Optical return-to-zero phase-shift keying with improved transmitters

A device 120 for modulating a digital optical signal 160, said device 120 comprising:

• a) an optical modulator with dual-drive design 104, having at input a substantially continuous flow of optical radiation, to modulate the optical power in said optical signal 160 with a train of optical pulses with repetition rate R (GHz) and to simultaneously modulate the optical phase in said optical signal 160 with a 4-levels phase driving signal Vφ 141 at symbol rate R (Gsymbol/s) and symbol period TB=1/R (s), representative of 2 input binary electrical tributaries 208, 209 having each the bit rate R,
• b) a clock signal 106 at frequency R/2 (GHz), delayed by (h+½)TB, respect to said phase signal 141, being h an integer number, and suitably amplified to produce the amplitude driving signal VA 140,
• c) an electrical pre-encoding circuit 130, having at input the 2 electrical tributaries 208, 209, generating the quaternary phase driving signal Vφ 141 suitable to be decoded by two DQPSK receivers,
• d) electrical circuits to analogically add 144 and subtract 145 said driving signals, according to the equations
• V1(t)=VA(t)+Vφ(t)+Vbias
• V2(t)=−VA(t)+Vφ(t) (7)
• whereas V1, V2 and Vbias are respectively the two driving voltages and the bias voltage applied to the modulator 104.

A method for transmitting a digital optical signal 160 with return-to-zero phase-shift keying format, comprising the steps of:

• a) modulating both the intensity and the phase of a substantially continuous flow of optical radiation using a single optical modulator with dual-drive design 104, to produce a train of optical pulses with M-ary phase modulation in said optical signal 160, being M an integer number,
• b) said train of optical pulses being generated using a clock signal 106 at frequency R/2 (GHz), having repetition rate R, period TB=1/R, and pulsewidth smaller than TB,
• c) said M-ary phase modulation assuming phase levels distributed between 0 and 2·π radians, being representative of n=log2M encoded input tributaries 108 with bit rate R (Gbit/s), having symbol rate R, symbol period TB=1/R and being synchronous to said train of optical pulses,
• d) obtaining said modulator 104 driving voltages 150, 151 by the analogical operations
• V1(t)=VA(t)+Vφ(t)+Vbias
• V2(t)=−VA(t)+Vφ(t), (9)
• with VA 140 and Vφ 141 respectively the amplitude and phase driving signals, and Vbias the modulator bias voltage.