An optical sensor is described for distinguishing between liquids of different refractive index, through strength of interference caused by an optical cavity having an exposed optical boundary in contact with such liquids. The sensor may be used, for example, to distinguish between water and aviatio
An optical sensor is described for distinguishing between liquids of different refractive index, through strength of interference caused by an optical cavity having an exposed optical boundary in contact with such liquids. The sensor may be used, for example, to distinguish between water and aviation fuel in an aircraft fuel tank.
대표청구항▼
1. An optical sensor for sensing a liquid, comprising: a sensor head comprising one or more optical cavities, including a first optical cavity constructed as a Fabry-Perot cavity and comprising a solid material, the first optical cavity arranged such that a liquid to be sensed contacts the solid mat
1. An optical sensor for sensing a liquid, comprising: a sensor head comprising one or more optical cavities, including a first optical cavity constructed as a Fabry-Perot cavity and comprising a solid material, the first optical cavity arranged such that a liquid to be sensed contacts the solid material at an external boundary of the first optical cavity;an optical source arranged to deliver probe light to the one or more optical cavities;a detector arranged to receive the probe light from the one or more optical cavities and to detect a magnitude of interference in the probe light caused by the first optical cavity, the magnitude of the interference being indicative of a refractive index of the liquid to be sensed; andan analyser arranged to generate an indication, based on the refractive index of the liquid to be sensed, of the liquid to be sensed at least in part by analyzing the magnitude of the detected interference caused by the first optical cavity. 2. The optical sensor of claim 1 wherein the one or more optical cavities includes one or more further optical cavities in addition to the first optical cavity, and the detector is arranged to detect separate interference in the received probe light caused by each of the further optical cavities. 3. The optical sensor of claim 2 wherein at least one of the further optical cavities is a pressure sensing cavity in the sensor head, and the analyser is arranged to generate an indication of pressure at the sensor head from an effect of changes in optical path difference of the pressure sensing cavity under changes in pressure on the detected interference in the received probe light caused by the pressure sensing cavity. 4. The optical sensor of claim 2 wherein at least one of the optical cavities is a temperature sensing optical cavity in the sensor head, and the analyser is arranged to generate an indication of temperature at the sensor head from an effect of changes in optical path difference of the temperature sensing optical cavity under changes in temperature on the detected interference in the received probe light caused by the temperature sensing optical cavity. 5. The optical sensor of claim 2 wherein the analyser is arranged to generate the indication of the sensed liquid based on relative magnitudes of the separate interference detected in the received probe light caused by two or more of the optical cavities respectively. 6. The optical sensor of claim 1 further comprising an optical fibre arranged to deliver the probe light to the sensor head, the optical fibre having formed therein proximal to the sensor head a Bragg grating, the optical sensor being arranged to detect variations in temperature at the sensor head from variations in a spectral characteristic of the Bragg grating. 7. The optical sensor of claim 6 wherein the optical sensor is arranged to detect variations in a characteristic of the Bragg grating using the probe light. 8. The optical sensor of claim 1 wherein the one or more optical cavities include a plurality of Fabry-Perot cavities. 9. The optical sensor of claim 1 wherein at least the first optical cavity is formed of one or more of sapphire, silica glass, and silicon. 10. The optical sensor of claim 1 wherein the detector comprises: a spectral engine arranged to detect in the received probe light an interference spectrum caused by the one or more optical cavities in the sensor head; anda transform function arranged to generate an optical path difference signal representing the magnitude of the detected interference for at least one or more optical path differences corresponding to the one or more optical cavities. 11. The optical sensor of claim 10 wherein the transform function is arranged to generate the optical path difference signal from the interference spectrum using at least one of a discrete Fourier transform and a cross-correlation of the interference spectrum with a set of periodic transfer functions. 12. The optical sensor of claim 10 wherein the magnitude of detected interference caused by any of the optical cavities is determined from a height of a corresponding peak in the optical path difference signal. 13. The optical sensor of claim 10 further arranged to detect, from the optical path difference signal, a measure of optical path difference of at least one of the one or more optical cavities, and to determine a parameter at the sensor head from the measure of optical path difference. 14. The optical sensor of claim 12 arranged to generate an indication of pressure at the sensor head from changes in optical path difference at a pressure sensing optical cavity of the sensor head, the changes in optical path difference being determined from the optical path difference signal. 15. The optical sensor of claim 12 arranged to generate an indication of temperature at the sensor head from changes in optical path difference at a temperature sensing optical cavity of the sensor head, the changes in optical path difference being determined from the optical path difference signal. 16. The optical sensor of claim 1 wherein the probe light comprises at least one of broadband light generated using one or more super-luminescent diodes and spread spectrum light generated using one or more tunable lasers. 17. The optical sensor of claim 1 wherein the sensor head is coupled to the optical source and the detector using an optical fibre carrying the probe light. 18. The optical sensor of claim 1 wherein the optical sensor is arranged to sense a refractive index of a liquid in contact with the optical boundary of the first optical cavity. 19. The optical sensor of claim 1 wherein the indication of the sensed liquid is an indication of a refractive index of the sensed liquid. 20. The optical sensor of claim 1 wherein the indication of the sensed liquid distinguishes between at least a first liquid and at least one other liquid or gas having a different refractive index to the first liquid at the boundary of the first optical cavity. 21. The optical sensor of claim 1, wherein the optical sensor is installed in a fuel tank of an aircraft fuel system. 22. The optical sensor of claim 21 wherein the indication of the sensed liquid, which the analyser is arranged to generate, distinguishes between water, fuel and gas at the boundary of the first optical cavity. 23. The optical sensor of claim 21 wherein the analyser is further arranged to generate an indication of at least one of temperature and pressure at the sensor head by interrogation of one or more of the optical cavities using the probe light. 24. The optical sensor of claim 1 wherein the analyser generates an indication of the presence or absence of the liquid to be sensed depending on the magnitude of the detected interference. 25. A method of sensing a liquid in contact with an external optical boundary, comprising: providing a sensor head comprising one or more optical cavities including at least a first optical cavity constructed as Fabry-Perot cavity and comprising a solid material having the external optical boundary;detecting reflection strength at the external optical boundary from a magnitude of interference caused by the first optical cavity in probe light delivered to and received back from the sensor head, the magnitude of the interference being indicative of a refractive index of the liquid to be sensed; andgenerating an indication, based on the refractive index of the liquid, of the liquid to be sensed at least in part by analyzing the magnitude of the detected interference caused by the first optical cavity. 26. The method of claim 25 wherein the indication of the liquid to be sensed distinguishes between contact with the external optical boundary of a gas and a liquid, and/or between different liquids, by detection of a refractive index of liquid or gas in contact with the external optical boundary. 27. The method of claim 26 wherein the indication of the liquid to be sensed further distinguishes between contact with the external optical boundary of water and aviation fuel. 28. The method of claim 25, further comprising providing an analyser arranged to detect the reflection strength based on relative magnitudes of the detected interference in the received probe light caused by the first optical cavity and a further optical cavity of the one or more optical cavities. 29. The method of claim 25 wherein the one or more optical cavities include a plurality of Fabry Perot cavities. 30. The method of claim 25 wherein the magnitude of interference caused by the first optical cavity in probe light delivered to and received back from the sensor head is detected from a transform of an interference spectrum of the received probe light. 31. The method of claim 30 wherein the transform of the interference spectrum of the received probe light is at least one of a Fourier transform of the interference spectrum of the received probe light and a cross-correlation of the interference spectrum of the received probe light with a set of periodic transfer functions, and in either case corresponding to a range of optical path differences giving rise to the interference. 32. The method of claim 25 further comprising detecting pressure and/or temperature at the sensor head from interference caused by one or more of the one or more optical cavities.
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Baker Peter D. (Basingstoke GB2), Liquid quantity gauging.
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