Method and apparatus for optical phase modulation

1. A method for modulating optical radiation, comprising the steps of:

phase-modulating the optical radiation with a modulation signal, by using a modulator, so as to obtain a multi-level phase shift key optical signal comprising a stream of optical pulses, wherein each of said optical pulses has a respective optical phase value related to said modulation signal; and

applying to each of said optical pulses a respective phase-shift, each respective phase shift having a substantially equal magnitude, each respective phase shift having a sign determined as a function of the respective optical phase value.

2. The method of claim 1 , wherein the magnitude of said phase-shift is substantially constant in each of said optical pulses.

3. The method of claim 1 , wherein the magnitude of said phase-shift is equal to or less than about π/10.

4. The method of claim 1 , wherein said modulator has an extinction ratio and the magnitude of said phase-shift is determined as a function of said extinction ratio.

5. The method of claim 4 , wherein the magnitude of said phase-shift is equal to about

arc

⁢

⁢

tg

⁡

(

1

ER

lin

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,

wherein ER lin is the extinction ratio.

6. The method according to claim 4 , wherein said function is a decreasing function with increasing said extinction ratio.

7. The method of claim 1 , further comprising the steps of:

providing a first and a second logical signal (S 1 , S 2 ) linked with said modulation signal;

generating a first and a second driving signal for said modulator from said first and second logical signal; and

establishing said sign as a logical function of said logical signals.

8. The method of claim 7 , wherein said logical function is equivalent to NOT[XOR(S 1 ,S 2 )].

9. The method according to claim 1 , wherein said multilevel phase shift key optical signal is a differential multilevel phase shift key optical signal.

10. A method of optical communication comprising transmitting an optical signal at a first location and receiving the optical signal at a second location different from the first location, wherein transmitting comprises modulating the optical signal by performing at least steps of:

phase-modulating the optical signal with a modulation signal, so as to obtain a multi-level phase shift key optical signal comprising a stream of optical pulses, wherein each of said optical pulses has a respective optical phase value related to said modulation signal; and

applying to each of said optical pulses a respective phase-shift, each respective phase shift having a substantially equal magnitude, each respective phase shift having a sign determined as a function of said respective optical phase value.

11. An electro-optical apparatus for modulating optical radiation based on a modulation signal, comprising:

an optical modulator capable of receiving optical radiation and generating a multilevel phase shift key optical signal including a stream of optical pulses, each having a respective optical phase value related to said modulation signal, said optical being capable of being driven by a first and a second driving signal (S 1 , S 2 );

a phase-shifter optically connected to the optical modulator, capable of applying to the phase of each optical pulse a phase-shift having a magnitude and a sign depending, for each of the optical pulses, on said respective optical phase value; and

a logical circuit capable of generating a third driving signal (S 3 ) logically related to said first and second driving signals, the logical circuit being logically connected to the phase-shifter for feeding said third driving signal to said phase-shifter.

12. The apparatus according to claim 11 , wherein said third driving signal (S 3 ) determines said sign of said phase shift.

13. The apparatus according to claim 11 , wherein said optical modulator comprises a dual-drive Mach-Zehnder modulator.

14. The apparatus according to claim 11 , wherein said optical modulator is an optical modulator capable of receiving optical radiation and generating a quadrature phase shift key optical signal.

15. The apparatus according to claim 11 , wherein said third driving signal (S 3 ) is logically related to said first and second driving signals (S 1 , S 2 ) through the logical relationship NOT[XOR(S 1 ,S 2 )].

16. An optical transmitter comprising an optical source optically coupled to an electro-optical apparatus for modulating optical radiation based on a modulation signal, the electro-optical apparatus comprising:

an optical modulator capable of receiving optical radiation and generating a multilevel phase shift key optical signal including a stream of optical pulses, each having a respective optical phase value related to said modulation signal, said optical modulator being capable of being driven by a first and a second driving signal (S 1 , S 2 );

a phase-shifter optically connected to the optical modulator, capable of applying to the phase of each optical pulse a phase-shift having a magnitude and a sign depending, for each of the optical pulses, on said respective optical phase value; and

a logical circuit capable of generating a third driving signal (S 3 ) logically related to said first and second driving signals, the logical circuit being logically connected to the phase-shifter for feeding said third driving signal to said phase-shifter.

17. An optical communication system comprising an optical transmitter for transmitting an optical signal, an optical receiver for receiving the optical signal, and an optical communication line connecting the transmitter to the receiver, wherein the transmitter comprises an electro-optical apparatus for modulating optical radiation based on a modulation signal, the electro-optical apparatus comprising:

an optical modulator capable of receiving optical radiation and generating a multilevel phase shift key optical signal including a stream of optical pulses, each having a respective optical phase value related to said modulation signal, said optical modulator being capable of being driven by a first and a second driving signal (S 1 , S 2 );

a phase-shifter optically connected to the optical modulator, capable of applying to the phase of each optical pulse a phase-shift having a magnitude and a sign depending, for each of the optical pulses, on said respective optical phase value; and

a logical circuit capable of generating a third driving signal (S 3 ) logically related to said first and second driving signals, the logical circuit being logically connected to the phase-shifter for feeding said third driving signal to said phase-shifter.