Abstract:
An apparatus for determining the velocity of sound waves which includes a liquid medium having a plurality of gas bubbles. In the liquid medium, a laser transmits a light pulse to interact with the bubbles excited by the sound wave. Backscattered light from the interaction of the light pulse is received. A processor is then responsive to the detector to provide detection of the acoustic wave through the fluid medium.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     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. 
    
    
     CROSS REFERENCE TO OTHER PATENT APPLICATIONS 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to acoustic wave systems and more particularly to means for detecting of acoustic waves by means of electromagnetic radiation. 
     (2) Brief Description of the Prior Art 
     The detection and measurement of sound using lasers is well known. Essentially, a light beam is caused to pass through a medium, which may be air or water, and detect and process reflections from particles in the medium. These particles will tend to have approximately the same velocity as the particle velocity associated with an acoustic plane wave propagating through the medium. The particle velocity for the plane wave is P/(ρ o c), where P is the pressure amplitude, ρ o  is the density and c is the speed of the sound in the medium. The elapsed time and the Doppler shift of the reflected beam indicate the locations and velocity of the scattering particle. This method is known as laser Doppler velocimetry and is reviewed by Vignola et al.  J. Acoust. Soc. Am.  90, 1275-1286, 1991. It has even been envisioned to create a “virtual array” by processing multiple returns from a single beam and then appropriately delaying them to achieve a gain against noise. 
     Various patents describe means for making use of light to measure properties of acoustic waves. U.S. Pat. No. 4,998,225 to Shajenko, for example, discloses a dual beam hydrophone wherein a reference laser beam and a signal laser beam are both modulated simultaneously by the movement of reflecting surfaces responding to pressure variations due to an impinging acoustic wave. Each beam, travels the same path length within the hydrophone before being detected, thus eliminating any otherwise needed signal compensation. The reference beam and signal beam are acoustically modulated 180° out of phase which reduces by one half the number of reflections normally required to produce the same sensitivity. 
     U.S. Pat. No. 5,379,270 to Connolly discloses an apparatus and method for determining the velocity of sound propagation in a fluid as a function of position in the fluid along an axis. A wave of acoustic energy is transmitted along the axis to produce a disturbance that moves in the medium at the velocity of sound. A laser generator transmits a light pulse substantially along the axis through the fluid medium. As the light passes through the disturbance, light backscatters in a characteristic pattern that a detector senses for analysis to provide information concerning the distance traveled and the time of travel for the acoustic wave through the fluid medium and to provide a profile of output characteristic, such as the speed of sound in the medium, as a function of position in the medium. 
     U.S. Pat. No. 5,504,719 to Jacobs discloses a system in which a hydrophone employs a laser beam which is focused upon a small “focal” volume of water in which natural light scattering matter is suspended and which matter vibrates in synchronism with any sonic waves present. The vibration produces a wave modulation of the scattered light, which may be recovered by optical heterodyne and sensitive phase detection techniques. The sonic waves are sensed at locations displaced from the focusing lenses. Because of this remote sensing capability, the physical hardware of an array of hydrophones may be confined to a small area comparable to the dimensions of the lenses themselves while the sensing of the sonic waves virtually occurs at widely spaced, remote focal volumes. Thus, by combining the signals from these remote focal volumes, a virtual array of hydrophones may be formed whose dimensions are large enough in relation to the sonic wavelengths of interest to achieve high directionality but without the penalties of hydrodynamic drag usually associated with large area arrays. 
     U.S. Pat. No. 5,610,704 to Berzins et al. discloses a probe which directs a light beam through a vapor plume in a first direction at a first angle ranging from greater than 0° to less than 90°, reflecting the light beam back through the vapor plume at a 90° angle, and then reflecting the light beam through the vapor plume a third time at a second angle equal to the first angle, using a series of mirrors to deflect the light beam while protecting the mirrors from the vapor plume with shields. The velocity, density, temperature and flow direction of the vapor plume may be determined by a comparison of the energy from a reference portion of the beam with the energy of the beam after it has passed through the vapor plume. 
     It will be appreciated that the measurement of same particle velocity is more effective in air than in water. The reason for this is that the ratio of the specific acoustic impedance, for the two mediums in approximately 4000. Therefore, the particle velocity of a scatterer will be 4000 times greater in air, leading to a much greater sensitivity. In water, Vignola et al. conducted experiments with standing waves that led to an estimate that particle displacements of 5 nm were detectable with this method. This is equivalent to a sound pressure level of 156 dB re: 1 μPa at a frequency of 1809 Hz. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to improve the efficiency of the measurement of the velocity of sound waves in a liquid medium by using lasers. 
     This invention makes use of a unique feature of water, i.e., entrained bubbles, to increase the Doppler shift of a scatterer by approximately three orders of magnitude. 
     Considering a single bubble in water, its resonant frequency f 0  is given by:              f   =       1     2      π                 a                3      γ                   P   o         ρ   0                   (   1   )                                
     where a is the radius of the bubble, γ is the ratio of specific heats of the air in the bubble ( ˜ 1.4), P o  is the steady-state pressure and ρ o  is the density. Thus, a 1-mm radius bubble in water has a resonant frequency of approximately 3300 Hz. The amplitude of radial velocity of such a bubble at resonant frequency f 0  is given by:                U   0     =       4      π                   a   2          P   i         Z   m               (   2   )                                
     At resonance Z m =R m +R r , where R r =4πa 2 ρ o c(ka) 2 , and R m   ˜ (1.6×10 −4 ) (4πa 3 ρ o ) (2πf 0 ) 1/2 . For the above bubble, the velocity amplitude is 3.49×10 −3 P i , compared to 6.7×10 −7 P i  for a plane wave in water. The velocity ratio is estimated to be 5200 or a 74 dB change on a sound pressure level basis. The above radial velocity is actually the same order of magnitude as the particle velocity associated with a plane wave propagating in air. 
     Another factor applicable to this invention is that detectability improves with optical scattering strength, which increases with particle size. Bubbles are often much larger than microparticles normally used for scattering. For example, bubbles may be about 1 mm which in turn, microparticles may be 0.01-10 μm. 
     In the present invention, it will be appreciated that the measurement of same particle velocity is more effective in air than in water. The reason for this is that the ratio of the specific acoustic impedance, for the two mediums in approximately 4000. Therefore, the particle velocity of a scatterer will be 4000 times greater in air, leading to a much greater sensitivity. In water, Vignola et al. conducted experiments with standing waves that led to an estimate that particle displacements of 5 nm were detectable with this method. This is equivalent to a sound pressure level of 156 dB re: 1 μPa at a frequency of 1809 Hz. 
     The presence of such a bubble therefore greatly improves the practicality of laser Doppler velocimetry detection of sound in water. The present invention makes use of this effect in two primary ways. The first way consists of directing multiple beams in the region near the water surface where most bubbles reside. The reflections from bubbles would be appropriately delayed and summed, effectively forming a virtual volumetric array. 
     The second way of improving detection of sound in water using laser Doppler velocimetry involves a towed array consisting of a gel-filled hose containing bubbles with a radius distribution having an appropriate mean and variance for the frequency band of interest. The bubbles would respond to an incident sound filed and a laser inside the hose would simultaneously illuminate them. 
     The gel and bubble radii distribution is selected such that the desired resonant frequency band is maintained at the towed array depth range causing compression of the bubbles. Such a towed array has the potential to achieve a good sensitivity in a compact hose. 
     In the present invention, an apparatus is provided for measuring the velocity of a wave of acoustic energy in a given bandwidth along an axis. In the liquid medium, a laser transmits a light pulse to interact with the sound wave. Backscattered light from the interaction of the light pulse is received. A processor is then responsive to the detector to determine a distance traveled and time of travel for the acoustic wave through the fluid medium. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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 drawing, wherein corresponding reference characters indicate corresponding parts in the drawing and wherein: 
     FIG. 1 is a schematic representation of the components of the laser source and receiver employed in the apparatus of present invention; 
     FIG. 2 is a schematic representation of a preferred embodiment of the apparatus of the present invention; and 
     FIG. 3 is a schematic representation of an alternate preferred embodiment of the apparatus of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a sound velocity profiler is constructed in accordance with this invention. Laser  14  transmits a light pulse along the laser pulse axis  16 . The laser pulse axis  16  passes through the acoustic field. Thus, this circuitry responds to the backscatter from predetermined positions within the acoustic field. The acoustic field at that position would modulate the backscatter light at any such position. 
     An optical signal generator  26  generates a signal that power amplifier  28  amplifies to enable a laser to fire along laser pulse axis  16 . 
     Optical sensors and receivers  32  that include light detector circuits  34  and a signal detector  36 , receive and analyze the backscattered light  38  using conventional processing techniques to provide input signals to a processor  40  that determines the distance traveled and the time of travel for the acoustic wave through the medium for visual presentation on display  42  or other output device. More specifically, the processor  40  includes a depth estimator  44  and a time estimator  46 . The depth estimator  44  uses the arrival time provided by the signal detector  36  and the clock pulse information representing the transmission of a laser pulse to determine the round trip time and estimate the depth of the wave front while a time estimator provides a corresponding time value. A sound velocity computer  48  in the processor  40  combine signals from the estimators  44  and  46  according to equation (1) to provide an output on a sound velocity profile display  42 . 
     Referring to FIG. 2, one embodiment of the present invention includes a laser source and receiver apparatus  50 , as was described combined in connection with either FIG.  1 . This laser source and receiver apparatus  50  is positioned in a body of water  52  which has an upper water line  54  that separates the water  52  from atmosphere  56 . Within the water adjacent the surface water line  54  there are a plurality of bubbles  58 ,  60  and  62 . Transmitted from the laser source and receiver apparatus  50  there is a laser beam  64 , which is reflected from bubble  58  in reflection  66 . Transmitted from laser source and receiver apparatus  52  at an angle from laser beam  64  there is a laser beam  68  which is reflected from bubble  60  in reflection  70 . At a still additional angler displacement there is laser beam  72 , which is transmitted from the laser source and receiver apparatus  50  and is reflected from bubble  62  in reflection  74 , which is received by the laser source and receiver apparatus. There is an acoustic plane wave  76  with axis  78  that moves through the positions of the bubbles  58 ,  60  and  62 , to cause these bubbles to resonate. This resonation is detected when the laser source and receiver apparatus  50  receive the reflection  66 ,  68  and  70  from respectably from bubbles  58 ,  60  and  62 . By measuring the amount of time which the acoustic plane wave  76  takes to progress from bubble  62  to bubble  60  and then to bubble  58 , the velocity of the acoustic plane wave  76  is calculated at the laser source and receiver  50 . 
     Referring to FIG. 3, another embodiment is shown in which the apparatus is submerged in water  80  and includes an elongated vessel  82  having a longitudinal axis  84  and which is filled with a gel  86  having a plurality of bubbles as at bubble  88  and  90  therein. A laser source and receiver apparatus  92  as was described above in conjunction in connection with either FIG. 1 is positioned at one end of the elongated vessel  82 . Laser beam  94  is generated by the laser source and receiver apparatus  92  and is reflected by bubble  88  in reflection  96  which is received at the laser source and receiver apparatus  82 . Laser beam  98  is generated by the laser source and receiver apparatus  92  and is reflected from bubble  90 . Reflection  100  is then received by the laser source and receiving apparatus  92 . There is also an acoustic plane wave  102  having an axis  104 , which moves transversely with respect to the longitudinal axis  84  of the elongated vessel  82 . This acoustic plane wave  102  causes bubbles as at bubble  88  and  90  in the gel  86  to resonate as a wave passes them. Since the bubbles  88  and  90  are transversely displaced from each other in the elongated vessel  82  the velocity of the acoustic plane wave  102  may be determined by measuring the time that it takes the wave to progress from bubble  88  to bubble  90 . 
     The presence of such bubbles therefore greatly improves the practicality of laser Doppler velocimetry detection of sound in water. The present invention makes use of this effect in two primary ways. The first way consists of directing multiple beams in the region near the water surface where most bubbles reside. The reflections from bubbles would be appropriately delayed and summed, effectively forming a virtual volumetric array. 
     The second way involves a towed array consisting of a gel-filled hose containing bubbles with a radius distribution having an appropriate mean and variance for the frequency band of interest. The bubbles would respond to an incident sound filed and a laser inside the hose would simultaneously illuminate them (the gel is selected such that the desired resonant frequency band is maintained at the towed array depth causing compression of the bubbles. Such a towed array has the potential to achieve a good sensitivity in a compact hose. 
     While the present invention has been described in connection with the preferred embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications and additions may be made to the described embodiment for performing the same function of the present invention without deviating therefrom. Therefore, the present invention should not be limited to any single embodiment, but rather construed in breadth and scope in accordance with the recitation of the appended claims.