Real-time spectroscopy of modulation-instability-mediated optical wave breaking in normal dispersion

For advanced laser micromachining, ultrashort laser pulses that can provide both high peak power (typically kilowatt to megawatt) and short duration (hundreds of femtoseconds to picosecond) have emerged as powerful sources. Self-phase modulation (SPM) is an important nonlinear effect of ultrashort pulses' fiber delivery for pratical laser applications. In the normal-dispersion regime, the overlapping of the front and rear parts of ultrashort pules can give rise to the optical wave breaking (OWB) effect, which may, particularly when interplaying with other nonlinear effects, lead to versatile spectraltemporal characteristics of ultrashort pulses' propagation in the presence of seed instability and external perturbation. Thus, real-time spectraltemporal observation of ultrashort pulses' propagation in the normal-dispersion regime involved with multiple nonlinear effects may create new insight into the SPM-based spectral broadening. Unfortunately, real-time observation of the OWB-associated spectraltemporal dynamics has rarely been studied so far. To this end, here we report the real-time SPM-based spectral broadening in a normal-dispersion optical fiber. The spectral intensity stability and correlation are experimentally and theoretically found to be degraded in the presence of modulation-instability-mediated OWB effect. Further studies show that, the periodic and burst spectral intensity oscillations exist in the real-time spectraltemporal evolution. The complementary theoretical analysis indicates that other nonlinear effects, particularly the cross-phase and polarization modulation instability, are potentially initiated and interplay with the OWB effect to render the spectraltemporal instability.