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Roland Diehl (Ed.)
High-Power Diode Lasers
Fundamentals, Technology, Applications
WithC ontributionsb yN umerousE xperts
With 260 Figures and 20 Tables
~ Springer
Dr. Roland Diehl (Ed.)
Fraunhofer-Institut f/Jr Angewandte Festk6rperphysik
Tullastrasse 72
79108 Freiburg
Germany
Email: [email protected]
Library of Congress Cataloging-in-Publication Data
High-power diode lasers : fundamentals, technology, applications, with contributions by
numerous experts / Roland Diehl, (Ed.).
p. cm. -- (Topics in applied physics ; v. 78)
Includes bibliographical references and index.
ISBN 3540666931 (alk. paper)
1. Semiconductor lasers. 2. Diodes, Semiconductor. I. Diehl, R. D. (Renee D.) II.
Series.
TAI700 .H52 2000
621.36'6--dc21
00-028510
Physics and Astronomy Classification Scheme (PACS): 42.55.-f; 42.6o.-v; 42.62.Cf
ISSN print edition: 0303-4216
ISSN electronic edition: 1437-0859
ISBN 3-540-66693-1 Springer-Verlag Berlin Heidelberg New York
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Preface
Indisputably, the laser is a key technology in highly industrialized economies.
In many industries, application of the laser in materials processing has mod-
ified fabrication processes with speed, quality, reliability, and flexibility of
manufacturing having been substantially increased. Nevertheless, with re-
spect to its technical potential and market diffusion, laser processing is still
far away from saturation. Among other things, this is due to process technolo-
gies not sufficiently adapted to laser employment and to the fact that today's
laser systems are generally heavy and voluminous with relatively high costs
for operation and maintenance.
This situation is going to change profoundly as the diode laser is on the
move to revolutionize laser technology, as the transistor did with electrical
engineering. Diode lasers have long been used as light emitters in fiber-optic
telecommunications, as barcode readers, and for implementing the write-read
functions of optical disks. Nowadays, diode lasers do not merely deliver bits
but also optical power. They are increasingly found in applications such as
materials processing (welding, cutting, drilling, derusting, surface harden-
ing, etc.) as well as in printing and graphical arts, in displays, and medical
applications.
In fact, since the advent of the high-power diode laser, laser technology
is experiencing a fundamental structural change, as this semiconductor de-
vice has become the key element of a new breed of laser systems that are
competing with gas lasers and lamp-pumped solid-state lasers. High-power
diode lasers are continuously making inroads into industrial applications, as
they are compact, easy to cool, yield a power efficiency beyond 50%, which
is about five times higher than any other kind of laser has to offer, and
their costs are becoming increasingly attractive. To exploit the tremendous
application potential of high-power diode lasers, research and development
(R & D) programs are performed in many industrial countries.
Such programs are operational in Germany as well. The idea to write this
book arose when R & D teams from research institutes and industry combined
their forces to cooperate in a network the objectives of which were to get ac-
VI Preface
cess to high-power diode lasers with increasing power, brightness and lifetime,
to reliable mounting technologies for fabricating high-brightness laser mod-
ules, and to demonstrate novel all-solid-state laser systems. As the general
coordinator of this network the editor accepted the responsibility to moti-
vate a team of authors who are actively researching the field of high-power
diode lasers. Their competence, knowledge, experience, viewpoints, ideas and
visions have been brought together to bring into existence this update mono-
graph on the principles, technology, and innovative applications of high-power
diode lasers, summarizing the state-of-the-art in a greatly dynamic field at
the threshold to a new millennium.
Readers who want a fundamental introduction to the underlying physical
principles, design considerations, and the basics of device technology will find
the first contribution by Peter Unger a compact stand-alone monograph on
diode lasers for high-power operation. Those who want to probe further will
find an extensive' theoretical treatment of the microscopic spatio-temporal
dynamics of large-active-area high-power diode lasers and their impact on the
macroscopic laser characteristics in the contribution by Edeltraud Gehrig and
Ortwin Hess. F~om a practical point of view, the analysis and prediction of the
temporal and spatial dynamics of broad-area lasers are extremely important
when designing high-power diode-laser systems.
Ideal material systems to fabricate high-power diode lasers are the III-
V compound semiconductors. Through solid solutions among various III-V
materials, a large spectrum of laser wavelengths has become accessible. It
ranges from the blue to the mid-infrared, being extended to the far-infrared
through the unipolar quantum cascade laser. Power performance is so far
restricted to wavelengths ranging from 630 nm to 1600 nm. One of the key
steps in the fabrication of high-power diode lasers is the epitaxial process.
Sophisticated stacks of III-V epilayers comprising up to 100 single layers have
to be deposited with high crystalline quality. Markus Weyers and coauthors
describe in detail the technologies and peculiarities of epitaxy for high-power
diode-laser layer structures.
Despite the tremendous progress III-V epitaxy has made in the last
decade, nothing would have been achieved if low-dislocation-density sub-
strate wafers were not to hand. As gallium arsenide (GaAs) is by far the
most important substrate material for high-power diode lasers, the contri-
bution by Georg Miiller et al. has been solely and comprehensively devoted
to the growth of GaAs-substrate crystals of high perfection, emphasizing the
importance of a sound materials base for long-lifetime devices.
Epiwafers of high crystalline perfection are patterned to laser structures
by applying semiconductor process technologies rather similar to processes
applied in microelectronics. High-power diode lasers usually come in the form
of bars (approximately 10mm x 0.6mm) comprising about 25 monolithic
groups of up to 20 parallel single-laser stripes. GStz Erbert and coauthors
present detailed design considerations, technologies and device characteris-
Preface VII
tics of single-diode lasers and bars with large optical cavities supported by
sections on the simulation of thermal behavior and on reliability considera-
tions.
A major drawback of high-power broad-area diode lasers is their unsatis-
factory beam quality. This allows only limited focusing of the total beam as it
is the addition of many single beams. Hence, power density at the workpiece
is limited as well, leaving high-power diode lasers with restricted application
opportunities. But this is changing as they are going to pick up brightness
to approach diffraction-limited performance by introducing special designs
such as, e.g., the tapered resonator/amplifier. Two contributions address this
important subject. Klaus-Jochen Boller et al. report on properties of high-
brightness diode-laser systems with their potential for frequency conversion,
whereas Michael Mikulla addresses the design and performance of tapered
diode lasers exhibiting both high power and high brightness.
Output power per bar is approaching the 100 W level, which substantially
reduces the cost per watt of delivered optical power. Crucial for reliability
and lifetime of bars is proper heat sinking. Although power efficiency is ex-
tremely high, one half of the absorbed pump power has to be removed as
waste heat. Mounting high-power diode-laser bars on cooling elements re-
quires high precision and the complete mastering of the electrical, thermal,
and mechanical junction process. Peter Loosen describes practical aspects of
heat sinking applying active coolers.
By stacking many such mounted bars, optical output powers in the kilo-
watt range can be achieved. This is the fundamental concept for direct-diode
applications. Stacked arrays integrating sophisticated micro-optical beam
shaping will push power densities to the 105 W/cm 2 realm with the prospects
to achieve even the MW/cm 2 level - sufficient for the welding of sheet
steel. For this purpose a large number of individual oscillators have to be
coupled to a single powerful beam. FollowingHans Opower's introduction,
Peter Loosen deals with currently applied methods of incoherent beam com-
bining whereas Uwe Brauch treats coherent beam combining, both to achieve
direct-diode-laser systems with power levels of some kilowatts.
Another approach besides coherent beam combining to achieve high
brightness is the use of high-power diode lasers to pump solid-state lasers
one of the earliest applications of these semiconductor devices and still
-
their predominant one- the solid-state gain medium acting as a brightness
transducer. Diode-Pumped Solid-State Lasers (DPSSLs) are extending their
presence at the expense of their lamp-pumped counterparts due to better
beam quality, reduced maintenance, and an improving cost-of-ownership sit-
uation. Classic DPSSLs will not be addressed in this book but rather novel
concepts that became feasible only through diode pumping. Such a novel
concept is the fiber laser. This type of laser promises innovative applications,
as it is particularly attractive to couple the laser output to an optical fiber so
that the optical power becomes available at the free end and can be directed
VIII Preface
wherever the flexible fiber is placed. This confers a great deal of flexibility
on the application of laser power previously unavailable to traditional lasers.
Moreover, using double-clad fibers, multimode input is transformed to single-
mode diffraction-limited output. Also, a new star in the laser sky is the disk
laser- a thin diode-pumped crystal plate (approximately 0.2mm thick and
up to 12 mm in diameter) with axial cooling. It provides an optimum combi-
nation of high beam quality, high efficiency, and high optical output power.
After their introduction to diode-laser pumping, Andreas Tiinnermann et al.
treat in detail the fiber laser; Adolf Giesen and Karsten Contag summarize
the still short history and current status of the disk laser.
Finally, the editor wants to express his sincerest thanks to all his col-
leagues who accepted the challenge and the burden to spend much of their
free time and effort to author the various chapters, responding so construc-
tively to the editor's criticism, suggestions, and recommendations. Special
thanks are due to the German Federal Ministry of Education and Research
(BMBF) and to the VDI Technology Center for consistent support of the joint
effort which yielded most of the results reported in this book. Furthermore
the team of authors owes thanks to Werner Skolaut and Claus Ascheron of
Springer-Verlag Heidelberg for their patience and continuous support. Last
but not least the editor thanks his wife Christel for forgiving her husband's
virtual absence during countless weekends, and for gallons of good coffee.
Freiburg, June 2000 Roland Diehl
Contents
Introduction to Power Diode Lasers
Peter Unger ............................................................. 1
1. Fundamental Aspects of Diode Lasers ................................. 1
1.1. Emission and Absorption in Semiconductors ..................... 1
1.2. Basic~Elements of Semiconductor Diode Lasers ................... 8
1.3. Optical Gain and Threshold Condition .......................... 10
1.4. Edge- and Surface-Emitting Lasers .............................. 13
1.5. Lateral Confinement ............................................ 15
1.6. Quantum-Well Structures ....................................... 18
2. Fabrication Technology .............................................. 20
3. Optical Waveguides and Resonators .................................. 24
3.1. Effective Refractive Index ....................................... 24
3.2. Normalized Propagation Diagrams .............................. 26
3.3. Optical Near- and Far-Field Patterns ........................... 28
3.4. Fabry-Perot Resonator .......................................... 31
3.5. Diode Laser Spectrum .......................................... 33
3.6. Mirror Coatings ................................................ 34
4. Rate Equations and High-Power Operation ........................... 37
4.1. Rate Equations for Electronic Carriers and Photons ............. 37
4.2. Electrical and Optical Characteristics of Power Diode Lasers .... 41
4.3. Design Considerations for High-Power Operation ................ 46
List of Symbols ......................................................... 50
List of Constants ....................................................... 51
Abbreviations for Indices ............................................... 51
References .............................................................. 52
