3 Advanced aspects of optical modulations in achieving high bit rates and immunity to signal degradation
3.9 Conclusion
- The most widely used formats are intensity (e.g. OOK, DB, CSRZ) and phase (e.g. BPSK, DPSK) modulation formats.
- Compared to intensity formats, phase-based modulation formats can perform better at the cost of increased transceiver's complexity.
- Above 10 Gbps, the only intensity format which can be potentially a good match between the transceiver complexity and system performance is DB. This format could for example transmit 40 Gbps data traffic even for 0.8 nm DWDM grid. Phase formats can perform well even for denser DWDM grids (0.4nm–0.1 nm spacing).
- However, multistage modulations using more than two levels (e.g. QPSK) or modulations based combining phase and intensity (e.g. QAM) or phase and polarization (e.g. PM-QPSK) become more promising for high-speed and/or long-haul systems.
- Formats using polarization multiplexing can benefit from higher spectral efficiency, optical reaches, optical to signal noise ratio and chromatic dispersion tolerances.
- The most promising format even for terabit transmission which can substantially be used in upgrading the current fibre infrastructure to higher bit rates than 40 Gbps is PDM-QPSK.
- The latest development is multicarrier modulations, such as OFDM and VDMT that found the application in digital television or LTE.