Wavelength correlation maps in Raman supercontinuum generation

Summary form only given. In the long pulse regime, supercontinuum (SC) generation is very sensitive to the input noise of the pump pulses resulting in poor coherence properties and stability [1]. Recently, the susceptibility of the dynamics to the input noise was shown to induce large shot-to-shot fluctuations when the initial stage of the SC mechanism is driven by modulation instability, leading to the generation of rare pulses with abnormally high intensity [2]. Recently, the noise amplification characteristics of modulation instability and subsequent stages of SC generation were observed in real time using a time-stretching technique which allows to capture single-shot SC spectra [2,3]. In this way, significant insight can be obtained into the SC dynamics that would be otherwise impossible to observe from average quantities, and indeed wavelength correlations in the SC have been shown to exhibit a rich structure that can be connected to the energy exchange dynamics along propagation [3].Here, we report on the experimental investigation of the spectral correlations in the SC regime of cascaded stimulated Raman scattering (SRS) where a large number of Stokes orders are generated leading to the formation of a broadband SC spectrum. We launch 355 nm nanosecond pulses from a 10 kHz repetition rate Q-switched laser deep into the normal dispersion region of highly nonlinear fiber single-mode at the pump wavelength and above. These pulses lead to the generation of a SC whose dynamics are mainly dominated by cascaded SRS [1]. Using and a direct single-shot detection technique where the output end of the fiber is connected to the entrance aperture of a spectrograph with an ICCD camera mounted on the spectrograph exit aperture, we can capture the spectra produced by individual pulses. Significant variations are observed and the captured single-shot spectra allow us to study in detail the SC spectral correlation map at the fiber output as shown in Fig 1.The correlation map analysis reveal how the pump and all the Stokes orders are generally highly correlated except at the long wavelength edge of the mean spectrum, beyond 400 nm, where the Stokes lines appears to be somewhat anti-correlated with the lower Stokes orders. Such correlations are indicative of the energy exchange dynamics between the different cascaded Raman orders and allow to interpret the noise dependence between the different Stokes components. Specifically, the region of high correlations are indicative of the saturated regime of cascaded SRS with little wavelength jitter from noise effects whilst the unsaturated regime of SRS at the edge of the SC is associated with a region of anti-correlations and correspond to large shot-to-shot fluctuations. Similar correlation/anti-correlation features were recently observed in modulation instability dynamics using [4]. We anticipate the new type of correlation analysis reported here to provide new insights into the study of broadband nonlinear propagation dynamics and noise amplification processes.