Sound-light interactions in PCF

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)].

Characterization of acoustic resonances

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)].

(Left) Scanning electron micrograph (SEM) of a PCF with a narrow hollow channel (diameter 350 nm) in the center of the core (diameter 1.7 µm). The white horizontal bar corresponds to 1 µm. (Right) Forward scattering spectra for two similar PCFs. In the PCF without the core hole, both low and high frequency ARs appear (red curve). In the PCF with a core hole shown as the left figure, no peaks are observed at the higher frequency (blue curve).

Coherent optoacoustic interactions

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)]

Coherent control of ARs trapped in a PCF core by means of precisely timed sequences of optical pulses. (a) SEM of a PCF with the core diameter of 1.27 µm. The white horizontal bar corresponds to 1 µm. (b) Excitation of AR by a single pulse (blue curve), enhancement of AR by two pulses whose temporal spacing is equal to the acoustic period (green curve), and suppression of AR by two pulses whose temporal spacing is the same as half of the acoustic period (red curve). The traces are offset for clarity. (c) Power spectra of the traces in (b). (d) 100-fold amplification of AR power by a train of 27 resonantly-timed pulses (black solid curve). The red dashed curve corresponds to single excitation (blue dashed curve in (c)). The arrows in (c) and (d) indicate the multiplication factor used in each curve for clarity.

Stimulated interpolarisation scattering

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)].

Stimulated Raman-like scattering (SRLS) and stimulated inter-polarization scattering (SIPS) mediated by ARs trapped in a PCF core. (a) Dispersion diagrams (not to scale) for the optical and acoustic modes, illustrating phase matching for SRLS and SIPS. (b) SEM of a PCF with the core diameter of 1.8 µm. The white horizontal bar corresponds to 1 µm. (c) Output spectrum as co-polarized pump and Stokes waves with the same optical power are launched. The red downward-pointing arrows indicate the two incident optical waves. (d) Output spectrum as orthogonally-polarized pump and Stokes waves with the same optical power are coupled into the different polarization eigenmodes.