Ewa Simpanen, Microtechnology and Nanoscience - MC2

​Title: Longer Wavelength GaAs-Based VCSELs for Extended-Reach Optical Interconnects


Ewa is a PhD student at the Photonics Laboratory
Faculty opponent: Dr. Nicolae Chitica, Finisar Sweden AB
Examiner: Prof. Anders Larsson
Main supervisor: Associate Professor Johan Gustavsson

​Abstract:
Data centers of today are increasing in size and are built to accommodate strong
trffic demands while providing sustainably by having clients sharing resources under
one roof. Their massive scale puts pressure on the server network topology and
has incited a need for data transmission links that are energy effcient and capable
of operation at high bit rates with reach up to a few kilometers. Optical interconnects
(OIs) offer large bandwidth and low attenuation at long distances, and are
therefore suitable for this task. The most commonly used OIs, with 850 nm GaAsbased
vertical-cavity surface-emitting lasers (VCSELs) and multi-mode fiber (MMF),
have a 25 Gb/s reach that is limited to a few hundred meters. However, the fiber
chromatic dispersion and attenuation that limit the OI reach can be reduced significantly
by increasing the wavelength of this very same technology. The upper limit
of the GaAs-based VCSEL technology, with strained InGaAs quantum wells (QWs),
is about 1100 nm.

With further improved OI performance, new hyperscale data center topologies
can be realized and explored. This will lead to a larger number of possible solutions
in traffic engineering as well as for power management. 1060 nm VCSELs could soon
open up for lane rates of 100+ Gb/s over distances up to 2 km and help reach the
Tb/s link speed aim of data center OI standards, in which capacity is built up mainly
by employing multiple parallel lanes, increasing symbol rate by going from binary
to four-level pulse amplitude modulation (PAM-4), and optimizing with electrical
mitigation techniques such as digital signal processing.

In this work we show that 1060 nm GaAs VCSELs are suitable light sources for
long-reach OIs by first demonstrating their overall stable performance and capability
of error-free data transmission up to 50 Gb/s back-to-back and 25 Gb/s over 1 km
of MMF. With PAM-4, we show 100 Gb/s error-free capability over 100 m of MMF,
suitable for wavelength division multiplexed OIs that can transmit data at several
wavelengths from 850 to 1060 nm over the same fiber channel. We also assemble
single-mode 1060 nm VCSEL and single-mode fiber links and demonstrate 50 Gb/s
error-free transmission over 1 km using pre-emphasis and 40 Gb/s over 2 km without
the use of any electrical mitigation techniques. These results stem from careful
VCSEL design, including strained InGaAs QWs with GaAsP barriers, doped AlGaAs
distributed Bragg reflectors, a short optical cavity and multiple oxide layers. In addition,
we show that the fabrication of such a device poses no increase in complexity
and can be realized using standard processing techniques.

Keywords: vertical-cavity surface-emitting laser, optical interconnect, chromatic dispersion,
attenuation, reach, high-speed modulation, single-mode, multi-mode

Category Thesis defence
Location: Kollektorn, lecture room, Kemivägen 9, MC2-huset
Starts: 20 March, 2020, 10:00
Ends: 20 March, 2020, 13:00

Published: Mon 30 Mar 2020.