Degenerate Four Wave Mixing (DFWM) and Nearly Degenerate Four Wave Mixing (NDFWM) have attracted a considerable attention in non-linear optics. DFWM has found several applications in phase conjugation, used in real time holography and adaptive optics. To realize a higher amplification of the conjugate wave, semiconductor amplifiers have been used. A theoretical analysis of the NDFWM process has been presented by Agrawal [1]. Here, an experimental and theoretical analysis of NDFWM in Distributed Feedback (DFB) lasers is proposed, where the FWM efficiency is enhanced by the cavity feedback mechanism.
We have investigated degenerate four-wave mixing (DFWM) in a strongly absorbing molecular gas. A theory taking into account saturable absorption of the interacting beams is used to explain the dependence of the DFWM reflectivity on laser intensity and on the molecular number density. These parameters have a great influence on the observed spectra, owing to linear absorption that suppresses the DFWM signal in the line center of the molecular transitions. The great sensitivity of the spectra to the experimental conditions limits the applicability of DFWM for spectroscopic studies in absorbing media.