Methods of and apparatus for optical signal transmission

A method of producing an amplitude modulated optical signal representing a binary signal, comprising the steps of:

• encoding the binary signal to produce a three-level encoded signal, the encoded signal having two outer levels representing a first state of the binary signal and having an intermediate level representing a second state of the binary signal;
• producing an optical signal with a polarization that is modulated in accordance with the three-level encoded signal;
• and
• filtering the optical signal in dependence upon its polarization to produce an amplitude modulated optical signal having first and second amplitudes representing the first and second states of the binary signal.

An optical signal transmitter comprising:

• an encoder for encoding a binary signal to produce a three-level encoded signal, the encoded signal having two outer levels representing a first state of the binary signal and having an intermediate level representing a second state of the binary signal;
• a semiconductor laser directly modulated by the encoded signal to produce a frequency modulated optical signal;
• and
• an optical interference filter to which the frequency modulated optical signal is supplied, the interference filter producing an amplitude modulated optical signal by constructive and destructive interference at different frequencies of the frequency modulated optical signal.

An optical transmission system comprising:

• a plurality of N encoders each for encoding a respective binary signal to produce a respective three-level encoded signal having reduced bandwidth and d.c. components compared with the respective binary signal;
• N semiconductor lasers each directly modulated by a respective encoded signal to produce a respective one of N frequency modulated optical signals having different frequency ranges;
• an optical combiner arranged to combine the frequency modulated optical signals in their respective frequency ranges;
• and
• an optical interference filter to which the combined frequency modulated optical signals are supplied, the interference filter producing in each of said respective frequency ranges an amplitude modulation of the optical signal by constructive and destructive interference at different frequencies of the respective frequency modulated optical signal, the amplitude modulation providing two states representing the binary states of the respective binary signal.

An optical signal transmitter comprising:

• an encoder arranged to encode a binary signal to produce a three-level encoded signal having reduced bandwidth and d.c. components compared with the binary signal;
• a modulator arranged to modulate polarization of an optical signal in accordance with the three-level encoded signal;
• and
• a polarization filter arranged to filter the optical signal in dependence upon its polarization to produce an amplitude modulated optical signal having first and second amplitudes representing the first and second states of the binary signal.