We present a new approach for the development of a highly stable optical fiber refractometer based on a path-matching differential interferometer. Exploiting a single-channel phase tracker and new synthetic heterodyne demodulations, one can eliminate the thermal drift on a piezoelectric transducer stack as a phase modulator by subtraction. A transducer in a differential Fabry–Perot refractometer is designed to compensate for the thermal effects not only from thermal expansion but also from the thermo-optic effect. The experimental data show that the refractive-index change in the sensing system can be kept at a level of approximately 5 × 10-4 without serious variations for a 1-h period of long-term monitoring associated with a temperature variation of from 25 to 50 °C. Accordingly, the proposed new system can be easily implemented and used as a long-term monitoring system for medical care applications such as monitoring patients during drug injection.
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