OPTICAL SYSTEMS FOR SENSING AND MEASURING PHYSICAL QUANTITIES

Abstract: An optical system according to the invention for sensing and measuring a physical quantity comprises a source of modulated electromagnetic radiation 1, a body of a phos­ phor 6 which is excitable by the said radiation to emit fluorescent radiation at a wavelength different from that of the exciting radiation, the fluorescent properties of the phosphor being dependent on the magnitude of the physical quantity to be sensed, means 5 for directing the exciting radiation from said source onto the body of the phosphor, said body of the phosphor being located so that its fluorescent properties are determined by the magnitude of the physical quantity to be measured and being disposed to receive radiation emitted from said source of electromagne­ tic radiation, and means for detecting 11 and measuring 12 variations in the lifetime of emission of radiation from the phosphor.

Description: This invention relates to systems for sensing and measuring physical quantities and more specifically to systems that make such measurements by optical techniques utilising phosphors which are sensitive to the said physical quantities. The term phosphor is intended to refer to any fluorescent compound, that is any compound which emits fluorescent radiation as a result of being irradiated at a different wavelength. Systems of this kind have been proposed, in particular, for remote sensing, since they have the advantages of noise immunity and safety in chemically unstable environments, they do not require remote power feeding, and are not subject to interference from local electronic installations. Systems have hitherto been considered which involve either the measurement of the absolute power transmitted by an optical fibre at one wavelength and modified by the transducer, or measurement of the rotation of the plane of polarisation of light transmitted by a monomode optical fibre: a system of either of these types would be subject to certain difficulties. Thus the absolute power level would be liable to be altered as a result of attenuation losses in a long length of optical fibre, and/or insertion losses at each connector incorporated in the system. The polarisation measurement technique, although substantially unaffected by optical losses, could give rise to mechanical and optical problems in forming connections with monomode fibres. There is disclosed in U.S. Patent Specification No. 4075493 a temperature measuring, system comprising a layer of phosphor material which when excited to luminescence emits detectable radiation within two or more distinct wavelength ranges with a relative intensity of emission in these wavelength ranges varying in a known manner as a function of the temperature of the phosphor. Such a system has a number of disadvantages. The emissions at different wavelengths are liable to differential refraction and attenuation by the optics of the system which therefore require sophisticated construction in order to be accurate. The apparatus for analysing the emissions in such a system is also necessarily complex. It is an object of the present invention to provide a system of the kind referred to which enables parameters other than absolute received power or polarisation plane rotation to be measured but which is simpler and obviates the disadvantages of the aforementioned system. According to the invention, an optical system for sensing and measuring a physical quantity comprises a source of modulated electromagnetic radiation, a body of a phosphor which is excitable by the said radiation to emit fluorescent radiation at a wavelength different from that of the exciting radiation, the fluorescent properties of the phosphor being dependent on the magnitude of the physical quantity to be sensed, means for directing the exciting radiation from said source onto the body of the phosphor, said body of the phosphor being located so that its fluorescent properties are determined by the magnitude of the physical quantity to be measured and being disposed to receive radiation emitted from said source of electromagnetic radiation, and means for detecting and measuring variations in the lifetime of emission of radiation from the phosphor. The detecting and measuring means may be calibrated to indicate the magnitude of the said physical quantity on which the fluorescent properties of the phosphor are dependent. The decay times of emissions from fluorescent materials are usually shorter than those of phosphorescent materials, and systems in accordance with this invention are advantageous in that their shorter timescale allows more information to be gathered in a given interval with a consequent increase in accuracy. The means for detecting and measuring variations in the lifetime of emission of radiation from the phosphor preferably determines the phase shift of the emitted radiation relative to the modulated radiation from the source. The phase shift is proportional to the lifetime of the emission modulations and the temperature may be conveniently determined from the phase shift by reference to a calibrated scale. An optical system according to the invention preferably incorporates one or more optical fibres along which either the radiation emitted by the phosphor or the radiation emitted by the source is or both are transmitted to the detecting and measuring means. Such a system is particularly suitable for remote sensing, since the optical fibre or fibres may be of any length required, and the operation of the phosphor is not affected by any attenuation in the fibre or fibres or insertion losses which may result from the use of connectors in the optical system. Optical fibres of either the monomode or multimode type may be employed; the fibres being preferably composed of silica containing one or more dopants for modifying the refractive index of the silica to impart either a step or a graded refractive index profile to the fibre. The system may be employed, for example, for measuring temperature or pressure, with an appropriate choice of a phosphor to function as a transducer converting the received radiation to radiation of a different wavelength and having fluorescent properties which vary in respect of the lifetime of the emission, with variations in the applied temperature or pressure. Examples of phosphors suitable for use in a temperature measuring system are barium chlorofluoride activated by divalent samarium, lanthanum aluminate activated with europium and yttrium vanadate. In the case of samarium activated barium chlorofluoride the emission lifetime decreases with increasing temperature. The barium chlorofluoride phosphor is preferred for use with silica optical fibres, since it is excited by radiation in the blue region of the visible spectrum which is efficiently propagated in silica. The type of radiation source employed will be determined by the wavelength, or range of wavelengths, of the radiation required to excite the particular phosphor used. In some cases, for example if the phosphor is excited by infrared radiation a semiconductor laser or other type of laser may be used; this is a convenient form of light source since the laser output can readily be coupled directly to an optical fibre or fibres. Moreover, for the measurement of the emission lifetime of the phosphor, pulsed excitation of the phosphor is required, and in such cases a laser is a convenient light source since it can be operated to give a pulsed output. However, in most cases the phosphor will be excitable by visible light or ultra violet radiation, and it will therefore be necessary to use a suitable lamp, for example a high pressure mercury vapour or xenon discharge lamp, and to modulate the lamp emission, preferably to produce radiation pulses which are focussed into the end or ends of the optical fibre or fibres. The fluorescent emission of the phosphor, and variations in its fluorescent properties, may be sensed and measured by any convenient known technique. The invention will now be further described by way of an example with reference to the accompanying drawings, Figure 1 of which shows a graph of the relationship between emission lifetime and temperature for a barium chlorofluoride phosphor. Figure 2 is a schematic representation of an apparatus in accordance with the invention. Thus, in the specific example of a system according to the invention for measuring temperatures or for sensing temperature changes, there is an optical fibre arrangement which consists of either a single fibre operating in a duplex mode or a pair of monomode fibres for transmitting light to and from the phosphor. The phosphor employed is barium chlorofluoride (Ba ClF) activated by divalent samarium. This phosphor is excited by pulsed radiation of wavelengths in the approximate range of 300 to 470 nm, and emits radiation in the red and infrared regions of the spectrum. The intensity and lifetime of the emission from the various levels produced by each pulse of exciting radiation decreases with increasing temperature. Measurements can be made up to about 4000C, at which temperature the emission is still detectable. s high pressure xenon arc or mercury vapour lamp may be employed for producing the exciting radiation, and the lamp output is modulated to produce pulsed emission by means of a suitable light chopper, for example a metal disc having a multiplicity of holes arranged in a circle, which is placed between the lamp and the input end or ends of the optical fibre or fibres and is rotated rapidly to give the desired pulse rate. The pulsed light beam is arranged to fall upon a convex lens which is built into the optical fibre termination and by means of which the pulses are focussed into the fibre end or ends. The phosphor, in a suitable physical form, is disposed in an optical cavity in which the output end or ends of the optical fibre or fibres terminate, this arrangement being placed in the location at which temperatures are to be measured. The lifetime of the phosphor emission is measured by a phase shift technique which depends upon the fact that the peak of the intensity of each emission pulse is shifted in phase from that of the exciting pulse of an amount proportional to the lifetime of the emission pulse. The temperature corresponding to any measured emission lifetime can be determined from a previously prepared graph, or the measuring apparatus may include an arrangement for applying the measurements to suitable recording or display means calibrated to indicate temperature. The accompanying drawing, Figure 1, shows a graph of the relationship between emission lifetime and temperature for the above-described barium chlorofluoride phosphor. The graph was drawn by plotting temperature against the logarithm of the lifetime of the 5Do - 7F emission line at wavelength 687.7nm, excited by blue light of wavelength 417 nm at a pulse rate of 150Hz, the lifetime being measured by the phase shift technique. The emission lifetime is approximately 2 milliseconds at room temperature (2000), and falls to approximately 50 0 microseconds at 350 C. Figure 2 shows an apparatus for measuring temperature in accordance with the invention. The apparatus comprises a tungsten halogne lamp 1 powered by a power supply 2. A rotating perforated disc 3 serves to chop the light into pulses which are then passed through a filter 4 which removes red light of the frequency at which the phosphor fluoresces. Two dichroic mirrors 8, 9 are used to further remove red light from the beam which is then focussed onto the end of an optic fibre 5 leading to the phosphor 6 which in this instance is barium chlorofluoride activated with samarium. The phosphor is contained in a heat cell 7 which is maintained at the temperature to be measured. The red fluorescent emission from the phosphor 6 passes through the dichroic mirror 9, through a red filter 10 to remove any residual existing radiation, and is focussed onto a silicion detector 11. The phase shift of the fluorescent radiation incident on the detector 11 is determined bya a lock-in amplifier 12 with reference to a signal taken from the light chopper 3. The output of the amplifier 12 is a measure of the temperature of the cell 7.

Claims: CLAIMS 1. An optical system for sensing and measuring a physical quantity characterised in comprising a source of modulated electromagnetic radiation 1, a body of a phosphor 6 which is excitable by the said radiation to emit fluorescent radiation at a wavelength different from that of the exciting radiation, the fluorescence of the phosphor being dependent on the magnitude of the physical quantity to be sensed, means 5 for directing the exciting radiation from said source onto the body of the phosphor, said body of the phosphor being located so that its fluorescent properties are determined by the magnitude of the physical quantity to be measured and being disposed to receive radiation emitted from said source of electromagnetic radiation, and means for detecting 11 and measuring 12 variations in the lifetime of emission of radiation from the phosphor. 2. An optical system according to Claim 1 wherein the means for detecting variations in the lifetime of emission of radiation from the phosphor determines the phase shift of the emitted radiation relative to the modulated radiation from the source. 3. An optical system according to Claims 1 or 2 incorporating one or more optical fibres 5 along which either the radiation emitted by the phosphor or the radiation emitted by the source is or both radiations are transmitted to the detecting 11 and measuring means 12. 4. An optical system according to any preceding claim for sensing and measuring either temperature or pressure, the phosphor being such that its fluorescence varies in response to changes in temperature or pressure as the case may be. 5. An optical system according to Claim 4 wherein the phosphor is composed of barium chlorofluoride activated with samarium, or lanthanum aluminate activated with europium, or yttrium vanadate. 5. An optical system according to Claim 3 wherein the said source of modulated electromagnetic radiation comprises a semiconductor laser. 7. An optical system according to Claim 3 wherein the phosphor is disposed within an optical cavity into which an optical fibre end or ends is or are coupled. 8. An optical system according to Claim 3 wherein the phosphor is incorporated in an optical waveguide which is coupled to the transmitting fibre or fibres. 9. An optical system according to any preceding claim substantially as hereinbefore described with reference to the accompany drawing.

Inventors: MCCORMACK, JOHN STEPHEN
Applicants: THE GENERAL ELECTRIC COMPANY, P.L.C.
Application year: 1980
Main classification: G01K11
IPC codes: G01D5, G01K11, G01K1