## 4 Generation of Polarization-Multiplexed Signal Using Intradyne Coherent Transmitter
The data is switched from the regular data to the pre-loaded data in memory. (**b**). The spectrum measured by Keysight OMA, clearly showing two different frequency peaks in orthogonal polarizations. the frequency response and other critical parameters of the ICR will not change drastically over this small range. Thus, we can use the average value of two frequency components, shown in the equation below, to represent the setting value of the heterodyne beating frequency. \[F_{set}=|\,LO_{tx}-LO_{rx}|+(F_{x}+F_{y})/2 \tag{2}\]  
In the receiver side of the DSP ASIC, the raw data from the ADC outputs is saved as a snapshot. We converted the ADC raw data from the time domain to the frequency domain through fast Fourier transform (FFT). From the results in the frequency domain, we can identify two peak frequencies in the spectrum, roughly corresponding to \(|LO_{tx}-LO_{rx}|+F_{x}\) and \(|LO_{tx}-LO_{rx}|+F_{y}\). The tuning of the carrier frequency of the local oscillator from the pre-defined wavelength grid is implemented through the temperature control of TLS. Thus, over the lifetime of the coherent transceiver, the actual detuning could be different from the set detuning. Instead of relying on the setting value of \(F_{set}\), we identify two peak frequencies within the frequency spectrum (\(f_{peak1}\) and \(f_{peak2}\)) and determine the actual heterodyne beating frequency \(f_{mass}=(f_{peak1}+f_{peak2})/2\). This will further improve the accuracy of calibration procedure. Figure 13 shows the two measured peak frequencies \(f_{peak1}\) and \(f_{peak2}\) versus the setting beating frequency \(F_{set}\). Typical FFT spectrums are also shown. As seen, an accurate linear relationship exists between \(F_{set}\) and \(f_{mass}\). Figure 14 shows the frequency response of a coherent transceiver on the Rx side, which includes the ICR integrated in the CFP2-ACO transceiver, the RF PCB traces, the pluggable connector and the DSP ASIC. In this figure, the measured value of \(f_{mass}\) is plotted along the X axis. For each value of \(f_{mass}\), we measure 16 times to improve the calibration accuracy. The peak-to-peak swing of ADC raw data for four tributaries (\(Xl_{swing}\), \(XQ_{swing}\), \(Yl_{swing}\), \(YQ_{swing}\)) are first determined in the time domain. Then,  
Figure 13: (**a**) The measured frequency peaks versus the set frequency \(F_{set}\). (**b**) and (**c**) The typical spectrums obtained through FFT. their distribution is shown as a boxplot. Some fluctuations in the peak-to-peak swing are observed. The likely root cause for fluctuations is that some high-order harmonics are generated when the fundamental sinusoidal wave is loaded and played from the memory in DSP ASIC. The measurement accuracy can be further improved by loading the memory with more complicated data to remove those high-order harmonics. The frequency response of four tributaries are determined from the median values of peak-to-peak swing. We used the median value instead of the average value. The reason is that the median value is very tolerant to any outliers in the measurements, while the average value is strongly influenced by the outlier measurements. In current setup, the maximum frequency difference between two LOs is 15 GHz, limited by how far the frequency of LO can be detuned from a pre-defined frequency grid. Our results indicate that the 3-dB bandwidth of this coherent transceiver on the Rx side is larger than 15 GHz for all tributaries. The IQ gain imbalance in X polarization is 0.29 dB. The IQ gain imbalance in Y polarization is 0.25 dB. Finally, the gain imbalance between polarizations is 0.25 dB. \[ \begin{split}& IMB_{IQ,X}=XQ_{swing\_median}/XI_{swing\_median}\\ & IMB_{IQ,Y}=YQ_{swing\_median}/Y_{swing\_median}\\ \end{split}  \tag{3}\]  
Usually, within a coherent transceiver, the coherent receiver shares the same TLS with the coherent transmitter as the LO for the benefit of cost reduction.