Phonon-photon interactions in photonic crystal fibre (PCF) depend strongly on the detailed fibre microstructure. Interactions can be strongly enhanced in small-core PCFs with high air-filling fractions due to the large discontinuity in acoustic properties between air and glass, which gives rise to phononic band gaps and families of multi-GHz core-guided acoustic modes, creating unusual forward and backward Brillouin spectra. The backward Brillouin spectra display multiple bands arising from different types of core-guided acoustic modes and show higher threshold intensities compared to conventional optical fibres [Dainese (2006)]. On the other hand, forward Brillouin scattering is Raman-like in character, acoustic resonances (ARs) trapped transversely in the core behaving much like artificial nonlinear oscillators whose frequency and lifetime can be designed to suit a particular application [Dainese (2006a)].
For characterization of ARs in fibres, we have developed pump-probe measurement techniques in which ARs are excited by short optical pulses and their acoustic frequency and lifetime measured by either polarization spectroscopy [Dainese (2006a)] or Sagnac interferometry [Kang (2008)]. We have also constructed another polarization spectroscopic setup for measurement of spontaneous forward Brillouin spectra. Using this setup, we have found out that forward scattering can be highly suppressed in a PCF with a sub-wavelength hole in the center of the core [Wiederhecker 2007] and that core-confined ARs become more apparent as the air-filling fraction increases [Brenn (2009)].
The amplitude of ARs can be coherently controlled by using trains of precisely timed optical pulses [Wiederhecker (2008)]. Such multi-pulse excitation allows preferential amplification or attenuation of selected ARs. The ARs can also mediate forward stimulated scattering of two-frequency laser light – pump and Stokes waves. Multiple side-bands can be efficiently generated from co-polarized pump and Stokes waves via stimulated Raman-like scattering (SRLS) [Kang (2009)]. The amount and direction of power transfer among the two input waves and side-bands can be arbitrarily controlled by means of sequences of optical pulses with precisely adjustable power and timing [Kang (2009a)].
Stimulated inter-polarization scattering (SIPS) can be observed when orthogonally-polarized pump and Stokes waves are coupled to different fibre polarization eigenmodes [Kang (2010)]. In contrast to SRLS, the generation of multiple side-bands are highly suppressed in SIPS, because the driven ARs do not have the correct wavevectors to cause scattering into side-bands. Furthermore, SIPS can be utilized in realization of optically controllable unidirectional attenuator/amplifier without use of magneto-optical elements [Kang (2010a); Kang (2011)].