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Fourier transform of the pulse train in Eq.(1). The Fourier transform reads 

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Here, J$_{n}$ (Φ 0 ) is the Bessel function of the order n and Φ 0 is the modulation index. When multiplied by exp ( − 1 / 2( ω$_{L}$ + n Ω − ω ) $^{2}$τ $^{2}$), it determines the shape of the power spectrum of the optical frequency comb. Depending on the value of Φ 0 , the power spectrum has different number of maxima as it is seen in Fig.5 for Φ 0 = 3 , 4 , 5 , 8. The increase in modulation index brings additional, intense peaks of modes into spectrum and broadens it. These maxima are located at different frequencies for different values of Φ 0 affecting the population dynamics in the λ -system. The power spectrum of the pulse train with the modulation amplitude Φ 0 =4 has three maxima, the highest one is at ω = 4 . 9 (which is in resonance with the ω 21 ), making the pulse train with Φ 0 =4, an optimal one for coherent accumulation of the population in the final state of the λ -system. It provides full population transfer to the ground electronic and rovibrational state and, thus, cooling the internal degrees of freedom 

