Is there Raman lasing in optical parametric oscillators based on periodically poled lithium niobate?

A different phase-matching period of 24.4 to 31.0 mum is used in singly-resonant optical parametric oscillator (OPO) leading to idler wavelengths from 2.33 to 5.32 mum. A frequency shift depending on the poling period would be a non-ambiguous proof that not a Raman effect is observed.