# Phase Alignment of an Array Optical Telescope System Using Balanced Detection
## 3 Simulation Experiment Process
### Transmitter  
Mach Zehnder Modulator (MZM) excels at modulating light intensity and phase because of a characteristic of wavelength independence. It is capable of high-data-rate modulation above 40 Gbps, which has become the basis for many advanced optical modulation formats [4]. Half-bit-rate NRZ-DPSK wave pattern was generated through the first modulator. The NRZ code enters the second modulator to obtain full-bit-rate RZ-DPSK by adding half-bit-rate clock signal (5 GHz). The process is shown in Figure 6.  
According to the value of bias voltage \(V_{bias1}\) and \(V_{bias2}\), MZM usually works at three bias points: peak, zero and quadrature. Signals with different duty cycles are generated at different bias points [22; 23]. This is shown in Table 1. \begin{table}
\begin{tabular}{c c c} \hline
**MZ Operating Points** & \(V_{bias1}\)**\(-\)**\(V_{bias2}\)** & **Different Duty Cycles of RZ Signals** \\ \hline Peak & 0 & 33\% \\ Quadrature & 0.5\(V_{\pi}\) or 1.5\(V_{\pi}\) & 50\% \\ Null & \(V_{\pi}\) & 67\% \\ \hline \end{tabular}
\end{table} 
Table 1: Generation of RZ signals with different duty cycles. Figure 5: Simulation of an array optical telescope system to compensate phase using balanced detection. Figure 6: The generation process of RZ-DPSK optical signal. Compared with NRZ-DPSK and other RZ-DPSK modulation patterns, CSRZ-DPSK modulation pattern at the transmitting end, as RZ-DPSK signal with 67% duty cycle, has lower requirements on system signal-to-noise ratio(SNR) and better suppression effect on nonlinear effects [24; 25]. Therefore, the CSRZ-DPSK signal is generated by cascading LiNbO\({}_{3}\) MZM in this simulation. Finally, the output signal of the transmitter is observed by the observation instrument, and the basis is provided for the comparison and analysis of the receiver. Table 2 shows the parameter values. Figure 5 and Table 2 show that the center wavelength was 1550 nm, which is widely used in Satellite-ground coherent communication systems. Furthermore, the optical fiber loss is the lowest to integrate with the ground optical fiber communication network simply when the working wavelength is 1550 nm [26]. [PERSON] et al. verified that the turbulence tolerance is widened and the detection efficiency of the deviation exceeds 95% when the communication rate reaches 2.5 Gbps and 10 Gbps [18]. Additionally, [PERSON] proposed that when the transmission rate is 40 Gbps, the signal will be distorted due to the fiber characteristics such as nonlinear effect, dispersion and polarization mode effect [27]. Therefore, the bit rate was defined as 10 Gbps. The input terminal uses a pseudo-random binary sequence (PRBS) as a code containing user information. It was copied into three channels through the cross-module, and the first channel was kept in the format of binary sequence; the second channel was input to the NRZ pulse generator and was maintained in a rectangular pulse format; the third channel was used for DPSK modulation. The extraction process is carried on as follows:  
Step 1. The binary sequence is inputted into the differential precoding system composed of a not gate, a delay and an exclusive OR gate. The conversion process of analytic code type is shown in Table 3. \begin{table}
\begin{tabular}{c c c} \hline
**Component** & **Parameter** & **Value** \\ \hline Bit Sequence Generator & Bit rate & 10 Gbit/s \\ NRZ Pulse Generator\_3 & Maximum & 1 a.u. \\ Minimum & 0 a.u. \\ NRZ Pulse Generator\_2 & Maximum & \(-\)4 a.u. \\  & Minimum & 0 a.u. \\  & Frequency & 5 GHz \\ Sine Generator\_1 & Amplitude & 2 a.u. \\  & Bias & 0 a.u. \\  & Frequency & 193.1 THz \\ CW Laser\_1 & Power & \(-\)20 dBm \\  & Linewidth & 10 MHz \\  & Extinction ratio & 100 dB \\  & Switching bias voltage & 4 V \\ LiNb MZ Modulator 1\_1 & Switching RF voltage & 4 V \\  & Bias voltage1 & 0 V \\  & Bias voltage2 & 0 V \\  & Extinction ratio & 50 dB \\  & Switching bias voltage & 4 V \\ LiNb MZ Modulator 2\_1 & Switching RF voltage & 4 V \\  & Bias voltage1 & \(-\)2 V \\  & Bias voltage2 & 2 V \\ \hline \end{tabular}
\end{table}