High-brightness laser diode module over 300 W
with 100 m / NA 0.22 fiber
Yohei Kasai*a, Shinichi Sakamotoa, Yukihiko Takahashia, Ken Katagiria, Yuji Yamagatab, Akira
Sakamotoa, and Daiichiro Tanakaa
aAdvanced Technology Lab. Fujikura Ltd. 1440, Mutsuzaki, Sakura, Chiba 285-8550, Japanb
Optoenergy Inc., 1440, Mutsuzaki, Sakura, Chiba 285-8550, JapanABSTRACT
High-brightness laser diode module over 300 W with 100 m core / NA 0.22 fiber has been developed by integrating theseveral tens of optimally designed single emitter laser diodes in a newly designed package. We employed theAsymmetric Decoupled Confinement Heterostructure (ADCH) and the wide strip width to increase the durability for thecatastrophic optical damage. High fiber-coupling efficiency was obtained with the uniquely designed micro-opticalsystem. In addition, low thermal resistance made it possible to operate higher power. As a result, 300 W power wasachieved without thermal rollover at 15.5 A with significantly high reliability. The high-brightness modules have a great
advantage for high power fiber lasers such as 10 kW and beyond.
Keywords: Laser diode module, high-brightness, high-power, single emitter, package, spatial multiplexing

1. INTRODUCTION
The market demand for fiber lasers has been increasing in a laser processing area because of their competitive properties
such as high conversion efficiency, good beam quality and small-footprint compare to CO2 lasers or YAG lasers [1]. The
fiber lasers with over kilowatt are widely used for the metal cutting or welding. Tens of laser diode modules are used inthose high-power fiber laser systems for pumping sources. So brightness of the module is the most critical issue toachieve the competitive fiber laser properties [2, 3, 4].
High-power, high-brightness fiber coupled modules are realized by means of the spatial multiplexing of beams frommulti emitters into the fiber. There are two methods to increase the power and the brightness of the modules - one is toincrease the number of emitters and the other is to increase output power of each emitter. However, as the number ofemitters increases, it becomes more difficult to focus the light from the laser diodes on the core of the fiber. On the otherhand, as the output power of each emitter increases, random failure rate raised because of Catastrophic Optical Damage(COD). In addition, high-power operation would cause the negative cycle of reduction of efficiency and elevation of heat.
As the results thermal rollover could happen. In either case high-efficiency and high-brightness laser diode modules aredifficult to be achieved.
To overcome these limitations, we have developed following technologies.
Laser diode chip; Single emitter diodes with ADCH structure and wide stripe width. Significantly high COD
level of the laser diodes was obtained and it became possible to operate at high current without degradation ofthe efficiency.

Spatial optics; Optical system for aligning the beams with high accuracy and high-precision alignment
machine realizing that system. The spatial density of the beams was maximized. Polarization multiplex
technique was also used to double the brightness.

Package; High efficient heat dissipation with the high-heat conducting materials for base materials of thepackage. Low thermal stress with the sub-mount multilayer structure. The thermal resistance of the moduleswas reduced and also the thermal stress to the laser diode chip was suppressed.

As a result of these developments, 300 W power and 50% efficiency was achieved without thermal rollover at 15.5 A
with 100 m core / NA 0.22 fiber. In this paper, detail of the module designs and their performance are discussed.

High-Power Diode Laser Technology and Applications XIV, edited by Mark S. Zediker, Proc. of SPIE
Vol. 9733, 973309 · © 2016 SPIE · CCC code: 0277-786X/16/$18 · doi: 10.1117/12.2211985

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2. MODULE DESIGN
2.1 Laser diode design
There are two general approach to increase output power of a laser diode. One is to elongate the length of the cavity. The
other is to make the emitter width wider. For both methods, the temperature of the active layer is lowered because thedecreasing of heat resistance and light density, and then reliability is improved. But in case of long cavity length, power
conversion efficiency generally decreases because a longer cavity length result in increasing of internal loss andthreshold current. Now then we adopted the design as widening the stripe width of a laser diode over 100 m while thecavity length remains conventional one. As stated above, it is possible to increase the output power by widening thestripe width. First, heat and electrical resistance decrease with increasing cross-section area of the active layer, sotemperature rising is suppressed, and then CW high-power operations can be possible without thermal rollover. Second,
because optical power density at the front facet of a diode decreases, COD level increases. These features brings higherpower operation with high reliability. These properties can be estimated by calculating mean-time-to-failure (MTTF)
represented by the following formulaE  1
MTTF1  SW0  P0 / SW0 1  exp  a    k T T 
MTTF0  SW1  P1 / SW1 B10 n(1)where SW is stripe width, P is output power, n is power acceleration multiplier, Ea is activation energy, kB is Boltzmann
constant, T is the temperature of the active layer, and subscript 0 means reference condition.
It should be noted that beam-parameter-products (BPPs = half stripe width times half beam divergence angle) becomelarger as widening stripe width. That means the amount of increased stripe width is greater than the reduction of beam
divergence angle due to thermal lens effect. In other words, the concept of high-power operation offers essentially rising
BPPs. We optimized laser stripe width giving the highest coupled power to 100 m core NA 0.22 fiber by our spatial
optical design and high-precision alignment techniques.
To increase the reliability at higher power operation, we have also utilized ADCH structure for laser diodes [5]. Opticalconfinement becomes small to the extent possible by changing the position of active layer. This is synonymous withp-side optical confinement becomes smallest, so internal loss is minimized with reducing absorption loss of p-doped
layer utmost. Significantly high COD level is also realized for optical density reduction.
ADCH structure and widened stripe width realize extremely high reliability. In addition, our laser modules do not use alaser bar but single emitter lasers, there is no thermal interference between the adjacent emitter. MTTF of developed laser
diodes is estimated by acceleration test to be over 1,000,000 hours at 15 W, 80 degrees C at p-n junction. Power
conversion efficiency is over 60%.

2.2 Spatial optical design
The originally designed micro-optics such as lenses and mirrors are used to couple the laser light emitted from wide
stripe single emitter laser diodes into the fiber most efficiently. The collimated beams from tens of laser chips are stacked
in the fast axis direction because a beam diameter of fast axis after the collimating lens is smaller than that of slow axis
direction reflected the near field pattern of a laser diode. Figure 1. shows a typical beam profile image of part of thebeam column in the module. Optimum value of beam alignment pitch along to the fast axis direction was decided under
the following reason. When the beam pitch is too narrow, laser light eclipses with the adjacent mirror. While on the other
hand if the beam pitch is too wide, fiber coupling loss arises because the focusing angle of the beam located on the edgeof the plurality of beams exceeds critical angle of the fiber. However it's difficult to achieve perfect optimal value ofbeam alignment pitch on the ray tracing simulation because very high alignment accuracy and dimensional precision of
the members are required. Then it is usually designed in consideration of the manufacturing tolerances, which means thatpower loss has been compromised. But we designed optical system that can correct the deviation of die-bonding position,
beam alignment, and dimension of the member by the mirrors. In addition, high-precision custom self-aligning machine
has been developed to support this optical system. Ideal layout of beam stack in the fast axis direction was realized. And
also optical axis of the beams were aligned to the fiber axis with high precision, i.e. center of the beams for the slow axis
direction were aligned as shown in Figure 1. It is possible to couple efficiently the large BPPs from wide stripe laser
diodes for the slow axis direction. Polarization multiplex technique was also used as well as spatial multiplexing to

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double the brightness.

Figure 1. Typical beam alignment profile image of part of the beam column in the module.

2.3 Package(thermal) design
It is very important to achieve following two properties at the same time in the module. One requirement is high thermal
conductivity from a laser diode chip to the heatsink. The other requirement is less stress design to the laser diode chip.
Coefficient of thermal expansion (CTE) of high-heat conductive materials usually doesn't match with that of a laser
diode. If a chip on sub-mount bonded to the CTE mismatch materials, thermal stress enlarges the point defects inherent
in the crystal, and then internal failure mode called dark line defect (DLD) may occurs. DLD have a concern about
degradation of the reliability of laser diodes. We managed the thermal stress by employing the sub-mount multilayer
structure using gold-tin solder and indium solder. Therefore the appearance of DLD has been suppressed.
Appearance of manufactured the 300 W module is shown in Figure 2. It is a compact size of about 10 cm×10 cm. And
also not necessary to provide a micro-water cooling channels because it uses single emitters rather than laser-bars. This
compact high-brightness module have a great advantage for reducing the size and cost of kilowatt fiber laser systems.

Figure 2. Appearance of the 300 W laser diode module.

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3.1 Optical and electrical characteristics
Optical power and forward voltage of the module as the function of current are shown in Figure 3. 300 W has beenachieved at 15.5 A at the heatsink temperature 25 degrees C. Linearity of the L-I curve is maintained to 16 A and there isvery few thermal rollover. Power conversion efficiency over 50% at 300 W was achieved as shown in Figure 4. The
reduction of the efficiency at high current range is markedly small. In addition, the capability of heat dissipating is moredirectly shown by center wavelength shit of the module as Figure 5. Shift amount is less than 1 nm/A even at 300 W.
That clearly shows a benefit of newly developed widening stripe width and high-thermal conductive package design.

350

50
45

300

40
250

35

150
100

50

E30

Pr

200

25+20
15

10
5

0002

4

6108

12141618

121416

Current (A)Figure 3. L-I and V-I characteristics of the module.

60%

a...

50%

40%
ó30%

v 20%
á10%

0%
024

681018Current (A)Figure 4. Power conversion efficiency of the module.

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Figure 5. Center wavelength shift of the module.

Figure 6 shows temperature dependences of L-I and power conversion efficiency on current. The characteristic
temperature of threshold current T0 of the module was 182 K, and that of slope efficiency T 1 was 1000 K obtained
respectively, that were almost similar value of laser diode chip's. That indicates the coupling efficiency of the module isstable at 15-45 degrees C range.

-15 deg. C.
-25 deg. C.
-35 deg. C.

-45 deg. C.

-15 deg. C.
-25 deg. C.
-35 deg. C.
-45 deg. C.

Figure 6. Temperature dependence of output power (left) and power conversion efficiency (right) of themodule.

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3.2 Reliability
To verify the reliability of developed high-brightness modules, we ran accelerated test by using several number ofmodules that include 158 chips inside. The test condition is 16.5 A driving current, 35 degrees C at heatsink temperature.
No change has been observed up to 1500 hours as shown in Figure 7. This results indicate that there are no effect to thereliability to the laser diode chip by packaging process even COD level nor DLD level.

120%

esitmassilansiassossallN0%
0300

600

900

1200

1500

1800

Time (hour)
Figure 7. Accelerated test result of the modules. 16.5 A, Heatsink temperature=35 degrees C.

4. CONCLUSION
High-power, high-brightness modules have been developed. The laser diodes design, the spatial optical design, thepackage(thermal) design are comprehensively optimized. Over 300 W output power was obtained with 100 m core NA
0.22 fiber, and the power conversion efficiency reached to 50% at 300 W, which is commercially available level. Theseultra-high-brightness pump modules contribute further to cost reduction and compactness of kilowatt fiber laser systems.
In addition, increasing a power of pump modules directly increase a power of a fiber laser, it would be expected to makeover 10 kW systems.

REFERENCES[1] Allen Nogee, [The Worldwide Market for Lasers: Market Review and Forecast 2015], Strategies Unlimited,
Mountain View, CA, Chapter 1-3 (2015)[2] Liu Rui, et. al.: "High brightness 9xxnm fiber coupled diode lasers," Proc. SPIE 9348, 93480V (2015)[3] M. Kanskar, et. al.: "High Brightness Diodes & Fiber-coupled Modules," Proc. SPIE 9348, 934804 (2015)[4] Richard Duesterberg, et. al.: "100 W High-Brightness Multi-Emitter Laser Pump," Proc. SPIE 7918, 79180V(2011)[5] Yuji Yamagata, et. al.: "915nm high power broad area laser diodes with ultra-small optical confinement basedon Asymmetric Decoupled Confinement Heterostructure (ADCH)," Proc. SPIE 9348, 93480F (2015)

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