Research Areas
Development of Femtosecond sources
Various types of femtosecond oscillators (ring dye laser,
antiresonant ring, Ti:sapphire linear and ring) have been
built and developed over the years in this group.
The lasers were intended as well as a research object
(to investigate various mechanisms of pulse formation
and pulse compression) as tools for various experiments.
A recent innovation of this group is to use
the laser to probe materials inside its cavity,
rather than a beam emitting blackbox.
Compact Oscillators
This group was one of the first to recognize the
essential role of dispersion in femtosecond laser cavities,
and to achieve intracavity pulse compression with prisms
[W. Dietel, J.J. Fontaine and J.-C. Diels, Optics Lett. 8,
4-6 (1983) ]. Recent work has concentrated on the control of
mode-locked combs. Various techniques of stabilization have been
devised, such as locking the repetition rate and the carrier
frequency to a reference cavity [j
Compact High Power Sources
A combination of Ti:sapphire oscillator at 744 nm,
pulse stretcher, regenerative
amplifier, post amplifier, pulse compressor, frequency trippling to 248 nm,
and an excimer amplifier provides us simultaneously with fs pulses
of millijoule energy at several wavelengths.
While being the main workhorse for a large variety of experiments,
such a laser system is complex, uneconomical, and takes a large
space in the laboratory.
We are presently developing alternate sources based on the traditional
flashlamp pumped Nd:YAG laser, and parametric oscillators and
amplifiers. The Nd:YAG laser, stabilized by passive negative feedback,
provides pulses of 10 ps duration. Synchronous
pumping of Optical Parametric Oscillators (OPO) can generate
tunable femtosecond pulses [A. Umbrasas, J. C. Diels, J. Jacob
and A. Piskarskas, "Parametric oscillation and compression in KTP crystals",
Opt. Lett. 19: 1753-1755 (1994)].
To further amplify this
tunable radiation, we have extracted a single pulse from
the Nd:YAG laser, and frequency doubled it to generate green pulses of
400 fs duration (our goal is to improve
this figure to 200 fs) of several mJ, through a simple saturated
nonlinear conversion in long (6 cm) KDP crystals
[A. Umbrasas, J. C. Diels, G. Valiulis, J. Jacob and A. Piskarskas,
"Generation of femtosecond pulses through
second harmonic compression of the output of a Nd:YAG laser",
Opt. Lett.,
20: 2228-2230 (1995)].
Stabilized Femtosecond lasers
We have shown that the spacing between modes of a fs laser
is rigourously constant over a bandwidth eceeding 250 nm.
Therefore, linear and ring femtosecond lasers are being locked
to stable reference cavities and to atomic lines.
Applications range from wavelength standard, to accurate atomic clocks,
to high precision gyroscopic measurements, to
the measurements of small magnetic fields and
femtosecond resolved measurements of small (10-9)
indices of refraction. We are developing methods to
lock the repetition rate of the laser to an integer number
of optical cycles.
The mode-locked atomic clock will
provide signals in various time ranges:
the optical frequency (wavelength standard),
the cavity repetition rate (time standard), and the beat note between
the counter-rotating beams of a ring cavity (another time standard).
In addition, the error signal will provide rotation sensing with hitherto
unprecedented sensitivity.
Sensors and Laser Gyros
Ultrashort pulse ring lasers can be viewed as active
differential interferometers: the two oppositely travelling
wave can have their repetition rate locked, but their
phase uncoupled. Applications range from the laser gyro
without dead band, to measuring displacements of less than
0.001 Å, to measuring changes in index of refraction
of less thant 10-9.
A practical laser gyro should be compact and economical.
We are investigating solid state diode pumped lasers, fiber lasers,
and synchronously pumped ring optical parametric oscillators.
A totally different approach uses multiple quantum well lasers to
create a pair of unidirectional (uncoupled) ring lasers
operating in opposite direction. Such type of ring lasers
have also been demonstrated in our laboratory.
Diagnostic Methods
All the source development cited above makes little sense without
accurate diagnostic method to analyze the amplitude and phase
of the pulses being generated.
We have developed over the years numerous types of diagnostic methods,
starting from the interferometric autocorrelation
[J.-C. Diels, J. J. Fontaine, I. C. McMichael, and F. Simoni,
Control and measurement of ultrashort pulse shapes (in amplitude and
phase) with femtosecond accuracy.
Applied Optics: 1270--1282 (1985)],
to the single shot ``femtonitpicker''
[J.-C. Diels, J. J. Fontaine, N. Jamasbi, Ming Lai and J.Mackey,
"The Femtonitpicker",
Conf. on Lasers and Electro-optics, Baltimore,
June 1987, (1987)],
to a diagnostic method
that does not use any nonlinear optics [Steffen Prein, Scott Diddams
and Jean-Claude Diels,
"Complete characterization of femtosecond pulses
using an all-electronic detector",
Optics Comm., 123: 567-573 (1996) ].
Laser Induced Discharges
We have demonstrated laser guiding across 50 cm gaps in air, at
less than 1/4 the self-breakdown voltage, with ultrashort UV laser
pulses of less than 50 mJ energy. We are investigating
a scaling up of these experiment across larger gaps.
An application being investigated is the triggering and guiding
of lightning.
Self-filamentation in air
The latter
topic led to the observation of self-filamentation in air:
the laser pulses creates its own waveguide, and propagate
without diffraction over tens of meters. We are investigating
comparatively
these effect at various wavelength ranging from the near IR to the UV.
The mechanism of formation of and stabilization of these filaments
as well as associated phenomena are totally different at 800 nm
and 248 nm.
Femtosecond Communication
Femtosecond pulses of less than 100 fs should enable digital communication
at a 10 THb rate. We have initiated experiments of time multiplexing
at the emitter,
and demultiplexing at the receiver. Fast electro-optic modulator arrays
(32 channel each) are being
tested for phase and amplitude modulation, in a pulse shaper that
transforms a single pulse of 100 fs in a train spanning less than 10 ps,
at a replenishment rate of 100 MHz.
Femtosecond communication requires accurate synchronization between
a laser at the emission and at the reception.
We are investigating optical synchronization scheme
for which we expect fs jitter, requiring only picowatt injection.
Diels' Group
Publications