Distributed electric field measurements along a hollow-core fiber by localizing a trapped charged particle with sub-mm spatial resolution using coherent optical frequency domain reflectometry

Optical sensors using conventional fiber can measure electric fields with high sensitivity while providing superior immunity to electromagnetic interference and low disturbance to the field under measurement compared to traditional field sensors based on metallic structures. With the development of hollow-core fibers (HCFs), entirely new sensing mechanisms are becoming available, leading to previously unfeasible measurement concepts. Such a concept is the flying particle sensor, which enables spatially resolved electric field measurements by localizing a trapped charged particle inside a HCF and its field dependent displacement. This displacement can be detected by measuring the transmitted power at the fiber output. In this work, we use coherent frequency domain reflectometry (COFDR) to obtain the absolute particle position, which in previous results could only be determined indirectly via the particle velocity using Doppler velocimetry. The sensor’s response to an electric field and its spatial resolution were characterized using a stationary particle in terms of electric field amplitude and frequency. The measurements resulted in a linear increase of 0.031 %m/kV of the modulation depth for sinusoidally modulated electric fields at 311 Hz up to 10 kV/m and a sub-mm spatial resolution over hours of optical trapping. Spatially distributed electric field measurements were demonstrated by obtaining repeatable results at multiple positions along the HCF and by validating the determined COFDR reflection profiles using a CCD directed to the trapped particle.