Self-Homodyne Coherent Systems using Advanced Modulation Formats

Licentiate thesis, 2010

Modulation formats that offer high spectral efficiency will be of great importance in future fiber optic communication systems. Currently, a lot of research efforts are ongoing to investigate the best ways of generating and detecting modulation formats in which both the phase and the amplitude of the light are used to carry information, such as 16-QAM. The conventional way of detecting the phase of a signal is known as intradyne coherent detection. The signal is mixed with light from a free running local oscillator laser in the receiver and the intermediate frequency between the two lasers is tracked with digital signal processing. This thesis deals with another coherent detection approach known as self-homodyne coherent detection, in which a polarization multiplexed pilot tone is co-propagated with the signal and used as local oscillator in the receiver. Self-homodyne systems offer an interesting alternative to intradyne and have some distinct advantages and disadvantages that are quantified to some extent in this work. The optical signal-to-noise ratio requirements of self-homodyne systems have been investigated and compared to intradyne systems. The beneficial effect of band-pass filtering of the pilot tone was demonstrated and it was shown that the performance limits for self-homodyne and intradyne systems become equal as the filter bandwidth is made small. Furthermore, we demonstrated the unique ability of self-homodyne systems to compensate for nonlinear phase distortion that distorts the received constellation diagram. We also demonstrated a novel interleaved polarization division multiplexing method to decrease the penalty in spectral effciency of self-homodyne systems compared with intradyne systems that is due to the fact that conventional polarization division multiplexing cannot be used in the former case. The performance of the scheme was evaluated and suggestions for future improvements were made. Finally, a novel 16-QAM transmitter was presented that uses two IQ-modulators in series. Based on this scheme, we generated the first 16-QAM signal at 40 Gbaud.