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A special issue of Sensors (ISSN 1424-8220). This special issue belongs to the section "Physical Sensors".

Deadline for manuscript submissions: closed (15 January 2020) | Viewed by 24556

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Prof. Dr. Sabina Merlo

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Department of Electrical, Computer and Biomedical Engineering, University of the Studies of Pavia, 27100 Pavia, Italy
Interests: MEMS; MOEMS; optical sensors; interferometry; microphotonics; biophotonics; biosensors; lab on a chip
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Dear Colleagues,

Optical techniques have currently several applications in various fields, from biomedical diagnoses and therapies to material analyses and shaping, from (bio)chemical sensing to environmental monitoring.

Optical properties like absorption, reflection, transmission and scattering of the fluids/materials under investigation strongly affect transmission and distribution of the optical beams, either used for passive readout or active interaction. As these properties depend on the refractive index of the materials crossed by light, in order to better understand and, possibly, foresee light-material interactions there is still an unsatisfied request of accurate data relative to the refractive index of a wide variety of substances and, as a consequence, of new sensing approaches for their detection.

Huge technological advances have been carried out in the past years in the field of passive and active photonic devices, working on spectral regions different from the classical visible range where most of the available refractive index data have been collected so far. It is time to fully exploit the potential offered by the amazing performances provided by these new technologies for further expanding the application horizons of optical techniques in gas/fluid/matter sensing and conditioning.

Refractive index sensing represents a starting point for new photonic applications as well as a self-standing research area leading to innovative instrumental configurations and/or new sets of data relative to still unexplored materials. The objective of this Special Issue is to present significant work in the field of refractive index sensors: authors from academia and industry are kindly invited to share their research innovations in this field. We welcome review articles and original research papers aiming to the related key issues of basic research, device development, system integration and data processing.

The topics of interest include, but are not limited to:

Innovative configuration for refractive index sensors
Fibre optic sensors for refractive index measurements
Refractive index sensing for environmental monitoring
New biomedical applications of refractive index sensing
Refractive index sensing in microfluidic devices

Prof. Dr. Sabina Merlo
Guest Editor

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12 pages, 2511 KiB

Open AccessArticle

Spectral Phase Shift Interferometry for Refractive Index Monitoring in Micro-Capillaries

by Valentina Bello, Alberto Simoni and Sabina Merlo

Sensors 2020, 20(4), 1043; https://doi.org/10.3390/s20041043 - 14 Feb 2020

Cited by 9 | Viewed by 3339

Abstract

In this work, we demonstrate spectral phase-shift interferometry operating in the near-infrared wavelength range for refractive index (RI) monitoring of fluidic samples in micro-capillaries. A detailed theoretical model was developed to calculate the phase-sensitive spectral reflectivity when low-cost rectangular glass micro-capillaries, filled with samples with different refractive indices, are placed at the end of the measurment arm of a Michelson interferometer. From the phase-sensitive spectral reflectivity, we recovered the cosine-shaped interferometric signal as a function of the wavelength, as well as its dependence on the sample RI. Using the readout radiation provided by a 40-nm wideband light source with a flat emission spectrum centered at 1.55 µm and a 2 × 1 fiberoptic coupler on the common input-output optical path, experimental results were found to be in good agreement with the expected theoretical behavior. The shift of the micro-capillary optical resonances, induced by RI variations in the filling fluids (comparing saline solution with respect to distilled water, and isopropanol with respect to ethanol) were clearly detected by monitoring the positions of steep phase jumps in the cosine-shaped interferometric signal recorded as a function of the wavelength. By adding a few optical components to the instrumental configuration previously demonstrated for the spectral amplitude detection of resonances, we achieved phase-sensitive detection of the wavelength positions of the resonances as a function of the filling fluid RI. The main advantage consists of recovering RI variations by detecting the wavelength shift of “sharp peaks”, with any amplitude above a threshold in the interferometric signal derivative, instead of “wide minima” in the reflected power spectra, which are more easily affected by uncertainties due to amplitude fluctuations. Full article

(This article belongs to the Special Issue Refractive Index Sensors)

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10 pages, 7713 KiB

Open AccessArticle

Ultra-Sensitive Fiber Refractive Index Sensor with Intensity Modulation and Self-Temperature Compensation

by Zhaojun Li, Liangtao Hou, Lingling Ran, Jing Kang and Jiuru Yang

Sensors 2019, 19(18), 3820; https://doi.org/10.3390/s19183820 - 04 Sep 2019

Cited by 16 | Viewed by 3202

Abstract

In this paper, a novel in-line modal interferometer for refractive index (RI) sensing is proposed and experimentally fabricated by cascading single-taper and multimode-double- cladding-multimode (MDM) fiber structure. Owing to evanescent field in taper area, the ultra-sensitive and linear intensity-responses to the varied surrounding RI are gained in both single- and double-pass structures. Moreover, the crosstalk from temperature can be effectively discriminated and compensated by means of the RI-free nature of MDM. The experimental results show that the RI sensitivities in single- and double-pass structures, respectively, reach 516.02 and 965.46 dB/RIU (RIU: refractive index unit), both with the slight wavelength shift (~0.2 nm). The temperature responses with respect to wavelength and intensity are 68.9 pm°C⁻¹/0.103 dB°C⁻¹ (single-pass structure) and 103 pm°C⁻¹/0.082 dB·°C⁻¹ (double-pass structure). So the calculated cross-sensitivity of intensity is constrained within 8.49 × 10⁻⁵ RIU/°C. In addition, our sensor presents high measurement-stability (~0.99) and low repeatability error (<4.8‰). On account of the ~620 μm size of taper, this compact sensor is cost-efficient, easy to fabricate, and very promising for the applications of biochemistry and biomedicine. Full article

(This article belongs to the Special Issue Refractive Index Sensors)

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16 pages, 12803 KiB

Open AccessArticle

D-Shaped POF Sensors for Refractive Index Sensing—The Importance of Surface Roughness

by Filipa Sequeira, Nunzio Cennamo, Alisa Rudnitskaya, Rogério Nogueira, Luigi Zeni and Lúcia Bilro

Sensors 2019, 19(11), 2476; https://doi.org/10.3390/s19112476 - 30 May 2019

Cited by 30 | Viewed by 4174

Abstract

In this study the influence of the surface roughness on the transmission capacities of D-shaped plastic optical fibers (POFs) and sensors performance was investigated. Five D-shaped POF sensors were produced and characterized for refractive index sensing between 1.33 and 1.41. The sensors were characterized using a low-cost optical sensing system based on the variation of the transmitted light though the POF with refractive index changes (RI). Higher surface roughness increases the scattering losses through the POF and influences the sensors’ performance; therefore, a balance must be attained. Generally, the best performance was achieved when the sensing region was polished with P600 sandpaper as a final polishing step. Polishing with sandpapers of lower grit size resulted in lower scattering, higher linearity of the sensor response and generally lower performance for RI sensing. A sensor resolution of 10⁻³–10⁻⁴ RIU, dependent on the value of the external refractive index, was obtained through simple and low-cost manufacturing procedures. The obtained results show the importance of surface roughness in the development of POF sensors which can be used in several applications, such as for water quality assessment. Full article

(This article belongs to the Special Issue Refractive Index Sensors)

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8 pages, 1762 KiB

Open AccessArticle

Polarization-Modulated, Goos–Hanchen Shift Sensing for Common Mode Drift Suppression

by Yuhang Wan, Mengxuan Cheng, Zheng Zheng and Kai Liu

Sensors 2019, 19(9), 2088; https://doi.org/10.3390/s19092088 - 05 May 2019

Cited by 6 | Viewed by 2806

Abstract

A polarization-modulation-based Goos–Hanchen (GH) sensing scheme leveraging the polarization-dependence of the Bloch surface wave enhanced GH shift is proposed and experimentally demonstrated. Based on a simple setup utilizing a liquid crystal modulator to switch the polarization state of the input beam periodically, the alternating positions of the reflected beam for both polarizations are monitored by a lock-in amplifier to handily retrieve the GH shift signal. The conventional direct measurement of the beam position for the target state of polarization is vulnerable to instabilities in the optomechanical setup and alignment. Our proposed scheme provides a sensitive yet robust GH shift-sensing setup where the common mode drift and noise could be suppressed to ensure better system stability. Full article

(This article belongs to the Special Issue Refractive Index Sensors)

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10 pages, 4837 KiB

Open AccessArticle

Tapered Fiber-Optic Mach-Zehnder Interferometer for Ultra-High Sensitivity Measurement of Refractive Index

by Vahid Ahsani, Farid Ahmed, Martin B.G. Jun and Colin Bradley

Sensors 2019, 19(7), 1652; https://doi.org/10.3390/s19071652 - 06 Apr 2019

Cited by 86 | Viewed by 7722

Abstract

A Mach-Zehnder interferometer (MZI) based fiberoptic refractive index (RI) sensor is constructed by uniformly tapering standard single mode fiber (SMF) for RI measurement. A custom flame-based tapering machine is used to fabricate microfiber MZI sensors directly from SMFs. The fabricated MZI device does not require any splicing of fibers and shows excellent RI sensitivity. The sensor with a cladding diameter of 35.5 µm and length of 20 mm exhibits RI sensitivity of 415 nm/RIU for RI range of 1.332 to 1.384, 1103 nm/RIU for RI range of 1.384 to 1.4204 and 4234 nm/RIU for RI range of 1.4204 to 1.4408, respectively. The sensor reveals a temperature sensitivity of 0.0097 nm/°C, which is relatively low in comparison to its ultra-high RI sensitivity. The proposed inexpensive and highly sensitive optical fiber RI sensors have numerous applications in chemical and biochemical sensing fields. Full article

(This article belongs to the Special Issue Refractive Index Sensors)

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