## 3 Innovative Calibration of Intradyne Coherent Receiver Using Polarization-Multiplexed Signal  
We propose a simple yet elegant solution to resolve the issue discussed in Section 2. The solution is to use a polarization-multiplexed signal to perform the heterodyne calibration of the ICR. Figure 5: Lissajous curve under different tests using single-polarization signal, whose SoP changes among different tests. The polarization-multiplexed signal contains two orthogonal polarization components with equal power. Even if one intentionally introduces some random polarization rotation in the SMF, an equal amount of power will always be projected into both X and Y polarizations. Thus, there is no need for the active stabilization of SoP, which greatly reduces the complexity of the calibration setup. Note that one needs to make sure that the two polarization components are not coherent with each other. Otherwise, a light with elliptical SoP will be generated. Its SoP cannot be maintained by the SMF. The degree of polarization (DoP) is the important characteristic in this case. For the polarization-multiplexed signal, the DoP should be very close to zero. There are two ways to generate the polarization-multiplexed signal. The first way is to combine the outputs of two separate TLSs with orthogonal polarizations through a polarization beam combiner (PBC). Two TLSs should have the same frequency and output power. One should note that, although two TLSs have the same frequency, their outputs are not coherent with each other since the outputs are coming from two individual TLSs. If the two TLSs have the same SoP initially (for example, along X polarization), one can insert a polarization rotator in one path to rotate the SoP along the Y polarization. A half-wave plate whose principal axis is aligned at either 45 degrees or 135 degrees can serve as the polarization rotator. The polarization-maintaining fibers are required before the PBC. The outputs from the PBC are effectively polarization-multiplexed signals, with the DoP being close to zero, as shown in Figure 6.  
To demonstrate the feasibility of this innovative method, we calibrate the same ICR integrated into the CFP2-ACO module using the polarization-multiplexed signal. We choose to form the polarization-multiplexed signal using two individual TLSs. We remove the automatic polarization controller, the polarimeter and the manual polarization controller from the setup since we do not need to precisely control the SoP anymore. No matter how the SMF changes the polarization of the signal, there will be always equal amounts of optical power projected into the X and Y polarizations. We illustrate this by showing the Lissajous curves in a few scenarios in Figure 7.  
During the calibration, the frequencies of two TLSs are simultaneously tuned to create the frequency sweeping. Similarly, the outputs of ICR are sampled by DSP ASIC and stored in the internal memory. The waveforms are further downloaded and post-processed. The critical parameters of ICR are measured. As seen, the outputs of ICR experience very small changes even without any SoP control, demonstrating the advantage of using a polarization-multiplexed signal as the calibration signal. Figure 6: Two TLS are used to generate the polarization-multiplexed signal. Figure 8 shows the comparison between the results obtained using two calibration methods. One method measures the bandwidth using the single-polarization configuration with active stabilization on, while the other method measures the bandwidth using the dual-polarization method  
Figure 7: Lissajous curves under different tests using polarization-multiplexed signal. No active control of SoP for input signal to ICR. with active stabilization off. As seen, the two methods give very close results, demonstrating that one can use the polarization-multiplexed signal to calibrate the coherent ICR. We notice a small deviation between the two results. The likely root cause is the drift of SoP in the single-polarization measurement.