Retarded molecular processes in liquid-core fibers have a decisive effect on the broadband, nonlinear light generation known as supercontinuum generation. A reduction in bandwidth while improving the coherence of the spectra is a clear signature of modified soliton dynamics, the understanding of which promises highly stable, dynamically tunable, broadband light sources.

By: Mario Chemnitz // Markus Schmidt  

Due to long propagation lengths and degrees of freedom in dispersion management of intensive light pulses, optical fibers have proven to be a highly efficient platform for nonlinear light generation. In particular, the phenomenon of supercontinuum generation, i.e. the strong spectral broadening of a pump laser pulse (in this case as a result of complex formation processes (soliton fission) of optical self-preserving states (solitons) and associated radiation of dispersive waves in the short-wave range) is the focus of research and laser development. Supercontinuum sources with spectral ranges from UV to the mid-IR have great potential for applications in biophotonics, medical technology, and material analysis.

Conventional, partially commercialized methods for the generation of supercontinua are based on instantaneous, nonlinear processes in optical fibers and impress with several octaves of spectral bandwidth (e.g., fused silica: 400-2400 nm, ~2.5 octaves). However, these methods have decisive disadvantages in terms of preserving the spectral pulse-to-pulse stability, also referred to as temporal coherence. This drastically limits the applicability of fiber-integrated supercontinuum sources, especially with regard to the recompressibility of spectra for generating attosecond pulses or the pulse-wise usability of spectra for ultrafast spectroscopy methods in the GHz range. The origin of this spectral instability lies in the amplification of minute noise effects at the quantum level by modulation instabilities of the input pulse in the anomalous dispersion range.

Liquid-core fibers could break this stability limit and promise a lot of potential for nonlinear light generation due to the reconfigurability of the core medium and external dispersion control. Our current studies on broadband, nonlinear light generation in CS2/silica step-index fibers using a thulium pump laser (1.95 µm, 450 fs) provide initial insight into the complex broadening processes in the anomalous dispersion range under the strong influence of molecular nonlinearities (see Fig. 1), which is not yet fully understood.

The retarded, nonlinear dynamics in liquid-core fibers (e.g., the molecular reorientation of carbon disulfide molecules (CS2) with a decay time of 1.6 ps) enable a significant reduction in the input noise and thus contribute to maintaining coherence during the broadening process. We derived this effect on the basis of measured supercontinuum spectra, which show clearly recognizable spectral modulations (interference structures; see Fig. 2) in the short-wave infrared. These modulations appear in the spectrometer’s long measurement process solely due to a high coherence between the individual pulses. This correlates with simulation results of broadband, nonlinear pulse propagation based on the latest models of the optical properties of CS2 (such as absorption, dispersion, and a nonlinear response).

The simulation also enables the direct comparison of a realistic liquid-core fiber and a hypothetical glass fiber to investigate the influence of non-instantaneous nonlinearity on supercontinuum generation. A direct comparison of the spectra averaged from 100 individual spectra (see Figs. 3a and c) reveals a significant reduction in the spectral bandwidth of liquid-core fibers. Similar to the experiment, dispersive wave generation (DWG) is observed as the dominant broadening effect in liquid-core fibers, which is opposed to the occurrence of modulation instabilities (MI) in the glass fiber, recognizable by characteristic spectral modulations (see markings in Figs. 3a and c).

The emergence of MI represents a well-known limit in pulse duration (<150 fs) and peak power (so that the number of solitons is less than 15) of the pump pulse, above which slight photon noise is sufficient to cause the pump pulse to break up chaotically into many solitons of different phases. This process can be understood as noise amplification. Despite the very broad and flat mean spectrum (see Fig. 3c), such supercontinua are only of limited use, since the spectral correlation between the pulses is lost due to the chaotic nature of MI, which is reflected in the drastic deterioration of coherence (see Fig. 3d). In the case of liquid-core fibers, however, successive individual spectra match, and the coherence improves significantly (see Fig. 3b), even at pulse widths (450 fs) well above the usual stability limit.

In summary, it can be concluded from the comparison of experimental and numerical results that non-instantaneous nonlinearities significantly reduce the influence of noise effects on spectral broadening. These extraordinary properties of supercontinuum spectra are clear signatures of modified soliton dynamics. A direct, experimental demonstration of the improved coherence properties of the supercontinua of liquid-core fibers is currently not possible due to a lack of suitable reference systems (i.e., fiber systems with similar dispersion properties but glass-like nonlinearity). For this reason, we are investigating other core fluids such as carbon tetrachloride (CCl4) and tetrachloroethylene (C2Cl4), which differ greatly in their nonlinear response behavior [2]. Both liquids enable efficient supercontinuum generation in dispersion-optimized step-index fiber designs, with C2Cl4 in particular proving to be a potentially more transparent alternative to CS2, which will enable more precise knowledge about the modified soliton dynamics in significantly longer propagation lengths in the near future.

Funded by: DFG, Free State of Thuringia, ESF, ERDF