Abstract:
A fiber optic sensor includes a housing having first and second end plates with a sidewall extending therebetween with the sidewall having an inwardly facing groove. A flexural disk having a central passage therethrough has an outer edge portion mounted in the inwardly facing groove in the housing. The flexural disk has a stepped thickness that is thinner at the outer edge portion than at a region spaced apart from the housing. A first fiber optic coil is mounted on a first side of the flexural disk in the thicker region of the flexural disk, and a second optical fiber is mounted on a second side of the flexural disk opposite the first fiber optic coil. The first and second fiber optic coils are optically coupled together by a fiber optic coupler to form an interferometer that produces an output signal in response to axial acceleration of the flexural disk.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to fiber optic sensors and particularly to fiber optic sensors for sensing acceleration. Still more particularly, this invention relates to a fiber optic sensor for measuring the particle velocity of an acoustic wave that is transmitted through water by direct measurement of the transferred acoustic energy imparted to the fiber optic sensor. 
     Most fiber optic acoustic sensors for underwater applications are pressure-type hydrophones that measure pressure variations in sound waves. 
     SUMMARY OF THE INVENTION 
     The present invention measures the particle velocity component of a sound wave when transmitted in water. A fiber optic sensor according to the present invention comprises a housing having first and second end plates with a sidewall extending therebetween with the sidewall having an inwardly facing groove therein. A flexural disk having a central passage therethrough has an outer edge portion mounted in the inward facing groove in the housing. The flexural disk is formed to have a stepped thickness such that the flexural disk is thinner at the outer edge portion than at a region spaced apart from the housing. A first fiber optic coil is mounted on a first side of the flexural disk in the thicker region of the flexural disk, and a second optical fiber is mounted on a second side of the flexural disk opposite the first fiber optic coil. The first and second fiber optic coils are optically coupled together by a fiber optic coupler to form an interferometer that produces an output signal in response to axial acceleration of the flexural disk. 
     The housing may be formed of magnesium using metal injection molding. The flexural disk may also be formed of magnesium by metal injection molding or it may be formed of aluminum machined to size by standard machining methods. 
     The fiber optic sensor according to the present invention preferably has a hub formed integrally with the flexural disk at the inner edge. The hub preferably is formed to have a first through-hole near a first side of the flexural disk and a second through-hole near a second side of the flexural disk. The first fiber optic coil preferably is formed to have a first mirrored fiber end secured within the first through hole and the second fiber optic coil preferably is formed to have a second mirrored fiber end secured within the second through hole. 
     The housing preferably has an angled fiber entry/exit passage that allows fiber optic leads to pass through the housing at an angle between 45° and 60° such that the leads can make a small transition to become aligned for entry into the fiber optic coupler, thereby minimizing fiber stress and optical bend loss. 
     The first and second end caps of the housing preferably have a plurality of radial stiffening ribs extending between the hub and the housing sidewall with each of the first and second end caps having an odd number of such ribs to reduce low frequency mechanical resonance vibrations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of the invention; 
         FIG. 2  is a top plan view of a portion of the invention; 
         FIG. 3  is a cross sectional view of the sensor structure taken along line  3 - 3  of  FIG. 2 ; 
         FIG. 4  is a cross sectional view of a flex disk that may be included in the sensor structure of  FIGS. 2 and 3 ; and 
         FIG. 5  is a cross sectional view of a housing that may be included in the sensor structure of  FIGS. 2 and 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a fiber optic sensor  10  that is formed as a Michelson interferometer. A laser source  12  produces optical signals that are input to an optical fiber  14  that guides the optical signals to a 2×2 optical coupler  16 . A first portion of optical signals input to the optical coupler  16  by the optical fiber  14  remains in the optical fiber  14  and propagates through the optical coupler  16  to a coil  18  formed in the optical fiber  14 . After passing through the coil  18 , the signals reflect from a mirrored end  20  of the optical fiber  14  and propagate through the coil  18  a second time before reaching the optical coupler  16  again. 
     The optical coupler  16  couples a second portion of optical signals input to the optical fiber  14  from the laser source  12  into an optical fiber  22  that has a coil  24  formed therein. After passing through the coil  24 , the optical signals reflect from a mirrored end  26  of the optical fiber  22  and propagate back through the coil  24  to the optical coupler  16 . The optical coupler  16  couples part of the reflected optical signal incident thereon via the optical fiber  14  into the optical fiber  22  so that the two reflected optical signals are combined in the optical fiber  22 . The combined optical signals then are incident upon a photodetector and demodulation module  28 . 
     The coils  18  and  24  are arranged so that the particle velocity of an incident underwater acoustic wave causes the length of the optical path through the coil  18  changes by an amount ΔL and the length of the optical path through the coil  24  changes by an amount −ΔL. These changes in the optical path lengths produce a phase difference in optical signals that have traversed the two optical paths. The phase difference may be expressed as 
               ΔΦ   =       2   ⁢     πη   ⁡     (     4   ⁢   Δ   ⁢           ⁢   L     )         λ       ,         
where η is the refractive index of the optical fibers  14  and  22  and λ is the wavelength of the optical signal output from the laser source  12 .
 
       FIGS. 2 and 3  illustrate a sensor module  30  according to the present invention. The coils  18  and  24  in the optical fibers  14  and  22  are formed on opposite sides  32  and  34 , respectively, of a stepped flex disk  36 , which is best shown in  FIGS. 3 and 4 . The stepped flex disk  36  is preferably formed of aluminum. The flex disk  36  is mounted in a housing  38  that is formed of a first section  40  and a second section  42 . 
     Referring to  FIGS. 3 and 4 , the stepped flex disk  36  has an outer rim  44  and a central passage  46 . A circular flange  48  extends around the periphery of the central passage  46 . The circular flange  48  has a thickness that is preferably greater than the thickness of the outer rim  44 . The flex disk  36  has a region  50  between the outer rim  44  and the circular flange  48  that has a thickness preferably greater than the thickness of the outer rim  44  and less than the thickness of the circular flange  48 . Referring to  FIGS. 3 and 4 , the fiber optic coils  18  and  24  are formed on the region  50  of the stepped flex disk  36 . 
     Referring to  FIGS. 2 ,  3  and  5 , the housing  38  includes a plurality of stiffening ribs  52 - 60  in the second section  42  of the housing  38 . The first housing section  40  includes similar stiffening ribs.  FIG. 3  shows two such ribs  62  and  63 . As shown in  FIG. 2 , the stiffening ribs  58  and  59  include slots  66  and  68 . The optical coupler  16  includes an elongate coupler housing  70  that is arranged to extend through the slots  66  and  68 . 
     As shown in  FIG. 3 , the housing section  40  includes a cavity  72  that is enclosed by an end wall  74  and a cylindrical sidewall  76 . A post  78  extends from the center of the end wall  74  into the region enclosed by the cylindrical sidewall  76 . The housing section  42  includes a cavity  80  that is enclosed by an end wall  82  and a cylindrical sidewall  84 . A post  86  extends from the center of the end wall  74  into the region enclosed by the cylindrical sidewall  84 . The housing sections  40  and  42  are formed so that when they are juxtaposed with the ends of the posts  78  and  86  in contact, a small gap  90  is formed at the facing edges of the cylindrical sidewalls  76  and  84 . The posts  78  and  86  include facing central recesses  92  and  94 , respectively. The posts  78  and  86  preferably are joined by a dowel pin  96  that is placed in the recesses  92  and  94  that serve to coaxially align posts  78  and  86  and housings sections  40  and  42  together. An adhesive may be applied to a bond line  100  at facing ends of the posts  78  and  86  to bond them together. Adhesive is preferentially also used to bond the outer rim  44  of the flex disk  36  to the housing sections  40  and  42 . 
     The outer rim  44  of the flex disk  36  is securely retained in the gap  90  when the sensor  10  is fully assembled. The posts  78  and  86  have outer diameters that are smaller than the inner diameter of the passage  46  in the flex disk  36  so that the flange  48  extends around the posts  78  and  86 . The circular flange  48  contains two diametrically opposed holes or slots  49  and  51  to allow passage of the mirrored fiber ends  20  and  26  respectively through the flange where it may be bonded securely to the inner diameter of the flange hub  48  as shown in  FIGS. 2 and 3 . 
     A portion  47  of the outer rim  46  of the flex disc  36  extends out of the gap  90  and acts as a hinge when the flex disc  36  is subjected to acceleration along a sensing axis that is perpendicular to the plane of flex disk  36 . 
     An incoming acoustic wave that impinges on the sensor  10  causes a very minute motion of the sensor housing  30  due to inertial forces. This motion in turn is imparted to the sensor&#39;s internal flex disk  36  that has the upper and lower optical fiber coil packs  18  and  24  attached thereto. The coil packs  18  and  24  are configured in a differential mode whereby any movement along the input axis of the sensor  10  causes one set of fiber coils to go into compression and the opposite set to undergo tensile expansion. When laser light is introduced into the optical fiber  14 , it splits at the optical coupler  16  and propagates along two separate paths through the two coils  18  and  24 . The end faces  20  and  26  of the fibers  14  and  22 , respectively, at the ends of the coils  18  and  24  have mirror coatings applied so that the laser beam is reflected back through the coils  18  and  24  and recombined in the coupler to form a Michelson interferometer. An interference pattern is generated whenever acoustic motion causes a differential change in optical path length between the coil  18  and the coil  24 . A signal indicating creation of an optical phase difference (or phase modulation) is fed by the output fiber  22  directly to an external demodulator  28  that recovers the original acoustic information. The theory and operation of the Michelson interferometer are well known to practitioners of the art within the fiber optic industry. 
     The principal end-use of this invention is for deployment in large aperture underwater two dimensional planar arrays (not shown) used in military systems. Such arrays require hundreds of individual sensing elements whose outputs are combined in a cooperative manner that permits creation of a highly-sensitive, steerable receiving beam that can be used for any type of sonar application including intelligence gathering, threat assessment, and targeting activities. To make such a system affordable and practical, it is necessary that the individual sensors be manufactured as nearly identical to one another and to be significantly less expensive than the current state-of-art designs. 
     The specific features of the sensor  10  according to the present invention that represent an improvement over previous velocity sensor designs and provide the potential to reduce sensor cost by half are detailed as follows: 
     1. Low-Cost Fabrication Techniques. 
     Typical fabrication of the sensor housings is currently done by Computer Numerical Control (CNC) machining to achieve near-identical reproduction of the sensor components. Although this methodology produces consistent hardware pieces, it is a relatively inefficient process since the parts often have to be machined from a solid block of material, which generates a large amount of material waste. In addition, it is a time-consuming process with several tool changes being required throughout the machining operation. The proposed method of fabrication for this invention is metal-injection molding (MIM), which has the ability to create final shape components that hold precision tolerances (±0.003″) at about a quarter of the recurring cost of similar CNC-machined components. An alternate cost-effective method is to cast the parts; but because of problems in strength, porosity, voids, and poor tolerances achieved by this method (typically ±0.020″), the casting process is deemed unsuitable for production of precision hardware. 
     2. Use of Alternate Materials of Construction. 
     The choice of magnesium as a sensor housing material has several advantages over standard aluminum, the material of choice for prior art devices. Whereas aluminum cannot be metal-injection molded, magnesium is quite amenable to this process. The cost of magnesium powder used in MIM-type processes is considered inconsequential since there is virtually no waste when a final molded product is produced. Under elevated heat and pressure, the magnesium powder develops thixotropic flow properties similar to plastic molding materials. In addition, magnesium has several advantages over other powder metallurgy candidates because it does not require the addition of an intermediate binder to retain the part shape until high temperature sintering is performed. 
     Typically, high temperature sintering is required to bake out the binder on powdered metals other than magnesium which results in a typical 17% shrinkage rate of the final part which makes it difficult to achieve uniform, repeatable tolerances. Magnesium, on the other hand, requires no binder addition (and subsequent sintering) and has less than 0.5% shrinkage with very little cleanup (e.g. typically sprue removal) required. The use of magnesium as a MIM material has an added benefit in that it has inherently higher internal damping characteristics over wrought or forged materials which results in a more desirable lower-Q structural characteristic. Concerns of potential corrosion are not warranted since a standard anodic protection coat per MIL-M-45202 can be added at very little cost during batch processing of hundreds of housings at one time. Additionally, although magnesium has about ⅔ the density of aluminum, the coefficient of thermal expansion (CTE) is nearly identical (14 μ-in/in/° F. for Mg versus 13 μ-in/in/° F. for Al) whenever a composite sensor requiring use of both materials is needed. A final advantage for the choice of magnesium over aluminum is that it allows overall reductions in sensor weight and size since there is a requirement for neutral buoyancy where the sensor has to closely match the density of water (or the encapsulating medium) to permit matched acoustic impedance. If a sensor&#39;s equivalent density or weight exceeds this requirement, the housing often has to be enlarged to create the additional volume needed to achieve the buoyancy target. 
     3. Stepped Flex Disk Construction. 
     This invention incorporates a novel outer diameter stepped flexure construction as shown in  FIGS. 3 and 4 . Computer analysis indicates an increase of nearly 1 dB in scale factor (SF) can be achieved by increasing the separation between upper and lower fiber coil packs from the neutral bend axis. The greater the distance from the neutral axis, the greater the compressive and tensile forces act to increase radial fiber strain and thus produce correspondingly greater path length differences. This cannot be achieved with simply increasing the thickness of the entire flex disk since the overall increased stiffness would reduce overall flex disk deflection (and scale factor sensitivity) while increasing F R  (sensor resonance) higher than desired. Normally, the flex disk and housing materials are manufactured from the same material (such as magnesium), but in some cases, such as this design, the computer analysis did indicate a higher scale factor could be achieved due to increased pendulosity effects of the heavier aluminum substrate driving the fiber harder. 
     As previously mentioned, a sensor combining magnesium and aluminum components would have little environmental sensitivity because of the closely matched CTEs. An added benefit of the step would be during the fiber winding process, wetted fiber would not deposit winding adhesive on the outer land surface  44  of the flex disk, which needs to be free of any buildup of cured epoxy prior to the critical flex disk-to-case bonding. 
     4. Dual-Winding Capability With Integral Flexure Hub. 
     An additional feature of the improved flex disk design is the incorporation of an integral hub or circular flange at the inner diameter as shown in  FIGS. 2 and 3 . Past practice of previous designs was to wind each coil pack separately with an individual start fiber and end fiber exiting the coil pack. The start leads are typically cleaved and mirrored at a later time following all winding operations. To prevent excessive micro-bend losses of the start fiber resulting from fiber crossovers, previous designs created a spiral groove on both sides of the flex disk to isolate the start lead from the remaining fibers of the coil pack. After curing of the first coil pack, the other side would be wound separately and cured as well with four fibers emanating from the wound flex disk at completion. The four fibers were extremely fragile and in many instances a fiber would be broken due to inadvertent handling damage, especially when feeding the fiber through the elongated cross-over slot prior to the second wind. The machining of the spiral groove, cross-over slot and subsequent burr removal of the previous flat flex disk design were labor-intensive operations that created needless additional cost. 
     This invention circumvents these previous problems by adding a mirrored start fiber through either through-holes  49  and  51  or slots (not shown) in the circular flange  48 . To protect the mirrored ends  20  and  26 , an overcoat of an UV-curable adhesive is applied to lock the fibers in place and to insulate the tip of the fragile mirrored ends  20  and  26  from incidental contact damage. In addition, by allowing the mirrored fiber ends  20  and  26  to be placed inside the hub cylinder  48  itself provides an additional degree of protection from physical handling of the flex disk assembly. With the elimination of the concern over routing and handling of the fiber start leads, it becomes relatively easy to design a winding fixture to allow simultaneous dual winding of both sides at once, cutting the processing time in half. 
     5. Angled Fiber Entry Into Housing. 
     Previous designs have the input and output fiber leads exiting the housing in a radial direction. Inside the sensor the fiber needs to make a relatively sharp turn and route along the internal peripheral wall until it reaches the fiber coupler. As can be seen in  FIG. 2 , this invention allows the fiber to enter the housing through a passage  108  at a 45° to 60° angle where it can make a small transition to become in line with the fiber entry point of the coupler  16  thereby eliminating a potential point of high fiber stress and optical bend loss. An additional benefit is gained with the angled entry in that an effectively longer through-hole length is created that increases the internal bonded surface area and enhances the sealing of the fiber-to-case seal. 
     6. Odd Number of Stiffening Ribs. 
     Earlier designs have featured a symmetrical rib layout that typically included an even number of ribs (e.g. 4, 6, or 8). Computer analysis has shown that incorporation of an even number of elements can inadvertently facilitate lower frequency structural bending modes to occur with an undesirable excitation or interaction within the acoustic band of interest. Utilizing an odd number of stiffening elements (e.g. −5, 7 or 9), makes the geometry less prone to introducing undesirable lower frequency structural resonances. This is shown in  FIG. 2  of this disclosure. 
     This invention incorporates other design features described in earlier patents (U.S. Pat. No. 6,650,418 B2 and U.S. Pat. No. 6,473,183 B1) such as direct wet winding, edge-support disk mounting, center post stiffener, dowel pin press fit, integral fiber holding tabs, radial rib stiffeners, integral coupler mounting notches, and thin floor thickness (for matched compressibility of water) and as such are mentioned as being relevant to this invention but are not added as specific claims to this invention. 
     The structures and methods disclosed herein illustrate the principles of the present invention. The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects as exemplary and illustrative rather than restrictive. Therefore, the appended claims rather than the foregoing description define the scope of the invention. All modifications to the embodiments described herein that come within the meaning and range of equivalence of the claims are embraced within the scope of the invention.