Optics: measuring and testing – Refraction testing
Patent
1995-10-06
1997-09-02
Pham, Hoa Q.
Optics: measuring and testing
Refraction testing
356133, 356361, G01N 2141
Patent
active
056637900
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a novel method for determining refractive index, and to the use of this method in determining qualitatively or quantitatively the presence of a substance in a gaseous, liquid or solid sample. The invention also relates to a sensor device for carrying out the method.
In its simplest form, a ring resonator is comprised of an open (normally straight) optical waveguide which is located closely adjacent to a closed (normally annular or ring-shaped) optical waveguide. When light from a single-mode laser, i.e. a laser which transmits laser light of single wavelength, is coupled into the open waveguide, some of this light will also be coupled to the annular waveguide in the region where the two waveguides lie close together. Normally, the light concerned in this regard will constitute only one or some percent of the total light. However, if the wavelength of the laser light is such that the waveguide ring is able to accommodate precisely a whole number of wavelengths, the resonance conditions of the ring for this wavelength are fulfilled and substantially more light will be coupled to the ring and be stored therein, which, in turn, results in a corresponding reduction in the amount of light that is transmitted at the output end of the open waveguide. Thus, when seen over a range of wavelengths, the resonance will be observed at the output end of the open waveguide as a sharp minimum in the power of the transmitted light. This phenomenon has been utilized in rotary sensors, such as the gyro, among other things, and also to determine very low propagation losses in waveguides.
The resonance condition for a ring resonator can be written as: index of the waveguide, m is an integer and .lambda. is the wavelength of the laser light. In order to be able to detect a resonance, it is necessary to vary the laser light wavelength .lambda. and to detect the effect upon the light coming from the open waveguide. It will be seen from the formula that when the effective refractive index n.sub.eff of the ring changes, the resonance wavelengths will also change. The effective refractive index (n.sub.eff) is, in turn, influenced by all media in which the wave-guided light propagates, mostly in the actual core of the waveguide although also in materials closely surrounding the core.
WO 91/03728 describes the use of a waveguide based sensor system for determining the concentration of a chemical substance, wherein there is produced two mutually separate evanescent fields having mutually different penetration depths. According to one embodiment, the system includes a ring resonator and there is determined a relative absorption change in relation to the concentration of the chemical substance.
SUMMARY OF THE INVENTION
The present invention is based on the concept of utilizing the dependency of the resonance wavelength in a ring-resonator type sensor on the surroundings of the waveguide core to determine refractive index or changes in refractive index in a gas, liquid or solid material, by placing such a sample in the immediate vicinity of the waveguide core. More specifically, the proposed measuring method is based on applying the sample onto a sample surface essentially on top of the closed waveguide itself, i.e. in the case of a simple ring-resonator on top of the waveguide ring, the resonance wavelength .lambda. of the waveguide ring being changed in proportion to the change in refractive index that is occasioned by the sample.
The present invention thus enables the bulk refractive index to be determined in primarily liquids or gases, although possibly also in solid materials, both statically and in flow cells, for instance for analysis and monitoring purposes, such as in FIA, liquid chromatography, electrophoresis, gas and liquid assays in process controls, in vehicle contexts, and so on. Furthermore, the interactions of different species with a derivatized sample surface or a sample surface pretreated in some other way can be detected via the change in refractive index
REFERENCES:
patent: 4240747 (1980-12-01), Harmer
patent: 4950074 (1990-08-01), Fabricius et al.
patent: 5173747 (1992-12-01), Boiarski et al.
patent: 5377008 (1994-12-01), Ridgway et al.
Ekstrom Bjorn
Oberg Magnus
Pham Hoa Q.
Pharmacia Biosensor AB
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