Pulse-to-pulse coherence between stokes pulses generated by stimulated Raman scattering in the transient regime

Abstract

Hollow-core photonic crystal fibers (HC-PCF) opened new perspectives towards multi-octave comb generation and waveform synthesis using stimulated Raman scattering (SRS) generation in gases introduced in its core [1, 2]. In the transient regime of SRS [3], a short pump pulse combined to a high gain can amplify a few spatial-temporal modes (STM) from the quantum noise. It is even possible to have only one coherent STM mode amplified to the macroscopic level if the net gain is high enough, which in HC-PCF configuration can reach extremely high values (>1000). Furthermore, the close-to-single mode guidance of these HC-PCF acts as spatial filter for the STM by leaving only the mode with the highest Raman gain coefficient. This process was proven to be an efficient means to generate a large Raman comb which spectral lines are phase coherent within a single pump pulse. On the other hand, previous works showed that in transient regime pulse-to-pulse phase locking between Stokes can be achieved for pulse-delay of 2.6 ns, which is ∼9 times longer than the molecular dephasing time [4]. Also, it was shown that this Raman coherence survival time is proportional to the initial Raman excited molecules which scales exponentially with the net gain. Consequently, the use of HC-PCF could enable to have pulse-to-pulse phase locking with much longer delay using the same configuration as in [1, 2]. Here, we report on a phase-coherence between two Stokes pulses generated in H2-filled HC-PCF with time-spacing as large as 28 ns. The experimental set-up is shown in Fig. 1(a) where two pulses, emitted from a 1064 nm microchip laser with a 500 Hz repetition rate and a pulse energy of 50 μΐ, are delayed by τ and coupled into a 4 m-long H2-filled Photonic Band Gap (PBG) HC-PCF to generate SRS (see Fig. 1 (c) for typical spectra for different pump powers). A pair of delayed first-order rotational Stokes pulses at 1135 nm are extracted and passed through a second delay stage to be temporally superposed [Fig. 1(b)], and sent to a CCD camera for interference fringes measurement. Figure 1(d) shows the measured visibility evolution with τ. The results show that interference fringes [insets in Fig. 1(d)] are observed even for τ = 28 ns. The fit of the τ-evolution of the visibility shows an exponential decrease.