OSA INTERVIEWS LEADERS IN PHOTONICS
Solid-state and fiber laser advances
lead to real-world applications
OSA: How did you get involved with
optics and lasers?
Andreas Tünnermann: I studied physics with Herbert Welling, a very inspiring professor who was a pioneer of photonics in Germany. When he worked
in the United States, he had supplied
ruby crystals to Theodore Maiman for
his laser experiments. Last December,
he presented a ruby crystal Maiman
gave him to the German Museum
For my post-PhD habilitation, I
developed an ultrastable diode-pumped
neodymium ring laser for inteferomet-ric detection of gravity waves. Bob Byer
at Stanford had introduced the technology, and we had access to the most
brilliant pump diodes then available, so
we teamed to make this very low-noise
OSA: Was that the same type of laser
used by Advanced LIGO to detect
AT: It was. Nobody had expected them
to see gravity waves so quickly. The signal really looked perfect—I had only
seen signals like that in numerical simulations. It was great.
OSA: What has made solid-state and
fiber lasers so successful?
AT: The advantages of near-infrared
solid-state lasers for non-contact materials processing were clear when I started in the early 1990s. But lamp pumping was inefficient and beam quality
was poor because of the thermal load
on the material.
Diode pumping reduced thermal
loading of the solid-state material,
but the thermo-optic effect still limited beam
quality. So we started working on different
geometries for the laser medium, including slabs
and fibers. The fiber laser’s waveguide proper-
ties are unique. The inner active core is doped
with a rare earth-like ytterbium, and defines
the beam quality. It is surrounded by a second
waveguide, which confines the pump light and
couples it into the active core.
The laser material is crucial, especially for
fibers where the power density is very high and
must withstand photodegradation. The Stokes
shift between the pump line and the laser line
in ytterbium is very small, so optical-to-optical
conversion efficiencies can reach the 90% range.
Thermal heating is so small that fiber lasers are
almost immune to thermal-optic effects.
Brilliant pump sources also are important.
Pumping fibers with high-power diodes resulted
in very good efficiency together with very good
beam quality, so fiber-laser development became
my main focus in lasers for the past 20 years. The physics is on your side in the
fiber geometry, but you also need very good engineering.
Fibers competed for high power with thin-disk lasers developed about the
same time at the German Aerospace Center and the University of Stuttgart. In
Germany, we love to play soccer, but we say you should play in the champion’s
league. In science, you need competition.
OSA: What interested you in micro- and nano-optics?
AT: Nanostructures let you optimize material properties for specific applications.
Fused-silica nanostructures called photonic-crystal fibers have been very successful in fiber lasers. Their geometry and fill factor control the refractive index,
avoiding the doping used in conventional fibers. That has a clear advantage in
is director of the Fraunhofer
Institute of Applied Optics and
Precision Engineering at
Friedrich Schiller University in
Jena, Germany. He was named a
fellow of The Optical Society for
his work and leadership in
high-power solid-state and fiber
laser technology, and his
contributions to laser
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