WO1997035180A1

WO1997035180A1 - Optical lightwave sensor based on resonant optical excitation of surface plasma waves

Info

Publication number: WO1997035180A1
Application number: PCT/DE1997/000575
Authority: WO
Inventors: Andreas Katerkamp; Maria Pellmann; Christian KÖTTER
Original assignee: Institut für Chemo- und Biosensorik Münster E.V.
Priority date: 1996-03-20
Filing date: 1997-03-20
Publication date: 1997-09-25
Also published as: DE19611025A1

Abstract

The invention relates to an optical lightguide sensor based on resonant optical excitation of surface plasma waves and to a device and process for operating the sensor. The use as per the invention of an alloy (15), preferably a heterogeneous alloy, instead of a pure metal as the carrier layer for monochromatically excited surface plasma wave results in sensitive sensors with adjustable operating points. Planar multimode waveguides in particular are suitable for sensors of the type claimed and can be used to create multisensor systems in a simple way. The core element of devices adapted to the operation of the claimed sensors is an optical interface with which the principal angle and angular distribution of the light coupled into the sensor can be set. This facilitates further improvement of the sensitivity of the sensor device. The sensor operating point can also be modified via the optical interface.

Description

Optical fiber optic sensor based on the resonant optical excitation of surface plasma waves

The invention relates to a sensor based on the resonant optical excitation of surface plasma waves and to a device for operating the sensor. Such a sensor can be used for substance detection or for determining the concentration of chemical substances.

Surface plasma vibrations at a metal interface are understood to mean oscillatory fluctuations in the electron charge density of the free metal electrodes. These can pass through at the interface between a thin metal layer and a dielectric

Light waves are excited. Part of the optical energy dissipates when a resonance occurs in plasmons, quantized units of the plasma oscillation. This dissipation of optical energy is a function of the dielectric constant of metal and dielectric. For this reason, the phenomenon of resonant surface plasma wave excitation is suitable for obtaining information about the dielectric via the detour of the dielectric constant.

In order for a coupling between light wave and plasma wave and thus dissipation of optical energy to be possible at all, a sharp resonance condition must be met: the light wave vector must match the wave vector of the plasma waves and the light wave energy must match the plasma wave energy. Changes in the dielectric constant of the dielectric due to, for example, changes in concentration lead to a change in the resonance conditions. A large number of sensor applications are based on the evaluation of this change in resonance.

As is generally known, a substance-recognizing (reactive) layer can be applied to the carrier metal to improve the selectivity of the surface plasma wave sensor and for signal amplification (Liedberg et al, "Surface Plasmon Resonance for Gas Detection and Biosensing", Sens. Actuators 4 (1983) pp . 299-304). Through a suitable choice of the substance-recognizing layer, strong changes in the dielectric constants and thus high signal amplification can be ensured even with small amounts of a substance to be detected.

A fiber-optic surface plasma wave sensor is known for example from US 5,359,681. In the case of an optical fiber, part of the sheathing was removed and a thin silver applied layer. Polychromatic light with a defined angular distribution is coupled into one end of the optical fiber. Light of a certain wavelength range fulfills the resonance condition and excites surface plasma waves on the metal layer surface with loss of intensity. The spectral intensity distribution of the light emerging at the other end of the fiber, determined with a spectrometer, has a resonance dip in a certain wavelength range. If the dielectric constant of the dielectric (eg of water) changes due to concentration changes (eg addition of a substance), the spectral center of gravity of the dip in resonance shifts in a characteristic manner depending on the current concentration.

A disadvantage of this arrangement is the fact that small changes in concentration cannot be detected due to the inherent noise of polychromatic light sources and the associated mode fluctuation or mode coupling in the light guide. Furthermore, the expenditure on equipment is very high due to the use of a spectrometer and a polychromatic light source.

Another surface plasma wave sensor based on a light guide is from C. Ronot-Trioli et al., "A monochromatic excitation of a surface plasmon resonance in an optical fiber refractive system", Transducers 95 and Eurosensors IX, Voi 2 pp. 783-796 known. A multimode fiber serves as the sensor element, and gold is used as a carrier for surface plasma waves. To meet the resonance condition, the monochromatic light from a laser is introduced into the optical fiber at a variable angle coupled. The intensity of the emerging light is then determined as a function of the coupling angle. The resonance condition is fulfilled at a characteristic angle. The dielectric constant of the medium can be determined via the angle at which the resonance occurs. The disadvantage of this measuring arrangement is that the laser has to be fastened on a complex precision turntable and the coupling angle between the fiber and the laser can only be determined imprecisely. The reduction in noise due to the use of monochromatic light is also unable to compensate for the resulting systematic measurement errors.

Another problem with surface plasma wave sensors is inadequate adhesion of the sensitive metal layers and, associated therewith, inadequate long-term stability.

The present invention is therefore based on the object of creating a sensor of the type mentioned at the outset which permits optimum adjustment of the working point and at the same time has a high level of sensitivity. In addition, a simple device for operating such a sensor is to be specified.

This object is achieved with regard to the sensor by the characterizing features of claim 1 and with regard to an apparatus-simple device for operating the sensor by the characterizing features of claim 16. Advantageous refinements and developments of the invention result from the respective subclaims . An optical sensor, in which one or more layers are applied to the light guide core of a light guide, of which at least one layer functioning as a support layer for surface plasma waves contains a metal alloy, the composition and / or position of the absorption band about its composition the surface plasma waves is set, has a drastically increased sensor sensitivity. At the same time, the expenditure on equipment for operating such a sensor can be reduced.

The coating according to the invention is applied directly to an area or to several separate areas of the light guide core between the end faces of the light guide and has direct contact with the medium to be analyzed with the other layer side.

The use of a metal alloy makes it possible to adapt the sensor according to the invention to commercial, inexpensive and handy, monochromatic light sources such as laser diodes. For a given emission wavelength of a laser and a given dielectric to be analyzed, the metal alloy composition is selected such that the absorption range of the surface plasma waves falls within the range of the emission wavelength of the laser or just outside of it. In this way, the position of the absorption band and thus also the sensor operating point is set. This possibility of an exact working point setting leads to an extraordinarily high sensor measuring accuracy. Another reason for this is that the noise of the monochromatic light source according to the invention is negligible. An absorption band is shown in FIG. The spectral position of the resonance curve curve 21 can be shifted by selecting a metal alloy composition in such a way that the excitation wavelength lies in particular in the steepest and therefore most sensitive flank region 23 of the resonance.

Depending on the application, in addition to the spectral focus 22, the full width at half maximum 24 as well as the slope of the resonance curve can be adapted to the requirements by selecting an appropriate metal alloy composition. In this way it is possible to influence the shape of the absorption band.

In contrast to the prior art, the resonance condition is not necessarily met with the aid of an extensive apparatus structure, but in a simple and inexpensive manner by a suitable choice of a metal alloy composition.

The shape and / or position of the absorption band can already be adjusted using homogeneous metal alloys. However, very decisive additional advantages of the invention develop when using a heterogeneous metal alloy. Heterogeneous metal alloys are understood to mean those in which the components form at least two phases which differ in their composition.

In principle, two types of heterogeneous alloys can be distinguished. On the one hand, at least one of the phases can be formed in at least one further phase in the form of separate areas, for example as spheres or ellipsoids, with dimensions of up to 200 nm and preferably between approximately 1 nm and 40 nm. On the other hand, at least one of the phases can be present in at least one further phase in the form of a network. This network arises, for example, if, due to the increasing number of originally separated areas, these come into contact with one another or if a closed layer of a pure metal is broken up. The breakup can take place, for example, thermally.

Two-component heterogeneous alloys are preferably used. The first phase, which is in the form of separate areas or as a network, should have direct contact, at least in some areas, with the wave guide core. The second phase, which surrounds the first phase, is embedded in the first phase and / or covers the first phase, has the following effects: an advantageously narrow and deep absorption peak and, in the form of a homogeneous surface, is particularly suitable for attaching, for example, components of a receptor-ligand complex. The second phase (e.g. gold) can also be used, for example, as a corrosion protection layer for the first phase (e.g. silver).

In the case of a sensor according to the invention with a heterogeneous alloy, excitation of surface plasma waves which is almost independent of the polarization of the coupled-in light is possible, ie surface plasma waves can be excited in a heterogeneous alloy layer both with TE and with TM polarized light waves. In contrast, in surface plasma wave sensors of the prior art, only TM-polarized light waves can contribute to the excitation. The sensor according to the invention therefore has essentially more distinctive measurement signal changes and thus a significantly higher sensitivity. The mode fluctuation noise is also negligible.

Gold or silver are preferably used in the alloy as the main component in connection with one or more further metals. Binary alloys made of gold and silver are particularly preferred. The mixing ratio is based on the desired selectivity and sensitivity of the sensor, i.e. matched to the desired shape and / or position of the absorption band.

Typical layer thicknesses for the alloy are in a range from 10 nm to 500 nm and preferably between 40 nm and 80 nm. Particularly preferred layer thicknesses are in the range between 50 nm and 60 nm.

The length over which the sensitive coating is applied to the light guide core also has an important influence on the absorption behavior. The length of the coating area can also influence the shape of the resonance curve and thus the working point and the sensitivity of the sensor. As the length of the selective metal alloy increases, the half-width of the resonance curve increases without, however, changing the slope.

The strength of the measurement signal can thus be influenced over the length of the metal alloy layer applied. The optimal length is a function of the dimensions of the light guide core and the angle or the angular range under which the light propagates in the light guide. Useful lengths are in a range between 0.1 mm and 100 mm, preferably between 10 mm and 20 mm.

To improve the adhesion of the sensitive metal layers and the long-term stability of the sensor, an adhesion promoter layer can additionally be arranged between the light guide core and the alloy layer. Metal layers which contain, for example, chromium, titanium, nickel, cobalt or vanadium are particularly suitable as adhesion promoters. The adhesion promoter layer should be very thin so as not to complicate the resonant optical excitation of surface plasma waves. Preferred layer thicknesses are in the range from 0.1 nm to 20 nm. In these layer thickness ranges, the adhesion promoter is usually present as a discontinuous layer. For this reason, the layer thickness information is to be understood as mean layer thicknesses. The islands of the discontinuous adhesion promoter layer can advantageously serve as condensation nuclei for the simplified production of a heterogeneous alloy.

In order to achieve a substance-specific selectivity of the sensor and for signal amplification, a substance-recognizing layer can be applied to the alloy layer, which has contact with the dielectric to be analyzed. The substance-recognizing layer preferably contains a component of a chemical or bio-chemical receptor-ligand complex with a high affinity for the corresponding partner in the medium to be analyzed. The substance-recognizing layer typically has a thickness of 0.1 nm to 10 μm.

An intermediate layer of chemically reactive can be placed between the substance-recognizing layer and the alloy layer. tive groups for attaching the receptor or the ligand to the alloy layer are applied. Suitable thicknesses of this intermediate layer are approximately 0.1 nm to 100 nm.

Although the use of monomode light guides known from communications technology is also conceivable, multimode light guides are preferably used for the sensor application, and in particular those which are in a wavelength range between 200 nm and 2400 nm multimode light guides are used. A multimode light guide allows a considerably simplified connection technology and coupling of the light due to the larger light guide dimensions. Whereas in the case of single-mode light guides at a given excitation wavelength, the resonance condition can be met solely by choosing the alloy composition, in the case of multimode light guides, light coupling angles and coupling angle distribution provide a further degree of freedom for fulfilling the resonance condition.

Preferred light coupling devices are presented below.

All known light guides, in particular optical fibers, ribbed light guides, buried light guides and film light guides, are suitable for sensor applications. Because of the advantages in terms of production technology, planar light guides are preferably used.

In a simple and inexpensive manner, a plurality of individually arranged sensors can also be integrated in a single planar light guide. Separate substance-recognizing layers can be arranged next to one another on a planar light guide core in the region of separate light paths of the individual sensors. One from a single planar light guide Existing multi-sensor system can be used, for example, for the simultaneous determination of the concentration of a plurality of chemical substances.

The coupling in and out of light in an optical sensor can take place via the end faces of the light guide core, for example the coupling in of light via one end and the coupling out via the second end of an optical fiber. It is also possible for one end of the light guide to be mirrored or to be provided with a reflective element. In this case, the light is coupled in and out via one and the same end face of the light guide. If the injected light is reflected at the end of an optical fiber, it can be used twice with the

Alloy layer interact. This allows the overall length of the sensor to be shortened or means a greater change in the measurement signal.

A metal layer or a metal layer system is suitable as the reflecting element, which has no plasma wave resonance at the wavelengths of the injected light used. An adhesion promoter layer can also be located between the metal layer or metal layer system and the light guide core.

A device according to the invention, which is simple in terms of apparatus, for operating the sensor while maintaining the high detection sensitivity, contains optical interfaces for coupling light into and out of the sensors and an optical measuring system. The optical measuring system contains one or two light sources and an optical detector arrangement. When using two light sources, the op- table measuring system also a mixer to combine the light of both light sources.

According to the invention, the light is coupled into the sensor with the aid of a first optical interface via a first surface of the sensor and coupled out via a second surface with the aid of an optional second optical interface or when a reflection device is attached on the second surface over the first surface using the first optical interface.

The optical interface allows optimal and reproducible light at a predetermined angle and with a predetermined angle distribution in the

Couple sensor in and / or out. In this way, the intensity occupation of the individual light guide modes can be defined. As a result, non-reproducible fluctuations in the mode distribution can be avoided and the detection sensitivity of the system can be increased. In addition, the optical interface makes it possible to specifically influence the shape of the absorption band and thus the working point.

The optical interfaces can contain diaphragms, which are preferably adapted to the shape of the coupling or decoupling surface. In addition, the interfaces can comprise optical lenses or imaging lens systems which map the output of the optical measuring system onto the first surface of the sensor. A diaphragm is introduced into the beam path so that only light at a certain angle and with a certain angle distribution can be coupled into the sensor. The optical measuring system contains a first, monochromatic light source for exciting the surface plasma waves and optionally a second, mono- or polychromatic light source with an emission wavelength or an emission wavelength range outside the absorption band. When using a second light source, the optical measuring system further includes a device for mixing the light of the first light source with the light of the second light source. The second light source preferably emits light with a very narrowly limited wavelength spectrum. In order to be able to use the second light source, for example a low-cost light-emitting diode, as a reference to the first light source, the light from both light sources must go through the same optical path. It is advantageous that both light sources can be permanently mounted. As a result, systematic measurement errors can be reduced and the detection sensitivity increased accordingly.

The optical measuring system also contains an optical detector arrangement which determines the optical power of the outcoupled light of the first light source and generates a first signal therefrom and, when using a second light source, determines the optical power of the outcoupled light of the second light source and generates a second signal therefrom . The output variable is then determined using the signal from the first light source or both signals.

The optical measuring system can contain a single optical detector for each sensor to be evaluated, which, when using two light sources on a multiplex basis, alternates the optical power of the decoupled determined light of the first light source and generates a first signal therefrom and then determines the optical power of the outcoupled light of the second light source and generates a second signal therefrom. This can happen, for example, in that the first signal is generated while the first light source is switched on and the second light source is switched off and the second signal is generated while the second light source is switched on and the first light source is switched off.

An alternative provides for the two light sources to be modulated sinusoidally with different frequencies, for example. In this case too, one detector is sufficient for each sensor to be evaluated, provided that the sensor comprises two demodulators, each of which demodulates a certain frequency component. In the simplest case, suitable demodulators are, for example, high, low or bandpass filters.

The optical measuring system can also contain two optical detectors, the first optical detector determining the optical power of the first light source and the second optical detector determining the optical power of the second light source. The detectors used are selected in accordance with the spectral range of the respective light sources. In order to increase the selectivity of the detectors, wavelength-selective components can be fitted in front of the detector inputs. Alternatively, the multiplex-based light sources can be switched on and off alternately.

With the help of the multi-sensor system according to the invention and the device for operation according to the invention The concentrations of several chemical substances can be determined simultaneously by sensors with little production technology and apparatus expenditure. The light can be coupled into the individual sensors, which can have different substance-recognizing layers, by means of a beam splitter and a first interface per sensor and can be coupled out again via corresponding second interfaces or the first interfaces. A separate output measurement variable is then determined for each sensor via an adapted optical detector arrangement.

The sensors can advantageously be designed as a single planar waveguide with substance-recognizing layers arranged at different locations on the waveguide. This application is of great practical importance, since a simultaneous determination of the concentration of several substances is possible on the basis of only a single planar waveguide.

If the second surface of a sensor is provided with a reflection element and the light is therefore coupled out via the first surface, the output light can optionally be supplied to the optical detector arrangement by a beam splitter of beam splitting 1 to 2 without the detector being attached ¬ order in the optical path between the light source and sensor must be attached.

Of course, the described device according to the invention for operating the sensor described can also be used for other optical sensors. Further details, advantages and features of the invention result from the following description of the exemplary embodiments and from the drawings, which represent preferred but, for example, configurations. Show it:

1 shows a film light guide, consisting of a light-guiding film, light guide support and cover, with a sensitive coating on the light-guiding film;

Figure 2 shows a film light guide with a reflective element on an end face;

FIG. 3 shows an optical fiber consisting of fiber core and sheathing with a sensitive coating on the fiber core;

FIG. 4 shows an optical fiber with a reflective element on an end face;

FIG. 5 shows a ribbed light guide, consisting of a laterally delimited light guide core, light guide carrier and cover, with a sensitive coating on the light guide core;

FIG. 6 shows a ribbed light guide with a reflective element on an end face;

FIG. 7 shows a buried light guide, consisting of a laterally delimited light guide core, light guide carrier and cover, with a sensitive coating on the light guide core; FIG. 8 shows a buried light guide with a reflecting element on an end face;

FIG. 9 shows a sensitive layer sequence consisting of an adhesion promoter, alloy, intermediate and substance-recognizing layer applied to the light guide core;

FIG. 10 shows an example of the spectral intensity distribution of the outcoupled light when surface plasma wave resonance occurs;

FIG. 11 shows an exemplary embodiment of an optical interface consisting of an aperture;

FIG. 12 shows an exemplary embodiment of an optical interface, consisting of an aperture and a lens;

FIG. 13 shows an exemplary embodiment of an optical interface consisting of a diaphragm and two lenses;

Figures 14-17 devices for operating a sensor;

Figures 18-19 devices for operating a multi-sensor system;

FIGS. 20-21 devices for operating a multi-sensor system based on a single planar waveguide;

FIG. 22 shows a further exemplary embodiment of a sensor according to the invention with a heterogeneous alloy layer; FIG. 23 the measured absorption band of a sensor according to FIG. 22; and

FIG. 24 shows a sensitivity comparison between a sensor with a silver layer and a sensor according to the invention with an alloy layer.

1 to 8 show exemplary embodiments of multimode optical waveguides which are suitable for a sensor according to the invention on the basis of the resonant optical excitation of surface plasma waves.

1 shows a film light guide consisting of a planar light-guiding film 3, which is surrounded by a planar light guide support 2 and a planar cover 1. 2 shows a film light guide with a reflective element 11 on one end face. In Fig. 3 is an optical fiber consisting of cylindrical symmetrical light-guiding

Fiber core 5 and sheath 4 shown. 4 shows an optical fiber with a reflective element 11 at one end. FIGS. 5 and 6 show a ribbed light guide and in FIGS. 7 and 8 a buried light guide, each consisting of a laterally delimited light guide core 6 surrounded by a planar light guide support 7 and a planar cover 8, and partially provided with a reflective element 11 , pictured. In the case of film, rib and buried light guides, the light guide support and cover can consist of the same or different material.

The light guides have a first surface 9 and a second surface 10. The second surface 10 can with be provided with a reflective element 11. One or more measuring surfaces are located on the light guide core between the two end faces 9 and 10. A sensitive coating 12 is applied to the measuring surfaces.

Suitable materials for light guide cores are, for example, glasses such as quartz, BK7, Pyrex and Tempax or breakaway polymers such as polycarbonate or PMMA.

Low-index glasses or polymers can be used as materials for the cover and the light guide support.

The reflective element 11 can be implemented, for example, by applying a metal layer or a metal layer system to the second surface 10. The metal layer or the metal layer system can be platinum, gold, silver, palladium,

Chrome or aluminum included. Suitable thicknesses of the metal layer or layers are 100 nm to 10 μm.

In addition, there can be an adhesive layer between the metal layer or metal layer system and the second surface 10. Chromium, titanium, cobalt, vanadium or nickel with layer thicknesses in the range from 0.1 nm to 300 nm are suitable as adhesion promoters. Both metal layers and adhesion promoters must be selected such that no surface plasma waves can be excited resonantly with the arrangement .

The measuring surface on the light guide core can be removed by mechanically loosening the cladding, wet chemical etching, reactive ion etching, plasma ashing, etc. can be realized. The sensitive coating, which comprises at least one alloy layer, is then applied to the measuring surface.

Suitable alloy compositions can be determined, for example, by analyzing the optical constants of the alloy, such as refractive index n and damping constant k. Surface plasma waves can only be excited if the relationship jn] <] k | is satisfied. The characterization of the alloy layers takes place e.g. by means of an ellipsometer. From the statements obtained with an Eilipsometer, the suitable parameters for the production and application of an alloy which has surface plasma wave resonances in a suitable wavelength range can then be reproducibly determined.

The application of the alloy can e.g. by vapor deposition, sputtering or by another coating method. Binary alloys of gold and silver are preferably used. In addition, e.g. Palladium or nickel can be introduced. When using a silver-gold alloy, it is possible to observe transmission minima in water in a wavelength range between approximately 550 nm (pure silver) to 660 nm (pure gold). In this area, semiconductor lasers can be used as the excitation light source.

A component of a chemical or biochemical receptor-ligand complex, such as, for example, a component of the systems antigen / antibody, lectin / carbohydrate, is suitable as the substance-recognizing layer. Nucleic acid / protein, nucleic acid / nucleic acid, hormone / receptor, enzyme / enzyme cofactor, enzyme / substrate, protein A or protein G / immunoglobulin or avidin / biotin. The substance to be detected is accordingly either a receptor or a ligand with a high level

Affinity for the corresponding component of the substance-recognizing layer. Optionally, the respective immobilized receptors or ligands of the substance-recognizing layer can be bound to the alloy via an intermediate layer made of suitable chemically reactive groups.

A first exemplary embodiment of a sensor according to FIG. 4 based on a multimode optical fiber made of quartz glass 5 with a core diameter of 400 μm and with a sheath 4 made of a polymer is described below. The end face 10 of the fiber is coated with a reflective metal layer system 11, subsequently 5 nm chromium, 400 nm silver and 100 nm platinum. The polymer sheathing 4 is removed by means of a chemical solvent over a length of 7 mm (measured from the non-reflecting end face). The structure of the sensitive coating is shown in FIG. 9. Thermal evaporation leads to the exposed end of the

Fiber core 13 first applied an adhesion promoter layer 14 made of chrome. Then an alloy 15, which consists of approximately 80% silver and approximately 20% gold, is evaporated thereon. An receptor or ligand present as a protein is immobilized thereon with the aid of an intermediate layer 18 made of dithiobis-εuc-cinimidyl-propionate (DSP). For this purpose, the sensor is first immersed in a solution of 0.01 M DSP in acetone and then rinsed with acetone. Then comes the coated area of the Sensors in contact with proteins 17, which adhere very strongly to the sensor. Only the surface 19 of the substance-recognizing layer is in contact with the sample to be analyzed.

With this sensitive layer sequence, the sensor can be used for selective substance detection. The sensor with the gold-silver alloy described has the spectral range of greatest slope in an aqueous solution, i.e. greatest sensitivity, at about 635 nm. A commercially available laser diode is thus suitable as the excitation light source for the surface plasma waves which spread on the surface 16 of the alloy.

A second exemplary embodiment of a sensor according to the invention, based on a heterogeneous alloy, is shown in FIG. 22. The basic structure of this sensor essentially corresponds to that of the sensor according to FIG. 9. Again, chromium is first applied to the waveguide core 13 as an adhesion promoter 14. A discontinuous chromium layer with an average layer thickness of approximately 5 nm is formed on the waveguide core 13. The island formation is typical for the application of such thin layers.

Following the application of the adhesion promoter 14, a silver-gold alloy 15, 16 with phases in the nanometer range is deposited over the adhesion promoter layer. The alloy layer according to the invention can be produced in different ways: a) A molar mixture of 30 to 5% silver and 70 to 95% gold (powder, granules, etc.) is deposited on the surface of the waveguide by thermal vapor deposition. Due to the different melting and boiling temperatures of

Silver and gold will initially deposit silver, which preferably condenses on the islands of the adhesion promoter layer. Gold 16 is deposited between and on the silver 15. With this method, in particular heterogeneous alloys with separate phases can be realized.

b) A first closed silver layer is thermally evaporated or sputtered onto the adhesion promoter layer 14. The thin silver layer is thermally treated for 10 seconds. up to 20 min. broken up at 100 ° C to 300 ° C under a protective gas atmosphere. A silver network is formed around the bonding agent islands 14. Gold 16 is then deposited in and around the silver network. This layered structure is then surface-homogenized by thermal treatment under protective gas for up to 5 hours at 40 to 180 ° C.

Following the application of the alloy layer, further sensitive layers 17, 18, 19 are applied analogously to FIG.

The decisive advantage of the heterogeneous alloy lies in the possibility of an almost polarization-independent excitation of surface plasma waves. While with a closed layer of one pure metal surface plasma waves can only be excited with TM polarized light, the surface plasma waves can be excited with both TM and TE polarized light in the layer structure described. Since each light polarization is a linear combination of TE and TM polarized light, the surface plasma waves are excited according to the invention independently of polarization. This has the very significant advantage, among other things, that the resonance peak (cf. measurement diagram Fig. 23) is almost the same

Value 0 assumes at least 22. Since the measurement is carried out with monochromatic light on one of the two flanks 23, a significantly more sensitive or larger change in the measurement signal is observed regardless of the polarization of the coupled-in light than is the case with conventional surface plasma wave sensors. This fact is outlined in FIG. 24.

FIG. 24 shows the sensitivities of a sensor coated with silver and a sensor coated with a heterogeneous gold-silver alloy as a comparison. In both cases, identical optical fibers were coated with 5 nm chromium each. The alloy fiber was produced in accordance with point a) of the second exemplary embodiment and coated in a molar ratio of 90% gold to 10% silver. The metal layer thicknesses on both fibers were approximately 60 nm in each case. A laser diode emitting at 635 nm and a light emitting diode emitting at 1300 nm were used as the monochromatic light source. The ratio of the light power signals laser diode to light emitting diode served as the output signal. The refractive index n of an alcohol / water solution was determined as a measured variable by varying the Alcohol concentration changed in a range from 1,333 to 1,360. It can be clearly seen that the alloy fiber in the interesting range between 1,340 and 1,360 shows a sometimes drastically higher sensitivity than the silver fiber.

FIGS. 14 to 17 show devices for operating a sensor 47 according to the invention. The devices contain a laser diode 44 with an emission maximum at approximately 635 nm as a monochromatic first light source and a light-emitting diode 45 with an emission maximum at approximately 1350 nm and a spectral half-width of 70 nm as a second light source. Because of the same optical path lengths in the sensor, the light from the second light source can be used as a reference to the first light source. The alloy composition, the coating length and the laser used are matched to one another in such a way that the emission wavelength of the laser lies in the absorption region of the surface plasma resonance, advantageously in the region of greatest slope. The emission spectrum of the light emitting diode lies far outside the absorption range of the plasma resonance and is very narrow-band. Instead of the light-emitting diode, a monochromatic light source (e.g. semiconductor laser) could also be used.

The light is coupled into the light guide via a device 48 for mixing the light of the first with the light of the second light source and an optical interface 46, which defines the distribution of the angles at which the light beams can propagate in the light guide core. The optical interface defines a reproducible mode occupation in the light guide core. Besides, can influence on the half-width 24 and the slope of the absorption band 20 of the surface plasma waves. This allows the operating point or the sensitivity of the sensor system to be influenced. The mode of operation of an optical interface will be explained in more detail below.

An exemplary embodiment of an optical interface is shown in FIG. In the beam path in front of the first surface 9 of the light guide core 3, 5, 6 there is an aperture 26 in the form of a thin plate or film made of non-transparent material. The arrangement is characterized by the dimensions of the exit opening 25 of the light rays, the maximum half opening angle α of the optical measuring system, the dimensions of the first surface 9 of the light guide core 3, 5, 6, the critical angle at which light is located in the light guide core 3 , 5, 6 can just spread out, the diaphragm dimensions and the distances 27 and 28 between the output of the optical measuring system 25 and diaphragm 26 and the first surface 9. These parameters are chosen so that only light at a certain main angle β with the distribution ± L \ ß / 2 in the light guide core 3, 5, 6 is coupled.

12 shows a further embodiment of an optical interface with a lens 29. The distances 31 and 33 from the lens 29 to the outlet opening 25 and to the first surface 9, the focal length and the diameter of the lens are chosen so that all Light rays that leave the outlet opening 25 are coupled into the light guide core. A diaphragm 30 is located behind or in front of the line at a fixed distance 32 from the lens. The diaphragm 30 can also be direct be applied to the lens. Through openings 34 in the diaphragm, which are adapted to the shape of the waveguide, with a fixed mean distance 35 from the optical axis 36, only light beams with a main angle β with a distribution of ± Δ / 3/2 are coupled into the light guide core 3, 5, 6.

13 shows a further embodiment of an optical interface with two lenses 37, 39. The distances 38, 40 between exit opening 25 and lens 37 or between lens 39 and first surface 9 are chosen so that all are from the optical Measuring system emerging light rays behind the lens 37 run almost parallel after passing through an aperture 41 by means of lens 39, all the parallel light rays are coupled into the light guide core. The aperture 41 has, for example, circular-ring-shaped openings 42 at a fixed mean distance 43 from the optical axis 36. Only light rays with a main angle β and a distribution of ± Δ / 3/2 are coupled into the light guide core 3, 5, 6.

The light coupled into the sensor with the aid of the first optical interface, which has passed through the sensitively coated light guide area and has been more or less attenuated in its intensity, is coupled out again via the surface 10 and then fed to an optical measuring system (FIG guren 14 and 15). If the light is coupled out via the first interface due to a reflection on the second surface 10 of the sensor (FIGS. 16 and 17), a beam splitter 53 of the beam widening 1 to 2 can be found between the interface and the optical measuring system. This arrangement allows the detector to take arrangement (49, 51, 52) from the beam path between light sources 44, 45 and sensor 47.

As shown in FIGS. 14 and 16, the optical measuring system can contain a single optical detector 49 which, in multiplex operation, first of all, while the second light source 45 is switched off, determines the power of the outcoupled light of the first light source 44 and generates a first signal therefrom and then while the first light source 44 is switched off, determines the power of the outcoupled light of the second light source 45 and generates a second signal therefrom.

The optical measuring system can be alternative, as in the

15 and 17, also contain a beam splitter 50 of beam splitting 1 to 2 and two optical detectors optimized with regard to the detection wavelength, for example a silicon photodiode 51 for the laser diode and a germanium photodiode 52 for the light-emitting diode. The optical measuring system can operate in multiplex mode, ie the first detector 51 generates the first signal while the first light source 44 is switched on and the second light source 45 is switched off and then the second detector 52 generates the second signal ¬ rend the second light source 45 and the first light source 44 is turned off. Alternatively, both light sources can be switched on at the same time and the two detectors 51, 52 each determine only the power of the outcoupled light of the first or the second light source 44, 45 due to a wavelength-selective component in front of the detector input. From the first signal or from the first signal in combination with the second signal, a variable can subsequently be generated which contains statements about the change in resonance and thus about changes in the dielectric.

In addition to silicon or germanium photodiodes, InGaAs photodiodes or photomultipliers are also suitable as optical detectors for the measuring system.

With the components mentioned above, it is possible to implement a multi-sensor system which simultaneously determines the concentrations of a large number of chemical substances. In the following, a system with m optical sensors (m> 1) for determining the concentrations of m substances should be considered.

Such a multi-sensor system is shown in FIG. 18. Each of the m optical sensors 47 is provided with a specific substance-recognizing layer for the detection of one of the m substances to be detected. The light from the two light sources 44, 45 is first combined (mixed) via a beam splitter 54 of the beam widening 2 to m and then fanned out again into m partial beams. The partial beams are then coupled into the m sensors 47 with the aid of m first optical interfaces 46. The light that has passed through the sensitive coating is then coupled out at the other end 10 of the sensor and fed to an optical measuring system consisting of either m or 2 × m, optionally provided with wavelength-selective components and optionally operating in multiplexing, optical detectors 55. Then the Detector arrangement generates m independent quantities which are characteristic of the detection result of the individual sensor.

As shown in FIG. 19, the m sensors 47 can be provided with a reflective element at the second end 10, so that the reflected light is in turn coupled out via the m first interfaces 46 and via beam splitters 53 of the beam widening 1 to 2 of the optical detector arrangement is fed.

20 shows a multisensor system in which the m independent optical sensors are formed by a single planar waveguide 57, for example a film waveguide. The m light beams coupled in by means of m first interfaces 46 pass through m different paths within the light guide. The m light rays come into contact with m different substance-recognizing layers 56 of the sensitive coating 12 applied at different locations of the light guide.

The light which came into contact with the sensitive coating can, as shown in FIG. 20, via the second surface 10 or, if a reflection device is attached to the second surface 10, as shown in FIG. 21, via the first surface 9 are coupled out. The optical detector arrangement corresponds to that from FIGS. 18 and 19.

Claims

claims

1. Optical sensor based on the resonant excitation of surface plasma waves with the help of monochromatic light

with an optical fiber, the part of which is covered by a cover (1,

2, 4, 7, 8) surrounded Licht¬ core (3, 5, 6) has a first surface (9) and a second surface (10), the surfaces (9, 10) each of the light coupling and / or serve to decouple light and / or to reflect light, and

with one layer or several layers, which are arranged on at least one area of the light guide core (3, 5, 6) between the first and second surface (9, 10),

wherein at least one of the layers contains a metal alloy and the shape and / or position of the absorption band of the surface plasma waves is set via the metal alloy composition.

Optical sensor according to claim 1, characterized in that the metal alloy is a heterogeneous metal alloy.

3. Optical sensor according to claim 2, characterized in that at least one phase is present in the heterogeneous metal alloy in the form of separate regions with dimensions of up to 200 nm.

4. Optical sensor according to claim 2, characterized in that at least one phase in the form of a network is present in the heterogeneous metal alloy.

5. Optical sensor according to one of the preceding claims, characterized in that the metal alloy contains at least one of the two components gold or silver.

6. Optical sensor according to one of the preceding claims, characterized in that the layer containing the metal alloy has a thickness between 10 nm and 500 nm and is applied to the light guide core over a length of between 0.1 mm and 100 mm.

7. Optical sensor according to one of the preceding claims, characterized in that the light guide is an optical fiber, a

Rib light guide, a buried light guide, a film light guide or another planar light guide.

8. Optical sensor according to one of the preceding claims, characterized in that the light guide is a multimode light guide iεt.

9. Optical sensor according to one of the preceding claims, characterized in that an adhesion promoter layer is applied between the layer containing the metal alloy and the light guide core.

10. Optical sensor according to claim 9, characterized in that the adhesion promoter layer is applied in the form of a discontinuous layer to the light guide core.

11. Optical sensor according to one of the preceding claims, characterized in that a substance-recognizing layer is applied to the layer containing the metal alloy.

12. Optical sensor according to claim 11, characterized in that the substance-recognizing layer contains a component of a chemical or biochemical receptor-ligand complex.

13. Optical sensor according to claim 11 or 12, characterized in that between the layer containing the metal alloy and the substance-recognizing layer there is a further layer with chemically reactive groups for binding the receptor or the ligand to the metal alloy layer.

14. Multi-sensor system consisting of several optical sensors according to one of claims 1 to 13, characterized in that several sensors (47) are arranged side by side in the beam direction.

15. Multi-sensor system according to claim 14, characterized in that the individual sensors (47) are integrated in a single planar waveguide (57) and each sensor has a separate substance-detecting end

Layer (56) is assigned.

16. Device for operating an optical sensor according to one of the preceding claims, including

a first optical interface (46), with the aid of which the light can be coupled into and / or out of the light guide via the first surface (9) of the sensor (47) at a defined angle with a defined angle distribution, and

an optical measuring system containing a first, monochromatic light source (44) with an emission wavelength in the range of

Absorption spectrum of the surface plasma waves and the optical power of the output

Determining light from the first light source (44), generating a first signal therefrom and under

Using the first signal, the output measurement variable-determining optical detector arrangement (49, 51, 52, 55).

17. The apparatus as claimed in claim 16, characterized in that a second optical interface, with the aid of which the coupled light can be coupled out via the second surface (10), is present.

18. The apparatus according to claim 16 or 17, characterized in that the optical interfaces contain at least one aperture (26, 30, 41) made of non-transparent material and in the optical beam path in front of the first surface (9) and / or after the second Surface (10) are arranged.

19. The apparatus according to claim 18, characterized in that the at least one aperture (26) is adapted to the geometry of the first and / or second surface (9, 10).

20. The apparatus according to claim 18 or 19, characterized in that the at least one aperture (30, 41) to the geometry of the first and / or second surface (9, 10) has recesses.

21. Device according to one of claims 18 to 20, characterized in that the optical interface further includes at least one optical lens (29, 37, 39) or an optical lens system and the at least one lens or the lens system in the beam path between the light source and the at least one diaphragm and / or between diaphragm and sensor and / or between diaphragm and optical measuring system are arranged.

22. Device according to one of claims 16 to 21 for operating a multi-sensor system according to one of claims 14 or 15, characterized in that a first optical element for each sensor (47)

Interface (46) is present and that the optical measuring system also contains a beam splitter (54) for distributing the light to be coupled into the individual first optical interfaces (46), with the aid of the optical detector arrangement (55) for each sensor ( 47) an output measurement variable can be determined independently.

23. Device according to one of claims 16 to 22, characterized in that the optical measuring system further comprises a second, mono- or polychromatic light source (45) with a wavelength or a wavelength spectrum outside the range of the absorption spectrum of the surface plasma waves, de¬ light passes through the same optical light path as the light from the first light source, and

a device (48) for mixing the light of the first with the light of the second light source,

wherein the optical detector arrangement (49, 51, 52, 55) additionally determines the optical power of the decoupled light from the second light source (45), generates a second signal therefrom and determines the output measurement variable using the first and the second signal.

24. The apparatus according to claim 23, characterized gekenn¬ characterized in that the optical measuring system for each sensor (47) alternately first determine the optical power of the outcoupled light of the first Lichtquel¬ le (44) and from it generate the first signal and then the optical Power of the decoupled light from the second light source (45) determining optical signal and generating the second signal therefrom

(49, 55).

25. The apparatus according to claim 23, characterized gekenn¬ characterized in that the optical measuring system for each sensor (47) one of the optical power of the decoupled light of the first frequency modulated with a first frequency (44) determining and therefrom with the aid of a first demodulator contains the first signal generating and the optical power of the outcoupled light of the second light source (45) modulated with a second frequency and with the aid of a second demodulator the second signal generating optical detector (49, 55).

26. The apparatus according to claim 23, characterized gekenn¬ characterized in that the optical measuring system for each sensor includes two optical detectors (51, 52), the first optical detector (51) independently of the second detector (52), the optical power of the decoupled light the first light source (44) is determined and the first signal is generated and the second optical detector (52) independently of the first detector (51) determines the optical power of the outcoupled light from the second light source (45) and the second signal is generated therefrom.

27. The apparatus according to claim 26, characterized in that a wavelength-selective component is attached in front of the first detector (s), which component is only permeable for the light coupled out of the first light source (44) and in front of the A wavelength-selective component is attached to the second detector (s), which is only used for the outcoupled light of the second light source

(45) is permeable.

28. Device according to one of claims 16 to 27, characterized in that each sensor (47) at least one, in

The light path between the sensor (47) and the optical detector arrangement of the beam splitter (50, 53) is assigned to the beam expansion 1 to 2.