# Two-Channel OTDM System for Data-Center Interconnects: A Review
## 4 Discussion  
It is expected that DCIs will be implemented with the 100, 200, and 400 Gb/s/\(\lambda\) IM/DD systems. Considering the limited bandwidth of commercially available transmitters, it is challenging to achieve high-speed DCIs operating at \(\geq\)100 Gb/s/\(\lambda\) with a single modulator. The use of the two-channel OTDM system enables sidestepping the insufficient bandwidth problem. It was demonstrated that the 200 Gb/s/\(\lambda\) PAM-4 signal can be implemented with 17.2 GHz MZMs. We believe that the 400 Gb/s/\(\lambda\) can also be realized using commercially available 40 GHz MZMs. An integrated transmitter on a chip should be developed to practically deploy a two-channel OTDM system for DCIs. It was previously demonstrated that this system could be implemented using a transmitter integrated into a silicon photonics (SiP) chip, as shown in Figure 14[21]. The 64 Gb/s OTDM-PAM-4 signal could be generated using a SiP chip  
Figure 12: (**a**) Experimental setup for the transmission of 200 Gb/s OTDM-PAM-4 signal. Eye diagrams of the 100 Gb/s (**b**) CSRZ- and (**c**) RZ-PAM-4 signals. Figure 13: BER curves of the 200 Gb/s OTDM-PAM-4 signal generated using the (**a**) CSRZ pulse and (**b**) RZ pulse. consisting of two optical couplers, two MZMs, an optical delay line, and a thermal phase shifter (TPS) (as a DS) [21]. More recently, the 300 Gb/s OTDM PAM-8 signal was generated using a SiP transmitter and successfully transmitted over the 500 m SSMF [31]. However, the previously developed SiP transmitter [21; 31] did not contain an optical pulse source due to its high insertion loss. In the future, it will be necessary to implement the OTDM system with the use of a SiP transmitter containing an optical pulse source. Furthermore, it is also necessary to develop a SiP OTDM transmitter integrating a polarization rotator (used as a DS) and a PBS [32]. Indium phosphide (InP) [28] and lithium niobate [33] -based modulators are also good candidates for the integrated photonics chip. The optical pulse source with a high repetition rate is a key component of the two-channel OTDM system. A technique wherein an MZM is biased at a sub-quadrature point was proposed to increase the ER of the optical pulse with a high repetition rate [34]. This technique could generate a 50 GHz optical pulse with 14.9 dB ER even when using a 17.2 GHz MZM, enabling the transmission of the 300-Gb/s OTDM-PAM-8 signal and a soft-decision FEC with 20% overhead (the FEC limit was BER \(~{}=~{}2.7\times 10^{-2}\)). The generated signal could be transmitted over a 1 km SSMF. If the phase encoding technique is applied to the optical pulse, the transmission distance may further increase. In addition, the optical pulse source can be implemented by a ring modulator (instead of an MZM) integrated into a silicon photonics chip [35]. Since the ring modulator is attractive in terms of footprint, power consumption, and bandwidth [35], it is necessary to demonstrate the feasibility of utilizing a ring modulator-integrated transmitter. The high insertion loss of the transmitter is one of the disadvantages of the system. Although EDFAs were used in the previous experiments [21; 30; 34], we believe that a cost-effective SOA is required to make this system more attractive, such that it is cheap and integrable with other components, including a PD [4]. Then, the SOA would be located on the receiver side since the input power should be reduced to reduce the BER degradation (especially for the PAM-\(N\) signal) due to the nonlinear transfer characteristics arising from gain saturation [36; 37]. The large noise figure of an SOA (compared to an EDFA) should also be considered [37]. Therefore, it is necessary to demonstrate a transmission experiment using a SiP transmitter and a receiver integrating an SOA and a PD. As described in Section 3, robustness to CD is crucial to a high-speed transmission system.