Ultracompact optical fiber sensors based on 3D µ-printed ferrule/fiber-top microstructures

Abstract

Optical fibers have become a versatile and enabling technology for development of various kinds of sensors because of their advantages such as small size, low weight, immunity to electromagnetic interference, remote sensing and multiplexing capabilities. Over the past decades, tremendous efforts have been made to develop new fiber-optic sensors. Notably, optical fiber-tip devices based on micromachined ferrule/fiber-top structures have become a very unique technology for development of miniature fiber-optic sensors. However, as the geometry of the ferrule/fiber-end surface is not compatible with conventional micro/nano-fabrication technologies, it remains challenges to fabricate 3D complex structures such as suspended mirrors to develop high-performance fiber-tip sensors for applications where miniaturization is critical. In this thesis, a novel optical 3D μ-printing platform established by our research team is introduced. With this printing technology, three kinds of ultracompact optical ferrule/fiber-top sensors are developed for measuring displacement, refractive index, and acoustic wave, respectively. Firstly, small optical fiber displacement sensors are fabricated by directly printing polymer suspended mirrors on the end surface of fiber-optic ferrules. With an own-established optical 3D μ-printing platform, three kinds of ferrule-top suspended-mirror devices (SMDs) are rapidly fabricated by using SU-8 photoresist. Optical reflection spectra of the fabricated SMDs are measured and then analyzed by using fast Fourier transform. The application of the ferrule-top SMD as a miniature displacement sensor is experimentally demonstrated. Secondly, ultrasmall optical fiber refractive-index sensors are developed by directly printing polymer suspended microbeams on the end surface of standard single-mode optical fibers. The reflection spectra of the fabricated fiber-tip devices have been measured and used to analyze the Fabry-Pérot (FP) cavities formed by suspended microbeams. The optical fiber-tip sensors can detect the RI changes of both liquid and gas and can also be used to measure the pressure of ambient environment via the detection of pressure induced refractive-index change. High sensitivities of 917.3 nm/RIU to RI change and 4.29 nm/MPa to gas-pressure change have been achieved experimentally. Such ultrasmall optical fiber-tip sensors with remote sensing capability are promising in microfluidic biosensing and environmental monitoring applications. Lastly, ultracompact fiber-optic acoustic sensors are presented by directly printing suspended optomechanical microresonator on the end-face of standard single-mode optical fiber. The suspended optomechanical microresonator acts as a reflection mirror and forms a Fabry-Pérot cavity together with the end-face of optical fiber. The mechanical resonance of the fiber-top optomechanical microresonator is analyzed by finite-element method (FEM). The response of the sensor to both acoustic waves and ultrasonic waves are experimentally tested and compared with simulation results. The optical fiber-tip sensor shows a high sensitivity of 118.3 mV/Pa and a corresponding noise equivalent acoustic signal level of 0.328 μPa/Hz1/2 at audio frequencies. With a resonance-enhanced mechanism, the sensitivity is further enhanced at the frequency of fundamental vibration resonance of the optomechanical resonator.