Phase meter based on zero-crossing counting of digitized signals

We developed a compact and easy-to-use phase meter based on a zero-crossing counting algorithm for digitized signals. Owing to the algorithm, the phase meter has low-noise and wide dynamic range. Low-noise differential phase measurements can be done for square waves (-204 $\mathrm{dBrad^{2}/Hz}$ for a 1-kHz, 1-$\mathrm{V_{p-p}}$ signal, 10 Hz-1 kHz offset, with cross-correlation) as well as sinusoidal waves, with a measurement error of $<1 \times 10^{-4}$ rad. We also demonstrated a direct phase measurement of an optical-beat note from a free-running laser over 10 decades (0.25 mHz-10 MHz) with a wide dynamic range of ~280 dB at 0.25 mHz. The phase meter can be an alternative for conventional phase meters and frequency counters in wide range of experiments.