Dual beam doppler shift hydrophone

A dual beam hydrophone wherein a reference laser beam and a signal laser m are both modulated simultaneosuly by the movement of reflecting surfaces responding to pressure variatrions 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 degrees out of phase which reduces by one half the number of reflections normally required to produce the same sensitivity.

BACKGROUND OF THE INVENTION 
The present invention generally relates to a device for sensing acoustic 
signals and more particularly relates to a dual beam doppler shift 
hydrophone wherein pressure variations due to an acoustic wave impinging 
on a series of mirrors are changed into modulations of light which are 
transmitted by fiber optic guides. 
A majority of the presently used hydrophones are piezo-electric transducers 
and their use imposes many difficulties. Each such hydrophone requires 
associated electronic circuitry which distributes power for its operation. 
There is also a limit as to the minimum of the size and weight of 
piezoelectric hydrophones. Thus, there is a need for a hydrophone that 
senses underwater sound with a light beam thereby not requiring telemetry 
electronics which are sensitive to electromagnetic interference and which 
can be smaller in size and simpler to operate. 
An optically operated hydrophone previously used frequency modulation of a 
light beam in response to acoustic pressure variations utilizing the 
doppler effect. This was fully described in my U.S. Pat. No. 4,188,096. 
Whenever the relative distance between a source of light and a receiver is 
varying in time, the light received is shifted in frequency in proportion 
to the relative velocity of the two. A device which provides a change in 
optical path length with the acoustic pressure wave can thus be employed 
as a light modulator. The device may include two mirrors between which a 
light beam propagates through a series of reflections while the distance 
between the mirrors is varying with pressure variations. It can also be a 
light transparent block of rubber whose dimensions vary with acoustic 
pressure variations or simply an optical fiber made of glass or other 
compressible optical material which changes in length due to applied 
acoustic pressure variations. The signal thus imposed on the light beam is 
then demodulated using techniques of interferometry, where the frequency 
shift of the light beam is detected by comparing it with a second light 
beam as a reference beam on the surface of a photodetector. Frequency 
stability, intensity and polarization of both beams are important for 
proper demodulation and it is thus desirable that the light beams be 
derived from the same optical source which can be a laser. Furthermore, 
the path length of both the beams should be matched to be within the 
coherent length of a laser used. For best performance, both the beams 
should be exposed to the same environmental conditions in order to balance 
modulating effects caused by static pressure, strain, etc. 
SUMMARY OF THE INVENTION 
An acoustic detector built according to the teachings of subject invention 
is a dual beam doppler shift hydrophone wherein both optical beams are 
modulated by the acoustic pressure variations. When both light beams are 
exposed to nearly the same environmental conditions, the design of the 
dual beam optical hydrophone is such that both beams are modulated by the 
same pressure variations due to the incident acoustic wave but 180.degree. 
out of phase with one another, i.e., when one of the beams see the maximum 
pressure the other beam sees the minimum pressure due to the acoustic 
wave. 
An object of subject invention is to have an acoustic transducer having a 
simple construction and relatively high sensitivity. 
Another object of subject invention is to have a dual beam doppler shift 
hydrophone wherein both beams are exposed to the same environmental 
conditions thus simplifying beam balancing. 
A further object of subject invention is to have a hydrophone wherein both 
beams are exposed to the same acoustic pressure variation at the same 
time. 
Still another object of subject invention is to have an optical hydrophone 
using optical fibers which simplifies interface problems. 
Other objects, advantages and novel features of the invention will become 
apparent from the following detailed description of the invention when 
considered in conjunction with the accompanying drawings wherein:

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings, FIG. 1 schematically shows a system 10 using a 
dual beam doppler shift hydrophone 12 built according to the teachings of 
subject invention. System 10 includes a transducer 12, an optical fiber 14 
carrying a beam of light 16 which is collimated by a lens 18 so as to fall 
on a beam splitter 20. The beam splitter 20 gives two light beams 22 and 
24. Beam 22 is reflected by mirror 26 so as to enter transducer 12, 
experiences multiple reflections between mirrors 28 and 30, and is finally 
reflected out of transducer 12 by means of mirror 32 so as to emerge as 
light beam 34. Beam 24 is reflected by mirror 36 and the reflected beam 38 
is reflected again by mirror 40 so as to fall on mirror 42 in transducer 
12. Beam 38 undergoes multiple reflections between mirrors 42 and 44 
emerging as beam 46 which is reflected by mirror 48 so as to obtain beam 
50. Beams 34 and 50 are focused by lens 52 and are carried by optical 
fiber 54. The frequency shift in light beams 34 and 50 is proportional to 
the velocity of mirrors 30 and 42 of transducer 12. This change in 
velocity of mirrors 30 and 42 is caused by their motion resulting from the 
pressure variations due to incident acoustic signals. 
FIG. 2 shows a transducer 55 built according to the teachings of subject 
invention. It includes a generally cylindrical enclosure, preferably made 
of a metallic piece having a suspended flexible disc 58 which is subjected 
to incident acoustic signals. Disc 58 is connected to the movably mounted 
mirror 60 through mirror 62. Fixed mirrors 62 and 64 are housed inside 
cylindrical enclosure 56 of transducer 55. Laser beams 66 and 68, 
emanating from laser 70, pass into enclosure 56 through windows 74 and 72 
respectively, undergo multiple reflections from mirrors 60, 62 and 64 and 
emerge through windows 76 and 78 as beams 80 and 82 respectively. An 
incident acoustic signal 83 impinging on disc 58 moves mirror 60 
accordingly causing light beams 80 and 82 to be modulated proportionally. 
Thus, pressure variations due to impinging incident signal 83 produce 
frequency changes in laser beams 80 and 82 which are proportional to the 
pressure variations due the incident acoustic signal. When these light 
beams fall on a photodetector, they produce an electrical signal 
containing information about the acoustic wave. 
FIG. 3 shows transducer 84 built according to the teachings of subject 
invention. Transducer 84 includes a generally cylindrical metallic box 85 
having a flexible disc 86 forming a part of one of the walls. Disc 86 is 
connected to movable mounted mirror 88. The inside of cylindrical box 85 
is partitioned by means of transparent plate 90. Two movably mounted 
mirrors 92 and 94 are installed inside the cylindrical box in the portion 
of box 85 which is opposite to the one where mirror 88 is housed. Laser 96 
produces beams 98 and 100 which pass through windows 102 and 104 
respectively, enter box 85, suffer multiple reflections at mirrors 88, 92 
and 94 as shown in FIG. 3 and emerge out of windows 106 and 108 as laser 
beams 110 and 112 respectively. Laser beams 110 and 112 are focused on and 
transmitted via a fiber optic guide to a photodetector. Incident acoustic 
signal 114 moves disc 86 and consequently mirror 88 due to the pressure 
variations of acoustic signal 114. The same pressure variations are 
transmitted to the region containing mirrors 92 and 94 through opening 115 
and cause relative motions thereof due to pressure variations from the 
incident acoustic signal. The modulation of laser beams 110 and 112 is 
dependent on the pressure variations of the acoustic signal and these 
beams can then be processed to extract information about the incident 
acoustic signal. 
FIG. 4 shows another embodiment of subject invention. Transducer 116 is 
shown in the form of a plate 118 supported by elements 120 and 122. Plate 
118 is capable of vibrating. Fiber optic guide 124 is placed on one side 
of plate 118 and fiber optic guide 126 is placed on the opposite side of 
plate 118. Plate 118 vibrates because of pressure variations due to the 
incident acoustic signal. As a result, one side of plate 118 is elongated 
while the opposite side is contracted. Consequently, both light beams 
passing through fiber optic guides 124 and 126 are modulated out of phase 
by the same incident acoustic signal. 
Still another embodiment is shown in FIG. 5. Transducer 129 includes a 
U-shaped metallic tube 130 which has a movably mounted disc 132 on one 
wall thereof. Disc 132 transmits pressure fluctuations due to incident 
acoustic signal 133 into tube 130. Laser beam 134 passes through beam 
splitter 136. The transmitted beam passing through beam splitter 136 is 
incident on beam splitter 138. The reflected beam 140 falls on reflector 
142. The transmitted portion 144 passing beam splitter 138 is reflected at 
mirror 146 to form laser beam 148. Beam 148 is reflected at mirror 150. 
Beams reflected by mirrors 142 and 150 emerge as beams 152 and 154 
respectively and they enter the respective sections of U-tube 130 through 
respective transparent ends 156 and 158 of U-tube 130. Beam 152 is 
reflected by mirror 160 and then is in turn reflected by mirror 162 which 
is attached to rubber piece 164. Beam 154 is reflected by mirror 166 and 
the reflected beam 168 is reflected again at mirror 170 attached to the 
opposite side of rubber piece 164. U-tube 130 is filled with a liquid and 
the pressure variations due to the impinging acoustic signal are 
transmitted to the liquid through movably mounted disc 132. Beam 172 
coming back is transmitted to a photodetector for processing to correlate 
the changes in frequency to the pressure variations caused by the incident 
acoustic signal. 
Briefly stated, a dual beam hydrophone built according to the teachings of 
subject invention provides two laser beams which are subjected to the same 
environmental conditions. The design of the hydrophone is such that both 
laser beams are modulated by the same pressure variations due to the 
incident acoustic wave but are 180.degree. out of phase with one another. 
The shifts in frequency of the two beams are measured and are then 
correlated to the incident acoustic signals. 
Obviously, many modifications and variations of the present invention are 
possible in the light of the above teachings. As an example, the structure 
of the transducer can be changed as long as the both light beams are 
modulated in the opposite sense by the incident acoustic signals. It is 
therefore understood that within the scope of the appended claims the 
invention may be practiced otherwise than specifically described.