Fig. 1: Ingredients for a high-performance χ⁽³⁾ microchip OPO.

a, b (Left) Schematic illustration of OPO in two configurations, based on a single mode family (a) and a hybrid mode family (b) scheme. In each, an input pump (frequency ν_p) is coupled into the microring to generate output signal (ν_s) and idler (ν_i) if the participating modes are phase- and frequency-matched. In a, all modes are chosen from the same transverse-electric (TE) mode family (TE₀ in this case). In b, the modes are chosen from different transverse mode families, including transverse-magnetic (TM) mode families ({TE₀, TE₀, TM₀} for {ν_i, ν_p, ν_s} in this case). (Right) Frequency mismatch Δν (top) and expected OPO spectrum (bottom) for the two approaches. In a, frequency matching (Δν = 0) is achieved through a dispersion balance that results in weakly normal dispersion near the pump, which allows for spurious competitive processes that saturate the nonlinear gain. In b, the use of mode families with different effective indices enables frequency matching while supporting strong normal dispersion around the pump to largely suppresses spurious effects (note the difference in y-axis scale between the two cases). c The waveguide-resonator coupling must be carefully controlled to optimize conversion efficiency while keeping the operating power within a desirable range. To optimize the conversion efficiency into one output channel (in this case the idler) while maintaining sufficiently high loaded quality factors, it is advantageous to weakly overcouple the input pump (K_p = κ_p,ext/κ_p,int > 1), strongly overcouple the output idler (K_i = κ_i,ext/κ_i,int ≫ 1), and undercouple the output signal (K_s = κ_s,ext/κ_s,int < 1), as qualitatively indicated by the weight of the arrows denoting κ_int and κ_ext for each mode. d OPO output power and conversion efficiency sharply rise once the system crosses threshold, with subsequent growth depending on how well various parasitic processes are suppressed and how well the coupling is engineered. At high enough input powers, the OPO output power will eventually go down once the system becomes frequency mismatched due to Kerr and thermal shifts. Modulation instability is abbreviated as MI.