Digital coherent optical receiver
A digital coherent optical receiver provided with a 90-degree optical hybrid circuit for detecting an in-phase signal and a quadrature signal of an input optical signal, includes first through fourth circuits. The first circuit calculates a square of a sum of the in-phase signal and the quadrature signal. The second circuit subtracts a squared value of the in-phase signal and a squared value of the quadrature signal from the calculation result of the first circuit. The third circuit detects a phase error of the 90-degree optical hybrid circuit based on the calculation result of the second circuit. The fourth circuit corrects at least one of the in-phase signal and the quadrature signal according to the phase error detected by the third circuit.
1. A digital coherent optical receiver provided with a 90-degree optical hybrid circuit for detecting an in-phase signal and a quadrature signal of an input optical signal, comprising:
a first circuit to calculate a square of a sum of the in-phase signal and the quadrature signal;
a second circuit to subtract a squared value of the in-phase signal and a squared value of the quadrature signal from the calculation result of the first circuit;
a third circuit to detect a phase error of the 90-degree optical hybrid circuit based on the calculation result of the second circuit; and
a fourth circuit to correct at least one of the in-phase signal and the quadrature signal according to the phase error detected by the third circuit.
2. The digital coherent optical receiver according to claim 1 , wherein the phase error of the 90-degree optical hybrid circuit is expressed by θ, and wherein
the third circuit calculates sin θ based on the calculation result of the second circuit, and
the fourth circuit comprises
a fifth circuit to calculate cos θ from the sin θ;
a sixth circuit to correct the in-phase signal using the cos θ; and
a seventh circuit to correct the quadrature signal using a product of the in-phase signal and the sin θ.
3. The digital coherent optical receiver according to claim 1 , further comprising
an addition circuit to calculate a sum of a squared value of the in-phase signal and a squared value of the quadrature signal, wherein
the second circuit subtracts the calculation result of the addition circuit from the calculation result of the first circuit.
4. A digital coherent optical receiver provided with a 90-degree optical hybrid circuit for detecting an in-phase signal and a quadrature signal of an input optical signal, comprising:
an amplitude error compensation circuit to generate a second in-phase signal by correcting the in-phase signal in such a way that amplitude information about the in-phase signal coincides with a target value and generate a second quadrature signal by correcting the quadrature signal in such a way that amplitude information about the quadrature signal coincides with the target value;
a first circuit to calculate a square of a sum of the second in-phase signal and the second quadrature signal;
a second circuit to subtract a squared value of the second in-phase signal and a squared value of the second quadrature signal from the calculation result of the first circuit;
a third circuit to detect a phase error of the 90-degree optical hybrid circuit based on the calculation result of the second circuit; and
a fourth circuit to correct at least one of the second in-phase signal and the second quadrature signal according to the phase error detected by the third circuit.
5. The digital coherent optical receiver according to claim 4 , wherein the phase error of the 90-degree optical hybrid circuit is expressed by θ, and wherein
the third circuit calculates sin θ based on the calculation result of the second circuit, and
the fourth circuit comprises
a fifth circuit to calculate cos θ from the sin θ;
a sixth circuit to correct the second in-phase signal using the cos θ; and
a seventh circuit to correct the second quadrature signal using a product of the second in-phase signal and the sin θ.
6. The digital coherent optical receiver according to claim 4 , wherein
the target value is 2 N where N is a integer,
the amplitude error compensation circuit corrects the in-phase signal and the quadrature signal in such a way that each of an average of a square of the in-phase signal and an average of a square of the quadrature signal coincides with the target value, and
the third circuit comprises
a first divider to divide the calculation result of the second circuit by 2;
an averaging circuit to average an output signal of the first divider; and
a second divider to detect the phase error by dividing the output signal of the averaging circuit by the target value.
7. The digital coherent optical receiver according to claim 6 , wherein
each of the first and second dividers is a bit shift circuit.
8. The digital coherent optical receiver according to claim 4 , wherein
the target value is 1,
the amplitude error compensation circuit corrects the in-phase signal and the quadrature signal in such a way that each of an average of a square of the in-phase signal and an average of a square of the quadrature signal coincides with the target value, and
the third circuit comprises
a divider to divide the calculation result of the second circuit by 2; and
an averaging circuit to detect the phase error by averaging the output signal of the divider.
9. The digital coherent optical receiver according to claim 8 , wherein
the divider is a bit shift circuit.
10. A digital coherent optical receiver provided with a 90-degree optical hybrid circuit for detecting an in-phase signal and a quadrature signal of an input optical signal, comprising:
a phase error compensation circuit to generate a first signal and a second signal, which represent the in-phase signal and the quadrature signal whose phase error of the 90-degree optical hybrid circuit is compensated for, according to the in-phase signal and the quadrature signal, wherein
the phase error compensation circuit comprises
a first calculation circuit to calculate a square of a sum of the first signal and the second signal;
a second circuit to subtract a squared value of the first signal and a squared value of the second signal from the calculation result of the first circuit;
a third circuit to detect the phase error of the 90-degree optical hybrid circuit based on the calculation result of the second circuit; and
a fourth circuit to generate the first signal and the second signal by correcting at least one of the in-phase signal and the quadrature signal according to the phase error detected by the third circuit.