WO2010030251A2

WO2010030251A2 - Integrated optical sensors operating in the frequency domain

Info

Publication number: WO2010030251A2
Application number: PCT/SG2009/000341
Authority: WO
Inventors: Carl Gang Chen; Bipin Sewakram Bhola; Seng-Thiong Ho
Original assignee: Agency For Science, Technology And Research
Priority date: 2008-09-15
Filing date: 2009-09-15
Publication date: 2010-03-18
Also published as: WO2010030251A3

Abstract

An integrated optical sensor operating in the frequency domain is disclosed together with a system and method for detecting the presence of a sample to be detected with the optical sensor. The optical sensor comprises a substrate for supporting the optical sensor, at least two substantially identical gratings or grating-based resonators, a waveguide directional coupler coupled to the gratings and arranged to receive light from a light source and propagate the light through to the gratings, and a flow cell for housing each of the gratings, each flow cell having a fluidic input and a fluidic output, one flow cell for receiving a reference fluid and another flow cell for receiving a fluid with the sample to be detected; the sample to be detected alters the effective refractive index of the propagating mode inside the grating by binding or being adsorbed by the grating to alter the optical frequency of the light that propagates through the grating. In an embodiment, the grating is surface functionalized with a sensitive layer to enable sample adherence or adsorption.

Description

INTEGRATED OPTICAL SENSORS OPERATING IN THE FREQUENCY DOMAIN

FIELD OF THE INVENTION

This invention relates generally to optical sensors operating in the frequency domain, and more particularly to grating-based integrated optical sensor devices for physical and biochemical sensing.

BACKGROUND

Physical or biochemical sensors based on optical transduction are well known in the art. Transduction mechanisms include fluorescence (Neuschafer 2003), surface plasmon resonance (United States Patent Number 5,478,755 to Attridge, and Homola 1999), Raman scattering (Vo-Dinh 1999), absorbance measurement (United States Patent Number 7.359,055 to Schneider), photon migration (United States Patent Number 6,771,370 to Sevick-Muraca), ellipsometry (Jin 1995), refractometry (Chaudhari 2002), reflectometry (Brecht 1995) and effective refractive index change in an integrated optical waveguide (Passaro 2007).

Generically, an integrated dielectric waveguide is made up of optically transparent dielectric material of high refractive index, and surrounding material of low index. The high index material forms the so-called "core" of the waveguide and the low index material forms the "cladding". Light is said to be in a guided mode if it is confined in the core due to the index contrast and can propagate freely along the length of the structure. The guided modes of a waveguide can be mathematically derived by applying Maxwell's Equations. A waveguide is said to be "single-mode" if its design only supports the propagation of the lowest order electromagnetic mode in quasi-transverse-electric (TE-like) and/or quasi-transverse-magnetic (TM-like) polarizations. The propagation speed relates to the so-called effective refractive index for the mode, which is determined by physical dimensions of the waveguide, its material composition and the wavelength of light used. Single mode is by far the most preferred configuration in the state of the art for sensing applications because it rids complications associated with light dispersion in a multi-mode structure.

Change in the single-mode effective index results from a sample analyte's modification to the waveguide's cladding layer. The cladding is usually formed by exposure to a gas or liquid phase that contains the sample analyte. An analyte can be a specific molecular layer adsorbed or bound, through antigen/antibody reaction for example (United States Patent Number 4,992,385 to Godfrey), to the waveguide core and cladding interface, or a homogeneous gas or liquid serving as the cladding. The evanescent electromagnetic field leaking from the core interacts with the modified cladding, leading to small but detectable changes in the effective index. Effective index variation can be measured using different integrated waveguide architectures. Architectures include interferometers such as the Mach- Zehnder type (United States Patent Number 4,515,430 to Johnson, and United States Patent Number 4,950,074 to Fabricius) and the Young type (Ymeti 2002, Ymeti 2003), integrated micro-cavity resonators such as micro rings and racetracks (Armani 2007), optical output grating couplers (Clerc 1993, United States Patent Number 4,815,843 to Tiefenthaler, United States Patent Number 5,071 ,248 to Tiefenthaler, United States Patent Number 5,033,812 to Yoshida) including athermal designs (United States Patent Number 7,203,386 to Krol), integrated Bragg gratings, integrated segmented waveguides (Van Lith 2005, United States Patent Number 6,956,982 to Heideman), integrated directional coupler (United States Patent Number 5,173,747 to Boiarski, and Luff 1996), integrated optical interference (United States Patent Number 5,120,131 to Lukosz), silicon slot waveguides (Almeida 2004), surface plasmon resonance-based waveguides (Harris 1999), anti-resonant reflecting waveguides (Benaissa 1998) and liquid-core hollow waveguides (Campopiano 2004). Both theoretical analyses and experimental results have demonstrated sensor resolution and detection limit relevant to real-world applications.

Presently, most of the state-of-the-art integrated optical sensors are based on interferometry, particularly the Mach-Zehnder interferometer (MZI) (Brosinger 1997, Luff 1998, United States Patent Number 6,429,023 to Gharavi US6429023, United States Patent Number 7,212,693 to Carr). MZI works by dividing incoming monochromatic light into two arms before combining the light again. The light interferes upon recombination, resulting in an output power signal dependent on the optical phase difference between the arms. If one of the arms— the measurement arm — encloses a sensing region where the mode's evanescent field interacts with a sample analyte, the output signal will acquire a phase originating from that specific interaction. MZI proves popular because its interaction length with an analyte is customizable. Longer interaction length translates into larger phase accumulation, therefore higher sensitivity. In particular, periodically segmented waveguide (PSW) MZI, a sub-class of the MZI sensor, uses periodically segmented waveguide in one or both arms of the interferometer to promote and enhance the interaction with the sample (Weissman 1997, Kinrot 2004). The two arms of a MZI, nominally balanced, also offer protection from environmentally-induced phase measurement errors. For example, temperature fluctuations can lead to small structural changes in the waveguide. In a two-arm balanced geometry, structural changes tend to be symmetric and resulting phase errors cancel out. Nevertheless, the very principle of phase detection, which gives MZI great sensitivity, suffers inherent drawbacks: fringe order and direction ambiguities, sensitivity fading near fringe extrema, phase errors induced by light source and detector noise, and temperature induced bias offset errors, to name a few (Heideman 1999). Phase modulating the MZI can fix some of these shortcomings but only at significantly increased sensor complexity and cost (Heideman 1999, United States Patent Number 6,618,536 to Heideman).

Integrated Bragg grating optical sensors exist in the art (Veldhuis 1998, United States Patent Number 7,212,693 to Carr). They differ in working principle from any phase-based intensity sensor. In simple terms, a Bragg grating is a one-dimensional diffraction grating that diffracts incoming light satisfying the so-called Bragg condition in the opposite direction. The Bragg condition arises from optical phase matching and can be expressed as P=mλO7(2Neff), where P is the grating period, λO is the free space wavelength of the light and Neff is the effective refractive index. The parameter m is the so-called grating order and is a positive integer: m≥1. The grating order reflects the fact that any integer multiple of the fundamental grating period when m=1 will cause light to diffract. Although higher order Bragg gratings (m>1) are less efficient than a first order device, because of their longer periods, they are comparatively easier to fabricate. As a result of the Bragg condition, an integrated Bragg grating sensor is a wavelength sensitive device and can convert a change in a waveguide's effective index (ΔNeff) to a corresponding shift in the light frequency (ΔλO): ΔλO=(2P/m)ΔNeff. The frequency shift can be detected with an optical spectrum analyzer (OSA) or other frequency detection means known to the art (Veldhuis 1998, and United States Patent Number 7,217,574 to Pien). Because Bragg grating sensors work in the frequency domain, they do not suffer the same drawbacks as phase-detection sensors. However, a sensor based on a single grating design can incur significant errors due to changes in the ambient environment as well as any frequency instability in the light source. For example, temperature drift can cause the grating structure to expand or contract, changing the grating period. The core/cladding material can experience the so-called thermal-optic effect whereby the material refractive indices change with temperature. Furthermore, any frequency instability in the light source may directly couple into the sensor output. Combined, these errors could match if not overwhelm the desired physical or biochemical parameter to be measured in magnitude. An integrated optical sensor comprising two cascaded Bragg gratings (International Patent Application Publication Number WO2006/008448 to Emmerson) provides partial protection against temperature induced errors. The cascade configuration allows changes in the Bragg frequency to be converted into changes in output optical power, detectable by a standard photodiode. However, the sensor's dynamic range is constrained by the available frequency overlap between the gratings, and its resolution and accuracy are limited by the light source and detector noise as well as the bandwidth of the individual grating. An integrated optical sensor comprising two parallel Bragg gratings is disclosed by Emmerson et al. in United States Application Number 2008/0204747 A1. The sensor is claimed to be temperature insensitive. However, by design, the two Bragg gratings used have characteristic wavelengths that are different from one another. Although a difference in Bragg wavelengths facilitates the detection of the back-diffracted light (also known as the reflected light), the difference translates into a mismatch in grating dimensions, in particular, the grating period. As a result, the insensitivity to environmental errors is not optimal. Furthermore, compared to resonator based sensors, sensors based on gratings alone suffer from inferior frequency resolution. The lack of spectral resolution is a problem that must be addressed if high resolution sensors are to be made from relatively low index contrast materials, such as polymers.

Therefore, there is a clear need for integrated optical sensors that are optimally insensitive to environment-induced errors; that offer spectral resolution at least one order of magnitude higher than the current state of the art; and that can be mass manufactured at low cost

SUMMARY

An aspect of the invention is an optical sensor comprising a substrate for supporting the optical sensor; at least two substantially identical gratings; a waveguide directional coupler coupled to the gratings and arranged to receive light from a light source and propagate the light through to the gratings; and a flow cell for housing each of the gratings, each flow cell having a fluidic input and a fluidic output, one flow cell for receiving a reference fluid and another flow cell for receiving a fluid with a sample to be detected; the sample to be detected alters the effective refractive index of the propagating mode inside the grating by binding or being adsorbed onto the grating to alter the optical frequency of the light that propagates through the grating.

An aspect of the invention is an optical sensor system comprising a light source for generating light for propagating to the optical sensor; a substrate for supporting the optical sensor; at least two substantially identical gratings; a waveguide directional coupler coupled to the gratings and arranged to receive light from the light source and propagate the light through to the gratings; and a flow cell for housing each of the gratings, each flow cell having a fluidic input and a fluidic output, one flow cell for receiving a reference fluid and another flow ceil for receiving a fluid with a sample to be detected; the sample to be detected alters the effective refractive index of the propagating mode inside the grating by binding or being adsorbed onto the grating to alter the optical frequency of the light that propagates through the grating. The optical sensor system may comprise an optical frequency detector such as an optical spectrum analyzer that is capable of detecting the optical frequency of the propagating light.

An aspect of the invention is a method of detecting the presence of a sample to be detected with an optical sensor, the method comprises generating light with a light source for propagating to the optical sensor, the optical sensor having at least two substantially identical gratings, a waveguide directional coupler coupled to the gratings and a flow cell for housing each of the gratings, the waveguide directional coupler arranged to receive light from the light source and propagate the light through to the gratings, each flow cell having a fluidic input and a fluidic output; receiving at one flow cell a reference fluid; receiving at another flow cell a fluid with a sample to be detected; and detecting the sample to be detected by the sample altering the effective refractive index of the propagating mode inside the grating by binding or being adsorbed onto the grating to alter the optical frequency of the light that propagates through the grating and by performing a differential frequency measurement between the reference and measurement frequency signals.

In an embodiment, the gratings are substantially identical in physical dimensions and material composition. The gratings may be uniform gratings with constant period. The gratings may be chirped gratings with period varying with position on the grating. The gratings may be apodized gratings with the grating strength parameter varying in amplitude with position on the grating. The gratings may be Bragg gratings. The gratings may be long period gratings. The long period grating may have a period in the range of 100 to 1000 microns. The gratings may be Bragg Grating Fabry-Perot (BGFP) resonators.

In an embodiment, the light source may be a tunable laser for generating a light with tunable wavelength. The light source may be a broadband source with a spectral bandwidth covering the frequency range of interest.

In an embodiment, the waveguide coupler coupling the light from the light source through to the gratings is a standard 4-port directional coupler, a Y-junction splitter, a multimode interference coupler or the like. The waveguide coupler may be an optical circulator coupled with a Y-junction splitter, a multimode interference coupler, or the like.

In an embodiment, each sensor may comprise a plurality of sensors multiplexed together with each sensor forming a sensor unit comprising two substantially identical gratings. The plurality of sensors, where each sensor unit of the plurality of sensor units may comprise two substantially identical gratings, may be multiplexed together in a cascade configuration. The plurality of sensor units, each comprising two substantially identical gratings, may be arranged in a parallel configuration. The waveguide directional coupler may comprise a multimode interference coupler for splitting the light that is propagated from the light source to the plurality of parallel sensor units. The waveguide directional coupler may be an arrayed waveguide grating for frequency selecting and channeling the selected light that is propagated from the light source through to the sensor unit operating at the same frequency. The waveguide directional coupler may comprise a Y-junction splitter for splitting the light that is propagated from the light source to the plurality of cascading sensor units. The waveguide directional coupler may comprise an optical circulator coupled with a Y-junction splitter for splitting the light that is propagated from the light source to the plurality of cascading sensor units. Each sensor unit, comprising two substantially identical gratings, operates at a separate frequency band to avoid signal crosstalk.

In an embodiment, the flow cell comprises a flow cell wall coming into contact with the waveguide and having a material refractive index that is less than the material refractive index of the waveguide. The material of the substrate, the waveguide directional coupler and the gratings may be polymers and the polymer material may be cyclic olefin copolymer (CXDC), polycarbonate (PC), polymethylmethacrylate (PMMA), polyethylene (PE), polyethylene terephthalate (PET), polystyrene (PS), polytetrafluoroethylene (PTFE), benzocyclobutene (BCB), polydimethylsiloxane (PDMS), SU-8, UV-15, NOA-61 , or Cytop. The distance between each flow cell may be less than 10mm. The surface of the grating may be functionalized with a sensitive layer to enable sample binding or adsorption. The gratings may be BGFP resonators having a cavity section and a mirror section wherein the surface of the entire resonator or the surface of the cavity section is functionalized with a sensitive layer to enable sample binding and adsorption. The gratings may be modulated in refractive index. The gratings may be modulated in amplitude. The amplitude modulation may take place on top of a ridge waveguide. The amplitude modulation may take place on sides of a ridge waveguide. The amplitude modulation may take place on top and the sides of a ridge waveguide simultaneously. The amplitude modulation may take place embedded at the interface between the substrate and the grating core. The gratings may have grating teeth embedded at the interface between the substrate and the grating core for amplitude modulation. The gratings with embedded amplitude modulation at the interface between the substrate and the grating core may be Bragg grating sensors or BGFP resonator sensors.

In an embodiment, the light transmitted is detected for transmission mode detection. The gratings may have an order (m) equal to or greater than 1 (m≥1 ). A material of the optical sensor may be a polymer material. The optical sensor may be for detection of samples in either gas phase or liquid phase. The frequency of the gratings of the optical sensor is designed for an operating wavelength of 1.31 microns, 1.55 microns, or the like.

In an embodiment, the optical sensor of any one of the preceding claims wherein the optical sensor is designed for single mode operation, and the light propagated through the gratings is for detecting the sample by differential frequency measurement. BRIEF DESCRIPTION OF THE DRAWINGS

In order that embodiments of the invention may be fully and more clearly understood by way of non-limitative examples, the following description is taken in conjunction with the accompanying drawings in which like reference numerals designate similar or corresponding elements, regions and portions, and in which:

FIG. 1 illustrates a schematic view of a Bragg grating based optical sensor in accordance with an embodiment of the invention;

FIG. 2A - 2E show various amplitude modulated gratings in accordance with embodiments of the invention including surface relief gratings of a single core layer (FIG. 2A) and dual layers (FIG. 2B), a double-sided lateral grating (FIG. 2C), an embedded grating (FIG. 2D) where the teeth of the grating are formed at the core-substrate interface, and an exemplary single-layer surface relief Bragg Grating Fabry-Perot (BGFP) resonator (FIG. 2E) in accordance with embodiments of the invention;

FIG. 3 shows a cross sectional view of the sensor of FIG. 1 in accordance with an embodiment of the invention;

FIG. 4A - 4C schematically illustrate a differential frequency measurement method in accordance with an embodiment of the invention;

FIG. 5 is a graph plotting the light frequency response curve for a Bragg grating-based optical sensor in accordance with an embodiment of the invention;

FIG. 6 illustrates a schematic view of a BGFP resonator based optical sensor in accordance with an embodiment of the invention;

FIG. 7 schematically illustrates the comparison of the light reflection frequency response of a BGFP resonator with that of a Bragg grating serving as one of two resonator mirrors in accordance with an embodiment of the invention;

FIG. 8A - 8B schematically illustrate embodiments of the invention where Bragg gratings (FIG. 8A) and BGFP resonators (FIG. 8B) are coupled with Y-junction splitters; FIG. 9A - 9B schematically illustrate embodiments of the invention where Bragg gratings (FIG. 9A) and BGFP resonators (FIG. 9B) are coupled with 1x2 multimode interference (MMI) couplers;

FIG. 10 schematically illustrates an embodiment of the invention with measurement and reference flow cells defined in the cavities of the BGFP resonators;

FIG. 11 schematically illustrates an embodiment of the invention whereby sensor multiplexing is achieved by cascading sensor units composed of Bragg gratings and/or BGFP resonators in a branching tree pattern;

FIG. 12 schematically illustrates an embodiment of the invention whereby sensor multiplexing is achieved with a 1xN multimode interference coupler coupling to multiple sensor units composed of Bragg gratings and/or BGFP resonators;

FIG. 13A - 13B schematically illustrate all (FIG. 13A) or partial (FIG. 13B) surface functionalization in accordance with an embodiment of the invention;

FIG. 14A - 14C schematically illustrate reference and measurement responses prior to (FIG. 14A), during (FIG. 14B), and after (FIG. 14C) sample introduction in accordance with an embodiment of the invention;

FIG. 15 shows a flow chart of a method in accordance with an embodiment of the invention; and

FIG. 16 illustrates schematically an exemplary optical sensor system in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

Embodiments of the invention relate to new grating-based integrated optical sensors that have high sensitivity, large dynamic range, as well as built-in insensitivity to light source instabilities and environmental disturbances such as temperature fluctuations. Implemented with polymer materials, the sensors are low-cost and can readily serve as disposable chips and cartridges for a portable diagnostic system. Embodiments of the invention fall into the category of effective refractive index change in an integrated optical waveguide. More specifically, embodiments of the invention operate on the principle of analyte-induced effective refractive index change in an integrated dielectric waveguide. These sensors share a common architecture. Light from a source with broad spectral bandwidth or a tunable laser with tunable wavelength is coupled into two Bragg grating waveguides. Both gratings are substantially identical in terms of physical dimensions_' as well as material composition. The identical design and construction lead to identical optical modal confinement within the waveguides and maximized insensitivity to environmental disturbances. One of the two gratings interacts with molecules from a sample analyte adsorbed or bound through chemical reaction onto the grating surface or homogeneous gas or liquid, and forms a measurement arm, while the other serves as a reference arm. Each arm selects and directs to the sensor output light of a particular frequency representative of the effective refractive index of that arm. The difference in output frequency between the two arms can be measured and is directly proportional to the physical or biochemical parameter to be sensed (the so- called measurand). Similar to MZI sensors, grating sensors based on this balanced two arm architecture are protected from certain environmental errors. Temperature drifts for example, result in equal frequency shifts in both arms. A differential frequency measurement ensures that errors due to temperature cancel out, leaving the measurand insensitive to temperature and resulting in a large dynamic range and an absolute measurement. The architecture also protects against errors induced by any source instability because drifts in input light frequency are equally experienced by both arms. Furthermore, the architecture is tolerant to fabrication imperfections, e.g., slight variations in grating period or duty cycle. In one embodiment selected for ease of manufacture, the Bragg grating is a higher-order grating (m>1 ) having a period of >1 micron, which can be patterned with a conventional UV mask aligner. The waveguide substrate and core are made from Pyrex glass and SU-8 photoresist, respectively. In an alternate embodiment, the substrate material is cyclic olefin copolymer (COC).

Bragg gratings used in all embodiments of the invention can be uniform gratings — gratings of constant period, or chirped gratings — gratings whose period varies with position (often linearly), or apodized gratings — gratings usually designed for minimal spectral side lobes with the grating strength parameter varying in amplitude with position (often decreasing towards the ends of the grating). They can be either amplitude-modulated or phase-modulated or both. The above sensor architecture is not limited to the use of Bragg gratings. Long-period gratings (Vengsarkar 1996) can replace Bragg gratings as frequency selective sensing elements. A nominal long-period grating (LPG) has a period in the range of 100 to 1000 microns. Because of the long period, unlike the Bragg grating, the LPG does not produce back-diffracted light. Through phase matching and resonant coupling, the LPG couples the fundamental guided mode of the core to the lossy forward propagating modes of the cladding, yielding a transmission spectrum with attenuated peaks at the resonant wavelengths. When the differential frequency measurement is performed on the transmission spectra from the reference and the measurement LPGs, the result is again insensitive to temperature induced errors. Another implementation of the invention replaces the Bragg grating in each of the sensor arms with a Bragg Grating Fabry-Perot (BGFP) resonator. A BGFP resonator uses Bragg gratings as wavelength selective mirrors at both ends of a Fabry-Perot cavity. For the optical resonance to occur, the Bragg mirrors are substantially identical. Optical resonance inside the FP cavity dramatically enhances the frequency response from that of an ordinary Bragg grating. A narrower frequency response translates directly into increased sensor performance. In a two-arm balanced geometry, these BGFP sensors are again insensitive to environmentally induced errors. In one embodiment for commercial manufacture and portable instrumentation, the bandwidth of the BGFP sensor is <20 GHz and preferably <10 GHz. Compared to the 200 GHz bandwidth offered by a grating-based sensor of similar polymer material system and grating dimensions, the BGFP-based sensor offers significant gains in sensor resolution and sensitivity.

Sensors based on the invention can be multiplexed readily. In one multiplexing scheme, individual sensors are cascaded one after another in a branching tree pattern while satisfying the physical constraints of the instrument system such as source bandwidth, intensity, detector bandwidth, device form-factor, etc. Each dual-arm sensor can be designed for a specific central frequency, all appropriately spaced from each other so as to avoid output frequency cross-contamination. In another multiplexing scheme, sensors are organized in a parallel architecture. Input light is split into multiple outputs by a multimode interference (MMI) power coupler. Each output is then guided into a sensor.

In order to be commercially viable, integrated optical sensors must be mass manufactured at low cost Because of their biocompatibility, low cost, ready availability, large selection, wide process latitude and existing technical know-hows, polymers will be the ideal material of choice for commercial sensor work. Polymers can be used as the substrate material, and to make up the core or the cladding. Furthermore, for an integrated optical sensor to be commercially successful, it most certainly will have to incorporate fluidic handling. Because polymer-based microfluidic devices are the norm today, it makes good sense to have the sensors made of the same material. All embodiments of the invention can be mass-manufactured at low cost using polymer materials. Polymers can form the sensor substrate, core, or cladding. The selection of a particular material system depends on the required index contrast, fabrication process and cost objective. Polymers include but not limited to cyclic olefin copolymer (COC), polycarbonate (PC), polymethylmethacrylate (PMMA), polyethylene (PE), polyethylene terephthalate (PET), polystyrene (PS), polytetrafluoroethylene (PTFE), benzocyclobutene (BCB), polydimethylsiloxane (PDMS), SU-8, UV-15, NOA-61 , Cytop, and the like.

Referring to FIG. 1 , one embodiment is shown where the sensor 10 is constructed with two Bragg gratings 18 and 20 connected to a standard 4-port waveguide directional coupler 12 on a substrate 11. All waveguides may operate in single mode. The Bragg gratings are identical not only in physical dimensions but also in material composition, and they are. placed in close proximity to minimize any potential thermal gradient and environmental dissimilarity. The reference arm 20 and the measurement arm 18 are shown as Bragg gratings in this embodiment. Each grating is completely enclosed in a flow cell 14, 16 with defined fluidic input 3Oa₁ 30b and output ports 32a, 32b. The cells can be filled with gas-phase samples and reagents, but in most of the applications envisioned, they will be used to accommodate liquid phase. Flow cells are bounded by walls 22 and 24. Parts of the flow cell walls 22 which come into direct contact with the grating cores are made of lower index material to reduce potential optical power loss. The grating whose flow cell is filled with the sample analyte solution constitutes the measurement arm and the grating whose flow cell is filled with a reference liquid constitutes the reference arm. For the sensor to be insensitive to thermal effects, the reference liquid is placed in thermal equilibrium with the sample solution prior to the start of any measurement. Depending on the fabrication process used, the gratings can be modulated either in amplitude (the so-called amplitude gratings) or directly in refractive index (the so-called phase gratings) or both. Amplitude modulation can be either atop (the so-called surface relief grating), or on the side(s) of the waveguide (the so-called lateral grating), or both. The light input 34 is shown with light output 1 (36), light output 2 (38), and light output 3 (40). The flow cells are provided with cover 26 and flow cell walls 22.

FIG. 2A - FIG. 2E shows examples of amplitude modulated gratings, of both the surface relief and the lateral types. Two kinds of surface relief gratings are shown: the single core layer 50 and the dual layer 52. In a single-layer surface relief grating, grating teeth are formed directly in the core material. In a dual-layer grating, teeth are not formed in the core but in the cladding atop the core. The refractive index of the cladding material is lower than that of the core. A double sided lateral grating 54 is shown in FIG. 2C and an embedded grating 56 with core 55 and substrate 57 is shown in FIG. 2D. FIG. 2E illustrates a BGFP resonator 58. A BGFP resonator has a cavity section 59 which is capped at both ends by Bragg gratings 53 serving as wavelength selective mirrors. For optical resonance to occur, the Bragg mirrors are substantially identical. While the illustrated Bragg mirrors are the single- layer surface relief type, other designs such as the dual-layer, the lateral or the embedded types are equally valid.

FIG. 3 shows a cross sectional view of the sensor embodiment from FIG. 1. The cross section is taken midway between the fluid input ports 30a, 30b and the fluid output ports 32a, 32b in a direction perpendicular to the two gratings, where n^ is the refractive index for the substrate material 12, n, is the material refractive index for the waveguide core 18 and 20, n₂ is the refractive index for the materials making up the walls 24 of the flow cells, n_ref is the material refractive index for the liquid in the reference flow cell 16 and n^ is the material refractive index for the liquid in the measurement flow cell 14. The flow cells are provided with cover 26. In an embodiment the material system is chosen for single mode operations for all waveguides, including the directional coupler and the Bragg gratings 18 and 20 (n_sub<n₁ and n^, rwdii). In particular, where the flow cell walls 24 come into contact with the grating cores, n₂ is less than n^ in order to, minimize any power loss during mode propagation through the walls. An embodiment of the invention operates in the single mode regime.

An optical sensor system 270 in accordance with an embodiment of the invention is shown in FIG. 16. The components of the system include a light source 272 optically coupled with an optical sensor 10 that is optically coupled with a detector 274. Further processing means such as a computer 276 may be provided with the detector 274 for processing the propagated light signals received at the detector from the optical sensor 10. Sample detection may be performed through a differential measurement method whereby the difference in light frequency between the two arms of the optical sensor 10 is used for detection. The detector 274 may be an optical spectrum analyzer for detecting the optical frequency of the propagating light and the processing means may be a computer 276 with additional processor 278, memory 280, input means 282 such as a keyboard and output means 284 such as a display. The computer 276 may control and interface with the components of the system such as the light source 272 and the detector 274; and there might be direct control/interface between the light source 272 and the detector 274. It will be appreciated that the system may have different configurations, for example, each component may be configured separately as separate modules or units. In other configurations, the components of the system 270 may be integrated together, for example, the light source and the detector may be integrated with the processing means together with the optical sensor to form a single unit and the like.

In a typical measurement run, light from a light source 272 such as for example a broadband source or a tunable laser is launched into one arm of the directional coupler of the optical sensor 10. Through evanescent field coupling, light exits the directional coupler in both arms before being guided into the gratings. In the measurement arm, the presence of the liquid analyte or molecular layer adsorbed/bound to the grating surface changes the effective refractive index of the propagating mode. The back-diffracted light from the Bragg grating in the measurement arm shifts in frequency according to the change in the effective index. Portion of this backward propagating light, after passing through the directional coupler, emerges at light output 3 (40) of FIG. 1. The forward propagating light in the measurement arm exits the sensor at light output 1 (36), missing the frequency component that has been back-diffracted. Similarly, the back-diffracted light from the Bragg grating in the reference arm also emerges from light output 3 (40), while the forward propagating light, missing the back- diffracted frequencies, exits via light output 2 (38). Frequency measurement can be conducted by the detector 274 (FIG. 16) either at output 1 and 2 or at output 3, the choice of which depends on the extent of the frequency overlap from the two arms as well as the desired signal-to-noise ratio. In all embodiments of the invention, the reference and the measurement gratings are designed to be substantially identical so as to ensure identical optical modal confinement and maximized insensitivity to environmental variations, such as for example, temperature fluctuations. The identical design can be implemented in reality via laser interference lithography for example. Because of this substantially identical construction, the transmitted light from output 1 and 2 (36, 38) is generally preferred since the reflected light from output 3 (40) may overlap sufficiently in frequency as to be unresolvable by the detector. While the reflection mode of detection may provide better signal-to-noise ratio when the sensor is composed of Bragg gratings, the transmission mode of detection provides superior signal-to- noise when the sensor is composed of BGFP resonators (FIG.6).

An exemplary device for the embodiment shown in FIG. 1 - FIG. 3 is as follows: the operating light wavelength is 1.31 microns. The substrate material is cyclic olefin copolymer (COC) with a refractive index of n^ = 1.53. The waveguide core is SU-8 photosensitive polymer with a refractive index of n^ = 1.57. Core width and height are 3 microns and 2 microns, respectively. Single-layer surface relief grating, a second-order (m=2) device, has a period of approximately 1 micron, duty cycle of 0.25, grating height of 150 nm and length of 2 mm. The directional coupler has a coupling length of 9 mm and a gap of 2 microns. The separation between the two grating arms is 2 mm and the dimensions of the flow cells are 5 mm in length, 300 microns in width, and 75 microns in height.

FIG. 4A - FIG. 4C schematically illustrate the differential frequency measurement method, using back-diffracted light from light output 3 (40) of FIG. 1 as an example. Differential measurement can be similarly implemented using the transmitted light spectra from light output 1 and 2 (36, 38) of FIG. 1. Optical frequency can be measured either with an optical spectrum analyzer or with other means known to the art. In frequency domain, the back-diffracted light from a Bragg grating exhibits a main lobe centered at a nominal design frequency, 1.31 microns for example, and numerous weaker side- lobes.

FIG. 4A shows two frequency peaks 60 after an experiment is conducted with both the measurement and the reference flow cells filled with the same liquid solution, for example, high purity de-ionized water. The left peak 61 is the readout from the reference arm and the right 63 from the measurement arm. The difference in frequency between the two peaks is denoted Δf. Ideally, for all embodiments of the invention, Δf should equal to zero because the gratings are identical. However, because of fabrication, material and measurement imperfections, Δf is unlikely to be zero even though the gratings are designed to be identical. As illustrated, FIG. 4A - FIG. 4C assume that the reflected spectra from both gratings are well separated and that Δf is resolvable by the detector. If however, the reflected spectral peaks at light output 3 (40) of FIG. 1 are substantially close to each other and they overlap at the detector as to be unresolvable, then the transmitted spectra from light output 1 and 2 (36, 38) are used instead. The illustration for the differential frequency measurement method using transmitted signals is essentially the same as FIG. 4A - FIG. 4C, except that the reflection spectral waveforms are now replaced by their transmission counterparts. In FIG. 4B, the temperature of the sensor has increased. As a result, both arms experience the same temperature-induced frequency shift and their respective peaks move in the same direction by the same amount Δf_τ, as compared to FIG. 4A. Nevertheless, the difference between the peaks remains unchanged at Δf, demonstrating the sensor's insensitivity to temperature variations. FIG. 4B also demonstrates graphically 62 how the two-arm balanced architecture protects against source instabilities as well as certain fabrication imperfections. If the light source, e.g., a tunable laser, undergoes any frequency drift, the drift is indistinguishable from an effective index change, but because both aims experience the same drift, errors cancel out. Fabrication imperfections, slight variations in grating period or duty cycle for instance, can give rise to measurement errors as the ambient environment varies. However, if imperfections are present symmetrically in both arms which are highly probable if for example laser interference lithography is employed to make the gratings, errors arising from the imperfections match and again cancel out. In FIG. 4C, the measurement flow cell is flushed and replaced with a sample analyte which causes the effective refractive index in the measurement arm to increase. The measured frequency red-shifts and the frequency difference becomes -Δf', with the negative sign denoting the direction of the shift as shown in the graph 64. The quantity M=Δf+Δf therefore is a true and absolute measurement of the analyte-induced effective index change. The scheme as outlined in FIG. 4A - 4C can be equally applied to the transmitted spectra from light output 1 and 2 (36, 38) as shown in FIG. 1 , leading to the discovery of the same quantity M. FIG. 5 plots a typical light frequency response curve 70 for the above disclosed Bragg grating-based optical sensor with light reflection wavelength versus analyte refractive index.

The sensor architecture illustrated by FIG, 1 is not limited to the use of Bragg gratings. Long-period gratings can replace the Bragg gratings as frequency selective sensing elements. The grating period of a LPG typically ranges from 100 to 1000 microns. Because of the long period, unlike the Bragg grating, the LPG does not produce back-diffracted light. However, through phase matching and resonant coupling, the LPG can couple the fundamental mode of the core to the lossy forward propagating modes of the cladding. The coupling and the subsequent energy loss give rise to a transmission spectrum with attenuated peaks at the resonant wavelengths. When the differential frequency measurement is performed on the transmission spectra from the reference and the measurement LPGs, the result is again insensitive to temperature and source induced errors.

FIG. 6 illustrates an embodiment of a BGFP resonator-based optical sensor 80. Both Bragg gratings in FIG. 1 are replaced by BGFP resonators 82 and 84. A BGFP resonator is a resonance cavity capped at both ends by substantially identical Bragg gratings. The cavity, as illustrated, is a section 83, 85 of a straight ridge waveguide without any grooves. The gratings serve as highly wavelength selective reflective mirrors, setting up the cavity for narrow-band optical resonance. As in FIG. 1, differential frequency detection can be done either by examining the back-diffracted light from the resonators or the forward transmitted light The transmitted spectrum is preferred because it has better signal-to-noise ratio and avoids potential signal interference if the frequencies from the two arms do overlap. Light coupling into the measurement and reference arms is achieved "with a standard 4-port integrated directional coupler and the two-arm balanced architecture again provides protection against common error sources. The Bragg mirrors — Bragg grating used as a cavity mirror — of a BGFP resonator can be amplitude gratings and/or phase gratings. As in FIG. 2, an amplitude-modulated Bragg mirror can take on a variety of different shapes. More specifically, it can be the surface-relief type, the lateral type, and/or the embedded type.

FIG. 7 is a graphic comparison 90 of the light reflection frequency response of a BGFP resonator 98 with that of a Bragg mirror 100. As shown, the full-width-half-max (FWHM) bandwidth 94 of a BGFP resonance peak is at least one order of magnitude narrower than the FWHM 102 of the Bragg mirror. The number of resonance peaks 92 inside the main lobe depends on the free spectral range of the resonator as well as the bandwidth of the Bragg mirrors (Yariv 1997). The spectral envelope 96 is shown and the bandwidth of the BGFP sensor is defined as the width of a single resonance peak, which depends on the reflectivity of the Bragg mirrors and the resonator loss. In an embodiment, the bandwidth of the BGFP sensor is <20 GHz and preferably <10 GHz.

FIG. 8A illustrates an alternative embodiment 110 of a Bragg grating-based sensor. Compared to the embodiment of FIG. 1, the 4-port directional coupler is replaced by an optical circulator 114 coupled with a Y-junction splitter 118. The optical circulator is a commercially available device which separates the forward and backward propagating light. The circulator directs the input light into the Y- junction which splits the light in two and guides them into the gratings, reference Bragg grating 126 and measurement Bragg grating 120, shown as a pair of Bragg gratings or a sensor unit 128. The back-diffracted light from the two arms, upon merging through the Y-junction and entering the circulator, gets directed to light output 3 (116). Transmitted light exit the sensor from light output 1 and 2 (122, 124), the same as in FIG. 1. It will be appreciated that an embodiment can be configured without an optical circulator, in particular when the method of detection is via transmission mode detection. FIG. 8B illustrates an alternative embodiment 130 of a BGFP resonator-based sensor, which is identical to FIG. 8A except that the Bragg gratings 120 and 126 forming the sensor unit 128 have been replaced by BGFP resonators, shown as a sensor unit 132. One more sensor geometry 140 is shown in FIG.9A where the only change from FIG.8A is the replacement of the Y-junction splitter with a 1X2 multimode interference (MMI) coupler 144. Similarly, FIG. 9B is another embodiment 150 like FIG. 8B with the replacement of the Y-junction splitter with a 1X2 MMI coupler 154. MMI devices are well known to the art (Soldano 1995). They can have multiple input and output ports and can be easily incorporated into an integrated planar waveguide structure.

The embodiments illustrated by FIG. 6, FIG. 8B and RG. 9B have both BGFP resonators completely enclosed by their respective flow cells. For each arm, analyte or reference solution covers both the Bragg mirrors and the resonant cavity. A changing refractive index causes the envelope as well as the FP resonant peaks of the frequency spectrum (as shown in FIG. 7) to shift in unison. In an alternative embodiment 160 shown in FIG. 10, only the resonator cavities where the grating grooves are missing are enclosed by flow cells 164, 166. The Bragg mirrors 162 and 168 are outside of the flow cells and therefore not exposed to any solution. As the refractive index varies, the spectral envelope, as defined by the Bragg mirrors, stays fixed. Due to the changing resonance condition however, the resonant peaks move with respect to the envelope.

The 4-port directional coupler (FIG. 1 and FIG. 6), the Y-junction splitter (FIG. 8) and the MMI coupler (FIG. 9) based sensor architectures disclosed in the present invention can be multiplexed to increase measurement throughput. FIG. 11 shows one embodiment 170 of a multiplexed sensor arranged in a cascading tree structure using the Y-coupler geometry. The sensor is made up of multiple sensor units 172. Each unit 172 is itself a sensor realized in embodiments disclosed in FIG. 8. To avoid signal crosstalk, each sensor unit is designed to operate at a separate frequency band centered on A_N (U=I , 2, 3, ...). Either the Bragg grating 176 or the BGFP resonator 174 can be used in each of the sensor units. FIG. 12 shows another embodiment 180 of a multiplexed sensor which utilizes a 1xN MMI coupler 182 to split the incoming source beam 188. Because the sensor units e.g. 184,186 are now arranged in parallel, signal crosstalk is no longer a concern, meaning that different sensor units can share the same center frequency; A₁ to A_N need no longer to be distinct. When implemented over a sufficiently compact form factor, environmental variations over different sensor units, thermal gradient in particular, can be sufficiently minimized. If this is the case and A₁ = A₂ =...= A_N, the reference arm can be removed from individual sensor units, and a common reference arm shared by all units. Yet another embodiment of a multiplexed sensor replaces the MMI device with an arrayed waveguide grating (AWG) which channels the input light at a particular center frequency A_N to the corresponding sensor unit with the same center frequency. Use of the AWG may boost the signal-to-noise ratio during detection.

In one embodiment, the form factor of the sensor is that of a microscope glass slide, approximately 75 mm by 25 mm. Both the substrate and the waveguide materials are polymers, including but not limited to cyclic olefin copolymer (COC), cycto olefin polymer (COP), polycarbonate (PC), polymethylmethacrylate (PMMA), polyethylene (PE), polyethylene terephthalate (PET), polystyrene (PS), polytetrafluoroethylene (PTFE), benzocyclobutene (BCB), polydimethylsiloxane (PDMS), Cytop and various photo-sensitive polymers such as SU-8, UV-15 and NOA-61. Both flow cells are also made of polymers and sealed with a polymer cover whose contact interface is functionalized with a hydrophilb layer to assist with fluid filling. In another embodiment, the substrate material is COC and the waveguides are made from SU-8 photoresist. The cover is a hydrophilic adhesive. The part of the flow cell wall in direct contact with the waveguide core is composed of UV or thermal curing epoxies. In one embodiment selected for low optical loss, the Bragg gratings or BGFP resonators are designed for an operating wavelength of 1.31 microns. In an embodiment selected for ease of manufacture, the Bragg grating has an order of m>1 and a period of approximately 1 micron or above. In another embodiment, the substrate material is glass, Pyrex 7740 for instance. Of course, the sensors may be configured for different operating wavelengths, for example 1.31 microns, 1.55 microns and the like.

In one embodiment, in order to minimize any thermal gradient and other environmental dissimilarity, the distance between the two flow cells is kept at <10 mm and preferably at <1 mm.

In one embodiment for detecting adsorbed or bound molecules, all or portions of the Bragg grating structures in FIG. 1 and FIG. 8A can be surface-functionalized with a sensitive layer, e.g., a specific antibody for example. In case of partial functionalization of the two gratings, the functionalized sections should be symmetric between the two arms in order to keep the material system in balance as to minimize any material mismatch induced errors. All or portions of the BGFP resonators in FIG. 6 and FIG. 8B can be similarly functionalized. In one embodiment, only the cavity sections of the resonators are functionalized. FIG. 13A and FIG. 13B schematically illustrate all 190 (FIG. 13A) or partial 200 (FIG. 13B) surface functionalization 192 with a "Y"-shaped antibody in accordance with the embodiment of the invention.

In a method of measurement embodiment, the reference and measurement gratings or resonators are both functionalized with a sensitive layer. A reference liquid which does not react with the sensitive layer is introduced into the reference flow cell. An analyte solution containing the molecules to be detected, a specific antigen for example, is introduced into the measurement flow cell. Adsorption or binding reactions take place causing the sensitive layer to be activated in the measurement arm. Both the measurement and the analyte flow cells are subsequently flushed with the non-reactive reference solution, leaving both arms in an identical state except for the already activated sensitive layer in the measurement arm. Results from a frequency measurement correspond solely to the index change effected by the adsorbed or bound analyte molecules. FIG. 14A - 14C schematically illustrate the reference and measurement responses 210, 220, 230 prior to (FIG. 14A) 212, 214, during (FIG. 14B) 222, 224, and after (FIG. 14C) 232, 234 sample introduction for reference and measurement flow cells, respectively, in accordance with the embodiment of the invention. In one embodiment of FIG. 1 and FIG. 8A, back-diffracted light from light output 3 is used because of inherently better signal to noise ratio. In yet another embodiment, light from light output 1 and 2 are used because the transmitted spectrum from each grating arm can be individually detected and resolved in case that the frequency components from the two arms overlap, leading to significant interference at light output 3. In one embodiment of FIG. 6 and FIG. 8B, transmitted light spectrum from each BGFP sensor arm is measured because of higher signal to noise ratio.

FIG. 15 shows a flow chart 250 of a method in accordance with an embodiment of the invention. The light is generated 252 with a light source for propagating to the optical sensor. The light is propagated 254 through the waveguide to two substantially identical gratings. A reference fluid is received 256 at one flow cell (reference flow cell) and the sample in a fluid is received at another flow cell (measurement flow cell). The sample is adsorbed or bound 258 to the sensitive layer functionalized on the grating surface. The sample is detected 260 by altering the effective refractive index of the propagating mode inside the grating by binding or being adsorbed onto the grating to alter the optical frequency of the light that propagates through the grating. Optical frequency detection is done via a detector such as an optical spectrum analyzer and/or the associated processing means such as a computer. The steps and procedures to functionalize polymeric surfaces are well known to those experienced in the art. An example procedure to functionalize a SU-8 surface with an antibody layer may include the following steps (Deepu 2008): 1. treatment of the SU-8 surface with oxygen plasma; 2. introduction of glycine molecules to the treated SU-8 surface to serve as crosslinkers; 3. activation of the sample with a mixture of 1 -Ethyl-3-[3-dimethyl amino]propyl carbodiimide hydrochloride (EDC) and N-Hydroxy Succinimide (NHS); 4. incubation and immobilization of the antibody (e.g., human IgG molecules) on the SU-8 surface; and 5. blocking the non-specific adsorption sites on the surface by using bovine serum albumin (BSA).

One of the high throughput low cost ways to manufacture the embedded Bragg gratings and/or the embedded BGFP resonators is by the so-called scanning beam interference lithography (SBIL) technique (Chen 2003, Konkola 2003, and United States Patent Number 6,882,477 to Schattenburg). SBIL may be employed to pattern the substrate polymer directly or to produce a stamping tool in silicon, quartz, glassy carbon or metal (e.g., nickel or nickel alloys) through lithographic patterning, pattern transfer through dry or wet etching, and/or electroplating. This stamping tool will preferably contain multiple sensor units for batch processing with all the requisite grating and resonator patterns. The stamping tool will facilitate pattern replication into a substrate polymer by serving as the master, where the fabrication technique of nano-imprint, hot embossing and/or compression injection molding, all well known to those experienced in the art, may be applied. Once the grating teeth have been patterned into the substrate polymer, the core polymer layer (e.g., SU-8) can be deposited and patterned on top in order to form the embedding grating design.

It will be appreciated that as discussed in the examples above, embodiments of the invention may take different form. For example, in an embodiment, depending on the fabrication process used, the Bragg gratings can be modulated either in amplitude or directly in refractive index. In one embodiment, amplitude modulation takes place on top of a straight ridge waveguide (FIG. 1 and FIG. 6). In another embodiment, amplitude modulation takes place on the sides of a straight ridge waveguide. In yet another embodiment, amplitude modulation takes place simultaneously atop and on the sides of a straight ridge waveguide.

In one embodiment, the Bragg gratings have a constant period. In another embodiment, the Bragg gratings are chirped so that the grating period varies slowly with position, often linearly. In a further embodiment, in order to minimize the sidelobes in the frequency spectrum, the Bragg gratings are apodized so that the amplitude of the grating strength varies with position, often made smaller towards the ends of the grating.

In embodiments of the invention, grating based integrated optical devices for physical and biochemical sensing are disclosed. Specific to embodiments of the present invention, sensing takes place in the frequency domain with frequency defined as the light reflection or transmission frequency at which the input light energy has significant reflection from a grating-based sensor or transmission through a grating-enabled resonator sensor.

An optical sensor realized in an embodiment comprises two separate Bragg grating waveguides or BGFP resonators that are substantially identical in physical dimensions as well as material composition. Having the gratings substantially identical both in physical dimensions as well as material composition ensures that the optical modal confinement is substantially identical as well as to ensure that the insensitivity to environmental disturbances, such as for example temperature variations, is maximized. Such identical construction and precise manufacturing is achievable by known techniques in the industry, such as for example with laser interference-lithography. In a typical sensing operation, light from a source with broad spectral bandwidth or a tunable laser with tunable wavelength is coupled into both Bragg gratings. One of the two gratings constitutes a measurement arm and interacts with a sample analyte of gas or liquid phase, while the other serves as a reference arm. Each arm selects and directs to the sensor output light of a particular frequency representative of the respective arm's effective refractive index. In particular, the presence of the physical or biochemical sample interacts with the evanescent field of the guided electromagnetic mode in the measurement arm, changing the mode's effective refractive index accordingly. The difference in output frequency between the two arms can be measured and is directly proportional to the measurand, i.e. the physical or biochemical parameter to be sensed. Unlike sensors based on single gratings which exist in prior art, Bragg grating sensors based on this balanced two arm design are insensitive to measurement errors induced by the ambient environment and the light source. Environmental disturbances such as temperature fluctuations as well as frequency instabilities of the light source are equally experienced by both arms. As a result, the differential frequency measurement method cancels out any related artifact, leading to improved device sensitivity and accuracy, and resulting in large dynamic range and absolute measurements.

Another embodiment of the invention replaces the Bragg grating in each of the sensor arms with a BGFP resonator. The resonator utilizes Bragg gratings as wavelength selective mirrors at the ends of a Fabry-Perot cavity. Compared to a grating of similar period and footprint, the BGFP resonator can have a frequency response that is narrower by an order of magnitude or more, giving rise to greatly improved sensor resolution and detection limit. In a two-arm balanced geometry, these BGFP resonator-based sensors are again immune to environmental disturbances as well as light source instabilities.

Measurement multiplexing can be achieved by cascading as many balanced sensors as possible in a branching tree pattern or arranging them in a parallel architecture, provided that they meet the physical constraints of the instrument system such as source bandwidth, power detection limit, detector bandwidth, device form factor, etc. Each sensor can be designed for a specific central frequency, all appropriately spaced from each other so as to avoid output frequency crosstalk.

Physical or biochemical sensors derived from embodiments of the present invention can be polymer based, and as a result, can be manufactured with low cost in large quantities through hot embossing, UV curing imprint lithography, and/or micro injection molding. These sensors have small form factors and can be used in portable analytical instruments as disposable chips or cartridges. They are also suited for integration into polymer-based microfluidic products, and can find applications in point-of- care medical diagnostics, real-time environmental monitoring, food and drug screening, security monitoring, drug discovery and military applications.

While embodiments of the invention have been described and illustrated, it will be understood by those skilled in the technology concerned that many variations or modifications in details of design or construction may be made without departing from the present invention. References:

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Claims

CLAIMS:

1. An optical sensor comprising: a substrate for supporting the optical sensor; at least two substantially identical gratings; a waveguide directional coupler coupled to the gratings and arranged to receive light from a light source and propagate the light through to the gratings; and a flow cell for housing each of the gratings, each flow cell having a fluidic input and a fluidic output, one flow cell for receiving a reference fluid and another flow cell for receiving a fluid with a sample to be detected; the sample to be detected alters the effective refractive index of the propagating mode inside the grating by binding or being adsorbed onto the grating to alter the optical frequency of the light that propagates through the grating.

2. The optical sensor of claim 1 wherein the gratings are substantially identical in physical dimensions and material composition.

3. The optical sensor of claim 1 or 2 wherein the gratings are uniform gratings with constant period.

4. The optical sensor of claim 1 or 2 wherein the gratings are chirped gratings with period varying with position on the grating.

5. The optical sensor of claim 1 or 2 wherein the gratings are apodized gratings with the grating strength parameter varying in amplitude with position on the grating.

6. The optical sensor of any one of the preceding claims wherein the light source is a tunable laser for generating a light with tunable wavelength.

7. The optical sensor of any one of claims 1 -5 wherein the light source is a broadband source with a spectral bandwidth covering the frequency range of interest.

8. The optical sensor of any one of the preceding claims wherein the gratings are Bragg gratings.

9. The optical sensor of any one of claims 1 -7 wherein the gratings are long period gratings.

10. The optical sensor of any one of claims 1 -7 wherein the gratings are Bragg Grating Fabry-Perot (BGFP) resonators.

11. The optical sensor of any one of the preceding claims wherein the waveguide coupler coupling the light from the light source through to the gratings is a standard 4-port directional coupler.

12. The optical sensor of any one of claims 1-10 wherein the waveguide coupler coupling the light from the light source through to the gratings is a Y-junction splitter.

13. The optical sensor of any one of claims 1 -10 wherein the waveguide coupler coupling the light from the light source through to the gratings is an optical circulator coupled with a Y-junction splitter.

14. The optical sensor of any one of claims 1-10 wherein the waveguide coupler coupling the light from the light source to the gratings is a multimode interference coupler.

15. The optical sensor of any one of claims 1 -10 wherein the waveguide coupler coupling the light from the light source through to the gratings is an optical circulator coupled with a multimode interference coupler.

16. The optical sensor of any one of the preceding claims wherein each sensor comprises a plurality of sensors multiplexed together with each sensor forming a sensor unit comprising two substantially identical gratings.

17. The optical sensor of claim 16 wherein the plurality of sensor units, each comprising two substantially identical gratings, are multiplexed together in a cascade configuration.

18. The optical sensor of claim 16 wherein the plurality of sensor units, each comprising two substantially identical gratings, are arranged in a parallel configuration.

19. The optical sensor of claims 16 or 18 wherein the waveguide directional coupler comprises a multimode interference coupler for splitting the light that is propagated from the light source to the plurality of parallel sensor units.

20. The optical sensor of claims 16 or 17 wherein the waveguide directional coupler comprises a Y-junction splitter for splitting the light that is propagated from the light source to the plurality of cascading sensor units.

21. The optical sensor of claims 16 or 17 wherein the waveguide directional coupler comprises an optical circulator coupled with a Y-junction splitter for splitting the light that is propagated from the light source to the plurality of cascading sensor units.

22. The optical sensor of claims 16 or 18 wherein the waveguide directional coupler is an arrayed waveguide grating for frequency selecting and channeling the selected light that is propagated from the light source through to the sensor unit operating at the same frequency.

23. The optical sensor of any one of claims 16-22 wherein each sensor unit, comprising two substantially identical gratings, operates at a separate frequency band to avoid signal crosstalk.

24. The optical sensor of any one of claims 16, 18 or 19, wherein any of the sensor units, each comprising two substantially identical gratings, operates at the same frequency band.

25. The optical sensor of any one of claims 16, 18, or 19, wherein any of the sensor units shares a common reference grating.

26. The optical sensor of claim 9 wherein the long period grating has a period in the range of 100 to 1000 microns.

27. The optical sensor of any one of the preceding claims wherein the flow cell comprises a flow cell wall coming into contact with the waveguide and having a material refractive index that is less than the material refractive index of the waveguide.

28. The optical sensor of any one of the preceding claims wherein the material of the substrate and the waveguide directional coupler are polymers and the polymer material is cyclic olefin copolymer (COC), polycarbonate (PC), polymethylmethacrylate (PMMA), polyethylene (PE), polyethylene terephthalate (PET), polystyrene (PS), polytetrafluoroethylene (PTFE), benzocyclobutene (BCB), polydimethylsiloxane (PDMS), SU-8, UV-15, NOA-61, orCytop.

29. The optical sensor of any one of the preceding claims wherein the distance between each flow cell is less than 10mm.

30. The optical sensor of any one of the preceding claims wherein a surface of the grating is functionalized with a sensitive layer to enable sample binding or adsorption.

31. The optical sensor of claim 30 wherein the gratings are BGFP resonators having a cavity section and a mirror section wherein the surface of the entire resonator or the surface of the cavity section is functionalized with a sensitive layer to enable sample binding and adsorption.

32. The optical sensor of any one of the preceding claims wherein the gratings are modulated in refractive index.

33. The optical sensor of any one of claims 1 -31 wherein the gratings are modulated in amplitude.

34. The optical sensor of claim 33 wherein the amplitude modulation takes place on top of a ridge waveguide.

35. The optical sensor of claim 33 wherein the amplitude modulation takes place on sides of a ridge waveguide.

36. The optical sensor of claim 33 wherein the amplitude modulation takes place on top and the sides of a ridge waveguide simultaneously.

37. The optical sensor of claim 33 wherein the afnplitude modulation takes place embedded at the interface between the substrate and the grating core.

38. The optical sensor of claim 37 wherein the gratings have grating teeth embedded at the interface between the substrate and the grating core for amplitude modulation.

39. The optical sensor of claim 37 or 38 wherein the gratings with embedded amplitude modulation at the interface between the substrate and the grating core are Bragg grating sensors.

40. The optical sensor of claims 37 or 38 wherein the gratings with embedded amplitude modulation at the interface between the substrate and the grating core are BGFP resonator sensors.

41. The optical sensor of any one of the preceding claims wherein the light transmitted is detected for transmission mode detection.

42. The optical sensor of any one of the preceding claims wherein the gratings have an order (m) equal to or greater than 1 (m≥1).

43. The optical sensor of any one of the preceding claims wherein a material of the optical sensor is a polymer material.

44. The optical sensor of any one of the preceding claims wherein the optical sensor is for detection of a sample in gas phase.

45. The optical sensor of any one of the preceding claims wherein the optical sensor is for detection of a sample in liquid phase.

46. The optical sensor of any one of the preceding claims wherein the frequency of the gratings of the optical sensor is designed for an operating wavelength of 1.31 microns.

47. The optical sensor of any one of the claims 1 -45 wherein the frequency of the gratings of the optical sensor is designed for an operating wavelength of 1.55 microns.

48. The optical sensor of any one of the preceding claims wherein the optical sensor is designed for single mode operation.

49. The optical sensor of any one of the preceding claims wherein the light propagated through the gratings is for detecting the sample by differential frequency measurement.

50. An optical sensor system comprising: a light source for generating a light; an optical sensor optically coupled to the light source for receiving light generated by the light source; the optical sensor comprising a substrate for supporting the optical sensor, at least two substantially identical gratings, a waveguide directional coupler optically coupled to the gratings and arranged to receive light from the light source and propagate the light through to the gratings, and a flow cell for housing each of the gratings, each flow cell having a fluidic input and a fluidic output, one flow cell for receiving a reference fluid and another flow cell for receiving a fluid with a sample to be detected, the sample to be detected alters the effective refractive index of the propagating mode inside the grating by binding or being adsorbed onto the grating to alter the optical frequency of the light that propagates through the grating.

51. The optical sensor system of claim 50 further comprising an optical frequency detector for detecting optical frequency of the propagating light.

52. The optical sensor system of claim 50 or 51 wherein the optical frequency detector receives the light propagated through the gratings for detecting the sample by differential frequency measurement.

53. The optical sensor system of any one of claims 50-52 wherein the gratings are substantially identical in physical dimensions and material composition.

54. The optical sensor system of any one of claims 50-53 wherein the gratings are uniform gratings with constant period.

55. The optical sensor system of any one of claims 50-53 wherein the gratings are chirped gratings with period varying with position on the grating.

56. The optical sensor system of any one of claims 50-53 wherein the gratings are apodized gratings with the grating strength parameter varying in amplitude with position on the grating.

57. The optical sensor system of any one of claims 50-56 wherein the light source is a tunable laser for generating a light with tunable wavelength.

58. The optical sensor system of any one of claims 50-56 wherein the light source is a broadband source with a spectral bandwidth covering the frequency range of interest.

59. The optical sensor system of any one of claims 50-58 wherein the gratings are Bragg gratings.

60. The optical sensor system of any one of claims 50-58 wherein the gratings are long period gratings.

61. The optical sensor system of any one of claims 50-58 wherein the gratings are Bragg Grating Fabry-Perot (BGFP) resonators.

62. The optical sensor system of any one of claims 50-61 wherein the waveguide coupler coupling the light from the light source through to the gratings is a standard 4-port directional coupler.

63. The optical sensor system of any one of claims 50-61 wherein the waveguide coupler coupling the light from the light source through to the gratings is a Y-junction splitter.

64. The optical sensor system of any one of claims 50-61 wherein the waveguide coupler coupling the light from the light source through to the gratings is an optical circulator coupled with a Y-junction splitter.

65. The optical sensor system of any one of claims 50-61 wherein the waveguide coupler coupling the light from the light source to the gratings is a multimode interference coupler.

66. The optical sensor system of any one of claims 50-61 wherein the waveguide coupler coupling the light from the light source through to the gratings is an optical circulator coupled with a multimode interference coupler.

67. The optical sensor system of any one of claims 50-66 wherein each sensor comprises a plurality of sensors multiplexed together with each sensor forming a sensor unit comprising two substantially identical gratings.

68. The optical sensor system of claim 67 wherein the plurality of sensor units, each comprising two substantially identical gratings, are multiplexed together in a cascade configuration.

69. The optical sensor system of claim 67 wherein the plurality of sensor units, each comprising two substantially identical gratings, are arranged in a parallel configuration.

70. The optical sensor system of claims 67 or 69 wherein the waveguide directional coupler comprises a multimode interference coupler for splitting the light that is propagated from the light source to the plurality of parallel sensor units.

71. The optical sensor system of claims 67 or 68 wherein the waveguide directional coupler comprises a Y-junction splitter for splitting the light that is propagated from the light source to the plurality of cascading sensor units.

72. The optical sensor system of claims 67 or 68 wherein the waveguide directional coupler comprises an optical circulator coupled with a Y-junction splitter for splitting the light that is propagated from the light source to the plurality of cascading sensor units.

73. The optical sensor system of claims 67 or 69 wherein the waveguide directional coupler is an arrayed waveguide grating for frequency selecting and channeling the selected light that is propagated from the light source through to the sensor unit operating at the same frequency.

74. The optical sensor system of any one of claims 67-73 wherein each sensor unit, comprising two substantially identical gratings, operates at a separate frequency band to avoid signal crosstalk.

75. The optical sensor system of any one of claims 67, 69 or 70, wherein any of the sensor units, each comprising two substantially identical gratings, operates at the same frequency band.

76. The optical sensor system of any one of claims 67, 69 or 70, wherein any of the sensor units shares a common reference grating.

77. The optical sensor system of claim 60 wherein the long period grating has a period in the range of 100 to 1000 microns.

78. The optical sensor system of any one of claims 50-77 wherein the flow cell comprises a flow cell wall coming into contact with the waveguide and having a material refractive index that is less than the material refractive index of the waveguide.

79. The optical sensor system of any one of claims 50-78 wherein the material of the substrate and the waveguide directional coupler are polymers and the polymer material is cyclic olefin copolymer (CCX)), polycarbonate (PC), polymethylmethacrylate (PMMA), polyethylene (PEi), polyethylene terephthalate (PET), polystyrene (PS), polytetrafluoroethylene (PTFE), benzocyclobutene (BCB), polydimethylsiloxane (PDMS), SU-8, UV-15, NOA-61, or Cytop.

80. The optical sensor system of any one of claims 50-79 wherein the distance between each flow cell is less than 10mm.

81. The optical sensor system of any one of claims 50-80 wherein a surface of the grating is functionalized with a sensitive layer to enable sample binding or adsorption.

82. The optical sensor system of claim 81 wherein the gratings are BGFP resonators having a cavity section and a mirror section wherein the surface of the entire resonator or the surface of the cavity section is functionalized with a sensitive layer to enable sample binding and adsorption.

83. The optical sensor system of any one of claims 50-82 wherein the gratings are modulated in refractive index.

84. The optical sensor system of any one of claims 50-82 wherein the gratings are modulated in amplitude.

85. The optical sensor system of claim 84 wherein the amplitude modulation takes place on top of a ridge waveguide.

86. The optical sensor system of claim 84 wherein the amplitude modulation takes place on sides of a ridge waveguide.

87. The optical sensor system of claim 84 wherein the amplitude modulation takes place on top and the sides of a ridge waveguide simultaneously.

88. The optical sensor system of claim 84 wherein the amplitude modulation takes place embedded at the interface between the substrate and the grating core.

89. The optical sensor system of claim 88 wherein the gratings have grating teeth embedded at the interface between the substrate and the grating core for amplitude modulation.

90. The optical sensor system of claim 88 or 89 wherein the gratings with embedded amplitude modulation at the interface between the substrate and the grating core are Bragg grating sensors.

91. The optical sensor system of claims 88 or 89 wherein the gratings with embedded amplitude modulation at the interface between the substrate and the grating core are BGFP resonator sensors.

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92. The optical sensor system of any one of claims 50-91 wherein the light transmitted is detected for transmission mode detection.

93. The optical sensor system of any one of claims 50-92 wherein the gratings have an order (m) equal to or greater than 1 (m≥1).

94. The optical sensor system of any one of claims 50-93 wherein a material of the optical sensor is a polymer material.

95. The optical sensor system of any one of claims 50-94 wherein the optical sensor is for detection of a sample in gas phase.

96. The optical sensor system of any one of claims 50-94 wherein the optical sensor is for detection of a sample in liquid phase.

97. The optical sensor system of any one of claims 50-96 wherein the frequency of the gratings of the optical sensor is designed for an operating wavelength of 1.31 microns.

98. The optical sensor system of any one of claims 50-96 wherein the frequency of the gratings of the optical sensor is designed for an operating wavelength of 1.55 microns.

99. The optical sensor system of any one of claims 50-98 wherein the optical sensor is designed for single mode operation.

100. A method of detecting the presence of a sample to be detected with an optical sensor, the method comprises: generating a light with a light source for propagating to the optical sensor, the optical sensor comprising at least two substantially identical gratings, a waveguide directional coupler coupled to the gratings and a flow cell for housing each of the gratings, the waveguide directional coupler arranged to receive light from the light source and propagate the light through to the gratings, each flow cell having a fluidic input and a fluidic output; receiving at one flow cell a reference fluid; receiving at another flow cell a fluid with a sample to be detected; detecting the sample to be detected by the sample altering the effective refractive index of the propagating mode inside the grating by binding or being adsorbed onto the grating to alter the optical frequency of the light that propagates through the grating; and determining the detected sample by differential frequency measurement of the light propagated through the gratings.

101. The method of claim 100 wherein the gratings are substantially identical in physical dimensions and material composition.

102. The method of claim 100 or 101 wherein the gratings are uniform gratings with constant period.

103. The method of claim 100 or 101 wherein the gratings are chirped gratings with period varying with position on the grating.

104. The method of claim 100 or 101 wherein the gratings are apodized gratings with the grating strength parameter varying in amplitude with position on the grating.

105. The method of any one of claims 100-104 wherein the light source is a tunable laser for generating a light with tunable wavelength.

106. The method of any one of claims 100-104 wherein the light source is a broadband source with a spectral bandwidth covering the frequency range of interest.

107. The method of any one of claims 100-106 wherein the gratings are Bragg gratings.

108. The method of any one of claims 100-106 wherein the gratings are long period gratings.

109. The method of any one of claims 100-106 wherein the gratings are Bragg Grating Fabry-Perot (BGFP) resonators.

110. The method of any one of claims 100-109 wherein the waveguide coupler coupling the light from the light source through to the gratings is a standard 4-port directional coupler.

111. The method of any one of claims 100-109 wherein the waveguide coupler coupling the light from the light source through to the gratings is a Y-junction splitter.

112. The method of any one of claims 100-109 wherein the waveguide coupler coupling the light from the light source through to the gratings is an optical circulator coupled with a Y-junction splitter.

113. The method of any one of claims 100-109 wherein the waveguide coupler coupling the light from the light source to the gratings is a multimode interference coupler.

114. The method of any one of claims 100-109 wherein the waveguide coupler coupling the light from the light source through to the gratings is an optical circulator coupled with a multimode interference coupler.

115. The method of any one of claims 100-114 wherein each sensor comprises a plurality of sensors multiplexed together with each sensor forming a sensor unit comprising two substantially identical gratings.

116. The method of claim 115 wherein the plurality of sensor units, each comprising two substantially identical gratings, are multiplexed together in a cascade configuration.

117. The method of claim 115 wherein the plurality of sensor units, each comprising two substantially identical gratings, are arranged in a parallel configuration.

1 18. The method of claims 115 or 117 wherein the waveguide directional coupler comprises a multimode interference coupler for splitting the light that is propagated from the light source to the plurality of parallel sensor units.

119. The method of claims 115 or 116 wherein the waveguide directional coupler comprises a Y-junction sputter for splitting the light that is propagated from the light source to the plurality of cascading sensor units.

120. The method of claims 115 or 116 wherein the waveguide directional coupler comprises an optical circulator coupled with a Y-junction splitter for splitting the light that is propagated from the light source to the plurality of cascading sensor units.

121. The method of claims 115 or 117 wherein the waveguide directional coupler is an arrayed waveguide grating for frequency selecting and channeling the selected light that is propagated from the light source through to the sensor unit operating at the same frequency.

122. The method of any one of claims 115-121 wherein each sensor unit, comprising two substantially identical gratings, operates at a separate frequency band to avoid signal crosstalk.

123. The method of any one of claims 115, 117 or 118, wherein any of the sensor units, each comprising two substantially identical gratings, operates at the same frequency band.

124. The method of any one of claims 115, 117 or 118, wherein any of the sensor units shares a common reference grating.

125. The method of claim 108 wherein the long period grating has a period in the range of 100 to 1000 microns.

126. The method of any one of claims 100-125 wherein the flow cell comprises a flow cell wall coming into contact with the waveguide and having a material refractive index that is less than the material refractive index of the waveguide.

127. The method of any one of claims 100-126 wherein the material of the substrate and the waveguide directional coupler are polymers and the polymer material is cyclic olefin copolymer (COC), polycarbonate (PC), polymethylmethacrylate (PMMA), polyethylene (PE), polyethylene terephthalate (PET), polystyrene (PS), polytetrafluoroethylene (PTFE), benzocyclobutene (BCB), polydimethylsiloxane (PDMS), SU-8, UV-15, NOA-61, or Cytop.

128. The method of any one of claims 100-127 wherein the distance between each flow cell is less than 10mm.

129. The method of any one of claims 100-128 wherein a surface of the grating is functionalized with a sensitive layer to enable sample binding or adsorption.

130. The method of claim 129 wherein the gratings are BGFP resonators having a cavity section and a mirror section wherein the surface of the entire resonator or the surface of the cavity section is functionalized with a sensitive layer to enable sample binding and adsorption.

131. The method of any one of claims 100-130 wherein the gratings are modulated in refractive index.

132. The method of any one of claims 100-130 wherein the gratings are modulated in amplitude.

133. The method of claim 132 wherein the amplitude modulation takes place on top of a ridge waveguide.

134. The method of claim 132 wherein the amplitude modulation takes place on sides of a ridge waveguide.

135. The method of claim 132 wherein the amplitude modulation takes place on top and the sides of a ridge waveguide simultaneously.

136. The method of claim 132 wherein the amplitude modulation takes place embedded at the interface between the substrate and the grating core.

137. The method of claim 136 wherein the gratings have grating teeth embedded at the interface between the substrate and the grating core for amplitude modulation.

138. The method of claim 136 or 137 wherein the gratings with embedded amplitude modulation at the interface between the substrate and the grating core are Bragg grating sensors.

139. The method of claims 136 or 137 wherein the gratings with embedded amplitude modulation at the interface between the substrate and the grating core are BGFP resonator sensors.

140. The method of any one of claims 100-139 wherein the light transmitted is detected for transmission mode detection.

141. The method of any one of claims 100-140 wherein the gratings have an order (m) equal to or greater than 1 (m≥1).

142. The method of any one of claims 100-141 wherein a material of the optical sensor is a polymer material.

143. The method of any one of claims 100-142 wherein the optical sensor is for detection of a sample in gas phase.

144. The method of any one of claims 100-142 wherein the optical sensor is for detection of a sample in liquid phase.

145. The method of any one of claims 100-144 wherein the frequency of the gratings of the optical sensor is designed for an operating wavelength of 1.31 microns.

146. The method of any one of claims 100- 144 wherein the frequency of the gratings of the optical sensor is designed for an operating wavelength of 1.55 microns.

147. The method of any one of claims 100-146 wherein the optical sensor is designed for single mode operation.