Magnetic field sensing with in-fiber cascaded Fabry-Perot interferometers (FPIs) using femtosecond laser direct writing

A magnetic field sensor made with femtosecond laser direct writing was built and characterized. The sensor makes use of the Vernier effect (VE) via a cascade of two Fabry-Perot interferometers (FPIs) with similar free spectra ranges (FSRs). The sensor cavity FPI 1 and the reference cavity FPI 2 are made by writing reflective surfaces directly onto the core of a single-mode fiber (SMF) with a femtosecond laser. FPI 1 is secured to a magnetostrictive material. A single FPI 1 has a sensitivity of 6 pm/mT in the 10–50 mT range, but two cascaded FPIs have a sensitivity of −117.1 pm/mT and an amplification of 19.5. The sensor's temperature properties are studied, and cross-sensitivity to temperature is eliminated. This sensor is easy to fabricate, compact, simple, and structurally stable, making it highly promising for magnetic field sensing applications.