Vadim Winebrand Faculty of Exact Sciences School of Physics and Astronomy Tel-Aviv University Research was performed under a supervision of Prof. Mark Shtaif

Outline Design of long haul fiber optic communication systems Signal propagation in the optical fiber Introduction to polarization effects in the systems Emulation with help of optical recirculating loop Simulations vs. Experiments Measurements performed to show Polarizations/Nonliniarities interactions Fiber optic DPSK systems

DCM Introduction to WDM long haul fiber optic communication systems TX MUX RX MUX Loss Dispersion Polarization Non-liniarities Noise

Degrees of freedom Transmitted waveform (modulation format) Optical power Dispersion management

Loss management 5 Q factor dB Input power dBm The Q factor grows linearly with input power ASE dominated But non-linear effects become significant Non-linear dominated

System design – Loss management 6 For given average optical power OSNR dB Number of amplifiers

Acc dispersion (ps/nm) Length (km) Dispersion management Acc dispersion (ps/nm) Length (km) Under-compensation Over-compensation Acc dispersion (ps/nm) Length (km) Exact-compensation

Propagation in optical fibers A is envelope of the signal Dispersion of the signal non-linear interaction Loss of the signal Non linear Schrödinger equation NLSE

NLSE Dynamics Characteristic length-scales Nonlinear length Dispersion length

Non-linear effect self phase modulation (SPM) With negligible dispersion SPM SPM induces chirp on the signal

Group velocity dispersion(GVD) When neglecting non-linearities GVD induces chirp as the pulse propagates Dispersion

When both Non-liniarities and Dispersion are present things cannot be described analytically. They get complicated…. Combined Effect of SPM and GVD

WDM system considerations – Four wave mixing Each 3 frequencies generate 4 th Power Spectrum 11  FWM noise

14 WDM system considerations – cross phase modulation(XPM) Phase of the signal depends on neighboring channels SPMXPM

15 WDM system considerations – cross phase modulation (XPM) XPM causes timing jitter and power fluctuations

16 WDM system considerations – Raman crosstalk Power Spectrum It depletes higher frequencies Amplifies lower ones

17 WDM system considerations – Raman crosstalk It depletes higher frequencies Amplifies lower ones It causes power fluctuations

Brillouin scattering The power is scattered back once the Brillouin threshold is passed Negligible in communication systems 18 Power Spectrum Brillouin threshold CW case Modulated signal case

Polarization and Nonlinearity In most of the existing literature – these two phenomena are separated. In the new generation of high-data-rate terrestrial systems this neglect is no longer possible. One of the goals of this work was to demonstrate and characterize polarization effects in long nonlinear systems.

Lack of cylindrical symmetry in fibers Polarization Mode dispersion (PMD) Polarization dependent loss (PDL) The outcome: Polarization effects

= To 1 st order in bandwidth Position dependent birefringence - PMD

NLSE with PMD In each segment the Coupled Nonlinear Schrödinger Equations (CNLSE) are solved:

Penalties of PMD/Non linear interactions Penalties are shown with cumulative Q distribution

Optical recirculating loop scheme

Measurement methods – Bit error rate BER = p(1)p(0/1)+p(0)p(1/0) PDF Voltage V0V1

Measurement methods – eye diagram Eye-diagram is a bit chain that is folded to a single bit slot

Measurement methods-optical spectrum Power spectral density provides significant information Power dB Spectrum Signal power Noise level OSNR Bandwidth

Simulations vs. Experiments Criterions for comparisons Bandwidth evolution Optical spectrum Eye-diagram - difficult. Q factor – difficult.

Comparisons results Comparison between theoretical and experimental spectrums 2dBm power and no precompensations 2dBm power and -precompensator of 290ps/nm 3dBm power and -precompensator of 290ps/nm

30 PMD/Non linear measurements – Idea Changes in dispersion map will worsen effects of PMD But will not affect average Q factor

PMD/Non linear interactions– experimental setup to measure penalties The Q statistics was gathered The Idea is to find that small change in dispersion map increases penalties

Difficulties measuring Q penalty of non-linear PMD Periodic PDL & EDFA amplifiers causes BER fluctuations 32 Periodicity does not allow true PMD measurement Requires high accuracy in measuring BER

PMD&PDL states in the recirculating loop are constant PMD states in the real system are random, but in the recirculating loop they are periodic 33 Real system case Recirculating loop case

Periodic PDL in the recirculating loop 34 Different states of polarizations lead to different OSNR levels Orthogonal noise is attenuated – increasing OSNR PDL element Orthogonal signal is attenuated – decreasing OSNR PDL element

Periodic amplifiers in the recirculating loop 35 PDL causes gain fluctuations PDL element Amplifiers experience polarization dependent gain Amplifiers are calibrated for the first cycle only

36 Solution (?) - Polarization scrambler - at the transmitter Polarization scrambler makes polarized light to un- polarized Effects of PDL are averaged out –but effects of PMD are unchanged OSNR variations transformed to amplitude jitter Gain and noise levels of the amplifiers are more stable Eye diagram at 1e-8Eye diagram at 1e-5

Solution (?) - Loop synchronous polarization controller Changes input polarization to a random state Break periodicity of the PMD and PDL states Problems with LSPC Does not break periodicity of the amplifiers and PDG

DPSK - introduction The data is stored in the phase of adjacent bits. Reception is performed with delay interferometer Modulation scheme of the signalScheme of the reception system OOKDPSK MZDIBalanced receiver Re{E} Im{E} Re{E} Im{E}

DPSK – transmitter Transmitter experimental setup Eye diagram at the output Scheme of the DPSK modulator Laser modulat or CarverDCA Bit stream Sinusoidal signal Re{E} Im{E} Requires additional bandwidth

DPSK reception system MZDI Frequency response of the interferometer Problems Exact one bit delay Phase mismatch Polarization match Controllable environment

DPSK – combining all the system together Output OOK vs. DPSK Laser modulat or CarverMZDI Bit stream Sinusoidal signal DCA

Many thanks to Prof. Mark Shtaif Many thanks for Prof. Moshe Tur Many thanks to Chen Rabiner and Efi Shahmon Many thanks to all members of the laboratory