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the volume Bragg gratings that narrow
the spectral line of the diode laser to a
range that is useful for the particular ap-
plication, custom package design that
permits tunability of the spectral line
emitted from the module, and the cus-
tom optics that form the beam to a precise output form also demanded by the
application.
Patterson provides background on
the MRI module (see Fig. 1), explaining
that it permits the hyperpolarization of a
particular isotope of xenon using a process called spin-exchange optical pumping; the spectrally narrowed and precise
wavelength of circularly polarized laser
light from the module excites rubidium
electrons inside a gas cell. The use of hy-perpolarized gas in MRI allows observa-
tion of otherwise inaccessible bodily pro-
cesses, such as pulmonary function and
blood profusion in the brain. The module itself emits 200 W of circularly polarized light at a 794.7 nm wavelength
and a linewidth of <0.3 nm; the opti-
cally isolated beam is tolerant to 100%
backreflection.
Direct diode
While laser-diode bars are used widely as laser pump sources, by using the
right optical techniques, light from the
same sorts of diodes can be directly
coupled into optical fibers to produce a
high-power “direct-diode” laser, notes
Tracey Ryba, product manager for la-
sers at the TRUMPF Laser Technology
Center (Plymouth, MI). For example, the
same 940 nm diode-bar platform used by
TRUMPF as pumps for its disk lasers is
the source for the company’s kilowatts-level direct-diode systems.
Optical techniques for combining laser-diode outputs to boost power while
maintaining brightness include spatial
and spectral beam combining. However,
the details, which are derived from care-
ful optical design and which make all the
difference in final beam brightness, are
usually proprietary.
The diode bars in TRUMPF’s direct-
diode systems are mounted on a pas-
sively cooled heat sink, eliminating the
need for high-quality deion-
ized (DI) water and elimi-
nating microchannel failure,
explains Ryba (see Fig. 2).
“The final fiber-coupled direct-diode la-
ser uses wavelength combining of one,
two, or three wavelengths depending on
power of the laser and varying in wave-
lengths from 920 nm to 1020 nm,” he
says. The company’s 150 and 300 W di-
rect-diode lasers have a beam-parame-
ter product (BPP) of 8 mm-mrad, while
the BPP for the 600 and 900
W lasers is 12 mm-mrad. The
high-power 2000–4500 W la-
sers have a BPP of 30 mm-mrad
and the highest-power 3000-
6000 W model has a BPP of
50 mm-mrad.
“Lasers ranging from 150 to
300 W typically are used for
plastic welding, soldering, and oth-
er low-power welding applications,”
says Ryba. “Between 600 and 900 W,
the ideal applications are thin-met-
al welding and low-volume cutting.
The 30 mm-mrad models are used for
deep-penetration and heat-conduction
welding, heat treating, and laser metal
FIGURE 2. Light from high-power laser-
diode bars is optically combined and fed into
optical fibers (yellow) in this direct-diode light
source unit from TRUMPF used for materials
processing. A 6000 W direct-diode laser is
shown in the inset. (Courtesy of TRUMPF)
