Communications – electrical: acoustic wave systems and devices – Signal transducers – Underwater type
Reexamination Certificate
1982-06-28
2002-05-07
Pihulic, Daniel T. (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Signal transducers
Underwater type
Reexamination Certificate
active
06385131
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to communications systems and more particularly to communications between an airborne or spaceborne vehicle and an underwater receiver.
BACKGROUND OF THE INVENTION
Submarines 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. The prior art utilized two methods for the transmission of information from an aircraft or satellite to points under water. The first method used electromagnetic energy transmitted from the aircraft or satellite to carry the signal. Electromagnetic (EM) energy does not propagate well in water, except in certain bands of the EM spectrum. Usable bands of the spectrum are the Extremely Low Frequency (ELF), Very Low Frequency (VLF) and the Visible Light bands (in the blue-green regime). The disadvantages of the foregoing method are that both VLF, ELF and blue-green signals are difficult to generate and transmit without heavy and massive equipment. The VLF and ELF communication schemes employ a very long and cumbersome antenna which must be deployed from the aircraft or satellite, and a similar antenna for receiving the electromagnetic signals must be deployed under water. An aircraft or submarine's agility is degraded by the deployment of such an antenna. The blue-green light communication scheme is very inefficient. Therefore, a very powerful laser must be used to transmit coherent blue-green light. The underwater receiver is a complex and highly sensitive light detector which employs very narrowband atomic transitions. The signal to noise ratio of the receiver at the receiving point is very low, since most of the light is scattered and attenuated as it propagates down from the water surface. Both low frequency electromagnetic techniques and blue-green communication require the receiver to be within at least 1,000 ft. of the water surface, which is not always practical. The second method used by the prior art for the transmission of information from an aircraft or satellite to an underwater receiver utilized the transmission of an RF signal to a surface ship or buoy. The buoy or surface ship then retransmitted the message underwater using acoustic energy. Acoustic energy in the sonic frequency regime can propagate miles underwater, thus making this scheme advantageous. However, some disadvantages of the foregoing method are that if the receiver is moving (e.g., if communication is to a moving submarine) the receiver may still move out of range of the acoustic transmitter, requiring the surface ship to move or deploy new expendable buoys. Furthermore, at distances of several miles from the acoustic transmitter, the transmitted information may arrive by several propagation paths, which are caused by refraction of acoustic energy by thermal gradients and reflections of the acoustic signal from the water surface and the ocean's bottom. This “multipath” phenomenon is similar to reverberation in a room of bad acoustic design, and can result in reduced intelligibility of the communication. It was also possible for an unfriendly power to intercept or jam the prior art methods which relied on ELF, VLF or RF transmission.
SUMMARY OF THE INVENTION
This invention overcomes the disadvantages of the prior art 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
≅(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;
&rgr;
) 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 (&dgr;) 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
r
) 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 the gas that was vaporized is approximately given by the ideal gas state equation:
E=3/2 P V
where E is the difference between the laser energy and the threshold energy required to boil the thin layer of water. The initial pressure in the gas bubble would approximately be given by
P
o
=
2
3
(
E
o
-
E
T
)
V
where:
E
o
=laser pulse energy
E
T
=Threshold for vaporization
V=Volume of fluid in which absorption of light occurs
V=A&dgr;=(spot area)×(laser light absorption depth)
Reasonable values for the spot area (A) and absorption lengths are:
A=spot area=1 CM
2
=10
−4
m
2
&dgr;=absorption length of fluid=10
−5
m at CO
2
laser wavelengths
The determination of allowable communication path length requires a knowled
Carey Charles A.
Jensen Richard A.
Woodsum Harvey C.
Asmus Scott
BAE Systems Information and Electronic Systems Integration Inc.
Maine Vernon
Pihulic Daniel T.
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