GB2222881A

GB2222881A - Guided optical wave chemical sensor

Info

Publication number: GB2222881A
Application number: GB8919797A
Authority: GB
Inventors: George Stewart; Douglas Fraser Clark; Ivan Andonovic; Brian Culshaw
Original assignee: Sira Ltd
Current assignee: Sira Ltd
Priority date: 1988-09-03
Filing date: 1989-09-01
Publication date: 1990-03-21
Also published as: GB8919797D0; GB8821862D0

Abstract

A guided wave chemical sensor system (10) comprises a sensor device (13) having a substrate (16) supporting an optical waveguide (17) which produces an evanescent field for penetrating an adjacent medium. The waveguide (17) forms part of or is coupled to an optical resonator (20) whereby the field strength of the evanescent field is increased when the light source (11) of the system (10) is tuned to the resonant frequency of the resonator (20). The resonator may comprise a Fabrey-Perot type with mirrors at each end of the waveguide (17), a Bragg type with a grating coated onto the waveguide (17), or a ring or loop resonator separate but coupled to the waveguide (17). <IMAGE>

Description

GUIDED OPTICAL WAVE CHEMICAL SENSOR SYSTEMS This invention relates to guided optical wave chemical sensor systems.

Guided wave chemical sensor systems are known and comprise a waveguide sensor device coupled via one or more waveguides to a light source and a light detector.

The sensor device comprises a substrate supporting an optical waveguide and, is constructed so that, when it is in use, the optical field has an evanescent portion which penetrates into the medium adjacent the sensor device as a result of which there is interaction with the chemical characteristics of the medium and this interaction manifests itself at the detector. Accordingly changes in the chemical characteristics produce changes in the detector reading.

The known sensor devices have proved very difficult to use particularly when the medium is gaseous and the system has been intended to identify the occurrence of a gaseous species. It is believed that this is due to relatively low concentration of the species at the sensor device, for example due to gaseous dispersion, or to inadequate sensitivity of the sensor device, or due to contamination of the sensor device.

It is an object of the present invention to provide a new and improved sensor device for use in a guided wave chemical sensor system.

According to the present invention there is provided a sensor device for use in a guided wave chemical sensor system, said sensor device comprising a substrate supporting an optical waveguide adapted to produce an evanescent field for penetrating an adjacent medium, and in which the-waveguide forms part of or is coupled to an optical resonator whereby the field strength of the evanescent field is increased when the light source of the system is tuned to the resonant frequency of the resonator.

By virtue of the present invention the physical dimensions of the sensor device can be retained essentially as compact as hitherto but the sensitivity of the device is substantially increased especially when the resonator is tuned with respect to the chemical characteristics of the medium to be sensed which interact with a corresponding spectroscopic match.

For aqueous and gaseous media where the refractive index is typically in the range 1.00 to 1.33 highest sensitivity is attained in practice with a sensor device having a substrate with a refractive index as close as possible to that of the medium (and, of course, the refractive index of the waveguide should be larger).

Thickness of the waveguide is also a factor that affects sensitivity. The waveguide may be embedded in the substrate or be of the ridge type, i.e., surface mounted on the substrate, the latter having greater sensitivity than the former. Also it is preferred that the waveguide be of the step index type rather than of the graded index type since the former provides more sensitivity than the latter.

The resonator may be formed as a Fabry-Perot cavity incorporating the waveguide or as a Bragg resonator incorproating the waveguide, both of which produce standing waves along the length of the waveguide.

Alternatively the resonator may be formed as a ring or loop secondary waveguide coupled at one point around its circumference to the principal waveguide which gives rise to a travelling wave in the secondary waveguide which like the principal waveguide is adapted to produce an evanescent field for penetrating the adjacent medium.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 schematically illustrates a guided wave sensor system; Figs. 2, 3 and 4 illustrate different forms of sensor device for use in the Fig. 1 system; Fig. 5 illustrates typical response characteristics of the Fig. 4 device; Fig. 6 illustrates the evanescent fields of two different optical waves propagating along a waveguide of a sensor device in accordance with the present invention; and Fig. 7 illustrates a modified form of the Fig. 6 waveguide.

The guided wave sensor system 10 shown in Fig. 1 comprises a light source 11 coupled via a waveguide 12 to a sensor device 13 the output of which is coupled via a waveguide 14 to a detector 15. The source 11 provides light output for the sensor device 13 at a wavelength which is appropriate to the medium to be sensed and typically is provided by a laser, preferably tunable, or a light emitting diode (LED) operating at an accurately stabilised wavelength. An alternative form of light source is an incandescent bulb with associated spectrometer which provides a less restricted range of wavelengths but imposes difficulties in launching the light into the guide 12. Guides 12, 14, are typically optical fibres and the detector 15 may be a colour sensitive diode or, a dichroic or absorption filter with matched detection elements which are wavelength sensitive.

Sensor device 13 in accordance with the present invention may take any one of the forms particularly illustrated in Figs. 2, 3 and 4 in each of which a substrate 16 supports a waveguide 17 forming part of or being coupled to a resonator 20.

In Fig. 2 the resonator 20 is of the Fabry-Perot type provided by dielectric mirrors 22A, 22B, at each end of the waveguide 17 so that according to the length of the waveguide 17 a standing resonance wave is established in the waveguide 17 when the source 11 is tuned to the resonant frequency of the resonator 20.

In Fig. 3 the resonator 20 is of the Bragg type provided by a distributed reflective transverse grating 23 coated onto the waveguide 17 along its length so that, like the Fig. 2 arrangement, a standing resonance wave is established in the waveguide 17.

In Fig. 4 the resonator 20 is of the ring or loop type separate from the waveguide 17 but coupled thereto at one point around its circumference and which gives rise to a travelling resonance wave in the loop guide which has the form illustrated at A in Fig. 5, the spacing Af between peaks being a function of the loop length (2) and the effective refractive index of the waveguide. The presence of the medium in contact with the sensor device 13 attenuates the amplitude of the resonance wave A to wave B shown also in Fig. 5, so that the Q-factor (or finesse) of the resonator 20, represented by the ratio Af/g f, is a measure of the nature of the medium.

In each resonator form the resonant wave may be tailored so that the resonant spacing bf matches the absorption characteristics of the chemical species being sensed for the purpose of enhancing sensitivity. For example if the chemical species is methane (CH4) which is known to have an absorption spectrum in the 1.66 micron band consisting of a series of nearly-equally spaced lines about 60 GHz apart the resonator can be designed so that fl f = 60 GHz by selecting the loop length for a given waveguide refractive index.

In practice the sensor device 13 would be used in combination with a similar device operating at a check wavelength to enable a ratiometric measurement to be taken between the detector 15 output for the tuned device 13 and the detector output for the untuned sensor device.

During use of the sensor device 13, a contaminant layer may form on the surface. The contaminant layer may be, for example, a thin (~ 100 nm) layer of water, oil or some other substance. The presence of surface contaminants modifies the output of the device 13 so that it is a function of contaminant layer thickness and the concentration of the chemical species. However there are various ways of correcting the measurements when surface contaminants are present now explained with reference to Figs. 6 and 7.

In Fig. 6, 24 and 25 are the evanescent fields of two different optical waves supported either simultaneously or otherwise by waveguide 17. The two optical waves are chosen so that the penetration depth of evanescent field 25 into the chemical species is greater than that of field 24. As is well known the penetration depth of the evanescent field is related to the propagation constant of the optical wave in the waveguide 17 and is independent of the nature of any external contaminant.

Examples of pairs of optical waves which have different penetration depths and which may be used are: (i) The zeroth-order and first-order modes of the waveguide 17; (ii) Two waves of different wavelengths supported by waveguide 17 and which correspond to absorption lines of the chemical species, e.g.

wavelengths of 1.33 and 1.66 microns for methane; and (iii) Two waves of different polarisation supported by waveguide 17, specifically the transverse electric (TE) and the transverse magnetic (TM) modes.

In order to compensate for surface contamination a ratiometric measurement is taken at the detector output for the two optical waves. For sensor device 13, it is known that, with no contamination present, the detector reading, D, (after suitable electronic processing) is proportional to the concentration c of the chemical species, ie., D = K.c. (1) where K is a constant determined by calibration of the sensor device for the optical wave.

When contamination is present, the detector output decreases exponentially with the (unknown) contaminant layer thickness t i.e.,

where d is the penetration depth of the evanescent field of the optical wave, the value of which is also obtained during calibration of the sensor device for the optical wave.

With two different optical waves, two detector readings are obtained such that:

From equations (3) and (4) we obtain the thickness of the contaminant layer as:

Hence t may be calculated from the measured ratio D of D1/D2 and the other (known) constants of the sensor device. This value of t can then be used in either equation (3) or (4) to obtain the correct measure of concentration (c) of the chemical species.

In order to further improve the sensitivity of the sensor device 13 and to improve the accuracy of the compensation for surface contamination, a composite waveguide (also known as a 4-layer waveguide) may be used as illustrated in Fig. 7. In Fig. 7, 26 is a thin (100-500 nm) film of high refractive index (n > 2) material. The ends 27, .28, of the film are tapered in thickness. Typical materials for 26 are titanium dioxide (TiO2), silicon nitride (Si3N4), niobium pentoxide (Nb205), tantalum pentoxide (Ta205) or arsenic trisulphide (As2S3). Depending on the thickness of the film 26, optical waves launched at 12 into waveguide 17 may be transferred in whole or in part into the overlay film 26 at taper 27. After travelling along the overlay film 26, the optical wave is coupled back to guide 17 at taper 28. The sensitivity of sensor device 13 may thereby be substantially improved.

In order to accurately compensate for surface contamination, unpolarised light may be launched into guide 17 and the output passed through a polariser device 29. The output at 30 is then measured for two rotary positions of the polariser device 29 (corresponding to polarisation directions parallel and perpendicular to the waveguide surface) and the ratio of the measurements may then be used to compensate for surface contamination as previously described.

Claims

1. A sensor device for use in a guided wave chemical sensor system, said sensor device comprising a substrate supporting an optical waveguide adapted to produce an evanescent field for penetrating an adjacent medium, and in which the waveguide forms part of or is coupled to an optical resonator whereby the field strength of the evanescent field is increased when the light source of the system is tuned to the resonant frequency of the resonator.

2. A device as claimed in claim 1, wherein the waveguide is embedded in the substrate.

3. A device as claimed in claim 1, wherein the waveguide is surface mounted on the substrate.

4. A device as claimed in any preceding claim, wherein the waveguide is of the step index type.

5. A device as claimed in any preceding claim, wherein the resonator is a Fabry-Perot cavity incorporating the waveguide.

6. A device as claimed in any one of claims 1-4, wherein the resonator is a Bragg resonator incorporating the waveguide.

7. A device as claimed in any one of claims 1-4, wherein the resonator is a ring or loop secondary waveguide coupled at its circumference to said waveguide.

8. A device as claimed in any preceding claim, wherein the waveguide carries a superstrate thin film of high refractive index material.

9. A guided wave chemical sensor system comprising a sensor device as claimed in any preceding claim and optically coupled to a light source and a light detector, the light source providing an output which is wavelength tuned to the resonant frequency of the resonator to establish a resonant wave in the waveguide.

10. A system as claimed in claim 9, wherein the light source is adapted to launch two different optical waves into the sensor device and the detector is adapted to measure the sensor device outputs arising from said different waves and to evaluate the concentration of the chemical species according to a predetermined algorithm.

11. A method of determining the concentration of a gaseous chemical species using a guided wave evanescent field optical sensor device, comprising establishing a resonant wave in an evanescent field supporting waveguide and tuning the resonant wave to the absorption characteristics of said chemical species.