GB2495518A

GB2495518A - Optical sensor in which a sensing waveguide is thermally isolated from a substrate

Info

Publication number: GB2495518A
Application number: GB1117611.2A
Authority: GB
Inventors: Arnold Peter Roscoe Harpin; Stephen James Fasham
Original assignee: Oxsensis Ltd
Current assignee: Oxsensis Ltd
Priority date: 2011-10-12
Filing date: 2011-10-12
Publication date: 2013-04-17
Anticipated expiration: 2031-10-12
Also published as: WO2013054077A3; GB201117611D0; WO2013054077A2; GB2495518B

Abstract

Optical sensor 10 comprises a substrate 12; sensing waveguide structure 16 and delivery waveguide 18, each formed integrally with the substrate; and an aperture 30 in the substrate under part of the sensing waveguide structure. The delivery waveguide allows delivery of interrogating probe light 22 into the sensing waveguide structure such that the optical sensor may sense an adjacent gas temperature. The aperture reduces thermal coupling between the substrate and the sensing waveguide structure, e.g. providing faster response to temperature changes. Alternatively a carrier chip (84, Fig.10) supports an optical fibre (25, Fig.10) in a groove (86, Fig.10) in its surface, the surface being in contact with a substrate (12, Fig.10) comprising an integrally formed waveguide such that the optical fibre core is aligned with an end of the waveguide. The sensing waveguide may be a Fabry-Perot or Bragg grating interferometer element.

Description

Optical Sensor The invention relates to optical sensors. In particular, but not exclusively, the invention relates to optical sensors for sensing temperature of an adjacent gas.

Introduction

Various applications would benefit from the more accurate response of a sensor to rapid changes in adjacent gas temperature. One such application is the determination of temperature in the exhaust manifold of an internal combustion engine, where exhaust gas temperatures cycle at rates corresponding to the engine strokes, and temperature detection rates of around 10 kHz may be required. Typical operational and measured temperatures in such environments may range over several hundred degrees, and in some cases may reach more is than a thousand degrees Celsius.

A suitable sensor for such applications should preferably read the gas temperature independently of the temperature of the bulk of the sensor body, which is likely to be mounted to a wall such as that of the exhaust manifold, or to some other engine component, and the temperature of such a component will also be fluctuating, although on timescales rather longer than the fluctuations in gas temperature.

As well as detecting adjacent gas temperature rapidly and independently from temperature of the bulk sensor and engine components to which is mounted, the sensor should also be robust and should be provided with a secure optical connection to an optical interrogator, typically by optical fibre.

Other applications where accurate response of a sensor to rapid changes in adjacent gas temperature would be useful include gas turbine and jet engine equipment, furnace technologies, and so forth.

Summary of the invention

The invention seeks to address problems and limitations of the related prior art. Accordingly, the invention provides an optical sensor for sensing temperature of an adjacent gas by interrogation using probe light, the sensor comprising a substrate; a sensing waveguide structure formed on or integrally with the substrate for interrogation by said probe light; a delivery waveguide formed on or integrally with the substrate, the delivery waveguide being arranged to deliver said probe light into the sensing waveguide; and an aperture in the substrate underneath at least part of the sensing waveguide structure.

The aperture is arranged to reduce thermal coupling between the substrate and the sensing waveguide structure, or equivalently to increase the thermal isolation of the sensing waveguide from the substrate not adjacent to the aperture. This is achieved by reducing thermal conduction between the sensing waveguide and the substrate beyond the aperture, and also by reducing the thermal inertia of the substrate adjacent to the aperture and the sensing waveguide structure. These aspects enable the sensor to respond more quickly to changes in the temperature of the adjacent gas.

is The aperture may, for example, be greater in extent than the sensing waveguide structure, and may for example account for more than 50% or more than 90% of the original thickness of the substrate.

The sensing waveguide structure may be implemented as various types of interferometer element. For example, the sensing waveguide structure may be provided by a Fabry Perot interferometer element having two end surfaces defining a Fabry Perot optical cavity therebetween for interrogation by said probe light, a termination of the delivery waveguide being arranged to deliver said probe light into the Fabry Perot interferometer structure through one of the end surfaces of the sensing waveguide structure.

The use of a Fabry Perot cavity formed by a waveguide on a substrate permits both a fast response time of the sensor to temperature changes because of the small size and exposure of the waveguide to the gas, and a fast measurement rate achievable by optical interrogation of the waveguide cavity.

However, similar effects can also be achieved using a Bragg grating interferometer element to provide the sensing waveguide structure.

The aperture may extend laterally in the plane of the substrate well beyond the footprint of the sensing waveguide structure, and may extend laterally in the plane of the substrate to lie underneath a termination of the delivery waveguide adjacent to the sensing waveguide structure. Typically, the lateral dimensions of the aperture in the plane of substrate maybe from about 100 pm to about 3000 pm. The aperture may be a blind aperture, or may include one or more vias s through the full depth of the substrate.

The sensing waveguide structure may also be provided by a Mach Zehnder interferometer element, or another branched waveguide interferometer element, with the aperture in the substrate provided beneath only one of the branches of the structure.

The sensor may also comprise a reference waveguide structure of the same interferometer element type as the sensing waveguide structure, but with the reference structure not disposed over the or a similar aperture in the substrate with the result that the reference structure is fully thermally coupled to the substrate. Interference signals from both the reference waveguide structure and the sensing waveguide structure can then be combined to provide a more accurate representation of fast changes in the adjacent gas temperature, for example by better accounting for residual thermal coupling between the bulk of the substrate and the sensing waveguide structure.

The aperture can be formed in several different ways and with different configurations and shapes. On the basis that the substrate has opposing top and bottom surfaces, and the sensing waveguide structure and delivery waveguide are formed at the top surface of the substrate (of course the terms top and bottom, above and underneath and similar can be exchanged without affecting the sense of this relationship and the invention and so should be read accordingly), and the aperture may be formed in the bottom surface of the substrate, or in the top surface of the substrate. If formed in the bottom surface, then the substrate between the aperture and the top surface may form a diaphragm carrying the sensing waveguide structure. If formed in the top surface then the sensing waveguide structure may be carried on an arm of the top surface of the substrate extending out over the aperture. Such an arm may be linked to a rim of the aperture by one or more further arms to provide a net for improved strength and support.

One or more of the waveguides may be channel waveguides, for example if the substrate is a silicon-on-insulator substrate then the delivery and sensor waveguides may be silicon on insulator rib waveguides. Other substrates and waveguide structures may be used, for example silicon nitride waveguides, or sapphire trench bulge geometry waveguides, strip loaded waveguides, photonic bandgap waveguides, ridge waveguides, and buried channel silicon on insulator ridge waveguides.

A silicon-on-oxide system may be used with various underlying bulk substrate materials including silicon and glass. Systems based on silicon nitride, sapphire, silicon carbide, silicon dioxide, silicon oxynitride, Ill-V semiconductors such as GaAs and lnP and other materials may be used. Subject to operational use temperatures, arrangements such as plastic embossed waveguides could also be used.

The sensor may further comprise a light source arranged to deliver said probe light to the sensing waveguide structure via the delivery waveguide, and an analyser arranged to receive light modified in the sensing waveguide structure back via the delivery waveguide or forward via a collection waveguide. The analyser is then arranged to detect interference properties of the light modified within the sensing waveguide structure. The analyser may be arranged to determine a temperature of a gas adjacent to the sensor from the detected interference properties. The analyser may provide a control signal or sensor signal or similar to other apparatus based on the interference properties, for example a sensor signal to an engine management unit.

If the sensing waveguide structure is a Fabry Perot interference element as mentioned above then the analyser may be further arranged to detect interference properties of light reflected from a further Fabry Perot cavity defined by the termination of the delivery waveguide and the proximal end face of the Fabry Perot interference element. This cavity may comprise or be a gas filled space, for example being exposed to the gas to be measured. Interference properties of light reflected from the further Fabry Perot cavity may then be used to compensate for thermal expansion properties of the solid Fabry Perot cavity defined by the sensor waveguide. The cavity may instead be filled with a solid.

The invention may provide various apparatus comprising the sensor discussed above, such as an internal combustion engine comprising an exhaust manifold, and the above sensor installed in the exhaust manifold so as to detect the temperature of gases in the exhaust manifold, a gas turbine with such a sensor installed, and so forth.

The invention also provides methods of fabricating an optical sensor for sensing temperature of an adjacent gas by interrogation using probe light, comprising: providing a substrate; forming a sensing waveguide structure and a delivery waveguide integrally with the substrate, the sensing waveguide structure optionally having two end surfaces defining a Fabry Perot optical cavity therebetween for interrogation by said probe light, or using some other interference element, the delivery waveguide being arranged to deliver said probe light into the sensing waveguide structure; and forming an aperture in the substrate underneath the sensing waveguide structure. The aperture may be formed before or after formation of the other elements, and may have properties as discussed elsewhere in this document. The aperture may be formed by a variety of techniques such as laser ablation, wet etching, dry etching, and powder blasting The invention also provides improved ways of aligning an optical fibre for delivery of probe light to and collection of reflected light from waveguides on a substrate such as the sensor substrate discussed above, or other substrates having one or more waveguides formed integrally thereon, such as rib waveguides. In particular, the invention provides a method of aligning the core of an optical fibre with the end of a waveguide formed integrally with a surface of a substrate, comprising: providing a carrier chip having a surface with a groove formed therein; disposing the optical fibre in the groove of the carrier chip; bringing the surface of the carrier chip into contact with the surface of the substrate; and aligning the core of the optical fibre and the end of the waveguide with each other by sliding the carrier chip and the substrate relative to each other.

The step of aligning may comprise monitoring optical coupling between the core of the optical fibre and the waveguide and sliding the carrier chip and the substrate until the optical coupling is optimized, for example using a coupling monitor. If used for a coupling to the sensor substrate discussed above, such a coupling monitor may measure the strength of interference fringes generated by a Fabry Perot cavity or other interference element defined by the sensing waveguide structure.

The method may further comprise using a spring clip to hold the carrier chip and the substrate together during the step of sliding, and may further comprise leaving the spring clip in place after the step of aligning is completed and during subsequent use of the substrate. The waveguide may be coupled to a sensing waveguide structure formed integrally with the substrate as discussed herein, for interrogation by probe light delivered along the waveguide, and the surface of the carrier chip may be provided with a carrier chip aperture arranged to be in confrontation with the sensing waveguide structure and/or the waveguide, and/or other structures on the surface of the substrate, to protect such structure from damage during the step of aligning by sliding and during subsequent use.

The invention also provides an optical device comprising: a substrate having a surface at which a waveguide is formed integrally with the substrate; a carrier chip having a surface with a groove formed therein; and an optical fibre disposed along said groove, wherein the surface of the substrate is in contact with the surface of the carrier chip, and a core of the optical fibre is aligned with the waveguide to provide an optical coupling therebetween.

This optical device may further comprise a spring clip holding the carrier chip and the substrate together, and the waveguide may be a rib waveguide.

The surface of the carrier chip may be provided with a carrier chip aperture in confrontation with one or more structures provided on the surface of the substrate, such as the delivery waveguide and sensing waveguide structures discussed herein, or other structures, so that such structures do not contact the carrier chip during and after the alignment process.

The invention also provides a sensor as set out variously above adapted for operation at temperatures in excess of 500 degrees Celsius.

The invention also provides a sensor as set out variously above adapted or arranged to provide temperature readings at rates of I kHz or above.

Although the invention is generally referred to with respect to detecting the temperature of an adjacent gas, it may also or instead be used to detect the temperature of other types of adjacent fluids including liquids and plasmas. The invention may particularly be arranged to detect adjacent fluid temperature at s high sampling rates, for example at rates of 1 kHz or more.

Brief description of the drawings

Embodiments of the invention will now be described, by way of example only, with reference to the figures, of which: Figure 1 shows a gas temperature sensor embodying the invention; Figure 2 is a sectional view showing a waveguide on the substrate of figure 1; Figure 3 is a plan view showing an example of delivery waveguide and sensing waveguide structure for the arrangement of figure 1; Figures 4A to 4C show another example delivery waveguide and sensing waveguide structure for the arrangement of figure 1; Figures 5A and 5B show how an aperture can be formed in the substrate on a lower surface of the substrate, beneath the sensing waveguide structure, to reduce thermal coupling between the sensing waveguide structure and the bulk of the substrate; Figures 6A, 6B and 7 show haw an aperture can be formed in the upper surface of the substrate, to reduce thermal coupling between the substrate and the sensing waveguide structure; Figures BA and 8B show another example of a delivery waveguide and sensing waveguide structure for use in the arrangement of figure 1; Figures 9A and 9B illustrate the additional use of a reference waveguide structure which is not subject to or arranged for reduced thermal coupling with the substrate; Figure 10 shows a way in which an optical fibre may be aligned with the delivery waveguide of figure 1 or with another waveguide formed at the surface of a substrate; and Figures hA and 11B show how an aperture may be provided in a surface of the carrier chip to protect structures on the surface of the substrate during relative sliding alignment of the substrate and carrier chip.

s Detailed description of embodiments

Figure 1 illustrates a sensor 10 embodying the invention. A sensing waveguide structurel6 is formed integrally at the top surface 14 of a substrate 12 (of course the orientation is arbitrary, and this could equally be at a bottom surface for example, but top will be used for convenience of description). An interrogator unit 20 comprises a light source 22 which is used to generate probe light which is delivered to the sensing waveguide structurel6 by a delivery waveguide 18 also formed integrally on and with the top surface of the substrate.

Light reflected back from the sensing waveguide structure via the delivery waveguide 18, or alternatively propagated onwards from the sensing waveguide structure through a collection waveguide 19 (not shown in this example) is detected and analysed by an analyser 24 of the interrogator 20. The light source 22 and analyser 24 may be coupled to the delivery waveguide, for example, by optical fibres 25, 26, 27 using an optical circulator 28 or coupler. Alternatively one or both of the light source 22 and analyser could be provided on or adjacent to the substrate 14.

The properties of the sensing waveguide structure 16 vary under changes in temperature of the material forming this structure and the material of the substrate, both through thermal expansion effects and the direct response of the refractive index of the sensing waveguide structure material to temperature. As the optical path length changes, the interference properties of the sensing waveguide structure 16 change and these changes are detected by the analyser 24.

The sensor is exposed to an adjacent gas, the temperature of which is to be measured, in such a manner that the optical properties of the sensing waveguide structurel6 are able to respond rapidly to changes in the gas temperature. To reduce the thermal inertia of the sensing waveguide structure 16 so that it can respond more rapidly to changes in adjacent gas temperature, an aperture 30 is provided in the substrate 12 underneath at least a part of the sensing waveguide structurel6. This aperture can be provided in various ways as discussed in more detail below. The aperture has the effect of reducing the thermal coupling between the substrate and the sensing waveguide structure.

S The sensor may, in particular, be adapted to operate at temperatures of 500 degrees Celsius or above, and may be arranged to provide temperature readings at a rate of 1 kHz or more. The sensor may be installed in an engine, such as an internal combustion engine, for example in an exhaust manifold, and may forward temperature readings to an engine management system for operation and control of the engine. A variety of other applications may be envisaged by the skilled person.

Figure 2 illustrates one way in which the delivery waveguide and sensing waveguide structure may be formed integrally on the substrate as channel waveguides. In figure 2 a silicon-on-insulator substrate 40 is about 500 pm thick.

is A thin layer 42 of silicon oxide, about 0.5 pm thick is grown on the top surface of the substrate. A further layer 44 of silicon is grown and processed on top of the silicon oxide to provide a layer about 2.8 pm thick with a superimposed rib structure 45 which is about 1.5 pm high and about 4 pm wide. However, dimensions of suitable waveguides could vary widely, with ridge heights as much as 20 pm or as little as 0.1 pm, or using buried or trench waveguides. The further layer of silicon 44 may be protected by a further layer of thermal silica cladding 48.

The rib structure 45, which extends across the surface of the substrate in a direction perpendicular to the plane of the page, acts with the buried silicon oxide layer 42 as a waveguide to trap propagating modes of light 46. In the sensor of figure 1 these propagating modes include the probe light delivered along the delivery waveguide 18 and into the sensing waveguide structure 16, and the light reflected back out of the sensing waveguide structure and back along the delivery waveguide 18 towards the interrogator 20, or forwards from the sensing waveguide structure to an interrogator via a collection waveguide.

Various substrates other than silicon-on-insulator substrates may be used, and other forms of waveguide known to the skilled person, including various other -10 -types of channel waveguide at the substrate surface, may also be used, depending on suitability with respect to the chosen substrate. A sapphire channel waveguide with a trench bulge geometry may be chosen, for example for use at high temperatures. Other waveguide types which may be used include strip S loaded waveguides, photonic bandgap waveguides, ridge waveguides, and buried channel silicon on insulator ridge waveguides.

A silicon-on-oxide system may be used with various underlying bulk substrate materials including silicon, and glass. Systems based on silicon nitride, sapphire, silicon carbide, silicon dioxide, silicon oxynitride, Ill-V semiconductors such as GaAs and InP and other materials may be used. Subject to operational use, temperatures arrangements such as plastic embossed waveguides could also be used.

One way in which the delivery waveguide and sensing waveguide structure may be laid out in detail on the substrate is shown in plan view in figure is 3. In this example, the sensing waveguide structure is provided by a Fabry Perot interferometer element 51 having end faces 50, 52 defining a Fabry Perot cavity therebetween. The Fabry Perot interferometer element may be interrogated by the interrogator in various ways familiar to the person skilled in the art. Some such schemes may track the movement of part or whole of a single interference fringe arising from probe light of a narrow frequency band of laser light, as the fringe shifts in wavelength due to changes in length of the Fabry Perot cavity.

Other schemes may make use of white light interferometry techniques using broader band probe light such as from a superluminescent diode.

WO2009/077727 discusses such a scheme in which the broad band light is conditioned used a Mach-Zehnder interferometer so as to match the Fabry Perot cavity, with the interrogator tracking the cavity length by adjusting the path length of one or both branches of the Mach-Zehrider interferometer, The end faces of the Fabry Perot interferometer element 51 are formed as 1-bar structures which extend the end faces laterally beyond the width of the main waveguide. The T-bar structures ensure that the ends of the Fabry Perot interferometer element 51 are fiat and not subject to rounding during the -11 -production process, for example due to intrinsic limits of lithographic or etching processes used to produce the waveguide.

Probe light is both delivered to the Fabry Perot interferometer element 51, and reflected light from the interferometer element is collected, through a s termination 54 of the delivery waveguide 18, which is also formed using a 1-bar structure to ensure flatness. The length of the spacing 56 between the termination end face of the delivery waveguide 18 and the adjacent end face 50 of the Fabry Perot interferometer element 51 may typically be about the same as the width of the waveguides, for example a few pm. Larger spacing lengths may be used at the risk of making alignment between the two waveguides less accurate and optical coupling less efficient.

The spacing 56 may be filled with a solid material suitably transparent to the probe light, such as silicon dioxide in the case of a silicon waveguide. This could be desirable for example if the adjacent gas was very opaque or there was is a risk that during operation the end faces of the optical cavities or access waveguides could be obscured by deposition from the gas. Alternatively, the spacing 56 may be left empty, for example being filled by the gas to which the sensor is exposed for measuring temperature.

The spacing 56 may form a further Fabry Perot cavity in addition to that formed by the two end faces 50, 52 of the Fabry Perot interferometer element 51, and if gas filled then the response of this cavity to temperature changes will be due essentially only to thermal expansion of the substrate, and not to refractive index changes of the gas in the cavity which will have a very small effect. The analyser 24 may be used to determine properties of the interference arising due to this spacing cavity. This could be carried out as an alternative to the interference cause by the Fabry Perot interferometer element 51, for example at high temperatures where the material of the waveguides might become more opaque (this happens to silicon above about 500 degrees Celsius), or in addition to the interference cause by the Fabry Perot interferometer element 51, for example in order to belier compensate for the thermal expansion of the substrate material in detecting temperature from the properties of interference of the Fabry Perot interferometer element 51.

-12 -The termination 54 end face of the delivery waveguide may be angled slightly, for example by about 1 degree, so as not to be exactly parallel with the end face 50 of the Fabry Perot interferometer element 51, so as to frustrate the formation of the further Fabry Perot cavity mentioned above which is defined by the spacing 56, and which could otherwise confuse or reduce the effectiveness of a sensor interrogation scheme not adapted to take this into account. The optical cavity of the Fabry Perot interferometer element 51 should then also be suitably angled so that probe light emerging from the surface 54 is aligned correctly with the axis of the cavity.

The length of the Fabry Perot cavity defined by the end faces 50, 52 of the Fabry Perot interferometer element 51 may typically be about 10 pm, although smaller or longer optical cavities may be implemented depending on the application. By way of example, if the sensor waveguide 16 is formed of silicon, which has a rate of change of refractive index with temperature of about 2 x 1 0, a 500 degree Celsius temperature range will give rise to an optical path length change of about 1 pm in a cavity of length 10 pm. For typical telecommunication laser wavelengths in the near infrared, the 500 degree temperature change will translate into an interference shift of about one interference fringe at the analyser which can therefore be easily detected and interpreted. If an interrogator which can track across multiple fringes is used, then a longer optical cavity can be used and/or a wider temperature range can be sensed. Generally, a longer cavity can be used to achieve higher temperature sensitivity at the expense of temperature range, and vice versa.

Another way in which the delivery waveguide and sensing waveguide structure may be provided on the substrate is shown in plan view in figure 4A. In this example, the sensing waveguide structure 16 is provided by a Bragg grating interlerometer element 58, for example structured on a single mode silicon ridge waveguide, although as discussed elsewhere other waveguide types may be used. The Bragg grating may be formed, for example, by varying the physical height of the ridge waveguide as illustrated in cross sectional figures 4B and 4C.

Figure 4B shows a cross section through a notch 59 in the waveguide, at which the height labelled (a) of the ridge is about 1.4 pm, compared with a full height -13 -labelled (b) of the ridge of about 1.5 pm as shown in figure 4C. Similar to the arrangement shown in figure 2, the thickness of the silicon-on-insulator substrate is about 500 pm, with a thin layer of silicon oxide, about 0.5 pm grown on the substrate. The layer 44 of silicon grown and processed on top of the silicon oxide S carrying the waveguide structures may in this case be about 5 pm thick shown by dimension (c).

The delivery waveguide 18 may join seamlessly with the Bragg grating interlerometer element 58, and indeed the Bragg grating structure may be superimposed on a part of the same waveguide structure as forms the delivery waveguide 18. Although figures 4A-4C show the Bragg grating structure formed by stepping the physical height of the waveguide, other techniques could also be used such as stepping the width of the waveguide, or using techniques where the refractive index within the waveguide is varied directly for example by photosensitive doping techniques.

In order to provide a Bragg grating in silicon with a reflection peak at about 1550 nm, convenient for use with telecommunications type laser diodes in the sensor, the pitch of the grating may be about 220 nm. Preferably, the grating should be designed to have a sharp reflection peak.

To interrogate the Bragg grating interlerometer structure 58, the interrogator may be arranged to collect and analyse light reflected back from the grating, or to collect and analyse light passed forward through the grating and through a collection waveguide (not shown). In either case, the adjacent gas temperature directly causes changes in the refractive index of the material of the grating and also causes some refractive index changes through thermal expansion, and these changes affect the frequency of light reflected and transmitted by the Bragg grating. Analysis of the spectrum of the collected light can therefore be used to determine the adjacent gas temperature.

Figures 5A and SB illustrate in plan view ways in which the aperture 30 in the substrate 12 as shown in figure 1 may be provided underneath the sensing waveguide structure 16. In these figures the sensing waveguide structure 16 is shown as a Fabry Perot interferometer element, but the Bragg grating interferometer element discussed above, or another type of interlerometer -14 -element could instead be used. The sensing waveguide structure 16 is formed at a top surface of the substrate. The dashed circle in figure 4 represents the outside boundary of an aperture 30 formed in an under surface of the substrate, so as to thin the substrate under the sensing waveguide structure. This aperture 30 preferably extends laterally beyond the footprint of the sensing waveguide structure, and may extend far enough to lie beneath the termination of the delivery waveguide 18, and even some way along the delivery waveguide if required. lithe sensing waveguide structure is of the order of 10 pm in length, then the lateral extent of the aperture might be around from around 100 to 3000 pm. A diameter at the upper end of this range will provide much more effective thermal isolation of the sensing waveguide structure from the substrate not forming part of the diaphragm, thereby reducing thermal coupling between the substrate and the sensing waveguide structure to a greater extent. Although a circular aperture is illustrated, any suitable shape may be used.

The underside of the substrate may be thinned in various ways to form the aperture 30, for example by wet or dry etching or by powder blasting, drilling, laser ablation or other techniques. The material of the substrate remaining between the aperture 30 and the top surface of the substrate may be quite thin, for example less than 10% or more preferably less than 2% of the bulk substrate thickness, for example between about 5 pm and 10 pm, noting that a typical silicon on insulator substrate cut from a larger wafer might be about 500 pm thick.

Where a buried oxide layer such as buried silicon oxide layer 42 shown in figure 2 is used, this buried oxide layer may conveniently used as an etch stop between the substrate and the layers carrying the waveguiding structures above.

The remaining material between the aperture and the top surface of the substrate effectively forms a diaphragm on which the sensing waveguide structurel6, and optionally a portion of the delivery waveguide, are supported.

The aperture 30 may be a blind aperture, with no through holes being formed in the diaphragm. Alternatively, to further reduce the thermal inertia at the sensing waveguide structure, and improve the response rate of the sensor to rapid changes in adjacent gas temperature, the aperture 30 may extend completely through the substrate in one or more places as illustrated by the vias 60 in figure -15 - 5, subject to retaining enough strength in the diaphragm for the sensor to be robust.

Figure 6a illustrates in perspective view another way in which the aperture in the substrate may be provided as a pit structure underneath the sensing S waveguide structure 16, again using a Fabry Perot iriterferometer element by way of example only. Figure 6b shows the arrangement in section view along the line A-A of figure 6a. The sensing waveguide structure 16 is formed at a top surface 14 of the substrate 12, and the aperture 30 is also formed in the top surface of the substrate 12, but so as to undercut the sensing waveguide structure, leaving it suspended over the aperture, extending out from a rim 63 of the aperture. This undercut aperture may extend far enough for the aperture 30 to also lie beneath the termination of the delivery waveguide 18, and even some way along the delivery waveguide if required. The sensing waveguide structurel6, and optionally some of the delivery waveguide 18 then lie upon an arm 64 overhanging the aperture 30.

If the sensing waveguide structure is of the order of 10 pm in length, then the lateral extent of the aperture might be from about 20 pm to about 1000 pm.

Although an approximately circular aperture is illustrated, any suitable shape may be used.

The aperture 30 in the top face of the substrate may be formed by wet etching, or other suitable techniques known to the skilled person. The material of the substrate remaining between the aperture 30 and the bottom surface of the substrate need not be very thick, and might be for example about a quarter to a half of the bulk substrate thickness, noting that a typical silicon on insulator substrate cut from a larger wafer might be about 500 pm thick. In some embodiments the aperture may extend all the way to the bottom surface of the substrate, rather than being blind. As mentioned above in connection with figures 5A and 5B, if a buried oxide layer such as buried silicon oxide layer 42 shown in figure 2 is used, this buried oxide layer may conveniently be used as an etch stop between the substrate and the layers above in which the waveguiding structures are formed.

-16 -In the arrangement illustrated in figures 6a and 6b, the sensing waveguide structure and some of the delivery waveguide lie upon the remaining arm 64 of the top surface 14 of the substrate 12. This arm may join the rest of the substrate 12 at only one point, as shown in figure 6a, or may form a bridge all the way across the aperture, or a more complex structure bridging over the aperture using three or more linked arms 66 as illustrated in plan view in figure 7. Such bridging arrangements can be used to improve the strength and robustness of the sensor without materially reducing the speed of response to rapid temperature changes.

The arm may typically be about ito 2 mm long, between about 50 and 100 pm wide, and between about 5 and 10 pm thick in order to accurately measure the adjacent gas temperature with a time constant of 10 milliseconds or less.

Referring now to figure 8A, an arrangement where the sensing waveguide structure 16 is implemented as a branched or Mach Zehnder interferometer is element is illustrated. The Mach Zehnder interferometer element is implemented as first 70 and second 71 waveguide branches. A substrate aperture 30 is provided underneath at least a part of the first waveguide branch 70 in order to reduce thermal coupling between that branch and the substrate, but there is no such aperture underneath the second branch 71. As for arrangements already described, the waveguide branches 70, 71 are formed as waveguide structures integrally with the surface of the substrate, and probe light is delivered to the Mach Zehnder interferometer element using delivery waveguide 18.

In the example of figure BA, light is delivered from the Mach Zehnder interferometer element in a forward direction to the analyser 24 via a collection waveguide 19. In contrast, figure 8B shows a reflection based Mach Zehnder interferometer element from which probe light modified by the element is returned to the analyser 24 through the delivery waveguide 18.

In operation, probe light is split and passes into each of the two branches 70, 71. Probe light either continuing on to the analyser 24 as in the example of figure 8A, or being reflected back to the analyser 24 as in the example of figure SB is subject to interference depending on optical path length difference between the two branches. Changes in the adjacent gas temperature affects the -17 -temperature of the first branch 70 in the vicinity of the aperture 30 more rapidly than the temperature of the second branch, due to lower thermal coupling of the first branch with the substrate. The different temperature changes lead to different changes in refractive index and optical path length between the two S branches which can be detected as shifts in phase difference and therefore the detected interference from the Mach Zehnder interferometer element, which can be interrogated in a variety of ways familiar to the person skilled in the art.

The Mach Zehnder interferometer element preferably has one arm longer than the other to provide a unique optical path length difference for interrogation.

Although in using the Mach Zehnder interferometer element arrangement it may be more difficult to derive the adjacent gas temperature because this effects both branches of the element, albeit at different rates because of the different degrees of thermal coupling with the substrate, an advantage over the Fabry Perot arrangement described above is that the change in optical path difference for a is given temperature change in the Mach Zehnder arrangement can be designed independently of the length or size of the interferometer element.

Independently of the type of interferometer element used in the sensing waveguide structure, a reference waveguide structure using the same interferometer type may be used in order to provide the interrogator with information about the temperature of the substrate. This information may be used to calibrate the information derived from the sensing waveguide structure.

Starting from the arrangement of figure 3, figure 9A shows the use of an additional Fabry Perot interlerometer element used as a reference waveguide structure 17. The length of the Fabry Perot cavity of the reference waveguide structure is preferably different to that of the sensing waveguide structure to assist the interrogator in detecting separate signals from the two structures.

Figure 9B shows a similar arrangement in which interrogation of the waveguide structures occurs via a collection waveguide 19 instead of by reflection back through the delivery waveguide 18.

As shown in figure 9A, the reference waveguide structure does not lie over the aperture 30, or over a similar aperture, so that the thermal coupling of the reference waveguide structure to the substrate is not thereby reduced. By -18 -combining the detected responses of the sensing and reference structures, and allowing for their different rates of response to changes in the adjacent gas temperature, a more accurate high frequency determination of the adjacent gas temperature can be made. The reference waveguide structure can also be used s to correct for any residual thermal conduction between the substrate and the sensing waveguide structure. This may be especially apparent at lower gas flow rates when the thermal coupling between the adjacent gas and the sensing waveguide structure may be reduced, and the interrogator may be arranged to take account of the gas flow rate in deriving the adjacent gas temperature.

The reference waveguide structure may be coupled to the interrogator by the same delivery waveguide 18 as the sensing waveguide structure, or by a separate delivery waveguide, and a separate interrogator may be used if desired Referring back to figure 1, the optical fibre 25 coupling the interrogator to the delivery waveguide 18 needs to be attached to the substrate 12 on which the waveguides are formed in such a way that the probe light is optically coupled reliably into the delivery waveguide 18 over the range of operating temperatures of the sensor. Techniques used in the prior art for connections of a similar type typically involve using adhesives such as UV cured epoxy to attach the fibre or a fibre block once it has been micro-positioned for optimum optical coupling.

However, such techniques may not be appropriate for a temperature sensor intended for use at temperatures of several hundred degrees Celsius or more, and subject to thermal cycling between these high temperatures and room temperatures.

Figure 10 illustrates an improved way of providing a coupling between the optical fibre 25 and the delivery waveguide 18. A carrier chip or block 84 is provided with a groove 86 in a top surface 87 of the chip, into which the optical fibre 25 is accepted and aligned. The groove 86 may be v-shaped or some other shape convenient to form accurately for precise positioning of the accepted optical fibre 25. The top surface 14 of the sensor substrate 12 is then brought into confrontation and contact with the top surface 87 of the chip 84 with the end of the optical fibre 25 adjacent to a probe light receiving end 88 of the delivery waveguide 18 to form the desired optical coupling. The groove 86 is proportioned -19 -so that the axis of the core 39 of the optical fibre 25 is in the same plane parallel to top surface 87 of the chip 84 as the propagation mode position 46 (see figure 2) in the delivery waveguide when the top surface 14 of the sensor substrate 12 is brought into contact with the top surface 87 of the carrier chip 84. This plane may typically lie within a few pm above or below the interface between the two surfaces.

To align the optical fibre 25 laterally with the receiving end 88 of the delivery waveguide, the carrier chip 84 and the sensor substrate 12 are slid relative to each other, for example using a micro-actuator, until the correct positioning for optimal optical alignment is achieved. The optimal optical alignment may be found when the core 89 and the receiving end 88 of the delivery waveguide are touching or within a few pm of each other, or a greater distance may be appropriate, for example to avoid risk of the two touching and damaging the end of the waveguide. The micro-actuator is then removed. This lateral adjustment process can be achieved by using a spring clip to hold the chip 34 and substrate together. When the micro-actuator is removed, the spring clip and friction hold the two components together.

This technique is advantageous in avoiding movement of the optical fibre relative to the delivery waveguide 18 when the micro-actuator is removed, and avoiding creep due to setting or curing of adhesives. Furthermore, if the chip 84 is made out of the same material as the optical fibre 25, for example both being made of silica glass, or if the thermal expansion coefficients of the two are otherwise closely matched, then the alignment of the fibre core 88 and the delivery waveguide 88 will be maintained irrespective of thermal expansion.

It may also be advantageous for rough or long term use to apply a glue or potting compound at the edges of the sensor chip to glue it to the carrier, but this should preferably be done while the micro-actuator is still in place so that no relative movement takes place while the glue is being applied.

The optical fibre 25 maybe provided with an end face for optical coupling with the waveguide which is angled slightly to frustrate the formation of a Fabry Perot cavity between the end face and the corresponding end face of the delivery waveguide. The end face of the sensor chip should be suitably angled also to -20 -allow the optical axis of the chip to be perpendicular to the lateral movement required to achieve alignment.

Figure 1 1A shows how a carrier chip aperture 90 may be provided in the surface of the carrier chip 84 so that a sensing waveguide structure on the substrate does not contact the surface of the carrier chip, with the result that the sensing waveguide structure is protected from damage during sliding alignment of the two components to optimise the optical coupling. Figure 1 Ia shows the carrier chip 84 in plan view, with the groove 86 on the left side of the figure.

Approximate location of the substrate 12 during and after the sliding alignment process is shown by a broken outline. The corresponding position of a substrate aperture 30 described, for example, in connection with figures SA-9B, is also shown as a broken line, and the delivery waveguide 18 also discussed above will typically be aligned along the shown axis A-A.

The sensing waveguide structure provided at the surface of the substrate 12 as variously discussed above will typically be positioned within the footprint of the shown substrate aperture 30. The carrier chip 84 is provided with a carrier chip aperture 90 in the surface brought into confrontation with the substrate 12.

This carrier chip aperture 90 is preferably arranged to be in confrontation with the sensing waveguide structure, such that the sensing waveguide structure does not contact the surface of the carrier chip 84 during or after the alignment process, including during the expected range of sliding which may be required for alignment, thereby protecting the sensing waveguide structure from possible damage. The carrier chip aperture 90 may also be arranged such that the delivery waveguide does not contact the surface of the carrier chip 84 during and after the alignment process for similar reasons, and as shown in figure 1 lAthe carrier chip aperture may therefore be connected to the groove 86. The carrier chip aperture 90 shown in figure 1 1A is rectangular, but other shapes may be used. Figure 11B shows cross sections through the carrier chip 84 along sections A-A and B-B. The carrier chip aperture 90 may be formed, for example, by etching, and may typically have a depth of a few pm or more, with the depth providing sufficient clearance from the delivery waveguide andlor sensing -21 -waveguide structures to ensure adequate protection from damage by contact and abrasion with the carrier chip surface.

The sensor of the invention may be used in a variety of applications, especially where rapid changes in gas temperature are to be measured. One such application is in measuring gas temperature in an internal combustion engine, for example in the exhaust manifold of a combustion engine. Other applications include gas turbine arrangements, furnace technologies and similar.

Although various embodiments have been presented above it should be understood that they have been presented by way of example only, and not limitation. It will therefore be understood by those skilled in the relevant art(s) that various changes in form and details may be made to the described embodiments without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

<claim-text>-22 -CLAIMS: 1. An optical sensor for sensing temperature of an adjacent gas by interrogation using probe light, comprising: a substrate; a sensing waveguide structure formed integrally with the substrate for interrogation by said probe light; a delivery waveguide formed integrally with the substrate, the delivery waveguide being arranged to deliver said probe light into the sensing waveguide structure; and an aperture in the substrate underneath at least part of the sensing waveguide structure, the aperture being arranged to reduce thermal coupling between the substrate and the sensing waveguide structure.</claim-text> <claim-text>2. The sensor of claim 1 wherein the sensing waveguide structure is a Fabry Perot interferometer element having two end surfaces defining a Fabry Perot optical cavity therebetween for interrogation by said probe light, a termination of the delivery waveguide being arranged to deliver said probe light into the Fabry Perot interferometer element through one of the end surfaces of the sensing waveguide structure.</claim-text> <claim-text>3. The sensor of claim 1 wherein the sensing waveguide structure is a Bragg grating interferometer element.</claim-text> <claim-text>4. The sensor of any preceding claim wherein the aperture extends laterally in the plane of the substrate to surround the footprint of the sensing waveguide structure.</claim-text> <claim-text>5. The sensor of any preceding claim wherein the aperture extends laterally in the plane of the substrate to lie underneath a termination of the delivery waveguide adjacent to the sensing waveguide structure.</claim-text> <claim-text>-23 - 6. The sensor of claim 1 wherein the sensing waveguide structure is a Mach Zehnder interferometer element comprising first and second waveguide branches, and the aperture in the substrate is beneath a part of said first waveguide branch such that the two branches are subject to different amounts of S thermal coupling with the substrate.</claim-text> <claim-text>7. The sensor of any preceding claim further comprising a reference waveguide structure of the same interferometer element type as the sensing waveguide structure, but without the or a corresponding aperture disposed underneath the reference waveguide structure such that thermal coupling of the reference waveguide structure with the substrate is not reduced.</claim-text> <claim-text>8. The sensor of claim 7 wherein the delivery waveguide is also arranged to deliver said probe light into said reference waveguide structure.</claim-text> <claim-text>9. The sensor of any preceding claim wherein the substrate has opposing top and bottom surfaces, the sensing waveguide structure and delivery waveguide are formed at the top surface of the substrate, and the aperture is formed in the bottom surface of the substrate.</claim-text> <claim-text>10. The sensor of claim 9 wherein the substrate between the aperture and the top surface forms a diaphragm carrying said at least part of the sensing waveguide structure.</claim-text> <claim-text>11. The sensor of claim 10 wherein the diaphragm is breached by one or more vias between the top surface of the substrate and the aperture.</claim-text> <claim-text>12. The sensor of claim 9 or 10 wherein the aperture is a blind aperture.</claim-text> <claim-text>13. The sensor of any of claims ito 8 wherein the substrate has opposing top and bottom surfaces, the sensing waveguide structure and delivery waveguide -24 -are formed at the top surface of the substrate, and the aperture is formed in the top surface of the substrate.</claim-text> <claim-text>14. The sensor of claim 13 wherein the said at least part of the sensing s waveguide structure is carried on an arm of the top surface of the substrate which extends over the aperture.</claim-text> <claim-text>15. The sensor of either claim 13 or 14 wherein the arm is linked to a rim of the aperture by one or more further arms of the top surface of the substrate, providing structural support to the arm.</claim-text> <claim-text>16. The sensor of any preceding claim wherein the delivery waveguide and sensing waveguide structure are channel waveguides.</claim-text> <claim-text>17. The sensor of claim 16 wherein the substrate is a silicon-on-insulator substrate and the delivery waveguide and sensing waveguide structure are formed from silicon on insulator rib waveguides.</claim-text> <claim-text>18. The sensor of any preceding claim wherein the waveguides are at least one of: sapphire waveguides; sapphire trench bulge waveguides; and silicon nitride waveguides.</claim-text> <claim-text>19. The sensor of any preceding claim further comprising a light source arranged to deliver said probe light to the sensing waveguide structure via the delivery waveguide, and an analyser arranged to receive said probe light after it has been modified in the sensing waveguide structure, the analyser being arranged to detect interference properties of the modified probe light.</claim-text> <claim-text>20. The sensor of claim 19 wherein the analyser is further arranged to determine a temperature of a gas adjacent to the sensor from the detected interference properties.</claim-text> <claim-text>-25 - 21. The sensor of claim 19 when dependent on claim 7or8wherein the analyser is arranged to also receive probe light after it has been modified in the reference waveguide structure, the analyser being arranged to detect reference interference properties of the probe light modified in the reference waveguide s structure, and to use the reference interference properties in the determination of temperature of the gas adjacent to the sensor.</claim-text> <claim-text>22. The sensor of claim 19 or 20 when dependent directly or indirectly on claim 2, wherein the analyser is further arranged to detect interference properties of probe light modified by a further Fabry Perot cavity defined by the termination of the delivery waveguide and the proximal end face of the sensing waveguide structure.</claim-text> <claim-text>23. The sensor of claim 22 wherein the optical path of the Fabry Perot cavity defined by the termination of the delivery waveguide and the proximal end face of the sensing waveguide structure comprises a gas filled space.</claim-text> <claim-text>24. The sensor of claim 22 or 23 wherein the analyser is arranged to use the interference properties of light modified by the further Fabry Perot cavity to compensate for thermal expansion properties of the Fabry Perot cavity defined by the sensing waveguide structure.</claim-text> <claim-text>25. An internal combustion engine comprising an exhaust manifold, and the sensor of any preceding claim installed in the exhaust manifold so as to detect the temperature of gases in the exhaust manifold.</claim-text> <claim-text>26. A method of fabricating an optical sensor for sensing temperature of an adjacent gas by interrogation using probe light, comprising: providing a substrate; forming a sensing waveguide structure and a delivery waveguide integrally with the substrate, the delivery waveguide being arranged to deliver said probe light into the sensing waveguide structure; -26 -forming an aperture in the substrate underneath the sensing waveguide structure.</claim-text> <claim-text>27. The method of claim 26 wherein the sensing waveguide structure is a s Fabry Perot interference element having two end surfaces defining a Fabry Perot optical cavity therebetween for interrogation by said probe light, and the method further comprises forming a termination of the delivery waveguide to deliver said probe light into the Fabry Perot interferometer element through one of the end surfaces thereof.</claim-text> <claim-text>28. The method of claim 26 wherein the sensing waveguide structure is a Bragg grating interferometer element.</claim-text> <claim-text>29. The method of claim 26 wherein the sensing waveguide structure is a Mach Zehnder interferometer element comprising first and second waveguide branches, and the method comprises forming the aperture underneath only one of the waveguide branches.</claim-text> <claim-text>30. The method of any of claims 26 to 29 wherein the substrate has opposing top and bottom surfaces, the sensing waveguide structure and delivery waveguide are formed at the top surface of the substrate, and the aperture is formed in the bottom surface of the substrate.</claim-text> <claim-text>31. The method of any of claims 26 to 29 wherein the substrate has opposing top and bottom surfaces, the sensing waveguide structure and delivery waveguide are formed at the top surface of the substrate, and the aperture is formed in the top surface of the substrate.</claim-text> <claim-text>32. The method of any of claims 26 to 31 comprising forming the aperture by one or more of: laser ablation, wet etching, dry etching, and powder blasting.</claim-text> <claim-text>-27 - 33 A method of aligning the core of an optical fibre with the end of a waveguide formed integrally with a surface of a substrate, comprising: providing a carrier chip having a surface with a groove formed therein; disposing the optical fibre in the groove of the carrier chip: bringing the surface of the carrier chip into contact with the surface of the substrate: and aligning the core of the optical fibre and the end of the waveguide with each other by sliding the carrier chip and the substrate relative to each other.</claim-text> <claim-text>34. The method of claim 33 wherein the step of aligning comprises monitoring optical coupling between the core of the optical fibre and the waveguide and sliding the carrier chip and the substrate until the optical coupling is optimized.</claim-text> <claim-text>35. The method of claim 33 or 34 further comprising using a spring clip to hold the carrier chip and the substrate together during the step of sliding.</claim-text> <claim-text>36. The method of claim 35 comprising leaving the spring clip in place after the step of aligning is completed and during subsequent use of the substrate.</claim-text> <claim-text>37. The method of any of claims 33 to 36 wherein the waveguide is coupled to a sensing waveguide structure formed integrally with the substrate for interrogation by probe light delivered along the waveguide.</claim-text> <claim-text>38. The method of claim 37 further comprising providing the surface of the carrier chip with a carrier chip aperture arranged to be in confrontation with the sensing waveguide structure during the step of aligning by sliding.</claim-text> <claim-text>39. The method of claim 38 wherein the carrier chip aperture is further arranged to be in confrontation with the waveguide during the step of aligning by sliding.</claim-text> <claim-text>40. An optical device comprising: -28 -a substrate having a surface at which a waveguide is formed integrally with the substrate; a carrier chip having a surface with a groove formed therein; and an optical fibre disposed along said groove, S wherein the surface of the substrate is in contact with the surface of the carrier chip, and a core of the optical fibre is aligned with the waveguide to provide an optical coupling therebetween.</claim-text> <claim-text>41. The optical device of claim 37 further comprising a spring clip holding the carrier chip and the substrate together.</claim-text> <claim-text>42. The optical device of claim 37 or 38 wherein the waveguide is a rib waveguide.</claim-text> <claim-text>43. The optical device of any of claims 40 to 42 wherein the substrate further comprises a sensing waveguide structure formed integrally with the substrate for interrogation by probe light delivered along the waveguide.</claim-text> <claim-text>44. The optical device of claim 43 wherein the surface of the carrier chip is provided with a carrier chip aperture arranged to be in confrontation with the sensing waveguide structure when the core of the optical fibre is in approximate alignment with the waveguide, such that the surface of the carrier chip does not contact the sensing waveguide structure.</claim-text> <claim-text>45. A method of fabricating the sensor of any of claims 1 to 24 comprising coupling an optical fibre for the delivery of probe light to the delivery waveguide using the method of any of claims 33 to 39.</claim-text> <claim-text>46. The optical sensor of any of claims 1 to 24 wherein the substrate is coupled to an optical fibre arranged to deliver probe light into the delivery waveguide using the arrangement set out in any of claims 40 to 44.-29 - 47. Apparatus substaritiafly as described herein with reference to the accompanying drawings.S</claim-text>