Patent Publication Number: US-2003227824-A1

Title: Optical hydrophone and array using bubble resonance for detecting acoustic signals

Description:
STATEMENT OF GOVERNMENT INTEREST  
     [0001] The invention described herein may be manufactured and used by or for the Government of the United States of America for Governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] (1) Field of the Invention  
       [0003] The present invention relates generally to optical hydrophones, and more particularly to an optical hydrophone in which light transmission through a material incorporating air-filled voids or bubbles is affected by the acoustic induced resonance of the air-filled voids or bubbles, the affected light beam serving as a means for measuring the acoustic waves causing such resonance.  
       [0004] (2) Description of the Prior Art  
       [0005] Towed acoustic receiver arrays are used in military and civilian applications to detect acoustic signals in the water. Traditional towed acoustic receiver arrays take the form of linear arrays of hydrophones mounted inside a flexible hose, the array being connected to a towing vessel by a tow cable. Although the data from these hydrophones may be transmitted through the tow cable optically, the measurement of the sound pressure levels is done electro-mechanically. More recently, innovative arrays developed for U.S. Navy applications use optical fibers wound around mandrels to detect sound. However, arrays made from either electro-mechanical hydrophones or mandrel-wound optical fiber hydrophones are fairly large in diameter. This leads to the use of larger, heavier and more expensive handling equipment (e.g., winches, motors and storage reels) for the storage, deployment and retrieval of the towed acoustic receiver array.  
       SUMMARY OF THE INVENTION  
       [0006] Accordingly, it is an object of the present invention to provide a linear hydrophone array that is small in diameter.  
       [0007] Another object of the present invention is to provide a hydrophone for use in a towed acoustic receiver array.  
       [0008] Still another object of the present invention is to provide a linear hydrophone array that is small in diameter.  
       [0009] Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.  
       [0010] In accordance with the present invention, an optical hydrophone includes a housing defining a chamber. The housing is acoustically transparent to acoustic waves at a frequency of interest. An optically transparent material fills the chamber and has air-filled voids (e.g., bubbles in the material, air-filled microspheres, etc.) defined therein that expand and collapse thus resonating within the material when the acoustic waves impinge on the material. Light is introduced into the material (e.g., a light source such as a light transmitting optical fiber) and passed therethrough. The light passed through the material is received/detected where the light passed through the material is affected by the air-filled voids resonating within the material. In terms of an optical hydrophone array, the light passed through the material is transferred into another optical fiber and then along the array of hydrophones. The array must be calibrated prior to use in order to measure the amount of light attenuation for a given acoustic signal level. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011] Other objects, features and advantages of the present invention will become apparent upon reference to the following description of the preferred embodiments and to the drawings, wherein corresponding reference characters indicate corresponding parts throughout the several views of the drawings and wherein:  
     [0012]FIG. 1 is a schematic view of an optical hydrophone according to the present invention;  
     [0013]FIG. 2 is a cross-sectional view of a microsphere that can be used to realize the air-filled voids in the present invention;  
     [0014]FIG. 3 is a side view of an embodiment of an optical hydrophone according to the present invention that is suitable for use in a towed acoustic receiver array; and  
     [0015]FIG. 4 is a side view of a towed acoustic receiver array that uses the optical hydrophone depicted in FIG. 3. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT(S)  
     [0016] Referring now to the drawings, and more particularly to FIG. 1, a schematic view of an optical hydrophone in accordance with the present invention is shown and referenced generally by numeral  10 . Optical hydrophone  10  will be used to explain the novel features and operating principles of the present invention. It will be readily understood by one of ordinary skill in the art that the features and operating principles disclosed herein can be practiced/realized by a wide variety of structural embodiments thereof without departing from the scope of the present invention. Accordingly, the embodiments to be described later below are provided as non-limiting examples of the present invention&#39;s features and operating principles.  
     [0017] Optical hydrophone  10  includes a housing  12  selected to be acoustically transparent with respect to incoming acoustic waves  14 . That is, housing  12  is a material that acoustically transparent (e.g., plastic, stiff rubber, etc.) across a broad frequency range or a narrower frequency range of interest. Housing  12 , or some portion thereof, defines a chamber filled with a material  16  that, in general, is optically transparent. Defined within material  16  are a number of air-filled voids  18  that can be air-pockets or bubbles in suspension within material  16 . Each air-filled void  18  could also be independently and positively defined by means of a microsphere  20  illustrated cross-sectionally in FIG. 2.  
     [0018] Material  16  is selected such that when acoustic waves  14  impinge thereon, air-filled voids  18  resonate within material  16 . To provide for such resonation, material  16  is typically a viscous liquid or gel, e.g., castor oil, glycerin, clear gels or jellies, etc. Note that if a liquid is used for material  16 , it may be necessary to make air-filled voids  18  neutrally-buoyant structures (i.e., via the use of air-filled microspheres  20 ).  
     [0019] Coupled to material  16  are a light source  22  and a light receiver  24 . Typically, light source  22  and light receiver  24  will be disposed opposite one another with material  16  being disposed therebetween. In operation, light source  22  transmits light (referenced by arrow  26 ) into material  16 . The amount of light  26  passing through material  16  will be affected by the resonance (i.e., expansion and contraction) of air-filled voids  18 , the amplitude and frequency of which is proportional the amplitude and frequency of acoustic waves  14 . Light (referenced by arrow  28 ) reaching light receiver  24  is altered relative to transmitted light  26  in proportion to the amplitude and frequency of resonating air-filled voids  18 . The size of air-filled voids  18  and the volume fraction thereof within material  16  can be used to tune optical hydrophone  10  to be sensitive to a particular frequency range and amplitude level of incoming acoustic waves  14 .  
     [0020] As mentioned above, the novel features and operating principles presented by optical hydrophone  10  can be realized in a variety of structures. By way of non-limiting example, one such structure is illustrated in FIG. 3 and is referenced generally by numeral  30  where like reference numerals are used for the elements common both of hydrophones  10  and  30 .  
     [0021] In general, optical hydrophone  30  utilizes optical fibers to transmit light into material  16  and to receive light passing through material  16 . More specifically, a fiber optic alignment sleeve  32  has optical fibers  42  and  44  fitted in either end thereof. Such fiber optic alignment sleeves are well known in the art. By way of illustrative example, optical fiber  42  transmits light to material  16  while optical fiber  44  receives light from material  16 . However, it is to be understood that this order can be reversed.  
     [0022] Sleeve  32  is made from a material that is acoustically transparent to acoustic waves  14  and is stiff enough to maintain alignment of lenses  46  and  48 . To achieve these functions, sleeve  32  is typically made of plastic or a stiff rubber. Each of optical fibers  42  and  44  has an optical fiber core  42 A and  44 A, respectively, for transmitting light therealong. Since optical fiber cores  42 A and  44 A are very small in diameter, it may be desirable to expand the light beam passing through material  16  to increase the cross-sectional area of the light exposed to the resonance of air-filled voids  18 . For example, a diverging lens  46  can be coupled to the terminal end of optical fiber core  42 A and a converging lens  48  can be coupled to the terminal end of optical fiber core  44 A. To utilize the full diameter of sleeve  32 , lenses  46  and  48  can span the full interior diameter of sleeve  32  as shown. The spacing or gap between lenses  46  and  48  is filled with material  16 . Operation of optical hydrophone  30  is the same as optical hydrophone  10 , i.e., light  26  is transmitted into material  16  and is affected by resonating air-filled voids  18  to yield acoustically affected light  28 .  
     [0023] Optical hydrophone  30  can be used as the basic hydrophone element for a towed acoustic receiver array  50  illustrated in FIG. 4. By way of non-limiting example, array  50  can be constructed with an outer flexible hose  52  (which can be reinforced with strength members) that is typically filled with oil  54  to protect each hydrophone  30  and reduce self-noise problems that occur due to flow noise. Note that the “receiving” optical fiber  44  for each hydrophone  30  becomes the “transmitting” optical fiber  42  for the next successive hydrophone  30 .  
     [0024] The advantages of the present invention are numerous. The optical hydrophone can be very small in diameter since it can be based on the diameter of an optical fiber. As a result, a towed acoustic receiver array constructed from the optical fiber-based embodiment of the present invention will yield a small diameter towed array. The reduction in overall size and weight of the towed array means that shipboard handling equipment can be smaller, lighter and, therefore, less expensive than current towed array handling systems.  
     [0025] It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.