Method for fiber optic transmission of a special coded measurement

• US 4,705,354 A
• Filed: 08/09/1985
• Issued: 11/10/1987
• Est. Priority Date: 08/09/1984
• Status: Expired due to Fees

• Current Assignee: Daimler-Benz AG (Mercedes-Benz Group AG)
• Original Assignee: Daimler-Benz AG (Mercedes-Benz Group AG)
• Inventors: Ulrich, Reinhard
• Primary Examiner(s): LaRoche, Eugene R.
• Assistant Examiner(s): Lee, James C.
• Application Number:
  US06/764,073
• Time in Patent Office: 823 Days
• Field of Search: 350/96.15, 350/96.29, 350/96.30, 250/226, 250/227, 250/231 R
• US Class Current: 356/477
• CPC Class Codes: G01D 5/268 using optical fibres G01D5/28 - G01D5/38 take precedence
• CPC Class Codes: G01D 5/34 the beams of light being detected by photocells

Abstract

A method wherein two light fluxes I₁ and I₂, spectrally encoded by interspersing or alternating line frequency spectra, are transmitted through an optical fiber from a transducer to an evaluation unit which generates an indication signal in relation to the ratio of the optical power of the two light fluxes. Both light fluxes I₁ and I₂ are generated with distinctly different narrow-bandline structures of the spectral distribution of their optical powers with the spectral separation of adjacent lines of the two line fluxes being greater than the linewidth of these lines. The optical power of the spectrally interposed light fluxes I₁ and I₂ are measured individually after spectral separation by filters which are selected to the specific distribution of the light fluxes I₁ and I₂.

Claims

1. A method for a fiber-optic transmission of the value of a spectrally encoded measurant representative of a variable physical quantity, from a transducer sensitive to that quantity, to an evaluation unit generating desired output signals which may be used for indication or for further processing, according to which method the optical power of at least a first flux I₁ is by means of the transducer, subjected to an intensity jodulation in a monotonous relation to the measurant, and the optical power of a second light flux I₂ used as a reference light flux, is by means of the evaluation unit, compared with said first light flux, whereby the desired output signal is generated as a signal characteristic of the power ratio of the two light fluxes, which by means of a fiber-optical arrangement are transmitted from the transducer to the evaluation unit, wherein both, the light flux I₁ and the light flux I₂ are transmitted from the transducer to the evaluation unit through one optical fiber forming the light path therebetween,

wherein both partial light fluxes I₁ and I₂ are generated each with distinctly different narrow-band line-structures of the spectral distributions of their optical powers, in such a manner, that a line of one of the two light fluxes is spectrally interposed between two lines of the other light flux, with a spectral interspersing of the light fluxes I₁ and I₂ resulting therefrom, and the spectral separation of adjacent lines of these light fluxes being greater than the line-widths of those lines, and wherein the optical powers of the spectrally interspersed light fluxes I₁ and I₂, for the purpose of comparing these optical powers in the evaluation unit, are measured individually after spectral separation from each other by means of filters which are selective to the specific sppectral distributions of the light fluxes I₁ and I₂.

2. The method according to claim 1, wherein one of the light fluxes comprises an even number of spectral lines of significant optical power, and the other light flux of an odd number of spectral lines, and wherein each line of the one light flux consisting of the smaller number of lines is spectrally interposed between two adjacent lines of the other light flux consisting of the larger number of lines.

3. The method according to claim 1, wherein both light fluxes I₁ and I₂ are derived from a single light source, these light fluxes being separated in the evaluation unit by spectral filters and directed to individual detectors, and the value of the measurant is determined from the ratio of the two detector output signals.

4. The method according to claim 1, wherein the light fluxes I₁ and I₂ are generated by means of individual light sources of a light supply unit of filters individually allocated to said light sources, and are coupled into an optical fiber defining a light path optically connecting the light supply with the transducer;

the light sources are alternatingly switched on and off, and the light fluxes I₁ and I₂ are fed through an optical fiber from the transducer and directed to a common optical rceceiver, the output signal of the receiver being an a.c. signal if the light fluxes I₁ and I₂ emerging from the transducer have different optical powers;

from an a.c. output signal of the receiver, by rectification synchronous with the switching of the light sources, an error signal is derived and fed to a control unit generating two control signals by which the output optical powers of the two light sources are controlled such a manner that the optical powers of the output light fluxes emerging from the tranducer are adjusted to minimal power-difference, and the two control signals which are measured for the intensity ratio of the light fluxes emitter from the light sources are evaluated in terms of the measurement.

5. A method according to claim 1, including comparing the optical powers of the two partial light fluxes emerging from a transducer which is modulating at least one of said partial light fluxes in a manner characteristic of the measurant, and generating a signal which is characteristic of said measurant, said partial light fluxes I₁ and I₂ emerging from the transducer having narrow-band line-shaped spectrally interspersed intensity distributions which are generated by spectral separation of a primary light flux generated by one light source, such that a line of one of the two partial light fluxes is spectrally interposed between two lines of the other light flux, the two partial light fluxes emerging from the transducer being transmitted through an optical fiber to an evaluation unit which performs the comparison of their optical powers;

spectral separating the two spectrally interspersed partial light fluxes I₁ and I₂ to be comparted with each other from the primary output light flux of the light source by a first pair of filters in the transducer, and

spectral separating and individually directing to an allocated one of two detectors light fluxes I₁ and I₂ emerging from the transducer by a second pair of filters in the evaluation unit.

6. A method according to claim 4, including comparing the optical powers of two partial light fluxes emerging from a transducer which is modulating at least one of said partial light fluxes in a manner characteristic of the measurant and generting a signal which is characteristic of said measurant, said partial light fluxes I₁ and I₂ emerging from the transducer having narrow-band line-shaped spectrally interspersed intensity distributions, which are generated by filtering of output light fluxes of two separate light sources in a manner that a line of one of the two partial light fluxes is spectrally interposed between two lines of the other partial light flux, the two partial light fluxes emerging from the transducer being fed through an optical fiber to an evaluation unit which performs the comparison of their optical powers,

defining the spectral composition of the inuput light fluxes fed to the transducer where the measurant-characteristic modulations occurs by a first pair of filters;

and providing for a separate modulation of the light fluxes and for their common coupling into the optical fiber which optically connects the tranducer with the evaluation unit by a second pair of filters having the same filter characteristics of the first pair of filters;

generating signals characteristic of the optical powers of the transducer output light fluxes by a detector, which detector is optically coupled with the output end of the fiber;

in dependence on the output signal of a clock generator, the light sources are alternatingly switched on and off;

a receiver unit is provided receiving the output signals of the detector and generating by rectification synchronous with the switching of the light sources, an error signal which is fed to a control unit designed to control the output optical powers of the light sources which are alternatingly switched on, in such a manner that the optical output powers of the light fluxes emerging from the transducer are adjusted to minimal power difference.

7. A method according to claim 5, wherein the first pair of filters is the input stage of a transducer separating the input light flux into the two partial light fluxes I₁ and I₂, at least one of which being modulated in the transducer;

the output stage of the transducer is a beam recombination unit which couples the two partial light fluxes into an optical fiber guiding the two partial light fluxes to the evaluation unit;

by a modulator between the pair of filters and the beam recombination unit, seen in the direction of light propapagation, is arranged, which, in modulating the optical power of at least one of the partial light fluxes I₁ and I₂ in dependence on the meausrant.

8. A method according to claim 7, wherein the modulator influences the optical powers of the two light fluxes in the opposite sense.

9. A method according to claim 5, by a beam splitter as an input stage of the transducer incoming light flux I is split into two partial light fluxes;

by a pair of filters as an output stage of the transducers, defining the spectral composition of the two partial light fluxes I₁ and I₂ which are guided to the evaluation unit, this pair of filters also acting as a beam recombination unit by which the two light fluxes are coupling into the optical fiber leading the evaluation unit.

10. A method according to claim 9, wherein influencing the optical power of at least one of the two partial light fluxes in dependence on the measurant is by a modulator arranged between the input stage and the output stage of the transducer.

11. A method according to claim 8, wherein the modulator influences the splitting ratio according to which optical power of the incoming light flux is distributed to the two partial light fluxes.

12. A method according to claim 5, wherein at least one of the pairs of filters consists of two transmission interference filters, and including selecting the spectral transmission characteristic of which at an average optical frequency of the light fluxes, to differ from each other by approximately an odd multiple of half an interferernce order.

13. A method according to claim 12, wherein the two interference filters are composed of at least two parallel partially transmissive reflectors having the total light flux cross section with about half of the reflecting surface of at least one of the reflectors reflecting the light with a phase of reflection which differs from the phase of reflection of the other half of the reflecting surface of said reflector by an odd multiple of π

/2.

14. A method according to claim 13, wherein the modification of the phases of reflection at the reflector is achieved by deposting at least one thin dieletric layer on one of said halves of the reflecting surface of the reflector.

15. A method according to claim 13, including modulating the intensity of the transducer by an intensity modulate designed as a shiftable mask which, in dependence of the value of the measurant, is more or less blocking or clearing, respectively, parts of the total light flux cross section within which the partial light fluxes I₁ and I₂ are propagating.

16. A method according to claim 15, including subdividing the reflecting surface of the reflector which is reflecting with different phases of reflection into stripes of equal widths alternatingly reflecting the incident light with different phases of reflection, and the modulator comprises a mask shiftable at right angles to the longitudinal direction of the reflector surface stripes reflecting with different phases of reflection, said mask having a stripe-shaped structure with alternatingly arranged opaque and transparent stripes and corresponding to the stripe-structure of the reflector surface reflecting with different phases of reflection.

17. A method according to claim 12, including designing the pair of interference filters as a polarization-dependent interference filter which for a first defined state of polarization has a first spectral interference-filter transmission-characteristics and which for the state of polarization orthogonal to the first-mentioned state of polarization has a second interference-filter transmision characteristics which is different from the first transmission characteristics by an odd multiple of half an interference order.

18. A method according to claim 17, including selecting the two polarization-eigenstates of the polarization-dependent interference-filter as the two states of orthogonal linear polarization.

19. A method according to claim 17, including selecting for the resonant volume of the polarization-dependent interference-filter, a birefringent material exhibiting for the two light fluxes I₁ and I₂ of orthogonal states of polarization, after one passage through the resonant volume, a phase retardation which approximately corresponds to an odd multiple of a quarter of the average wavelength of the light fluxes.

20. A method according to claim 17, wherein at least one of the reflectors of the polarization-dependent interference-filter is selected to exhibit a polarization dependent of the phases of reflections occurring at this reflector, the total phase difference resulting from a reflection of light fluxes of orthogonal polarization states at the reflecting surfaces of the interference filter corresponding to an odd multiple of π

/2.

21. A method according to claim 20, wherein the reflector exhibiting the polarization-dependence of the phase of reflection comprises a relief laminar-grating acting on a phase grating, the grating constant of which is selected to be smaller than the wavelength of light used for the transmission of the measurant-value.

22. A method according to claim 17, wherein the modulator influencing the light fluxes I₁ and I₂ used for the transmission of the measurant value in a monotonous relation thereto is a polarizer and including adjusting the polarizer to different polarization states in dependence of the measurant.

23. A method according to claim 22, wherein the polarizer is a linear polarizer and including controlling the azimutal orientation of said polarizer in dependence of the measurant.

24. A method according to claim 17, wherein the modulator comprises a polarizer and a birefringent element and including controlling the birefringence of said polarizer in dependence of the measurant.

25. A method according to claim 5, including designing the transducer as a reflection unit optically connected to the evaluation unit by an optical fiber, the fiber end arranged far from the transducer forming a common input- output-interface at which incident light, which is to be modulated in the transducer, is coupled into the fiber and into the transducer, respectively, and at which the light fluxes I₁ and I₂ emerging from the transducer in the reverse direction are coupled out and thereafter directed to the evaluation unit, between said interface and the light supply unit and the evaluation unit, respectively, a partially transmissive mirror being arranged through which a fraction of the light fluxes I₁ and I₂ emerging from said fiber end is directed to the detection unit of the evaluation unit.

26. A method according to claim 25, including by a pair of filters as an input and an output stage of the transducer spatial separating the two light fluxes subjected within the transducer to an individual or common measurant dependent modulation;

at least one reflector is provided reflecting said light fluxes I₁ and I₂ back to the pair of filters, the modulator, seen in the direction of light propagation, being arranged between said pair of filters and said reflector.

27. A method according to claim 25, including by a beam splitter as an input and output stage of the transducer, splitting an incoming light flux in accordance with a splitting ratio, which is determined by the measurant, into two partial light fluxes which are guided through a pair of filters defining the spectral composition of the light fluxes returning to the interface fiber end to a least one reflector which is arranged to reflect an incident light flux back into itself.

28. A method according to claim 25, including by a beam splitter as an input and an output stage of the transducer, splitting an incoming light flux in accordance with a splitting-ratio, which is determined by the measurant, into two partial light fluxes, these partial light fluxes being guided to separate reflection filters each of which defining the spectral composition of one of the partial light fluxes I₁ and I₂ which are reflected back to the far interface fiber end.

29. A method according to claim 27, wherein the reflection filters each comprise a serial arrangement of partially transmissive reflectors and including arranging said reflectors equidistantly along the direction of light propagation and oriented at right angles thereto, the optical distances d₁ and d₂ between the reflectors of said serial arrangements being different from each other and defined in such a manner that the interference orders of the light fluxes I₁ and I₂ reflected by these reflection filters are differing from each other by approximately an odd multiple of half an interference order.

30. A method according to claim 25, wherein the transducer comprises a polarization-dependent reflection filter selected to exhibit for each one of two orthogonal states of polarization a specific line-shaped reflection-characteristics resulting in the desired spectrally interspersed line-shaped structures of the spectral intensity-distributions of the reflected partial light fluxes I₁ and I₂ having the said orthogonal polarization states and;

wherein between the reflection filter and the near end of the optical fiber optically connecting the transducer with the evaluation unit arranging a polarizer, which, in dependence on variations of the measurant, is adjusted to different states of polarization of light which is transmitted to the far fiber end interface.

31. A method according to claim 30, wherein the polarization dependent reflection filter is formed by arranging an optical series-connection of partially reflective birefringent layers of equal thickness and of equidistant along the direction of light propagation.

32. A method according to claim 25, wherein the transducer comprises a polarization-dependent reflection filter selected to exhibit for each one of two orthogonal states of polarization specific line-shaped reflection-characteristics resulting in the desired spectrally interspersed line-shaped structures of the reflected partial light fluxes reflection filter exhibiting for each one of two orthogonal states of polarization specific line-shaped reflection-characteristics resulting in the desired spectrally interspersed line-shaped structures of the reflected partial light fluxes I₁ and I₂ having the said orthogonal polarization states, wherein further arranging a polarizer defining a selected state of polarization between said filter and the near end of the optical fiber optically connecting the transducer with the evaluation unit, and

wherein arranging between said polarizer and said reflection filter a transparent body which exhibits a varying birefringence dependent on variations of the measurant.

33. A method according to claim 32, wherein the transparent body consists of an elasto-optic material, and including imparting by a force varying in relation to variations of the measurant and acting on the body in a transverse direction with respect to the direction of light propagation, a birefringence in relation to said force.