Optimizing anti-Stokes Raman scattering in gas-filled hollow-core photonic crystal fibers

Anti-Stokes Raman scattering in gas-filled hollow-core photonic crystal fibers is discussed. It is shown that the efficient anti-Stokes generation observed under conditions of significant wave mismatch is caused by phase locking of the interacting fields. This leads to the establishment of a phase difference that is independent of the optical path. An optimization technique, based on the adjustment of the wave mismatch along a gas-filled hollow fiber using pressure control, is proposed. Anti-Stokes conversion efficiencies close to the theoretical maximum of 50% are predicted.

Figure 1

The initial stage of a phase-locked resonant ASRS. The amplification of the anti-Stokes field intensity $I_a=a_a^2$ [the lines labeled with (a)–(c)] and the corresponding locked phase difference $\overline{\theta }$ as a function of the normalized wave mismatch $\Delta K{L}_0$ for increasing values of pump intensity: (a) $a_p^2/a_0^2=0.8$; (b) $a_p^2/a_0^2=1.6$; and (c) $a_p^2/a_0^2=2.7$. The corresponding gain factors are: (a) $g_{0}z=5$; (b) $g_{0}z=10$; and (c) $g_{0}z=18$.

Figure 2

(Color online) Vibrational ASRS in a Kagome-type HC-PCF filled with hydrogen at 17 atm. (a) Intensities of the pump, Stokes and anti-Stokes field (in inset) vs fiber length (the circles, triangles, and diamonds show experimental data from [2]); (b) evolution of the phase difference (solid line). For comparison, the anti-Stokes field (inset) is also shown, calculated neglecting the nonlinear term in Eq. (2d) that gives rise to the phase-locking effect.

Figure 3

An optimized scheme for ASRS: (a) gas pressure variation along a PCF to provide optimum $\Delta k\left(z\right)$. The inset is a scanning electron micrograph of a typical Kagome PCF (core diameter $26\mu \text{m}$). (b)–(d) The results of optimization of ASRS in a Kagome-type HC-PCF filled with hydrogen. The input pump field intensity is as in experiment [3]. The dotted, dashed, and solid line shows the results for different lengths ($L_1=18$, 23.5, and 21 cm) of the high-pressure region. The solid line corresponds to the optimum regime of ASRS with conversion efficiency of 27%.