Abstract:
A fiber-optic temperature sensor assembly comprises a cap with an inner cavity. A sensor substance is received loosely in the inner cavity of the cap, the sensor substance having light-emitting properties adapted to change with specific temperature variations. An optical fiber has a first end received in the inner cavity of the cap and fusion spliced thereto, and a second end of the optical fiber being adapted to be connected to a processing unit for transmitting light signals from the sensor substance to the processing unit when the fiber-optic temperature sensor assembly is subjected to specific temperatures. A method for manufacturing the fiber-optic temperature sensor assembly is defined.

Description:
FIELD OF THE APPLICATION 
       [0001]    The present application relates to temperature sensors, and more particularly to a fiber-optic temperature sensor assembly. 
       BACKGROUND OF THE ART 
       [0002]    Fiber-optic temperature sensors are commonly used in given applications as an advantageous alternative to thermocouples and the like. Fiber-optic temperature sensors are immune to electromagnetic interference (EMI)/radio-frequency interference (RFI). Moreover, fiber-optic temperature sensors are relatively small, and can withstand hazardous environments, including relatively extreme temperatures. 
         [0003]    Fiber-optic temperature sensors have an optical fiber extending from a processing unit to the measurement location. A sensor member (e.g., a semiconductor sensor) is provided at an end of the optical fiber. Present fiber-optic temperature sensors use an adhesive or solder to connect the sensor member to the end of the optical fiber. 
         [0004]    However, the presence of an adhesive limits the uses of the fiber-optic temperature sensors. For instance, the range of temperature to which the fiber-optic temperature sensor may be exposed is reduced by the reaction of the adhesive to higher temperatures. Also, the strength of the connection between the sensor member and the optical fiber is not optimal. There also have been some shortcomings in uniformly producing fiber-optic temperature sensors of suitable strength at the fiber/sensor member connection. These problems affect the reliability of current fiber-optic temperature sensors. Unreliable temperature sensors are impractical in constraining environments (e.g., nuclear power plants), or concealed systems (e.g., industrial transformers). 
       SUMMARY OF THE APPLICATION 
       [0005]    It is therefore an aim of the present application to provide a fiber-optic temperature sensor assembly that addresses issues associated with the prior art. 
         [0006]    Therefore, in accordance with the present application, there is provided a fiber-optic temperature sensor assembly comprising: a glass cap with an inner cavity; a sensor substance received loosely in the inner cavity of the cap, the sensor substance having light-producing properties adapted to change with specific temperature variations; and a glass optical fiber having a first end received in the inner cavity of the cap and fused without adhesive to the cap, and a second end of the optical fiber being adapted to be connected to a processing unit for transmitting light signals from the sensor substance to the processing unit when the fiber-optic temperature sensor assembly is subjected to specific temperatures. 
         [0007]    Further in accordance with the present application, there is provided a method for manufacturing a fiber-optic temperature sensor assembly comprising: fusing without adhesive a first end of a glass cap having an inner cavity to an end of a glass optical fiber; inserting a sensor substance having light-emitting properties adapted to change with temperature variations into the inner cavity of the cap; and closing a second end of the cap to seal the sensor substance in the inner cavity of the cap. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic sectional view of a fiber-optic temperature sensor assembly in accordance with a first embodiment of the present disclosure; 
           [0009]      FIG. 2A  is a schematic sectional view of a cap and optical fiber of the fiber-optic temperature assembly of  FIG. 1 , prior to assembly; 
           [0010]      FIG. 2B  is a schematic sectional view of the cap of  FIG. 2A , with the optical fiber connected to an end of the cap; 
           [0011]      FIG. 2C  is a schematic sectional view of the cap and optical fiber assembly of  FIG. 2B  with a sensor substance being inserted in the cap; and 
           [0012]      FIG. 2D  is a schematic sectional view of the fiber-optic temperature sensor assembly with a second end of the cap being closed. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]    Referring to the drawings and more particularly to  FIG. 1 , a fiber-optic temperature sensor assembly in accordance with a first embodiment is generally shown at  10 . The fiber-optic temperature sensor assembly (hereinafter temperature sensor assembly) is of the type having an optical fiber  12  connected to a processing unit (not shown), with a sensor substance  13  being provided at the sensor end  14  of the optical fiber  12  as accommodated in a cap  16 . The optical fiber  12  may consist of a micro-structured optical fiber, or any other suitable type of optical fiber. 
         [0014]    The sensor substance  13  may be of the type producing a light signal as a function of the temperature, which light signal is transmitted to the processing unit through the optical fiber  12 . In an embodiment, the sensor substance  13  transforms an excitation signal received from a source connected to the optical fiber  12 , into light of different characteristics, such as a modified wavelength (e.g., fluorescent substance). According to an embodiment, the sensor substance  13  is typically a fluorophore in a granular or powdery state, loosely received in the inner cavity  18  of the cap  16 . When referring to the sensor substance  13  received loosely, it is understood that the sensor substance  13  is simply deposited in the inner cavity  18 . The sensor substance may subsequently be restricted from moving in the inner cavity  18  by the insertion of the optical fiber  12  or the closing of the inner cavity  18 . For instance, the fluorophore may be fluorogermanate (Mg4FGeO6:Mn) for given applications, with the granular size being within selected ranges. As an alternative, the fluorophore may be LuPO4:Dy, among other possibilities. Fluorogermanate may be used for applications ranging between −260° C. to 725° C. LuPO4:Dy may be used as sensor substance  13  for higher temperature measurements, for instance up to 1500° C. 
         [0015]    Other sensor substances  13  may be used as well, for instance substances having a light-absorption spectrum variable as a function of the temperature, or substances whose birefringence varies as function of the temperature. 
         [0016]    The cap  16  defines an inner cavity  18 , in which the sensor substance  13  is received. A first end  20  of the cap  16  receives the sensor end  14  of the optical fiber  12 . The second end  22  of the cap  16  is closed, whereby the sensor substance  13  is sealingly enclosed in the cap  16 . 
         [0017]    According to an embodiment, the optical fiber  12  and the cap  16  are all-glass components, for instance using silica. Accordingly, the optical fiber  12  may be fusion spliced to the cap  16  in the manner illustrated in  FIG. 1 , whereby no bonding agent is required therebetween. In this embodiment, the fiber-optic temperature sensor assembly  10  is mainly fused silica. As a result, the fiber-optic temperature sensor assembly  10  has a matched coefficient of thermal expansion. 
         [0018]    As an example, it is considered to use the optical fiber  12  and capillary  16  having the range of dimensions set forth below for the temperature sensor assembly  10 : optical fiber  12  at 50/125 μm, the cap  16  at 75/175 μm and 50/125 μm; also, the optical fiber  12  at 105/125 μm for the cap  16  at 150/350 μm. 
         [0019]    Although not shown, the optical fiber  12  may be covered with a jacket of protective material, such as polyimide or PTFE. The protective material (if needed) is selected as a function of the contemplated use of the temperature sensor assembly  10 . 
         [0020]    The cap  16  is typically a capillary having the end  22  being collapsed or closed by way of a plug. In the instance of a plug, the plug may also be a glass plug that is compatible with a remainder of the cap  16  for fusion splicing. 
         [0021]    Referring to  FIGS. 2A to 2D , a sequence of operations is illustrated for the manufacture of the fiber optic temperature sensor assembly  10  of  FIG. 1 . 
         [0022]    In  FIG. 2A , there is provided the cap  16 . It is observed in  FIG. 2A  that the cap  16  has both ends opened. The cap  16  may be cut or cleaved to suitable dimensions. 
         [0023]    In  FIG. 2B , the sensor end  14  of the optical fiber  12  is inserted in the inner cavity  18  of the cap  16 . As mentioned above, the optical fiber  12  and the cap  16  may be of the same material and thus fused or fusion spliced to achieve the configuration of  FIG. 2B . In order to perform the fusion splicing, it is considered to use a commercial fusion splice apparatus or CO 2  laser. As an alternative, the cap  16  may be collapsed onto the sensor end  14  of the optical fiber  12 . 
         [0024]    In  FIG. 2C , the sensor substance  13  is inserted in the inner cavity  18  of the cap  16 . The insertion of the sensor substance  13  is typically performed by the micro encapsulation of a minute amount of the substance (e.g., fluorophore) in the inner cavity  18 . For instance, it is considered to use mechanical pressure on the sensor substance  13  to ensure that the sensor substance  13  is lodged in the inner cavity  18 . 
         [0025]    In  FIG. 2D , the second end  22  of the cap  16  is closed. According to one embodiment, the second end  22  is collapsed to seal the inner cavity  18  shut as illustrated in  FIG. 2D . According to another embodiment, a plug (e.g., a piece of optical fiber) is used to close the second end  22 . The plug may then be fusion spliced to close off the second end  22 . In such a case, precautions are taken to keep the sensor substance  13  at a given distance from the fusion spliced zone during the fusion splicing to avoid exposing the sensor substance  13  to heat. It may be required to cut off an excess portion of the cap  16  after  FIG. 2D , for example to facilitate the thermal contact between the sensor substance  13  and the measured environment. For instance a cleavage process may be used to remove an excess portion. 
         [0026]    A specific sequence of steps is illustrated following  FIGS. 2A to 2D , it is pointed out that a different sequence may be performed. For instance, the second end  22  of the cap  16  may be closed prior to the insertion of the sensor substance  13  therein, or prior to the connection with the optical fiber  12  (with the sensor substance  13  already in the cap  16 ). According to an embodiment, once the sensor substance  13  is inserted in the capillary, the capillary  16  may be cleaved so as to have a proper length (e.g. reduced cavity thickness) prior to the insertion of the optical fiber  12  therein. If the end  22  is cleaved with the sensor substance  13  enclosed in the cap  16 , the fusion of the optical fiber  12  to the cap  16  at the end  20  is performed at a suitable minimum distance from the sensor substance  13  so as not to damage the sensor substance  13  with the heat released by the fusion step. 
         [0027]    The fused silica embodiment of the fiber-optic temperature sensor assembly  10  is well suited for extreme temperature range measurements, such as cryogenics, nuclear, microwave, strong RF applications, patient monitoring under MRI or intense electromagnetic field, aerospace applications and direct winding temperature measurements in high voltage transformers, among other possibilities. The temperature range of the fiber-optic temperature sensor assembly  10  will be dependent on the types of sensor substances  13  used. The temperature sensor assembly  10  may be used for long fiber link at extreme temperatures.