Broadband electric field sensor with electro-optic polymer micro-ring resonator on side-polished optical fiber

Abstract

A novel broadband E-field sensor based on electro-optic polymer micro-ring resonator directly coupled to the core of optical fiber is proposed and demonstrated. A flat is made on the side of the optical fiber by polishing and an electro-optic polymer waveguide in the shape of a ring is placed on the polished flat. One side of the ring is directly above the core of the fiber and light is evanescently coupled between the fiber and the micro-ring. External electric fields change the index of refraction of the ring resonator and therefore its resonant wavelengths. The sensor is all dielectric without metal layers to distort the measured E-field. The resonance structure allows the sensor to potentially have much higher sensitivity than other electro-optic sensors based on interferometry or polarization modulation. Since electro-optic polymers have higher electro-optic coefficients, lower dielectric constants and faster electro-optic responses than inorganic crystals, higher sensitivity, lower invasiveness and higher bandwidth of E-field sensing can be expected. The sensor with EO polymer micro-ring directly coupled to side-polished fiber eliminates unreliable and possibly lossy fiber to waveguide butt coupling as well as the high propagation loss which comes from the long straight EO polymer waveguides. Unlike devices based on waveguide technology, a supporting substrate is not necessary in this device. This leads to sensors of small size and low disturbance to the measured electric field. In the proof-of-concept experiment, a sensitivity of 100 mV/m has been achieved at frequencies up to 550 MHz (limited by the measurement system) using AJLS103 electro-optic polymer.