A Free-Space Interferometer for Phase-Delay Measurements in Integrated Optical Devices in Degenerate Pump-and-Probe Experiments

The authors report on a free-space interferometer for simultaneous phase and transmittance measurements of an optical signal in an integrated photonic circuit during degenerate pump-and-probe experiments. Differentiation of the weak probe signal from the strong pump relies on a lock-in amplifier. The interferometer shows high flexibility in terms of operating wavelengths, tested devices, and optical powers; it works with any integrated optical device provided with optical input-output channels; it can operate both with high input power (up to 100 mW at the sample input port) and low output power ( < 1~\mu \text{W} at the detection stage). Measuring both the transmittance and the phase of the optical signal allows using the phasor representation in the analysis. A dynamic operating mode allows to perform power and/or wavelength scan of the input signal, obtaining transmittance and phase spectra of the tested device. Remote control of the system and insulation from the external environment brings to more stable operating conditions. Characterization of the system properties with photonic-integrated structures (waveguide and ring resonator) is reported, with observed phase stability as high as 0.25°/min and average phase noise below 1°.