Long period grating-based humidity sensor for potential structural health monitoring

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Abstract

This paper reports the work on the development of novel humidity sensors using long period grating (LPG) technology in an optical fibre. The sensors are created through coating a thin layer of polyvinyl alcohol (PVA), whose refractive index varies as a function of humidity level, onto an LPG. The LPG written in a single mode optical fibre provides a means to change the coupling between the core and cladding modes thus to induce a wavelength shift in the attenuation bands of the transmission spectrum of the optical sensor device created. In this work different sensor configurations were created by coating various LPGs and then characterizing the resulting sensor performance. The work carried out has shown that, compared to the more familiar fibre Bragg grating (FBG)-based humidity sensors, LPG sensors show higher measurement sensitivity, with more relaxed requirements for the coating thickness and uniformity, with the potential for their development as chemical sensors through optimization of the design of the coating materials chosen. In this research, the sensors which were designed and fabricated were created for future applications in determining moisture ingress in a range of concrete materials when subjected to various environmental conditions.

Introduction

Fibre optic-based humidity sensors have shown real promise due to the attractive features they possess such as small size, geometric versatility, multiplexing capability and resistance to corrosive and hazardous environments [1]. The influence of humidity on a polymer-coated fibre Bragg grating (FBG) was first reported by Giaccari et al. [2] and the technique described for humidity detection has been further explored by several research groups [1], [3], [4]. Important applications arise from the need for monitoring of structural deterioration, which often is linked to the ingress of water which acts as a transport medium for aggressive ions, such as chloride, sulfate, carbonate and ammonium [4].

The long period grating (LPG) has demonstrated its sensitivity to the refractive index value of the surrounding material of the grating [5], [6], [7], [8], [9], [10] and thus has been explored as a refractive index sensor for various applications. The relationship between the LPG resonance wavelength (λ) and the grating periodicity (Λ) is given by the following [7]:λ=(ncoeffncl,meff)Λ,(m=1,2,)where ncoeff and ncl,meff are the effective indices of the fundamental core mode and the mth cladding mode respectively and the effective refractive index of the cladding mode, however, can be modulated by the refractive index variation of the surrounding material.

In this paper, building upon prior work by some of the authors and other published data on fibre Bragg grating and long period grating technologies, research into the use of LPGs coated with a thin layer of polyvinyl alcohol (PVA) has been carried out to explore the measurement of humidity through the refractive index change of the coating material caused by the swelling effect on the coating on the LPG in the fibre when it is exposed to different humidity conditions. LPG-based humidity monitoring based on the resonance wavelength shift has been recently reported by several researchers [11], [12]. Important amongst these is the work of Konstantaki et al. [11] who have reported a PEO/CoCl2-coated LPG-based humidity sensor. In this work an interesting result has been reported in that when the humidity level was increased from 52%RH to 77%RH, a blue shift in the resonance wavelength was observed and when the humidity level was increased from 77%RH to 95%RH a red shift in wavelength was then observed. This could, however, allow the system to give ambiguous RH readings. In further reported work by Liu et al. [12], a shift in the resonance wavelength of a hydrogel-coated LPG in response to various humidity levels is seen. The work in this paper takes such sensor systems forward reporting interesting performance data.

Section snippets

Theoretical background

Extensive research has been carried out to investigate the dependence of resonance wavelength as a function of the refractive index change of the surrounding media [5], [6], [7], [8], [9] and a typical calibration curve (shown in Fig. 1) shows the resonance wavelength change as a function of the refractive index (RI) of the surrounding medium (or coating) with the cladding refractive index of the LPG being ∼1.45. This indicates that when the coating refractive index is lower than that of the

Experimental setup

The experimental setup used in this work is reported in detail below.

Experimental results

Experimental results are reported on devices prepared using the techniques above.

Summary and discussion

This paper has presented detailed results of work carried out on realising two LPG-based polymer-coated humidity sensors and reported on their characterisation. Following consideration of the properties of several candidate materials, PVA was chosen as the moisture sensitive coating material and the LPG sensors created with thin layer coatings have shown good response when they are exposed to various humidity conditions.

Both sensors evaluated have shown a non-linear response to the humidity

Acknowledgements

The authors would like to thank the Engineering and Physical Sciences Research Council in the UK and Hong Kong CERG grant (grant number: Polu Y 5296/06E) for their support.

T.Venugopalan received the B.Eng. (Hons) degree in computer systems engineering from the Division of Electrical, Electronics and Information Engineering, SEMS, City University, London, U.K., in 2002. He is currently working towards the Ph.D. degree in the area of optical fibre sensors at City University, London.

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    T.Venugopalan received the B.Eng. (Hons) degree in computer systems engineering from the Division of Electrical, Electronics and Information Engineering, SEMS, City University, London, U.K., in 2002. He is currently working towards the Ph.D. degree in the area of optical fibre sensors at City University, London.

    T. Sun received the B.Eng., M.Eng., and Dr. Eng. Degrees in mechanical engineering from the Department of Precision Instrumentation, Harbin Institute of Technology, Harbin, China, in 1990, 1993, and 1996, respectively. She received the Doctor of Philosophy degree at City University, London, U.K., in applied physics in 1999. She was an assistant Professor at Nanyang Technological University, Singapore, from year 2000 to 2001 before she rejoined City University in 2001 as a Lecturer. Subsequently, she was promoted to a Senior Lecturer in 2003 and to a Reader in 2006 at City University. Dr. Sun is a member of the Institute of Physics and the Institution of Engineering and Technology. She is a Chartered Physicist and a Charted Engineer in the United Kingdom. She has authored or co-authored some 130 scientific and technical papers. Her research interest is in optical fibre sensors, instrumentation, laser engineering and optical communications.

    K.T.V. Grattan was born in Co. Armagh, Northern Ireland, on 9 December 1953. He received the B.S. degree in physics (First Class Hons) and the Ph.D. degree from the Queen's University, Belfast, Ireland, in 1974 and 1978, respectively. He received the Doctor of Science degree from City University, London, U.K., in 1992. In the same years, he became a Postdoctoral Research Assistant at Imperial College, London. His research during that period was on laser systems for photophysical systems investigations. His work in the field continued with research using ultraviolet and vacuum ultraviolet lasers for photolytic laser fusion driver systems and studies on the photophysics of atomic and molecular systems. He joined City University in 1983 after 5 years at Imperial College, undertaking research in novel optical instrumentation, especially in fibre-optic sensor development for physical and chemical sensing. The work has led into several fields including luminescence-based thermometry, Bragg-grating-based strain sensor systems, white light interferometry, optical system modelling and design and optical sensors for water quality monitoring. The work has been extensively published in the major journals and at international Conferences in the field, where regularly he has been an invited speaker, and over 600 papers have been authored to date. He is currently Deputy Dean of Engineering at City University having from 1991 to 2001 been Head of the then Electrical, Electronic and Information Engineering Department. He was Chairman of the Applied Optics Division of the U.K. Institute of Physics and President of the Institute of Measurement and Control in 2000. He is Editor of the Journal of the International Measurement Confederation, Measurement, published by Elsevier.

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