Abstract:
In a Fabry-Perot interferometer based sensor if insufficient light reflected from the sensor re-enters the fiber, the results from the Fabry-Perot interferometer-based sensor are compromised. Accordingly, a sensor assembly is provided that comprises an optical fiber having an optical axis, a lens in optical communication with the optical fiber, the lens having an optical axis and the lens capable of transmitting a beam of light, a reflective surface, the reflective surface spaced from the lens such that the beam of light transmitted from the lens is capable of reflecting from the reflective surface back to the lens, and an alignment device capable of aligning the beam of light transmitted from the lens substantially perpendicular with the reflective surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority from U.S. Provisional Patent Application No. 60/662,202 filed on Mar. 16, 2005, which is hereby incorporated by reference, and claims priority from U.S. Provisional Patent Application No. 60/774,289 filed on Feb. 17, 2006, which is also hereby incorporated by reference. 
     
    
     FIELD OF INVENTION  
       [0002]     The present invention relates to sensors for measuring the absolute length of a gap in a Fabry-Perot interferometer, and more particularly to a Fabry-Perot sensor that provides a more intense signal.  
       BACKGROUND OF THE INVENTION  
       [0003]     The use of Fabry-Perot interferometers to measure the absolute length of a gap is known. Use of a ball lens to collimate light shining on a Fabry-Perot interferometer is needed for sensors measuring gaps exceeding about 30 μm in order to maintain a uniform optical path length for all light rays and to assure a high percentage of the light reflected by the interferometer is captured by the fiber. However, if the light delivery fiber is not precisely centered on the ball lens axis or if the interferometer is not precisely perpendicular to the axis of the incident light transmitted by the ball lens, then the reflected light from the diaphragm does not re-enter the fiber because the reflected light spot that is re-imaged by the ball lens is not centered on the end of the input fiber. As a result, if insufficient light reflected from the sensor re-enters the fiber, the results from the Fabry-Perot interferometer-based sensor are compromised.  
         [0004]     Accordingly, a Fabry-Perot interferometer-based sensor with a ball lens and alignment scheme that reflects high intensity light signals would provide benefits such as improved power budget, improved signal to noise ratio, and would be welcomed by the industry.  
       SUMMARY OF THE INVENTION  
       [0005]     The present application discloses a sensor assembly that comprises an optical fiber having an optical axis, a lens in optical communication with the optical fiber, the lens having an optical axis and the lens capable of transmitting a beam of light, a reflective surface, the reflective surface spaced from the lens such that the beam of light transmitted from the lens is capable of reflecting from the reflective surface back to the lens, and an alignment device capable of aligning the beam of light transmitted from the lens substantially perpendicular with the reflective surface.  
         [0006]     Another embodiment discloses a Fabry-Perot sensor assembly that comprises an optical fiber, a ball lens in optical communication with the optical fiber the ball lens capable of transmitting a beam of light, a window having a first surface and a second surface, a diaphragm spaced from and parallel to the second surface of the window, the diaphragm having a partially reflective surface, and an alignment device capable of aligning the beam of light transmitted from the ball lens substantially perpendicular with the partially reflective dielectric coating of the diaphragm.  
         [0007]     In yet another embodiment, a sensor assembly comprises a body having a socket, a ball rotatably positioned in the socket of the body, an optical fiber, at least a portion of the optical fiber positioned in the ball, a ball lens attached to the optical fiber, the ball lens capable of transmitting a beam of light, a diaphragm having a reflective surface, the diaphragm spaced from the ball lens such that the beam of light transmitted by the ball lens is capable of reflecting from the surface of the diaphragm back to the ball lens, and wherein rotation of the ball aligns the beam of light transmitted from the ball lens substantially perpendicular with the reflective surface of the mesa diaphragm. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0008]     Operation may be better understood by reference to the following detailed description taken in connection with the following illustrations, wherein:  
         [0009]      FIG. 1A  is a concept drawing of a Fabry-Perot interferometer based sensor assembly with a ball lens and Fabry-Perot gap, wherein the window is a wedge with nonparallel surfaces.  
         [0010]      FIG. 1B  is a concept drawing of a Fabry-Perot interferometer based sensor assembly with a ball lens, a Fabry-Perot gap, and a wedge-shaped spacer that is used with a window having parallel surfaces.  
         [0011]      FIG. 2  is a concept drawing of sensor assembly with ball lens and Fabry-Perot gap, wherein the two surfaces of the window are plane parallel (where one surface is a first reflector in a Fabry-Perot interferometer) and the transducer body is machined at the desired angle to maximize the reflected light signal.  
         [0012]      FIG. 3  is a ray trace drawing of a 2 mm diameter ball lens with de-centering of the delivery fiber relative to the ball lens and tilt of the diaphragm relative to the fiber, where the lens-to-window spacing=0.1 mm; the window thickness=0.7 mm; the window-reflector spacing (gap)=1 mm; the fiber de-center=0.05 mm; and the tilt angle=0.5°.  
         [0013]      FIG. 4  is a ray trace drawing of a 2 mm diameter ball lens with de-centering of the delivery fiber relative to the ball lens and no tilt of the diaphragm relative to the fiber (i.e., reflected rays do not re-enter fiber), where the lens-window spacing=0.1 mm; the window thickness=0.7 mm; the window-reflector spacing (gap)=1 mm; the fiber de-center=0.05 mm; and the tilt angle=0.  
         [0014]      FIG. 5  shows a cross section of a second reflector in the Fabry-Perot interferometer with a mesa diaphragm configuration.  
         [0015]      FIG. 6  shows a cross section of a second reflector in the Fabry-Perot interferometer with a plug diaphragm configuration.  
         [0016]      FIG. 7  shows a cross section of a second reflector in the Fabry-Perot interferometer with a bellows diaphragm configuration.  
         [0017]      FIG. 8  shows a cross section of a second reflector in the Fabry-Perot interferometer with a spherical depression.  
         [0018]      FIG. 9  shows a flexible transducer incorporating an embodiment of a Fabry-Perot interferometer based sensor.  
         [0019]      FIG. 10  shows a cross-section drawing of ball and socket alignment device of an optical fiber with fused ball lens.  
     
    
     DETAILED DESCRIPTION  
       [0020]     While the present invention is described with reference to the embodiments described herein, it should be clear that the present invention should not be limited to such embodiments. Therefore, the description of the embodiments herein is illustrative of the present invention and should not limit the scope of the invention as claimed.  
         [0021]     To obtain the maximum light intensity using a Fabry-Perot interferometer based sensor, it is necessary to assure the optical fiber is precisely centered on the lens optical axis and the second reflector in the Fabry-Perot interferometer is precisely perpendicular to the beam of light transmitted from the lens. Since these conditions cannot be met precisely in manufacturing practice, certain adjustments are necessary to achieve these results. An embodiment of a Fabry-Perot interferometer based sensor  10  is shown in  FIG. 1A . In this embodiment, a wedge shaped window assembly  15  is used rather than a plane-parallel window as an alignment device. The Fabry-Perot interferometer based sensor  10  comprises a transducer body  11 , a ferrule  20 , an optical fiber  25  having an optical fiber axis  27 , a lens  30  having a lens optical axis  32 , and a Fabry-Perot sensor  40 . Despite only a ball lens being shown in  FIG. 1 , any sort of lens that focuses and collimates light can be used, e.g., a graded index lens or a ball lens. The Fabry-Perot sensor  40  comprises a wedge shaped window assembly  15  and a diaphragm  42  having an optical axis  45  and a reflective surface  49 . The wedge shaped window assembly  15  comprises one surface  52  that serves as the first partially reflector in a Fabry-Perot interferometer where the window assembly  15  is located between the lens  30  and a second reflector  49  in the Fabry-Perot interferometer, which allows for proper operation of the invention with long gaps. Rotation of the wedge-shaped window assembly  15  causes a change in the angle of refraction into and out of the window assembly  15  until the window assembly  15  is in the precise rotational location where the column or beam of light transmitted from the lens  30 , is perpendicular to the first reflective surface  52  on the window assembly  15 . Additionally, the lens optical axis  32  is perpendicular to the surface  49  of the diaphragm  42 , as well as the optical axis  27  of the optical fiber  25  being perpendicular to the first reflective surface  52  of the window assembly  15  and the surface  49  of the diaphragm  42 .  
         [0022]     Alternatively, the window surfaces  51 ,  52  can be maintained parallel to each other and parallel to the second reflector surface  49  in the Fabry-Perot sensor. Plane-parallel windows are easier to manufacture. In this embodiment, the alignment device comprises a wedge-shaped spacer  61  located between the lens and the reflective surface as shown in  FIG. 1B . Accordingly, to provide the angle tuning, the wedge-shaped spacer  61  is inserted until the column or beam of light transmitted from the lens  30 , is perpendicular to the reflective surface on the diaphragm. Spacers  61  with different wedge angles can be matched to different transducer bodies to correct for variation in manufacturing tolerances of the transducer bodies and to optimize light transmission.  
         [0023]     As shown in  FIG. 2 , another alternative embodiment of a Fabry-Perot interferometer based sensor  210  is shown. In this embodiment, the Fabry-Perot interferometer based sensor  210  maintains the window surfaces  251 ,  252  parallel to each other and parallel to the second reflector surface  249  in the Fabry-Perot sensor  240 . To provide the angle tuning, the alignment device comprises a surface  213  of the transducer body  211  that mates with the window assembly  215  that is machined at the desired angle after the ball lens  230  and optical fiber  225  assembly are bonded. In this alternative embodiment, the window assembly  215  does not need to be rotated to bring the window  215  into precise alignment with the transducer  211 . It is simply attached to the transducer body  211  at any rotational position. The transducer body  211  is machined at a predetermined angle to produce the desired tilt angle of the Fabry-Perot interferometer based sensor. In other words, the alignment device comprises the transducer body  211  having its end surface or face  213  machined at an angle relative to its axis to align the beam of light transmitted from the lens perpendicular with the reflective surface  249  of the diaphragm. The desired tilt angle of the transducer body  211  is also when a light beam transmitted from the ball lens  230  is perpendicular to the end face  213  of the transducer body  211 . This ensures the light beam is perpendicular to the diaphragm surface  249 , as shown in  FIG. 2 . This approach can also be used even when there is no ball lens and no window, to correct for any misalignment of the light beam with the transducer body and second reflector of the Fabry-Perot sensor, i.e. the diaphragm surface.  
         [0024]     In the yet another embodiment, the method for pointing the light beam to achieve perpendicularity with the diaphragm is to use a metal ball-and-socket assembly shown in  FIG. 10 . In this embodiment, the Fabry-Perot interferometer based sensor  1000  comprises a ferrule  1020 , an optical fiber  1025 , a lens  1030 , a Fabry-Perot sensor  1040 , and an alignment device. The alignment device comprises a body  1060  having a socket  1065 , and a ball  1070 . The Fabry-Perot sensor  1040  comprises a window assembly  1015  and a diaphragm  1042 . The window assembly  1015  comprises one surface  1052  that serves as the first reflector in a Fabry-Perot interferometer where the window  1015  is between the lens  1030  and a second reflector  1049  in the Fabry-Perot interferometer. This allows for proper operation of the embodiment with long gaps. The window assembly  1015  also includes another surface  1051  parallel to the surface  1052 . The ball  1070  can be a metal ball, but is not limited to such. It can be of any material. The ball  1070  is rotatably attached in the socket  1065 . Held inside the metal ball  1070  is the ferrule  1020  that holds the optical fiber  1025  and lens  1030 . The metal ball  1070  can be rotated in its mating socket  1065  through two degrees of freedom about the center-of-rotation  1072 , as shown by the arrow. In this manner the light beam angle transmitted from the lens  1030  is fine-tuned to be perpendicular to the diaphragm  1042  surface  1049 .  
         [0025]     Various alternatives have been modeled using optical ray tracing software. In one embodiment, a  2  mm diameter ball lens that is configured according to the drawing in  FIG. 3  has the design parameters presented in Table 1.  
                                                                                                     TABLE 1                       Parameter definitions for design in  FIG. 3 . For       a lens with 2 mm FS ball w 0.7 mm thick window                                            APERTURE           SRF   RADIUS   THICKNESS   RADIUS   GLASS               OBJ   —   0.530000   0.025000   AIR       AST   1.000000   2.000000   0.119528 AS   FK3 (fused silica)       2   −1.000000   0.100000   0.418365 S   AIR       3   —   0.700000   0.420979 S   BK7 (glass)       4   —   1.000000   0.433099 S   AIR       5   —   −1.000000   0.459240 S   REFL_HATCH *       6   —   −0.700000   0.485381 S   BK7       7   —   −0.100000   0.497501 S   AIR       8   −1.000000   −2.000000   0.500115 S   FK3       9   1.000000   −0.530000   0.221589 S   AIR       IMS   —       0.060116 S                    * TILT/DECENTER DATA                                0   DT   1   DCX   —   DCY   0.050000   DCZ   —                   TLA   —   TLB   —   TLC   —       3   DT   1   DCX   —   DCY   —   DCZ   —                   TLA   −0.500000   TLB   —   TLC   —       5   DT   1   DCX   —   DCY   —   DCZ   —                   TLA   −0.500000   TLB   —   TLC   —       7   DT   1   DCX   —   DCY   —   DCZ   —                   TLA   −0.500000   TLB   —   TLC   —                  
 
         [0026]     The tilt angle is an input parameter to the ray trace. The same tilt angle is applied to each window surface C and D and the reflector E.  FIG. 4  shows what happens if the fiber de-center remains 0.05 mm and the tilt angle is set to 0. The reflected rays miss the end of the fiber. Compare  FIG. 4  with  FIG. 3 , where the reflected rays re-enter the fiber. The object and image size in  FIG. 3  is roughly 0.065 mm (total spot size, not rms). In  FIG. 4 , the size of the image (reflected spot) is roughly 0.115 mm and is not centered about the object (fiber end).  
         [0027]     As previously discussed, a configuration to collimate light shining on the diaphragm of a fiber optic Fabry-Perot pressure sensor is shown in  FIGS. 1 and 2 . A light delivery fiber and a ball lens are not attached to one another.  
         [0028]     In addition to the alignment issues caused by the non-attached ball lens and fiber, the non-attached case results in two unwanted reflective surfaces (the fiber end and the ball lens input surface) that could interfere with the desired signal from the Fabry-Perot sensor. In the embodiment shown in  FIG. 10 , the ball lens  1030  is attached to the optical fiber  1025 . More specifically, the ball lens  1030  is fused and centered on the end of the optical fiber  1025  minimizing the de-centering problem and eliminating two unwanted reflective surfaces. A ball lens is fused to the silica optical fiber by heating the end of the fiber to the melting point. During melting of the fiber, surface tension produces a sphere of transparent silica, and when the melted silica refreezes, the ball lens is permanently fused to the end of the fiber. Alternatively, the ball lens  1030  can be bonded to the optical fiber  1025  using an adhesive. The typical diameter of the ball lens formed in this manner is 340 μm.  
         [0029]     An additional way to improve the performance of the Fabry-Perot interferometer based sensor is to machine a feature (such as a circular groove) into the diaphragm that causes the surface of the diaphragm to translate without bending as the diaphragm deflects. This feature could be configured as a mesa  500  (which is the circular groove cut into the diaphragm substantially surrounding the flat mesa reflective surface of the diaphragm), a plug  600 , or a bellows  700  as depicted in  FIGS. 5, 6 , and  7 , respectively. As shown in  FIG. 5 , the mesa diaphragm  500  includes a circular groove  510  cut therein. As shown, the circular groove  510  surrounds the reflective surface  549  of the diaphragm  500 .  
         [0030]     Another way to improve the performance of the Fabry-Perot interferometer based sensor  10  is to attach a glass plate and/or dielectric coating  49  to the surface of the diaphragm  42  that allows the reflectance of the diaphragm  42  to be optimized and to remain uniform with time and temperature.  
         [0031]     Additionally, the performance of the Fabry-Perot interferometer based sensor could be improved by machining a concave spherical depression  810  as depicted in  FIG. 8  in the center of the diaphragm  800  to provide modal control of the Fabry-Perot gap. The depth of the spherical depression must be less than the minimum gap that is to be measured with the Fabry-Perot sensor.  
         [0032]     The features of the second reflector in the Fabry-Perot interferometer based sensor combine to enable a transducer head  900  to be fabricated that is very short and very small in diameter. The small size allows the transducer head  900  to be placed on then end of a flexible probe  910  for use in locations where space and access are very limited, forming a flexible transducer. In gas turbine applications where pressure pulsations in the combustor are to be monitored, it is desirable to install the pressure transducers and other sensors as close as possible to the combustion zone. Combustor baskets in Siemens Westinghouse turbines contain J-tubes used to examine the combustor basket with a boroscope. A flexible transducer may be installed in this location but there are physical limitations to the size of the transducer head and the pigtail assembly that contains the leads. It is straightforward to design and build a fiber optic transducer that fits within the size envelope defined by the gas turbine combustor basket J-tube. One design is shown in  FIG. 9 . The size constraints include the diameter and length of the transducer and the flexibility of the pigtail assembly that must be pressure sealed.  
         [0033]     While the present invention has been described with reference to the preferred embodiment, obviously other embodiments, modifications, and alternations could be ascertained by one skilled in the art upon reading the present disclosed. The present invention is intended to cover these other embodiments, modifications, and alterations that fall within the scope of the invention upon a reading and understanding of this specification.