A design for improving the sensitivity of a Mach–Zehnder interferometer to chemical and biological measurands

Integrated optical Mach–Zehnder interferometers (MZIs) composed of graded-index channel waveguides are often used as chemical/biological sensors. Such MZIs have a relatively low sensitivity because the graded-index active arms have a weak evanescent field. To improve the sensitivity, a channel-planar composite optical waveguide (COWG) is proposed as a substitute for the graded-index active arm. An actual channel-planar COWG was fabricated by sputtering a tapered TiO2 film onto a straight glass channel waveguide prepared by the potassium ion exchange method. Measurement of the evanescent absorption of the dye solution demonstrated a significantly enhanced evanescent field over the TiO2 film region caused by adiabatic transition of the guided mode between the channel waveguide and TiO2 film. Theoretical calculations show that the sensitivity of a glass-based MZI can increase 71 times when the COWG active arm contains a 25 nm thick and 5 mm long tapered TiO2 film. The use of a COWG as the active arm of a glass-based MZI also allows for elimination of the low-index buffer layer because of a large difference in evanescent field between the COWG and channel waveguide.