Penn-Ohio Chapter Training September 20, 2012. Harmonic Confidential Introduction Review of optical...

Forward Laser Set up & Maintenance

Penn-Ohio Chapter TrainingSeptember 20, 2012

Harmonic Confidential

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


Typical networking link

O/T

O/RRFin RFout

Splices

Connectors

• Transmitter• fiber• splice/connector• Optical amplifier• Receiver


Typical networking link

O/T

O/RRFin RFout

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, ……


1310nm Transmitter Direct Fed Applications

Transmitter (PWL)

Receiver


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


Attenuation versus wavelength

0.0

0.50

1.0

1.5

2.0

2.5

800 1000 1200 1400 1600

Atte

nuat

ion

(dB

/km

)

Wavelength, nm


Dispersion characteristic of single mode fiber

Wavelength, nm

-120

-100

-80

-60

-40

-20

0

20

40

800 1000 1200 1400 1600

Dis

pe

rsio

n [p

s/(n

m*

km)]

Standard

Dispersion Shift

Dispersion Flat


Single mode lasers: Key parameters

Linewidth

RIN noise

Wavelength

• Center wavelength (nm)

• Power (dBm or mW)

(0dBm=1mW, 10dBm=10mW, 20dBm=100mW)

• Linewidth (typical MHz)

• RIN noise (typical 155dB/Hz)

• Chirp (MHz/mA)


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.


Optical transmitter: Intensity modulation

Directly modulated

Externally modulated

Laser

RFPre-distortion

Bias circuit

Optical OutputRF Input

Laser Modulator

RF Pre-distortion

Bias circuit



Directly modulated: L-I curve

Ith

Bias Point

Ligh

t Int

ensi

ty

RF Drive current

L I Curve

DC Bias Current

• curve is non linear• Wavelength depends on current chirp


Optical Modulation Index (OMI)

Time

Op

tica

l Lev

el

(Po

we

r)

Transmitter DC output power, P0

Modulation index per single channel, msingle

ch. =

PPP0

(msingle ch. 100 % , otherwise clipping)


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%


Performance vs. received power

Pin

RIN limited (flat)Shot noiseLimited (1dB/dB)

Thermal noiseLimited (2dB/dB)

The higher the received power the better the CNR

Not applicable to direct-mod

1550nm FS trransmitters


Performance vs. OMI with analog channel only

OMI

Per

form

ance

CNR increases 1dB per dB

CSO degrades 1dB per dB

CTB degrades 2dB per dB

The higher the OMI the better the CNR but the worst the distortion


Performance vs. OMI with analog + QAM channels

OMI

Per

form

ance

CNR has an optimum point

CSO degrades 1dB per dB

CTB degrades 2dB per dB


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


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.


Link demonstration


1550nm Transmitter Broadcast and Long-Haul Applications

Externally modulated. Transmitter

Rx

Rx

EDFA Optical

Amplifier

Optical Receiver


Optical transmitter: Intensity modulation

Directly modulated

Externally modulated

Laser

RFPre-distortion

Bias circuit


Laser Modulator

RF Pre-distortion

Bias circuit

OpticalOutputRF Input


Stimulated Brillouin Scattering

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

Pin

Pout

Prefl

Ptrans

Acoustic wave

light


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.


1550nm Transmitter Broadcast / Narrowcast Applications


BC/NC Architecture: Overview

Headend

1550-nm BC Tx

l1

l2

lN

Hub

NodesOptical filter

BCNC

NC

NC • Important parameters- Channel loading- link noise- Optical Rx power- Optical delta- Drive levels


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 QAMsCNR BC aloneCNR BC+NC

NC number of QAM

BER QAM

−Must decrease BC/NC optical delta

−Dual receivers offer advantage


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)


With 10 dB Optical Delta


With 7 dB Optical Delta


Dual Receiver Option

Headend

1550-nm BC Tx

l1

l2

Hub

Nodes

Optical Filter

BCNC

NC

+

RF filter +RF combiner

• 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


WDM Full-Band Transmitters O Band / C Band Applications


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


Broadcast-Narrowcast vs. Full Spectrum


Direct Fed Nodes with Full Spectrum 1550nm TX


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,

traditional

Operating region,

DWDM 1550 nm


The overall CNR of a fiber optic communication system from all the noise sources:

m Modulation Index Per Channelr Detector Responsivity [A / W], 1310nm: 0.85, 1550nm: 1.0Pr 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 Various Sources.

Signal

Relative Intensity Of Light

Shot Noise Thermal Noise

CNR of Optical Link


Laser RIN - Typically Small ContributionEDFA 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

RIN Sources

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


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


Thank You

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