Coherence and noise structure of the octave-spanning IR supercontinua

Abstract: Noise structure and coherence of mid-IR supercontinua in fibers with strong Raman nonlinearity were investigated. It was established that the octave-spanning spectra retain very high degree of coherence in the presence of the enhanced nonlinearity. 2006 Optical Society of America OCIS codes: (190.4370) Nonlinear optics, fibers; (320.7140) Ultrafast processes in fibers Photonic crystal fibers (PCFs) provide extremely broad supercontinuum (SC) generation at sub-nJ input pulse energies in the near-infrared wavelength range. This was possible due to the mechanism of the so-called high-order soliton fission in the vicinity of zero-dispersion wavelength [1]. However, this mechanism loses its efficiency in IR wavelength range. This results from the decrease of the effective electronic nonlinearity, which scales approximately as 31 / λ ∝ [2]. Therefore, in order to provide more than octave-spanning SC, it is necessary to use the high-nonlinear fibers like SF6 [3,4] pumped by the femtosecond pulses from the fiber or Cr:YAG laser [2]. Our Raman spectrum measurements have shown that such fibers (SF6 and SF57) possess higher Raman gain and twice higher Raman frequency shift in comparison with that in fused silica [5]. This can provide an addition spectral extra-broadening in mid-IR, but can also affect the coherence and the noise structure of SC. The aim of this paper was to investigate the coherence of octave-spanning IR SC from the highly-nonlinear PCFs and to demonstrate that the enhanced nonlinearity (in particular, stimulated Raman scattering (SRS)) does not reduce the SC coherence. Our treatment was based on the Wigner phase-space representation of the quantized field evolution in fiber [6]