## 4 Generation of Polarization-Multiplexed Signal Using Intradyne Coherent Transmitter
Following the same methodology, we loaded the memory into the DSP ASIC as follows:  
\[ \begin{split} XI(n)=DAC_{\max}*\cos(2\pi*F_{x}*n)\\ XQ(n)=DAC_{\max}*\sin(2\pi*F_{x}*n)\\ YI(n)=DAC_{\max}*\cos(2\pi*F_{y}*n)\\ YQ(n)=DAC_{\max}*\sin(2\pi*F_{y}*n)\\ \end{split}  \tag{1}\]  
where \(DAC_{max}\) is the maximum value in the output of DAC, \(n\) is the index within the memory. For an 8-bit DAC with the most significant bit (MSB) being the sign bit, \(DAC_{max}\) is equal to 128. The value is rounded to the nearest integer and loaded into the memory in the two's complement fashion. In the ICT, the bias points of the MZMs should be at the null points. In normal operation, automatic bias-control loops (ABC) are enabled to prevent the bias points from drifting away. The ABC loops work as desired when the regular data (with a broad spectrum) path is chosen. We observed that the ABC loop could lose its tracking when the memory data path is chosen. This is due to the fact that the sinusoidal function within the memory has a narrow spectrum. To prevent this from happening, the MZM is firstly driven by regular data traffic so that the ABC loop locks the bias to the null point. Next, ABC loops are frozen so that the bias points do not change further. Finally, the data path is switched from regular data traffic to the sinusoidal wave within its memory. Also, the ICT should use a linear RF amplifier instead of a limiting RF amplifier. The reason is that the limiting RF amplifier will distort the sinusoidal wave and create undesired high-order harmonics. Similar to TIA in the ICR, there are two operational modes for the RF amplifier: the AGC mode and MGC mode. In the normal operation using regular data, the RF amplifier should be in AGC mode to dynamically compensate any variation in the data path. In the calibration process using data in memory, the RF amplifier should be in MGC mode so that its gain remains constant when the frequency  
Figure 10: Block diagram of coherent DP-IQ transmitter. of the sinusoidal waveform changes during the process. The reason behind this is the same as the reason for the mode of the TIA, as described in Section 2. Figure 11 shows the measured DoP versus the frequency difference between two polarizations \(|F_{x}\)-\(F_{y}|\). As seen, the DoP keeps decreasing when \(|F_{x}\)-\(F_{y}|\) increases. When \(|F_{x}\)-\(F_{y}|\) is larger than 0.94 GHz, the DoP is very close to zero. The residual DoP is due to the power imbalance in ICT. Figure 12a shows the measured DoP and output power versus time. During this measurement, we switch from the regular data to the calibration waveform loaded in memory. After the switching, the DoP is very stable and close to zero. Figure 12b shows the spectrum measured by Keysight OMA N4392. We choose OMA to perform this measurement since the polarization-demultiplexing algorithm within OMA can separate the signal into two orthogonal polarizations. One can clearly identify the frequency shift between two polarizations. Small side lobes are noticeable in this spectrum due to the fact that only a fundamental frequency component is used to modulate the MZMs for simplicity. To eliminate those side lobes, one needs to include the high-order harmonics of fundamental frequency, which can cancel out those side lobes. Having generated the polarization-multiplexed signal from the ICT, we further adjusted the carrier frequencies of the TLS in the ICT (defined as \(LO_{tx}\)) and the carrier frequency in the ICR (defined as \(LO_{rx}\)). This creates a frequency difference and generates a heterodyne beating signal in the outputs of the ICR. By sweeping the frequency difference (\(|LO_{tx}-LO_{rx}|\)) over the baseband of the ICR, one can determine many critical parameters. Although two frequency components are 0.94 GHz apart,  
Figure 11: (**a**) The measured DoP versus the frequency difference between two polarizations. (**b**) The measurement DoP and SoP with 0.94-GHz frequency difference between two polarizations. Figure 12: (**a**) The DoP and power versus time.