KenIchi Suzuki, Youichi Fukada, Koichi Saito, and Yoichi Maeda

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Long-reach PON system using Gain-clamped Optical
Amplifier
Ken-Ichi Suzuki, Youichi Fukada, Koichi Saito,
and Yoichi Maeda
NTT Access Network Service Systems Laboratories,
NTT Corporation
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Contents
I. Background II. Requirements of Burst-mode
Optical Amplifier for PON System III.
Optical-amplifier-based PON repeater IV.
Optical-amplifier-based PON repeater for giga-bit
PON application V. Conclusion
IV-I Upstream allowable loss measurement of
optically-amplified PON repeatered system
IV-II S/N evaluation and long reach PON
experiment
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Contents
I. Background II. Requirements of Burst-mode
Optical Amplifier for PON System III.
Optical-amplifier-based PON repeater IV.
Optical-amplifier-based PON repeater for giga-bit
PON application V. Conclusion
IV-I Upstream allowable loss measurement of
optically-amplified PON repeatered system
IV-II S/N evaluation and long reach PON experiment
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Background(1)

• PONs provide cost effective optical access
  systems because the transmission fiber and the
  central office equipment can be shared by several
  customers.
• PON systems have grown popular throughput the
  world as the preeminent FTTx system.

Central office
User premise
Access network
UNI
ONU
Optical splitter
NNI
OLT
...
Transmission fiber
OLT Optical Line Terminal ONU Optical Network
Unit NNI Network Node Interface UNI User
Network Interface
UNI
ONU
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Background(2)
Demands placed on PON systems.
Higher speed
Wider area

• OLT must accommodate as many customers as
  possible and as efficiently as possible.
• Enlarge the transmission distance in rural areas
  that have relatively lower service demand.
• Increase the allowable splitting number in urban
  areas that have relatively higher densities.

• Explosive spread of Internet applications
• Advent of new services such as VoIP, VoD, and the
  home network.

Higher speed

• Degrades the receiver sensitivity.

• It is important to improve the allowable
  attenuation range between the OLT and ONUs to
  offer the services to many more customers.

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Background(3)
How to enlarge the allowable attenuation range
PON repeater with 3R functions
needs a dedicated PON repeater for each bit-rate
or system.
The use of optical amplifiers as PON repeaters
can amplify optical signals regardless of the
transmission bit-rate and/or protocol.
Focus on optical-amplifier-based PON repeaters
Much more longer distance
Central office
User premise
Access network
UNI
ONU
Optical splitter
Increase the splitting number
NNI
OLT
...
Transmission fiber
OLT Optical Line Terminal ONU Optical Network
Unit NNI Network Node Interface UNI User
Network Interface
UNI
ONU
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Contents
I. Background II. Requirements of Burst-mode
Optical Amplifier for PON System III.
Optical-amplifier-based PON repeater IV.
Optical-amplifier-based PON repeater for giga-bit
PON application V. Conclusion
IV-I Upstream allowable loss measurement of
optically-amplified PON repeatered system
IV-II S/N evaluation and long reach PON experiment
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Requirements of Burst-mode Optical Amplifier for
PON System(1)

• Upstream and downstream wavelengths are 1.3 mm
  region and 1.49 mm region. So we can employ a
  PDFA as an upstream PON repeater, and a TDFA as a
  downstream repeater.

Optical-amplifier-based PON repeater
OLT
ONU
Upstream Wavelength 1.31 mm region
Optical splitter
ONU
Downstream Wavelength 1.49 mm region
Differential distance between ONUs
Transmission distance between OLT and ONU
Transmission distance logically depends on
ranging method, and physically depends on the
splitting number and the laser type.
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Requirements of Burst-mode Optical Amplifier for
PON System(2)

• The PON repeater based on an optical amplifier is
  a promising approach to achieving both longer
  transmission distance and higher splitting
  numbers.

• The maximum splitting number of branches
  continues to increase.

Optical-amplifier-based PON repeater
OLT
ONU
Upstream Wavelength 1.31 mm region
Optical splitter
ONU
Downstream Wavelength 1.49 mm region
Differential distance between ONUs
Transmission distance between OLT and ONU
Transmission distance logically depends on
ranging method, and physically depends on the
splitting number and the laser type.

• Transmission distance logically depends on
  ranging method, and physically depends on the
  splitting number and the laser type.

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Requirements of Burst-mode Optical Amplifier for
PON System(3)

• Downstream signals are continuous waves, so we
  can employ an optical amplifier with conventional
  techniques for downstream amplification.
• Upstream signals are burst signals, so the
  optical-amplifier-based PON repeater requires
  some form of gain control to amplify optical
  burst signals.

Optical-amplifier-based PON repeater
OLT
ONU
Downstream signals
Upstream signals
Upstream Wavelength 1.31 mm region
Optical splitter
ONU
The OLT receives burst signals.
The ONU receives continuous signals
Gain-clamping is employed.
Downstream Wavelength 1.49 mm region
Differential distance between ONUs
Transmission distance between OLT and ONU
Transmission distance logically depends on
ranging method, and physically depends on the
splitting number and the laser type.
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Gain-clamping and Optical Surge Suppression(1)

• Gain-clamping causes gain suppression, but
  gain-clamping drastically improves PDFA
  linearity. The optical gain deviation is held to
  less than 0.6 dB over the optical input power
  range of -40 dBm to -10 dBm shown in Fig. (a).
• In Fig. (b), we use the optical band-pass filter
  with 1300 nm center wavelength and and 20 nm
  bandwidth.

Gain
Gain
Optical gain, NF (dB)
Optical gain, NF (dB)
NF
NF
Optical input power (dBm)
Optical signal wavelength (nm)
(a)
(b)
(a)Optical gain and NF as a function of optical
input power. Closed and open circle are
with/without GC.
(b)Optical gain and NF as a function of optical
signal wavelength. Open and closed circle are
-35 dBm and -15 dBm of optical input power.
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Gain-clamping and Optical Surge Suppression(2)

• In burst mode amplification, optical surges are
  generated by gain dynamics of optical amplifiers.
  (Estimated gain relaxation time constants are 8
  ms for S-M transition and 76 ms for M-S
  transition.)
• Gain-clamping can improve gain dynamics
  properties and can suppress optical surges.

200 ms
T
a.u.
-7dBm
Without GC
17 dB
-24 dBm
Normalized optical surge intensity(dB)
(b)
With GC
200 ms
a.u.
Without GC
measurements
simulations
Repetition rate (kHz)(1/T)
(a)
(c)
(a) Numerical and experimental results of
normalized optical surge intensity as a function
of burst repeated frequency. Examples of optical
signal trace at repetition rate of 1 kHz
(b)without GC and (c)with GC.
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Contents
I. Background II. Requirements of Burst-mode
Optical Amplifier for PON System III.
Optical-amplifier-based PON repeater IV.
Optical-amplifier-based PON repeater for giga-bit
PON application V. Conclusion
IV-I Upstream allowable loss measurement of
optically-amplified PON repeatered system
IV-II S/N evaluation and long reach PON experiment
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Configuration of optical-amplifier based PON
repeater

• The PON repeater consists of a 0.98 ?m pumped
  GC-PDFA and a 1.40 ?m pumped TDFA. All pump
  powers are set at around 300 mW and optical power
  level of the gain-clamp light is set at about 5
  mW.

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Optical-amplifier-based PON repeater

• The physical dimensions of the developed PON
  repeater interface are 62-mm wide, 220-mm deep,
  and 337-mm high.

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Gain and the NF in the upstream direction

• Although the bidirectional configuration degrades
  the NFs because of the additional WDM couplers,
  especially, in the upstream direction, the PON
  repeater achieved 17 dB gain and good gain
  linearity because the gain compression is less
  than 1 dB at input powers smaller than 10 dBm.

Gain and NF as functions of optical input power
to PON repeater in the upstream direction.
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Gain and the NF in the downstream direction

• Sufficient gain and NF are obtained because the
  14 dB gain is achieved at the input power of -3
  dBm and NFs below 7 dB are achieved at the range
  of -40 dBm to 0 dBm.

Gain and NF as functions of optical input power
to PON repeater in the downstream direction.
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Contents
I. Background II. Requirements of Burst-mode
Optical Amplifier for PON System III.
Optical-amplifier-based PON repeater IV.
Optical-amplifier-based PON repeater for giga-bit
PON application V. Conclusion
IV-I Upstream allowable loss measurement of
optically-amplified PON repeatered system
IV-II S/N evaluation and long reach PON
experiment
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Upstream allowable loss measurement of
optically-amplified PON repeatered system

• The experimental setup of upstream allowable loss
  measurement of optically-amplified PON repeatered
  system consists of our developed GC-PDFA and a
  commercial G-PON system.
• The downstream signals were bypassed by WDM
  couplers to evaluate only upstream signal
  amplification.

Downstream Bit-rate 2.48832 Gbit/s Wavelength
1.49 mm region
ATT
Upstream averaged transmitted power 1 dBm
Downstream averaged transmitted power 6 dBm
WDM (1.31/1.49)
WDM (1.31/1.49)
Optical splitter
ATT
OLT
ONU1
Optical-amplifier-based PON repeater
10/15dB
ATT
V-ATT1
ONU2
15dB

• Maximum level difference from ONUs was set to 10
  dB and 15 dB.

Upstream Bit-rate 1.24416 Gbit/s Wavelength
1.31 mm region
ATT Optical attenuator V-ATT Variable ATT WDM
WDM coupler
Experimental setup of upstream allowable loss
measurement of optically-amplified PON repeatered
system.
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Upstream bit error rate as a function of optical
input power to PON repeater

• 22.5 dB dynamic range is achieved because
  sensitivity and overload to the PON repeater (at
  BER10-10) are -32.5 dBm and -10 dBm,
  respectively.

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Upstream bit error rate as a function of optical
received power at the OLT

• The OLT could not receive optical signals that
  did not exceed the overload power so that optical
  powers above the 1-dB gain suppression point
  cause optical surge.

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Summary to here

• In our experiment, we concluded that optical
  powers above the 1-dB gain suppression point
  cause optical surges which interferes with normal
  reception of optical signals at the OLT. Thus the
  1 dB gain suppression point is an important
  factor in determining the gain linearity of a
  burst-mode optical amplifier.
• We confirmed a 17 dB improvement in the upstream
  allowable attenuation range and raised the
  attenuation range to 48 dB at the ONU power
  difference of 15 dB.

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Contents
I. Background II. Requirements of Burst-mode
Optical Amplifier for PON System III.
Optical-amplifier-based PON repeater IV.
Optical-amplifier-based PON repeater for giga-bit
PON application V. Conclusion
IV-I Upstream allowable loss measurement of
optically-amplified PON repeatered system
IV-II S/N evaluation and long reach PON
experiment
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S/N of optically-amplified PON repeatered system
(1)
Upstream Wavelength 1.31 mm region
Received power
20 km
Transmitted power
Optical splitter
OLT
ONU
Optical amplifier based PON repeater GainG filter
bandwidthBopt
ONU
PON repeater-OLT distancevaried Span loss L
ONU-PON repeater distance fixed at 20 km Span
loss M including splitter loss
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S/N of optically-amplified PON repeatered system
(2)

• The PONs level of split of over 70 can be
  achieved at a transmission distance of 60 km.

ONU-PON repeater distance 20 km Parameter PON
repeater-OLT distance
0 km
10 km
SNR (dB)
BER10-10
20 km
30 km
40 km
Splitting number
SNR as a function of splitting number with the
parameter of PON repeater-OLT distance and the
fixed ONU-PON repeater distance of 20 km.
Attenuation constant is fixed at 0.4 dB/km
(_at_1280nm-1335nm in Fig. A.1/G.957).
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S/N of optically-amplified PON repeatered system
(3)

• Transmission distance of over 67 km can be
  achieved with the attenuation constant of 0.4
  dB/km in the PONs 64 way level of split.

SNR as a function of PON repeater-OLT distance
between with the parameter of attenuation
constant, ONU-PON repeater distance is fixed at
20 km. Splitting ratio is fixed at 64.
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Long reach PON experiment

• The experimental setup of a PON repeatered system
  consists of our developed optical-amplifier-based
  PON repeater and a commercial G-PON system.
• We have successfully demonstrated over 20
  minutes error free transmission for each
  direction in the optically-amplified PON
  repeatered system. This error free time
  corresponds to the bit error rate (BER) of under
  10-11 in each direction thus confirming the
  validity of our optical-amplifier-based PON
  repeater.

Upstream Wavelength 1.31 mm region
20 km
40 km
64-split way
Optical splitter
OLT
ONU
Optical-amplifier-based PON repeater
ONU
10 km
Downstream Wavelength 1.49 mm region
ONU
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Contents
I. Background II. Requirements of Burst-mode
Optical Amplifier for PON System III.
Optical-amplifier-based PON repeater IV.
Optical-amplifier-based PON repeater for giga-bit
PON application V. Conclusion
IV-I Upstream allowable loss measurement of
optically-amplified PON repeatered system
IV-II S/N evaluation and long reach PON
experiment
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Conclusion

• We proposed an optical-amplifier-based PON
  repeater to improve the allowable attenuation
  range between an OLT and ONUs regardless of
  transmission bit-rate and/or transmission
  protocol.
• We clarified the optical gain and the NF of the
  proposed PON repeater.
• We showed the practicality of the
  optical-amplifier-based PON repeater by
  demonstrating error free transmission over a 60
  km long reach optically-amplified PON repeatered
  system with a splitting ratio of 64 that was
  established on a commercial PON system.

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Thank you !