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There have been significant recent developments in the field of integrated optical Bragg grating sensors for use in the biological domain, where changes in the thickness of a surface layer upon specific binding of biological targets allows quantitative detection. However in the chemical domain less work has been reported. We present here an integrated optical Bragg grating sensor, capable of evanescently detecting small changes in refractive index down to 10(-6) RIU at infrared wavelengths, within a microfluidic system. The high spectral fidelity of the Bragg gratings combined with precise thermal compensation enables direct monitoring of the surface throughout the experiment. This allows the sensor to probe surface changes in situ and in real-time, from preparation through to chemical modification of the surface, so that the progress of dynamic surface-localized interactions can be followed. Here we describe confirmatory studies to validate this approach, including a comparison with the modelled optical system, before assessing the ability to detect binding of Group I cations at a crown ether-functionalised supramolecular surface. Unlike larger biological entities, for these small chemical species, simple additive changes in film-thickness no longer prevail.

An exposed Bragg grating incorporated into a planar waveguide forms an optical device that acts as a refractive index sensor. The exposed evanescent field causes the Bragg peak to be sensitive to the refractive index of its surroundings and can be used to detect changes in this environment. The method reported is able to provide accurate temperature compensation by applying a scaling factor derived from measurement of the birefringence of the transverse electric (TE) and transverse magnetic (TM) modes of the Bragg reflection. For a fluid sensor, fluctuations in ambient temperature can be both accounted for and compensated to produce a temperature-insensitive sensor.

An overview of the fabrication and geometries of integrated planar Bragg sensors will be described including refractive index sensors for chemical detection and applications of micro-structured devices for physical detection.

A planar Bragg grating device has been developed that is capable of detecting changes in the refractive index of a wide range of fluids including solvents, acids and bases. The integration of this high precision refractive index sensor within a chemically resistant microfluidic flow system has enabled the investigation of diverse fluid interactions. By cycling between different solvents, both miscible and immiscible, within the microfluidic system it is shown that the previous solvent determines the nature of the refractive index profile across the transition in composition. This solvent dispersion effect is investigated with particular attention to the methanol–water transition, where transients in refractive index are observed that are an order of magnitude larger in amplitude than the difference between the bulk fluids. The potential complications of such phenomenon are discussed together with an example of a device that exploits this effect for the unambiguous composition measurement of a binary solvent system.

Oxygen sensing is required for the understanding of many chemical processes across a diverse set of fields including medicine, environmental science and chemical synthesis. Oxygen sensing can be achieved through the use of electronic sensors. However, there are limitations associated with electronic sensors including susceptibility to electromagnetic interference and presenting a spark risk in flammable environments. Optical fiber and integrated optical chemical sensors overcome these limitations of electrical based sensing methods.

A photoactive material has been synthesised in which azobenzene units are attached to a methyl methacrylate/2-hydroxyethyl methacrylate co-polymer and shows repeated photoresponsive switching upon irradiation at 365 nm and 254 nm with long term thermal stability. In combination with a direct UV-written silica-on-silicon Bragg grating, this has been used to fabricate a prototype optical device which undergoes reversible refractive index changes at telecom wavelengths. The 63 GHz tuning response demonstrated by this device has potential applicability for reconfigurable dispersion compensation for use in optical networks.

Multiplexed direct UV written planar Bragg gratings are integrated into a compact silica-on-silicon chip, with the ability to locally monitor temperature, chemical species, pressure and flow.

Strong transitional mixing effects were observed by a planar Bragg grating sensor within a microfluidic system. This property was used to develop an integrated optofluidic sensor for detection of the composition of mixed solvent systems.

UV written planar Bragg grating sensors have been shown to form effective refractometers. Here we show that by using the birefringence of an integrated waveguide a temperature insensitive Bragg grating refractometer can be realised.

Planar waveguides can be written with a UV-laser into photosensitized silica to produce a wide range of optical devices. Careful modulation of two interfering beams allows Bragg gratings to be directly written into the channel (Figure 1.a). These Bragg gratings are inherently sensitive to temperature and strain. However etching away the surface exposes the mode within the grating to its surroundings. The corresponding observed shift in Bragg wavelength can be used to detect changes in this environment [1]. Figure 1.b shows a large wavelength shift due to two analytes of markedly different refractive index. This demonstrates the apodised spectral response that in practice allows resolution of sub-picometer shifts in wavelength.

An exposed Bragg grating incorporated into a planar waveguide was used to form a refractive index sensor. The high sensitivity to subtle changes allowed the study of surface functionalisation and binding within a microfluidic system.

Integrated photonics is a proven platform for physical and chemical sensing. It offers miniaturised solutions that are suited for use in extreme environments, including strong EM-fields, EM-pulses and contact with flammable materials, often far exceeding electronic sensors in this regard. This review looks into direct UV-written planar Bragg grating technology and its application to integrated photonic sensors. The platform has been demonstrated widely for measurement of physical properties such as temperature, pressure and strain. In addition, by using an evanescent interaction, refractive index can be measured allowing for chemical and biochemical detection. Further to this, the platform has recently been utilised in quantum information processing, where quantum gate operations and single photon detection has been shown.

The negative thermo-optic coefficient of water has been used to fabricate an athermal Bragg grating device. Glucose was used to modify the refractive index of water, allowing this response to be tuned to the desired operating temperature range. Over a 10°C temperature range the Bragg response was found to vary by 3.7pm, an improvement of nearly two orders of magnitude over the unetched Bragg grating. The low cost of fabrication compared to traditional athermal Bragg devices makes them ideal for incorporation within integrated photonic networks.

We present the direct measurement and modeling of the sensitivity limit of an integrated refractive index sensor for the detection of molecular monolayers. Direct UV writing can be used to fabricate a wide range of integrated optical devices particularly Bragg gratings. These Bragg gratings are inherently sensitive to temperature and strain. By etching a portion of the cladding, the mode within the grating region can be exposed to an analyte (Figure 1.a). The spectral response of the grating provides information about the refractive index of the analyte [1].