measurements of vcsel mode delays implications...

Measurements of VCSEL Mode Delays &

Implications for System Performance

Dr. Abhijit Sengupta

CommScope Labs

1300 E. Lookout Drive, Richardson, TX 75082

+1-972-792-3198 · [email protected]

July 15-20, 2012

San Diego, California USA

IEEE-802.3 Plenary Session

2Date: July 10, 2012

Author: Dr. Abhijit Sengupta

Motivation

• Pattern Dependent BER shoulders have been observed for transmission over

MMF links with Multimode VCSELs

• BER shoulder related penalty divergence is caused by RIN and MPN

• In multimode VCSEL, MPN is related to spatial hole burning

• The question is: does spatial dependence of gain switching cause observable

VCSEL modal delays (LMD)

• If LMD is pattern dependent, the deterministic jitter has to be accounted for in

the penalty

• Since MPN in VCSEL is related to transverse modes, is there a spatial

dependence of spectral width?

• How to account for the spatial dependence of spectral width and LMD in the

traditional MPN model?

• Is it possible to design VCSEL with low MPN and characterize MPN

performance from spatial measurements?



Patter Dependent Penalty

Tranceiver Effect

1.00E-16

1.00E-15

1.00E-14

1.00E-13

1.00E-12

1.00E-11

1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1.00E-02

1.00E-01

1.00E+00

-17 -16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2

RxPower (dBm)

BER

D_Stand_ F14_PRBS31

D_Stand_F14_PRBS23

OFS_BIMMF_F6_PRBS23

OFS_BIMMF_F6_PRBS31

D-TMS1_F8_PRBS23

D-TMS1_F8_PRBS31

• Both the Standard and BI OM4 fibers show the pattern dependent BER shoulder with 10G PRBS

• What is the source of pattern dependent penalty?

• Whatever the cause is, the errors occur rather infrequently (BER<10E-8)

BW related

Noise

related

Stand_F14_PRBS31

Stand_F14_PRBS23

BIMMF_F6_PRBS23

BIMMF_F6_PRBS31

BIMMF_F8_PRBS23

BIMMF_F8_PRBS31



RIN and MPN related Penalty Divergence

• MMF Link penalty diverges at different characteristic critical lengths for VCSELs with different RIN and MPN

• Simulations show that the point of penalty divergence can be pushed out to longer length by reducing the RIN and MPN

Reference: A. Sengupta, Proc. IWCS, Vol. 57, p. 320 (2008)



Measurement Methods

SMF

PDVC

SE

L

MMFLMD(r)

PG Sampling

Oscilloscope

Pattern trigger

OSA Spectral width

∆λ (r)

Jitter(r)

RIN (ω,r)



Radial Dependence of VCSEL Spectra

-26-24-22-20-18-16-14-12-10

-8-6-4-202468

101214161820222426

846 847 848 849 850 851 852 853

center wavelength_prms (nm)

r (um)

QSFP-28-1 QSFP-34-1 QSFP-34-2 QSFP-34-3 QSFP-34-4 SFP+123

SFP+C15 SFP+C16 SFP+J13 SFP+J14 SFP+AT17 SFP+AT18

-26-24-22-20-18-16-14-12-10

-8-6-4-202468

101214161820222426

0 0.5 1 1.5 2

prms width

r (um)

QSFP-28-1 QSFP-34-1QSFP-34-2 QSFP-34-3QSFP-34-4 SFP+123SFP+C15 SFP+C16SFP+J13 SFP+J14SFP+AT17 SFP+AT18

• Clearly, there is an observable radial dependence of the average spectral width

• Spectral width may also vary from one bit to another but the bit level spectral width measurement is difficult



• In absence of chromatic dispersion over a short length of fiber, we should see no temporal changes in the bits even though the spectral width has a radial dependence.

• If the spatial dependence of gain switching cause observable VCSEL modal delays (LMD), can we observe a radial dependence of the LMD?

• Is LMD pattern dependent?

VCSEL Mode Delay or LMD measurements



Pattern Dependent Spatio-Temporal Effect

0

0.05

0.1

0.15

0.2

0.25

0 100 200 300 400 500 600 700 800 900

time / ps

power (linear)

+25 um

+15 um

0 um

-25 um

Fiber length =1m

20psError @ bit “0”

Bits 0 0 0 1 0 1 0 0 1 0

No

rma

lize

d A

mp

litu

de

(a

rb.

un

it)



MMF-SMF Radial Scan (Short Length)

0

0.05

0.1

0.15

0.2

0.25

0 100 200 300 400 500 600

time (ps)

Amplitude (arb. unit)

0 um radial offset

10um radial offset

Fiber length =1m

20ps 20ps

Higher order modes are delayed relative to low order VCSEL modeN

orm

aliz

ed

Am

plit

ud

e (

arb

. u

nit

)

10Date: July 10, 2012


MMF-MMF Radial Scan (Short Length)

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

1000 1050 1100 1150 1200 1250 1300 1350 1400 1450 1500

time (ps)

-17.5um

-2.5um

+7.5um

+17.5um

Fiber length =1m

No significant changes are noticed due to limited mode filtering by the MMF

Bits 0 1 1 0 N

orm

aliz

ed

Am

plit

ud

e (

arb

. u

nit

)

11Date: July 10, 2012


Angle and Radial Offset Dependence

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 20 40 60 80 100 120 140 160 180 200

time (ps)


angle high

angle low

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 50 100 150 200 250

time (ps)


0um

10um

Fiber length =1m

The temporal delay or “phase lag” is similar for higher radial offsets

and higher angular offsets

No

rma

lize

d A

mp

litu

de

(a

rb.

un

it)

No

rma

lize

d A

mp

litu

de

(a

rb.

un

it)

12Date: July 10, 2012


Pattern A (Angle Dependence)

0

0.05

0.1

0.15

0.2

0.25

0.3

0 100 200 300 400 500 600 700 800 900 1000

time (ps)


angle low

angle highFiber length =1m

10ps

20ps

10ps

10ps

The relative delay or “phase lag” of the VCSEL modes depends on

the bit sequence

Bits 0 1 0 1 1 0 1 1 1 1 0 N

orm

aliz

ed

Am

plit

ud

e (

arb

. u

nit

)

13Date: July 10, 2012


Pattern A (Radial Offset Dependence)

0

0.05

0.1

0.15

0.2

0.25

0.3

0 100 200 300 400 500 600 700 800 900 1000

time (ps)


0um

10umFiber length =1m

20ps

10ps

10ps

10ps

• The relative delay of the VCSEL modes show similar dependence on radial and angular offsets.

• There is a finite and noticeable delay between when the VCSEL modes are turned “on and off”

Bits 0 1 0 1 1 0 1 1 1 1 0 N

orm

aliz

ed

Am

plit

ud

e (

arb

. u

nit

)

14Date: July 10, 2012


Pattern B (Angle Dependence)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 100 200 300 400 500 600 700 800 900 1000

time (ps)


angle low

angle highFiber length =1m

10ps 40ps

Noise at bit 0

5ps

Bits 0 1 0 0 1 1 0 1 1 0 1 N

orm

aliz

ed

Am

plit

ud

e (

arb

. u

nit

)

15Date: July 10, 2012


Pattern B (Radial Offset Dependence)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 100 200 300 400 500 600 700 800 900 1000

time (ps)


0um

10umFiber length =1m

5ps

40ps10ps

Noise at bit 0

Bits 0 1 0 0 1 1 0 1 1 0 1 N

orm

aliz

ed

Am

plit

ud

e (

arb

. u

nit

)

16Date: July 10, 2012


Summary

• The relative delay of the VCSEL modes show similar dependence to

radial offsets and angular offsets.

• The relative delay of the VCSEL modes depends on the bit sequence

• The occurrence of the observable LMD is limited to a few bits in a long

PRBS, and therefore the BER shoulder related to the effect remains

below BER~10E-8

• There is a finite and noticeable delay in VCSEL modes “on and off”

and there is a spatial dependence

• The relative delay can be up to 20-40ps and impact the signal

irrespective of the fiber length

• MPN has a square law dependence on the source spectral width, and

radial dependence of spectral width may translate into increased

modal noise in the different mode groups of the MMF

17Date: July 10, 2012


Further Measurements

VC

SE

L

MMFPG RINL=0 (ω)

VC

SE

L

MMFPG RINL=x (ω)

RINL=0 (ω)

RINL=x (ω)M((ω) =

• RINL=0 (ω) is measured at with a very short length of MMF , such that the “anti-

correlation” of the noise in individual competing VCSEL modes are maintained

• RIN measurement is repeated with sufficiently long length (X) of MMF, so the

anticorrelation of the noise in the modes are lost due to the temporal separation caused

by the DMD

• M can be used as a measure of MPN

18Date: July 10, 2012


Concluding remarks

• Recent research shows spatial hole burning effect increases the MPN among

modes that overlaps spatially

• Simulations based on spatial hole burning also confirm that there are

significant delays in the gain switching of different transverse modes and the

delays depend on electrode configurations

• May be it would be possible to design the electrodes and cavity to reduce the

overlap of the transverse modes and possibly reduce the MPN

• In the frequency domain, radial dependence of RIN can be applied to

characterize mode power fluctuations in individual modes

• Radial dependent measurements of RIN, LMD, spectral width may be used to

characterize VCSEL for MPN performances

• Possibly, the radial dependence can be incorporated in the existing model of

MPN or k-factor for better prediction of link performance.

19Date: July 10, 2012


References

• A. Sengupta, Proc. IWCS, Vol. 57, p. 320 (2008)

• J. Y. Law, G. H. M. van Tartwijk and G. P. Agrawal, Quantum Semiclass. Opt.

Vol. 9, 737 (1997)

• Dellunde et al., IEEE J. Quant. Elec. Vol. 33, p. 1197 (1997)

• J. Y. Law and G. P. Agrawal, IEEE Phot. Tech. Lett. Vol. 9, p.437 (1997);

• A. Valle, IEEE J. Quant. Elec. Vol. 34, p1924 (1998)

• A.Valle and L. Pesquera, IEEE Phot. Tech. Lett. Vol. 13, p. 272 (2001);

• Tayahi et al., Proc. SPIE Vol. 5737, p.129 (2005);

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