EP1540315A1

EP1540315A1 - Optical waveguide interferometer comprising a laminate structure with a first planar waveguide monolayer and a second sandwich layer

Info

Publication number: EP1540315A1
Application number: EP03790990A
Authority: EP
Inventors: Neville John Freeman; Graham Gross; Gerard Anthony Ronan
Original assignee: Farfield Sensors Ltd
Current assignee: Farfield Group Ltd
Priority date: 2002-08-29
Filing date: 2003-02-21
Publication date: 2005-06-15
Also published as: WO2004020987A1; GB0220058D0; AU2003215719A1

Abstract

The present invention relates to an optical waveguide interferometer comprising a laminate structure with a monolayer constituting a first planar waveguide (2) and a sandwich layer constituting a second planar waveguide (4), wherein the laminate structure is integrated with a lowermost substrate (5) and comprises the second planar waveguide (4) located above and spaced apart from the first planar waveguide (2) by a spacer monolayer (3). The sandwich layer (4) comprises a first layer (4a) exhibiting a first refractive index spaced apart from a second layer (4c) exhibiting a second refractive index by a spacer (4b), wherein the refractive index of the spacer (4b) is less than that of the first refractive index and of the second refractive index.

Description

OPTICAL WAVEGUIDE INTERFEROMETER COMPRISING A LAMINATE STRUCTURE WITH A FIRST PLANAR WAVEGUIDE MONOLAYER AND A SECOND SANDWICH LAYER

The present invention relates to an optical waveguide interferometer including a monolayer constituting a first planar waveguide and a sandwich layer constituting a second planar waveguide.

Optical waveguide interferometers and other integrated optical devices often exhibit an undesirable response to changes in temperature that can complicate their operation and packaging. Several attempts have been made to eliminate the complications arising from these thermal effects.

Athermal waveguides have been proposed in Kokobun et al, IEEE Phot. Tech. Lett., 5, 1297- 1300 (1993) and Kokubun et al, Electronics Letters, 30, 1223-1224 (1994) and generally use a combination of materials with differing thermo-optic coefficients so that the if light is distributed through these materials in the correct proportion, the net thermal effects cancel out. For example, a combination of glasses (having positive thermo-optic coefficients) with a polymer (having a negative thermo-optic coefficient) can produce the desired cancellation.

Such a combination of materials may be used only in special cases where the restrictions imposed in manufacture are few. More commonly, the choice of materials is constrained by the manufacturing processes required for reliable and reproducible mass production of devices. Since these materials are usually members of a family (eg glasses), they often have thermo-optic coefficients that are of the same sign even if they are of different magnitude. As an example, optical waveguides may be fabricated from silicon and silicon oxide (see Weiss et al, IEEE Phot. Tech. Lett., 3, 19-21 (1991)). These silicon-on-insulator structures rely on optical confinement in layers of high refractive index silicon sandwiched between low refractive index silicon oxide (Soref et al, IEEE Phot. Tech. Lett., 3, 22-24 (1991)). The thermo-optic coefficients of these two materials differ markedly making it quite likely that changes in temperature will affect significantly the propagation of light. Nevertheless, the athermal operation of an optical waveguide filter based on quaternary compound semiconductors has been reported in Tanobe et al, IEEE Phot. Tech. Lett., 8, 1489-1491, 1996.

In one type of optical waveguide interferometer, the input light is split into two separate paths, each transmitting a single propagating mode. The light in the two propagating modes may be recombined into a single output so that if the relative phase of the light between the two propagating modes changes (eg due to a change in the wavelength of input light or in the localised environment,), a response may be measured in the output. If the two waveguide paths are not designed precisely alike, a change in temperature can also contribute to the phase change. In some cases, the response of the interferometer might be used to measure the temperature change but more commonly it is an unwanted side effect. O-A-98/22807 and WO-A-02/08736 disclose an optical waveguide interferometer having a laminate structure. Strict temperature control is usually needed to keep the device operating within its design specification and for this purpose a sophisticated temperature controller has been proposed in WO-A-01/22068.

WO-A-02/093115 discloses an optical waveguide interferometer whose two waveguides are structurally symmetric monolayers made as dimensionally and compositionally asymmetric as possible so as to produce a large sensitivity to changes in the wavelength of input radiation. When dimensional or compositional asymmetry is large, the wavelength dispersion difference of the propagating optical waveguide modes is also large. Such a device may be used to track and correct wavelength shifts in the output of telecommunication and other laser sources. However the dimensional and compositional asymmetry of the waveguides leaves the interferometer prone to an exceptionally large thermo-optic effect that may completely mask the measurement of the change in the wavelength of input radiation.

Looked at in more detail, WO-A-02/0931 15 discloses an optical waveguide interferometer having two slab waveguides which are fabricated in a laminate fashion on a substrate and which can act as separate optical paths. The two waveguides are illuminated equally so that (after propagating through the interferometer) the light is allowed to diffract out onto a viewing screen or measuring device to obtain an interference pattern. Movements in the spatial intensity distribution of the interference pattern signify relative phase changes between the light travelling in the two optical paths and can be related back to relative wavelength shifts. However the device is also sensitive to thermal shifts and it is not straightforward to distinguish thermal shifts from true wavelength shifts. Indeed it is a significant technical challenge to achieve sufficient difference in the wavelength dispersion characteristics of the two waveguide modes whilst simultaneously minimizing thermal asymmetry.

A schematic representation of this conventional optical waveguide interferometer is given in Figure 1. The interferometer comprises five layers 1-5 of optically transparent material deposited in a laminar fashion onto a substrate S. Each of layers 2, 4 is of higher refractive index than that of layers 1, 3 and 5 and thus each constitutes an optical slab waveguide. Light from a source 9 may be transmitted to the input end 8 of the device so as to equally illuminate layers 2 and 4. The two waveguide modes are thus excited equally and propagate through the length of the device accumulating different phase retardation as they travel. At the output end 6, the light diffracts from the end face onto a screen or measuring device 7 (such as a photodiode array) and the intensity distribution is representative of the relative phase retardation which accumulates. If the wavelength of the input light changes, a different relative phase retardation accumulates and the intensity distribution shifts.

The drawbacks of the coexistence of thermal asymmetry and wavelength dispersion asymmetry may be illustrated with specific reference to an interferometer of Figure 1 in which layers 2 and 4 are composed of silicon and silicon dioxide. The refractive index of each of these two materials has its own dispersion with wavelength but this may be neglected in a calculation which models the effects using only small wavelength perturbations. The greatest wavelength dispersion difference for the device shown in Figure 1 is found by making the thickness of each of the two layers 2 and 4 markedly different. The thickness is constrained to lie between (at the lower limit) the cut-off thickness for the lowest order (zero order) mode and (at the upper limit) the cut-off thickness for the first order mode. Calculations are made at the important telecommunications wavelength 1.55μm and to a few nm either side at which the refractive index for silicon is 3.5 and for silicon dioxide is 1.457. The design thicknesses for the layers 2 and 4 are 0.1 and 0.24μm and in order to keep the two waveguide modes sufficiently separate, the thickness of layer 3 is set at 4μm.

Calculations of the change in relative phase retardation upon changes in wavelength are obtained by normalizing the propagation distance to 1mm. The results are therefore presented in units of radians/nm.mm. With these parameters there was found to be a wavelength dispersion difference that yields a change in relative phase retardation (Δφ_λ) of 1.49rad/nm.mm. The thermo-optic coefficients of the two materials and their thermal expansion will contribute to the relative phase retardation. Literature values for the thermo- optic effect at 1.55μm are for silicon (1.818 * 10^"4K ') and for silicon dioxide (22.3 x lO^K"¹) and the thermal expansivities are for silicon (4.15 x 10^'6K^"') and for silicon dioxide (0.55 x 10" ⁶K^~'). Using these values gives a relative phase retardation (Δφ_τ) due to thermo-optic and - thermal expansion effects of -177.8mrad/K.mm. Thus an assumed wavelength shift of O. lnm/K that typically occurs in semiconductor diode lasers and temperature change of IK in a package that contains both the laser and the interferometer leads to a measured phase change of 150mrad/mm and -178mrad/mm respectively. From this it will be apparent that the wavelength effect is completely obscured.

The reason for the excessively large thermal effect is that the power distribution between layers 2 and 4 is markedly different. Optical waveguide modes have an electric field that is distributed between the layers of the waveguide structure. The relative amount of power contained in the various layers determines the "effective refractive index" of the waveguide mode. The effective refractive index of a mode determines its speed of propagation and therefore the extent of phase retardation that can accumulate with distance. If the temperature changes, the distribution of the field changes which results in a change in the effective refractive index. If the thermo-optic properties of the layers are also different then the changes can be quite large.

The present invention seeks to overcome certain deficiencies of conventional optical waveguide interferometers by exploiting a novel laminate structure in whose waveguides a different relative phase retardation accumulates when subjected to a non-thermal change whilst exhibiting thermal symmetry. More particularly, the present invention relates to an optical waveguide interferometer incorporating structurally asymmetric waveguides which are thermo-optically balanced.

Thus viewed from a first aspect the present invention provides an optical waveguide interferometer comprising: a laminate structure in which a monolayer constitutes a first planar waveguide and a sandwich layer constitutes a second planar waveguide, wherein the first planar waveguide and second planar waveguide are spaced part and are adapted so as in use to transmit a substantially equal proportion of incident electromagnetic radiation, wherein the first planar waveguide and second planar waveguide are capable of exhibiting a measurable relative response to a change in the wavelength of incident electromagnetic radiation or in the localised environment.

By "planar waveguide" is meant a waveguide which permits propagation of incident electromagnetic radiation in any arbitrary direction within a plane. Preferably the (or each) planar waveguide is a slab waveguide.

The insensitivity to temperature fluctuations of such an athermal structure means that associated packaging (such as temperature controllers) may be relatively unsophisticated. Moreover the optical waveguide interferometer of the invention may be manufactured advantageously using any suitable combination of materials even if these materials have a thermo-optic coefficient of the same sign but different magnitude.

To render the laminate (ie multi-layered) structure sensitive to changes in wavelength or to changes in the localised environment, the composition, number, dimension (eg thickness) and separation of layers of the laminate structure (ie of the monolayer, sandwich layer and any additional layers) may be chosen judiciously (although it will be appreciated that in practice constraints will be imposed by the precision of the process for manufacturing the laminate structure). Generally this is achieved in accordance with familiar fabrication methods such as CVD (eg PECVD or LPCVD). In this manner (for example), the refractive index of a silicon oxynitride planar waveguide (at a constant thickness) may be selected at any level in the range 1.457 to 2.008. Typically the laminate structure is of thickness in the range 0.2-10 microns. Preferably the monolayer and sandwich layer are each adapted to support only a single propagating mode. Preferably the sandwich layer is adapted to support a supermode, particularly preferably a supermode with an electric field distribution substantially as illustrated in Figure 3.

In a preferred embodiment, the sandwich layer comprises: a first layer exhibiting a first refractive index spaced apart from a second layer exhibiting a second refractive index by a spacer, wherein the refractive index of the spacer is less than that of the first refractive index and of the second refractive index. Preferably the first refractive index and the second refractive index are substantially equal.

Preferably the laminate structure is integrated with a lowermost substrate (typically a silicon or indium phosphide substrate) and comprises the second planar waveguide located above and spaced apart from the first planar waveguide by a spacer monolayer (eg a silicon dioxide spacer monolayer).

In an embodiment of the invention, the first and/or second planar waveguide is composed of silicon, silicon oxynitride or silicon nitride.

In a first preferred embodiment, the dispersion characteristics (ie the change in the effective refractive index of the propagating mode vs wavelength) of the first planar waveguide mode are of different magnitude to the dispersion characteristics of the second planar waveguide mode. Thus the first planar waveguide and second planar waveguide are capable of exhibiting a measurable relative response to a change in the wavelength of incident electromagnetic radiation. By being adapted to be sensitive to small changes in wavelength, the optical waveguide interferometer may be used as a wavelength monitor. Preferably the laminate structure further comprises a capping monolayer adapted to isolate the second planar waveguide from the environment. An example of suitable capping monolayers is given in GB0203581.4 (Farfield Sensors Limited).

In a second preferred embodiment, the first planar waveguide and second planar waveguide are capable of exhibiting a measurable relative response to a change in the localised environment caused by the introduction of or changes in a stimulus of interest. As a consequence of the introduction of or changes in a physical, biological and/or chemical stimulus of interest in the localised environment (ie a change in the refractive index of material in the localised environment), the first and second planar waveguides accumulate different relative phase retardation causing a measurable relative response.

Thus the optical waveguide interferometer of the second preferred embodiment of the invention may advantageously be used to detect the presence of or changes in a chemical or biological stimulus in an analyte which is introduced into the localised environment (ie a chemical sensor waveguide interferometer). The interaction of the stimulus with the first planar waveguide and second planar eguide may be a binding interaction or absorbance or any other interaction. For example, a gaseous or liquid phase analyte comprising chemical stimuli may be introduced into the localised environment of the optical waveguide interferometer. Alternatively, a chemical reaction may take place which effects changes in the nature of the chemical stimuli in situ and causes a change in the localised environment.

The second preferred embodiment may be used to measure inter alia pressure, position, temperature or vibration in relation to the presence of or changes in a physical stimulus (ie a physical optical waveguide interferometer). The physical stimulus may be applied to the first or second planar waveguide of the optical waveguide interferometer via an impeller (for example) located on the first or second planar waveguide to enable the measurement of (for example) pressure or precise position.

In the second preferred embodiment, the optical waveguide interferometer may be used in whole waveguide mode wherein the second planar waveguide constitutes a sensing waveguide and the first planar waveguide constitutes a reference waveguide, wherein the optical waveguide interferometer is arranged so as to expose to the localised environment at least a part of the second planar (sensing) waveguide. In the whole waveguide mode, the first planar (reference) waveguide may be an inactive (eg deactivated) planar waveguide substantially incapable of exhibiting a measurable response to a change in the localised environment caused by the introduction of or changes in the stimulus of interest. To optimise the performance of the second preferred embodiment in the whole waveguide mode, the physical, biological and chemical properties of the second planar (sensing) waveguide and first planar (reference) waveguide are as similar as possible (with the exception of the response to the change in the localised environment caused by the introduction of or changes in the stimulus of interest).

Alternatively, in the second preferred embodiment, the optical waveguide interferometer may be used in evanescent mode wherein the laminate structure further includes one or more sensing layers capable of inducing in the second planar waveguide a measurable response to a change in the localised environment caused by the introduction of or changes in a stimulus of interest, wherein the optical waveguide interferometer is arranged so as to expose to the localised environment at least a part of the (or each) sensing layer and the first planar waveguide is a reference waveguide. In the evanescent mode, the first planar waveguide may be an inactive (eg deactivated) planar secondary waveguide substantially incapable of exhibiting a measurable response to a change in the localised environment caused by the introduction of or changes in the stimulus of interest. To optimise the performance of the second preferred embodiment in the evanescent mode, the physical, biological and chemical properties of the second planar waveguide and first planar (reference) waveguide are as similar as possible (with the exception of the response to the change in the localised environment caused by the introduction of or changes in the stimulus of interest). It is preferred that the second planar waveguide and first planar (reference) waveguide have identical properties.

The sensing layer may comprise an absorbent material (eg a polymeric material such as polysiloxane) or a bioactive material (eg containing antibodies, enzymes, DNA fragments, functional proteins or whole cells). The absorbent material may be capable of absorbing gases, liquids or vapours containing a chemical stimulus of interest. The bioactive material may be appropriate for liquid or gas phase biosensing.

The sensing waveguide may comprise an absorbent material (eg a polymeric material such as polymethylmethacrylate, polysiloxane, poly-4-vinylpyridinε) or a bioactive material (eg containing antibodies, enzymes, DNA fragments, functional proteins or whole cells). The sensing waveguide may comprise a porous silicon material optionally biofunctionalised with antibodies, enzymes, DNA fragments, functional proteins or whole cells.

In the second preferred embodiment, the optical waveguide interferometer may comprise one or more means for intimately exposing to the localised environment at least a part of the (or each) sensing layer or the sensing waveguide (and optionally at least a part of the (or each) inactive layer or the inactive waveguide), said means being optionally integrated onto the optical waveguide interferometer. By way of example, suitable means are disclosed in WO- A-01/36945. Typically the measurable response of the optical waveguide interferometer (to a change in the localised environment or in the wavelength of the incident electromagnetic radiation) manifests itself as movement of the fringes in an interference pattern. The relative phase shift of the radiation in the optical waveguide interferometer may be calculated from the movement in the fringes. In turn, the amount of or changes in a chemical, biological or physical stimulus in the localised environment or the change in wavelength may be calculated from the relative phase shift. An interference pattern may be generated when the electromagnetic radiation from the optical waveguide interferometer is coupled into free space and the pattern may be recorded in a conventional manner (see for example WO-A-98/22807) either using a single detector which measures changes in the intensity of electromagnetic radiation or a plurality of such detectors which monitor the change occurring in a number of fringes or in the entire interference pattern. The one or more detectors may comprise one or more photodetectors. Where more than one photodetector is used this may be arranged in an array eg a two- dimensional photodiode array (or the like).

Electromagnetic radiation generated from a conventional source may be propagated into the first and second planar waveguides in a number of ways. In a preferred embodiment, radiation is simply input via an end face of the optical waveguide interferometer (this is sometimes described as "an end firing procedure"). Preferably (but not essentially), the electromagnetic radiation source provides incident electromagnetic radiation having a wavelength falling within the visible range. Preferably the optical waveguide interferometer comprises: propagating means for substantially simultaneously propagating incident electromagnetic radiation into the first and second planar waveguides. For example, one or more coupling gratings or mirrors may be used. A tapered end coupler rather than a coupling grating or mirror may be used to propagate light into the lowermost first planar waveguide or equally into the first and second planar waveguide. Preferably the electromagnetic radiation source (eg laser) is integrated with the laminate structure on the common substrate (typically a common silicon or indium phosphide substrate).

The incident electromagnetic radiation may be oriented (eg plane polarised) as desired using an appropriate polarising means. The incident electromagnetic radiation may be focussed if desired using a lens or similar micro-focussing means.

Viewed from a further aspect the present invention provides a method for monitoring the wavelength of electromagnetic radiation comprising:

(A) providing an optical waveguide interferometer as hereinbefore defined;

(B) propagating electromagnetic radiation of a first wavelength into the first planar waveguide and the second planar waveguide in the laminate structure;

(C) measuring a measurable relative response being the response of the first planar waveguide relative to the second planar waveguide; and

(D) relating a change in the measured measurable relative response to a change in the wavelength of the electromagnetic radiation from the first wavelength to a second wavelength.

In a preferred embodiment, step (C) comprises:

(Cl) generating a pattern of interference fringes; and

(C2) measuring a movement in the interference pattern; and step (D) comprises: relating the movement in the interference pattern to a change in the wavelength of the electromagnetic radiation from the first wavelength to a second wavelength.

In a particularly preferred embodiment, step (C) further comprises: (C3) calculating the phase shift in the first planar waveguide relative to the phase shift in the second planar waveguide ("the relative phase shift") from the movement in the interference pattern; and step (D) comprises: relating the relative phase shift to the change in the wavelength of electromagnetic radiation from a first wavelength to a second wavelength.

In a particularly preferred embodiment, the method further comprises:

(E) generating an adjustment signal dependent on the movement in the interference pattern measured in step C2 (or the relative phase shift calculated in step C3); and

(F) applying the adjustment signal to the source of electromagnetic radiation whereby to adjust the wavelength of the electromagnetic radiation from the second wavelength to the first wavelength.

Step (E) may be carried out by a comparator which generates an adjustment signal dependent on the magnitude and/or direction of the movement in the interference pattern (or preferably of the relative phase shift).

Step (F) may be carried out thermo-optically. For example, a conventional temperature controller may be used to thermo-optically tune the source of electromagnetic radiation. Step (F) may be carried out by adjusting the electromagnetic radiation source current using (for example) a tuning element such as a tunable filter (eg Bragg grating filter).

In an embodiment of the invention, step (D) comprises: deducing the wavelength shift from the measured measurable relative response.

Viewed from a still further aspect the present invention provides the use of an optical waveguide interferometer as hereinbefore defined for monitoring the wavelength of incident electromagnetic radiation, wherein the first planar waveguide and second planar waveguide are capable of exhibiting a measurable relative response to a change in the wavelength of incident electromagnetic radiation.

Viewed from a yet still further aspect the present invention provides a process for detecting the introduction of (eg the amount or concentration of) or changes in a chemical, biological or physical stimulus of interest in a localised environment, said process comprising:

(a) providing an optical waveguide interferometer as hereinbefore defined in a localised environment;

(b) introducing or causing changes in the chemical, biological or physical stimulus of interest in the localised environment;

(c) irradiating the optical waveguide interferometer with electromagnetic radiation;

(d) measuring a measurable relative response being the response of the first planar waveguide relative to the second planar waveguide; and

(e) relating the relative response to the presence of or changes in the chemical, biological or physical stimulus of interest.

In a preferred embodiment, step (d) comprises:

(dl) generating a pattern of interference fringes; and

(d2) measuring a movement in the interference pattern; and step (e) comprises: relating the movement in the interference pattern to the presence of or changes in the chemical, biological or physical stimulus of interest. In a particularly preferred embodiment, step (d) further comprises:

(d3) calculating the phase shift in the first planar waveguide relative to the phase shift in the second planar waveguide ("the relative phase shift") from the movement in the interference pattern; and step (e) comprises: relating the relative phase shift to the presence of or changes in the chemical, biological or physical stimulus of interest.

Methods for performing this calculation will be familiar to those skilled in the art. The phase shift data may be related to the amount (eg concentration) of or changes in the chemical stimulus of interest by comparison with standard calibration data.

Preferably the process of the invention is carried out in evanescent or whole waveguide mode.

In a preferred embodiment, the process comprises: continuously introducing the analyte containing a chemical stimulus of interest. In a particularly preferred embodiment, the process comprises: continuously introducing the analyte containing a chemical stimulus of interest in a discontinuous flow (eg as a train of discrete portions).

Preferably the process further comprises: inducing a chemical reaction in the analyte which is static in the localised environment.

Viewed from an even still further aspect of the present invention there is provided the use of an optical waveguide interferometer as hereinbefore defined for detecting the presence of or changes in a chemical, biological or physical stimulus of interest in a localised environment, wherein the first planar waveguide and second planar waveguide are capable of exhibiting a measurable relative response to a change in the localised environment.

The invention may be exploited for any type of electromagnetic radiation including optical, UV, IR and microwaves.

The present invention will now be described in a non-limitative sense with reference to the accompanying figures 2-5 in which:

Figure 2 illustrates an embodiment of the optical waveguide interferometer of the invention;

Figure 3 illustrates the mode field profile for the supermode of the optical waveguide interferometer of the invention:

Figure 4 illustrates the far field diffraction pattern at 1 mm from the end facet of an optical waveguide interferometer of the invention with optogeometrical parameters as given in Table

1 ; and

Figure 5 illustrates the ratio of wavelength response to thermal response versus thickness of layer 2 (for a constant set of thickness values for the remaining layers of Table 1 ).

The laminate structure of an embodiment of the optical waveguide interferometer of the invention is shown in Figure 2. A monolayer 2 and a sandwich layer 4 act as first and second planar waveguides separated by a spacer monolayer 3. The sandwich layer 4 comprises two high refractive index monolayers 4a and 4c separated by a spacer 4b. A capping monolayer 5 isolates sandwich layer 4 from the environment. The monolayers 1, 2, 3 and 5 and sandwich layer 4 are fabricated on a silicon substrate S.

The optogeometrical parameters (ie thicknesses and refractive indices) of each of monolayers 1 , 2, 3 and 5 and sandwich layer 4 used in the followed calculation are given in Table 1 : Table 1

By bringing two thin, high refractive index layers 4a and 4c into close proximity sandwiching a spacer 4b, they form a sandwich layer that can support so called 'supermodes'. For a representative combination of thickness and separation for two layers 4a and 4c, the electric field distribution is as shown in Figure 3. The field peaks in the vicinity of the high refractive index layers 4a and 4c and exhibits a small dip in the spacer 4b. The field to either side decays with distance according to an exponential law whose decay constant depends on the effective refractive index of the supermode. When combined by diffraction interference with the single peaked field distribution of the high refractive index monolayer 2, the power distribution is as shown in Figure 4. This power distribution is almost identical in appearance to that which would be generated from the interference of two single peaked field distributions (such as is the case with conventional optical waveguide interferometers). This means that conventional methods of analysis that quantify the phase shifts may be applied without modification.

Using the parameters given in Table 1 , the calculated effect of the difference in wavelength dispersion is Δφ_λ = -95mrad/nm.mm which is smaller than the optimum value calculated above for the conventional device. The thermo-optic effect gives a value of Δφ_τ=-0.1mrad/K.mm. Close to this value, the thermal response passes through zero and the device is wholly insensitive to temperature changes. The calculated ratio of wavelength response to thermal response around this special region is shown in Figure 5. Thus provided that the optical waveguide interferometer can be fabricated to the specifications given in Table 1, the wavelength response can be substantially decoupled from the thermal response. In practice, fabrication tolerances to either side of this optimum structure must be accounted for. Repeating the calculations for small changes in the optimum thickness for the single waveguide layer it was found that within ±lnm, the thermo-optic effect lies within maximum limits of ±2mrad/K.mm.

The advantages of the structure of the optical waveguide interferometer of the invention may be illustrated firstly by considering a IK change in temperature of the laser/interferometer package. The laser wavelength output would vary by 0.1 nm and provide a phase response of -9.5mrad/mm. The thermal effect would lead to a maximum phase shift of 2mrad/mm thereby providing a minimum discrimination between pure wavelength effect and pure thermal effect of about 4.5. In practice, simple temperature control of the laser/interferometer package would achieve a temperature variation of less than ±lOOmK. Pure thermal shifts in the interferometer would then be limited to a maximum of ±0.2mrad/mm. The active length of the optical waveguide interferometer may be increased to 5mm at which the pure thermal excursions would not exceed ±lmrad. However, for wavelength shifts of 1pm (0.12GHz at 1550nm), a phase shift of -2.45mrad would be recorded ie almost twice the thermal noise level. Such a device could then act to correct the laser wavelength by a feedback loop. With better thermal control this baseline discrimination may be further improved.

Claims

1. An optical waveguide interferometer comprising: a laminate structure in which a monolayer constitutes a first planar waveguide and a sandwich layer constitutes a second planar waveguide, wherein the first planar waveguide and second planar waveguide are spaced part and are adapted so as in use to transmit a substantially equal proportion of incident electromagnetic radiation, wherein the first planar waveguide and second planar waveguide are capable of exhibiting a measurable relative response to a change in the wavelength of incident electromagnetic radiation or in the localised environment.

2. An optical waveguide interferometer as claimed in claim 1 wherein the (or each) planar waveguide is a slab waveguide.

3. An optical waveguide interferometer as claimed in either of claims 1 or 2 wherein the monolayer and sandwich layer are each adapted to support only a single propagating mode.

4. An optical waveguide interferometer as claimed in any preceding claim wherein the sandwich layer is adapted to support a supermode.

5. An optical waveguide interferometer as claimed in claim 4 wherein the supermode has an electric field distribution substantially as illustrated in Figure 3.

6. An optical waveguide interferometer as claimed in any preceding claim wherein the sandwich layer comprises: a first layer exhibiting a first refractive index spaced apart from a second layer exhibiting a second refractive index by a spacer, wherein the refractive index of the spacer is less than that of the first refractive index and of the second refractive index.

7. An optical waveguide interferometer as claimed in claim 6 wherein the first refractive index and the second refractive index are substantially equal.

8. An optical waveguide interferometer as claimed in any preceding claim wherein the laminate structure is integrated with a lowermost substrate and comprises the second planar waveguide located above and spaced apart from the first planar waveguide by a spacer monolayer.

9. An optical waveguide interferometer as claimed in any preceding claim wherein the dispersion characteristics of the first planar waveguide mode are of different magnitude to the dispersion characteristics of the second planar waveguide mode.

10. An optical waveguide interferometer as claimed in any of claims 1 to 8 wherein the first planar waveguide and second planar waveguide are capable of exhibiting a measurable relative response to a change in the localised environment caused by the introduction of or changes in a stimulus of interest.

1 1. An optical waveguide interferometer as claimed in claim 10 wherein the second planar waveguide constitutes a sensing waveguide and the first planar waveguide constitutes a reference waveguide, wherein the optical waveguide interferometer is arranged so as to expose to the localised environment at least a part of the second planar waveguide.

12. An optical waveguide interferometer as claimed in claim 1 1 wherein the first planar waveguide is an inactive planar waveguide substantially incapable of exhibiting a measurable response to a change in the localised environment caused by the introduction of or changes in the stimulus of interest.

13. An optical waveguide interferometer as claimed in claim 10 wherein the laminate structure further includes one or more sensing layers capable of inducing in the second planar waveguide a measurable response to a change in the localised environment caused by the introduction of or changes in a stimulus of interest, wherein the optical waveguide interferometer is arranged so as to expose to the localised environment at least a part of the (or each) sensing layer and the first planar waveguide is a reference waveguide.

14. An optical waveguide interferometer as claimed in claim 13 wherein the first planar waveguide is an inactive planar secondary waveguide substantially incapable of exhibiting a measurable response to a change in the localised environment caused by the introduction of or changes in the stimulus of interest.

15. A method for monitoring the wavelength of electromagnetic radiation comprising:

(A) providing an optical waveguide interferometer as defined in any preceding claim;

16. A method as claimed in claim 15 wherein step (C) comprises: (Cl) generating a pattern of interference fringes; and

17. A method as claimed in claim 16 wherein step (C) further comprises:

(C3) calculating the phase shift in the first planar waveguide relative to the phase shift in the second planar waveguide from the movement in the interference pattern; and step (D) comprises: relating the relative phase shift to the change in the wavelength of electromagnetic radiation from a first wavelength to a second wavelength.

18. A method as claimed in any of claims 15 to 17 further comprising:

(E) generating an adjustment signal dependent on the movement in the interference pattern measured in step C2 or the relative phase shift calculated in step C3; and

19. Use of an optical waveguide interferometer as defined in any of claims 1 to 15 for monitoring the wavelength of incident electromagnetic radiation, wherein the first planar waveguide and second planar waveguide are capable of exhibiting a measurable relative response to a change in the wavelength of incident electromagnetic radiation.

20. A process for detecting the introduction of or changes in a chemical, biological or physical stimulus of interest in a localised environment, said process comprising: (a) providing an optical waveguide interferometer as defined in any of claims 1 to 15 in a localised environment;

21. A process as claimed in claim 20 wherein step (d) comprises: (dl) generating a pattern of interference fringes; and

(d2) measuring a movement in the interference pattern; and step (e) comprises: relating the movement in the interference pattern to the presence of or changes in the chemical, biological or physical stimulus of interest.

22. A process as claimed in claim 20 or 21 wherein step (d) further comprises:

(d3) calculating the phase shift in the first planar waveguide relative to the phase shift in the second planar waveguide from the movement in the interference pattern; and step (e) comprises: relating the relative phase shift to the presence of or changes in the chemical, biological or physical stimulus of interest.

23. Use of an optical waveguide interferometer as defined in any of claims 1 to 15 for detecting the presence of or changes in a chemical, biological or physical stimulus of interest in a localised environment, wherein the first planar waveguide and second planar waveguide are capable of exhibiting a measurable relative response to a change in the localised environment.