Optical signal modulation

A 2n quadrature amplitude modulation optical modulator comprising:

• an optical input for receiving an optical signal;
• a first optical splitter coupled to the optical input, the first optical splitter having a first output and a second output;
• a first optical modulation apparatus coupled to the first output of the first optical splitter which is arranged to apply a modulation scheme having 2n-2 constellation points to produce a first modulated optical signal representing an in-phase component;
• a second optical modulation apparatus coupled to the second output of the first optical splitter which is arranged to apply a modulation scheme having 2n-2 constellation point to produce a second modulated optical signal, representing a quadrature component;
• an optical combiner for combining the first modulated optical signal and the second modulated optical signal to produce an output modulated optical signal which is modulated with a modulation scheme having 2n constellation points, wherein each of the first optical modulation apparatus and the second optical modulation apparatus comprises a dual-drive Mach Zehnder modulator having an input optical splitter with an unequal split ratio.