Free-space optical communication

Free-space optical communication (FSO) is an optical communication technology that uses light propagating in free space to wirelessly transmit data for telecommunications or computer networking. "Free space" means air, outer space, vacuum, or something similar. This contrasts with using solids such as optical fiber cable. The technology is useful where the physical connections are impractical due to high costs or other considerations. History Optical communications, in various forms, have been used for thousands of years. The ancient Greeks used a coded alphabetic system of signalling with torches developed by Cleoxenus, Democleitus and Polybius. In the modern era, semaphores and wireless solar telegraphs called heliographs were developed, using coded signals to communicate with their recipients. In 1880, Alexander Graham Bell and his assistant Charles Sumner Tainter created the photophone, at Bell's newly established Volta Laboratory in Washington, DC. Bell considered

Monte Carlo simulations of ultra high vacuum and synchrotron radiation for particle accelerators

Márton Ady

With preparation of Hi-Lumi LHC fully underway, and the FCC machines under study, accelerators will reach unprecedented energies and along with it very large amount of synchrotron radiation (SR). This will desorb photoelectrons and molecules from accelerator walls, which contribute to electron cloud buildup and increase the residual pressure - both effects reducing the beam lifetime. In current accelerators these two effects are among the principal limiting factors, therefore precise calculation of synchrotron radiation and pressure properties are very important, desirably in the early design phase. This PhD project shows the modernization and a major upgrade of two codes, Molflow and Synrad, originally written by R. Kersevan in the 1990s, which are based on the test-particle Monte Carlo method and allow ultra-high vacuum and synchrotron radiation calculations. The new versions contain new physics, and are built as an all-in-one package - available to the public. Existing vacuum calculation methods are overviewed, then the steady-state and time-dependent algorithms behind the ultra-high vacuum simulator Molflow are presented. Some practices to tackle the most common problems that arise when simulating large systems are also discussed. Results are compared to theory, and validated through two experiments. Next the the main steps of synchrotron radiation simulations are presented. Properties of SR are summarized, along with optimizations that allow simulating the rather complex underlying physics at a higher speed. The resulting software's photon generation algorithm is benchmarked against published data. The phenomenon of photon stimulated desorption and its literature is overviewed, then two dedicated photodesorption experiments carried out in KEK (Tsukuba, Japan) are presented: one with six room-temperature samples and an other at liquid nitrogen temperature. A simple synchrotron radiation calculation is performed for the LHeC interaction region, allowing to compare Synrad+ results with published analytic calculations. Then the calculations are repeated for a more precise geometry description. The pressure profile of a crotch absorber of the recently started Max IV light source is calculated using Molflow+ and Synrad+ together. Finally the pressure analysis of the SuperKEKB interaction region is presented, consisting of modeling the vacuum chamber and the optics, calculating synchrotron radiation, then performing vacuum simulations. It is confirmed that pressure is expected to meet the design requirements during operation of the machine.

EPFL2016

Ultrafast dynamic switching of optical response based on nonlinear hyperbolic metamaterial platform

Qian Li, Jiaye Wu

The pursuit of high-speed and on-chip optical communication systems has promoted extensive exploration of all-optical control of light-matter interactions via nonlinear optical processes. Here, we have numerically investigated the ultrafast dynamic switching of optical response using tunable hyperbolic metamaterial (HMM) which consists of five pairs of alternating layers of indium tin oxide (ITO) and SiO2. The nonlinearity of the HMM is analyzed by the ultrafast dynamics of the hot electrons in the epsilon-near-zero (ENZ) ITO. Our approach allows large and broad all-optical modulation of the effective permittivity and topology of the HMM on the femtosecond time-scale. Based on the proposed HMM platform, we have shown considerable tunability in the extinction ratio and Purcell enhancement under various pump fluence. In addition, we have achieved all-optical control of the coupling strength through depositing plasmonic resonators on the HMM platform. A significant tuning of the coupled resonance is observed by changing pump fluence, which leads to a switching time within 213 fs at a specific wavelength with a relative modulation depth more than 15 dB. (C) 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

Optica Publishing Group2022

Efficient error correction solutions for OFDM based digital video

Mauro Lattuada

The robustness of the delivery of digital compressed video streams has always been one of the main topic of the standardization bodies to assure the Quality of the Service (QoS). The MPEG standard which is nowadays the reference for digital television defines how to format the various component parts of a multimedia program and how they are combined into a single transmission bit stream. Although the MPEG stream may be directly used over a wide variety of media, it may also be used over a communication network and it is already packetized into short data packets. The multiplexed stream can be transmitted over a variety of communication networks such as Radio Frequency links (UHF/VHF), Digital Satellite links, Cable TV Networks, Standard Terrestrial Communication Links (PDH, SDH) or Packet/Cell Links (ATM, IP, IPv6, Ethernet). In critical and error prone channels the robustness of the stream has to be granted by employing strong error correction schemes. Depending on the used media the transmission has its own coding scheme and modulation methods designed for the particular environment. For example, in the case of IP based links the transfer protocol accepts loss of data and the problem can be easily overcome by retransmission. Wireless simplex transmission cannot exploit the same approach. Wireless transmission always means variable channel conditions especially when one or both terminals are moving on the outfield. The channel fading evolution and the signal impairments increase the error rate and corrupt the data. In the recent past the standardization of the DVB-T shows the possibility to provide a robust service for digital video delivery. This standard provides superior picture quality with the opportunity to view pictures in standard format or wide screen (4/3 or 16/9) formats, services including subtitling, multiple audio tracks, interactive content, multimedia content where, for instance, programs may be linked to world wide web material. Beside the provided services and features the standard offers also a modulation and transmission scheme capable to cope with the impairments of the terrestrial channel and to provide a BER after forward-error correction below 10-12. Unfortunately, these performances are guaranteed only for fixed reception, since the standard was originally aimed to replace classical analog television broadcasting in the UHF band. This thesis addresses the study of enhanced error-correction schemes in the scope of a compact, flexible and robust high-throughput real time video transmission system able to cope with high mobility channels. The study of a transmission system and error correction scheme tailored for this kind of application is here presented. The system exploits the OFDM modulation technique. This modulation scheme has already proven to be very effective in hostile environments. A flexible modulation scheme is employed to obtain variable bandwidth transmission and enhanced performances especially for mobile applications. The main topic of this research is the study of an effective error correction scheme adapted to this application. While nowadays the digital television standards use a concatenated error correction scheme based on convolutional inner coder, in this work the proposed solution is based on a turbo inner coder. Turbo codes has already shown good performances compared to convolutional codes. These results are here applied to a concatenated scheme to obtain very low BER. A comparison of the performances of various turbo coding techniques under certain implementation constraints is done in this work.The second element of the error correction scheme is the channel interleaver used to average the channel fading in time and frequency. This mechanism is conceived to be adapted to the modulated bandwidth and is optimized for high throughput real time video. This study shows that the proposed system proves to be very effective in high mobile channels because of the deep averaging strategies in conjunction with the high efficiency turbo based concatenated error correction scheme.

EPFL2005