0 200 400 600 800 t 0 0.2 0.4 0.6 0.8 1 population FIG. 6: Population transfer in the three-level λ -system using a standard optical frequency comb characterized by the radio-frequency equal to 5 GHz and zero offset frequency. Black curve shows population dynamics of the Feshbach state, red - same for the excited electronic state, green - same for the final, ultracold state. Parameters of the pulse train are the carrier frequency ω$_{L}$ = ω 32 =434.8 THz, and the pulse duration τ 0 =3 fs. For the λ -system, we apply a set of parameters that correspond to experimental data on molecular rovibrational cooling of loosely bound KRb molecules from the Feshbach states presented in [9]. In the described experiments, the STIRAP scheme was implemented to coherently transfer population from Feshbach states to the ground electronic triplet or the ground electronic singlet state with zero rovibrational quantum number. These deeply bound states were achieved with two µs pulses that were in resonance with the one-photon transi- tions between the molecular states involved into the process, which are the Feshbach state, the 2 $^{3}$Σ electronically excited state, and the triplet or singlet ground electronic states. In our model, we address the fundamentally cold state of the KRb molecule by using parame- ters that correspond to the experiment involving the singlet ground electronic state. These parameters are the ω 21 =309.3 THz, and the ω 32 =434.8 THz, giving the frequency of the two- photon transition ω 31 =125.5 THz. Fig.6 shows the population dynamics in the λ -system for Ω$_{R}$ = 0 . 1 ω 31 = 12.56 THz and T=0.2 ns, which results in full population transfer to the ultracold state within 238 pulses during 47.6 ns. Next 238 pulses restore full population in the initial state | 1 > - the Feshbach state - and the dynamics repeats. Within the lifetime of the Feshbach state, it is possible to transform the medium from highly vibrationally excited 10