Presentazione di PowerPoint

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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
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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ERBIUM ION ENERGY LEVELS
Three level laser system
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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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Effect of the lasing single stable longitudinal
mode very narrow linewidth
Fiber laser linewidth lt 5 kHz
??laser lt 410-8 nm (??Bragg 0.2 nm)
Coherence length gt 50 km
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FIBER LASER TYPICAL CHARACTERISTICS

• ED Fiber characteristics
• ED fiber absorption _at_ 980 nm 14 dB/m
• ED fiber absorption _at_ 1530 nm 18 dB/m
• 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 ED 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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ADVANTAGES

• Intrinsic safety and immunity from
  electromagnetic fields
• the fiber is realized entirely with dielectric
  materials (glass and plastic)
• Very low invasivity
• 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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ADVANTAGES
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) 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 from the laser emission power, the
OPD, and the emission wavelength ?(Pascal)
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FIBER LASER VS. CONVENTIONAL HYDROPHONE
Calibration of the first developed single mode FL
(10 ?W output power) September 2004
Response to loud-speaker amplitude modulation at
16.625 kHz
The fit is very good
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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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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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FL DEPENDENCE FROM ACOUSTIC FREQUENCY
Output power 600 ?W
Not completely understood fenomenum.
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FL DEPENDENCE FROM ACOUSTIC FREQUENCY
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 FLMZI depends greatly on
the frequency
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FL DEPENDENCE FROM ACOUSTIC FREQUENCY
ENVIRONMENTAL AND EXPERIMENTAL SETUP DRAWBACKS
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 Its impossible to discriminate
the direct acoustic waves from the reflected
waves. The mechanical frame is not optimized.
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POSSIBLE IMPROVEMENTS
Recoating of the fiber
Increase the FL pressure sensitivity
Flats the frequency respons
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FL RECOATING
Cylindrical recoating (6 mm ? x 100 mm) of the
fiber with epoxy resin
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STATIC PRESSURE SENSITIVITIES
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FUTURE DEVELOPMENTS

• Flat emission loud-speaker
• Time domain signal acquisition (ADC sampling _at_ 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)

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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?)