Special IssueLaser Processing for Copper

Special Contribution

Development of High-Power Blue Laser Diodes
Principal Researcher
LD Business Division,
Optoelectronics Products Business Unit,NICHIA CORPORATION

Ph. D. (Engineering) Shinichi NagahamaWith the aim of applying the blue laser diode to the laser processing, we havedeveloped the high-power Gallium Nitride-based (GaN-based) blue laser diodes.
Starting from the technical findings of the existing GaN laser diodes for display applications, weachieved the high output power of 11.2 W, the highest value in the world for an optical output
power per single emitter of a GaN-based blue laser diode, by optimizing the device structure
and adapting a new package with low-thermal resistance. In addition, from the results of eachreliability evaluation, we confirmed that the Blue Laser Diode which we have developed is promising for its greatly high optical output stability and low failure rate, and therefore the device thatis sufficiently practical as the light source for the laser processing.

ABSTRACTIn the laser processing field, the visible-light laser of blue
to green color, which has a high absorbing ratio in themetallic materials, such as pure copper, is the focus ofour attention. There are some candidates for the materials and device configurations to obtain the blue to green
color laser. However the direct emission by a laser diodeusing GaN-based materials being the most high efficiency
and a simple configuration, its development is intensified
in recent years1)-3). For the GaN-based laser diodes, its
epitaxial layers can be made from AlxGayIn1-x-yN and its
band gap at room temperature is from 0.8 eV of InN to
6.2 eV of AlN, therefore it is theoretically a direct-transition semiconductor which can cover not only over therange from blue to green but also over all ranges of visible light. Since 1995 when the laser emission (wavelength: 410nm)4) by current injection was achieved for thefirst time with this material, its performance as a device
has been drastically improved, and this laser became
widely used as the light source for large capacity opticaldisk devices as represented by Blue-ray DiscTM. In recentyears, the range of emission wavelength is expanding
from blue 5) to green 6), and it is widely used as the lightsources for displays such as projectors. Since 2000, we
have been developing the high output and high efficiency
blue and green GaN-based laser diodes as light sourcesfor displays. Figure 1 shows the optical output and Wall
Plug Efficiency (WPE) improvement of our developed blue
laser diodes for displays. The optical output and WPE ofthe blue laser diode were 0.5 W and 20%7) in 2006 andthen we have improved up to approximately 10 times at5.67 W optical output and approximately 2.5 times at
48% WPE in 2020 with the introduction of several newelement technologies for efficiency improvement 8).
With the aim of the laser diode’s application to the optical light source of laser processing, this time, we considered, based on the technical findings of the GaN-baselaser diodes for displays, to increase its output power. As
a result, we were able to develop the blue laser diodewhich has an optical output over 10 W. In this section, we
report the manufacturing method and characteristics ofthis high output blue laser diodes.

5550Wall-plug eﬃciency (%)

INTRODUCTION
1.1. INTRODUCTION

454035302520

W.P.E. @CW 25℃
455 nm (48.1%, 5.67 W)〇 Products◆ Development Sample

455 nm (43.4%, 5.25 W)

455 nm (41.5%, 5.05 W)
455 nm (39%, 4.7 W)
455 nm (37%, 4.6 W)
455 nm (37%, 3.85 W)
448 nm (35%, 3.5 W)
448 nm (28%, 1.6 W)
445 nm (22%, 1.0 W)
445 nm (20%, 0.5 W)

152004 2006 2008 2010 2012 2014 2016 2018 2020 2022 2024

YearFigure 1 Output power and WPE improvement of the blue laser
diodes.

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10Development of High-Power Blue Laser Diodes

MANUFACTUREOF
OF THETHE HIGH-POWER
HIGH-POWER
2.2. MANUFACTURE
BLUELASER
LASERDIODE
DIODE
BLUE
Figure 2 shows the schematic structure of the blue laser
diode which we have developed. It consists of anAlxGayIn1-x-yN crystal and an InGaN multiple quantum welllayer which is used as an emission layer. Using a highquality C-plane GaN free-standing substrate, it is possible to grow the epitaxial layers with low dislocation density similar to the substrate. Since the c-plane GaN substrate has cleavableness, the front and rear facets of thelaser diode can be manufactured easily by cleaving thesubstrate. The structure of the laser device is a generalindex-guided ridge stripe structure, and by covering theouter laterals of the ridge stripe with dielectric insulating
film, the device controls its transverse mode with arefractive index difference in the horizontal direction. We
set the ridge stripe width at 90 μm to obtain the optical
output of more than 10 W with one single emitter. Thatmeans, we considered the coupling to the optical fiberwith a 100 μm diameter core of a general delivery fiber,
while reducing the optical density by widening the output
beam width of the laser to avoid the occurrence of errors
by the Catastrophic Optical Damage (COD) which is aparticular failure mode of general laser diodes. On theother hand, to control the vertical lateral mode, weemployed the Separate-Confinement Heterostructure(SCH) which efficiently confines carriers and optical lightrespectively to the InGaN multi-quantum well emission
layer and to the n- and p-guide layers.
Isolation ﬁlm
Ridge stripe structure(ZrO 2, SiO 2, etc.)
p-electrodep-cladding layer
p-guide layer(Multi-quantum well)

Lightemission
pointn-guide layern-cladding layer

WindowLead

H: 6.55 mm

W: 6.9 mm

D: 6.0 mm

Figure 3 Presentation of the SLP.

CHARACTERISTICS OF
OF THE HIGH3.3. CHARACTERISTICSHIGHPOWERBLUEBLUELASER
LASER DIODESDIODES
POWER
3.1

p-contact layerp-electron-blocking
layerLight-emitting layer

optical output or its lifetime, we developed a new package, the Side Lead Package (SLP) which has a heat resistance reduced. The presentation of the SLP is shown inFigure 3. The SLP is a metal package using copper material and is hermetically sealed by a seam welding of theupper lid. The outside dimensions are 6.9 mm (width), 6.0mm (depth) and 6.55 mm (height). Placing the feeding
lead pins on the lateral side of the package, it is easy toexhaust heat from the bottom of the package. The heat
resistance value of the blue laser diode using this SLP is2.3 K/W. This value has been reduced drastically in comparison to a blue laser diode of displays’ use (theTO-CAN Package of 9 mm-outer diameter, 5.6 K/W heatresistance) which is commercialized currently.

GaN substraten-electrodeFigure 2 Schematic structure of a blue laser diode.

With respect to the package which the laser device isassembled, the TO-CAN Package of 9 mm-outer diameter is often used for a display purpose. The design of thispackage was optimized with the goal of down-sizing,
therefore its heat exhausting resistance is high, and theincreasing junction temperature causes a problem when ahigh output laser diode with more than 10 W optical output is driven. Since the laser diode’s increasing temperature generates harmful effects such as degradation of theBasic Characteristics of the High-Power Blue
Laser Diodes
Figure 4 shows the typical temperature dependency ofthe light-current (L-I) characteristic and voltage-current(V-I) characteristic of the high-power blue laser diode.
Under the continuous wave (CW) driving, we measured
them with varying the temperature of the SLP’s bottom.
From the dependency of the L-I characteristic, a drasticdecrease of the slope efficiency (η s =ΔL / ΔI) is not seen
until a 10 A of injection current even though the package’s temperature becomes as high as 80°C. Especially
under a driving when the package’s temperature is 40°Cor less, it is found that incredibly good temperature characteristics is achieved. The optical output, at the room
temperature with the rated current value (Iop=8.5 A), is11.2 W and the WPE is 33.8%. With optimization of thelaser chip and Introduction of the low-heat resistancepackage, a high optical output value of 11.2 W isobtained. As far as we can tell, this is the world’s best
optical output per one single device of one single emitterwith a GaN-based blue laser diode.
Figure 5 shows the wavelength spectra of the blue laser
diode. The peak wavelength is 465 nm. The longitudinal
Furukawa Electric Review, No. 52 202111

Development of High-Power Blue Laser Diodes

14(a) Horizontal direction
Relative intensity (a. u.)1086
Tc=20℃
Tc=40℃
Tc=60℃
Tc=80℃

2002

684Injection current (A)10-20125Voltage (V)43-1002684Injection current (A)101020-200Angle ( °)2040

Figure 6 FFPs of the blue laser diode in (a)Horizontal direction
and in (b)Vertical direction.

Tc=20℃
Tc=40℃
Tc=60℃
Tc=80℃

10Angle ( °)(b) Vertical direction

-40

20

Relative intensity (a. u.)

412Figure 4 Typical L-I and V-I characteristics of the blue laser
diode.

Relative intensity (a. u.)

Output power (W)12(a) Horizontal direction-80 -60 -40 -20020406080

234Relative intensity (a. u.)

Relative intensity (a. u.)Distance (µm)

460 461 462 463 464 465 466 467 468 469 470
Wavelength (nm)(b) Vertical direction-4-3-2-1

01

Distance (µm)

Figure 5 Wavelength spectra of the blue laser diode.

Figure 7 NFPs of the blue laser diode in (a)Horizontal direction
and in (b)Vertical direction.

mode is multimode and the envelope of the spectra isapprox. 2 - 3 nm. We controlled the In composition ratioof the InGaN multi-quantum well.
The Far Field Pattern (FFP) and the Near Field Pattern(NFP) of the blue laser diode are shown in Figure 6 andFigure 7 respectively. In Figure 6, the beam intensity provides a Gaussian-like shape distribution because the vertical lateral mode (b), in a perpendicular direction toward
the epitaxial surface, is a single mode. However, thebeam intensity provides a rectangular-like shape because
the horizontal transverse mode (a), in a horizontal direction, is multi-mode. As defining a beam divergence angle
as the angle where the beam becomes 1/e 2 of the peakintensity, the angle is 44° in a vertical direction and 10° ina horizontal direction. From the intensity distribution ofthe NFP in Figure 7, the size of the light emission point isapprox. 1 μm in a vertical direction and approx. 89 μm,
which corresponds to the ridge stripe width of the laser
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Development of High-Power Blue Laser Diodes

Normalized output power

1.10
1.05
1.00
0.95
(a) Iop=8.5A, Tc=40℃
n=10Optical output deterioration (Ave.)
＝1.6%

0.90
0.85
0.80

01000

2000
3000
time (hour)

4000

5000

Normalized output power

1.10
1.05
1.00
0.95(b) Iop=8.5A, Tc=60℃
n=9Optical output deterioration (Ave.)
＝1.3%

0.90
0.85
0.80

01000

2000
3000
time (hour)

4000

5000

Normalized outout power

1.10
1.05
1.00
0.95(c) Iop=10.5A, Tc=40℃
n=10Optical output deterioration (Ave.)
＝1.4%

0.90
0.85
0.80

01000

2000
3000
time (hour)

4000

5000

Figure 8 Lifetime test results of the blue laser diodes.

1000000
Estimated COD life time (hour)chip, in a horizontal direction, and it is found that the optical intensity especially in a horizontal direction is emitting
almost uniformly. By adapting a ridge stripe structure of
the laser diode and maintaining stable high order modes
with refractive index differences in a horizontal direction,
the laser diode is designed so that its NFP in a horizontal
direction becomes uniform. Generally, a non-uniform distribution of a NFP induces a sudden COD where the optical power intensity is locally high and it leads to a failureof the laser diode in a short time. The NFP control for auniform distribution is the one of the important technology aspects to ensure a high reliability.
3.2 Reliability of the High-Power Blue Laser Diodes
With respect to degradation modes of the GaN-basedlaser diode, there are two modes namely: a gradual degradation and a sudden failure. Gradual degradation is asimilar degradation mode as the one which general emission devices have and its life prediction is available fromthe Arrhenius Model as an acceleration from the junction
temperature of the device. In order to measure the optical
output deterioration from the gradual degradation, reliability tests on the blue laser diodes have been conducted and results are shown in Figure 8. In the tests, thelaser diodes were driven with the CW and an Auto
Current Control (ACC) and the temporal change of its
optical output is monitored over 4000 hours. In Figure 8,
test conditions are (a) rated condition (current = 8.5 A,
package temperature = 40°C), (b) temperature accelerating condition of injection current (current =8.5 A, package
temperature = 60°C), and (c) current accelerating condition of the rated current (current =10.5A, package temperature = 40°C). Under all the test conditions, the opticaloutput decrease is within 1 to 2% of the initial value after
4000 hour-driven and an extremely high stability isobtained. Under the rated condition drive, the deterioration percentage of the optical output is significantly small
and the optical output under the ACC drive is stable.
Next, we describe the sudden failure of the developed
blue laser diodes. The main cause of a sudden failure of a
GaN-based laser diode is the COD. The COD is a particular phenomenon of general laser diodes and it occurs
with causes such as a deterioration of the passivation
layer at the front facet, a carrier recombination at a surface, and a temperature increase by the optical absorption around the front facet. As defining the time until theCOD occurs as the COD lifetime, the COD lifetime
strongly depends on the optical power density at the front
facet. From our previous findings, it is found, as a rule ofthumb, that the COD lifetime of the GaN-based blue laser
diodes is proportional to the optical output power density
at the front facet. The graph in Figure 9 shows an estimation for the COD lifetime of the blue laser diodes which
we developed on the basis of the findings of blue laser
diodes for display applications. In Figure 9, the horizontal
axis shows the optical output power density at the front
facet and the vertical axis shows the COD lifetime. Since
the optical output power density at the front facet of the100000

10000

1000

1000

1020
30
40
50
60
70
Optical output power density (MW/cm2)

80

Figure 9 Relationship between COD lifetime and optical output
power density on the front facet of the blue laser
diodes.

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Development of High-Power Blue Laser Diodes

blue laser diode developed this time is approx. 14 MW/
cm2, the estimated average of the COD lifetime is approx.
130,000 hours. In consideration of the mounting to a laser
processing machine, it seems that the probability of COD
occurrence of the blue laser diodes in the field isextremely low.
Regarding a light source for the laser processing, a highstability of optical output power and a small failure rate
are required. From the results of the these reliability tests,a considerably high optical output stability and a low failure rate are promising in the developed blue laser diodes
and it is considered that these blue laser diodes can besignificantly sustainable for the field use as the laser processing light source.

REFERENCES
1) Y. Ishige, et. al., Proc. SPIE 11668 (2021) 116680M.
2) H. König, et. al., Proc. SPIE 11262 (2020) 112620Q.
3) S. Nozaki, et. al., Proc. SPIE 11262 (2020) 112620S.
4) S. Nakamura, et. al., Jpn. J. Appl. Phys. 35, (1996) L74-L76.
5) S. Nagahama, et. al., Appl. Phys. Lett. 79, (2001) 1948-1950.
6) T. Miyoshi, et. al., Appl. Phys. Exp. 2, (2009) 062201-062203.
7) T. Miyoshi, et. al., SID Digest (2006) 1915-1917.
8) Y. Nakatsu, et. al., Proc. SPIE 11280, (2020) 112800S 1-7.

CONCLUSION
4.4.CONCLUSION
With aim of mounting on a laser processing machine, wedeveloped high-power blue laser diodes by using GaNbased semiconductor materials. By optimizing epitaxial
layers and a device structure, and mounting it in the newpackage, SLP, with a low thermal-resistance, we achieved
the blue laser diode which has the high optical output of
112 W and the high WPE of 33.8%. From the results of
the reliability tests on the developed blue laser diodes, wealso confirmed that these laser diodes have sufficient reliability for the field use as the light source for the laser
processing. The application of the blue laser diodes tothe laser processing for hard-to-work materials such ascopper has started only just recently. We would like toaccelerate the application of the blue laser diodes to laser
processing by achieving much higher output power andmuch higher efficiency thereafter.

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