## 2 Traditional Calibration Method of Intradyne Coherent Receiver Using Single-Polarized Signal  
The ICR can be divided into two parts: an analog front end and a digital back end. The analog front end of ICR is composed by a PBS, a 90-degree optical hybrid, four pairs of balanced photo diodes, and four sets of TIAs. The digital back end of the ICR is composed by analog digital converters (ADC) and a DSP application-specific integrated circuit (ASIC). The interface between the analog front end and digital back end may include high-speed radiofrequency (RF) traces or RF connectors. A block diagram of ICR is shown in Figure 1.  
Frequency response, bandwidth, skew, gain imbalance, and quadrature angle are the critical parameters of analog front end of the ICR. Smooth frequency responses without abrupt changes for all tributaries are desired. The bandwidth of the ICR should be sufficient, otherwise a significant penalty in the required optical signal-to-noise ratio (OSNR) is introduced. The bit error rate (BER) will suffer if the skew among different tributaries of the ICR is large. The impact is even more severe for the skew between the I tributary and Q tributary [15]. Distortion in the constellation diagram is introduced when the gain imbalances among different tributaries are not compensated, leading to reduced system performance. The ideal quadrature angle should be 90 degrees. Any deviation from ideal value leads to the crosstalk between the in-phase signal and the quadrature signal. Those parameters discussed above should be carefully measured during the initial calibration. Once characterized, many impairments due to non-ideal parameters can be compensated through DSP [16; 17; 18; 19; 20; 21; 22]. Many critical parameters of the ICR can be calibrated through the exploitation of the heterodyne beating between two continuous-wave (CW) signals. One signal is from the internal LO within the ICR. The other signal is generated from an external TLS. The heterodyne beating creates a sinusoidal signal at the outputs of TIAs, whose frequency is the frequency difference between TLS and LO. By sweeping the frequency of the TLS and maintaining the frequency of the LO, the frequency of the sinusoidal beating signal is continuously varied. During this process, both TLS and LO should maintain a constant output power. In this way, frequency sweeping in the baseband of ICR can be performed, and many critical parameters of ICR can be derived from this frequency sweeping. The curve of amplitude versus frequency determines the frequency response and the signal bandwidth. The difference in amplitude response among four tributary channels corresponds to the gain imbalance. The phase difference between the I tributary and Q tributary is used to calculate the quadrature angle. The skew can be derived by examining the curve of phase versus frequency. One important, yet subtle, aspect during the calibration of coherent ICR is to set the appropriate operational mode of TIA within the ICR. In the normal operation, the TIAs should be in the automatic gain control (AGC) mode to ensure that the electrical outputs of TIAs remain constant even when there is any change or transient of receiving optical power. It is natural to assume that the TIAs should be in AGC mode during the calibration of ICR. However, there is a big difference in the characteristics of the input signal to the ICR. During normal operation, the input signal is broadband with multiple frequencies. The AGC mode is used to compensate the change in the optical power, while the gain across the whole spectrum remains constant. In a stark contrast, the input signal contains a single frequency during the calibration through heterodyne beating. When the frequency difference between TLS and LO is large, the outputs from the TIAs will be reduced due to the limited bandwidth. If one  
Figure 1: The block diagram of the intradyne coherent dual-polarization in-phase-and-quadrature (DP-IQ) receiver.