Abstract:
A displacement sensor includes a first optical fiber for radiating light to a target, and a second optical fiber for receiving light from the target. The end of the first fiber is adjacent and not axially aligned with the second fiber end. A lens focuses light from the first fiber onto the target and light from the target onto the second fiber.

Description:
The United States Government has rights in this invention pursuant to Department of Energy Contract No. DE-AC04-94AL85000 with Sandia Corporation. 
    
    
     CROSS REFERENCE TO RELATED APPLICATIONS 
     (Not Applicable) 
     BACKGROUND OF THE INVENTION 
     There are many needs for sensors that optically couple a laser output to a target and also couple the reflected light from the target to a measuring device. The principal of operation of such devices is that light from a laser is coupled through an optical fiber and focused on a target. Light reflecting from (or emitted by) the target is coupled through an optical fiber to the measuring device. Subject to limitations of cost, size, and efficiency, such devices can be used for applications such as compact disk read or read/write devices, for spectral analysis of samples, and for measuring shock waves. 
     One approach to this problem is to use a single optical fiber to couple the light both to the target and, after reflection, back to a sensor. Such as system is shown in U.S. Pat. No. 4,154,529 of Dyott. There are several disadvantages to such a system. Since the light is sent and received on the same fiber, there is a major problem of signal mixing, that is, some of the outgoing light is detected as a return signal before it gets to the target. One major contributor to this phenomenon is Fresnel reflections which occur at every juncture where the light leaves the fiber and goes into air or some other medium with a different index of refraction. Typical Fresnel reflections are about 4% of the incident light per surface, so a connector junction where two fibers are butted together would have nearly 8% of the initial laser power mixed with the light returning from the target. Add that signal to other Fresnel reflections in the system and it is immediately apparent that this phenomenon can obscure any true return light from a system. There are many instances where the return light is less than 1% of the input light signal which would be swamped by the Fresnel reflections. In addition, the system for sending and collecting is much more complex and expensive than a dual fiber system. Since the input and return light are emanating from the same place, i.e. the end of the fiber that is coupled to the laser, separation equipment, and expensive optical components must be used and precisely aligned and maintained. 
     Another approach is to use two parallel optical fibers that are aligned perpendicular to a target, as shown in U.S. Pat. No. 4,739,161 of Moriyama et al. This patent does not suggest the use of a lens to concentrate the light, but keeps the fiber ends close enough to the target that sufficient reflected energy is received. Since the simple form of this invention (shown in FIGS. 10 and 11 of the &#39;161 patent) does not have high sensitivity, the preferred embodiments use multiple optics and complicated signal processing to enhance sensitivity 
     U.S. Pat. No. 4,801,799 of Tromborg et al. shows at FIG. 3 an alternative approach using two parallel optical fibers located at the focal point of a lens, so collimated light from a first fiber is transmitted from the lens to a reflective area on a vibrating surface, and reflected collimated light is received by a different portion of the lens and transmitted to the second fiber. The principle of operation of Tromborg&#39;s system is believed to be different from that stated by that patent, because movement of the vibrating stage towards and away from the lens would not change the focal position to ‘b’ and ‘c’ as shown in FIG. 3 of that patent, as the collimated light emanating from the reflective surface and passing through the lens will always return to the focal point ‘a’ of the lens. One explanation for the operation of Tromborg&#39;s system is that the vibrating mirror 34 is at an angle other than perpendicular to the path of light from fiber 14, so that the reflection is directed towards output fiber 16. In this event, the mirror or the optics assembly must be on a precise tilt stage in order to reflect the light back into the return fiber, and the reflective surface can only be a high quality mirror. This is a serious limitation in that a very narrow range of reflectors can be used and if that reflector is slightly misaligned either during operation or during setup, it will not work. 
     U.S. Pat. No. 5,202,558 of Barker shows as sensor that has a first optical fiber coupled from a laser to a graded index (GRIN) lens mounted axially in a housing. Light from the GRIN lens reflects off a target and is collected by a lens system axially mounted in the housing behind the GRIN lens. The lens system focuses the received light on an output optical fiber at the rear of the housing. The efficiency of this device is compromised by the shadow cast by the GRIN lens on the reflected light. The device is also incapable of being reduced in size as the housing must be large enough for most reflected light to pass around the GRIN lens. Furthermore, the GRIN lens has a short focal length compared to the collection lens. This short focal length is mandatory since the lens must capture the diverging light from the fiber, then focus it onto the target. Image magnification (in this case the diameter of the fiber is the image size) can be simply stated as the ratio of the focal lengths of the sending lens (GRIN) and the collecting lens. The Barker design will expand the image (spot diameter on the target) by approximately 6, which means that if a pair of 200 um diameter fibers were used, the spot on the target would be six times larger, or 1200 um diameter. The collecting lens takes that image diameter and tries to focus it into the 200 um receiving fiber and severely overfills the fiber by a factor of six. Using the area ratio of the diameter of the fiber versus the overfill diameter of 1200 um, the light collection is 36 times less efficient than the instant invention. 
     SUMMARY OF THE INVENTION 
     The present invention may comprise a sensor comprising a first optical fiber for receiving optical radiation from a radiation source and for radiating radiation transmitted through the fiber to a target. A second optical fiber receives radiation transmitted from the target and couples this radiation to a detector. The first output end of the first fiber is adjacent and not axially aligned with the second input end of the second fiber. A lens focuses the radiation from the first output end onto the target and focuses the radiation emitted by the target onto the second input end. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and form part of the specification, illustrate an embodiment of the present invention and, together with the description, serve to explain the principles of the invention. 
     FIG. 1 shows a preferred embodiment of the invention used as a shock sensor. 
     FIG. 2 shows test results from the preferred embodiment. 
     FIG. 3 shows an alternative embodiment of the invention. 
     FIGS. 4 a - 4   c  show alternative details of the alternative embodiment of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in FIG. 1, the invention includes a sensor  10  for transmitting optical radiation from a source  4  through a first optical fiber  12  to a target  30 , and for receiving optical radiation from the target for transmission through a second optical fiber  16  to a detector  8 . The optical radiation may be radiation of any wavelength capable of transmission through optical fibers and focussing by a lens. Preferably, it is radiation within the range extending from ultraviolet into the infrared. In one preferred embodiment, source  4  is a laser, the radiation is visible light, and detector  8  is any detector of change in applied visible light. 
     Sensor  10  preferably includes a case  20  having a generally cylindrical outer wall  22  enclosing a bore extending from a target end  24  to a fiber end  26 . A lens  50 , which may comprise a single lens or a plurality of lenses, is fastened within target end  24  of the case bore. A fiber housing  40  containing adjacent first and second optical fibers  12 ,  16  is adjustably positioned within fiber end  26  and fastened in place as discussed hereinafter. 
     A target  30  is mounted at target end  24  of case  20 . In order for the energy which displaces target  30  to be concentrated at a spot  32  on target  30 , the light from fiber end  14  is focused by lens  50  on spot  32  through lens assembly  50 , and the reflected light from spot  32  is focused on fiber end  18 . Movement of spot  32  toward lens  50 , which may be caused by impinging target  30  with a high energy pulse, causes a perturbation in the received light at detector  8 . 
     To accomplish this result, a portion  15  of first optical fiber  12  extending from output end  14  is held parallel to and adjacent a portion  19  of second optical fiber  14  extending from input end  18 . This construction may be provided for as shown in FIG. 1, where fiber housing  40  has a generally cylindrical outer wall  42  enclosing an axial bore  46  having a diameter at end  44  of approximately the combined diameters of fibers  12  and  16 . When these fibers are placed within bore  46 , then the portions of fibers  15 ,  19  are parallel to each other and housing axis  48 . 
     To ensure that light from source  4  is focused on spot  32 , lens  50  is a positive lens, i.e., one which focuses to a point, and output end  14  is placed a greater distance from lens  50  than its focal length. This device would not be efficient if the point source of light from first fiber  12  was transmitted along a path perpendicular to target  30 , as the principle of reversibility would place the energy from target  30  back on the source, fiber end  14 , and not adjacent fiber end  18 . Accordingly, the structure must be modified in order that the concentrated light through lens assembly  50  is neither shadowed nor misdirected. 
     As shown in FIG. 1, housing  40  is mounted within case  20  such that housing axis  28  is at a slight angle with respect to case (and lens) axis  28 . Light striking at an angle at an off-center location on lens  50  is focused on spot  32 , and light emanating from spot  32  strikes another off-center location on lens  50  and is focused on second fiber  18 . The ends  14  and  18  of fibers  12  and  16  are preferably adjacent case and lens axis  28  to minimize longitudinal aberrations and coma that results from their being off-axis. 
     To ensure the proper alignment of housing  40  within case  20 , a steady-state light is applied from source  4  and the amplitude is detected at detector  8 . Housing  40  is moved within case  20  until the amplitude is maximized, and then housing  40  is fixed in position with respect to case  20 . 
     To provide inexpensive structure to accomplish the aforementioned alignment, outer surface  42  of housing  40  may have a circumferential groove  43  adjacent end  44 . A resilient o-ring  56  in groove  43  slides along the inner surface of the bore in case  20 . The o-ring may be replaced by any frictional boss which maintains the fiber pair housing in central alignment with the bore. When the position of housing  40  within case  20  has been adjusted to maximize the signal at detector  8  from a source  4 , the friction of o-ring  56  will maintain housing  40  on this optimum position until an epoxy  58  or other quick setting filler can be placed between housing  40  and case  20  and subsequently harden. 
     In cases where the invention may need to be refocused due to a change in target-lens distance, the filler may be replaced by a mechanical holding device such that it holds the back of the fiber holder shaft in alignment for optimal signal collection. 
     It should be appreciated that this invention may be made very small. In one recent test, the length of case  20  was 10 mm and the outer diameter was 5 mm. One small probe had a body diameter of 3.5 mm and a 7 mm length and used a 2.5 mm diameter lens. The diameter of bore  46  of housing  40  at end  44  was 0.5 mm, and each of optical fibers  12  and  16  had a diameter of 200 μm. 
     Among the advantages of this design are the fact that its few components have a low material cost and easy fabrication. In addition, the sensor can be prefocused for plug-in use with a predetermined target in a variety of applications. It also can easily be hermetically sealed and fabricated with a variety of materials such as brass, stainless steel, and plastics. System efficiency is increased because it focuses the radiant energy to a very small spot on the target. 
     Optical fiber size and placement is easily controlled by the size and placement of the bore or bores in housing  40 . For the illustrated configuration, larger fibers may be utilized with a larger bore. However, other configurations which have ends  14  and  18  adjacent on another and not casting a shadow on each other are also possible. For example, separate bores could be provided for each of fibers  12  and  16 . If these bores were each parallel to axis  48 , operation would be similar to the embodiment of the Figure. However, these bores could also be at an angle to each other and to axis  48 , so long as their relationship with lens assembly  50  permits the output of first fiber  12  to be focused first on spot  32  and then on the end of second fiber  16 . 
     Because of the unique arrangement of the fibers  12 ,  16 , and lens  50 , target  30  does not have to be a highly reflective surface and may disperse the reflected light. However, because the light from fiber  14  is focused on spot  32 , whatever reflected light falls on lens  50  will be directed back to second fiber  16 . This system can send and collect light off of any target, e.g. aluminum foil, liquids, paper, and is not limited to mirror surfaces as is the Tromborg device. 
     This invention has been observed to have much greater efficiency than the GRIN device of Barker, referenced above. Comparison tests were made using a reflective surface that was adjusted to maximize the return signal. For each device, the input laser  4  was a frequency doubled Nd:YVO laser operating with an output power of 1 mW injected into the send fiber  12 . For the first test, the commercially available Valyn Fiber Optic Probe (Valyn International, Albuquerque, N.Mex.), the subject of U.S. Pat. No. 5,202,558, had a standard 50 μm input fiber and a 300 μm output fiber. (The device is designed to have a smaller input fiber to more accurately focus the transmitted spot. However, the small fiber design makes it very difficult to couple larger amounts of light from the diode through the fiber.) The return signal was 20 μW, or 2% system efficiency. In a second test, this device had 200 μm input and output fibers. This test yielded a return signal of 0.19 μW, or 0.19% efficiency. However, the test embodiment of the invention, using 200 μm input and output fibers, had a return signal of 180 μW, or an efficiency of 18%. 
     FIG. 2 shows the velocity profile of a surface containing spot  32  moving toward the probe over a distance of about 1 μm, where the vertical axis is in km/s. This movement resulted from hitting target  30  with a high intensity X-ray pulse. The GRIN device would not be able to make such a measurement because the smaller input fiber and the plastic lenses darken and fluoresce during the X-ray pulse. 
     FIG. 3 shows a second embodiment of fiber housing  60  which replaces o-ring  56  of the embodiment of FIG. 1 with a flexible flange  62  extending preferably from end  64  of housing  60 . The diameter of flange  62  is slightly greater than the diameter of the bore of case  20 , so the natural springiness of flange  62  will hold housing  60  in position as discussed above. Flange  62  may have any cross-section shape, such as circle  62  (FIG. 4 a ), portions of a circle  62 ′ (FIG. 4 b ), or spokes  62 ″ (FIG. 4 c ), so long as there is sufficient friction with case  20  to hold housing  60  in position. 
     While lens  50  is indicated as being symmetrical about axis  28 , it is also contemplated that some or all of the lens which make up assembly  50  may be asymmetrical with respect to axis  28 . 
     The invention has been illustrated utilizing a target  30  that moves under the impact of a high energy beam. It is also contemplated that target  30  could be a material that is vaporized by focussed energy from first optical fiber  12 . Since a large area of the lens is used to focus the outgoing laser light, high energy radiation may be transmitted without fear of damaging the lens, due to the spreading of the radiation over a large area of the lens. If the frequency of the transmitted beam as received by fiber  16  is filtered out, the spectrum of the resulting vapor may be analyzed at detector  8 . 
     The particular sizes and equipment discussed above are cited merely to illustrate a particular embodiment of this invention. It is contemplated that the use of the invention may involve components having different sizes and shapes as long as the principle of having light that is focused on a target and also on an output fiber, without having a shadow cast by one fiber on the other, is followed. For example, a combination of fiber optic diameters can be used for optimal performance: a small diameter fiber optic can be used for the sending fiber which will allow the lens to focus a smaller and brighter spot on the target, while the return fiber can be quite large which gives the advantages of a larger tolerance zone for focusing on the target, and can collect more light from the target due to the larger core diameter of the fiber. It is intended that the scope of the invention be defined by the claims appended hereto.