Electro-optical transducer based on indium-tin-oxide-coated optical fiber for analysis of ionized media

The paper introduces a concept of an optical fiber based electro-optical transducer for monitoring of ionized media, such as low-temperature plasma. It utilizes optical fiber with a section of a core coated with tailored indium tin oxide (ITO) thin film and thus combines the optical phenomena of lossy-mode resonance (LMR) with the electrostatic probe. ITO is an optically transparent and electrically conductive material and if its properties such as thickness, as well as optical and electrical properties are optimized, the ITO-LMR sensor is sensitive to changes in properties of both the film and its close vicinity. The ITO-LMR sensor, with its conductive surface, corresponds well by shape and geometry to electrostatic Langmuir probe. In this work, ITO-LMR sensor was employed for simultaneous optical (LMR spectrum) and electrical (current-voltage) interrogation of ionized media, where low-temperature plasma was used as an example. It has been found that optical and electrical responses highly correspond to one another and are attributed to changes in the properties of a plasma sheath surrounding the sensor. The charged species in the sheath influence not only its electrical and optical properties, but also optical properties of ITO. Thus, the sensor allows for real-time, optical monitoring of properties of the ionized media and obtaining optical readout that can be delivered directly to distant locations via an optical fiber without interference of electromagnetic noise.