WO2010150118A1

WO2010150118A1 - Single-waveguide bio-sensor based on propagating modes in a grating waveguide

Info

Publication number: WO2010150118A1
Application number: PCT/IB2010/052532
Authority: WO
Inventors: Shlomo Ruschin; Tomer Yehezkel
Original assignee: Ramot At Tel Aviv University Ltd.
Priority date: 2009-06-08
Filing date: 2010-06-08
Publication date: 2010-12-29

Abstract

A device for sensing based on a thin high-index waveguide (100) over which a periodic coupling grating (110) is disposed. The grating acts as a coupling mechanism between the guided modes propagating in the waveguide, which is very sensitive to thickness variations in the grating. The step height difference in segments of the grating is dependent on a substance to be sensed. When the grating is exposed to the substance to be sensed, a change in the step height difference affects an exchange of optical power between the guided modes, which can be measured to determine the presence of the substance to be sensed.

Description

SINGLE-WAVEGUIDE BIO-SENSOR BASED ON PROPAGATING MODES IN A GRATING WAVEGUIDE

FIELD QF THE INVENTION

The present embodiment generally relates to sensors, and in particular, it concerns a biological sensor based on an optical waveguide.

BACKGROUND OF THE INVENTION

Sensors based on optical waveguides of various types have been exieiisively studied in recent years, since the use of optical waveguides in biosensors offers numerous benefits. By using integrated optics configurations, optical waveguide sensors have the potential for being smaller, portable, immune to electromagnetic interference, and rugged. Refer to G. Robinson, Optical immunosensing systems - meeting the market needs, Biosensors and Bioelectronics, 6 (1991) 183-191, for additional information. In particular, sensors based on interferometric principles are highly sensitive to changes both in the refractive index of the cover, and in the thickness of an overlaying thin film (also known as a cover having a cover thickness, or overlay thickness). This same property is found in sensors based on Surface Plasmon Resonance (SPR) where optical environment variations also affect the resonance condition. These changes can be interrogated by different methods, and are influenced by both the overlay thickness and the superstate environment. The need to differentiate between these two influences was pointed out by Kunz and Cottier {Optimizing integrated optical chips for label-free (bio-) chemical sensing, Anal Bioanal C hem (2006) 384: 180-190), who discussed in detail this point and presented methods for distinctly optimizing the sensitivity to these two effects. The strive for high sensitivity is driven by the need to identify low concentrations of hazardous chemicals and bio-molecules in a label-free fashion. The attainment of low sensitivities (10^"5 refractive index units (RIUs) and below) presented on the other hand a problem of sensitivity to other environmental effects like pressure and temperature, raising the need for external stabilization of other environmental effects in order not to interfere with the measurement of the desired parameter(s) for the substance to be sensed. There is therefore a need for an optical waveguide-based sensing device that can sense with increased sensitivity using variations of overlay thickness, but is less sensitive to changes in refractive index in the optical medium in which the sensor is immersed (super strate).

SUMMARY

According to the teachings of the present embodiment there is provided, a device for sensing including: a waveguide; a periodic coupling grating including a plurality of non- overlay segments and a plurality of overlay segments, the overlay segments of a given step height and step length, the overlay segments placed at a given period, the periodic coupling grating disposed on a given section of the waveguide, wherein the period satisfies a phase matching condition for an exchange of power between at least two types of propagation, wherein a step height difference between the step height and a height of the non-overlay segment is dependent on a substance to be sensed, and wherein the step height difference affects an exchange of optical power between the types of propagation; a light input system for exciting at least one type of propagation into the waveguide on a first side of the periodic coupling grating towards the periodic coupling grating; and a light measurement system for coupling out at least one type of propagation from the waveguide and measuring at least one parameter of at least one type of propagation coupled out from the waveguide, to determine the presence of the substance to be sensed.

BRIEF DESCRIPTION OF FIGURES

The embodiment is herein described, by way of example only, with reference to the accompanying drawings, wherein:

FIGURE 1 is a diagram of one implementation of a basic structure of a sensor.

FIGURE 2A is a plot of the output power of the zero order mode TEO as a function of the growing layer thickness. FIGURE 2B is a plot of the output power of the zero order mode TEO as a function of the change of refractive index.

FIGURE 3 represents kh_mm and An_min (for sensor length L_g) as a function of the grating tooth height.

FIGURE 4 represents L₀ as a function of the grating tooth height.

FIGURE 5 represents the products Ah_mmxL_g and An_1nJnXL_g as a function of the grating tooth height.

DETAILED DESCRIPTION

The principles and operation of the device according to the present embodiment may be better understood with reference to the drawings and the accompanying description. The present embodiment is a device for sensing, also referred to as a sensor. The device facilitates sensing comparable to conventional sensors, using variations of overlay thickness, while being more specific, in other words less sensitive to changes in the refractive index of the optical medium in which the sensor is immersed (superstrate). The device is based on a coupling mechanism provided by periodic perturbation, and acts directly on this mechanism. The device provides increased sensitivity to overlay thickness changes at specific segments for short overall sensor lengths, by way of a preferred example a thickness-length product down to 2x 10^" nm*mm (nanometer*mϊllimeter). The example thickness-length product means that a 10 mm sensor is sensitive to overlay thickness variations on the order of 2x 10^"4 ran or 0.2 pm

(picometer). While providing increased sensitivity to the overlay thickness for short sensor lengths, the superstrate refractive index sensitivity remains low, which in this context refers to typically about two orders of magnitude lower than a conventional SPR sensor.

Referring now to the drawings, FIGURE 1 is a diagram of one implementation of a basic structure of a sensor according to the present embodiment. The sensor consists of a waveguide 100 of relatively high refractive index, which supports at least two types of propagation, in this case two guided modes, (for example TEo and TEi) each guided mode in a given polarization. The thickness of the waveguide is shown as 101. A waveguide is also referred to as a waveguide layer, or a waveguiding layer. In this context, a relatively high refractive index is a refractive index that is higher than the refractive indexes of the substrate 102 and environment 104. In the context of this document, the term light generally includes electromagnetic radiation at diverse sections of the frequency spectrum including visible, infrared, ultra-violet and microwave and tera-herz range. Depending on the application, various frequencies of light can be used in the sensor.

The set of light wave patterns, or field distributions, that can propagate along a waveguide are known as modes, also referred to as "guided modes". A transverse guided mode is a particular electromagnetic field pattern of radiation measured in a plane perpendicular (that is, transverse) to the propagation direction of the light. Transverse modes occur because of boundary conditions imposed on a light wave by a waveguide and are characterized by an amplitude profile characteristic of the specific mode. In transverse electric (TE) modes, there is no electric field in the direction of propagation. In transverse magnetic (TM) modes, there is no magnetic field in the direction of propagation. The lowest order transverse electric mode is referred to as the fundamental mode, TEQ. Higher order modes are referred to as TEi, TE₂, and so forth. In the present embodiment, one of the modes should preferably be distinctly excited, which can be achieved by means including, but not limited to, a prism, by butt-coupling the light at a specific angle, mode combining/dividing, or etching a waveguide interface periodically (grating). A light input system excites at least one type of propagation into a waveguide on a first side of a periodic coupling grating towards the periodic coupling grating. In FIGURE 1 a non-limiting example of a prism is used as an in-coupler 106 to excite a guided mode 107 into waveguide 100. Couplers for exciting a guided mode by coupling-in radiation shined on a waveguide surface are known in the art, and based on this description, one skilled in the art will be able to choose an implementation for a given application.

A normal mode of light is an oscillation in which the phase velocity of the electromagnetic wave is well defined and the transverse amplitude distribution of light retains its shape during propagation. The light is mostly confined to the 'guiding' layer of relatively high refractive index waveguide, surrounded above and below by relatively lower index cladding materials. All the normal modes are normalized and orthogonal to each of the other normal modes. An unperturbed waveguide can transmit any of the supported normal modes of the waveguide without converting power to any of the other possible normal modes. A perturbation of the waveguide may couple a particular normal mode to other normal modes, resulting in an exchange of power between the modes. When a phase matching condition (also referred to as a resonance condition) is satisfied, a perturbation of the waveguide can cause a large exchange of power between the modes of the unperturbed waveguide.

At a given section of the waveguide a periodic coupling grating 110 is disposed on the waveguide, contacting or being in connection with the waveguide. A periodic coupling grating is also referred to a simply a grating. The periodic coupling grating components include overlay segments 111 and non-overlay segments 114. Overlay segments and non-overlay segments are collectively referred to as segments. Overlay segments 111 include one or more overlay layers (112, 116, and 117, as described below). One period of a grating generally includes two segments, an overlay segment and a non-overlay segment. One technique for disposing a periodic coupling grating on a waveguide is to deposit a selected material on the waveguide to construct a series of overlays 112 with a given overlay refractive index (n_gr). The height d of the overlay 112 and step height 124 of the overlay segment 111 are discussed below. Overlay segments are also referred to as steps, strips, teeth, or grating teeth, for example when referring to the grating tooth height. The distance between a given location (such as the leading edge) of an overlay segment and the same given location on an adjacent overlay segment is referred to as the period Λ of the grating, grating period length, or simply period. Segments may be equal in length (50% duty cycle), or different lengths depending on the application. The length of a grating L_g is discussed below. The period of the grating is determined by a phase matching condition between the two guided modes.

Metallic and non-metalϋc materials can be used for overlays. In a preferable implementation, a metallic grating is used due to the fact that well known techniques exist for selectively depositing materials on metal, distinct from techniques for depositing materials on a dielectric. This allows an additional thin film linking layer 116 to be deposited on the overlays 112 of the grating (and not on the non-overlay segments 114). This is an important feature of the device, since the high sensitivity performance requires changing the step height difference between adjacent segments. In the context of this document, step height difference is the difference in height between the step height of an overlay segment and a height of a non- overlay segment. In other words, the difference between the height of the overlay segments (steps) and the non-overlay segments (space between the steps). Depending on the application, various techniques can be used for constructing overlay segments of a given height. The below description uses a metallic grating on which an additional linking layer 116 of a given thin film height (ha) and refractive index (ny) is grown to provide a given total step height. The linking layer is also referred to as a thin film linking layer, and the material of the linking layer is referred to as a linker. The linker can be chosen depending on the application to link (in other words to attract and bind molecules of) a substance to be sensed.

When the sensor is exposed to an environment (superstrate) 104 containing a substance to be sensed, the substance links to the linking layer 116, thereby increasing the thin film height (ha) by the height (d_s) of the linked substance film 117, which in turn changes the total step height 124. A change in step height 124 changes the step height difference, effectively causes a modification of the effective waveguide's thickness, which effects the propagation of light through the waveguide. Note that to simplify the below example calculations, linking layer height (h_d) is used with the overlay refractive index (n_gr). It is foreseen that more accurate calculations including the linked substance height with the linking layer height (d_s + h_<j) and the refractive indices of the overlay (π_gr), linking layer (n_<j), and linked substance can be used to furnish more accurate calculations. The use of simplified calculations to assist in the description of this embodiment should not detract from the utility and basic advantages of the invention.

In another embodiment, a linking layer (not shown) is disposed on the non-overlay segments 114. Similar to the process described above, when the sensor is exposed to an environment (superstrate) 104 containing a substance to be sensed, the substance links to the linking layer on the non-overlay segments 114. The linking layer increases by the height of the linked substance, thereby changing the step height difference.

Propagation through the given section of the waveguide on which a periodic coupling grating has been disposed will cause a periodic exchange of optical power between the types of propagation. A light measurement system for coupling out at least one type of propagation from the waveguide can be used to measure at least one parameter of at least one type of propagation coupled out from the waveguide, to determine the presence of a substance to be sensed. The point of maximum sensitivity can be calculated and a out-coupler 108 facilitates at least one guided mode (TEo, TEi) to be coupled out (from the waveguide into free-space) radiating light 118, at the appropriate point to measure a parameter change of the at least one guided mode to determine the presence of a substance to be sensed. Light measurement systems for measuring parameters of light, for example a light detector, are known in the art, and based on this description, one skilled in the art will be able to choose an appropriate light measurement system for a given application. In one embodiment, a single guided mode is coupled out and a change in power of the single guided mode is measured. In another embodiment, two or more guided modes are coupled out and changes in the power of each guided mode are measured and used to make a preliminary determination of the presence of a substance to be sensed. Typically, both modes are coupled out and the observer can choose whether to measure the power of one mode or both). Preliminary determinations are used to make a final determination of the presence of a substance to be sensed. In another embodiment, two or more guided modes are coupled out, and a difference in power between guided modes measured. Based on the above description, one skilled in the art will be able to select parameters, measuring methods, and implementations for a specific application. Coupling out can be done using similar techniques as used for coupling in. An example of coupling out technique is to use a prism, shown in FIGURE 1 as out-coupler 108. Means for coupling out light from a waveguide are known in the art, and based on this description, one skilled in the art will be able to choose an implementation for a given application.

Note that FIGURE 1 is not to scale. Typical sizes for some of the components are prisms from several millimeters to several centimeters high, a waveguide thickness from sub- micrometer to several micrometers and an overlay height of about several nanometers. Typically, linking layer and linked substance film heights are even thinner (less high) than the overlay height, although implementations where the overlay layer is thinner than the linking and linked layers are possible. Calculations with specific example sizes are discussed below. Depending on the application, areas 120, 122A, and 122B of the sensor can be immersed in a liquid or gas environment 104. In a case where one or more components, such as couplers 106 and 108, are immersed (in this example in areas 122A and 122B, respectively), the environment needs to be sufficiently transparent to allow propagation of the light (in this example 107 and 118, respectively) through the environment. In some applications, a preferred method of operation is to limit immersion of the sensor to the area 120 of the grating. On the basic level, this is not strictly an evanescent wave sensor, since the overlay thickness growth may affect the core size and the mode there. Changes in the evanescent part of the field will inevitably take place, but the influence of such changes should be preferably minimized in this device. The realistic case where the specific binding takes place on top of the linking layer can be similarly analyzed and is expected to have the same qualitative features.

Since it is increasingly realized that most biological processes occur at interfaces, the study of grating couplers for sensing application has attracted attention in the literature. In most cases, the gratings acted as coupling means between free propagating radiation and a waveguide mode. Grating- Assisted Directional Couplers (GADC) have been widely used as elements in integrated optoelectronics, but such surface gratings have not been studied insofar as a binding biosensor mechanism. The coupled-mode theory (CMT) used here for analysis has been extensively used for describing the interaction between the waveguide modes in a variety of configurations. The simulation formalism described below was validated by comparison with work done by J. Hong (J Hong, W. Huang, T. Makino, On the Transfer Matrix Method for Distributed-Feedback Waveguide Devices, J Lightwave Technol, 10(12) (1992) 1860-1868) who analyzed a similar structure used as a DFB device in a contra-directional coupling scheme.

In another embodiment, the periodic coupling grating can be used with a contra- directional scheme for sensing. One implementation of a contra-directional scheme uses a single mode of light propagating in a first direction, and the reflection of the light propagating in a reverse (contra) direction from the first direction within a waveguide, resulting in two types of propagation (first/forward and reverse/contra) at a given section of the waveguide where a periodic coupling grating is placed. The reflected light provides a second type of propagation that can be coupled out of a waveguide using the same coupler used to couple in a first type of propagation. In this case, the in-coupler and out-coupler are implemented using a shared coupler, for example using a shared prism. Because a contra-directional scheme can be implemented using a single guided mode, a single-mode waveguide can be used, in comparison to a scheme using at least two guided modes that requires a multi-mode waveguide. Typically, a contra-directional implementation requires a shorter period for the periodic coupling grating, compared to an implementation using two guided modes. Contra-directional propagation is known in the art, and based on this description, one skilled in the art will be able to implement an appropriate coupling in, propagation, and coupling out scheme to function with an embodiment of a periodic coupling grating.

The following analysis is provided to facilitate an understanding of the mechanisms believed to underlie the present embodiment. Note however, that the embodiment as described has been found to be of utility, independent of the accuracy or otherwise of this theoretical analysis.

For simplicity, we assume a planar waveguide and TE polarization for all the guided modes supported by the waveguide. Either TM or TE polarization can be used. The total electric field in the y direction can be then expanded as:

£/r,θ 4∑4(2)<V^(*-^fe) +_C.c. (Equation 1)

where Λ_m(z) is the complex amplitude of the normalized mode amplitude a/^m)(x), and β_m is the propagation constant of the m^th mode. A perturbation that is periodic along the z-axis can be expanded by Fourier series as:

.²II₁.

An² (x, z) = An² (x) ^c_?e ^Λ (Equation 2)

9=-

where q is the grating order, A is the grating period length and An(x), the refractive index difference, is defined below. For a rectangular tooth shape grating, with 50% duty cycle, (equation 2) takes the form:

Δn² (x, z) = Λn^? (x>| - + - ^' s .in ( —2% z λ + 1 . Λ, 2π 1 2 % U — sin 3 — z + . (Equation 3) J 3 { A )

The coupling coefficients, between the zero and first mode for a first grating order (q=l), are then given by

ωε₀ f_{Λ w}2 / N (1,0) / >, (O₁I) / w

^10,01 - ~T } ^Δn \^X)^ay \^X)^ay W"* (Equation 4)

Aπ

and the propagation expression to be minimized for phase matching condition is Δ = |A - A_>| — -^- (Equation 5)

where phase matching means thai delta in equation 5 is zero or close to zero. We assume that the waveguide supports only two modes and only one mode is excited at the grating input, so the initial conditions will be

A₀(O) = I, 4(O) = O (Equation 6)

The solutions for the coupled equations, for a co-directional interaction when the modes are strictly phase matched (A=O), for a unit value of input power, are simply:

P₀ (z)

(Equation 7.1)

P₁ (z) = |4 (z)f = sin(|/ψ)² , (Equation 7.2)

where

K ≡

(Equation 8)

The minimum length required for a total power exchange between the two modes is defined as the coupling length and (according to equations 7.1 and 7.2) will be given by;

L ≡^_j (Equation 9)

2*j

Maximum sensitivity will be achieved at a working point where the power slope as a function of K is maximal. At this point, the power is equally distributed between the two modes. The grating minimal length for maximum slope is then

L_g(min) - -^ ^

(Equation 10)

For a given grating length L₃, a change in either grating height or in the environment refractive index will change the coupling length and will cause a deviation in the power distribution between the two modes at the grating output. Equations 7.1 and 7.2 show that since Po(z) and Pj(z) are approximately linear in the perturbation parameter around the optimal L_g, the induced changes will accumulate and the sensitivity will be increased by factor 2m +1 for a grating of the length

L_g = LJ m + - ) ;m = 0,1,2... (Equation 11)

The following modeling and calculations are provided as a non-limiting description of one implementation of the device. The resolution of the different sensor configurations were numerically examined using a rigorous computer program, solving for the stable modes inside the integrated optical waveguide and calculating the coupling constant as detailed in the following. The configuration shown in FIGURE 1 was used in the computer simulation with the following parameters: a substrate 1Θ2 having a refractive index (n_s) of 1.457 (SiO₂) and a 0.2μm (micrometer) thick waveguide 100 with a high refractive index (n_g) of 2.38 (TiO₂) supporting two TE modes (TEo, TEi). The grating period length (A) needed for phase matching is then 0.96[μm]. A gold grating 110 of length L_g is laid on top of the waveguide. The gold grating has a complex refractive index (n_gr) of 0.2-332i. All above refractive index values refer to a wavelength (X) of 632.8 [nm] (for example provided by a He:Ne laser). When the sensing process takes place, an additional linked substance film 117 of the sensed sample is grown over the linking layer 116 having a changing thickness d_s.

One possibility of exciting a distinct mode at the input and monitoring the distinct mode at the output is to use prism couplers (for example, GaP with (n_p) of 3.314 and an angle (θ_p) of 40.5°).

In order to ease the modeling, the width of the waveguide is assumed to be relatively wide compared to the height of the waveguide, preferably about two orders of magnitude wider. Based on this description, one skilled in the art will be able to select a waveguide width appropriate for an application.

The refractive index difference for a perturbation grating of height d is Λ^{nl ~} < ° ^{≤ X ≤ d} (Equation 12)

0 elsewhere

where n_e is the environment refractive index value. Hence the coupling coefficients as given by equation 4 will be

(Equation 13)

The integral value (in equation 13) should be significant to get high sensitivity, which strongly depends on the coupling strength. In order to achieve high sensitivity, the field overlap between the two modes within the grating region should be high too. This requirement can be accomplished in either of the following situations:

Option 1) a thin guiding dielectric layer with a high refractive index resulting in high power density inside that layer and near surroundings.

Option 2) a thin metal layer to form a SPW (Surface Plasmon Wave) for a p-polarized (TM) mode. SPW is a strongly localized electromagnetic surface wave that propagates along an interface between metallic and dielectric media. In this case, TM polarization is preferred.

We choose the first option for the guiding layer since the first option implies lower propagation losses. As mentioned, the grating is chosen to be metallic to facilitate differentiation in the linking chemistry. The losses introduced by the metallic segments can be neglected since the thickness of the metallic segments is assumed to be on the order of few nanometers and the polarization chosen is TE. The choices used to assist in the description of this embodiment should not detract from the validity and utility of the invention. It is foreseen that more general choices including, but not limited to, materials and polarizations can be used, depending on the application.

Now is described the outcomes of two simulations that illustrate the main characteristics of the sensor. The first simulation simulates the growth of a dielectric substance (njrl .43) of thickness h^ above the grating teeth (on the overlay) in an aqueous environment (n_e=l.33) acting as an added layer. As described elsewhere in this description, using different materials for the guiding layer and for the grating is preferable in order to support a chemical technique, which enables the selective growth of a linking layer. In the second simulation, the refractive index of the medium above the waveguide (referred to as the superstrate or environment), changes in the range of 133-1.43 RIU. For this range of environmental changes, the assumption is the basic modal shapes remain unchanged. This assumption was verified by the computer simulation. FIGURE 2 A and FIGURE 2B represent the output power of the zero order mode TEo as a function of the growing layer thickness and the change of refractive index respectively. The calculations correspond to an initial grating tooth height (d) of 5nm and a grating length of L_g = L/2

The sensitivity of the sensor σ is defined by the differential change in the normalized power of the mode exiting at the grating output, Po(Lg), as a function of the thickness of the grown dielectric sensing layer QtJ) or as a function of the environment refractive index (n_e):

dP d?

⁰W_ACE = "Z-; σ_BUUC = — (Equation 14) dh_ά dn_e

The simulations correspond to a value of Lc = 66 [μm] = 69Λ which is short as compared to the lengths of conventional waveguide sensors. The value we get for Lc is therefore significant: a sensor as short as 33[μm] could be realized, and according to (equation 1 1) the sensitivity of the sensor can be enhanced 2m+l folds by adding to the sensor length an integral number of coupling lengths Lc.

In order to determine the detection resolution of the device and as a basic design aid, we have repeated the steps mentioned above for different grating tooth heights in the range of 1 - 50 [nm]. The threshold resolutions are calculated by finding the maximum slopes of the output power graphs and multiplying the output power graphs' inverses with the resolution of practical measuring apparatus often quoted in the literature, ( — AP = 0.2% in all of our

calculations). Explicitly for the minimum delectability of the two parameters discussed:

ΔP ΔF

_Ah (Equation 15)

hence Δh_mn is the minimum resolvable sensing layer height and An_mm is the minimum resolvable sensing medium refraction index change. FIGURE 3 represents Ah_mm and An_mm (for sensor length L_g) as a function of the grating tooth height. The graphs show that the surface sensitivity is high whereas the bulk sensitivity is rather poor, differing considerably with the relation between bulk refractive index to film thickness detection limits shown in SPR sensors. This is a desirable situation: The fluid environment where the sensor is immersed may be noisy or non-homogeneous and we would prefer the non-specific components of the environment to interfere with the selective linking process as little as possible. Furthermore, one can see that the threshold sensitivity to superstrate changes An_mm remains constant and rather low as the grating tooth height changes. On the other hand, the minimum detectable thickness variation Ah_mm increases with metallic tooth height, favoring shallow gratings, this will come at the expense of larger values of the coupling length Lc and longer sensors.

FIGURE 4 represents L₀ as a function of the grating tooth height. As expected, the coupling length decreases as the grating tooth height increases. The validity of the coupling- mode formalism should be questioned as the number of periods within a coupling length decreases down to a few units, and an arbitrary line was placed at the length of / OA.

Since the sensitivity of the sensor analyzed here increases with length (a property common also to other waveguide sensors), the actual figure of merit for the sensor is the product of threshold sensitivity times the device's length. This product should preferably be as small as possible. FIGURE 5 represents the products Ah_mm *L_S and An_mm *L_g as a function of the grating tooth height. The graphs show that these two parameters have a distinct functional dependence: Ah_mm ^χL_g becomes higher whereas An_mm*L_g becomes lower as the grating tooth height increases. A preference in many applications is for the sensor to be more sensitive in the former and less sensitive in the later, the sensor performance can be optimized for the lowest grating tooth height that can be manufactured. In most of the analysis above the grating tooth height was set to 5[nm], which in the non-limiting example results in the value of Ah_mm ^y-L_g being 2^χlO^"3 [ran x mm]. In practical terms, this analysis means that a 10 [mm] sensor will be able to resolve thickness variations of the order of 0.2fpm]. The sensor performance can be further enhanced by lowering the metallic tooth height. Uniform films of gold have been achieved in the art with heights down to 1 nm. The above description shows that the GADC sensor presents good performance, comparable to top- sensitivity published thickness sensors, with the advantage of less sensitivity to changes in the superstrate environment. The configuration of a single- waveguide Integrated Optic GADC sensor disclosed here facilitates sensing surface-linking processes with increased sensitivity and specificity. If the sensor is designed to an optimal length (L_g), a high sensitivity will be achieved at the largest value of this length, meaning a thin metallic grating layer. The sensitivity to bulk superstrate changes will remain constant under this condition, and therefore a longer device is favorable. The sensitivity of the device can be further enhanced by enlarging the length of the device to a larger optimized coupling length value namely: L_g=(2m+l)(L_c/2.

The description above includes specific implementations of the present embodiment. It should be appreciated that based on the present description, one skilled in the art will be able to design other implementations depending on the specific application. It is foreseen that parameters can be changed, modified, and/or controlled by design, manufacture, or implementation to achieve specific operational characteristics, including, but not limited to: number of waveguide layers, characteristics (material, thickness) of waveguide layers, grating characteristics (material, tooth height, duty cycle, and length), light source wavelength, light source power and polarization, and method of coupling light into the waveguide.

In general, some of the features of the present embodiment include:

1. The sensor is based on coupling between two guided modes by a periodic modification of the waveguide's thickness.

2. Within each period there are at least two segments (exemplified above by a region with an overlay layer and a region without an overlay layer). The process of fabrication of the waveguides is such that linking of the sensed material takes place in one of the segments and not in the other segment.

3. The linking process affects the effective thickness of the segment that is linked to, and therefore the amount of power transferred between the modes is affected.

4. The measurement of the power change (or another related parameter) in one mode or both modes is the signal used for sensing.

5. The modes may be propagating in the same direction and thus the coupling is co- directional, or in opposite directions, making the coupling counter-directional The first option allows the input and output ports to be different, and the periods to be substantially shorter.

It will be appreciated that the above descriptions are intended only to serve as examples, and that many other embodiments are possible within the scope of the present invention as defined in the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A device for sensing comprising:

(a) a waveguide;

(b) a periodic coupling grating including a plurality of non-overlay segments and a plurality of overlay segments, said overlay segments of a given step height and step length, said overlay segments placed at a given period, said periodic coupling grating disposed on a given section of said waveguide, wherein said period satisfies a phase matching condition for an exchange of power between at least two types of propagation, wherein a step height difference between said step height and a height of said non-overlay segment is dependent on a substance to be sensed, and wherein said step height difference affects an exchange of optical power between the types of propagation;

(c) a light input system for exciting at least one type of propagation into said waveguide on a first side of said periodic coupling grating towards said periodic coupling grating; and

(d) a light measurement system for coupling out at least one type of propagation from said waveguide and measuring at least one parameter of at least one type of propagation coupled out from said waveguide, to determine the presence of said substance to be sensed.

2. The device of claim 1 wherein one of the types of propagation is distinctly excited.

3. The device of claim 1 wherein each of said plurality of overlay segments include a metallic overlay.

4. The device of claim 1 wherein each of said plurality of overlay segments include a gold overlay.

5. The device of claim 1 wherein said periodic coupling grating is disposed on said given section of said waveguide by a depositing technique,

6. The device of claim 1 wherein each of said plurality of overlay segments includes an overlay disposed on said given section of said waveguide and a linking layer joined to a face of said overlay opposite to said waveguide.

7. The device of claim 6 wherein said Unking layer is deposited on said overlay.

8. The device of claim 6 wherein a height of said linking layer changes based on said substance to be sensed.

9. The device of claim 1 wherein each of said plurality of non-overlay segments includes a linking layer disposed on said waveguide.

10. The device of claim 9 wherein said linking layer is deposited on said waveguide.

11. The device of claim 9 wherein a height of said linking layer changes based on said substance to be sensed.

12. The device of claim 1 wherein said light input system includes a prism to excite one of the types of propagation into said waveguide.

13. The device of claim 1 wherein said light input system uses a mode combining/dividing technique to couple one of the types of propagation into said waveguide.

14. The device of claim 1 wherein said light input system uses a grating to couple one of the types of propagation into said waveguide.

15. The device of claim 1 wherein said at least two types of propagation includes two guided modes, each guided mode in a given polarization.

16. The device of claim 1 wherein said waveguide is a multi-mode waveguide.

17. The device of claim 1 wherein said waveguide is a single-mode waveguide.

18. The device of claim 1 wherein said at least two types of propagation includes a single guided mode propagating in a forward direction to provide a first type of propagation and the single guided mode's reflection propagating in a reverse direction to provide a second type of propagation.

19. The device of claim 18 wherein said light input system and said light measurement system are implemented using a shared coupler.

20. The device of claim 18 wherein said light input system and said light measurement system are implemented using a shared prism.

21. The device of claim 1 wherein said light input system couples out at least one type of propagation from said waveguide, thereby providing at least part of said light measurement system.

22. The device of claim 1 wherein said light measurement system is placed on a second side of said periodic coupling grating opposite to said first side of said periodic coupling grating.

23. The device of claim 1 wherein said light measurement system uses a prism to couple out at least one type of propagation from said waveguide.

24. The device of claim 1 wherein said light measurement system uses a grating to couple out at least one type of propagation from said waveguide.

25. The device of claim 1 wherein said light measurement system is configured to measure the power of at least one type of propagation to determine the presence of said substance to be sensed.

26. The device of claim 1 wherein said light measurement system is configured to measure the difference in the power between at least two types of propagation to determine the presence of said substance to be sensed.

27. The device of claim 1 wherein said substance to be sensed is in a liquid environment and said device is immersed in said liquid environment.

28. The device of claim 1 wherein said substance to be sensed is in a gaseous environment and said device is immersed in said gaseous environment.

29. A device for sensing comprising:

(a) a waveguide;

(c) an in-coupler for exciting at least one type of propagation into said waveguide on a first side of said periodic coupling grating towards said periodic coupling grating;

(d) an out-coupler for coupling out at least one type of propagation from said waveguide; and

(e) a light measurement system for measuring at least one parameter of at least one type of propagation coupled out from said waveguide, to determine the presence of said substance to be sensed.

30. A device for sensing comprising:

(a) a waveguide;

(b) a periodic coupling grating including a plurality of non-overlay segments and a plurality of overlay segments, said overlay segments of a given step height and step length, said overlay segments placed at a given period, said periodic coupling grating disposed on a given section of said waveguide, wherein said period satisfies a phase matching condition for an exchange of power between a single guided mode propagating in a forward direction to provide a first type of propagation and the single guided mode's reflection propagating in a reverse direction to provide a second type of propagation, wherein a step height difference between said step height and a height of said non-overlay segment is dependent on a substance to be sensed, and wherein said step height difference affects an exchange of optical power between the types of propagation;

31. A method for sensing comprising the steps of:

(a) exciting at least one type of propagation into a waveguide on a first side of a periodic coupling grating towards said periodic coupling grating, wherein said periodic coupling grating includes a plurality of non-overlay segments and a plurality of overlay segments, said overlay segments of a given step height and step length, said overlay segments placed at a given period, said periodic coupling grating disposed on a given section of said waveguide, wherein said period satisfies a phase matching condition for an exchange of power between at least two types of propagation, wherein a step height difference between said step height and a height of said non-overlay segment is dependent on a substance to be sensed, and wherein said step height difference affects an exchange of optical power between the types of propagation;

(b) coupling out at least one type of propagation from said waveguide; and

(c) measuring at least one parameter of at least one type of propagation coupled out from said waveguide, to determine the presence of said substance to be sensed.

32. The method of claim 31 wherein one of the types of propagation is distinctly excited.

33. The method of claim 31 wherein each of said plurality of overlay segments includes an overlay disposed on said given section of said waveguide and a linking layer joined to a face of said overlay opposite to said waveguide..

34. The method of claim 33 wherein said linking layer is deposited on said overlay.

35. The method of claim 33 wherein a height of said linking layer changes based on said substance to be sensed.

36. The method of claim 31 wherein each of said plurality of non-overlay segments includes a linking layer disposed on said waveguide.

37. The method of claim 36 wherein said linking layer is deposited on said waveguide.

38. The method of claim 36 wherein a height of said linking layer changes based on said substance to be sensed.

39. The method of claim 31 wherein said at least two types of propagation includes two guided modes, each guided mode in a given polarization.

40. The method of claim 31 wherein said waveguide is a multi-mode waveguide.

41. The method of claim 31 wherein said waveguide is a single-mode waveguide.

42. The method of claim 31 wherein said at least two types of propagation includes a single guided mode propagating in a forward direction to provide a first type of propagation and the single guided mode's reflection propagating in a reverse direction to provide a second type of propagation.

43. The method of claim 42 wherein exciting at least one type of propagation into said waveguide and coupling out at least one type of propagation from said waveguide are implemented in part using a shared coupler.

44. The method of claim 31 wherein measuring includes measuring the power of at least one type of propagation to determine the presence of said substance to be sensed.

45. The method of claim 31 wherein measuring includes measuring the difference in the power between at least two types of propagation to determine the presence of said substance to be sensed.

46. The method of claim 31 wherein said substance to be sensed is in a liquid environment.

47. The method of claim 31 wherein said substance to be sensed is in a gaseous environment.