Quadrature amplitude modulation of optical carriers

1. An apparatus, comprising:

a Mach-Zehnder interferometer for encoding three or more data bits onto an optical carrier during each coding interval for a symbol, comprising:

an optical splitter having an input port and first and second output ports,

an optical combiner having first and second input ports and an output port,

a first optical path coupling the first output port of the optical splitter to the first input port of the optical combiner; and

a second optical path coupling the second output port of the optical splitter to the second input port of the optical combiner; and

a digital signal generator; and

wherein at least one of the first and second optical paths in the Mach-Zehnder interferometer includes an electro-optical phase shifter and an electro-optical attenuator; and

wherein the digital signal generator is configured to apply a first set of binary-valued voltage signals to a control input of the phase shifter and a second set of binary-valued voltage signals to a control input of the attenuator.

2. The apparatus of claim 1 , wherein each electro-optical phase shifter is configured to function as a binary phase key encoder.

3. The apparatus of claim 2 , wherein the other one of the optical paths includes an electro-optical attenuator and the digital signal generator is configured to apply binary-valued voltage signals to a control input of the attenuator of the other one of the optical paths.

4. The apparatus of claim 3 , wherein the apparatus is configured to modulate an optical carrier received at the input of the optical splitter according to a QAM constellation in response to the signals applied by the signal generator.

5. The apparatus of claim 4 , wherein the QAM constellation has, at least, 16 signal points.

6. The apparatus of claim 3 , further comprising a laser positioned to transmit laser light to the input port of the optical splitter; and wherein in response to said digital voltage signals, each of the electro-optical phase shifters is configured to phase shift received light from said laser light between two values that differ by about π.

7. The apparatus of claim 3 , wherein the first and second optical paths are configured such that light beams sent by the optical splitter to the first and second optical paths receive a relative phase shift of about π/2 up to integer multiples of π between being produced in the optical splitter and being recombined in the optical combiner.

8. The apparatus of claim 1 , wherein the digital signal generator is configured to cause the Mach-Zehnder interferometer to encode four data bits onto the optical carrier during each coding interval.

9. The apparatus of claim 1 , wherein the digital signal generator is configured to cause the Mach-Zehnder interferometer to encode more than four data bits onto the optical carrier during each coding interval.

10. A method for modulating an optical carrier, comprising:

splitting an input light beam into mutually coherent first and second light beams;

modulating the first light beam to encode, at least, two data bits thereon per symbol coding interval;

modulating the second light beam to encode one or more data bits thereon per the symbol coding interval; and

then, recombining the modulated first and second light beams to produce a modulated output light beam, the recombining producing a nonzero relative phase shift modulo integer multiples of π between the first and second light beams; and

wherein the modulating is such that a value of a first of the data bits determines a modulated phase of one of the first and second light beams during the symbol coding interval and a value of a second of the data bits determines a modulated amplitude of the one of the first and second light beams during the symbol coding interval.

11. The method of claim 10 , wherein the modulating is such that a value of a third of the data bits determines a modulated phase of the other one of the first and second light beams during the symbol coding interval and a value of a fourth of the data bits determines a modulated amplitude of the other one of the first and second light beams during the symbol coding interval.

12. The method of 10 , wherein the two values of the first of the data bits modulate the phase of the one of the first and second light beams by phases that differ by about π.

13. The method of claim 10 , wherein the output light beam is a quadrature amplitude modulated version of the input light beam.

14. The method of claim 13 , wherein the modulated output light beam is quadrature amplitude modulated according to a QAM constellation with 16 signal points.

15. The method of claim 13 , wherein the modulated output light beam is modulated according to a QAM constellation with more than 16 signal points.

16. The method of claim 10 , wherein the nonzero relative phase shift between the first and second light beams is π/2 modulo integer multiples of π.