# X-Band Photonic-Based Pulsed Radar Architecture with a High Range Resolution
## 2 Principle of the Proposed Architecture  
Figure 1 shows the proposed photonic-based pulsed radar architecture. In figures of this paper, the solid-arrow line and the dot-arrow line mean the electrical signal path and the optical signal path, respectively. As shown in Figure 1, the intermediate frequency (IF) signal from the arbitrary waveform generator (AWG) enters to the RF ports of a dual-parallel Mach-Zehnder modulator (DPMZM) through a 90\({}^{\circ}\) hybrid coupler. As studied in Reference [25], the frequency and bandwidth of the input IF signal with a 90\({}^{\circ}\) phase difference is quadrupled by a DPMZM under the proper bias conditions. The quadrupled output signal of a DPMZM is sent to a Mach-Zehnder modulator (MZM) to produce a pulse waveform, which can be able to operate the conventional FMCW radar as a pulsed radar. Figure 2 shows the signal waveforms for each node in the proposed architecture in the time domain. It is noted that (ii) and (iv) in Figure 2 is actually the optical domain signals but they are represented as the electrical domain signals to express clearly the procedure of the pulse signal generation. In Figure 2, the linear frequency modulation (LFM) signal, (i), from an AWG is quadrupled, (ii), by a DPMZM. By controlling the applied voltage of an MZM as represented at (iii), pulse waveform generation, (iv), can be performed using the quadrupled LFM signal applied to the MZM. As shown in (iv) of Figure 2, it is confirmed that the output signal of an MZM has both high frequency and broadband characteristics of the signal quadrupled by a DPMZM. In addition, since the pulse repetition frequency (PRF) of the pulse generated by the MZM is determined by the period of the applied signal by a gating signal generator (GSG) of the MZM, therefore, the PRF of the pulsed radar waveform can be changed  
arbitrarily for applying in the various circumstances. The gated LFM signal by the MZM is converted to the electrical signal through a photodetector (PD) and transmitted to space to detect a target by an antenna. The operation of the receiver is almost the same as the conventional photonic-based FMCW radar except the adopting a balanced photodetector (BPD). The reflected signal by the target is applied to the phase modulator (PM) with the output signal of a DPMZM, and the phase of output signal of a DPMZM is modulated by the reflected signal. The phase-modulated signal is applied to a dual optical band-pass filter (DOBF) through an optical coupler (OC) and the signal with a beat frequency is filtered. The output signal of DOBPF is passed to the BPD for eliminating common-mode noise and enhancing the amplitude of the signal. The BPD output signal is converted to a digital domain through the analog-to-digital converter (ADC) and processed to measure the positions of the target and visualize the target movements in real-time. For the theoretical analysis of the proposed architecture, the CW laser source, \(E_{\text{in}}\), in Figure 1 can be expressed mathematically as  
\[E_{\text{in}}=E_{\text{c}}\cos(\omega_{\text{c}}t), \tag{1}\]  
Figure 1: The proposed photonic-based pulsed radar architecture. AWG: Arbitrary waveform generator, GSG: Gating signal generator, CW laser: Continuous-wave laser, DPMZM: Dual-parallel Mach-Zehnder modulator, OC: Optical coupler, MZM: Mach-Zehnder Modulator, PD: Photodetector, CIR: Circulator, BPD: Balanced photodetector, DOBPF: Dual Optical Bandpass filter, PM: Phase modulator, ADC: Analog-to-digital converter, DSP: Digital signal processor. Figure 2: Signal waveforms for each node in the proposed architecture in the time domain. where \(E_{c}\) and \(\omega_{c}\) are the amplitude and angular frequency of the CW laser source, respectively. By an AWG, the LFM signal with the initial frequency of \(f_{0}\) with a bandwidth of \(kt\) is generated and sent to a \(90^{\circ}\) hybrid coupler to replicate two signals that have \(90^{\circ}\) phase difference.