Download Optical noise of a 1550 nm fiber laser as an underwater acoustic

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 d’Electronique du Sud
(IES), CNRS UMR 5214 / University Montpellier 2, CC 084, Place Eugène
Bataillon, F-34095 Montpellier Cedex 05, France
** Société d’études, 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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UPON’2008 ENS Lyon 2-6 June 2008
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
• Sensor:Distributed 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 δΦ 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): δΦDSSO
– RIN Detected Phase Noise:δΦRIN
– Frequency Detected Phase Noise: δΦfreq
Laser Noise Equivalent Pressure: δPNEP
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
lB=2neffL where L
Pump light at
1480nm
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Bare DFB FL without acoustic amplifier
Principle
external
Pressure
p(t)
Path variation
DL(t)
Fiber mechanical.
strain e(t)
Optomechanical
coupling
Wavelenght Variation
of laser signal
Dl (t)
Mechanical Sensitivity e / Dp
( Frequency dependance)
-acousto-optique Sensitivity : Dl / Dp of bare DBF FL
without acoustic amplifier.
Dl 1  2
=
DP
E


n2
 1  (2 p12  p11 )l nm/Pa
2


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
lB
A
lB
= ( ).(0,78.
)
p
k
LFL
where A is the sensitive surface area, k is the equivalent fiber sensor
stiffness, lB 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 Δl and of the optical path difference OPD = neff. L,
where L is the length unbalance of the two interferometer arms.
D = neff kL
=
2neff L
Dl
l
•The wavelength modulation is analyzed by means of a FFT spectrum
analyzer coupled with a phase meter.
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D = s  
Where  s 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
I = I 0 (1  V cos(D))
I max  I min
I 0 is the mean optical power , V =
is the visibility .
I max  I min
• It is important that the interferometer is in quadrature (multiples of
/2) to have linear responses; hence we can use a sinusoidal phase
carrier signal to carry the phase delay created in the interferometer
2neff L
D =
sin (t )  D
c
I oV cos( sin t  D ) = I oV (cos( sin t ) cos(D ) sin ( sin t )sin( D ))



 

= I oV . J 0( ) 2 J 2k( ) cos(2kt ) cos(D ) I oV 2 J 2k  1( )sin ((2k  1)t )sin( D )
k =1


 k =1

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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 δΦ
versus acoustic frequency f.
 sin( ) 
 = 
arctan 
 cos () 
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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.
RIN ( f , l ) =
SP ( f , l )
P (l )
2
• is the spectral density of the optical power fluctuations and is the mean optical power
generated by laser near l = 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:
RIN = RIN ( f , l )
•Measurements carried out on a single DFB FL pumped at 1480 nm with a power of 140 mW:
RIN ( f , l ) dB / Hz = 10 log(
SP ( f , l )
P (l )
2
)
•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,l) 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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DFB Fiber Laser frequency noise:
A typical frequency noise S(f,l) is shown at 1552.06 nm.
The frequency noise of the laser was measured using experimental
set. S(f,l) is related to Laser linewidth δυ1/2 by the ralationship:
1/ 2 =  .S 2f ( f ) where S ( f , l ) is shown in this picture
Optical frequency noise Sf(Hz/√Hz)
1,0E+02
1,0E+01
1,0E+00
1,0E+00
1,0E+01
1,0E+02
1,0E+03
1,0E+04
Acoustic frequency (Hz)
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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:
 DSS 0 = P0 .102, 2  (
f 0 0,85
)  S AO  GMZI
f
where f is the acoustic frequency and GMZI is the gain of imbalanced interferometer given by the relationship:
GMZI =
 2 neff L
=
l
l2
with the values l= 1552 nm, neff =1,465, L= 300m, GMZI =1,149. 106 rad/nm.
1,0E+01
Phase fluctuations (rad/√Hz)
1,0E+00
Interferometric Phase Resolution
1,0E-01
1,0E-02
1,0E-03
1,0E-04
1,0E-05
1,0E-06
1,0E+00
1,0E+01
1,0E+02
1,0E+03
1,0E+04
Acoustic frequency (Hz)
δΦ Freq (rad/√Hz)
δΦ DSSO (rad/√Hz)
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δΦ RIN (rad/√Hz)
δΦ ambiant (rad/√Hz)
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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.
Phase fluctuations (rad/√Hz) for different acousto-optic
sensitivities
1,0E+01
1,0E+00
1,0E-01
1,0E-02
1,0E-03
1,0E-04
1,0E+00
1,0E+03
1,0E+02
1,0E+01
1,0E+04
Acoustic frequency (Hz)
Sao=1,5E-5 (nm/Pa)
δΦ Freq (rad/√Hz)
Sao = 4,0E-5 (nm/Pa)
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Sao=0,75E-5 (nm/Pa)
δΦ ambiant for Sao = 4E-5 (rad/√Hz)
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Laser Noise Equivalent Pressure
• We compute the laser noise equivalent pressure (Pa/Hz) given by the model:
PPEB =
equ
S AO  Gint
=
é
RIN
  éfreq
S AO  Gint
• In order to compare with see noise equivalent pressure (Pa/Hz)
PDSSO =
DSSO
S AO  Gint
• 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.
PPEB
Sao(nm/Pa)
f(Hz)
δΦfreq = δΦdsso
(rad/√Hz)
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
(µPa/√Hz)
17
4,00E-05
9000
0,0012
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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 < 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 (> 1.5. 105 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 < 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.
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