“…If two pulses at WL and U>R are present the nondegenerate fourphoton interaction (II) w L + WR -* w 3 + w 4 is possible as well. If no signal at w 3 and u/4 is entered to the fibre, one speaks of stimulated four-photon mixing [4,5,13,[40][41][42][43], stimulated parametric four-photon interaction [44,45], stimulated four-wave parametric emission [46], or parametric four-photon generation (start from quantum noise and black-body radiation [37]). If a weak signal at either w 3 or w 4 is already present, one speaks of four-photon parametric amplification [4,5,10,47].…”