Communications – electrical: acoustic wave systems and devices – Signal transducers – Underwater type
Reexamination Certificate
1982-06-28
2002-05-28
Pihulic, Daniel T. (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Signal transducers
Underwater type
Reexamination Certificate
active
06396770
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to systems for the generation of acoustic signals and more particularly to the generation of Steerable Thermoacoustic Signals using thermoacoustic effects.
BACKGROUND OF THE INVENTION
A submarine is a ship that can operate both on the surface of the water and completely submerged. In order to avoid detection by radar, surface ships and air patrol, a submarine is usually submerged. Modern submarines have the capability of remaining submerged for long periods of time. In fact, a modern submarine can circumnavigate the earth while running submerged. Thus, modern submarines may complete large portions of their missions while being submerged.
Occasionally, while the submarine is submerged, an airborne vehicle may want to communicate with the submarine. Prior art communications systems used buoys. The airborne vehicle dropped a buoy into the water and the submarine either surfaced to receive the message from the buoy (the message was transmitted to the buoy on an RF Frequency) or the airborne vehicle communicated with the buoy on an RF Frequency and the buoy emitted a sound wave which propagated through the water. The submarine would detect the aforementioned sound waves with sound detection systems like sonar. One buoy would produce omnidirectional sound and directional sound was needed for good communications and detection. The prior art produced directional sound by utilizing an array of buoys which were dropped in the water by an airborne vehicle. The buoys were precisely spaced in the water and beamforming equipment was used to properly phase the beam. Some of the disadvantages of the foregoing systems were that: the buoys had to be carried by an aircraft and the buoys would require space aboard the vehicle and add to the weight of the aircraft, which would reduce the amount of other equipment the aircraft would carry and/or reduce the aircraft's range; the buoys might be detected by a foreign power and disclose the relative location of the submarine; the buoys had a limited range and as the submarine proceeded on its mission the submarine might travel away from the buoy, necessitating the dropping of another array of buoys so that the transmission between the aircraft and the submarine might be continued; the buoys used an active transducer to convert the signals it received from the airborne vehicle into acoustic noise (the acoustic noise levels were high, which is undesirable from a covertness standpoint); the buoys required beamforming equipment; and the buoys were expendable, which meant that the transmission of a message to a submarine was relatively expensive.
Another method utilized by the prior art for the transmission of messages between an airborne vehicle and a submarine employed the use of a very low frequency antenna. The airborne vehicle would extend a low frequency antenna from its fuselage. This method proved to be disadvantageous, since it changed the performance characteristics of the aircraft and made the aircraft less maneuverable. Furthermore, low frequency antennas were not capable of being installed aboard all types of airborne vehicles.
SUMMARY OF THE INVENTION
This invention overcomes the disadvantages of the prior art by creating a Steerable Thermoacoustic Array that is completely mobile by having its transmission equipment aboard an airborne or spaceborne platform and its receiving equipment aboard a device that operates underwater, e.g., submarine, torpedo, mine, underwater oil exploration equipment, etc. The laser or particle beam that is used to produce thermoacoustic signals is steered so that a beam will be produced which traces a path along the water at speed C
o
/sin &thgr; where C
&thgr;
is the speed of sound in water and sin &thgr; is the steering angle. Thus, the foregoing system produces directional signals without any expendable components or easily detectable components that float on the surface of the water. Furthermore, since the receiving equipment can function underwater, a submarine would not have to interrupt its mission (surface) to receive a message from an airborne vehicle or satellite.
The apparatus of this invention achieves the above by utilizing the direct conversion of EM or particle kinetic energy into acoustic energy. The foregoing is accomplished by using either a pulsed infrared wavelength laser or particle beam which is fired into the water from an aircraft or satellite. The physical mechanisms producing sound are of two kinds: (1) thermal expansion of the water from heat generated by medium attenuation of a pulse of laser light or impinging particles, or (2) explosive vaporization of a small volume of water when the heat deposited by the laser or particle beam is large enough to raise the local water temperature above boiling threshold. Infrared laser light is usually used because of its high attenuation coefficient in water which causes high thermal densities. The level of sound produced by infrared lasers is sufficient for communications at expected ranges of communication buoys. Infrared lasers may be controlled (modulated) to the extent required for an underwater communications system. Typical data rates are ~1-10 bits per second.
Modulation schemes which may be employed are on-off keying (OOK), pulse duration modulation (PDM), pulse amplitude modulation (PAM), and frequency shift keying (FSK). The foregoing modulation schemes may be used for lasers and particle beams.
When the density of the heat energy deposited by laser beam absorption is less than that required to vaporize a local volume of water (~2500 joules/cm
3
) the acoustic pressure at radial distance R and polar angle &thgr; from the beam impact point at the water surface is given by the following expression:
P
(
R
,
T
,
θ
)
=
k
2
π
∫
-
∞
+
∞
ⅆ
ω
M
(
ω
)
ω
2
exp
[
-
j
(
ω
t
-
R
/
c
o
)
]
·
sin
θ
where k=&bgr;I
o
/(4&pgr;Rc
o
C
p
)
C
o
=speed of sound
C
p
=specific heat of water
I
o
=laser power output
t=time
&bgr;=thermal expansion coefficient of water
Here M(&ohgr;) is the Fourier transform of the modulation, and I
o
the laser power output prior to modulation. The above expression assumes that the useful portion of the acoustic signal is transmitted at a frequency with wavelength smaller than either the beam spot size or absorption depth.
If the modulation is a gaussian pulse
M
(
t
)
=
M
o
2
π
σ
t
exp
[
-
t
2
/
σ
t
2
]
where &sgr;
t
{tilde over (=)}(one-half of the laser pulse width). The Fourier transform of P(R,&thgr;) is proportional to the function F(&ohgr;)=&ohgr;
2
exp[−&ohgr;
2
&sgr;
t
2
]. The frequency (&ohgr;)
p
when the spectral energy is the acoustic pulse peak is
ω
p
=
1
3
σ
t
-
1
as can be found by setting the derivative of F(&ohgr;) equal to zero.
Thus, the duration of the laser pulse (2&sgr;
t
) controls the spectral W
p
. The bandwidth of the signal can be controlled by firing the laser a number of times at a repetition interval less than or equal to the duration of an acoustic pulse produced by a single laser pulse, or by simply lengthening the pulse duration for a single pulse. The pulse amplitude may be controlled and varied by changing the laser power output.
The extremely short 1-10&mgr; absorption length for certain infrared light frequencies in water makes an explosive vaporization mode of thermoacoustic generation attractive. Incident light with a fluence of >3 J/cm
2
(E
T
) at 10&mgr; wavelength, for instance, will instantaneously boil the 10 micron layer in which most of the light is absorbed. This rapid vaporization produces an explosive stress or shock wave (with Fourier transform S(&ohgr;)) which eventually propagates through the water as a soundwave (with Fourier transform proportional to &ohgr;S(&ohgr;)). The internal energy (E) contained in t
Carey Charles A.
Jensen Richard A.
Woodsum Harvey C.
Asmus Scott J.
BAE Systems Information and Electronic Systems Integration Inc.
Maine Vernon C.
Pihulic Daniel T.
LandOfFree
Steerable thermoacoustic array does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Steerable thermoacoustic array, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Steerable thermoacoustic array will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2841277