A Michelson interferometer is designed for the refractive index detection of nanolitre samples in a microfluidic chip. A He-Ne laser beam (power of 0.5mW) is divided into a detection beam and a reference beam by a beam splitter. Equal inclination interference patterns (circular fringes) formed by the reference beam and the detection beam passing through the chip channel (width of 65μm and depth of 50μm) are observed clearly. The intensity of the central part of the interference pattern changes while rotating the chip. Signals of the light intensity versus the rotation angle are collected by a detector (consisted of a fiber and a photomultiplier) and recorded by a computer. By rotating the chip from 0° to +17°, signals are measured when the chip channel is filled with air, water and sodium hydroxide solution NaOH (with a refractive index of 1.3450). Fitting the signal curves with Origin program in a theoretical equation, the refractive indices of air, water and NaOH solution in the chip channel are derived. The results coincided basically with those measured by an Abbe refractometer.
By use of common optical equipment, a laser Michelson interferometer system that can be used for the detection of microfluidic chip is designed. A He-Ne laser beam (power of 0.5mW and diameter of about 0.7mm) is divided into a detection beam and a reference beam by a beam splitter. The reference beam is slightly expanded with a lens and has a diameter of about 1.5mm. A microfluidic chip with channel diameter of 60μm is placed in the detection arm of the interferometer. The detection beam falls on the channel of the chip. On the screen 1.6 meters far from the splitter, an interesting pattern containing both circular fringes and linear fringes is observed. Experiment shows obviously that the patterns on the screen are different when the channel is filled with air, pure water and the sodium hydroxide solution (with a refractive index of 1.343). It is indicated that the interferometer is effective for the detection of solutions as tiny as 3 nano liters with different refractive indices.
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