Abstract:
A fiber optic acoustic sensor that detects pressure waves from all incident angles features multiple mandrels orthogonally disposed to provide omni-directional sensing capability. The topology of the mandrels prevents frequency response drop-off characteristic of cylindrical sensors at wavelengths smaller than ½ the length of the acoustic cylinder. The larger operating bandwidth and omni-directional sensing capability makes this fiber optic acoustic sensor a suitable choice for a wide range of applications.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to acoustic sensors. More particularly, the invention relates to acoustic sensors that feature optical fiber wound around a compliant structure. 
     2. Description of the Related Art 
     Conventional fiber optic acoustic sensors frequently feature a sensing arm and a reference arm that terminate at an optical coupler. The sensing arm consists of a first optical fiber wound tightly around a compliant mandrel. The reference arm consists of a second optical fiber of fixed length disposed in an environment where stresses are minimal. Under quiescent conditions (no acoustic wave) light introduced into both the sensing arm and the reference arm travels through the respective fibers and arrives at the coupler. The path length of the sensing arm and the reference arm are fixed, thus light from each arm will arrive at the coupler with a time invariant phase difference. Under these conditions, the output of the coupler (mixed light) will be a light wave of constant amplitude. 
     If an acoustic wave is introduced into the environment of the sensing arm, the compliant mandrel will respond to the acoustic wave by expanding and contracting, stressing the sensing fiber. The stress on the sensing fiber changes the path length of light traveling through the fiber, modulating the light, accordingly. At the same time, the reference arm&#39;s path length remains unchanged in response to the acoustic wave. When light from both arms is mixed the light amplitude will varies proportionally with the incident acoustic wave. 
     One of the problems with conventional fiber optic acoustic sensors is that the sensitivity and directivity of the acoustic measurement is dependent on the size and orientation of the compliant mandrel. As the acoustic wavelength approaches the length of the mandrel, the sensor&#39;s sensitivity rapidly decreases. To maintain a constant frequency response over the entire band of frequencies of interest, hydrophone designers generally limit the length (and the diameter) of mandrels to half the wavelength of the highest frequency of interest. 
     This size limitation on the length of the mandrels imposes a practical limit on the operating band of fiber optic acoustic sensors. As the frequency of interest increases, the wavelength of interest decreases, requiring the use of very small mandrels. With very small mandrels, the number of fiber optic windings that can be formed around the mandrel decreases resulting in less sensitivity. In practice, this problem makes fiber optic acoustic sensors based on fiber wound mandrels a poor choice for sensing frequencies above 50 KHz. 
     Another problem with conventional fiber optic acoustic sensors is the frequency response is dependent on the direction of arrival of the acoustic wave. Compliant mandrels often are more responsive to acoustic waves that impact the mandrel broadside (radial direction of the mandrel) and less responsive to acoustic waves that impact the endfire (longitudinal direction of the mandrel). This is because the mandrel diameter is usually smaller than its length. 
     There currently is a need for fiber optic acoustic sensors that can detect frequencies up to 100 KHz with a constant sensitivity over the entire frequency band. There is also a need for a fiber optic acoustic sensor that is able to detect acoustic waves arriving at the sensor from any spatial direction. 
     SUMMARY OF THE INVENTION 
     The invention accomplishes these goals by using multiple mandrels oriented along three orthogonal axes. A single optical fiber is tightly wound around all the mandrels to form a sensing arm. Light is introduced into the sensing arm and the phase shift is measured at the output of the sensing arm to determine strain in the fiber. Acoustic waves incident upon any one of the mandrel segments will induce a strain in the fiber resulting in a proportional phase shift in the light at the output of the sensing arm. The orthogonal arrangement insures that a significant component of any acoustic wave incident upon the sensing arm will be incident upon the broadside of at least one of the mandrels. This makes the invention less vulnerable to the frequency response drop-off that is characteristic of conventional sensors at wavelengths shorter than one half the mandrel length. The orthogonal arrangement of the present invention also increases the sensitivity of the sensor. Multiple mandrels allow more optical fiber windings over more compliant structure surface area. The topology of the mandrel layout also makes it inherently omni-directional. The present invention&#39;s frequency response is largely independent of the direction of the acoustic wave with the sensitivity being relatively constant over the sensors entire bandwidth. The length of the compliant mandrels can be increased to at least the wavelength of the highest frequency of interest without experiencing the sensitivity drop-off normally encountered at half the wavelength. Expansion of the sensitivity envelope obtained with the present invention makes fiber optic acoustic sensors a suitable choice for sensing acoustic frequencies up to at least 100 KHz. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The exact nature of this invention as well as its objects and advantages will be readily understood upon consideration of the following specification as related to the attendant drawings wherein like reference numeral throughout the drawings indicate like parts, and wherein: 
         FIG. 1  is a perspective view of a preferred embodiment of the present invention. 
         FIG. 2  is a perspective view of an alternate embodiment of the present invention. 
         FIG. 3  is a perspective view of the midpoint sensor of the preferred embodiment of  FIG. 1 . 
         FIG. 4  is a side plan view of a mandrel 
         FIG. 5  is a bottom plan view of a mandrel. 
         FIG. 6  is a cross section of  FIG. 5  taken along line A-A. 
         FIG. 7  is a perspective view of an endpoint sensor, an alternate preferred embodiment of the sensor of the present invention. 
         FIG. 8  is a perspective view of the preferred embodiment of a base for mounting the mandrels. 
         FIG. 9  is schematic diagram of the optical fiber windings around the mandrels of a midpoint sensor. 
         FIG. 10  is a plan form diagram of the optical fiber windings around the mandrels of a midpoint sensor. 
         FIG. 11  is a schematic diagram of the optical fiber windings around the mandrels of the endpoint sensor. 
         FIG. 12  is a plan form diagram of the optical fiber windings around the mandrels of the endpoint sensor. 
         FIG. 13  is a schematic diagram of the sensing and reference arms of the preferred embodiment. 
         FIG. 14  is a schematic diagram of the sensing arm in another embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In this description, “optical fiber” includes all flexible optical waveguides. An “optical coupler” includes optical beam splitters, combiners, and Bragg gratings. An “acoustic wave” means all pressure waves. An “acoustic sensor” includes hydrophones and pressure transducers. 
       FIG. 1  shows the preferred embodiment of the present invention. A housing  102  holds a reference arm  104  and associated optical couplers and splices (not shown). The housing  102  is attached to a midpoint sensor  106 . The housing unit  102  is composed of a durable noncompliant material such as stainless steel to minimize stress on the reference arm  104 . 
       FIG. 2  shows an alternate embodiment of the invention. A housing  102  holds a reference arm  104  and associated optical couplers and splices (not shown). The housing  102  is attached to an endpoint sensor  206 . 
       FIG. 3  shows the midpoint sensor  106 . The first mandrel  302 , the second mandrel  304 , and the third mandrel  306  are nearly cylindrically shaped with the longitudinal axis of each mandrel orthogonal to the other mandrels. Extensions of the longitudinal axes meet at the center of the base  312 . The fourth mandrel  308 , the fifth mandrel  310  and sixth mandrel  314  are cylindrically shaped and mounted opposite the first mandrel  302 , the second mandrel  304 , and the third mandrel  206 , with reflective symmetry. All six mandrels are mounted on the base  312  such that the longitudinal axes of the mandrel pairs lie on the coordinate axes of a Cartesian coordinate system with its center at the center of the base  312 . 
     The benefit of sensor  106  can be seen most clearly for the case when an acoustic wave with a wavelength equal to the mandrel length impinges the endfire direction of a single mandrel. In a conventional sensor most of the acoustic energy is damped with the sensor being insensitive at that frequency and above. Using the present invention, an acoustic wave of that frequency incident on the endfire of a single mandrel must also be incident on the broadside of the other two mandrels. The response of the endfire will be damped but the other two mandrels will comply with the wave inducing strain in the optical fiber making the sensor sensitive at that frequency. 
       FIG. 4  shows a side view of the mandrel  302 , which is the same as the other mandrels  304 ,  306 ,  308 ,  312 ,  314  of the midpoint sensor  106 . A cover  402  of cylindrical shape defines the top of the mandrel. The bottom  404  is cylindrical. The longitudinal axis of the cover  402  and the bottom  404  are coincident. 
     All the mandrels  302 ,  304 ,  306 ,  308 ,  312 ,  314  are composed of any compliant material that allows the mandrels to expand and contract sympathetically with incident acoustic waves. The mandrels may be solid, air-backed, or fluid filled. The mandrels may be cylindrical spool shaped or any other shape that accommodates the windings of an optical fiber. 
     The mandrels may be larger than one half the wavelength of the maximum design frequency. For instance, a sensor with a maximum design frequency of 75 KHz (2.0 cm wavelength) might feature mandrels with lengths of 2.0 cm. 
       FIG. 5  shows a bottom view of the mandrel  302 . The cover  402  has a slightly larger circumference than the bottom  404  with coincident center points. 
       FIG. 6  shows a cross section of the mandrel  302 . The cover  402  and the bottom  404  define an inner cylindrical cavity. 
       FIG. 7  shows the endpoint sensor  206 . The first mandrel segment  502 , the second mandrel segment  504  and the third mandrel segment  506  are nearly cylindrically shaped and mounted on a base  508  with the longitudinal axes of each of the mandrels orthogonal to each other. Extensions of the longitudinal axes meet at the center of the base  508 . 
     The mandrel segments  502   504  and  506  are preferably two cylindrical mandrels of the first embodiment joined with a binding  510 . The binding may be composed of any material that joins the mandrels. Other embodiments may feature mandrel segments  502   504  and  506  consisting of a single mandrel but twice as long as the six mandrels. 
     In the midpoint sensor  106 , ( FIG. 3 ) the mandrels  302   304   306   308   312   314  are mounted along orthogonal axes to minimize the spatial sensitivity variation common in many conventional sensors. Acoustic waves incident on the less sensitive endfire (longitudinal axis) of one mandrel impact the more sensitive broadside (radial axis) of the four other mandrels. Acoustic waves arriving from any direction will have a significant broadside component on at least two of the mandrels making the midpoint sensor  106  sensitive to acoustic waves arriving from any direction. 
       FIG. 3  shows a midpoint sensor  106 .  FIG. 7  shows an endpoint sensor  206 . Other embodiments may contain sensors with any number of mandrels arranged in a variety of topologies. Although  FIG. 3 and 7  show mandrels mounted with the longitudinal axes of the mandrel forming a tri-axis orthogonal topology, other embodiments may feature mandrels that are mounted on non-orthogonal axes. Embodiments featuring only two mandrel sensors at oblique angles offer considerable advantage over conventional sensors. The mandrels may be formed in a variety of shapes. Alternate embodiments feature mandrels that are cylindrical, spool shaped, conical, hour glass shaped, tapered with a varying perimeter length, or any other shape that is reactive to acoustic waves are contemplated. 
       FIG. 8  illustrates the base  212  of the midpoint sensor  106 . The base  212  is formed to have surfaces that accommodate six mandrels. The first mandrel  302  mates with a first surface  602 , the second mandrel  304  with a second surface (not shown), the third mandrel  306  with a third surface  604 , the fourth mandrel  308  with a fourth surface (not shown), the fifth mandrel  310  with a fifth surface  606 , and the sixth mandrel  314  with a sixth surface (not shown). The order of the mandrels is not important. 
       FIG. 9  shows the fiber optic winding of the midpoint sensor  106 . A single sensing fiber  702  (fiber optic) is tightly wound around the first mandrel  302 , the fourth mandrel  308 , the third mandrel  306 , the sixth mandrel,  314 , the fifth mandrel,  310  and the second mandrel  304 . 
     With a conventional sensor, optical fiber windings are limited to a single structure with a length less than one half the maximum sensing frequency. With the present invention, not only may the mandrel length be increased accommodating more windings per mandrel but the number of sensing mandrels is increased also. An acoustic wave having a significant component incident upon the broadside of single mandrel will also have a significant component incident on at least one other mandrel. If an acoustic wave is incident upon the endfire of a single mandrel it will also be incident on the broadside of the other two mandrels. Both compliant mandrels will react to the broadside wave inducing strain in the respective segments of the optical fiber. In this case, the sensitivity of the present invention would be twice that of a conventional sensor. 
       FIG. 10  shows the topology of the windings around the midpoint sensor  106 . The sensing fiber  702  is wound about three orthogonal axes. The first mandrel  302  and fourth mandrel  308  are mounted on opposite sides of the base  212  forming the first axis. The sixth mandrel  314  and the third mandrel  306  are mounted on opposite sides of the base  312  forming the second axis. The fifth mandrel  310  and second mandrel  304  (not shown) are mounted on opposite sides of the base  212  forming the third axis. 
     Preferably there are multiple windings around each of the mandrels with the windings tight enough to slightly strain the wave conducting portion (the fiber) of the sensing fiber  702  at each of the mandrels. Each of the windings increases the sensitivity of the sensor. The winding tension and the number of windings of the sensing fiber  702  at each of the mandrels is preferably identical making the sensitivity of the acoustic measurements similar for acoustic waves arriving from any direction. 
     The sensing fiber  702  may vary in composition in different embodiments. An optical fiber generally consists of an optical fiber surrounded by cladding. A tough buffer layer may be added with a plastic jacket to add strength to the fiber. Alternate embodiments may feature watertight coverings, casings, or shells that surround sensing fiber  702  while allowing the mandrels to deform with the applied acoustic wave. A secondary elastic covering may be applied over the midpoint sensor  106  or the sensing fiber  702  windings. 
       FIG. 11  shows the fiber optic windings around the mandrels of an endpoint sensor  206 . A single sensing fiber  902  (fiber optic) is tightly wound around the first mandrel segment  502  shown as two joint mandrels, the second mandrel segment  504  shown as two joint mandrels, and the third mandrel segment  506  shown as two joint mandrels. Preferably, there are multiple windings around each of the mandrel segments with the windings tight enough to slightly strain the wave conducting portion (the fiber) of the sensing fiber  902  at each of the mandrels. 
       FIG. 12  shows the topology of the windings around the endpoint sensor  206 . The sensing fiber  902  is wound about three orthogonal axes defined by the first mandrel segment  502 , the second mandrel  504  segment and the third mandrel segment mounted on base  508 . 
       FIG. 13  shows the sensing arm  702  and the reference arm  104  of the preferred embodiment in a Michelson interferometer configuration. The sensing fiber  702  is wound around each of the mandrels with one end terminating at a first mirror  1102  and the other end terminating at an optical coupler  1104 . The reference fiber  1   106  is also an optical fiber of fixed length preferably isolated in a housing  102  to minimize strain on the light conducting portion (fiber) of the reference fiber  1106 . One end of the reference fiber  1106  terminates at a second mirror  1108  and the other end terminates at the optical coupler  1104 . 
     Components in the housing  102  may be potted with a hard resin epoxy and the entire invention may be molded over with a water resistant material such as polyurethane. 
     A collimated or laser light  1110  is introduced into the coupler  1104 . Light traveling through the sensing arm will travel through the sensing fiber  702  to the first mirror  1102  and will be reflected back through the sensing fiber  702  to the coupler  1104 . Light traveling in the sensing fiber  702  will be modulated by strains in the fiber caused by deformation or bending of any of the mandrels. Light from the reference arm will travel through the reference fiber  1106  to the second mirror  1108  and be reflected back to the coupler  1104 . At the coupler  1104 , light modulated from the sensing fiber  702  will mix with reference light from the reference fiber  1106  producing an interference pattern. The mixed light&#39;s  1112  intensity is proportional to the strain on the sensing fiber  702  and the acoustic forces acting on the mandrels. 
       FIG. 14  shows the present invention without a housing or a reference unit. The optical fiber  702  contains a first Bragg grating  1202  at one end of the sensing arm and a second Bragg grating  1204  at the other end of the sensing arm. Laser or collimated light traveling through the optical fiber  702  is introduced into the first Bragg grating  1202 . Some of the light is reflected. The remainder of the light travels through the remainder of the optical fiber  702  and is reflected at the second Bragg grating  1204  returning through the sensing arm to the first Bragg grating  1202  where it is mixes with the reflected light. The mixed light&#39;s intensity is proportional to the strain on the sensing fiber  702  and the acoustic forces acting on the mandrels. 
     The invention thus expands the operating bandwidth of fiber optic acoustic sensors. The unique topology of the mandrels allows more windings and a flatter frequency response over the sensing range. The invention is more sensitive to acoustic waves and provides omni-directional sensing capability.