High-power visible spectrum diode lasers for display and medical

High-power visible spectrum diode lasers for display and medical
applications – beam sources with tailored beam quality and spectral
characteristics
Andreas Unger∗, Bernd Köhler, Jens Biesenbach
DILAS Diodenlaser GmbH, Galileo-Galilei-Str. 10, 55129 Mainz-Hechtsheim, Germany
ABSTRACT
In this paper we report on the further progress of fiber coupled high power diode lasers in the visible spectral range with
regard to beam quality and spectral characteristics. Improved beam shaping concepts allow coupling of red and blue
diode lasers into smaller fibers. For medical applications beam sources with narrow wavelength distribution in the blue
spectral region were developed. Modules up to 100W in a 400µm NA0.22 fiber were realized. Progress in manufacturing
technologies allows for coupling of more than 25W into a 200µm NA0.22 fiber in the blue wavelength range.
Keywords: Visible diode laser, fiber coupling, display applications
1. INTRODUCTION
Diode lasers in the visible spectral range are currently a highly active field of research. While the development of red and
blue diode lasers is driven by the goal to replace arc lamps in cinema or home projectors and enable mobile projectors
that can be incorporated into smartphones, the availability of high power diode lasers in the red and blue spectral region
also enables other new applications. The discovery that blue light with wavelengths smaller than 450nm can be used to
fight pathogens1 opens up markets for fiber coupled blue lasers in the medical area. Blue Lasers with a wavelength of
444nm can also be used as a pumping source for Pr3+ and enables the creation of several laser lines in the blue to red
spectral range without frequency doubling2. On the other hand red lasers at wavelengths of ~638nm can also be used to
several semiconductor and solid state lasers which is interesting e.g. for LIDAR applications. Of course the application
for large venue and cinema projectors is still highly relevant, and it is expected that the first cinema projectors operating
with laser light sources will be released in 2014 to public cinemas.
To serve the needs of these emerging markets, DILAS has developed a range of fiber coupled and free space lasers in the
blue and red spectral range. In the red the well-established T-Bar concept developed at DILAS was employed3,4. The
resulting electro optical base plate which is capable of delivering 50W of polarized light at 638nm into a 400µm NA0.22
fiber was published previously5. Polarization coupling can be used to further scale the output power up or increase the
brightness and couple into a 200µm fiber. Further upscaling of the output power can only be done by increasing the
beam parameter product of the resulting laser source. Since the T-Bar concept is modular, beam sources incorporating up
to 8 base plates are available and can be adapted for the baseplate at 638nm wavelength. This paper will focus on the
development of a polarization coupled beam source with up to 100W output power from a 400µm fiber and a 40W beam
source with increased brightness by coupling to a 200µm fiber.
In the blue spectral range single emitters are employed and coupled side by side to enable high power output. Currently
available high power multi-mode single emitters in the blue have a beam parameter product of 1-2mm mrad in the slow
axis and an output power of more than 1.5W6. Based on these data beam sources with several 100W into a 400µm
NA0.22 fiber seem possible but require a high number of single emitters. At DILAS a modular combining concept for
packaged single emitters was developed. This concept, published previously, enables output powers up to 100W from a
400µm fiber5. Based on this concept 10W and 25W beam sources were recently introduced into the market.
Improvement of the fabrication technologies enables further scaling of the brightness of these modules and coupling into
∗
a.unger @dilas.de, tel. +49 (0)6131 9226 459; fax +49 (0)6131 9226 255; www.dilas.de
200µm fibers. In addition, spectral selection of single emitters enables tighter wavelength tolerances and tailored spectral
characteristics for special applications, like medical use or pumping.
2. LASER MODULE CONCEPTS
2.1 Fiber coupled 100W red laser diode module
In this section the setup of the fiber coupled 100W laser source at 638nm wavelength is described. The beam source is
based on two electro optical base plates. These base plates, as published previously5, combine up to 7 laser diode bars
and incorporate collimation and stacking optics as well as a cooling structure for efficient heat removal. The base plate
assembly and all optics alignment are done fully automated enabling cost-efficient mass production. The output of a
single base plate is a square collimated laser beam with roughly 5x5mm2 and a beam parameter product of 45 mm mrad
in the slow axis and 10 mm mrad in the fast axis, respectively. The divergence in the slow axis is quite large (~40mrad
full angle) and requires special attention due the accordingly short Rayleigh length.
Two of these base plates are assembled together in a readily available package developed for infrared direct diode
applications (Fig. 1).
Fig. 1 Package for coupling of 2 T-bar base plates into a single fiber. The laser module has a size of 200x200x50mm3.
The beam path of the polarization combining and fiber coupling section is shown in Fig. 2. The beam paths of the two
plates are of equal length and redirected with folding mirrors towards a thin film polarizer (TFP). To optimize the
combining efficiency the TFP is hit under an angle of 56°. Due to the good degree of polarization of the single beams
and the optimized TFP combining efficiency is ~97%. The long beam path, which is necessary due to the mechanical
constraints, would lead to inefficient fiber coupling when using a single focusing lens, because the high slow axis
divergence in combination with a long beam path would introduce considerable astigmatism. To cope with this problem
and adapt the different divergence angles in the fast and slow axis, an imaging and magnifying relay optic is employed.
This optic is interleaved with the TFP and consists of three plano-convex cylindrical lenses, two before the TFP (one in
each beam path) which form an intermediate image and one after the TFP for the combined beam which recollimates the
beam and forms a magnified image of the collimated laser diode bars in front of the fiber coupling lens. The fiber
coupling optics finally consists of a single aspheric lens.
Fig. 2 Simulated beam path from exit of the base plates to the fiber. On the top right the exit of the two base plates is shown
in different beam color. Please see text for a description of the optical elements.
Dividing the beam in the slow axis and overlaying both parts with polarization coupling optics can lead to a beam source
with half the beam parameter product in the slow axis with almost the same optics than the previous two plate module.
For coupling this module into a 200µm fiber NA0.22 the same optics were used as previously for a single base plate.
Instead of the TFP an integrated optical element behind the second telescope lens was used for polarization coupling. An
additional telescope with 2:1 magnification of the beam in the slow axis was used to be able to use the same fiber
coupling lens.
2.2 Fiber coupled blue laser diode module
The beam quality of a current state-of-the-art single emitter at 450nm is around 1-2mm mrad in the slow axis, thus
allowing in principle to couple more than 100 emitters into a 400µm NA0.22 fiber. Practically it is challenging to align
many pre-packaged single emitters in a cost-efficient way for highly efficient fiber coupling. Previously a 25W module
with a beam quality of 31mm mrad in the slow axis was published5. This BPP was sufficient for coupling into a 400µm
NA0.2 fiber. However, the achieved beam quality was dominated by fabrication imperfections due to glue shrinkage in
the alignment. This problem is much more severe for the blue single emitters than for a conventional laser diode bar
because the small BPP and correspondingly small divergence of a single collimated diode requires an extremely precise
pointing alignment. To improve the beam quality a new submodule for 6 single emitters aligned along their fast axis was
developed (Fig. 3). In this module 6 diodes and an aspheric collimation lens for each diode are fit into a common heat
sink. A turning prism for each single emitter allows for an increase in fill factor and enables precise pointing alignment
for each single laser beam. Up to 4 of these modules can be fit onto a water cooled heat sink and coupled side by side
along the fast axis giving a total raw output power of up to 38W at 1.6W per single emitter. The resulting raw beam has
an asymmetrical far field divergence and can be coupled to a 200µm fiber with two cylindrical lenses.
Based on this concept modules with 10W and 25W Output power were fabricated (Fig. 4). The common heat sink can
also be used for TEC-cooling and does not have to be water cooled.
Fig. 3 Submodule for 6 pre-packaged single emitters on a water cooled heatsink for up to 4 submodules. The single emitters
are arranged in the fast axis. For the increase of the fill factor in the fast axis and pointing alignment turning prisms are
glued to the black triangular holding structure.
Fig. 4 Single emitter module for up to 24 single emitters. The module has a size of 120x150x65mm3
3. RESULTS
In this section the performance of the realized submodules and fiber coupled sources will be presented in detail.
3.1 Fiber coupled red laser diode modules
Fig. 5 shows the measured output power and power conversion efficiency (PCE) from the built 100W module. The
targeted output power of 100W is reached at a current of 14A. From the raw output power of a single base plate (Fig. 6)
a total coupling efficiency of 75% is calculated. At 100W output power and 20°C cooling plate temperature a PCE of
24% is achieved. This is only 3% lower compared to the previously published data of a fiber coupled single base plate at
50W output power and shows the good combining efficiency by means of polarization coupling. The remaining losses
are mainly due to Fresnel losses at the uncoated fiber ends and a larger BPP of the single base plate in the slow axis
compared to the fiber as published previously. The latter problem could be overcome if a slightly larger fiber, e.g.
500µm is permissible in a specific application. Fig. 7 shows the wavelength spectrum of the 100W module. A line width
of 1.7nm is achieved.
Fig. 5 Output power from a 400µm NA0.22 fiber and power conversion efficiency of the 100W 638nm laser source. At 14A
an output power of 100W with a conversion efficiency of 24% is achieved.
Fig. 6 raw output power from a typical single base plate.
Fig. 7 wavelength spectrum of the 100W 638nm laser source at 80W output power. At 20°C the central wavelength is
639nm and the 90% line width is 1.7nm
A prototype module for coupling one base plate to a 200µm NA0.22 fiber was built and characterized. Fig. 8 shows the
obtained output power and PCE. Although the polarization coupling works well with low power losses a significant
degradation in coupling efficiency compared to the previously shown 400µm 50W and 100W modules was observed.
One reason for this loss in coupling efficiency is that only in the slow axis the beam parameter product is halved whereas
the fast axis beam parameter product stays the same, which leads to increased coupling losses in the diagonal axis. The
position of the polarization combining optics and the second telescope also can still be optimized by using a different
housing which provides more space for the optics. Despite this optimization potential more than 40W in the 200µm
NA0.22 fiber with a power conversion efficiency of 20% was achieved.
Fig. 8 Output power from a 200µm NA0.22 fiber and power conversion efficiency of the one base plate polarization coupled
module. At 14A an output power of 42W with a conversion efficiency of 20% is achieved.
3.2 Fiber coupled blue laser diode module
Fig. 9 shows the fiber coupled output of a 25W module coupled into a 400µm NA0.22 fiber. At a nominal current of
1.2A an output power of 29W is reached. The optical coupling efficiency is 84%, the remaining coupling losses are
mainly Fresnel losses at the uncoated fiber ends and absorption, scattering and reflection losses at the optical elements
inside the module. Due to the short wavelength and therefore increased absorption and Rayleigh scattering these losses
are actually more pronounced than at the longer red and infrared wavelengths.
Fig. 9 Output power and efficiency of the blue laser module with 24 single emitters coupled into a 400µm NA0.22 fiber.
A 10W module was built for 200µm fiber coupling and characterized. Fig 10 shows the PI curve and power conversion
efficiency. Compared to 400µm fiber coupling the WPE is reduced to 18% which is a decrease of 3%. 10W of output
power are reached at a current of 1A, which is very well within the specification of the used diodes. However, from the
design of the module and single diode measurements a 10% higher coupling efficiency is expected. Since in the 25W
module only the NA in one axis increases but stays well below the fiber NA, equal coupling efficiency but a higher NA
filling is expected. The NA filling of the 10W module is actually only 0.16.
Fig. 10 Output power and efficiency from a module with 12 single emitters from a 200µm NA0.22 fiber. The NA of these
modules is actually only 0.16 (95%)
To further investigate the reasons for the unclear loss in coupling efficiency the laser beam profile at the optical fiber was
investigated with a CCD camera. Fig. 11 shows the obtained beam profile. From the camera a focal spot with 144µm
width in the slow and 172µm in the fast axis (95% values) was obtained. The NA of the module was measured from the
laser beam size before the focusing optics and the focal width of the focusing optics and was found to be 0.16 over the
diagonal of the beam. Based on these measurement data the fiber coupling efficiency should be higher. Possible
explanations for the unclear additional losses are stray light caused by aberrations or the single emitters themselves,
which is not captured by the dynamic range of the camera but contributes to the power within the 400µm fiber diameter
or additional absorption or scattering losses inside the used 200µm fibers.
Fig. 11 Focal Spot of a 10W modul (12 emitters coupled) on the coupling side of the fiber in false color (middle ,the circle
has a 200µm diameter) and integrated cross section for slow axis (left) and fast axis (right). The 95% spot sizes
measured are 144µm in the slow axis and 172µm in the fast axis, respectively.
For many applications the typical spectral distribution of multiple single emitters is too broad and wavelength selected
modules are requested. For example in medical use a wavelength distribution <450nm is desired. Fig. 12 shows the
spectrum of a 10W module for medical use with a central wavelength of 446nm and a spectral width of 3.2nm
Fig. 12 wavelength spectrum of the 10W module intended for medical use (wavelength <450nm). The central wavelength is
446nm and the spectral width is 3.2nm (90%).
4. SUMMARY
In this paper we reported on further progress regarding the development of fiber coupled visible spectrum diode lasers.
Polarization coupling was used to realize a module with over 100W output power at 639nm from a 400µm NA0.22 fiber
and over 40W from a 200µm NA.022 fiber.
Improvements in packaging lead to fiber coupled modules in the blue spectral range of up to 25W which can be coupled
to a 200µm fiber. Wavelength selection of diodes can be used to obtain wavelength distributions tailored to specific
applications.
The developed scalable and modular diode laser concepts can be used to further scale up power and brightness of fiber
coupled visible diode lasers if demanded. For example instead of coupling two base plates at 638nm into a 400µm fiber
polarization coupling can also be used to couple one plate to a 200µm fiber. All modules in the blue spectral range are
still polarized, which means the brightness can immediately be doubled by polarization combining.
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