Cosimo Trono - Arena Workshop 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” Pisa Istituto di Fisica Applicata “ Nello Carrara”, IFAC-CNR INFN Sez. Pisa Dipartimento di Ingegneria dell’Informazione - Pisa

Cosimo Trono - Arena Workshop May 2005 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

Cosimo Trono - Arena Workshop May 2005 FIBER BRAGG GRATINGS WHAT 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 n eff, the resonance condition is given by: Bragg = 2n eff  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

Cosimo Trono - Arena Workshop May 2005 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)

Cosimo Trono - Arena Workshop May 2005 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

Cosimo Trono - Arena Workshop May 2005 ERBIUM ION ENERGY LEVELS Three level laser system

Cosimo Trono - Arena Workshop May 2005 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. 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.

Cosimo Trono - Arena Workshop May 2005 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

Cosimo Trono - Arena Workshop May 2005 FIBER LASER TYPICAL CHARACTERISTICS ED Fiber characteristics ED fiber 980 nm: 14 dB/m ED fiber 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

Cosimo Trono - Arena Workshop May 2005 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).

Cosimo Trono - Arena Workshop May 2005 FIBER LASER SENSORS Physical elongation (strain), temperature and pressure variations, which changes the FBG pitch , the cavity length, and fiber refractive index n eff, produce a shift in the fiber laser emission line. STRAIN [  ] ~1.2 pm/ 1550 nm TEMPERATURE ~ nm PRESSURE ~ nm TYPICAL SENSITIVITIES FOR A BARE FIBER LASER

Cosimo Trono - Arena Workshop May 2005 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

Cosimo Trono - Arena Workshop May 2005 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

Cosimo Trono - Arena Workshop May 2005 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.

Cosimo Trono - Arena Workshop May 2005 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.

Cosimo Trono - Arena Workshop May 2005 DETECTION APPARATUS SENSITIVITY Proportional to It depends from the laser emission power, the OPD, and the emission wavelength (Pascal)

Cosimo Trono - Arena Workshop May 2005 FIBER LASER VS. CONVENTIONAL HYDROPHONE Response to loud-speaker amplitude modulation at kHz The fit is very good Calibration of the first developed single mode FL (10  W output power) September 2004

Cosimo Trono - Arena Workshop May 2005 FIBER LASER VS. CONVENTIONAL HYDROPHONE 100  W output power; OPD=15m Response to a sinusoidal acoustic signal (60 KHz) Spectrum Analyzer bandwidth: 125 Hz

Cosimo Trono - Arena Workshop May 2005 FIBER LASER VS. CONVENTIONAL HYDROPHONE 100  W output power; OPD=15m Spectrum Analyzer bandwidth: 125 Hz Response to a sinusoidal acoustic signal (60 KHz)

Cosimo Trono - Arena Workshop May 2005 FL DEPENDENCE FROM ACOUSTIC FREQUENCY Not completely understood fenomenum. Output power: 600  W

Cosimo Trono - Arena Workshop May 2005 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 FL DEPENDENCE FROM ACOUSTIC FREQUENCY

Cosimo Trono - Arena Workshop May 2005 FL DEPENDENCE FROM ACOUSTIC 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: ~ 1m 3 It’s impossible to discriminate the direct acoustic waves from the reflected waves. The mechanical frame is not optimized. ENVIRONMENTAL AND EXPERIMENTAL SETUP DRAWBACKS

Cosimo Trono - Arena Workshop May 2005 POSSIBLE IMPROVEMENTS Recoating of the fiber Flats the frequency respons Increase the FL pressure sensitivity

Cosimo Trono - Arena Workshop May 2005 FL RECOATING Cylindrical recoating (6 mm  x 100 mm) of the fiber with epoxy resin

Cosimo Trono - Arena Workshop May 2005 STATIC PRESSURE SENSITIVITIES

Cosimo Trono - Arena Workshop May 2005 FUTURE DEVELOPMENTS Flat emission loud-speaker Time domain signal acquisition (ADC 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)

Cosimo Trono - Arena Workshop May 2005 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?)