WO2000020848A1

WO2000020848A1 - Integrated optical waveguide chemical sensor with bragg grating

Info

Publication number: WO2000020848A1
Application number: PCT/GB1999/003262
Authority: WO
Inventors: Neil Blackie; Noori Nourshargh; Thomas Butler; Eishi Igata; Stephen John Sheard; Peter Alfred Payne
Original assignee: Marconi Electronic Systems Limited; Osmetech
Priority date: 1998-10-02
Filing date: 1999-10-04
Publication date: 2000-04-13
Also published as: GB9821424D0; GB2342440B; AU6110099A; GB2342440A; EP1117988A1; CA2346137A1

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

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.