Forward Path Laser Setup - SCTE Penn

Report
Penn-Ohio Chapter Training
September 20, 2012
Harmonic Confidential
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
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RFin
O/T
O/R
Splices
Connectors
• Transmitter
• fiber
• splice/connector
• Optical amplifier
• Receiver
RFout
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RFin
O/T
O/R
RFout
Splices
Connectors
• Transmitter
Performance
is going to depend on:
• fiber
RF drive level, launched power, laser RIN, number of channels, reflection parameters,
splice/connector
EDFA •noise
figure, EDFA input power, received power, receiver quantum efficiency, receiver
• Optical
amplifier
Thermal
noise, Input
performance, receiver output power, optical modulation index, number of
• Receiver
wavelength
in the system, flatness of the filters, transmitter linearization quality, splice
quantity, SBS parameters, laser chirp, type of fiber, connector cleanliness, ……
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Transmitter (PWL)
Receiver
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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
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Attenuation (dB/km)
2.5
2.0
1.5
1.0
0.50
0.0
800
1000
1200
Wavelength, nm
1400
1600
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Dispersion [ps/(nm* km)]
40
Standard
20
0
Dispersion
Shift
-20
-40
-60
Dispersion
Flat
-80
-100
-120
800
1000
1200
Wavelength, nm
1400
1600
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• Center wavelength (nm)
• Power (dBm or mW)
Linewidth
(0dBm=1mW, 10dBm=10mW,
20dBm=100mW)
• Linewidth (typical MHz)
• RIN noise (typical 155dB/Hz)
• Chirp (MHz/mA)
RIN
noise
Wavelength
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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.
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Directly modulated
RF Input
RF
Pre-distortion
Optical Output
Laser
Bias circuit
Externally modulated
RF
Pre-distortion
Optical Output
RF Input
Laser
Bias circuit
Modulator
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L I Curve
• curve is non linear
• Wavelength depends on
current chirp
Light Intensity
Bias Point
DC Bias Current
Ith
RF Drive current
Optical Level
(Power)
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Transmitter DC output power, P0
Time
Modulation index per single channel, msingle ch. =
(msingle ch.  100 % , otherwise clipping)
PP
P0
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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= N
1/2
x
(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%
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Shot noise
Limited (1dB/dB)
RIN limited (flat)
Thermal noise
Limited (2dB/dB)
Pin
The higher the received power the better the CNR
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Performance
CNR increases 1dB
per dB
CSO degrades 1dB
per dB
OMI
CTB degrades 2dB
per dB
The higher the OMI the better the CNR but the worst the distortion
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Performance
CNR has an
optimum point
CSO degrades 1dB
per dB
OMI
CTB degrades 2dB
per dB
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I
Current
Chirp + dispersion creates distortion
- No full band directly modulated system at 1550nm only at 1310nm
- Externally modulated system at 1550nm for analog
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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 reoptimize the RF input to the laser.
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Rx
Rx
Externally
modulated.
Transmitter
EDFA
Optical
Amplifier
Optical
Receiver
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Directly modulated
RF Input
RF
Pre-distortion
Optical Output
Laser
Bias circuit
Externally modulated
RF
Pre-distortion
RF Input
Optical
Output
Laser
Bias circuit
Modulator
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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
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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.
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1550-nm BC Tx
BC
Optical
filter
Nodes
l1
NC
l2
NC
lN
NC
Headend
Hub
• Important parameters
- Channel loading
- link noise
- Optical Rx power
- Optical delta
- Drive levels
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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
−Must decrease
BC/NC optical delta
−Dual receivers offer
advantage
CNR BC+NC
CNR BC alone
BER QAM
NC number of QAM
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(1)
(2)
(3)
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.
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Optical
Filter
1550-nm BC Tx
BC
Nodes
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
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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 electroabsorbtion 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
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Broadcast-Narrowcast vs. Full Spectrum
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Direct Fed Nodes with Full Spectrum 1550nm TX
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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
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CNR of Optical Link
The overall CNR of a fiber optic communication system from all the
noise sources:
Signal
Shot Noise
m
r
Pr
B
q
Ith
RIN
Thermal Noise Relative Intensity Of
Light
Modulation Index Per Channel
Detector Responsivity [A / W], 1310nm: 0.85, 1550nm: 1.0
Detected Average Optical Power [W]
Noise Equivalent Bandwidth, Video BW For TV system [Hz]
Electron Charge [Coulomb], 1.6 * 10-19
Receiver Thermal Noise [A/Hz 0.5]
Relative Intensity Noise [Hz-1] From Various Sources.
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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
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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
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