Fig. 1

Principle of the 4MI phenomenon in the frequency domain. a Electrical field \(\tilde E(f)\) of a CW pump at frequency fpump when the 4MI phenomenon used for converting frequency combs occurs. The dashed lines represent the two amplification bands of bandwidth Δf and centered at a frequency detuning ±fopt from the pump. b 4MI phenomenon associated with a spectrally wide pump showing the broadening of the amplification bands compared to CW case (see also “Methods”)