We propose coating side-polished optical fiber (SPF) with epoxy polymer to form a fiber-optic sensor for cryogenic temperature measuring applications. The thermo-optic effect of the epoxy polymer coating layer enhances the interaction between the SPF evanescent field and surrounding medium, considerably improving the temperature sensitivity and robustness of the sensor head in a very low-temperature environment. In tests, due to the evanescent field-polymer coating interlinkage, transmitted optical intensity variation of 5 dB and an average sensitivity of -0.024 dB/K were obtained in the 90-298 K range.
Keywords: cryogenic temperature; epoxy; optical sensor; refractive index; side-polished optical fiber.