WO2001090712A1

WO2001090712A1 - Device for determining or for monitoring the pressure or differential pressure of at least one process medium

Info

Publication number: WO2001090712A1
Application number: PCT/EP2001/005825
Authority: WO
Inventors: Dietfried Burczyk; Thomas Velten; Jörg Müller; Manfred Kasper
Original assignee: Endress + Hauser Gmbh + Co. Kg.
Priority date: 2000-05-19
Filing date: 2001-05-21
Publication date: 2001-11-29
Also published as: AU2001263937A1; DE10192126D2; DE10024588A1

Abstract

The invention relates to a device for determining/monitoring the pressure or differential pressure of at least one process medium. The aim of the invention is to provide a device for measuring pressure or differential pressure extremely precisely. To this end, an energy supply unit (4), a medium with a laser capacity (7) introduced into a resonator, a pressure-stress converter (11) and a receiving/evaluating (19) unit are provided. Said energy supply unit (4) pumps said medium (7), exciting the medium to produce laser activities. Without the exertion of a load, when this medium with laser capacity or the other medium is subjected to the same pressure on all sides, it has isotropic properties. The laser formed by the resonator and the medium with laser capacity emits radiation whose frequency depends on the physical properties of said medium (7). The pressure-stress converter (11) transfers the pressure or differential pressure to be measured of the at least one process medium to at least one partial area of the medium with laser capacity (7). This defines a state in which the medium (7) is subject to a load and in which the isotropic properties of said medium (7) are disturbed. The radiation emitted when the medium with laser capacity (7) is subject to a load has different frequencies in different directions of polarisation and the receiving/evaluating unit (19) determines the pressure or differential pressure of the at least one process medium using the different frequencies in the various directions of polarisation.

Description

Device for determining or monitoring the pressure or differential pressure of at least one process medium

The invention relates to a device for determining or monitoring the pressure or differential pressure of at least one process medium.

From DE 43 22 291 C2 an optical force measuring device has become known, the main component of which is a laser, that is to say a laser-capable crystal, in particular a neodymium-doped YAG crystal, with mirrored end faces. The laser is optically pumped using a laser diode. At the same time, the laserable crystal has photoelastic properties: if no force acts on the YAG crystal, it shows an isotropic behavior. In this case, isotropic behavior means that the light in the crystal 'sees' a refractive index that is independent of the polarization direction of the light. The laser then emits coherent light of one wavelength. If a force acts on one side of the YAG crystal, the crystal lattice is distorted; the crystal properties become anisotropic. The consequence of this is that the refractive indices and therefore also the optical resonator length and consequently also the frequencies of the radiation emitted in different directions of polarization are different from one another. The frequency difference between the radiation emitted in two different polarization directions is proportional to the force acting on the crystal.

In DE 43 22 291 C2 it is mentioned succinctly that the force measuring device can also be used to measure variables derived from the force, such as acceleration, pressure or mass. However, the patent does not provide any information as to how the measuring device can be designed in concrete terms if it is to be used, for example, for pressure measurement. The invention has for its object to propose a device for high-precision pressure or differential pressure measurement.

The object is achieved in that the device for determining / monitoring the pressure or differential pressure of at least one process medium has an energy supply unit, a laser-capable medium introduced into a resonator, a pressure-voltage converter and a reception / evaluation unit, the energy supply unit having the laser-capable one Pumps medium and stimulates laser activities, whereby either the laserable medium or, in addition to the laserable medium, a further medium introduced into the resonator in the unloaded state, if the same pressure acts on the laserable medium or the further medium on all sides, has isotropic properties and does so the resonator, the laser-capable medium and possibly the further medium introduced into the resonator are emitted laser radiation with a frequency dependent on the physical properties of the laser-compatible medium or the further medium, the pressure-voltage converter transmits the pressure or differential pressure to be measured of the at least one process medium to at least a portion of the laserable medium or the further medium, thereby defining a loaded state of the laserable medium or the further medium in which the isotropic properties of the laserable medium or further medium are disturbed, the radiation emitted in the loaded state of the laserable medium having different frequencies in different polarization directions, and the receiving / evaluation unit using the different frequencies in the different polarization directions the pressure or the differential pressure of the at least one process medium certainly. According to an advantageous development of the device according to the invention, in the case of a differential pressure measurement, the pressure-voltage converter is designed such that the two pressures act on the laserable medium or on the further medium at an angle of essentially 90 ° to one another.

The pressure-voltage converter can be, for example, a plunger and / or an ideally incompressible liquid. If a plunger is used, the pressure or differential pressure acts on at least one membrane to which the plunger is attached, which then, depending on the pressure present, presses more or less strongly on the medium showing the photoelastic effect and induces mechanical stresses there. It is also possible that the pressure acts on a membrane which separates the process medium from the incompressible liquid, e.g. B. silicone oil separates. Then the pressure of the process medium is transferred to the incompressible liquid or the incompressible medium and from the incompressible medium to the medium which shows the photoelastic effect.

In addition or separately from the pressure-voltage converters already mentioned, a membrane as such can also take on the function of a pressure-voltage converter. This configuration is a particularly preferred variant, which will be discussed in more detail below.

The laserable medium is, for example - as described in DE 43 22 291 C2 - a laserable Nd: YAG crystal, ie a neodymium-doped yttrium aluminum garnet crystal. Incidentally, a Nd.YAG crystal has the required photoelastic properties. However, the laserable material is preferably a laserable semiconductor material. Suitable semiconductors are materials which, in accordance with a preferred embodiment of the device according to the invention, additionally show the photoelastic effect. In particular, the semiconductor materials can be constructed on a gallium or silicon basis. Another preferred laser-compatible material is LiNb0 ₃ . Although it is favorable if the laserable medium also shows the photoelastic effect at the same time, this is nevertheless not a necessary requirement for the design of the pressure or differential pressure sensor according to the invention. It is also entirely possible to use a medium with photoelastic properties in addition to the laserable medium. The laserable material is preferably arranged in the form of a resonator, as will be explained in more detail below.

The photoelastic medium can have very different shapes: it can be cylindrical or rod-shaped with a rectangular, square or triangular cross-section. Other shapes can of course also be used. Furthermore, the photo-elastic medium can also be composed of individual partial components which are held together, for example, by soldering or gluing. In addition, the photoelastic medium composed of several partial components can also be constructed in such a way that it has a cavity which is possibly evacuated.

Depending on the laser-compatible medium used, the energy supply unit is, for example, a light source, e.g. B. a laser diode, or an electrical voltage. In the first case, the laserable material is pumped optically, while in the second case it is pumped electrically. This last variant is particularly inexpensive in that the light source can be dispensed with, which saves space and money. In addition, in this configuration, the energy requirement for the pressure or differential pressure sensor significantly reduced. This is because a z. B. with a laser diode pumped laser-capable material only achieves an efficiency of at most 10%, while the efficiency of an electrically pumped semiconductor diode laser has an efficiency of almost 50%. It can therefore be roughly said that in the case of a pressure sensor or a differential pressure sensor to achieve comparable signal amplitudes, the power consumption in electrical pumps is an order of magnitude smaller than in optical pumps. If the excitation of the laserable medium occurs optically, then the pump light can be irradiated, incidentally longitudinally or transversely - that is from the side.

As already mentioned above, the laser-capable medium, and possibly the medium exhibiting the photoelastic effect, is preferably designed as a resonator. For this purpose, either two opposite end regions are mirrored, or the resonator mirrors are designed as discrete components and positioned adjacent to the corresponding end regions or side regions of the laserable material. Any other configuration is of course also possible, as long as it is only ensured that the laserable medium and possibly the medium showing the photoelastic effect are subsequently in a resonator.

In particular, resonators can also be implemented as so-called ring resonators, ie, as resonator arrangements with a self-contained optical path into which light is coupled in or out in a suitable manner. A ring resonator can, for example, be circular, elliptical, polygonal, or any other self-contained path, the light being guided in the resonator at the corners of the polygon, for example by total reflection and / or by suitable mirroring of the corners. In the case of a rectangular resonator, corners etched at an angle of 45 ° are sufficient without additional mirroring to guide the light in the resonator. A sensor arrangement with a ring resonator has the advantage over linear resonators that the very complex mirroring of the end faces can be omitted.

The coupling or decoupling of the light for supplying energy to the ring resonator or for analysis can take place, for example, by means of suitable Y branches and / or by means of light guides guided next to the resonator. The proportion of the extracted energy can e.g. be influenced by the distance between the ring resonator and the coupling-out light guide. The quality of the resonator can also be controlled in this way.

Details of the various resonator configurations are explained using the exemplary embodiments.

According to a particularly advantageous development of the device according to the invention, it is proposed that the laserable medium and the resonator or the laserable medium designed as a resonator, which may also show the photoelastic effect at the same time, be arranged on a corresponding membrane or in the immediate vicinity of the membrane / are that the pressure or the pressure across the membrane acts or act on the at least a portion of the laserable medium and interferes with the isotropy of the laserable medium. When the membrane is pressurized, it is deflected. As a result, mechanical stresses are generated in the laser-capable medium which shows the photoelastic effect. The isotropy of the medium is disturbed by the introduced mechanical stresses. The introduction of the mechanical stress is particularly effective if the thickness of the medium which shows the photoelastic properties is small compared to the thickness of the membrane. As a result of the mechanical stresses occurring in the medium, the radiation in different polarization directions no longer 'sees' the same refractive index, as a result of which the radiation emitted in different polarization directions has different frequencies. The pressure or differential pressure can be determined on the basis of the frequency difference between the two different frequencies, since the frequency difference has a highly linear behavior with respect to the pressure or differential pressure that acts on the photoelastic medium over many orders of magnitude.

A particularly advantageous development of the device according to the invention provides that the membrane is a thinned partial area of a wafer, the laserable medium and / or the resonator preferably being arranged on the membrane using the methods of micromechanics and / or the integrated optics , When using this manufacturing method, it is possible to design the pressure sensor or differential pressure sensor according to the invention as an integrated microchip with the well-known advantages.

According to a favorable embodiment of the device according to the invention, the laserable medium and / or the medium with the photoelastic properties is bonded or preferably grown on the membrane or in the immediate vicinity of the membrane. By the way, the preferred position of the grown medium or the grown media is an edge region of the membrane. In this embodiment, no discrete crystal is on the membrane, the z. B. is formed from a wafer, but the crystal is directly connected to the wafer at the crystal lattice level. The applied crystal is preferably a grown monocrystalline layer. The direct connection of the crystalline medium to the wafer ensures optimal transfer of the pressure from the membrane to the photoelastic medium. A particularly simple and inexpensive embodiment of the device according to the invention proposes that the membrane or a thinned partial area of the wafer or an area of the wafer adjacent to the thinned partial area be designed as a laser-capable medium. Individual components of the device according to the invention can then be z. B. form an etching process from the wafer material. In this context, an advantageous development of the device according to the invention provides that at least some of the components of the device for determining / monitoring the pressure or the differential pressure of the at least one process medium form an integral unit.

According to a variant of the device according to the invention it is provided that at least the energy supply unit and / or the receiving

/ Evaluation unit of the laserable medium a predetermined spatial

Have or has distance and that at least one connecting line is provided, via which the excitation energy and / or the emitted

Radiation to / from the laserable medium or are guided away from the laserable medium. At least one optical fiber is used as the preferred connecting line for optical pumping of the laserable material. Different designs of the device according to the invention are explained in detail in conjunction with the drawings. If, on the other hand, the laserable medium is pumped electrically, the connection between energy supply unit and photoelastic medium takes place via electrical

Cables. All variants known in connection with chip production can be used here.

Another aspect of the present invention takes into account the phenomenon that the wavelength of the pump light changes, for example of temperature fluctuations can change. This change may result in the laser medium no longer being able to be pumped.

According to one approach of the invention, this problem can be alleviated by the pressure sensor according to the invention having an amorphous laser medium. An amorphous laser medium, preferably amorphous YAG, has wider pump bands, so that even with a temperature-related change in the pump wavelength, this laser medium or a resonator made of this material can still be supplied with the amount of energy required for laser operation. However, it is important to ensure that the mechanical properties of the laser material continue to meet the requirements for a pressure sensor. Plastic flow of the laser material must be avoided in particular.

In order to control the mechanical properties of the laser material, it can be annealed, whereby a long-range order can be formed in addition to the short-range order of the atoms which is characteristic of amorphous materials. In addition to the currently preferred Nd: YAG, basically all materials are suitable that show the photoelastic effect and can be stimulated to perform laser work.

Apart from the problem described above, temperature fluctuations can lead to thermally induced stresses in the resonator material if its thermal expansion coefficient differs from that of the carrier material (e.g. Nd: YAG with a thermal expansion coefficient α = 7.8 * 10 ^'6 K ^{' 1} on a silicon substrate α = 2.56 * 10 ^"6 K ^{" 1} ). Such thermally induced voltages can be present especially after the deposition of the resonator material, if the deposition temperature is not is equal to the operating temperature. The mechanical stresses caused in this way lead to an offset of the pressure sensor signal. If the resonator material can deform not only purely elastically but also plastically, temperature changes result in a temperature hysteresis of the pressure sensor zero point signal.

In order to be able to compensate for the influence of thermally induced mechanical stresses, a pressure sensor with two essentially identical resonators can be provided. The resonators are arranged in such a way that both identical thermal conditions are exposed, but only the first resonator or measuring resonator is exposed to voltages dependent on the measuring pressure. The second resonator or reference resonator is arranged so that it is independent of pressure fluctuations.

The sensor signal results e.g. from the difference between the signal of a first measuring resonator and the signal of the reference resonator. Changes in the resonator signals, which are not due to a change in pressure, but to a change in another influencing variable - such as Temperature - are due, affect the identical resonators equally and are eliminated by the difference.

Another approach to avoiding or reducing temperature hysteresis and other thermal influences provides single-layer or multi-layer intermediate layers between the substrate and the resonator material, the intermediate layer or the intermediate layers being chosen such that the expansion coefficient α of a layer in each case between the expansion coefficients α of the adjacent materials. In this way the mechanical stresses are distributed over several interfaces, namely 1 + N, where N is the number of interlayers (α ^ <o ^ <α _{i + 1} 1 j = 1 ... N) This results in a low temperature dependency of the pressure sensor signals as well to a low temperature hysteresis.

Materials for forming interlayers between an Nd: YAG resonator on a silicon substrate are given in the table below. Suitable layer sequences for intermediate layers can be produced with a selection from the materials listed in the table. In addition to the aforementioned, other materials can of course also be used which have suitable coefficients of thermal expansion.

Further aspects and advantages of the invention will now be explained on the basis of some exemplary embodiments with reference to the drawings. It shows:

1: a pressure sensor with a discrete laser-capable crystal with a membrane as a pressure-voltage converter,

Fig. 2: a relative or absolute pressure sensor with a plunger as

Pressure-voltage converter,

3: a differential pressure sensor with a plunger as a pressure

Voltage converters,

4: a relative or absolute pressure sensor with an incompressible liquid as a pressure-voltage converter,

5: a differential pressure sensor with an incompressible

Liquid as pressure-voltage converter,

6: a differential pressure sensor with a floating laser-capable crystal,

7: a differential pressure sensor with a plunger and an incompressible liquid as a pressure-voltage converter,

8: a relative or absolute pressure sensor, in which the laserable crystal is suspended from a tube and has a cavity and a membrane, 9: a differential pressure sensor in which the laserable crystal is suspended from a tube and has a cavity and a membrane.

10: a first embodiment of a pressure sensor chip with integrated laser-capable crystal,

Fig. 10a: a schematic representation of the one shown in Fig. 10

Design in top view,

11: a second embodiment of a pressure sensor chip with integrated laser-capable crystal,

12: a third embodiment of a pressure sensor chip with integrated laser-capable crystal,

13: a fourth embodiment of a pressure sensor chip with integrated laser-capable crystal,

14: a fifth embodiment of a pressure sensor chip with integrated laser-capable crystal,

15: a schematic representation of the invention

Device with a semiconductor material as a laser-capable, photo-elastic medium,

16 a: a schematic view of a first exemplary embodiment of a ring resonator, a Y-branching being provided for coupling in the pump light, 16 b: a schematic view of a second one

Embodiment of a ring resonator, wherein for

Coupling of the pump light to a pump light source (not shown) arranged outside the plane of the ring resonator is provided,

16 c: a schematic view of a third exemplary embodiment of a ring resonator, wherein a light guide guided parallel to the ring resonator is provided for coupling in the pump light,

17 a: a schematic view of a circular ring resonator arranged on a chip, the ring resonator being arranged along the edge of the sensor membrane,

17 b: a schematic view of a square ring resonator arranged on a chip, the ring resonator being arranged along the edge of the sensor membrane,

17 c shows a schematic view of a square ring resonator arranged on a chip, one side of the ring resonator running along a section of the edge of the sensor membrane, and

18: shows a schematic view of a resonator arrangement arranged on a sensor chip with a measuring resonator and a reference resonator and a common light guide for supplying the pump light and for deriving the

Measurement signal. Different embodiments of the device according to the invention are shown in FIGS. 1 to 9. Common to all of the embodiments is that the laser-capable medium 7, which in all cases additionally has photoelastic properties, is designed as a discrete component. FIGS. 10 to 15 show different configurations of a pressure sensor chip 3 with an integrated or grown laser-capable medium 7 which shows the photoelastic effect. The figures 16 ac, 17 ac and 18 show different exemplary embodiments with ring resonators, the ring resonators in FIGS. 17 ac and 18 likewise having a laser-capable medium which shows the photoelastic effect and which is grown on or integrated in a pressure sensor chip.

1 shows in cross section a pressure sensor 1 for measuring relative pressure or absolute pressure. The pressure sensor 1 is composed of three layers: a first section 16a of the wafer 16, a second section 16b of the wafer 16 and a layer 37. The two sections 16a, 16b of the wafer 16 are connected to one another via a bond 39. Silicon is preferably used as the wafer material; layer 37 is made, for example, from Pyrex or likewise from silicon.

The essential component of the pressure sensor 1 is the discrete laser crystal 7, which is arranged in a recess 36 of the section 16a of the wafer 16. The laser crystal 7 in the case shown has an elliptical or round cross section. As already mentioned several times, the laser crystal 7 used is, for example, a neodymium-doped YAG crystal. The pressure of the process medium to be determined is transmitted to the membrane 14 via the pressure feed 29 and thus acts indirectly on the laser crystal 7 via the membrane 14. The isotropic properties of the laser crystal 7 are more or less disturbed under the influence of the pressure of the process medium.

In the case shown, the membrane 14 is made of the same material as the wafer 16. The membrane 14 is integrated into the wafer 16 in a simple manner in that it is formed by a thinned region 17 of the wafer 16. In order that the pressure transfer can effectively take place from the membrane 14 to the laser crystal 7, the area of the membrane 14 which is in direct contact with the laser crystal 7 is stiffened.

2 shows a further embodiment of the device according to the invention. In this relative or absolute pressure sensor 1, the pressure-voltage converter 11 is designed as a plunger 12. The laser crystal 7 is positioned in a laser crystal housing 36. The pressure of the process medium is applied to the laser crystal 7 via the process membrane 38 and the plunger 12. A predetermined pressure acts on the laser crystal 7 via the pressure feed 31 perpendicular to the process pressure. Depending on the type of pressure present, one speaks of a relative pressure sensor or an absolute pressure sensor, the feed 31 being connected to a vacuum in the latter case.

3 shows a differential pressure sensor 2, in which a plunger 12 with an upper and a lower partial area is used as the pressure-voltage converter 11. A first pressure p1 of the process medium acts on the upper part of the plunger 12 via the upper process membrane 38, while a second pressure p2 of the same or a different process medium acts on the lower part of the plunger 12 via the lower process membrane 38. The difference between the two pressures is transferred to the laser crystal 7 via the plunger 12. The laser crystal 7 is - just as in the previously described configurations - arranged in a laser crystal housing 36. The Incompressible liquid, in which the plunger 12 and the laser crystal 7 are embedded, ensures that the membranes 38 do not lie in the wave bed.

The embodiment of the device according to the invention shown in FIG. 4 is again a relative or absolute pressure sensor 1. The relative or absolute pressure is applied to the laser crystal 7 via the feed 31. An incompressible liquid 13 is used as the pressure-voltage converter 11. The laser crystal 7 is arranged in a laser crystal housing 36. The section 16b of the laser crystal housing 36 is connected to the first section 16a via a z. B. welded connection 34 connected. Any other pressure-resistant connection can of course also be used. The section of the laser crystal 7 to which the pressure is transferred via the process membrane 38 and the incompressible liquid 13 is defined by the arrangement of the seals 32.

5 shows a differential pressure sensor 2, in which an incompressible liquid 13 is also used as the pressure-voltage converter. The two pressures p1, p2 are each transferred to the laser crystal 7 via two feeds, the pressure p1 and the pressure p2 each being transferred to two regions of the laser crystal 7 which are arranged at an angle of essentially 90 degrees to one another. The laser crystal 7 is positioned in a laser crystal package 36, as in some of the previously described embodiments. The sections 16a, 16b of the housing 36 are in turn connected to one another via a welded connection 34. The partial areas of the laser crystal 7 to which the pressures p1, p2 are transmitted via the incompressible liquid 13 are, as already described in FIG. 4, defined by the arrangement of the seals 32.

6 shows a further embodiment of a differential pressure sensor 2 according to the invention. Here, the pressures p1, p2 are not only one

Side, but transferred from two sides to the laser crystal 7. As Pressure-voltage converter 11, in turn, an incompressible liquid 13 is used. The laser crystal 7 is suspended in the recess 36 by means of the seals 32.

FIG. 7 shows a variant of the embodiment shown in FIG. 5. In addition to the incompressible liquid 13 as a pressure-voltage converter 11, a plunger 12 is also used here.

FIG. 8 shows a further embodiment of the device according to the invention, which is designed as a pressure sensor 1 for the relative or absolute pressure measurement. The laserable crystal 7 consists of two sections 7a, 7b, whereby - roughly speaking - the section 7b has the shape of a pot and the section 7a has the shape of a lid. The two sections 7a, 7b are connected to one another via a gas and pressure-tight connecting layer 39. A cavity 35 is formed in the inner region of the laser crystal 7 due to the shape of the sections 7a, 7b. In the case of a relative pressure sensor, there is a reference pressure in the cavity 35; in the case of an absolute pressure sensor, the cavity 35 is evacuated. The laser crystal 7 again consists of two sections 7a, 7b and is suspended from a tube. It is preferably mirrored on two opposite side surfaces. The laser crystal 7 is arranged in the incompressible liquid 13, which in the present case takes over the function of the pressure-voltage converter 11. To minimize the volume of the incompressible liquid, the oil reservoir can be attached to the tube. As indicated in the figure by the reference numeral 21, the laser beam generated by the energy supply unit 4 (not shown separately) preferably passes through the area of the laser crystal 7 in which the greatest mechanical stresses are induced by the pressure to be measured. The construction of the differential pressure sensor 2 shown in FIG. 9 is very similar to the embodiment shown in FIG. 8. A repeated description of the analog components is therefore omitted.

The pressure p1 of a first process medium is introduced into the cavity 35 via the first process membrane 38 and the incompressible liquid 13, which, like the variant shown in FIG. 8, takes over the function of the pressure-voltage converter. The pressure p2 of a second process medium is introduced via the second process membrane 38 and the incompressible liquid 13 into the space between the housing 36 and the laser crystal 7.

10 shows a first embodiment of a pressure sensor chip or differential pressure sensor chip 3 with an integrated laser crystal 7. For the realization of the resonator properties of the laser crystal 7, the essentially vertical end faces of the laser crystal 7 are mirrored, for example. Alternatively, the optical waveguides 28 can be mirrored in their end regions facing the optical fibers 27. Of course, it is also possible to mirror the end faces of the optical fibers 27. As shown in FIG. 11, the resonator mirrors 23, 24 can also be separately formed components. The resonator mirrors 23, 24 are produced, for example, as follows: Thin walls made of the material of the wafer 16 are produced using a micromechanical method, e.g. B. via an anisotropic, wet chemical etching process or an anisotropic plasma etching process. Here, a few micrometers of the surface of the wafer 16 are etched away, so that only two thin walls remain on both sides of the laser crystal 7. These are subsequently mirrored by means of a dielectric layer or by means of a thin metal layer and form the two resonator mirrors 23, 24. The laser crystal 7 is preferably designed as a monocrystalline layer 8. This is formed epitaxially on the wafer material. In addition, it is possible to manufacture a laser crystal in the form of a wafer 16 and to connect it to the wafer 16. The wafer 16 consisting of the laserable medium 7 is then structured using methods of micromechanics. However, the laser crystal 7 can also be a discrete crystal with, for example, a round or rectangular cross section. This is preferably positioned in a guide trench. The wafer 16, on which the laser crystal 7 is arranged, has a thinned, preferably central region 17 which takes over the function of the membrane 14. Incidentally, the pressure sensor chip 3 can be produced using all common methods of micromechanics and / or the integrated optics. It preferably consists of a semiconductor material, e.g. B. made of silicon.

As can be seen from the plan view shown in FIG. 10 a, the laser crystal 7 is positioned on a membrane edge 15. Furthermore, as already mentioned above, the thickness of the laser crystal 7 is small in relation to the thickness of the membrane 14. Due to the special arrangement of the laser crystal 7 on the membrane 14 and / or due to the small thickness of the membrane 14 Laser crystal 7 optimal measurement results are achieved. This is due to the fact that the stresses which occur as a result of the bending of the membrane 14 are particularly pronounced if the two conditions described above are observed.

In the embodiment shown in FIG. 10, the laser crystal is - as already mentioned - an integral part of the pressure sensor chip 3. Only the optical waveguides 28 are arranged on the pressure sensor chip 3. By the way, these are optional. The light can also be coupled directly from the optical fiber 27 into the laser crystal 7. For an optimal coupling between the optical fibers 27 or the optical fiber 27 and the optical component / the optical To achieve components, each optical fiber 27 is preferably guided on the chip 3 in a guide trench. The guide trench can have a V-shaped cross section, for example. In this case, it is usually generated by KOH etching of (100) oriented silicon. Of course, a guide trench can also have a rectangular or circular cross section.

The further components: pump laser 5, polarizer 20 and photodiode 22 (whose respective functions are adequately described in the patent already cited and are hereby explicitly attributed to the disclosure content of the present invention) are in the configurations shown in FIGS. 10 and 11 not arranged on the pressure sensor chip 3. The radiation from the pump laser 5 is coupled via the optical fiber 27 and the light guide 28 to the laser crystal 7 or to the monocrystalline layer 8. The radiation emitted by the laser crystal 7 or the monocrystalline layer 8 is guided to the polarizer 20 via the optical fiber 27. Incidentally, the optical fibers 27 can have a length of several kilometers. The radiation emerging from the polarizer 20 is detected by the photodiode 22. The frequency of the photodiode signal represents a measure of the pressure of the process medium. The pressure or the differential pressure is determined in the evaluation unit 19.

12 relates to a variant of the pressure sensor chip 3 with an integrated laser-capable medium 7, in which the radiation is coupled in and out via a common optical fiber 27. The pump light of the pump laser 5 is coupled onto the optical fiber 27 via a beam splitter 25 (or mirror) that is transparent to the pump light. Since the frequency of the radiation emitted by the laser crystal 7 has a different wavelength from the pump light, this does not pass through the beam splitter through, but is directed from the beam splitter 25 to the polarizer 20. The photodiode 22 detects the radiation supplied by the polarizer 20.

In the variant shown in FIG. 13, the pump laser 5, which is preferably a laser diode, is integrated directly on the pressure sensor chip 3. The laser diode can be applied to the surface of the wafer 16, for example, by a self-adjusting soldering process.

14 shows a particularly advantageous compact embodiment in which all components, that is to say pump laser 5, laser crystal 7, polarizer 20, photodiode 22 and optical waveguide 28, are arranged on the wafer 16.

While in connection with the previous embodiments reference is always made to a laser crystal 7 or a monocrystalline layer 8 which is / are optically pumped, FIG. 15 shows a schematic illustration of the device according to the invention, in which a laser-capable, photoelastic medium 7 is used Semiconductor laser 40 is used. This electrically pumped variant of the invention has the advantage over the aforementioned configurations that its efficiency is significantly higher.

Instead of discrete resonator mirrors 23, 24, the further developments mentioned above can also be used here. The semiconductor laser 40 is preferably applied to the edge of the membrane 14. When the diaphragm 14 is pressurized, it is deflected and generates mechanical stresses in the semiconductor laser 40. The semiconductor laser 40 is also made from a material which exhibits the photoelastic effect. The isotropy of the optical properties of the semiconductor laser 40 is disturbed by the introduced mechanical tension, so that laser light polarized in different directions no longer the same Refractive index 'sees' and thus the optical length of the laser resonator is of different sizes for light polarized in different directions.

Gallium arsenic compounds, for example, can be used as the material for the semiconductor laser 40. It is also possible to use silicon-based semiconductor lasers 40, which in many respects brings outstanding advantages for the invention.

In the following, with reference to FIGS. 16-18 exemplary embodiments for pressure sensors with ring resonators discussed.

Although not shown in detail, the ring resonators can be pressurized according to the principles described above with a suitable pressure transducer.

To extract the laser light from a ring resonator 70; 170; 270; 370, 376, the exemplary embodiments have optical waveguides 71; 171; 271; 371, which are arranged next to the resonators, with part of the laser light entering the optical waveguides 71; 171; 271; 371 is decoupled. The proportion of the extracted energy can e.g. can be influenced by the distance between the ring resonator and the coupling-out optical waveguide, whereby the quality of the resonator can also be adjusted.

The figures 16 a-c show various arrangements for coupling in the pump light, which are described below.

In the arrangement according to FIG. 16 a, the coupling takes place via a Y branch 73. That is, the light from a pump light source 75 is fed to the ring resonator 70 via a light guide section 72 which opens tangentially into the ring resonator at the Y branch 73. When designing a Y-branch, the following points should generally be observed:

If the light guide section 72 consists of Nd: YAG, a considerable proportion of the pump light can already be absorbed in this section. Correspondingly less energy is then available for pumping the actual annular laser resonator.

In order to counter this problem, the light guide section can, on the one hand, have a material other than material than Nd: YAG. The other material preferably has approximately the same refractive index as Nd: YAG at the wavelength of the pump light. However, this solution has the disadvantage that reflections take place at the interface of the two different materials, which also reduce the amount of pump light available in the actual resonator.

To avoid absorption on a larger scale, on the other hand, the section of the light guide section 72 of the Y branch 73 can also be made as short as possible, and / or the proportion of Nd atoms in the entire (resonator + Y branch) YAG host material is chosen to be lower than in Nd: YAG lasers common. Neodymium doping of 1.0% to 1.5% is customary. The lower the proportion of neodymium and the shorter the length of the light guide section 72, the more pump energy can reach the actual laser resonator.

In a further embodiment of the Y branch, the light guide section 72 contains no neodymium. This can be achieved by the following manufacturing process: The ring resonator including Y-branching initially consists only of YAG. The neodymium is only subsequently introduced by means of implantation or diffusion. The light guide section 72 of the Y- Branching is covered by means of suitable layers during diffusion or implantation in order to prevent the penetration of neodymium. The result of this is that practically no pump light is absorbed in the light guide section 72.

In Figure 16 b, the pumping light is coupled in via a pumping light source (such as a laser diode or LED or optical fiber into which the light of a laser diode or LED is coupled), which is arranged above the ring resonator 70. The pump light thus reaches all areas of the ring resonator 70 directly. A Y-branching with the above-mentioned disadvantages of light absorption is not required in this case.

In Fig. 16 c there is a first optical fiber directly next to the ring resonator, into which the pump light is initially coupled. Because the first optical waveguide 74 is very close to the ring resonator, part of the pump light is coupled over from the first optical waveguide to the ring resonator.

In a preferred embodiment of the invention, the system of the first optical waveguide / pump light source can be designed in such a way that little or no pump light is reflected back from the first optical waveguide into the pump light source.

FIGS. 17 a - c show arrangements of ring resonators on a sensor chip.

In Figs. 17 a, b is the resonator 70; 170 arranged along the preferably thinned membrane edge. The greatest mechanical stresses arise here when the membrane is deflected. Therefore, the greatest possible sensor sensitivity is achieved with an arrangement according to 17 a, b. The ring resonator 170 of FIG. 17 b is arranged essentially square. This form of the resonator is preferably used for square measuring membranes. The ring resonator preferably runs along the area of the membrane edge. The corners of the resonator are etched away at an angle of 45 °, as shown in FIG. 17b.

A further special embodiment of a rectangular, in particular square, ring resonator 270 is shown in FIG. 17c. A part of the square ring resonator 270 runs along a section of the membrane edge 215 or along a section of a section of the membrane edge 215 of the measuring membrane 214. This is advantageous in the case of membranes with a large circumference, which are preferably used in small measuring ranges, because the length of the resonator is small and therefore the longitudinal modes of the resonator are far apart in frequency. This large separation of the longitudinal modes is desirable to ensure that only one longitudinal mode can oscillate in the resonator. To further shorten the ring resonator, a rectangle should be chosen in which the sides that run perpendicular to the pressurized resonator section are as short as possible.

In principle, with each configuration of a pressure sensor according to the invention, pump light and measuring light can be transported to the chip or away from the chip in different ways. The pump light source can be arranged directly on the chip, as a hybrid on the chip, or the pump light is supplied via an optical fiber.

A polarizer can be arranged on the chip, in a fiber or after a fiber. A photodiode can be arranged directly on the chip, as a hybrid on the chip (for example with a polarizer in front) or after a fiber.

In a particularly preferred embodiment, pump light and measuring light are transported over a common fiber, i.e. there are no electrical connections to the chip, only an optical connecting fiber.

In this embodiment shown in FIG. 18, a fiber is used to supply the pump light and to remove the measurement light. Electrical connections to the sensor chip are not required. This is particularly advantageous for Ex applications. In addition, this sensor chip can also be used at high temperatures (approx. Up to 250 ° C).

The preferably circularly arranged ring-shaped measuring resonator 370 runs along the edge region of the membrane 302. A second, structurally identical resonator 376 is located on the sensor chip 303 and serves as a reference. The reference resonator 376 is located on a solid part of the sensor chip 303. However, in order to increase the structural similarity between the measuring resonator 370 and the reference resonator 376, the reference resonator can also extend along the edge of a second membrane 301, whereby the second membrane is not exposed to a pressure difference.

In this configuration, only one mode is formed per resonator, namely the TE mode, in which the direction of the electric field lies in a plane parallel to the chip surface. The TM mode, in which the electric field strength is perpendicular to the chip surface, is suppressed. This is done by means of polarizers 320, 321, which preferably have a thin dielectric / metal layer which is applied to part of the resonator. The metal layer acts like a polarizer that weakens the TM mode. This means that the ring laser can only laser on TE mode. If the measuring membrane 302 is deflected by a measuring pressure, radial mechanical stresses arise in the membrane, the maximum of which lies in the area of the membrane edge. The direction of the mechanical stresses coincides with the direction of polarization of the TE mode in the ring resonator 370. The greater the mechanical stresses, the more the refractive index of the resonator material changes (photoelastic effect) and thus the optical resonator length. This results in a change in the laser frequency. The laser frequency of the reference resonator 376 is not influenced by the measuring pressure. Changes in the laser frequency, which can be attributed to other influencing factors, such as temperature, affect the measuring resonator and the reference resonator equally and can be eliminated by mixing the laser frequencies of the two resonators using an optical detector (e.g. photodiode) and only the difference between Laser frequencies ajs evaluates a signal proportional to the measuring pressure.

The pump light passes from an optical fiber onto a first waveguide 328. The first waveguide 328 branches into two broad branches 305, 306 and a very narrow branch 377. Almost the entire pump light is distributed symmetrically over the two broad branches 305, 306. The broad branches feed the pump light to the two resonators, preferably via Y branches. A laser mode is formed in each resonator, which rotates in the direction of the arrows. No mode can develop in the opposite direction, since this mode would suffer excessive losses due to the Y-branching for coupling in the pump light.

Coupling light waveguides 371 are provided in addition to the resonators 370, 376 for coupling out part of the laser light. The two outcoupling optical waveguides are brought together to form an optical waveguide which represents the thin branch 377 of the first optical waveguide described above. This comes from this optical fiber Measuring light into the optical fiber 327. A polarizer 322 of the type described above can also be located on the outcoupling light waveguide, which ensures that only the TE mode is present in the waveguide, ie that the light has only a single polarization direction. This is the prerequisite for mixing the laser frequencies in a downstream (not shown) optical detector.

At the not shown end of the fiber 327 there is an optical element which separates the measuring light from the pump light. This task can e.g. be fulfilled by a mirror arranged at 45 °, which allows the pump light to pass through unhindered and reflects the entire measuring light (wavelength measuring light ≠ wavelength pump light).

In a further preferred variant, the pressure sensor has two resonators with two modes per resonator. A mixed signal between the two modes is generated for each resonator. The difference between the two mixed signals is the actual pressure-dependent measurement signal.

The measuring light then passes from the mirror to the optical detector, for example a photodiode. The pump light from a suitable pump light source, for example an LED or laser diode, is coupled into the fiber by means of a suitable device.

Finally, possible arrangements of the functional components of the pressure sensor according to the invention should be mentioned again, the subject matter of protection being defined only by the claims and not being limited to these arrangements:

a) Polarizer and photodiode are an integral part of the pressure sensor chip. b) The polarizer is an integral part of the pressure sensor chip, the photodiode is applied to the pressure sensor chip in a hybrid construction.

c) The polarizer is an integral part of the pressure sensor chip; the light is guided to a separate photodiode by means of an optical fiber.

d) By means of a fiber, the light is guided from the coupling-out waveguide to the separate polarizer / photodiode unit.

e) By means of a fiber that contains a polarizer, the light is guided from the coupling-out waveguide to a separate photodiode.

f) By means of a fiber, the light is guided from the coupling-out waveguide to a separate photodiode. It is a polarization-maintaining fiber, which is arranged so that the polarization direction of the fiber is at an angle of 45 ° with the polarization directions of the two electromagnetic waves in the

Coupling light waveguide forms.

LIST OF REFERENCE NUMBERS

1 pressure sensor

2 Differential pressure sensor 3 Pressure sensor / differential pressure sensor chip

4 power supply unit

5 pump lasers

6 Electrical voltage source

7 Laser-compatible medium or laser crystal 7a Section of the laser-compatible medium or laser crystal

7b section of the laser-capable medium or the laser crystal

8 monocrystalline layer

9 mirrors

10 mirrors 11 pressure-voltage converters

12 pestles

13 Incompressible liquid

14 membrane

15 membrane edge 16 wafers

16a section of a wafer

16b section of a wafer

17 Thinned area

18 Stiffened area 19 Receiver / evaluation unit

20 polarizer

21 laser beam

22 photodiode

23 resonator mirrors 24 resonator mirrors

25 beam splitters connecting line

Optical fiber

optical fiber

pressure feed

Relative pressure feed

poetry

tube

soldered point

cavity

Gehäusung

Material layer a process membrane b process membrane c process membrane d process membrane

link layer

Semiconductor laser

tube

ring resonator

optical fiber

Optical waveguide section

Y-branch

Pump light source 3 sensor chip 0 ring resonator 1 light guide 3 sensor chip 4 measuring membrane 5 membrane edge 0 ring resonator 271 light guide

301 reference membrane

301 measuring membrane

303 sensor chip

304 branching

305 wide left branch

306 wide right branch

320 polarizer

321 polarizer

322 polarizer

327 optical fiber

328 light guide

370 measuring resonator

371 light guide

376 reference resonator

377 narrow light guide

Claims

claims

1. Device for determining / monitoring the pressure or differential pressure of at least one process medium with an energy supply unit (4), a laser-capable medium (7) arranged in a resonator, a pressure-voltage converter (11) and a reception / evaluation unit (19), wherein the energy supply unit (4) pumps the laserable medium (7) and stimulates laser activities, either the laserable medium (7) or, in addition to the laserable medium (7), another medium introduced into the resonator in the unloaded state, if the same on all sides Pressure acts on the laserable medium (7) or the other medium, has isotropic properties and the laser radiation built up from the resonator, the laserable medium and possibly the further madium introduced into the resonator with a physical properties of the laserable medium ( 7) or the other medium dependent frequency is emitted, the pressure-voltage converter (11) transmits the metering pressure or differential pressure of the at least one process medium to at least a portion of the laserable medium (7) or the further medium, thereby defining a loaded state of the laserable medium (7) or the further medium, in which the isotropic properties of the laserable medium (7) or the further medium are disturbed, the radiation emitted in the loaded state of the laserable medium (7) having different and different frequencies in different polarization directions, and the receiving / evaluation unit (19 ) determines the pressure p or the differential pressure p1, p2 of the at least one process medium on the basis of at least one of the changed frequencies in the different polarization directions.

2. Device according to claim 1, wherein the receiving / evaluating unit (19) determines the pressure p or the differential pressure p1, p2 of the at least one process medium on the basis of the different frequencies in the different polarization directions.

3. Apparatus according to claim 1 or 2, characterized in that the pressure-voltage converter (11) is designed in the case of a differential pressure measurement such that the two pressures act at an angle of substantially 90 ° to each other on the laser-capable medium (7).

4. Device according to one of claims 1 to 3, characterized in that the pressure-voltage converter (11) is a plunger (12).

5. Device according to one of claims 1 to 3, characterized in that the pressure-voltage converter (11) is an ideally incompressible liquid (13).

6. Device according to one of claims 1 to 5, characterized in that taken on its own or in addition to the plunger (12) and / or the incompressible liquid (13) at least one membrane (14) is provided as a pressure-voltage converter (11) ,

7. The device according to claim 6, characterized in that that the laserable medium (7) or the resonator on a corresponding membrane (14) or in the immediate vicinity of the membrane (14) are arranged such that the pressure or the pressure across the membrane (14) on the at least a portion of the acts or act on laser-compatible medium (7) and interferes with the isotropic properties of laser-compatible medium (7).

8. The device according to claim 1, 6 or 7, characterized in that the membrane (14) is a thinned portion (17) of a wafer (16), the laserable medium (7) and / or the resonator preferably are arranged on the membrane (14) via the methods of micromechanics and / or the integrated optics.

9. The device according to claim 6, 7 or 8, characterized in that the laserable medium (7) on the membrane (14) or in the immediate vicinity of the membrane (7) has grown.

10. Device according to one of claims 6 to 9, characterized in that the thickness of the laserable medium (7) is small compared to the thickness of the membrane (14).

11. The device according to claim 6 or 10, characterized in that the membrane (14) or a thinned portion (17) of the wafer (16) or an adjoining the thinned portion of the wafer is designed as a laser-capable medium (7) ,

12. The device according to claim 1, characterized in that at least some of the components (4, 7, 9, 10, 14, 19, 23, 24, 25, 26) of the pressure / differential pressure sensor chip (3) form an integral unit.

13. The apparatus according to claim 1 or 12, characterized in that at least the energy supply unit (4) and / or the receiving / evaluation unit (19) from the laserable medium (7) have or has a predetermined spatial distance and that at least one connecting line (26) is provided, via which the excitation energy and / or the emitted radiation are directed to the laserable medium (7) or away from the laserable medium (7).

14. Device according to one of the preceding claims, wherein the resonator is a ring resonator.

15. The apparatus of claim 14, wherein the ring resonator is substantially circular.

16. The apparatus of claim 15, wherein the ring resonator is rectangular.

17. The apparatus of claim 15 or 16, wherein the ring resonator extends along the edge of a membrane.

18. The apparatus of claim 16, wherein one side of the rectangular ring resonator extends along at least a portion of a membrane edge of a measuring membrane.

19. The apparatus of claim 18, wherein the geometric path length of the ring resonator is smaller than the length of the membrane edge.

20. Device according to one of claims 14 to 19, wherein the device comprises two substantially identical ring resonators, the first

Ring resonator is arranged as a pressure-dependent measuring resonator and the second ring resonator as a pressure-independent reference resonator.

21. Device according to one of the preceding claims, wherein the resonator has an amorphous laser-capable material.

22. The apparatus of claim 21, wherein the amorphous laserable material of the resonator is annealed.

23. Device according to one of the preceding claims, wherein the resonator is arranged on a sensor chip, and the laserable material of the resonator has a different coefficient of thermal expansion than the material of the sensor chip, wherein at least one intermediate layer is further arranged between the sensor chip and the laserable material has a coefficient of thermal expansion that lies between the coefficient of thermal expansion of the material of the sensor chip and the coefficient of thermal expansion of the laser-capable material.