Development of interferometer for refractive index measurement of aqueous solution in a microfluidic chip

Abstract

This study presents the design and development of an interferometer for the measurement of the refractive index or concentration of a sub-microliter volume of aqueous solution in a microfluidic chip. This proposed interferometer is manifested by the successful measurement of the refractive index of a sugar-water solution, using a He-Ne laser as a light source and robust instrumentation for practical implementation. The measurement principle and the feasibility of the system are analyzed theoretically. The experimental device is constructed with a He-Ne laser, two lenses, two optical plates and a complementary metal oxide semiconductor detector. The refractive index change is determined by measuring the positional changes of the interference fringes. A refractive index change of 10 −4 is inferred from the measured image data.

A miniaturized system for measurement of the refractive index of sub-microliter liquid

“…Such a placement method can improve detection sensitivity. This is different from the placement method reported elsewhere [20] because the He-Ne laser has a very long coherence length, and, although the two glass plates double the optical path difference, interference fringes still appear. The coherence length of the laser diode is much smaller, so the placement method of parallel plates increases detection sensitivity.…”

“…For the miniaturization of the instrument system, a laser diode, rather than a He-Ne laser, was used as the light source [20] . Two beams were first generated through parallel plates.…”

“…Firstly, a double-beam interferometer using a He-Ne laser as its light source was used [20] . Owing to the He-Ne laser (Melles Griot, USA) showing high coherence and a long coherence length, there was enough range of optical path difference for interference between the two beams.…”