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In-band OSNR Monitoring Technique based on Brillouin Fiber Ring Laser
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In-band OSNR Monitoring Technique based on Brillouin Fiber Ring Laser

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We propose an improved technique for in-band OSNR monitoring based on a Brillouin fiber ring laser seeded by the optical channel to be monitored. This technique shows a reduction of the required input …

We propose an improved technique for in-band OSNR monitoring based on a Brillouin fiber ring laser seeded by the optical channel to be monitored. This technique shows a reduction of the required input power into the monitor along with a large and tunable dynamic OSNR monitoring range. It is demonstrated experimentally and numerically for various bit rates and modulation formats

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  • 1. July 2nd, 2012Brillouin Fiber Ring Laser based In-Band OSNR Monitoring Method for Transparent Optical Networks David Dahan, Uri Mahlab, Yuval Shachaf ECI TelecomNetwork Division Solutions
  • 2. Motivations : Requirement of in-band OSNR monitor Deployment of high speed transparent andreconfigurable optical networks requireseffective, flexible and robust Optical PerformanceMonitoring techniques The most common method to monitor the OSNRderives the OSNR level by estimating the in-bandnoise level using the out-of-band noise levelmeasurement However out-of band OSNR approaches lead tovery large underestimation of real OSNR level inROADM based networks There is a strong requirement in developingefficient in-band OSNR monitor techniques Confidential , not for distribution 2
  • 3. MotivationsSeveral in band OSNR monitoring techniques have been proposedsuch as : PMD and PDL sensitive and not Polarization nulling techniques compliant with polarization multiplexed modulation formats Delay tap asynchronous sampling Nonlinear transfer functions using an CD & PMD sensitive optical parametric amplifier Nonlinear loop mirror CD & PMD insensitive* Stimulated Brillouin scattering* Compliant with Polarization multiplexing** *M.D. Pelusi et al., “Multi channel in band OSNR monitoring using Stimulated Brillouin Scattering” , Opt. Express,18(9), 9435-9446, 2010 **Dahan et al. , “Stimulated Brillouin Scattering based in-band ONSR monitoring technique for 40 Gbps and 100 Gbps optical transparent networks”, Opt. Express, 18(15), 2010not for distribution Confidential , 3
  • 4. Stimulated Brillouin Scattering (SBS) processBrillouin scattering is the interaction between light and sound waves in thematter. The propagating light beam in the fiber generates a propagating soundwave which creates a periodic variation of the fiber refractive index. Thisgenerates a Fiber Bragg Grating that backscatters the light through Braggdiffraction process . The back scattered wave , called “Stokes wave” isdownshifted by ~10 GHz with regard to the incident wave frequencyWhen increasing the launched power of the optical beam, the reflected powerincrease linearly due to back Rayleigh scattering effect in the fiber.Above a given threshold, the reflected power increases exponentially ; this isdue to the stimulated Brillouin scattering effect Confidential , not for distribution 4
  • 5. SBS based in band OSNR monitoring technique : principle & challenges EDFA Power Meter For a given fixed input power, the back-reflected power is OSNR dependent For bit rates higher than 10 Gb/s , OSNR requirements at the RX become strongerand links should be planned to meet OSNR>15dB. Therefore, the in band noise isnot high enough to cause a significant change of the back reflected power in theOSNR monitor, limiting the accuracy of the OSNR measurement. Beyond 10 Gb/s, the optical channels present very high SBS threshold due to theuse of carrier-less modulation formats (DQPSK, PM-QPSK,PM-16QAM). This requiresthe use of long and expensive nonlinear fiber along with high power optical amplifier togenerate the SBS effect: prohibitive cost of the monitor unit! Confidential , not for distribution 5
  • 6. Brillouin Fiber Ring laser based in band OSNR monitoring technique• A novel, relatively low cost technique for SBS based in-band OSNR monitoring, compliant with very high bit rates and various modulation formats.• Enabling to increase and tune effectively the OSNR sensitivity monitoring range• This technique is based on the lasing process of a Brillouin Fiber Ring Laser (BFRL) where the optical seed is the modulated signal to be monitored A 6km DCF is used in the fiber ring to stimulate the SBS process The feedback section loss R is defined as RdB  ILOC  ILOC  IL feedbackfiber  ILsplitter 1 2 Because of the optical circulator configurations, only the Stokes waves undergoes multiple round trip into the ring Confidential , not for distribution 6
  • 7. Power equations of the BFRLAssuming parallel SOP of the signal and Stokes waves, steady states differentialequations governing the signal, Stokes and Rayleigh backscattering powers inthe DCF are : Simulation parameters Value dPsig gB   Psig  DCF Length 6.1 km Psig PStokes  B g B Psig PStokes L α DCF loss coefficient 0.75 dB/km dz Aeff αR Rayleigh backscattering 2.7 10-3 dB/km dPStokes gB coefficient   PStokes  Psig PStokes  B g B Psig PStokes gB Brillouin gain coefficient 1.65 10-11 m/W dz Aeff B Spontaneous Brillouin 8.5 10-3 W m3 dP scattering noise coefficient Aeff DCF effective mode area 16 μm2 Rayleigh   PRayleigh   R Psig Feedback loss (open loop) ∞ dz R Feedback loss (close loop) 4.4 dBWith feedback loss R, DCF length L, the boundary condition are   Psig  0   P0   P PStokes  L   Stokes  0   R  PRayleigh  0   PRayleigh  L    R Confidential , not for distribution 7
  • 8. Principle of operation Experimental & numerical results for CW signal Close loop Open loop RdB=4.4 dB RdB=∞The power threshold is defined as theinput power that leads toPTH=Pout=PStokes+PRayleigh=2PRayleigh PTH Pout=PTH+20dB Pin in close loop 0.3 dBm 0.95 dBm Pin in open loop 6.5 dBm 9.5 dBm Confidential , not for distribution 8
  • 9. Principle of operation Experimental & numerical results for 10.7 Gb/s OOK NRZ signal 10.7 Gb/s NRZ OOK without frequency dithering 10.7 Gb/s NRZ OOK with 10 kHz frequency dithering PTH Pout=PTH+20dB PTH Pout=PTH+20dBPin in close loop 4.2 dBm 5.6 dBm 10.2 dBm 14 dBmPin in open loop 10.7 dBm 12.9 dBm 17.2 dBm 22.6 dBm Without frequency dithering With 10 kHz frequency dithering 9 Confidential , not for distribution
  • 10. In Band OSNR monitor Experimental results for 10.7 Gb/s OOK NRZ signal with frequency ditheringClose loop configuration (RdB=4.4 dB) Power dynamic range= Pout variations over a given OSNR range variations OSNR range Pin=11.5 dB Pin=11.9 dB Pin=12.3 dBm Pin=13.1dBm 10 dB - 15 dB 0 dB 2 dB 8.6 dB 8.2 dB 15 dB - 20 dB 5 dB 8.5 dB 4.5 dB 2.2 dB 20 dB - 30 dB 5.2 dB 3.3 dB 0.7 dB 0.6 dB Confidential , not for distribution 10
  • 11. Principle of operation Experimental results for 44.6 Gb/s RZ-DQPSK signal Carrier-less modulation formats such as DQPSK exhibit a very high SBS threshold leading to a very high required optical launched power. In order to reduce the required launched power, a small power fraction of an optical pilot tone is inserted to a 44.6 Gb/s RZ-DQPSK signal at the output of the transmitter We define Optical Signal to Pilot tone Ratio (OSPR) as : Psignal OSPR  PpilotToneAn optimal frequency detuning, Δf=fsig-fpilotTonecan be found with reduced the pilot tone inducedpenalty at the receiver thanks to the transferfrequency response of the DLI at the receiver andthe balanced detectionFor 44.6 Gb/s RZ-DQPSK signal, OSPR level of 13 dB and frequency offset Δf=-12.3 GHz, givean OSNR penalty of 0.3 dB Confidential , not for distribution 11
  • 12. In Band OSNR monitor Experimental & numerical results for 44.6 Gb/s RZ- DQPSK signalOSNR=24 dB, OSPR=13 dB, 44.6 Gb/s RZ-DQPSK signal with OSPR=13 dB, offset Δf=-12.3 GHz offset Δf=-12.3 GHz Confidential , not for distribution 12
  • 13. Numerical results : 120 Gb/s PM-QPSK signalSince the PM-QPSK modulation format presents carrier-less spectrumcharacteristics, an optical tone is added at the signal carrier frequency. 6dBOSNR penalty < 0.5 dB at BER=1.5E-2 is With OSPR =16dB , the pilot toneachieved for OSPR =16dB in the case of peak is 6 dB above the signaltransmission over a CD uncompensated spectrumlink of 1000km. Confidential , not for distribution 13
  • 14. In Band OSNR monitor Numerical results for 120 Gb/s PM-QPSK signal Close loop configuration – 120 Gb/s DP-QPSK signal with OSPR=16 dB, RdB=4.4 dB offset Δf=0 GHz OSNR range 10 dB -15 dB 15 dB -20 dB 20 dB -30 dB Optimum Pin 17.5 dBm 16.9 dBm 16.6 dBmPower dynamic range 10.8 dB 6.2 dB 3.2 dB Confidential , not for distribution 14
  • 15. Numerical results : 224 Gb/s PM-OFDM signalThe 224 Gb/s PM-OFDM signal iscomposed by 128 subcarriers with cyclic 8dBprefix of 12.5%.Some subcarriers are used as pilot tones forequalization purposes at the receiver whilethe modulated subcarriers use a 16-QAMmodulation scheme.The OFDM signal presents an RF pilot toneat the optical carrier frequency for blindphase noise compensation purposes at thereceiver :this is the main contributor of theSBS effectThe RF pilot tone peak is 8 dB above the other subcarrier components Confidential , not for distribution 15
  • 16. In Band OSNR monitor Numerical results for 224 Gb/s PM-OFDM signal Close loop configuration RdB=4.4 dB OSNR range 10 dB -15 dB 15 dB - 20 dB 20 dB - 30 dB Optimum Pin 19 dBm 16 dBm 15 dBmPower dynamic range 16 dB 14 dB 11.3 dB Confidential , not for distribution 16
  • 17. In Band OSNR monitor System calibrations Estimated OSNR measurement uncertainty for power monitoring accuracy of +/- 0.1dB 44.6 Gb/s 120 Gb/s 224 Gb/s OSNR [dB] Min Max Min Max Min Max 10 9.6 10.4 9.5 10.5 9.8 10.2 12 11.4 12.5 11.4 12.5 11.8 12.3 Good 15 14 16 14 16.1 14.7 15.3 18 16.3 20.3 16.3 20.3 17.5 18.6 20 17.5 24.4 17.5 24.3 19.3 20.7 25 19.8 30 19.9 30 23.2 27.1 Not good Such an increase in the OSNR inaccuracy is caused by :  The power dynamic range decreases at high OSNR  Optimum input power into the monitor approaches the lasing threshold level where the Brillouin laser is very sharp and power monitoring inaccuracies might lead to large errors. MUX PC PS Solution : TX 50% 50% OSA  Working in the optimized OSNR range of 50% 50% ASE source VOA 10 dB -15 dB by adding a known level of ASE noise before the monitor Monitored optical signal  Deriving the altered OSNR level PD0 PD1 OTF OC1 DCF OC2 PC VOA (Pin)  With the knowledge of the ASE added level, EDFA Psig,out Stokes the real OSNR level is estimated * signal 50%*Dahan et al. , “Stimulated Brillouin Scattering based in-band ONSR ASE source OSNR range 50% VOA PSmonitoring technique for 40 Gbps and 100 Gbps optical transparent shifter PD2 (Pout)networks”, Opt. Express, 18(15), 2010 17 Confidential , not for distribution
  • 18. ConclusionsWe have proposed a novel and improved approach for in-bandOSNR monitoring based on Brillouin fiber ring laser seeded by thesignal to be monitoredWe have demonstrated experimentally and numerically that such atechnique enable to reduce drastically the required input power intothe OSNR monitor and provided a large OSNR dynamic powervariations for acceptable monitoring accuracyIn order to provide acceptable monitoring accuracy, the OSNRmonitor should be operated in the optimized OSNR range of 10-15dBby adding a known ASE level into the signal if neededFor carrier-less modulation formats, a relative low power pilot tonecan be inserted into the signal at the transmitter to reduce the SBSthreshold to acceptable values while leading to relative low OSNRpenalty Confidential , not for distribution 18