Nonlinear integrated waveguide characterization by generalized heterodyne interferometry

Nonlinear heterodyne interferometry is a sensitive, quantitative method for measuring the Kerr nonlinearity of integrated photonic waveguides. Using a frequency-shifted Mach–Zehnder interferometer to detect the amplitude and phase of femtosecond pulses enables the direct retrieval of the nonlinear phase shift within integrated waveguides. This method can accurately determine the nonlinear coefficient gamma over five orders of magnitude, ranging from silica fibres to chalcogenide glass and silicon waveguides. When combined with numerical modelling of the nonlinear Schrödinger equation, this approach can be extended to strong nonlinear regimes involving free carrier dynamics, where pulse reshaping, absorption, and carrier dispersion compete with the Kerr effect. This unified framework bridges nonlinear metrology and ultrafast dynamics, offering a direct alternative to Z-scan and four-wave-mixing techniques. It establishes nonlinear heterodyne interferometry as a robust diagnostic tool for next-generation photonic integrated circuits and emerging high-nonlinearity materials.