Temperature-stress phase-matching tuning in BBO crystals for high-efficiency quantum radiometric calibration

Entangled photon pairs generated via spontaneous parametric down-conversion (SPDC) in beta-barium borate (BBO) crystals are fundamental to the realization of anchor-free quantum metrology and spaceborne radiometric standards. High-precision calibration strictly necessitates rigorous spectral and spatial matching of these entangled photons. However, the spatial distribution of entangled photons exhibits a profound dependence on the phase-matching angle. Due to the combined effects of crystal fabrication tolerances, environmental perturbations, and the extremely low intensity of entangled signals (∼pW level), conventional mono-dimensional angle tuning struggles to balance the conflicting requirements of spectral bandwidth and spatial distribution matching. To address this bottleneck, a synergistic tuning strategy integrating angle, temperature, and stress is proposed in this work. A multi-channel, high-precision temperature control system based on series-connected thermoelectric coolers (TECs) was developed, achieving a control precision superior to 0.01 ℃ at a setpoint of 50 ℃. By combining theoretical derivations with thermo-structural coupled simulations, the tuning regularities of temperature (30–70 ℃) and stress on the photon output angle were quantitatively characterized. Experimental measurements on the multi-channel BBO crystal configuration (comprising eight crystals) achieved a maximum tuning range (Δ θ _OA ) of 2.941° and identified a non-destructive stress threshold of 312.5 N. This work elucidates the intrinsic mechanisms of temperature and stress in BBO phase-matching, providing critical engineering methodologies and theoretical underpinnings for enhancing quantum calibration accuracy while offering a paradigm for the multi-dimensional tuning of nonlinear optical crystals.