Raman-Kerr comb generation based on parametric wave mixing in strongly driven Raman molecular gas medium

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 Kerr-type nonlinearity. This combination enabled us to transform a conventional quasiperiodic Raman comb into a comb with a single and smaller frequency spacing. This phenomenon is achieved 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 Schrödinger 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 preexisting spectral lines, mediated by the large Raman-induced optical nonlinearity. The use of such a technique for coherent comb generation is discussed. The results show a route to the generation of optical frequency combs that combine large bandwidth and high power controllable frequency spacing.

Figure 1

(a) Scanning electron microscope image of the inhibited-coupling single-ring tubular-lattice HC-PCF used for this experiment. (b) Evolution of losses (black curve) and effective refractive index in the vacuum (blue curve) and in hydrogen (red curve) as a function of wavelength. (c) Evolution of group index as a function of the wavelength in the vacuum (blue curve) and in hydrogen (red curve). (d) Evolution of dispersion as a function of wavelength in the vacuum (blue curve) and in hydrogen (red curve). (e) Output spectrum after a 16 W power pump propagation in a 3 m long HC-PCF filled with 40 bars of deuterium. The pump laser line is identified by black color, the vibrational Stokes and anti-Stokes lines by purple, the Stokes and anti-Stokes lines due to ortho-ro-vibrational Raman resonance by blue, the Stokes and anti-Stokes lines due to para-ro-vibrational Raman resonance by green, and four-wave-mixing generated sidebands by red. Inset: Near-field intensity distribution of the output beam. (f) Zoomed-in spectrum of the black-dashed rectangle.

Figure 2

Schematic sequence of the processes during the comb generation. The pump laser line is identified by black color, the vibrational Stokes and anti-Stokes lines by purple (not visible), the Stokes and anti-Stokes lines due to ortho-ro-vibrational Raman resonance by blue, the Stokes and anti-Stokes lines due to para-ro-vibrational Raman resonance by green, and four-wave-mixing generated sidebands by red.

Figure 3

The phase-matching lengths for the ${\nu }_{FWM2}=2{\nu }_{0,-2,1}-{\nu }_{0,-1,-1}$ type of FWM process (red curve), for the ${\nu }_{FWM1}={\nu }_{0,-2,0}+{\nu }_{0,-1,-1}-{\nu }_{0,-2,+1}$ type of FWM process (green curve), and for the ${\nu }_{FWM1}={\nu }_{0,-2,0}+{\nu }_{0,0,-1}-{\nu }_{0,-1,1}$ type of FWM process (blue curve). The dashed black line indicates the 3 m fiber length.

Figure 4

(a)–(c) Evolution of the numerical output spectra for the cases of coupled pump power of 12.8 W (a), 14.4 W (b), and 16 W (c). The fiber length was 3 m, the ${\mathrm{D}}_2$ gas pressure was 40 bars. The pump laser line is shown with black color, the ortho-ro-vibrational shifts with blue color, the para-ro-vibrational shifts with green color, and all the FWM-generated sidebands with red color.