Duobinary coding and modulation technique for optical communication systems

A method for encoding a binary input sequence x(0,1) to obtain a duobinary output sequence y(+1,0,-1), comprising the steps of:

• preparing from said input sequence x(0,1) a binary switch signal Q(0,1) which maintains a current logical value when the input sequence x(0,1) comprises one of:
• a succession of bits x.sub.k =0, a succession of bits x.sub.k =1, and a succession of bits x.sub.k =0, x.sub.k =1, and switches to the opposite logical value when the input sequence x(0,1) comprises a succession of bits x.sub.k =1, x.sub.k+1 =0;
• logically adding said binary input sequence x(0,1) and said switch signal Q(0,1) to obtain a first binary sequence a(0,1);
• logically adding said binary input sequence x(0,1) and the complement Q(1,0) of said switch signal to obtain a second binary sequence b(0,1);
• and
• summing said first and second binary sequences for obtaining said duobinary output sequence y(+1,0,-1).

A coding device for encoding a binary input sequence x(0,1) to a duobinary output sequence y(+1,0,-1), comprising:

• means for generating a binary switch signal Q(0,1);
• a first AND circuit for receiving said binary input sequence x(0,1) and said switch signal Q(0,1), and providing a first binary sequence a(0,1);
• a second AND circuit for receiving said binary input sequence x(0,1) and the complement Q(1,0) of said switch signal to obtain a second binary sequence b(0,1);
• and
• a summer for processing said first and second binary sequences to obtain said output sequence y(+1,0,-1) on an output terminal.

A Mach-Zehnder (M-Z) interferometer for modulating a continuous wave (CW) optical carrier with a duobinary encoded driving signal, said M-Z interferometer having a first and a second travelling wave-guide, a splitter between an input port and said first and second travelling wave-guides, a combiner between said first and second travelling wave-guides and an output port, a first and a second travelling wave electrode, each associated with said respective first and second travelling wave-guide, and a control electrode, said M-Z interferometer further comprising:

• means for applying a bias voltage V.sub.Bias between said first and said second travelling wave electrode;
• means for providing said duobinary encoded driving signal to said control electrode;
• means for coupling said CW optical carrier signal to said input port;
• and
• means for coupling a duobinary modulated optical signal from said output port into an optical fiber.

A Mach-Zehnder (M-Z) interferometer for modulating a continuous wave (CW) optical carrier with a duobinary encoded differential driving signal, said M-Z interferometer having a first and a second travelling wave-guide, a splitter between an input port and said first and second travelling wave-guides, a combiner between said first and second travelling wave-guides and an output port, a first and a second travelling wave electrode, each associated with said respective first and second travelling wave-guide, said M-Z interferometer further comprising:

• means for applying said modulating signal to a splitter side of said first travelling wave electrode and for applying said complementary modulating signal to a splitter side of said second travelling wave electrode;
• a first matched impedance for connection with a first end to a combiner side of said first travelling wave electrode and with a second end to ground;
• a second matched impedance for connection with a first end to a combiner side of said second travelling wave electrode and with a second end to ground;
• means for coupling said CW optical carrier signal to said input port;
• and
• means for coupling a duobinary modulated optical signal from said output port into an optical fiber.