An optical transmission system comprising:
- an optical field receiver;
- and
- an optical field transmitter,
- wherein the optical field transmitter includes:
- a light source;
- one or more DA converters;
- an optical field modulator;
- a complex information multilevel signal generator circuit;
- and
- a phase pre-integration circuit,
- wherein the complex information multilevel signal generator circuit converts an information signal of a plurality of bits input at given time intervals into a complex multilevel information signal to output the complex multilevel information signal,
- where the phase pre-integration circuit receives the complex multilevel information signal output from the complex information multilevel signal generator circuit and sampled at the given time intervals, and outputs phase pre-integration complex information obtained by integrating a phase component of the complex multilevel information signal by a given time interval in advance,
- wherein the one or more DA converters each convert the complex multilevel information signal including the phase pre-integration complex information output from the phase pre-integration circuit into an analog signal, and outputs the converted analog signal to the optical field modulator,
- wherein the optical field modulator modulates light output from the light source into a optical field signal by using the analog signal, and transmits the modulated optical field signal,
- wherein a phase angle of the complex multilevel information signal at a complex signal point is any one of values of integral multiples obtained by dividing 360 degrees by a positive integer N,
- wherein an amplitude value of the complex multilevel information signal at the complex signal point is any one of values of a positive integer M,
- wherein a total number of the complex signal points that the complex multilevel information signal may take is lower than a product of N and M,
- wherein the optical field receiver includes:
- an optical splitter;
- two or more optical delay detection receivers different in interference phase from each other by a given delay amount;
- two or more AD converters;
- and
- a digital phase processing circuit,
- wherein the optical field receiver receives a phase pre-integrated optical field signal output from the optical field transmitter,
- wherein the optical splitter splits the received optical field signal into at least two optical field signals,
- wherein the plurality of optical delay detection receivers receive the split optical field signals at the same time,
- wherein the two or more AD converters each convert signals output from the plurality of optical delay detection receivers into respective digital signals, and
- wherein the digital phase processing circuit calculates a differential phase between a received complex optical field signal and a previously received complex optical field signal.