Towards attosecond 4D imaging of atomic-scale dynamics by single-electron diffraction

Many physical and chemical processes which define our daily life take place on atomic scales in space and time. Time-resolved electron diffraction is an excellent tool for investigation of atomic-scale structural dynamics (4D imaging) due to the short de Broglie wavelength of fast electrons. This requires electron pulses with durations on the order of femtoseconds or below. Challenges arise from Coulomb repulsion and dispersion of non-relativistic electron wave packets in vacuum, which currently limits the temporal resolution of diffraction experiments to some hundreds of femtoseconds. In order to eventually advance the temporal resolution of electron diffraction into the few-femtosecond range or below, four new concepts are investigated and combined in this work: First, Coulomb repulsion is avoided by using only a single electron per pulse, which does not repel itself but interferes with itself when being diffracted from atoms. Secondly, dispersion control for electron pulses is implemented with time-dependent electric fields at microwave frequencies, compressing the duration of single-electron pulses at the expense of simultaneous energy broadening. Thirdly, a microwave signal used for electron pulse compression is derived from an ultrashort laser pulse train. Optical enhancement allows a temporal synchronization between the microwave field and the laser pulses with a precision below one femtosecond. Fourthly, a cross-correlation between laser and electron pulses is measured in this work with the purpose of determining the possible temporal resolution of diffraction experiments employing compressed single-electron pulses. This novel characterization method uses the principles of a streak camera with optical fields and potentially offers attosecond temporal resolution. These four concepts show a clear path towards improving the temporal resolution of electron diffraction into the few-femtosecond domain or below, which opens the possibility of observing electron densities in motion. In this work, a compressed electron pulse's duration of 28±5 fs full width at half maximum (12±2 fs standard deviation) at a de Broglie wavelength of 0.08 Å is achieved. Currently, this constitutes the shortest electron pulses suitable for diffraction, about sixfold shorter than in previous work. Ultrafast electron diffraction now meets the requirements for investigating the fastest primary processes in molecules and solids with atomic resolution in space and time

XUV frequency comb metrology on the ground state of helium

The operation of a frequency comb at extreme ultraviolet (XUV) wavelengths based on pair-wise amplification and nonlinear upconversion to the 15th harmonic of pulses from a frequency comb laser in the near-infrared range is reported. Following a first account of the experiment [Kandula et al., Phys. Rev. Lett. 105, 063001 (2010)], an extensive review is given of the demonstration that the resulting spectrum at 51 nm is fully phase coherent and can be applied to precision metrology. The pulses are used in a scheme of direct-frequency-comb excitation of helium atoms from the ground state to the 1s4p and 1s5p 1P_1 states. Laser ionization by auxiliary 1064 nm pulses is used to detect the excited state population, resulting in a cosine-like signal as a function of the repetition rate of the frequency comb with a modulation contrast of up to 55%. Analysis of the visibility of this comb structure yields an estimated timing jitter between the two upconverted comb laser pulses of 50 attoseconds, whch indicates that extension to even shorter wavelengths should be feasible. The helium metrology investigation results in transition frequencies of 5740806993(10) MHz and 5814248672(6) MHz for excitation of the 1s4p and 1s5p 1P_1 states, respectively. This constitutes the first absolute frequency measurement in the XUV, attaining unprecedented accuracy in this windowless part of the electromagnetic spectrum. From the measured transition frequencies an eight-fold improved 4He ionization energy of 5945204212(6) MHz is derived. Also a new value for the 4He ground state Lamb shift is found of 41247(6) MHz. This experimental value is in agreement with recent theoretical calculations up to order m\alpha^6 and m^2/(M\alpha^5), but with a six times higher precision, therewith providing a stringent test of quantum electrodynamics in bound two-electron systems.Comment: 18 pages, 13 figure

Optical Communication with Semiconductor Laser Diode

Theoretical and experimental performance limits of a free-space direct detection optical communication system were studied using a semiconductor laser diode as the optical transmitter and a silicon avalanche photodiode (APD) as the receiver photodetector. Optical systems using these components are under consideration as replacements for microwave satellite communication links. Optical pulse position modulation (PPM) was chosen as the signal format. An experimental system was constructed that used an aluminum gallium arsenide semiconductor laser diode as the transmitter and a silicon avalanche photodiode photodetector. The system used Q=4 PPM signaling at a source data rate of 25 megabits per second. The PPM signal format requires regeneration of PPM slot clock and word clock waveforms in the receiver. A nearly exact computational procedure was developed to compute receiver bit error rate without using the Gaussion approximation. A transition detector slot clock recovery system using a phase lock loop was developed and implemented. A novel word clock recovery system was also developed. It was found that the results of the nearly exact computational procedure agreed well with actual measurements of receiver performance. The receiver sensitivity achieved was the closest to the quantum limit yet reported for an optical communication system of this type

Noise model for a dual frequency comb beat

Cette thèse porte sur le raffinement d’un modèle du bruit utilisé pour des mesures spectroscopiques réalisées avec des peignes de fréquences optiques. La majorité des travaux antérieurs utilisaient des peignes de fréquences où le glissement (chirp) est minimisé, en supposant que tout glissement différentiel entre les peignes allait réduire le rapport signal sur bruit. L’hypothèse sous-jacente était que l’impact du bruit multiplicatif serait augmenté, le glissement lui permettant d’agir plus longtemps sur le signal d’interférence. Cependant, d’autres recherches indiquaient plutôt contraire : le chirp pourrait améliorer la mesure. Cette thèse cherche à augmenter la compréhension du comportement du bruit lorsque les peignes ont des glissements différentiels. De plus, celle-ci apporte de nouvelles évidences sur l’utilité du chirp dans ce type de mesure. À cet effet, nous avons fait une révision bibliographique des modèles du bruit dans les peignes de fréquences optiques. Ensuite, du point de vue théorique, nous avons analysé les effets du chirp sur les bruits additifs et multiplicatifs. Pour le bruit d’intensité, nous avons proposé un modèle phénoménologique décrivant le comportement de l’émission spontanée amplifiée (ASE) dans un laser à verrouillage de mode par rotation non linéaire de polarisation. Les spectres des peignes et leurs battements ont été caractérisés en portant une attention particulière à leur relation avec l’ASE. La thèse permet de conclure que le chirp différentiel n’affecte pas les niveaux des densités spectrales de bruit. Grâce au glissement différentiel de fréquence, il est possible d’envoyer plus puissance à l’échantillon et ainsi améliorer le rapport signal sur bruit des instruments à peignes de fréquence. D’un autre côté, la caractérisation de l’ASE a établi sa nature non-stationnaire. Elle a aussi expliqué des attributs spectraux qui sont observés régulièrement dans les signaux de battement des peignes. Finalement, en supposant que l’ASE circule largement dans une cavité opérée sous le seuil, sa caractérisation fournit une méthode pour estimer le déphasage non linéaire que subit le train d’impulsions femtosecondes.This thesis proposes a noise model refinement for spectroscopic measurements using dual optical frequency combs. Until now most studies centered their efforts on noise characterization using chirp free combs based on an unproved hypothesis: measurements would get worse with chirped combs since multiplicative noises would be present over a longer duration on the interference pattern thus leading to a greater impact. However, at least one experimental result hinted to the contrary: differential chirp would actually improve the signal to noise ratio. This thesis therefore aims at increasing the understanding of noise when a differential chirp is present in a dual comb measurement. The specific goal is to provide new insights about the usefulness of chirp in this kind of measurement. With this in mind, we conducted a literature review of noise models in optical frequency combs. We subsequently analyzed the chirp’s effect in the presence of both additive and multiplicative noise. The thesis also proposes a phenomenological model to describe the amplified spontaneous emission - ASE in short pulse lasers mode locked using non linear polarization rotation. Finally the comb spectra and their beat notes are characterized putting special attention to their relation with the ASE components. As conclusions, we can report that noise power spectral density levels do not change with a differential chirp. Chirping allows sending a greater optical power through the sample, such that the measurement signal to noise ratio can be improved. On the other hand, the ASE characterization established its non-stationary nature and explained very well characteristic features routinely observed in dual comb beat notes that were not fully understood. Finally, assuming the ASE experiences a sub threshold linear cavity allows using theses features to estimate the non linear phase shift experienced by the modelocked pulse train in the laser cavity

The study of an adaptive bit rate modem for meteor scatter communications

Includes bibliographical references

Performance evaluation of currently available VLSI implementations satisfying U-interface requirements for an ISDN in South Africa.

A project report submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree of Master of Science in Engineering.This project report examines the performance of three VLSI U-interface implementations satisfying the requirements of Basic Access on an ISDN. The systems evaluated are the Intel 89120,Siemens PEB2090 and STC DSP144, operating on 2BIQ, MMS4J and SU32 line codes respectively. Before evaluating the three abovementioned systems, a review of the underlying principles of U-interface technology is presented. Included in the review are aspects of transmission line theory, line coding, echo-cancellation, decision feedback equalisation, and pulse density modulation. The functional specifications of the three systems are then presented followed by a practical evaluation of each system. As an aid to testing the transmission systems, an evaluation board has been designed and built. The latter provides the necessary functionality to correctly activate each system, as well as the appropriate interfacing requirements for the error-rate tester. The U-interface transmission systems are evaluated on a number of test-loops, comprising sections of cable varying in length and gauge. Additionally, impairments are injected into data-carrying cables, in order to test the performance of each system in the presence of noise. The results of each test are recorded and analysed. Finally, a recommendation is made in favour of the 2BIQ U-interface. It is shown to offer superior transmission performance, at the expense of a slightly higher transmit-power level.Andrew Chakane 201

Quantum Communication, Sensing and Measurement in Space

The main theme of the conclusions drawn for classical communication systems operating at optical or higher frequencies is that there is a well‐understood performance gain in photon efficiency (bits/photon) and spectral efficiency (bits/s/Hz) by pursuing coherent‐state transmitters (classical ideal laser light) coupled with novel quantum receiver systems operating near the Holevo limit (e.g., joint detection receivers). However, recent research indicates that these receivers will require nonlinear and nonclassical optical processes and components at the receiver. Consequently, the implementation complexity of Holevo‐capacityapproaching receivers is not yet fully ascertained. Nonetheless, because the potential gain is significant (e.g., the projected photon efficiency and data rate of MIT Lincoln Laboratory's Lunar Lasercom Demonstration (LLCD) could be achieved with a factor‐of‐20 reduction in the modulation bandwidth requirement), focused research activities on ground‐receiver architectures that approach the Holevo limit in space‐communication links would be beneficial. The potential gains resulting from quantum‐enhanced sensing systems in space applications have not been laid out as concretely as some of the other areas addressed in our study. In particular, while the study period has produced several interesting high‐risk and high‐payoff avenues of research, more detailed seedlinglevel investigations are required to fully delineate the potential return relative to the state‐of‐the‐art. Two prominent examples are (1) improvements to pointing, acquisition and tracking systems (e.g., for optical communication systems) by way of quantum measurements, and (2) possible weak‐valued measurement techniques to attain high‐accuracy sensing systems for in situ or remote‐sensing instruments. While these concepts are technically sound and have very promising bench‐top demonstrations in a lab environment, they are not mature enough to realistically evaluate their performance in a space‐based application. Therefore, it is recommended that future work follow small focused efforts towards incorporating practical constraints imposed by a space environment. The space platform has been well recognized as a nearly ideal environment for some of the most precise tests of fundamental physics, and the ensuing potential of scientific advances enabled by quantum technologies is evident in our report. For example, an exciting concept that has emerged for gravity‐wave detection is that the intermediate frequency band spanning 0.01 to 10 Hz—which is inaccessible from the ground—could be accessed at unprecedented sensitivity with a space‐based interferometer that uses shorter arms relative to state‐of‐the‐art to keep the diffraction losses low, and employs frequency‐dependent squeezed light to surpass the standard quantum limit sensitivity. This offers the potential to open up a new window into the universe, revealing the behavior of compact astrophysical objects and pulsars. As another set of examples, research accomplishments in the atomic and optics fields in recent years have ushered in a number of novel clocks and sensors that can achieve unprecedented measurement precisions. These emerging technologies promise new possibilities in fundamental physics, examples of which are tests of relativistic gravity theory, universality of free fall, frame‐dragging precession, the gravitational inverse‐square law at micron scale, and new ways of gravitational wave detection with atomic inertial sensors. While the relevant technologies and their discovery potentials have been well demonstrated on the ground, there exists a large gap to space‐based systems. To bridge this gap and to advance fundamental‐physics exploration in space, focused investments that further mature promising technologies, such as space‐based atomic clocks and quantum sensors based on atom‐wave interferometers, are recommended. Bringing a group of experts from diverse technical backgrounds together in a productive interactive environment spurred some unanticipated innovative concepts. One promising concept is the possibility of utilizing a space‐based interferometer as a frequency reference for terrestrial precision measurements. Space‐based gravitational wave detectors depend on extraordinarily low noise in the separation between spacecraft, resulting in an ultra‐stable frequency reference that is several orders of magnitude better than the state of the art of frequency references using terrestrial technology. The next steps in developing this promising new concept are simulations and measurement of atmospheric effects that may limit performance due to non‐reciprocal phase fluctuations. In summary, this report covers a broad spectrum of possible new opportunities in space science, as well as enhancements in the performance of communication and sensing technologies, based on observing, manipulating and exploiting the quantum‐mechanical nature of our universe. In our study we identified a range of exciting new opportunities to capture the revolutionary capabilities resulting from quantum enhancements. We believe that pursuing these opportunities has the potential to positively impact the NASA mission in both the near term and in the long term. In this report we lay out the research and development paths that we believe are necessary to realize these opportunities and capitalize on the gains quantum technologies can offer

All-semiconductor High Power Mode-locked Laser System

All-optical synchronization and its application in advanced optical communications have been investigated in this dissertation. Dynamics of all-optical timing synchronization (clock recovery) using multi-section gain-coupled distributed-feedback (MS-GC DFB) lasers are discussed. A record speed of 180-GHz timing synchronization has been demonstrated using this device. An all-optical carrier synchronization (phase and polarization recovery) scheme from PSK (phase shift keying) data is proposed and demonstrated for the first time. As an application of all-optical synchronization, the characterization of advanced modulation formats using a linear optical sampling technique was studied. The full characterization of 10-Gb/s RZ-BPSK (return-to-zero binary PSK) data has been demonstrated. Fast lockup and walk-off of the all-optical timing synchronization process on the order of nanoseconds were measured in both simulation and experiment. Phase stability of the recovered clock from a pseudo-random bit sequence signal can be achieved by limiting the detuning between the frequency of free-running self-pulsation and the input bit rate. The simulation results show that all-optical clock recovery using TS-DFB lasers can maintain a better than 5 % clock phase stability for large variations in power, bit rate and optical carrier frequency of the input data and therefore is suitable for applications in ultrafast optical packet switching. All-optical timing synchronization of 180-Gb/s data streams has been demonstrated using a MS-GC DFB laser. The recovered clock has a jitter of less than 410 fs over a dynamic range of 7 dB. All-optical carrier synchronization from phase modulated data utilizes a phase sensitive oscillator (PSO), which used a phase sensitive amplifier (PSA) as a gain block. Furthermore, all-optical carrier synchronization from 10-Gb/s BPSK data was demonstrated in experiment. The PSA is configured as a nonlinear optical loop mirror (NOLM). A discrete linear system analysis was carried out to understand the stability of the PSO. Complex envelope measurement using coherent linear optical sampling with mode-locked sources is investigated. It is shown that reliable measurement of the phase requires that one of the optical modes of the sampling pulses be locked to the optical carrier of the data signal to be measured. Carrier-envelope offset (CEO) is found to have a negligible effect on the measurement. Measurement errors of the intensity profile and phase depend on the pulsewidth and chirp of the sampling pulses as well as the detuning between the carrier frequencies of the data signal and the center frequency of the sampling source. Characterization of the 10-Gb/s RZ-BPSK signal was demonstrated using the coherent detection technique. Measurements of the optical intensity profile, chirp and constellation diagram were demonstrated. A CW local oscillator was used and electrical sampling was performed using a sampling scope. A novel feedback scheme was used to stabilize homodyne detection

Matter-wave Atomic Gradiometer Interferometric Sensor (MAGIS-100)

MAGIS-100 is a next-generation quantum sensor under construction at Fermilab that aims to explore fundamental physics with atom interferometry over a 100-meter baseline. This novel detector will search for ultralight dark matter, test quantum mechanics in new regimes, and serve as a technology pathfinder for future gravitational wave detectors in a previously unexplored frequency band. It combines techniques demonstrated in state-of-the-art 10-meter-scale atom interferometers with the latest technological advances of the world's best atomic clocks. MAGIS-100 will provide a development platform for a future kilometer-scale detector that would be sufficiently sensitive to detect gravitational waves from known sources. Here we present the science case for the MAGIS concept, review the operating principles of the detector, describe the instrument design, and study the detector systematics.Comment: 65 pages, 18 figure