Method and apparatus for optical phase modulation
A method of phase modulating optical radiation by the steps of phase modulating the optical radiation by using a modulator having an extinction ratio in order to provide a multilevel phase shift key signal, and applying to each optical pulse a phase-shift having an absolute value depending on the extinction ratio and a sign depending, for each of the optical pulses, on the respective optical phase value. An apparatus implementing the method is also disclosed.
1. An optical system comprising:
an optical modulator being configured to output a multilevel phase shift keyed optical signal including a stream of optical pulses, the optical modulator having an extinction ratio (ER lin ); and
a phase shifter optically coupled to the output of the optical modulator for receiving the multilevel phase shift keyed optical signal, the phase shifter applying to each of the optical pulses a phase shift having an equal magnitude and a sign, the magnitude of the phase shift selected based on the extinction ratio of the optical modulator.
2. The optical system of claim 1 , wherein the magnitude of the phase shift is inversely proportional to the extinction ratio.
3. The optical system of claim 1 , wherein the magnitude of the phase shift is approximately equal to
arc
tg
(
1
ER
lin
)
.
4. The optical system of claim 1 , wherein the magnitude of the phase shift is less than about π/10.
5. The optical system of claim 1 , wherein a sign of the phase shift is determined for each optical pulse based on a phase value of the optical pulse.
6. The optical system of claim 1 , wherein the optical modulator is a dual-drive Mach-Zehnder modulator.
7. The optical system of claim 1 , wherein the multilevel phase shift keyed optical signal is a quadrature phase shift keyed optical signal.
8. The optical system of claim 1 , wherein the multilevel phase shift keyed optical signal is an eight-level phase shift keyed optical signal.
9. A method of operating an optical phase-shifting device, the method comprising:
determining an extinction ratio (ER) of an optical modulator disposed upstream of the optical phase-shifting device;
receiving at the optical phase-shifting device a multilevel phase shift keyed optical signal output by the modulator, the multilevel phase shift keyed optical signal including a stream of optical pulses; and
calibrating the optical phase-shifting device to apply to each of the optical pulses a phase shift having an equal magnitude and a sign, the magnitude selected based on the extinction ratio.
10. The method of claim 9 , wherein the magnitude of the phase shift is selected in inverse proportion to the extinction ratio.
11. The method of claim 9 , wherein the magnitude of the phase shift is approximately equal to
arc
tg
(
1
ER
lin
)
.
12. The method of claim 9 , wherein the magnitude of the phase shift is less than about π/10.
13. The method of claim 9 , wherein a sign of the phase shift is determined for each optical pulse based on a phase value of the optical pulse.
14. The method of claim 9 , wherein the multilevel phase shift keyed optical signal is a quadrature phase shift keyed optical signal.
15. The method of claim 14 , wherein the quadrature phase shift keyed optical signal includes pulses having a first pair of phase values separated by π and pulses having a second pair of phase values separated by π, the second pair of phase values being separated from the first pair of phase values by π/2,
wherein a phase shift having positive sign is applied to optical pulses having a phase value in the first pair of phase values; and
wherein a phase shift having negative sign is applied to optical pulses having a phase value in the second pair of phase values.
16. The method of claim 9 , wherein the multilevel phase shift keyed optical signal is an eight-level phase shift keyed optical signal.