Temperature sensor with the Vernier effect based on two parallel-connected in-line Michelson interferometers

A high-sensitivity temperature sensor based on two parallel-connected in-line Michelson interferometers (MIs) and the Vernier effect is proposed and demonstrated. Each MI is a segment of a specially designed inner-cladding fiber spliced with a lead-in fiber. The interference occurs between the core and cladding modes of inner-cladding fiber. Due to the special waveguide properties of the inner-cladding fiber, the 3 mm-long MI exhibits a regular reflection spectrum with a low free spectral range (FSR) of approximately 9 nm. The high regularity and low FSR of individual interferometer allows us to obtain a superimposed spectrum with a clearly defined envelope, calculated using approximately two dozen points. The temperature sensitivities of a single MI and the Vernier temperature sensor were measured to be 54.29 ± 0.22 pm/°C (in the temperature range of 23–200 °C) and 1.012 ± 0.017 nm/°C (in the temperature range of 23–160 °C), respectively. Due to the high sensitivity of the individual interferometer, an enhanced sensitivity of the Vernier sensor was achieved with an average amplification factor of M = 18.7.