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Femtosecond soliton fibre lasers
A/P Tang Dingyuan
Fiber lasers are a new type of laser system made of
special optical fibre. They have the characteristics of
compact size, ultra stable operation and low cost. Various
modes of operation can be achieved in the lasers: they can
emit either high power continuous light beams, or singlefrequency
ultra-stable optical waves, or ultra stable, ultrashort
optical pulse trains.
A femtosecond soliton fibre laser generates high quality
special optical pulses with pulse duration of merely hundreds
of femtosecond. Unlike conventional optical pulses,
these special pulses, known as solitons, maintain their
pulse shape and width without any distortion in propagating
through long lengths of optical fibre, a property
that is very useful for fibre-optic communication.
If the soliton laser is further operated in combination
with chirp pulse amplification and nonlinear wavelength
conversion techniques, ultra-high peak power, frequencytunable
optical pulses can be generated. This kind of
optical pulses have wide-ranging applications in scientific
research as well as military devices and medical surgery.
Researchers in my group have conducted intensive theoretical
and experimental research on passively modelocked
soliton fibre lasers. They have developed compact,
ultra-stable femtosecond soliton fibre lasers of various
configurations and achieved transform-limited soliton
pulses of duration as short as 250 fs, and high-energy
femtosecond optical pulses of single pulse energy as large
as several tens of nJ. These lasers can be used in a number
of applications, e.g. the injection seeding, two-photon
microscopy and laser radar systems.
Beside the conventional soliton pulse laser our group is
also the first to discover a novel twin-pulse soliton emission
in the lasers. This observation not only confirms
experimentally the existence of the multi-hump temporal
optical solitons, but also makes available for the first
time a unique novel laser system that generates closely
paired optical pulses. A laser beam with such a property
may have a number of special applications, e.g. in
triggered ultrafast photography, laser spectroscopy, and
time-division multiplexing optical communication systems.
In addition, our group has also studied other features
of the soliton fibre lasers, such as the noise-like pulse
10 hotonics'a
emission in which the emitted pulses have very large energy
and ultra-broad optical spectrum. Lasers of such
property are an ideal high power broadband light source,
which may be used for testing fibre optic communication
systems and for optical metrology.
Theoretically, we have developed a model that can accurately
reproduce all features of the lasers observed experimentally.
With this model we can not only understand
the physical mechanism of a specific experimental
effect observed, but also predict and design new features
of the laser and other ultrashort pulse laser systems.
Figure 1: Femtosecond soliton fibre laser developed in NTU.
Figure 2: Optical spectrum of a double-pulse soliton laser
emission.
Figure 3: Double-pulse soliton emission of the lasers numerically
simulated.