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108 5 Discussion of the Experimental Results The last row of Figs. 5.10 and 5.11 evaluate the symmetry quantitatively by plotting the difference between the first derivative and its symmetric inversion Θ S (∆ν,∆ν S ) = ∂ ∆ν T(∆ν) − ∂ ∆ν T(−(∆ν − 2∆ν S )). (5.10) By minimizing ∫ |Θ S (∆ν,∆ν S )|d∆ν with respect to ∆ν S , the symmetry point ∆ν S = −150 MHz results. The plot shows, that even in the worst-case scenario only fluctuation of 4.6 × 10 -5 MHz -1 and 3.8 × 10 -5 MHz -1 arise for pump detunings of ∆ν P = 0.58(2) GHz and ∆ν P = 0.98(2) GHz respectively. These values are already a factor of 1.75 better than employing only one single edge. However, evaluating the same entity between -1.3 GHz and 0.9 GHz, where the highest degree of symmetry has been achieved, reveals maximum transmission fluctuations of only 6 × 10 -6 MHz -1 and 4 × 10 -6 MHz -1 for the same pump detunings. Hence, a significant increase in signal accuracy results intrinsically from the symmetric transmission edges. Precisely speaking, the signal accuracy increases by a factor of at least 12.5 and 17.5 for the same pump detunings. Conversely, when considering the achieved transmission accuracy of 2.5 × 10 -4 , these values translate to a tolerable laser frequency jitter of 42 MHz and 63 MHz without degrading the current accuracy limitations. The demonstrated symmetry of the ESFADOF transmission spectra and the accompanied increase in signal accuracy, when exploiting the symmetry of the Brillouin-doublet is an important result. The above discussion proves, that laser frequency fluctuations can be neglected as long as they do not extend the values of tolerable laser frequency jitter. This result is very comfortable, as no sophisticated laser stabilization is necessary to guarantee the achieved transmission accuracy of 2.5 × 10 -4 . Current available distributed feedback laser diodes provide sub-MHz linewidths when temperature and injection current stabilized without any other external locking mechanism [179]. However, the current design specifications of the Brillouin-lidar demand for 10 ns, Fourier-limited pulses in order to guarantee a spatial resolution of 1 m within the water column. Due to the Fourier-limit the frequency bandwidth of the laser pulses is at least 44 MHz [16, 54], which lies well within the discussed accuracy. 5.3 ESFADOF Operational Limits The last section discussed the influence of the accessible control parameters in order to optimize the ESFADOF transmission spectrum. As expected, the pump geometry has an enormous influence on the population of the lower ESFADOF state and thus on the ESFADOF transmission spectrum. Within certain limits, which depend on the vapor cell temperature, the injected pump intensity and the spectral overlap between the pump laser and the 5S 1/2 → 5P 3/2 pump transition, stable ESFADOF operation with considerable transmissions of up to 17% has been demonstrated. As it has been shown, exceeding these limits results in a collapse of the ESFADOF spectra. Beyond the mentioned limits a laser-induced plasma is maintained by the pump laser and the plasma considerably consumes
5.3 ESFADOF Operational Limits 109 the 5P 3/2 population, which explains the transmission breakdown. The possibility of inducing a plasma in alkali-metal vapors is particularly remarkable, as it demands only relatively low laser intensities of the order of a few hundred W/cm 2 , a fact, which has been noted and elaborated already by Measures in 1969. His proposed theoretical description of the laser-induced plasma in alkali-metal vapors [180] is the basis of current theoretical predictions [181, 182]. Lucatorto and McIlrath demonstrated in 1976 the formation of a laser-induced plasma in Sodium vapor [183, 184]. They have been unaware of the work of Measures. However, Measures subsequently published in 1977 a theoretical description of the observations of Lucatorto and McIlrath [185]. In addition, Chéret, Barbier and their co–workers investigated the laser-induced ionization of Rb vapors systematically [140, 186–188]. Besides these first investigations a large amount of authors contributed to the understanding of the physics behind laser-induced plasma in alkali-metal vapors throughout the decades. It would certainly exceed the purpose of this thesis to reference every single contribution. However, to the best knowledge of the author, the review articles of Bahns et al. [189] and Stwalley and Bahns [190] offer the best overview on this topic and a very extensive bibliography. In addition, A. C. Tam extensively reported on plasma formation in alkali metal vapors in 1982 [191] and Papas et al. published some recent results [192]. However, this highly nonlinear coupled system is still an interesting research field and the development of ultra cold atomic gases also made the physics of ultra cold neutral plasma visible. This is a new research field along the frontiers among plasma, atomic and solid state physics [193]. The main purpose of the following discussion is an overall understanding of the ESFADOF operational limits, which are dominated by the laser-induced plasma. Sec. 5.3.1 presents that the ESFADOF transmission breakdown is accompanied by a pronounced hysteresis; Sec. 5.3.1 will explain the physical processes which maintain the plasma and Sec. 5.3.3 ends with an overview of the ESFADOF operation along the plasma maintenance threshold. By operating the ESFADOF device below this threshold it was possible to measure the highest so far realized ESFADOF transmissions on the Rubidium 5P 3/2 → 8D 5/2 resonance. They represent a milestone towards a practicable realization of the Brillouin-lidar detector. Furthermore, Sec. 5.3.3 will also discuss the observed functional behavior, which separates the ESFADOF operation from the laser-induced plasma phase. 5.3.1 ESFADOF Hysteresis The induction of the plasma phase is accompanied with a hysteretic behavior. Figs 5.12– 5.15 show a compilation of several ESFADOF spectra, which demonstrate the hysteresis. The spectra have been recorded while adiabatically increasing and afterwards decreasing the pump power, meaning that the pump power changes on time scales, which are orders of magnitude slower than the inner pro-
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INSTITUT FÜR ANGEWANDTE PHYSIK TEC
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Curriculum Vitae Alexandru Lucian P
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Supervised Theses Denise Stang, Wei
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184 1 Danksagung Bei Herrn Arno Wei
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