An intrinsic biochemical concentration sensor using a polymer optical fibre Bragg grating

A new type of fibre-optic biochemical concentration sensor based on a polymer optical fibre Bragg grating (POFBG) is proposed. The wavelength of the POFBG varies as a function of analyte concentration. The feasibility of this sensing concept is demonstrated by a saline concentration sensor. When polymer fibre is placed in a water based solution the process of osmosis takes place in this water-fibre system. An osmotic pressure which is proportional to the solution concentration, will apply to the fibre in addition to the hydraulic pressure. It tends to drive the water content out of the fibre and into the surrounding solution. When the surrounding solution concentration increases the osmotic pressure increases to drive the water content out of the fibre, consequently increasing the differential hydraulic pressure and reducing the POFBG wavelength. This process will stop once there is a balance between the osmotic pressure and the differential hydraulic pressure. Similarly when the solution concentration decreases the osmotic pressure decreases, leading to a dominant differential hydraulic pressure which drives the water into the fibre till a new pressure balance is established. Therefore the water content in the polymer fibre - and consequently the POFBG wavelength - depends directly on the solution concentration. A POFBG wavelength change of 0.9 nm was measured for saline concentration varying from 0 to 22%. For a wavelength interrogation system with a resolution of 1 pm, a measurement of solution concentration of 0.03% can be expected.

Hydrostatic pressure sensing using a polymer optical fibre Bragg gratings

The sensitivity of a fibre Bragg grating (FBG) sensor fabricated in polymer optical fibre (POF) to hydrostatic pressure was investigated for the first time. In this initial investigative work a reflected Bragg response of a FBG fabricated in multimode microstructured POF (MMmPOF) was monitored, whilst the hydrostatic pressure was increased up to 10MPa. Positive sensitivities were observed, meaning a positive wavelength shift to increasing pressure, as opposed to negative sensitivities monitored when using a FBG sensor fabricated in silica optical fibre. The FBG sensors fabricated in the MMmPOF gave fractional changes in wavelength and hence sensitivities of at least 64.05×10-6/MPa, which is some 25 times larger than the -2.50×10-6/MPa sensitivity of a FBG sensor fabricated in silica optical fibre that was measured in this work. Furthermore this work highlighted a decrease in sensitivity of the FBG sensor fabricated in the MMmPOF by some 50% by sealing the holes of the mPOF at the tip of the fibre with an adhesive. This offers the potential to tailor the response of the sensor to hydrostatic pressure.

Nonlinear response in polymer optical fibre Bragg grating based sensors

PMMA based polymer optical fibre Bragg gratings have been used for humidity, temperature and concentration sensing. Due to the water affinity of PMMA, the characteristic wavelength of the grating is largely modulated by the water content in the fibre. The rate of water transportation between fibre and surrounding depends on the permeability coefficient for PMMA, which is a function of surrounding temperature and humidity. This leads to increased water content with increasing humidity and temperature. Consequently the wavelength of the grating shows a nonlinear change over varying humidity and temperature. This nonlinearity needs to be calibrated prior to sensor application.

Non-linear temperature response of Bragg gratings in doped and un-doped holey polymer optical fibre

We present measurements on the non-linear temperature response of fibre Bragg gratings recorded in pure and trans-4-stilbenemethanol-doped polymethyl methacrylate (PMMA) holey fibres.

Fibre Bragg gratings recorded in microstructured polymer optical fibre

We present results obtained since recording the first FBGs in microstructured polymer optical fibre (mPOF) and discuss the relative merits of Bragg grating based sensing with polymer optical fibre in general and mPOF in particular. © 2006 OSA/OFS 2006.

Optical bend sensor for vector curvature measurement based on Bragg grating in eccentric core polymer optical fibre

We present an optical bend sensor based on a Bragg grating written in an eccentric core polymer optical fibre. The grating wavelength shifts are studied as a function of bend curvature and fibre orientation and the device exhibits strong fibre orientation dependence, wide bend curvature range of ± 22.7 m-1 and high bend sensitivity of 63 pm/m-1, which is 80 times higher than the reported sensor based on an offset-FBG in standard single mode silica fibre.

Grating based devices in polymer optical fibre

We describe recent research into devices based on fibre Bragg gratings in polymer optical fibre. Firstly, we report on the inscription of gratings in a variety of microstructured polymer optical fibre: single mode, few moded and multimoded, as well as fibre doped with trans-4-stilbenmethanol. Secondly, we describe research into an electrically tuneable filter using a metallic coating on a polymer fibre Bragg grating. Finally we present initial results from attempts to produce more complex grating structures in polymer fibre: a Fabry-Perot cavity and a phase-shifted grating.

Development of an electrically tuneable Bragg grating filter in polymer optical fibre operating at 1.55 νm

We present a thorough study on the development of a polymer optical fibre-based tuneable filter utilizing an intra-core Bragg grating that is electrically tuneable, operating at 1.55 νm. The Bragg grating is made tuneable using a thin-film resistive heater deposited on the surface of the fibre. The polymer fibre was coated via the photochemical deposition of a Pd/Cu metallic layer with the procedure induced by VUV radiation at room temperature. The resulting device, when wavelength tuned via Joule heating, underwent a wavelength shift of 2 nm for a moderate input power of 160 mW, a wavelength to input power coefficient of -13.4 pm mW-1 and time constant of 1.7 s-1. A basic theoretical study verified that for this fibre type one can treat the device as a one-dimensional system. The model was extended to include the effect of input electrical power changes on the refractive index of the fibre and subsequently to changes in the Bragg wavelength of the grating, showing excellent agreement with the experimental measurements. © 2007 IOP Publishing Ltd.

Water detection in jet fuel using a polymer optical fibre Bragg grating

Water is a common impurity of jet fuel, and can exist in three forms: dissolved in the fuel, as a suspension and as a distinct layer at the bottom of the fuel tank. Water cannot practically be eliminated from fuel but must be kept to a minimum as large quantities can cause engine problems, particularly when frozen, and the interface between water and fuel acts as a breeding ground for biological contaminants. The quantities of dissolved or suspended water are quite small, ranging from about 10 ppm to 150 ppm. This makes the measurement task difficult and there is currently a lack of a convenient, electrically passive system for water-in-fuel monitoring; instead the airlines rely on colorimetric spot tests or simply draining liquid from the bottom of fuel tanks. For all these reason, people have explored different ways to detect water in fuel, however all these approaches have problems, e.g. they may not be electrically passive or they may be sensitive to the refractive index of the fuel. In this paper, we present a simple, direct and sensitive approach involving the use of a polymer optical fibre Bragg grating to detect water in fuel. The principle is that poly(methyl methacrylate) (PMMA) can absorb moisture from its surroundings (up to 2% at 23 °C), leading to both a swelling of the material and an increase in refractive index with a consequent increase in the Bragg wavelength of a grating inscribed in the material.

Connectorisation of fibre Bragg grating sensors recorded in microstructured polymer optical fibre

We describe the production and characterization of FC/PC connectorised fibre Bragg grating sensors in polymer fibre. Sensors were recorded in few-moded and single mode microstructured fibre composed of poly (methyl methacrylate). © 2013 SPIE.

Polymer optical fibre Bragg gratings

This paper provides a review of the current state of research and development into polymer fibre Bragg gratings, along with a description of some current challenges. © 2012 OSA.

Reliability of fibre Bragg gratings in polymer optical fibre

In this paper we report on investigations of some of the factors that have a bearing on the reliability and repeatability of polymer fibre Bragg gratings. The main issues discussed are the fibre preform composition, the fibre drawing conditions and the thermal history of the fibre grating.

Strain and temperature sensitivity of a singlemode polymer optical fibre

We report experimental measurements of the strain and temperature sensitivity of the optical phase in a singlemode polymer optical fibre. These values were obtained by measuring optical path length change using a Mach-Zender interferometer.

An investigation into the wavelength stability of polymer optical fibre Bragg gratings

The inscription of Bragg gratings has been demonstrated in PMMA-based polymer optical fibre. The water affinity of PMMA can introduce significant wavelength change in a polymer optical fibre Bragg grating (POFBG). In polymer optical fibre losses are much higher than with silica fibre. Very strong absorption bands related to higher harmonics of vibrations of the C-H bond dominate throughout the visible and near infrared. Molecular vibration in substances generates heat, which is referred to as the thermal effect of molecular vibration. This means that a large part of the absorption of optical energy in those spectral bands will convert into thermal energy, which eventually drives water content out of the polymer fibre and reduces the wavelength of POFBG. In this work we have investigated the wavelength stability of POFBGs in different circumstances. The experiment has shown that the characteristic wavelength of a POFBG starts decreasing after a light source is applied to it. This decrease continues until equilibrium inside the fibre is established, depending on the initial water content inside the fibre, the surrounding humidity, the optical power applied, and the fibre size. Our investigation has shown that POFBGs operating at around 850 nm show much smaller wavelength reduction than those operating at around 1550 nm in the same fibre; POFBGs with different diameters show different changes; POFBGs powered by a low level light source, or operating in a very dry environment are least affected by this thermal effect.