Accurate characterization of nonlinear photonic integrated circuits is fundamentally limited by the inability of linear transmission measurements to distinguish input and output on/off-coupling efficiencies, which enter only through their product. We show that this degeneracy arises from an inherent input-output symmetry of linear optics and can be broken by exploiting the directional asymmetry of nonlinear processes. We introduce bidirectional nonlinear optical tomography (BNOT), which combines forward and backward pumping of complementary nonlinear interactions to uniquely infer individual on/off-coupling efficiencies from off-chip measurements. By jointly modeling the nonlinear response and detection statistics, BNOT enables unbiased reconstruction of on-chip nonlinear figures of merit with significantly reduced uncertainty, whereas conventional calibration yields systematically biased estimates. Monte Carlo simulations demonstrate reliable convergence of the inferred efficiencies across realistic noise and fluctuation regimes. Our results establish nonlinear directionality as a general symmetry-breaking resource for coupling-resolved characterization in integrated photonics, with immediate implications for scalable quantum light sources, frequency conversion, and precision optical metrology.

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• Integrated optics
• Nonlinear optics
• Optical quantum information processing
• Quantum optics
• Tomography