Spatio-temporal cleaning of a femtosecond laser pulse by a filament conjugate mirror

Summary form only given. Optical phase conjugation is well known for its time reversal properties [1]. A phase conjugate mirror is formed when two laser beams of same frequency w meet from opposite directions in a transparent nonlinear medium. When a probe beam is retro-reflected by an optical phase conjugate mirror, wavefront distortions induced by a phase object are removed after re-crossing this object [2, 3]. In this presentation, we use two counter-propagating filaments to create a conjugate mirror with air as a non linear medium [4]. We demonstrate that it operates differently from a usual conjugate mirror. With a filament conjugate mirror, the reflected signal has essentially a perfect spatial mode independent of the initial probe beam profile.The principle of the experiment is represented on figure 1: two separate ultra-short pump laser pulses 1, 2 of equal peak power > 10 GW undergo filamentation in air and then meet from opposite directions to form a middle of the common two signal beams 4, 4´, filament phase conjugate mirror (FCM). A weaker fs probe pulse 3 is focused onto the region ionized by the filaments and one co-propagating with the probe beam and amplifying it, the other counter-propagating. Figure 2(a) shows the fluence profile of the incident probe 3. After reflection on the FCM, the pulse 4 displays a high quality beam profile as shown in figure 2(b). Temporal characterizations of the conjugate beam have also been performed. The FCM acts as a plasma mirror and increases the temporal contrast of the pulse (ratio between the peak intensity and any precursor light). is time coincident with the pump pulses, generating In conclusion, filament conjugate mirrors display unusual properties. They should find applications in situations where femtosecond laser pulses with high quality beam are required, for instance in the stage amplifier femtosecond laser system. early stages of a multi-stage amplifier femtosecond laser system.