All-Optical Control of Gigahertz Acoustic Resonances by Forward Stimulated Interpolarization Scattering in a Photonic Crystal Fiber

We report the observation of a novel nonlinear optoacoustic phenomenon, that we name forward stimulated interpolarization scattering. When two frequency-offset laser signals are colaunched into orthogonally polarized guided modes of a birefringent small-core ($1.8\mu \mathrm{m}$ diameter) photonic crystal fiber, a pattern of axially moving polarization fringes is produced, with a velocity and spacing that depends on the frequency offset. At values of frequency offset in the few-GHz range, the pattern of moving fringes can perfectly match the phase velocity and axial wavelength (3.9 mm) of the torsional-radial acoustic mode tightly guided in the core. An intense optoacoustic interaction ensues, leading to efficient nonlinear exchange of power from the higher frequency (pump) mode to the orthogonally polarized lower frequency (Stokes) mode. A full-vectorial theory is developed to explain the observations.

Figure 1

Dispersion diagrams (not to scale) for the optical modes and the TR acoustic mode, comparing SIPS (a) and SRLS (b). The orientations of the TR-AR displacements causing SIPS and SRLS are also shown below the corresponding dispersion diagrams, where the dotted circles and the gray ellipses represent the original core and the vibrating one. The horizontal and vertical lines correspond to the fiber eigenaxes ($x$ and $y$). In (a), two different colors (green and violet) of the circles and arrows in the dispersion diagram correspond to two configurations of SIPS. In (b), the optical mode can be either the fast or the slow mode.

Figure 2

Schematic diagram of the experimental setup. ECDL: external-cavity diode laser, PC: polarization controller, SSBM: single-sideband modulator, EDFA: erbium-doped fiber amplifier, TF: tunable filter, VOA: variable optical attenuator, FPI: Fabry-Pérot interferometer. The inset is a scanning electron micrograph of the PCF used in the experiment. The white horizontal bar corresponds to $1\mu \mathrm{m}$.

Figure 3

(a),(b) Power conversion from pump to Stokes as a function of (a) frequency difference ($f_P-f_S$) around the ${\mathrm{TR}}_{21}$-like AR frequency at 1.50 GHz, and (b) input power of each wave ($P_{P0}=P_{S0}$). The vertical axes represent the relative difference of output power between the pump and Stokes waves. The dots are experimental results and the lines are fits to the analytical theory. In (a), the input power of each wave is fixed at 42 mW. In (b), the frequency difference is tuned to the resonant frequency for each case, i.e., 1.5015 GHz for SIPS and 1.497 GHz for SRLS. (c),(d) Output spectra at a total input power of $2\times 150\mathrm{mW}$ for SIPS (c) and SRLS (d). These spectra correspond to the two points indicated by the green dashed circle in (b). Zero relative frequency corresponds to the pump wave.

Figure 4

(a) Power conversion from pump to Stokes as a function of frequency difference ($f_P-f_S$) around the ${\mathrm{R}}_{01}$-like AR frequency at 1.80 GHz. The input power of each wave is fixed at $42\mathrm{mW}$. Dots are experimental results and the curve is a fit assuming a Lorentzian gain profile. (b) Output spectrum for SRLS corresponding to the point indicated by the green dashed circle in (a).