Vadim Winebrand Faculty of Exact Sciences School of Physics and Astronomy Tel-Aviv University

1. Vadim Winebrand Faculty of Exact Sciences School of Physics and Astronomy Tel-Aviv University Emulation of penalties in fiber-optic communications systems with the help of a recirculating loop Research was performed under a supervision of Prof. Mark Shtaif

2. 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

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

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

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

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

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

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

9. NLSE Dynamics Characteristic length-scales Nonlinear length Dispersion length

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

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

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

13. WDM system considerations – Four wave mixing Each 3 frequencies generate 4th FWM noise Power w1 w2 w3 w4 w5 Spectrum

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

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

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

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

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

19. 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.

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

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

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

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

24. Optical recirculating loop scheme

25. Measurement methods – Bit error rate BER = p(1)p(0/1)+p(0)p(1/0) PDF V0 V1 Voltage

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

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

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

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

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

31. 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

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

33. 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 Real system case Recirculating loop case

34. Periodic PDL in the recirculating loop 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

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

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 • Gain and noise levels of the amplifiers are more stable • OSNR variations transformed to amplitude jitter Eye diagram at 1e-8 Eye diagram at 1e-5

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

38. DPSK - introduction • The data is stored in the phase of adjacent bits. • Reception is performed with delay interferometer DPSK OOK Re{E} Re{E} MZDI Balanced receiver Im{E} Im{E} Modulation scheme of the signal Scheme of the reception system

39. DPSK – transmitter Transmitter experimental setup Scheme of the DPSK modulator Laser modulator Carver DCA Re{E} Requires additional bandwidth Bit stream Sinusoidal signal Im{E} Eye diagram at the output

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

41. DPSK – combining all the system together Output Laser modulator Carver MZDI DCA Bit stream Sinusoidal signal OOK vs. DPSK

42. 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

43. Thank you