- spiral launch method for enhanced mmf...

1

Spiral Launch Method for Enhanced MMF Bandwidth

D. Vernooy and H. BlauveltXponent Photonics

March 2004IEEE 802.2 10Gb/s on FDDI-grade MM fiber Study Group

hblauvelt@xponentinc.com

2

Outline

I. Overview of the problem and proposed solution• Overlap of various multimode fiber modes to center and outer index

defects• Launched mode populations for conventional offset launch (OSL) and

spiral launch (SL)• Encircled flux characteristics of offset and spiral launch

II. Bandwidth simulations for offset launch and spiral launches• Fibers with single index defect• Fibers with combinations of index defects

III. Simulation of impact of misaligned connectors on offset andspiral launch

IV. Implementation methods for spiral launchV. Initial test results with spiral launch mode conditioning

patchcord and comparison to offset launch and overfilled launch (OFL)

VI. Summary

3

I. Overview of Modal Dispersion Problem and Proposed Solution

4

Offset vs Spiral Launch

» In offset launch, a beam (often from single mode fiber) is launched into MMF at a point that is radially offset from the center of the core. Beam is launched parallel to optical axis

»From a ray optics perspective, offset launch excites rays that periodically cross the center of the fiber with outer turning points equal to the initial radial offset

» In a spiral launch a beam is launched into the MMF with a radialoffset and an angle in the azimuthal direction

»From a ray perspective, spiral launch excites rays that travel in a spiral path down the fiber and never cross the center of the core.

5

Conventional Offset Launch

Optical axis

Beam launch+

6

Spiral Launch

Optical axis

Circular or Elliptical beam launch

+

z

x

y∆y

∆y

7

Differential Mode Delay (DMD) Limited Bandwidth and Enhancement via Selective Mode Launch

» Deviations from optimum index profile can significantly increase DMD and decrease bandwidth

» Three main categories of index profile defects1. Dip or peak at center of core: Can be addressed by not

launching modes that overlap the center of the core2. Dip or peak near core-cladding interface: Can be addressed by

not launching modes that overlap the core-cladding interface. 3. Deviation from optimum power law profile across core: Can be

minimized by launching modes with a reduced range of principle mode numbers

» Selective mode launch technique should address all three categories

» Conventional offset launch is very good at category 3, but not as good at simultaneously meeting categories 1 and 2

» Spiral launch can simultaneously minimize the impact from all three categories of index profile defect

8

Overview of Mode Characteristics

» Fibers have truncated power law index profilen2 = n2

core[1-2∆(r/a)α] r<a= n2

core[1-2∆] r>a» Modes can be characterized by an azimuthal mode number, L

and a radial mode number, M; L=0, M=1 is fundamental mode» For infinite quadratic profile, mode field has dependence

E ~ rLLLM-1 (Vr2)exp(-Vr2/2)exp(iLΦ)

• All modes of order L>0 have nulls in amplitude at center, exclusion from center increases with L

• All L=0 modes have maximum amplitude at the center• Radial “center of mass” depends on principle mode number, m

m = 2M+|L| - 1

» Real multimode fiber modes are very similar except for modes approaching cut-off

9

Lowest Azimuthal Order Modes: 62.5 Micron Fiber

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1

Normalized Radius

Mod

e Am

plitu

de

L=0 M=1L=0 M=2L=0 M=3L=0 M=4L=0 M=5L=0 M=6L=0 M=7

Central Defect Region Outer Defect Region

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Lowest Radial Order Modes: 62.5 Micron Fiber

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.2 0.4 0.6 0.8 1

Normalized Radius

Mod

e Am

plitu

de

L=0 M=1L=1 M=1L=2 M=1L=3 M=1L=4 M=1L=5 M=1L=6 M=1L=7 M=1L=8 M=1

Central Defect Region Outer Defect Region

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Mode overlap to Central Defect: 62.5 Micron Fiber

Azimuthal Order [L]

RadialOrder [M]

9

8

7

6

5

4 2-4

3 4-8

2 >8

1

-16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

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Mode Overlap to Outer Defect Region: 62.5 Micron Fiber

9

8 5-10

7 10-20

6 >20

5

4

3

2

1

-16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Modes beyond cut-off

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Combined Center and Outer Defect Exclusion Zone: 62.5 Micron Fiber

9

8

7

6

5

4

3

2

1

-16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

14

Exclusion Zones for 50 Micron Core Fiber

9

8

7

6

5

4

3

2

1

-16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

15

Mode Excitation for 20 Micron Offset Launch 4.5x4.5 Micron Beam: 62.5 Micron Fiber

9

8

7

6 1-2

5 2-3

4 >3

3

2

1

-16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

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Mode Excitation for Spiral Launch 4 Degree Angle, 12 Micron Offset, 4.5 x 4.5 Micron Beam

9

8

7

6 >10

5 5-10

4 2.5-5

3

2

1

-16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

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Spiral Launch 3 Degree Angle, 12 Micron Offset 4.5 x 12 Micron Beam

9

8

7

6

5 >10

4 5-10

3 2.5-5

2

1

-16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

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E n c i rc le d F lu x fo r d i ffe re n t l a u n c h e s i n to 6 2 . 5 m ic ro n s M M F

0

0 .2

0 .4

0 .6

0 .8

1

0 5 1 0 1 5 2 0 2 5 3 0 3 5

R a d iu s [m ic ro n s ]

2 0 m ic r o n s o f f s e t,0 d e g r e e s , 4 .5 x4 .5

1 2 m ic r o n s o f f s e t,3 d e g r e e s , 4 .5 x4 .5

1 2 m ic r o n s o f f s e t,3 d e g r e e s , 4 .5 x1 2

Offset Launch Spiral Launch

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II. Bandwidth Simulations for Offset Launch and Spiral Launch

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Initial Simulation Methodology

»Simulation were done using a subset of the modes of an infinite quadratic index profile fiber

• All modes with neff< ncladding were discarded• Modes of infinite quadratic fiber are very similar to truncated clad

fiber except for those modes very close to cut-off

» Index defects are analyzed as perturbations of baseline fiber»Mode power distribution calculated for each launch method»Group velocities calculated for each mode in the presence of

the index defects»Fiber impulse response and bandwidth calculate based on

population of modes launched and the modal delays»No mode dependent loss included

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Fiber Index Defects

»Central peak or dip: Index defect assumed to have Gaussian shape with FWHM of 3 microns

»Outer Peak or Dip: Index dip is abrupt drop to cladding index at lower than normal radius, peak is 3 micron FWHM increase

»Profile error: Deviation from optimum power law index variation

»Defect amplitudes set so that when only one defect is present, the OFL bandwidth is 500 MHz-km

»27 fiber combinations evaluated»Most fibers with multiple defects have OFL bandwidths less

than 500 MHz, but no scaling adjustments were made

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Conventional Offset Launch: 62.5 Micron Fiber

Fibers with only single defect All fibers

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 5 10 15 20 25 30 35

Offset [microns]

BW

[MH

z*km

]

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 5 10 15 20 25 30 35

Offset [microns]

BW

[MH

z*km

]

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Spiral Offset Launch: 4.5x4.5 Micron Beam, 3 Degree Angle, 62.5 Micron Fiber

Fibers with only single defect All fibers

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 5 10 15 20 25 30 35

Offset [microns]

BW

[MH

z*km

]

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 5 10 15 20 25 30 35

Offset [microns]

BW

[MH

z*km

]

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Spiral Offset Launch: 4.5x12 Micron Beam, 3 Degree Angle, 62.5 Micron Fiber

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 5 10 15 20 25 30 35

Offset [microns]

BW

[MH

z*km

]

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 5 10 15 20 25 30 35

Offset [microns]

BW

[MH

z*km

]

Fibers with only single defect All fibers

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Conventional Offset Launch: 50 Micron Fiber

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 5 10 15 20 25 30

Offset [microns]

BW

[MH

z*km

]

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 5 10 15 20 25 30

Offset [microns]

BW

[MH

z*km

]

Fibers with only single defect All fibers

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Spiral Offset Launch: 4.5x12 Micron Beam, 3 Degree Angle, 50 Micron Fiber

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 5 10 15 20 25 30

Offset [microns]

BW

[MH

z*km

]

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 5 10 15 20 25 30

Offset [microns]

BW

[MH

z*km

]

Fibers with only single defect All fibers

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III. Simulations of the Effects of Misalignments at Connectors

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Effect of Connector Misalignments on Modal Power Distribution

»Fiber bandwidth determined by the modal power distribution through main length of transmission fiber

»Modes couple at misaligned connectors and due to fiber micro and macro bends

»Net effect is a diffusion of the modal population from that of the initial launch

»Width of distribution of principle mode numbers will tend to increase, decreasing bandwidth from profile error defects for both offset and spiral launch

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Diffusion of Power from L=2 M=1 Mode at Offset Connector

3 micron Offset

5 micron Offset

987654321

-16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

5-10% 10-20% 20-40% >40%

9

8

7

6

5

4

3

2

1

-16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

x

x

x: initial mode L order

M order

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Diffusion of Power from L=5 M=1 Mode at Offset Connector

987654321

-16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

5-10% 10-20% 20-40% >40%

9

8

7

6

5

4

3

2

1

-16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

3 micron Offset

5 micron Offset

x

x

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Diffusion of Power from L=0 M=5 Mode at Offset Connector

987654321

-16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

5-10% 10-20% 20-40% >40%

987654321

-16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

3 micron Offset

5 micron Offset

x

x

constant principle mode number

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987654321

-16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

1-5% >80%

Diffusion of Power from L=0, M=5 Mode at 1 µm Offset Connector

x

Power couples to L,M orders +/- 1 from original

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Summary of Mode Coupling Characteristics

»Mean azimuthal mode number, L, is unchanged after mode coupling at offset connector

»Standard deviation of distribution of L and M values after connector proportional to the connector offset and proportional to the square root of the initial principle mode number

»For weak coupling, power couples primarily to modes with +/-1 change in L and/or M number

»Diffusion of modal power distribution is biased towards modes with similar principle mode index

»Diffusion of modal power moves spiral launch distribution closerto exclusion zones, but low L, M order modes with power near central defect diffuse less

»Bandwidth advantage of spiral launch is expected to decrease, but still remain significant, in the presence of multiple misaligned connectors and other mode coupling mechanisms

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IV. Selected Implementation Methods for Spiral Launch

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Spiral Launch Implementations: Elliptical Beam

Fiber CoreLaunch spot

Vertical offset implemented by V-groove widthAzimuthal angle implement by PLC waveguide anglingElliptical beam implemented by PLC waveguide spot size converter

+

36

PLC Implementation Compatible with Spiral Launch

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Stand Alone PLC Spiral Launch Mode Conditioner: Elliptical Beam Launch

PLC

SMF InputMMF Output

Optical waveguidewith mode size converter

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Spiral Launch Silicon Optical Bench Implementation: Circular Beam only

Angle cleaved SMF Straight cleaved MMF

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V. Initial Test Results for Spiral Launch

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Initial Test Result Overview and Limitations

» A primary objective of the spiral launch is to suppress DMD related to central index defects

» None of the fibers used for initial tests have a significant central index defect

» Results for spiral launch are representative of what is expected in the presence of a central defect

» Initial results are for spiral launch with a circular beam. Spiral launch with an elliptical beam is predicted to be substantially better

» Bandwidth and impulse response were measured for three 62.5 micron core fiber reels (two 1.1 km and one 550 m)

» Tests done for four launch conditions• Overfilled with step index type mode scrambler• 4 µm offset launch: Strong indicator of presence of central defect• 20 µm offset launch• 12 µm, 3.5 degree spiral launch with circular beam

» Conditioned launch versions implemented with silicon optical bench designs

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Fiber #1: 1100 Meters

-30

-25

-20

-15

-10

-5

0

0 500 1000 1500 2000 2500 3000

Frequency MHz

Res

pons

e d

B OFLspiral 12OSL 4OSL 20

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Impulse Response Fiber 1

OFL 500 ps/div Spiral launch 200 ps/div OSL 200 ps/div

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Fiber #2: 1100 Meters

-30

-25

-20

-15

-10

-5

0

0 500 1000 1500 2000 2500 3000

Frequency MHz

Resp

onse

dB OFL

spiral 12OSL 4OSL 20

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Impulse Response Fiber 2

OFL 200 ps/div Spiral launch 200 ps/div OSL 200 ps/div

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Fiber #3 550 Meters

-12

-10

-8

-6

-4

-2

0

0 500 1000 1500 2000 2500 3000 3500 4000

Frequency MHz

Resp

onse

dB OFL

spiral 12OSL 4OSL 20

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Test Result Summary

»Key test of suppression of DMD related to central index defects by spiral launch not possible because test fibers lacked central index defects

»Both offset launch and spiral launch substantially outperformed overfilled launch

»Spiral launch bandwidth comparable to slightly better than offset launch for these test fibers

»Significant improvements for spiral launch compared to offset launch expected for fibers with central index defects and for spiral launch with elliptical beam shape

47

Planned Next Steps

»Continue simulation work• Analysis using exact modes of fibers with defects• Expand range of defect fibers in simulation• Modeling of link bandwidth including mode coupling and

mode dependent loss• Modeling of potential spiral launch compliance test

methods

»Experimental work• Link bandwidth measurements on “worst case” fibers with

central index defect• Build TOSA with elliptical spiral launch• Encircled flux measurements, before and after fiber

transmission• Coupled power ratio (CPR) measurements

48

Summary

»Spiral launch method has the potential for significantly mitigating adverse effects of index profile defects

»Spiral launch predominantly excites modes that have negligible overlap to central and outer index defects

»Simulation results indicate an enhancement of bandwidth by 1.6x for 50 micron fiber and >2x for 62.5 micron fiber compared to conventional offset launch

»A single spiral launch condition appears possible for use with both 50 and 62.5 micron fibers while maintaining bandwidth greater than that of separately optimized offset launches; would enable integrated TOSA launch

»Spiral launch can be implemented at low cost»Xponent will make available spiral launch components to

interested parties for evaluation