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

Stimulated Brillouin scattering is well-known for providing distributed measurements in the field of optical fiber sensing. For the conventional backward stimulated Brillouin scattering, the sensing capability is nevertheless limited to temperature and mechanical strain in the core of an optical fiber. Forward stimulated Brillouin scattering (FSBS), due to the participating transverse acoustic waves, can be used to directly measure the mechanical properties of the material that surrounds a standard telecom fiber. We review our work on harnessing FSBS to extract the acoustic impedances of different liquid materials. The acoustic impedance mismatch between silica optical fiber and its surroundings determines the decay rate or the corresponding resonance linewidth of the transverse acoustic modes. We address the measurement with both time and frequency domain techniques, demonstrating accuracy of measurements up to 95%. This work could be potentially extended to realize fiber based ultrasonic chemical analysis.