For many applications a frequency stabilized beam source with high output power and a good beam quality is
needed. Tapered lasers and amplifiers can provide a high output power, whereas they have a slightly lower beam
quality than ridge lasers. In a single mode fiber (SMF) coupled module, the beam quality provided by the
module is predetermined by the fiber. The technological progress of tapered lasers should allow a high enough
coupling efficiency to give SMF coupled modules using a tapered laser or amplifier the potential for a higher
output power than modules using a ridge laser.
It will be shown how this potential can be exploited by using different coupling systems for example with
cylindrical lenses either crossed or in combination with rotational lenses. The advantages, problems and coupling
results of those systems will be illustrated.
Two approaches of frequency stabilization will be shown. To stabilize a tapered amplifier the external cavity has
been set up by a fiber bragg grating on the backside of the amplifier. A volume holographic grating, which is
written in the fast axis collimation lens of the coupling system, was used to stabilize a tapered laser.
Laser modules for single mode fiber (SMF) coupling of frequency stabilized diode lasers are so far mainly
realized with ridge lasers due to their good beam quality. Tapered lasers are beam sources with a beam quality
which is nearly as good as that of a ridge laser but with a higher optical output power. Therefore they have the
potential for a higher SMF-coupled power than ridge lasers. It will be shown how the radiation of a tapered laser
or amplifier can be frequency stabilized and coupled into a SMF in a compactly build module.
To couple a tapered laser different coupling systems, using cylindrical lenses either crossed or in combination
with rotational lenses are possible. The advantages, problems and coupling results of those systems will be
For many applications it is necessary to stabilize the frequency of the laser. This can be achieved for example by
a fiber bragg grating, written in the SMF in which the laser is coupled or by a volume holographic grating,
applied to a lens in the coupling system. Another possibility is the use of a tapered amplifier, which is stabilized
by a fiber bragg grating on the backside of the amplifier.
The common wavelength regime for high-power diode laser modules is the range between 800 nm and
1000 nm. However, there are also many applications that demand for a wavelength of around 2 &mgr;m.
This wavelength range is extremely interesting for applications such as the processing of plastics,
medical applications as well as environmental analytics. The interest in lasers with this wavelength is
based on the special absorption characteristics of different types of material: Numerous plastics
possess an intrinsic absorption around 2 &mgr;m, so that the use of additives is no longer necessary. This is
of great value especially for medical-technical products, where additives require a separate approval.
Furthermore the longer wavelength allows the processing of plastics which are clear and transparent at
the visible. In addition, water, which is an essential element of biologic soft tissue, absorbs radiation at
the wavelength about 2 &mgr;m very efficiently. As radiation of this wavelength can be guided by glass
fibers, this wavelength may be very helpful for laser surgery.
Currently available lasers at the spectral range about 2 &mgr;m are solid-state lasers based on Ho- and Tmdoped
crystals. These systems suffer from high purchase costs as well as size and weight. In contrast
to this, diode lasers can be built more compact, are much cheaper and more efficient.
For this background, GaSb based high-power laser diodes for the wavelength regime of 1.9 - 2.3 &mgr;m
are developed at the Fraunhofer Institute for Solid State Physics (IAF). At the Fraunhofer Institute for
Laser Technology (ILT), fiber-coupled laser diode modules based on these laser bars are designed and
realized. A first module prototype uses two laser bars with a wavelength of 1.9 &mgr;m to provide an
output power of approx. 15 W from a 600 &mgr;m, NA 0.22 fiber. The module setup as well as the
characteristics of the laser bars at 1.9 &mgr;m wavelength are described in this paper.