A new continuous wave 2500W semiconductor laser vertical stack

Abstract

With the increasing applications of high power semiconductor lasers in industrial, advanced manufacturing, military, aerospace, medical systems, display, entertainment. etc., semiconductor lasers with high power and high performances are required. The performance of semiconductor lasers is greatly affected by packaging structure, packaging process and beam shaping. In this work, a high power semiconductor laser vertical stack was successfully fabricated. A series of techniques such as spectrum control and beam control were used to achieve marrow spectrum and high beam quality. The performances of the semiconductor laser vertical stack were characterized. A high power of 2500 W, a narrow spectrum of 3.11 nm and an excellent rectangular beam shape were obtained. The lifetime of the vertical stack laser was tested as well.

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A New Continuous Wave 2500W Semiconductor Laser Vertical Stack

Xiaoning Li1,2, Chenhui Peng1, Yanxin Zhang1, Jingwei Wang1, Lingling Xiong1, Pu Zhang1, Xingsheng Liu1,3
1 State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics
Chinese Academy of Sciences
No. 17 Xinxi Road, New Industrial Park, Xi'an Hi-Tech Industrial Development Zone, Xi'an, Shaanxi, 710119, P.R. China ,
tel. 8629-88880786, fax.8629-88887075, smto@opt.ac.cn
2 Key Laboratory for Physical Electronics and Devices of the Ministry of Education & Shaanxi Key Lab of Information
Photonic Technique, Xi’an Jiaotong University,
No.28, Xianning West Road, Xi'an, Shaanxi, 710049, P.R. China
3 Xi’an Focuslight Technologies Co., LTD
No. 60 Xibu Road, New Industrial Park, Xi'an Hi-Tech Industrial Development Zone, Xi'an, Shaanxi, 710119, P.R. China

Abstract
With the increasing applications of high power
semiconductor lasers in industrial, advanced manufacturing,
military, aerospace, medical systems, display, entertainment.
etc., semiconductor lasers with high power and high
performances are required. The performance of semiconductor
lasers is greatly affected by packaging structure, packaging
process and beam shaping. In this work, a high power
semiconductor laser vertical stack was successfully fabricated.
A series of techniques such as spectrum control and beam
control were used to achieve marrow spectrum and high beam
quality. The performances of the semiconductor laser vertical
stack were characterized. A high power of 2500 W, a narrow
spectrum of 3.11 nm and an excellent rectangular beam shape
were obtained. The lifetime of the vertical stack laser was
tested as well.
Introduction
High power semiconductor lasers have been found
increasing applications in pumping of solid state laser systems
and fiber amplifiers, frequency doubling, medical systems and
material processing, such as welding, cutting and surface
treatment [1]. Although the output power of a single emitter
has been improved significantly in recent years, the power is
still generally limited to below 20 watt for a 9xxnm laser,
which is unable to satisfy new applications with higher power
demands obviously. Laser bar can provide a magnitude of
output power of single emitter by integrating the multiple
individual emitters. To further increase the output power,
several packaging technologies have been developed,
including multiple single-emitter modules, horizontal bar
arrays, and area bar arrays [2]. Naturally, integration of single
bar unit as a vertical stack is another choice to increase the
output power. The multiple bars are electrically connected in
series and can be operated at continuous wave (CW) or quasi-
continuous wave (QCW) mode.
In this work, the traditional packaging structure has been
optimized and improved. Based on the new packaging
structure, a new 976 nm semiconductor laser vertical stack
with 2500 W in CW mode was fabricated and its
performances were characterized. A series of techniques such
as spectrum control and beam control was used to improve the
performance of the vertical stack.
Structure design of single bar
The performance and reliability of a semiconductor laser
are greatly affected by its packaging structure and thermal
properties. Since the vertical stack is assembled by single bars,
the structure of the single bar has a significant impact on the
performance of the vertical stack. Therefore, not only the
structure of the vertical stack but also that of the single bar
should be considered.

Fig. 1 The traditional (a) and double-sided (b) cooling
package structure
Fig. 1(a) shows a traditional packaging structure. The heat
generated from the laser bar is one-sided conducted to heat
sink with a low cooling efficiency. Fig. 1 (b) shows the
double-sided cooling packaging structure we designed. The
heat can be conducted through both the anode and cathode;
therefore, the thermal dissipation efficiency is improved
significantly. The transient thermal behavior of double-sided
cooling packaging was studied using finite element analysis
(FEA), as shown in Fig. 2. The simulation result shows that
the heat is not only conducted to the heat sink from bottom
directly, but also conducted by the cathode, which improves
the cooling efficiency. Moreover, according the double-sided
cooling packaging design, the heat dissipation efficiency from
the cathode can be up to 20%.

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Fig.2 Thermal flux vector graph of CW micro-channel
water cooled semiconductor laser
Performance of the single bar
Fig. 3 shows the LIV curve of the single bar under the
condition of CW current measure at room temperature. The
output power of 150 W at a drive current of 144 A is obtained.
Although the output power can be up to 150W, the single bar
we used was operated at 100W in order to keep reliable
performance of the device.

Fig. 3 LIV curve of single bar 976nm CW Micro-channel
water cooled semiconductor laser
The Full Width at Half Maximum (FWHM) of the single
bar we designed is only 2.79nm and 90% energy width is 4.07
nm after using the spectral control technology of the single
bar, as shown in Fig. 4.

Fig. 4 Spectrum character of single bar 976 nm CW
Micro-channel water cooled semiconductor laser
The constant power lifetime test (25℃) of single bars
under output power of 100W at CW mode is shown in Fig. 5.
The chart exhibits that the power degradation of most devices
is less than 5% after working 2700 hours, which indicates a
good reliability. The lifetime test is still ongoing.
050010001500
Time/h
200025003000
0.0
0.2
0.4
0.6
0.8
1.0
1.2


Intensity/a.u.

Fig. 5 Lifetime curve of a single bar 976 nm CW micro-
channel water cooled semiconductor
Near-field beam can directly reflect the performance of
each individual emitter. Therefore, checking the necrosis and
the uniformity of emitters is useful for studying overall
performance of the semiconductor lasers. Accurately testing
equipments were used to test the near-field beam of the single
bar. Fig. 6 shows the near-field beam spot of 976nm CW
micro-channel water cooled single semiconductor laser bar.
Obviously, the output light intensity of each emitter in the
laser bar is uniform, which is contributed from the
homogeneous distribution of drive current on individual
emitter in the laser bar.


Fig. 6 Laser spot of 976nm CW micro-channel water
cooled single semiconductor laser bar at near field
The Near-field Non-linearity (“smile”) is another key
parameter of laser array products and improving the Near-
field performance is especially important. The Near-field Non-
linearity of laser diode array is caused by the coefficient of
thermal expansion (CTE) mismatch among the different layers
of a bare bar, the packaging process and CTE mismatch
between the laser bar and the bonding heat sink [3].
Using advanced packaging technology, a good Near-field
linearity of the laser bar was obtained, as shown in Fig. 7.

Fig. 7 The “smile” image of a typical good semiconductor
laser bar

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Nearly 900 single bars were tested. It was found that
about 99% Near-filed nonlinear were less than 1μm, as
illustrated in Fig. 8. This result shows that the linearity of
near-filed optical cavity of the laser bars and its uniformity are
both good.

1.81.51.2 0.90.60.30.0
USL
Process Data
Sample N
StDev(Within)
StDev(Overall) 0.195016
898
0.150383
LSL
Target
USL
Sample Mean
*
*
1
0.527984
Potential (Within) Capability
Cp
CPL
CPU
Cpk
Overall Capability
Pp
PPL
PPU
Ppk
Cpm
*
*
0.81
0.81
*
*
*
1.05
1.05
Observed Performance
PPM < LSL
PPM > USL 7795.10
PPM Total 7795.10
*
Exp. Within Performance
PPM < LSL
PPM > USL 848.34
PPM Total 848.34
*
Exp. Overall Performance
PPM < LSL
PPM > USL 7751.72
PPM Total 7751.72
*
Within
Overall
Process Capability of SMILEProcess Capability of SMILE值值

Fig. 8 Smile statistics of the single bar 976 nm CW
micro-channel water cooled semiconductor laser
Design of the vertical stack laser
In the design process of the vertical stack laser, one of the
main problems is the thermal crosstalk, which seriously affects
the cooling efficiency. In order to avoid thermal crosstalk, a
parallel format of liquid cooling is designed to overcome the
heat unevenness between the bars, which can effectively
improve the thermal dissipation. Fig. 9 shows the design of
parallel format of liquid cooling.

Fig. 9 the design of parallel format of liquid cooling
A vertical stack laser with 20 bars was simulated for the
thermal design and the structure optimization, which can also
be used in 25 bars vertical stack laser design. The simulation
results are shown in Fig. 10. Most of the heat is dissipated via
the cooling flow liquid. There is no large accumulation of heat
and the temperature gradient of each bar is uniform. The
maximum temperature is 36.92 ℃, which has only 11.92 ℃
difference with the water temperature in the entrance.

6
8
10
12
14
16
051015 20
Temperature Rise/℃℃
BarNo.

Fig. 10 The thermal simulation results of the 20 bar
semiconductor laser vertical stack
Based on the thermal simulation, the structure is optimized
from cooling water flow, micro-channel cooler selection,
water distribution and other aspects. The heat can be taken
away as quickly as possible by the cooling water, which can
ensure that no thermal accumulation exists between the bars.
Based on the double-sided cooling packaging technology
and the parallel format of liquid cooled structure, a 2500W
semiconductor laser vertical stack with 25 bars was fabricated,
as shown in Fig. 11.

Fig. 11 Semiconductor laser vertical stack
Fig. 12 shows the LIV curve of the 25 bar semiconductor
laser vertical stack at 976 nm under the condition of CW
current measure at room temperature. The output power of
2500 W at a drive current of 99 A was obtained. The
photoelectric conversion efficiency is more than 61%.

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Fig. 12 LIV curve of the 25 bar semiconductor laser
vertical stack
The FWHM of the 25 bar semiconductor laser vertical
stack is only 3.11nm, and 90% energy width is 4.15 nm, as
shown in Fig. 13.

Fig.13. spectral result of 25 bar semiconductor laser
vertical stack
The Near-field beam spot is shown in Fig. 14, which
shows a good rectangular beam shape.

Fig. 14 Laser beam of vertical stack
Discussion
The major challenges in vertical bar stack packaging are
the spectrum control and beam control [2].
1. spectral control technology
With the increasing demand on the high power laser
performance, commercial semiconductor laser bar products
are required to have narrow spectral width for applications.
The challenge of spectral control of single bar is how to
achieve the temperature uniformity and stress uniformity
across the laser bar to eliminate the thermal effect and stress
effect [4]. Local temperature rise is mainly caused by solder
voids in bar bonding interface. So we used the advanced
process which can minimize solder voids or even achieve void
“free” bonding to reach temperature uniformity across the bar
[5]. For the CTE mismatch problem, many methods were
used, such as: using a higher CTE matched materials as
packaging materials, design of a suitable heat sink structure,
application of new technologies such as the "void free" patch
technology and vacuum solder reflow system [6-7]. Based on
the spectral control technology, the FWHM of the single bar is
only 2.79nm, as shown in Fig.4.
Spectral width is also one of the key parameters of
semiconductor laser vertical stack products and it is very
important to improve the spectral performance to improve
production yield, reduce cost and gain competitiveness.
Although the laser bars in the vertical stack are cooled in
parallel in both conduction cooled and micro-channel liquid
cooled configurations, there remains temperature non-
uniformity among the bars due to thermal crosstalk and/or
liquid flow non-uniformity. This would alter the wavelength of
the bars and broaden the spectrum of the stack, as shown in
Fig.15.

Fig.15 The broadened wavelength of semiconductor laser
vertical stack
In this work, advanced packaging process was used to
maintain temperature distribution uniformity. First, total
temperature distribution was simulated and calculated, as
shown in Fig. 10. Second, the wavelength of each chip was
selected to match the temperature distribution based on the
simulation results. The third step was using optimized
packaging technology to achieve the same output wavelength.
Using this method the spectrum broadening of vertical stack
can be controlled effectively, as shown in Fig. 13.
2. Beam control technology
For semiconductor laser vertical stack, beam control
includes beam size, intensity uniformity and pointing direction
control [5].
Because the vertical stack we designed is assembled by 25
single bars, the initial beam size is large. Therefore, beam
shaping optical systems need to be designed and assembled to
control beam size. First, fast-axis collimation components are
added to keep each bar pointing direction consistent. And then
using advanced real-time monitoring equipment, the position
of each bar is fine-tuned from vertical and horizontal
orientation to ensure accurate positioning.

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Fig. 16 The process of pointing direction control
Near-field beam spot is shown in Fig. 14. It was found that
the output beam spot of each bar was very uniform and the
directivity was excellent.
Conclusions
In a summary, based on the double-sided cooling
packaging technology and parallel format of liquid cooled
structure, a 2500 W semiconductor laser vertical stack with 25
bars was fabricated. A series of techniques such as spectrum
control and beam control was developed. The FWHM is only
3.11nm and a rectangular beam shape was obtained. The
lifetime testing result shows that the power degradation of
most devices is less than 5% even after working 2700 hours.
Acknowledgments
This work was supported by the project of the “Hundred
Talents Research Fund of Chinese Academy of Sciences”, the
Instrument Developing Project of the Chinese Academy of
Sciences, and the National High Technology Research and
Development Program of China, Grant No. 2009AA032704.
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