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

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114 5 Discussion of the Experimental Results 0.25 780 nm transmission 0.20 0.15 0.10 0.05 ESFADOF Breakdown T ESFADOF = 165 °C - decreasing P Pump T ESFADOF = 173 °C - decreasing P Pump T ESFADOF = 173 °C - increasing P Pump 0.00 0 100 200 300 400 500 P Pump / mW Fig. 5.16: Transmission of the 780 nm pump beam through the Rb vapor cell I for two different cell temperatures: The magnetic field strength was B z = 270 mT and a parallel polarized pump beam with a detuning of ∆ν P = −1.76(3) GHz has been injected into the cell while adiabatically changing the pump power P Pump . The arrows indicate the direction of power change. At low pump powers a linear behavior of the transmission can be observed, whereas the transmission shows pronounced kinks and saturates at high pump powers. The T ESFADOF = 173 ◦ C case reveals two different hystereses. The first one, around a pump power of 200 mW, indicates feedback driven optical bistability [194] and the second one, marked by the circled data points, has its origins in the onset of a laser-induced plasma inside the Rb vapor. formation of a laser induced plasma could not be observed any more. This can be attributed to the lack of reinjected pump laser power by the tilted feedback mirror. However, the situation changes dramatically when increasing the temperature of the vapor cell to T ESFADOF = 173 ◦ C, which results also in a Rb vapor density increase. Although the feedback mirror remained tilted a plasma has been induced inside the vapor cell. The circled data points in Fig. 5.16 mark the observed laser induced plasma. Striking is the fact that the data reveals two different hystereses. The downward directed branch corresponds to a pump power decrease (blue dashed curve in Fig. 5.16) and the upward directed one to a pump power increase (red dot dashed curve in fig, 5.16). Both curves follow the same path up to a pump power of P Pump = 160 mW and differ significantly beyond this value. In contrast to the colder cell, a first kink appears at approx. P Pump = 45 mW and, despite the coarse resolution, indicates a first nonlinear behavior. For higher pump powers the following observations result: 1. The red upward directed curve shows a higher transmission than the blue downward directed curve between P Pump = 160 mW and P Pump = 240 mW.
5.3 ESFADOF Operational Limits 115 The latter exhibits a clear kink between these limits. Noticeable is the fact that within the errors of the measurement the turning points of this kink correspond to the one already observed for T ESFADOF = 165 ◦ C. Unfortunately the upward directed case for T ESFADOF = 165 ◦ C has not been measured. Due to the lack of optical feedback by the tilted mirror, optical bistability has not been expected [194, 195]. However, the measurements with T ESFADOF = 173 ◦ C indicate optical bistability for this pump power range. This hysteresis resembles the one reported by Ackemann et al. [194]. Although direct feedback was not present, it is reasonable to assume that the strong nonlinear coupling of the pumped Rb atoms with the trapped radiation together with the Fresnel reflections of the window panes is responsible for the encountered optical bistability. This assumption is corroborated by several other publications [177, 178, 196, 197]. However, Ackemann et al. emphasized, that the encountered resonatorless optical bistability emerges from transversal magnetic fields, which allow for light induced level crossings [194]. Thus, the recorded optical bistability has its origins in those parts of the vapor cell, where transverse magnetic fields dominate, i.e. apart from the pump beam injection, where the permanent ring magnets reside. On the other hand, the ESFADOF spectral characteristics emerge from the longitudinal magnetic field components, which dominate the first part of the Rb vapor cell (cf. fig. 4.2(a)). Further investigations of this topic have been omitted as they do not contribute directly to the main purpose of this thesis. 2. Increasing the pump power further leads to a saturation of the red upward directed curve and at a pump power of P Pump = 300 mW a sudden change in transmission occurs. At this value the cell transmission increases stepwise by 2.6(3)% after slightly increasing the pump power. Any further pump power increment leads only to small changes in transmission. After reaching P Pump = 420 mW the pump power has been decreased subsequently and the blue downward directed curve results. Compared to the red upward directed one, this curve shows an overall higher transmission up to P Pump = 240 mW. Decreasing the pump power further again results in a stepwise change of transmission, which decreases approximately by the same amount, i.e. by 2.4(4)%. At this point both curves intersect. Again, together with the stepwise changes in pump power transmission, which lies 60 mW apart, the laserinduced plasma suddenly sets in or ceases, when increasing or decreasing the pump power respectively. The circled data points in Fig. 5.16 mark the observation of the laser-induced plasma by the emitted fluorescence spectrum. A detailed discussion of the emitted fluorescence will be given in Sec. 5.3.2. In conlusion, both hysteresis are particularly remarkable: The first develops due to the feedback action of the trapped radiation inside the high opaque Rb vapor in combination with the presence of transverse magnetic fields [194, 195]. The second hysteresis dramatically affects the population of the pumped level and hence the ESFADOF transmission. But, in contrast to the first discussed above, the second does not emerge from external feedback. It is rather an intrinsic property of the laser-induced plasma inside the hot high density vapor. Sec. 5.3.3 will elab-
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INSTITUT FÜR ANGEWANDTE PHYSIK TEC
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Abstract Global and local climate c
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II Contents 4.3 Measurement Unit .
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2.2 System Requirements 23 Fig. 2.6
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164 References [69] Claude C. Leroy
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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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