Sam PalermoAnalog & Mixed-Signal Center

Texas A&M University

Lecture 8: VCSEL TXs

ECEN689: Special Topics in Optical Interconnects Circuits and Systems

Spring 2020

Announcements• Reading

• Sackinger Chapter 8

2

• Light Amplification by Stimulated Emission of Radiation

• Light Oscillation by Stimulated Emission of Radiation• Lasers are optical oscillators that emit coherent light through the

process of stimulated emission• 3 Elements in all lasers

• Amplifying Medium• Pumping Process• Optical Feedback (Cavity)

What is a Laser?

3

[Fejer]

Semiconductor Diode Lasers

4

• Can be made with simple p-n junction• Based on transitions between bands

• Direct bandgap materials necessary• Si isn’t GaAs, InP

• Pumped electrically with current source• Efficient device requires confinement of both carriers

and photons• Leads to the use of heterostructures

[Verdeyen]

Edge Emitters & VCSELs

5

• Edge Emitters• Advantage

• Historically easier to manufacture• Disadvantages

• Emit light in an elliptical mode• Higher testing and packaging costs

• VCSELs – Vertical Cavity Surface Emitting Lasers• Advantages

• Can make 2-D arrays• Emit light in a circular output mode• Smaller device Lower operating

currents• Lower testing and packaging costs

• Disadvantage• Hard to manufacture due to growth of

high reflective mirrors

[Verdeyen]

VCSEL Light-Current-Voltage (LIV) Curve

6

ITH = 700A = 0.37mW/mA

AW Efficiency Slope

IPIIP THo

VCSEL Model

7

Ca Ra

RmCpp‐DBR

n‐DBR

Anode

Cathode

OxideQW 1

cP

LossOutput mirror

Back mirror

aGain

cN

2 g

Output power

aI

1opt cP P

Cp(Ts) Ca(Ts, Ib)

Rm(Ts, Ib)

Ra(Ts, Ib)V

I Ia

+

-

Va

+

-+-

Electrical Input Stage

aI

Optical Output Stage

Vdc(Ts, Ib)

copt PP 1

i a thc cg

T I I TdN PTdt q h

c cg c sp

c

dP PT P Tdt

• Capture thermally-dependent electrical and optical dynamics• Provide dc, small signal, and large-signal simulation capabilities

10Gbps VCSEL Electrical Model

Models finite Q pad capacitance, mirror series resistance, and junction RC

Frequency response dominated by current dependent RJCJ f3dB about 6.5GHz with I≥3mA

RJ Current

Laser Rate Equations

Two coupled differential equations describe the electron density (N) and the photon density (Np) interaction

psp

GNNNqVI

dtdN

:Equation RateDensity Electron

Rate of electrondensity change

Injected electronsgeneration

Non-radiative & spontaneousemission recombination

Stimulatedemission

recombination

p

p

spspp

p NNGNNdt

dN

:Equation RateDensity Photon

Rate of photondensity change Stimulated

emission Spontaneous emission

Photons loss dueto optical

absorption andscattering

lifetimephoton

tcoefficienemission sspontaneou tcoefficienemission stimulated

umecavity vol current injected

p

sp

GVI

VCSEL Rate EquationFrequency Response

p

pdc

sppdc

pdcmgac

GNGNj

GNq

hvvIP

121

)I tonal(Proportio avgp

pdcR

GN

p

pdc

sppdc

GN

GN

2

1

tcoefficien lossmirror

velocitygroup g

m

10Gb/s VCSEL Frequency Response

THavg IIBW

D. Bossert et al, "Production of high-speed oxide confined VCSEL arrays for datacom applications," Proceedings of SPIE, 2002.

VCSEL Reliability

Mean Time to Failure (MTTF) is inversely proportional to current density squared

Failure time modeled with lognormal distribution

Higher mechanical stress reduces flip-chip bond reliability

Trade-off between reliability and bandwidth

37311

2j

ATk

E

ejAMTTF

4

1BW

MTTF

M. Teitelbaum and K. Goossen, "Reliability of Direct Mesa Flip-Chip Bonded VCSEL’s," LEOS, 2004.C. Helms et al, “Reliability of Oxide VCSELs at Emcore,” Proceedings of SPIE, 2004.

Laser Rate Equations – Transient Response

• Laser step response displays relaxation oscillations due to low damping

• Turn-on delay (td) occurs if the laser is biased below threshold– Causes data-dependent jitter

TH

spd IIIIt

1

01ln

Turn-on Delay

[Honeywell]

• VCSELs also have additional unwanted frequency modulation called chirp

• The linewidth in this case can be approximated as

Chirp

14

.or theis where

122

t factorenhancemenlinewidth meterchirp para

Bc

• The parameter relates the change in optical frequency to the change in optical power

tPdtdtf outln

4

• Directly modulated lasers have positive values, implying that for a rising edge the laser will blue-shift (higher frequency/shorter ) and red-shift for a falling edge

• VCSELs have occasional spontaneous emissions which add amplitude and phase noise to it’s coherent light output

• The resulting intensity fluctuations are known as relative intensity noise (RIN)

• At the receiver, this will get converted to an equivalent electrical noise component by the photodetector which is approximately proportional to the received signal power

Relative Intensity Noise (RIN)

15

power!laser theincreasingby SNR improvet Can'

1

is SNR resulting The dB/Hz.in measured noise RINlaser thezingcharacteriparameter a is Here

,2

2

2,

2

nRINn

PIN

nPINRINn

BWRINiISNR

RIN

BWIRINi

RIN Power Penalty

16

dB

GHziISNR

HzdBRIN

HzdBRINn

PIN 351016.3

1010

1

is SNR thebandwidth, noisereceiver 10GHz a and /135 laser with a Assuming

3

10/135

,2

2

• This SNR is fine for digital NRZ signaling, but may be an issue for analog optical links for applications such as cable TV

• RIN noise does introduce an additional power penalty

dBPP

BWRINQPP

n

069.0016.1

1016.31035.71

1ispenalty power 10BER theexample, above For the

11

32

12-

2

Temperature-Dependent Performance

• Optical power-current-voltage (L-I-V) response is temperature-dependent

17

23oC

80oC

25Gb/s

• Bandwidth is bias and temperature dependent

18

Ib=6mA

Ib=5mA

Ib=4mA

Ts=23oC

Ib=6mA

Ib=5mA

Ib=4mA

Ts=80oC

Measured and Simulated 25Gb/s Eye Diagrams

VCSEL Performance Issues• Threshold Current

– Reduced with smaller devices, better electron/photon confinement (quantum wells)

• Bias Dependent Frequency Response– Proportional to Sqrt(Ibias)– Low damping factor causes relaxation oscillations in step response

• Turn-On Delay– Leads to data-dependent jitter if biased below threshold

• Chirp– Direct amplitude modulation also modulates the frequency of the

optical carrier– Leads to dispersion in optical fiber– Reason why most long-haul systems use external modulation

Laser Drivers

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• Current-mode drivers are often used due to the laser’s linear L-I relationship

• In addition to the high-speed modulation current IM, laser drivers must also supply a bias current IB to ensure a minimum frequency response and/or eliminate turn-on delay

Current-Mode VCSEL Driver

Laser Drivers

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• The total laser current depends on whether the high-speed modulation current is DC- or AC-coupled

• DC-coupled case

Current-Mode VCSEL Driver

MBLBL IIIII 1,0, current level-0 theiscurrent bias The

• AC-coupled case

2

2

current average theiscurrent bias The

1,0,M

BLM

BLIIIIII

Termination Strategies

22

• The laser interface with the driver determines whether double or “back” termination is necessary

• Reflections from bondwires and any laser/transmission-line mismatch can degrade high-speed performace

• Driver on-die termination improves this at a power cost

Active Back Termination

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• While not a major issue for relatively low-power VCSELs, the lost current with on-die driver termination is a concern for high-power (long-haul) lasers

• This motivates the use of active back termination circuitry where the termination resistor is connected to an AC voltage generated by a replica stage

• Ideally, without reflections, no voltage drop is across RT

25Gb/s VCSEL Link

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• Current-mode output driver• Bandwidth extension achieved with on-die shunt-peaking

termination in the output stage and with Cherry-Hooper preamplifier stage

[Proesel ISSCC 2012]

Multiplexing FIR Output Driver

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• 4-tap FIR filter implemented in driver to compensate for VCSEL frequency response

S. Palermo and M. Horowitz, “High-Speed Transmitters in 90nm CMOS for High-Density Optical Interconnects," ESSCIRC, 2006.

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Tap Mux & Output Stage

5:1 multiplexing predriver uses 5 pairs of complementary clock phases spaced by a bit time

Tunable delay predriver compensates for static phase offsets and duty cycle error

VCSEL TX Optical Testing

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Wirebonded 10Gb/s VCSEL

VCSEL 16Gb/s Optical Eye Diagrams

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Iavg=6.2mA, ER=3dB

w/ Equalization

IDC = 4.37mAIMOD = 3.66mA

IDC = 3.48mAI= -0.70mAI0 = 4.36mAI1 = -0.19mAI2 = 0.19mA

Equalization increases vertical eye opening

45% at 16Gb/s

No Equalization

Equalization Performance

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ER=3dB ER=6dB

14Gb/s35% less Iavg

138% increase in MTTF4Tap Eq

No Eq

• Maximum data rate vs Average current• Min 80% eye opening & <40% overshoot

• Equalization allows lower average current for a given data rate

• Linear equalizer limited by VCSEL nonlinearity

PAM2 VCSEL Driver w/ 2-Tap Nonlinear FFE

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• VCSEL’s bias-dependent frequency response results in nonlinear transient pulse responses

• A 2-tap non-linear equalizer with different equalization taps for high and low pulses provides performance improvement

[Raj CICC 2015]

PAM4 VCSEL Driver w/ 2.5-Tap Nonlinear FFE

• A 2.5-tap nonlinear equalizer, with the first pre-cursor weight only dependent on the MSB, is a good compromise between complexity and performance 31

1,11,00,10,0S-1

1,11,00,10,0S0

1,11,00,10,0S+1

3-tap equalizer

1,11,00,10,0S-1

1,11,00,10,0S0

1,X

0,X

S+1

2.5-tap equalizer with the pre-cursor weight only a function of the MSB

Serializing VCSEL TX & Output Stage

• VCSEL transmitter serializes 16 bits or 8 PAM-4 symbols

• Output stage is a 5-bit non-uniform current-mode DAC• MSB and MSB-1 set the main PAM-4 symbol levels• 3 LSB currents implement the 2.5-tap equalizer with the symbol

pattern selecting the weighting from the 32X3 LUT32

VDD VCSEL

VCSEL

DummyLoad

SYDAC Code

EQDAC Code

000

111000

111000

111000

111

00

SYDAC Code

EQDAC Code

Incr

easi

ng V

CSEL

Cur

rent

IBLEED

EQD

AC

DnDbn

VCAS

VBIAS

DnDbn

VCAS

VBIAS

DnDbn

VCAS

VBIAS

DnDbn

VCAS

VBIAS

SYDA

C

0110

11

0001

1011

Uniform DAC Nonuniform DAC

000

111000

111000

111000

111

I0

Output Driver

32x3 lookup table

8:1 Serializer

SYDAC<2>x20

SYDAC<1>x10

EQDAC<3>x4

EQDAC<2>x2

EQDAC<1>x1

Output

8x3

Serializer Clocks

3 bit32:1 MUX

3 bit32:1 MUX

3 bit32:1 MUX

3 bit32:1 MUX3 bit32:1 MUX

3 bit32:1 MUX

3 bit32:1 MUX

3 bit32:1 MUX

8x5

8:1 Serializer

LUT & EQ

PRBS-15

Sel

16 2 bit delay logic

2 bit delay logic

2 bit delay logic

2 bit delay logic

2 bit delay logic

2 bit delay logic

2 bit delay logic

2 bit delay logic

32x3

8x5

16

8x2

8x2 2

3

Clocking UnitCLKNC

LKIN

Shunt-Peaking CML Input & Buffer Stages

CML-to-CMOS

CLKP

Fixed Pattern

Serializing VCSEL TX 5-b Non-Uniform Current DAC Driver

50Gb/s PAM4 Experimental Results

• Core transmitter area is 0.2mm2

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No Equalization 2-Tap Linear

2.5-Tap Linear 2.5-Tap Nonlinear

• 2.5 tap nonlinear equalizer improves eye height and timing alignment of the 3 PAM4 eyes

[Tyagi PTL 2018]

Next Time• Mach-Zehnder Modulator (MZM) TX

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