# The Design of a Long-Distance Signal Transmission System in a Bistatic RCS Testing System
## 2 System Principle and Simulation
### Selection of an Electro-Optic Modulator  
Optical modulators encode an electrical signal onto an optical wave, known as the optical carrier. This encoding can manipulate the amplitude, phase, frequency, polarization, or any combination of these properties of the optical carrier [25]. To meet the demand for long-distance and high-capacity service in modern optical transmission networks, high speed, compactness, and low power have become the mainstream development themes for future electro-optical modulators [26; 27]. Among the various electro-optical modulators, the Mach-Zehnder modulator has become one of the most widely used types in recent years due to its advantages: high speed, low cost, and low complexity [28]. The Mach-Zehnder modulator (MZM) is an interferometric device constructed from a material exhibiting a strong electro-optic effect (such as LiNbO\({}_{3}\), GaAs, InP). Lithium niobate (LiNbO\({}_{3}\))-based MZMs are commonly used for modulation in communication systems with a capacity of up to 40 Gb/s. However, their application is constrained by high drive voltages, a large physical footprint, and a lack of integrability [29]. In contrast, indium phosphide (InP)-based MZMs offer higher 3 dB bandwidths and require lower drive voltages [30]. But they are also more expensive than the former. The structure of the MZM is illustrated in Figure 4. The optical input \(E_{in}\) is divided between the upper and lower modulator arms, where phase modulation occurs via two phase shifters driven by the electrical signals \(V_{1},V_{2}\). The modulated signals are then recombined to produce the optical output \(E_{out}\). Figure 3: Schematic diagram of the signal transmission system. Figure 2: Schematic diagram of the bistatic testing system. The MZM has three operating modes: Maximum Transmission Point (MATP), Quadrature Point (QTP), and Minimum Transmission Point (MITP). By setting an appropriate DC bias voltage, the operating mode of the MZM can be selected. When the bias voltage is set to 0, the MZM is in the MATP mode, and the output optical intensity is at its maximum. In this mode, since the bias voltage is 0, the refractive indices of the upper and lower arms do not change, and the optical signals in both arms are not modulated. When the bias voltage is set to half of the modulator's half-wave voltage, the MZM is in the QTP mode, and the output optical intensity is half of the maximum optical intensity. The externally applied voltage alters the refractive indices of the upper and lower arms, causing the optical signals in each arm to undergo varying levels of phase modulation. This process induces a phase shift between the optical signals in the two arms, resulting in a coherent superposition within the optical coupler. When the bias voltage is set to the modulator's half-wave voltage, the MZM is in the MITP mode, and the output optical intensity is 0. In this mode, when the DC bias voltage is equal to the MZM's half-wave voltage, the optical amplitudes in the upper and lower arms cancel each other out in the optical coupler, resulting in an output optical field of 0. As the MZM is one of the more mature technologies among the various optoelectronic modulation methods, with relatively simple principles and moderate hardware costs, we employed an MZM operating in single-drive push-pull mode, which modulates the signal in terms of amplitude. In the push-pull-mode MZM, there is only one radio frequency (RF) electrode and one direct current (DC) electrode, as depicted in Figure 5. Two RF signals of equal amplitude and opposite phases are applied to the upper and lower arms of the MZM. The MZM operates in the QTP state to avoid the need for carrier and phase recovery [31]. In this state, only the intensity modulation of the input optical signal occurs.