Chromatic Dispersion Monitoring for WDM Systems using VestigialSideband Optical Filtering

1
Outline
1. Introduction to Reconfigurable
Networks 2. Degrading Effects in
Systems 3. Optical Amplifiers 4. Dispersion
Compensation 5. Polarization Mode
Dispersion 6. Modulation Formats 7. Performance
Monitoring 8. Optical Switching
2
Modulation Formats

• Motivation to explore different modulation
  formats
• Amplitude-Shift-Keying (ASK)
• On-Off-Keying (OOK)
• Non-Return-to-Zero (NRZ),
  Return-to-Zero (RZ)
• Modified RZ formats
• Phase-Shift-Keying (PSK)
• Differential PSK

3
Modulation Formats

• To achieve higher spectral efficiency
• (decrease the cost/bit)
• To make transmission more robust to
• chromatic dispersion
• polarization mode dispersion
• fiber nonlinearities
• channel crosstalk
• To support more low speed end users
• secure transmission

4
On-Off Keying (OOK)
(a) Non-Return-to-Zero (NRZ)
(b) Return-to-Zero (RZ)
5
NRZ vs. RZ Modulation Format
Worst WDM Channel _at_ 40 Gbit/s
16 channels
RZ has less phase matching for long strings of
1 bits
RZ increases transmission distance
M. I. Hayee et al, PTL, 1999
6
Modified RZ Formats

• Chirped RZ (C-RZ)

Optical Spectrum
fclcok
fclcok
frequency
7
Comparison of CRZ, RZ and NRZ
When accumulated dispersion and nonlinear ISI is
large, CRZ provides considerably better
performance.
B. Bakhshi et al, OFC, 2001
8
Carrier Suppressed RZ (CS-RZ)

• CS-RZ format

Data
Clock/2
Intensity Modulator
Intensity Modulator
LD
Output Optical Pulses (Clock)
Optical Spectrum
fclcok
M-Z Output
time
V?
time
Driving Signal (Clock/2)
frequency
9
CS-RZ vs. RZ
RZ In-Phase
CS-RZ Anti-Phase
Pulse width of CS-RZ
K. Sato, IEICE, 2002
10
Vestigial-Sideband RZ (VSB-RZ)
VSB Filtering
11
Duobinary Coding
Generation of Duobinary Signal
Duobinary Output c k
Binary Input a k
2
Coding
1
0

• Benefits
• Minimum-bandwidth signal
• High dispersion tolerance
• Suppresses nonlinear effects

12
Duobinary vs. Binary
10-3
10 Gb/s NRZ
? Binary Back-to-Back ? Duobinary
Back-to-Back ? Duobinary 100 km SMF ? Binary
100 km SMF
10-5
Bit-Error-Rate
10-7
10-9
10-11
-22
-26
-34
-30
Received Optical Power (dBm)
X. Gu, IEE Proceedings-Optoelectronics, 1996
13
Differential Phase-Shift-Keying (DPSK)
DPSK
1
1
0
1
0
0
t
Constant optical power
RZ-DPSK
1
1
0
1
0
0
t
Pulse appears in every bit
14
DPSK Transmitter and Receiver
XOR
Data
Fiber Link
Clock
Decoding
Direct Detection
T
Intensity Modulator
Phase Modulator
Laser
Rx
MZI
DPSK

• Benefits
• Improve receiver sensitivity
• Suppresses nonlinear effects

15
DPSK vs. IM
K. Yonenaga, OFC, 1997
16
Modulation Formats

• PMD impairments depend on the data formats and
  transmitter/receiver designs
• Short pulse may be more robust to PMD
• - need more DGD to cause outages
• - RZ works better than NRZ without PMD
  compensation
• Wide spectrum is more susceptible to
  higher-orders of PMD
• - higher-order PMD decreases the tolerance
  of RZ systems
• - NRZ works better than RZ with simple
  PMD compensation

Is there any modulation format good for PMD?
17
Chirped Pulses in High PMD Fibers
a) PMD
b) Dispersion
t0
t lt t0
t0
t gt t0
t
t
t
t
unchirped pulse
pulse broadening
chirped pulse
pulse compression
c) PMD and Dispersion
t0
t t0
t
t
chirped pulse
PMD compensation
Pulse Compression (Chirp Dispersion)
Pulse Spreading (PMD)
18
Format Dependence in High PMD Fiber
a) Total PMD 0 ps
NRZ
RZ
Soliton
Chirped-RZ
b) PMD 1.7 ps/(km)1/2, total PMD 40 ps
NRZ
RZ
Soliton
Chirped-RZ
10 Gb/s, 570 km

• NRZ pulses distort

• Solitons loose integrity

• RZ pulses split

• CRZ pulses relatively unaffected

R. Khosravani et al, OFC, 2000
19
Modulation Format

• W/o PMD compensation, shorter pulse-widths
  formats perform better
• W/ 1-stage PMD compensation, narrower bandwidth
  formats perform better

• After compensation, higher orders of PMD become
  important!
• Spectral efficiency concerns will drive new
  modulation formats!

(C. Xie et. al., OFC, 2003)