Appl. Phys. Lett. 97, 151101 (2010); https://doi.org/10.1063/1.3499361 97, 151101
© 2010 American Institute of Physics.
Highly temperature-stable modulation
characteristics of multioxide-aperture
high-speed 980 nm vertical cavity surface
emitting lasers
Cite as: Appl. Phys. Lett. 97, 151101 (2010); https://doi.org/10.1063/1.3499361
Submitted: 09 June 2010 • Accepted: 20 September 2010 • Published Online: 11 October 2010
A. Mutig, J. A. Lott, S. A. Blokhin, et al.
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Highly temperature-stable modulation characteristics of multioxide-
aperture high-speed 980 nm vertical cavity surface emitting lasers
A. Mutig,1,a兲J. A. Lott,2S. A. Blokhin,1,b兲P. Wolf,1P. Moser,1W. Hofmann,1
A. M. Nadtochiy,1,b兲A. Payusov,1,b兲and D. Bimberg1
1Institut für Festkörperphysik und Zentrum für Nanophotonik, Technische Universität Berlin,
Hardenbergstrasse 36, 10623 Berlin, Germany
2VI Systems GmbH, Hardenbergstrasse 7, 10623 Berlin, Germany
共Received 9 June 2010; accepted 20 September 2010; published online 11 October 2010兲
We present multioxide-aperture 980 nm-range vertical cavity surface emitting lasers 共VCSELs兲with
highly temperature stable modulation characteristics operating error-free at 25 Gbit/s at 25 and
85 °C. We perform small signal modulation experiments and extract the fundamental physical
parameters including relaxation resonance frequency, damping factor, parasitic cut-off frequency,
D-factor, and K-factor, leading to identification of thermal processes and damping as the main
factors that presently limit high speed device operation. We obtain very temperature-insensitive
bandwidths around 13–15 GHz. Presented results clearly demonstrate the suitability of our VCSELs
for practical and reliable optical data transmission systems. © 2010 American Institute of Physics.
关doi:10.1063/1.3499361兴
Human society’s constantly increasing demand for more
information, the on-going scaling of silicon-based integrated
circuits to ever smaller dimensions, and the intrinsic limita-
tions of copper-based electrical data links have led to our
current need for lower cost, power efficient, readily manu-
facturable, reliable photonic devices. The unstoppable and
progressive penetration of optical communication links into
traditional copper interconnect markets greatly expands the
applications of vertical cavity surface emitting lasers 共VC-
SELs兲for next-generations of board-to-board, module-to-
module, and chip-to-chip interconnects.1,2The continued
down-sizing of integrated photonics systems and quest for
broader bandwidths leads naturally to interest in very short-
reach 共VSR兲data transmission systems residing, e.g., among
microprocessors, where the high temperature stability of the
lasers is indispensable. In the recent years, great progress
was achieved in increasing both the data rate and the tem-
perature stability of VCSELs. At room temperature bit rates
of 32 共Ref. 3兲to 38 Gbit/s 共Refs. 4and 5兲共850 nm兲,35
Gbit/s 共980 nm兲,6and 40 Gbit/s 共1100 nm兲7have been dem-
onstrated. While 850 nm is the current standard wavelength
for local and storage area networks and other short-reach
optical link systems, potential competitive standards at 980
and 1100 nm have many critical advantages for VSR systems
including: smaller operational voltages, deeper potential
wells, and transparency of the GaAs substrate, which is im-
portant for bottom-emitting VCSELs. Hereby the wavelength
of 980 nm is preferable for Si-based photodetectors, since
the absorption coefficient of Si is much larger at 980 nm as
compared to longer wavelengths.
Because the free carrier absorption coefficient increases
with the wavelength, tolerances for adjustment of doping
levels in the epitaxial structure for 980 nm are more strictly
as compared to 850 nm. Additionally the application of
highly strained materials for the active region and thus the
proper strain handling creates new challenges, which are nor-
mally absent in 850 nm devices. While at elevated tempera-
tures of 85 °C or even above bit rates of 25 Gbit/s have been
demonstrated for VCSELs operating at 850 共Ref. 3兲and
1100 nm,7however, at 980 nm the highest bit rate at 85 °C
was still limited to 20 Gbit/s,8where VCSELs with InGaAs
layers grown in the submonolayer growth mode by molecu-
lar beam epitaxy were applied. Here we indeed present 980
nm-range oxide-confined VCSELs operating error-free at
data bit rates of up to 25 Gbit/s at temperatures as high as
85 °C, which were grown by metal-organic chemical-vapor
deposition 共MOCVD兲with conventional multiple quantum
wells 共QWs兲, thus applying the mature and well approved
large-scale mass production techniques. To facilitate our
analysis of the limiting physical processes inside the VC-
SELs we perform small and large signal modulation experi-
ments at various temperatures enabling us to determine the
dominant factors that control the VCSEL’s speed, notably at
elevated temperatures.
The VCSEL structure is grown on an 兵001其-oriented 2°-
off n-doped GaAs-substrate using MOCVD. Doped
Al0.12Ga0.88As/Al.90Ga0.10As distributed Bragg reflectors
共DBRs兲with 20 nm thick linear graded interfaces with 24/37
periods for the top/bottom mirror are employed. The active
region contains five 4.2 nm thick compressively strained
In0.21Ga0.79As QWs with 6 nm thick GaAs0.88P0.12 tensile
strained barrier layers. The 4.7% smaller lattice constant of
GaAs0.88P0.12 as compared to GaAs partially compensates the
larger lattice constant of In0.21Ga0.79As and the correspond-
ing lattice constant mismatch of 1.55%. Carefully designing
of the layer thicknesses leaves both the local and the global
strain well below values critical for the high quality growth.
To improve the temperature stability of the gain and differ-
ential gain optimized detuning of 15 nm between the QW
photoluminescence peak and the cavity resonance was ap-
plied, helping to compensate the gain decrease at elevated
temperatures and leading to the very temperature insensitive
a兲Electronic mail: [email protected].
b兲On leave from the St. Petersburg Physical Technological Centre for Re-
search and Education RAS, Khlopina 8/3 and Ioffe Physical-Technical
Institute RAS, Polytekhnicheskaya 26, Saint-Petersburg, 194021 Russian
Federation.
APPLIED PHYSICS LETTERS 97, 151101 共2010兲
0003-6951/2010/97共15兲/151101/3/$30.00 © 2010 American Institute of Physics97, 151101-1
threshold current behavior. To facilitate electrical and optical
confinement and to reduce electrical parasitics two 30 nm
thick Al0.98Ga0.02As oxide aperture layers3with tapered fin-
gers are formed by selective wet oxidation. The VCSELs are
planarized with ⬃10
m thick bisbenzocyclobutene to re-
duce the parasitic capacitances. Ground-source-ground metal
contact pads are evaporated for ease of on-wafer high-
frequency probe characterization.
We show in Fig. 1the measured output power-current-
voltage characteristics for a VCSEL with a 10
m diameter
oxide aperture and the corresponding values for differential
efficiency and threshold current. The maximum output power
is 4.3 mW at 20 °C and decreases little to 2.6 mW at 85 °C.
The maximum differential efficiency decreases by only 18%
from ⬃0.34 W/Aat20°Cto⬃0.28 W/A at 85 °C. The
applied redshifted detuning of nominally 15 nm between the
QW photoluminescence peak wavelength 共⬃965 nm兲and
the cavity resonance wavelength 共⬃980 nm兲resulted in a
minimum threshold current appearing at a temperature
around 50 °C as seen in Fig. 1共b兲. The threshold current of
⬃584
A at 20 °C, decreasing to ⬃504
A at 50 °C, and
again increasing to ⬃579
A at 85 °C. The variation in the
threshold current across the whole range from 20 to 85 °C is
less than ⬃16% of the minimum value. The detuning effect
decisively contributes to the improved temperature stability
of the device by keeping the threshold current density to a
minimum. The small threshold current density helps to main-
tain a large differential gain9and thus high speed VCSEL
operation. The differential resistance of the VCSEL is
⬃45 ⍀at 12 mA, practically perfectly matched to the stan-
dard impedance of 50 ⍀. The thermal resistance is
⬃2.2 K/mW, which can be further improved in the future
by using, e.g., binary alloys in the bottom DBR.
To determine the factors that control high speed opera-
tion, we measure the small signal modulation response 共S21兲
and input impedance 共S11兲using a network analyzer and a
calibrated photodetector at three temperatures of 20, 50, and
85 °C. These data are used to extract the relaxation reso-
nance frequency fr, damping factor
␥
,D-factor, K-factor,
parasitic cut-off frequency fp, and –3 dB frequency f−3 dB.
These key physical parameters are commonly used to char-
acterize the high speed properties of VCSELs and are based
on the standard laser diode rate equation formalism.9
Small signal modulation response for different currents
at 20 and 85 °C with corresponding curve fits are shown in
Fig. 2. All fits show excellent agreement with the measured
data. The maximum parasitic cut-off frequency extracted
from the measured S11 data is at all temperatures larger than
22 GHz. In Fig. 3we present the extracted values of the
relaxation resonance frequency frand the –3 dB-frequency
f−3 dB as a function of the forward bias current for 20, 50,
and 85 °C. The maximum bandwidth is 15.3 GHz at 20 °C
and decrease to 14.5 GHz at 50 °C. At 85 °C the maximum
bandwidth is still 13.2 GHz, which is only by ⬃2 GHz
lower than the value at 20 °C. The overall change in the
maximum bandwidth within the investigated temperature
range is only ⬃2 GHz, which is a result of the gain-cavity
detuning and the highly temperature stable multi-QW active
region. The maximum relaxation resonance frequency is
12.9 GHz at 20 °C and decreases to ⬃9.8 GHz at 85 °C.
This decrease corresponds to higher temperatures inside the
VCSEL at higher ambient temperatures 共internal temperature
at 12 mA at 85 °C is ⬃140 °C兲, leading to saturation of the
relaxation resonance frequency at lower currents.
The K-factor that characterizes the damping limitation
can be extracted by fitting a linear function to a plot of the
damping factor as a function of the squared relaxation reso-
nance frequency,9as shown in Fig. 3共b兲. The K-factor is in
the range of 0.4 ns between 20 and 85 °C and is temperature
insensitive. One of the main reasons for the relatively large
K-factor is a relatively high mirror reflectivity, since the
K-factor increases with the photon life time.9On the other
hand, the high mirror reflectivity reduces the mirror losses
and thus the threshold carrier density, which increases the
differential gain and thus the relaxation resonance frequency.
Since the relaxation resonance frequency is lower than the
bandwidth 关Fig. 3共a兲兴, especially at 85 °C, a proper compro-
mise between the acceptable high damping and the reason-
FIG. 1. 共Color online兲Output power-current-voltage 共L-I-V兲characteristics
of a 980 nm double oxide-aperture VCSEL at temperatures from 20 to
85 °C 共a兲, and the corresponding extracted values for the maximum differ-
ential efficiency and threshold current 共b兲.
FIG. 2. 共Color online兲Magnitude of the small signal modulation response
共2-port scattering parameter S21兲for different applied forward bias currents
and the corresponding curve-fits at 20 °C 共a兲, and at 85 °C 共b兲.
151101-2 Mutig et al. Appl. Phys. Lett. 97, 151101 共2010兲
able high relaxation resonance frequency has been found.
The extracted values of the K-factor correspond to a maxi-
mum damping limited bandwidth of ⬃21–23 GHz, clearly
demonstrating its noticeable limiting effect.
The important D-factor can be obtained by fitting a lin-
ear function to the extracted squared relaxation resonance
frequencies as a function of the current above threshold.9The
D-factor is practically constant over the whole temperature
range, and is ⬃3.5 GHz/sqrt共mA兲. The high temperature
stability of the D-factor is caused by the gain—cavity detun-
ing, which prevents the decrease in the differential gain at
higher temperatures.
Next, we performed data transmission experiments using
a nonreturn to zero 共NRZ兲data format with a 共27−1兲pseu-
dorandom binary sequence 共PRBS兲in a standard back-to-
back measurement configuration 共BTB兲共⬃3 m fiber兲.In
Fig. 4we show the measured bit error rates 共BER兲at the data
bit rate of 25 Gbit/s and temperatures of 25 and 85 °C. The
drive current and the peak-to-peak modulation voltage Vp-p
are held constant in both data transmission experiments at 12
mA and 0.8 V, respectively. This makes the expensive driver
electronics redundant, reducing the application costs. At both
temperatures error-free data transmission with BERs smaller
than 10−12 is demonstrated. Received optical power for the
lowest BER is at both temperatures smaller than –1 dBm,
well below 1 mW. The power penalty for the lowest BER for
the higher temperature of 85 °C in respect to 25 °C is
smaller than 0.8 dBm and is caused by the reduction of the
output power at higher temperatures 关Fig. 1共a兲兴. These results
clearly demonstrate the maturity of the presented devices for
future high speed VSR interconnects, where high tempera-
ture stability is indispensable.
To summarize and conclude, 980 nm VCSELs with
highly temperature stable modulation characteristics are pre-
sented. We demonstrate maximum bandwidths around 13–15
GHz that are very temperature insensitive. By using a stan-
dard small signal analysis, we identify a saturation of the
relaxation resonance frequency, due to the limits of heat dis-
sipation in our current VCSELs, and damping as the stron-
gest factors that limit high speed operation. We measure
error-free data transmission at data bit rates of up to 25
Gbit/s at temperatures of up to 85 °C, and thus demonstrate
the suitability of our VCSELs for practical high speed and
high temperature stable short-reach optical link applications.
We acknowledge funding by the EU FP7 program under
Agreement No. 224211, the Deutsche Forschungsgemein-
schaft 共Sfb 787兲, the State of Berlin 共100⫻100 Optics兲and
grants of St. Petersburg Government and Ministry of Educa-
tion and Science of Russia.
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FIG. 3. 共Color online兲Relaxation resonance frequency frand –3 dB fre-
quency f−3 dB vs VCSEL forward bias current 共a兲, and damping factor
␥
vs
fr-squared with corresponding linear fits 共b兲, both at temperatures of 20, 50,
and 85 °C.
FIG. 4. 共Color online兲Bit error rates at 25 Gbit/s at 25 and 85 °C at the
corresponding VCSEL drive current of 12 mA and VP-P of 0.8 V for a BTB
measurement configuration with a 共27−1兲PRBS in the conventional NRZ
excitation scheme.
151101-3 Mutig et al. Appl. Phys. Lett. 97, 151101 共2010兲
