Waveguide structures

Details Waveguide structures Waveguide structures

2000-10-19

2002-11-19

Sugarman, Scott J. (Department: 2873)

Optical waveguides

Optical waveguide sensor

C385S005000, C385S141000

Reexamination Certificate

active

06483959

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
This invention relates to waveguide structures, and particularly though not exclusively to waveguide structures suitable for use as optical sensors.
BACKGROUND OF THE INVENTION
Sensors which are capable of monitoring biological interactions in real time and with high sensitivity are of considerable importance in life science research. Several sensors exist which monitor changes in the refractive index (or other parameters) of a biological sample, caused by molecular interactions. In a typical sensor an evanescent wave associated with an optical mode existing in a high refractive index dielectric layer of a waveguide extends into a biological sample, which is held in a gel. A change of the refractive index of the sample will modify an optical property of the waveguide mode, and detection of this change will provide dynamic information relating to interactions occurring within the biological sample.
Known optical evanescent sensors include those based on surface plasmon resonance and those based on dielectric waveguiding techniques (see for example Welford, K (1991) Surface plasmon-polaritons and their uses—
Optical and Quantum Electronics,
23, 1-27; Smith, A. M. (1987) Optical waveguide immunosensors,
Proc. SPIE
798
Fibre Optic Sensors II,
206-213); R. H. Ritchie, Phys. Rev. 106, 874 (1957).
Sensors which use surface plasmon resonance comprise a thin metal layer (typically a few tens or hundreds of Angstroms thick) deposited onto a dielectric prism or grating, and a sensing layer (or a fluid) whose optical properties are of interest provided at an opposite surface of the metal layer. Measurements are made by directing light via the prism or grating onto that side of the metal layer which is not in contact with the sensing layer, and detecting light which is reflected from the same side of the metal layer. A surface plasmon resonance excited by the incident light will result in the absorption of that incident light, and a consequent dip in the reflected light intensity. The condition for exciting a resonance (i.e. the angle of incident light which will excite a resonance) is sensitive to changes in the optical properties of the sensing layer. The optical properties of the sensing layer may be monitored by detecting changes in the angle of incidence which excites a resonance
The resolution, and hence the sensitivity, of sensors which utilise surface plasmon resonance is limited by the resonance width (i.e. the range of angles of incident light which will excite resonance). This width is determined ultimately by the amount of absorption of incident light into the metal layer. Absorption is considerable at wavelengths commonly used for biological measurements, and the maximum resolution of surface plasmon sensors is correspondingly restricted.
The angle of incident light which excites a surface plasmon resonance will alter if the wavelength of the incident light is changed. Variations in the wavelength of incident light will thus introduce an error into measurements. This is a further limitation of surface plasmon resonance sensors, since wavelength-stabilised sources of incident light are needed to allow accurate measurement.
A waveguide structure, based upon the surface plasmon resonance structure and known as a leaky mode waveguide, is described by R. P. Podgorsek, H. Frarke and J. Woods (1998) Monitoring of the Diffusion of Vapour Molecules in Polymer Films using SP-Leaky-Mode Spectroscopy,
Sensors and Actuators B-Chemical,
Vol.51, No.1-3, pp.146-151. The waveguide comprises a substrate, a thin metal layer disposed on top of the substrate, and a sensing layer whose optical properties are of interest disposed as a further layer on top of the layer of metal. The sensing layer has optical properties which change if the medium is exposed to conditions to be sensed, and may be for example dextran gel.
The leaky mode excited within the sensing layer is of a type known in the art as a bulk mode. This contrasts with the mode which is excited by surface plasmon resonance sensors, which mode is known in the art as a surface mode. Generally only one mode may be excited in surface plasmon sensors (the mode must be a TM mode), whereas the leaky mode waveguide allows the excitation of a series of modes (the modes may be any combination of TE and TM).
A leaky mode of the waveguide, i.e. a bulk mode which is centred on the sensing layer, is excited by directing light towards the layer of metal or metal alloy through the substrate over a range of incident angles. The presence of an excited leaky mode is determined by detecting the intensity of light returned from the waveguide over a range of angles. When light is coupled to a leaky mode of the waveguide this is seen as a dip in the intensity of light emitted from the waveguide. A change of an optical property of the sensing layer will modify the angle of incident light required to excite the leaky mode. The angle at which the dip of intensity is returned from the waveguide will change accordingly.
The leaky mode waveguide is advantageous compared to surface plasmon resonance because a bulk mode of the waveguide is excited rather than a surface mode. This bulk mode is centred on the sensing layer of the waveguide and is therefore considerably more sensitive to changes of the optical properties of the sensing layer than the surface mode provided by surface plasmon resonance.
A disadvantage of known leaky mode waveguides is that detection optics are required to detect a dip in the intensity of light returned from the waveguide, and to follow angular movement of that dip. The absence of light is inherently more difficult to detect than a peak of light intensity.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a leaky mode waveguide which will return a peak of intensity when a leaky waveguide mode is excited.
According to a first aspect of the invention there is provided an optical sensor comprising a waveguide having a substrate, a layer of metal or metal alloy disposed on top of the substrate, and a medium disposed as a sensing layer on top of the layer of metal or metal alloy, the medium having optical properties which change if the medium is exposed to conditions to be sensed, the sensor further comprising means for directing light towards the layer of metal or metal alloy through the substrate over a range of incident angles, and detection means for detecting the intensity of light returned from the waveguide over a range of detection angles, the means for directing light being configured to direct light such that a leaky waveguide mode is excited within the sensing layer, and the means for detecting the intensity of light being arranged to detect variations with detection angle in the intensity of returned light resulting from the excitation of the leaky waveguide mode; characterised in that the waveguide is configured such that the overlap of the optical field with the layer of metal or metal alloy is less for light incident at an angle which results in excitation of a leaky waveguide mode than for light incident at an angle which does not result in excitation of a leaky waveguide mode, whereby the detected intensity peakswat a detection angle related to an incident angle which results in excitation of a leaky waveguide mode.
The invention is advantageous because it allows for the easy detection of a waveguide mode.
The term metal alloy is intended to include mixtures of metals and mixtures of two or more elements which include at least one metal. Metals or metal alloys are used because they have a sufficiently high imaginary part of refractive index that an optical field extending into the metal or metal alloy suffers significant loss. The term metal or metal alloy is therefore intended to include any material having an imaginary part of refractive index comparable to that of a metal or metal alloy.
Preferably, the substrate comprises a prism or grating for coupling light into the waveguide mode.
An optical source comprising a laser, a light emitting diode or a source capable of

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