Self-Coherent Optical Transmission System and Receiver

An optical receiver comprising:

• a polarisation controller arranged to receive as its input a first modulated optical signal having a first polarisation and an unmodulated optical carrier signal polarisation aligned with the first modulated optical signal, wherein the first modulated optical signal has negligible spectral power density within a predetermined bandwidth around an optical spectrum of the unmodulated optical carrier signal;
• an optical filter apparatus having a main polarisation mode;
• and
• a coherent optical receiver apparatus, wherein:
• the polarisation controller is arranged to apply polarisation rotations to the first modulated optical signal and the unmodulated optical carrier signal such that their polarisation is aligned to the main polarisation mode of the optical filter apparatus,
• the optical filter apparatus is arranged to receive the unmodulated optical carrier signal and the first modulated optical signal from the polarisation controller and is arranged to separate the unmodulated optical carrier signal from the first modulated optical signal, and
• the coherent optical receiver apparatus is arranged to receive said separated signals from the optical filter apparatus and to perform coherent detection of the first modulated optical signal by using the unmodulated optical carrier signal as a local oscillator signal.

A method of coherent optical detection comprising steps of:

• receiving a first modulated optical signal having a first polarisation and an unmodulated optical carrier signal that is polarisation aligned with the first modulated optical signal, wherein the first modulated optical signal has negligible spectral power density within a predetermined bandwidth around an optical spectrum of the optical carrier signal;
• then, applying polarisation rotations to the first modulated optical signal and the unmodulated optical carrier signal such that their polarisation is aligned to a main polarisation mode,
• then, acting on said signals aligned to the main polarisation mode, separating the unmodulated optical carrier signal from the first modulated optical signal;
• and
• then, using said separated signals, performing coherent detection of the first modulated optical signal using as a local oscillator signal the unmodulated optical carrier signal.