Optical modulator for higher-order modulation

1. An optical device, comprising:

a Mach-Zehnder modulator (MZM) adapted to modulate a phase and an amplitude of an optical input signal; and

a first drive circuit adapted to drive the MZM based on j+1 electrical binary signals so as to cause the MZM to impose either a first phase shift or a second phase shift that differs from the first phase shift by about π radians, wherein the optical device is adapted to apply 2 k−1 different phase shifts based on k−1 additional electrical binary signals to further modulate the phase and produce a modulated optical output signal corresponding to a star M-QAM constellation, where j is a positive integer, k is an integer greater than one, and M=2 j+k .

2. The invention of claim 1 , wherein the first drive circuit is adapted to drive the MZM based on the j+1 electrical binary signals so as to produce 2 j different amplitudes in the modulated optical output signal.

3. The invention of claim 1 , wherein said MZM is adapted to apply said 2 k−1 different phase shifts and produce the modulated optical output signal.

4. The invention of claim 3 , wherein:

the MZM is a dual-drive MZM;

the first drive circuit is further adapted to drive the MZM based on said k−1 additional electrical binary signals; and

the optical device further comprises a second drive circuit adapted to drive the MZM based on said j+1 electrical binary signals and said k−1 additional electrical binary signals.

5. The invention of claim 4 , wherein the first and second drive circuits are adapted to:

drive the MZM to enable push-pull modulation using said j+1 electrical binary signals; and

further drive the MZM to enable push-push modulation using said additional k−1 electrical binary signals.

6. The invention of claim 1 , further comprising a phase shifter external to the MZM, which phase shifter is adapted to apply said 2 k−1 different phase shifts.

7. The invention of claim 6 , wherein:

the MZM is adapted to generate an intermediary modulated optical signal corresponding to an intermediary constellation consisting of 2 j+1 constellation points that are arranged on a straight line in a corresponding in-phase/quadrature-phase plane; and

the application of said 2 k−1 different phase shifts to the intermediary modulated optical signal results in incremental rotation of the intermediary constellation, said rotation producing the star M-QAM constellation.

8. The invention of claim 1 , wherein j=1 and k=3.

9. The invention of claim 1 , further comprising a logic circuit adapted to process some or all of the j+1 electrical binary signals to derive one or more identifiers for one or more circles of the star M-QAM constellation, wherein the optical device is further adapted to apply to the phase an additional phase shift based on said one or more identifiers.

10. The invention of claim 9 , wherein:

different identifiers correspond to different additional phase shifts; and

the logic circuit comprises a logic element adapted to apply an exclusive OR function to two of the j+1 electrical binary signals.

11. The invention of claim 1 , further comprising a pulse carver adapted to form a pulse train from the modulated optical output signal.

12. The invention of claim 1 , wherein:

the first drive circuit comprises a plurality of amplifiers, each adapted to amplify a respective one of said j+1 electrical binary signals to produce a corresponding amplified electrical signal; and

the first drive circuit is adapted to combine the resulting j+1 amplified electrical signals to produce a multilevel drive signal for driving the MZM.

13. The invention of claim 12 , wherein the first drive circuit is further adapted to:

superimpose the multilevel drive signal onto a bias voltage corresponding to an optical null of the MZM; and

drive the MZM with the resulting superimposed signal.

14. A method of modulating an optical signal, comprising:

modulating a phase and an amplitude of an optical input signal using a Mach-Zehnder modulator (MZM) and based on j+1 electrical binary signals, wherein the MZM imposes either a first phase shift or a second phase shift that differs from the first phase shift by about π radians, where j is a positive integer; and

further modulating the phase by applying 2 k−1 different phase shifts based on k−1 additional electrical binary signals to produce a modulated optical output signal corresponding to a star M-QAM constellation, where k is an integer greater than one and M=2 j+k .

15. The invention of claim 14 , wherein the step of further modulating the phase is implemented using the MZM.

16. The invention of claim 14 , wherein the step of further modulating the phase is implemented using a phase shifter external to the MZM, wherein:

the step of modulating the phase and the amplitude comprises (i) generating an intermediary modulated optical signal corresponding to an intermediary constellation consisting of 2 j+1 constellation points that are arranged on a straight line in a corresponding in-phase/quadrature-phase plane and (ii) applying the intermediary modulated optical signal to the phase shifter;

the step of further modulating the phase results in incremental rotation of the intermediary constellation, said rotation producing the star M-QAM constellation.

17. The invention of claim 14 , further comprising:

based on some or all of the j+1 electrical binary signals, deriving one or more identifiers for one or more circles of the star M-QAM constellation; and

applying to the phase an additional phase shift based on said one or more identifiers.

18. An optical device, comprising:

an optical modulator adapted to modulate a phase and an amplitude of an optical input signal; and

a first drive circuit adapted to drive the optical modulator based on j+1 electrical binary signals so as to cause the optical modulator to impose either a first phase shift or a second phase shift that differs from the first phase shift by about π radians, wherein the optical device is adapted to apply 2 k−1 different phase shifts based on k−1 additional electrical binary signals to further modulate the phase and produce a modulated optical output signal corresponding to a star M-QAM constellation, where j is a positive integer, k is an integer greater than one, and M=2 j±k .