DE3920840C1 - Integrated optical sensor detecting temp. and refraction index changes - has double refractive waveguide in substrate extending along X or Y axis of cut lithium-niobate crystal - Google Patents

Abstract

The sensor simultaneously detects temp. and refractive index variations. The LiNbO3 substrate (12) has a monomode double refraction wave-guide (14) extending between its opposing end faces (18, 19). A side surface of the wave-guide lies in contact with a measuring fluid (58) with a high reflection coupling surface (16, 17) at the opposite ends of the waveguide. One coupling surface (16) is supplied with laser light, the other coupling surface (17) used to supply a measuring signal to a photodetector (69, 70). The waveguide extends along the X or Y axis of the LiNbO3 crystal. with proton exchange at the waveguide side surface (22) via a thin proton exchange layer (56). ADVANTAGE - Temp. changes of fluid measurable independently and refraction index in same vol. Measuring accuracy of more than 0.0001 Kelvin with resolution of phase of transmitted optical intensity of 1 millirad.

Description

The invention relates to an integrated optical Sensor for simultaneous detection of temperature and changes in refractive index with a substrate body Lithium niobate, in which a single-mode waveguide is provided, which is between two end faces of the substrate body along its surface stretches, with a lateral boundary surface of the Waveguide can be brought into contact with a measuring fluid is and coupling surfaces of the waveguide on the forehead surfaces are highly reflective and where a coupling surface with laser radiation and a first one Light detector with that in reflection or transmission measurement obtained from a corresponding coupling surface signal can be applied.

Such an integrated optical sensor is from the Article "Integrated optical temperature sensor" by L.M. Johnson et al. from appl. phys. Lett. 41, page 134 (1982), in which the waveguide is within of the lithium niobate substrate as an asymmetrical mach Zehnder interferometer is formed. The optically change transmitted intensity of the interferometer changes sinusoidally with the temperature of the inter ferometers and thus with the temperature of the Boundary surface contacting measuring fluid.

From the article "Optical π -arc waveguide interfero meter in proton-exchanged LiNbO ₃ for temperature sensing" by Masamitsu Haruna et al. another asymmetrical Mach-Zehnder interferometer is known from Applied Optics 24, page 2483 (1985), in which the waveguide is made from proton-exchanged lithium niobate. A phase shift between the optical paths of different lengths of the π interferometer arms is achieved with temperature changes of around 0.3 degrees Kelvin. With a resolution of the phase measurement of the transmitted optical intensity of 1 millirad, a measurement accuracy of a few 10 ^-4 Kelvin can be achieved.

In the summary by U. Hollenbach et al. "Integrated optical refractive index sensor by ion exchange in glass" in the Technical Abstracts of the International Congress on Optical Science and Engineering, September 19-25, 1988 in Hamburg, paper 1014 describes a symmetrical waveguide Mach-Zehnder interferometer which only one of the waveguide arms can be contacted by the measuring liquid. Due to the symmetrical waveguide arms of the same length, the output signal of the interferometer is independent of temperature fluctuations. The portion of the guided wave modes protruding from the waveguide into the measuring liquid is changed in phase by the measuring liquid, so that the refractive index of the measuring liquid can be detected with an accuracy of more than 10 ^-4 .

Based on this state of the art Invention based on the task of an integrated to create an optical sensor that allows Changes in temperature and refractive index of a measurement fluids independently in the same volume measure up.

The object is achieved in that the birefringent waveguide in the substrate body along the X or Y intersection axis of the ge cut lithium niobate crystal that  a thin one, not enough to run a fashion thick layer of proton-exchanged lithium niobate is provided in the lateral boundary surface that the laser beam acting on the coupling surface lung is linearly polarized, the polarisa direction between the Z axis and the one on it the plane of the coupling surface standing at right angles Cut axis of the lithium niobate crystal predetermined and that a second light detector with the in Reflection or transmission from the corresponding Coupling surface received measurement signal can be acted upon, the light detectors each for 90 degrees differently polarized laser radiation sensitive are.

The thin proton-exchanged layer, which by itself is not capable of guiding a mode, causes a shift in the normal fashion, which is polarized at right angles to the Z direction and parallel to the surface of the lithium niobate crystal, in the direction of the substrate and a shift in the center of gravity of the extraordinary fashion polarized along the Z-cut axis of the crystal into the measurement fluid. The fact that the dental mode has only very small electromagnetic field components in the measuring fluid means that its optical path in the Fabry-perot interferometer can mainly be changed only by temperature fluctuations. The extraordinary fashion, however, sees the refractive index fluctuations of the measuring fluid in superposition with a phase change from the temperature fluctuation. The relative fluctuations in the refractive index can be calculated from the measurement information contained in the extraordinary mode with the help of the temperature information which can be detected by the ordinary mode. As a result, measurements of temperature changes in the range of 10 ^-3 Kelvin and of changes in refractive index in the range of 10 ^-4 to 10 ^{-5 can be} carried out.

The thickness of the proton exchanged layer becomes selected depending on the wavelength and is at one Laser light wavelength of 0.84 microns between 50 and 350 nanometers.

Further embodiments of the invention are in the Subclaims marked.

The following is an embodiment of the invention explained in more detail with reference to the drawing. Show it:.

Fig. 1 is a schematic view of a wave guide in a crystal substrate,

Fig. 2 is a plan view of a symmetric waveguide Mach-Zehnder interferometer,

Fig. 3 is a plan view of a single-ended waveguide Mach-Zehnder interferometer,

Fig. 4 is a schematic view of an in Trans mission operated integrated optical sensor according to an embodiment of the invention,

Fig. 5 shows a cross section through the waveguide of the sensor along its longitudinal axis with schematically drawn modes and energy distribution and energy

Fig. 6 is a schematic view of an integrated optical sensor operated in reflection according to the embodiment.

Fig. 1 shows a prior art integrated optical sensor 10 to measure temperature variations. The integrated optical temperature sensor 10 according to the prior art has a lithium niobate crystal 12 in which a waveguide 14 is introduced, for example by titanium diffusion. The waveguide 14 has a semi-cylindrical shape and forms a Fabry-perot interferometer between coupling surfaces 16 and 17 in the end surfaces 18 and 19 of the substrate 12 . A boundary surface 22 of the waveguide 14 is aligned with a surface 24 of the substrate 12 .

The surface 24 of the substrate 12 and thus also the boundary surface 22 can be acted upon with a measuring fluid, not shown in FIG. 1. To detect changes in temperature and refractive index, a coupling surface 16 of the waveguide 14 can be acted upon by mono-chromatic laser radiation. Much of this laser radiation is guided in the waveguide 14 ; a smaller portion protrudes from the boundary surface 22 into the measuring fluid. A change in the temperature of the measuring fluid and / or a change in the refractive index of the measuring fluid leads to phase shifts within the Fabry-perot interferometer formed by the waveguide 10 due to a change in the effective refractive index for a guided mode and thus to a changed output transmission or reflection the same.

Fig. 2 shows a plan view of a known integrated optical sensor 30 which is designed as a symmetrical waveguide Mach-Zehnder interferometer. This integrated optical sensor 30 has two interferometer arms 31 and 32 of equal length. The lithium niobate crystal of the substrate 12 is covered with a cover layer 34 indicated by hatching. Only the interferometer arm 32 is not covered by the covering layer 34 in a measuring window 36 . In the measuring window 36 , a part of the mode guided in the interferometer arm 32 of the waveguide 14 can protrude into the measuring fluid contacting the cover layer 34 and the measuring window 36 and can be influenced by changes in refractive index. Because the interferometer arms 31 and 32 are of equal length, the transmission or reflection output signal of the interferometer is not influenced by temperature fluctuations in the measuring fluid. Relative fluctuations in refractive index, but no temperature fluctuations can be detected.

Fig. 3 shows in plan view a prior art integrated optical sensor 40 in the form of a single-ended waveguide Mach-Zehnder interferometry meters that has a shorter interferometer arm 41 and a longer interferometer arm 42nd Because the interferometer arms 41 and 42 are of unequal length, an optical path length difference occurs within the interferometer. When the temperature of the measuring fluid changes, the optical path length changes in the interferometer arms 41 and 42 of different lengths, so that the output signal of the interferometer depends on the temperature of the measuring fluid. With the integrated optical sensor 40 , only temperature fluctuations can be detected.

Fig. 4 shows a schematic view of an integrated optical sensor 50 according to an exemplary embodiment of the invention, which is operated in transmission. The substrate 12 in the form of a lithium niobate crystal is cut such that the Z axis 52 points in the direction of the surface 24 of the substrate 12 . A waveguide 14 is introduced into the substrate 12 from the surface 24 . The semi-cylindrical birefringent waveguide 14 has been formed, for example, by titanium diffusion. The waveguide 14 extends in a straight line between the end faces 18 and 19 of the lithium niobate crystal and thus parallel to the X and Y axes 54 of the lithium niobate crystal of the substrate 12 .

The birefringent waveguide 14 has an essentially semicylindrical shape with a length of a few centimeters, for example between three and four centimeters, the semicylinder being delimited by the boundary surface 22 toward the surface 24 . In the uppermost layers of the substrate 12 and the boundary surface 22 , a proton-exchanged layer 56 is provided, the thickness of which is not sufficient to guide an independent mode. The layer 56 preferably has a thickness between 50 and 350 nanometers and is almost twice as wide as the diameter of the semi-cylindrical waveguide 14 . The layer 56 has the shape of a along the waveguide 14 in the upper layers of the sub strate 12 extending band with a quadratic volume. Layer 56 can be produced, for example, by immersing substrate 12 in a hot acid, for example in phosphoric or benzoic acid. A measuring fluid 58 can be applied to the surface 24 , into which the thin proton-exchanged layer 56 is placed, which lies on the boundary layer 22 .

The coupling surface 16 on the end face 18 is polished and preferably provided with a dielectric or metallic reflection layer. With the Kop pelfläche 16 a polarization-maintaining fiber 60 is connected in a butt. In another embodiment, the polarization-maintaining fiber 60 can also be glued onto the coupling surface 16 . The polarization-maintaining fiber 60 is acted upon by a polarized mono-chromatic laser beam, for the wavelength of which the lithium niobate substrate 12 is transparent. The direction of polarization 61 of the laser light is preferably inclined by 45 degrees in the plane of the end face 18 with respect to the Z-axis 52 , so that both basic modes, both an ordinary and an extraordinary mode, are excited simultaneously.

Between the highly reflecting coupling surfaces 16 and 17 , a Fabry-perot interferometer is formed in the waveguide 14 , the intensity of which is coupled out on the coupling surface 17 can be fed into a second polarization-maintaining fiber 62 . The coupling surface 17 is polished and preferably provided with a dielectric or metallic reflection layer. The second polarization-maintaining fiber 62 is connected to the coupling surface 17 in an abutting or glued manner. The polarization-maintaining fiber 62 is divided into two sub-fibers 65 and 66 in a Y-coupler 64 . The light guided in the partial fibers 65 and 66 acts on polarizers 67 and 68, detectors 69 and 70 . The first polarizer 67 is set such that the first detector 69 is acted upon by the ordinary beam, the direction of polarization of which is perpendicular to the intersection axes 52 and 54 and is identified by 71 .

The second polarizer 68 is arranged so that the second detector 70 receives light whose polarization is 90 degrees different with respect to the polarization of the light received by the first detector 69 . The polarization of the light detected by the detector 70 corresponds to the polarization of the extraordinary beam in the substrate 12 , which is polarized in the direction of the Z axis 52 of the substrate 12 . Instead of the Y-coupler 64 and the two polarizers 67 and 68 , a fiber polarization divider can also be provided.

Fig. 5 shows a cross section through the waveguide 14 of the sensor 50 along its longitudinal axis with schematically drawn mode field distributions and illustrates the operation of the sensor 50th The birefringent waveguide 14 , which is covered by the thin proton-exchanged layer 56 , is arranged in the substrate 12 . The thin proton-exchanged layer 56 with an exaggerated thickness of 50 to 350 nanometers is not capable of carrying out a mode.

The field distributions 73 and 75 are also shown in FIG. 5. The field distribution 73 characterizes the field course of the extraordinary fashion and the field distribution 75 that of the ordinary fashion. The proton-exchanged layer 56 has the property that in the birefringent waveguide 14 the center of gravity of the ordinary mode 75 is shifted into the substrate 12 , so that only a small proportion of the ordinary mode 75 outside the layer-exchanged layer 56 and thus above the surface 24th is guided in the measuring fluid 58 . The proton-exchanged layer 56 also has the property that the extraordinary mode 73 propagates before preferably in the proton-exchanged layer 56 , and thereby the extraordinary mode 73 is pulled out of the substrate 12 and further shifted into the measuring fluid 58 .

As a result, the ordinary mode 75 is only sensitive to temperature fluctuations of the measuring fluid and the temperature fluctuations of the waveguide 14 , while the extraordinary mode 73 also reacts to fluctuations in the refractive index of the same volume of the measuring fluid 58 . The orderly mode 75 acting on the first detector 69 , with which temperature fluctuations can be detected, allows the signal of the second detector 70 comprising the temperature fluctuations and refractive index fluctuations to be returned to a pure measure of fluctuations in the refractive index.

FIG. 6 is a view showing the integriert- optical sensor 50, which is operated in reflection. A laser diode 80 emits a monochromatic laser radiation schematically indicated by reference numeral 82 , which is collimated by a lens arrangement 84 . The laser radiation 82 passes through a beam splitter 86 and is focused on the coupling surface 16 with the aid of a microscope objective 88 . The polarization of the laser beam 82 has an angle of 45 degrees with the axis 52 in the end face 18 .

The Fabry-Perot interferometer formed by the sensor 50 is operated in reflection, which means that the laser light of the Fabry-Perot interferometer leaking from the coupling surface 16 is used to detect the refractive index and temperature changes. The divergent laser beam 90 emerging from the coupling surface 16 is limited by the microscope objective 88 and is deflected with the aid of the beam splitter 86 . A polarization beam splitter 92 is arranged in such a way that the ordinary beam 75 and the extraordinary beam 73 are directed to the first detector 69 and the second detector 70 , respectively.

This optical arrangement can be replaced as follows, for example. The laser radiation 82 runs in a polarization-maintaining fiber, the beam splitter 86 is replaced by a fiber Y-splitter and the polarization beam splitter 92 is replaced by a fiber polarization splitter.

The transmission curves for the ordinary mode 75 and the extraordinary mode are usually shifted ben in the frequency range of the laser 80 ben. At a predetermined wavelength of the laser 80 there is, for example, a high transmitted intensity of the ordinary mode 75 and thus a steep measuring edge of the Airy function of the interferometer, whereas the transmitted wavelength of the extraordinary mode 73 is relatively small for the same wavelength and thus the sensitivity to fluctuations in the refractive index detected by the extraordinary mode 73 is low.

The laser 80 is connected to a modulator which, in the case of a semiconductor diode laser, can be designed as a supply current modulator. The modulator, not shown in the drawings, switches the wavelength of the laser light with a right angular modulation frequency of, for example, a few kilohertz between two corresponding frequencies in such a way that both the uncoupled extraordinary mode 73 and the uncoupled ordinary mode 75 have an intensity that is as close as possible to half the maximum transmitted intensity. Then the integrated optical sensor 50 is very sensitive both to changes in refractive index and to temperature fluctuations around this state of equilibrium. The use of the sensor 50 in time multiplexing with a few kilohertz permits a quasi-simultaneous recording of changes in the refractive index and temperature fluctuations of the measuring fluid 58 .

Claims (9)

1. Integrated optical sensor for the simultaneous detection of temperature and refractive index changes with a substrate body ( 12 ) made of lithium niobate, in which a single-mode waveguide ( 14 ) is provided, which is located between two end faces ( 18 , 19 ) of the substrate body ( 12 ) extends along its surface ( 24 ), a lateral boundary surface ( 22 ) of the waveguide ( 14 ) being able to be brought into contact with a measuring fluid ( 58 ) and coupling surfaces ( 16 , 17 ) of the waveguide ( 14 ) on the end faces ( 18 , 19 ) are highly reflective and a coupling surface ( 16 ) with laser radiation ( 82 ) and a first light detector ( 69 ) with the reflection signal or trans mission from a corresponding coupling surface ( 17 or 16 ) received measurement signal ( 73 & 75 ) acted upon is characterized in that the birefringent waveguide ( 14 ) in the substrate body by ( 12 ) along the X or Y cutting axis ( 54 ) of the cut lithium iobat crystals ( 12 ) extends that a thin layer ( 56 ) of proton-exchanged lithium niobate, which is not sufficiently thick for guiding a mode, is provided in the lateral boundary surface ( 22 ) such that the laser radiation ( 82 ) acting on the coupling surface ( 16 ) is linearly polarized, the direction of polarization ( 61 ) between the Z axis ( 52 ) and the cut axis ( 71 ) of the lithium niobate crystal ( 12 ) which is perpendicular to it in the plane of the coupling surface, and that a second light detector ( 70 ) can be acted upon by the measurement signal ( 73 , 75 ) obtained in reflection or transmission from the corresponding coupling surface ( 17 or 16 ), the light detectors ( 69 or 70 ) in each case for laser radiation ( 75 or 73 ) are sensitive. 2. Sensor according to claim 1, characterized in that the proton-exchanged layer ( 56 ) has a thickness between 50 and 350 nanometers. 3. Sensor according to claim 1 or claim 2, characterized in that the proton-exchanged layer ( 56 ) has the shape of a band in the lateral boundary surface ( 22 ) provided with a width of approximately twice the width of the waveguide ( 14 ). 4. Sensor according to one of claims 1 to 3, characterized in that the waveguide ( 14 ) is made of titanium or vanadium-doped lithium niobate. 5. Sensor according to one of claims 1 to 4, characterized in that the coupling surfaces ( 16 , 17 ) are polished. 6. Sensor according to one of claims 1 to 4, characterized in that the coupling surfaces ( 16 , 17 ) are vapor-coated with a dielectric or metallic mirror material. 7. Sensor according to one of claims 1 to 6, characterized in that the laser radiation acting on the second light detector ( 70 ) is polarized in the waveguide ( 14 ) in the direction of the Z-axis ( 52 ). 8. Sensor according to claim 7, characterized in that the coupling surface ( 16 ) acting laser radiation in the end face ( 18 ) is polarized by 45 degrees relative to the Z-axis ( 52 ). 9. Sensor according to one of claims 1 to 8, characterized characterized in that the laser radiation over a Wavelength in the range from 450 to 1600 nanometers disposes.