380 research outputs found
Using ultra-short pulses to determine particle size and density distributions
We analyze the time dependent response of strongly scattering media (SSM) to
ultra-short pulses of light. A random walk technique is used to model the
optical scattering of ultra-short pulses of light propagating through media
with random shapes and various packing densities. The pulse spreading was found
to be strongly dependent on the average particle size, particle size
distribution, and the packing fraction. We also show that the intensity as a
function of time-delay can be used to analyze the particle size distribution
and packing fraction of an optically thick sample independently of the presence
of absorption features. Finally, we propose an all new way to measure the shape
of ultra-short pulses that have propagated through a SSM.Comment: 15 pages, 29 figures, accepted for publication in Optics Express will
update with full reference when it is availabl
Surface acoustic waves for acousto-optic modulation in buried silicon nitride waveguides
We theoretically investigate the use of Rayleigh surface acoustic waves
(SAWs) for refractive index modulation in optical waveguides consisting of
amorphous dielectrics. Considering low-loss SiN waveguides with a
standard core cross section of 4.40.03 m size, buried 8 m
deep in a SiO cladding we compare surface acoustic wave generation in
various different geometries via a piezo-active, lead zirconate titanate film
placed on top of the surface and driven via an interdigitized transducer (IDT).
Using numerical solutions of the acoustic and optical wave equations, we
determine the strain distribution of the SAW under resonant excitation. From
the overlap of the acoustic strain field with the optical mode field we
calculate and maximize the attainable amplitude of index modulation in the
waveguide. For the example of a near-infrared wavelength of 840 nm, a maximum
shift in relative effective refractive index of 0.7x10 was obtained for
TE polarized light, using an IDT period of 30 - 35 m, a film thickness of
2.5 - 3.5 m, and an IDT voltage of 10 V. For these parameters, the
resonant frequency is in the range 70 - 85 MHz. The maximum shift increases to
1.2x10, with a corresponding resonant frequency of 87 MHz, when the
height of the cladding above the core is reduced to 3 m. The relative
index change is about 300-times higher than in previous work based on
non-resonant proximity piezo-actuation, and the modulation frequency is about
200-times higher. Exploiting the maximum relative index change of
1.210 in a low-loss balanced Mach-Zehnder modulator should allow
full-contrast modulation in devices as short as 120 m (half-wave voltage
length product = 0.24 Vcm).Comment: 19 pages, 8 figure
Supercontinuum generation in media with sign-alternated dispersion
When an ultrafast optical pulse with high intensity is propagating through
transparent material a supercontinuum can be coherently generated by self-phase
modulation, which is essential to many photonic applications in fibers and
integrated waveguides. However, the presence of dispersion causes stagnation of
spectral broadening past a certain propagation length, requiring an increased
input peak power for further broadening. We present a concept to drive
supercontinuum generation with significantly lower input power by counteracting
spectral stagnation via alternating the sign of group velocity dispersion along
the propagation. We demonstrate the effect experimentally in dispersion
alternating fiber in excellent agreement with modeling, revealing almost an
order of magnitude reduced peak power compared to uniform dispersion.
Calculations reveal a similar power reduction also with integrated optical
waveguides, simultaneously with a significant increase of flat bandwidth, which
is important for on-chip broadband photonics.Comment: Main text and supplementary informatio
Electron bunch injection at an angle into a laser wakefield
External injection of electron bunches longer than the plasma wavelength in a
laser wakefield accelerator can lead to the generation of femtosecond
ultrarelativistic bunches with a couple of percent energy spread. Extensive
study has been done on external electron bunch (e.g. one generated by a
photo-cathode rf linac) injection in a laser wakefield for different
configurations. In this paper we investigate a new way of external injection
where the electron bunch is injected at a small angle into the wakefield. This
way one can avoid the ponderomotive scattering as well as the vacuum-plasma
transition region, which tend to destroy the injected bunch. In our
simulations, the effect of the laser pulse dynamics is also taken into account.
It is shown that injection at an angle can provide compressed and accelerated
electron bunches with less than 2% energy spread. Another advantage of this
scheme is that it has less stringent requirements in terms of the size of the
injected bunch and there is the potential to trap more charge
Ultrafast, low-power, all-optical switching via birefringent phase-matched transverse mode conversion in integrated waveguides
We demonstrate the potential of birefringence-based, all-optical, ultrafast
conversion between the transverse modes in integrated optical waveguides by
modelling the conversion process by numerically solving the multi-mode coupled
nonlinear Schroedinger equations. The observed conversion is induced by a
control beam and due to the Kerr effect, resulting in a transient index grating
which coherently scatters probe light from one transverse waveguide mode into
another. We introduce birefringent phase matching to enable efficient
all-optically induced mode conversion at different wavelengths of the control
and probe beam. It is shown that tailoring the waveguide geometry can be
exploited to explicitly minimize intermodal group delay as well as to maximize
the nonlinear coefficient, under the constraint of a phase matching condition.
The waveguide geometries investigated here, allow for mode conversion with over
two orders of magnitude reduced control pulse energy compared to previous
schemes and thereby promise nonlinear mode switching exceeding efficiencies of
90% at switching energies below 1 nJ
Photo-electric effects in X-ray preionization for excimer laser gases
We present detailed measurements on the x-ray preionization electron density in a discharge chamber filled with different gases relevant to discharge pumped high-pressure excimer lasers. By comparing experimental results with the theoretical electron densities, we conclude that the observed preionization is inconsistent with the standard picture of direct ionization through x-ray absorption in the gas. We conclude that depending on the gas pressure, type of gas, and the gap length between the discharge electrodes used, x-ray preionization in excimer gases is, to a significant extent or even dominantly, based on a different mechanism. We identify that this mechanism is based on fast photoelectrons emitted by the cathode into the discharge chamber
Frequency stability of a self-phase-locked degenerate continuous-wave optical parametric oscillator
The properties of a self-phase-locked by-2-divider optical parametric oscillator are presented. A locking range of up to 156 MHz is measured, and the divider's relative frequency stability is shown to be better than 6/spl times/10/sup -14/
A gain-coefficient switched Alexandrite laser
We report on a gain-coefficient switched Alexandrite laser. An electro-optic
modulator is used to switch between high and low gain states by making use of
the polarization dependent gain of Alexandrite. In gain-coefficient switched
mode, the laser produces 85 ns pulses with a pulse energy of 240 mJ at a
repetition rate of 5 Hz.Comment: 8 pages, 5 figure
Storage by trapping and spatial staggering of multiple interacting solitons in -type media
In this paper we investigate the properties of self induced transparency
(SIT) solitons, propagating in a -type medium. It was found that the
interaction between SIT solitons can lead to trapping with their phase
preserved in the ground state coherence of the medium. These phases can be
altered in a systematic way by the application of appropriate light fields,
such as additional SIT solitons. Furthermore, multiple independent SIT solitons
can be made to propagate as bi-solitons through their mutual interaction with a
separate light field. Finally, we demonstrate that control of the SIT soliton
phase can be used to implement an optical exclusive-or gate.Comment: 7 pages, 7 figure
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