1 / 18

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

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***

zinna
Download Presentation

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

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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 . UPON’2008 ENS Lyon 2-6 June 2008

  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

  4. 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 UPON’2008 ENS Lyon 2-6 June 2008

  5. external Pressure p(t) Wavelenght Variation of laser signal Dl (t) Path variation DL(t) Fiber mechanical. strain e(t) Optomechanical coupling Bare DFB FL without acoustic amplifier Principle • MechanicalSensitivity e / Dp • ( Frequency dependance) • acousto-optiqueSensitivity : 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). 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). UPON’2008 ENS Lyon 2-6 June 2008

  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. UPON’2008 ENS Lyon 2-6 June 2008

  8. Experimental Set Up UPON’2008 ENS Lyon 2-6 June 2008

  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 UPON’2008 ENS Lyon 2-6 June 2008

  10. 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 UPON’2008 ENS Lyon 2-6 June 2008

  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. UPON’2008 ENS Lyon 2-6 June 2008

  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. UPON’2008 ENS Lyon 2-6 June 2008

  13. 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. UPON’2008 ENS Lyon 2-6 June 2008

  14. DFB Fiber Laser frequency noise: 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: UPON’2008 ENS Lyon 2-6 June 2008

  15. Interferometric Phase ResolutionWhen 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. Interferometric Phase Resolution UPON’2008 ENS Lyon 2-6 June 2008

  16. Phase noise resolution versus acoustic frequencyThe 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. UPON’2008 ENS Lyon 2-6 June 2008

  17. 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.

  18. 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. 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. UPON’2008 ENS Lyon 2-6 June 2008

More Related