Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2001-05-11
2004-01-13
Porta, David (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S227160, C356S073100, C385S012000
Reexamination Certificate
active
06677576
ABSTRACT:
The present invention relates to sensing apparatus employing optical fibres, and in particular, although not exclusively, to sensing apparatus incorporating optical fibre sensors mounted in catheter probes for medical applications such as in-vivo measurement of pressure.
Sensors for particular measurands, such as pressure, temperature, and strain, are often required to be as small as possible, and this is particularly true for in-vivo medical applications. Sensors are also often required to be passive, ie. requiring no electrical power input in order to function, or to dissipate negligible or zero power during operation.
Optical sensors are good candidates for applications having these requirements, and indeed optical sensors that exploit optical fibre technology are most attractive devices for application in medical procedures associated with diagnosis and intervention. They have a number of important advantages over more classical sensors (eg. electronic), namely, their small size, immunity to EM-noise, high degree of bio-compatibility, high sensitivity, ease of sterilisation, and passive operation. However they often necessitate the use of complex and expensive optics and electronics as is the case with interferometric based optical fibre sensors. It is therefore desirable to reduce the complexity and cost of fibre optic sensing apparatus.
Fibre optic sensors based on numerous principles of operation are well-known, including those based on interferometry, non-linear effects, fluorescence (e.g. as a function of temperature), dimensional changes of in-fibre Bragg gratings, and amplitude modulation of light signals. Both extrinsic and intrinsic amplitude modulation sensors are known. In extrinsic amplitude sensors, light exits from an optical fibre and the sensor is configured such that a varying amount of light is recaptured in another or the same fibre, the amount being dependent on the particular measurand. Some of the light input to the device is therefore lost, reducing the power of the recaptured signal. Furthermore, the power of the recaptured signal, rather than being a function of the measurand only, is instead dependent on the input light power, which may vary.
Intrinsic amplitude sensors have typically involved the measurand interacting with an optical fibre and leading to a variation in light loss in the fibre. Such interaction usually takes the form of squeezing or flexing the fibre such that micro bending loss occurs, and again input light power is lost and the output is affected by fluctuations in the input light power.
Any fibre optic sensors which rely on the intensity of the output signal have the inherent disadvantage of being sensitive to variations in the power level of the light source.
It is desirable, therefore, to provide sensing apparatus and a measurement method which address the problems associated with the prior art.
According to a first aspect of the present invention there is provided Sensing apparatus including:
a sensor comprising
a fused tapered fibre optic coupler formed of two optical fibres fused together to provide a fused portion which is drawn down to form a taper waist portion, the coupler having an input end comprising an input unfused portion of one of said two optical fibres and an output end comprising an output unfused portion of one of said two optical fibres;
a light source arranged to input light to the taper waist portion along the input unfused portion; and a light detector arranged to generate a signal indicative of a parameter of the light transmitted to the output unfused portion from the taper waist portion,
characterised in that the taper waist portion is formed as a loop and at least part of the loop is arranged to bend in response to a measurand.
The sensor may further comprise bending means arranged to bend at least part of said loop according to the measurand.
The parameter of the light, of which the generated signal is indicative, may for example be the light power, intensity, or wavelength.
The fused tapered coupler may be a typical 2×2 device comprising two unfused input portions and two unfused output portions, or may be formed from three or more optical fibres.
Alternatively, the fused tapered coupler may have only one unfused input portion. Such an arrangement may be formed by cutting off or otherwise removing one of the input portions of a 2×2 coupler, or by suitable coupler fabrication.
Similarly, the output end may comprise a single infused output portion, or two or more unfused output portions.
The light source may be a simple light source, such as a LED. The taper waist portion typically has a substantially uniform cross sectional area along its length, and the drawing down process, which, in the art, is also referred to as “tapering” or “elongation” or “pulling”, results in that cross sectional area being smaller than the sum of the cross sectional areas of the unfused fibres. Fusing together and drawing down (i.e. pulling in a controlled fashion) the optical fibres enables optical interaction between them. Thus, although in certain embodiments light is input to the sensor along only one unfused “input” fibre, in general not all of the light reaching the output end will be transmitted to one output unfused portion. In embodiments where the output end comprises two unfused output portions, in general the total light power emerging from the device along the output fibres will be shared between them. A splitting ratio may be defined as the ratio of the light powers propagating in the two unfused output fibres, but is often defined in terms of the light power in one output fibre expressed as a fraction or is percentage of the total emerging power.
The optical field within the tapered portion is very sensitive to changes in geometry, and bending the loop will, in general, result in a change in the splitting ratio. The term “bending” is used to denote any action resulting in deformation, deflection, distortion or change in the curvature of the loop, in part or as a whole.
In embodiments where the output end comprises a single unfused portion, deformation of the loop, in general, results in a change in a parameter of the light transmitted to the output portion, for example a change in its intensity.
The sensor is arranged so that the bend applied to the loop is in accordance with the quantity being measured by the sensing apparatus, i.e. the measurand. Thus the applied bend is a substantially reproducible function of the measurand. For example, the end of the loop may be deflected sideways by a distance proportional to the magnitude of the measurand.
Changes in the measurand result in changes in the bend applied to the loop, and hence changes in the splitting ratio. This in turn leads to a change in the signal generated by the light detector, which can therefore be used to monitor the measurand.
This first aspect of the present invention provides numerous advantages including:
a) By arranging the taper waist portion as a loop, the sensor can have a probe-like form, with input and output fibres at the same “end” of the loop;
b) An indication of the measurand can be obtained by simply monitoring the magnitude or other aspect of the signal from the light detector, using for example a photodiode. The sensing apparatus may thus have low complexity and cost;
c) As the parameter of the light transmitted to the output unfused portion (or the splitting ratio) is very sensitive to changes in the geometry of the taper waist region, the bending means may be engineered in a wide variety of ways, to suit particular applications. Providing that the bend applied to the loop is in accordance with the measurand, i.e. a substantially reproducible function of the measurand, then the generated signal will be a useful indication of the measurand. Thus, there is considerable design freedom. In addition, reproducable bending may be easier to engineer than the application of positive axial strain. Also, the loop need not be encapsulated in a holding medium. This produces the advantage that light propagating in the taper wai
Hussey Conleth D.
Kenny Robert P.
Lucia Alfredo C.
O'Brien Elaine M.
O'Sullivan Paul F.
European Community represented by Commission of the European Com
Lee Patrick J.
Porta David
Westman Champlin & Kelly P.A.
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