## 1 Introduction  
In contrast to the previous generation of optical transceivers relying on direct detection [1; 2; 3], the current generation of optical transceivers utilizes coherent detection. Coherent detection allows for the full recovery of the electrical field, including both amplitude and phase. This property allows many linear impairments, like chromatic dispersion (CD), polarization modal dispersion (PMD) and polarization dependent loss (PDL), to be compensated through digital signal processing (DSP) [4; 5]. The coherent transmission exploits many degrees of freedom of photons, such as phase, polarization, amplitude, and wavelength. The state-of-the-art coherent transmitter modulates information on two orthogonal phase domains: in-phase (I) and quadrature (Q). Two orthogonal polarization domains, X and Y, are also utilized to carry the information. Thus, four tributary channels (XI, XQ, YI, and YQ) can carry modulated data. High-order modulation format, like quadrature amplitude modulation (QAM), further boosts the total capability of the coherent transceiver [6; 7; 8; 9; 10]. With so many advantages, the arrival of a coherent optical transceiver could revolutionize the optical communication industry. The history of the optical communication system could be split into two eras: before coherent detection (BC) and after direct detection (AD) [11; 12]. Inevitably, the advantage and convenience provided by a coherent transceiver are accompanied by many complicated and delicate components. Here, intradyne coherent transmitters (ICT) and intradyne coherent receivers (ICR) are two essential components of the coherent transceiver. Both are much more elaborate than their counterparts used in direct detection. They require many intricate and sophisticated calibration procedures. Many parameters of the ICR must be carefully calibrated to minimize any impairment. In this paper, our focus is to reduce the complexity for this calibration. Conventionally, this calibration is performed through heterodyne beating between the internal local oscillator (LO) and the external signal generated from a tunable laser source (TLS) [13; 14]. The external signal, being coherent, has a single state of polarization (SoP). To ensure that an equal amount of optical power is injected into both X polarization and Y polarization, the SoP of the TLS signal should be aligned either at 45 or 135 degrees, with respect to the principal axes of the polarization beam splitter (PBS) within the ICR. In general, a single mode fiber (SMF) is used at the input port of the ICR since both polarizations are carrying data. It is well known that SoP cannot be maintained by SMF. Thus, a critical step in this calibration is to maintain and stabilize the SoP of TLS through SMF, which requires an active control loop. However, active polarization control leads to a very complicated setup. In addition, this method is also not suitable for calibrating the coherent transceiver in field operations. Over the lifetime of coherent transceivers and under different environments (temperature, elevation, humidity, etc. ), the critical parameters of ICRs can change. It would be beneficial to perform periodic calibration of the ICR to improve the performance of coherent optical communication system. The need for an external TLS and the requirement for active SoP control prevent the in-field calibration of ICRs from being realized. In this paper, we develop an innovative method to address the problems aforementioned. To reduce the complexity of the setup, we propose to calibrate the ICR using a polarization-multiplexed signal. Precise and complicated control of the SoP is not required in this setup, leading to a significant reduction in complexity. Furthermore, we show that the internal components of the ICT can be used to generate the polarization-multiplexed signal. One can connect the ICT to the ICR using an optical cable and perform the calibration of the ICR. The calibration of the coherent transceiver is achieved without any external components.