An optical probe based on forward Brillouin scattering for multidimensional sensing of temperature and concentration in microscale liquid environments

Conventional temperature sensing based on forward Brillouin scattering (FBS) typically relies on the frequency shift of acoustic modes in tens-of-meters-long optical fibers, making it ill-suited for microscale microfluidic or bio-integrated environments due to spatial intrusiveness and limited adaptability. To overcome these limitations, we propose and experimentally validate an innovative FBS optical probe designed for in situ, multidimensional sensing of temperature and an osmotic pressure model in microscale liquid environments. The probe employs a 1-m-long uncoated highly nonlinear fiber as the core sensing element, integrated into a Sagnac interferometer, with the trace liquid placed in a capillary tube for interaction. This configuration enables direct and efficient opto-acoustic coupling with microscale liquid samples, overcoming the spatial limitations of conventional fiber sensors. Experimental results show a frequency shift-temperature coefficient of 38.8 kHz/°C and a superior linewidth-temperature coefficient of 57.0 kHz/°C for the R0,12 mode. A key advancement is the implementation of a dual-dimension temperature validation scheme, which achieves measurement consistency within ±0.09 °C and significantly enhances system reliability in complex media. For concentration sensing, the linewidth of the TR2,23 mode exhibits a sensitivity of 14.38 kHz/% to NaCl concentration, serving as an effective proxy. The uncoated, short-length fiber probe, which can be further tapered, offers a noninvasive, label-free solution for real-time monitoring and may be useful in applications such as organ-on-a-chip systems, single-cell analysis, and microfluidic diagnostics. This work thus establishes a viable pathway toward minimally invasive sensing in confined microenvironments and provides a proof-of-concept platform for future biophysical applications.