Frequency-domain technique to measure the inertial response of forward stimulated Brillouin scattering for acoustic impedance sensing

Abstract

A frequency-domain method is proposed to measure the inertial response of forward stimulated Brillouin scattering, so that the acoustic impedance of a liquid medium outside an uncoated 30 m standard single-mode fibre can be sensed. The proposed technique overcomes the linewidth distortion present in the time-domain approach due to Kerr effect and reduce time and complexity for data post-processing. Experimental results demonstrate that acoustic impedances of water and ethanol agree well with the expected values.

Opto-acoustic chemical sensor based on forward stimulated Brillouin scattering in optical fiber (Invited)

“…Because of Kerr effect giving a similar signature in phase quadrature, the detected spectrum shows Fano resonance shape that makes the retrieval of the acoustic resonance linewidth challenging. Taking advantage of the contrast between the long tail of FSBS response and the instantaneous nature of Kerr effect, transverse acoustic wave is stimulated within a long pulse, yet only the acoustic response outside of the pulse duration is selectively gated for detection [4]. Using both time and frequency domain techniques, the acoustic impedances of water and ethanol are measured and agree with the reported standard values.…”

“…For time-domain measurement technique [3], a short pulse of 1 ns is shaped from the incoming distributed feedback (DFB) laser by an electrooptic modulator (EOM) whereas for frequency-domain method [4], a long pulse of 500 ns is first shaped from the laser by a semiconductor optical amplifier (SOA) and subsequently intensity-modulated by an EOM at FSBS frequency where the modulation signal is provided by a vector network analyzer (VNA). The purpose of introducing the long pulse is to suppress the unwanted response due to Kerr cross-phase modulation by means of selectively gating away the affected time duration in the response trace before detection [4]. The generated optical pulse in both techniques is amplified by an Erbium doped fiber amplifier (EDFA) to produce a peak power of around 1 W. A continuous wave from another DFB laser at a disparate wavelength of 1534 nm is used for probing the induced phase modulation.…”

“…For the frequency domain technique [4], a continuous wave (CW) light is intensity-modulated at the FSBS resonance frequency to stimulate a specific transverse acoustic mode. The resonance spectrum is then obtained by sweeping the modulation frequency across the resonance frequency.…”

Forward Brillouin Scattering-Based Sensing Outside Polyimide-Coated Standard Fiber

“…(4), m  denotes the linewidth of F-SBS. This parameter is of particular significance, since it varies with the mechanical impedance of surrounding media [16][17][18][19].…”

“…Over the last two years, this attribute of guided acoustic waves has been used in a new class of fiber sensors, which are based on F-SBS [16][17][18][19]. The mechanical impedances of water and ethanol outside the cladding of unmodified, standard fiber were measured with 1% accuracy, based on the linewidths of guided acoustic modes that were driven through F-SBS [16].…”