Optics: measuring and testing – Refraction testing
Patent
1997-01-10
1998-09-22
Rosenberger, Richard A.
Optics: measuring and testing
Refraction testing
356136, 356445, G01N 2141, G01N 33487, G01N 2175
Patent
active
058122559
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a Process and to a device for carrying out the Process, by means of which the refractive index of various media can be measured. For this purpose there is utilised the excitation of surface plasmons in a boundary layer metallic dielectric and the close dependence of the resonance conditions on the refractive index of the layer or of the coating system adjoining the metallic layer. The invention relates in particular to the emission and derivation of signals by fibre optics. Possible applications of the invention lie in the construction of fibre-optic sensors for measuring physical, chemical and biological magnitudes, which may be indirectly detected through an alteration in refractive index.
2. Description of Background Information
Fibre optic SPR sensors have been intensively investigated for some years. The first attempts (EP-PS 0 410 505) proceeded in this respect from the conventional Kretschmann arrangement with a solid glass prism, to multi-mode fibres being used for coupling and decoupling the monochromatic light. The intensity of the radiation reflected at the receiving fibre is measured, the operating point being set by displacement of this fibre.
A sensor is known from the publication "Fibre-optic sensor based on surface plasmon interrogation", Letizia De Maria, et. al. Sensors an Actuators B, 12 (1993) 221-223, which uses the obliquely-ground and coated end face of a mono-mode fibre as a sensor head.
Here also operation was with monochromatic light from a He-Ne-laser and the polarisation was set by means of a rotating .lambda./2 delay plate. The sinusoidal signal arising at the detector has a minimum and a maximum, as the surface plasmons are only excited by the TM-wave, whereas the TE-wave serves as a reference signal. Precise measurement in this arrangement requires optimum polarisation and a high degree of stability of the wavelength. A disadvantage in this case is the necessary mechanical system for rotation of the .lambda./2 plate and the low intensity diffused back by the sensor head.
A further SPR-sensor with a glass fibre is known from EP-PS 0 326 291, in which however the fibre-optic derivation of the signal obtained is not possible.
A fibre-optic SPR-sensor proposed in DE-PS 43 05 830 is adjustable only with difficulty due to the use of two fibres for exciting and retroreflection of the coupled light.
A further SPR-sensor on the basis of monomode fibres is described in New "in-line" optical-fibre sensor based on surface plasmon excitation, R. Alonso, F. Villuendas, J. Tornos and J. Pelayo, Sensors and Actuators A, 37-38 (1993) 187-192. In this case excitation of the surface plasmon resonance is effected with a thin gold layer (15-35 nm) on the ground periphery of a monomode fibre embedded in an epoxy resin.
Here also linearly polarised, monochromatic light is coupled into the fibre and the transmission is evaluated as a measurement magnitude.
Due to the use of coherent sources and the necessary stabilisation of wave length, good-value sensors are difficult to manufacture in this variant.
In the publication A Novel Surface Plasmon Resonance based Fiber Optic Sensor Applied to Biochemical Sensing, R. C. Jorgenson et. al., SPIE VOL. 1886 (1993), p. 35-48, the vapour-coated cylinder surface of the core of a multi-mode glass fibre serves to excite surface plasmons with broad-band light. The signal reflected at the point of the fibre is analysed with the aid of a spectrometer. The wavelength of the damping maximum is in this case a measure for the refractive index of the medium adjoining the metallic layer. A serious disadvantage of this arrangement resides in the intense propagation of the resonance due to the number of modes, and thus the multiple excitation of surface plasmons at different wavelengths.
The object of the invention is to propose a fibre-optic SPR-sensor which is favourable in terms of energy and which is suitable for use with multi-mode fibres, without accepting the disadvantageous propagation o
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Kirschner Uwe
Lau Matthias
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