Patent ID: 8358936

Claim:
An optical apparatus comprising: a quadrature phase shift keying modulator for generating a non-return-to-zero quadrature phase shift keyed NRZ-QPSK signal from a received lightwave, the modulator being driven by a radio frequency RF signal; an intensity modulator for carving the NRZ-QPSK signal to return-to-zero RZ pulses with a different duty cycle than that of the NRZ-QPSK, the intensity modulator being driven by a radio frequency RF clock signal to change the NRZ-QPSK signal; and an optical filter with a narrow band for filtering the signal from the intensity modulator; and an optical digital coherent detection system which receives the filtered signal; wherein the duty cycle of the RZ-QPSK signal is smaller than that of the NRZ-QPSK signal, the duty cycle of the RZ-QPSK signal being smaller than 0.5.