1. Introduction
The optical output power of semiconductor laser diodes
(LDs) and fiber coupled modules (LDMs) at 9xx-nm
wavelengths have increased remarkably in the last 10
years. That is because the kilowatt-class fiber lasers have
replaced CO
2
lasers in the laser processing market
1)
, and
thus the demand for LDs and LDMs as a pump source has
also increased. Furthermore, because of the increase in
both optical output power and brightness of LDs and
LDMs, direct diode lasers (DDLs), which directly use
output power from these devices as a heat source, are also
in increasing demand. Therefore, each company can be
said to have made significant progress through active
development efforts in parallel with the rapid growth of
the market
2-7)
. The Fujikura Group also started
developing LDs and LDMs more than 10 years ago and, in
terms of optical output performance, has always been in
the top group in the world
8, 9)
. Fig. 1 shows the
improvement in optical output power of our LDs and
LDMs. Compared to the initial development stage, the
power has increased three times in the LDs and five times
in the LDMs. The long cavity structure and the
optimization of the epitaxial-layer design are the main
reasons for the improved output power of the LDs. These
LDs also have achieved a significantly high power
conversion efficiency (PCE) value under high-power
operating conditions as well as at peak. Such high-power,
high-efficiency LD technologies are described in Chapter
2. For LDMs , improvements in beam multiplexing
technology have enabled the devices to have many
emitters in the package and to combine the beams into a
fiber with a higher output power. Combining polarization
beams is another technique utilized to improve beam
brightness, and it has become commonplace for dozens of
beams to be combined into a single optical fiber. Heat
dissipation technology is also an important factor that
contributes to maximizing LD performance considering
several hundred watts of heat could be generated in a
The Fujikura Group’s High-power Semiconductor
Laser Technologies
Yohei Kasai,
1
and Toshiyuki Kawakami
1
The Fujikura Group has accumulated in-depth technology and know-how to develop products
such as high-power semiconductor lasers and laser modules. In addition, we also have expertise
to grow high-quality III-V compound semiconductor crystals, enhance power and efciency of
laser diodes by optimizing the structure, multiplex beams, mount parts on substrates, and
dissipate heat. This paper introduces the technologies and performance of high-power
semiconductor lasers developed by the Fujikura Group.
1 Optical Device Research Department Optical Technologies R&D Center
Fig. 1. Improvement in optical output power of the Fujikura
Group’s LDs and LDMs.
palm-sized package. These packaging technology for
high-power LDMs is described in Chapter 3. Furthermore,
since the reliability of laser devices is an extremely
important characteristic that determines the product
operational period in industrial applications, the reliability
of LDs will be discussed based on long-term accelerated
aging test data in Chapter 4. The conclusion is given in
Chapter 5.
2. Technologies on high-power high-efciency
operation of laser diodes
Since an extremely large number of pumping LDMs are
used in fiber laser systems, the fundamental characteristics
of the LDs, such as output power per single emitter and
PCE, have a significant impact on the total system
performance. Therefore, improvements in output power
and PCE of LDs have always been a high-priority
development issue. Fujikura has worked on the
development of the 9xx-nm-wavelength LDs with the
world’s highest output power and efficiency in cooperation
with our group company, Optoenergy Inc.
11-13)
. Recently,
we have successfully developed the 9xx-nm LDs with the
world’s highest level PCE at over 20 W output power. This
section reviews the approaches to improve the power and
58
efficiency of LDs, and then explores the possibility of
design optimization according to each application field.
2.1 Basic structure of LDs
Fig. 2 (a) and (b) show a schematic of structure of our
LDs in cross section and a perspective view, respectively.
The LD structure composed of InGaAs/AlGaAs-based
materials with an emission wavelength of 9xx-nm was
epitaxially grown by MOCVD on an n-GaAs
single-crystal substrate. A self-aligned stripe (SAS)
structure consisting with an n-type current-blocking layer
was used for controlling the stripe width of the emitter.
The merits of SAS structure include increased polarization
purity of emitting light by the use of a flattened surface,
which avoids mounting stress
14)
, simultaneous formation
of a lateral current confinement structure and
non-current-injection window at the facet
15)
, reductions in
the resistivity of p-type ohmic contacts. These are
important factors that affect the performance of our LDs.
Fig. 2 (c) shows a schematic of an index guide structure
and mode profile of our LDs for the vertical structure. By
the use of an asymmetric decoupled confinement
heterostructure (ADCH), the optical confinement ratio of
the p- and n-doped layers (Γp, Γn) and the confinement
factor of the active layer (Γwell) can be optimized
individually. This allows us to flexibly design a low-loss
waveguide, the optimal Γwell value to achieve high-power,
high-efficiency operation in long cavity LDs. For an
InGaAs/AlGaAs-based material system, it is essential to
suppress the p-type doping level because the free carrier
loss of the p-doping layer is more than twice that of the
n-doping layer. On the other hand, since the carrier
mobility of the p-type layer is one order of magnitude
lower than that of the n-type layer, reducing the p-type
doping simply increases the operating voltage and results
in a low efficiency of LDs. Therefore, the basic strategy is
to reduce the free carrier loss by decreasing the Γp-Γn
ratio, and at the same time, to decrease the electrical
resistance by reducing the thickness of the p-type layer or
increasing the amount of the p-type dopant in the layer far
from the mode distribution peak. In order to further
improve the PCE, it is also effective to increase the Γwell
value within an appropriate range taking into account the
trade-off with reliability, and to eliminate the excess
voltage components generated at the ohmic contacts and
interlayer regions to reduce the operation voltage.
2.2 Improvement of efciency and output power of
LDs
In accordance with the strategy explained in section 2.1,
the following investigations were conducted to improve
the performances of the LD: 1. reductions in internal loss
by adjusting the Γp-Γn ratio, 2. efficiency improvement by
adjusting the Γwell value and balancing gain with loss, 3.
resistance reductions by minimizing the p-type layer
thickness and increasing the doping level for the p-type
layer far from the mode distribution peak, 4. reductions in
excess voltage due to the band-discontinuity between each
epitaxial layer
8,13,16)
. Fig. 3 shows the trend of the PCE
improvement in our LDs. Each data was obtained by
measuring the LDs with a cavity length of 4 mm at 25 deg.
C. The PCE values at peak and an output power of 20 W
were plotted. As can be seen from the plot, the peak PCE
value has increased significantly from 67.0% in 2016 to
73.5% in 2020, and PCE at 20 W increased from 63.0% to
69.0%, the highest level in the world.
High reliability of LDs is also required for high-power
fiber lasers. Fig. 4 shows the optical output power versus
the drive current (L-I) characteristics of the newly
developed LDs (cavity length of 4 mm) driven in pulsed
Fig. 2. Schematic LD structure (a) cross section, (b) perspective view, (c) Index guide structure and mode prole.
LD
−
Laser Diode
CoS
−
Chip on Submount
LDM
−
Laser Diode Module
DDL
−
Direct Diode Laser
PCE
−
Power Conversion Eciency
SAS
−
Self-Aligned Stripe
ADCH
−
Asymmetric Decoupled Connement
Heterostructure
FAC
−
Fast-axis Collimation Lens
SAC
−
Slow-axis Collimation Lens
BPP
−
Beam Parametar Product
NA
−
Numerical Aperture
FFP
−
Far-Field Pattern
NFP
��
Near-Field Pattern
AR coat
−
Anti-Reection Coat
VBG
−
Volume Bragg Grating
SWaP
−
Size, Weight, and Power consumption
Panel 1. Abbreviations, Acronyms, and Terms.
59
Fujikura Technical Review,2022