摘要： Fluoride crystals (CaF2, SrF2) doped with neodymium Nd3+ and codoped with non-active ions as Y3+, La3+, or Gd3+ for breaking clusters of active ions, have become recently interesting active media for the diode-pumped mode-locked lasers. They possess broader emission spectra as well as longer fluorescence lifetime in comparison with widely used crystals such as Nd:YAG or Nd:YVO4. The femtosecond Nd,Y:SrF2 laser generating 97 fs pulses was reported in the Ti:sapphire pumped system [1]. We have reported continuously mode-locked operation of this laser pumped by a 2 W multimode laser diode and mode-locked using a semiconductor saturable absorber generating pulses with the duration of 258 fs [2]. In laser systems with La3+ codoped crystals, which have wider tunability than Y3+ codoped crystals, passively mode-locked 354 to 1200 fs and continuous-wave 1044 to 1081 nm tunable operation was reported in the system pumped by 4 W laser diode [3]. Recently a new Nd,Gd:SrF2 crystal has been investigated and the dual wavelength mode-locked picosecond operation was reported for pump power higher than 3.8 W [4]. We reported femtosecond operation of laser with this crystal and obtained pulses as short as 228 fs with output power of 2 x 50 mW in system pumped by 2 W laser diode for absorbed pump power of 1320 mW [5]. In the present contribution we report on operation of this novel laser with a Brewster cut crystal in passively mode locked regime for extremely low pump power. Laser system schematic is similar as in ref. [5] and is shown in Fig. 1. A 5 mm long Brewster cut 0.5%Nd, 5%Gd:SrF2 crystal was pumped by a multimode 2 W laser diode at 796 nm and passively mode-locked using a semiconductor saturable absorber mirror (SAM). Using a chirped mirror M3 with GDD equal to -650 fs2 it was possible to achieve continuously mode locked regime at absorbed pump power of only 320 mW, with output power of 2 x 12 mW behind the mirror M4 in linearly polarized beams with nearly Gaussian spatial profile and pulse duration of 740 fs (assuming sech2 pulse shape). For higher absorbed pump power of 632 mW (905 mW incident on the crystal) the total output power of 2 x 20 mW was achieved with a pulse duration of 600 fs. The autocorrelation curve is shown in Fig. 2 together with the measured spectrum 3.3 nm wide, and centered at 1051.5 nm. The absorbed pump power is 2 - 4 times smaller in comparison with our previous results. The calculated time-bandwidth product of 0.536 gives potential to get shorter pulses (down to 352 fs) with optimum GVD compensation.