DE102015007206A1 - Optical sensor - Google Patents

Abstract

The optical sensor is used to measure trace constituents in liquids and / or gases and has at least one optical resonator, which has at least one porous layer whose pores serve to receive moisture particles. The resonator is a ring resonator, which is associated with at least one optical waveguide. In the waveguide light is fed, which is partially decoupled in the ring resonator. In the ring resonator arise due to its sometimes very high finesse interference with the waveguide, in which a light spectrum is irradiated. From the transmission spectrum a comb of wavelength minima can be filtered out. If trace constituents are deposited in the pores of the ring resonator, the refractive index and thus the optical path length are changed, which leads to a wavelength shift of the wavelength minima filtered out. From the wavelength shift, the content of trace constituents can be determined.

Description

The invention relates to an optical sensor according to the preamble of claim 1.

Optical sensors are used to measure trace moisture in gases and liquids. They consist of several porous and dielectric layers, in the pores of which any moisture particles are deposited. If light is radiated by the moisture sensor, the refractive index of the incident light changes as a result of the embedded moisture particles. This leads to a wavelength shift in the optical sensor which is proportional to the humidity in the medium to be measured. From the wavelength shift, the content of moisture in the medium to be measured can be detected.

The measuring principle is based on the basic principle of a Fabry-Perot interferometer. However, for many applications it has too slow a response to changes in humidity. The cause of this relatively slow reaction behavior lies in the structure of the layer system and in the thicknesses and porosities of the individual layers. The thicker the layer system, the slower it dries in a gas stream. On the other hand, the thickness of the layer system is critical to the finesse of the Fabry-Perot interferometer and thus also determines the half-width at the wavelength minimum. This significantly influences the accuracy of the signal analysis.

The known optical sensor has a layer system of eleven alternating low and high refractive porous layers, which consist of SiO ₂ , ZrO ₂ . The layer system has a total thickness of more than 2 μm. Such a design of the optical sensor represents a compromise between the lowest possible thickness and a high finesse. The measured with such an optical sensor response times at a dew point change between + 10 ° C and -60 ° C at a gas temperature of 30 ° C vary due different porosity of the layers between 1 and 7 hours.

The invention has the object of providing the generic optical sensor in such a way that the measurement times are significantly reduced without affecting the measurement accuracy.

This object is achieved in the generic optical sensor according to the invention with the characterizing features of claim 1.

In the sensor according to the invention, the resonator is formed by a ring resonator. In the ring resonator due to its sometimes very high finesse interference with the waveguide, in which a light spectrum is irradiated. From the transmission spectrum a comb of wavelength minima can be filtered out. If trace constituents are deposited in the pores of the ring resonator, the refractive index and thus the optical path length are changed. This leads to a wavelength shift of the wavelength minima filtered out. From the wavelength shift can be determined in a known manner, the content of trace constituents in the medium to be measured with high accuracy. With the sensor according to the invention results compared to the known sensor a greatly reduced thickness of the moisture layer, whereby a much faster reaction time can be made possible.

Generally, a modems spectrum is filtered out of the transmission spectrum, which can be used to determine the content of trace constituents. The shift of the wavelength minima offers the possibility to calibrate highly accurate gas characteristics.

The ring resonator is self-contained, but may have any outline shape.

In an advantageous embodiment, the ring resonator is assigned a further waveguide. The modems spectrum or the discrete wavelength maxima running in the ring resonator are partially decoupled into it. The decoupled light can be fed separately to a spectrometer, with which the decoupled light is evaluated.

In an advantageous embodiment, a plurality of ring resonators are arranged one behind the other along the waveguide. Such a design allows a better measurement of the trace moisture content. Also, the plurality of ring resonators may be formed so that they have different sized pores, so that when several ring resonators and different trace constituents can be detected in a measurement process. The pore size is adapted to the trace constituent to be detected. Thus, for example, by corresponding pore size of the ring resonators simultaneously different concentrations of H ₂ O and / or other molecules can be detected in sample gases.

In an advantageous embodiment, the plurality of ring resonators may be located between the two waveguides, so that the ring resonators at least partially decouple the coupled light in the other waveguide. The further waveguide thus makes it possible to add additional discrete wavelength maxima to the mode spectra of the further ring resonators and to feed them to a spectrometer or a similar measuring device.

In a further advantageous embodiment, two adjacent ring resonators are located between the two waveguides.

In a further embodiment, a plurality of such annular resonators arranged in pairs may be provided one behind the other between the two waveguides.

The ring resonator (s) and the waveguide (s) may be provided on a carrier. It can be made of glass or crystal, for example.

However, it is also possible for the ring resonator (s) to be located on the carrier while the waveguide (s) are embedded in the carrier. The waveguide is thus protected in the sensor.

In an advantageous embodiment, the ring resonator (s) may be wholly or partially surrounded by a cladding layer on the outer diameter. This cladding layer is advantageously at least one optical PVD layer. The material of the cladding layer has a refractive index which is smaller than the refractive index of the material of the ring resonator. The cladding layer can be compared to the gas to be measured or the liquid to be measured both as a protective layer and, with a defined pore size, which is smaller than the pore size of the material of the ring resonator, as a filter layer and with a defined pore size, which is greater than the pore size of the material of the ring resonator, serve as a storage layer.

The waveguide may be made of ion-implanted glasses or crystals or PVD optical layers. Such PVD layers may for example consist of SiO _{2 ,} ZrO ₂ , TiO ₂ or TaO ₅ .

Advantageously, the ring resonator on optical PVD layers, the z. As SiO ₂ , ZrO ₂ , TiO ₂ or TaO _{. 5} could be.

The subject of the application results not only from the subject matter of the individual claims, but also by all the information and features disclosed in the drawings and the description. They are, even if they are not the subject of the claims, claimed as essential to the invention, as far as they are new individually or in combination over the prior art.

Further features of the invention will become apparent from the other claims, the description and the drawings.

The invention is explained below with reference to some embodiments shown in the drawings. Show it

1 a schematic representation of a first embodiment of a sensor according to the invention,

2 from the transmission spectrum of the sensor according to 1 filtered wavelength minima,

3 in a representation accordingly 1 a second embodiment of a sensor according to the invention,

4 in a representation accordingly 2 the wavelength minima resulting from the sensor according to 3 assigned transmission spectrum are filtered out,

5 an enlarged section of the diagram after 4 .

6 to 10 more diagrams accordingly 5 to explain a wavelength shift,

11 to fourteen in representations accordingly 1 further embodiments of sensors according to the invention,

15 a top view of a further embodiment of a sensor according to the invention,

16 a side view of the sensor according to 15 .

17 a front view of the sensor according to 15 .

18 to 21 each in section further embodiments of sensors according to the invention,

22 the sensor according to 15 to 17 which is connected to an LED light source and a polychromator.

The sensors are used to measure trace moisture or trace gases in gases and / or liquids. For example, the sensor determines the moisture content in natural gas. For this purpose, the sensor protrudes into the natural gas pipeline so that it is detected by the natural gas flowing through it. The sensor is connected to a measuring device (not shown) which detects the signals coming from the sensor and evaluates them in a known manner. The determined moisture content is assigned to a dew point by means of an evaluation unit of the measuring device.

The optical sensor will be described below using the example of a humidity sensor. It can also be designed as a gas sensor with which trace gases in the medium can be detected and measured.

The humidity sensor has in the embodiment according to 1 a ring resonator 2 , which serves as optical measuring medium. The ring resonator is a self-contained waveguide, which has a circular shape in the exemplary embodiment. The ring resonator 2 but may have any other suitable geometric shape.

The ring resonator 2 has a porous coating, which is preferably at least one PVD layer vapor-deposited on a base support. This porous layer consists for example of SiO ₂ , ZrO ₂ or Ta ₂ O ₅ . The porosity of this layer is matched to the trace constituent to be detected. If this trace constituent to be detected is water, then the pores of the porous layer typically have a diameter of about 3 Å. If the medium to be measured contains water, then water molecules, the diameter of which is slightly smaller than 3 Å, are deposited in the pores of the porous layer. The water molecules incorporated in the porous layer change, as will be described, the refractive index of an incident light, which results in a wavelength shift which is proportional to the moisture content in the medium to be measured.

The light necessary for measuring the moisture content is transmitted through a waveguide 1 in the ring resonator 2 coupled. The waveguide 1 may be made of suitable materials, such as ion-implanted glasses and crystals, or of PVD optical layers, which may be, for example, SiO ₂ , ZrO ₂ , TiO ₂ , Ta ₂ O _5, and the like. In the waveguide 1 is by an arrow 8th indicated light initiated. The light source used is preferably LED, which has a long service life, is inexpensive and requires no maintenance. Further sources of light are halogen lamps or, for example, also laser LEDs or laser diode-pumped phosphor layers or pumped Ti: sapphires.

The thickness of the porous layer is for example some 100 nm. However, this value is not to be understood as limiting. The layer has such a thickness that an optimally fast adaptation to changing moisture conditions in a gas or liquid stream is possible.

The ring resonator is a ring waveguide in which a continuous wave is formed due to the total reflection capability of the ring waveguide.

The smallest distance 9 between the waveguide 1 and the ring resonator 2 is the measurement distance that determines the size of the coupling factor. It is a measure of the decoupling of part of the light from the waveguide 1 ,

The ring resonator 2 sometimes has a very high finesse. This has the consequence that in the ring resonator 2 Interference with the waveguide 1 arise. The proportion of the ring resonator 2 coupled portion of the light 8th can be represented in a transmission spectrum in which the intensity against the wavelength of light 8th is applied. From this transmission spectrum, the wavelength minima can be filtered out. An example of this shows 2 in which, from the transmission spectrum, a comb of wavelength minima, which at the coupling of the light from the waveguide 1 in the ring resonator 2 arise, have been filtered out.

If there is moisture in the medium to be tested, in this case water, then the water molecules are stored in the porous layer of the ring resonator 2 from. The incorporated water molecules change the refractive index of the ring resonator 2 coupled light. This leads to a wavelength shift that is proportional to the moisture content in the medium to be measured.

On the basis of the wavelength shift, which is visible in the transmission spectrum or in the filtered-out wavelength minima, the moisture content in the analyst can be determined with high accuracy. This will be explained in more detail with reference to the following embodiment of the sensor.

In the embodiment according to 3 are the waveguide 1 two ring resonators 2 . 3 assigned. They are advantageously designed differently and with the measuring distance 9 next to the waveguide 1 arranged in which the light 8th is initiated. In the manner described, a part of the light is in the ring resonators 2 . 3 coupled. From the associated transmission spectrum two combs of wavelength minima can be filtered out. The corresponding diagram shows 4 ,

5 shows the wavelength range between 1,300 and 1,307 nm 4 in an enlarged view.

6 shows in each case a wavelength minimum of two ring resonators, which have been filtered out of a transmission spectrum, which result when entering the porous layer of the ring resonator 2 Store water molecules in the non-porous layer or in the impermeable protective layer coated porous layer of the ring resonator 3 no water molecules get. A comparison of 5 and 6 shows that due to the embedded water molecules, the one wavelength minimum from the ring resonator 2 has shifted from the wavelength of 1.301.45 nm to the wavelength of 1.301.68 nm. This wavelength shift, caused by the stored water molecules, is proportional to the measured moisture in the medium. The wavelength minimum of the ring resonator 3 , which is in the embodiment at a wavelength of 1.301,1 nm, serves as a reference value and can compensate for temperature influences. The evaluation unit in the measuring device to which the sensor is connected, can determine the dew point and thus the proportion of moisture in the medium to be tested due to the difference between the two wavelength minima and at a known temperature of the medium.

The temperature of the medium would be exemplary, as in 13 shown with another additional ring resonator of a non-porous layer of a material with a high thermo-optical coefficient dn / dt, which is strongly dependent on the thermo-optical coefficient of the two ring resonators 2 and 3 differs, determine, if with this a difference value of its wavelength minimum to the reference value of the ring resonator 3 measured and this difference value would be assigned a temperature value based on a calibration curve.

The 7 to 10 show examples of how the wavelength shift depends on the moisture content in the medium to be measured. Exemplary is in 7 shown that the two wavelength minima are at a wavelength of 1,300 and 1,303.1 nm. Water molecules are stored in the porous layer of the ring resonator (s) 2 This leads to a wavelength shift, as shown in the diagram 8th evident. Both wavelength minima have shifted to 1,303.1 and 1,303.4 nm.

With an even greater amount of water shifts the in 9 right wavelength minimum to a wavelength value of 1,303.65 nm. With an even greater water content, this wavelength minimum shifts to a wavelength of almost 1.304 nm ( 10 ).

These examples show that the wavelength shift is detected with high accuracy and accordingly the moisture content in the medium to be measured can be determined very accurately.

11 shows a further embodiment of a sensor. In contrast to the previous embodiments, two waveguides 1 . 5 provided between which is the ring resonator 2 located. Both waveguides 1 . 5 are advantageously made of the same material, such as from ion-implanted glasses or crystals, or of PVD optical layers consisting, for example, of SiO ₂ , ZrO ₂ , TiO ₂ or Ta ₂ O ₅ . In the waveguide 1 becomes the light 8th introduced, in the manner described partially in the ring resonator 2 is coupled. The in the ring resonator 2 forming mode spectra or discrete wavelength maxima are in the waveguide 5 decoupled, the decoupled light 8th' forwards. The decoupled and through the waveguide 5 forwarded light 8th' can be fed separately to a spectrometer.

The two waveguides 1 . 5 are advantageous with the same measuring distance 9 next to the ring resonator 2 arranged.

From the transmission spectrum, which is when coupling the light 8th from the waveguide 1 in the ring resonator 2 results, with a occurring wavelength shift in the manner described, the moisture content can be determined.

In the embodiment according to 12 are in turn the two waveguides 1 . 5 provided, of which the waveguide 1 the light 8th in the ring resonator 2 coupled, as has been described with reference to the previous embodiments. Next to the ring resonator 2 there is a second ring resonator 10 , with the measuring distance 9 next to the ring resonator 2 lies. This can cause light from the ring resonator 2 in the ring resonator 10 be coupled. The waveguide 5 makes sure that in the ring resonator 10 current mode spectrum or the discrete wavelength maxima in the waveguide 5 be decoupled. The decoupled light 8th' in turn can be fed to a spectrometer.

The use of two adjacent ring resonators 2 . 10 offers the advantage that the second ring resonator 10 could be so dimensioned that he z. B. only every second wavelength maximum from the ring resonator 2 and thus allows twice as large free spectral range. This principle is helpful if the wavelength shift between maximum dryness and maximum humidity is greater than the free spectral range in the first ring resonator 2 , In this way, the entire humidity range can be detected without ambiguity.

13 shows the possibility of multiple ring resonators 2 to 4 one behind the other between the two waveguides 1 and 5 to arrange. The light 8th that the waveguide 1 is supplied in the manner described in the respective ring resonators 2 to 4 coupled and into the waveguide 5 decoupled. The decoupled light 8th' can with the waveguide 5 be supplied to the spectrometer, for example. In addition, the waveguide serves 5 in this embodiment, in addition thereto, further discrete wavelength maxima from the mode spectra of the various ring resonators 2 to 4 to add. In this way, for. B. several to many ring resonators come with different pore sizes from fine to coarse and thus different gases are distinguished from each other. Also, a ring could act as a reference signal that would compensate for interference, and a ring made of another material with a high thermo-optic coefficient, which could serve to determine the gas temperature.

In 13 is through the between the ring resonators 3 and 4 indicated points that the number of ring resonators may vary depending on the application of the sensor.

According to the embodiment fourteen are between the two waveguides 1 . 5 the ring resonators 2 to 4 and 10 to 12 in pairs. That over the waveguide 1 supplied light 8th gets into the respective ring resonators 2 to 4 coupled. This coupled light is in turn in the ring resonators 10 to 12 coupled to the light in the waveguide 5 couple out. This decoupled light 8th' In turn, for example, is fed to a spectrometer. As in the embodiment according to 13 explained, the wavelength maxima from the mode spectra of the individual ring resonators 10 to 12 the waveguide 5 supplied, which thus adds up these wavelength maxima.

Also in this embodiment, as indicated by the points, further ring resonators can be provided in both vertical rows, depending on the application of the sensor.

The 15 to 17 show a concrete embodiment of a sensor. The waveguide 1 is in a carrier 13 , which may be made of glass or crystal, for example. The two ends fourteen . 15 of the waveguide 1 open into a frontal side 16 of the carrier 13 , The at the ends fourteen . 15 subsequent sections 17 . 18 of the waveguide 1 initially run parallel to each other and go over a loop part 19 into each other. The waveguide 1 is located at a short distance below the top 20 of the carrier 13 ,

On the carrier top 20 are the two ring resonators 3 and 4 arranged at the level of the loop part 19 of the waveguide 1 seen in plan view of the sensor. The ring resonators 3 . 4 are spaced next to each other and so are in relation to the waveguide 1 arranged them according to plan view 15 seen, overlap (see also 16 ).

That in the waveguide 1 fed light 8th gets into the ring resonators 3 . 4 coupled in the manner described. The ring resonators 3 . 4 come as they are at the top 20 of the carrier 13 are arranged, with the medium to be measured in contact. The molecules of any moisture contained in the medium are deposited in the porous layer of at least one ring resonator 3 and or 4 in the manner described, lead to a wavelength shift of the wavelength minima, as has been explained with reference to the previous exemplary embodiments.

18 shows a section through the sensor according to 11 , On a carrier 6 , which can be made of glass or crystal, are the two waveguides 1 and 5 applied, between which is the ring resonator 2 located. The waveguides 1 . 5 and the ring resonator 2 can be at the top 21 of the carrier 6 very easy to apply. During the measurement, the sensor is arranged so that the medium to be measured with the ring resonator 2 comes into contact, allowing moisture molecules in the pores of the porous layer of the ring resonator 2 can settle.

The embodiment according to 19 differs from the previous embodiment in that the two waveguides 1 . 5 not at the top, but at a short distance from the top 21 inside the vehicle 6 are arranged. The distance between the waveguides 1 . 5 and the ring resonator 2 is chosen so that over the waveguide 1 fed light into the ring resonator 2 coupled and from the ring resonator in the waveguide 5 can be disconnected. The measuring distance between the waveguides 1 . 5 and the ring resonator 2 is in this case perpendicular to the carrier top 21 available. The location of the waveguides 1 . 5 is chosen so that it is sectioned below the ring resonator 2 lie.

20 shows an embodiment of a sensor, which basically has the same structure as the sensor according to 19 Has. The ring resonator 2 is of a cladding layer 7 surrounded by a material having a smaller refractive index n than the coated ring resonator 2 , The coat layer 7 is advantageous an optical PVD layer. It can for example consist of SiO ₂ , which has a refractive index n of 1.45. The ring resonator 2 may in this case for example consist of ZrO ₂ with a refractive index n of 2.14.

The coat layer 7 can the ring resonator 2 completely or partially surrounded. The coat layer 7 may be used both as a protective layer and as a filter layer or as a storage layer with respect to the gas to be measured or the liquid to be measured. In the case of a filter layer, the cladding layer has 7 a defined pore size that is smaller than the pore size of the material of the ring resonator 2 , In the case of an intercalation layer, the cladding layer has 7 a defined pore size that is greater than the pore size of the material of the ring resonator 2 ,

The training according to the 18 to 20 may be provided in the sensors, which are the two located on either side of the ring resonators or the waveguide 1 . 5 having ( 11 to fourteen ).

21 shows a sensor with a layer structure. The carrier 6 is unlike the embodiments according to the 18 to 20 designed as a ring. On the carrier 6 is the ring resonator 2 attached, according to the embodiment according to 20 from the cladding layer 7 is surrounded. Besides, on the carrier 6 the waveguide 1 attached. He is in the coat layer 7 embedded the ring resonator 2 surrounds. The waveguide 1 and the ring resonator 2 with the cladding layer 7 lie in a common plane and form a first layer 22 on the carrier 6 is attached.

On the shift 22 is with the interposition of an insulating layer 23 a second layer 24 applied, which is the same design as the first layer 22 , The insulating layer 23 prevents the two layers 22 . 24 interfere with each other during the measurement process.

This way are on the carrier 6 one above the other with the interposition of an insulating layer 23 more layers 25 . 26 applied. The dots above the sensor indicate that additional layers may be provided.

In the embodiments with multiple ring resonators, it is possible to tune the ring resonators to different trace constituents. Thus, for example, with the help of one ring resonator water and with the help of the other ring resonator ammonia can be detected in the medium in one measurement. The pore size is adapted to the molecular size of the trace constituents to be detected. The molecular sizes of some trace constituents are given below: water 0.28 nm ammonia 0.30 nm carbon monoxide 0.32 nm carbon dioxide 0.33 nm Hydrochloric 0.35 nm hydrogen sulfide 0.36 nm methanol 0.38 nm methyl mercaptan 0.45 nm

The pores of the ring resonator are adapted to the molecular sizes given above so that the corresponding molecules can be deposited in the pores.

The ring resonators 2 in the different layers can be matched to different components to be detected, so that the corresponding trace components can be detected in the medium to be measured.

22 shows the sensor according to the 15 to 17 with the waveguide 1 that is connected to a light source 27 connected. It is in the illustrated embodiment, an LED light source with which the light in the waveguide 1 over a fiber cable 28 is introduced. That in the waveguide 1 fed light passes through the waveguide 1 in the manner described and ends up 15 in another fiber cable 29 that the decoupled light is a polychromator 30 supplies. The light passes over a slit diaphragm 31 in the polychromator 30 and enters a concave grid 32 at which the light to a CCD detector 33 is reflected. It is connected to the meter (not shown), that of the detector 33 evaluates incoming signals in the manner described.

Claims (14)

Optical sensor for measuring trace constituents in liquids and / or gases, comprising at least one optical resonator having at least one porous layer, the pores of which serve to receive wet particles, characterized in that the resonator is a ring resonator ( 2 to 4 . 10 to 12 ), to which at least one optical waveguide ( 1 . 5 ) is assigned to the light ( 8th ) fed into the ring resonator ( 2 to 4 . 10 to 12 ) is partially decoupled. Sensor according to claim 1, characterized in that the ring resonator ( 2 to 4 . 10 to 12 ) a further waveguide is assigned, in which the in the ring resonator ( 2 to 4 . 10 to 12 ) coupled light ( 8th ) is at least partially decoupled. Sensor according to claim 2, characterized in that in the further waveguide ( 1 . 5 ) decoupled light ( 8th' ) is fed to a spectrometer. Sensor according to one of claims 1 to 3, characterized in that a plurality of ring resonators ( 2 to 4 . 10 to 12 ) in succession along the waveguide ( 1 . 5 ) are arranged. Sensor according to claim 4, characterized in that the plurality of ring resonators ( 2 to 4 . 10 to 12 ) the coupled light in the further waveguide ( 1 . 5 ) decouple at least partially. Sensor according to one of claims 2 to 5, characterized in that between the two waveguides ( 1 . 5 ) two adjacent ring resonators ( 2 to 4 . 10 to 12 ) are provided. Sensor according to claim 6, characterized in that a plurality of annular resonators arranged in pairs ( 2 to 4 . 10 to 12 ) are arranged one behind the other. Sensor according to one of Claims 1 to 7, characterized in that the ring resonator (s) ( 2 to 4 . 10 to 12 ) and the waveguide (s) ( 1 . 5 ) on a support ( 6 . 13 ) are provided. Sensor according to one of Claims 1 to 7, characterized in that the ring resonator (s) ( 2 to 4 . 10 to 12 ) on a support ( 6 . 13 ) and the waveguide (s) ( 1 . 5 ) in the carrier ( 6 . 13 ) is / are embedded. Sensor according to one of Claims 1 to 9, characterized in that the ring resonator (s) ( 2 to 4 . 10 to 12 ) of a cladding layer ( 7 ) is wholly or partly surrounded, whose material has a smaller refractive index ( 4 ) than the material of the ring resonator. Sensor according to one of Claims 1 to 10, characterized in that the ring resonator (s) ( 2 to 4 . 10 to 12 ) of a filter layer ( 7 ) is or are completely or partially surrounded, whose material for filtering molecules has a smaller pore size than the material of the ring resonator. Sensor according to one of claims 1 to 10, characterized in that the / the ring resonator / s is surrounded by a storage layer with a corresponding pore size for molecules / are. Sensor according to one of claims 1 to 12, characterized in that the waveguide ( 1 . 5 ) consists of ion-implanted glasses or crystals or optical PVD layers. Sensor according to one of Claims 1 to 13, characterized in that the ring resonator (s) ( 2 to 4 . 10 to 12 ) has at least one optical PVD layer, such as SiO ₂ , ZrO ₂ , TiO ₂ or TaO ₅ .

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