Page 132 - High Power Laser Handbook

102 Diode Lasers Semiconductor Laser Diodes 103

direct-diode sources, where they are replacing traditional laser techno-
logies, such as flash lamp–pumped or diode-pumped solid-state lasers
and carbon dioxide (CO ) gas lasers.
2
This chapter introduces the key attributes of the semiconductor
laser diode to form the backdrop to its ubiquity. Although an over-
view of the physical mechanism of lasing in semiconductors is
briefly presented, it is not the focus of this work. The wafer fabrica-
tion processes used to create the semiconductor laser chip are
described, and the key processes that enable high-power laser
performance are noted. State-of-the-art performance values for
single-emitter lasers, both single spatial mode and multiple mode,
are detailed. Understanding these values at the single-emitter level
allows understanding of their scaling to one-dimensional and two-
dimensional laser arrays, which are respectively known as laser
“bars” and “stacks” and which are covered in Chap. 6. Basic single-
emitter assembly concepts, fiber-coupled packaging, and reliability
metrics and methods are also presented here.

5.2 Historical Growth of Power
The birth of the modern semiconductor laser took place in 1963 with
two independent proposals for the double heterostructure laser design
1
from Alferov and Kazarinov and from Kroemer. Advances in two
epitaxial growth techniques in the 1970s—molecular beam epitaxy
(MBE) and metal organic chemical vapor deposition (MOCVD)—were
important enabling technologies that allowed the creation of tightly
controlled layer thickness and atomic composition, which are needed
to grow quantum well (QW) active layers and which have the associ-
ated benefits of gain and reduction in threshold current. Significant
commercialization of high-power laser diodes started in 1983, with
2
the formation of Spectra Diode Labs. The literature provides several
excellent reviews of these early days. 1-3
Continuous improvements in crystal growth technologies and
the purity of materials sources drove improvements in the 1980s and
1990s. Advancements over the past 10 years have been driven by fur-
ther refinements in laser design, which are focused on increased effi-
ciency, improved facet passivation technology, robust die attach, and
advanced heat sinking. As shown in Fig. 5.1, the past 17 years have
seen steady growth in the reliable optical output power of commer-
cial products. Both multimode and single-mode laser diode powers
have increased by about 15 percent per annum. This growth rate is
likely a function of increased investment in the required technolo-
gies. During the dot-com and telecom frenzy of the late 1990s, the
advancement in 980-nm single-mode power increased to double the
historic rate.