Combined phase and intensity modulation in optical communication systems

A device (105) for modulating a digital optical signal (120), the said device (105) comprising:

• a) an optical intensity modulator (104), having at input a substantially continuous flow of optical radiation with linear polarization, to produce an encoded sequence of dark pulses (410) in the said optical signal (120), the said encoded sequence of dark pulses (410) being representative of a first data sequence (131) and having bit rate R and bit period TB=1/R,
• b) an electrical driving circuit (130) for the said optical intensity modulator (104), having at input a first electrical data signal (131) of type Non Return Zero (NRZ) at bit rate R, and a clock signal (132) at the same bit rate R having a delay ΔtAclock respect the said first electrical data signal (131),
• c) an electrical bias potential difference VA bias (106) applied to the said optical intensity modulator (104),
• d) an optical phase modulator (108) to produce an encoded phase modulation (420) in the said optical signal (120), the said encoded phase modulation (420) being representative of a second data sequence (141) and having bit rate R, amplitude Δφ, bit period TB=1/R and delay τφ−τA respect to the said encoded sequence of dark pulses (410),
• e) an electrical driving circuit (140) for the said optical phase modulator (108), having at input a second electrical data signal (141) of type Non Return Zero (NRZ) at the bit rate R, and a clock signal (142) at the same bit rate R having delay Δtφclock respect to the said second electrical data signal (141).

An apparatus (250) to receive a digital optical signal (225), the said apparatus (250) comprising:

• g) a non polarizing beam splitter (217) to divide the said optical signal (225) and apply the two portions to one or more polarization controllers (118),
• h) one or more polarization controllers (118) capable to substantially recover the initial power distribution between the two linear polarization components of the said optical signal (225),
• i) a first linear analyzer (122) to select a first of the two linear polarization components on which the said optical signal (225) power is distributed,
• l) a second linear analyzer (222) to select the second of the two linear polarization components on which the optical power of the said optical signal (225) is distributed,
• m) two non-polarizing beam splitters (124) to split the optical power of the said linear polarization components and to apply the two portions respectively to two optical intensity receivers (126) and two optical phase receivers (128),
• n) two optical intensity receivers (126) of the IM-DD type, for the detection of the optical intensity of the two linear polarization components of the said optical signal (225), comprehensive of electrical circuits for reversing the polarity of the received electrical signals,
• o) two optical phase receivers (128), for the detection of the optical phase of the two linear polarization components of the said optical signal (225), comprehensive of electrical circuits for the polarity inversion of the received electrical signals.

A method for transmitting a digital optical signal (120) comprising the steps of:

• a) modulating the intensity of a substantially continuous flow of optical radiation with linear polarization, to produce an encoded sequence of dark pulses (410) in the said optical signal (120), the said encoded sequence of dark pulses (Amplitude Tributary) being representative of a first data sequence (131) and having:
• a1) bit rate R and bit period TB=1/R,
• a2) reduced or zero optical intensity over a certain portion Δt1 (412) of the bit period TB, in correspondence of the bits of the said first data sequence (131) which are related to the said dark pulses,
• a3) nearly unperturbed intensity, in correspondence of the bits of the said first data sequence (131) which are not related to the said dark pulses,
• a4) in every case, nearly unperturbed intensity on a portion TB−Δt1 (414) of the bit period TB;
• b) modulating the optical phase in the said optical signal (120), to produce an encoded phase modulation (420), the said encoded phase modulation (Phase Tributary) being representative of a second data sequence (141) and characterized by:
• b1) having bit rate R, amplitude Δφ, bit period TB=1/R and delay τφ−τA respect to the said encoded sequence of dark pulses (410),
• b2) using the said portion TB−Δt1 (414) of the bit period TB to add a phase code representative of the said second data sequence (141).