Passive mode-locking with an intracavity
saturable absorber was first demonstrated in 1966 , six years after
the first laser was discovered. However, passive modelocking in solid-state
lasers suffered from a fundamental problem: the so-called Q-switched
modelocking behavior, where an overlying large Q-switched pulse modulated
the train of clean mode-locked pulses. This meant that the laser would
turn itself off at regular intervals. The underlying mode-locked pulses
could only be used in special circumstances and with very limited applications.
The semiconductor saturable absorber mirror (SESAM) solved this problem
more than 25 years later in 1992 for the first time for diode-pumped
solid-state lasers .
I invented and led the development of SESAMs, which are a novel family
of optical devices that allow for very simple, self-starting, passive
modelocking of ultrafast solid-state and semiconductor lasers. I invented
the first member of the SESAM family in 1992 when I was still at Bell
Labs in New Jersey, USA . This breakthrough device allowed the first
demonstration of a passively mode-locked neodymium:YLF laser - without
Q-switching. After moving to ETH Zurich in Switzerland, we developed
the theoretical underpinnings of the performance of SESAMs in solid-state
lasers, worked out design guidelines for application to practical laser
systems, and took this know-how to demonstrate unprecedented laser performance
improvements in several key directions: shortest pulse widths (in the
5-fs regime with only about two optical cycles), highest average and
peak power from a passively mode-locked laser (nanojoules extended to
more than 10 microjoules), and highest pulse repetition rate to (~1
GHz extend to >160 GHz) . Today, SESAM modelocked solid-state
lasers fulfill the requirements for industrial applications and are
being used for many different applications (see for example overview
Table 2 in Ref ).
With the help of Dr. Thomas Südmeyer (senior postdoc in my group)
and Sergio Marchese (graduate student), we were able to increase the
pulse energy of passively modelocked solid-state lasers by more than
four orders of magnitude: We demonstrated more than 10 µJ directly
out of a SESAM modelocked diode-pumped solid-state laser oscillator
, and we believe that we can scale this concept even further, well
above the 100 µJ regime. This pulse energy will enable material
processing and high field laser physics at very high pulse repetition
rates (1 – 100 MHz). Previously, such pulse energies were only
obtained with modelocked oscillators, followed by one or several amplifier
stages at much lower pulse repetition rates of ~ 1 kHz. A higher pulse
repetition rate increases the possible scan rate in material processing
(e.g. marking, waveguide writing etc.) and increases the signal-to-noise
ratio in high field physics experiments. This feature will enable many
new measurements that were not previously possible because space charge
effects smeared out the important dynamics.
This JSTQE publication was the first longer review paper describing
SESAMs and also introduced the acronym which contains all relevant information:
first it is based on a semiconductor saturable absorber material (which
is ideally suited for this application) and second the semiconductor
saturable absorber is embedded into a mirror structure. In the simplest
case this is a Bragg mirror (which was first demonstrated with a single
quantum well absorber in 1995 ). However, the SESAM concept is much
broader and can be extended to any other mirror structure. A more recent
review of SESAM design concepts is given in Ref. 7.
 A. J. D. Maria, D. A. Stetser, and H. Heynau, “Self mode-locking
of lasers with saturable absorbers,” Appl. Phys. Lett., vol. 8,
pp. 174-176, 1966.
 U. Keller, D. A. B. Miller, G. D. Boyd, T. H. Chiu, J. F. Ferguson,
and M. T. Asom, “Solid-state low-loss intracavity saturable absorber
for Nd:YLF lasers: an antiresonant semiconductor Fabry-Perot saturable
absorber,” Opt. Lett., vol. 17, pp. 505-507, 1992.
 U. Keller, “Recent developments in compact ultrafast lasers,”
Nature, vol. 424, pp. 831-838, 14.08.2003 2003.
 U. Keller, “Ultrafast solid-state lasers,” Progress
in Optics, vol. 46, pp. 1-115, April 2004.
 S. V. Marchese, S. Hashimoto, C. R. E. Bär, M. S. Ruosch, R.
Grange, M. Golling, T. Südmeyer, U. Keller, G. Lépine, G.
Gingras, B. Witzel, “Passively modelocked thin disk laser reach
10 µJ pulse energy at megahertz repetition rate and drive high
field physics experiments”, CLEO Europe 2007, Munich, Germany,
June 17-22, 2007
 L. R. Brovelli, I. D. Jung, D. Kopf, M. Kamp, M. Moser, F. X. Kärtner,
and U. Keller, “Self-starting soliton modelocked Ti:sapphire laser
using a thin semiconductor saturable absorber,” Electron. Lett.,
vol. 31, pp. 287-289, 1995.
 G. J. Spühler, R. Grange, L. Krainer, M. Haiml, V. Liverini,
M. Golling, S. Schön, K. J. Weingarten, and U. Keller, “Semiconductor
saturable absorber mirror structures with low saturation fluence,”
Appl. Phys. B, vol. 81, pp. 27-32, July 2005.
Biography: Ursula Keller
Ursula Keller joined ETH as an associate professor in 1993 and has been
a full professor of physics since 1997. She received the Ph.D. in Applied
Physics from Stanford University in 1989 and the Physics “Diplom”
from ETH in 1984. She was a Member of Technical Staff (MTS) at AT&T
Bell Laboratories in New Jersey from 1989 to 1993. Her research interests
are exploring and pushing the frontiers in ultrafast science and technology:
ultrafast solid-state and semiconductor lasers, ultrashort pulse generation
in the one to two optical cycle regime, frequency comb generation and
stabilization, reliable and functional instrumentation for extreme ultraviolet
(EUV) to X-ray generation, attosecond experiments using high harmonic
generation, and attosecond science. She has published more than 260
peer-reviewed journal papers and 11 book chapters and she holds or has
applied for 18 patents. She was a “Visiting Miller Professor”
at UC Berkeley in 2006 and a visiting professor at the Lund Institute
of Technologies in 2001. She received the Philip Morris Research Award
in 2005, the first-placed award of the Berthold Leibinger Innovation
Prize in 2004, and the Carl Zeiss Research Award in 1998. She was the
“2006 Ångström lecturer” supported by the Royal
Swedish Academy of Sciences and the LEOS Distinguished Lecturer for
modelocked solid-state lasers in 2000. The Thomson Citation Index highlighted
her as the third-place top-cited researcher during a decade (1991-1999)
in the field of optoelectronics in 2000. She is an OSA Fellow and an
elected foreign member of the Royal Swedish Academy of Sciences.