A coupling-in apparatus is provided for coupling light from a light-emitting diode into a fiber entry end of at least one optical fiber. The headset device includes a first headset, a second headset, and a transmission line. Determining the phase difference can comprise digitally determining a timing difference between the input signal and the feedback clock signal, digitally determining a transition of the input signal to produce a transition detect signal, and digitally updating the timing difference based on the transition detect signal and the feedback clock signal. The process is repeated for the highest body voltage. The disclosed coil isolators provide high voltage isolation and high voltage breakdown performance characteristics in small packages that provide a high degree of functionality at a low price.