Sensitivity Improvement in Vernier-Effect Demodulation Based on Frequency Doubling of Sensing Spectrum

We propose that the sensitivity can be effectively improved by spatial-frequency doubling of the sensing signal before the signal processing assisted Vernier-effect demodulation via a single fiber interferometer, which has been proven to have the same effect as physically increasing the optical path difference (OPD) in an interferometer but keeps the sensor small in size and does not introduce any additional crosstalk and cost. In the proposed scheme, only one fiber interferometer is used for sensing. The sensing spectra are squared to double their spatial frequencies, and then, an artificial reference spectrum (ARS) is extracted from one of the squared spectra and multiplied by each squared spectrum to form the Vernier effect. To validate the principle, a widely used Sagnac interferometer (SI) with a conventional polarization-maintaining fiber (PMF) of only 10 cm in length is designed for temperature sensing. Significantly, by simply doubling the spatial frequency of the sensing signal, the magnification of Vernier effect is improved from 4.96 to 74.82, resulting in an ultrahigh sensitivity of 117.17 nm/°C.