# Integrated Components and Solutions for High-Speed Short-Reach Data Transmission
## 3 Receiver for Short-Reach Transmission Systems
### Single-End Photodetector under Kramers-Kronig Direct Detection  
As mentioned above, single-end PD in conventional IM/DD transmission systems is only used to detect signal intensity. Therefore, even various complex equalization techniques are used to assist signal demodulation; the single-end PD based transmission systems are still limited to tens of kilometers, since the CD effect would lead to serious inter-symbol interference. An effective way to compensate the dispersion is to insert an optical dispersion compensation module in the transmission link, but extra insertion loss and component cost will be introduced for cost-sensitive short-reach transmission systems. Currently, an alternative approach, Kramers-Kronig direct detection [18; 92; 93; 94; 95; 96; 97; 98; 99; 100], has been investigated, as it can reconstruct the intensity and phase of the received signal \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**T\({}_{\textbf{x}}\)** & **\(\lambda\)** & **MF** & **BW** & **Rate** & **Reach** & **Fiber** & **FEC** & **Ref.** \\
**Device** & **(nm)** & **MF** & **(GHz)** & **(Gbits)** & **(cm)** & **Type** & **Limit** & **Ref.** \\ \hline SiP-based DDTW-MZM & 1546.98 & PAM-4 & 22.5 & 176 & 1 & SMF & \(1.5\times 10^{-2}\) & [81] \\ SiP-based DDTW-MZM & 1546.98 & PAM-6 & 22.5 & 200 & 1 & SMF & \(1.5\times 10^{-2}\) & [82] \\ SiP-based DDTW-MZM & 1546.98 & SSA-4 & 22.5 & 176 & 400 & SMF & \(4\times 10^{-2}\) & [83] \\ SiP-based DDTW-MZM & 1546.98 & PAM-4 & 22.5 & 184 & 320 & SMF & \(2.4\times 10^{-2}\) & [84] \\ SiP-based ME-MZM & O-band & PAM-4 & -35 & 112 & 10 & SMF & \(2.4\times 10^{-2}\) & [85] \\ SiP-based TW-MZM & O-band & PAM-8 & 47 & 255 & 2 & SMF & \(3.8\times 10^{-3}\) & [86] \\ SiP-based FOH-MZM & 1541.9 & OOK & \(>\)70 & 222 & 0.12 & SMF & \(2.24\times 10^{-2}\) & [87] \\ SiP-based 4 ch-MZM & –1540 & PAM-4 & 40 & 200 & 1 & SMF & \(3.8\times 10^{-3}\) & [88] \\ SiP-based MRM & 1541.57 & PAM-4 & 47 & 160 & 1 & SMF & \(2.12\times 10^{-3}\) & [89] \\ \hline \hline \end{tabular}
\end{table} 
Table 5: _Cont._from the intensity measurement. Figure 5 shows the Kramers-Kronig direct detection scheme. The received signal, which consists of an optical carrier \(c\) and a modulated signal under bandwidth \(B\), is directly detected by a single-end PD. The output analog signal is proportional to the optical intensity. After the conversion from analog signal to digital signal by ADC, the complex digital signal can be reconstructed by digital signal processing (DSP). Once the phase and amplitude information are reconstructed, the conventional DSP algorithms (e.g., CD compensation, fiber nonlinearity compensation, polarization demultiplexing, carrier phase recovery, et al.) based on coherent receiver can be applied to mitigate channel distortions. Therefore, the use of the Kramers-Kronig direct detection in transmission system would be able to extend the transmission distance and transmission rate. At present, many research groups around the world have implemented Kramers-Kronig direct detection in short-reach transmission systems, even in metro and long-haul transmission systems. [PERSON] et al. achieved four-channel 168-Gbit/s 64-QAM Nyquist-SCM transmission using a 40-GHz single-end PIN photodiode [96]. [PERSON] et al. demonstrated 125-km SMF transmission in a four-channel 240-Gbit/s polarization division multiplexing (PDM) 32-QAM system by using a 24-GHz photodiode [97]. [PERSON] et al. transmitted a line rate of 450-Gbit/s 32-QAM signal detected by a ~85-GHz bandwidth photodiode over 480-km SMF [98]. Assisted by a sparse I/Q Volterra filter algorithm, [PERSON] et al. experimentally demonstrated 112-Gbit/s 16-QAM transmission over 960-km SSMF based on a 50-GHz bandwidth photodiode [99]. Additionally, [PERSON] et al. demonstrated 1.6-Tbit/s 8-channel 16-QAM transmission with 1200-km Corning@TXF\({}^{\text{TM}}\) fiber link by using a 3-dB bandwidth of ~80-GHz photodiode [100].