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Optical noise of a 1550 nm fiber laser as an
underwater acoustic sensor B. Orsal,
N.P.Faye, K. Hey Tow, R. Vacher, D.
Dureisseix Research team Bruit
Optoélectronique, Institut dElectronique du Sud
(IES), CNRS UMR 5214 / University Montpellier 2,
CC 084, Place Eugène Bataillon, F-34095
Montpellier Cedex 05, France Société
détudes, de recherche et de développement
industriel et commercial (Serdic), 348 avenue du
Vert-Bois, F-34090 Montpellier, France
Research team Systèmes Multi-contacts,
Laboratoire de Mécanique et de Génie Civil
(LMGC), CNRS UMR 5508 / University Montpellier 2,
CC 048, Place Eugène Bataillon, F-34095
Montpellier Cedex 05, France
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Introduction

• The goal of this presentation is to provide the
  first results we got concerning the optical noise
  of a distributed feedback fiber laser (DFB FL )
  used as an underwater acoustic sensor. The main
  sensor characteristics are
• - A sensitivity allowing detection of all signal
  levels over background sea noise (the so-called
  deep-sea state 0). Among other applications, one
  may mention seismic risk prevention, oil
  prospection, ship detection, etc.
• - An optical noise reduced to its minimal value
  it is the lower bound below which no acoustic
  pressure variation is detectable.
• - To show that sufficiently low Relative
  Intensity Noise (RIN) can be obtained from DFB FL
  with a good choice of 1480 nm pump lasers powered
  with a very low noise current source in order to
  minimize Phase Noise detection due to DFB FL .

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Outline

• Introduction
• SensorDistributed Feedback fiber laser (DFB
  FL)
• DFB FL with acoustic amplifier
• AcoustoOptic Sensitivity SAO
• Experimental Set Up
• Détection Unit
• The optical intensity detected by the photodiodes
• Detected phase noise dF versus acoustic frequency
  f
• Optical sensor noise sources
• DFB Fiber Laser intensity noise
• DFB Fiber Laser frequency noise
• Detected Phase noise resolution versus acoustic
  frequency f
• Deep Sea State Zero Noise (DSS0 Sea Noise)
  dFDSSO
• RIN Detected Phase NoisedFRIN
• Frequency Detected Phase Noise dFfreq
• Laser Noise Equivalent Pressure dPNEP
• Conclusion

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Bare Distributed Feedback fiber laser (DFB
FL)and Acoustic Amplifier
Laser Cavity Lenght L 5 cm with distributed
BRAGG reflector GainErbium Doped Gain Zone
lB2neffL where L
Pump light at 1480nm
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Bare DFB FL without acoustic amplifier
Principle

• Mechanical Sensitivity e / Dp
• ( Frequency dependance)
• acousto-optique Sensitivity Dl / Dp of bare
  DBF FL
• without acoustic amplifier.
• The deformation of the DFB fiber laser is small
  when a bare fiber laser is placed directly in
  water. It is not sufficient to detect Deep See
  State Zero Noise (DSS0).

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Bare DFB FL with acoustic amplifier
Its sensitivity can be increased by using an
acoustic amplification. Typically we have
calculated for underwater surveillance
applications, an amplification of the sensitivity
of about 500 1000 times is required to approach
the deep sea state zero noise level (DSS0).
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AcoustoOptic Sensitivity SAO
where A is the sensitive surface area, k is
the equivalent fiber sensor stiffness, ?B is the
wavelength and LFL is the cavity DFB laser length
equal to 5 cm.
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Experimental Set Up
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Détection Unit

• The unbalanced in-fibre Mach-Zender
  interferometer(MZI) converts the pressure induced
  wavelength shift of the radiation emitted by the
  DFB fiber laser, into a phase delay which is a
  function of the FL output wavelength shift ?? and
  of the optical path difference OPD neff. L,
  where L is the length unbalance of the two
  interferometer arms.
• The wavelength modulation is analyzed by means of
  a FFT spectrum analyzer coupled with a phase
  meter.
• Where is the phase delay which corresponds to
  the pressure induced wave length shift and is
  the noise component associated with the signal

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The optical intensity detected by the photodiodes

• It is important that the interferometer is in
  quadrature (multiples of p/2) to have linear
  responses hence we can use a sinusoidal phase
  carrier signal to carry the phase delay created
  in the interferometer

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The optical intensity detected by the photodiodes

• The even harmonics of the carrier are all
  amplitude modulated by the cosine of the phase
  delay while the odd harmonics are amplitude
  modulated by the sine of phase delay.
• The phase meter gives Y(t) and X(t) as output.
  Both signals can be connected to two channels of
  a FFT analyser from which the phase delay can be
  extracted both in time and frequency domain in
  order to plot frequency noise dF versus acoustic
  frequency f.

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Optical sensor noise sources

• A noise source refers to any effect that
  generates a signal which is unrelated to the
  acoustic signal of interest and interferes with
  precise measurement.
• In the remote interrogated optical hydrophone
  sensors, there are several optical noise sources
  that contribute significantly to the total sensor
  noise.
• i) laser intensity noise, ii) laser frequency
  noise.
• Other noise sources such as optical shot noise,
  obscurity current noise, oscillator phase noise
  and fiber thermal noise and input polarization
  noise are generally less significant and will be
  ignored.

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DFB Fiber Laser intensity noise

• Fluctuations in the intensity of the laser
  contribute to the sensor noise and generate a
  noise current on the detection indistinguishable
  from the sensor phase signal.
• is the spectral density of the optical power
  fluctuations and is the mean optical power
  generated by laser near ? 1.55?m.
• For the case where the RIN occupies a bandwidth
  much less wide than the homodyne beat frequency,
  RMS induced phase noise is given by
• Measurements carried out on a single DFB FL
  pumped at 1480 nm with a power of 140 mW
• A typical spectrum is shown in figure 4.The
  noise of the DFB fiber laser was found to exhibit
  an f -? relationship where ? 0.5 for
  frequencies up to 10 kHz. Our measurements have
  given that RIN(f,?) levels less than 110 dB/Hz
  between 10kHz and 100kHz thanks to a RINPump lase
  r is lower than 10-13 s.
• This behavior proves that sufficiently low RIN
  can be obtained from DFB FL with a good choice of
  pump lasers powered with a very low noise current
  source.

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A typical frequency noise S(f,?) is shown at
1552.06 nm. The frequency noise of the laser was
measured using experimental set. S(f,?) is
related to Laser linewidth d?1/2 by the
ralationship
DFB Fiber Laser frequency noise
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Interferometric Phase Resolution When a
hydrophone is placed in the ocean, the background
acoustic noise will contribute to the total
detected phase noise. The phase noise generated
due to sea state is given by where f
is the acoustic frequency and GMZI is the gain of
imbalanced interferometer given by the
relationshipwith the values ? 1552 nm, neff
1,465, L 300m, GMZI 1,149. 106 rad/nm.
Interferometric Phase Resolution
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Phase noise resolution versus acoustic
frequency The acoustic pressure resolution of
the hydrophone can be computed for the two cases
limited by the sensor self noise (red) and
ambient acoustic noise (green) in the ocean
versus frequency for different DFB FL sensitivity
SAO.
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Laser Noise Equivalent Pressure

• We compute the laser noise equivalent pressure
  (Pa/?Hz) given by the model
• In order to compare with see noise equivalent
  pressure (Pa/?Hz)
• When sensitivity is high, we can detect the DSSO
  noise on all acoustic frequency range.
• When sensitivity is lower than 1,5 10 -6 nm/Pa,
  laser noise is detected on all range.

Sao(nm/Pa) f(Hz) dFfreq dFdsso (rad/vHz) (µPa/vHz)
1,50E-06 1 0,1 58479
3,00E-06 10 0,03 87781
5,00E-06 38 0,015 2631
7,50E-06 100 0,01 1169
1,50E-05 800 0,0032 187
4,00E-05 9000 0,0012 26
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Conclusion

• In this paper, we have shown the first frequency
  noise measurements of a single mode DFB FL used
  as an underwater hydrophone which is pumped with
  a 1480 nm laser with a very low RINPump lt 10-13
  s.
• .
• The low frequency pressure resolution in water
  becomes limited by Deep See State zero ambient
  acoustics if the acousto-optic sensitivity is
  sufficiently high (gt 1.5. 10-5 nm/Pa).
• If the sensitivity is lower than 1.5. 10-6 nm/Pa,
  then the frequency resolution is limited by DFB
  FL noise which is nearly equal to frequency
  noise.
• The phase noise related to relative Intensity
  noise is negligible because the DFB fiber laser
  is pumped with a 1480 nm laser with a very low
  RINPump lt 10-13 s
• .
• This type of system can be adapted for any
  applications requiring networks of sensor
  elements to be efficiently multiplexed. In
  particular, for seismic surveying arrays such as
  those positioned on ocean floor, for instance
  plugged to the Deep Sea Net used by Ifremer.

UPON2008 ENS Lyon 2-6 June 2008