Multimode fiber-optic hydrophone based on aschlieren technique
W. B. Spillman, Jr.
A multimode fiber-optic hydrophone is described which is based upon a schlieren acoustooptic intensitymodulation mechanism. Computer modeling of critical device parameters was experimentally verified andused to indicate ultimate attainable device performance. The device was shown to be able to detect theKnudsen noise level for frequencies up to 1 kHz, to have a dynamic range of 125 dB, to have an omnidirec-tional receiving response, and to be able to detect displacements as small as 3.4 X 0-3 A. The device is notsusceptible to phase noise, is relatively insensitive to static pressure head variations and is electrically pas-sive.
1. Introduction
During the past few years, considerable effort hasbeen expended to utilize the rapidly developing field offiber optics to enhance the technology involved inacoustic sensing. This has meant not only the use offiber-optic data links to return information from thesensors to the data processing point, but also the de-velopment of electrically passive sensors which containtransduction mechanisms that directly convert acousticwaves into modulation of the light passing through anoptical fiber. Previous fiber-optic acoustic sensor workfalls into two categories, the first of which is based upona single-mode interferometric approach- 4 in whichacoustic waves induce phase variations in the lightpassing through a single-mode fiber. However, tem-perature and pressure ambients create a severe phasenoise problem in devices of this type. Also, since in-creasing the fiber-optic path length of the interferom-eter arms causes a concomitant increase in sensitivityto fluctuations in ambient conditions, no increase in theobtainable SNR occurs beyond lengths of some fixedvalue.5 Attempts have been made to circumventthe phase noise problem with varying degrees ofsuccess. 6 -8
The author is with Sperry Research Center, Sudbury, Massachu-setts 01776.
The second category of fiber-optic acoustic sensorwork includes all devices which utilize intensity mod-ulation acoustooptic transduction mechanisms. Thesensitivity of the interferometric approach to drift andphase noise due to environmental ambients and therelatively primitive state of single-mode fiber-optictechnology have forced a reevaluation of intensitymodulation mechanisms. A number of approacheshave been proposed, and encouraging progress has beenreported. 9 -1 1
This paper presents a detailed analysis of a multi-mode fiber-optic acoustic sensor based upon a schlierenacoustooptic transduction mechanism and representsan extension of results previously published.12 Thedevice has already been shown to be sensitive andcompatible with the multimode fiber-optic technologythat presently exists. The present work includes acomputer modeling of critical device parameters anda comparison of those results with experimental dataobtained from a particular device. The computermodeling then allows the ultimate performance of de-vices of this type to be determined.
11. Theory
In this section, the basic principle of operation of thedevice are described and critical parameters identified.Optical loss in the device as a function of these param-eters are then determined via computer modeling.
A. Device Description
A schematic of the device as constructed is shown inFig. 1. Light from one multimode optical fiber is col-limated by a graded-index rod microlens (GRINrod)and passed through an opposed grating structure after
coaxial and that the fibers were butt-coupled to theGRINrod end surfaces with no loss. The GRINrods
ENS were assumed to be standard SLS Selfoc1 3 microlenseswith a 2-mm diameter. A sample of 441 rays, approx-
CAL imating a completely filled fiber N.A., was traced fromFt input to output fiber for each value of GRINrod-
LIGHT IN GRINrod separation. The angular range of the inputLE rays was -2 TN.A. < T < + 2 TN.A., where TN.A. is the
angle corresponding to the fiber N.A. Each ray wasESSURE given a weight W according to.E
W = IyI exp[-2.3(T2/T2 A)] (graded-index fiber), (1)
Fig. 1. Schematic diagram of schlieren multimode fiber-optichydrophone.
WATER IS ASSUMED TO BE/THE MEDIUM BETWEEN THE
4 GRINRODS
3 0.25N.A.0.4 N.A. ST E
° 2 - Op. CORE,/X 2 0.2 N.A., GRADED
00 1 2 3 4 5 6 7 8
G RINROD-GRINROD SEPARATION (cml
Fig. 2. Calculated coupling loss as a function of GRINrod-GRINrodseparation for various multimode fiber types with water assumed
between the GRINrods.
which it is focused down onto an output fiber by a sec-ond microlens. The opposed grating structure is cou-pled to two flexible diaphragms which are attached tothe top and bottom of the hydrophone housing. De-flection of the diaphragms by ac pressure changes causesrelative motion of the two gratings and thereby modu-lates the amount of light coupled between input andoutput fibers. Critical parameters are the GRINrod-GRINrod spearation x, the relative grating displace-ment x, and the separation of grating planes d. Thegrating period D used in the device structure was 10 Aum.Optical throughput loss in the device can be describedas the product of three factors: (1) the fractionalamount of light which is initially in the input fiber whichis coupled to the output fiber with the grating structureremoved (a function of x,); (2) the fractional amount oflight incident upon the grating structure which istransmitted through it (a function of x and d); and (3)the fractional amount of light transmitted through thegrating structure in the zeroth diffraction order relativeto the total amount of transmitted light (a function ofx and d).
B. GRINrod-GRINrod Coupling Loss
A computer model was developed using the matrixdescription of GRINrod properties developed by theNippon Sheet Glass Co., Ltd.1 4 This model determinedthe fiber-GRINrod-GRINrod-fiber coupling loss as afunction of GRINrod-GRINrod separation x. Themodel assumed that the fibers and GRINrods were all
or
W y ITI TN.A. (step-index fiber),W=0 IT I > TN.
(2)
where y is the distance between the GRINrod axis andthe point where the ray enters the GRINrod.
The results of this calculation are shown in Fig. 2 forvarious multimode fiber types: 200-,um core; 0.4-N.A.step-index fiber; 100-,im core; 0.3-N.A. step-index fiber;and 50-,um core; 0.2-N.A. graded-index fiber. As canbe seen, the smaller core diameter fibers allow for cou-pling losses of <0.5 dB for GRINrod-GRINrod sepa-rations of up to 2 cm if an index-matching medium isused between the GRINrods.
C. Computer Model
The total fractional intensity of light transmittedthrough the grating structure Ig(x,d), as a function ofrelative grating displacement and grating plane sepa-ration, was calculated using a 2-D computer model inwhich each transparent stripe was approximated by aseries of eleven-point sources equally spaced 0.5 mapart. Each series of point sources was separated by5 m. The intensities reaching the second grating weredetermined at 0.25-Am intervals and summed over the5-Mm open stripe width. The calculations were carriedout for relative grating displacements from 0 to 14 ,umand for grating plane separations of 0.1, 0.3, 0.5, 0.7, 1.0,1.5, and 2.0 m. Interference effects were not consid-ered in modeling the total light throughput of the op-posed grating structure. Figure 3 shows the modulationdepth obtainable by relative grating displacement as a
100
Z50
25__
.5 t.0 1.5 2.0
SEPARATION OF GRATING PLANES (m)
Fig. 3. Calculated opposed grating structure modulation depth asa function of grating plane separation.
function of grating plane separation. As can be seen,the attainable modulation depth decreases as a functionof grating plane separation, i.e., even with completegrating overlap, some light passes through the struc-ture.
For an observer looking at the grating structure in thefar-field region of the transmitted radiation, it wouldseem that perfect grating overlap is never obtained,since some light always leaks through the structure.This behavior may be modeled by assuming that thetransparent stripe width varies from a 0.5 grating periodat zero relative grating displacement down to somesmaller finite minimum value at a relative grating dis-placement equal to 0.5 grating period. Considerationof this point is important, as it bears directly upon thefraction of the light transmitted through the gratingstructure in the zeroth diffraction order relative to thetotal amount of light transmitted through the opposedgrating structure.
For small grating periodicities, only the light in thezeroth order will be captured by the output fiber. Theratio of light in the zeroth order to the total amount oftransmitted light from a transmission diffractiongrating of period D and transparent stripe width w isgiven by 15
I(O)/IT = w/D (3)
To use this expression with the opposed grating struc-ture and to obtain a value for it in terms of relativegrating displacement x and the separation of gratingplanes d, the apparent width of the transparent stripeswhen viewed from the far-field region was assumed tovary approximately as the total transmitted intensity,i.e.,
where Md is the modulation depth of the total amountof the light transmitted through the opposed gratingstructure obtainable by relative grating displacement.By inserting Eq. (4) in Eq. (3), Eq. (3) may be redefinedas
S(x,d) = w(x,d)/D
consists of three types of subunit: ( two fiber-fer-rule-GRINrod structures; (2) one hydrophone housingwith rubber diaphragms; and (3) one opposed gratingstructure. Use of these three subunits allowed for finalassembly of the optical hydrophone without the needfor close assembly tolerances. The fiber used was Du-pont PiFax S120 type 30 200-,um core plastic-clad op-tical fiber with a 0.4 N.A. Standard pressure reliefmechanisms were omitted in this experimental model,which is a free-flooded design. As the final step beforetesting, the interior of the hydrophone was filled withdistilled water through the pressure relief hole; 1.2 mWof light power from a Hughes model 3222H-PC He-Nelaser at 0.63 Am was coupled into the input fiber usinga 1oX microscope objective. The light leaving theoutput fiber was detected by an RCA C30808 photo-diode with a 200-kQ load resistor.
Four separate types of measurement were performed:(1) The GRINrod-GRINrod coupling loss was deter-mined as a function of GRINrod-GRINrod separation.(2) Relative light power coupled through the opposedgrating structure as a function of static displacementwas determined prior to inserting it into the hydrophonehousing. (3) The actual sensitivity of the device toacoustic waves in water was determined using a NavalResearch Laboratory type G19 hydrophone calibrator.(4) Finally the directionality of the device receivingresponse was determined.
IV. Results
In this section, the results of device testing are pre-sented. These results are then used to determine howwell the computer modeling previously described agreeswith actual device performance.
(5)
The total intensity transmitted through the structurein the zeroth order as a function of relative grating dis-placement and separation of grating planes is then givenby
I(x,d) = Iogg(x,d)S(x,d), (6)
where Io is the light intensity incident upon the opposedgrating structure. The computer model results fortransmitted intensity as a function of relative gratingdisplacement and grating plane separation are shownin Fig. 4.
Ill. Experimental: Device Fabrication and Testing
In this section, the process of device fabrication isdiscussed, and various aspects of device testing are de-scribed.
A description of the device fabrication has been givenin a previous paper. 1 2 As shown in Fig. 1, the device
0
'C0
C
0 2 4 6 8 10
RELATIVE GRATING DISPLACEMENT (MICRONS)
Fig. 4. Fractional light power transmitted through the opposedgrating structure in the zeroth diffraction order as a function of rel-
ative grating displacement and grating plane separation.
Before the two fiber-ferrule-GRINrod units are in-serted into the hydrophone housing, the optical cou-pling loss between them in air was determined as afunction of separation for separations ranging from 0to 7 cm. These results are shown in Fig. 5 together withvarious computer model predictions. Curve (b) indi-cates the predicted behavior of the coupling using per-fect quarter pitch GRINrod lenses, and curve (a) showsthe predicted behavior assuming 0.254 pitch lenses and1-dB excess loss. The 1-dB excess loss may be attrib-uted to scattering and small misalignments in the regionwhere the GRINrods were bonded to the optical fiber.As can be seen, with this loss component added in, curve(a) agrees very well with the experimental data. Theassumed pitch of 0.254 lies within the stated tolerancesof the manufacturer for quarter pitch Selfoc lenses.
When the fiber-ferrule-GRINrod units were per-manently bonded into the hydrophone housing at aseparation of 1.5 cm, the coupling loss was found to be3.1 dB. This agrees with the results shown in Fig. andindicates that no additional loss was incurred in matingthe fiber-ferrule-GRINrod structures to the housing.Additional optical loss was incurred due to use of theplastic-clad optical fiber. During the process of pol-ishing the fibers within their ferrules, the cladding wasfound to separate from the fiber core near the fiber endsurface, and optical loss was introduced. The addi-tional loss in this case was 4.0 dB. The total insertionloss of the device with the grating structure removed wasthen 7.1 dB.
B. Opposed Grating Structure Optical ThroughputBefore the opposed grating structure was inserted
into the hydrophone housing, its optical throughput asa function of static displacement was determined.Light from a Hughes 2-mW He-Ne laser was passed
a
120
0 1 2 3 4 5 6 7 8
GRINROD-GRINROD SEPARATION (cm)
Fig. 5. Coupling loss as a function of GRINrod-GRINrod separation,comparison of computer model predictions with experimental data:(a) calculated loss assuming 0.254 pitch GRINrods and 1-dB excessloss; (b) calculated loss assuming perfect quarter pitch GRINrods and
no excess loss.
.20
CD
I
2
CD'
<S
I:
u.
.15
.10
.05
0 2 4 6 8 10
RELATIVE GRATING DISPLACEMENT (MICRONS)
Fig. 6. Functional light power transmitted through the opposedgrating structure in the zeroth diffraction order as a function of rel-ative grating displacement and comparison of computer model pre-
dictions with experimental data.
through the opposed grating structure at normal inci-dence, and the amount of light in the zeroth order rel-ative to the incident optical intensity was determinedas a function of static relative grating displacement.The results of these measurements are shown in Fig. 6where it is seen that the experimentally determined datacorrespond quite well to computer model predictionsif the two grating planes are assumed to be separatedby 0.7-1.0 m.
C. Minimum Detectable DisplacementAfter the hydrophone assembly, with the diaphragm
tension adjusted to put the opposed grating structureat the 50% bias point, the light intensity leaving theoutput fiber was found to be 19 MmW. In the shot noiselimit, the minimum detectable optical power may bedetermined using standard expressions16 and the statedcharacteristics of the photodiode detector. In this case,Pm = 3.4 X 10-12 W. From Fig. 6, the quantityR, de-fined as
R = -L[dI/dx)] at 50% bias point, (7)
may be determined. In this instance, R = 4.8 X 105m-1. By equating I with the power striking the pho-todiode and Pm with d the minimum detectable dis-placement may be determined. For this particulardevice, xmin = 3.4 X 10-3 A with 19 MW of detectedpower.
D. Minimum Detectable Pressure
To determine the minimum detectable pressure ofthe hydrophone, it was flooded with distilled water andimmersed in water in a NRL G19 hydrophone calibratorand subjected to acoustic waves of known intensity from100 Hz to 5.5 kHz. A calibrated Gould CH-17UT
Fig. 7. Minimum detectable pressure as a function of frequency,measured behavior, and shot noise limited behavior.
U ~~~~~~~~(a) COUSTI ITENSITY
+170 dB RE 1 pPa
SIGNAL LEVEL = +5 dBVNOISE LEVEL = -120 dBV
UIIEUIIEEUE II ~~(b) COUSTIC INTENSITY =-XilllW -fln +174 dB RE1 pla
Fig. 8. (a) Optical signal produced by an acoustic intensity of +170dB re 1 AtPa. (b) Optical signal produced by an acoustic intensity of
+174 dB re 1 ytPa.
electrical hydrophone was used to determine theacoustic intensities. The SNR from the detector wasmeasured by a Hewlett-Packard 3580A spectrum ana-lyzer. The minimum detectable pressure (SNR = 1)was then determined from the known acoustic pressure.These results are displayed in Fig. 7. Also shown is thecalculated minimum detectable pressure using themeasured signal level and assuming the noise level tobe equivalent to the photodetector shot noise. Mea-surements on the laser source indicated that, within themeasurement accuracy of the spectrum analyzer, it wasessentially shot noise limited for frequencies above 500Hz. As can be seen, the device is sensitive enough todetect sea state zero or below for frequencies up to 1 kHzand appears to be shot noise limited above 500 Hz. Theactual device minimum detectable pressure below 500Hz is probably lower than shown, but the true valuecould not be accurately determined because the domi-nant noise level in that frequency range is the noise in-herent in the spectrum analyzer.
The device sensitivity to ambient conditions in thelaboratory appeared minimal. No elaborate isolationprocedures and/or complicated detection schemes hadto be used to obtain the best device sensitivity. Indeed,after applying the correct tension to the diaphragms toobtain the maximum sensitivity bias point, no changein device sensitivity was observed over the period ofseveral weeks during which measurements weremade.
E. Dynamic Range
To determine the dynamic range of the device, it wasplaced under water in the NRL G19 calibrator anddriven to the point where the signal began to distort.The SNR was measured at that point and identifiedwith the device dynamic range. This process is dis-played in Fig. 8. Trace (a) shows the signal output atjust below the distortion level, which occurred for anacoustic pressure of 170 dB re 1 MPa. Trace (b) showsthe result of overdriving the device. The driving fre-quency was 1 kHz. In this case, the SNR obtained fromthe output shown in trace (a) was 125 dB, which is thenthe dynamic range of this particular device.
F. Directionality of Device Response
The directionality of response of the device was de-termined by placing it under water in a test tank at aconstant distance from a low level acoustic source ra-diating acoustic power at a frequency of 1300 Hz. Thedevice was rotated in the acoustic field about an axiscoplanar with the rubber diaphragms and passingthrough the center of the device. The signal level wasdetermined for several angular orientations of the deviceabout this axis. Figure 9 shows the determined re-ceiving response in dB re 1 V/MPa as a function of angle.The average value of this response over the anglesmeasured was 178.4 ± 1.6 dB re 1 V/MPa, which indi-cates the essential omnidirectionality of the device re-sponse.
* ~~~~~~~~~~0.
\-120 -140 -160 -180 -200
180°
Fig. 9. Device receiving response in dB re 1 V//iPa as a functionangular orientation.
In general, as seen in Sec. IV, there is a good corre-spondence between the computer modeling of deviceproperties and the experimentally determined values.This allows the use of the computer modeling to deter-mine the optimum performance that could be expectedfrom this hydrophone design.
The total insertion loss for this device was -18 dB,which can be broken into three distinct components:(1) a 4.0-dB loss from the fibers used; (2) a 3.1-dB lossfrom the GRINrod-GRINrod coupling; and (3) an11.0-dB loss incurred in passing through the opposedgrating structure. The use of glass-on-glass opticalfiber instead of the plastic-clad fiber presently usedshould reduce the fiber loss from 4.0 dB to <0.5 dB. Asshown in Fig. 2, by proper choice of fiber, the GRIN-rod-GRINrod coupling loss can be reduced from 3.1 to0.4 dB or less for a separation of 1.5 cm. Finally, byreducing the grating plane separation to 0. m throughthe use of optically polished substrate materials, the lossincurred, as shown in Fig. 4, in passing through the op-posed grating structure with a 50% bias can be reducedto 9.5 dB. These considerations indicate that the op-tical throughput of this device can be improved by 7.6dB. In the shot noise limit, this improvement in opticalthroughput translates into a 3.8-dB (optical) improve-ment in SNR or a 7.6-dB decrease in the minimum de-tectable pressure. In addition, as may also be seen fromFig. 4, the response of the device to static displacementschanges from R = 4.8 X 105 m-1 for 0.7 Mm < d < 1.umto R = 5.1 X 105 m-1 at d = 0.1 m or a 0.26-dB (optical)improvement in sensitivity. This translates into a0.5-dB decrease in the minimum detectable pressure.The optimum sensitivity for this device configurationin the shot noise limit is then 8.1 dB below the shot noiselimit curve shown in Fig. 7, which would place it wellbelow sea state zero for frequencies up to 1 kHz. Theseimprovements can be incorporated into the presentdesign by more careful selection of materials and closerassembly tolerances.
Other improvements, which would require modifi-cations to this design, could reduce the minimum de-tectable pressure even further. For example, throughthe use of well-known techniques of pressure amplifi-cation from standard electrical hydrophone technology,reductions of as much as 20 dB could be obtained in theminimum detectable pressure. Reduction of thegrating stripe width from 5 m to some smaller valuewould also improve the sensitivity of the device, al-though the precise nature of the improvement is notclear, since interference effects would become more andmore important as the width was reduced, and thecurrent computer modeling process would become lessand less valid. Finally, the use of shot noise limitedoptical sources which could couple more power into thedevice would also result in a reduction of the minimumdetectable pressure.
It should be emphasized that the device describedhere is merely an experimental test vehicle for use in thelaboratory. Advanced engineering development of thismultimode optical hydrophone will be required to de-
termine and minimize the effects of environmentalambients before it is suitable for incorporation intooperational hydrophone arrays. The results to date are,however, encouraging.
VI. Summary and Conclusions
A multimode fiber-optic hydrophone has been de-scribed that is sensitive enough to detect sea state zerofor frequencies up to 1 kHz. Computer modeling ofcritical device parameters has been shown to be insubstantial agreement with experimentally determinedvalues. Extrapolation of device performance using theresults of the computer modeling indicates that thesensitivity of the device can be improved by -8 dB bya more careful selection of materials and closer fabri-cation tolerances. Modification of the design, to in-corporate standard pressure amplification techniquestogether with the use of a higher power shot noise lim-ited optical source, should improve device sensitivitya further 20 dB or more. The device is electricallypassive and responds to ac variations in pressure. It isrelatively insensitive to static pressure head due to itsfree-flooded design and will function over large staticpressure ranges using simple pressure relief mecha-nisms. Finally, ease of construction due to the use ofmodular components that are assembled separately,compatibility with currently existing multimodefiber-optic technology, sensitivity, smoothness of re-sponse, and potential low cost make devices of this typeattractive for incorporation into practical optical hy-drophone arrays.
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