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A few mode fiber temperature sensor filled with PDMS based on vernier effect is proposed. It uses single mode fiber (SMF), few mode fiber (FMF), and polydimethylsiloxane (PDMS) to form the SMF-air microcavity-FMF-PDMS-FMF structure. It is to slot the glass capillary tube, place the two FMFs in the slot at a suitable distance, and fill the slot with PDMS multiple times and seal it. The PDMS is used as a sensing microcavity. We insert the SMF from the other end of the glass capillary tube, adjust the distance, fix the SMF and seal the connection. Two end faces of the SMF and the first FMF form an air reference microcavity. Since the refractive index of each medium is different, multiple reflective surfaces are formed to reflect. Because the second FMF is infinitely long, whose right end is beveled, it is considered that the sensor has four reflected beams. When the temperature changes, the length and refractive index of the PDMS microcavity will change, which will cause a change in the interference spectrum of the sensor. And because the vernier effect is formed by the superposition of the two microcavities, the temperature sensitivity can be amplified and the temperature can be measured with high sensitivity. The experimental results show that in the range of 40-56°C, the transmission spectrum of the sensor appears red shift with the increase of temperature, and the temperature sensitivity can reach 3.89nm/°C. The sensor is simple to manufacture, has high sensitivity, and has good application prospects.

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