Bias compensated optical grating hydrophone

An optical grating hydrophone employs a dual grating construction and two pairs of optical fibers to provide a reliable output signal without the need for mechanically adjusting the system bias. A bias compensation circuit selects the output signal to be received based upon the output signals from the two optical cable pairs.

BACKGROUND OF THE INVENTION 
This invention relates to optical grating hydrophones, and, more 
particularly, to a bias compensation technique for optical grating 
hydrophones. 
An optical grating hydrophone is a device employed to convert time varying 
acoustic waves under water into optical signals, which can be used for 
detection of a sound source within the water. The hydrophone includes two 
optical waveguides (such as light fibers) in axial alignment with a narrow 
gap separating their ends. The optical grating hydrophone utilizes a pair 
of gratings located in the gap between the optical waveguides, which 
consist of equal width opaque and transparent stripes as a controllable 
aperture between the waveguides. When the opaque stripes of one grating 
coincide with the transparent stripes of the other grating the net 
transparent area is zero. When the transparent stripes of both of the 
gratings coincide, the net transparent area is at the maximum, one half 
the total area. The optical transmission from one waveguide to the other, 
therefore, varies from 0-50%. An acoustic signal received by a compliant 
part of the hydrophone supporting one of the gratings moves that one 
grating with respect to the other, which results in the modulation of the 
intensity of a light beam passing through the gratings. Such hydrophone 
systems require a static setting of the gratings relative to each other, 
the bias of the system, which establishes the light beam intensity when no 
acoustic wave is being received. This intensity is the base line against 
which the light beam intensity is measured to determine the 
characteristics of the received acoustic wave. To properly interpret the 
intensity modulation produced by a received acoustic wave, the bias must 
be within a known range. Especially in an array of hydrophones, where 
different biases occur from hydrophone to hydrophone as a result of 
manufacturing tolerances and operating parameters, some technique for bias 
compensation is very important. For the information produced by the 
intensity modulation to be useful, each hydrophone must be operating in a 
linear region of the transfer function which relates the received acoustic 
signal to the output optical signal. Mechanical adjustment of each pair of 
gratings to establish a common bias is especially difficult when high 
density gratings are used, since the small dimensions of the grating 
stripes require a very fine adjustment. This limits the applicability of 
this type of intensity modulated optical hydrophones to small arrays. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide an optical 
grating hydrophone construction and accompanying circuitry to provide a 
bias compensation technique for an optical grating hydrophone. 
The optical grating hydrophone of the present invention includes a 
two-chambered housing having a pair of light sources entering one chamber 
of the hydrophone and a pair of detectors in optical alignment with 
respective ones of the sources. A pair of optical gratings is disposed in 
optical alignment with each source-detector pair to modulate the light 
beam passing between each source-detector pair. The optical gratings are 
so constructed that the intensity of the light beam received by one of the 
two detectors will always be linearly related to the intensity of a 
received acoustic wave. A compensating circuit is provided to determine 
which of the source-detector pairs is operating in the linear region.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a pressure sensitive optical grating hydrophone 10 built 
according to the present invention. Hydrophone 10 includes a housing 12 
divided into chambers 14 and 16. The housing 12 includes an outer 
cylindrical wall 18 having a thick walled section 20, a thinner walled 
section 22 and a plate 24 located between the thick walled and thin walled 
sections and dividing the interior space of the housing. The housing is 
closed at one end by a flexible member 26, made of, for example, rubber. 
The chambers 14 and 16 are filled with a fluid, such as castor oil, which 
has suitable light transmission properties. The pressure inside the 
hydrophone is equalized to the outside pressure by flexible member 26. A 
capillary tube 28 is inserted through the dividing plate 24 to provide 
pressure equalization between the two chambers. The opposite end of the 
housing is closed by a cylindrical cap 30 fitted to the wall 22 at a 
circumferential notch 32. Cap 30 includes a thick walled ring 34 tapered 
inwardly from a shoulder 36 to which a flexible diaphragm 38 is attached. 
The taper of the wall 34 provides a gap between the diaphragm 38 and the 
surface 40 to allow vertical motion of the diaphragm. A pair of light 
conductors 42, 44, which serve as light sources, pass through the ring 34 
and are supported by a light conductor terminator 46 which supports the 
light conductors 42, 44 within the hydrophone. A second pair of light 
conductors 48, 50, which serve as light receivers, pass through ring 34 
and are supported by a light conductor terminator 52 to be in optical 
alignment with light conductors 42, 44, respectively. The ends of the 
light conductor terminators 46, 52 are spaced to have an axial gap 
therebetween. 
Positioned in the gap between the respective ends of the light conductor 
terminators 46 and 52 is a grating support structure, as shown in FIG. 2, 
which includes a disc 54 attached to the diaphragm 38 by, for example, 
adhesive, and a vertical support member 56 extends between the end 58 of 
the terminator 46 and the end 60 of the terminator 52. Support member 56 
includes opening 57 for transmission of light beams 43, 45 therethrough, 
and attached to one surface 62 of support member 56 is a first optical 
grating 64, and attached to surface 60 of terminator 52 is a second 
optical grating 66. 
The optical grating pairs of the present invention are shown greatly 
enlarged in FIGS. 3 and 4. Grating 64 comprises a series of alternating 
opaque stripes 68 and transparent stripes 70 and is divided into upper and 
lower gratings 64a and 64b as shown in FIG. 4. Grating 66 comprises 
alternating opaque stripes 72 and transparent stripes 74 and is divided 
into individual gratings 66a and 66b. The opaque stripe 72a at the center 
of grating 66 has a vertical dimension of approximately half that of the 
other opaque stripes. The effect of the narrow center stripe 72a is to 
produce a grating pair 64a, 66a which is spatially shifted 90 degrees with 
respect to the grating pair 64b, 66b. It will be appreciated by those 
skilled in the art that the vertical dimension of the opaque and 
transparent stripes are greatly exaggerated in the figures; a typical 
vertical dimension would be in the range of 0.001 inches to 0.004 inches. 
The configuration shown in FIG. 1 employing the grating structure 
illustrated in FIGS. 3 and 4 provides two optical channels, the first 
using fibers 42 and 48, and the second using fibers 44 and 50 as shown in 
FIG. 3. The 90 degree shift of grating 66b relative to grating 64b 
produces a 90 degree phase shift in the optical transfer functions of the 
two optical channels. The light intensity transfer function for each 
grating pair is defined as the amount of light transmitted through the 
grating pair as a function of the relative spatial displacement of the 
gratings of the grating pair. With transparent and opaque stripes of equal 
width, maximum transmission occurs when the transparent and opaque stripes 
exactly overlap, so that by establishing this as 100 percent of possible 
light output, unity corresponds to a beam intensity at one of the light 
detectors of half the total light incident upon the grating pair from the 
optical source. The transfer functions are illustrated in FIG. 5, in which 
relative displacement of the gratings of a grating pair is along the 
horizontal axis and light output received by the detector is along the 
vertical axis for each trace with unity representing maximum transmission. 
The transfer function for optical channel a of the grating pair 64a, 66a 
is shown by trace 80. The transfer function for optical channel b of the 
grating pair 64b, 66b is shown by trace 82. As shown by the displacements 
of the minimums of the two transfer functions, the outputs are shifted 90 
degrees relative to each other. The sum of the outputs is shown at 84 and 
the difference is 86. The optical transfer functions can be divided into 
four basic quadrants A, B, C and D, which then repeat. 
In order to avoid fringing effects which interfere with the transfer 
function, it is highly desirable to operate in the linear regions of the 
transfer functions; e.g., quadrant B or D of transfer function 80 of 
grating pair 64a, 66a, and quadrant A or C of transfer function 82 of 
grating pair 64b, 66b. This bias point may reside within any of the four 
quadrants. If knowledge can be gained of the quadrant in which the system 
is operating (i.e., where the bias point is located), then compensation 
for the bias can readily be made. For example, it may be assumed that it 
is desirable to operate at, or near, the center of the positive slope of 
the optical transfer function. If it is determined that the bias is 
located in quadrant B, then all that is required is to choose the signal 
in channel a. Similarly, if it is determined that the bias is located in 
quadrant C, then channel b would be preferred. If it is determined that 
bias resides in quadrant A, the inverse in the signal in channel b would 
be used; and if the bias were determined to be located in quadrant D, the 
inverse of the signal in channel a would be preferred. Employing this 
selection of signals will ensure that the hydrophone is always operating 
at a bias level of 50 percent plus or minus 25 percent of maximum output 
intensity and that the output will always have the same phase relationship 
to the acoustic wave input to the hydrophone, i.e., that the output signal 
amplitude will increase with increasing amplitude of the incident acoustic 
wave. By comparing the sum signal 84 with a threshhold value corresponding 
to unity (i.e., the level corresponding to half of the total light 
incident upon one set of gratings) and producing a signal whenever the sum 
exceeds 1, the comparator output shown at 88 is provided. By comparing the 
difference signal 86 with a threshhold value of 0 and producing a signal 
whenever the difference is greater than 0, the output signal shown at 90 
is obtained. By comparing the outputs 88 and 90 the preferred quadrant of 
operation can be identified. As shown in FIG. 5, if both the sum 
comparator output 88 and difference comparator output signal 90 are low, 
the hydrophone is operating in quadrant A. If the comparator output 88 is 
low and the comparator output 90 is high, the hydrophone is operating in 
quadrant B. If both outputs 88 and 90 are high, the system is operating in 
quadrant C, and if the comparator output 88 is high, and the comparator 
output 90 is low, the system operates in quadrant D. This selection of 
quadrant provides a determination of which signal a, signal b, the inverse 
of signal a and the inverse of signal b may be used as the output signal 
for the hydrophone. 
The circuitry for accomplishing this signal processing is shown 
schematically in FIG. 6. A light source 92 provides light inputs to the 
hydrophone 10. Photo detectors 94, 96 provide modulator output signals 98 
and 100 for light channels a and b, respectively, representative of the 
intensity of light output by each receiver after modulation by the grating 
pair in the associated light channel a or b. Signals 98 and 100 are input 
to amplifiers 102, 104, respectively, and the amplified output signals 
107, 109 are input to analog multiplexer 106. Signals 107, 109 are input 
to inverters 103, 105, respectively, and the inverted output signals 108, 
a, and 109, b, are input to the multiplexer 106. The signals 98 and 100 
are also input to a summer 112 and the sum 113 compared with a reference 
voltage by comparator 119 to provide an output 114 to the multiplexer 
control input 115 if the sum is greater than the reference voltage; i.e., 
the signal shown at 88 on FIG. 5. The difference of output signals 98 and 
100 is produced in a comparator 116, and the difference signal 117 is 
compared to a reference voltage 0 in comparator 120. If the difference 
signal 117 is greater than the reference voltage 0, a control signal 118 
is provided to the multiplexer 106 from the difference comparator 120; 
i.e. the signal shown at 90 in FIG. 5. These two control signals 114, 118 
determine which of inputs 107, 108, 109 or 110 is provided as output 
signal 122 to a detector to indicate the presence of an acoustic wave. The 
detector will receive signals from each hydrophone in an array of 
hydrophones to provide sensing covering an area larger than that which can 
be monitored by a single hydrophone. Each hydrophone will provide a 
reliable output regardless of the quadrant of its operation, since the 
compensation circuit determines which quadrant will produce a useful 
output. 
Although dimensions for a hydrophone may vary widely typical dimensions are 
a diameter of 2 inches and a total length of 23/8 inches. Typical optical 
fiber size is 400 micrometers in diameter. A variety of compensation 
circuits may be employed, so long as the compensation function is 
accomplished. Further modifications in the optical system can be made 
within the concept of the present invention. For example, a single optical 
fiber could provide an input to the hydrophone with a beam splitter, such 
as a prism, located within the hydrophone to provide two optical sources. 
Locating the beam splitter within the hydrophone would add only a small 
complication. As will be appreciated by those skilled in the art other 
modifications to the specific features of the present invention may be 
made within the scope of the present disclosure.