An electro-optic device, comprising: a Mach-Zehnder modulator; and one or more components to: apply a child DC bias with dither to arms (115-I L,115-I R) of a first branch of the MZM (110-I) and to arms (115-Q L,115-Q R) of a second branch of the MZM (110-Q); determine a second harmonic of a first return signal associated with applying the child DC bias with dither to the arms (115-I L,115-I R) of the first branch (110-I) and to the arms (115-Q L,115-Q R) of the second branch (110-Q); apply a child DC bias with phase-shifted dither to the arms (115-I L,115-I R) of the first branch (110-I) or to the arms (115-Q L,115-Q R) of the second branch (110-Q), the phase-shifted dither being out of phase from the dither; determine a second harmonic of a second return signal associated with applying the child DC bias with phase-shifted dither to the arms (1 15-I L,115-I R) of the first branch (110-I) or to the arms (115-Q L,115-Q R) of the second branch (1 10-Q); determine, based on the second harmonic of the first return signal and the second harmonic of the second return signal, whether the first branch and the second branch (110-Q) are operating at quadrature; and characterised to selectively adjust parent DC biases, applied to the first branch (110-I) and the second branch (110-Q), based on whether the first branch and the second branch (110-Q) are operating at quadrature. a Mach-Zehnder modulator; and one or more components to: apply a child DC bias with dither to arms (115-I L,115-I R) of a first branch of the MZM (110-I) and to arms (115-Q L,115-Q R) of a second branch of the MZM (110-Q); determine a second harmonic of a first return signal associated with applying the child DC bias with dither to the arms (115-I L,115-I R) of the first branch (110-I) and to the arms (115-Q L,115-Q R) of the second branch (110-Q); apply a child DC bias with phase-shifted dither to the arms (115-I L,115-I R) of the first branch (110-I) or to the arms (115-Q L,115-Q R) of the second branch (110-Q), the phase-shifted dither being out of phase from the dither; determine a second harmonic of a second return signal associated with applying the child DC bias with phase-shifted dither to the arms (1 15-I L,115-I R) of the first branch (110-I) or to the arms (115-Q L,115-Q R) of the second branch (1 10-Q); determine, based on the second harmonic of the first return signal and the second harmonic of the second return signal, whether the first branch and the second branch (110-Q) are operating at quadrature; and characterised to selectively adjust parent DC biases, applied to the first branch (110-I) and the second branch (110-Q), based on whether the first branch and the second branch (110-Q) are operating at quadrature. apply a child DC bias with dither to arms (115-I L,115-I R) of a first branch of the MZM (110-I) and to arms (115-Q L,115-Q R) of a second branch of the MZM (110-Q); determine a second harmonic of a first return signal associated with applying the child DC bias with dither to the arms (115-I L,115-I R) of the first branch (110-I) and to the arms (115-Q L,115-Q R) of the second branch (110-Q); apply a child DC bias with phase-shifted dither to the arms (115-I L,115-I R) of the first branch (110-I) or to the arms (115-Q L,115-Q R) of the second branch (110-Q), the phase-shifted dither being out of phase from the dither; determine a second harmonic of a second return signal associated with applying the child DC bias with phase-shifted dither to the arms (1 15-I L,115-I R) of the first branch (110-I) or to the arms (115-Q L,115-Q R) of the second branch (1 10-Q); determine, based on the second harmonic of the first return signal and the second harmonic of the second return signal, whether the first branch and the second branch (110-Q) are operating at quadrature; and characterised to selectively adjust parent DC biases, applied to the first branch (110-I) and the second branch (110-Q), based on whether the first branch and the second branch (110-Q) are operating at quadrature.