Optical Signal Property Synthesis at Runtime – An new approach for coherent transmission stress testing

Optical Signal Property Synthesis at Runtime

Andy DobersteinKeysight, GermanyMay 2015

A new Approach for Coherent Transmission Stress Testing

Optical Signal Synthesis at Runtime

Page 2Outline

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

PageOptical Signal Synthesis at Runtime

3

Test strategies for coherent optical receivers

5/27/2015


4

Next generation optical transmission systemsChallenges and requirements

5/27/2015

ROADM: reconfigurable optical add-drop multiplexerEDFA: Erbium-doped fiber amplifierWXC: Wavelength cross connect

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

Key element: Coherent optical receiverBasic block diagram

5/27/2015


6

- 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 testingTraditional test setup

5/27/2015

Reference TX or golden

line card

Pattern generator

Fiber testbed

Coherent Receiver

Error Detection

DUT

Page

Coherent receiver testingAWG based optical signal synthesis


line card

Pattern generator

Fiber testbed

Coherent Receiver

Error Detection

Optical Signal Synthesizer

Coherent Receiver

Error Detection

DUT

Laser source MZ

MZPBS PBC

E/O

Memory

D/AD/A

D/AD/A

AWG

Example: 256Gb/s channel- 64GSa/s sampling rate- 32GBaud data rate - DP-QAM16

Page

Coherent receiver testingAWG based optical signal synthesis


line card

Pattern generator

Fiber testbed

Coherent Receiver

Error Detection


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

AWG based signal synthesisLimitation in current architecture

5/27/2015

– 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


10

Introducing DSP processing into AWG based optical signal synthesizer

5/27/2015


11

AWG based signal synthesisAdvanced real-time processing

5/27/2015


line card

Pattern generator

Fiber testbed

Coherent Receiver

Error Detection


Coherent Receiver

Error Detection

DUT

D/AD/A

D/AD/A

AWG Laser source MZ

MZPBS PBC

E/O

DS

P

Mem

ory

NEW


12

- 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 architectureAdding real-time processing into signal synthesizer

5/27/2015


13

Comparison Waveform playback vs. real-time processing

5/27/2015

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-calculationCoefficients / pattern update at

runtime

Continuous playWaveform loops at end requiring matching

begin and endSymbol 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

AWG with advanced Tx-DSP functionalityArchitectural overview

5/27/2015

Architecture of AWG with real-time processing using the example of a Keysight M8195A prototype


15

Clean signal generation

5/27/2015


16

Clean signal generationBasic blocks

5/27/2015

byp

asse

d

bypa

ssed



17

Clean signal generationBasic blocks

5/27/2015

byp

asse

d

bypa

ssed



18

Encoding examples:

QPSK QAM16 QAM16

QAM64 QAM128

Clean signal generationSymbol encoding

5/27/2015

- Encoding up to QAM256

QPSK


19

Clean signal generationPulse shaping (interpolation filter)

5/27/2015

- 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 10

0.2

0.4

0.6

0.8

1

Normalized frequency

Pow

er s

pect

rum

(lin

ear)

Raised Cosine Filter

= 1 = 0.7 = 0.35 = 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

Pow

er s

pect

rum

(lo

garit

hmic

)

Raised Cosine Filter

= 1 = 0.7 = 0.35 = 0.05


20

Generation of optical signal properties and impairments

5/27/2015


21

Optical signal property synthesisPhase noise & carrier frequency offset

5/27/2015



22

Electrical field of unmodulated carrier signal (single-mode semiconductor laser)

Adding artificial phase noise / frequency offset (t)


5/27/2015

𝑐 (𝑡 )=𝐸 ∙𝑒𝑥𝑝 ( 𝑗 [2𝜋 𝑓 0 𝑡+𝜑 (𝑡 ) ] )

Amplitude of electrical field

Carrier frequency Intrinsic phase noise

𝑐 (𝑡 )

𝑒 𝑗 𝜃 (𝑡 )

𝑐 ′ (𝑡 )


23

- Pre-programmed pattern contains rotation angles calculated on basis of laser phase noise model

• Laser linewidth• Flicker noise / Random walk noise


5/27/2015

Clean signal Emulated phase noiseExemplary phase pattern emulating desired phase noise parameters


24

Optical signal property synthesisPolarization control/rotation

5/27/2015



25

- 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


5/27/2015

[ 𝑋 𝐼 ′+ 𝑗𝑋𝑄 ′𝑌 𝐼 ′+ 𝑗𝑌𝑄 ′ ]=[𝑊 𝑥𝑥 𝑊𝑥 𝑦

𝑊 𝑦 𝑥 𝑊 𝑦𝑦] ∙ [𝑋 𝐼+ 𝑗𝑋𝑄

𝑌𝐼+ 𝑗𝑌𝑄 ]Formula of 1-tap butterfly filter with scalar, complex coefficients Wxx, Wxy, Wyx and Wyy


26


5/27/2015

Examples of polarization rotation patterns with exemplary histogram of SOP change rate

- Great circle pattern

- „Slicer“ pattern

SOP rate of change distribution


27


5/27/2015

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

Optical signal property synthesisPMD emulation

5/27/2015



29


5/27/2015

- 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


5/27/2015

Addressable PMD space with shown modelat 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 2200

2000

4000

6000

8000

10000

12000

First-order PMD [ps]

Sec

ond.

orde

r P

MD

[ps

2 ]

-50 -40 -30 -20 -10 0 10 20 30 40 500

50

100

150

200

DG

D (

ps)

-50 -40 -30 -20 -10 0 10 20 30 40 500

5000

10000

SO

PM

D (

ps2 )

-50 -40 -30 -20 -10 0 10 20 30 40 50-4000

-2000

0

2000

4000

Relative frequency (GHz)

PD

CD

(ps

2 )

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

Summary

5/27/2015

- 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.


32

Question & Answers

Thank you!

5/27/2015

Technology

Optical Signal Property Synthesis at Runtime – An new approach for coherent transmission stress testing