Assessment of a Rubidium ESFADOF Edge-Filter as ... - tuprints

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26 2 Remote Sensing of the Water Column: The Brillouin-Lidar the Brillouin-peaks. However, a second master oscillator is required then. Its emitted radiation has to be amplified and frequency converted as described. But, as the frequency of the local oscillator has to be known and held fixed on a MHz scale with respect to the frequency of the injected laser pulses, an additional locking–mechanism is required, which considerably increases the complexity of the detection scheme. A promising alternative to actively shifting the emission wavelength of the local oscillator is the employment of a temperature and salinity controlled water sample on board the aircraft. This makes the need of a second master oscillator obsolete, in case the cw output of the initial master oscillator is amplified and frequency converted. By injecting this green cw output into the calibrated water sample, the spontaneous Brillouin–scattering inside the water sample can be exploited, as it is presicely known. The so generated known Brillouin– scattering can be successively mixed with the unknown Brillouin–scattering signal from the ocean. The resulting beat–signal on the photodetector, can be evaluated as above in order to extract the temperature profile of the oceanic water column. In addition to the Brillouin–shift, the heterodyne technique delivers also the width of the Brillouin–lines, so that the above mentioned temperature and salinity dependency of the Brillouin–width (cf. Eq. 2.23) can be explored. Schemes 2 and 3 are referred to as direct detection schemes and scheme 4 as heterodyne or coherent detection scheme and a thorough comparison of these schemes can be found in Ref. [45]. There it is shown that, regardless whether scheme 2 or 3 is employed as edge-filter, the accuracies of the direct and coherent detection scheme are nearly equivalent, when employing state-of-the-art technologies. They almost reach the shot–noise limit for a wide range of signal intensities and the accuracy is dominated by the spectral width and the intensity of the return signal. The two intensity extrema are of particular interest: Coherent– lidars profit from an enhancement of the beat–signal due to the presence of the local oscillator, so that they are advantageous for low intensities. Direct detection lidars are superior when high return intensities lead to disturbing coherent speckle–formation [45]. In conclusion, as a practical receiver system has to measure the Brillouin– shift ν B on a ns time scale, while being able to reliably operate within the harsh environment on board an aircraft, only direct detection schemes, which employ edge-filters, or the heterodyne technique are accurate and fast enough, while they are sufficiently insensitive to vibrations. However, in view of the discussed complexity of the heterodyne technique, when regarding the measurement of the Brillouin–shift, a direct detection scheme seems more advantageous. This work is embedded in the framework of the proposal of Th. Walther et al. and investigates the employment of an excited state Faraday anomalous dispersion optical filter (ESFADOF) as edge–filter within the direct detection scheme. This approach offers several advantages: (1) The employment of an ESFADOF allows to tailor the edge-filter characteristics to the needs of the measurement, so that the symmetry
2.3 Ideal Edge–Filter 27 of the Brillouin–backscatter can be exploited, which increases the signal/noise– ratio considerably. (2) The system is insensitive to vibrations, as resonators are not required, and exhibits a high light gathering power. (3) The signal processing allows single shot profiling on a ns time scale, while keeping the complexity reasonable. 2.3 Ideal Edge–Filter The previous section discussed the system requirements towards a practical Brillouin–lidar remote sensing technique of oceanic temperature profiles. Particular focus has been spent on the Brillouin–scattering and the spectral profile of the backscatter, which can be explored as a temperature tracer. Different detection schemes are conceivable in order to extract the Brillouin–shift and hence the temperature information (cf. Sec. 2.2.4 and Fig. 2.6). This subsection is dedicated to the direct detection scheme, which is one of the most promising techniques. Suitable edge-filters are excited state Faraday anomalous dispersion optical filters (ESFADOFs). Their working principle, their implementation and assessment are the subject of this work. However, a thorough investigation of the ideal edge–filter reveals important insight for an optimized system design. The symmetric square well transmission spectrum, is the ideal edge–filter, as the dynamic range of the Brillouin–shift ν B barely extends to twice the Brillouin– width δν B for water temperatures of interest (0 ◦ C–40 ◦ C; compare Figs. 2.3–2.5). The symmetry cancels small frequency fluctuations of the probe laser. The transmission spectrum of this ideal edge–filter follows F(∆ν) = { Θ(−(∆ν − ∆ν Edge )) −Θ(−(∆ν − 2∆ν Edge )) } + (2.24) { Θ(∆ν − ∆νEdge ) −Θ(∆ν − 2∆ν Edge ) } and is depicted in the inset of Fig. 2.7. Θ(∆ν) represents the Heaviside unit step function [89] and the terms in curly brackets correspond to boxcar functions, such that the overall transmission beyond the region of interest of [−2∆ν Edge ,2∆ν Edge ] vanishes. The position of the transmission edges has been chosen to ∆ν Edge = 7.5 GHz, which lies well within the region of interest of ±6.8 − 7.8 GHz and delivers the steepest transmission signal with respect to a temperature change. According to Fig. 2.6, the signal of the direct detection scheme, which employs an edge–filter, is the ratio between the transmitted and collected intensity S 1 = T(ν B ) = I PMT-2 /I PMT-1 . Any corrections, as e.g. the sensitivities of the employed photomultiplier tubes, have been omitted for clarity. They can be easily integrated [45]. However, any photomultiplier tube spectrally integrates the recorded intensity and the signal S 1 becomes S 1 (T,S) = 1 ∫ F(∆ν) S L B 2I (∆ν,ν B(T,S),δν B (T,S),δν L ) d∆ν. (2.25) B δν L = 63 MHz accounts, according to Eq. 2.20, for the spectral width of the probe laser pulses. ν B (T,S) and δν B (T,S) follow Eqs. 2.9, 2.12, 2.13 and 2.21
Page 1 and 2: INSTITUT FÜR ANGEWANDTE PHYSIK TEC
Page 3: Abstract Global and local climate c
Page 7: Pentru Alexandru şi Florina
Page 10 and 11: II Contents 4.3 Measurement Unit .
Page 12 and 13: 2 1 Introduction wide sea currents
Page 15 and 16: 2 Remote Sensing of the Water Colum
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Page 21 and 22: 2.2 System Requirements 11 operatin
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Page 25 and 26: 2.2 System Requirements 15 Table 2.
Page 27 and 28: 2.2 System Requirements 17 1. Rayle
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76 4 Experimental Investigations of
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5 Discussion of the Experimental Re
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5.1 Overview of the Experimental Pa
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5.1 Overview of the Experimental Pa
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5.2 Vapor Cell I: 270 mT 95 5.2 Vap
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5.2 Vapor Cell I: 270 mT 97 0.20 5.
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5.2 Vapor Cell I: 270 mT 99 Fig. 5.
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5.2 Vapor Cell I: 270 mT 101 introd
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5.2 Vapor Cell I: 270 mT 103 Fig. 5
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5.2 Vapor Cell I: 270 mT 105 which
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0.15 ∆ν P = 0.98(2) GHz 5.2 Vapo
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5.3 ESFADOF Operational Limits 109
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cesses of the atomic vapor. This al
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Fig. 5.18: Rubidium Grotian energy
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5.4 Vapor Cell II: 500 mT 133 maxim
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5.4 Vapor Cell II: 500 mT 137 such
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6 Conclusion and Outlook The presen
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6 Conclusion and Outlook 141 pulses
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6 Conclusion and Outlook 143
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Appendix
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148 A Rubidium Atom 2. Relevant ato
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B Manufacturing process of Rb Vapor
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B Manufacturing process of Rb Vapor
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C Magnetic Field Strengths of the E
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D Tripod ECDL Fig. D.1: Schematic o
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160 References [10] G.L. Mellor and
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162 References [41] E.S. Fry, J. Ka
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164 References [69] Claude C. Leroy
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166 References [97] L. Goldberg, D.
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168 References [125] Cord Fricke-Be
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170 References [158] Triad Technolo
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172 References [186] M. Cheret, L.
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Curriculum Vitae Alexandru Lucian P
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178 1 Publications Conference Proce
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Supervised Theses Denise Stang, Wei
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184 1 Danksagung Bei Herrn Arno Wei
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