Temperature-Stable, Energy-Efficient,
and High Bit-Rate 980 nm VCSELs
vorgelegt vom
Master of Science Physik
Hui Li
geb. in Rizhao
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. Michael Lehmann
Berichter/Gutachter: Prof. Dr. Dieter Bimberg
Berichter/Gutachter: Prof. Dr. Gadi Eisenstein
Berichter/Gutachter: Prof. Dr. James A. Lott
Tag der wissenschaftlichen Aussprache: 30. Juli 2015
Berlin 2015
D 83
Abstract
For over 30 years, vertical-cavity surface-emitting lasers (VCSELs) have been the
subject of intensive worldwide research, due to their many applications in optical
data communications, optical and spectroscopic sensing, printing, and displays.
Most notably VCSELs are the key enabling technology for short-reach optical inter-
connects (OIs) across multimode fiber in modern data centers and petaflop-scale to
exaflop-scale supercomputers. VCSELs have replaced edge-emitting laser diodes
as the preferred light sources for short-reach OIs due to their significant advantages,
including high bit-rates, low energy consumption, high beam quality, low manufac-
turing cost, and more. Optical communications provide the only reliable means of
transferring large volumes of data at the ultra-high bit-rates needed in data centers.
Considering cost, long-term system sustainability, and reliability, future OIs must
be suited for operation without extra cooling, implying the VCSELs must be capable
of operating perpetually and reliably at elevated temperatures (e.g. at 85 °C). Tem-
perature stability can also contribute to the low energy consumption of OIs, because
high bit-rate operation at constant current and voltage driving parameters provides
the opportunity to dispose of cooling systems and to use simpler driver circuits. In
addition to temperature insensitivity one also seeks to concurrently improve the
energy efficiency and to increase bandwidth via an increased single-channel bit
rate to reduce the total life cycle cost of a given VCSEL-based OI system. Future
exaflop-scale supercomputers will require billions of OIs and are predicted to require
high bit-rate interconnects operating at 25 Gb/s per channel or beyond. This leads
to the firm requirement for future OI systems of increased bit rate and lower energy
dissipation.
This work experimentally demonstrates that 980 nm VCSELs can achieve high
bit-rate, temperature-stable, and energy-efficient operation concurrently with one
epitaxial wafer design for the first time. It is shown that this is a result of high-speed
device fabrication and careful wafer design, including the active region design,
the quantum well gain-to-etalon wavelength offset design, the distributed Bragg
reflector design, and a careful thermal design. Systematic experimental temperature-
dependent and oxide-aperture diameter-dependent characterization are presented,
including static measurements, small-signal analysis, and data transmission experi-
ments. It is also demonstrated that VCSELs with oxide-aperture diameters between
~3 and ~4 µm are most suitable to achieve energy-efficient, temperature-stable, and
high bit-rate operation at the same time. Error-free data transmissions at 38 Gb/s
at 25, 45, 65 and 85 °C are achieved without any change of working point and
modulation condition by using VCSELs with oxide-aperture diameters smaller than
5 µm. Moreover, error-free data transmission at a bit rate of 42 Gb/s at room tempera-
ture is achieved, as is 38 Gb/s at 85 °C by using small oxide-aperture VCSELs. These
maximum achievable data transmission bit rates match very well with the prediction
from small-signal analysis. Record low energy dissipation of 139 and 177 fJ/bit for
35 and 38 Gb/s error-free data transmission at 85 °C are achieved by using ~3 µm
oxide-aperture diameter VCSELs. These VCSELs are the most energy efficient
VCSELs operating at 85 °C at any wavelength to date. At room temperature, only 145,
147, and 217 fJ/bit of dissipated heat energy per transferred bit are needed for 35, 38,
and 42 Gb/s error-free data transmission by using a ~3 µm oxide-aperture diameter
VCSEL, which are all record low heat energy dissipation for 980 nm VCSELs. A
temperature-dependent and oxide-aperture diameter-dependent impedance analysis
are performed to better understand the data bit rate limitations and to understand
what improvements should and can be made for the next generation 980 nm VCSEL
device design. Relative intensity noise (RIN) values are also given, which are low
enough to satisfy the application requirements of the 32 GFC Fibre Channel standard.
During the course of this dissertation, small oxide-aperture diameter (smaller than
5 µm) 980 nm VCSELs are demonstrated to be especially well suited for use in short-
reach optical interconnects in high performance computers, and in board-to-board
and chip-to-chip integrated photonics systems.
Table of Contents
Chapter 1 ...........................................................................................................1
Introduction
1.1 A Brief History of VCSELs ..........................................................................1
1.2 VCSELs for Short-Reach Optical Communication ......................................3
1.2.1 Advantages of VCSELs ....................................................................5
1.2.2 High Bit-Rate VCSELs .....................................................................6
1.2.3 High Operating Temperature VCSELs .............................................7
1.2.4 Energy-Efficient VCSELs .................................................................9
1.3 More Applications of VCSELs ...................................................................10
1.4 Dissertation Overview ................................................................................11
Chapter 2 .........................................................................................................13
Design and Modeling of 980 nm VCSELs
2.1 Theoretical Background ..............................................................................14
2.1.1 Static VCSEL Properties .................................................................14
2.1.2 Dynamic VCSEL Properties ...........................................................16
2.2 Active Region Design .................................................................................19
2.2.1 Critical Layer Thickness.................................................................19
2.2.2 Compressively strained InGaAs QWs ............................................20
2.2.3 Strain-Compensated InGaAs/GaAsP QWs ....................................26
2.2.4 Summary ........................................................................................28
2.3 DBR Design ................................................................................................28
2.3.1 Electrical Design of DBR Mirrors ..................................................29
2.3.2 Optical Design of the DBR Mirrors ...............................................32
2.4 QW Gain-to-Etalon Wavelength Offset Design .........................................33
2.4.1 The Effect on Static Properties .......................................................33
2.4.2 The Effect on High Bit-Rate Modulation Properties ......................35
2.4.3 Summary ........................................................................................37
2.5 Thermal Design ..........................................................................................37
2.5.1 Theoretical Background .................................................................37
2.5.2 Thermal Simulation ........................................................................39
2.5.3 Summary ........................................................................................43
Loading more pages...