Photonic Hydrophones based on Coated Fiber Bragg Gratings

Transcription

1 Photonic Hydrophones 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

2 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

3 Comparison with Traditional Hydrophones Marine environmental monitoring Traditional hydrophones Electromagnetic sensitivity Water seepages Heavy and bulky Military applications Fiber optic hydrophones Immunity to Electromagnetic Interferences Light and small No electric connections Remote monitoring Multiplexing capability Medical

4 Photonic hydrophones: State of the art Interferometric detection DFB Laser Photonic crystal mirror J.A. Bucaro et al.(1977) Intensity modulation S.Goodman et al.(29) Very high sensitivities Complex configuration O.Kilic et al.(27) W.B. Spillman et al.(198) Acoustic detectors based on bare FBGs Cheap and well assessed technology Limited by the high Young s modulus of glass LOD ~ MPa N.Takahashi et al., Opt. Rev. 4, (1997)

5 Fiber Bragg Grating (FBG) A Fiber Bragg Grating is a longitudinal periodic variation of the refractive index in the core of an optical fiber The maximum reflectivity occurs at the wavelength that matches the Bragg condition: where n eff is the effective refractive index of the guided mode and Λ 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 p11 x p12 z y, 2

6 Optoelectronic Division, Engineering Department The Project Idea Acoustic wave detection using coated FBGs FBG The optical hydrophone is based on standard FBG coated by a ring shaped overlay ACOUSTICAL PLANE WAVE COATING WATER 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) h FEM geometry y z x 2*R f 2*R C The acoustical frequency range is.5 3 khz y z x 2*R w

7 Optoelectronic Division, Engineering Department Numerical Results: Resonant Behavior S P n z 2 1 eff P 2 p 11 x p 12 z y Sensitivity Gain 2* log S S BARE S BARE MPa -1 Pressure distribution in water and z-strain distribution on the cylinder surface 2kHz) Sensitivity gain h= 4 cm ; R C /R f = 2 Resonant modes khz 18.7 khz 25 khz E= 78 MPa n= 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 5.7 khz Resonances are associated to resonant longitudinal modes of the cylindrical hydrophones 15 khz 21.9 khz 27.9 khz AWARD: Optical Fiber Sensors 21, Ottawa (Canada) M.Moccia et al., Opt. Express 19 (2), 211

8 Optoelectronic Division, Engineering Department R /R C R = 2 C f /R = 2 f Cylinder height E=78 MPa E=97 MPa 2 data Frequency data4 [khz] data5 Numerical Results: Parametric Analysis Elastic data6 modulus Poisson ratio Damping data7 h= 4cm h= 1cm data3 data4 data h= 4 cm R C /R f =1 R C /R f =2 h= 4 cm 2 data data data5 Cylinder data6 radius data7 h= 4cm ; R /R =2 C f n =.3 n =.4 data3 data4 data5 data6 data By acting on the geometrical size and elastic properties, it is possible to design and tailor the sensor performance for a specific application h= 4cm ; R C /R f =2 undamped =.1

9 Optoelectronic Division, Engineering Department Sensor Design Project ASSO defines among the needs and requirements two operating frequency ranges for the underwater acoustic sensors: a low frequency range -15 khz and a high frequency range 15-3 khz DAMIVAL 1365 Thermosetting polyurethane resin 7 5 h=4cm; h = 4 cm D; R C =5mm = 5 mm C E= 2 MPa n=.4 r=118 kg/m 3 = ARALDITE DBF Epoxy adhesive resin h=4cm; h = 4 cm D; C R=5mm = 5 mm C E= 2.9 GPa n=.345 r=11 kg/m 3 =

10 FBG Hydrophone Fabrication A TEFLON modular holder was properly designed to fabricate DAMIVAL cylindrical coatings with different sizes. Assembled holder Optical Fiber DAMIVAL FBGs D-5 : D C = 5mm ; h = 4 cm D-1 : Screw TEFLON modulus D C = 1 mm ; h = 4 cm A cylindrical holder was properly designed to fabricate ARALDITE coatings. Araldite Coating Perfored plate Plastic mould A-5 : D C = 5mm ; h = 4 cm Optical fiber

11 Vac [V] Vac [V] Vac [V] Optoelectronic Division, Engineering Department Experimental Setup CONDITIONING CIRCUIT DATA ACQUISITION 2x1 GAIN SIGNAL GENERATOR TUNABLE LASER Telescopic pole 3 m coated FBG PZT Acoustic source 7 m Optical fiber 1 m 11 m 2 m 5 m PZT reference hydrophone Fiber optic hydrophone PZT FBG Time [ms] Time [ms] weight

12 Optoelectronic Division, Engineering Department Experimental Validation D-5 sensor D-1 sensor A-5 sensor Numerical prediction Experimental data Elaborated Numerical prediction Experimental data Elaborated Numerical prediction Experimental data Elaborated 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

13 Sensitivity [db re Volt/ Pa] Optoelectronic Division, Engineering Department Sensor Performances 7 D-5 D-1 A D-5 D-1 A PZT Sensors with Damival coating are more sensitive than PZT in the frequency range 4-2kHz 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 1mPa/(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

14 Sensor Array: Fabrication Flexible array : Material : Damival Array : 4x1 Sensors : D-5 PHONO-ABSORBER SPACERS FBG SENSORS Phono-absorber spacers keep FBG sensors mechanically and acoustically separated Rigid array : Material : Araldite Array : 4x4 Sensors : A-5 STEEL HOLDER SENSORS

15 Vac (V) Vac (V) Optoelectronic Division, Engineering Department Offshore Preliminary Test Port of Baia, Napoli FBG Sensor: D-5 Sea environment with high noise Sensor time response PZT FBG Sensor ,6 Time (ms),3 Reference Sensor, -,3 -, Time (ms)

16 Conclusions A full 3-D numerical analysis of an FBG coated by a ring-shaped material in the frequency range.5-3 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

17 Thanks