How to Measure and Manage for Optimal Performance Wajih Daab - - PowerPoint PPT Presentation

Polarization in Fiber Optic Systems: How to Measure and Manage for Optimal Performance

Wajih Daab Product Line Manager May 19th, 2020

Me

• Introduction to Polarization
• Polarization Related Issues in Fiber Optic Systems
• Methods for Measuring Polarization Parameters
• Polarization Management Technologies
• Polarization Mitigation Techniques
• Summary

Outline

3

Fundamental Parameters of Light Waves

Simple electric field representation of a light wave:

Amplitude Frequency Phase (constant)

Time Amplitude Time Amplitude Time Amplitude

) / ( f c   

4

Polarization is Also an Important Property Light Waves

Polarization describes the oscillation direction of an electric field. A polarized wave may be expressed as a sum of two orthogonal waves

        

y x

i y i x

e A e A E

 

http://www.radartutorial.eu/06.antennas/pic/zirkulanim.gif via de.wikipedia.org

5

State of Polarization (SOP)

Linearly Polarized Light Circularly Polarized Light Elliptically Polarized Light

, or , or

) cos( ) , ( Kz t A x t z E   

) cos( ) , ( Kz t A y t z E   

) cos( ) cos( ) , ( Kz t A y Kz t A x t z E

y x

      ) sin( ) cos( ) , ( Kz t A y Kz t A x t z E      

(Left-Circularly Polarized), or

) sin( ) cos( ) , ( Kz t A y Kz t A x t z E      

(Right-Circularly Polarized)

) sin( ) cos( ) , (         Kz t A y Kz t A x t z E

y x

x

E0

y

E0

6

State of Polarization (SOP)

Linear and Circular polarization states are special cases of the generalized elliptical polarization states Note: If δ varies randomly with time, then the light is unpolarized

7

Poincare Sphere Presentation of Polarization

Any SOP can be represented as a point on a sphere with spherical coordinates defined by the

• rientation angle Y and ellipticity angle c .

In the Poincaré Sphere representation, the Cartesian coordinates of the point are the Stokes parameters S1, S2, and S3.

                          c  c  c 2 sin 2 sin 2 cos 2 cos 2 cos 1 S S S S S

3 2 1



x

 2

c 2

RHC LHC L45

L-45 LH

LV

(c=-p/4, =0) (c=p/4, =0) (c=0, =3p/4)

(c=0, =p/2)

(c=0, =p/4)

(c=0, =0)

8

Degree of Polarization (DOP)

DOP is defined as the fraction of the power of the light signal that is polarized DOP = 0: Unpolarized light  Natural light DOP < 1: Partially polarized light.  Reflected natural light, SLED, ASE… DOP = 1: 100% polarized light  Laser light DOP (instantaneous) = 1, <DOP>t ~ 0  Scrambled light

Polarization Filter

• Introduction to Polarization
• Polarization Related Issues in Fiber Optic Systems
• Methods for Measuring Polarization Parameters
• Polarization Management Technologies
• Polarization Mitigation Techniques
• Summary

Outline

10

Polarization is Time Varying in Fiber Systems

Free space: Polarization does not change with time Fiber: Stresses, Temperature, imperfections → fiber birefringence variation Sources of fiber stress

• Temperature (Slow)
• Wind caused vibration (Fast)
• Train induced acoustic vibration (Faster)
• Lightning/ electromagnetic field (Ultra fast)

11

Polarization Related Issues in Fiber Optic Systems

Polarization Dependent Loss (PDL)

• Difference in maximum and minimum IL due to polarization effects as a function of wavelength.
• Different polarization states suffer different attenuations

Polarization Mode Dispersion (PMD)

• The difference in propagation time between fastest-travelling and the slowest-travelling polarization modes.
• Sometimes called differential group delay (DGD).

 

min max

) ( ) (       d d d d ps PMD

i i i

 

         

i i i dB

P P PDL

min, max, ,

log 10

Component

DGD

Component

12

Polarization Related Issues in Fiber Optic Systems

Polarization Extinction Ratio (PER):

• The ratio between the optical power in the principal linear polarization component and that in the orthogonal linear

polarization component at the point of measurement (typically after propagating through a system)

Polarization Dependent Gain (PDG):

• Difference in maximum and minimum Gain due to polarization effects
• The stronger component may experience faster gain saturation

Fast Axis

Polarization Maintaining Component

Slow Axis

         

i fast i slow i dB

P P PER

, , ,

log 10

          

    || || , , || , , || ,

log 10 G G PDG P P G

dB in

• ut

G

13

Polarization Affects the Performance of Coherent Systems

Fast changes in SOP, high PDL and PMD are limiting factors in high-speed transmission systems Polarization mainly affects the following polarization related functions in the receiver:

• Polarization tracking and demultiplexing
• Polarization Mode Dispersion (PMD) Compensation
• Polarization Dependent Loss (PDL) Compensation/Mitigation

TL QPSK QPSK PBC

TX processor

TX

PDLC algorithm PMDC algorithm DeMUX algorithm PBS TL 90° hybrid DSP

RX

• SOP Variation
• PDL Variation
• PMD Variation

14

Optical Component and System Characterization is Essential

Optical components and devices modify the light propagating through them Industry needs to know whether components meet spec Researchers are interested in evaluating / discovering the optical characteristics of devices

Input Distinguishable Output Output error

• Introduction to Polarization
• Polarization Related Issues in Fiber Optic Systems
• Methods for Measuring Polarization Parameters
• Polarization Management Technologies
• Polarization Mitigation Techniques
• Summary

Outline

16

SOP Measurement Methods

Real observables

• Electrical field cannot be measured.
• What can be measured is optical power

Stokes Parameters method

POD-201 PSGA-101

                                                              

2 1 2 1 4 2 1 2 1 3 2 1 2 1 ' 3 ' 2 ' 1 2 1 4 2 1 3 2 1 2 1 3 2 1

)] ( 2 [ )] ( 2 [ ) ( 2 ) ( 2 P P P P P P P P P P P P P P S S S S P P P P P P P P P P S S S S S

• 



/4

plate Circular polarizer

P1 P2 P3 P4

17

DOP Measurement Methods

Polarimeter Polarization Scrambling Maximum/minimum Search

DOP S S S So   

1 2 2 2 3 2

min max min max

P P P P DOP   

POD-201 MPC-201 DOP-201

min max min max

P P P P DOP   

Polarization Scrambler

Photodetector

Electronics

Polarizer

Time Power

P

• ,max

P

• , min

Polarization Scrambler Polarization Scrambler Laser

Polarization Scrambler

Photodetector

Electronics

Polarizer

Polarization Control

min max min max

P P P P DOP   

Time

Power

Po, max Po, min

Feedback

Laser 90 45 RHC

Electronics

18

PER Measurement Methods

Polarimeter Rotating Polarizer Distributed Polarization Cross-talk

        

min max

log . 10 P P PER

A D C B B’ C’

Power in slow axis Power in fast axis

X-talk at B X-talk at C PM fiber

Stress Stress LB LC

DZB=LBDn

POD-201

         

fast fast

P P P PER log . 10

PXA-1000 ERM-202

Photodetector

Electronics

Rotating Polarizer

Time Power

P

• ,max

P

• , min

Broadband Light Source DUT DUT

19

PDL Measurement Methods

Polarization scrambling (PDL only) Maximum/minimum search (PDL only)

        

min max

log . 10 P P PDL

MPC-201 PDL-201

        

min max

log . 10 P P PDL

Polarization Scrambler

Photodetector

Electronics Polarization Control

Time

Power

Po, max Po, min

Feedback

DUT Laser Polarization Scrambler

Photodetector

Electronics Polarization Scrambler

Time Power

P

• ,max

P

• , min

DUT Laser

20

PMD and PDL Measurements Using Matrices Method

Jones and Muller Matrix Methods (PDL and PMD)

The polarization transfer matrix of an optical device under test (DUT) can be described by a 2x2 complex Jones transfer matrix Γ, or by a 4x4 Mueller Matrix M.

                         

PSG y PSG x PSA y PSA x

J J c J J 1

10 01 00 *

Polarization State Generator Tunable Laser Polarization State Analyzer                                            

PSG i PSG i PSG i PSG i PSA i PSA i PSA i PSA i PSA i

S S S S m m m m m m m m m m m m m m m m S S S S S

3 2 1 33 32 31 30 23 22 21 20 13 12 11 10 03 02 01 00 3 2 1

PSGA-101 OVA 5000 OCA-1000 PSG-001

DUT

• X. Steve Yao, Xiaojun Chen, and Tiegen Liu, "High accuracy polarization measurements using

binary polarization rotators," Opt. Express 18, 6667-6685 (2010)

• Introduction to Polarization
• Polarization Related Issues in Fiber Optic Systems
• Methods for Measuring Polarization Parameters
• Polarization Management Technologies
• Polarization Mitigation Techniques
• Summary

Outline

22

Polarization Controller

Converts any given polarization state to any desired polarization state

Control

F1 F2 F3 F4

Fiber Fiber O 45 O 45

F1

Pressure applying actuator

Fiber

Rotate-able fiber squeezer

Control MPC-201,202, 203

23

Polarization Scrambler

It is a Polarization Controller + Algorithm

scrambler MPC-201/202/203 PCD-104

24

Polarization Stabilizer

It is a Polarization Controller + Algorithm + Feedback signal (Internal or external)

POS-203 POS-202 stabilize

25

Polarization Switch

Rotates the polarization state by ±45/90 deg along the latitude line on which the input polarization state falls Uses Magneto Optic (MO) crystal technology

90 deg Polarization Switch Case 1: Linear input SOP →output states A and B Case 2: Elliptical input SOP →.output states C and D 45 deg Polarization Switch Case 1: Linear input SOP →output states A and B Case 2: Elliptical input SOP →.output states C and D

PSW-002 Switch

26

Polarization Generator

Deterministic generation of 6 states of polarization (L-45°, L0°, L45°, L90°, RHC & LHC)

Polarizer 22.5 polarization Rotator (MO Crystal) /4 plate PM or SM fiber Collimator

Drive circuit 1 2 3 4 5 6

Generate PSG-001 PSGA-101 PSY-201

Deterministic generation of any states of polarization using a polarization control and feedback signal from a polarimeter

Generate

27

Live Demonstration of Polarization Related Functions

Polarization Synthesizer/ Analyzer Laser Source @ 1550 nm Manual Polarization Controller

28

• Introduction to Polarization
• Polarization Related Issues in Fiber Optic Systems
• Methods for Measuring Polarization Parameters
• Polarization Management Technologies
• Polarization Mitigation Techniques
• Summary

Outline

30

Polarization Mitigation Techniques

Full understanding of polarization behavior in fiber optic systems is critical to optimize system design and improve measurement accuracies Low insertion loss, low PDL, low activation loss, low back reflection, high speed, and high-power handling polarization management products are essential

scrambler Stabilize Control Depolarize Switch

31

Optical Coherence Tomography (OCT) Application

In order to produce a good interferometric signal, the state of polarization (SOP) of the two interfering arms should be aligned. When DOP<100%, the depth resolution becomes dependent on the polarization mismatch of the sample and reference arms*

Reference Path Swept Laser Source Sample Path Balanced Detector Narrow Linewidth t

Manual or electrically driven polarization controller

*Jiao, Shuliang, and Marco Ruggeri. “Polarization effect on the depth resolution of optical coherence tomography.” Journal of biomedical optics vol. 13,6 (2008): 060503. doi:10.1117/1.3037341 PLC or MPC-3x

32

FBG Based Fiber Optic Sensor Application

Due to Birefringence effects, the reflected wavelength becomes dependent on the polarization state of the incident light FBG can be quantified using the Polarization Dependent Frequency Shift (PDFS)* In order to mitigate polarization sensitivity, a high-speed polarization scrambler or passive fiber optic depolarizer can be used

Broadband Light Source

1 2 3

1 2 3 Passive Fiber- Optic Depolarizer Active Polarization Scrambler

Wavelength detection system

*(Ref. FBGS website)

DEP-002/003 PSM-002 PCD-003/005

33

High Power Laser Application

Controlling laser beam polarization vector during the drilling or cutting process has a significant impact

• f accuracy, efficiency and quality of processing*.

High speed polarization tracker can be used to avoid any unwanted power fluctuations and get rid of polarization dependent gain in the system

Laser

G1 G2 Gn Sample

Multi-stage Amplifiers

Feedback signal Polarization Tracker Polarization Control

*Wyszyński, Dominik & Grabowski, Marcin & Lipiec, Piotr. (2017). Design of instrumentation and software for precise laser machining. AIP Conference Proceedings.

• 1896. 180008. 10.1063/1.5008213.

34

Fiber Optic Manufacturing Test Application

The presence of high PDL in the test setup may degrade the measurement accuracy The effective PDL experienced by a light signal is, in general, given by: A polarization scrambler randomizes the SOP over time. Although the instantaneous DOP of the signal is still 100%, the effective DOP measured at the detector can be lowered to 5% by an appropriate choice of scrambling rate and averaging time.

dB in is PDL and 1, DOP where PDL DOP (dB) PDL

actual effective eff

   

@ 700KHz scrambling frequency

5%

35

Coherent Detection Systems Application

Polarization Emulation sources are used to quantitatively characterize all polarization-related functions

• f coherent receivers

TX

BER Tester

PC

SOP t PMD t PDL t SOP t PMD t PDL t SOP generation

1st order PMD generation 2nd order PMD generation

PDL generation

Tracking Speed Tests Recovery Time Tests

DSP Circuit DeMUX algorithm

RX

Tolerance Range Tests

PMDC algorithm PDLC algorithm MPC-202/203 as SOP emulator PMD-1000/DGD-1000 as PMD emulator PDLE-101 as PDL emulator

36

PMD Effect on Signals is Highly Dependent on the SOP

Power time DOP = 0% Power time DOP = 100% Power time DOP = 100%

“Worst case” “Best case” “Best case”

DGD

Slow axis ny Fast axis nx <ny

If DGD > bit-width

Slow axis ny Fast axis nx <ny

If DGD > bit-width

Fast axis nx <ny Slow axis ny

If DGD > bit-width

37

PDL Effect on Signals is Highly Dependent on the SOP

Optical component with PDL acts as a partial polarizer with two orthogonal axes One axis will exhibit the max loss while the other will exhibit the minimum loss Example:

“All possible SOPs” Pin Po,max Po,min PDL 10 mw 9 mw 7.2 mw 1 dB

Max loss axis Min loss axis Fixed PDL

Power time

Pin Po,max Po,min

38

PDL Effect on Signals is Highly Dependent on the SOP

“Random case” 0<θ<90 “Worst case” “Best case”

Power time

Pin Po,max Po,min

Power time

Pin Po,max Po,actual = Po,min

Power time

Pin Po,actual = Po,max Po,min Po,actual

Max loss axis Min loss axis

Fixed PDL

Max loss axis Min loss axis

Fixed PDL

Max loss axis Min loss axis

Fixed PDL

39

Summary

Polarization is a fundamental property of light Understanding and managing polarization helps improve system performance and measurement accuracies Different techniques can be used to manage and mitigate polarization effects in fiber systems Luna offers a complete range of polarization management solutions

• Modules for system integrations
• Benchtop instruments for building test stations

Luna supplies multiple polarization test and measurement systems for a complete characterization of optical components

​Wajih Daab

Product Line Manager Luna Innovations Inc, Lightwave Division Chino office: 14351 Pipeline Ave., Chino, CA 91710, USA Phone: +1 (909) 590-5473

www.generalphotonics.com | www.lunainc.com/