INTEGRATED OPTICAL WAVEGUIDE CHEMICAL SENSOR WITH BRAGG GRATING

Abstract: In an optical device including a Bragg grating (4), such as a gas or vapour sensor, a planar optical waveguide layer (2) is carried on a substrate (1) and has a cladding layer (3). Light from a source (7) is transmitted via an input fibre (6) along the waveguide layer (2) to be incident on the Bragg grating (4) at an angle which results in Bragg scattering from it. The scattered radiation is monitored to determine the concentration of the gas to which the sensor is exposed, the effective index of the waveguide (2) varying with changes in concentration of the gas. To obtain high sensitivity, edge surface (5) of the substrate (1) and waveguide layer (2) is curved, permitting the input fibre (6) to be adjusted in position to satisfy the Bragg angle at the grating (4) for a particular device geometry. Thus variations in dimensions of the device because of manufacturing tolerances may be accommodated. Also, the same device may be used with different light sources by suitably positionning the input fibre (6) relative to the grating (4).

Description: INTEGRATED OPTICAL WAVEGUIDE CHEMICAL SENSOR WITH BRAGG GRATING This invention relates to optical devices and more particularly to those involving Bragg reflection or scattering. The invention is particularly, but not exclusively, applicable to optical sensors suitable for identifying and/or measuring the concentration of gases or vapours. In a device making use of Bragg reflection, light is directed onto a grating which consists of a regular array of etched lines, for example. When the light is incident on the grating at a particular angle, termed the Bragg angle, a strong reflection of the incident propagating light occurs when reflections from each line of the grating add up in phase with each other to give constructive interference. The conditions required for Bragg reflection are a function of the incident angle and the wavelength of the incident light. Any changes will result in a change in the intensity of light scattered by the grating and this may be monitored to give a measurement of certain parameters, for example, when the device is used in sensing applications. The present invention is particularly concerned with an optical sensor capable of measuring the concentration of gases or vapours, particularly hydrocarbons such as toluene and benzene at low ppm levels. However, it is envisaged that the invention may also be applicable to other sensor and non-sensor applications in which Bragg gratings are employed. According to the invention, there is provided an optical device comprising: a light source; means for coupling light from the source into an optical waveguide; and a Bragg grating located such that light transmitted along the waveguide interacts with the Bragg grating; and characterised in that the waveguide is planar and the waveguide surface at which light is coupled into the waveguide is shaped such that light can be directed normal to the surface into the waveguide in a selected one of a plurality of directions whereby the angle at which light is incident on the Bragg grating is adjustable to obtain Bragg scattering for different conditions. In a conventional device, it is difficult to fabricate an input channel waveguide and the grating structure to the high tolerances required and correctly align them to achieve Bragg scattering. The Bragg scattering condition is highly sensitive to the angle at which light is incident on the grating and difficulty in tolerancing and achieving alignment results in loss of sensitivity in the completed device. By using the invention, it is possible to adjust the position at which light is directed into the planar waveguide and onto the Bragg grating until the correct input angle is found to achieve Bragg scattering. As the waveguide itself is planar, problems in achieving alignment between a channel waveguide and a grating are obviated. Although some power is lost from the light beam launched into the planar waveguide because of the lack of lateral confinement, the divergence of a light beam in the plane of the waveguide may be made such that there is still significant scattering from the grating when the Bragg condition is satisfied. The nature of this device also allows for a convenient method of monitoring the intensity of the light source to compensate for drift and other changes. This may be achieved by monitoring the unreflected portion of the light after it has passed directly through the grating region. Alternatively the source power can be monitored by any suitable means prior to entering the waveguide. Another advantage of the present invention is that the same optical device may be used for light sources of different wavelengths as the light may be launched into the waveguide at respective different ones of the plurality of directions to achieve Bragg scattering. Thus, for example, devices may be manufactured in relatively large quantities to a nominally identical design and then input couplings added at different points on the shaped surface to give a plurality of sensors having different characteristics. Preferably, the waveguide is planar over an entire substrate on which it is supported as this is straightforward to fabricate. However, it may be useful in some circumstances to include some lateral confinement to the planar waveguide whilst still permitting the necessary freedom for positioning the point at which light is launched into the waveguide and onto the grating. Optical or electrical components may be carried on the substrate for integration into circuits. The substrate may be of glass or a semiconducting material for example. Preferably, the waveguide surface at which light is coupled into the waveguide is shaped as a curve in the plane of the waveguide and advantageously the curve is part of the circumference of a circle with the Bragg grating being located at its centre. This shape is a simple structure and also permits a large continuous range of input angles onto the grating to be achieved. More complex shaping may be introduced but this leads to additional complexity without necessarily providing any additional benefits. The Bragg grating is preferably an etched grating in the planar waveguide, in its substrate or in a cladding layer covering the waveguide. The grating may be fabricated using an image reversal resist process and a reactive ion etching step to give better quality gratings than is achievable using a standard positive resist process and with a greater tolerance to changes in exposure or development conditions for example. In other embodiments, the grating may be defined by other methods, for example, by periodic changes in the refractive index of the material of the waveguide, substrate or cladding layer. The light source may be one of a laser, LED or incandescent light source, for example. An optical output fibre may be arranged to receive light after it has undergone Bragg reflection and transmit it to a detector, such as an avalanche photodiode. In another embodiment, a detector might be located on the substrate itself. The invention is particularly advantageously used for sensing gases or vapours at low concentrations and identifying the substances involved. Even very low concentrations of toxic chemicals may damage the environment and human health. By employing the invention, good sensitivity may be achieved as deviations from the ideal design parameters are accommodated by adjustment of the input along the shaped waveguide surface. This enables the advantages of an optical device to be exploited, such as immunity to electrical interference, distribution of remote signals via optical fibre links and the possibility of using spectroscopic analysis to identify substances. The cladding layer covering the planar waveguide may be tailored such that it exhibits a large change in its refractive index when exposed to molecules of a particular substance which is to be sensed. The change in the refractive index of the cladding layer in the region of the grating causes the effective index of the planar waveguide to alter. The extent of this change is then monitored by monitoring changes in the intensity of light scattered from the grating. Identification of specific species may be achieved by using an array of sensors each having a cladding material tuned for enhanced absoφtion of different species, or by exploiting wavelength sensitive variations in the amount of Bragg scattering due to the influence of the species infra-red absorption spectra on the waveguide mode, if the concentration of the influencing species is sufficient. The sensor is particularly useful for measuring and identifying hydrocarbons but other substances may also be monitored. In one preferred embodiment, the waveguide layer is coated with a cladding layer which is a polysiloxane polymer, although it may be non-polymeric. Some ways in which the invention may be performed are now described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic perspective view of a sensor in accordance with the invention; Figure 2 is a schematic transverse section of part of the sensor shown in Figure 1; Figures 3 and 4 are graphs schematically illustrating operation of the device shown in Figures 1 and 2; and Figure 5 schematically illustrates a sensor array in accordance with the present invention. With reference to Figures 1 and 2, an optical sensor in accordance with the present invention comprises a substrate 1 of silicon on which is fabricated a silica buffer layer of 10 microns thickness followed by a doped silica layer of 4 microns thickness which defines a planar optical waveguide 2 which is single mode at a wavelength of 850 nm. A cladding layer 3 having a thickness of 5 microns is deposited on the waveguide layer 2. In this embodiment, the cladding layer 3 is a polysiloxane polymer having a refractive index of 1.40 at a wavelength of 850 nm and which readily allows molecules to diffuse through its structure when subjected to a changing partial pressure of gas at its surface. Polysiloxanes exhibit a particularly high rate of diffusion for toluene, for example. A Bragg grating 4 is fabricated, prior to the cladding layer being spun on, using an image reversal resist process and a reactive ion etching step to give a surface relief grating at the boundary between the waveguide layer 2 and the cladding layer 3. The pitch of the grating is 0.6 microns and the depth is approximately 0.4 microns. An edge surface of the substrate 1 and waveguide and cladding layers 2 and 3 is formed into a curve in the plane of the waveguide layer 2 as shown at 5. This curve may conveniently be formed by polishing or in some other way. An optical input fibre 6 is placed adjacent the waveguide layer 2 to permit coupling of light travelling along it from a 10 mW laser diode 7 having a wavelength of 855 nm into the waveguide layer 2 via which it is directed onto the Bragg grating 4. An output optical fibre 8 is placed adjacent the waveguide layer 2 to receive light after it has been scattered from the Bragg grating 4. Light travelling along the output fibre 8 is detected at 9 by an avalanche photodiode to give an output representative of the intensity of the reflected light. This is applied to a processor 10 for evaluation. During operation of the sensor, the sensor is exposed to a gas or vapour the concentration of which it is wished to monitor. Light from the source 7 is transmitted along the input fibre 6 to the waveguide layer 2. Light entering the waveguide layer 2 may be regarded as confined and referred to as an optical mode. The properties of the optical mode are determined from the characteristics of the propagating light and the refractive indices of the waveguide layer 2, the cladding layer 3 and the substrate 1. The refractive index of the waveguide layer is higher than that of the adjacent layers. Light travelling along the waveguide layer 2 is confined in that layer but also has an evanescent field that extends into the cladding layer 3 and substrate 1. The shape of the optical mode in the waveguide layer 2 and adjacent layers of lower refractive index is illustrated in Figure 2 by the line 11 representing the optical wave front of light travelling from left to right. Propagation of the confined mode may be defined unambiguously by a property of the mode called its effective index. The effective index changes if the refractive index of any of the waveguide layer 2, the cladding layer 3 or substrate 1 changes. Light from the input fibre 6 is arranged to strike the Bragg grating at a particular angle, called the Bragg angle. At this angle, a strong reflection of the incident propagating beam occurs. It is this Bragg scattering reflection which is detected by the detector 9. The Bragg condition is highly resonant and depends on the incident angle of the light, its wavelength and the effective index in the waveguide layer 2. During use, when the sensor is exposed to a hydrocarbon gas (shown schematically at 14) in this embodiment, some of the vapour molecules are absorbed into the cladding layer 3. Establishment of an equilibrium of concentration of volatile hydrocarbon molecules in the cladding layer 3 is a function of the vapour pressure of the hydrocarbon species to which the sensor is exposed. The presence of the vapour molecules causes the cladding layer 3 to swell in thickness, resulting in a change in its density and thus a change in the optical refractive index of the cladding layer 3. This therefore results in a change in the effective index of the guiding waveguide mode in the waveguide layer 2. The alteration in the Bragg condition because of the change in the effective index leads to a change in the intensity of light reflected by the Bragg grating 4 into the output fibre 8. By monitoring the change in intensity, it is possible to monitor the change in the effective index and thus gain an indication of the concentration of the vapour to which the sensor is exposed. The cladding material may be chosen such that it is particularly responsive to the gas or vapour to be monitored. The curved surface 5 of the device allows the positioning of the input fibre 6 to be adjusted so as to ensure that the Bragg condition is fulfilled to give a strong Bragg reflection at the grating 4. By monitoring the light detected at 9, the position of the input fibre 6 can be adjusted around the curve for the particular device geometry and light source. Thus any deviation from the ideal dimensions and characteristics of the materials are not significant and the sensor is able to sense low concentrations of vapour. Figure 3 schematically shows a response achieved when the sensor is exposed to 100 ppm toluene in nitrogen. Sensors in accordance with the invention have detected lower concentrations down to 10 ppm toluene in nitrogen and with further optimisation, it should be possible to achieve higher sensitivities. Figure 4 schematically shows the response of sensors having different thicknesses of cladding layers, from which it can be seen that a larger response is obtained when the cladding layer is thicker. The Bragg condition is, as explained above, also dependent on the wavelength of the light which is incident on the Bragg grating. When it is wished to use the sensor with light of a different wavelength, the input fibre 6 may be adjusted to a new position along the curve 5 and the output of the radiation scattered from the grating 4 observed to determine when the Bragg condition obtains. Thus one sensor design may be used for a range of input wavelengths as the input fibre 4 may be moved between the limits shown by the dotted lines along the curve 5 (Figure 1) and still allow light to be coupled efficiently into the waveguide layer 2. Another light detector (not shown) may be included to receive light which does not undergo Bragg scattering. This may be used to compensate for any changes in the output of the light source 7. Figure 5 schematically illustrates an array 12 of sensors similar to that shown in Figures 1 and 2 in which different cladding materials and light sources are used. The output signals are transmitted to a neural net processor 13 to give a characteristic response for the gas or vapour being sensed and a measure of its concentration.

Claims: CLAIMS 1. An optical device comprising: a light source; means for coupling light from the source into an optical waveguide; and a Bragg grating located such that light transmitted along the waveguide interacts with the Bragg grating; and characterised in that the waveguide is planar and the waveguide surface at which light is coupled into the waveguide is shaped such that light can be directed normal to the surface into the waveguide in a selected one of a plurality of directions whereby the angle at which light is incident on the Bragg grating is adjustable to obtain Bragg scattering for different conditions. 2. A device as claimed in claim 1 wherein the plurality of directions consists of a continuous range between two limits. 3. A device as claimed in claim 1 or 2 wherein the surface is shaped as a curve in the plane of the waveguide. 4. A device as claimed in claim 3 wherein the curve is part of the circumference of a circle with the Bragg grating being located at its centre. 5. A device as claimed in any preceding claim wherein the optical waveguide comprises a waveguide layer of relatively high refractive index supported by a substrate and covered by a cladding layer both of relatively low refractive index. 6. A device as claimed in claim 5 wherein the planar waveguide is extensive over substantially the whole substrate. 7. A device as claimed in claim 5 or 6 wherein the Bragg grating is located in at least one of the waveguide layer, substrate and cladding layer. 8. A device as claimed in any preceding claim wherein the Bragg grating comprises an etched grating. 9. A device as claimed in any preceding claim and including a detector arranged to detect light after it is incident on the Bragg grating. 10. A device as claimed in any preceding claim wherein the Bragg grating is defined in a material which is sensitive to a substance to be sensed such that characteristics of the Bragg condition change on exposure to the substance. 11. A device as claimed in claim 10 wherein the substance is a vapour or gas. 12. A device as claimed in claim 11 wherein the vapour or gas is a hydrocarbon. 13. A device as claimed in any preceding claim and including means for monitoring scattered light from the Bragg grating to identify and/or monitor the concentration of a substance to which the device is exposed. 14. A device as claimed in any preceding claim wherein the output of the light source is monitored before it enters the waveguide layer. 15. A device as claimed in any preceding claim wherein non-reflected light from the Bragg grating is monitored. 16. A device as claimed in claim 14 or 15 and including means for using the monitored output of the light source and/or the monitored non-reflected light from the Bragg grating to compensate for changes in ambient conditions and/or the light source output. 17. A device as claimed in any preceding claim wherein the light source is one of a laser, LED or incandescent light source. 18. A sensor arrangement comprising a plurality of devices as claimed in any preceding claim and including processing means for monitoring light scattered by the Bragg grating of each device to identify and/or monitor the concentration of a substance to which the arrangement is exposed. 19. A device substantially as illustrated in and described with reference to the accompanying drawings. 20. A sensor arrangement substantially as illustrated and described with reference to the accompanying drawings.

Inventors: BLACKIE NEIL, BUTLER THOMAS, IGATA EISHI, NOURSHARGH NOORI, PAYNE PETER ALFRED, SHEARD STEPHEN JOHN, BLACKIE, NEIL, BUTLER, THOMAS, IGATA, EISHI, NOURSHARGH, NOORI, PAYNE, PETER, ALFRED, SHEARD, STEPHEN, JOHN
Applicants: BLACKIE NEIL, BUTLER THOMAS, IGATA EISHI, MARCONI ELECTRONIC SYST LTD, NOURSHARGH NOORI, OSMETECH, PAYNE PETER ALFRED, SHEARD STEPHEN JOHN, BLACKIE, NEIL, BUTLER, THOMAS, IGATA, EISHI, MARCONI ELECTRONIC SYSTEMS LIMITED, NOURSHARGH, NOORI, PAYNE, PETER, ALFRED, SHEARD, STEPHEN, JOHN
Application year: 1999
Main classification: G01N21
IPC codes: G01N21, G02B6