Nach oben pdf Satellite Laser Ranging at 100 kHz Pulse Repetition Rate

Satellite Laser Ranging at 100 kHz Pulse Repetition Rate

Satellite Laser Ranging at 100 kHz Pulse Repetition Rate

The limit of ambiguity applies to any ranging experiment using a regularly pulsed source: If the repetition interval is shorter than the ToF (time of flight), an assumption of the expected distance must be used to properly correlate time- stamps of outgoing and incoming pulses. In satellite laser ranging, the expected ToF is usually derived from CPF pre- dictions available from the ILRS website [5]. Their accuracy is often better than ten meters for regularly tracked targets, but may become larger than 100 meters for less tracked tar- gets or objects at very low altitudes that are seriously af- fected by atmospheric drag. If no ILRS predictions are avail- able, two line element (TLE) predictions can be used [8]. In this case, the range uncertainty is often several hundred me- ters.
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High-repetition-rate picosecond pump laser based on an Yb:YAG disk amplifier for optical parametric amplification

High-repetition-rate picosecond pump laser based on an Yb:YAG disk amplifier for optical parametric amplification

Employing new few-cycle laser systems generating pulse energies clearly exceeding the mJ- level in the near infrared, the XUV radiation will be shifted towards higher photon energies enabling experimental studies on previously unexplored intermolecular- and atom-dynamics. Reaching the “water window”, a wavelength range where water is transparent while carbon structures can be imaged, between 2.2 and 4.4 nm, would allow in situ analysis of biological relevant molecules. The free electron laser in Hamburg (FLASH) generates currently about 50 fs long pulses at wavelengths down to 6.9 nm with a flux of about 10 13 photons per pulse [115]. Currently these pulses are too long and have too low photon energy to observe fast mo- tions in long biological molecules via diffraction and allow only observation on frozen sam- ples. Therefore short attosecond pulses with high photon flux are required to measure in a single shot the diffraction pattern delivering the information of the molecule allowing obser- vations simultaneously in space and time with sub-atomic resolution. Currently the standard driving source for HHG and pump-probe experiments is a Ti:sapphire laser based laser system (Femtopower) from Femtolasers Produktions GmbH delivering 10 7 to 10 8 XUV photons per shot which is currently not sufficient for XUV diffraction measurements on biological tissues. Additional the pulse energy is not efficient to generate two attosecond XUV-beams to use them in pump-probe experiments due to the low count rate monitored in such experiments. Ishii et el demonstrated in 2005 a near infrared OPA [20] operating at 20 Hz with an conver- sion efficiency of 16 % from the 50 mJ pump pulse at 532 nm to the compressed 10 fs short NIR pulse amplified to 8 mJ. Although the repetition rate of the presented system is too low for pump-probe experiments it showed the feasibility of a multi-millijoule few-cycle OPA. Such a system pumped with the in this work developed regenerative amplifier could achieve few cycle pulses with pulse energies above 2.5 mJ at 3 kHz allowing XUV-XUV pump-probe experiments and would abolish the current limitation on observations of systems witch are sensitive to the NIR infrared and the XUV-beam at the same time.
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High-Power High-Repetition-Rate 1-µM Fiber Laser System for Strong-Field Physics and Mid-Infrared Generation

High-Power High-Repetition-Rate 1-µM Fiber Laser System for Strong-Field Physics and Mid-Infrared Generation

The schematic construction of the repetition rate reduction module is shown in Figure 2.12. The input 28-MHz, 800-ps, 1-mW, 1-µm beam was amplified by two stages PMSMF pre-signal-amplifiers to about 100 mW before the AOM. For each stage of pre- signal-amplifiers, the PMSMF amplifier consists of a polarization maintaining wavelength division multiplexer (PMWDM), 976-nm pump laser, and 0.5-m high Yb- doped gain fiber (PM YB401). The electronic pulse picker circuits were used to reduce the repetition rate of the electronic signal from the APD shown in Figure 2.4 from 28 MHz to 1 MHz. This 1 MHz electronic signal was used to be the carrier envelope modulation signal to modulate the generated 200 MHz radio frequency signal from the AOM driver to make the AOM become an optical pulse picker. The insertion loss of the fiber AOM was about 4 dB, so that the output power of the 1 MHz pulse train was about 1 mW. The raise time of the AOM was <5 ns. The peak power ratio of the main pulse and satellite pulse was >20 dB. The PMSMF pre-power-amplifier consisted of the PMWDM, 976-nm single-mode laser diode and 0.75 m PM-YB-501 fiber. The PM large-core diameter fiber pre-power-amplifier consisted of the PM large-core diameter fiber combiner (25-µm core-diameter), 20 W multi-mode 976-nm pump diode, and 4 m Nufern PLMA-YDF-25/250 gain fiber. The generated high order modes inside the 25-µm core-diameter fiber were inhibited by tight coiling the fiber on the 8-cm diameter aluminum column. After these two pre-power-amplification stages, the 1-MHz, 800-ps, 1-µm laser were amplified to about 3 W at fundamental mode.
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Satellite Laser Ranging with a fibre-based transmitter

Satellite Laser Ranging with a fibre-based transmitter

The experimental satellite laser ranging station in Stuttgart has commenced operations in January 2016. Its modular, flexible and cost-efficient design uses only readily available components and is therefore well suited for an upgrade of existing astronomical observatories to SLR stations. One of its key features is the laser light transmission via an optical fibre, thus avoiding the need for a coudé path mount. Currently, the transmitter achieves an output pulse energy of about 25 µJ at 5 kHz (125 mW) and is operated at the fundamental Nd:YAG wavelength of 1064 nm. The complete system, including IT hardware and observer workplaces, is fitted into a 12 feet dome. With the current configuration, many cooperative targets in LEO and beyond (up to LAGEOS) have successfully been observed, with usual return rates of several hundred counts per second. Since the tracking relies on visual guiding, no accurate CPF predictions are needed, and out-of- service SLR targets like GEOS 3 can be observed as well using public TLE data.
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Design and Construction of an Infrared Laser Transmitter for a Compact Satellite Laser Ranging System

Design and Construction of an Infrared Laser Transmitter for a Compact Satellite Laser Ranging System

However, a retroreflector on every single space object would facilitate and improve the quality of SLR. Ranging to objects without a reflector (uncoorperative objects) requires high pulse energies (∼ 50 mJ [2]), as only scattered light can be received on Earth. High energy Lasers constitute complete systems, as they need supplies for large cooling and power systems [8]. The low return rate of photons require a larger diameter of the receiving optics [2]. All in one, ranging to uncoorperative objects lead to a more complex SLR system set-up. This problem can be solved with a retroreflector, which is easily integrated into a satellite or rocket body, as they are light and small. This method is already successfull in use with several microsatellites, e.g. Technosat. The position determination happens with accuracy in the millimeter range [4], [2], trajectories are predicted with uncertainties of a few meters [6]. Summing up, the demand for more SLR stations rises. That’s why several stations are built up at the moment. Though, installation and operation of a ground station leads to high costs for the responsible agency, as a site for the station, high energy lasers, sensitive electronics an on-site staff are required. Nevertheless, the technical approach offers new possibilities.
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Chirped probe pulse femtosecond coherent anti-Stokes Raman scattering thermometry at 5 kHz in a Gas Turbine Model Combustor

Chirped probe pulse femtosecond coherent anti-Stokes Raman scattering thermometry at 5 kHz in a Gas Turbine Model Combustor

advantages of fs-CARS over traditional ns- and ps-CARS techniques [1]. Solid-state femtosecond lasers operate at high repetition rates and offer excellent shot-to-shot spectral stability, nearly eliminating the shot-to-shot spectral fluctuations encountered with typical broadband dye lasers used for nanosecond CARS. In addition, the nearly Fourier-transform limited broadband femtosecond laser pulses excite many Raman transitions at once, creating a strong coherence in the sample medium [2,3]. This allows fs-CARS temperature measurements based on the frequency-spread dephasing rate after initial excitation of the Raman coherence of the molecule by the pump and Stokes beams. If one uses a chirped-probe-pulse (CPP), a method introduced by Lang et al. [4], the temporal decay of the Raman coherence can be mapped with linear and nonlinear contributions onto the frequency of the CARS signal allowing single-laser-shot measurements. Experiments have shown the CARS signal is nearly independent of molecular collisions after the initial excitation of the coherence and the initial decay rate of the Raman coherence in gas phase measurements depends only on temperature [5,6]. Focusing on the first few picoseconds after excitation will result in collision-free measurements for pressures up to 20 bar [3]. This simplifies significantly the theoretical modeling of fs-CARS spectra and improves accuracy by eliminating the need for Raman linewidth information [1].
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Moderate High Power 1 to 20 µs and kHz Ho:YAG Thin Disk 
Laser Pulses for Laser Lithotripsy

Moderate High Power 1 to 20 µs and kHz Ho:YAG Thin Disk Laser Pulses for Laser Lithotripsy

An acousto-optically or self-oscillation pulsed thin disk Ho:YAG laser system at 2.1 µm with an average power in the 10 W range will be presented for laser lithotripsy. In the case of cw operation the thin disk Ho:YAG is either pumped with InP diode stacks or with a thulium fiber laser which leads to a laser output power of 20 W at an optical-to-optical efficiency of 30%. For the gain switched mode of operation a modulated Tm-fiber laser is used to produce self- oscillation pulses. A favored pulse lengths for uric acid stone ablation is known to be at a few µs pulse duration which can be delivered by the thin disk laser technology. In the state of the art laser lithotripter, stone material is typically ablated with 250 to 750 µs pulses at 5 to 10 Hz and with pulse energies up to a few Joule. The ablation mechanism is performed in this case by vaporization into stone dust and fragmentation. With the thin disk laser technology, 1 to 20 µs- laser pulses with a repetition rate of a few kHz and with pulse energies in the mJ-range are available. The ablation mechanism is in this case due to a local heating of the stone material with a decomposition of the crystalline structure into calcium carbonate powder which can be handled by the human body. As a joint process to this thermal effect, imploding water vapor bubbles between the fiber end and the stone material produce sporadic shock waves which help clear out the stone dust and biological material.
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First successful satellite laser ranging with a fibre-based transmitter

First successful satellite laser ranging with a fibre-based transmitter

Satellite Laser Ranging (SLR) is an established technology used for geodesy, fundamental science and precise orbit determination. This paper reports on the first successful SLR measurement from the German Aerospace Center research observatory in Stuttgart. While many SLR stations are in opera- tion, the experiment described here is unique in several ways: The modular system has been assembled completely from commercial off-the-shelf components, which increases flexibility and significantly re- duces hardware costs. To our knowledge it has been the first time that an SLR measurement has been conducted using an optical fibre rather than a coud´ e path to direct the light from the laser source onto the telescope. The transmitter operates at an output power of about 75 mW and a repetition rate of 3 kHz, and at a wavelength of 1064 nm. Due to its rather small diameter of only 80 µm, the receiver detector features a low noise rate of less than 2 kHz and can be operated without gating in many cases. With this set-up, clear return signals have been received from several orbital objects equipped with retroreflectors. In its current configuration, the system does not yet achieve the same performance as other SLR systems in terms of precision, maximum distance and the capability of daylight ranging; however, plans to overcome these limitations are outlined.
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High repetition rate laser driven proton source and a new method of enhancing acceleration

High repetition rate laser driven proton source and a new method of enhancing acceleration

It is worthwhile to recall the physical image of the target normal sheath acceleration (TNSA) mechanism here. After ionization of the target, some hot electrons are transported in the lon- gitudinal direction and reach the back side of the target where they set up a strong field that accelerates ions but also reflects the majority of electrons back into the target. The hot electrons can also move along the vacuum-target surface and this way their kinetic energy spreads over a wide region (> 100 µm), resulting in the attenuation of the electric field [171, 172]. Therefore, reducing the lateral dimension of the target, i.e., achieving a mass-limited target, can effec- tively prevent the transfer of energy from the hot electrons to a distant location because the hot electrons remain macroscopically bound to the center of the target [70]. Theoretical and experimental studies have reached a consensus that the optimal result of laser-accelerated ions occurs when the size of the target is comparable to or smaller than the size of the laser focal spot, which also depends on the pulse duration and target shape [72, 70, 173]. The QIMT provided by the donut-shape pre-pulse seems to indicate such an enhancement that is expected from this simple picture. The difference is that the QIMT is surrounded by a thin plasma cloud. The size of the QIMT can be estimated, but the density distribution of the plasma cloud around it at the time when the main pulse arrives is inaccessible. Therefore, we carried out fluid dynamics simulation with the software MULTI-1D and MULTI-2D to study the evolution of the plasma expansion that is triggered by the interaction of the LG-pre-pulse.
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Simultaneous Three Component PIV / OH-PLIF Measurements of a Turbulent Lifted, C3H8-Argon Jet-Flame at (Sustained) 1.5 kHz Repetition Rate

Simultaneous Three Component PIV / OH-PLIF Measurements of a Turbulent Lifted, C3H8-Argon Jet-Flame at (Sustained) 1.5 kHz Repetition Rate

Cinematographic OH-PLIF image sequences were acquired in the plane intersecting the jet- axis over the region of the lifted flamebase. The imaged region extended from approximately 19 to 54mm beyond the jet-exit and was positioned to the right of the jet-centerline. The cinematographic PLIF system used a frequency-doubled dye laser (Sirah, Cobra-Stretch) pumped with a frequency-doubled, diode-pumped solid state Nd:YLF laser (Edgewave IS- 811E). The dye laser is modified from its original design to operate efficiently with the relatively low pulse energies produced by the pump laser at high repetition rates (7 - 12mJ, 8.5ns pulse duration). The modifications include use of only the oscillator and pre-amplifier (in a dual-level dye couvette), a high flow-rate dye circulator pump and a low-loss resonator cavity. The dye used was Rhodamine 6G and the solvent Ethanol (conc. 0.09gm/L). Under optimal operating conditions, the laser produces 0.82W (time-averaged) power at 283nm. In the present work the laser delivered 0.3W at 1.5 kHz, or 0.2mJ/pulse at 283nm. The reduced energy is believed to have resulted from a combination of sensitivity of the laser alignment and depletion of the dye solution as a result of sustained, high intensity laser-pumping. Three cylindrical lenses were used to form a sheet overlapping the imaging region. Based on a photographic paper burn pattern, the thickness of the laser sheet is estimated to have been 0.5mm in the imaging region.
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GNSS Satellite Orbit Validation Using Satellite Laser Ranging

GNSS Satellite Orbit Validation Using Satellite Laser Ranging

BeiDou) to 20 cm (QZSS). Mean biases between GNSS-based orbits and SLR observations are presently at the 5 cm level for the aforementioned constellations. SLR tracking will also help to assess future improvements from refined orbit dynamics models, improved GNSS tracking coverage and refined processing concepts (e.g., ambiguity resolution). The diversity of orbits and spacecraft models within even a single GNSS constellation suggests a need for comprehensive SLR tracking of “all” new GNSS satellites until the GNSS only orbit determination accuracy is compatible with that of existing systems (GPS, GLONASS). Beyond this immediate goal, SLR tracking of GNSS satellites will contribute to the harmonization of GNSS- and SLR-based reference frames. The development of a consolidated SLR tracking concept for GNSS satellites within the ILRS is therefore encouraged. As part of this, special consideration should be given to the potential increase in overall tracking capacity provided by high-rate kHz laser systems. Build-up of such systems appears of particular interest for the Asia-Pacifica region, which hosts a large number of geosynchronous GNSS satellites that are less well covered than the more common GNSS satellites in medium Earth orbit.
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Pulse Metrology Tool and Burst-Mode Laser Amplifier for the Free-Electron Laser in Hamburg

Pulse Metrology Tool and Burst-Mode Laser Amplifier for the Free-Electron Laser in Hamburg

Worldwide, FLASH is the only operational high repetition rate FEL, delivering pulse trains of 800 pulses with a repetition rate of 10 Hz. Future high repetition rate FELs are under con- struction, such as FLASH II and the European XFEL. The increased number of pulses of these facilities allow for a better data quality in terms of higher statistical significance and better signal-to-noise ratio. However, the bottleneck are outdated optical lasers. Low repetition rate Ti:sapphire systems are limited to pulse durations of about 20 fs. At FLASH, the burst-mode amplifier is limited to 70 fs pulse duration and 10 µJ pulse energy, which is not sufficient for ex- ternal FEL seeding. In this work, this bottleneck has been overcome by means of a high-power optical parametric chirped-pulse amplifier for the application as seeding and pump-probe laser. The realized prototype deliveres pulse trains of 22 pulses, supporting sub-7 fs pulse duration at 1.4 mJ pulse energy. A total energy conversion efficiency of 20% from the SHG pump pulses to the OPCPA output pulses is achieved. The system is capable to approach optical peak powers in the terawatt regime. With the short pulse duration, the pump-probe resolution is potentially in- creased by one order of magnitude. Currently, a 100 kHz version is being developed as seeding laser for FLASH-2. Recently, the target parameters, 1 mJ pulse energy and sub-30 fs pulse dura- tion, have been achieved. To reach the final repetition rate of 1 MHz, further high-power pump amplifiers are under development. The OPCPA output energy and the spectral bandwidth are highly sensitive to pump-to-signal drifts. A synchronization technology for OPCPA long-term stability improvement has been investigated. The generation of a white-light supercontinuum in YAG was utilized for broadband OPCPA seeding with an intrinsic synchronization to the OPA pump. A stable operation of a white-light source over a period of 28 hours was demonstrated with a signal-to-pump jitter of 7 fs rms, which is an outstanding performance compared to active methods. However, this introduces small additional jitter in the FEL-to-signal synchronization. Furthermore, a stable white-light amplification and temporal compression was demonstrated. The scaling possibilities towards continuously operating kilowatt-pumped high-power OPCPAs was theoretically evaluated. It was shown that with adequate spectral clipping of the signal seed, the major contribution to the heat load is introduced by the pump absorption, rather than signal or idler absorption. The simulations confirmed the preservation of the broad gain bandwidth in 1 kW pumped BBO and LBO crystals. Assuming a pump-to-signal conversion efficiency of up to 20%, possible output powers of up to 200 W are expected. Recent absorption measurements of LBO revealed that these crystals might even be applicable for OPCPAs with kW of output power, provided that tens of kW pump amplifiers are available.
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Chirped Pulse Oscillators: Generating microjoule femtosecond pulses at megahertz repetition rate

Chirped Pulse Oscillators: Generating microjoule femtosecond pulses at megahertz repetition rate

where e, E, m, ω, I, and λ are the electron charge, laser field amplitude, electron mass, laser frequency, intensity and wavelength, respectively. The highest photon energy therefore scales linearly with the laser intensity. Although, the single-atom effect of high harmonic generation helps to understand many important features of this process, applications of high harmonic radiation as a coherent XUV source depend upon optimizing the total flux gen- erated from an ensemble of atoms [91, 93, 97, 98]. For optimizing the efficiency of high harmonic generation different factors has to be taken into account: The high harmonic gener- ation process necessarily ionizes the gas, generating a free-electron plasma. The dispersion of the plasma causes a mismatch in the phase velocities of the fundamental an harmonic light, significantly reducing the amount of signal produced. Also, the plasma can defocus the laser beam, decreasing the peak intensity and limiting the maximum harmonic energy [99]. The conversion efficiency can also be reduced due to re-absorption of the harmonic radiation by the emitting medium. Generating high-harmonics generally implies focusing the fundamen- tal beam which introduces a geometrical phase shift on the fundamental beam, the so-called Gouy shift [2]. For a fundamental Gaussian beam passing through a waist the phase fronts will shift forward by a total amount of half a wavelength compared to an ideal plane wave. This geometrical effect represents another source of dephasing between the fundamental beam and the harmonic beam [43, 100]. To date, the highest harmonic energy has been achieved using short-duration laser pulses and noble gases with high ionization potentials that can reduce the amount of ionization for a given peak intensity [101, 102]. Limitations arising from geometric effects such as dephasing by the Gouy phase shift and defocusing depend on the confocal pa- rameter and can be minimized by increasing the pulse energy and increasing the beam radius (focusing the beam gentler) such as the peak intensity remains constant [43, 103]. Another important parameter to consider is the extreme sensitivity of high harmonic generation on the ellipticity of the fundamental pulse. For slightly elliptically polarized laser pulse, high-order harmonic generation is strongly suppressed [91].
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High repetition rate, phase-stable, infrared OPCPA for strong-field experiments

High repetition rate, phase-stable, infrared OPCPA for strong-field experiments

In chapter 2 the development of the Innoslab based amplifier laser chain is de- scribed, being a powerful laser source at 100 kHz. It produces stable short laser pulses around 1 ps with 1030 nm central wavelength with an energy of up to 5 mJ (uncompressed). A central point within this thesis was the development of a reliable seed generation, circumventing common difficulties in OPCPA de- sign. Therefore, the possibilities of utilizing supercontinuum generation with comparably long laser pulses were experimentally studied. The results prove, that the generated white light is stable on short and long time scales, which is the key requirement for the following parametric stages. Consequently, it was used for broadband amplification in BBO with a frequency doubled fraction of the fundamental beam of the 1030 nm amplifier. The good efficiency of this stage, together with the results of the following DFG stage, shows the potential of generating the seed for OPCPA with our approach. The DFG stability is on the same level as the Innoslab amplifier and the spectrum supports few-cycle pulses. The generated pulses were compressible close to the Fourier limit and the pulse energy was sufficient for OPCPA seeding.
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Optical Properties of Pulse Laser Heated Soot

Optical Properties of Pulse Laser Heated Soot

peak  at  404.977 nm  [34]  and  graphene  would  also  interact  strongly  with  405 nm  radiation.    The  formation of secondary species would take some time and thus the time delay between the material  loss and extinction enhancement.  Conversion of amorphous into crystalline sub‐units (termed mode A  and  mode  B  in  [33],  and  distinguished  by  physical  properties),  being  one  potential  realization  of  “annealing”  is  another  effect  expected  for  high  temperatures,  having  a  different  time  constant  than  the  chemical  effects  mentioned  above  and  influencing  absorptive  properties  as  detailed  in  [33]  and  relevant  for  different  profiles  monitoring  at  830  and  405 nm,  respectively.    Once  the  material  is  completely  transformed,  or  once  the  soot  temperature  drops  sufficiently,  the  extinction  coefficient  stops  rising  and  begins  to  drop  as  the  gas  phase  material  cools  and/or  condenses.    For  the  characteristic decay time of about 100 µs, and for typical gases at a temperature of 533 K, it is noted  that the mass diffusion length is about 150 µm which is too short to explain a significant diffusive loss  of desorbed species from the laser heated volume.  Another possible loss mechanism for reactive C n  
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Double pulse laser-induced breakdown spectroscopy on manganese

Double pulse laser-induced breakdown spectroscopy on manganese

The figures illustrate that mean intensity values for series taken inside the cavitation bubble were higher. However, so was intensity of background signals. Difference between manganese emission lines and average background level was similar for series of 100 pulses. As mentioned above, spectra received from plasma creation inside a cavitation bubble varies largely, and thus the plotted deviations in Figure 4.27 are stronger than the signal intensity itself. Nevertheless, if single pulses were extracted and compared with the CCD image of the plasma, it became obvious that it was possible to receive good and stable data. Figure 4.28 shows some examples of the image and corresponding emission spectra. Plasma, that can be seen to the left of the cavitation bubble represents plasma, which was most often found in this series (about 40 to 50%). A more evolved plasma was created only on few occasions and, due to settings used, led to a signal overload.
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Erzeugung intensiver Pulse im Terahertzfrequenzbereich mittels laser-generieter Plasmen

Erzeugung intensiver Pulse im Terahertzfrequenzbereich mittels laser-generieter Plasmen

deromotiven Kräfte, radialsymmetrisch sein. Damit kann keine THz-Dipol-Emission in Vorwärts- richtung erfolgen. Wie Hamster et al. jedoch durch numerische Rechnungen zeigen, wird auf Ba- sis der erzeugten raum-zeitlichen Ladungsverteilung in bestimmten anderen Richtungen als in der Vorwärtsrichtung eine THz-Emission erfolgen. Diese Emissionsrichtungen liegen typischer Weise seitwärts zur Strahlachse. Die Emission erfolgt also in einem kegelförmigen Bereich rund um die Strahlachse. Hamster et al. verwenden für ihre Experimente Laserpulsenergien im Bereich von bis zu 50 mJ (was etwa um einen Faktor 100 über den im Rahmen dieser Arbeit verwendeten Laserpul- senergien liegt). Damit konnten THz-Pulsenergien von etwa 300 pJ erzeugt werden [121]. Nimmt man einen Brennpunktdurchmesser der THz-Strahlung am Ort des Detektors von etwa 300  m an 8 und berücksichtigt die von Hamster et al. gemessene THz-Pulslänge von 300 fs, entspricht diese Energie einer THz-Amplitude von etwa 20 kV/cm. Bei den im Rahmen der vorliegenden Arbeit verwendeten Laserpulsenergien von 500  J ergeben sich auf Basis des von Hamster et al. gefun- denen quadratischen Zusammenhangs zwischen THz- und Laserpulsenergie [121] Werte für die zu erwartenden THz-Pulsenergien von etwa 30 nJ, was einer THz-Amplitude von etwa 200 V/cm entsprechen würde. Emissionen auf Basis ponderomotiver Kräfte sind auch im Rahmen der vorlie- genden Arbeit beobachtet worden (vergl. Abb. 4.12 in Kap. 4.4). Dabei weicht der Wert der experi- mentell bestimmten THz-Amplitude mit ca. 15 V/cm von der Abschätzung auf Basis der Angaben von Hamster et al. ab. Diese Abweichung ist vermutlich auf Unterschiede in der Laserpulslänge und die von Hamster et al. verwendete bolometrische Detektionsmethode zurückzuführen. Diese erfasst den gesamten THz-Spektralbereich, während bei der im Rahmen dieser Arbeit verwende- ten elektro-optischen Methode nur Signalbeiträge mit Frequenzen von unterhalb von etwa 2,5 THz berücksichtigt werden.
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Slow Pulse Repetition Interval Variation for High-Resolution Wide-Swath SAR Imaging

Slow Pulse Repetition Interval Variation for High-Resolution Wide-Swath SAR Imaging

sampling of the image in azimuth, this represents an orbital accuracy in the order of 3 cm or 0.006 pixels. According to [36], this is in the order of the best-case 3D 1𝜎 accuracy achieved by TerraSAR-X and approximately a factor of 4 looser than the requirements of the TOPS mode described therein. [34] also reports an accuracy requirement of 1 cm for Sentinel-1 TOPS interferometry, achieved through the procedure described in [36]. Though challenging, this accuracy is deemed achievable with state- of-the-art compensation methods as described in [36], [34], which could be adapted to the slow PRI variation mode by, e.g., exploiting the different looks (see Section II.B.3). Furthermore, the ranges in which no blockage occurs (cf. e.g. Fig.4 (a)), though limited, could provide additional information for the corregistration. Note that, as discussed in [33], the range co-registration is not a concern even for a more demanding TOPS mode with an overall Doppler centroid variation in the order of 10 kHz.
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Experimental Characterization of the Photocathode Laser System for Advanced 3D Pulse Shaping at PITZ

Experimental Characterization of the Photocathode Laser System for Advanced 3D Pulse Shaping at PITZ

Moderne Freie Elektronen Laser (FEL) umgehen diese Einschränkungen indem sie ohne die Verwendung von Materialien zur Lichterzeugung auskommt. Als Medium für die Erzeugung von kohärenten und ultrakurzen Lichtpulsen dienen stattdessen Pakete relativistischer Elektronen, die von Teilchenbeschleunigern bereitgestellt werden. Durch die Ablenkung dieser Pakete in periodisch wechselnden magnetischen Feldern wird anstelle reiner spontaner Emission von Synchrotronstrahlung sogenannte Selbstverstärkung durch stimulierte Emission (engl. Self Amplification by Stimulated Emission, SASE) hervorgerufen. Dies ermöglicht die Bereitstellung von Lichtpulsen mit Lasereigenschaften im kurzwelligen UV- und weichen Röntgen-Bereich bis hin zu noch kleineren Wellenlängen ohne die Verwendung eines optischen Resonators und damit ohne Materialeinschränkungen. Da Pulswiederholrate, Betriebszyklus, Pulsdauer, Wellenlänge und Kohärenz der erzeugten Lichtpulse vom verwendeten Elektronenstrahl abhängen bietet der FEL einzigartige Flexibilität in der Lichterzeugung.
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The role of attitude determination for inter-satellite ranging

The role of attitude determination for inter-satellite ranging

The first absolute gravity measurements were carried out around the turn of the 17th and 18th century by means of diverse types of pendulum. The first spring-based relative gravimeters were developed in the first half of the 20th century and later on the absolute gravimeters based on rise-and-fall and free-fall methods were built, which were significantly more accurate than the pendulum gravimeters (Torge, 1989). The technology development in the last few decades allowed to build highly precise relative superconducting gravimeters (Goodkind, 1999), and currently absolute quantum gravimeters are under development (de Angelis et al., 2009). With these instruments it is possible to perform the gravity measurements only pointwise. Later, regional measurements have been possible by means of airborne and ship- borne instruments. However, the determination of the global gravity field was not possible before using measurements from space. The very first satellite, Sputnik 1, was launched in 1957 and since then the number of launched satellites has raised exponentially. The satellite orbits are perturbed by the inhomogeneous mass distribution within the central body and so it is possible to recover the gravity field from the orbit tracking data. One of the first global Earth’s gravity field models was presented in 1966 by Lundquist and Veis (1966). The accuracy of the first models was rather low, though.
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