Cosimo Trono - Arena Workshop 17-19 May 2005 Fiber laser hydrophones as pressure sensors P. E. Bagnoli, N. Beverini, E. Castorina, E. Falchini, R. Falciai,

Cosimo Trono - Arena Workshop 17-19 May 2005

Fiber laser hydrophones as pressure sensors

P. E. Bagnoli, N. Beverini, E. Castorina, E. Falchini,R. Falciai, V. Flaminio, E. Maccioni, M. Morganti,

F. Sorrentino, F. Stefani, C. Trono

Dipartimento di Fisica “E. Fermi” PisaIstituto di Fisica Applicata “ Nello Carrara”, IFAC-CNR

INFN Sez. PisaDipartimento di Ingegneria dell’Informazione - Pisa


SUMMARY

•What is a Fiber Bragg Grating (FBG)

•What is a Distributed Bragg Reflector Fiber Laser (DBR-FL)

•Fiber Laser sensor working principle

•Fiber laser as acoustic sensor (hydrophone)

•Future R&D

•Conclusions


FIBER BRAGG GRATINGSWHAT IS A FBG: a periodic perturbation of the core refractive index of a

monomode optical fiber.

If the grating pitch is , and the core effective refractive index is neff, the

resonance condition is given by:

Bragg = 2neff

HOW DOES IT WORK: when the radiation generated by a broad source is injected into the fiber and interacts with the grating, only the wavelength in a “very narrow band” (~0.2 nm) can be back-reflected without any perturbation in the other wavelengths.

A FBG is a narrow-band mirror


FBG LASER FABRICATION TECHNIQUE

FBGs reflectors are fabricated in our labs by using the phase-mask technique. A phase-mask is a diffractive optical element, which spatially modulates the UV

writing beam (typically at = 248 nm, where the photosensitivity of the fiber is at its best). The near-field fringe pattern, which is produced behind the mask, photo-imprints a refractive index modulation on the core of the photosensitive

fiber. The grating pitch depends on the phase-mask pitch M (= M/2)


THE DISTRIBUTED BRAGG REFLECTORFIBER LASER (DBR-FL)

Two Bragg gratings with identical reflection wavelength directly inscribed on an erbium doped (active medium) optical fiber. This structure forms a Fabry-Perot laser cavity which, when pumped at 980 nm, lases with emission at ~1530 nm


ERBIUM ION ENERGY LEVELS

Three level laser system


0123456789

101112131415161718

1500 1510 1520 1530 1540 1550 1560 1570 1580 1590 1600

Wavelength (nm)

Em

issi

on

(d

B/m

)TYPICAL AMPLIFIED SPONTANEOUS EMISSION

SPECTRUM OF AN ERBIUM DOPED OPTICAL FIBER

A cavity enclosed between two mirrors forms an optical resonator. Only a discrete set of resonant longitudinal modes can be allowed.

FSRv/2L

Amplified spontaneous emission

Laser cavity longitudinal modes

FBG reflection spectrum

Bragg gratings reflectivities and cavity length are choosen in order to select a single stable longitudinal mode.


Effect of the lasing: single stable longitudinal mode+

very narrow linewidth

Fiber laser linewidth: < 5 kHz laser < 4*10-8 nm

(Bragg = 0.2 nm)

Coherence length > 50 km


FIBER LASER TYPICAL CHARACTERISTICS

ED Fiber characteristics

•ED fiber absorption @ 980 nm: 14 dB/m

•ED fiber absorption @ 1530 nm: 18 dB/m

FL characteristics

•Bragg reflectors length: 1cm

•Rear FBG reflectivity: >99%

•Output FBG reflectivity: ~ 90%

•Cavity length (distance between gratings): 1-3 cm

•Optical power emitted: 500 W – 2 mW (pump power 300 mW)

•Stable single longitudinal mode


The ED fiber is cut and spliced to a standard fiber (low loss <0.3 dB/Km) very close to the cavity.

DBR FIBER LASER

Photo of one of several DBR lasers realized. The green light is due to “up conversion” (collision of two erbium ions with energy jump at higher levels).


FIBER LASER SENSORSPhysical elongation (strain), temperature and pressure variations, which changes the FBG pitch , the cavity length, and fiber refractive index neff,

produce a shift in the fiber laser emission line.

STRAIN []

~1.2 pm/ @ 1550 nm

TEMPERATURE

~ 10 pm/°C @ 1550 nm

PRESSURE

~ -4.6 pm/MPa @ 1550 nm

TYPICAL SENSITIVITIES FOR A BARE FIBER LASER


ADVANTAGES

•Intrinsic safety and immunity from electromagnetic fieldsthe fiber is realized entirely with dielectric materials (glass and

plastic)

•Very low invasivityvery small dimensions of the optical fiber (~ 125 m)

•ED fiber is compatible with standard telecom fibers (very low signal attenuation ~0.3 dB/km)

the optoelectronics control unit can be placed several km far from the measurement point


ADVANTAGESFBG reflects the light in a narrow band: possibility of multiplexing for a quasi-distributed measurement configuration, by using a single pump and a single interrogation system

Several lasers (an array of hydrophones) can be “written” on the same optical fiber

Optical Spectrum Analyzer

0.1 nm optical resolution


FL HYDROPHONE EXPERIMENTAL SETUP

The acoustic wave produces the modulation of the laser wavelength.The interferometer converts wavelength modulation into a phase-shift.

The pump and laser radiations travel both into the core of the ED optical fiber. The WDM coupler acts the spectral separation and the optical isolator avoid unwanted feedback into the cavity.


MACH-ZENDER INTERFEROMETER (MZI)

Quadrature detection

The laser signal is splitted and then recombined thus obtaining a signal proportional to a raised cosine function at the MZI output. The phase depends on the laser frequency c/ (which depends on the acoustic signal) and on the Optical Path Difference (OPD).

The MZI is locked, at low Fourier frequencies (< 5 kHz), at one side of a fringe in the middle point, where the sensivity has its maximum value, by using a servo loop that acts on the length of one arm of the interferometer by stretching the fiber through a piezoelectric actuator.


DETECTION APPARATUS SENSITIVITY

Proportional to

laser

OPDP and

Pascal

It depends from the laser emission power, the OPD, and the emission wavelength (Pascal)


FIBER LASER VS. CONVENTIONAL HYDROPHONE

Response to loud-speaker amplitude modulation at 16.625 kHz

The fit is very good

y = 0.2458x + 0.0109

R2 = 0.9996

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Pa read from a conventional hydrophone(response 1.778 mV/Pa)

Fib

er la

ser

resp

on

se (

mV

)

Calibration of the first developed single mode FL (10 W output power)September 2004



100 W output power; OPD=15m

Response to a sinusoidal acoustic signal (60 KHz)

Spectrum Analyzer bandwidth: 125 Hz

-140

-130

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-110

-100

-90

-80

-70

-60

-50

0 10 20 30 40 50 60 70 80 90 100

acoustic signal (KHz)

dB

V

FLhydrophone

Peak pressure value: 20 mPaPa



100 W output power; OPD=15m

Spectrum Analyzer bandwidth: 125 Hz

Response to a sinusoidal acoustic signal (60 KHz)

-150

-140

-130

-120

-110

-100

-90

-80

-70

-60

-50

0 10 20 30 40 50 60 70 80 90 100

acoustic signal (KHz)

dB

V

FLhydrophonePeak pressure value: 430 Pa


0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

20 30 40 50 60 70 80 90 100

Acoustic signal (KHz)

FL

re

sp

on

se

(m

V/P

a)

FL DEPENDENCE FROM ACOUSTIC FREQUENCY

Not completely understood fenomenum.

Output power: 600 W


EFFECTS OF THE ACOUSTIC WAVE ON THE DBR-FL

The wavelength of a 50 KHz acoustic signal in water is 3 cm

Comparable with the laser cavity and FBG length

Power modulation of the laser emission, induced by the acoustic signal, is superimposed to the wavelength modulation

The sensitivity of the FL+MZI depends greatly on the frequency




The PZT loud-speaker is not calibrated and is driven with sinusoidal continuous wave (no short pulse source at present).

No time domain analysis

The water tank has finite (small) dimensions: ~ 1m3

It’s impossible to discriminate the direct acoustic waves from the reflected waves.

The mechanical frame is not optimized.

ENVIRONMENTAL AND EXPERIMENTAL SETUP DRAWBACKS


POSSIBLE IMPROVEMENTS

Recoating of the fiber

Flats the frequency respons Increase the FL pressure sensitivity


FL RECOATING

Cylindrical recoating (6 mm x 100 mm) of the fiber with epoxy resin


y = -69.991x - 7E-05

R2 = 0.9998

y = -4.5699x + 4E-06

R2 = 0.999

-70

-60

-50

-40

-30

-20

-10

0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Static Pressure (MPa)

wa

ve

lng

th s

hif

t (p

m)

recoated

bare

STATIC PRESSURE SENSITIVITIES


FUTURE DEVELOPMENTS

•Flat emission loud-speaker

•Time domain signal acquisition (ADC sampling @ 1 MHz)

•Sensor calibration in a wide tank (equipped pool)

•Sensor test at high pressure (300 Atm)

•Production of a bipolar acoustic signal (-like)

•Study of the FL mechanical frame

•Measurements of directional sensitivity

•Tests in ice

•Many sensors on the same fiber (multiplexing)


CONCLUSIONS

•We have the technology for producing DBR Fiber laser

•We have developed the apparatus for acoustic wave detection in water

•The comparison with conventional hydrophone showed a very good fit between the two sensors

•The FL sensitivity is better than that of the standard hydrophone, but depends on the acoustic frequency

•The fiber laser based technology seems to be adequate for neutrino acoustic detection in water (in ice?)

Documents

Cosimo Trono - Arena Workshop 17-19 May 2005 Fiber laser hydrophones as pressure sensors P. E. Bagnoli, N. Beverini, E. Castorina, E. Falchini, R. Falciai,