· Tu2G.4.pdf OFC 2016 © OSA 2016 Requirements and Results for Practical VCSEL Transmission using...

Tu2G.4.pdf OFC 2016 © OSA 2016

Requirements and Results for Practical VCSEL Transmission using PAM-4 over MMF

GaTech: Justin Lavrencik, Shriharsha Kota Pavan, Aliro Melgar, Varghese A Thomas

GaTech Research Support: GaTech Terabit Consortium; Adva Optical Networks, Avago, Inphi, OFS, Keysight, Harris, Picometrix, Synopsys

Slides: Balemarthy, Bhoja, Cunningham, Giovane, Kolesar, Kuchta, Lingle, Owens, Tatum


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Outline

Evolution of VCSEL Links Impairments and Noise of VCSEL/MMF Links Simulation and Modeling tools VCSELs Fiber Noise

RIN MPN MPN Revisited

Recent PAM-4 Transmission Results OM4 Wideband Fiber

Summary

2 OFC 2016 Tutorial - PAM-4 and VCSEL MMF Links S. E. Ralph


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MMF Ethernet Standards

IEEE 802.3ae 10Gb/s (2002) 10GBase SR 300m on OM3 10GBase-LX4 300m on OM3

IEEE 802.3aq 10Gb/s (2007) 10GBase LRM 220m on OM2

IEEE 802.3ba 40 Gb/s and 100 Gb/s (2010) Introduced 4 x 25 Gb/s as fundamental building block 40GBase SR4 150m on OM4 (8 fibers duplex) 100GBase SR10 150m on OM4 (20 fibers duplex)

IEEE 802.3bm 40 Gb/s and 100 Gb/s (2015) 100GBase SR4 100m on OM4 (8 fibers duplex) Read-Solomon FEC Uncorrected BER ~5x10-5 Single mode variant: CWDM w/ 2 fibers

IEEE 802.3bs 400 GbE (planned 2017) At least 100 m over OM4 Baseline 100GBase SR16 Re-use of 100GBASE-SR4 specifications

Uncorrected BER ~2x10-4


http://www.ethernetalliance.org/roadmap/

Core data rates are now 25Gbps and moving to 50Gbps


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The Ethernet Alliance roadmap shows the evolution of data rates

• Client Optics – needed soon to connect IP routers to 400G DWDM gear for long haul transport

• Switch Ports – 100G data

center switches deployed in 2016 in hyper-scale, but 40G will remain common in enterprise

• Server I/O – planning for 50G,

deploying 25G servers now, though 10G has high volume still

OFC 2016 Tutorial - PAM-4 and VCSEL MMF Links S. E. Ralph 4

Slide Courtesy: Robert Lingle Jr.

Moving to Terabit


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400G: Prospects for 50 Gb/s per laser w/MMF

100G MMF links have 100m reach using 4 fibers x 25G 400G requires16 fibers x 25 Gb/s (32 distinct fibers for duplex)

Alternatives for 8 x 50 Gb/s per fiber 400G solution NRZ: Much faster VCSELs and/or strong pre emphasis PAM-4: increases RIN and MPN requirements Shortwave WDM requires wideband MMF fiber

Challenges for both NRZ and PAM-4 RIN: Continue improvement of VCSEL RIN MPN: Much better understanding of mode

partition noise and characterization Higher speed VCSELs at new wavelengths


Clear roadmap forming(ed) within IEEE for 50GbE, 200GbE and 400GbE

Scalability: Eyes on 1 Terabit

Paul Kolesar,– CommScope IEEE 802.3 50G & NGOATH Study Group January 2016, Atlanta GA


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Deployment Opportunities

DCs optimized for cloud computing require more interconnections and redundancy


SMF < 2km

MMF <500m

Cu Cable <5m

DC Architecture for Cloud Computing

Standards Based Building Wiring Link Lengths

LAN Architecture

Number of DCs world wide will continue to grow exponentially Data Centers are a key part of Web 2.0 networks Data Centers provide two key functions:

Content Storage and Computing

Data Centers and High Performance Computing require high data rate short distance links

100 m reach covers >80% of data center cabling

Cisco Global Cloud Index: Forecast and Methodology, 2014–2019

25% CAGR 2014 - 2019


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Early Optical Demonstrations

Experimental demonstration of 4-ASK over 225 SSMF (1997) S. Walklin, J. Conradi, "A 10 Gb/s 4-ary ASK lightwave system," in Integrated Optics and Optical Fibre

Communications, 11th International Conference on, and 23rd European Conference on Optical Communications (Conf. Publ. No.: 448) , vol. 3, pp. 255-258 Sep. 1997

Multilevel Signaling and Equalization over Multimode Fiber at 10 Gbit/s (2003) C. Pelard, E. Gebara, A. J. Kim, M. Vrazel, E. J. Peddi, V. M. Hietala, S. Bajekal, S. E. Ralph, and J. Laskar

Gallium Arsenide Integrated Circuit (GaAs IC) Symposium, 25th Ann. Tech. Digest 2003. IEEE , 9-12 Nov. 2003 Experimental demonstration of analog FFE using PAM-4 in OM2 MMF using VCSEL10 Gbps,150 m MMF

Experimental sensitivity analysis of PAM4/VCSEL/MMF links 100 m 5 Gbps (2005) J. E. Cunningham, D. Beckman, D. Huang, T. Sze, K. Cai, and A.V. Krishnamoorthy, "PAM-4 signaling over

VCSELs using 0.13 μm CMOS," in Information Photonics, 2005. OSA Topical Meeting, 6-8 June 2005

2 µm GaAs HBT 5 Gsym/s w/o FFE 150m OM2 5 Gsym/s w/ FFE 150m OM2



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VCSEL Data Rates

8

IBM OOK 71Gb/s, 7m 64Gb/s, 57m 62Gb/s, 7m

Chalmers PAM-4 70Gb/s, btb Offline equalization

OFS PAM-4 51.56Gb/s, 150m, 850nm

Georgia Tech PAM-4 51.56Gb/s,100m

IBM Chalmers

IBM OFS

PAM-4 w/DSP

850nm OOK w/DSP

850nm OOK w/o DSP

GT

OFC 2016 Tutorial - PAM-4 and VCSEL MMF Links S. E. Ralph


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Evolution in PAM-4 Transceivers

9

[1] R. Farjad-Rad et al., “A 0.3-µm CMOS 8-Gb/s 4-PAM Serial Link Transceiver," in Solid-State Circuits, IEEE Journal of, May 2000

[2] J. T. Stonick et al., "An adaptive PAM-4 5-Gb/s backplane transceiver in 0.25-μm CMOS," in Solid-State Circuits, IEEE Journal of, Mar. 2003

[3] J. L. Zerbe et al., "Equalization and clock recovery for a 2.5-10-Gb/s 2-PAM/4-PAM backplane transceiver cell," in Solid-State Circuits, IEEE Journal of, Dec. 2003

[4] T. Toifl et al., "A 22-Gb/s PAM-4 receiver in 90-nm CMOS SOI technology," in Solid-State Circuits, IEEE Journal of, April 2006

[5] J. Lee et al., "Design and Comparison of Three 20-Gb/s Backplane Transceivers for Duobinary, PAM4, and NRZ Data," in Solid-State Circuits, IEEE Journal of, Sept. 2008

[6] Broadcom unveils 40G/50G PAM-4 physical layer chip, (http://www.lightwaveonline.com/articles/2014/12/broadcom-unveils-40g-50g-pam-4-physical-layer-chip.html), Dec. 2014

[7] P. Khandelwal et al., "100Gbps Dual-channel PAM-4 transmission over Datacenter Interconnects," DesignCon, Jan. 2016

Complete Tx and Rx Transceivers All employed SiCMOS as the material platform 5 Gb/s

250 nm [2]

10 Gb/s 130 nm [3]

22 Gb/s 90 nm [4]

20 Gb/s 90 nm [5]

50 Gb/s 28 nm [6]

56 Gb/s (dual channel) 28 nm [7]

8 Gb/s 300 nm [1]



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PAM-4 Transceiver Architectures

Transmitter Binary or gray mapping Precoder implements a 1/(1+D) filter to reduce DFE

burst-error length 3-tap FIR filter & line driver


Courtesy: Pulkit Khandelwal, InPhi)

Dual 56Gbaud PAM-4 transceivers are

commercially available

Receiver Continuous-Time Linear Equalizer (CTLE) 7-bit ADC at 28GSamples/s Digital FFE for equalization 1-tap Decision Feedback Equalizer Inverse of Tx Precoder


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Impairments and Noise of VCSEL/MMF Links

Quantifying and Minimizing



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Impairments

PAM-4 Waveform

Source Driver

VCSEL

TIA

PIN

PAM-4 Receiver

MMF

Tx Limited bandwidth of

transmitter o Components not scaling w/ bit rate

Tx nonlinearity o Driver and VCSEL

Finite extinction ratio (ER)

Fiber Fiber attenuation and

connector losses Chromatic Dispersion

(CD) o VCSEL RMS spectrum

(~0.5nm) determines spectral content

Modal dispersion (DMD) Multi-Path Interference

(MPI) o Multiple optical reflections

Rx Baseline Wander

o AC-coupling induces pattern dependent “dc” point

Limited Bandwidth of the receiver o Components not scaling w/ bit

rate

Rx nonlinearity

Optical Coupling



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Noise

PAM-4 Waveform

Source Driver

VCSEL

TIA

PIN

PAM-4 Receiver

Tx Relative Intensity Noise (RIN)

o Random amplitude fluctuations at the output of VCSEL

Fiber Mode partition noise (MPN)

o Different delay values for different modes resulting in timing jitter

Modal noise (MN) o Different fiber modes have

different attenuation (fiber DMA or connectors) resulting in amplitude noise

Rx Receiver noise

o Thermal noise o Shot noise

All noise is evaluated at the receiver

Jitter o Clock recovery

MMF

Optical Coupling



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Cen

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End-to-end MMF-VCSEL Physical link model

Complete link model including computation of fibers mode profiles and group velocities, coupling optics, RIN and MPN Transmits bit stream and counts errors Cj(λi): Coupling coefficient of ith VCSEL mode into jth fiber mode group δτj(λi): Modal and Chromatic Delays of jth fiber mode group with wavelength λi


MMF Optical Link Simulation Model

MPN factor kmpn

Random variables generated:

{a1(λ1), a2(λ2), a3(λ3),…} (∑ 𝑎𝑖𝑛

𝑖=1 = 1)

x

λ1 λ2 λn

a1(λ1)

I1(r), λ1

an(λn)

MM VCSEL

RIN

time

VCSEL output (No RIN)

time

VCSEL output (with RIN)

Fiber illuminating lens assembly

In(r), λn x

MMF Model C1(λ1), δτ1(λ1)

CK(λ1), δτK(λ1)

GT MMF Mode Solver

GT MMF Mode Solver

C1(λ1), δτ1(λn)

CK(λ1), δτK(λn)

Order of mode {l, m}, Mode field radius (MFR)

Order of mode {l, m}, Mode field radius (MFR)

VCSEL Spectrum

I1(r), λ1

In(r), λn

(1xK)

(1xK)

Collection lens assembly

K. Balemarthy, et al., J. Lightwave Technol., V24, pp4885-4894, 2006.


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End-to-end link analytic model: Overview

Analytic model calculates BER directly from 𝑄 factor:

𝑸 =𝑹𝑷𝒐𝒐𝒐

𝝈𝑹𝑹𝟐 + 𝑹𝒓𝒐𝒎𝒎𝑷𝒐𝒐𝒐

𝟐 𝑬𝑹𝑬𝑹 − 𝟏

𝟐+ 𝑹𝒓𝑹𝑹𝑹𝑷𝒐𝒐𝒐

𝟐 𝑬𝑹𝑬𝑹 − 𝟏

𝟐+ 𝝈𝑹𝑹

𝟐 + 𝑹𝒓𝒐𝒎𝒎𝑷𝒐𝒐𝒐𝟐 𝑬𝑹𝑬𝑹 − 𝟏

𝟐+ 𝑹𝒓𝑹𝑹𝑹𝑷𝒐𝒐𝒐

𝟐 𝟏𝑬𝑹 − 𝟏

𝟐

𝑬𝑹: Extinction Ratio (linear units)

𝒓𝑹𝑹𝑹: Normalized std dev. due to RIN {𝑟𝑅𝑅𝑅 = Δ𝑓 ∗ 10(𝑅𝑅𝑅𝑑𝑑/𝐻𝐻10 )}

𝒓𝒐𝒎𝒎: Normalized std dev. due to MPN 2 {𝑟𝑚𝑚𝑚 = 𝑘2

(1 − 𝑒−𝜋𝜋𝜋𝜋σλ2)}

𝑷𝒐𝒐𝒐: Optical Power OMA reduced by ISI eye closure (Watts)

Extended to PAM-4 Noise variances equivalent to the IEEE 802.3bm link budget model

𝑸 =𝑹𝑷𝒐𝒐𝒐𝝈𝟏 + 𝝈𝟎

𝝈𝑹𝑹: Receiver noise std. dev. in Amps (Thermal, Shot etc.,) 𝑹: Responsivity of photodiode (A/W)

𝑷𝒐𝒐𝒐 includes all ISI effects: Laser and receiver bandwidths Chromatic Dispersion (CD) Modal Dispersion (DMD) Baseline wander (BLW) Any additional ISI causing effects



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12.5 Gbps (6.25 GBaud), PRBS7 Tx with 3-tap FFE

PAM-4 at Rx

Simulated Tx Waveform to AWG

Compare simulation with measurement

Keysight ADS 2016.01 Channel Simulation Tx Rx

DUT

AWG Tx with FFE PAM-4 at DCA Rx

AWG

QSFP28 3m DUT

PAM-4 Test and Measurement


ADS PAM-4 Channel Simulator DesignCon 2016 Keysight Technologies

17

Slide courtesy Keith Owens, Keysight


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Simulation Model - Analytic Model

Link Parameters Tx

10Gbit/s, PRBS-7, ER=3.5dB, RIN=-133dB/Hz

Δλrms ~ 0.55nm, λ0 ~ 849nm Inset depicts simulated Gaussian

VCSEL spectrum

Fiber D = -117ps/(nm.km); EMBc~10GHz.km Ogawa - Agarwal MPN model

Rx NF = 4dB BWRx = 34GHz Responsivity = 0.4A/W Tx filter LCF = 40KHz (for BLW effect)

Simulation results match very well with IEEE based analytic model results at 10Gbit/s



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25Gbit/s: Simulation Model - Analytic Model(No MPN)

25Gbit/s PRBS-7, ER = 4.5dB,

RIN = -140dB/Hz

D=-117ps/(nm.km); EMBc~6GHz.km

Δλrms ~ 0.45nm, λ0 ~ 849nm

No MPN

Receiver specifications used NF = 4dB

Temp = 300K BWrx = 28GHz Responsivity = 0.38A/W

L-MMF performs 0.5dB better than the R-MMF: MCDI impact is noticeable at 25Gbit/s

IEEE 802.3 based analytic model slightly overestimates the fiber ISI penalty

Performance depends on the exact alpha profile of the fiber used



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0

5

10

15

20

25

30

35

1991 1996 2001 2006 2011 2016

Ban

dwid

th (G

Hz)

Year

VCSEL Bandwidth Evolution

21

Chalmers, Tokyo Inst. of Technol.

Chalmers



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Volume Manufacturing of High-Speed VCSELs


Volume Manufacturing of 25-28Gb/s VCSELs has been enabled by developing a robust InGaAs QW based process

High Bandwidth VCSELs with well behaved frequency response are essential to generate quality PAM-4 waveforms

Figure Courtesy Laura Giovane, Avago

Tu3D.5. High Speed Transmitters (5:30pm) Volume Manufacturable High speed 850nm VCSEL for 100G Ethernet and Beyond Laura Giovane; Jingyi Wang; MV Ramana Murty; Ann Lehman Harren; Hsu-Hao Chang; Charlie Wang; David Hui; Zheng-Wen Feng; Thomas Fanning; Aaditya Sridhara; Sumitro-Joyo Taslim; Jason Ch


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1D VCSEL Rate Equations

1D VCSEL model Coupled carrier and photon rate equations

Can be modified to include polarization effects Requires electric field rather than power

Can be modified to support transverse modes Requires a 2D model to support spatial overlaps

See “A Simple Rate-Equation-Based Thermal VCSEL Model” by P.V Mena for example rate equations

VCSEL nonlinearities Significant overshoot and ringing Not often measured experimentally



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1D VCSEL Rate Equations

Fabry-Perot lasers exhibit large ringing consistent with 1D rate equations

Measured Transients of 25G 850nm VCSEL


Application Note: Modulating VCSELs – Finisar, 2007

Multiple transverse modes give rise to extra features in the power transients

1D VCSEL rate equations do not capture the full effects of VCSELs at 25G

24

2D Laser Model with Multiple Transverse Modes are needed to simulate VCSEL more accurately


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Amplitude and Timing Penalty


Amplitude Penalty HPAM4 ≅

13*HOOK

PAM-M amplitude penalty = M-1 PAM-4 eye closure penalty ~ 4.77 dB

compared to OOK Optical sensitivity penalty may be less

due to the reduced electrical bandwidth

Timing Penalty Expect: WPAM4 ≈2*WOOK

Observe: WPAM4 < 2*WOOK

Reason: 12TPAM4< WPAM4 <

23TPAM4

Additional transitions narrow eye width: 8 (OOK), 64 (PAM 4)

y

TOOK

WOOK

HOOK

TPAM4

Time (s)

y

-2

2

Time (s)2.04e-9 2.1975e-9

S1

WPAM4

HPAM4

Timing Penalty Cannot be Dismissed


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Transmitter Signal Quality

Generating and maintaining high quality PAM-4 signals Electrical noise figure Impedance mismatch: Driver-VCSEL VCSEL nonlinearities


25.78 Gbaud

No pre-emphasis With pre-emphasis

Q-Factor ~ 9.5 Q-Factor ~ 8.7

Vmax,pp= ~600mV

PAM-4 Waveform

Source Driver

VCSEL

Electrical Optical

Pre-emphasis is essential


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Fiber: Short Wavelength Regime

Fiber loss o Ethernet standard is <3.5 dB/km @

850nm o Typical is <2.5 dB/km @850nm

Dispersion

o ~100 ps/nm-km @850nm o Prior to 25G symbol rates, dispersion

in the 850 regime has not been a significant impairment

Chromatic dispersion limited reach o Source limited spectral content o ∆λ~ 0.4nm RMS o @850nm 0.1nm = 37GHz


Chromatic Dispersion is a primary impairment for symbol rates >10Gbps

760 800 840 880 920 960 1000 1040 1080-160

-140

-120

-100

-80

-60

-40

-20

0

Atte

nuat

ion

[dB/

km]

Ethe

rnet

Disp

ersio

n [p

s/nm

-km

]

Wavelength [nm]

0

1

2

3

4

5

6

Typical

IEEE standard


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Modal Dispersion in Graded Index MMF

Effective Modal Bandwidth – Calculated (EMBc) Standardized VCSEL flux distributions weight a sum of DMD impulse responses Method standardized by the Telecommunications Industry Association (TIA)

10 VCSEL Radial power distributions

Measured Fiber DMD

-3dB

f

H

f3dB Impulse Response


FOTP-220 Differential Mode Delay Measurement of Multimode Fiber in the Time Domain”, TIA-455-220-A, Jan. 2003.

MMF

Differential Modal Delay (DMD) Measure impulse response for a range of launch offset conditions DMD metric: delay between 25% of trailing edge of slowest pulse

and 25% of leading edge of fastest pulse

SMF

EMBc = worst case f3dB


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Differential Mode Delay (DMD)

Current MMF manufacturing techniques allow precise control of the index profile yielding near optimum profile and excellent DMD

Fiber Type

Wavelength (nm)

Max Loss (dB/km)

Minimum Bandwidth

(MHz-km)

OFL EMBc

62.5µm (OM1)

850 1300

3.5 1.5

200 500

-

50µm (OM2)

850 1300

3.5 1.5

500 500

-

50µm (OM3)

850 1300

3.5 1.5

1500 500

2000

50µm (OM4)

850 1300

3.5 1.5

3500 500

4700


Evaluate link performance on multiple examples of the same class of fiber

DMD=22ps


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Impact of DMD

51.56Gbs PAM-4 OM4 100m With Pre emphasis 3 Tap T-spaced

FFE

DMD results in measurable penalty

10 GHz-km: ISI penalty ~1.5dB 6 GHz-km:, ISI penalty ~1.9dB

S. Kota Pavan et. al., ECOC Cannes, France

Sept. 22-25, 2014


Inherent back-to-back penalty arises primarily from impedance mismatch between driver and laser For unpackaged devices it is best to compare results against back-to-back


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Mode Spectral Bias

VCSEL transverse modes have increasing power away from center


Mode Spectral Bias Longer wavelengths couple predominantly to lower order

fiber modes

Transverse modes have decreasing wavelength with increase mode

VCSEL modes couple to MMF modes with similar mode spatial patterns: higher VCSEL modes couple to higher fiber modes

Two polarizations each

LP01

LP11

LP02

LP21

LP31


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Modal and Chromatic Dispersion Interaction (MCDI)

Ideal GI fiber profiles α = αopt Least delay between lowest and highest order modes Performs best in the absence of CD

L-MMF (over-compensated) α < αopt Higher order modes arrive before lower order modes Compensates for CD (Since higher-order VCSEL modes

couple mostly into higher-order fiber modes) Performs the best in presence of CD

R-MMF (under-compensated) α > αopt Higher order modes arrive after lower order modes Performs the worst in presence of CD

MCDI: Chromatic dispersion (λ<1300nm) always results in delays for

the shorter λ (lowest-order modes) with respect to longer λ (higher-order modes)

CD and hence interaction depends on VCSEL spectral content


CD and DMD effects may either add or subtract

L-MMF

R-MMF


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Left and Right Fiber

All EMBc is Not Equal


EMBc GHz-km

DMD slope

ISI penalty

4.8 L 2.3

5.6 R 2.4

6.0 N 1.9

10.0 N 1.5

Left Sloped DMD compensates CD

51.56Gbs PAM-4 OM4 100m With Pre emphasis 3 Tap T-spaced FFE

S. K. Pavan, J. Lavrencik, S.E. Ralph, "Experimental demonstration of 51.56 Gbit/s PAM-4 at 905nm and impact of level dependent RIN," in Optical Communication (ECOC), 21-25 Sept. 2014


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Cen

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PAM-4 and RIN

Effect of RIN Imposes power penalty at lower power Results in BER floor


Captures all laser intensity noise Fundamental Spontaneous emission Shot noise Mixing of spontaneous emission with

the lasing field Environmental Electrical noise Back reflections Vibrations

Varies with Bias i.e. laser power Frequency

Strongly connected with MPN

RIN parameter Simulation Parameters Ith = 2.2x10-6Amp; Responsivity = 0.4A/W Extinction Ratio (ER) = 6dB 25.78 Gbaud

2

210 log /avg s

RIN dB HzP fσ

=

Effective RIN should be <140 dB/Hz 36


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Relative Intensity Noise (RIN)-Bias Dependence

RIN exhibits strong bias and frequency dependence

RIN penalty (IEEE)

𝑃𝑅𝑅𝑅 = 10 log 1

1− 𝑄∙𝜎𝑅𝑅𝑅𝑅𝐼𝑅𝑟

2



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L-I-RIN

Increasing output power decreases average RIN RIN decreases by ~ 2 dB/Hz per 1mA Relaxation oscillation freq and hence modulation bandwidth also increase with

power

IEEE model currently uses single fixed RIN Proper Effective RIN allows good predictions for OOK Effective RIN for PAM-4, presumes unequal levels and may have unknown

cross terms

Average RIN vs bias




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Variable RIN for PAM-4, 50Gbit/s

Using separately measured RIN for each PAM-4 level yields better modeling than use of average fixed RIN which under estimates RIN penalty Back-to-back (btb): ~0.5dB OM4: 0.6dB Higher RIN yields larger discrepancy

OM4 (10 GHz-km), 100m: ISI penalty ~1.5dB

RIN is not a fixed parameter




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Cen

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Mode Partition Noise and Modal Noise

Mode partition noise Time-varying mode power distribution

of laser sources Differential path delay

VCSEL mode power fluctuates among different modes which couple to different fiber modes

MPN is phase jitter, appearing as amplitude noise, due to variation in group velocities for each VCSEL mode group

Modal noise Time-varying mode power distribution in optical sources

and mode selective losses in the link High-speed VCSELS (>10Gbps)

may exhibit chaotic behavior Differential path attenuation Combination of VCSEL mode power fluctuation and mode

selective losses in the link

Fraction of power reaching receiver is time dependent

Arrival time of energy is dependent on current



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Mode Partition Noise

The delay among the VCSEL spectra is Where D = dispersion, L = fiber length Typical value is 5ps for 100m and 0.5nm rms

Relating this to an amplitude noise requires many assumptions

Basic Idea of Ogawa-Agrawal model used in IEEE 802.3 standards

Modes are strictly anti-correlated

Developed for FP lasers with longitudinal modes that share the same gain region Not appropriate for VCSELs with multiple transverse modes

RMSD Lτ λ∆ = ⋅ ⋅ ∆


If one mode experiences an increase in power, the other modes decrease in power


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MPN: Ogawa - Agrawal

Ogawa-Agrawal model assumptions: (1) No RIN: ∑ ai

Ni=1 = 1 (assumes RIN and MPN are separable)

(2) Any two modes exhibit the same constant correlation

Covariance is strictly negative: a decrease in one mode is exacly balanced by increases in other modes

Covariance varies between 0 and -1 γcc varies between 1 and 0, 1=>no variation

(3) MPD ai is constant for entire bit duration

Using the IEEE model, we determine a kmpn from mode noise

measurements using these assumptions, which we use to calculate an MPN penalty in end-to-end models


,covariance1 = for all i jγ

−− = ≠i j i j i j

cci j i j

a a a a

a a a a

Strongly constrains fluctuation statistics of the VCSEL modes

Not true for VCSEL transverse modes

Not true in general


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MPN

The variance of the ith mode is

Noise variance can be determined, using additional assumptions of the IEEE modeling efforts Raised cosine pulse shape at receiver Temporal delay results exclusively from CD i.e. single mode fiber Continuous Gaussian spectrum

The amplitude variance is approximately given by

kmpn is a scaling metric that depends on the specific power variation and correlations among the VCSEL modes

Calculated heuristically var(ai) = k𝟐mpn(< ai> −< ai > 2)

var(ai) = (1 −𝛾cc)(< ai> −< ai > 2)

22 2

2 12

λπ σ− = − mpn BLD

mpn

kr e

B = bit rate L = transmission length D = Dispersion σλ = rms spectral width


Exponential Increase with Fiber length Dispersion Baudrate


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Mode Partition Noise Penalty

-10 -5 014131211109 8 7

6

5

4

3

2

Pave(dBm)

-log(

BER

)

BER Vs Rx Power

Rx Thermal Limit0m25m50m75m100m

Kmpn = 0.3

Impact of MPN Creates noise floor Imposes significant power penalty

-10 -5 014131211109 8 7

6

5

4

3

2

Pave(dBm)

-log(

BER

)

BER Vs Rx Power

Rx Thermal Limit0m25m50m75m100m125m150m

Kmpn = 0.1

IEEE standards require link performance with KMPN = 0.3



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MPN Limited Reach

MPN is a limiting penalty for VCSEL-MMF links Kmpn = 0.3 limits PAM 4 links to <100m 25G VCSELs are better than IEEE assumptions

Figure Courtesy David Cunningham, Avago

Kmpn=0.3, with 3 Tap T spaced FFE OM4 fiber @ 850nm

Isolated MPN penalty OM4 fiber @ 850nm

Realistic Assessment of VCSEL MPN is Needed



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Cen

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Direct Assessment of kMPN

Directly measure mode correlations real-time 1) MMF efficiently coupled into spectrometer: matched f/# 2) Grating spatially separates mode sets Grating chosen to minimize polarization sensitivity 3) Modes spatially separated and individually collimated

RF cable VCSEL Lensed Fiber

probe Bias-tee SHF 12124A 30G

BPG

Spectrometer Triax 550

+5.00 dBm

1.40 dBm

SHF 827

1

2 3

Mirror MMF

Shorter λ



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Direct Assessment of kMPN

Inphi TIA

SHF 807

50Ω Termination

Inphi TIA

LeCroy Real-Time Scope

SHF 807

50Ω Termination

LeCroy Real-Time Scope

GaTech Custom InGaAs MMF Receivers

28GHz BW Top-Illuminated PDs 20μm diameter

Lensed fiber

Lensed fiber


Filtering VCSEL spatial modes retains all spectral components

Unfiltered VCSEL spectrum (black) Individual filtered mode sets (colors)


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kmpn

Mode separation enables the direct assessment of the variance and power of each mode and the covariance between each mode pair

Continuing with the Ogawa-Agrawal theory we can determine kmpn with two methods:

Two-mode correlation Single mode variance

𝑘𝑚𝑚𝑛 𝜏 = −𝑅𝑖𝑖 𝜏𝜎𝑖𝜎𝑗𝑎𝑖 𝑎𝑗

= 1 − 𝛾𝑐𝑐 𝑘𝑚𝑚𝑛 = ( 𝑎𝑖2 − 𝑎𝑖 2)( 𝑎𝑖 − 𝑎𝑖 2)

where, 𝑅𝑖𝑖 𝜏 =𝑎𝑖(𝑡)𝑎𝑗(𝑡+𝜏) − 𝑎𝑖 𝑎𝑗

𝜎𝑖𝜎𝑗=

𝑐𝑐𝑐𝑖𝑗(𝜏)𝜎𝑖𝜎𝑗

Where kmpn is now in terms of the measured parameters

If the assumptions of the Ogawa-Agarwal model hold, specifically assumption (2),

then these two methods will yield the same results

Reference: J. Lavrencik, et al., "Direct Measurement of Transverse Mode Correlation and MPN using 900nm VCSELs," Optical Fiber Communication Conference, paper W2A.55, 2015.


The covariance and kmpn depend on the relative delay between the VCSEL modes


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Kmpn for Modulated Signals

Partial spectral filtering significantly increases kmpn VCSEL modes are mode sets


Critical to capture entire VCSEL spectra

Single mode determination of kmpn allows evaluation of modulated signals

Mode 2: Average kmpn ~ 0.06

Pattern: 0011011000100111 8mA bias


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Directly Measured kmpn

Measured kmpn consistently < 0.1 For both static and modulated

VCSELs

Single mode method and two mode method yield similar but not identical results Demonstrates limits of Ogawa-

Agarwal model

kmpn is not a fixed parameter

Unmodulated signals

Two-Mode Correlation

Single Mode Variance

VCSEL Mode Pairs kmpn Mode kmpn

3.5mA 1,2 0.042 1 0.045 2,3 0.047 2 0.04 - - 3 0.034

8mA 1,2 0.053 1 0.022 2,3 0.039 2 0.059 - - 3 0.043

10mA

1,2 0.035 1 0.031 2,3 0.015 2 0.029 3,4 0.018 3 0.016 - 4 0.012




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Cen

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Direct Measurement of Mode Correlation

Cross correlation Rij versus delay Modes are correlated for less than ~500ps Longer than symbol duration Maybe shorter than delay due to dispersion

Adjacent mode pairs strongly anti-correlated Mode pair 1, 3 is positively correlation Contradicts Ogawa-Agarwal model assumptions



54

𝑅𝑖𝑖(𝜏) =𝑃𝑖(𝑡)𝑃𝑖(𝑡 + 𝜏) − 𝑃𝑖 𝑃𝑖

𝜎𝑖𝜎𝑖


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RIN Enhancement Through Fiber

The aggregate RIN depends on the degree of correlation among VCSEL modes at the receiver Received RIN depends on the measured variances and cross-correlations and

hence delay τ: The RIN coefficient RIN[dB/Hz] is

10𝑅𝑅𝑅 [𝑑𝜋 𝐻𝐻]⁄

10 ∙ ∆𝑓 = �𝜎𝑎𝑖2

𝑅

𝑖=1

+ ��𝜎𝑎𝑖𝜎𝑎𝑗𝑅𝑖𝑖𝑖≠𝑖

𝜏

where 𝜎𝑎𝑖

2 is the variance of mode i normalized to the mode power, ∑ 𝑎𝑖 = 1 The lasing transverse modes are orthogonal to each other Noise variance is measured over bandwidth ∆f which is limited by the receiver The noise is white and Gaussian

J. Lavrencik, et al., "Direct Measurement of Transverse Mode Correlation and Fiber-Enhanced RIN through MMF using 850nm VCSELs," Optical Fiber Communication Conference, 2016. Th3G.1


22

2

σσ =i

ia

ia


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RIN Enhancement Through Fiber

Fiber dispersion increases the received RIN regardless of modulation by reducing the cross-correlations between modes, e.g.

lim𝑅𝑖𝑗→0,

��𝜎𝑖𝜎𝑖𝑅𝑖𝑖𝑖≠𝑖

𝜏 = 0

The effect is observed as a higher RIN induced noise floor in R-MMF fiber which exhibits larger total dispersion compared to L-MMF

(a) Measured RIN Parameter of back-to-back vs.100m MMF; (b) 34 Gbps OOK link BER analytic model vs. experiment; (c) Eye diagrams of 100m L-MMF and 100m R-MMF



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Cen

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MPN: New Model

Mode fluctuation statistics completely described by a single Covariance matrix:

COV ai =

var1 cov12 . .

cov1Mcov21

var2 . . cov2M

.

.covM1

covM2 . . varM

where vari = <ai2> − <ai>2 and covij = <aiaj> − <ai><aj>

For instance, ∑ aiMi=1 ≠ 1 though Σ<ai> = 1

Captures both RIN and MPN while retaining their independence

Composite power fluctuations measured as RIN Correlation in fluctuations define extent of MPN

We derive vari and covij from the VCSEL physical dynamics

Formalism explicitly allows for both positive and negative cross−correlations



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Closed-Form Expressions for VCSEL Mode Correlations: Time Domain

Equivalent time-domain expressions VCSEL-specific parameters 𝜶, 𝜞𝒎, and 𝒌𝒎 defined over the measurement bandwidth

Composite RIN for the VCSEL derived as

Variance due to MPN derived as

rik is the normalized received waveform at the optimum sampling instance

𝑣𝑎𝑟𝑖 = 𝜶𝟐 ai�

cov𝑖𝑖 = ai�aj� 𝜞𝒎𝟐 − 𝐶𝑖𝑖𝑖𝒌𝒎𝟐

𝑅𝑅𝑅𝑐𝑐𝑚𝑚,𝑎𝑐𝑖 =1

2𝜋Δ𝑓� vari

𝑀

𝑖=1

+ �� covij

𝑀

𝑖≠𝑖

σMPN2 = 𝛂2 ∑ rik2 − 1 a𝑖� M

i=1 + ∑∑ {rikrjk𝑖≠𝑖 − 1} {𝜞𝒎𝟐 − 𝒌𝒎𝟐𝐶𝑖𝑖𝑖}a𝑖� a𝑖�



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Simulation Results: Individual Mode RIN and CSD

Average RIN in a single VCSEL mode, 𝑹𝑹𝑹𝒊,𝒐𝒂𝒂 vs. 𝑨i 𝑴

Average Cross spectral density, 𝑹𝑹𝑹𝒊𝒊,𝒐𝒂𝒂vs.𝑪𝒊𝒊𝒂

Cross-correlation coefficient, 𝑹𝒊𝒊(𝟎) vs.𝑪𝒊𝒊𝒂

(a): 𝑅𝑅𝑅𝑖 ω� =𝑣𝑎𝑟𝑖 ω�

ai�2

(b): 𝑅𝑅𝑅𝑖𝑖 ω� =cov𝑖𝑖 ω�

ai� aj�

(c): Rij(τ = 0) = 𝑐𝑐𝑐𝑖𝑗 0𝑐𝑎𝑣𝑖(0)𝑐𝑎𝑣𝑗(0)

(a)

(b)

(c)



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Features of the new MPN model

Robust and accurate Allows for a customized COV matrix that is measured directly for a VCSEL Can be readily included in the IEEE spreadsheet model Representation of RIN and MPN requires 3 VCSEL-specific parameters

New MPN model includes VCSEL relevant features not found in the standard O-A model VCSEL mode correlation statistics depend on the spatial overlap integral 𝑪𝒊𝒊𝒂 Mode pairs may have positive cross-correlations

Direct measurements on mode fluctuations of VCSELs validate the new model


S. K. Pavan, J. Lavrencik, S.E. Ralph, “New Model for Mode Partition Noise in VCSEL-MMF Links Based on Langevin-driven Spatio-Temporal Rate Equations,” Submitted to JLT


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Cen

ter Recent PAM-4 Transmission Results



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Comparison with OOK modulation

Measured sensitivity at BER=10-12 , theoretical sensitivity at BER=10-9

and measured sensitivity at BER=1.8 × 10-4 , with data rates reduced by the 7% FEC overhead, measured using 100 m of fiber

K. Szczerba, P. Westbergh, E. Agrell, M. Karlsson, P. A. Andrekson, and A. Larsson, "Comparison of Intersymbol Interference Power Penalties for OOK and 4-PAM in Short-Range Optical Links," in Lightwave Technology, Journal of , vol. 31, no. 22, pp. 3525-3534, Nov.15, 2013

16 GHz VCSEL 100 m OM4 fiber Photoreceiver with 10 GHz -3dB

bandwidth

At higher bit rates the sensitivity of PAM 4 is better than OOK

The cross over is due to limited link bandwidth



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80 Gbps PAM-4

64

80 Gb/s over 3 m of OM4 Separate coding for LSB and

MSB Effective bit rate of 70 Gb/s

Offline 5 tap FIR equalization Component bandwidths

VCSEL: 25 GHz Photoreceiver: 22 GHz

K. Szczerba, P. Westbergh, M. Karlsson, P. A. Andrekson, and A. Larsson., "70 Gbps 4-PAM and 56 Gbps 8-PAM Using an 850 nm VCSEL," in Lightwave Technology, Journal of , vol. 33, no. 7, pp. 1395-1401, April1, 2015



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PAM-4: 850nm @51.56Gbps

Error-free performance demonstrated for variety of 100m OM4 fibers

Near error free demonstrated for OM3 Pre-emphasis required ISI penalties with pre-emphasis

All fibers 100m OM4 fibers ~ 0.8dB OM3 fibers ~ 1.4 to 2.2dB

No measurable influence of MPN Noise/ISI metrics in analytic model

Noise Eq Curr Ith = 2.2x10-6Amp Responsivity = 0.3A/W Extinction Ratio (ER)

6dB RIN < -150dB/Hz

PAM-4 ISI penalties: OM4 fibers ~ 0.8dB

OM3-1 ~ 1.4dB, OM3-2 ~ 2.2dB OFC 2016 Tutorial - PAM-4 and VCSEL MMF Links S. E. Ralph 65


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VCSEL Data Rates




OFS PAM-4 51.56Gb/s, 150m, 850 nm

Georgia Tech PAM-4 51.56Gb/s,100m

IBM Chalmers

IBM OFS

PAM-4 w/DSP

850nm OOK w/DSP

850nm OOK w/o DSP

GT 880nm-1100nm w/DSP

GT



OFS PAM-4 51.56Gb/s, 150m 850nm-940nm CWDM four λ @ 51Gbps each

Georgia Tech PAM-4 51.56 Gb/s, 100m 50Gb/s, 200m 62 Gb/s, 100m


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VCSEL MMF links: Speed vs. Reach

Filled markers indicate error-free performance achieved without FEC correction



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Wideband MMF

OM4 performance is possible over a wide range of wavelengths

Standard graded index MMF is not wideband due to wavelength dependence of index and dispersion n = n(λ) |dn/dλ| >0


Figure Courtesy Y. Sun, OFS


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WDM in Wideband MMF

70

51.56 Gb/s per wavelength, 4 channels, 150 m fiber BER below the KP4 FEC threshold (2 ×10-4)

Without linear equalization With linear equalization (4 taps and 1 precursor)


Figure Courtesy Y. Sun, OFS

Y. Sun, R. Lingle, R. Shubochkin1, K. Balemarthy, D. Braganza, T. Gray, W. J. Fan, K. Wade, D. Gazula, and J. Tatum, "51.56 Gb/s SWDM PAM4 Transmission over Next Generation Wide Band Multimode Optical Fiber, " Optical Fiber Communications Conference, Tuesday, 22nd March, 2:45pm-3pm


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50Gbps 1060nm PAM-4

Wideband fiber capacity Robust error free performance beyond 150m Beyond 300m with low latency FEC

“wideband”


IBM has demonstrated 1530 nm VCSELs at 56 Gb/s OOK ECOC 2015 PDP Figure courtesy Dan Kuchta, IBM

S. K. Pavan, J. Lavrencik, R. Shubochkin, Y. Sun, J. Kim, D. Vaidya, R. Lingle, T. Kise, and S.E. Ralph, "50Gbit/s PAM-4 MMF transmission using 1060nm VCSELs with reach beyond 200m," in Optical Fiber Communications Conference and Exhibition (OFC), 2014, 9-13 Mar. 2014


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PAM-4, 50Gbit/s at 1060nm: OM3 and OM4

72

OM3, 100m 2.86 GHz-km: ISI penalty ~2.1dB 2.05 GHz-km: ISI penalty ~2.6dB

OM3, 200m

2.86GHz-kmz: ISI penalty ~4.2dB

OM3, 150m

2.05GHz-km: ISI penalty ~4.2dB

OM4, 100m 5.6 GHz-km: ISI penalty ~0.8dB 10 GHz-km: ISI penalty ~0.8dB

Noise/ISI metrics RIN < -150dB/Hz MPN k < 0.1

ISI penalties 100m OM3 (2.86GHz-km) ~ 2.1dB

100m OM4 ~ 0.8dB



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PAM-4, > 31GBaud, 1060nm: Wideband Fiber

31GBd btb ISI penalty ~1.5dB

31GBd 100m ISI penalty ~2.2dB

33GBd btb ISI penalty ~2.2dB

Noise/ISI metrics RIN < -150dB/Hz kmpn < 0.1

~ 1. 5dB

~ 2.2dB



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Summary

Components and test/measurement tools are available to support wide deployment of VCSEL-MMF based links employing PAM-4 signaling High capacity, low power, SiCMOS transceivers with digital and analog filtering High speed VCSELs Low DMD OM4 fiber

MPN of current generation InGaAs based VCSELs exhibit low noise so that MPN is not a limitation up to and beyond 100m High bandwidth low RIN VCSELs are moving to large scale production

Multimode VCSEL-based technology (850-1100 nm) will likely continue to be the lowest cost and lowest power solution for short reach links

PAM-4 modulation allows links to maintain reach to 100m and beyond

Wideband MMF together with PAM-4 modulation enables deployment of short wavelength WDM systems that support 400G and higher connections



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GaTech Terabit Optical Networking Group


Back Row: Siddharth Varughese, Pierre Isautier, Jerrod Langston, Edward Tan, Justin Lavrencik Front Row: Mike Pratt, Antony Thomas, Stephen Ralph, Aliro Melgar

Thanks to my team at GaTech

· Tu2G.4.pdf OFC 2016 © OSA 2016 Requirements and Results for Practical VCSEL Transmission using...

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