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An investigation into relative humidity measurement using an aluminosilicate sol-gel thin film as the active layer in an integrated optical Bragg grating refractometer

An investigation into relative humidity measurement using an aluminosilicate sol-gel thin film as the active layer in an integrated optical Bragg grating refractometer
An investigation into relative humidity measurement using an aluminosilicate sol-gel thin film as the active layer in an integrated optical Bragg grating refractometer
We have demonstrated that a planar integrated optical Bragg grating sensor chip, fabricated using a direct UV-writing approach, can perform as an all-optically accessed relative humidity sensor. Using a simple mesoporous silica thin film enabled operation across the range 0-100%RH, however a thickness decrease of 25% caused by hydrolytic degradation of the silica thin film resulted in a permanent decrease in sensitivity after 1-2 weeks use. Grafting the thin-film with aluminium oxide is shown here to prevent hydrolytic degradation with long-term sensitivity maintained. Aluminium oxide grafting offers the additional benefit of enhanced sensitivity, up to 3.5 times that of the silica analogue; the sensitivity of the aluminium oxide modified thin film device = 0.69 ± 0.05%RH/pm, in the range 0-60%RH within a gas flow system, whilst the sensitivity of the silica thin film sensor device = 2.47 ± 0.18%RH/pm. Long-term measurements, in static ambient lab conditions, were in excellent agreement with a commercial humidity sensor and also indicated that the thermal compensation afforded by the on-chip reference gratings was sufficient to prevent any drift in calibration. As with all humidity sensors, contamination of the sensor occurred. But for the aluminium oxide device, this was easily removed by rinsing with common organic solvents and the sensitivity was repeatedly and reproducibly restored. The device is accessed using optical fibre technology and is intrinsically safe in flammable environments.
bragg grating, refractive index sensor, relative humidity, integrated optics, sol–gel
0925-4005
857-866
Wales, Dominic J.
c001ff27-d3ac-4d2e-859f-9e963fc0374c
Parker, Richard M.
b052ca4d-b6c7-4fdd-a2f9-45032f0ff13f
Gates, James C.
b71e31a1-8caa-477e-8556-b64f6cae0dc2
Grossel, Martin C.
403bf3ff-6364-44e9-ab46-52d84c6f0d56
Smith, Peter G.R.
8979668a-8b7a-4838-9a74-1a7cfc6665f6
Wales, Dominic J.
c001ff27-d3ac-4d2e-859f-9e963fc0374c
Parker, Richard M.
b052ca4d-b6c7-4fdd-a2f9-45032f0ff13f
Gates, James C.
b71e31a1-8caa-477e-8556-b64f6cae0dc2
Grossel, Martin C.
403bf3ff-6364-44e9-ab46-52d84c6f0d56
Smith, Peter G.R.
8979668a-8b7a-4838-9a74-1a7cfc6665f6

Wales, Dominic J., Parker, Richard M., Gates, James C., Grossel, Martin C. and Smith, Peter G.R. (2013) An investigation into relative humidity measurement using an aluminosilicate sol-gel thin film as the active layer in an integrated optical Bragg grating refractometer. Sensors and Actuators B: Chemical, 188, 857-866. (doi:10.1016/j.snb.2013.07.089).

Record type: Article

Abstract

We have demonstrated that a planar integrated optical Bragg grating sensor chip, fabricated using a direct UV-writing approach, can perform as an all-optically accessed relative humidity sensor. Using a simple mesoporous silica thin film enabled operation across the range 0-100%RH, however a thickness decrease of 25% caused by hydrolytic degradation of the silica thin film resulted in a permanent decrease in sensitivity after 1-2 weeks use. Grafting the thin-film with aluminium oxide is shown here to prevent hydrolytic degradation with long-term sensitivity maintained. Aluminium oxide grafting offers the additional benefit of enhanced sensitivity, up to 3.5 times that of the silica analogue; the sensitivity of the aluminium oxide modified thin film device = 0.69 ± 0.05%RH/pm, in the range 0-60%RH within a gas flow system, whilst the sensitivity of the silica thin film sensor device = 2.47 ± 0.18%RH/pm. Long-term measurements, in static ambient lab conditions, were in excellent agreement with a commercial humidity sensor and also indicated that the thermal compensation afforded by the on-chip reference gratings was sufficient to prevent any drift in calibration. As with all humidity sensors, contamination of the sensor occurred. But for the aluminium oxide device, this was easily removed by rinsing with common organic solvents and the sensitivity was repeatedly and reproducibly restored. The device is accessed using optical fibre technology and is intrinsically safe in flammable environments.

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Published date: November 2013
Keywords: bragg grating, refractive index sensor, relative humidity, integrated optics, sol–gel
Organisations: Optoelectronics Research Centre, Organic Chemistry: Synthesis, Catalysis and Flow

Identifiers

Local EPrints ID: 356427
URI: https://eprints.soton.ac.uk/id/eprint/356427
ISSN: 0925-4005
PURE UUID: cb230744-e58c-4794-b3cc-4cd7a25909c3
ORCID for James C. Gates: ORCID iD orcid.org/0000-0001-8671-5987
ORCID for Martin C. Grossel: ORCID iD orcid.org/0000-0001-7469-6854
ORCID for Peter G.R. Smith: ORCID iD orcid.org/0000-0003-0319-718X

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Date deposited: 26 Sep 2013 16:10
Last modified: 03 Dec 2019 02:03

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Contributors

Author: Dominic J. Wales
Author: Richard M. Parker
Author: James C. Gates ORCID iD

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