L f r |1> |2> |3> FIG. 1: The three-level λ -system modeling the molecular energy levels, involved into cooling dynamics. State | 1 > is the Feshbach state, state | 2 > is the transitional, electronic excited state, and state | 3 > is the cold molecular state. to the final state | 3 > , Fig.(1). We assume that state | 1 > is the Feshbach state, state | 2 > is the transitional, electronic excited state, or state manifold, and state | 3 > is the ultracold molecular state. For Fechbach molecules, the energy splitting is large enough to justify the validity of three-level model for the description of their interaction with the frequency comb aiming to coherently create ultracold molecules. Parameters of the λ -system correspond to data on molecular rovibrational cooling of loosely bound KRb molecules from the Feshbach states, discussed in [8]. Coherent popula- tion accumulation was achieved in the deeply bound ground electronic state with vibrational quantum number v=22. The related parameters of the molecular system, implemented in our model, are ω 21 =340.7 THz, ω 32 =410.7 THz, and ω 31 =70 THz. In the discussion below, all frequency and time parameters will be given in the units of [ ω 31 ] or [ ω − 1 31 ] for convenience of interpretation, then, e.g., ω 21 = 4 . 9, and ω 32 = 5 . 9. The field frequencies are in resonance with the one-photon transitions in the three-level λ -system, the carrier frequency ω$_{L}$ = ω 32 , and the modulation frequency Ω = ω 21 . The peak Rabi frequency is Ω$_{R}$ = ω 31 , the pulse duration is τ =0.25, (3 fs), and the pulse train period is T = 6400 τ , (20 ps), giving the radio frequency f$_{r}$ = 6 . 25 · 10 − $^{4}$, (43 GHz). A single pulse area is estimated to be 0.2 π . The mod- ulation of the carrier frequency having value 410.7 THz assumed to be done at frequency 340.7 THz. To achieve IR modulation, an approach, described in [14], on efficient generation of a Raman-type optical frequency comb in an enhancement cavity may be applied. The technique provides with the whole comb bandwidth covering 300-900 THz. 4