Temperature-Independent Micro-Refractometer Based on Cascaded In-Fiber Air Cavities With Strain-Error Correction

We proposed a new fabrication technology of in-fiber air-cavity by splicing single-mode fiber and hollow core fiber (HCF) filled with the glycerol solution in this paper. A fiber-optic interferometer composed of two cascaded ellipsoidal in-fiber air cavities was fabricated by the above-mentioned method. The two air cavities are the mode splitter and the mode combiner of the transmission-type multimode interferometer (MMI), respectively. The collapsed HCF with the length of 973 μmu between them can be used as the refractive index (RI) sensing area of the MMI, and the temperature sensitivity of the MMI can be ignored compared with the RI sensitivity. Owing to the RI insensitivity and temperature insensitivity, the air cavity with the minimum wall thickness of 6.2 μm is used as the reflection-type Fabry-Perot interferometer to compensate the strain of the MMI. Research shows that the sensor can eliminate the effects of temperature and strain in the measurement of RI.