Spectral coherence properties of continuum generation in bulk crystals

Abstract

The stability of the phase difference between two white-light continua, generated from the same 180-fs pulses at ≃ 1035 nm, is assessed by a modified Bellini-Hänsch interferometer. Mutual spectral phase stability is studied and quantified as a function of several parameters: pulse energy, position of the nonlinear crystal with respect to the beam waist and interaction length. Our results show that intrapulse decoherence may significantly contribute to the measured CEP noise floor. In addition, spectrally-resolved intensity-to-phase coupling coefficients are measured and stability regions are identified.

Spatial-to-spectral phase coupling mechanisms in bulk continuum generation

“…Figure 4(c) shows κ(λ) for the four aberrations. As observed in our previous study [8], the spectral phase of the lower wavelengths is more sensitive to spatial phase fluctuations. Focus (i) is the lowest order aberration (second order) and has the largest transfer coefficients : κ > 50 at 750 nm.…”

“…A first explanation to the high values of κ(λ) measured in the previous section could be based on phase-amplitude coupling mechanism, induced by changes in peak power due to modifications of spatial phase. However, the latter are very small, and the transfer coefficient values reported by [8,10] (a few hundreds of mrad per% of input power) do not explain the observed large phase shifts.…”

“…As was reported before, the input face of the YAG crystal placed slightly before the focal point of the driving pulse favors the generation of a broad and flat spectrum in the visible [8,11,19]. In practice, the working point for WLG is, here, z 0 = −1.5 mm.…”

“…In the considered spectral range, the relative phase stability is assessed for all the newly generated wavelengths by retrieving the standard deviation of the phase of the spectral fringes over 1000 single shot consecutive spectra. The standard deviation of the phase at each wavelength, σ(λ), is then used as a metric to assess the intrapulse coherence [8]. Without crystals, the typical measured standard deviation is 5 mrad at 1030 nm, which provides a ground floor for the phase noise measurement.…”

“…In a recent study, we have proposed a variant of the Bellini-Hänsch interferometer, coined as double white-light interferometer, to experimentally characterize the intrapulse coherence of a supercontinuum [8]. Complementing the few previous studies, these measurements were quantitative and frequency-resolved [9,10].…”