Patent Grant
7209605
The present invention generally relates to optical fibers, and more particularly concerns an optical fiber sensor integrated on a preferred azimuthal portion of the side of an optical fiber. An exemplary application of this invention is a temperature measurement in vivo.
Optical fiber sensors are well known and their application fields cover a broad area ranging from physical parameter measurement to chemical and biochemical parameter measurement.
Patents and scientific papers have also been published in the field of chemical and biochemical measurement through luminescent optical fiber sensors. These cover the biomedical field through the measurement of physiological parameters such as pH, O2, glucose, and CO2 concentration in blood.
Another major area involved by the luminescent detection through optical fiber sensors is the biomedical diagnostic domain through optical biopsy. This area involves the evaluation of biological tissues through the measurement of a tissue's auto-fluorescence or through induced fluorescence by specific markers revealing the presence or absence of pathological tissues. These techniques are currently under development but some have reached the clinical level.
Of particular interest is the measurement of temperature through luminescent optical fiber sensors since optical fibers, unlike thermistors and thermocouples, are not affected by microwaves used in thermal treatment of cancers.
Luminescent optical fiber sensors usually work as follows: an excitation wavelength is directed into the optical fiber entrance with appropriate optical components. The excitation light travels through the fiber up to the other end of the fiber, where a luminescent material has been packaged at the fiber tip. The incoming light excites the luminescent material which in turn emits its luminescent light. The material is chosen such that its luminescent light properties (intensity, spectral content, lifetime decay) vary with the parameter to be measured. The luminescent light follows the optical fiber path down to the fiber entrance and is then collected and filtered against the excitation wavelength with proper optics and electronics. Finally, the luminescent properties of the collected light are analysed to deduce the parameter value to be measured.
Most or all of these luminescent optical fiber sensors are packaged at one end of the fiber. Thus, few or none allow distributed measurements, either by spatially distributing the measurement of one parameter or through simultaneous measurement of many parameters, through only one fiber. Furthermore, in some cases, the fact that the sensor is placed at the end of the fiber renders its use less attractive.
For example, it is known that the temperature measurement of intra-arterial walls can be used as a diagnostic tool to detect active arteriosclerotic plaque at risk of disrupting. These active plaques have a temperature which is higher (from 0.1 to 1.5° C.) than normal arterial walls, and the temperature measurement of intra-arterial walls can then be used to detect these plaques. If one measures the temperature of intra-arterial walls with a luminescent optical fiber temperature sensor placed at the end of the fiber, one will use the small and potentially piercing sensing end of the fiber to make contact with the arterial wall. This is a serious disadvantage, since one can accidentally pierce the artery or worse, the active arterial plaque can be broken, which can result in a cardiac stroke.
The same configuration, i.e. the use of the sensor at one end of the fiber, could be used to measure the fluorescence coming from the arterial wall. In this case, the optical fiber is used as a light pipe to make the excitation light reach the arterial wall and to gather part of the luminescent light from the wall and guide it down to the fiber entrance. The luminescent light can then be analysed to identify the type of biological tissue and eventually diagnose the presence of plaques at risk of disrupting. However, to excite and collect the maximum of light level, one needs to put the fiber end in contact with the arterial wall, which can lead to the problems described above. This is also true for any optical fiber extrinsic spectroscopic sensor which collect light (luminescent or not) from biological tissue or from an optical sensing material making contact with that tissue.
Thus the use of conventional optical fiber sensor packaged at one end of the fiber should be prohibited in cases where biological tissue damage can cause health problems.
Therefore, there is a need for a sensor better adapted for a safe in vivo spectroscopy. Moreover, it would be desirable to provide a sensor offering more precise measurement.
It is an object of the present invention to provide an optical sensor on a preferred azimuthal portion of the side of an optical fiber.
In accordance with the invention, this object is achieved with an optical sensor including an optical fiber for conveying a light beam. The optical fiber is provided with a first end for receiving the light beam and a second end opposed thereto, a core and a cladding surrounding the core. The optical fiber also includes at least one claddingless portion having a longitudinal, a radial and an azimuthal extent. Each of the claddingless portions defines a shaped cavity extending in the optical fiber between the two opposite ends. The optical sensor further includes a sensing material extending in each of the cavities for forming a sensing area therein in contact relationship with the core.
It is a preferable object of the present invention to provide a sensor well adapted for spectroscopic measurement in the biomedical and biotechnological fields.
Accordingly, there is provided an optical sensor wherein the optical fiber has a predetermined permanent shape for projecting the sensing area outwardly, thereby allowing a safe use of the optical sensor for in vivo measurement. This also allows a better contact of the sensor with the area to be sensed.
According to another aspect of the present invention, there is provided an optical sensor having a plurality of sensing areas separated to each others by a predetermined distance. In a preferred embodiment, each of the sensing area extends in a longitudinal alignment relationship with each others. In another preferred embodiment, each of the sensing area extends in an azimuthal alignment relationship with each others.
According to yet another aspect of the present invention, there is provided an optical sensor wherein the sensing material can be either a luminescent material either a transparent material.
It is another object of the present invention to provide an optical sensing system including at least one optical sensor. Each of the optical sensors is provided with an optical fiber for conveying a light beam. The optical fiber has a first end for receiving the light beam and a second end opposed thereto, a core and a cladding surrounding the core, and at least one claddingless portion having a longitudinal and a radial extent. Each of the claddingless portions defines a shaped cavity extending in the optical fiber between the two opposite ends. The optical sensor further includes a sensing material extending in each of the cavities for forming a sensing area therein in contact relationship with the core. The optical sensing system also includes a light source for injecting light into the first end of the optical fiber of each of the optical sensors. The system also includes a detector operatively connected to one of the ends of the optical fiber of each of the optical sensors for detecting light coming from each of the sensing areas. The system is further provided with an analyser operatively connected to the detector for analysing light coming from each of the sensing areas.
It is a preferable object of the present invention to provide an optical sensing system for temperature measurement.
The present invention and its advantages will be more easily understood after reading the following non-restrictive description of preferred embodiments thereof, made with reference to the following drawings in which:
a is a schematic cross sectional side view of an optical sensor according to a preferred embodiment of the present invention.
b is a cross sectional front view of the optical sensor of
a to 3h are schematic cross sectional side views of different optical sensors according to different preferred embodiments of the present invention; each of the optical sensors having a portion of its cladding and of its core removed with different shapes.
a is a schematic representation of an optical fiber provided with a plurality of sensors longitudinally arranged thereon according to another preferred embodiment of the present invention.
b is a graph illustrating an optical technique known as Optical Time Domain Reflectometry.
a is a cross-sectional front view of an optical sensor in contact relationship with an arterial wall according to another preferred embodiment of the present invention.
b is a cross sectional side view of the fiber of
a is a schematic cross sectional side view of a fiber including a reflection splice proximate the sensor and extending perpendicularly to the fiber optical axis.
b is a schematic cross sectional side view of a fiber including a reflection splice proximate the sensor and extending with an angle with respect to the fiber optical axis.
While the invention will be described in conjunction with an example embodiment, it will be understood that it is not intended to limit the scope of the invention to such embodiment. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included as defined by the appended claims.
As mentioned previously, the present invention relates to optical fiber sensors and more particularly to optical fiber sensors packaged on the side of optical fibers. The application domain covers a large area but mainly aims the field of luminescent and spectroscopic optical fiber sensors with application possibilities in the field of telecommunications.
More specifically, the packaged sensor application field aims the measurement of temperature and the spectroscopic measurement in the biomedical and biotechnological domains.
The spectroscopic sensors cover the identification and concentration measurement of biological, biochemical and chemical compounds aiming applications in the biomedical, biotechnological, chemical, environmental and industrial domains. More specifically, the concentration measurement of pH, O2, CO2, and glucose are of interest. Identification of biological pathologic tissues through spectroscopy is more specifically aimed.
The present invention alleviates the most glaring problem of the prior art, viz. the placement of the sensor at the tip of a fiber, since the sensing material is placed on one side of the fiber. The sensing material could then make a gentle contact with the surface to be measured through the side of the fiber, which is not as piercing as its tip. Thus, this will prevent the fiber from damaging for example, the arterial wall in the example described above.
The present invention concerns the placement, in the side of an optical fiber or any other appropriate waveguide, of a sensing material. Referring to
The claddingless portion 22 is obtained by removing a longitudinal, a radial and an azimuthal part of the optical fiber 12. In the preferred embodiment illustrated in
The sensing material extending in the cavity 28 of the optical fiber 12 can be a luminescent material or can also be a transparent material. It is to be understood that throughout the present description, the expression “sensing material” is intended to specifically cover such materials as well as absorbing, reflecting or semi-transparent materials, or even any sensing material that is able to change properties of light reaching it. For example, according to a particular application, the sensing material may be chosen to have spectral properties changing with the presence and/or the concentration of chemical or biochemical compounds.
One can use a transparent material in order to make a window on the side of the fiber. This window can then transmit an excitation light from the fiber entrance to a luminescent material or a biological tissue and the luminescent light from the material or tissue back to the fiber entrance. The collected luminescent light can then be analysed to measure different desired parameters. In the case one uses a transparent material, such a transparent material preferably has an index of refraction which is greater than or equal to that of the core of the fiber. The window can also be used to transmit light having a wide spectral range to biological tissue or chemical compounds. The collected light could then be analysed to measure its spectral content and to deduce physical or chemical properties of the reflecting compounds or tissues.
Referring now to
From a theoretical point of view, it is known that the light rays going through an optical fiber core have an angular content comprised between rays going parallel along the fiber and rays reflected on the fiber core walls at a determined angle known as the critical total internal reflection angle (θc). This critical angle is determined by the following relation:
θc=sin−1(n2/n1) Equation 1
where n1 and n2 are the refractive indexes of the fiber core 18 and cladding 20 respectively. This angle is measured between a ray of light and the normal to fiber core walls. Rays having angle equal to or above this value (θc) will be reflected on the fiber core wall and the ones having angle lower than this value will partly go through the wall. If one is to get light going to the sensing area without being totally reflected, he should either allow the light to hit the sensing area at an angle lower than this critical angle (θc) or to choose the refractive index of the sensing material in order to change this critical angle, or both.
To get fiber core rays of light hitting the sensing area at an angle lower than the critical angle (θc), one can shape the sensing area in order to lower the angle of rays hitting it, as illustrated in
The shape of the removed part of the fiber is advantageously adapted to the application in order to get the optimum amount of light coupled from the fiber core to the sensing area and from the sensing area back to the fiber core.
If the sensing material extending in the shaped cavity 28 of the fiber 12, has a refractive index (n3) equal to or less than the refractive index of the cladding (n2), then the amount of light coupled from the fiber core to the sensing area will be the same for embodiments illustrated by
On the other hand, if the sensing material extending in the shaped cavity 28 of the fiber 12, has a refractive index (n3) greater than the refractive index of the fiber core (n1), the light will more easily couple from the fiber core to the sensing material, but could suffer total internal reflection when going from the sensing material back to the fiber core. This will make coupling from the sensing material to the fiber core less efficient for embodiment illustrated by
To form the cavity 28, the removal of the portion of the fiber can be done by a chemical etching process or by laser processing, preferably a laser ablation. The chemical etching process could be done through masking of the fiber except for the part to be etched. The masked fiber is then exposed to HF that dissolves the glass and shape the fiber in a rather linear way by removing progressively the exposed glass. This can be used to create square-like shaped cavity 28, as illustrated on
The integration of the sensing material in the shaped cavity of the fiber can be done in many ways. In the case where the sensing material is in the form of a powder, it can be inserted by integrating it with an epoxy glue, preferably having a high thermal conductivity coefficient if the sensor is to be a temperature sensor. The sensing material could also be integrated into a silica powder which can be melted into the opening by heating it with a laser. In the case where the sensing material can be melted with a laser without losing its sensing properties, it could be directly melted into the opening. Yet alternatively, the sensing material can be included into a paste, which can be cured by UV or laser illumination. In the case where the sensing material can not be down sized to a powder or a paste, it can be chemically etched or laser machined to match the shape of the cavity 28 of the fiber. This sensing part can then be glued in place with epoxy for example or joined to the fiber with a melting material placed between the sensing material and the shaped cavity of the fiber, and then heated in an oven or with a laser beam to complete the joining process. These techniques could be used as well for many types of sensing materials such as, for example, but not limited to, luminescent, absorbing, non-linear, transparent, polarizing, porous, sol-gel or even birefringent materials.
As previously said and according to Equation 1, in order to obtain better results, the coupling of the light from the optical fiber core to the sensing area can be optimized through the proper choice of the sensing area shape and/or of its refractive index with respect to the one of the fiber core. This could be achieved by making the refractive index of the sensing material higher than the refractive index of the fiber core by a proper choice of the sensing material or by including a material with a refractive index significantly higher than the one of the fiber core to the sensing material.
In the case where the shaped cavity of the fiber does not extend in the fiber core, as illustrated in
Advantageously, a fiber with a core occupying a greater proportion of the section of the fiber is used. This reduces the risk of compromising the mechanical integrity of the fiber, since less material needs to be removed to reach the core. Furthermore, the manufacture of the sensor is simplified, since it is not necessary to penetrate deeply into the fiber. Another advantage is that the sensing material will be located closer to the surface of the fiber, which will promote a better reading of the parameter to be sensed (e.g. temperature), since the contact point will be less affected by the ambient environment (or average temperature) of the fiber. This could also lead to a faster response of the sensing element to sensed parameter (or temperature) change. Finally, the core of the fiber being greater, it will be easier to couple light from the sensing material back into the fiber core. For example, a fiber having a total diameter of 125 μm having a core diameter of 100 μm (a standard multimode fiber) requires the removal of 13 to 25 μm. However, in the case of luminescent intensity time decay measuring techniques, a fiber having a greater core has a high modal dispersion, which can negatively impact on the measurements if luminescent lifetime decay of the order of a nanosecond is used. A fiber having a core of 100 μm and an index jump of 0.015 has a modal dispersion estimated to be 0.05 ns/m. Obviously, a monomode fiber or any convenient waveguide could also be envisaged, according to a particular application.
According to another object of the present invention and with reference to
Thus, in order to measure a parameter with a sensor made according to the present invention, such as temperature, a sensing light is directed into the optical fiber entrance with appropriate optical components. The sensing light travels through the fiber up to the area where the sensing material has been packaged. The sensing light reaches the sensing material which in turn modifies its optical properties. The sensing material is chosen such that its optical properties (luminescence, intensity, spectral properties, absorption, reflection, lifetime decay, . . . ) vary with the parameter to be measured in a well known manner. The returned light is collected and filtered against the sensing light with proper optics and electronics. Finally, the optical properties of the collected light are analysed to deduce the parameter values to be measured.
In some cases, care must be taken to minimise the light reaching the area to be sensed through the sensing material, which could cause a parasitic signal induced on the area to be sensed. This can be obtained by deposing an opaque or reflecting film 32 above the sensing material 24, as illustrated in
Referring now to
The presence of a reflector 34 near the sensing material allows maximizing the return of the luminescent light towards the excitation source. This can be done by placing on a section of the fiber a reflecting material such as, for example TiO2. The proximity of this reflector 34 is important in order to minimise the temporal shift induced by differences in optical path produced between the sensing signal directly reaching the sensing material and the one produced by reflection, and also between the signal coming from the sensing material directly towards the source and the one reaching it after reflection. It is even more advantageous to place the reflector 34 at an angle in order to collect more light, as illustrated in
It should be noted that the use of the reflector 34 is optional, but preferable in order to increase the sensing light back to the entrance of the optical fiber. In some applications, it is also possible to inject light at one end of the fiber, to detect the parameter to be sensed at an intermediate position, and to detect the sensing signal at the other end of the fiber. In the case where the opposite end of the fiber is nor used and is not provided with a reflector, it is preferable to place an absorbent material or an index-matching material 42 (as illustrated in
Referring again to
Alternatively, referring now to
Referring now to
Although the optical sensing system according to the present invention has been described in details for the particular application of temperature measurement, it should be understood that such a sensing system could also be useful in many other applications such as, for non-restrictive example, the measurement of a pH concentration, O2, CO2 concentration or glucose concentration. Such a sensing system may also be used as a biological tissue identifying system. Moreover, although the present invention has been explained hereinabove by way of a preferred embodiment thereof, it should be pointed out that any modifications to this preferred embodiment within the scope of the appended claims is not deemed to alter or change the nature and scope of the present invention. More specifically, the present invention is not limited to temperature measurement, but can be used for any parameter measurement, in vivo or not, where small sensors are required, or where insensitivity to EM radiation is required (for example in nuclear reactors). Also specifically, the present invention is not limited to the use of luminescent sensing materials, but can use any sensing materials (such as absorbing, reflecting, transparent, semi-transparent, non-linear, porous, sol-gel, polarizing, electro-optical, birefringent, . . . materials) that change properties of light (such as wavelengths or spectral content, temporal properties, polarisation, relative intensity or power, . . . ) impingent on it through absorption, reflection, radiation (or emission), non-linear effects, guiding properties, . . . .
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