# Three-Dimensional Broadband Electric Field Sensor Based on Integrated Lithium Niobate on Insulator
## 2 Design and Simulation  
Figure 1 expresses the diagrammatic drawing of the EFS for a single polarization direction. The sensor chip is fabricated on an x-cut LNOI, where the optical waveguide has an asymmetrical MI architecture (\(\Delta L=16\)\(\upmu\)m), and the light propagates along the y direction. An optical circulator is used to inject the continuous-wave (CW) light into the sensor chip and extract the modulated light. The operation principle of the EFS is described as follows. The CW light injected into the sensor chip is divided into two branches. In the lower branch, a pair of printed inclined tapered dipole antennas are placed on both sides of the straight waveguide, whose bottom width and height are 0.5 mm and 2 mm, respectively. The EM wave is received by the antennas and is then loaded onto the CW light to achieve phase modulation via a pair of modulation electrodes with a length of 5 mm, as shown in Figure 1. The phase-modulated light is reflected by a high-reflective film at the end of the chip and experiences second-round modulation in reverse propagation. In the upper branch, the light is directly reflected by the high-reflective film at the end of the chip. Hence, the output light from the chip is intensity-modulated light, where the intensity envelope carries the information of the EM wave and can be demodulated through photoelectric detection. Benefiting from the asymmetrical MI architecture, the direct-current bias control of the sensor chip can be achieved by tuning the wavelength of the CW light from the tunable laser source, which is favorable for realizing remote sensing without an electrical power supply. By setting the sensor chip to be at its quadrature transmission point, the electric field of the EM wave can be obtained without distortion. In addition, the modulation electrodes are placed as close as possible to the high-reflective film to reduce the traveling difference and the phase mismatch between the electric wave and optical wave, which is beneficial for realizing a large operational bandwidth. Figure 1: Diagrammatic drawing of the EFS based on LNOI for a single polarization direction. In order to achieve 3D sensing, three sensor chips are placed on the three faces of a triangular-prism-type clamp, as shown in Figure 2a. Conventionally, in order to ensure the polarization orthogonality of the antennas, the three sensor chips should be distributed along the principal axes of the rectangular coordinate system, which leads to a relatively large size. The proposed triangular prism package has a volume decrease of 30% for the identical sensor chips. To guarantee that the three pairs of antennas are mutually orthogonal, the tapered dipole antennas are designed to have an incline angle of 54.7\({}^{\circ}\) from the horizontal plane, as shown in Figure 2b. The polarization directions of the tapered dipole antennas are strongly relative to the direction of the antennas. Figure 2b shows the orthometric direction of the antennas, where the dashed lines represent the directions of the antennas on the other two sensor chips placed on the other two faces. The details of this geometrical proof can be found in [22]. The polarization simulation model of the inclined tapered dipole antennas is shown in Figure 3a, where \(\varphi\) is the degree versus to z-axis and is set to be 90\({}^{\circ}\). \(\theta_{1}\) and \(\theta_{2}\) are the degrees of the two tapered antennas versus to x-axis. The simulation result using the High-Frequency Structure Simulator (HFSS) software is shown in Figure 3b. The axial ratio at the line of the antennas is over 37.5 dB, which indicates that the inclined tapered dipole antennas have an absolute linear polarization. Although the tapered dipole antennas are mutually orthogonal, they have an inevitable interaction due to the close distance. The effect of the electric field reflection between the sensor chips in the proposed clamp is qualitatively revealed by carrying out a simulation via HFSS.