# Integrated Components and Solutions for High-Speed Short-Reach Data Transmission
## 4 Some Integrated Solutions for Short-Reach Transmission Systems  
### Simplified Detection Scheme  
A simplified detection scheme consisting of a Stokes vector receiver, an optical band-pass filter, and a single-end PD had been proposed to simultaneously detect two channels PDM-PAM4 transmission signals [103]. The simplified scheme can reduce the number of receivers by half without sacrificing transmission capacity compared to Ref. [21], and the spectral efficiency can be doubled compared to Ref. [108]. The principle of the simplified detection scheme was shown in Figure 8. In the transmitter side, four independent PAM-4 signals with two optical carriers were combined to generate the 2 \(\times\) PDM-PAM4 signals, whose state of polarizations (SOPs) consisted of 0\({}^{\circ}\), 90\({}^{\circ}\), \(-\)45\({}^{\circ}\), and 45\({}^{\circ}\). Here, the vector \(E_{1}\), \(E_{2}\), \(E_{3}\), and \(E_{4}\) represented the signal of 0\({}^{\circ}\), 90\({}^{\circ}\), 45\({}^{\circ}\), and \(-\)45\({}^{\circ}\) SOP respectively. It is notable that there was a 90\({}^{\circ}\) phase difference between two orthogonal PDM signals to mitigate beating crosstalk at the receiver. After fiber transmission, the intensity signals that were received by Stokes vector receiver were \(I_{01}\), \(I_{02}\), \(I_{03}\), and \(I_{04}\). Two channels of transmission signals cannot be demultiplexed by a Stokes vector receiver. Here, an optical band pass filter (OBPF) and additional single-end PD were introduced to obtain intensity information \(I_{5}\) of laser 1. Two channels of PDM-PAM4 transmission signals can be achieved for demultiplexing through these Stokes parameters and additional information.  
The transmission performance of the 2 \(\times\) PDM-PAM4 and the single PDM-PAM4 systems were measured as depicted in Figure 9. Here, the 2 \(\times\) PDM-PAM4 signals transmitted over 25-km NZDSF link, and the transmission rate was set to 4 \(\times\) 20-Gbit/s and 4 \(\times\) 40-Gbit/s, respectively. Figure 9a showed the performance of 4 \(\times\) 20-Gbit/s signals over 25-km NZDSF link. About 0.8-dB power penalty was observed between the 2 \(\times\) PDM-PAM4 system and the single PDM-PAM4 system. As depicted in Figure 9b, about 1-dB power penalty can be obtained when transmission rate was up to 4 \(\times\) 40-Gbit/s. We found that the performance degradation was limited as the bitrate increased from 4 \(\times\) 20-Gbit/s to 4 \(\times\) 40-Gbit/s over 25-km. In addition, the linear relationship between BER and received power is destroyed after  
Figure 7: (**a**) Schematic top-view (XZ-plane) of the integrated Stokes vector receiver on InP; (**b**) symmetric waveguide (SW) and (**c**) asymmetric waveguide (ASW) used in the receiver; (**d**) illustrations of four basis vectors on the Poincaré sphere; MMI: multimode interference; SV: Stokes vector; PS-PD: polarization-selective PD; TE: transverse-electric; TM: transverse-magnetic; (From [106] © 2021 OSA).  
transmission. It may be that the received power was close to or beyond the saturation power of single-end PD. On the other hand, the performance difference between two orthogonal PDM-PAM4 signals became more apparent for 4 \(\times\) 40-Gbit/s signals. The most likely reason was the influence of CD in spite of NZDSF used. Experimental demonstration had verified the effectiveness of the simplified detection scheme after 25-km fiber transmission and in the presence of polarization scrambling. We believe that such a scheme might be applied in short-reach transmission systems such as data center, access or metro network.