Photonic Hydrophones Based on Coated Fiber Bragg Gratings.

Optoelectronic Division, Engineering Department 

University of Sannio, Benevento (Italy) 

SOCIETÀ ITALIANA DI FISICA 

XCVIII CONGRESSO NAZIONALE 

Napoli, 17-21 Settembre 2012 

<strong>Photonic</strong> <strong>Hydrophones</strong> 

based on Coated Fiber Bragg Gratings 

M. Pisco, M. Moccia, M. Consales, V. Galdi, 

A. Cutolo, A. Cusano 

Optoelectronics Division, Engineering Department, 

University of Sannio, Benevento, Italy. 

A. Iadicicco 

Department for Technologies, University Parthenope, Napoli, Italy 

S. Passaro, E. Marsella, S. Mazzola 

Istituto per l'Ambiente Marino Costiero, CNR, Napoli, Italy



Research Project “ASSO” 

Supported by Italian Ministry of University and Research 

Funding : 7 M€ 

Duration: 3 years 

In collaboration with: 




Aim: To get high-performance opto-acustic antennas, based on Fiber Bragg Grating (FBG) 

technology, for military, environmental and industrial underwater applications



Traditional hydrophones 

Electromagnetic sensitivity 

Water seepages 

Heavy and bulky 




Comparison with Traditional <strong>Hydrophones</strong> 

Fiber optic hydrophones 

Immunity to Electromagnetic Interferences 

Light and small 

No electric connections 

Remote monitoring 

Multiplexing capability 

Marine environmental monitoring 

Medical 

Military applications


Interferometric detection 

J.A. Bucaro et al.(1977) 

Intensity modulation 

W.B. Spillman et al.(1980) 





<strong>Photonic</strong> hydrophones: State of the art 

DFB Laser 

S.Goodman et al.(2009) 

Very high sensitivities 

Complex configuration 

• Acoustic detectors based on bare FBGs 

Cheap and well assessed technology 

Limited by the high Young’s modulus of glass 

LOD ~ MPa 

<strong>Photonic</strong> crystal mirror 

O.Kilic et al.(2007) 

N.Takahashi et al., Opt. Rev. 4, 691-694 (1997)



Fiber Bragg Grating (FBG) 

The maximum reflectivity occurs at the wavelength that matches the Bragg condition: 




A Fiber Bragg Grating is a longitudinal periodic variation of the refractive index in the core of an optical fiber 

where n eff0 is the effective refractive index of the guided mode and Λ 0 is the FBG period 

Any effect able to modify the physical and geometrical features of the grating can lead to a shift in the Bragg wavelength 

A strain variation shifts the Bragg wavelength through dilating or compressing the grating and changing 

the effective index (via the elasto-optic effect) 

2 

 

neff 

z p11xp12zy, 2 

0


ACOUSTICAL PLANE 

WAVE 


WATER 

2 

1.8 

1.6 

1.4 

1.2 

1 

0.8 

0.6 

0.4 

0.2 

0 

1546 1546.5 1547 1547.5 1548 1548.5 1549 1549.5 1550 

-0.05 

1546.5 1547 1547.5 1548 1548.5 1549 1549.5 1550 1550.5 1551 1551.5 

FBG 

COATING 

0.3 

0.25 

0.2 

0.15 

0.1 

0.05 

0 

-0.05 

1546.5 1547 1547.5 1548 1548.5 1549 1549.5 1550 1550.5 1551 1551.5 




Dynamic Analysis (Acoustic Wave Detection) 

• COMSOL Multiphysics is used as FEM solver 

The 3D geometry is composed by an inner cylinder 

(fiber), an outer cylinder (coating) and a sphere 

(truncated water domain) 

• The acoustical frequency range is 0.5 – 30 kHz 

0.3 

0.25 

0.2 

0.15 

0.1 

0.05 

0 

The Project Idea 

Acoustic wave detection using coated FBGs 

The optical hydrophone is based on standard FBG 

coated by a ring shaped overlay 

y 

z 

x 

h 

2*R w 

FEM geometry 

y 

z 

x 

2*R f 

2*R C



2 

 

1 neff 

S 

z 11 x 12 z y 

P0 

P0 

 

2 



Numerical Results: Resonant Behavior 

p p 

 

Pressure distribution in water and 

z-strain distribution on the cylinder surface 

(i.e. @ 20kHz) 

Sensitivity Gain [dB] 

120 

100 

80 

60 

40 

 

 

 

Sensitivity Gain 20* 

log 

 

 

E= 78 MPa 

n=0.3 

Sensitivity gain 

h= 4 cm ; R C /R f = 20 

20 

0 3 6 9 12 15 18 21 24 27 30 

Frequency [kHz] 


The coated FBG responds to the impinging acoustic plane wave through a mechanical deformation. 

The resulting strain at the FBG location determines a Bragg wavelength shift 

Resonances are associated to resonant longitudinal modes of the cylindrical hydrophones 

S 

SBARE 

 

 

 

 

5.7 kHz 

SBARE 

10.8 kHz 18.7 kHz 

15 kHz 

-6 -1 

-2.7610 

MPa 

Resonant modes 

AWARD: Optical Fiber Sensors 21, Ottawa (Canada) M.Moccia et al., Opt. Express 19 (20), 2011 

21.9 kHz 

25 kHz 

27.9 kHz



120 

100 

120 

100 

80 

60 

40 

80 

60 


120 

100 

80 

60 



R /R 

C R = 20 

C f /R = 20 

f 

40 

0 3 6 9 12 15 18 21 24 27 30 100 

40 

0 3 6 9 


12 15 18 21 24 27 30 


80 

E=78 MPa 

20 

0 3 6 

E=970 MPa 

data3 

9 12 15 18 21 

Frequency data4 [kHz] 

data5 

24 27 30 



Numerical Results: Parametric Analysis 

Cylinder height Cylinder data6 radius 


20 

0 3 6 9 12 15 18 21 24 27 30 


Elastic data6 modulus Poisson ratio Damping 

data7 

h= 4cm 

h= 1cm 

data3 

data4 

data5 


120 

60 

40 

110 

20 

0 3 

100 

100 

6 9 12 15 18 21 24 27 30 

90 Frequency [kHz] 




120 

100 

80 

80 

70 

60 

60 

50 

80 

60 

40 

h= 4 cm 

R C /R f =10 

R C /R f =20 

h= 4 cm 

20 

data3 

0 3 6 9 12 15 18 21 24 27 30 

data4 


data5 

data7 h= 4cm ; R /R =20 

C f 

n = 0.3 

n = 0.4 

data3 

data4 

data5 

data6 

data7 

40 

40 0 3 6 9 12 15 18 21 24 27 30 

12 15 18 21 24 27 30 




By acting on the geometrical size and 

elastic properties, it is possible to 

design and tailor the sensor 

performance for a specific application 

120 

100 

80 

60 

40 

h= 4cm ; R C /R f =20 

undamped 

=0.1



DAMIVAL 13650 

Thermosetting polyurethane resin 

E= 200 MPa 

n=0.4 

r=1180 kg/m 3 

=0.1 

Sensor Design 




Project “ASSO” defines among the needs and requirements two operating frequency ranges for the underwater 

acoustic sensors: a “low” frequency range 0-15 kHz and a “high” frequency range 15-30 kHz 


70 

60 

50 

40 

30 

20 

10 

h=4cm; h = 4 cm D; RC=5mm = 5 mm 

C 

0 

0 5 10 15 20 25 30 35 



70 

60 

50 

40 

30 

20 

10 

ARALDITE DBF 

Epoxy adhesive resin 

h=4cm; h = 4 cm D; C=5mm 

R = 5 mm 

C 

E= 2.9 GPa 

n=0.345 

r=1100 kg/m 3 

=0.02 

0 

0 5 10 15 20 25 30 35 

Frequency [kHz]



FBG Hydrophone Fabrication 

Optical Fiber 




A TEFLON® modular holder was properly designed to fabricate DAMIVAL cylindrical coatings with different sizes. 

Assembled 

holder 

Optical fiber 

Screw 

Perfored plate 

DAMIVAL FBGs 

A cylindrical holder was properly designed to fabricate ARALDITE coatings. 

Plastic mould 

TEFLON modulus 

Araldite Coating 

D-5 : 

D C = 5mm ; h = 4 cm 

D-10 : 

D C = 10 mm ; h = 4 cm 

A-5 : 

D C = 5mm ; h = 4 cm

Vac [V] 

CONDITIONING 

CIRCUIT 

TUNABLE LASER 

Vac [V] 

Vac [V] 

2 

0 

-2 

1 

0 

-1 

2 

1 

0 

-1 

-2 


2x1 


3 m 

coated 

FBG 

1 m 

11 m 

GAIN 

PZT 

2 m 

Experimental Setup 

SIGNAL 

GENERATOR 

Acoustic 

source 

5 m 

DATA ACQUISITION 

1 

1 

1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 

0.5 

0 

-0.5 

-1 

PZT 

FBG 

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 

1 2 3 Time [ms] 4 5 6 

Time [ms] 

7 m 

PZT reference 

hydrophone 




Telescopic 

pole 

Optical fiber 

Fiber optic 

hydrophone 

weight


70 

60 

50 

40 

30 

20 

10 


Numerical prediction 

0 

Experimental data 

Elaborated 

-10 

0 5 10 15 20 




Experimental Validation 

70 

60 

50 

40 

30 

20 

10 


0 


Elaborated 

-10 

0 5 10 15 20 





D-5 sensor D-10 sensor A-5 sensor 


70 

60 

50 

40 

30 

20 

10 

0 



Elaborated 

-10 

0 5 10 15 20 25 30 35 


• Experimental data confirm the resonant behavior of the underwater acoustic sensor outlined by the numerical analysis 

• Acceptable prediction capabilities both in terms of resonant frequencies and sensitivity values have been obtained 

• The slight disagreements between experimental and numerical data can be attributed to second order effects (i.e. 

fabrication imperfections), which have not been taken into account in the simulations


70 

60 

50 

40 

30 

20 



Sensor Performances 

D-5 

D-10 

A-5 

5 10 15 20 25 30 35 


Sensitivity [dB re Volt/ Pa] 

-180 

-190 

-200 

-210 

-220 

-230 




D-5 

D-10 

A-5 

PZT 

5 10 15 20 25 30 35 


• Sensors with Damival coating are more sensitive than PZT in the frequency range 4-20kHz 

• Sensors with Araldite coating are more sensitive than PZT in the frequency range 15-35kHz 

• Sensitivity improvement with respect to bare FBG of 2-3 order of magnitude 

• Sensitivity comparable or higher than traditional PZT and other photonic technologies at the state of the art 

• Resolution: about 10mPa/(Hz) 1/2 at the resonances -It depends also on the interrogation strategy. It can be improved- 

• Frequency selectivity is desired in active sensing applications 

whereas flatness is typically required in passive sensing applications



PHONO-ABSORBER SPACERS 

FBG SENSORS 

Sensor Array: Fabrication 




Flexible array : Material : Damival Array : 4x1 Sensors : D-5 

Phono-absorber spacers keep FBG sensors mechanically and acoustically separated 

Rigid array : Material : Araldite Array : 4x4 Sensors : A-5 

STEEL HOLDER 

SENSORS

PZT 



Offshore Preliminary Test 

Vac (V) 

Vac (V) 

4 

2 

0 

-2 

-4 

0,3 

0,0 

-0,3 




• Port of Baia, Napoli 

• FBG Sensor: D-5 

• Sea environment with high noise 

Sensor time response 

0 2 4 6 8 10 

0,6 

Time (ms) 

-0,6 

0 2 4 6 8 10 

Time (ms) 

FBG Sensor 

Reference Sensor



Conclusions 




A full 3-D numerical analysis of an FBG coated by a ring-shaped material in the frequency range 0.5-30 kHz 

The resonant behavior of such underwater acoustic sensor has been reported for the first time 

Numerical analysis demonstrated that the sensing performances can be tailored for a specific application by 

a proper selection of the coating features 

Experimental analysis of fabricated optical hydrophones 

A good agreement between the experimental characterizations and the numerically predicted sensitivity gains 

has been obtained, confirming the correct modeling of the hydrophone as well as its prediction capability 

Excellent capability to detect acoustic waves in the frequency range 4–35 kHz, extremely high sensitivity, 

resolutions of the order of a few Pascal, and good linearity without using active configurations. 

Sensor array and offshore preliminary testing highlighted the strong potential of FBG hydrophones to be 

employed for in-field trials and industrial applications



Thanks 



Napoli, 17-21 Settembre 2012

Photonic Hydrophones Based on Coated Fiber Bragg Gratings.

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