Novel ultrafast vertically emitting semiconductor lasers

Abstract

We demonstrate the highest average power of any modelocked semiconductor laser: the optically-pumped MIXSEL generates 6.4-W in 28.1-ps pulses at 2.5-GHz repetition rate. Furthermore, we discuss electrically-pumped VECSELs optimized for passive modelocking and present initial cw-lasing with >100-mW.

Full text

Page 1

Novel Ultrafast Vertically Emitting Semiconductor Lasers
W.P. Pallmann(1), M. Hoffmann(1), V. J. Wittwer(1), B. Rudin(1), P. Kreuter(2), B. Witzigmann(2),(3),
M. Golling(1), Y. Barbarin(1), T. Südmeyer(1) and U. Keller(1)
(1) Dep. of Physics, Ultrafast Laser Physics Lab, ETH Zurich, Switzerland, pallmann@phys.ethz.ch
(2) Dep. of Information Tech. and Elec. Eng., Integrated Systems Laboratory, ETH Zurich, Switzerland
(3) Dep. of Elec. Eng. and Comp. Sc., Comp. Elec. and Phot. Group, University of Kassel, Germany

Abstract We demonstrate the highest average power of any modelocked semiconductor laser: the
optically-pumped MIXSEL generates 6.4-W in 28.1-ps pulses at 2.5-GHz repetition rate. Furthermore,
we discuss electrically-pumped VECSELs optimized for passive modelocking and present initial cw-
lasing with >100-mW.
Introduction
Today,
femtosecond lasers are based on standard ion-
doped laser materials in combination with an
intra-cavity component for pulse formation. Their
high cost and complexity are the key issues for
bringing ultrafast lasers to wide-spread mass
applications such as biomedical imaging or
optical clocking. Modelocked semiconductor
lasers have the potential to strongly improve this
situation. Picosecond edge emitters made an
impressive progress in the last years, achieving
up to 250 mW of average power at 4.3 GHz1.
But it is very challenging to increase the power
levels into the watt regime, because the long
interaction length in edge-emitters introduces
significant dispersion and nonlinearities, and
end facet damage can occur at high peak
powers.
Vertical external cavity surface emitting
lasers (VECSELs2) appear better suited for the
generation of ultrashort pulses with high peak
power. Optically pumped (OP) VECSELs using
a semiconductor saturable absorber mirror
(SESAM3) for passive modelocking have
generated shorter pulses and higher average
powers than any
semiconductor laser (135-fs pulses at 35 mW
power4, and 2.1 W of average power in 4.7 ps
pulses5). In 2007, the integration of both SESAM
and VECSEL into one single semiconductor
element was demonstrated. The optically-
pumped MIXSEL (modelocked
external-cavity surface emitting laser6) enables
modelocking in a simple straight cavity and the
possibility of a quasi-monolithic
However, the first MIXSEL results were limited
to 40 mW of average output power, and the
growth tolerance was limited due to a strong
field enhancement in the absorber section
necessary for achieving stable modelocking.
Here we present a novel MIXSEL design
based on a low-saturation fluence quantum dot
most commercial picosecond or
other modelocked
integrated
design.
(QD) layer in an anti-resonant structure7, which
substantially improves growth tolerances. We
demonstrate successful power-scaling up to
6.4 W of average power, which is three times
higher than any other modelocked semi-
conductor laser (see Fig. 1). The pulse duration
of the generated pulses is 28.1 ps at a repetition
rate of 2.5 GHz in a circular beam with excellent
transverse mode quality (M2 < 1.05). This is
achieved in a simple straight cavity with only two
intracavity elements, the MIXSEL chip and the
external output coupler.
Furthermore, we discuss the progress
towards electrical pumping (EP). So far,
modelocking of an electrically pumped VECSEL
with 40 mW of average power in 57-ps pulses
has been demonstrated8. Ultrafast electrically
pumped VECSEL or MIXSEL designs require an
optimized balance between the competing
requirements of high electric conductivity, low
optical losses and controlled dispersion. Our
detailed numerical study of EP-VECSELs9
enabled us to develop an optimized design
compatible with passive modelocking. We
discuss our design approach and present initial
EP-VECSEL devices generating >100 mW cw
power. Homogenous
achieved even for large devices, showing very
good agreement with our simulations.
current injection is
Fig. 1: Output power and repetition rate of different
types of lasers in the gigahertz regime
ECOC 2010, 19-23 September, 2010, Torino, Italy
978-1-4244-8535-2/10/$26.00 ©2010 IEEE
P2.10

Page 2

MIXSEL design and fabrication
Fig. 2a shows the basic concept of the MIXSEL.
The structure contains two distributed Bragg
reflectors (DBRs), the QD saturable absorber
layer, the active quantum well (QW) region and
an antireflection (AR) section. The bottom DBR
reflects the laser light and forms the laser cavity
together with an external output coupler. The
second DBR reflects only the pump light to
prevent bleaching of the absorber. The structure
was grown by MBE in reverse order to enable
substrate removal. This results in a reduced
thickness of the device and leads to a nearly
one-dimensional heat
semiconductor material to the heat sink, making
power-scaling possible.
flow from the
Experimental results
The MIXSEL setup (Fig. 2b) for the modelocking
experiments consists of a simple straight cavity
made up of the MIXSEL gain structure and a
curved output coupler with a radius of curvature
of 500 mm and a transmission of 0.7%. The
cavity length of 60 mm leads to a repetition rate
of 2.5 GHz. The MIXSEL was pumped at
808 nm under an angle of 45° with a spot radius
of 215 µm, matched to the laser mode on the
structure to ensure fundamental transverse
mode operation. Pumping at 37 W, stable
modelocking was achieved with 6.4 W of
average output power in 28.1 ps pulses at a
center wavelength of 959.1 nm, as shown in
Fig. 3. The temperature of the heat sink was
cooled to -15°C. An optical-to-optical efficiency
of 17% was reached. At slightly lower output
power of 5.3 W, the beam quality was measured
and showed an excellent M2 value of <1.05 in
both directions. In a previous experiment using a
smaller pump spot radius of 80 µm and a copper
heat sink, 660 mW were achieved in 23 ps
pulses at 10°C pumping with up to 4 W.
EP-VECSEL design and fabrication
The design of our EP-VECSELs has been
simulated previously and the trade-off between
electrical and optical properties has been
discussed9. The detailed numerical study
enabled us to develop design guidelines for EP-
VECSELs compatible with passive modelocking.
The resulting design is shown in Fig. 4.
A good confinement of the injected carriers is
ensured by an optimized p-doped DBR on the
bottom, a small bottom disc contact and a thick
GaAs current spreading layer (CSL) with a top
electrode. The low mobility of the holes confines
the carriers to the middle of the device, whereas
the highly mobile
recombination in the center of the device9.
Our simulations predict
transverse inversion profile for such a device,
which should enable TEM00 mode control. The
low reflectivity AlAs/GaAs n-DBR and AR
section on top are introduced to enhance the
gain and to reduce cavity losses in the doped
top layers. The AR section and the moderate
thickness of the CSL also help reduce
dispersion, favoring
electrons allow for
a homogeneous
stable modelocking.
Fig. 4: Sketch of the EP VECSEL gain structure
Fig. 2: a) MIXSEL concept; b) MIXSEL cavity
Fig. 3: a) Measured autocorrelation fitted with the autocorrelation of an ideal 28.1 ps sech2-pulse;
b) Optical spectrum centered at 959.1 nm; c) RF-spectrum
P2.10

Page 3

To reduce the electrical and optical losses of the
p-DBR, a compositional Al(1-x)GaxAs “staircase
grading” between each AlAs/GaAs interface was
inserted and the doping profile was tuned to
have lower doping concentrations in layers of
high field intensity. The active region consists of
two groups of each three InGaAs quantum
wells. 61 EP-VECSELs with different bottom and
top contact size have been fabricated on a
single chip. For efficient heat removal, this chip
is soldered to a CuW wafer, which has the same
coefficient of thermal expansion as GaAs. The
structure is MBE-grown in reverse order for
substrate removal and double-sided processing.
Experimental results
The 61 different EP-VECSELs have been
characterized at 3°C in a simple straight cavity
consisting of the gain structure and an output
coupler with a radius of curvature of 25 mm and
a transmission of 10%. The results are shown in
Fig. 5a, the scalability of the output power up to
120 mW with increasing
diameter is clearly visible. Fig. 5b shows an
L-I-V (light-current-voltage) curve of an EP-
VECSEL with a bottom contact diameter of
140 µm, over 100 mW are reached without
thermal rollover. The differential resistance of
this device is 2.1 Ω at lasing threshold. Fig. 5c
shows spatially resolved electroluminescence
profiles recorded at high current injection of EP-
VECSELs with different back contact diameters.
Devices up to 120 µm in diameter are favorable
for TEM00 beams, as predicted by our numerical
simulations.
bottom contact
Conclusion and outlook
We have
MIXSEL, a passively modelocked VECSEL with
the saturable absorber directly integrated into
the semiconductor gain
antiresonant approach
constraints substantially. Improved thermal
management enables 6.4 W in 28.1 ps pulses

demonstrated an antiresonant
structure.
relaxes
The
growth
achieved in modelocked operation in a simple
straight cavity. Further scaling of the output
power and of the repetition rate is expected. In
addition, we present the design and first
experimental results of electrically pumped
VECSELs suitable for passive modelocking.
Homogenous current injection is achieved even
for large devices, showing very good agreement
with our numerical simulations. Power scalability
up to 120 mW of cw output power is
demonstrated. In a next step, the device will be
modelocked. Ultimate
combination of electrical pumping and SESAM-
integration to yield an electrically pumped
MIXSEL. This will enable a robust, cost-efficient,
ultra-compact multi-GHz laser source producible
on a high-volume wafer-scale basis.
goal will be the
Acknowledgments
This work is supported by ETH Zurich and the
FIRST clean-room facility. Financial support is
granted by the Intel Corporation through a
university sponsored research agreement and
by the Swiss Confederation Program Nano-
Tera.ch.
References
1 Plant, J.J., et al., Opt. Lett., 2006. 31(2):
p. 223-225.
2 Kuznetsov, M., et al., IEEE Photon. Technol.
Lett., 1997. 9(8): p. 1063-65.
3 Keller, U., Nature, 2003. 424: p. 831-838.
4 Quarterman, A.H., et al., Nat. Photonics,
2009. 3(12): p. 729-731.
5 Aschwanden, A., et al., Opt. Lett., 2005. 30:
p. 272-274.
6 Maas, D.J.H.C., et al., Appl. Phys. B, 2007.
88: p. 493-497.
7 Bellancourt, A.-R., et al., Opt. Express, 2009.
17: p. 9704-9711.
8 Jasim, K., et al., Electron. Lett., 2003. 39(4):
p. 373-375.
9 Kreuter, P., et al., Appl. Phys. B, 2008. 91: p.
257-264.

Fig. 5: a) Output power of EP-VECSELs with different bottom contact diameter measured in a straight
cavity; b) L-I-V curve of an EP-VECSEL with a bottom contact of 140 µm using a 5% output coupler;
c) Electroluminescence profiles of devices with different bottom contact diameters
P2.10