Femtosecond laser fabricated Fabry-Perot sensors on optical fiber tip for acoustic sensor

Smart sensors based on optical fibers are popular in recent years, and one of them is optical fiber-based acoustic/ultrasonic sensors, which plays a vital role in areas from scientific research to nondestructive testing. However, as the sensitivity of traditional fiber optic sensors is limited, it is necessary to develop highly sensitive optical fiber-based sensors for ultrasound detection. Here, we present a 3D printed polymer-based Fabry-Perot interferometer (FPI) directly on a single mode fiber tip. The fabrication is based on femtosecond laser writing through two-photon polymerization. The resolution can reach up to ∼100 nm, which is less than 1/10 wavelength within the C-band. The spectral characteristics of the sensors are presented. Due to the properties of polymer materials, the devices have a higher sensitivity to acoustic waves that can modify the length of the cavity, which can be utilized for designing ultrasonic sensors. However, the optical quality of the fabricated FP sensor is lower, which is not suitable for high-frequency ultrasound detection. In this research, we propose a tunable erbium-doped fiber ring laser with the 3D printed FPI, which acts a wavelength filter and a reflector of the fiber ring laser. The stability and thermal variations around the modal interferometers are investigated. The spectra are symmetric with a maximal power difference about 35 dB between the lasing modes and the average of the side mode suppression ratio, which is tuned into the C-band with a resolution of 0.02 nm. An unbalanced interferometer-based demodulator using a PID controller is presented to demodulate the ultrasonic signal, which is applied directly on the fabricated FPI. The results show that this sensing scheme offers low wavelength drift, good signal to noise ratio and high-power stability, and can therefore be used for acoustic sensing applications.