INTRODUCTION

Compact diode-pumped sources of the femtosecond pulses tunable within the wavelength range between 2 and 3 $\mu$m are of interest for various applications, such as laser surgery, remote sensing and monitoring, spectroscopy of semiconductors etc. To date only cryogenically operated Pb-salt diode lasers, optical parametrical oscillators and difference-frequency convertors were available for the operation in this spectral range. Therefore the possibility of the direct mid-IR lasing from the new class of the transition-metal doped chalcogenides  [1,2,3,4] has attracted much attention. The impressive advantages of these media are room-temperature operation between 2 and 3 $\mu$m, possibility of direct diode pumping, high emission and absorption cross-sections, negligibly low excited-state absorption and, as consequence, low thermal load (the basic laser material characteristics will be described in the next section). The most remarkable examples of the lasers under consideration are Cr$^{2+}$-doped ZnSe, ZnS and ZnTe. To date the following achievements for these media have been demonstrated: 1) for Cr:ZnSe CW operation with over 1.7 W power  [5], over 1100 nm tunability  [6], over 350 nm tunable diode-pumped CW operation  [7], active mode locking  [8] and active modulator assisted passive mode locking  [9] were achieved; 2) for Cr:ZnS pulsed  [2,10] and tunable CW operation  [11] were obtained. Cr:ZnTe, which is a member of the considered media class, remains unexplored. In spite of the numerous advantages, there exist some obstacles for femtosecond pulse generation from these lasers. As they are the semiconductors, i.e. possess a comparatively narrow band-gap, the nonlinear refraction in the active crystal is extremely large (see below). Hence the self-focusing has a low threshold which in the combination with the strong self-phase modulation produce a tendency to multiple pulse operation in the Kerr-lens mode locking (KLM) regime  [9,12]. So there is a need in the study of KLM stability limits and methods of stability enhancement in the lasers under consideration. In this paper we present the results of the numerical optimization of KLM aimed at the multipulsing suppression and taking into account a strong saturation of the Kerr-lens induced fast absorber. As a result, we demonstrate the possibility of few optical cycle generation from Cr-doped Zinc-chalcogenodes. The presented model is quite general and can be applied to the overall optimization of the different KLM lasers.

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