Optical Signal Property Synthesis at Runtime – An new approach for coherent transmission stress testing
1. Optical Signal Property Synthesis at
Runtime
Andy Doberstein
Keysight, Germany
May 2015
A new Approach for Coherent Transmission Stress Testing
2. PageOutline
1. Test strategies for coherent optical receivers
• Requirements and challenges of next generation optical transmission
networks
• Optical receiver stress testing and current limitations
2. Introducing DSP processing into AWG based optical signal synthesizer
• Overview of real-time processing architecture
• Clean signal generation / Generation of optical signal properties
3. Summary
5/27/2015
Optical Signal
Synthesis at
Runtime 2
3. Page
Test strategies for coherent optical
receivers
Optical Signal
Synthesis at
Runtime 3
5/27/2015
4. Page
Next generation optical transmission systems
Challenges and requirements
5/27/2015
Optical Signal
Synthesis at
Runtime
ROADM: reconfigurable optical add-drop multiplexer
EDFA: Erbium-doped fiber amplifier
WXC: Wavelength cross connect
4
Increasing demand:
end-user services (e.g. streaming), cloud
computing applications with different QoS
requirements
Transmission impairments
(stochastic/deterministic nature) :
e.g. CD, PMD, ASE noise,
non-linear distortions, filtering/
ROADM concatenation
Needs network management:
- Cognitive and adaptive optical networks
- Condition monitoring to guarantee QoT
- Reconfigurable transmitter and receivers
5. Page
Key element: Coherent optical receiver
Basic block diagram
5/27/2015
Optical Signal
Synthesis at
Runtime 5
6. Page
- Traditional receiver stress testing has different drawbacks:
• All optical fiber testbed subject to stochastic processes
• Lack of well-defined and reproducible worst-case stress
conditions for coherent receiver testing
• Inflexible, costly structure
Coherent receiver testing
Traditional test setup
Optical Signal
Synthesis at
Runtime
5/27/2015
6
Reference
TX or golden
line card
Pattern
generator
Fiber
testbed
Coherent
Receiver
Error
Detection
DUT
7. Page
Coherent receiver testing
AWG based optical signal synthesis
Reference
TX or golden
line card
Pattern
generator
Fiber
testbed
Coherent
Receiver
Error
Detection
Optical Signal
Synthesizer
Coherent
Receiver
Error
Detection
DUT
Laser
source MZ
MZ
PBS PBC
E/O
Memory
D/A
D/A
D/A
D/A
AWG
Example: 256Gb/s channel
- 64GSa/s sampling rate
- 32GBaud data rate
- DP-QAM16
8. Page
Coherent receiver testing
AWG based optical signal synthesis
Reference
TX or golden
line card
Pattern
generator
Fiber
testbed
Coherent
Receiver
Error
Detection
Optical Signal
Synthesizer
Coherent
Receiver
Error
Detection
DUT
- Benefits of AWG based signal synthesis
• Complementary setup for development of receiver algorithms
• Flexible structure allows switching of signal parameters (modulation formats,
optical impairments, etc.)
• Deterministic and repeatable generation of stress conditions (incl. corner cases)
• Increased test coverage at reduced test time (→ Importance sampling)
9. Page
AWG based signal synthesis
Limitation in current architecture
5/27/2015
Optical Signal
Synthesis at
Runtime
– Conventional AWGs generate signal from pre-calculated waveform
memory (in the range of several GBytes)
→ Waveform exactly repeats after each memory loop iteration making it
unfeasible to synthesize slow optical effects
Example:
16GByte waveform memory (single channel)
with 64GSa/s sampling rate = 250ms play length
– Pre-calculated waveform exhibits strong correlation between undistorted
data signal and emulated impairment
– Cumbersome pre-calculation / upload into AWG waveform memory
(large amounts of data) making it impossible to quickly change signal
and impairment parameters
9
11. Page
AWG based signal synthesis
Advanced real-time processing
Optical Signal
Synthesis at
Runtime
5/27/2015
11
Reference
TX or golden
line card
Pattern
generator
Fiber
testbed
Coherent
Receiver
Error
Detection
Optical Signal
Synthesizer
Coherent
Receiver
Error
Detection
DUT
D/A
D/A
D/A
D/A
AWG Laser
source MZ
MZ
PBS PBC
E/O
DSP
Memory
NEW
12. Page
- Similar structure as in receiver
but in reverse order
- Blocks enabled or bypassed
individually
- Impairments generated
independently of „clean“ signal
→ better decorrelation
- Coefficient banks and pattern
memories can be programmed at
run-time
Novel architecture
Adding real-time processing into signal synthesizer
Optical Signal
Synthesis at
Runtime
5/27/2015
12
13. Page
Comparison
Waveform playback vs. real-time processing
Optical Signal
Synthesis at
Runtime
5/27/2015
13
Operation
Waveform mode
(conventional operation )
Real-time processing
mode
Memory usage Pre-calculated samples representing waveform Only symbol pattern stored
Signal parameter
change
Complete re-calculation Coefficients / pattern update at runtime
Continuous play
Waveform loops at end requiring matching begin
and end
Symbol pattern loops at end
independently on real-time blocks
Signal pre-
distortion
Pre-distortions (frequency response, skew, etc.)
calculated into waveform
Filter coefficients update at runtime
14. Page
AWG with advanced Tx-DSP functionality
Architectural overview
Optical Signal
Synthesis at
Runtime
5/27/2015
14
Architecture of AWG with real-time processing using the example of a Keysight M8195A prototype
16. Page
Clean signal generation
Basic blocks
Optical Signal
Synthesis at
Runtime
5/27/2015
16
Architecture of AWG with real-time processing using the example of a Keysight M8195A prototype
17. Page
Clean signal generation
Basic blocks
Optical Signal
Synthesis at
Runtime
5/27/2015
17
Architecture of AWG with real-time processing using the example of a Keysight M8195A prototype
18. Page
Encoding examples:
QPSK QAM16 QAM16
QAM64 QAM128
Clean signal generation
Symbol encoding
Optical Signal
Synthesis at
Runtime
5/27/2015
18
- Encoding up to QAM256
QPSK
19. Page
Clean signal generation
Pulse shaping (interpolation filter)
Optical Signal
Synthesis at
Runtime
5/27/2015
19
- Generic FIR filter with 32 taps (at 32GBaud symbol rate)
- Time-domain pulse shaping for increased spectral efficiency
Exemplary constellation and eye diagrams for QAM-16 signal filtered with raised cosine filter
a = 0.05 (Narrowest spectrum) a = 1.0 (Best Q-Factor)
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
0
0.2
0.4
0.6
0.8
1
Normalized frequency
Powerspectrum(linear)
Raised Cosine Filter
a = 1
a = 0.7
a = 0.35
a = 0.05
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
-20
-15
-10
-5
0
5
Normalized frequency
Powerspectrum(logarithmic)
Raised Cosine Filter
a = 1
a = 0.7
a = 0.35
a = 0.05
20. Page
Generation of optical signal properties
and impairments
Optical Signal
Synthesis at
Runtime 20
5/27/2015
21. Page
Optical signal property synthesis
Phase noise & carrier frequency offset
Optical Signal
Synthesis at
Runtime
5/27/2015
21
Architecture of AWG with real-time processing using the example of a Keysight M8195A prototype
22. Page
Electrical field of unmodulated carrier signal (single-mode semiconductor laser)
Adding artificial phase noise / frequency offset (t)
Optical signal property synthesis
Phase noise & carrier frequency offset
Optical Signal
Synthesis at
Runtime
5/27/2015
22
𝑐 𝑡 = 𝐸 ∙ 𝑒𝑥𝑝 𝑗 2𝜋𝑓0 𝑡 + 𝜑 𝑡
Amplitude of
electrical field
Carrier frequency Intrinsic phase noise
𝑐 𝑡
𝑒 𝑗𝜃 𝑡
𝑐′ 𝑡
23. Page
- Pre-programmed pattern contains
rotation angles calculated on basis of
laser phase noise model
• Laser linewidth
• Flicker noise / Random walk noise
Optical signal property synthesis
Phase noise & carrier frequency offset
Optical Signal
Synthesis at
Runtime
5/27/2015
23
Clean signal Emulated phase noiseExemplary phase pattern emulating desired
phase noise parameters
24. Page
Optical signal property synthesis
Polarization control/rotation
Optical Signal
Synthesis at
Runtime
5/27/2015
24
Architecture of AWG with real-time processing using the example of a Keysight M8195A prototype
25. Page
- Generation of polarization rotation
patterns based on underlying optical
model
- Pattern parameters and stored in
pattern memory and played repetitively
- Adjustable pattern advance rate to meet
required SOP change rate from few
rad/s to Mrad/s
Optical signal property synthesis
Polarization control/rotation
Optical Signal
Synthesis at
Runtime
5/27/2015
25
𝑋𝐼′ + 𝑗𝑋𝑄′
𝑌𝐼′ + 𝑗𝑌𝑄′
=
𝑊𝑥𝑥 𝑊𝑥𝑦
𝑊𝑦𝑥 𝑊𝑦𝑦
∙
𝑋𝐼 + 𝑗𝑋𝑄
𝑌𝐼 + 𝑗𝑌𝑄
Formula of 1-tap butterfly filter with scalar,
complex coefficients Wxx, Wxy, Wyx and Wyy
26. Page
Optical signal property synthesis
Polarization control/rotation
Optical Signal
Synthesis at
Runtime
5/27/2015
26
Examples of polarization rotation patterns with
exemplary histogram of SOP change rate
- Great circle pattern
- „Slicer“ pattern
SOP rate of change distribution
27. Page
Optical signal property synthesis
Polarization control/rotation
Optical Signal
Synthesis at
Runtime
5/27/2015
27
Examples of polarization rotation patterns with
exemplary histogram of SOP change rate
- Great circle pattern
- „Slicer“ pattern
Measured trajectories on optical
modulation analyzer (N4391A)
28. Page
Optical signal property synthesis
PMD emulation
Optical Signal
Synthesis at
Runtime
5/27/2015
28
Architecture of AWG with real-time processing using the example of a Keysight M8195A prototype
29. Page
Optical signal property synthesis
PMD emulation
Optical Signal
Synthesis at
Runtime
5/27/2015
29
- Model: concatenation of 7 birefringent segments with same retardation but
variable axes orientation and residual phase
- Retardation = 2 / samplerate
(i.e. 31.25ps @64GSa/s) resulting in
max. first-order PMD of 218ps
- Digital time-domain representation as
7-tap butterfly FIR structure, with
programmable, complex impulse
responses hxx, hxy, hyx and hyy
30. Page
Optical signal property synthesis
PMD emulation
Optical Signal
Synthesis at
Runtime
5/27/2015
30
Addressable PMD space with shown model
at 64GSa/s sampling rate:
• First order PMD: up to 218ps
• Second order PMD: up to 11500ps2
Addressable PMD space
0 20 40 60 80 100 120 140 160 180 200 220
0
2000
4000
6000
8000
10000
12000
First-order PMD [ps]
Second.orderPMD[ps2]
-50 -40 -30 -20 -10 0 10 20 30 40 50
0
50
100
150
200
DGD(ps)
-50 -40 -30 -20 -10 0 10 20 30 40 50
0
5000
10000
SOPMD(ps2)
-50 -40 -30 -20 -10 0 10 20 30 40 50
-4000
-2000
0
2000
4000
Relative frequency (GHz)
PDCD(ps2)
PMD spectra (exemplary settings)
32G channel spectrum
- Red dots: selected states from
7 segment model
- Green dots: selected states
from 6 segment model
- Blue dots: simulated random
states
31. Page
Summary
Optical Signal
Synthesis at
Runtime
5/27/2015
31
- Next generation optical transmission systems require cognitive networks and
network condition monitoring to meet demanded quality of service (QoS) for
future applications.
- Powerful DSP algorithms in coherent receivers are one key element for
mitigating optical signal impairments occurring in transmission path and
ensuring required quality of transmission (QoT).
- Real-time processing features in enable deterministic and repeatable stress
conditions for development and tolerance testing of coherent receivers, thus
increasing test coverage at reduced test time.