Mode-Iocking of a Cr 2+: ZnSe laser using a PPLN non linear mirror: theoretical modelling and cavity design J.-B. Dherbecourt, J.-M. Melkonian, M. Raybaut, A. Godard, M. Lefebvre and E. Rosencher ONERA, The French Aerospace Lab, Chemin de la Huniere, F-9176i Palaiseau, France
[email protected]
Since its first demonstration as a solid optical gain medium [1], Cr+:ZnSe (A = 2 .5~m) has shown interesting lasing properties, such as a wide emission spectrum, and has become a promising mid-infrared source. Until now, wide tunability (2000nm - 3000nm) has been obtained in CW regime, and ultrashort-pulse generation has been demonstrated from SESAM [2] or self-starting Kerr lens mode-lock [3]. These techniques generally require high pump power (several Watts in CW regime), or specific devices (SESAM based on multiple InAs/GaSb quantum wells). Here, we propose to apply to Cr+ :ZnSe an original mode-locking technique, using an intracavity second order nonlinear mirror (Fig. 1), proposed by K. A. Stankov [4], and efficiently demonstrated on near-infrared (A = 1 urn) laser sources. Indeed, efficiency and reliability reached by second order non linear crystals, such as PPLN, allow a transposition of the nonlinear mirror concept from near-infrared to mid-infrared laser sources.
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of Stankov non linear mirror (a) : the laser wave is converted forward by SHG, second harmonic is completely reflected whereas fundamental is partially reflected , the two waves undergo a 1t phase-shift so that the SHG wave is back-converted to the laser wave. The laser wave thus sees an effective nonlinear reflectivity. Nonlinear reflectivity as a function of incident pump intensity for a 20 mm long PPLN and different output couplers (b).
We investigate numerically Cr+:ZnSe mode-locking by intra-cavity nonlinear mirror, based on a PPLN crystal. We explore the effects of different parameters (output coupling, non linear efficiency ... ) on the laser temporal dynamics, and show that, depending on various parameters , the laser can display a CW, or pulsed (Qswitched and/or mode-locked) behaviour. The simulation is carried out for the characteristics of a Tm 3+:YLF pumped CW Cr+:ZnSe laser available in our lab. We numerically show the possibility of mode-locking in the picosecond regime, with pulse durations around 100 ps, and study the dynamics and stability of this behaviour (Fig. 2). 1.00
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