Optical Parametric Amplifier

Physics: OPA: pump photon ω_p splits into signal ω_s + idler ω_i with energy conservation ω_p = ω_s + ω_i and momentum conservation k_p = k_s + k_i (phase matching). Parametric gain g ∝ χ⁽²⁾·E_pump. Crystal angle-tuning adjusts signal/idler wavelengths continuously. At low pump power (SPDC): spontaneous parametric down-conversion generates entangled photon pairs — cornerstone of quantum optics (Bell tests, quantum cryptography, quantum teleportation). Type II SPDC produces polarization-entangled pairs. OPO: add cavity mirrors for continuous-wave tunable output across visible/IR.