optical noise of a 1550 nm fiber laser as an underwater acoustic sensor

1

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

UPON’2008 ENS Lyon 2-6 June 2008

2

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 .


3

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

4UPON’2008 ENS Lyon 2-6 June 2008

Bare Distributed Feedback fiber laser (DFB FL)and Acoustic Amplifier

Pump light at 1480nm

Laser Cavity Lenght L= 5 cm with distributed BRAGG reflector GainErbium Doped Gain Zone neffwhere

5

externalPressure p(t)

Path variation(t)

Fiber mechanical.strain (t)

Optomechanicalcoupling

Wavelenght Variation of laser signal

(t)

Mechanical Sensitivity / p( Frequency dependance)-acousto-optique Sensitivity : / p 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).

Bare DFB FL without acoustic amplifier

nm/Pa )2(2

121

1112

2

ppn

EP

Principle

UPON’2008 ENSLyon 2-6 June 2008

6

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).


7

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.

).78,0).(k

A(

FL

BB

Lp


8

Experimental Set Up


9

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 Δ 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

2

2 Ln

kLn

eff

eff

s

s


10

The optical intensity detected by the photodiodes

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

))cos(1(0 VII

.visibilitytheis,poweropticalmeantheisminmax

minmax0 II

IIVI

)(sin2

tc

Lneff

)sin())12((sin)(122)cos()2cos()(22)(0.

))sin()sin(sin)cos()sin(cos()sin(cos

11

tkkJVItkkJJVI

ttVItVI

ko

ko

oo


11

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.

)(cos

)sin(arctan


12

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.


13

•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 = 1.55m.

•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.

DFB Fiber Laser intensity noise:

2)(

),(),(

P

fSfRIN P

),( fRINRIN

))(

),(log(10),(

2/

P

fSfRIN P

HzdB


14

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 δυ1/2 by the ralationship:

1,0E+00

1,0E+01

1,0E+02

1,0E+00 1,0E+01 1,0E+02 1,0E+03 1,0E+04

Acoustic frequency (Hz)

Op

tic

al f

req

ue

nc

y n

ois

e S

f(H

z/√H

z)

DFB Fiber Laser frequency noise:

picturethisinshownis),(where)(. 22/1 fSfS f


15

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 relationship:

with the values = 1552 nm, neff =1,465, L= 300m, GMZI =1,149. 106 rad/nm.

1,0E-06

1,0E-05

1,0E-04

1,0E-03

1,0E-02

1,0E-01

1,0E+00

1,0E+01

1,0E+00 1,0E+01 1,0E+02 1,0E+03 1,0E+04


Ph

as

e f

luc

tua

tio

ns

(ra

d/√

Hz)

δΦ Freq (rad/√Hz) δΦ DSSO (rad/√Hz) δΦ RIN (rad/√Hz) δΦ ambiant (rad/√Hz)

MZIAODSS GSf

fP 85,002,2

00 )(10.

2

2

Ln

G effMZI

Interferometric Phase Resolution


16

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.

1,0E-04

1,0E-03

1,0E-02

1,0E-01

1,0E+00

1,0E+01

1,0E+00 1,0E+01 1,0E+02 1,0E+03 1,0E+04


Ph

ase

flu

ctu

atio

ns

(rad

/√H

z) f

or

dif

fere

nt

aco

ust

o-o

pti

c se

nsi

tivi

ties

Sao=1,5E-5 (nm/Pa) δΦ Freq (rad/√Hz) Sao = 4,0E-5 (nm/Pa) Sao=0,75E-5 (nm/Pa) δΦ ambiant for Sao = 4E-5 (rad/√Hz)


17

• 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.

Laser Noise Equivalent Pressure

PEBPSao(nm/Pa) f(Hz)

δΦfreq = δΦdsso (rad/√Hz) (µPa/√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

4,00E-05 9000 0,0012 26

intint GSGSP

AO

éfreq

éRIN

AO

equPEB

intGSP

AO

DSSODSSO

18

• 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. 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 < 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.

Conclusion


optical noise of a 1550 nm fiber laser as an underwater acoustic sensor

Documents

optical noise

noise dss0 sea noise

phase noise detection

background sea noise

noise level dss0

acoustic amplification

bare fiber laser

underwater acoustic