# Two-Channel OTDM System for Data-Center Interconnects: A Review
## 3 Two-Channel OTDM System with Phase-Alternating Pulse  
### Phase-Alternating Pulse Generation  
Figure 9a shows the structure of the OTDM transmitter that uses a phase-alternating pulse generated by an MZM. The MZM1 is biased at the null point and modulated using an electrical clock with a frequency of \(0.5B\) and \(\Delta V\) of \(2V_{\pi}\). Then, a phase-alternating pulse with a 66% duty ratio is generated, as shown in Figure 9b. When generating the OTDM signal using the phase-alternating pulse, the maximum power of the transition region is larger than that of the symbol center due to the pulse's large duty ratio, as shown in Figure 9c. Since the optical field of the phase-alternating pulse has a zero mean, the carrier of the phase-alternating pulse disappeared--resulting in a carrier-suppressed RZ (CSRZ) pulse. The carrier of the OTDM signal generated by the CSRZ pulse is also suppressed, as shown in Figure 9d. While the phase-alternating pulse has a broader pulse width compared with the constant-phase pulse, its optical spectrum is narrower, as shown in Figure 9d,g, making the generated OTDM signal robust to CD. Furthermore, the CSRZ pulse is generated using the half-rated electrical clock. Consequently, the required bandwidth for generating the optical pulse decreases.  
Alternatively, a CSRZ pulse can be generated using a phase modulator (PM) and a periodic optical filter [30]. When the light is phase-modulated using an electrical clock with a frequency of \(0.5B\), multiple harmonics are generated because of the nonlinearity of the PM. By applying an optical filter with a free-spectral range of \(B\) to suppress the carrier frequency (as shown in Figure 10b), a CSRZ pulse with a repetition rate of \(B\) is generated, as shown in Figure 10c. The resulting CSRZ pulse and a half-rated electrical clock are used to generate an OTDM signal with a baud rate of \(2B\). The optimal \(\Delta V\) of the PM is \(1.2V_{\pi}\). In this method, the pulse can be generated without requiring an auto-bias controller.  
Figure 9: (**a**) OTDM transmitter consisting of a phase-alternating pulse generator and an MZM. Eye diagrams of the (**b**) CSRZ-PAM-4 signal; (**c**) OTDM-PAM-4 signal based on the CSRZ pulse; (**e**) RZ-PAM-4 signal; and (**f**) OTDM-PAM4 signal based on the RZ pulse. Optical spectra of the OTDM-PAM-4 signal generated using a (**d**) CSRZ pulse and (**g**) RZ pulse.  
As illustrated in Figure 9d,g, the phase-alternating pulse makes the OTDM signal robust to CD. Numerical simulations were performed to identify the robustness to CD from eye diagrams of 200 Gb/s OTDM-PAM-4 signals. A constant-phase and a phase-alternating pulse were generated using an MZM, as illustrated in Figures 1 and 9a, respectively. The following assumptions were made: (1) the light had a wavelength of 1550 nm; (2) the pulse ER was infinite; (3) the two generated (CS)RZ-PAM-4 signals in different polarization domains were combined; (4) the CD parameter of the SSMF was 16 ps/(nm\(\cdot\)km); and (5) the PMD parameter was 0.5 ps/km\({}^{0.5}\). Each level of the OTDM signal generated using a constant-phase pulse was not distinguishable after 1 km transmission, but each level generated using a phase-alternating pulse was distinguishable after 1 km transmission, as shown in Figure 11. The phase-alternating pulse made the OTDM signal robust to CD because the adjacent bits in each multiplexed channel were subjected to destructive interference.