Hybrid sensor based on a hollow square core fiber for temperature independent refractive index detection

Abstract

In this work, a hybrid sensor based on a section of hollow square core fiber (HSCF) spliced between two single mode fibers is proposed for the measurement of refractive index of liquids. The sensor, with a length of a few millimeters, operates in a transmission configuration. Due to the HSCF inner geometry, two different interferometers are generated. The first, a Mach-Zehnder interferometer, is insensitive to the external refractive index, and presents a sensitivity to temperature of (29.2 ± 1.1) pm/°C. The second one, a cladding modal interferometer, is highly sensitive to the external refractive index. An experimental resolution of 1.0 × 10−4 was achieved for this component. Due to the different responses of each interferometer to the parameters under study, a compensation method was developed to attain refractive index measurements that are temperature independent. The proposed sensor can find applications in areas where refractive index measurements are required and the control of room temperature is a challenge, such as in the food and beverage industry, as well as in biochemical or biomedical industries.

Optical Fiber Sensor for Monitoring the Evaporation of Ethanol–Water Mixtures

“…Figure 4 presents the spectral responses of the sensor (left) with the respective fast Fourier transform (FFT) graphs (right) when surrounded by air, water, and ethanol. As previously reported [ 25 ], the sensor response in liquid media is modulated by two interferometric phenomena, the CMI (corresponding to the low frequencies located at ~0.1 nm −1 ) and the MZI (corresponding to the high frequencies located at ~0.6 nm −1 ). The spectral response of the sensor changes significantly when there is a transition from air medium to liquid media, which is also observable in the FFT (air), where more peaks, with a higher amplitude, appear in the low frequencies.…”

“…Furthermore, a temperature compensation technique can also be established [ 25 ], diminishing the associated errors when considering the influence of temperature in the optical sensor measurements.…”

“…In the first region, from 1.313 RIU to 1.336 RIU, a sensitivity of (229 ± 16) nm/RIU was attained, whilst for the second region, from 1.336 RIU to 1.350 RIU, a sensitivity of (387 ± 14) nm/RIU was estimated. Notice that the refractive index was estimated in the 1550 nm region, following the same procedure reported in [ 25 ].…”

“…The hollow square-core fiber (HSCF) is an antiresonant fiber whose light guidance relies on the antiresonance reflection within the core [ 24 ]. However, when this fiber is spliced between two single-mode fibers (SMF), other paths can be generated, giving rise to a Mach–Zehnder interferometer (MZI) and a cladding modal interferometer (CMI) [ 25 ], as depicted in Figure 1 . When the light that comes from the input SMF reaches the interface of the HSCF, it will be coupled to its hollow core, enhancing multiple core modes, and will also be coupled to the silica strands that surround the core.…”

Real-Time Measurement of Refractive Index Using 3D-Printed Optofluidic Fiber Sensor

“…From this more concentrated solution, working solutions were prepared with concentrations ranging from 0 to 40 wt.%, with intervals of 5 wt.%, by successive dilutions with deionized water. The refractive indices of these samples were extrapolated according to Pereira et al [ 25 ] because of the fact that the interrogation setup was set at 1550 nm. To confirm that the solutions were correctly prepared, the refractive indices were also measured in a refractometer Easy R40 (Mettler Toledo, Columbus, OH, USA).…”