## 4 Generation of Polarization-Multiplexed Signal Using Intradyne Coherent Transmitter  
The setup shown in the previous section is mostly suitable for the initial calibration of ICR in the laboratory environment or on the manufacturing floor. The reason is that the external TLSs are used to create the polarization-multiplexed signal. It is highly desirable to further simplify the setup and eliminate any usage of external components. By generating the polarization-multiplexed signal, relying only on the internal components within the ICT, one can implement the calibration of the ICR, providing significant advantages and flexibility. To implement this, we examine the building blocks of a coherent IQ transmitter, as shown in Figure 10. It is composed of an optical front end and an electrical back end. In the electrical back end, a block of forward error correction (FEC) firstly encodes the data. Next, a finite impulse response (FIR) filter in the tap-and-delay structure compensates many impairments. Then, a high-speed digital-to-analog converter (DAC) converts the output of the FIR filter from the digital domain to the analog domain. Finally, the analog electrical signal goes through RF traces on print circuit board (PCB) and the pluggable RF connector (in some implementations). Four linear RF amplifiers boost the signal and drive the Mach-Zehnder modulators (MZM). In the optical front end, the output from a CW TLS is firstly split into four tributaries and fed into the MZMs. Next, the optical signal is modulated by MZM based on the output of the RF amplifier. Then, a phase modulator (PM) is used to introduce a 90-degree difference between the I tributary and Q tributary. On one optical path, the polarization is rotated by 90 degrees by using a polarization rotator (Pol-Rot). Finally, a PBC is used to combine the outputs from two polarizations. As seen, all components for the generation of a polarization-multiplexed signal, using the second way described in Section 3, have been built in an ICT. However, some additional steps are required. In order to minimize the skew between polarizations, the length of optical path in the X polarization is the same as that in the Y polarization. If the modulation signal is not applied to MZM, the signals from two polarizations will be coherent with each other. The DoP of the combined output from the PBC is close to one, and the SoP of the combined output can drift and fluctuate over time. To prevent this from happening, one can apply one small frequency shift (\(F_{x}\)) through the MZMs in the X polarization, and another small frequency shift (\(F_{y}\)) through the MZMs in the Y polarization. If the absolute difference between \(F_{x}\) and \(F_{y}\) is larger than the linewidth of TLS, the signal from the X polarization will no longer be coherent with the signal from the Y polarization. Moreover, the DoP of the combined output from PBC is close to zero. In this manner, a polarization-multiplexed signal is generated from an ICT. Figure 9: One external TLS is used to create the polarization-multiplexed signal. Two data paths exist in the DSP ASIC before the ICT. One path is the regular data traffic, which passes through the FEC block, the FIR filter and the DAC. The other path is through a built-in memory within the DSP ASIC. A user-defined data pattern can be loaded into this memory. The output of DAC will correspond to the data within the memory. Once the end of the memory is reached, the output will go back to the beginning of the memory in a round-robin way. The outputs of DAC are first amplified and then modulate the optical signal through the MZM. One can switch the data path between regular data traffic and the memory in ASIC. In this work, we load the ASIC memory with the desired data pattern to create the necessary frequency differences for the polarization-multiplexed signal. In [25, 26, 27, 28, 29, 30], a method to apply a frequency shift to the output of ICT has been demonstrated through applying particular driving signals to the MZM.