Various embodiments include methods, apparatus, and systems to operate a tool downhole in a well, where the tool has sensing system to determine different properties of downhole structures. Such an apparatus can include a received signal that is input to a beam splitter with 5 a local oscillator signal. The beam splitter outputs light signals to first and second photodetectors that convert the respective signals to electrical signals. The electrical signals are input to differential amplifier that generates an amplitude representative of the phase difference between the two input signals. A feedback path converts that 10 amplitude to a phase adjustment signal that is couple to the local oscillator.