Fig. 2: Raman emission for TE-polarised light from an X-cut LN racetrack microresonator.

a Three Raman spectra (from top to bottom) at pump powers of 20, 200, and 400 mW, which correspond to the threshold power for the Raman oscillation with frequency shifts of 250 cm⁻¹, 628 cm⁻¹, and 875 cm⁻¹, respectively. The Stokes and pump are TE-polarised and are along the LN polar z-axis. Both the forward (blue) and backward (red) spectra are shown. The cascaded Raman peak from the 1st Stokes field is also observed at 1691 nm. b Raman emission power (top to bottom: 1st to 3rd Stokes) as a function of the pump power. The threshold for the 1st Stokes peak in the backward direction is 20 mW with a high differential conversion efficiency of 46% (linear fit shown with the black dashed line). The deviation from linear growth above a pump power of 60 mW is due to mini-comb formation and other Raman oscillations.