Common-path dual-probe spectroscopic interferometry for ultrafast differential-phase diagnostics

Ultrafast phase spectroscopy requires interferometric stability at the milliradian level, yet conventional two-arm geometries suffer from thermal drift and acoustic noise, while scanning-wedge common-path designs introduce mechanical jitter. We present a passive common-path dual-probe interferometer in which a single fixed birefringent quartz crystal encodes the interpulse delay, achieving a root mean square (RMS) phase noise of ∼ 1.16 mrad under full amplified laser conditions, without active feedback or lock-in amplification. The orthogonal polarization geometry enables simultaneous detection of two polarization channels without waveplate reconfiguration, allowing direct decomposition of the nonlinear polarization response. The framework is benchmarked via polarization-dependent optical Kerr effect (OKE) measurements in sapphire, two-component relaxation dynamics in molecular liquids (CS₂, CCl₄, ethanol), and group delay dispersion (GDD) characterization cross-validated against frequency-resolved optical gating (FROG). This platform enables high-sensitivity ultrafast differential-phase diagnostics across nonlinear optics and molecular dynamics.