## 2 Traditional Calibration Method of Intradyne Coherent Receiver Using Single-Polarized Signal
sets the TIA to AGC mode, the gain in TIAs will be adjusted to compensate the limited bandwidth. However, this is not the intended usage of the AGC mode. Thus, it is critical to set the TIA to manual gain control mode (MGC). This will ensure that the gain of the TIAs remains constant during the frequency sweeping. In certain aspects, the MGC mode in the heterodyne measurement resembles the AGC mode in normal operation mode. The performance of a standalone individual ICR is measured using the heterodyne beating method described above. The outputs of TIAs are sampled and post-processed by an optical modulation analyzer (OMA, Keysight N4392, Santa Rosa, CA, USA). The results in both the MGC mode and the AGC mode are shown in Figure 2. In the MGC mode, the gain of the TIAs remains constant during the calibration. The peaks in the frequency response are located at approximately 17.5 GHz. To determine a 3-dB bandwidth, we measure the response at 0.5 GHz. Then, we find the frequency where the response is 3 dB lower than that at 0.5 GHz. The YI and YQ tributaries have a 3-dB bandwidth of approximately 25 GHz, and the XI and XQ tributaries have a 3-dB bandwidth of approximately 26 GHz. In the AGC mode, the gains of the TIAs are adjusted to maintain constant peak-peak swing at outputs of TIAs for different frequencies. An 'artificial' flat frequency response from 5 GHz to 17.5 GHz is observed. However, a real broadband signal with multiple frequencies will not experience this frequency response. One can also notice that any intrinsic gain imbalance among different tributaries is removed in the AGC mode. It is important to operate TIA in the MGC mode during the calibration, which represents the real application scenario. As discussed in the introduction, a critical step is to control the SoP of TLS launching into the ICR. Ideally, the SoP of TLS should be aligned at either 45 or 135 degrees with respect to the principal axes of the PBS in the ICR. This will guarantee that equal amounts of optical power are sent to both the X polarization and Y polarization. An active control loop for SoP, formed by an automatic polarization controller and a polarimeter, is needed to maintain the SoP at the input port of ICR. Figure 3 shows the typical configuration using the heterodyne beating method. Because SMF is used to connect the output of polarization control to the input of ICR, one needs to add a manually adjustable polarization controller in between. This is to ensure that the SoP at the input to the ICR is the same as the SoP at the input of the polarimeter. Figure 2: Frequency response of a stand-alone ICR under different operational modes of TIA. (**a**) MGC mode; (**b**) AGC mode. The typical steps to set up the polarization controller are as follows: first, one sets the automatic polarization controller so that the SoP to the polarimeter is linearly polarized along one polarization (for example, X polarization). We use a Keysight N7788 (Santa Rosa, CA, USA) component analyzer, which contains both the automatic polarization controller and the polarimeter. Next, one adjusts the manual polarization controller so that the outputs of TIAs from the other polarizations (for example, Y polarization) are minimal. This will ensure that the inputs to both polarimeters and the ICR have the same SoP. Finally, one sets the automatic polarization controller so that the SoP is linearly polarized at 45 degrees or 135 degrees. The automatic polarization controller will maintain a constant SoP based on the internal feedback from the polarimeter. Figure 4 shows the results obtained with OMA N4392 with active stabilization turned on. In the subfigure (a), the frequency difference between the TLS and the LO is 2.5 GHz, showing the response of the ICR near the direct current (DC) frequency. In the subfigure (b), the frequency difference between the TLS and the LO is 22.5 GHz, showing the response of the ICR at a high frequency.