Passive optical network comprising an optical exchange linked by at least one bidirectional optical fibre (20, 40 1, 40 N) to at least one line termination device (50 1, 50 N) of said network, said optical exchange comprising first means of transmitting a downlink optical data signal (SO D) phase-modulated in NRZ-DPSK format and first means of receiving an uplink optical signal (SO r), said line termination device (50 i) comprising coupling means (52 i) able to divide the optical power of the transmitted downlink optical signal (SO D) so as to obtain a first and a second downlink optical signal (SO D,i1, SO Di2), second optical means (55 i) of receiving and means (53 i) of generating and transmitting an uplink optical signal (SO Ri) from the second downlink optical signal (SO Di2), characterized in that: - said line termination device (50 i) furthermore comprises first means (54 i) of converting said first downlink optical signal (SO D,i1) phase-modulated in NRZ-DPSK format into an optical data signal (SO D,i1,duo) amplitude-modulated in duobinary modulation format, said second optical reception means (55 i) being able to receive said amplitude-modulated downlink optical data signal (SO D,i1, duo) amplitude-modulated in duobinary modulation format; said means (53 i) of generating and transmitting the uplink optical signal are able to generate and transmit an uplink optical signal phase-modulated in NRZ-DPSK format from said second downlink optical signal (SO Di2); said optical exchange comprises, prior to said first reception means, second means of converting said uplink optical signal (SO r) phase-modulated in NRZ-DPSK format into an optical data signal (SO R, duo) amplitude-modulated in duobinary modulation format, said first reception means being able to receive said amplitude-modulated uplink optical signal (SO R, duo); the first transmission means and the first reception means of the optical exchange are coupled to a same bidirectional fibre; and the generation and transmission means (53 i) and the second reception means (55 i) of the line termination device (50 i) are coupled to a same bidirectional fibre (40 i). - said line termination device (50 i) furthermore comprises first means (54 i) of converting said first downlink optical signal (SO D,i1) phase-modulated in NRZ-DPSK format into an optical data signal (SO D,i1,duo) amplitude-modulated in duobinary modulation format, said second optical reception means (55 i) being able to receive said amplitude-modulated downlink optical data signal (SO D,i1, duo) amplitude-modulated in duobinary modulation format; said means (53 i) of generating and transmitting the uplink optical signal are able to generate and transmit an uplink optical signal phase-modulated in NRZ-DPSK format from said second downlink optical signal (SO Di2); said optical exchange comprises, prior to said first reception means, second means of converting said uplink optical signal (SO r) phase-modulated in NRZ-DPSK format into an optical data signal (SO R, duo) amplitude-modulated in duobinary modulation format, said first reception means being able to receive said amplitude-modulated uplink optical signal (SO R, duo); the first transmission means and the first reception means of the optical exchange are coupled to a same bidirectional fibre; and the generation and transmission means (53 i) and the second reception means (55 i) of the line termination device (50 i) are coupled to a same bidirectional fibre (40 i).