High speed VCSELs for optical interconnects [Elektronische Ressource] / vorgelegt von Alex Mutig

Julia Und Alex

Sujets

• Physik

Informations

• Publié par: technische_universitat_berlin
• Publié le: 01 janvier 2010
• Nombre de lectures: 28
• Langue: English
• Poids de l'ouvrage: 10 Mo

Extrait

High Speed VCSELs for Optical Interconnects
vorgelegt vom
Diplom-Physiker Alex Mutig
aus Semipalatinsk
von der Fakultät II - Mathematik und Naturwissenschaften
der Technischen Universität Berlin
zur Erlangung des akademischen Grades
Doktor der Naturwissenschaften
Dr. rer. nat.
genehmigte Dissertation
Promotionsausschuss:
Vorsitzender: Prof. Dr. Andreas Knorr
Berichter/Gutachter: Prof. Dr. Dieter Bimberg Prof. Dr. Shun Lien Chuang (University of Illinois)
Tag der wissenschaftlichen Aussprache: 15.07.2010
Berlin 2010
D 83To my grandfather Omar Baizaurov
IIIAbstract
The forecast for the serial transmission speeds used for data communication systems is a
continued exponential increase with time, directly in concert with silicon integrated circuit
scaling and in response to human society’s perpetual hunger for massive increases in
bandwidth. Electrical interfaces for single channel bit rates beyond 10 Gbit/s are being
standardized for a variety of applications, including for example (with an expected data-rate):
Fibre Channel FC32G (34 Gbit/s), InfiniBand (20 Gbit/s), common electrical interface CEI
(25-28 Gbit/s), and universal serial bus protocols USB 3.0 (up to 25 Gbit/s). As a result, the
fundamental electro-magnetic limitations of copper wire-based links at bit rates >10 Gbit/s
make fiber based optics for data communication indispensable at distances >1 m. At shorter
distances, problems associated with electrical transmission lines at such high frequencies, e. g.
high power consumption, strong signal attenuation, signal distortion and electromagnetic
interference, lead to unstoppable and progressive penetration of optical communication links
into traditional copper interconnect markets. These trends greatly expand the applications of
vertical cavity surface emitting lasers (VCSELs) and VCSEL arrays as very inexpensive,
efficient, reliable, readily manufacturable and compact laser light sources for next-generations
of fiber-optic, free-space, board-to-board, module-to-module, chip-to-chip and on-chip
interconnects and related information systems and networks.
Already today oxide-confined GaAs-based VCSELs emitting at 850 nm are the key
components for low cost high speed local and storage area network (LAN/SAN) data
communication systems. Furthermore, active optical cable links for short-reach computer and
consumer applications, for example USB, DisplayPort, and HDMI standards, are increasingly
based on VCSELs operating in the near-infrared spectral range. Immense research and
development effort all around the world resulted in the great progress in the area of
highspeed GaAs-based VCSELs in the last few years. At the standard wavelength of 850 nm room
temperature error free data transmission at the bit rate of 32 Gbit/s has been demonstrated in
2009. Also at longer wavelengths bit rates of 35 Gbit/s (980 nm) and 40 Gbit/s (1100 nm)
have been achieved at room temperature using GaAs-based VCSELs, while the latter device
was based on the buried tunnel junction, requiring additional epitaxial growth step and
making the laser fabrication more complicated. While the wavelength of 850 nm is the current
standard for LAN/SAN applications, potential competitive standards at 980 and 1100 nm have
many critical advantages for very and ultra short reach systems. This includes smaller
operational voltages due to the lower photon energy, which are decisive for complementary
metal oxide semiconductor (CMOS) drivers, transparency of the GaAs substrate, which is
important for bottom-emitting lasers, and deeper potential wells, suppressing the escape of
injected non-equilibrium carriers and increasing the temperature stability of the devices. The
latter is especially important for the module-to-module, chip-to-chip and on-chip optical
interconnects, since they residing among other close to or even directly on chips, where
elevated temperatures are common. Reliable operation of a module at 85 °C requires full
functionality of the laser chip at 100 °C or even higher temperatures. The highest temperature
reported so far for VCSEL-based high speed data transmission was 100 °C with the bit rate of
25 Gbit/s using oxide-confined VCSELs emitting at 1100 nm.
In this work high speed GaAs-based oxide-confined VCSELs emitting at 850 and 980 nm
were developed. While the main goal for 850 nm was the increase of the data
transmission speed at room temperature towards 40 Gbit/s, high speed high temperature stable
operation at elevated temperatures of higher than 100 °C was aimed with 980 nm VCSELs.
The main challenge was, however, to reach the goals by the development of straightforward,
inexpensive, scalable growth and fabrication processes avoiding complicated steps, to make
the manufactured VCSELs capable to large-scale mass production. Simultaneously, reliable
laser operation should be guaranteed, setting additional requirements, e. g. low operation
Vcurrent densities, increased mechanical stability etc. Thus, the general task within the scope of
the present dissertation was to develop inexpensive, reliable VCSELs capable to large-scale
mass production, achieving record bandwidths and highest operation temperatures for future
high speed short and very short reach optical interconnects.
Carefully optimization of the laser epitaxial structure and device fabrication process has
led to worldwide first demonstration of 850 nm VCSELs operating error free at the bit rate of
38 Gbit/s. These were also the worldwide first oxide-confined VCSELs operating at 38 Gbit/s.
2The operation current density was as low as 14 kA/cm , which is sufficient low to ensure
reliable and stable device operation. The increase of the maximum transmission data bit rate
by more than 5 Gbit/s, compared to the previous worldwide record of 32 Gbit/s for 850 nm
VCSELs, decisively improved the chances to demonstrate error free data transmission at 40
Gbit/s with inexpensive and reliable GaAs-based oxide-confined VCSELs at the
commercially mostly relevant wavelength of 850 nm. To reach such high bit rates, physical
processes limiting high speed laser operation have been carefully studied and optimization
concepts have been consequently developed. First of all, electrical parasitic elements inside of
the fabricated VCSELs have been consequently suppressed by reducing the laser diameters
and electrical contact dimensions, applying multiple oxide apertures and utilizing thick
dielectric layers. This resulted in the electrical parasitic cut-off frequencies larger than 27
GHz, decisively contributing to the record high speed device performance. Carefully
optimized epitaxial layout and device fabrication concept based on two mesas with different
diameters, together with the introduction of compressive strain into the active region by
utilizing of multiple InGaAs quantum wells (QWs), have significant increased the maximum
achievable relaxation resonance frequency to values larger than 22 GHz, decisively increasing
the laser speed. Additionally, reduced damping has enabled to reach bandwidths of larger than
20 GHz, sufficient large to achieve error free transmission at 38 Gbit/s.
With the 980 nm oxide-confined VCSELs developed in the present work, worldwide first
high speed operation at the elevated temperatures of up to 120 °C at the bit rate of 20 Gbit/s
has been demonstrated. These were worldwide first of any VCSELs operating at 20 Gbit/s at
120 °C. Carefully optimization of the laser structure and device design, utilization of the
highly strained multiple InGaAs layers grown in the submonolayer growth mode for the
active region, and introduction of the optimized detuning of 15 nm between the peak gain and
the cavity dip wavelengths resulted in the very temperature stable static and dynamic
characteristics. Hardly temperature dependent bandwidths of 11 – 13 GHz in the whole
temperature range from 25 up to 120 °C were measured, consequently leading to a insensitive large signal modulation at the bit rate of 20 Gbit/s. Further
improvements of the laser structure and device design enabled an increase of the data
transmission bit rate to 25 Gbit/s at elevated temperatures of up to 85 °C, using 980 nm
VCSELs based on highly strained multiple InGaAs QWs.
The lasers developed in the scope of the present dissertation have increased both the
maximum achieved data transmission bit rate and the maximum operational temperature of
oxide-confined VCSELs, confirming their immense potential to serve as very inexpensive,
compact, low power consuming, highly efficient and reliable laser light sources for future
short and very short reach optical interconnects.
VICurriculum Vitae
Alex Mutig
Personal
thBirthday: October 20 , 1978
Birth place: Semipalatinsk, Kazakhstan
School and High School
1985 – 1993 9 years middle school, Semipalatinsk, Kazakhstan
1993 – 1995 10. and 11. year at the Pedagogical Institute of
Semipalatinsk, Semipalatinsk, Kazakhstan
University
September 1995 – July 1999 Study of physics at the University „Semej“, Semipalatinsk,
Kazakhstan
October 1999 Move to Germany
February 2000 – January 2000 Language course (German) at GFBM, Berlin, Germany
October 2000 – May 2004 Study of physics at the Technical University of Berlin,
Berlin, Germany
Dissertation
June 2004 – July 2010 PhD student in the group of Prof. Dr. D.