Bright dispersive waves in dual-core microstructured fiber under different laser pumps

Summary form only given. Dispersive wave (DW) generation in microstructured optical fibers (MOFs) is a hotly debated topic in supercontinuum generation. DWs are a typical outcome of strong nonlinear wave interactions, however their spectral positions are mainly ruled by the linear dispersive properties of the fiber: DWs appear at those spectral positions where wavevectors of DWs and soliton-like pulses are matched [1]. In this work we experimentally and numerically discuss how super-bright sideband generation around 1550 nm can be achieved in dual concentric core MOF (DCC-MOF) with long pulses typically delivered by subnanosecond lasers working in the near infrared. Strong conversion efficiencies (up to 30% of the injected input power) were observed thanks to the exotic guiding properties of these DCC-MOFs [2]. Differently from conventional MOFs, whose dispersion is in general anomalous in the optical communications band, the linear coupling of DCC-MOF drastically changes the guiding properties of the fundamental mode, thus causing an abrupt variation of the group delay between the pumping and the DW generation spectral regions. In this work we emphasize the role of group velocity difference between DWs and solitons. For this reason we have carried out a time-resolved spectral analysis of the supercontinuum generated by a 4-m long DCC-MOF by combining a diffractive grating and a single-slit giving a spectral resolution of 6 nm, and two 12.5 GHz bandwidth photodiodes. The laser pump was a microchip laser with pulse duration of 900 ps at 1064 nm and 4 kW peak power. We show on the left panel of Fig. 1 the pulse-width measured at different spectral regions (finest temporal structures cannot be unveiled with this method). We observe a negative time delay for the pulse center of mass when moving the 6 nm spectral window toward the infrared side, which qualitatively agrees well with the theoretical group delay and it is confirmed by our numerical predictions. In our DCC-MOF the DW overtakes the supercontinuum between 1064 nm and 1200 nm, in striking contrast with the case of single-core conventional MOFs, where the infrared part of the spectrum is in the trailing edge of the pulse envelope. Another key point discussed in the present work is the role of pump wavelength and pulse duration. On the top right panel of Fig. 1 we show the spectrum obtained with the 1064 nm microchip laser: an intense and spectrally wide DW around 1575 nm is easily recognized. Whereas the spectrum in the bottom right panel was obtained with a mode-locked laser emitting 120 ps pulses at 1030 nm. The pulse peak power had the same order of magnitude in both cases. In this case the DW is shifted towards 1505 nm, and its maximum spectral density is about 20 dB lower than in the previous case. Numerical simulations confirm the formation of the DW starting from the solitons bunch generated by the quasi-CW pump pulse break-up.