Cascaded RZ and NRZ laser modulators having RZ/NRZ phase alignment bias control

摘要

An optical transmitter architecture has a return-to-zero (RZ) laser modulator coupled in cascade with a non-return-to-zero (NRZ) laser modulator. DC bias and modulation signal drive levels of the two modulators are controlled using dither-based feedback, so as to align the phase of the RZ signal with that of the NRZ data. A first method uses an RZ quadrature phase dither or an RZ hillclimber phase dither, to apply a relatively low frequency dither signal to a variable RF delay for the RZ clock signal applied to the RZ modulator. A second method is an RZ quadrature phase dither scheme that applies equal amplitude and opposite phase dither signals to perform complementary amplitude modulation of both the RZ and NRZ MZ modulators simultaneously.