Optical Acoustic Sensor

This case is related to U.S. Pat. No. 4,299,490, filed Dec. 7, 1978, entitled "Phase Nulling Optical Gyro" by Richard F. Cahill and Eric Udd. The teachings of that application are hereby incorporated by reference as though fully set forth hereinbelow.

Extremely sensitive, low-cost acoustic sensors are required for a wide range of applications, many of which involve operation under hostile environmental conditions. Typical examples are under water acoustic sensors for detection of deep diving submarines and geophones used to record the response of the earth to acoustic impulses applied thereto in the search for minerals and petroleum. It has been recognized that fiber optics may allow the construction of rugged, high performance microphones and, in particular, the United States Navy has successfully developed an approach based on the comparison of light passed through a reference optical fiber shielded from acoustic noise with light passed through a second signal fiber. This approach is reported in the paper entitled, "Fiber-Optic Hydrophone" by J. A. Bucaro, et al.

Unfortunately, the available devices are orders of magnitude less sensitive than what is desired for some applications. Also, many sensitive acoustic devices have the need, heretofore, unfulfilled, for automatic noise suppression, low cost, and ease of incorporation in acoustic sensor arrays.

In its simplest form, the present invention includes a source of light whose output is split by a beamsplitter and conducted into the opposite ends of a fiber optic coil. Acoustic noise impinging on the fiber coil causes nonreciprocal phase differences between the beams propagating through the fiber optic coil in the counter-clockwise and clockwise directions. The nonreciprocal phase differences are superimposed when the beams are recombined in the beamsplitter. The recombined beam is directed back to the light source. Consequently, the light beam directed back to the light source is modulated at the acoustic frequency of the noise applied to the fiber coil. It is preferable that the light source be a laser diode operating near threshold so that the small amounts of feedback due to back-scattered light will result in large fluctuations in the output of the laser diode. Thus, the laser diode acts as a high gain amplifier of the acoustic signal from the fiber coil. Similar results can be obtained with light sources such as a gas laser or light-emitting diode, although the gain of light-emitting diodes is not as large. A portion of the output of the light source is split by another beamsplitter onto a detector whose electrical output becomes the output of the sensor for display or use.

In more complex embodiments, extra beamsplitters and mirrors can be used to redirect the frequency-modulated light back to the light source more intensely for higher gain. This also can be accomplished by inserting a phase shifter in the light beam that adjusts the relative phase of the counter propagating beams upon emergence from the fiber coil so that the maximum amount of modulated light is directed toward the light source. Hybrid units can be constructed using fiber optic beamsplitters rather than the more conventional type to assure that more light is fed back to the light source. A feedback loop, including a notch or bandstop filter, a signal inverter, and a phase shifter, also can be applied between the detector output and the modulated light beam to filter out noise from the desired signal frequencies. More elaborate versions of this system can be used to suppress all but a specific frequency. It also is possible to use the sensors in an array to obtain more sensitivity or directional information. An alternative embodiment operates the light source at high power levels well above threshold and uses a beamsplitter to direct the return beam from the fiber coil onto the detector. The techniques described above for enhancing the intensity of the return light beam and for noise suppression also apply.

It is therefore an object of the present invention to provide an optical acoustic sensor which can be used as a replacement for microphones on a one-to-one basis.

Another object is to provide an acoustic sensor with high sensitivity, immunity to electromagnetic interference, and the ability to operate under extreme environmental conditions.

Another object is to provide an acoustic sensor which has low power consumption.

Another object is to provide an optical acoustic sensor which can be incorporated conveniently into large arrays.

Another object is to provide an acoustic sensor into which noise suppression can be incorporated easily.

These and other objects and advantages of the present invention will become apparent to those skilled in the art after considering the following detail specification in conjunction with the accompanying drawings wherein :

FIG. 1 is a diagrammatic view of a basic optical acoustic sensor constructed according to the present invention;

FIG. 1A is a diagrammatic view of means to optimize the acoustic sensitivity of a fiber optic coil used in the sensor of FIG. 1 by acoustically isolating one half of the coil;

FIG. 1B is a diagrammatic view of other means to optimize the sensitivity of the fiber optic coil by removing one half of the coil from the sound field;

FIG. 1C is a diagrammatic view showing the effect of an acoustic wave on a fiber optic coil;

FIG. 1D illustrates graphically the effect of acoustic energy on an unshielded fiber optic coil;

FIG. 1E illustrates graphically the effect of shielding a portion of the fiber optic coil;

FIG. 2 is a diagrammatic view of the acoustic sensor of FIG. 1 including means for optimizing performance by increasing the light fed back to the light source;

FIG. 3 is a diagrammatic view of another modification of FIG. 1 employing a phase shifter to optimize feedback into the light source;

FIG. 3A is a diagrammatic view of a modification of FIG. 1 that employs a polarizing element to enhance sensitivity;

FIG. 3B is a diagrammatic view of an intensity maintenance device placed at one end of the fiber optic coil to ensure maximum acoustic signal;

FIG. 4 is a diagrammatic view of a modified embodiment of FIG. 1 employing fiber optic beamsplitters;

FIG. 4A is a diagrammatic view of a Y type fiber optic beamsplitter;

FIG. 4B is a diagrammatic view of an X type fiber optic beamsplitter;

FIG. 5 is a diagrammatic view of a modification of FIG. 1 where the output beamsplitter has been reoriented;

FIG. 6 is a diagrammatic view of a modification of FIG. 4 where the output fiber optic beamsplitter has been reoriented;

FIG. 7 is a diagrammatic representation of a modified embodiment of FIG. 1 including a noise suppression system employing a notch filter;

FIG. 8 is a diagrammatic view of a modified version of FIG. 1 designed to detect a signal of a specific frequency;

FIG. 9 is a diagrammatic view of modified devices of FIG. 1 arranged in an array;

FIG. 10 is a diagrammatic view of an array of optical acoustical sensors similar to those of FIG. 4 arranged in an array;

FIG. 11 is a diagrammatic view of a continuous output scheme for an array of optical acoustic sensors employing correlators and delay lines;

FIG. 12 is a diagrammatic view of a sequential output scheme for optical acoustic sensor arrays constructed according to the present invention; and

FIG. 13 is a diagrammatic view of a continuous output scheme for an array of optical acoustic sensors using frequency modulated fiber optic coils.

Referring to the drawings, more particularly the reference numbers, number 20 in FIG. 1 refers to an acoustic sensor constructed according to the present invention. This sensor 20 includes a light source 22 such as a laser, a laser diode, or a light-emitting diode, which emits a beam of light 24, a portion of which 26 passes through a detector beamsplitter 28 and is split by a second beamsplitter 30 into a clockwise beam 32 and a counter-clockwise beam 34. The beams 32 and 34 are focused by means of suitable lenses 36 and 38 respectively into the opposite ends 40 and 42 of a fiber optic coil 44. Even if the light is not what is generally known as coherent, the ends 40 and 42 can act as spatial filters to produce a coherent characteristic in the beams 32 and 34.

The coil 44 normally is a multi-turn coil. The choice of number of turns depends upon the sensitivity required (more turns, more sensitivity) and the amount of light attenuation that can be accommodated by the length of the optical fiber 45 (more length, more attenuation). The clockwise light beam 32 enters the end 40 of the coil 44 and exits the coil at its end 42 as beam 32a while the beam 34 enters the coil 44 at its end 42 and exits the coil 44 at its end 40 as beam 34a. The beamsplitter 30 then recombines the beams 32a and 34a into a return beam 46. If the fiber optic coil 44 is subjected to acoustic noise, the coil 44 causes nonreciprocal phase differences between the light beams 32a and 34a which are superimposed on the return beam 46 to modulate it. The beam 46 passes through the beamsplitter 28 and is redirected into the light source 22, the beam 46 being modulated at the acoustic frequency seen by the fiber coil 44.

It is preferable that the light source 22 be a laser diode operating near its threshold. When this is true, small amounts of feedback due to the back-scattered light in the beam 46 cause large fluctuations in the output thereof so that the light source 22 acts as a high gain amplifier of the acoustic signal from the fiber coil 44. Similar results can be obtained when the light source 22 is a gas laser or light-emitting diode operated under the same conditions, although in the case of a light-emitting diode, the gain that can be expected is not large.

The light output beam 24 of the light source 22 is split by the beamsplitter 28 as aforesaid so that a portion 48 of the beam 24 is applied to a suitable detector 50, which, in response thereto, produces an electrical signal 52 for transmission to an oscilloscope or other readout device 54.

In order to optimize the sensitivity of the optical acoustic sensor 20, it is necessary to isolate one half 44a of the fiber coil 44 from acoustic signals and sensitize the other half 44b. This may be accomplished by jacketing one half 44a of the fiber 45 starting at one end, as in FIG. 1A, or moving one half 44c of the fiber coil 44 out of the sound field, such as by placing it in an acoustic isolation chamber 44d as shown in FIG. 1B.

To understand why this procedure enhances sensitivity, consider for example, that the speed of sound in water is 1500 m/sec, and thus at 1.5 KHZ the wave is one meter in length. Even at 15 KHZ, which is beginning to approach the upper range of the acoustic frequencies normally of interest, the wavelength of the sound is 10 cm, which is long compared to the fiber coil height that usually ranges from 0.1 mm to 1 cm. Thus the wave can be thought of as an instantanious linear pressure gradient across the fiber coil 44 as shown in FIG. 1C for times t.sub.1, and t.sub.2. The pressure difference seen by the point (A) of the coil between the cw and ccw light beams 32 and 34 is given by P.sub.A =P.sub.cw (t.sub.1)-P.sub.ccw (t.sub.2) where the cw beam 32 arrives at t.sub.1 and the ccw beam 34 at t.sub.2. Now on the other half of the coil 44 there is a corresponding point (B) such that the ccw beam 34 arrives at t.sub.1, and the cw beam 32 at t.sub.2, and P.sub.B =P.sub.ccw (t.sub.1)-P.sub.cw (t.sub.2). The center point (C) remains stationary because it takes the beams 32 and 34 the same time to reach (C) no matter what the applied pressure. In the absence of acoustic shielding for this situation, P.sub.A +P.sub.B =0, and there is no net phase difference between the two beams. FIGS. 1D and 1E plot the relative phase shift induced by a pressure gradient across a coil of length L with and without acoustic insulation over one half the coil, the center of the coil 44 being considered position zero. From FIG. 1E it is apparent that the largest net phase shift between the cw and ccw beams 32 and 34 occurs when one half of the coil is shielded starting at one end, i.e., P.sub.B =0 so that P.sub.A +P.sub.B =P.sub.A.

The embodiment as shown in FIG. 1 does not couple all the light circulating through the fiber coil 44 back to the light source 22, whereas FIGS. 2, 3, and 4 show modified devices for optimizing the light feedback.

In the sensor 55, shown in FIG. 2, a third beamsplitter 56 and two mirrors 58 and 60 are added to redirect light 62 from the beamsplitter 30 that would otherwise not reach the light source 22. This is accomplished by providing the mirrors 58 and 60 in positions to reflect the light 62 back to the beamsplitter 56 wherein the light 62 is combined into the beam 46.

In FIG. 3, the sensor 63 includes a phase shifter 64 positioned between the second beamsplitter 30 and the lens 36. The phase shifter 64 adjusts the relative phase of the counterpropagating beams 32 and 34 upon emergence from the fiber coil 44 so that the maximum amount of modulated light power is directed toward the light source 22.

The exact nature of the phase shifter 64 depends upon the type of single-mode optical fiber 45 employed. If the fiber 45 is polarization preserving, a single drop off due to changing birefringence of the fiber 45 is not a serious problem, and a simple phase shifter 64 is adequate. However, conventional single-mode fiber is subject to birefringence noise, that is, the relative state of polarization of the beams 34a and 32b may be altered due to temperature and pressure effects on the fiber coil 44. This phenomenon in itself may be used to enhance the magnitude of the returning acoustic signal impressed on the light beam. That is, since the relative polarization state of the two beams changes at a rate proportional to the acoustic frequency of the impinging pressure wave, the depth of modulation can be enhanced by defining a particular state of polarization by inserting a polarizing element 65 in the beam path, as shown with sensor 63a of FIG. 3A. In particular, the light beam 34 enters the fiber coil 44 in a polarization state defined by the polarizing element 65. The action of an acoustic wave impinging on the fiber optic coil 44 is to rotate the state of polarization of the circulating beam 34 so that when it leaves the coil 44 as beam 34a, it does not have the same polarization state as beam 34. Consequently, when beam 34a passes back through the polarizing element 65, it is attenuated by an amount dependent upon the degree of polarization state rotation between beams 34 and 34a, which in turn is proportional to the acoustic frequency. The degree of polarization rotation can be controlled by using a sequence of elements that exert force on the fiber, changing the birefringence locally and causing the state of polarization to rotate to the desired position. Such a system is shown in FIG. 3B where two pairs of elements 66a and 66b exerting compressive force on the fiber 45 are aligned at 45.degree. with respect to each other. In this manner two adjustable fiber optic "waveplates" are established whose retardance can be controlled by adjusting the degree of compression. In particular the compression can be adjusted to move an incoming state of polarization toward the desired outgoing polarization state.

In FIG. 4, a further modified version 67 is employed in which the beamsplitters 28 and 30 of FIG. 3 as well as the lenses 36 and 38 are eliminated by providing fiber optic beamsplitters 68 and 70 as substitutes for the beamsplitters 28 and 30 respectively. The construction of such beamsplitters 68 or 70 is shown in FIGS. 4A and 4B wherein the fiber 72 with its cladding 74 is bifurcated into a fused Y or X so that the fiber 72 continues as two fibers 76 and 78. Light passing from fiber 72 past the juncture 80 of the fibers 76 and 78 is split so that about half goes through fiber 76 and the rest goes through fiber 78. When operating in the other direction, light from fibers 76 and 78 combine at the juncture 80 to pass in the opposite direction on fiber 72. Although such fiber optic beamsplitters are difficult to construct, they ensure a minimum of light loss due to splitting and recombination of the light beams. Otherwise, the sensor 67 operates identically to the sensor 63.

The orientation of the beamsplitter 28 or 68 of FIGS. 1 through 4 may be changed as in the beamsplitters 28a and 68a of FIGS. 5 and 6 respectively. These configurations are useful as the light source 22 is isolated from back reflections by an element that may be a Faraday rotator. For these cases, the light source 22, which may be a laser or laser diode, can operate well above threshold, increasing the amount of light introduced into the system 20a or 67a and the intensity of the return signal 48, although the light source 22 would not act as a gain element. Therefore, it should be understood that the beamsplitters 28 and 68 or 28a or 68a may be oriented to receive an output signal from the light source 22 as in FIGS. 1 through 4, or from the fiber coil 44 as in FIGS. 5 and 6 and in each of the embodiments and arrays described below.

In FIG. 7 a noise suppression system 82 has been added to the sensor 63 of FIG. 3, although the sensor 67 of FIG. 4, 20a of FIG. 5, or 67a of FIG. 6, could just as well be shown. The noise suppression system 82 includes a notch or bandstop filter 84 to which is fed the output 52 of the detector 50. The notch or bandstop filter 84 is constructed to block out the desired signal frequencies while letting noise pass from its output 86 to a signal inverter 88 where the noise signal is inverted and fed via the output 90 of the signal inverter 88 to the phase shifter 64. Since the noise is inverted in the signal inverter 88, the noise is nulled out in the phase shifter 64.

A somewhat more complex noise suppression system 92 is shown in FIG. 8 in association with sensor 63. The noise suppression system 92 is particularly useful when it is desired to look for a particular acoustic frequency and detect the amplitude of that signal. For that reason, the output 52 from the detector 50, in addition to being fed to the output device 54, is also fed to a correlator 94 which is used to maximize signal strength at the desired acoustic frequency by delay. The correlator 94 also is connected to one output 96 of an acoustic frequency generator 98 which is chosen to generate the acoustic frequency or frequencies, if more than one frequency is to be sampled, which is to be detected. The signal on the output 96 from the generator 98 is compared to the output signal 52 of the detector 50 in the correlator 94 which produces an output signal on line 100. This compared output signal is fed to a mixer 102. The mixer 102 also receives the signal from the second output 104 of the generator 98 after it has passed through a delay line 106 so that the original signal level of the desired frequency is restored at the output 108 of the mixer 102 for feeding to a signal inverter 110. The delay line 106 is used to compensate for delays caused within the correlator 94 so that phasing problems are not present at the mixer 102. The output of the detector 50 also is passed through a second delay line 112 similar to delay line 106 and from there to the signal inverter 110 where the signal 52 as delayed by the delay line 112 and as processed through the correlator 94 and mixer 102 is subtracted from the signal on the output 111 of the delay line 112. This causes the signal on the output 113 of the signal inverter 110 to be the same as the output of the detector 50 except for the frequency of interest. Therefore, when the output signal of the signal inverter 110 is applied to the phase shifter 64, the phase shifter 64 nulls out all but the desired frequency by the nulling action of the system 92 operating closed loop.

It is also possible to extend the sensors 20, 55, 63, with or without noise suppression systems such as systems 82 and 92 into array systems 114 as shown in FIG. 9, or 115 as shown in FIG. 10. In FIG. 9, a second fiber coil 44', having identical components as the first fiber coil 44 numbered with a prime (') is shown as representative of the array. The array system 114 has in addition third beamsplitters 116 and 116' positioned between the second beamsplitter 30 or 30' and the lens 36 or 36'. The beamsplitter 116 feeds a portion of the light present in beam 32 via a mirror 118 as the input beam 26' to the beamsplitter 30' of the coil 44'. The coil 44' operates on the beam 26', as does the coil 44 on beam 26, to generate a return beam 46' modulated in accordance with acoustic energy being applied to the beam coil 44'. As can be seen, this can be continued through the use of beamsplitter 116' and mirror 118' so that the serial array system 114 is created. A similar serial array also can be created using multiple sensors 67 as shown in FIG. 10 wherein multiple coils 44, 44', and 44" are connected via fiber optic beamsplitters 70, 70', 70" and so on, are positioned in serial fashion with the beamsplitters 120, 120', and 120" corresponding to beamsplitters 116, 116' and mirrors 118 and 118' shown in FIG. 9. The systems of FIGS. 9 and 10 can be operated in a number of ways, perhaps the simplest involving pulsing the light source 22, which may be a laser diode operating near threshold. The pulses are applied periodically at high power levels. The detector 50 is electronically gated by the timer 121 so that it receives acoustic signals from each sensor coil, 44, 44', 44" and so on in turn for processing.

Another array system 122 is shown in FIG. 11 where the arrays of FIG. 9 or 10 can be used and the light source 22 is operated continuously. The output from the fiber coils 44, 44', and 44" is fed to correlators 123, 124, and 126. Delay lines 128, 130, and 132 are placed between the correlators 123, 124 and 126 and a reference generator 133 to maximize the signal from each fiber optic coil and so that each acoustic sensor coil output may be monitored continuously. A variation 134 is shown in FIG. 12 where an auto correlator consisting of a correlator 136 and a variable delay line 138 is used instead of the correlators 123, 124, or 126 and their associated delay lines 128, 130, and 132. The arrangement 134 shown in FIG. 12 allows each acoustic sensor coil 44, 44' and 44" to be sampled in sequence.

A third array scheme 140 is shown in FIG. 13. Compression, strain, or bending phase shifter elements 142, 144 and 146 are driven by drivers 148, 150 and 152 so they each exert a force on a portion of each fiber optic coil 44, 44', 44" at frequencies F.sub.i, i=1, 2, 3, . . . where the F.sub.i are chosen so that they do not have related harmonics. In essence this imposes a different carrier frequency on the output of each coil 44, 44', 44". The acoustic output of each fiber coil 44, 44' and 44" is monitored continuously through the use of bandpass filters 154, 156 and 158 tuned to be selective to the carrier frequency of the associated coil. After passing through the correct bandpass filter, the signal is sent to an associated mixer 160, 162 or 164 which mix the outputs of the associated driver and filter to restore the original acoustic signal on outputs 166, 168 and 170.

Therefore, there has been shown and described novel acoustic sensors which fulfill all the objects and advantages sought therefore. Many changes, modifications, variations, other uses and applications of the subject invention will, however, become apparent with those familiar in the art after considering the foregoing specification together with the accompanying drawings. All such changes, modifications, variations, and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention, which is limited only by the claims which follow.