their importance to military applications. Through rational design, radomes can introduce an effect of transparency to the design frequency band^[20]. To some extent, the design procedure of the hybrid radome can be considered an artificial modification of the transmission properties of the dielectric radome. It has a similar goal of improving the microwave transmittance of the beam splitters. The design of the optics/microwave beam splitter can benefit from it.

In this paper, we propose an optics/microwave beam splitter combined with an FSS structure, as shown in Fig. 1. First, a theoretical analysis was performed by calculating transfer matrix methods corresponding to different beam splitter structures to validate the feasibility of introducing FSS structures into beam splitters. Subsequently, a beam splitter with a physical thickness of 20 mm, whose substrate material is quartz glass, was designed and simulated. Finally, based on the design results, we have fabricated a sample and measured its optical reflectivity and microwave transmittance. The results were basically consistent with the simulation values. The FSS-based optics/microwave beam splitter is expected to play a key role in the hybrid optics/microwave communication systems.

In actual systems, to ensure structural strength, surface flatness, and other factors, the beam splitter is usually designed to have a certain thickness. Its corresponding electrical thickness is of the order of several tens of millimeters, which corresponds to the microwave operating wavelength. As shown in Fig. 2, this has an obvious interference effect, which leads to a decrease in the transmittance in the microwave working frequency band.

Equation (1) describes the relationship between the transmission peak positions and the substrate thickness under the multi-beam interference effect, and Eq. (2) is a variation of Eq. (1)^[28], $$2nh\cos \phi =m\lambda ,m=0,1,2,\dots ,$$(1)$$2nh\cos \phi =m\frac{c}{f},\mathrm{\Delta }f=\frac{c}{2nh\cos \phi }.$$(2)where $n$ and $h$ represent the refractive index and physical thickness of the substrate; $\phi$ and $\lambda$ are the incident angle and wavelength of the incident light; $c$ is the speed of light; and $f$ is the frequency. Reducing the electrical thickness of the beam splitter and adjusting the position of the transmission peak to the center frequency of the microwave signal is thus a direct method for increasing the microwave transmittance. However, due to mechanical constraints, the physical thickness and material category of the beam splitter in practical systems cannot usually be adjusted freely.

Figure 3 shows a schematic of the equivalent circuit corresponding to the beam splitter model. To improve the microwave transmittance, the beam splitter substrate was first divided into multiple blocks to reduce the electrical thickness of the sub-unit. They were then boned, as shown in Fig. 3(c).

According to the network theory, the dielectric substrate transfer matrix can be expressed as^[29]$$M_{\mathrm{TL}}=\left(\begin{array}{cc}\cos \theta & j{Z}_1\sin \theta \\ \frac{j}{Z_1}\sin \theta & \cos \theta \end{array}\right),$$(3)where $\theta =2\pi nh/\lambda$ is the phase shift generated by the microwave signal propagating through the substrate, and $Z_1$ is the characteristic impedance of the substrate^[29]. For oblique incidence, $Z_{\mathrm{TE}}=Z_1/\cos \phi$ for TE polarization, and $Z_{\mathrm{TM}}=Z_1\cos \phi$ for TM polarization^[30]. Because beam splitters are usually designed to work at specific angles, we omit the subscripts TE or TM for simplicity and still use $\theta$ to represent the phase shift at oblique incidence. As shown in Fig. 3(d), the total transfer matrix corresponding to Model c [Fig. 3(c)] can be expressed as $$\begin{gathered} M_{\text{total}}=M_{\mathrm{TL}}^{\alpha }{M}_{\text{bond}}{M}_{\mathrm{TL}}^{\beta } \\ =\left(\begin{array}{cc}\cos {\theta }_{\alpha }& j{Z}_1\sin {\theta }_{\alpha }\\ \frac{j}{Z_1}\sin {\theta }_{\alpha }& \cos {\theta }_{\alpha }\end{array}\right)\left(\begin{array}{cc}\cos {\theta }_2& j{Z}_1\sin {\theta }_2\\ \frac{j}{Z_2}\sin {\theta }_2& \cos {\theta }_2\end{array}\right)·\left(\begin{array}{cc}\cos {\theta }_{\beta }& j{Z}_1\sin {\theta }_{\beta }\\ \frac{j}{Z_1}\sin {\theta }_{\beta }& \cos {\theta }_{\beta }\end{array}\right). \end{gathered}$$(4)

Because the electrical thickness of the bonding layer is much smaller than the microwave wavelength, $\cos {\theta }_2\approx 1$, $\sin {\theta }_2\approx 0$, and its transfer matrix is approximate to the identity matrix. The total thickness of the substrates remains stable, ${\theta }_1={\theta }_{\alpha }+{\theta }_{\beta }$, so $M_{\text{total}}$ can be calculated as $$M_{\text{total}}\approx \left(\begin{array}{cc}\cos {\theta }_1& j{Z}_1\sin {\theta }_1\\ \frac{j}{Z_1}\sin \theta & \cos {\theta }_1\end{array}\right).$$(5)

After multi-layer bonding, the total transfer matrix remains almost unchanged and does not significantly impact the transmittance curve, as shown in Eq. (5). Therefore, simple multi-layer bonding cannot effectively improve the transmittance of the microwave operating frequency bands.

The equivalent circuit model corresponding to the addition of the FSS to the interface between the substrates is shown in Fig. 3(f), and its transfer matrix can be calculated as^[19]$$M_Y=\left(\begin{array}{cc}1& 0\\ Y& 1\end{array}\right),$$(6)$$\begin{gathered} M_t=M_{\mathrm{TL}}^{\alpha }{M}_Y{M}_{\mathrm{TL}}^{\beta } \\ =\left(\begin{array}{cc}\cos {\theta }_1+\frac{1}{2}j{Z}_{1}Y\sin {\theta }_1& j{Z}_1\sin {\theta }_1-Y{Z}_1^2\frac{1-\cos {\theta }_1}{2}\\ \frac{j}{Z_1}\sin {\theta }_1+Y\frac{1+\cos {\theta }_1}{2}& \cos {\theta }_1+\frac{1}{2}j{Z}_{1}Y\sin {\theta }_1\end{array}\right). \end{gathered}$$(7)

According to the matrix transformation relationship^[29], the corresponding $S_{12}$ and $S_{21}$ values were calculated as $$S_{12}=S_{21}=\frac{2}{j\left(Z_{1}Y+\frac{Z_1}{Z_C}+\frac{Z_C}{Z_1}\right)\sin {\theta }_1+[\frac{Y}{2}(\frac{Z_1^2}{Z_C}+Z_C)+2]\cos \theta {}_1+\frac{Y}{2}\left(Z_C-\frac{Z_1^2}{Z_C}\right)}.$$(8)

It can be seen from Eq. (8) that the microwave transmittance of the beam splitter is modulated by the equivalent admittance of the FSS added to the interface. Through the rational design of the FSS structure, that is, to regulate the frequency-response characteristics of the equivalent admittance of the FSS, the microwave transmittance of the beam splitter can be effectively improved. On the other hand, the design of FSS structure can be seen as the process of shaping the resonant curve, and a single periodic surface of slots provided with dielectric slabs of equal thickness and dielectric constants on each side produces a bandwidth that is considerably larger than that of a single layer of the FSS structure^[20]. Based on the above analysis, first, the beam splitter substrate was divided into two parts of equal thickness. Second, we added the FSS structure, which has wide passband characteristics, to the interface between the substrates. Finally, through parameter optimization, equivalent capacitance and inductance of the FSS were adjusted to satisfy the requirements of a wide passband.

According to this idea, a simulation analysis was performed using CST Microwave Studio software. Here, the microwave operating frequency band is 35–36.5 GHz. Owing to some constraints, the substrate material of the beam splitter is limited to quartz glass with a relative permittivity of 3.87 and a loss tangent of 0.001 in the Ka band. The EVA adhesive film was chosen as the bonding layer with a thickness of 40 µm and a relative permittivity of 2.45. Focusing on the design of the FSS structure, a simulation model in which the quartz glass was divided into two parts of equal thickness, and the FSS was located at the interface between them, as shown in Fig. 3(e), was established to perform the optimization calculation. The final parameters are shown in Fig. 4(a). In Fig. 4(b), the capacitance and inductance properties of the FSS structure are observed from the resonant current distribution at 35.75 GHz. Meanwhile, to verify the above perspective, the Model a [Fig. 3(a)] and Model c [Fig. 3(c)] were simulated separately using the same material parameters. The simulation results are shown in Fig. 4(c). Curve 1 corresponds to a single piece of quartz glass with a thickness of 20 mm, Curve 2 corresponds to the bonding of two pieces of 10-mm-thick quartz glass with a single-layer of EVA adhesive film, and Curve 3 corresponds to the bonding of two pieces of quartz glass with a single-layer of EVA adhesive film after the FSS was added to the interface. From Figs. 4(c) and 4(d), it is observed that the transmittance in the working frequency band significantly improved after the introduction of the FSS, and the transmittance in the entire working frequency band exceeded 90%. The simulation results demonstrate the effectiveness of this method for improving the transmittance in the microwave frequency band by adding an FSS to the interface.

The optical reflection mechanism of the proposed beam splitter results from a highly reflective film system deposited on the outer surface of the optical glass, as shown in Fig. 1. The optical reflection band is 450–900 nm and 7.7–10.5 µm. The ZnS/YbF₃ film stack was designed as the highly reflective film system according to the $\lambda /4$ regular membrane stack parallel design principle. In general, the total thickness of an optical film system is significantly less than the microwave wavelength^[31,32]. In the microwave band, the transfer matrix of the dielectric optical film is approximately equal to that of the identity matrix, and this can be expressed as follows: $$M_{\mathrm{opticalfilm}}=\left(\begin{array}{cc}\cos \theta & j{Z}_1\sin \theta \\ \frac{j}{Z_1}\sin \theta & \cos \theta \end{array}\right)\approx \left(\begin{array}{cc}1& 0\\ 0& 1\end{array}\right).$$(9)

Therefore, the microwave transmittance of the dielectric high-reflection film system on one side of the beam splitter remained unchanged. From the perspective of the theoretical analysis, the high-efficiency optics/microwave beam splitter introduced in this study is reasonable.

In order to verify the validity of the design method, the beam splitter sample was fabricated, and the fabrication process is illustrated in Fig. 5. First, the quartz glass substrate was divided into two equal parts with 10-mm equal thickness. Then, the FSS structure on the substrate surface was obtained using photolithography combined with vacuum evaporation. The metal target material is aluminum, and the deposition thickness is 0.4 µm. The FSS structural parameters were measured by utilizing the optical microscope (Zeta-300, Zeta), and the maximum structural deviation is less than 0.1 mm among the five randomly selected test positions. Subsequently, a ZnS/YbF₃ film stack was deposited on one side of the outer surface of the beam splitter to reflect the target optical signal. Finally, the sample was obtained by bonding the two processed substrates with a single-layer of EVA adhesive film.

Figure 6(a) shows a fabricated beam-splitter sample ($350\mathrm{mm}\times 210\mathrm{mm}$). Figure 6(b) illustrates the microwave measurement process. The microwave transmittance of the sample was measured over a frequency range of 26–40 GHz in an anechoic chamber using two gain horn antennas (GW2665, Gwave Technology) for transmission and reception in the far-field condition. The Agilent N5244A network analyzer was used to measure the sample transmission. The visible reflectance was measured using a UV-2500 spectrophotometer (LAMBDA 1050, PerkinElmer) in the wavelength ranges of 450–900 nm. The IR reflectance of the sample was measured using an FT-IR spectrometer (INVENIO-S, Bruker) at wavelengths of 7.7–10.5 µm. The microwave transmittance and optical reflectance were measured at an incidence angle of 45° under TE polarization. The measurement results are presented in Fig. 7.

Figure 7 shows that the measured results are consistent with the simulation values, verifying the validity of the proposed method. The results indicate that the minimum TE polarization transmittance between 35 and 36.5 GHz under oblique incidence angle of 45° is 86.43%, and the mean reflectance values at 450–900 nm and 7.7–10.5 µm exceed 96%. The fabricated beam splitter has good optical reflectance and microwave transmission.

This paper proposes a novel optics/microwave beam splitter design based on the frequency selective surface principle, which aims to meet the application requirements of transmitting microwave signals and reflecting optics signals. The ability of the FSS to shape a resonant curve forms the basis of this design idea. A theoretical analysis based on the transfer matrix method from network theory and simulation calculations were performed using CST Microwave Studio. The sample was fabricated according to the design results, and the measurement results are consistent with the design values. The minimum TE polarization transmittance between 35 and 36.5 GHz under oblique incidence angle of 45° is 86.43%, and the mean reflectance values at 450–900 nm and 7.7–10.5 µm exceed 96%. This idea is expected to contribute to the beam splitter design in hybrid optics/microwave communication systems.