VCSELs and Optical Interconnects

Anders Larsson

Chalmers University of Technology

ADOPT Winter School on Optics and Photonics

February 4-7, 2016

Outline

Part 1 VCSEL basics

- Physics and design

- Static and dynamic properties

- Applications

Part 2 Research highlights – VCSELs for optical interconnects

- Optical interconnects

- High speed and energy efficient VCSELs

- VCSEL arrays and integration

Semiconductor laser basics

Current injection in a forward biased pin-junction population inversion

optical gain through stimulated emission

Band gap wavelength:gE

hc0

Semiconductor Fabry-Perot laser:

Gain medium (active region) = smaller band gap material

Pump = current

Optical feedback = cleaved facets (mirrors)

Heterostructure confinement of carriers and photons (waveguide)

Optical confinement factor: Material gain: Modal gain:

n-type, larger band gap

p-type, larger band gap

undoped, smaller band gap

insulating, larger band gap

cleaved facet

coherent outputcoherent output

cleaved facet

n-contact

p-contact +V

gain (active) region

I

n-type p-type

EC

EV

electrons

holes

optical amplification

through stimulated

emission

n = n0+n

p = p0+p

Egn = p

everywhere

y

regionactive

y

dxdyy,xE

dxdyy,xE

2

2

holes

electrons

optical field distribution active region

ggm g

VCSEL vs. EEL

Edge emitting laser (EEL) Vertical cavity surface emitting laser (VCSEL)

Horizontal resonator

Edge emitting (cleaving needed)

Elliptical beam

”Large” gain and optical mode volumes (~102 µm3)

”High” current and ”high” power (~102 mW) operation

Vertical resonator

Surface emitting (on-wafer testing and screening)

Circular beam

”Small” gain and optical mode volumes (~100 µm3)

”Low” current and ”low” power (~100 mW) operation

VCSELEEL

Beam profiles

VCSEL building blocks

distributed

Bragg reflector (DBR)

distributed

Bragg reflector (DBR)

active (gain) region

embedded in pin-junction

Vertical resonator created by two DBRs separated by the active region

Current injection through doped DBRs

Aperture for transverse current and optical confinement

transverse current and

optical confinement

transverse refractive index profile

vertical refractive index profile

Active (gain) region

n-type p-type

EC

EV

n = n0+n

p = p0+p

energy

momentum

E2

E1

EFn

EFp

absorp

tion

Eg

gain

absorption under thermal equilibrium (EFn = EFp = EF)

gain under full inversion (EFn - EFp = )

I1

(EFn-EFp) at I1

(EFn-EFp) at I2

I2 > I1

ntr,bulk

gmax

n = p

ntr,QW

Thin active layer (QW) quantization of states modified density of states

Strained (lattice mismatched) active layer modified density of states

Lower transparency carrier density (ntr), higher differential gain at low carrier density

Bulk Quantum well (QW)

Gain vs. carrier density:

Differential gain:

trmax nngng 0 tr

maxn

nlngng

0

0gnd

dgmax n

g

nd

dgmax

0

Carrier injectionQuasi-Fermi level separation

Optical gain spectrum

Eg

N pairs of layers with different refractive index n1 and n2

Each layer is a quarter wavelength thick at the Bragg wavelength:

Reflections at all interfaces add in phase constructive interference

large reflectivity at the Bragg wavelength

The maximum reflectivity increases with the index contrast (n = n1-n2) and the number of pairs (N)

The spectral width (bandwidth) increases with the index contrast

Distributed Bragg reflectors

R

nin

n1

n1

n1

n1

n1

n1

n1

n2

n2

n2

n2

n2

n2

nout

d1

d21

2

.

.

.

N

101 4n/d 202 4n/d

2

2

2

1

2

2

1

1

1

N

in

out

N

in

out

max

n

n

n

n

n

n

n

n

R

and

Reflection c

hara

cte

ristics o

f an

Al 0

.90G

a0

.10A

s/A

l 0.1

2G

a0

.88A

s D

BR

(0

= 8

50 n

m)

0 = 850 nm

650 700 750 800 850 900 950 1000 10500

10

20

30

40

50

60

70

80

90

100

(nm)

Po

we

r re

fle

ctivity

(%)

bandwidth

N = 21

650 700 750 800 850 900 950 1000 1050-4

-3

-2

-1

0

1

2

3

4

(nm)

Refle

ctio

nphase

(ra

d)

N = 21

0 25 50 75 100 125 150-1

0

1

2

3

4

5

6

7

8

9

Position (nm)

Na

-N

d(1

018

cm

-3)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Alu

min

um

conte

nt

(-)

90% Al

12% Al

0.8

6.0

4.0

2.5∙1018 cm-3

Field penetration and phase of reflection are represented by a

hard mirror placed at a distance Leff from the start of the DBR

Distributed Bragg reflectors (cont’d)

For highly reflecting DBRs:

where: B = Bragg wavelength

nL B

eff

4

R

Leff

Hard mirror

Semiconductor DBRs have graded composition interfaces

and are modulation doped to reduce resistance and optical

loss (free carrier absorption)

graded

interfaces

modulation

doping

Al0.90Ga0.10As/Al0.12Ga0.88As

p-DBR

0 25 50 75 100 125 1500

1

2

3

4

5

6

7

8

9

10

Position (nm)

Hole

concentr

ation (

101

8cm

-3)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Norm

aliz

ed f

ield

square

d

R

The vertical resonator

Leff1

Leff2

Lactive

DBR1

DBR2

active region

Effective optical length of the resonator: where in the DBRs2211 effDBRactiveactiveeffDBRopt LnLnLnL

21

11

2

11

nnn

Lopt very small (~ one wavelength) large longitudinal mode spacing only one longitudinal mode under the gain

spectrum single longitudinal mode laser !

Small longitudinal optical confinement factor (1-2%), thin gain region (QWs) high DBR reflectivities needed (close to 100%)

Field antinodes (maxima) strong interaction with the gain region (multiple QWs), resonant gain enhancement

Field nodes (minima) weak interaction with the medium (regions with high optical loss)

resonatorentire

y

QWs

y

longdzzE

dzzE

2

2

Longitudinal optical confinement factor:

y

z

longitudinal

optical

field

effective

resonator length

Transverse current and optical confinement

p-contact

p-semiconductor DBR

oxide apertureactive region

n-semiconductor DBR

n-substrate

n-contact

GaAs-based VCSELs (670 – 1100 nm)

neffneff2

neff1

dox

The number of transverse modes depends on the aperture size and the effective index difference

Multiple transverse modes multiple emission wavelengths (multimode VCSEL)

Single transverse mode (LP01) single emission wavelength (single mode VCSEL)

The transverse confinement factors are large (>95%)

Transverse optical confinement factor:

everywhere

y

aperturewithin

y

transdxdyy,xE

dxdyy,xE

2

2

Multimode

VCSEL

y

zdox = 6 µm

fundamental

mode

LP01 LP11 LP21

LP02 LP31 LP12

Transverse current and optical confinement (cont’d)

InP-based VCSELs (1300 – 2000 nm), GaSb-based VCSELs (2000 – 3500 nm)

n-semiconductor DBR

n-substrate

n-contact

active region

dielectric DBR

n-contact (intra-cavity)

n-type material

p-type material

buried tunnel

junction (n+/p+)

blocking pn-junction

neffneff2

neff1

dBTJ

Threshold current, slope efficiency and power efficiency

From optical resonator simulations: Transmission loss through top DBR (tr,top)

Transmission loss through bottom DBR (tr,bottom)

Absorption loss in DBRs (abs)

Material gain at threshold (gth)

Threshold carrier density (QWs):

Spontaneous recombination rate at threshold:

Threshold current (injection balances recombination): where

External differential quantum efficiency:

Slope efficiency (W/A):

Power efficiency:

32ththththsp nCnBnAnR

thspact

thi nRqV

I

thsp

i

actth nR

qVI

(no stimulated emission)

VI

Pp

current

po

we

r

vo

lta

ge

Ith I

V

P

P

I

trn

nlngg

0

0g/gtrth

thenn

gain balances loss

free carrier absorption

4

2dLNV QWQWact

absbottom,trtop,tr

top,trid

absbottom,trtop,tr

top,tri

q

hc

I

PSE

0

defect recombination spontaneous emission Auger recombination

i = internal quantum efficiency

NQW = number of QWs

LQW = QW thickness

d = aperture diameter

0 5 10 15 200

2

4

6

8

10

Current (mA)

Pow

er

(mW

),V

oltage

(V)

dox = 10 µm

Detuning and temperature dependence

Emission wavelength set by the resonance wavelength

Resonance wavelength and gain spectrum redshift with temperature

Gain peak shifts ~3 times faster than resonance

Small temperature variation of the threshold current with proper detuning

T3 > T2

T2 > T1

T1 = RT

gain spectrum resonance

detuning

-20 0 20 40 60 80 1000.5

1

1.5

2

2.5

Temperature (C)

Th

resh

old

cu

rre

nt (m

A)

sv

0Z

pi ci

parasitics intrinsic laser

pad chip

ai

ppCmC

mR

thr IIDf

)n/g(

qV

vD

a

gi

2

1;

)n/g(vK

gp

240

2 rfK ;

2

22

2

fjff

fconst

i

)f(p)f(H

r

r

aint

Resonance frequency and D-factor:

Damping rate and K-factor:

Optical confinement ()

Differential gain (g/n)

Active volume (Va)

Photon lifetime (p)

Gain compression ()

VCSEL dynamics – damping

frequencyresonancefr ratedamping

Kf damping,dB

223

Intrinsic small signal modulation response:

;

Damping limited modulation banwidth:

Carrier transport ()

I1 < I2 < I3 < I4

I1

I3

I4

I2

VCSEL dynamics – thermal effects and parasitics

)T(II)T(Df thr

)n/g(

qV

vD

a

gi

2

1;

p

maxg

max,r

Sn

gv

f

2

1

Increasing current increasing temperature (self heating) saturation of photon density and resonance frequency

thermally limited bandwidth

Major heat sources:

DBR resistance

Free-carrier absorption

max,rf

Increasing frequency modulation current shunted past active region parasitics limited bandwidth

pr

r

s

a

a

f

fj

fjff

fconst

v

i

i

)f(p)f(H

1

1

2

22

2

intrinsic response parastics

Major parasitics:

DBR resistance

Capacitance over oxide layer (GaAs) or reverse biased pn-junction (InP)

Frequency [GHz]

0 10 20 30 40 50 60 70

Mo

du

latio

n r

esp

on

se

[d

B]

-20

-15

-10

-5

0

5

10

intrinsic response

with thermal effects

with thermal

effects and

parasitics

VCSEL dynamics – large signal modulation

current

outp

ut pow

er

Ith

modulation

current

modulated

output power

bias

current

Digital modulation: biasing above threshold + current pulses (”1” and ”0”)

on-level (binary 1)

off-level (binary 0)

current

pulse

optical

pulse

time

ringing

ringing

90%

10%

rise time fall time

bias current

“Eye-diagram” VCSEL operating at 2 Gbit/s

VCSEL applications – datacom

Lane rate up to 25 - 28 Gbit/s (Ethernet, Fiber Channel)

Aggregate capacity up to 400 Gbit/s (16 x 25 Gbit/s) in parallel fibers

Reach typically 1-100 m

Multimode optical fiber (50 µm core)

Wavelength: 850 nm (GaAs-based VCSELs)

10-2 10-1 100 101 102 103

Distance (m)

rack - rack

shelve - shelve

board - board

module - module

Multimode optical fiber

Polymer optical waveguide

Datacenters

High performance computing

Facebook

IBM

2000 2005 2010 2015

Year

2020

1000

100

10

1

0.1

La

ne

rate

(G

bit/s

)

Ethernet

Fiber Channel

Infiniband

Consumer

electronics

Mid-board optical modulePluggable tranceiver Active optical cable

TE Connectivity

Corning

TE Connectivity TE Connectivity

VCSEL applications – sensing

Major applications:

- Optical navigation (computer mouse, finger navigation modules, guesture recognition, …)

- Spectroscopy (e.g. gas sensing and analysis)

- Optical encoding (digital encoding of e.g. linear and rotary positions)

- ……

Many sensing applications require spectral purity, coherence and/or focusing capabilites

single-mode and polarization stable VCSELs

It is expected that a smartphone will contain up to ~10 VCSELs !

VCSEL applications – high power

2D integration of 1000´s of VCSELs enables >100 W low brightness VCSEL arrays

Major applications:

- Surface treatment (heating)

- Illumination (night vision)

- Laser pumping

VCSEL array top view 2D array of VCSEL arrays

400 W VCSEL emitter

8 W VCSEL array

Beam profile of a 100 W VCSEL array

9.6 kW VCSEL module

Philips Photonics

Princeton Optronics

Lasertel

Optical interconnects – developments

Higher capacity and bandwidth density (SDM, WDM)

- Lane rates: 25-28 Gbit/s 50, 56, 64, 100 … Gbit/s

- Aggregate capacity: 400 Gbit/s (16 x 25 Gbit/s) multi-Tbit/s

Higher efficiency

- 10’s of pJ/bit 1 pJ/bit

High operating temperature (85°C, at least)

Longer and shorter reach

- Datacenters are growing in size (~ 103 m)

- Migration closer to the onboard circuits (~ 10-3 – 100 m)

10-2 10-1 100 101 102 103

Distance (m)

rack - rack

shelve - shelve

board - board

module - module

Multimode optical fiber

Polymer optical waveguide

VCSEL-based optical interconnects

Datacenters

High performance computing

Facebook

IBM

2000 2005 2010 2015

Year

2020

1000

100

10

1

0.1

La

ne

rate

(G

bit/s

)

Ethernet

Fiber Channel

Infiniband

Consumer

electronics

Mid-board optical modulePluggable tranceiver Active optical cable

TE Connectivity

Corning

TE Connectivity TE Connectivity

Optical interconnects – developments (cont’d)

Higher speed optoelectronics and electronics + electronic compensation + multilevel modulation higher lane rates

Spatial and wavelength division multiplexing (SDM, WDM) higher aggregate capacity and higher bandwidth density

TE Connectivity

Single fiber

Single core

Single wavelength

Multiple fibers

Single core

Single wavelength

Multiple fibers

Multiple cores

Single wavelength

~100 Gbit/s

~1 Tbit/s

~10 Tbit/sMultiple fibers

Single core

Multiple wavelengths

~100 Tbit/s

Multiple fibers

Multiple cores

Multiple wavelengths

VI Systems

1 2 3 4

IBM

High speed 850 nm VCSELs

Speed optimized with respect to damping, thermal effects and parasitics

Modulation bandwidth = 28 GHz @ 25°C and 21 GHz @ 85°C

Data rates up to 47 (57) Gbit/s with limiting (linear) optical receiver

4 µm

7 µm

7 µm

47G

25C40G

85°C

40G

25C

4 µm

high thermal

conductance

n-DBR

low resistance, low optical loss DBRs

strained InGaAs/AlGaAs QWs

short cavity, reduced photon lifetime (reduced damping)

multiple oxide

layers

BCB

undoped GaAs substrate

3.3 ps

1.2 ps

5.3 ps

6.4 ps

Impact of damping on VCSEL dynamics

Modulation bandwidth: trade-off between resonance frequency and damping rate

Eye quality and BER: trade-off between rise/fall times, modulation efficiency and jitter

K = 0.35 ns

3.3 ps5.3 ps 1.2 ps

K = 0.30 ns K = 0.20 ns K = 0.14 ns

6.4 ps

)n/g(

qV

vD

a

gi

2

1

)n/g(vK

gp

24

10 Gbit/s

40 Gbit/s

K = 0.4 ns K = 0.3 ns K = 0.2 ns K = 0.1 ns

Highest bandwidth (K = 0.2 ns)= best receiver sensitivity

Transmitter integration – equalization

Driver and receiver circuits with two-tap feed forward equalization (FFE)

IBM SiGe BiCMOS 8HP (130 nm)

26 GHz VCSEL (Chalmers), 22 and 30+ GHz photodiode (Sumitomo)

Error-free transmission over MMF up to 71 Gbit/s at 25°C and 50 Gbit/s at 90°C

56 Gbit/s 60 Gbit/s 64 Gbit/s

56 Gbit/s, 107 m 60 Gbit/s, 107 m 64 Gbit/s, 57 m

71 Gbit/s, 7 m

VCSEL

Driver

25 - 71 Gbit/s, 7 m

-12 -10 -8 -6 -4 -2

-3

-4

-5

-6

-7

-8

-9

-10

-11

-12

-13

OMA (dBm)

log

10(B

ER

)

25G

32G

40G

50G

60G

64G

68G

71G

Onboard polymer waveguide interconnect

Polymer waveguide embedded in backplanes and circuit boards to interconnect boards and modules

25 GHz high power/high slope efficiency VCSEL

40 Gbit/s NRZ and 56 Gbit/s PAM-4 transmission over 1 m polymer waveguide

Record speed-distance products (40 and 56 Gbit/s∙m)

Back-to-back

Waveguide

25 Gbit/s 36 Gbit/s 40 Gbit/s 56 Gbit/s PAM-4

0.04 dB/cm @ 850 nm

Speed and efficiency enhancement by strong confinement

Half-wavelength optical resonator, oxide apertures close to and on either side of the active region

Modulation bandwidth = 30 GHz

Transmission at 25-50 Gbit/s with energy dissipation < 100 fJ/bit

0 1 2 3 4 5 60

0.5

1

1.5

2

2.5

Bias current (mA)

Outp

ut pow

er

(mW

)

0 1 2 3 4 5 60

1

2

3

Voltage

(V)

840 844 848-80

-60

-40

-20

0

Wavelength (nm)

Rela

tive inte

nsity

(dB

)

2 mA

3.5 µm50 Gbit/s

2.5 mA

95 fJ/bit

25 Gbit/s

1.3 mA

85 fJ/bit

-16 -12 -8 -4 0 4

-12

-10

-8

-6

-4

-2

Received optical power (dBm)lo

g(B

ER

)

40 Gbit/s

1.8 mA

78 fJ/bit

0 5 10 15 20 25 30 35

-18

-15

-12

-9

-6

-3

0

3

6

9

12

Frequency (GHz)

Modula

tion r

esponse

(dB

)

0.5 mA1 mA

1.7 mA4 mA

3.5 µm

high thermal

conductance

n-DBR

low resistance, low optical loss DBRs

strained

InGaAs/AlGaAs

QWs

short cavity, reduced photon lifetime (reduced damping)

multiple oxide

layers

BCB

undoped GaAs substrate

Transverse and polarization mode control

Surface micro/nano structures for transverse and polarization mode control

Multi-mW single mode output power

20 dB orthogonal polarization suppressionSub-wavelength

surface grating

Oxide aperture 6 µm

Mode filter 3 µm

120 nm

grating period

20 Gbit/s, 2000 m (40 Gbit/s∙km)

High speed VCSEL for extended reach

VCSEL with integrated mode filter

26 Gbit/s, 1000 m (26 Gbit/s∙km) 25 Gbit/s, 1300 m (32.5 Gbit/s∙km)

Integrated mode filter for single mode emission

Reduced impact of chromatic dispersion

Extended reach: 25 Gbit/s over 1300 m MMF, 20 Gbit/s over 2000 m MMF

without mode filter

with mode filter

RMS = 0.25 nm

RMS = 0.80 nm

Oxide aperture 6 µm

Mode filter 3 µm

19 GHz bandwidth

Multilevel modulation over VCSEL-MMF link

PAM

signal

generator

VCSEL

optical

receiver

error

analyzer

MMF

VOA20 GHz

Back-to-back 50/100 m OM4 fiber

Increased capacity through improved spectral efficiency

Pulse amplitude modulation (PAM) – low complexity, low power consumption, good receiver sensitivity

PAM-n modulation (n levels, log2(n) bits/level)

30 Gbaud PAM-4 (60 Gbit/s) transmission over a 20 GHz link (no equalization, no FEC)

40 Gbaud PAM-4 (80 Gbit/s ) with equalization and FEC (70 Gbit/s)

20 Gbaud PAM-8 (60 Gbit/s) with FEC (56 Gbit/s)

40 Gbaud PAM-4

(80 Gbit/s)

20 Gbaud PAM-8

(60 Gbit/s)

30 Gbaud PAM-4

(60 Gbit/s)

VCSEL arrays for multicore fiber interconnects

Multicore, multimode optical fiber for increased capacity and bandwidth density

6-channel circular VCSEL array

40 Gbit/s/channel up to 80°C, 240 Gbit/s aggregate capacity

No crosstalk penalty

0 2 4 6 8 10 12 14 16 18 200

2

4

6

8

10

Current (mA)

Pow

er

(mW

)

0

2

4

6

8

10

Voltage (

V)

Ch 1

Ch 2

Ch 3

Ch 4

Ch 5

Ch 6

25°C 85°C

25°C 85°C

Ch 1

Ch 2

Ch 3

Ch 4

Ch 5

Ch 6

40 Gbit/s, 25°C 40 Gbit/s, 85°C

26 µm

39 µm

Ch5

Ch4

Ch3

Ch2

Ch1

Ch6

25°C

85°C25°C

85°C

Integrated transmitters for WDM optical interconnects

Integration platform

- Short wavelength compatible (high speed and high efficiency GaAs-based optoelectronics)

- Wavelength multiplexing, drive electronics

Monolithic multi-wavelength VCSEL array

- Wavelength setting in a post-growth process

- Hybrid (flip-chip) or heterogeneous integration

- In-plane emission

Heterogeneous integration:

multi-channel

driver

multi-wavelength

VCSEL array

silicon nitride waveguide PIC

1

2

3

4

wavelength

multiplexer1, 2, 3, 4

silicon

”half” III-V VCSEL

SiN HCG and waveguide on Si

bonding

interface

silicon

dielectric DBR and SiN grating/waveguide on Si

Multi-wavelength high-contrast grating VCSEL array

High contrast grating (HCG) for wavelength setting in a planar post-growth fabrication process

High, broadband and mode/polarization selective reflectivity

Phase of reflection (resonance wavelength) controlled by grating parameters (duty cycle, period)

4 channels with ~5 nm channel spacing at 980 nm

Single transverse and polarization mode

1 2 3 4

HCG (GaAs) mirror

active (gain) region

distributed Bragg reflector (DBR)

oxide

apertures

Si-integrated short wavelength VCSEL with a hybrid cavity

”half” III-V VCSEL

dielectric DBR on Si

bonding interface

Si substrate

GaAs-based half-VCSEL attached to a dielectric DBR on Si using adhesive BCB bonding

Hybrid III-V/Si cavity

Sub-mA threshold current, high slope efficiency

20 Gbit/s modulation

AlGaAs DBR

SiO2/Ta2O5 DBR

Si substrate

p-contact

n-contact

oxide aperture

bonding interface

0 1 2 3 4 5 6 70

1

2

Pow

er

(mW

)

Current (mA)

1

2

3

4

Voltage

(V)

0

3 µm

5 µm

7 µm

9 µm

830 840 850 860

Wavelength (nm)

10G

-20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0-14-12

-10

-8

-6

-4

-2

Received optical power (dBm)

log 1

0(BE

R)

15G

20G

10G

15G

20G

5 µm