Abstract
We report on experimental and theoretical demonstration of an optical comb spectrum based on a combination of cascaded stimulated Raman scattering and four-wave mixing mediated by Raman-induced nonresonant Kerr-type nonlinearity. This combination enabled to transform a conventional quasi-periodic Raman comb into a comb with a single and smaller frequency spacing. This new phenomenon is realized using a hollow-core photonic crystal fiber filled with 40 bars of deuterium, and pumped with a high-power picosecond laser. The resultant comb shows more than 100 spectral lines spanning over 220 THz from 800 nm to 1710 nm, with a total output power of 7.1 W. In contrast to a pure Raman comb, a 120 THz wide portion of the spectrum exhibits denser and equally-spaced spectral lines with a frequency spacing of around 1.75 THz, which is much smaller than the lowest frequency of the three excited deuterium Raman resonances. A numerical solution of the generalized nonlinear Schr\"odinger equation in the slowly varying envelope approximation provides very good agreement with the experimental data. The additional sidebands are explained by cascaded four-wave mixing between pre-existing spectral lines, mediated by the large Raman induced optical nonlinearity. The results open a new route to the generation of optical frequency combs that combine large bandwidth and high power controllable frequency spacing.
Highlights
The generation of optical combs is turning into an extremely important topic in several scientific and technological fields, such as waveform synthesis [1], optical clocks [2], communication systems [3], and biomolecular imaging [4]
We report on experimental and theoretical demonstrations of an optical comb spectrum based on a combination of cascaded stimulated Raman scattering and four-wave mixing mediated by Raman-induced nonresonant Kerrtype nonlinearity
The results show a route to the generation of optical frequency combs that combine large bandwidth and high power controllable frequency spacing
Summary
The generation of optical combs is turning into an extremely important topic in several scientific and technological fields, such as waveform synthesis [1], optical clocks [2], communication systems [3], and biomolecular imaging [4]. Generation of optical combs was associated with mode-locked lasers [5] or with high harmonic generation using high-peak-power and ultrashort-pulse laser sources [1,6]. The former has matured to be the cornerstone in optical frequency metrology thanks to the outstanding frequency stability of the comb spectral components [7], and the latter is a powerhouse in synthesizing attosecond pulses in vacuum ultraviolet (VUV) and soft x-ray experiments thanks to its multioctave bandwidth [8].
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