Multiparameter Sensing Using Photonic Crystal Sensors on a Multicore Fiber-Tip

We demonstrate an approach to integrate multiple transducers on the tip of an optical fiber, leading to multiplexed and multiparameter sensing. The sensor consists of four photonic crystals that are placed on the tip of a cleaved 125 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>m diameter four-core fiber. Each photonic crystal has a different design which ensures that the sensitivities to temperature and refractive index of the solutions are different, enabling the simultaneous measurement of temperature and concentration. The sensor was calibrated in solutions of deionized water and ethylene glycol in a temperature range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$25\,\boldsymbol{^\circ }$</tex-math></inline-formula>C to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$65\,\boldsymbol{^\circ }$</tex-math></inline-formula>C by fitting using a two-dimensional second order polynomial. Non-linear least square estimation was applied to determine concentration and temperature based on test data. The estimation has a high coefficient of determination where both parameters have a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$R^{2}&gt;0.998$</tex-math></inline-formula>. The root mean square errors were found to be <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{RMSE_\mathbf {T}=0.45\,^\circ }$</tex-math></inline-formula>C and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{RMSE_\mathbf {c}=0.20}$</tex-math></inline-formula>% for the temperature and concentration estimation respectively. The demonstrated sensor represents a scalable platform for multiplexed sensing on the tip of an optical fiber, with unprecedented spatial density. It could open the way to referenced biosensing to multiple analytes.