7 research outputs found
High gain ultra-short laser pulse raman amplification in plasma
No full textThis thesis presents an experimental study of broad-band Raman chirped pulse amplification in plasma. Amplification. of a short seed pulse occurs through stimulated Raman backscattering in the presence ofa counter-propagating chirped pump pulse in a preformed plasma waveguide channel. 300 and 200 Jlm diameter, 40 rom long, hydrogen-filled capillary discharge waveguides have been investigated. The plasma channel has the dual purpose of guiding the laser pulses over distances of many Rayleigh lengths and acting· as the nonlinear medium for the Raman instability. Laser energy transmission through the plasma channel, in single mode propagation, is observed ~o be better than 80 % for the pulses with intensities up to 5.8 X 1014 W cm-2. The dependence of the gain on pump intensity and plasma density is experimentally studied and confirms theoretical predictions. It is shown that higher pump intensities inside the capillary can be realised using unmatched guiding due to periodic oscillations ofthe laser beam waist along the plasma channel. Amplification of the seed pulse in the linear regime is obtained when the frequency detuning between the seed and the pump matches th~ plasma frequency. Thus the frequency difference between the pump and gain peaks enables to estimate the plasma density, measured around 1018 cm-3. The high gain regime has been investigated showing that peak gains of over 1200 % can be routinely realised. A peak gain of almost 3000 % has 'been observed under optimal conditions with the highest densities and pump energies. Several diagnostic systems have been developed for simultaneous detection of the spectral and energy gain together with the duration of the seed. Ultrashort pulse high gl;lin amplification of 80 fs, 5 mJ pulse has been demonstrated, giving a peak value of 800% and energy gain of 40 % together with 40 % increase in pulse duration. Amplification ofthe whole seed pulse spectra has been demonstrated using an optical cut-off filter to provide a sufficient detul1ed seed pulse. Future increase of the efficiency will occur in the nonlinear regime of the stimulated Raman backscattering resulting in compression ofthe seed pulse.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
Chirped pulse Raman amplification in plasma
No full textRaman amplification in plasma has been proposed to be a promising method of amplifying short radiation pulses. Here, we investigate chirped pulse Raman amplification (CPRA) where the pump pulse is chirped and leads to spatiotemporal distributed gain, which exhibits superradiant scaling in the linear regime, usually associated with the nonlinear pump depletion and Compton amplification regimes. CPRA has the potential to serve as a high-efficiency high-fidelity amplifier/compressor stage
Spectral sensitivity data for various wearable light sensing actigraphy devices
No full textRelative sensitivity to spectral irradiance with narrowband (monochromatic) ligh
Measurements of UV—A Exposure of Commercial Pilots Using Genesis-UV Dosimeters
No full textA number of studies suggest that pilots are at twice the risk of melanoma and keratinocyte skin cancers than the general population, and that they have a raised mortality from melanoma. No conclusive links with in-flight exposure to ionising radiationor circadian rhythm disruption due to the pilots’ shift work were found. Possible over-exposure to ultraviolet radiation (UVR) may be implicated as pilots may be exposed to higher UV-A levels at cruise altitude compared with those at ground levels. The direct method of making in-flight spectral measurements has been carried out on a limited number of flights, but this technique is challenging; the use of small wearable sensors may be more appropriate but there are a few issues that should be addressed for their use in cockpit measurements. While the spectral response of sensors for erythema effective values usually closely matches the corresponding weighting function, the response of UV-A sensors may not be spectrally flat, which, if not corrected to account for the transmission of the aircraft windshield, could potentially result in large errors. In this paper, the spectral correction method was applied to the UV-A sensor of the Genesis-UV unit to measure UVR exposure of commercial pilots on 312 flights to a range of destinations from four UK airports from September 2016 to August 2017
