A device includes an optical modulator configured to produce first and second modulated optical signals in response to a first electrical signal and a reference optical signal. The device further includes a phase detector configured to receive first and second encoded optical signals and the first and second modulated optical signals and to produce a second electrical signal indicating a phase difference between the first encoded optical signals and the first modulated optical signals. The device further includes a voltage-controlled oscillator configured to produce electrical pulses in response to the second electrical signal, and a counter configured to count a number of the electrical pulses, producing a count. The first electrical signal to the optical modulator is the count or a derivative of the count.