A temperature-insensitive optical fiber refractometer, based on multimode interference in no-core square fibers, has been proposed and experimentally demonstrated. The refractometer is formed by a single-mode fiber sandwiched between two segments of no-core square fibers through cleaving and fusion splicing. The transmission spectra characteristic of refractive index (RI) and environmental temperature have been investigated. Experimental results show that a transmission dip exhibits a redshift as large as about 25 nm when the ambient RI increases from 1.3424 to 1.4334. Within the RI range of 1.4033 to 1.4334, the RI sensitivity reaches 474.8189 nm/RIU. A temperature sensitivity of 0.00639 nm/℃ is experimentally acquired between 20℃ and 85℃, showing a low temperature cross-sensitivity of about 1.35 × 10~(-5) RIU/℃. The proposed refractometer has several advantages, such as low cost, simple structure, and compact size. Therefore, it is also expected to be employed in chemical and multi-parameter sensing applications.
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