High-Speed Measurement of Refractive Index Using Dielectric Multilayer Films Deposited on Optical Fiber End

We have recently developed a refractive index (RI) sensor using a band-pass filter (BPF) composed of dielectric multilayer films deposited on an optical fiber end. An optical spectrum analyzer was, however, conventionally employed in this sensor to obtain the reflection wavelength, which limited the total measurement speed due to its second-order sweep time. In this study, we enhance its measurement speed by use of a high-speed demodulator where the optical wavelength information is converted into electrical voltage using the linear slopes of the reflection spectra. The response time of the sensor, which is influenced by the dynamic behavior of the sample liquid, is found to be as short as several tens of micro-seconds. We also compare its RI sensitivity with that of a simple Fresnel-based RI sensor.