Multi-kW high-brightness fiber coupled diode laser - DILAS

Multi-kW high-brightness fiber coupled diode laser
Bernd Köhler * , Armin Segref, Paul Wolf, Andreas Unger, Heiko Kissel, Jens Biesenbach
DILAS Diodenlaser GmbH, Galileo-Galilei-Str. 10, 55129 Mainz-Hechtsheim, Germany
ABSTRACT
Fiber coupled diode laser devices are attractive light sources for applications in the area of solid-state laser pumping and
materials processing. The ongoing improvement in the brightness of diode lasers, which means power per beam quality,
makes more and more industrial applications accessible to diode lasers. For many applications in materials processing
multi-kW output power with a beam quality of better than 30 mm x mrad is needed.
Previously we have reported on a modular diode laser platform based on a tailored bar design (T-Bar) and have
demonstrated an output power of up to 785 W out of a 200 µm NA 0.22 fiber at a single wavelength of 976 nm. We have
now extended that tailored bar platform to different wavelengths in the range from 900 nm to 1100 nm. At each single
wavelength efficient fiber coupling into a 200 µm NA 0.22 fiber will be demonstrated.
One important concept for power scaling is coarse wavelength multiplexing with a spectral separation of typically about
40 nm. Combining of different wavelengths enables scalable multi-kW high-brightness diode laser units. Further power
scaling can be achieved by dense wavelength multiplexing with a spectral separation of only about 5 nm. In this paper
we report on a diode laser unit with 3.5 kW output power and a beam quality of 25 mm x mrad.
Keywords:
High-power diode laser, high-brightness, fiber coupling, fiber laser pump source,
materials processing, wavelength multiplexing, multi-kW
1 INTRODUCTION
The performance of high-power and high-brightness fiber coupled diode laser devices has been continuously improved
within the last few years, which was mainly pushed by two fields of application, namely solid-state laser pumping and
materials processing. The rapid progress in the fiber laser area has increased the demand for pump lasers in the
wavelength range of 900 - 1030 nm. For pumping standard Ytterbium (Yb)-fiber lasers around 1080 nm fiber coupled
diode laser systems at 915 nm, 940 nm and 980 nm are required. Especially the pump region around 980 nm is important
because of the high absorption coefficient in combination with a small absorption bandwidth. A typical brightness
requirement for fiber laser pump sources is 200 W with a beam quality of about 20 mm x mrad.
For materials processing the main goal is an output power of about 4 kW in combination with a beam quality of at least
30 mm x mrad, which is needed to replace inefficient lamp pumped solid-state lasers. Laser systems based on highpower
diode laser devices can benefit from high wall-plug efficiency, high optical power, reliability, robustness against
environmental conditions, small footprint and potentially low weight.
In addition to these technical requirements economic aspects become more and more important in order to reduce the
costs per laser unit. The basic design concept of a scalable diode laser system has to take account for the technical issues
as well as for economic aspects. As a result DILAS consistently pursued the development of tailored minibars, which is
the heart of the modular concept 1-3 . The second main design aspect of the modular concept is the possibility of
automation of production steps to allow for a stable and cost-efficient production process.
* b.koehler@dilas.de, tel. +49 (0)6131 9226 133; fax +49 (0)6131 9226 255; www.dilas.de

2 MODULAR DIODE LASER CONCEPT
In this section we present some general design aspects of the modular diode laser concept which enables power scaling
of fiber coupled high-brightness diode laser systems with output powers from 100 W up to several kW.
2.1 Basic laser unit
The modular concept has already been described in previous publications 1 . The basic subunit is a tailored minibar, whose
pitch, emitter size and number of emitters is chosen such that a desired beam quality in slow-axis direction can be
realized without using sophisticated beam shaping optics. The optical design concept is based on fast-axis collimator
(FAC) and slow-axis collimator (SAC) lenses followed by only one additional focusing optic for efficient coupling into a
fiber with 200 µm core and a numerical aperture of 0.22. The lateral structure of the tailored minibar is defined by
5 emitters with an emitter width of 100 µm spaced by a pitch of 1000 µm.The advantage of such a low fill factor bar is a
reduced thermal crosstalk between the emitters, which is illustrated in the left part of Fig. 1. The thermal simulation for
the low fill factor bar shows negligible thermal crosstalk between the emitters which increases reliability and potential
output power per emitter.
The next step of the modular concept is to arrange seven tailored minibars on one baseplate in combination with FAC
and SAC for each bar. In addition mirrors are implemented on the baseplate to build an optical stack. All optical
components are mounted automatically to ensure very high reproducibility and an efficient production process. As a
result pointing errors are minimized which is important for beam quality with regard to fiber coupling or wavelength
stabilization, which is possible by using only one volume holographic grating (VHG) for the whole baseplate. Another
important design aspect is that the cooling strategy allows the use of industrial water for the bottom-cooled baseplate.
Fig. 1: Thermal simulation of a tailored minibar with reduced number of emitters (left diagram). Measurement of the beam caustic for
the baseplate with resulting beam quality of 16.4 mm x mrad (M 2 =53) in slow-axis direction and 6.6 mm x mrad (M 2 =21) in fast-axis
direction, respectively (right diagram).
The baseplate with seven minibars is characterized by an output power of about 260 W at a current of 40 A and an
overall beam quality of better than 20 mm x mrad enabling efficient coupling into a 200 µm NA 0.22 fiber. The right
part of Fig. 1 shows a measurement of the beam quality for the baseplate. The resulting beam qualities are
16.4 mm x mrad (M 2 = 53) for the slow-axis direction and 6.6 mm x mrad (M 2 = 21) for the fast-axis direction,
respectively. The central wavelength of the base plate is 976 nm with a spectral width of about 4 nm (90 % power
content). Optionally - by using one common VHG for the whole base plate - the central wavelength can be fixed and the
spectral width can be reduced down to 0.5 nm (90 % power content). Furthermore it is worth noting that the laser light of
the base plate is linearly polarized.

2.2 Modular Laser Concept
The basic idea of the modular laser concept is to use the standard laser unit described in the previous section as a
building block for a variety of lasers with different output powers and beam qualities. According to the modular design
principle the baseplates can be easily combined to scale output power, which is realized optically by spatial and / or
polarization multiplexing. The advantage of a common baseplate as basic building block for the modular system is that
the baseplate can be produced in high volume. The production process for the baseplate is highly automated which leads
to a cost-efficient and reliable building block with high repeatability regarding optical properties.
The modular concept is schematically shown in Fig. 2. Starting with a one-plate unit with 200 W output power for a
200 µm fiber (NA 0.22) we end up with a laser system consisting of 8 baseplates resulting in 1.7 kW output power for a
400 µm fiber (NA 0.22) at one single wavelength.
.
Fig. 2: Schematic drawing of modular diode laser concept based on one common baseplate.
Detailed investigations for the different configurations have already been presented 1 . The results have shown that
combining four baseplates still allows efficient fiber coupling into a 200 µm fiber with NA 0.22. However, further power
scaling by adding even more baseplates is at the cost of the overall beam quality, which reduces the coupling efficiency
for a 200 µm fiber with NA 0.22. Power scaling without affecting the beam quality is possible by using wavelength
multiplexing to combine multiple units. The basic principle of wavelength combining is shown in Fig. 3 for several
four-plate units. The drawback of the concept is that the overall spectral brightness is reduced, which would be
disadvantageous for pumping applications but which is not important for many applications in materials processing.
Fig. 3: Schematic setup of wavelength multiplexing of different laser units

Wavelength combination is realized by coarse multiplexing with appropriate dielectric mirrors. For power scaling it is
desirable to keep the wavelength separation between different units small, because the minimum wavelength separation
defines the number of units that can be combined within a given spectral range. Parameters which are important for the
efficiency of wavelength combination are the spectral width of the individual modules and the transmission
characteristics of the dielectric mirrors. Additional parameters are the angular divergence of the laser units in
combination with the angle of incidence for the dielectric mirror.
One example of a typical transmission curve for such a dielectric mirror is shown in Fig. 4. It is important to notice that
the transmission curve is dependent on the polarization. The transmission characteristics are shown as a function of
wavelength for both polarization directions. The split of the transmission curves defines the minimum allowable
wavelength separation for combining unpolarized laser sources. In addition the wavelength shift as a function of current
and temperature further limits the minimum possible wavelength separation. The example of the transmission curve
shown in Fig. 4 would allow the combination of two laser units with a wavelength separation of about 30 nm. Typical
spectral distributions of the four-plate units are also illustrated in Fig. 4 for two different wavelengths of 940 nm and
980 nm. The edge steepness of the transmission curves can be optimized by reducing the angle of incidence, but for
small angles of incidence the mechanical setup becomes more complex.
Fig. 4: Example of a dielectric mirror for wavelength combination. Transmission values are shown for both polarization values as a
function of wavelength (black). Typical spectra of unstabilized laser sources at two different wavelengths of 940 nm (green) and
980 nm (red) in comparison to a spectrum of a wavelength stabilized laser source at 974 nm (blue).
Increasing the number of spectrally combined units within a given wavelength range can be achieved by using
wavelength stabilized systems, which benefit from a reduced spectral width and a significantly reduced sensitivity of the
wavelength to temperature and current. One example of a spectrally stabilized laser unit is shown in Fig. 4 (blue) in
comparison to spectra of unstabilized laser units. In combination with appropriate dielectric mirrors or volume Bragg
gratings dense wavelength multiplexing with up to four channels within 12 nm can be realized 4,5 . Dense wavelength
beam combining can also be realized with an external cavity consisting of a lens, a diffraction grating and an output
coupler. Such diode laser systems have been reported recently with output powers in the kW range in combination with
very high brightness 6 .

3 RESULTS
In this section we will present a detailed characterization of the multi-kW high-brightness diode laser system, which
includes the results of the individual subunits at different wavelengths, as well as the investigation of the complete
system with wavelength combination. The subunits consist of four baseplates with 7 bars per plate. In each case two
plates are stacked optically by means of several folding mirrors. Afterwards the two resulting beams are combined by
polarization coupling to enhance the brightness. As mentioned in the previous section the overall brightness of the fourplate
unit still allows efficient fiber coupling into a 200 µm fiber with NA 0.22. Such units have been built at five
different wavelengths of 910 nm, 940 nm, 976 nm, 1020 nm and 1060 nm.
3.1 Laser units at different wavelengths
At a central wavelength of 976 nm we have already presented results for the four-plate unit 2 . We have demonstrated
775 W of optical output power for a 200 µm fiber (NA 0.22) with a spectral linewidth of 5.1 nm (90 % power content).
Wavelength stabilization was possible by using only one common VHG per baseplate. The output power of the fourplate
unit with VHG was 690 W with a central wavelength of 976.8 nm and a spectral width of only 0.7 nm (90 % power
content) 2 . For many industrial applications a numerical aperture below 0.22 is desirable, so we have realized an optional
design for a numerical aperture of 0.12. One advantage of low-NA systems is the reduction of aberrations in the optical
system. Of course, the NA-reduction is at the cost of the fiber diameter because the overall beam parameter product
remains unchanged.
Fig. 5 shows the experimental results for the four-plate unit at a wavelength of 976 nm with a low numerical aperture.
We have realized an output power of 840 W out of a 400 µm fiber with a numerical aperture of 0.12. The corresponding
electro-optical efficiency was 47% at an operating current of 40 A. The right part of Fig. 5 shows the mechanical layout
with dimensions of 285 x 280 x 100 mm 3 and a weight of 9.7 kg 1 .
Fig. 5: Output power and electro-optical efficiency of a unit with four baseplates as function of current at a temperature of 25°C (left).
The fiber is a 400 µm fiber with numerical aperture of 0.12 and the central wavelength of the laser unit is 976 nm. Mechanical
dimensions are 285 x 280 x 100 mm 3 with a weight of 9.7 kg (right).
For further power scaling by wavelength multiplexing we have adapted the basic concept to different wavelengths in the
range from 900 nm to 1100 nm. The beam quality of the four-plate unit is about 22 mm x mrad, which allows efficient
fiber coupling into a 200 µm fiber with NA 0.22 or alternatively into a 400 µm fiber with lower NA. We have used a
400 µm fiber with NA 0.12, which corresponds to a beam parameter product of 24 mm x mrad. The results are
summarized in Fig. 6. The left part shows the output power as a function of current for each unit. The maximum output
power was achieved for the 1020 nm unit with 950 W at an operating current of 45 A. Similar output power has been
measured for the 980 nm unit with 934 W, whereas the other units show slightly less power down to 775 W for the
940 nm unit. The reason for the variation in output power is explained by different electro-optical characteristics of the
diode laser bars at each wavelength, like threshold current, slope efficiency and far field divergencies.

In the right part of Fig. 6 typical spectra are shown exemplarily for the 940 nm unit and the 980 nm unit, respectively.
Typical spectral widths of such units without additional wavelength stabilization are about 5 nm (90 % power content).
Fig. 6: Output power of different four-plate units as function of current at a temperature of 20°C (left). The wavelengths of the
individual units are 915 nm, 940 nm, 980 nm, 1020 nm and 1060 nm, respectively. The fiber has a 400 µm core with a numerical
aperture of 0.12. Typical spectra of the 940 nm unit and the 980 nm unit with a spectral width of 4.8 nm and 5.5 nm (90% power
value) (right).
3.2 Wavelength multiplexing of base units
The basic principle of beam combination by coarse wavelength multiplexing has already been described in Sect. 2.2. In
this section we will present the experimental results of power scaling by spectrally combining the individual units
described in the previous section. One main advantage of fiber coupled subunits is that the beam quality at the combiner
input is well defined. Furthermore adding or removing subunits for power scaling is easy because of the fixed fiber
interface. The disadvantage of individual fiber coupled units is that the overall power loss is increased due to the optical
elements for fiber coupling and the incoupling losses at the fiber entrance plane. Therefore also solutions without fiber
coupling of the subunits have to be considered. After spectrally combining the subunits the beam is coupled into a
500 µm fiber with low NA 0.12, corresponding to a beam quality of 30 mm x mrad. The overall output power is shown
in Fig. 7 as a function of current for 3 different duty cycles of 100 %, 50 % and 25 %.
Fig. 7: Output power as a function of current of the complete system based on spectrally combined subunits The wavelengths of the
individual units are 915 nm, 940 nm, 980 nm, 1020 nm and 1060 nm, respectively. The fiber has a 500 µm core with a numerical
aperture of 0.12.

The maximum output power for continuous wave (cw) operation (100 % duty cycle) was 3835 W at an operating current
of 45 A. For lower duty cycles thermal effects are reduced leading to a slightly higher output power of 3916 W for
50 % DC and 3960 W for 25 % DC, respectively. The minor variation in output power for different duty cycles is only
valid at very high output power above a current of about 40 A. For a second unit we have measured a slightly increased
output power of 3921 W for continuous wave operation with an overall efficiency of nearly 40 %.
Typically the fibers which are used for low NA sources of about 0.12 are characterized by a larger NA of up to 0.22.
Therefore it is important to check the real NA of the laser beam that exits the fiber end, because the NA could be
increased by misaligned incoupling optics and also by bending of the fiber. Such effects would worsen the beam quality
and cause potential losses in subsequent processing heads. We have measured the actual beam quality by inserting
various mechanical apertures into the collimation optics after the exit of the 500 µm fiber.
The results of these measurements are shown in Fig. 8. The upper left part of Fig. 8 shows the output power of the
complete system with a 500 µm fiber as function of the numerical aperture. The output power decreases from 3960 W at
a numerical aperture of 0.13 down to 3460 W at a numerical aperture of 0.1. The upper right part of Fig. 8 is based on
the same data but illustrates the relative power content as a function of the beam quality defined by the beam parameter
product. More than 97 % of the power is included within a beam parameter product of 30 mm x mrad. For a beam
quality of 25 mm x mrad the value is reduced down to about 87 %, which corresponds to an output power of nearly
3.5 kW with a beam quality of 25 mm x mrad.
Additional measurements of the beam quality have been performed by scanning the laser beam with a rotating pinhole
(FocusMonitor, PRIMES). The measured beam caustic is shown in the lower part of Fig. 8. The beam caustic
measurement has been done in cw-operation at a power of 3835 W with a resulting beam quality of 26.5 mm x mrad.
This value is in good agreement with the previous measurement by means of mechanical apertures.
Fig. 8: Output power of the complete system with a 500 µm fiber as function of NA (top left). Relative power content of the fiber
coupled system with 500 µm fiber as a function of the beam quality (top right). Beam caustic measurement of the complete system at a
current of 45 A in cw-operation (bottom).

SUMMARY AND CONCLUSION
In conclusion we have presented a modular diode laser concept for building highly efficient fiber coupled diode laser
units with multi-kW output power and a beam quality of 25 – 30 mm x mrad. The main idea of the concept is to use the
previously presented basic building block for all laser units, which enables automated and cost-efficient production with
very high repeatability of optical properties. Power scaling in the multi-kW range was achieved by extending the concept
of the basic building block to other wavelengths and combining the laser units by coarse wavelength multiplexing.
In detail we have presented five different fiber coupled laser units (400 µm NA 0.12) within a wavelength range of
900 - 1100 nm with output powers from 775 W at a wavelength of 940 nm up to 950 W at 1020 nm. Power scaling in the
multi-kW range was realized by coarse wavelength multiplexing with dielectric mirrors resulting in a maximum output
power of 3921 W for continuous wave operation (500 µm NA 0.12). Measurements of the beam quality have shown that
more than 97 % of the power was included within a beam parameter product of 30 mm x mrad. For a beam quality of
25 mm x mrad we have measured an output power of 3460 W.
Further power scaling is possible by using dense wavelength multiplexing which will enable the building of laser units
with even higher output power while maintaining the beam quality.
ACKNOWLEDGEMENT
A part of this work was sponsored by the German Federal Ministry of Education and Research (BMBF) within the
German national funding initiative “Integrated Optical Components for High Power Laser Beam Sources (INLAS)”. In
addition part of this work was supported with funding from the “Leadership in Fibre Laser Technology (LIFT)” project
under the 7 th Framework Programme of the European Commission.
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