Backward Stimulated Brillouin Scattering in a Air-filled Hollow-core Fiber

Abstract: Backward stimulated Brillouin scattering is observed in air-filled hollow-core photonic crystal fiber, showing a clear resonance at 440 MHz. This opens a new field for Brillouin gas lasers and novel distributed fiber sensors.

Introduction: Hollow-core photonic crystal fiber (HC-PCF) provides an efficient platform for light-matter interaction and gas-based nonlinear optics [1]. The optomechanical properties of the HC-PCF have been recently studied [2, 3]. Forward stimulated Brillouin scattering (SBS) resonances in air-filled HC-PCFs have been demonstrated by measuring the acoustic resonance of the honey-comb cladding [2] and the hollow-core tube [3]. Backward SBS in air using free-space optics was first observed in 1965 [4]. By using HC-PCF, backward SBS is substantially enhanced compared to free-space optics, due to the tight optical mode confinement over a long interaction length.

A first observation of backward SBS in atmospheric air-filled HC-PCF (NKT Photonics HC-1550-02) is here reported. The actual gain coefficient is 760 times smaller than in silica, but it can be significantly increased by filling the HC-PCF with a higher gas pressure since it scales with the square of the molecular density in gases [5]. The feasibility of a gas Brillouin laser can thus be considered, as well as strain-insensitive distributed temperature sensing.

Results: Pump and probe waves counter-propagate along a 50 m HC-PCF with a frequency difference producing a longitudinally moving fringe pattern (Fig. 1(a)). When the phase matching condition is met, electrostriction gives rise to a travelling compression wave in the gas, causing a Bragg-type coupling between pump and probe. The small sound velocity in air makes the frequency difference between pump and probe less than 1 GHz, so that it turns very challenging to optically filter the reflected pump wave. To minimize the impact of the residual pump reflection, both pump and probe are intensity modulated at distinct frequencies (Fig. 1(b)), so that only the nonlinear response proportional to the product of the pump and probe intensities is detected. Figure 1(c) shows the measured backward SBS amplification spectrum, when the pump laser wavelength is scanned over a range around the probe line. The measured gain (3.3×10^-4 W^-1m^-1), frequency (440 MHz), and linewidth (~400 MHz) are in good agreement with theory [5].

References

[1] P. St. J. Russell, et al., Nature Photonics 8, 278-286 (2014).

[2] W. H. Renninger, et al., New Journal of Physics 18, 025008 (2016).

[3] W. H. Renninger, et al., Optica 3, 1316-1319 (2016).

[4] E. E. Hagenlocker and W. G. Rado, Apply Physics Letters 7, 236-238 (1965).

[5] R. W. Boyd, “Nonlinear Optics”, Third Edition, Chapter 9.5 (2008).

Presenting author email: flavien.gyger@epfl.ch