Juraj Poliak

Analytical expression of FSO link misalignments considering Gaussian beam

In the paper, an optical wireless link for point-to-point communication is analyzed. Its main focus is the analysis of the influence of various misalignments on the received power. Analysis includes influence of the tilt of the transmitter and the receiver as well as their mutual misalignment respecting the Gaussian intensity distribution of the transmitted beam in terms of geometrical optics. As a result of the analysis, expression for influence of the various misalignments on the received power are presented. For the simulation of the link, a MATLAB environment was used. The conclusion contains synthesis of the simulations results. The main contribution is in consideration of the Gaussian beam distribution of the transmitted beam and its analysis for precise prediction of geometrical losses for individual free-space optical links.

Link budget for high-speed short-distance wireless optical link

The paper discusses analyses of high-speed wireless optical link where various effects influence the detected optical power. For purpose of the analyses, Gaussian optical intensity distribution is considered. Further mathematical analysis leads to analytical expressions of attenuation of detected optical power under all common conditions (e.g. misalignment, tilts). All analyses are based on exact mathematical derivation and are modeled using a computer. In conclusion, results of simulations are described and individual influences of various effects are discussed and compared. Emphasis on exact mathematical analysis leads to thorough understanding of a system and allows us to design optical links more effectively with respect to the performance and also the price.

Integration of FSO in local area networks - Combination of Optical Wireless with WLAN and DVB-T for last mile internet connections

Nowadays FSO (Free Space Optics) also called Optical Wireless, is a well-established technology for closing the last mile gap. Optical Wireless is best suited to be directly connected to fibre optics, like the extension of a fibre cable for access networks. But also combinations with RF- and wireless networks are of main interest in current applications. Various technologies have different influences on weather conditions, because of the used wavelengths (frequencies) and the differences in the propagation behaviour and the transmission channel. This contribution will show at first the selection of best suited wavelengths for optical wireless communications (OWC) for the last mile access in relevance to the weather effects and the influences of beam divergence and beam shaping. The second part is showing the usage of FSO in combination with other wireless technologies (as a special scenario with WLAN and DVB-T), available for regions of low Internet coverage. FSO allows a broadband access with high data rate for users not connected so far by fibre cables. It is a well-known fact that peripheral regions lack of reasonable broadband Internet services. WLAN and FSO are license-free transmission technologies and DVB-T has become a geographically widespread technology. This paper will show an alternative Internet access method using WLAN and FSO systems as a data uplink and DVB-T as a downlink channel for broadband data access. The work was carried out within international co-operations in the frame of a special programme of European Union called SEE (South-East-Europe).

On the derivation of exact analytical FSO link attenuation model

The development of semiconductor technology has led to an increase of the usable bandwidth of light-emitting diodes LED, which are suitable for indoor high-speed communications or free-space optical FSO communications over a relatively short distance. The high-speed long-distance FSO links, where turbulence effects are considerably high, are a domain of laser sources. Hence, the paper covers the study of an exact analytical model of FSO link attenuation and the benefits of using an elliptically symmetrical Gaussian beam rather than a circularly symmetrical one.

Modeling of the multichannel optical wireless link

This paper deals with multi-channel wireless links with a higher range designated for free-space optical (FSO) communications in an atmospheric media. Propagation of the optical beam in the atmosphere is analyzed and a novel approach of the photonic transmitter was proposed and analyzed. The simulation is carried out for the dual optical link. The link budget was analyzed for both wavelengths in the atmospheric windows - 850 nm and 1550 nm. Optical intensity distribution of the transmitted beam was also examined. At the end, measurements and simulations were carried out to verify the accuracy of the simulation of individual components used in the wireless link.

Extended model of restricted beam for FSO links

Modern wireless optical communication systems in many aspects overcome wire or radio communications. Their advantages are license-free operation and broad bandwidth that they offer. The medium in free-space optical (FSO) links is the atmosphere. Operation of outdoor FSO links struggles with many atmospheric phenomena that deteriorate phase and amplitude of the transmitted optical beam. This beam originates in the transmitter and is affected by its individual parts, especially by the lens socket and the transmitter aperture, where attenuation and diffraction effects take place. Both of these phenomena unfavourable influence the beam and cause degradation of link availability, or its total malfunction. Therefore, both of these phenomena should be modelled and simulated, so that one can judge the link function prior to the realization of the system. Not only the link availability and reliability are concerned, but also economic aspects. In addition, the transmitted beam is not, generally speaking, circularly symmetrical, what makes the link simulation more difficult. In a comprehensive model, it is necessary to take into account the ellipticity of the beam that is restricted by circularly symmetrical aperture where then the attenuation and diffraction occur. General model is too computationally extensive; therefore simplification of the calculations by means of analytical and numerical approaches will be discussed. Presented model is not only simulated using computer, but also experimentally proven. One can then deduce the ability of the model to describe the reality and to estimate how far can one go with approximations, i.e. limitations of the model are discussed.

Influence of optical elements on the laser beam profile

The resistance of signal transmission to atmospheric phenomena is possible to solve partially by means of the special optical intensity distribution at the beam spot, i.e. by means of the special beam profile. In practice the resultant beam profile can take users of the free space optical link by surprise if only computer models of the optical beams are used and any diffraction effect at the transmitting lens is not considered. Two models of diffraction of optical beam radiated from the optical transmitter are presented and two methods of the beam modeling are clarified (the method based on Bessel function integrating and the method based on FFT). Confirmation of the models elaborated is a part of the contribution.

New research areas in the field of terrestrial optical wireless links

The paper deals with the terrestrial optical wireless links (TOWL) operating with coherent laser beams. The contribution discusses two main types of TOWL: data link and test link. Main aspects of constructions are shown. Purely photonic concept of given links is presented and justified. For the TOWL, applications not only in the field of communications, but also in environmental informatics or for synchronisation impulse distribution (time transmission) are given. Paper further discusses phenomena (standard and non-standard) deteriorating the TWOL availability and their monitoring, evaluation and reduction are explained. TOWL modelling is divided with respect to the stationary and statistical point of view and standardisation of the optical link and the atmosphere are proposed. The topic presented in the paper are independent from modulation formats and coding scheme.

Detection of non-standard atmospheric effects in FSO systems

Modern free-space optical (FSO) communication systems in many aspects overcome wire or radio communications. They offer a license-free operation and a large bandwidth. Operation of outdoor FSO links struggles with many atmospheric phenomena that deteriorate phase and amplitude of the transmitted optical beam. Thanks to the recent advancing development, these effects are more or less well understood and described. Goal driven research increased the link availability. Besides increasing the availability of data links it is necessary to focus on the accuracy and reliability of testing optical links. Research of the data optical links is focused on the transmission of a large amount of data whereas the testing FSO link is designed to achieve maximal resolution and sensitivity thus improving accuracy and repeatability of the atmospheric effects measurement. Given the fact that testing links are located in the measured media, they are themselves influenced by it. Phenomena such as the condensation on transceiver windows (rain, frost) and the deviation of the optical beam path caused by the wind are referred to as non-standard effects. Non-standard effects never occur independently; therefore we must always verify the cross-sensitivity of the testing link. In the paper we respond to an increasing number of articles dealing with influence of the atmosphere on the link but ignoring the cross-sensitivity of the testing link on other variables than tested. In conclusion, we carry out qualitative and quantitative analysis of self-identified non-standard effects.

Measurement system for analyzing the spectral radiation pattern of light sources for FSO systems

For the verification of Free Space Optics (FSO) systems it is common to determine optical power measured depending on illuminated area at a certain wavelength. In the context of this work a new measurement system was developed, which is able to measure not only the optical power but also the complete spectral distribution of the light beam. FSO is becoming more and more important, because of the high usable bandwidth (leading to high data-rates) in optics compared to radio frequency (RF). The measurement system consists of a mechanical structure, where a collimator is mounted on. This collimator can be moved in two dimensions to enable the measurement of predefined points. It is connected with a spectrum analyzer using an optical fiber. A computer controls the position of the collimator and initiates the spectrum analyzer to record the spectral curves, the maximum peak of power, the 3dB bandwidth and the total power of spectrum. The obtained data covers a measurement area of 0.91 m by 0.77 m which can be surveyed at a high local resolution of 0.7 mm. Collimated beams in the range between 600 and 1750 nm are analyzed. The measurement results discussed in this contribution are shown on the example of light sources, used in at TU Graz developed FSO-systems.