The evanescent field coupling resonances between a single-mode optical fiber and a multimode planar waveguide overlay are sensitive in position to the refractive index of the superstrate material in contact with the top surface of the overlay. By using lithium niobate and zinc selenide in the role of the overlay and Cargille refractive-index oil as the superstrate, this principle has been investigated for use in refractometry. The ability to resolve index changes of <1 × 10−5 has been clearly demonstrated for an open-loop mode of operation by using intensity modulation, and a method of closed-loop operation is proposed by using active materials such as lithium niobate in the role of the overlay to provide independent feedback control of the resonance position.
© 1992 Optical Society of America
Original Manuscript: July 7, 1992
Published: November 1, 1992
W. Johnstone, G. Thursby, D. Moodie, and K. McCallion, "Fiber-optic refractometer that utilizes multimode waveguide overlay devices," Opt. Lett. 17, 1538-1540 (1992)