Patent ID: 8565621

Claim:
A method for detecting time skew between in-phase (I) and quadrature (Q) components of a quadrature phase shift keying (QPSK) signal comprising: at a coherent optical receiver, receiving an optical QPSK test signal; mixing the optical QPSK test signal with an oscillator signal, generated by an oscillator local to the coherent optical receiver, said mixing to produce a detected signal; sampling I and Q components of the detected signal with analog to digital converters and computing Fourier transforms of the I and Q components; filtering the Fourier transformed I and Q components; and correlating the filtered I and Q components to obtain the time skew as a result of said correlating, wherein the optical QPSK test signal comprises a first test signal having a first period longer than twice its bit period and a second test signal having a second period equal to twice its bit period, and where said receiving, said mixing, said sampling, said filtering and said correlating are performed on the first test signal to resolve multi-bit-period skew, and on the second test signal to resolve sub-bit-period skew.