Coherent optical transmission is a most promising transmission scheme, enabling data to be modulated simultaneously in the phase and the amplitude of the optical carrier. This makes advanced modulation formats, e.g. quadrature amplitude modulation (QAM), as well as electronic impairment compensation possible. The drawback is the complex receiver, where the relative phase between a local oscillator and the signal laser needs to be tracked continuously for the data to be retrieved. This is usually done by the intradyne principle, which means that the phase tracking is done via digital signal processing after detection, sampling and analog-to-digital conversion. The numerous issues related to synchronization and efficient algorithms in this detection process is done in a separate sub-project (2.4) discussed below. This sub-project deals with experimental realization of intradyne systems, and investigations of their robustness to distortions and transmission impairments. It also deals with self-homodyne systems, in which the local oscillator is transmitted together with the signal, in the orthogonal polarization state. In this scheme the local oscillator and the phase tracking algorithms are obsolete, which leads to a significant reduction of the receiver complexity, as well as an increased tolerance towards laser phase noise.
Some specific objectives are
- Comparison of intradyne and self-homodyne transmission with respect to complexity, sensitivity, and stability.
- Investigation of nonlinear tolerances for intradyne and self-homodyne receivers, and realization of formats and compensating schemes that can increase nonlinear tolerances.
- Experiments on transmission of multi-level coherent modulation formats over long distances, and development of transmitter and receiver technologies for such systems.
- Realization of transmitters and receivers for high (>40 Gbaud) baud rates, including implementation of algorithms and digital signal processing.
Vinnova IKT no. 2007-02930