Measuring the Thermo-Optic Coefficient of Liquids With Athermal Multimode Interference Devices

Abstract

We demonstrate the use of thermally insensitive fiber optic multimode interference (MMI) devices to measure the thermo-optic coefficient (TOC) of liquids. Such athermal MMI devices are all-fiber, easy to fabricate and, in general, consist of a cascade of several multimode sections. Removing the inherent thermo-optical response of the MMI device is achieved passively in a material-based fashion. Starting from the general theory, we derive simple analytical expressions for the design of two-element MMI cascaded devices with arbitrary materials. Moreover, we verify that those expressions are valid irrespectively of the self-imaging operation. We designed and fabricated an athermal MMI device with a simple architecture consisting of a so-called no-core optical fiber (NCF), which is partly coated by PDMS, and we used it to measure the TOC of several standard liquids. Our results are in good agreement with the values of the TOC reported in the literature. By removing the thermal cross-sensitivity in our device, only one calibration measurement is necessary, which allows for straightforward information retrieval.