All-fiber microfluidic multimode Mach–Zehnder interferometers as high sensitivity refractive index sensors

Abstract

We present microfluidic multimode fiber Mach---Zehnder interferometers (MMF-MZIs), suitable for refractive index (RI) measurement. The compact and low-cost all-fiber MMF-MZIs were made by forming two coupling points in series at a localized short section (about 1---5 cm) of a multimode optical fiber, using an electric-arc system, thus allowing the excitation and recombination of core and cladding modes. The MMF-MZIs have been integrated with a home-made microfluidic chip, used as a high sensitivity RI sensor to ascertain the surrounding RI variations. This type of microfluidic lab-on-a-chip MZI-based sensor was shown to be effective in sensing very small quantities of samples and its transmitted light signal could easily be measured. The spectra of all MMF-MZIs were Fourier transformed to obtain and the spatial frequency spectra. The MMF-MZIs exhibited many dominant spatial frequencies for all interferometer lengths and interference was mainly constructed by the core mode and dominant lower order cladding modes when the sensing length was 2---5 cm as well as distributed by the core mode and dominant higher order cladding modes when the sensing length was 1 cm. The transmitted spectra of the MMF-MZIs exhibited various sinusoidal-like interference fringes' amplitudes and peaks changed differently by immersing the MMF-MZIs in different concentrations of sucrose solutions. The microfluidic MMF-MZI sensors have a high resolution of 3.1 × 10ź5 to 7.5 × 10ź5 RIU and a high sensitivity of 160---347 dBm/RIU within RI range from 1.333 to 1.370.