laser beam sources
High-Brightness Diode Lasers and
New Wavelengths
The Next Generation of Applications
Design considerations for highbrightness laser diodes in various new
wavelength ranges are described, together with achieved results based on laser diode arrays, in the following referred
to as laser bars. In addition, the new applications for laser diodes with such new
wavelenghts are presented.
High-Brightness Diode Lasers
More and more applications ask for easeof-use, hence requiring fiber-coupled diode
laser modules. Some important design rules
have to be considered when high-brightness
diode laser modules shall be built, especially
when these are based on non-standard
wavelengths:
In principle, the minimum diffraction
limited beam parameter product (BPP)
is proportional to the wavelength, which
means that the beam quality gets worse
with increasing wavelength (~ λ). Fibercoupled modules require a specific beam
parameter product, meaning that the
number of emitters that can be coupled
into an optical fiber decreases with the
square of the wavelength (~ λ-2). For example, at 1940nm the number of emitters which can be coupled into a given fiber core is 4 times lower when compared
to 970nm.
The slow-axis divergence typically increases with the wavelength, which
means that the focal length of the slowaxis collimation lenses (SACs) has to be
adapted to avoid losses at the SAC.
For some bars with non-standard wavelengths, the divergence in the fast-axis direction is relatively high with a full angle
of up to 90°, which requires fast-axis collimating lenses (FAC) with a high numerical aperture and good quality.
Additionally, the losses in the optical elements themselves have to be con­sidered.
Especially above 2200nm, significant
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
The Authors
NADINE MUY
Nadine Muy achieved a
degree in American and
Hispanic studies, as well
as in economic studies
with focus on marketing from the University in Mainz. Afterwards she held a position
as event manager at a medical publishing
house. Since June 2008 she is responsible
for marketing and communications at Dilas
Diodenlaser GmbH.
JöRG NEUKUM
Dr. Jörg Neukum studied
Physics at Technische
Universität Darmstadt,
Germany, and did his
doctor’s degree in the field of rare-earth spectroscopy. After positions as product manager
at a Japanese laser diode manufacturer and
several years as European sales manager for
Coherent Semiconductor, he is now responsible for marketing and the worldwide sales
at Dilas Diodenlaser GmbH since 2004. He
is furthermore in charge of the business unit
Dilas Industrial Laser Systems.
JENS BIESENBACH
Dr. Jens Biesenbach
earned a Master’s degree in Mechanical Engineering from Rheinisch
Westfälische Technische
Hochschule
(RWTH)
Aachen, Germany, and
obtained his PhD in the field of precision
packaging of high-power diode laser bars.
Before joining Dilas, Dr. Biesenbach was
R&D Engineer at Solar Turbines Incorporated and was in charge of a research and
development group for diode lasers at the
Fraunhofer Institute for Laser Technology
in Aachen, Germany. Since 1998 he serves
as technical director for Dilas Diodenlaser
GmbH.
BERND KOEHLER
Dr. Bernd Köhler received
his diploma in Physics
from the University of
Kaiserslautern, Germany.
He spent five years as a
research assistant with
the Laser and Nonlinear
Optics Group of Prof. R. Wallenstein and received his PhD for his work on the generation
of high-power picosecond light pulses in the
ultraviolet and infra-red spectral range. Subsequently he joined the R&D Department of
Dilas Diodenlaser GmbH, Germany, in 2001.
Since 2009 he is responsible for Diode Laser
Beam Sources and Optics.

Dilas Diodenlaser GmbH
Mainz, Germany
E-mail: [email protected]
E-mail:[email protected]
E-mail:[email protected]
E-mail: [email protected]
Website: www.dilas.com
www.laser-journal.de LTJ 45 Laser beam sources Table A: Realized optical output powers for a variety of fiber-coupled modules.
*: Significant power loss due to intrinsic absorption in the micro-optics.
wafers allow the production of high-power
laser diodes at power levels of several watts
at 630nm, up to ~20W at 680nm.
Applications for such wavelengths can be
found in the fields of Photodynamic Therapy (PDT), optical pumping of Cr3+:LiCAF
or Cr3+:LiSAF solid-sate lasers for ultra-short
pulse generation, as well as in illumination,
holography and display. For display applications, the light is very often mixed with
green and blue light sources to achieve
a white light impression. The chosen red
wavelength is therefore dependent on the
wavelengths of the green and blue parts.
Photodynamic Therapy works with photo
sensitizers, which are injected and are used
to treat human tissue. After a short period
of time (normally some fractions of an hour)
the photo sensitizer accumulates in particular parts of the body (e.g. in a tumor). The
various photo sensitizers are wavelength
selective. By using high intensity light, the
photo sensitizer molecule is excited and,
upon returning into its ground state, can
generate free oxygen radicals which are
highly reactive and hence destroy surrounding (tumor) cells.
The range of wavelengths between
808nm and 976nm is considered standard
for high-power diode lasers. This range has
been investigated for a long time, resulting
in various wavelengths for optical pumping
of solid-state laser materials (see Table B).
The development and optimization in
optical power generation, and hence the
availability of wavelengths, has been pretty
much focused on the aforementioned wavelengths, which fit various absorption lines of
active ions (in most cases rare-earth ions) in
various host crystals (e.g. Nd:YAG).
Aside from optical pumping of solid-state
material, a further application has been developed over the last years. It began with
the optical pumping of alkaline gases for the
production of spin-polarized noble gases
used for medical diagnostics in magnetic
resonance imaging (MRI). In such applications, a gas mixture of Rubidium and Xenon
isotopes (129Xe) for example is put into an
optical cell under high pressure and under
the influence of a magnetic field. By illuminating the cell homogeneously with circular
polarized light at 794.8nm, the Rubidium is
excited which, by collision, transfers its spin
to the nucleus of the Xenon isotope. The
spin-polarized Xenon isotopes can then be
frozen out, conserving the spin polarization.
This process is utilized in MRI to visualize
heart or lung activity. Temperature tuning is
used in order to reach the necessary wave-
46 LTJ April 2010 No. 2
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
water absorptions occur due to the
OH-stretching. These absorptions affect
nearly all materials used for micro-optics
today.
An overview of the realized optical output
powers for a variety of fiber-coupled modules (Fig. 1.) can be seen in Table A.
New Wavelengths and Their
Applications
With research in all fields, various new wavelengths have been developed for a variety
of applications. Very often, this starts in the
low power range as in the case with 405nm
for Blue Ray Disk for example. If one is able
to get larger pieces of such GaN-based wafers, e.g. 10mm width and 1mm resonator
length, the power scaling just by the high
number of emitters on such a chip allows the
generation of several watts (~ 4W) of optical
power in this wavelength range. Such highpower diode lasers in the 405nm to 440nm
range can then be used in applications like:
Epoxy curing in screen printing,
Lithography in the printing and semiconductor industry,
Optical pumping of praseodymiumdoped crystals and fibers.
Unfortunately, investigation of semiconductor laser structures in the green wavelength range has been stopped some years
ago, with the advent of blue lasers. However,
Wavelength
Range
Figure 1: Fiber-coupled diode laser modules, based on 1, up to 3, up to 6 and up
to 12 laser diode bars.
recently some results at 515nm in the low
power range have been reported. Hence, we
expect to see first green high-power diode
lasers in a few years.
Another interesting wavelength range
in the visible spectrum is between 630nm
to 690nm. Whereas these wavelengths are
available for pointers and for DVD applications in the low power range, laser bars
based on such InGaAlP-structures on GaAs
Number of Bars per
Fiber-Coupled
Module
Fiber Core and
Cladding at
NA 0.22
Output Power ex
Fiber [W]
1
200µm modestrip
3.2
638nm
1
400µm modestrip
5.5
2
400µm modestrip
13
650nm
1
400µm/480µm
5
670nm
1
400µm/480µm
12
1
400µm/480µm
35
2
400µm/480µm
65
1
400µm/480µm
13
1
400µm/480µm
15
2
400µm/480µm
30
3
600µm/660µm
45
6
600µm/660µm
90
1
600µm/660µm
6
2
600µm/660µm
12
3
600µm/660µm
18
1
600µm/660µm
2.2*
1064nm
1210nm
1470nm
1550nm
1940nm
2200nm
51nanoFI-... / 51nanoFCM-...
Laser Diode Beam Sources
laser beam sources
with singlemode and polarization-maintaining fiber cables
P1
938nm
Yb3+:YAG
976nm
Yb3+:glass
Table B: Important diode laser wavelengths for the optical pumping of solidstate lasers (DPSSLs).
length, whereas the small line width required
for the gaseous transition can be achieved
using line narrowing of the laser diode by a
virtual bragg grating (VBG).
What has been demonstrated with Rubidium and Xenon isotopes, also works with
other alkaline gases such as Cesium, and
other noble gases such as 3He. The alkaline
gases are only used to generate a spin-polarized noble gas, which is the only element
introduced into the patient, and which can
leave the human body without any negative
impact after the diagnostic treatment.
Another important application is based on
similar optical pumping as described above,
but used for the pumping of gas lasers. As
surprising as this may sound, the concept is
pretty straightforward to understand.
When trying to build 50kW or 100kW lasers for ballistic-missile defense systems, the
first choice was the use of diode-pumped
solid-state lasers. However, by scaling up
the power, heat becomes an issue in the laser gain media itself. Instead of waiting for
the crystals to cool down for the next laser
the company
Dilas Diodenlaser GmbH
Mainz, Germany
Dilas, the diode laser company, is focused on delivering the most innovative
technologies and advanced product
solutions for the industrial, defense,
graphic arts, and medical markets.
Founded in 1994 in Mainz, Germany,
with operations in North America and
China, Dilas designs, develops and
manufactures high-power semiconductor laser components, modules and systems, including fiber-coupled products
for worldwide distribution.
www.dilas.com
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
51nanoFI-...
low
Noise
51nanoFCM-...
low
•Coherencelength300µm
•Noise<0.1%RMS(<1MHz)
•Spectralrange405nmto1080nm
•Beamprofilerotationalsymmetrywith
Gaussian intensity distribution P1
for increased
•Laseroutputpowerupto30mW
accuracy and
reproducibility in laser •FC-APCconnector (8°-polish),
metrology and
optional DINAVIOorE2000
CoHeReNCe
and
reduced
Speckle
y
Yb3+:glass
an
915nm
5
rm
Nd3+:YAG,
Nd3+:YVO4, etc.
+
4
Ge
880nm-range:
Appl.: Fig.
n
Nd3+:YAG,
Nd3+:YVO4, etc.
ei
808nm-range:
nanotechnology
ad
Nd3+: YLF
New
51nanoFI-...
with integrated
Faraday isolator
M
793nm, 797nm:
pulse, the idea came up to simply exchange
the laser gain media. This is best achieved
by using a gaseous laser medium and high
flow rate pumps, resulting in the concept of
diode-pumped alkaline vapor lasers based
on Rubidium (pumped at 794.8nm, for example) or Cesium (pumped at 780nm or
852nm). Other alkaline vapor lasers are also
under investigation. Again, the difficulty is
the small absorption line width of the optical transitions of these gases, even when
using pressure-broadening of the absorption
lines by a buffer gas. Hence, line narrowing
techniques, such as the use of VBGs or internal line narrowing by distributed feedback
(DFB) structures, have to be applied [1].
The demand from defense-related pro­
jects is increasing and the targeted power
as well as the number of systems planned,
allows for bespoke development of highpower diode lasers with the required specification.
Another wavelength for high-power
diode lasers is 1064nm. Aside from the replacement of Nd:YAG lasers, however, there
is not much commercial interest in this. Since
high-power diode lasers normally suffer
from a poorer beam quality than solid-state
lasers and an inability to create super pulses,
semiconductor-based lasers at 1064nm are
only able to replace Nd:YAG lasers in low
brightness applications as in the case of
fiber-coupled, cw applications in the medical field.
The really new wavelength range for
high-power diode lasers starts beyond
1064nm. Applications range from 1210nm
which is used in laser-assisted liposuction, a
technique that supposedly destroys fat cells
and tightens skin simultaneously. This is followed by the range of 1320nm to 1380nm
(based on InP-wafers) which is essentially a
Nd:YAG wavelength formerly used in medical applications. The effect on human tissue
is based on water absorption. With the now
well-established 1470nm (also based on InP)
high-power diode lasers, such applications
show comparable, if not better results when
compared with the Nd:YAG wavelengths.
The light is absorbed by the water inside
the cells and heats the water until the cell
bursts. This is an effect which is used for tissue removal in prostate treatment (BPH =
Benign Prostatic Hyperplasia) for example.
By flushing cold water during the treatment,
the pain for the patient is reduced and the
cell debris is removed [2].
Further applications of 1470nm are in all
medical treatments which rely on water absorption of laser light. Aside from the fact that
Advantagesoflaserbeamsources 51nanoFCM-... / 51nanoFI-...
LaserSpeckles
LaserSpectrum
Noise
2.0
1.6
1.2
0.8
50%
0.4
0
0
Pnoise peak value in % 60 min
FWHM
1.5 nm
Low speckle contrast
from reduced coherence length: uniform
illumination of 4-quadrant diodes for AFM
with improved edge
detection
Broadened spectrum
(~1.5 nm FWHM) with
reduced coherence
length (~ 0.3 mm) as a
result of rF modulation. No mode hopping
occurs
rF
modulation
results in constant
mean laser power
with noise < 0.1%
rMS (<1MHz)
Disadvantagesofconventionallaserdiodesources
Noise
2.0
LaserSpectrum
LaserSpeckles
Mode hopping: temporal shifts between
modes. The coherence length changes over time. It can
be > 1 m.
The laser spot produced by a standard
laser diode beam has
a speckle pattern, increasing the statistical uncertainty in position determinations
1.6
1.2
0.8
0.4
0
0
Pnoise peak value in % 60 min
Power noise resulting from laser diode
which generates an
external cavity with
the fiber coupling
Applications
51nanoFI-...
51nanoFCM-...
A tomic
F orce
icroM scopy
1
51nanoFI-...
Back-reflection
particlemeasurement
Lasers for Adjustment
and Alignment
FIBEROPtIC
Fabry-Perot
Interferometer
2
3
Scratch Detector
51nanoFI-...
4
5
A tomic
F orce
icroM scopy
Application: Fiber-optic Fabry-Perot Interferometers with 51nanoFI-... 5
FIBER-OPtIC
Fabry-Perot
B
D
A
Interferometer
Interferometer
C
F
E
Signal (V)
785nm:
Tm3+:YAG
time (ms)
(ms)
time
Schemeofafiber-opticinterferometer
Schemeofafiber-opticinterferometer
A laser beam source
D Fc-pc fiber
51nanoFl-...
E cantilever
B Singlemode fiber
F Detector and interferometric
signal
C Fiber-optic beam splitter
For measurement of the cantilever movement, the interference
is evaluated between the partially reflected light at the fiber end
facette (approx. 4%) and the light reflected at the cantilever.
Accessories
Fiber collimators 60Fc-...
Micro focus
optic 5M-...
Vacuum Feed-throughs
V-KF-...
Fiber-optic Beam
Splitters
FBS-...
For more information see www.SuKHamburg.de/dl/fibercat_e.pdf
www.laser-journal.de LTJ 47 Kieler Straße 212 • d-22525 Hamburg
[email protected]
www.SuKHamburg.de
630 – 635, 652, 668
√
670
√
689, 730
√
Semiconductor
Lithography
√
Avionics
√
Instrumentation
Materials
Processing
√
Defense
Printing
405
DPSSL
λ [nm]
Medical
Laser beam sources √
√
√
780, Δλ<1
√
√
785, 792, 797
795, Δλ<1
√
√
805 / 808
√
√
810± 10
√
√
√
√
830
852 Δλ<1, 868-888
√
901
√
√
√
905
√
915
√
√
940
√
√
968, 973 – 976
√
√
980 ± 10
√
1064
√
1210
√
1330 – 1380
√
1450 – 1470
√
√
√
√
1530, 1650, 1700
√
√
√
√
1850-2200
√
√
√
√
√
√
Conclusion
√
Table C: Summary of high-power diode laser wavelengths and applications.
this wavelength was originally developed to
use the minimum optical absorption of glass
fibers in order to reach longer distance data
transfer in telecommunications, the usage
of high-power diode lasers in the 1470nm
range opens up applications such as plastics
welding of white polymers [3] in medical device manufacturing, as well as applications
in defense, such as turbulence-detection
in front of airplanes or optical pumping of
Er3+-doped crystals to generate laser wavelengths in the 2µm range.
Further wavelengths generated from InPbased semiconductor structures, which become especially interesting for defense applications, are 1550nm as well as 1650nm.
Such laser wavelengths are sometimes
described with the misleading term “eyesafe”, since these wavelengths are already
absorbed in the tear fluid. The point should
be made that any high-power diode laser
48 LTJ April 2010 No. 2
A series of new wavelengths for highpower diode lasers has been developed in
the last years by m2k-Laser, a spin-off company of the Fraunhofer Institute for Applied
Solid State Physics, but now part of Rofin Sinar Technologies Inc. Here, GaSb-wafers are
used to generate edge emitting solid-sate
laser structures in the wavelength range of
1800nm to 2300nm.
A variety of applications can be addressed,
ranging from welding of transparent plastics for medical device manufacturing using
the intrinsic vibronic absorption of polymer
chains to direct medical applications in surgery, relying on the even larger absorption
of water in human tissue (about three orders
of magnitude higher than at 980nm) [4].
Defense applications such as IRCM or laser range gated imaging can also be found
in this wavelength range.
The wavelength 1940nm can directly
be used for illumination, replacing Tm3+based solid-state lasers. Additional interest
is raised by the fact that optical pumping
at 1908nm of Ho3+-doped solid-state laser
crystals will result in an emission wavelength
of >2100nm, which is of high interest in defense applications.
will do harm to human tissue due to the
aforementioned water absorption.
These wavelengths can be used for il­
lumination purposes or infra-red counter
measures (IRCM), in which the infra-red
target acquisition system of an incoming
missile is misled by an active high-intensity
signal, which will override the steering cues
from the target. A further interesting application is laser range gated imaging, in
which laser pulses at 1550nm for example
are used with a gated camera system, sensitive for this wavelength range. Subsequent
pictures created by the back-scattered light
from different distances are gathered. Here,
the time-of-flight information of the light,
together with the photographic sampling
allows to “look through” clouds of smoke
or through a camouflage net, by only taking
pictures whose back-scattered time-of-flight
is related to scenes behind the obstacle.
The novel wavelengths available for highpower diode lasers open up new applications (see Table C). All power values mentioned in the achieved results are based on
application-relevant lifetime expectations.
Further investigation, improvement and
optimization for such new wavelengths is
expected to lead to an additional enhancement of the optical output power or to result
in longer lifetimes.
References
[1] Bernd Köhler et al.: “Wavelength stabilized high-power diode laser modules”;
Proc. of SPIE Vol. 7198 (2009)
[2] biolitec AG: www.biolitec.com
[3] Treffert GmbH & Co. KG: www.treffert.
org
[4] George M. Hale and Marvin R. Querry:
„Optical Constants of Water in the 200nm to 200-µm Wavelength Region,“
Appl. Opt. 12 (1973) 555
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim