Simultaneous measurement of refractive index and temperature using multimode interference inside a high birefringence fiber loop mirror

A fiber optic sensor for simultaneous measurement of refractive index and temperature is presented. The sensing probe is realized by introducing a multimode interference device inside a high birefringence fiber loop mirror resulting in a configuration capable of refractive index and temperature discrimination. The multimode interference peak is sensitive to the surrounding refractive index (90 nm/RIU) and slightly responsive to the temperature (0.01 nm/°C). On the other hand, the birefringent fiber loop mirror is highly sensitive to temperature (2.36 nm/°C) and it has almost no response to refractive index. Using a power ratiometric peak detection scheme, a temperature independent refractive index measurement can be achieved with a resolution of ±2.25 × 10−5 RIU.