Threshold Characteristics of 1.56-to-2.84 μm SRS Conversion in Methane under Broadband Pumping by High-Power Frequency-Modulated Pulses of an Erbium Fiber Source

We study the threshold characteristics of SRS generation at a wavelength of 2.84 μm in a hollow-core revolver fiber filled with methane depending on the duration of positively chirped broadband pump pulses at a wavelength of 1.56 μm. It is experimentally proven that the average effective SRS gain increases from 0.2 × 10 –2 cm/GW to 3.3 × 10 –2 cm/GW while the threshold pump intensity decreases from 36 GW/cm 2 to 1.4 GW/cm 2 as the effective pump pulse duration increases in the range of 10 to 321 ps (and with a corresponding dimensionless linear chirp of ~10 to 300). An analysis of threshold characteristics showed that SRS conversion exhibits high nonstationarity even at the maximum width of the pump pulse envelope. According to the results of numerical simulation, the effect of dispersion and SPM in a hollow-core fiber contributes to a significant increase in the SRS threshold in pump pulse energy, especially for positively chirped pulses lasting less than 100 ps.