DE3528294A1 - Method for the fibre-optical, spectrally coded transmission of the value of a variable physical measured quantity - Google Patents

Abstract

In simple fibre-optical pickups, in which the measured quantity to be detected modulates the power of a first light flux and in which a second light flux is transmitted as reference power for approximate compensation of the fibre losses, a high independence of the transmission of the measured value from the characteristics of the fibre and their fluctuations is achieved by arranging that the measured quantity modulates the power ratio of the two light fluxes, and that the latter are then transmitted, together with mutually interleaved comb spectra consisting of many lines, over this fibre from the measurement sensor to the evaluation device. Because the two light fluxes thereby have the same mean optical frequencies, and thus also suffer the same losses, it is possible to use fibre plugs and fibres of different length, without influencing the display of measured values. The required filtering of the light fluxes in the measurement sensor and in the evaluation unit can be undertaken with different interference filters, in which the light fluxes are advantageously also interleaved spatially or in their polarisations. Modulation is possible by means of movable masks of polarisers, or else by means of elasto-optical members.

Description

Daimler-Benz Daim 16 β5Γ |? / 4

Aktiengesellschaft, \ [q. 07/31/1985

7000 Stuttgart 60

Process for fiber-optic, spectrally coded transmission of the value of a variable physical measured variable as well as Facility for carrying out the procedure.

The invention relates to a device for fiber-optic, spectrally coded transmission of the value of a variable physical measured variable with the generic features mentioned in the preamble of claim 1, and a device suitable for carrying out such a process.

Such transmission methods and devices should, if possible, depend on the properties of the optical fibers used be independent, so in the context of a fiber optic transducer to connect the sensor and evaluation device intended fibers with different lengths, cross-sections, losses, curvatures, etc. the evaluation of the Impact spectrally and with regard to their intensity coded useful light signals as little as possible.

Under fiber optic transducers are understood in the following arrangements that consist of a sensor and a Evaluation devices exist that have one or more fiber optics

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fibers are optically interconnected. In the sensor a physical measurand to be recorded (e.g. pressure, temperature, Force, position, angle, etc.) into an optical signal converted or "encoded", which is sent to the evaluation device via the optical fiber. There the decoding takes place, i.e. the value of the measured variable effective at the sensor is determined from the optical signal. This value can can then be displayed, registered or otherwise processed in a suitable form, for example in a Regulator.

Because of the excellent insulation properties of conventional optical fibers, there is such fiber optic transducers a complete galvanic separation between the sensor and the display device. You can thus without special Protective measures are used in high-voltage technology (monitoring of transformers), in medicine (no Risk of electric shock for the patient), as well as in plants in the petrochemical and mining industries (no short circuit / risk of ignition) or in automotive engineering.

A large number of functional principles have therefore become more fiber optic Measured value transducers proposed and demonstrated, for example in the article "Optical Fiber Sensor Technology" by T.G. Giallorenzi et al. in IEEE Journal of Quantum Electronics, Vol. QE - 1-8 (1982), pp. 626-655. Among these transducers, a subgroup is characterized by particular structural simplicity in which the The coding mentioned takes place by means of an intensity modulation, for example by means of a light barrier. The one from one

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-to

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Daim ίδ * J July 31, 1985

Light source usually accommodated in the evaluation device and guided to the sensor via an optical fiber Luminous flux is weakened there to a greater or lesser extent, according to the value of the currently effective one Measurand. The remaining luminous flux runs over the same or a different fiber to a detector in the evaluation device. The detector signal proportional to the light power arriving there is thus a measure of the value of the Measurand.

One advantage of this functional principle is that the analog intensity modulation used here for coding technically very simple by shifting or twisting diaphragms that can be carried out in the luminous flux are attached and allow a fraction of it to pass, depending on their position. Other important benefits lie in the possibility of being able to use inexpensive multimode optical fibers for these transducers and operate them with luminescent diodes as reliable, small light sources. It can also be done through appropriate design the modulator diaphragm achieves a very high absolute measurement sensitivity with regard to the diaphragm position will. A typical one for this group of transducers Example can be found in that of W.B. Spillmann and D.H. McMahon described "Schlieren Multimode Fiberoptic Hydrophones" (Applied Physics Letters, Vol. J7 '1980, pp. 145 ff.) will. With this transducer, the intensity modulation is achieved in that the luminous flux successively penetrated by two closely adjacent, parallel arranged line gratings, which form a kind of "moiré" modulator.

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The displacement of one grating against the other by a line width (typically: 5 μΐη) causes / that this modulator is controlled from the state of maximum light transmission to the state of minimum transmission. This arrangement therefore primarily represents a displacement transducer. In the special case of this example, it becomes a hydrophone (underwater sound pressure transducer) in that one of the grids is connected to an elastic membrane on which the pressure to be detected acts and deflects. The membrane thus functions as a pressure * displacement transducer. In the same way, the mentioned modulator could also be used for the construction of fiber optic thermometers, dynamometers, accelerometers and sensors for other quantities by using other transducers instead of the membrane, for example a bimetallic body (temperature path), a spring body (force - * ■ distance), or a test mass on a spring body (acceleration · * ■ force - * ■ distance).

Another simple fiber optic transducer that also belongs to the group of analog intensity modulation working transducer is the fiber optic thermometer according to A.J. Rogers (Applied Optics Vol. 21, 1982, Pp. 882-885). In this instrument, the light flux arriving at the sensor via a fiber is not measured by means of a mechanically moved diaphragm, but by means of a temperature-dependent, polarization-optical arrangement in its intensity attenuated before it is fiber optically returned to the evaluation device and a signal corresponding to the detector signal Display causes.

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The two mentioned and numerous other prior art fiber optic transducers of the Subgroups with coding by intensity modulation have the fundamental disadvantage that the evaluation device cannot distinguish between changes in the luminous flux received by the detector due to changes in the Measured variable on the one hand and changes due to fluctuations in fiber losses on the other. The latter can for example occur when the fiber is bent, or when in a fiber cable its temperature or its mechanical Voltage vary. Another disadvantage is that these transducers are generally associated with permanently Fixed length fiber lines have to work. The use of fiber connectors is ruled out because they usually do not cause precisely reproducible losses. Actuating the plug could then result in a corresponding variation or uncertainty in the display. In the same sense, the installation of fibers of different lengths or attenuation would also be problematic affect the calibration of the transducers of this subgroup.

For this reason, a second transmission channel is introduced in some improved fiber optic transducers. which transmits a reference luminous flux that is not modulated by the measured variable or is modulated in opposite directions. For determination the measured variable in the evaluation device is then no longer an absolute output, but the ratio of the outputs of the received luminous fluxes in the signal and in the reference channel. If the mentioned changes in fiber losses are the same in both channels, the absolute power of the luminous fluxes change with the fiber losses,

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but their relationship remains unaffected, and the display is independent of these changes .. Such a coding the measured variable in the intensity ratio of two luminous fluxes is used, for example, in the by H. Dötsch et al. specified fiber optic transducers (IEE Conference * Proceedings No. 221, "Optical Fiber Sensors", London 1983, pp. 67-71). A slidably arranged lens in the probe here couples the incoming luminous flux into two outgoing optical fibers leading to the evaluation device. The measurand influences the position of the lens and thus couples the light correspondingly more strongly into one or the other of the two fibers in such a way that the ratio of the two partial luminous fluxes clearly represent the measured variable should.

In fact, this compensation for variable fiber losses is only effective to a limited extent, because the signal and the Reference channels are routed to the evaluation device via two different optical fibers and are therefore not subject to them exactly the same influences. Against a broad practical applicability stands next to the costly necessity the additional reference fiber but above all still the mentioned disadvantage of non-reproducible connector losses, which in general will be significantly different in the two channels. This disadvantage precludes the use of So far, plugging in this type of transducers has been practically unplugged, but this considerably limits their possible uses will. A solution to this problem that initially seems worth considering can be seen in the signaling and the reference luminous flux over the same optical fiber lead and these two luminous fluxes at two different

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to transmit optical frequencies, for example in the green and red spectral range. Such a solution using light fluxes with a relatively large spectral spacing cannot, however, be satisfactory in practice, since most types of fiber losses depend heavily on the optical frequency (or wavelength), as do the detector sensitivities, which can result in additional calibration problems. A transmission with two spectrally far apart optical frequencies can therefore not be independent of the fiber properties, whereby the expression "independent of the fiber properties" is intended to mean that the two luminous fluxes are subject to all types of fiber losses in the same way as possible during their transmission, ie absorption and loss Scattering losses, curvature losses and coupling losses from connectors, splices and changes in the cross-section of the fiber. For a transmission that is independent of the fiber properties in the sense explained above, it would at least be necessary to choose the two optical frequencies very closely adjacent, ie so that their frequency spacing ΔV from the mean frequency y of the two light fluxes is very small. Furthermore, the spectral line width OV of the two luminous fluxes would also have to be small, for example üV sr AV.

Correspondingly narrow-band optical filters are possible, but very complex and they would be removed from the continuous emission spectrum of a light emitting diode only a very small one, the filter line width Let the corresponding performance share through, which in turn leads to problems with regard to the detection sensitivity or accuracy could result.

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The object of the invention is therefore to specify a method of the type mentioned at the outset which has one of the properties a fiber optic transmission link largely independent transmission for the measured value more characteristic allows optical signals and their simple evaluation, as well as a device for performing this method.

With regard to the method, this object is achieved by the characterizing features of claim 1.

_{Thereafter, two luminous fluxes I 1} and I ₂ 1 , which are spectrally encoded by interleaved line spectra, are coupled into the evaluation device via a single optical fiber that marks the light path from the sensor to the evaluation device, through which the output ratio is evaluated in units the measured variable takes place. The two luminous fluxes are, as they are conducted via the same optical fiber and the same connecting elements between it and the measuring sensor on the one hand and the evaluation device on the other, e.g. fiber plug connections, subject to exactly the same influences of the fiber optic transmission path, which are therefore subject to evaluation through ratio formation as it were, so that optimal conditions are given for a transmission of the measured value information independent of the configuration of the fiber-optic transmission path. The linear structure of the two luminous fluxes used according to the invention, which can be described as spectrally interlocking or nested comb spectra, is achieved in a simple manner

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that the two luminous fluxes to be compared are practically the same, at least in a very good approximation

- Middle - have wavelengths, so that the - wavelength-dependent - influences of the fiber optic transmission path on the in this propagating luminous fluxes I, and I- is given and thus Overall, the transmission of measured values is largely independent of the properties of the fiber and its fluctuations is achieved. A possibly required dependency of the detector sensitivities on the wavelength can also arise

- because of the interleaved spectral structure of the luminous fluxes I, and I, - do not interfere with the measurement result. In addition, the process according to the invention also makes it possible, depending on the number of in the individual core spectra contained a relatively large fraction of the power from the continuous lines Use the spectrum of the light source used, e.g. a luminescent diode. The method according to the invention can also be carried out with simply designed optical filter devices.

In the procedure outlined by the features of claim 2 can be an optimal approximation to the ideal case of absolute equality of the mean wavelengths of the partial luminous fluxes I and I_ are achieved, for example, in that the spectrum of a partial luminous flux is only one includes the single line and the spectrum of the other partial luminous flux two lines, the mean wavelength of which corresponds to that of the one line of the first-mentioned partial luminous flux c. Such a procedure is possible, for example, using laser light sources.

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ORIGINAL INSPECTED

Daim 16 β5β / 4 July 31, 1985

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The features of claims 3 and 4 indicate alternatively applicable types of execution of the method according to the invention, with one type of execution - according to FIG interleaved transmission of the partial luminous fluxes I ₁ and I ₂ and thus synchronized reception of the same with the help of only one detector is worked.

Devices suitable for carrying out these alternative procedures are their basic structure according to the features of claims 5 and 6, through which the so far underlying the invention Subtask is solved.

Due to the features of claims 7 to 24, alternatively or in a logical combination, usable design and construction Functional features within the framework of devices according to claims 6 and 7 which can be used for detecting the measured variable, Advantageously simply designed measuring sensor specified, which are designed for use in transmission. Such sensors are then expediently not only connected to the evaluation device, but also to the respective light source coupled via an optical fiber.

The features of claim 25 provide the basic structure of a method for carrying out the method according to the invention suitable, further device indicated in which the sensor used to detect the measured variable

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is designed as a reflection unit. For the optical coupling of the sensor to the light source or a light supply device on the one hand and to the evaluation device on the other hand, only a single optical fiber is required here, as well as a partially transparent mirror through which part of the light fluxes I and I ₂ returned through the optical fiber to the detection device of the evaluation device can be deflected.

The features of claims 26 to 33 make it possible to implement designs of with simple technical means Specified measuring sensors which can be used in the context of a device designed according to claim.

Further details and features of the invention emerge from the following description of implementation examples of the method according to the invention and of exemplary embodiments for the implementation of suitable facilities. Show it:

Fig. 1 shows the basic structure of an inventive, fiber optic transducer as a device for carrying out an inventive Method for the spectrally coded transmission of a physical measured variable, in a simplified, schematic block diagram representation,

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ORIGINAL INSPECTED

Dam, 16- ₆ Λ /? 52829

07/31/1985

Figure 2 (a) shows a typical spectral distribution of the intensity of one in the transducer according to Fig.l usable light source,

Fig. 2 (b) transmission characteristics in the context of the transducer transmission filter that can be used according to FIG. 1,

Fig. 2 (c) the spectral composition for detecting and 2 (d) _{measuring the} g _{r e} 5ß-utilized partial luminous fluxes I ₁

and I-,

to explain the method according to the invention,

Fig. 3 simplified block diagram representations ^un alternative embodiments of fiber optic transducers alternative to illustrate the ways of performing the inventive method using sensors, which are designed for transmission operation,

FIG. 5 shows alternative design options within the framework of measured value pickups according to FIG. 3 or FIG. insertable measuring probe,

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FIG. 9 is suitable for transmission mode sensor with ^un-designed reference Filter

pair to determine the spectral composition of the partial luminous fluxes I and I_ as well as with sliding screens for the intensity modulation of these partial luminous fluxes,

11 measuring sensors which can also be used in transmission ^and with a polarization-dependent division of a

Primary luminous flux into the measurement value acquisition utilized partial luminous fluxes and with polarization-dependent modulation devices,

Fig. 13 alternative designs according to the invention ^un fiber optic transducer having Refle

xion mode operable sensors,

15 shows the basic structure of various ^1S sensors that can be used in reflection mode,

18 shows a special design of a basic structure according to the sensor Fig. 17 corresponding sensor with as Bregg reflectors trained interference filters for the spectral coding of the partial luminous fluxes I. and I2 / as well as

Fig. 9 functionally ^un the sensors according to Figures 11 and 12 corresponding, however, be in reflection

drivable sensors.

In Figures 1 and 3 to 20 of the drawing, structurally and functionally identical or analogous elements are each with the same Reference symbol occupied.

Daim 16 βΛ / 4 yt (- 07/31/1985

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With reference to Figures 1 and 2, all details of which are expressly referred to, on the one hand the general construction principle of a transducer designated as 100 is explained below, which is a device for carrying out a method according to the invention for the spectrally coded transmission of the value of a represents continuously variable physical measured variable "x", which will be explained in more detail with reference to FIG. FIG. 1 shows a fiber-optic measured value pick-up 100 which works with coding of the measured variable χ in the intensity ratio of two luminous fluxes I ₁ and I ₀ . The transducer 100 includes a light source 14 with a continuous emission spectrum, a sensor 11, which transmits the coding of the measured variable χ in the ratio of the intensities of the two luminous fluxes I ₁ and I ₂ , which from a - spectral - branch of one of the light source 14 in the Sensor 11 coupled primary luminous flux I are generated, as well as an evaluation stage 10, which a decoding of the intensity ratio of the two luminous fluxes I and I "mediates in units of the measured variable χ and generates a signal characteristic of the measured variable χ, which by means of a display instrument 22 in units of the Measured variable can be displayed or used for further processing, e.g. to activate a pump that charges a pressure accumulator, the pressure level of which must not fall below a lower limit value, in which case the measured variable χ is the pressure in this pressure accumulator.

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From the light source 14, for example a luminescent diode emitted light is fed to the sensor 11 via a first optical fiber 12 as a primary luminous flux I

guided. In this sensor 11, a first pair of filters 15 is provided as an input stage, which divides _{the primary luminous flux I into the two partial luminous fluxes I and I 2.}

Due to the filter pair 15, which is only indicated schematically in FIG. 1, the partial luminous fluxes I and I ₂ resulting from the division of the primary luminous flux I are impressed with a characteristic - different - spectral intensity distribution, each with a narrow-band, line-shaped structure of the spectral distribution of the light outputs, such that a The spectral line of one luminous flux I- lies as a spectral between two lines of the other luminous flux I _2; This achieves a spectral interleaving of the two luminous fluxes I and I ₂ , the spectral distance of a line of the one luminous flux I ₂ from a spectrally adjacent line of the other luminous flux I _{2 being} greater than the line width of these spectral components of the two luminous fluxes I- and I ₂ .

In the exemplary embodiment of a transducer 100 shown in FIG. 1, chosen to explain the principle of the invention, the output partial _{luminous fluxes I 1} and I _{2 of} the first filter pair 15 are spatially separated from one another, and within the scope of the sensor 11, for example, as transverse to Direction of propagation of a luminous flux I ₁ displaceable shutter 17 provided modulation device by means of which the light output of the one

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from the pair of filters 15 exiting light current I ₁ with the displacement of the diaphragm 17 correlated values is variable I _1, whereas in the embodiment according to FIG 1, the light output of the other, emerging from the pair of filters 15 partial light current I ₂ by the modulating means 17 is not affected. The two partial luminous fluxes are combined with one another again in a combination element provided as the output stage of the measuring sensor 11 in the sense of a superimposition and fed together to the evaluation device 10 via a second optical fiber 13.

In the evaluation device 10 a second filter pair 18 is provided, optically connected to the output of the beam combining element 16 via the optical fiber 13, which has the same overall transmission characteristics as the first filter pair 15 provided in the context of the sensor 11. The filter pair 18 of the evaluation device 10 mediates thus, again, separation of the joint through the second optical fiber 13 to the evaluation unit 10 supplied to the partial luminous fluxes I ₁ and I _2, the intensities of which are detected by two each of the two light currents I ₁ and I ₂ associated detectors 19 and 20, wherein the output of the a detector, eg the detector 19, the intensity I _{1 of} the partial luminous flux I ₁ modified by the modulation device 17 and the output signal of the other detector 20 which is proportional to the _{intensity of the other partial luminous flux I 2, unchanged in the illustrated example.} For further evaluation, an electronic stage 21 is provided in the evaluation device, which is a

Daim 16 p

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• 3a ·

forms the output signal corresponding to the ratio R = S / S ₂ ^ he signal level of the detector output _{signals S 1} and S ₂ , which output signal can be displayed by means of the measuring instrument 22. If the partial luminous flux I- is modulated in the sensor 11, for example by being weakened from its original output I- more or less to an output I- by the movably arranged diaphragm 17, the display of the measuring instrument 22 is reduced accordingly. The fiber optic arrangement shown in FIG. 1 thus represents a measured value pick-up for the measured variable χ if this measured variable controls the position of the diaphragm 17 and the measuring instrument 22 is calibrated in units of the measured variable χ.

In FIG. 2, the method of spectral coding is illustrated, which effects a transmission independent of the properties of the optical fibers 12 and 13 used for the measured variable of characteristic optical signals between the measuring sensor 11 and the evaluation device 10 ) schematically the spectral power density distribution "I ₀ (V) of a light source 24 with a continuous emission basic spectrum as a function of the optical frequency The bandwidth of the basic spectrum, measured between the frequency values Vl / 2 and γΉ / 2, where the power density of the basic spectrum is half of the maximum value I _Q is denoted by By.

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Daim 16 65 ^ / 4 07/31/1985

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Furthermore, FIG. 2 (b) shows spectral transmission characteristics T ₁ (V) and T ₂ (ν) which each of the two filter pairs 15 and 18 must have. Each of these two transmission characteristics corresponds to a line spectrum with a typical distance Ay between adjacent lines of the same filter 15 or 18 and with a typical line width 6v . For one partial luminous flux I _1, the transmission characteristic T ₁ (γ)

effective, while the transmission characteristic T ₂ (V) is effective for the other partial luminous flux I _2. In the figure 2 ^ b). are, for the sake of simplicity,

The spectral transmission ranges of the filters 15 and 18 are shown at the same spectral intervals as being “wide” and being “high”. These equations, although advantageous for the effectiveness of the method according to the invention, do not have to be strictly fulfilled over the entire bandwidth By. Rather, it is sufficient if they exist approximately over larger partial areas of the half-value width By of the basic spectrum, for example in the sense that the spectrum T ″ (V) emerges from T ₁ (V) by a shift of approximately ΔΥ / 2 all that both spectra or transmission characteristics T ₁ (V) and T_ (V) are nested in one another so that they overlap as little as possible, and that a large number of lines _{fall within the bandwidth By of the basic spectrum I 0} (V) .

The following conditions should therefore apply:

(S V * AV / 2 (1;

AV «By (2).

Daim 16

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_{The spectra I- (V) and I 2} (V) of the luminous fluxes I ₁ and I, resulting from these filter characteristics and the basic spectrum I _Q (V) of the light source 24, are shown in Figures 2 (c \ _ and 2 ( It can be seen that because of condition (2) and the steady course of the basic spectrum over the frequency scale at de spectral distributions I- (V) and I «(V), the same mean optical frequency V ₀ and also to a good approximation have the same total bandwidth By, which _{are specified by the basic spectrum I Q} (V). As a result, the two luminous fluxes I and I "always experience the same losses caused by the optical fibers 12 and 13. This advantage can be achieved through the design of the filter pairs 15 and 18 can still be achieved even if the fiber losses change relatively drastically as a function of the optical frequency V, such as, for example, the scattering losses which

2
typically increase proportionally to V. The spectral line spacing AV must then be selected to be sufficiently small that the loss-related differences in the spectral intensities in two spectrally adjacent lines are smaller than the display accuracy required for the transducer. Because of the continuity of all real loss spectra, this is always possible.

The fact that the two in the inventive Way, spectrally coded luminous fluxes X- and I_ have practically the same mean frequency V "and bandwidth By, is also advantageous in the detection of these luminous fluxes, especially in the event that their Performances are to be measured with the same detector as it is expedient in self-adjusting evaluation processes is. With the same values of V and By for both

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Daim 16 65 * 0/4 July 31, 1985 35

Partial luminous fluxes I and I ₂ are namely the detector sensitivities for both luminous fluxes the same, even if they should both be rapidly changing functions of the frequency.

Another important advantage of the coding provided according to the invention is that a relatively large fraction of the light power available in the basic emission spectrum of the light source 24 can be used. This fraction is roughly given by the spectral transmission characteristics T. (V) and T _{2 of} the filter pairs 15 and 18 averaged over the bandwidth By approximately the value T ₁ = T ₂ = Ov / Δν. It is assumed here that the peak transmission values of the transmission characteristics T. (V) and T ₂ (y) are close to the value 1. The stated values τ 1 and T ₂ can be orders of magnitude higher than in cases in which partial _{luminous fluxes I 1} and I _{2 are} only obtained by filtering out a single line of spectral width flV.

Finally, the fact is even more advantageous that filters 15 and 18 having spectral transmittance characteristics as shown in Figure 2 (b) _(shown, are relatively easy to construct in the form of interference filters, which will be discussed in more detail below.

Daim 16 $ 6 € / 4 July 31, 1985

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z. for a more detailed explanation of the so far explained The following numerical example is considered for the conditions that occur in practice »

A gallium arsenide luminescent diode with an average optical wave number y = 12,000 cm, corresponding to a wavelength of 0.83 µm, serves as the light source. The bandwidth By of the emission spectrum of this luminescent diode has a typical value of 400 cm. The conditions (1) and (2) are in any case complies with safety, line widths of 2 cm oV be used when line spacings of 40 cm and Δ V. This can be achieved in practice with interference filters of order m = 300 and finesse F =

The clearing process explained so far can be carried out in practice in various ways, which will now be explained in more detail with reference to the transducers 100 and 100 'shown in FIGS. For the sensors 11, which can be of identical design for the two transducers 100 and 100 ', it is initially only assumed that they allow the light currents I ₁ and I ₂ supplied and discharged via the optical fibers 12 and 13 to pass through to different degrees, such that the ratio T ₁ / T ₂ ^{ü represents the} current value of the measured variable χ. In the implementation of the method according to the invention represented by the measured value sensor 100 according to FIG. 3, these two luminous fluxes I ₁ and I _{2 are} branched off from a primary luminous flux 1 of a light source 14. As a result, fluctuations in the power of this light source 14 have no influence on the display of the measured value.

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As a result of the intimate spectral interleaving of the two luminous fluxes I and I ₂ , which are contained in the exemplary embodiment according to FIG that they also have the same optical performance. _{The luminous fluxes I- and I 2 carried} jointly by the sensor 11 via the second optical fiber 13 to the evaluation device 10, in which the light source can be integrated, are, as already explained in connection with FIG separate detectors 19 and 20 converted into electrical signals S ^ and S ₂ . The ratio S./S _{2 of} the detector signals is equal to the transmission ratio T ../ T, because the two luminous fluxes started with the same power in the fiber 12 and, apart from the sensor Ί1, were subject to the same losses everywhere and were also subject to the same sensitivities the detectors 19 and detected.

Figure 4 illustrated by the Meßwertaufnenmer 100 'according way of performing the method according to the invention provides far as a reversal of the procedure described above is, as here, the two light currents I ₁ and I ₂ having the same performance of an evaluation device 10 · arrive at a receiver 35, but with different, correspondingly modified according to the reciprocal of the transmission ratio explained above in that of one

Daim 16 65 ^ / 4 38 July 31, 1985

Light supply device 14 'to the sensor 11 leading optical fiber are coupled. For this purpose, two separate but essentially identical light sources 30 and 31 are provided within the scope of the light supply device 14 ', the output powers of which can be controlled by electronic elements 13 and 33. A pair of filters 18, which is operated in the opposite direction compared to the transducer 100 according to FIG. 31 and at the same time combines these luminous fluxes I- and I ₂ t which are coupled into the optical fiber leading to the sensor 11. The required equality of the power of the luminous fluxes I- and I returned via the second optical fiber 13 from the measuring sensor 11 to the evaluation device 10 ' At the decoupling end of this optical fiber 13, the light sources 30 and 31 are alternately switched on and off by a clock 34, the two light currents I and I _{2 being} received by the same detector 35 after passing through the entire optical system will. With the same power of the luminous fluxes 1 and I ₂ , this detector 35 emits an output signal that is constant over time. In the event that the light outputs of the two light fluxes I and I ₂ should deviate somewhat from one another, the detector 35 generates an alternating signal which, after amplification and synchronous rectification, is fed to a controller as an error signal by means of an amplifier 36 acting in the manner of a phase-sensitive rectifier, which influences the electronic control elements 13 and 33 in such a way that the required equality of the light outputs of the two light fluxes I and I _{2 at the detector-side output of the optical fiber 13}

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adjusts. The above-explained influencing of the light sources 30 and 31 can consist in the fact that their output light power is influenced; But it is also possible, within a clock period of the clock generator 34, which controls the light sources 30 and 31 alternately in their light-emitting states, the periods of time are changed within which each - within a clock period - the one and the other light source or light source. 31 is operated with a certain output power. With this type of control of the light sources 31 and 31, the required equality of the light outputs or intensities of the light fluxes I ₁ and I «when they strike the detector 35, thus by a variation of the within one clock period light energy emitted by the individual light sources 30 and 31. From the resulting ratio of the signals controlling the light sources 31 and 31 with regard to their intensity or their lighting duration, an electronic stage 21 'again calculates the transmission ratio T-1 / T2 sought and, if necessary, effects after evaluation with a calibration function to be determined beforehand, also a measurement value see with the help of Änzeigeinstruments 22. It is to be executed noted that takes place in the above-reference to the Figure 4 method ultimately .Bestimmung the value aer measurement variable from the ratio of the powers of the light currents I ₁ and I ₂ at the extraction end of the optical fiber 13, also if this ratio has the value 1 after the control loop has settled.

I.

Daim 16 6Λ / 4

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• 1 * 0 ·

The construction within the scope of transducers 100 or 7. 100 'as in Figures 3 and 4 only schematically indicated, can be done essentially according to four different construction principles, which are shown in Figures 5 to 8, reference is now made to the details of which, schematically, in the manner suitable for comparison are shown.

The sensor 11 shown in FIG. 5 corresponds to that already shown in FIG. 1:

From the light flux I arriving via the optical fiber 12, the spectral composition of which is determined by the emission characteristics of the light source 14, the two light fluxes 1- and Ι are separated by means of the filter pair 15, modulated by the measured variable and combined again in the beam combiner 16 and coupled into the optical fiber 13 leading to the evaluation device 10. It is essential here that the modulation acts differently on the two luminous fluxes I- and I ~. In Figure 5, this is indicated by a displaceable damping filter 17 with location-dependent transmission, a so-called gray wedge; alternatively, a diaphragm which can be displaced transversely to the direction of light propagation could also be provided, or any other known method for attenuating a light output could be used. While only one measurand-dependent modulation of a light current is I-provided in the probe 11 according to Figure 5, it is practically advantageous, and the other light flux Ι ~ / these, however, in opposing senses for the modulation of the luminous flux I _1, to modulate,

'■ u

Daim 16 630/4 July 31, 1985

that is, in such a way that there is a decrease in the intensity of the luminous flux 1- with an increase in the intensity of the luminous flux I2 goes hand in hand. The sensor 11 required for such a modulation of the two luminous fluxes I and Ι, ίη Separation of the same is indicated schematically in FIG. 5 as a real spatial separation.

It is understood, however, that other types of separation of two light currents can also be used, for example a separation with regard to the directions of propagation of the light currents I ₁ and I 2 or with regard to different polarization states of the same. In both of the last-mentioned cases it is advantageously possible to have both luminous fluxes I. and I ₂ flow essentially within the same spatial cross-section, which guarantees particularly good equality of all losses caused, for example, by diaphragm edges or scattered reflections.

Numerous arrangements known to the person skilled in the art can be used as the beam combining element 16, which are otherwise used as beam splitter devices working with wavefront or amplitude splitting, for example fiber optic Y-couplers, partially transparent mirrors or the like. In the event that the difference to be united, that is coupled into the optical fiber 13A of the light currents I ₁ and I ₂ with respect to their polarization, a polarization analyzer can be used as beam combiner 16 when a between the polarization states of the luminous fluxes I ₁ and I ₂ lying polarization state can pass.

¹ zn

- U-

Daim 16 07/31/1985

In the modification shown in FIG. 6 of a sensor 100 or 100 ', as shown in FIGS. 4, the usable measuring sensor 11 is, in contrast to the measuring sensor according to FIG - Fulfills. Depending on the value of the measured variable χ, the modulator 16 couples 11 according to Figure 6 is a more or less large proportion of the power of the Liehtstromes I ₁ and a complementary portion of the power of the Liehtstromes I ₂ into the secondary to the evaluation unit 10 optical fiber of the probe. This type of beam combination in combination with opposing modulation of the intensities of the luminous fluxes I 1 and I ₂ can be implemented, for example, by a movably arranged lens or, in the case of different polarizations of the luminous fluxes I and I ₂ ^, by a lens that can be adjusted depending on the measured variable χ Polarization analyzer.

In FIGS. 7 and 8, further advantageous designs of measuring sensors 11 are shown, which convey the different characteristic modulations required for coding the measured variable χ - relative changes in the intensities of the partial luminous fluxes I and I ₂ . The sensor 11 shown in FIG. 7 is derived from that according to FIG. 5 simply by interchanging the order of the filter pair 15 and

'■ · 2t

Daim 16 07/31/1985

of the beam combiner 16 with respect to the direction of propagation of the partial luminous fluxes, with now the according to FIG. 5 as a beam combiner v / acting element acts as a beam splitting element (Figure 7).

The same applies mutatis mutandis to the design of a further measuring sensor 11 shown in FIG from that according to FIG. 6 by corresponding interchanging of the luminous fluxes in the direction of propagation viewed one another, film-based optical elements 16 - measured variable-controlled beam splitter - and a pair of filters 15, the pair of filters 15 in turn acting as a beam combining element.

For a more detailed explanation of applicable modulation methods, reference is now made to the details of FIGS. 9 and 10, in which it is shown schematically how the _{performance of the two partial luminous fluxes 1 and I 2} can be modulated in opposite directions by moving a diaphragm.

According to FIG. 9, the luminous flux I arriving via the optical fiber 12 is directed, for example by means of a collimation lens 54, onto a pair of filters 15, which consists of two interference filters 52 and 53, which are of the Fabry-Perot interferometer type. These interference filters 52 and 53, which each have a transmission characteristic with narrow-band pass bands arranged equidistantly in the frequency scale, filter out the luminous fluxes I ₁ and I ₂ with the narrow-band linear spectral structure used according to the invention, which

2 ₃

Daim 16 6SO / A July 31, 1985

in the explanatory example according to FIG. 9, run side by side in two spatially separate areas. The diaphragm 51 shades from the two luminous fluxes I ₁ and I ₂ , as can be seen from FIG. 9, depending on its position, more or less large areas of the light flux cross section. When the diaphragm 51 is displaced as a function of the measured variable x, transversely to the direction of discharge of the partial luminous fluxes I ₁ and I ₂ , the transmitted portion of the light current I _{1 increases} , while the portion passed on _{by the partial luminous flux I 2 decreases.} It goes without saying that the width of the diaphragm measured in the plane of the drawing must be selected appropriately, expediently equal to the width of the light flux cross-sections of each of the partial luminous fluxes I and I _{2 measured transversely to the direction of propagation} - and which are impressed by the interference filters 52 and 53 with alternately interleaved, narrow-band, line-shaped spectral structure, is achieved by a converging lens 55, which transfers the two partial _{luminous fluxes I 1} and I ₂ or their fractions not shaded by the aperture 51 to the sensor side The input end of the optical fiber 13 leading to the evaluation device 1Θ is focused and coupled into it. One advantage of this simultaneous, opposing modulation of the partial _{luminous fluxes I 1} and I ₂ is that the power ratio used for the evaluation is ₁ A ₂ ^{as a} function of the Displacement of the Aperture 51 changes twice as quickly as if only one of the two partial luminous fluxes

would be modulated.

'■ 30

"J

07/31/1985

Also in the embodiment shown in the Figure 10 design of a sensor 11 already explained with reference to Figure 9 basic principle is a modulation spectrally interleaved partial luminous fluxes I and I- with the aid of a transversely movable shutter 51 utilized *, where the spectral composition of these partial luminous fluxes I ₁ ¹ and I ₂ ^{1 is} again determined by interference filters 52 'and, each of which has the structure characteristic of a Fabry-Perot interferometer.

In the X-direction transverse to the direction of light propagation, in sensor 11 according to FIG. 10, filters 52 'of one type and interference filters 53 "of the second type are arranged next to one another in an alternating sequence. The interference filters "52 ¹ ^" each have the same transmission characteristic, by which the spectral composition of the partial luminous fluxes I- ¹ transmitted by these filters 52 'is determined, and the filters 53' in turn each have the same transmission characteristic, through which the spectral composition of the partial luminous fluxes I ₂ ^{1 is} determined, which in turn have the same spatially "nested" distribution as the interference filters 52 'with respect to the interference filters 53'. As in the exemplary embodiment according to FIG. 9, in the exemplary embodiment according to FIG. 10, the interference filters 52 'and 53' corresponding in their effect to a filter pair 15 are arranged between a collimation lens 54 and a converging lens 55, which acts as a beam combining element. The measuring sensor 11 according to FIG. 10

¹ Sf

ORIGINAL INSPECTED

üaim 16 65f / 4 07/31/1985

has the advantage that a possible misalignment of the collimator lens 54 cannot cause a disruptive inequality of the illumination of the interference filters 52 'and 53, the effect of which cannot be distinguished from a change in the measured variable. If, as indicated schematically in FIG. 10, the sub-filters 52 'and 53' have a narrow strip-shaped structure or are sector-shaped and, viewed in the direction of the optical axis of the measuring sensor 11, are nested in the azimuthal direction, in such a way that always one If a larger number (in a typical case not less than 5) of sub-filters 52 'or 53 "of each type _{f are} illuminated, the disruptive influence of possible misadjustments is greatly reduced 'and 53' is the simpler way of producing such a filter arrangement 52 ', 53'.

If only two interference filters 52 and 53, like the Figure 9 can be removed, are used, they must be made so that the transmission maxima of the one Filters 52, which define the spectral position of the lines of the luminous flux I-, with the transmission minima of the other interference filter 53 coincide, as shown in Figure 2 (b). So that these conditions over the entire spectral range By of the light source is sufficiently well met, it is necessary that the Interference orders of the sub-filters 52 and 53 at a mean optical frequency y by half an order or an odd multiple of half an interference order differentiate. If such partial filters 52 and 53 are manufactured separately, however, the

Daim 16 65 «/ 4 07/31/1985

Keeping this condition a very precise control of the reflector spacing of the interference filters 52 and 53.

In contrast to this, the nested arrangement of the sub-filters 52 'and 53' provided according to FIG two reflectors, according to FIG. 10, the reflector 43 _f reflects light with a different reflection phase on strip-shaped areas of its reflective surface than in the strip-shaped areas in between. This can be achieved, for example, in that before the application of the partially transparent reflective layer, which covers the entire reflective surface, e.g. of the reflector 43, in the surface of which the reflective layer is supported, strips with a depth of / 4 (or an odd multiple thereof)

to be etched. "X" is the _{wavelength corresponding to the mean optical frequency V 0} according to FIG. 2. Another option, equivalent to this, is, after the reflective layer has been applied, to cover the reflector surface with a transparent layer with a thickness of 0/25 ^ / in-1), where η denotes the refractive index of this layer and the term (n-1) takes into account the optical delay of the light in the coated strips relative to the uncoated strips in between

Daim 16 δΛ / 4 07/31/1985

Vs

Possibilities to modify the reflection phases, with which light is reflected on the different strip-shaped areas apply to the Case that the substrates carrying the reflective layers outside the resonance volume of the "Fabry-Perot interference filter" are arranged, that is, the reflective layers on the inner sides facing one another the reflectors 42 and 43 are arranged. Modifications that are necessary for the Fa] I, that the reflective layers on the opposite outer sides of a transparent carrier, e.g. a substrate made of quartz glass, which contains the resonance volumes of the individual Partial filter 52 'or 53' fills are considered in the area of professional skill lying down. In the case of the sensor 11 according to FIG. 10, the sub-filters 52 ″ are each through the raised strip-shaped areas 44 of one reflector 43 and the these oppositely arranged partial areas of the reflector 42 are formed with a flat reflective layer, while the sub-filters 53 'are arranged between the raised, strip-shaped areas 44, recessed strip-shaped regions 46 and the surface regions arranged opposite them the flat reflective layer of the reflector 42 are formed. The overall combination of the sub-filters 52 'des The effect of the measuring sensor 11 according to FIG. 10 corresponds to that of the filter 52 according to FIG. 9. The same applies mutatis mutandis to the combination of the sub-filters 53 ′ according to FIG. 10 and the filter 53 according to Figure 9.

Daim 16 65-0 / 4

07/31/1985

ι.

Likewise, the totality of the partial luminous fluxes I ^ ', the spectral composition of which is determined by the partial filters 52', corresponds to the partial luminous flux I ₁ and the totality of the partial luminous fluxes I ₂ ¹ r, the spectral composition of which is determined by the partial filters 53 ', corresponds to the partial luminous flux I ₂ / the spectral composition of which is determined by the filter 53 of the measuring sensor 11 according to FIG. Corresponding to the strip-shaped structure of the filter arrangement 52 ', 53' of the measuring sensor 11 according to FIG. 10, there is also a strip-shaped lattice structure with alternating opaque strips 47 and transparent strips for its transversely displaceable screen 51 '

■ provided. The type of modulation of the partial _{luminous fluxes 1 .. 'and I 2} ' provided according to FIG. 10 is particularly suitable for measuring sensors which primarily detect a geometric displacement in the X direction. With a sector-shaped design of the partial filters 52 'and 53' and azimuthal grouping of these around the central longitudinal axis 41 of the sensor 11 and a corresponding sector-shaped design and arrangement of the opaque and permeable areas of the diaphragm b1 'and rotatable arrangement of the same around the longitudinal axis 41 an angle of rotation can be detected with such a sensor. Such measuring sensors can also be used in a known manner for the acquisition of other physical measured variables: if, for example, in the case of measuring sensor 11 according to FIG. Probe

Daim 16 6 ft / 4-

07/31/1985

_ 50-

suitable. If the rotary movement takes place in a sector-shaped manner designed aperture in a sensor with sector-shaped interference filters against the restoring force of a torsion spring, then such is primarily for the detection of an angle of rotation suitable sensor can be used as a torque sensor.

Numerous sensors for other variables such as temperature, pressure, electrical or magnetic fields can be implemented in a corresponding manner by using expansion bodies, membranes or piezoelectric or magnetostrictive bodies, the diaphragm movements can be controlled by their deformation proportional to the measured variables ¹ .

In the case of the sensor 11 shown in FIG. 11, the details of which are now referred to, a filter pair 15 'which corresponds functionally to the filter pairs explained above with partial filters 52 and 53 or 52' and 53 'is realized with the aid of a single structural element which is used as Interference filter m is designed as a Fabry-Perot interferometer. This interference filter 15 ', like the pair of interference filters 15, 52, 53 according to FIG. 9 and the interference filter combination 52', 53 'according to FIG. 10, is arranged between a collimator lens 54 and a collecting lens 55. _e ui the 62 Res'onatorraum limiting, partially transparent reflectors and 64 have flat reflection surfaces 66 and 67. The ER- located between these reflection surfaces 66 and 67

Daim 1 3528294 -54- 07/31 6 6Ϊ0 / 4 1985

stretching resonator chamber is with a plate-shaped Body made of a transparent, birefringent material • filled. The seen in the direction of light propagation 41 the thickness of the birefringent body, which determines the distance between the reflective surfaces 66 and 67, is selected so that that for two mutually orthogonal polar i- sationsbedingungen of a light flux passing through the birefringent body 68 in one of them Polarization state has a delay of about a quarter wavelength relative to the other of the two States of polarization. consists. The birefringent body dimensioned in this way then represents a so-called quarter-wave plate. This has the consequence that 'of this body 68 and the outside arranged Reflection layers 66 and 67 an interference filter is formed, its transmission characteristics differ by just half an order for two orthogonal polarization states, as in Figure 2 (b) shown. It is understood that instead of a birefringent Body 68, which between orthogonal polarization states a delay of a quarter wavelength mediated, a birefringent body 68 can also be used, which mediates a delay, which corresponds to an odd multiple of a quarter wavelength.

The combination of two Fabry-Perot interference filters explained so far to a single with orthogonal polarization eigenstates can be both with linear as well as with circular birefringence of the resonator

ρ- c \ Daim 16 δΛ / 4

07/31/1985

space 62 filling, birefringent body 68 is achieved where the eigen-states correspond to these polarizations. The practical, particularly simple one The case of linearly polarized eigenstates can also be achieved without using birefringent materials are such that at least one of the reflectors and / or 64 of the interference filter 15 'is formed is that the reflection phase with normal incidence of light becomes polarization dependent. This is possible, for example, that on this reflector dielectric or metallic relief grating is applied, the grating period of which is smaller than the light wavelengths used for the transmission of measured values. The modulation depth and the period of such Line grids must be chosen so that the difference in reflection phases for light that is parallel or is polarized perpendicular to the grating lines, a total of 180 ° to shift the for the two polarization states have effective transmission characteristics by half an interference order to ensure. Are both reflectors 63 and 64 of the interference filter 15 'with such line gratings provided, they can be designed so that each of the two line grids with, for example 90 ° contributes to the difference in the reflection phases.

To further explain the type of modulation that can be carried out by means of the measuring sensor according to FIG it is assumed that the interference filter 15 'has two eigenstates of linear polarization, whose Oscillation directions parallel and perpendicular to

/ 29

Daim 16 sSu / 4

07/31/1985

Are oriented to the drawing plane. Correspondingly, the partial luminous fluxes I- and 1 ~ emanating from this filter 15 'are then also polarized linearly and parallel to these directions, but are not spatially separated, but spread in the same spatial area and in the same direction. A linear polarization analyzer 60, which is arranged between the interference filter 15 'and the converging lens 55 of the measuring sensor 11, is provided for modulating these partial luminous fluxes I and I "in opposite directions. If this polarizer adjusted with respect to the plane on an azimuth cC, he leaves from the two partial light currents I _n and I ₀ only fractions of COS t> C or Sm ck happen and thus encodes its azimuthal angle position in the power ratio of luminous flux. The sensor shown in FIG. 11 is therefore particularly suitable for a fiber optic angle measurement. As already stated, the measurement of numerous other physical quantities can be carried out by combining them with transducer elements that are sensitive to the measured quantities.

The one in FIG. 12, the details of which are now referred to The sensor shown is particularly suitable for measuring forces. He is different of the sensor according to Figure 11 essentially in that between the interference filter 15 'and the Analyzer 60, a stress-optical element 61 is arranged, which by the action of a force with. defined direction of attack becomes birefringent. The direction of application of the force and the arrangement of this optical tension element 61 is expedient

Daim 16

SV-

07/31/1985

chosen so that the double refraction main axes have azimuths of + 45 ° under the action of the force, the analyzer expediently remaining fixed to the azimuth oC = O. If the elasto-optical body 61 is free of birefringence in the force-free case, it does not influence the two orthogonally polarized light fluxes, with the result that one partial light flux I- can pass through the analyzer 60 without being weakened, while the other partial light flux I ₂ is blocked. If a force P acts in the direction of the arrow 69 on the elasto-optical body 61, the two partial luminous fluxes I and I _{2 are} more and more elliptically polarized, with the result that the analyzer 60 increasingly weakens _{one partial luminous flux I 1,} but lets the other partial luminous flux I ₂ pass increasingly. If the force reaches such a size that the retardation in the elasto-optical body 61 has the value TT, the partial luminous flux I is completely blocked and the partial luminous flux I _{2 is} completely allowed through. The sensor 11 according to FIG. 12 is thus suitable for encoding a force P in the ratio of the powers of partial luminous fluxes I and 1 ″ _{2 of} different polarization.

In the alternative types of transducers 100 or 10 described above with reference to FIGS. 100 ', within the framework of such transducers, depending on the intended use the sensors 11 explained with reference to FIGS. 5-12 can be used, these sensors are shown in FIG Transmission operated, what.in practice has the consequence that a first optical fiber 12 must be provided over

^{Daim 16 6} ^ ^tf / ⁴

07/31/1985

the output luminous fluxes of a light source 14 or a light supply device 18, 30, 31 can be fed to the sensor 11, as well as a second optical fiber 13, via which partial _{luminous fluxes I. and I 2} define different spectral compositions, the intensity ratio of _{which is 1./I 2,} the measured value information contains, can be fed to an evaluation stage 10 or 10 · provided for their evaluation in units of the measured variable.

As an alternative to such transducers 100 or 100 ″, transducers 110 or 110 ¹ can also be used as devices for carrying out the method according to the invention, the basic structure of which is shown in FIGS Measurement sensors 111 which can be operated in reflection mode and which can be implemented with the designs and functional properties that can be taken from FIGS. 15-20, the details of which are likewise referred to.

To the extent that reference numerals are used in FIGS are identical to the reference symbols used in FIGS. 1-12, should thereby also be used at the same time to avoid repetitions to avoid referring to the parts of the description belonging to the identically labeled elements in FIGS. 1-12 be referenced, so that the description of the transducers 110 and 110 'shown in Figures 13 and 14 as well as the measuring sensor 111 shown in FIGS. 15-20 essentially for functional properties of the illustrated devices and a comparison of the same with the functionally analogous ones described above Facilities can be restricted. In the difference

Daim 16 07/31/1985

^{to the transducers 100 and 100 1} shown in Figures 3 and 4, which have a coupling interface between the light source 14 or the light _{supply device 18 f} 31, 30 on the one hand and the respective sensor 11 on the other hand, as which, for example, the sensor-side end of the first optical fiber 12, and in which between the respective measuring sensor 11 and the evaluation device 10 or 10 'a further optical interface, referred to as a decoupling interface, is provided, as which, for example, the end of the second, remote from the respective measuring sensor, from the sensor 11 to the evaluation device 10 or 10 'leading optical fiber 13 is, in the case of the transducers 110 and 110 ¹ shown in FIGS. 13 and 14, between the light source 14 according to FIG. 13 and the light supply device 18, 30, 31 according to FIG. 14 and the respective measuring sensor 111 on the one hand, and between this and the evaluation device 10 or 10 '' on the other hand, in each case n Only a common optical coupling / decoupling interface is provided, which is formed here by the end 1Φ3 of a single optical fiber 112, which is arranged at a distance from the respective measuring sensor 111, which is designed as a reflection unit, via which both light can be coupled into the respective measuring sensor 111 and that from These back-reflected, intensity-modulated partial light currents I ₁ and I _{2 can be} coupled out to the evaluation device 10 or 10 · nin.

The optical "separation" of the coupling and decoupling interfaces that coincide in this way is in itself known arrangement a partially transparent mirror 23 is provided, through which, in the case of FIG.

Daim 16 - 5f - 07/31/1985

source 14 outgoing Brimärlichtstrora I - can partially pass through and be coupled into the optical fiber 112, and by means of which at the fiber end 113 from the sensor 111 coming partial luminous fluxes I ₁ and I ₂ , in whose intensity ratio the measured value information is coded, to the evaluation device 10 can be diverted.

In the case of the embodiment according to FIG. 14, spectrally interleaved teal light fluxes emanating from the light supply device 18, 30, 31 are deflected by the partially transparent mirror 23 to the fiber end 113 immediately adjacent to it and coupled into the optical fiber 112, while emanating from the respective sensor 111 Partial luminous fluxes I. and I ₂ , in whose intensity ratio the measured value information is coded, pass through the partially transparent mirror 23 and can impinge on the detector 35 of the evaluation device 10 '.

Thus, general terms and conditions, there are special designs of the the reflection units forming the respective measuring sensors 111, the structural differences of the measuring transducers and 110 'according to Figures .13 and 14 compared to the functionally analog transducer 100 or 100 'according to Figures 3 and 4 exhausted. In particular, those provided within the scope of the transducers 110 and 110 ″ Evaluation devices 10 and 10 'have the same structure as described with reference to FIGS. 3 and 4.

Further explanations can therefore be drawn on the special designs of sensors 111, as described in detail in FIG shown in Figures 15-20, are limited:

/ H3

Daim 16 65 * / 4

07/31/1985

In the case of the sensor 111 shown in FIG. 15, a pair of filters denoted as a whole by 15 serves both for the spectral splitting of a primary luminous flux I emitted by the light source 14 into the two spectrally interleaved partial luminous fluxes I ₁ and I ₂ as well as their reunification after these partial luminous fluxes I ₁ and I ₂ have been thrown back into themselves at these individually assigned reflectors and 116 and at least one of the two partial luminous fluxes, e.g. the partial luminous flux I- has been subjected to a modulation of its intensity corresponding to the measured variable χ, e.g. by means of a gray wedge 17 or a diaphragm . The construction and function of the measuring sensor 111 is largely analogous to the measuring sensor 11 shown in FIG.

The sensor 111 shown in FIG. 16 is after Structure and function comparable to the sensors 11 shown in FIGS. 6 or 8 for transmission operation:

A modulation device 16 controls the division of a primary luminous flux I into partial luminous fluxes 1 'and I ^{11 as a} function of the measured variable χ. These pass twice each, ie once in the forward and once in the reverse direction, a transmission _{filter pair 15, the common arm 117 of which, in which the partial currents I 1} and propagate with a spectrally interleaved composition, is closed off by means of a reflector 1T8.

_{The luminous fluxes I 1} and I ₂ returning to the modulator 16 in the "upper" and "lower" according to FIG

Daim 16 65 «/ 4 July 31, 1985

.59-

Filter pairs 15 defined, in the frequency scale, nested spectral compositions. In the modulator 16, these luminous fluxes I ₁ and I - are combined again with one another in accordance with the division ratio of this modulator and are coupled into the optical fiber 112.

The one in the fog. 17 is structurally and functionally largely the same as that according to FIG. 16, but in contrast to this, instead of a pair of filters 15 with a common arm 117, individual filters 119 and 121 are provided, which reflect the partial luminous fluxes I ₁ and I _{2 thrown back to the modulator 16} imprint said, nested, spectral compositions, these individual filters each being individually closed by a reflector 114 ¹ or H6 ¹ integrated into the respective filter.

In Fig. 18 is a specific design of a sensor 111 shown, its basic structure according to the construction and funxtionsprinzip shown in FIG is equivalent to.

One from the optical fiber 112 into the probe 112 according to FIG 18 coupled in primary luminous flux I is determined by means of the lens b4 collimates. Depending on the measured variable x transversely to the optical axis 41 of: Sensor 111 displaceable slit or perforated diaphragm gives an opposite direction depending on the measured variable χ Variation of the intensities of the "upper" and "lower" areas of the light flux cross section according to FIG spreading luminous fluxes. Each of these

Daim 16 07/31/1985

Partial areas are assigned a reflection filter 119 or 121, which reflects the incident light flux back into itself. In the special embodiment shown, the reflection filters 119 and 121 are designed as high-order Bragg reflectors. Such a reflector consists of a larger number (at least five, but possibly 20 and more) weakly reflecting partial reflectors 123 or 124, which, viewed in the direction of propagation of the incident light, are arranged one behind the other at equal intervals d. Or d ". If the distance between these reflectors, for example the reflectors 123, has the value d .. and the refractive index of the medium between these reflectors 123 has the value n .. then the distance AV between spectrally adjacent reflection maxima is (cf. FIG. 2)

where AV is expressed in wave numbers (cm). The required spectral interleaving of the partial night currents I ₁ and I "thrown back by the reflection filters 119 and 121 is achieved in that - given the same values of the refractive indices of the media arranged between the reflectors 123 and 124 of the reflection filters 119 and 121 - the distances d of the partial reflectors 123 des a reflection filter 119 have a different value than the distances
filters.

Distances d _{2 of} the partial reflectors 124 of the other reflection

It goes without saying that it is also possible to achieve the desired different reflection characteristics of the two reflection filters 119 and 121 in that, given the same reflector spacings, d. and d ₂ the refractive indices n.

Daim 16 6ffO / 4 July 31, 1985

• 6A-

and n _{2 of} the media disposed between reflectors 123 and 124, respectively, are different.

In the case of the measuring sensor 111 shown in FIG. 19 and acting as a reflection unit, a primary luminous flux I coupled in from the optical fiber 112 is converted into a parallel bundle by means of a collimation lens 54, which is reflected back into itself by means of a reflection filter 126 designed as a Bragg reflector . The partial reflectors 127 of this reflection filter are made of a birefringent material so that light with linear polarization parallel and perpendicular to the plane of the drawing "sees" different refractive indices and thus the partial luminous fluxes I and I _2, which are characterized by different polarization states and reflected by the reflection filter 126, have nested spectral composition in the frequency scale. Between the reflection filter 126 and the collimation lens 54 there is a polarizer 128 which can be rotated around the central axis / rotated depending on the measured variable and determines the transmission ratio of the transmitted components of different polarizations, the azimuthal position of this polarizer 128 being variable in proportion to the measured variable χ.

The sensor 111 is its function according to that in the tig. 11 shown, polarization optical sensor 11 analog.

The sensor 111 shown in FIG. 20 differs from that according to FIG. 19 only in that the polarizer 128 has a predetermined azimuthal setting

Daim 16 6Λ / 4 July 31, 1985

ment (e.g. 45 with respect to the plane of the drawing), and that between the polarizer 128 and deir »again as a Bragg reflector formed reflection filter an elasto-optical body 129 is arranged, the one for the measured variable proportional birefringence unfolds and thereby a modulation of the polarization of that transmitted by the polarizer 128 Conveys light. It goes without saying that with the given orientation of the polarizer the optical axes of the reflection filter 126 must also run at 45 with respect to the plane of the drawing. Of the Sensor 111 according to prop. 20 is functionally appropriate to the one. 12 analogous and in particular for acquisition a pressure that acts on the elasto-optical body 129 is suitable.

It should be mentioned that the methods and device variants described can be combined, modified and, if necessary, refined in many ways by a person skilled in the art. For example, the variants explained with reference to FIGS 11 and 12 Wollaston prisms or other polarizers with two output beams can be used. Instead of the simple interference filters considered by way of example, it can also be useful to switch two or more such interference filters in series or to use such interference filters that two or more _{overflows to suppress a possibly disruptive crosstalk between the partial light flows I 1 and I_}

/ kB

Daim 16 6§ψ / Α A ^ July 31, 1985

partially transparent reflection layers coupled resonance volumes own. Such filter combinations have relatively broader transmission maxima and lower minima.

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Claims (1)

Daimler-Benz Aktiengesellschaft Daim 16 6 $ ύ / 4 7000 Stuttgart 60 July 31, 1985 Claims 1 Process for the fiber-optic, spectrally-coded transmission of the value of a variable physical measured variable from a measuring sensor responsive to the measured variable to an evaluation device that generates characteristic display signals or output signals suitable for further processing as useful signals, after which the light output of a first luminous flux , which by means of the sensor is subjected to a power modulation which is monotonously related to changes in the measured variable and the light power of a second luminous flux used as a reference luminous flux is subjected to a comparison that can be carried out by means of the evaluation device, and from this the mentioned useful signal as one for the ratio of the intensities of the two Luminous flux characteristic signal is generated, the; two luminous fluxes are fed from the measuring sensor to the evaluation device by means of a fiber optic arrangement, characterized in that the two luminous fluxes I- and I ~ via one of the light paths from the measuring sensor (11; 111) to the evaluation device. (10) forming optical fiber (13; 112) are coupled into the evaluation device (10) so that the two partial luminous fluxes I and 1. / 2 Jul 31, 1985 - 2 - can also be generated with a different, each narrowband line-shaped structure of the spectral distribution of their light output, such that a line of one luminous flux lies spectrally between two lines of the other luminous flux and thus a spectral interleaving of the two luminous fluxes I ₁ and I _{2 is} achieved, with the The spectral distance of a line of the one luminous flux from a spectrally adjacent line of the other luminous flux is greater than the line width OV of these spectral components of the two partial luminous fluxes, and that the powers of the spectrally nested luminous fluxes I, and I ₂ with regard to the in the evaluation device (10) performance comparison by means of filter arrangements (15,18; 15 ';119,121; 126) selectively adjusted to the specific spectral distributions of the two luminous fluxes from- can be measured spectrally separated from each other. 2. The method according to claim 1, characterized in that one luminous flux comprises an even number of spectral lines and the other luminous flux an odd number of spectral lines which contribute significantly to the total output of these luminous fluxes, with each a spectral line of one luminous flux lies spectrally between two lines of the other luminous flux. 3. The method according to claim 1 or claim 2, characterized in that the two light fluxes come from the same light source (14) that these light fluxes in the Evaluation device (10) separated by spectral filter (18) and two detectors (19 and 20) are supplied, and that off the ratio of the output signals of the two detectors (19 * and 20) the value of the measured variable is determined. '/ 3 ORIGINAL INSPECTED Since in 16 63 0/4 07/31/1985 4. The method according to claim 1 or claim 2, characterized in that the two luminous fluxes I ₁ and I "by means of a light source provided in a light supply device (30 and 31) and the light sources individually assigned filters of a filter pair (18) generated and in a formed by an optical fiber (12; 112), the light supply device (18,30,31) optically with the sensor (11; 111) connecting light path are coupled that these two light sources (30 and 31) alternately in time-periodic sequence and be switched off that the light fluxes fed via the optical fiber (13; 112) from the sensor (11; 111) to the evaluation device (10) are fed to a receiver (35) synchronized with the light source switch, the output signal of which is thus synchronized with this clock periodic alternating signal when the i from the sensor (11; 111) exiting the partial luminous fluxes I. ^ and I_ have different performance, that of an AC Ausga ngssignal of the receiver (35) an error signal for controlling a controller (37) is generated which emits control signals by means of which the output powers of the two light sources (30 and 31) are readjusted in such a way that the powers of the output luminous fluxes of the sensor (11; 111) are regulated to a minimum difference, and that the control signals, which are a measure of the intensity ratio of the light fluxes emitted by the light sources (30 and 31), are evaluated in units of the measured variable. / 4 352829A Daim 16 07/31/1985 Device for carrying out the method according to one of the preceding claims 1 to 3, which generates a signal characteristic for the measured variable from a power comparison of two output luminous fluxes of a measuring sensor which transmits a power modulation characteristic of the measured variable of at least one of the two partial luminous fluxes, these output luminous fluxes I ₁ and I _{2 of} the sensor have a narrow-band, linear, spectrally interleaved intensity distribution, which is achieved by spectral splitting a primary luminous flux emitted by a single light source in such a way that a line of one luminous flux always lies spectrally between two lines of the other luminous flux, and the both partial luminous fluxes are fed via an optical fiber to an evaluation unit provided for the performance comparison, characterized in that the measuring sensor (11; 111) has a first pair of filters (15; 15 ';119,121; 126,128) which mediates the spectral distribution of the light output of the light source (14) to the spectrally interleaved partial luminous fluxes to be compared, and that A second pair of filters (18) is provided in the evaluation device (10), which according to the spectral composition of the two output _{luminous fluxes I 1} and I «of the measuring sensor (11; 111)> their spatial division and separate feed to each one of the luminous fluxes ( I ₁ and I ~) provided detectors (19 and 20) mediated. / 5 1528294 Daim 16 6αΦ / 4 Jul 31, 1985 - 5 - 6. Device for performing the method according to claim 4, which consists of a performance comparison of two output luminous fluxes a measuring sensor / the power modulation characteristic of the measured variable of at least one of the two luminous fluxes conveyed, a characteristic signal for the measured variable is generated, this output luminous flux of the sensor being a narrow-band linear spectrally interleaved intensity distribution, which is achieved by filtering output light fluxes two light sources is achieved in such a way that a line of a luminous flux is always spectrally between two lines of the other luminous flux lies, and the two output luminous fluxes of the sensor via an optical Fiber to an evaluation device intended for the performance comparison are forwarded, characterized in that the two output light fluxes defined in terms of their spectral composition by a first filter pair (18) arranged between the light sources (30 and 31) and which are subject to the measured variable-characteristic modulation in the sensor (11; 111) by means of an im Frame of the measuring sensor (11; 111) provided second filter ^{pair (15; 15 1} ; 119,121; 126,128), which has the same filter characteristics as the first filter pair (18) separately subjected to the modulation and into which the measuring sensor (11 111) with the evaluation device (10) optically connecting fiber (13; 112) can be coupled in that a detector (35) optically coupled to the output of the fiber (13; 112, 113) is provided to generate signals characteristic of the output luminous flux of the sensor (11; 111), that the light sources (30 and 31) controlled by means of a clock generator (34), can be switched on and off alternately / 6 Daim 16 65Ϊ> / 4 July 31, 1985 - 6 - are, and that a by means of the clock (34) synchronized / receiving the output signals of the detector (35), in the manner of a phase-sensitive rectifier working electronic stage (36) is provided, with the output signal of which can be controlled by a controller (37) which controls the output light powers of the two light sources (30 and 31) for the minimal intensity difference of the output light fluxes received by the detector (35) and I- of the sensor (11; 111) controls. 7. Device according to claim 5 or claim 6, characterized in that the filter pair (15) forms an input stage of a sensor (11) which separates the input light current into the two Teillicrvt.strömme I ₁ and, of which at least one of the modulation that the output stage of this measuring sensor (11) is formed by a beam combiner (16) which couples the two partial light currents into an optical fiber leading to the evaluation device (10), and that a modulation device (17), which is dependent on the measured variable the power of at least one of the two partial light currents (I ₁ and / or I ₂ ^ seen in the direction of propagation of these partial light currents, is arranged between the filter pair (15) and the beam combiner (16) (Figure 5). 8. Device according to claim 7, characterized in that the modulation device modulates both luminous fluxes in the sense of a mutual change in intensity (Figure 6). / 7 Daim 16 659/4 July 31, 1985 9. Device according to claim 5 or claim 6, characterized in that the input stage of the sensor (11) a beam splitter (16) is provided, which divides an incoming luminous flux into two partial luminous fluxes, that as the output stage of this sensor (11) a pair of filters (15) is provided which defines the spectral composition of the two partial luminous fluxes forwarded to the evaluation device (10) and is also designed as a beam combiner that feeds the two luminous fluxes into the one leading to the evaluation device optical fiber (13) is coupled. 10. Apparatus according to claim 9, characterized in that a modulation device (17), which, depending on the measured variable, influences the power of at least one of the two partial luminous fluxes between the Input stage (16) and the output stage (15) of the sensor (11) is arranged. 11. Device according to claim 8, characterized in that the modulation device (16) changes the division ratio with which the light output of the incoming luminous flux is divided between the two partial luminous fluxes (Figure 8). 12. Device according to one of the preceding claims, characterized in that at least one of the filter pairs (15 or 18) consists of two interference filters (52 and 53) operated in transmission, whose spectral Transmission characteristics are medium /8th Daim 16 6 ^ / 4 07/31/1985 optical frequency of the luminous flux approximated by one distinguish odd multiples of half an interference order. 13. Device according to claim 12, characterized in that the two interference filters (52 'and 53') of at least two are parallel to one another arranged, partially transparent reflectors (42 and 43) are formed and that of the within the Cross-sectional area of the light flux lying area of the reflective surface of one of the reflectors (43) about half gives a reflection with a reflection phase that is an odd one Multiples of 7Γ / 2 of that reflection phase differs with which the other half of the reflective surface of this reflector (43) is reflected. 14. Device according to claim 13, characterized in that the reflection phases of the reflector (43) are modified by applying at least one thin dielectric layer is achieved on one of the two partial areas of the reflector (43). 15. Device according to claim 13 or claim 14, characterized in that a modulation device provided for power modulation of the luminous fluxes is used as a movable mask (51; 51 ') is formed, which in The fractional parts of the partial luminous fluxes I- and I_ which are dependent on the value of the measured quantity of the Light flux cross-section shaded more or less (Figures 9 and 10). / 9 Daim 16 07/31/1985 16. Device according to claim 15, characterized / that the reflective surface of a reflector (43), which is partially with different Reflection phases reflected, divided into strips of equal width, reflect the incident light reflect with the different reflection phases, and that the modulation device is transverse to the longitudinal direction of the strip displaceable strip mask (51 '), the structure of which is determined by alternating transparent and opaque areas corresponds to the strip structure of the reflector (43) reflecting with a different reflection phase (Figure 10). 17. Device according to claim 12, characterized in that a polarization-dependent interference filter (15 ¹ ) is provided as an interference filter pair, which has a spectral interference filter transmission characteristic for light of a first, defined polarization state and also an interference filter for light of the polarization state orthogonal to the first-mentioned polarization state. Has transmission characteristic which is offset from the first-mentioned transmission characteristic by an odd multiple of half an interference order (FIGS. 11 and 12). 18. Device according to claim 17, characterized in that the two polarization eigenstates of the polarization-dependent interference filter (15 ¹ ) are the two orthogonal states of linear polarization (Figures 11 + 12). / 10 Daim 16 SSV / 4 July 31, 1985 - 10 - 19. A device according to claim 17 or claim 18, characterized in that the resonance volume (62) of the polarization-dependent interference filter (15 ¹ ) contains a birefringent medium (68) whose phase delay for the two luminous fluxes of orthogonal polarization with a single light passage (transmission) through the Resonance volume is approximately an odd multiple of a quarter of an average Lijcht wavelength. 20. Device according to claim 17 or claim 18, characterized in that in at least one of the reflectors (63 and / or 64) of the polarization-dependent ^; Interference filter (15 *), the phase with which the reflection takes place on this is polarization-dependent, the phase difference that results overall when two orthogonally polarized light fluxes are reflected on the reflecting surfaces (66 and 67) of the interference filter (15 ¹ ), corresponds to an odd multiple of ΊΓ / 2. 21. Device according to claim 20, characterized / that the reflector with polarization-dependent Phase contains a relief grating acting as a phase grating, the grating constant of which is smaller as the wavelength of the light used for the transmission of measured values. / 11 Daim 07/31/1985 ie 6 ^ 28294 - 11 - 22. Device according to one of the preceding claims 17 to 21, characterized in that _{a polarizer (60) is provided as the modulation device, which influences both luminous fluxes I 1} and I _{0 used} for transmission in a relation monotonous with the measured variable, whose excellent polarization state can be changed as a function of the measured variable. 23. Device according to claim 22, characterized in that the polarizer (60) is a linear polarizer whose azimuthal orientation can be controlled by the measured variable. 24. Device according to one of the preceding claims 17 to 23, characterized in that the modulation device comprises a polarizer (60) and a birefringent element (61), whose birefringence can be influenced by the measured variable. 25. Device according to claim 5 or claim 6, characterized in that the measuring sensor (111) is designed as a reflection unit, which for the coupling of the modulation subject light and for the decoupling of the evaluation device (10) supplied luminous fluxes I ₁ and I _{3 has} a common optical coupling / decoupling interface (113) which is formed by the end of an optical fiber (112) remote from the sensor (111), via which light can be coupled into the sensor (111) and from the sensor in the opposite direction back to the evaluation / 12 ORtGlHAL ie eSi / 528294 07/31/1985 - 12 - device (10) can be supplied, and that between this interface (113) and the light supply device or the evaluation device (10) a partially transparent mirror (23) is arranged is / through which part of the light fluxes I- and I_ returned through the optical fiber (112) for Detection device of the evaluation device (10) can be deflected. 26. Device according to claim 25 _; characterized in that the measuring sensor (111) has a filter pair (15) as input and output stage, which provides a spatial separation of the two luminous fluxes I ₁ and I ₂ which can be individually or jointly subject to a measured variable-dependent modulation, and that at least one reflector is provided which reflects these light fluxes in itself back to the filter pair (15), and that the modulation device (17), _{viewed in the direction of propagation of the light fluxes I 1} and I ₂ , is arranged between the filter pair (15) and the reflector (Figure 15) . 27. Device according to claim 25, characterized in that a beam splitter (16) is provided as the input and output stage of the measuring sensor (111) is an incoming luminous flux in a division ratio determined by the measured variable in two Luminous fluxes branched out, which have a spectral composition at least one filter pair (15) defining the luminous fluxes returning to the interface Reflector (118) are fed, which reflects the light hitting it back into itself. / 13 ORIGINAL INSPECTED ■ "" Dairn 16 β5 <> / 4 Jul 31, 1985 - 13 - 28. Device according to claim 25, characterized in that a beam splitter (16) is provided as the input stage and output stage of the measuring sensor (111), which branches an incoming light flux into two light fluxes in a division ratio determined by the measured variable, and that these two light fluxes each have a reflection filter (119 and 121) which define the spectral distribution in the light fluxes I ₁ and I ₂ reflecting back to the interface (113) (FIG. 17). 29. Device according to claim 25, characterized in that the reflection filter (29) each from a sequence, seen in the direction of propagation of the incident light, arranged equidistantly and Partially transparent reflectors (123, 124) running perpendicular to the direction of propagation exist, the optical distances in the two sequences of reflectors are different and are chosen such that the interference order of the reflected light in one reflection filter (119) differs from that in the other Reflection filter (121) differs approximately by an odd multiple of a half order (FIG. 18). 30. Device according to claim 25, characterized in that the sensor (111) is a contains polarization-dependent reflection filter (126), one each for two orthogonal polarization states has specific, linear reflection characteristics such that the spectral intensity distributions the luminous fluxes reflected with one of the stated polarization states each have the spectrally different / 14 Daim 16 65 «/ 4 07/31/1985 14 - have nested linear structure, and that between this reflection filter (126) and the interface (113) a polarizer (128) is arranged, which is dependent on the measured variable for different polarization states the luminous fluxes transmitted to the interface (113) are permeable. 31. Device according to claim 30, characterized in that the polarization-dependent reflection filter (126) is a series of equidistantly arranged, partially reflective birefringent layers is formed. 32. Device according to claim 25, characterized in that the sensor (111) is a contains polarization-dependent reflection filter (126), one for each of two orthogonal polarization states has specific linear reflection characteristics, such that the spectral intensity distributions of the each with one of the mentioned polarization states reflected light fluxes the spectrally interleaved, linear Have a structure that a polarizer (128) is arranged between this filter (126) and the interface (113) is, and that between this polarizer (128) and the reflection filter (126) a for the useful light transparent body (129) is arranged, which develops a birefringence that varies with the measured variable. / 15 Daim 16 GUI / 4 July 31, 1985 - 15 - 33. Device according to claim 32, characterized in that the transparent body (129) consists of an elastic-optical material, the through a force acting transversely to the direction of light propagation and varying with the measured variable a force varying with the latter Birefringence can be imprinted.