A Bio-Compatible Fiber Optic pH Sensor Based on a Thin Core Interferometric Technique
<p>Principle of a single mode-multi mode-single mode (SMS) thin core Mach-Zehnder interferometer. Light from the core of the left SMF28 fiber is coupled to cladding modes in the short (TCF), which can interact with the surrounding liquid depending on the liquid refractive index. For a broadband light source, a constructive or destructive interference pattern is created at the right (TCF)/SMF28 interface, which is guided in the core of the right SMF28 fiber.</p> "> Figure 2
<p>A COMSOL multiphysics simulation of an inline interferometer at the wavelength 1550 nm and an external RI 1.33. Inset (<b>a</b>) shows the whole simulated structure, whereas the insets (<b>b</b>,<b>c</b>) show the input and output part of the thin core fiber (TCF) respectively.</p> "> Figure 3
<p>Electric field along a center line running through the fiber.</p> "> Figure 4
<p>Electric field in the center of fiber, at wavelength 1550 nm and different surrounding refractive indexes, RI 1.33, 1.37 and 1.41. The inset shows an enlargement of the destructive interference around 540 um where a small shift is observed for increasing RI.</p> "> Figure 5
<p>Typical transmission spectrum measured on a TCF inline sensor with a length of 24.2 mm held in air (<b>a</b>) and center wavelength for the left minima in (<b>a</b>) as a function of TCF length (<b>b</b>).</p> "> Figure 6
<p>The wavelength of the transmission minima for a 20.0 mm long TCF sensor submerged in solutions with different RI (<b>a</b>) and the sensitivity in nm per RIU as a function of RI (<b>b</b>).</p> "> Figure 7
<p>TCF coated with a thin, ∼0.5 <math display="inline"><semantics> <mi mathvariant="sans-serif">μ</mi> </semantics></math>m layer of pH-sensitive coating based on 1.3-BDDA and PIP (<b>a</b>). Large volume expansion of the polymer creating problem with adhesion to the glass fiber when the coated fiber is submerged in a liquid (<b>b</b>).</p> "> Figure 8
<p>Spectral response from a 1.4-BDDA/PIP polymer coated inline sensor when submerged in liquids with pH-levels ranging from 1.95 to 11.89. The wavelength of the central minima as a function of pH is shown in the inset for pH 6.12 to 11.89.</p> "> Figure 9
<p>Schematic picture showing the interaction of the evanescent wave with the hydrogel coated layer on the fiber cladding. For thin hydrogel layers, the evanescent wave will be influenced not only by the RI of the hydrogel but also the RI of the surrounding liquid (<b>a</b>). For thicker layers the evanescent wave will only be influenced by the effective index of the hydrogel (<b>b</b>).</p> "> Figure 10
<p>Wavelength of the central minima as a function of time for a sensor repeatedly submerged in pH-levels of 7.10 and 8.15.</p> ">
Abstract
:1. Introduction
2. Theory and Simulations
2.1. Sensor Concept
2.2. Simulation of the Inline Sensor
2.3. Simulation Results
3. Results and Experimental Verification of Sensor Concept
3.1. Sensor Design and Construction
3.2. Polymer Preparation and Coating Procedure
3.3. Experimental Evaluation of the pH Sensors
4. Discussion
5. Conclusions
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
References
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Engholm, M.; Hammarling, K.; Andersson, H.; Sandberg, M.; Nilsson, H.-E. A Bio-Compatible Fiber Optic pH Sensor Based on a Thin Core Interferometric Technique. Photonics 2019, 6, 11. https://doi.org/10.3390/photonics6010011
Engholm M, Hammarling K, Andersson H, Sandberg M, Nilsson H-E. A Bio-Compatible Fiber Optic pH Sensor Based on a Thin Core Interferometric Technique. Photonics. 2019; 6(1):11. https://doi.org/10.3390/photonics6010011
Chicago/Turabian StyleEngholm, Magnus, Krister Hammarling, Henrik Andersson, Mats Sandberg, and Hans-Erik Nilsson. 2019. "A Bio-Compatible Fiber Optic pH Sensor Based on a Thin Core Interferometric Technique" Photonics 6, no. 1: 11. https://doi.org/10.3390/photonics6010011