Spectrometer-Free Vernier-Effect-Based Sensor

Vernier effect has been captivated as a promising approach to achieve high-sensitivity optical sensors. Nevertheless, their practical applications are hindered by the ultra-fine manufacturing process and spectrum-based demodulation method. Since the optical path differences (OPDs) of the sensing and reference interferometer need to be extremely close to each other, very accurate fabrication technique is required that is quite difficult to satisfy in many laboratories. Additionally, the spectrum-based demodulation method requires a broadband light source and an optical spectrum analyzer to acquire its spectrum in a broad wavelength range, which is not readily applicable. In this work, we demonstrate an easy-to-fabricate and spectrometer-free Vernier-effect-based optical fiber sensor. A short section of polarization maintaining fiber (PMF) is inserted into one arm of Mach-Zehnder interferometer to form sensing area and an all-fiber variable optical delay line (VODL) is utilized to drive OPD scan. Thanks to the birefringence effect of PMF, two slightly detuned interferometric signals can be generated and overlap at the output of sensor during OPD scanning process. In our design, the instantaneous interference intensity and OPD are obtained in an accurate way via three photodetectors so that the detuned interferograms caused by Vernier effect can be worked out. Our device is independent of the precise and complicated fabrication procedures, the sensing signal can be demodulated without broadband light source and OSA. We envision that the proposed structure will inspire a new concept for constructing simple and cheap Vernier-effect based sensors that are well suited for practical applications.