Forward Laser Set up & Maintenance

1. Forward Laser Set up & Maintenance Penn-Ohio Chapter Training September 20, 2012

2. AGENDA • Introduction • Review of optical components and their impact on system performance • Direct fed 1310 TX • Long haul 1550 TX • 1550nm Broadcast / narrowcast • Full band TX (O-band, C-band, EM, EAM) • Summary

3. Typical networking link RFin O/T RFout O/R Splices Connectors • Transmitter • fiber • splice/connector • Optical amplifier • Receiver

4. Typical networking link RFin O/T RFout O/R Splices Connectors • Transmitter • fiber • splice/connector • Optical amplifier • Receiver Performance is going to depend on: RF drive level, launched power, laser RIN, number of channels, reflection parameters, EDFA noise figure, EDFA input power, received power, receiver quantum efficiency, receiver Thermal noise, Input performance, receiver output power, optical modulation index, number of wavelength in the system, flatness of the filters, transmitter linearization quality, splice quantity, SBS parameters, laser chirp, type of fiber, connector cleanliness, ……

5. 1310nm Transmitter Direct Fed Applications Transmitter (PWL) Receiver

6. Single mode fiber characteristic • Attenuation • 1310 nm: < 0.35 dB/km • Minimum loss near 1550 nm: < 0.22 dB/km • Standard design value @ 1550 nm: 0.25 dB/km • Dispersion • Dispersion: Traveling speed of a lightwave in a medium varies with wavelength • Dispersion parameter for SMF-28 fiber • Zero near 1310 nm • +17 [ps/(nm*km)] @ 1550 nm

7. Attenuation versus wavelength 2.5 2.0 1.5 Attenuation (dB/km) 1.0 0.50 0.0 800 1000 1200 1400 1600 Wavelength, nm

8. Dispersion characteristic of single mode fiber 40 Standard 20 0 Dispersion Shift -20 Dispersion [ps/(nm* km)] -40 -60 Dispersion Flat -80 -100 -120 800 1000 1200 1400 1600 Wavelength, nm

9. Single mode lasers: Key parameters Linewidth • Center wavelength (nm) • Power (dBm or mW) • (0dBm=1mW, 10dBm=10mW, 20dBm=100mW) • Linewidth (typical MHz) • RIN noise (typical 155dB/Hz) • Chirp (MHz/mA) RIN noise Wavelength

10. Distributed FeedBack laser (DFB)(usually uses an Isolator) • Uncooled DFB • No temperature control Wavelength varies with temperature • Cheaper • Used for non-WDM application or CWDM application • Cooled DFB • Uses a TEC to keep the temperature constant. • Wavelength stays constant with outside temperature • Used for DWDM • More expensive.

11. Optical transmitter: Intensity modulation RF Pre-distortion RF Pre-distortion RF Input Optical Output Optical Output RF Input Laser Laser Modulator Bias circuit Bias circuit • Directly modulated • Externally modulated

12. Directly modulated: L-I curve • curve is non linear • Wavelength depends on • current chirp

13. Optical Modulation Index (OMI) Optical Level (Power) TransmitterDCoutput power, P0 Time PP P0 Modulation index per single channel, msinglech. = (msingle ch. 100 % , otherwise clipping)

14. Composite modulation index • For a multichannel system, the RF carriers are uncorrelated and the effective modulation index is the root mean square (rms) sum of the indexes of each channels. • Composite OMI= N1/2x (OMI/ch) • where N is the total channel number, msingle is the modulation index of a single channel. Total RMS modulation should be limited to 25-30%. Example: for 80a, OMI per channel= 3.5%

15. Performance vs. received power RIN limited (flat) Shot noise Limited (1dB/dB) Thermal noise Limited (2dB/dB) Pin The higher the received power the better the CNR Not applicable to direct-mod 1550nm FS trransmitters

16. Performance vs. OMI with analog channel only CNR increases 1dB per dB Performance CSO degrades 1dB per dB CTB degrades 2dB per dB OMI The higher the OMI the better the CNR but the worst the distortion

17. Performance vs. OMI with analog + QAM channels CNR has an optimum point Performance CSO degrades 1dB per dB CTB degrades 2dB per dB OMI

18. Chirp in Directly Modulated Systems I Current • Chirp + dispersion creates distortion • No full band directly modulated system at 1550nm only at 1310nm • Externally modulated system at 1550nm for analog

19. Setting up your 1310 Link • Initial setup • Verify RF input is the correct level. • RF input should be flat. • Note: Factory Settings (Harmonic) • 80 unmodulated carriers 45 to 550 MHz. • Above 550 is 450 MHz digital -6db down from analog. • RF input level is 15dbmv. • If the channel load is different adjust RF input accordingly. • Run Auto Setup (Harmonic) • Fine Tune the transmitter by manually adjusting the internal RF pad. • Periodically • Verify RF input is flat and the correct level. • Verify delta between the analog and digital channels. • If the transmitter is in MGC and the channel load has changed re-optimize the RF input to the laser.

20. Link demonstration

21. 1550nm Transmitter Broadcast and Long-Haul Applications Rx Rx Externally modulated. Transmitter EDFA Optical Amplifier Optical Receiver

22. Optical transmitter: Intensity modulation RF Pre-distortion RF Pre-distortion Optical Output RF Input Optical Output RF Input Laser Laser Modulator Bias circuit Bias circuit • Directly modulated • Externally modulated

23. Stimulated Brillouin Scattering Pout Ptrans Acoustic wave light Prefl Pin • Non-linear effect in fiber that limits the amount of light that can be launched into fiber to about 7dBm per 20MHz BW) • Special technique are used to limit the effect of SBS in externally modulated system allow launch of 17dBm with one wavelength • Beating between incoming & reflected laser beams introduce additional CSO & CTB distortions

24. Setting up your 1550 Link • Initial setup • Verify RF input level of 18 dBmV (Harmonic) • RF input should be flat. • Turn Switch to Factory Settings in AGC (Harmonic) • Note: Factory settings • RF input 18dBmv • MGC- 80 NTSC Channels - Set pilot pads accordingly. • Check for SBS and adjust accordingly. • SBS Adjustment (Harmonic) • Under Transmitter adjustments • Select Dual tone for links less than 85km. Select Single tone for links longer than 85 km. In single tone max optical launch power is 14dBm. Adjust SBS 1 or SBS2 as necessary.

25. 1550nm Transmitter Broadcast / Narrowcast Applications

26. BC/NC Architecture: Overview Optical filter 1550-nm BC Tx Nodes BC l1 NC l2 NC lN • Important parameters • Channel loading • link noise • Optical Rx power • Optical delta • Drive levels NC Headend Hub

27. Well Served by this Solid Architecture (but …) • Good performance (>51 dB CNR) using fewer fibers • Good fiber reach (50 km or more) • Now possible to use O-Hubs instead of buildings • Some limitations starting to become apparent • Older narrowcast transmitters limited to 8 QAMs • Newer transmitters support up to 50 QAMs CNR BC alone CNR BC+NC BER QAM • Must decrease BC/NC optical delta • Dual receivers offer advantage NC number of QAM

28. Setting up your BC/NC Link 1- Setup the BC transmitter at the right level (not overdriven) 2- Setup the optical delta between BC and NC. -10 for 64 QAM and -6 for 256 QAM. 3 -Adjust RF pad on NC TX to have the proper level for the QAM NC compared to the BC. (1) (2) (3)

29. With 10 dB Optical Delta

30. With 7 dB Optical Delta

31. Dual Receiver Option Optical Filter 1550-nm BC Tx Nodes BC l1 NC + l2 NC RF filter + RF combiner Headend Hub • Removes the NC noise on the BC • Removes the BC beat term below the NC (if BC Tx is overdriven) • Optical delta is not so important anymore • Level of NC QAMs are adjusted in the node

32. WDM Full-Band Transmitters O Band / C Band Applications

33. WDM Full Spectrum Transmitters • O-Band (1260nm – 1360nm) is older technology limited by Raman Crosstalk. • Large wavelength separation causes a problem … trade off between number of wavelengths and launched power • Two competing technologies at C-Band (1530-1565nm) • Low chirp laser sources such as external modulation or electro-absorbtion modulator (EAM) • Widely available laser sources using newest predistortion technology to control dispersion • FS Transmitters offer segmentation options never before possible and have advantages over BC/NC architectures

34. Broadcast-Narrowcast vs. Full Spectrum

35. Direct Fed Nodes with Full Spectrum 1550nm TX

36. Full Spectrum Performance Considerations • Is it time to re-think our node input levels ?? • Traditionally, we have targeted 0 dBm or higher • Modeling shows that levels of -5 to +3 dBm offers flat MER performance with mostly QAM loading RIN limited (flat) Operating region, DWDM 1550 nm Operating region, traditional

37. CNR of Optical Link • The overall CNR of a fiber optic communication system from all the noise sources: m Modulation Index Per Channel r Detector Responsivity [A / W], 1310nm: 0.85, 1550nm: 1.0 Pr Detected Average Optical Power [W] B Noise Equivalent Bandwidth, Video BW For TV system [Hz] q Electron Charge [Coulomb], 1.6 * 10-19 Ith Receiver Thermal Noise [A/Hz 0.5] RIN Relative Intensity Noise [Hz-1] From VariousSources. Signal Shot Noise Thermal Noise Relative Intensity Of Light

38. RIN Sources • Laser RIN- Typically Small Contribution • EDFA Noise - Small or large depending on optical input power (per wavelength) into the EDFA and number of EDFAs in the link. • Fiber Noise - Depends on the technology and fiber length. Large contribution with long fibers with SPL; small contribution with HLT and PWL. • CIN (Intermodulation Noise) - Depends on QAM load, fiber length, technology,.. • Four Wave Mixing (FWM) - Depends on number of optical channels, wavelength separation between channels, optical power into fiber,… IF link noise is dominated by RIN noise, then… CNR doesn’t improve much with increased received power • RIN noise behaves like this:  1dB increase of optical received power translates into 2dB increase in RF carrier level and 2dB increase in noise power translating into RIN generated CNR independent of received power Raising the node optical levels may actually decrease the CNR/MER because you have increased the RIN as a result of increased power in the fiber

39. Full Spectrum Performance Considerations • Is it time to re-think our node input levels ?? • Traditionally, we have targeted 0 dBm or higher • Modeling shows that levels of -5 to –3 dBm offers optimum performance • What should the performance target be for MER ?? • Today, operators strive for 38-39 dB MER • Studies suggest that with all QAM networks, 35-36 dB MER offers great performance and plenty of margin • Some say that BER is a better performance indicator

40. Thank You