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DEVELOPMENT OF AN ERBIUM-DOPED FIBER LASER AS A
DEEP SEA HYDROPHONE
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
Pisa Istituto di Fisica Applicata Nello
Carrara, IFAC-CNR INFN Sez. Pisa Dipartimento di
Ingegneria dellInformazione - Pisa
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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 RD
• Conclusions

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FIBER BRAGG GRATINGS
WHAT IS A FBG a periodic perturbation of the
core refractive index of a monomode optical fiber.
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.
If the grating pitch is ?, and the core effective
refractive index is neff, the resonance condition
is given by ?Bragg 2neff?
A FBG is a narrow-band mirror
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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)
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THE DISTRIBUTED BRAGG REFLECTOR FIBER 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
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TYPICAL AMPLIFIED SPONTANEOUS EMISSION SPECTRUM
OF AN ERBIUM DOPED OPTICAL FIBER
Amplified spontaneous emission
Laser cavity longitudinal modes
FBG reflection spectrum
A cavity enclosed between two mirrors forms an
optical resonator. Only a discrete set of
resonant longitudinal modes can be allowed.
Bragg gratings reflectivities and cavity length
are choosen in order to select a single stable
longitudinal mode.
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DBR FIBER LASER single stable longitudinal
mode very narrow linewidth
??laser lt 410-8 nm (??Bragg 0.2 nm)
Fiber Laser linewidth lt 5 kHz
Coherence length gt 50 km
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FIBER LASER TYPICAL CHARACTERISTICS

• FL characteristics
• Bragg reflectors length 1cm
• Rear FBG reflectivity gt99
• 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

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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).
The Erbium Doped fiber is cut and spliced to a
standard fiber (low loss lt0.3 dB/Km) very close
to the cavity.
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FIBER LASER SENSORS
Physical 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.
TYPICAL SENSITIVITIES FOR A BARE FIBER LASER
STRAIN ? 1.2 pm/me _at_ 1550 nm
PRESSURE -4.6 pm/MPa _at_ 1550 nm
TEMPERATURE 10 pm/C _at_ 1550 nm
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FIBER LASER SENSORS CHARACTERISTICS

• Intrinsic safety and immunity from
  electromagnetic fields
• the fiber is realized entirely with dielectric
  materials (glass and plastic)
• Very 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

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FIBER LASER SENSORS CHARACTERISTICS
FBG 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
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FL HYDROPHONE EXPERIMENTAL SETUP
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.
The acoustic wave produces the modulation of the
laser wavelength. The interferometer converts
wavelength modulation into a phase-shift.
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MACH-ZENDER INTERFEROMETER (MZI) Balanced
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 (lt
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.
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DETECTION APPARATUS SENSITIVITY
Proportional to
It depends on the laser emission power, on the
Optical Path Difference (OPD), and on the
emission wavelength ? (which is a function of the
pressure).
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EXPERIMENTAL CONDITIONS

• First comparisons, with a standard PZT
  hydrophone, made with cw sinusoidal waves (to
  allow a rough response evaluation) and by means
  of a home-made (not-calibrated) in-water emitter.
• Use of a calibrated acoustic transducer (with a
  known in-water transfer function).
• Bare and recoated FL.
• Sinusoidal Burst Excitation.
• White Noise Excitation.

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FIBER LASER VS. CONVENTIONAL HYDROPHONE
Comparison with the first developed single mode
FL (10 ?W output power) September 2004
Response to a loud-speaker test tone of 16.625
kHz and variable amplitude
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FIBER LASER VS. CONVENTIONAL HYDROPHONE
Response to a sinusoidal acoustic signal (60 KHz)
100 ?W output power OPD15m
Spectrum Analyzer bandwidth 125 Hz
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Weak structureFrequency response not homogeneous
(probably because of output power modulations and
resonances of the Bragg gratings and FL cavity
lengths)
BARE FL SENSOR
FL RECOATING (by means of acrylate, PVC, epoxy
resin...)
Increases the response omogenity
Increases the FL pressure sensitivity
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FL RECOATING Epoxy Rod
Cylindrical recoating (6 mm ? x 100 mm) of the FL
with epoxy resin
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STATIC PRESSURE SENSITIVITIES Epoxy Rod
FL Measurements in a small pressure controlled
vessel, observing the frequency shifts by means
of a Fabry-Perot optic interferometer.
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LP AND HP FILTERING

• To cut-off low frequency components, a High Pass
  filter has been set at the output of the
  Mach-Zender locking loop. Its 3 dB point is at
  about 16 kHz about the same low frequency limit
  imposed by the dimensions of the vessel.
• A Low Pass filter (- 3 dB point at 100 kHz) cuts
  off high frequency.

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BURST EXCITATION

• Needed to avoid interference effects due to waves
  reflections inside the vessel.
• Sinusoidal burst length limit imposed by the
  dimensions of the vessel and by the
  emitter-receiver distance.
• Burst cycles (no of waves in the wave packet)
  enough to allow the FL sensor reaching a stable
  response.

• Chosen burst characteristics max. duration 0.5
  ms, 10 wave cycles, rep. rate 10 Hz.
• Low freq. limit 20 kHz (determined by the vessel
  dimensions).
• High freq. limit 100 kHz (reference hydrophone
  limit).

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WHITE NOISE EXCITATION

• By means of an arbitrary waveform generator, the
  in-water transducer is activated with a white
  noise function of chosen amplitude.
• The experimental results are equivalent to those
  obtained with burst excitation, but with a less
  cumbersome procedure.

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FL RECOATINGAcrylate Coating

• By means of a recoater machine, it is possible to
  recoat the FL up to the standard diameter of 250
  mm.
• The coating makes the FL structure stronger and
  improves the homogeneity of the frequency
  response.
• Also the sensitivity to static and dynamic
  (acoustic and ultra-sound) pressure is enhanced.

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Bare to Acrylate Coated FL Comparison
The response is flatter and the background lower
for the coated FL
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Bare to Acrylate Coated FL Comparison
The Signal to Background ratio is flatter for the
coated FL
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Nominal PZT hydrophone sensitivity ( 29 dB of
amplification), taken from its datasheet
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Acrylate FL to Hydrophone Comparison
The Signal to Background is higher for the FL
The blue line is simply an eye-guide
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Minimum Detectable Signal with the actual
experimental set-up
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FILTERED RESPONSE OF AN ACRYLATE COATED FL The
comparison with the interrogation system
background noise, shows that the sensitivity
could be improved by 25/30 dB operating in an
acoustically clean environment
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FUTURE DEVELOPMENTS

• Time domain signal acquisition (ADC sampling _at_ 1
  MHz).
• Sensor calibration in a wide tank (equipped
  pool).
• Functionality test at high pressure (300 Atm) a
  suitable vessel is under construction.
• Study of the FL mechanical frame (in progress).
• Measurements of directional sensitivity.
• Many sensors on the same fiber (multiplexing)
• a double sensor is already under development.
• High frequency (MHz regime) test.

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CONCLUSIONS

• We have the technology for producing DBR Fiber
  Lasers.
• We have developed the apparatus for acoustic
  wave detection in water.
• The comparison with conventional hydrophones
  shows that is possible to achieve a greater
  sensitivity.
• This kind of sensor may be used in the place of
  the conventional PZT hydrophones in a wide range
  of applications, spanning from the marine
  environment acoustic monitoring, to the deep-sea
  study and survey of dolphins and whales. Another
  possible application is as receivers for the
  in-water acoustic positioning (range finding).
• Medical Ultrasonic applications.