Threshold characteristics of the 1.56 → 2.84 μm Raman transformation in methane under broadband pumping by powerful frequency-modulated pulses of erbium fiber source

The threshold characteristics of Raman lasing at 2.84 μm wavelength in a hollow revolver fiber filled with methane are studied as a function of the duration of broadband pump pulses with positive frequency modulation (chirp) at 1.56 μm wavelength. It was experimentally shown that the average effective Raman gain increases from 0.2 × 10^–2 to 3.3 × 10^–2 cm/GW, while the threshold pump intensity decreases from 36 to 1.4 GW/cm² with increasing effective pump pulse duration within the range of 10 – 321 ps (and the corresponding dimensionless linear chirp ∼10 – 300). The analysis of the threshold characteristics showed that the SRS conversion had high degree of non-stationarity even at the maximum width of the pump pulse envelope. According to the results of numerical simulation, the influence of dispersion and self-phase modulation in a hollow fiber contributes to significant increase in the SRS threshold in terms of the energy of the pump pulses, especially for positively chirped pulses with the duration of less than 100 ps.