Limits of coherent supercontinuum generation in normal dispersion fibers

Abstract

We study the largely unexplored transition between coherent and noise-seeded incoherent continuum generation in all-normal dispersion (ANDi) fibers and show that highly coherent supercontinua with spectral bandwidths of one octave can be generated with long pump pulses of up to 1.5 ps duration, corresponding to soliton orders of up to N = 600. In terms of N, this corresponds to an approximately 50 times increase of the coherent regime compared to anomalous dispersion pumping. In the transition region between coherent and incoherent spectral broadening we observe the manifestation of nonlinear phenomena that we term incoherent cloud formation and incoherent optical wave breaking, which lead to a gradual or instantaneous coherence collapse of SC spectral components, respectively. The role played by stimulated Raman scattering and parametric four-wave mixing during SC generation in ANDi fibers is shown to be more extensive than previously recognized: their nonlinear coupling contributes to the suppression of incoherent dynamics at short pump pulse durations, while it is responsible for non-phasematched parametric amplification of noise observed in the long pulse regime. We further discuss the dependence of SC coherence on fiber design, and present basic experimental verifications for our findings using single-shot detection of SC spectra generated by picosecond pulses. This work outlines both the further potential as well as the limitations of broadband coherent light source development for applications such as metrology, nonlinear imaging, and ultrafast photonics, amongst others.

Amplitude noise and coherence degradation of femtosecond supercontinuum generation in all-normal-dispersion fibers

“…2(a)). In particular, good coherence is seen to require pulse durations below 100 fs, which is an order of magnitude shorter than the corresponding limit found without this even weak laser amplitude noise of 0.3% [12,14].…”

“…In ANDi SC generation, it has been shown that the pulse duration, fiber length, and peak power have a critical influence on the noise properties [12,14], e.g., the anticipated coherent spectra are only obtained for sufficiently short pulse durations and fiber lengths. To see the effect of pump laser amplitude noise on the coherence and the requirements on the fiber length and pulse parameters, we simulate a wide parameter space and calculate for each case the spectrally averaged coherence given by [12,20]:…”

“…These results show that the limits for high coherence, in terms of pulse duration and fiber length, suggested in previous publications, change substantially when technical pump laser noise is included. Indeed, we find that while a high coherence can be maintained for a pulse duration below 1.2 ps in a system without technical pump laser noise and only with one polarization [12], a pulse duration of ∼ 50 fs will begin to lose coherence even when a relatively low noise pump laser is properly described.…”

“…The reason for this high coherence comes from the coherent spectral broadening mechanisms of self-phase modulation (SPM) and optical wave-breaking (OWB) [12,13]. In fspumped ANDi SC generation these mechanisms are considerably more efficient than the incoherent nonlinear effects of noise-seeded modulation instability and stimulated Raman scattering [1], allowing for octave spanning SC generation in which these incoherent effects are suppressed for sufficiently short fiber lengths and input pulse durations [12,14].…”

“…Several theoretical studies about SC coherence have been reported in anomalous dispersion fiber, covering pulse durations from fs up to continuous wave, and including several kinds of noise, such as one-photon-per-mode (OPM) quantum noise added as pure phase noise in the frequency domain, Wigner-representation based quantum noise added in the time domain as amplitude and phase noise, Raman noise, and polarization noise [15][16][17][18][19][20][21][22][23][24][25][26]. This is not the case for theoretical studies of ANDi SC generation, which have mostly considered only the OPM noise [1,12,20], except for a recent work of Gonzalo et al [14], in which relative intensity noise (RIN) was experimentally and numerically compared, for ANDi SC generation with 170 fs pump pulses. In this study, it was found that OPM noise was too weak to describe the experimentally observed noise, whereas adding 1 % amplitude fluctuations of the laser gave better agreement with the experimental results.…”