671 research outputs found
Diacritization as a machine translation problem and as a sequence labeling problem
In this paper we describe and compare two techniques for the automatic diacritization of Arabic text: First, we treat diacritization as a monotone machine translation problem, proposing and evaluating several translation and language models, including word and character-based models separately and combined as well as a model which uses statistical machine translation (SMT) to post-edit a rule-based diacritization system. Then we explore a more traditional view of diacritization as a sequence labeling problem, and propose a solution using conditional random fields (Lafferty et al., 2001). All these techniques are compared through word error rate and diacritization error rate both in terms of full diacritization and ignoring vowel endings. The empirical experiments showed that the machine translation approaches perform better than the sequence labeling approaches concerning the error rates
Spectral broadening of 2 mJ femtosecond pulses in a compact air-filled convex-concave multi-pass cell
Multi-pass cell (MPC) based temporal pulse compressors have emerged in the
last years as a powerful and versatile solution to the intrinsic issue of long
pulses from Yb-based high-power ultrafast lasers. However, the spectral
broadening of high-energy (typically more than 100 uJ) pulses has only been
realized in complex setups, i.e., in large and costly, pressure-controlled
vacuum chambers to avoid strong focusing, ionization, and damage on the
mirrors. Here, we present spectral broadening of 2 mJ pulses in a simple and
compact (60 cm long) multi-pass cell operated in ambient air. Instead of the
traditional Herriott cell with concave-concave (CC/CC) mirrors, we use a
convex-concave (CX/CC) design, where the beam stays large at all times allowing
both to minimize damage and operate in ambient air. We demonstrate spectral
broadening of 2.1 mJ pulses at 100 kHz repetition rate (200 W of average power)
from 2.1 nm (pulse duration of 670 fs) to a spectral bandwidth of 24.5 nm,
supporting 133 fs pulses with 96% transmission efficiency. We show the
compressibility of these pulses down to 134 fs, and verify that the spectral
homogeneity of the beam is similar to previously reported CC/CC designs. To the
best of our knowledge, this is the first report of a CX/CC MPC compressor,
operated at high pulse energies in air. Because of its simplicity, small
footprint and low cost, we believe this demonstration will have significant
impact in the ultrafast laser community.Comment: The following article has been submitted to Optics Letters. 4 pages,
4 figure
Temperature-Dependent THz Properties and Emission of Organic Crystal BNA
As high-average power ultrafast lasers become increasingly available for
nonlinear conversion, the temperature dependence of the material properties of
nonlinear crystals becomes increasingly relevant. Here, we present
temperature-dependent THz complex refractive index measurements of the organic
crystal BNA over a wide range of temperatures from 300 K down to 80 K for THz
frequencies up to 4 THz for the first time. Our measurements show that whereas
the temperature-dependent refractive index has only minor deviation from room
temperature values, the temperature-dependent absorption coefficient decreases
at low temperature. We additionally compare these measurements with conversion
efficiency and spectra observed during THz generation experiments in the same
temperature range and using the same crystal, using an ultrafast Yb-laser for
excitation. Surprisingly, the damage threshold of the material does not improve
significantly upon cooling, pointing to a nonlinear absorption mechanism being
responsible for damage. However, we observe a significant increase in THz yield
at lower temperatures, which is most likely due to the reduced THz absorption.
These findings will be useful for future designs of high average power pumped
organic-crystal based THz-TDS systems
Photoconductive receivers at 1030 nm for high average power pulsed THz detection
In the last few years, many advances have been made in the demonstration of
high-average power pulsed THz sources; however, little effort has been made to
study compatible sensitive field-resolved detectors. Here, we investigate
ErAs:InAlGaAs photoconductive receivers optimized for a probe wavelength of
1030 nm and thus suitable for the new class of high-power ultrafast
Ytterbium-based laser sources for THz generation and detection. The performance
of the receiver is tested with a few-cycle THz source with high average power
up to 20 mW and the dynamic range and saturation behavior of the receiver is
thoroughly characterized. Under optimized settings, a dynamic range of more
than 115 dB is reached in a 120 s measurement time with 20 mW of THz average
power, which is among the highest reported values to date. By reviewing the
state-of-the art in TDS measurement and post-processing technology, we identify
current limitations and guidelines for further increasing the dynamic range
towards 150 dB in short measurement times using high average power THz systems.Comment: 14 pages, 7 figures plus biograph
Broadband THz-TDS with 5.6 mW average power at 540 kHz using organic crystal BNA
We demonstrate efficient optical rectification in the organic crystal BNA
(N-benzyl-2-methyl-4-nitroaniline), driven by a temporally compressed,
commercially available industrial Yb-laser system operating at 540 kHz
repetition rate. Our THz source reaches 5.6 mW of THz average power driven by
4.7 W, 45 fs pulses and the resulting THz-TDS combines a very broad bandwidth
of 7.5 THz and a high dynamic range of 75 dB (in a measurement time of 70 s).
The conversion efficiency at maximum THz power is 0.12%. To the best of our
knowledge, this is the highest THz power so far demonstrated with BNA, achieved
at a high repetition rate, and enabling to demonstrate a unique combination of
bandwidth and dynamic range for THz-spectroscopy applications.Comment: 5 pages, 5 figures. Submitted to APL Photonic
Stakeholders' perceptions on sustainability transition pathways of the cocoa value chain towards improved livelihood of small-scale farming households in Cameroon
Given the persistent poor livelihood of cocoa-farming households, future climate predictions and the worldwide demand pressure for higher cocoa quality and productivity, there is still a strong need to find new approaches that guarantee a sustainable cocoa future in cocoa-producing countries amongst which Cameroon is one of them. This exploratory research investigates potential future pathways for the cocoa sector in Cameroon by mapping the perceptions of actors involved in the socio-technical regime. Qualitative expert interviews, structured questionnaires and field observation, as well as a focus group discussion have been applied to understand how a sustainability transition can be triggered. The study shows that actors envisage a sustainability change which determines their actions; however, their perceptions towards future transitions are not actively coordinated. Actors are not finding a way of adopting new organizational structures and letting a transition occur effectively, like in the case of certification standards. An alignment of perceptions and activities, and a stronger cooperation between the private and public are strongly recommended. The study encourages to consider a coordination of actors' perceptions towards future scenarios as a starting point to study sustainability transitions
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