Fiber-optic refractometer shows promise for measuring biological SRI

A reflective fiber-optic refractometer with a thin-core fiber (TCF) section achieves a sensitivity of 133/26 dB/refractive-index unit for temperature immune surrounding-refractive-index (SRI) measurement. The device has potential for measurement of biological samples, which have refractive indices ranging from about 1.33 to 1.41. Developed by researchers at Northwest University (Xi’an, China) and Jinan University (Guangzhou, China), the refractometer can also potentially measure temperature as well as SRI.

A 10 mm section of TCF is connected on one end to a standard single-mode fiber (SMF), and on the other end to a fiber stub containing a fiber-Bragg grating (FBG). Incoming core-mode light in the SMF excites both core and cladding modes in the TCF due to the mismatch in core diameters. A strong evanescent field on the outer surface of the TCF interacts with the surrounding medium under test. The resulting cladding modes enter both the core and cladding of the FBG. Upon reflection and propagation back along the SMF, an irregular waveband structure with wavelengths shifted due to the SRI modifies the power of the reflected core mode; monitoring the power variation of the core reflection at a given wavelength produces the SRI measurement. Simultaneous wavelength and power detection would enable temperature measurements, too. Contact Yue Ma at [email protected].