vcsels and optical interconnects - kth · dbr2 active region effective optical length of the...
TRANSCRIPT
-
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
0g
nd
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 Beff
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:
32 ththththsp 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
ppC mCmR
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
23
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(B
ER
)
15G
20G
10G
15G
20G
5 µm