Abstract
Most fiber optic interferometric refractometers suffer from large temperature sensitivities, especially those that employ long-period gratings. This paper presents results on a compound-cavity Michelson interferometer whose phase shift is only dependent on the refractive index of the analyte surrounding the fiber probe. This single probe Michelson interferometer uses mode coupling in a long-period grating to establish the two optical paths in a single fiber, and therefore presents a compact sensor for measuring the refractive index and other related properties (such as chemical concentration and composition) of various types of substances. It can also be used to measure the level of liquid in a container. Experimentally, we measured a phase change of 1.374 rad for a change in refractive index of 0.0132. The temperature sensitivity of one such compensated device is only -0.01 rad/°C, which is less than the temperature gradient of the refractive index of solutions of glycerine in water.
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