Diffraction Effects in Transmitted Optical Beam

Dizertačná práca pojednáva o vlnových a elektromagnetických javoch, ku ktorým dochádza pri zatienení eliptického Gausovského zväzku kruhovou apretúrou. Najprv boli z Huygensovho-Fresnelovho princípu odvodené dva modely Fresnelovej difrakcie. Tieto modely poskytli nástroj pre zavedenie kontrastu difrakčného obrazca ako veličiny, ktorá kvantifikuje vplyv difrakčných javov na prevádzkové parametre optického spoja. Následne, pomocou nástrojov elektromagnetickej teórie svetla, boli odvodené štyri výrazy (dva presné a dva aproximatívne) popisujúce geometrický útlm optického spoja. Zároveň boli skúmané tri rôzne prípady odsmerovania zväzku - priečne posunutie a uhlové odsmerovanie vysielača, resp. prijímača. Bol odvodený výraz, ktorý tieto prípady kvantifikuje ako útlm elipticky symetrického Gausovského zväzku. Všetky vyššie uvedené modely boli overené v laboratórnych podmienkach, aby sa vylúčil vplyv iných javov. Nakoniec práca pojednáva o návrhu plne fotonického optického terminálu. Najprv bol ukázaný návrh optického vysielača nasledovaný vývojom optomechanickej sústavy prijímača. Pomocou nástrojov geometrickej a maticovej optiky boli vypočítané parametre spoja a odhad tolerancie pri zamierení spoja.The thesis was set out to investigate on the wave and electromagnetic effects occurring during the restriction of an elliptical Gaussian beam by a circular aperture. First, from the Huygens-Fresnel principle, two models of the Fresnel diffraction were derived. These models provided means for defining contrast of the diffraction pattern that can be used to quantitatively assess the influence of the diffraction effects on the optical link performance. Second, by means of the electromagnetic optics theory, four expressions (two exact and two approximate) of the geometrical attenuation were derived. The study shows also the misalignment analysis for three cases -- lateral displacement and angular misalignment of the transmitter and the receiver, respectively. The expression for the misalignment attenuation of the elliptical Gaussian beam in FSO links was also derived. All the aforementioned models were also experimentally proven in laboratory conditions in order to eliminate other influences. Finally, the thesis discussed and demonstrated the design of the all-optical transceiver. First, the design of the optical transmitter was shown followed by the development of the receiver optomechanical assembly. By means of the geometric and the matrix optics, relevant receiver parameters were calculated and alignment tolerances were estimated.

A free-space optical communications link to an unmanned aerial vehicle.

Free-space optics (FSO) is an optical communications technology in which information is transmitted through the atmosphere on modulated optical beams. At the time of the development of FSO, it was envisioned to be a possible solution for provision of connectivity between customer premises and the optical backbone of today's telecommunications infrastructure. Due to limitations in the performance of FSO technology under adverse weather conditions, it is unable to provide sufficiently reliable connectivity for permanent telecommunications and networking applications. This dissertation investigates the deployment of FSO communications links to provide a temporary high-bandwidth communication line between a ground station and an unmanned aerial vehicle (UAV). The deployment of FSO links for ground-to-UAV communications is analyzed from the standpoint of feasibility based on beam steering tolerances and beam divergences, along with platform vibration analysis and active beam steering element comparisons. A mathematical model of a ground-to-UAV communication link is developed. Furthermore, an experimental analysis of a mechanical gimbal's accuracy and repeatability is performed. A wavelength diversity scheme in the presence of atmospheric turbulence is proposed to improve the alignment process. Finally, a wavelength diversity scheme is discussed which allows for transmission through radiation fog

Subcarrier intensity modulated free-space optical communication systems

This thesis investigates and analyses the performance of terrestrial free-space optical communication (FSO) system based on the phase shift keying pre-modulated subcarrier intensity modulation (SIM). The results are theoretically and experimentally compared with the classical On-Off keying (OOK) modulated FSO system in the presence of atmospheric turbulence. The performance analysis is based on the bit error rate (BER) and outage probability metrics. Optical signal traversing the atmospheric channel suffers attenuation due to scattering and absorption of the signal by aerosols, fog, atmospheric gases and precipitation. In the event of thick fog, the atmospheric attenuation coefficient exceeds 100 dB/km, this potentially limits the achievable FSO link length to less than 1 kilometre. But even in clear atmospheric conditions when signal absorption and scattering are less severe with a combined attenuation coefficient of less than 1 dB/km, the atmospheric turbulence significantly impairs the achievable error rate, the outage probability and the available link margin of a terrestrial FSO communication system. The effect of atmospheric turbulence on the symbol detection of an OOK based terrestrial FSO system is presented analytically and experimentally verified. It was found that atmospheric turbulence induced channel fading will require the OOK threshold detector to have the knowledge of the channel fading strength and noise levels if the detection error is to be reduced to its barest minimum. This poses a serious design difficulty that can be circumvented by employing phase shift keying (PSK) pre-modulated SIM. The results of the analysis and experiments showed that for a binary PSK-SIM based FSO system, the symbol detection threshold level does not require the knowledge of the channel fading strength or noise level. As such, the threshold level is fixed at the zero mark in the presence or absence of atmospheric turbulence. Also for the full and seamless integration of FSO into the access network, a study of SIM-FSO performance becomes compelling because existing networks already contain subcarrier-like signals such as radio over fibre and cable television signals. The use of multiple subcarrier signals as a means of increasing the throughput/capacity is also investigated and the effect of optical source nonlinearity is found to result in intermodulation distortion. The intermodulation distortion can impose a BER floor of up to 10-4 on the system error performance. In addition, spatial diversity and subcarrier delay diversity techniques are studied as means of ameliorating the effect of atmospheric turbulence on the error and outage performance of SIM-FSO systems. The three spatial diversity linear combining techniques analysed are maximum ratio combining, equal gain combining and selection combining. The system performance based on each of these combining techniques is presented and compared under different strengths of atmospheric turbulence. The results predicted that achieving a 4 km SIM-FSO link length with no diversity technique will require about 12 dB of power more than using a 4 × 4 transmitter/receiver array system with the same data rate in a weak turbulent atmospheric channel. On the other hand, retransmitting the delayed copy of the data once on a different subcarrier frequency was found to result in a gain of up to 4.5 dB in weak atmospheric turbulence channel

Studies of atmospheric effects on free-space THz pulse propagation and applications

Within THz frequency range, free-space propagated EM wave and the related applications have attracted a lot of attention, due to promising solutions for new types of remote sensing, wireless communications and security applications. However, the characteristics of free-space THz wave are degraded by several atmospheric conditions, such as water vapor absorption and pulse distortion.In this thesis, a comprehensive study on atmospheric effects on free-space THz signal and its potential applications by using the state-of-art Long-Path THz-TDS system is presented. Two types of absorptions from the ambient water vapor have been investigated: the resonant absorption which is associated with strong phase shifts and the continuum absorption. The molecular response theory is used as the lineshape function based on parameters from JPL and HITRAN databases for simulation to water vapor resonant absorption and phase shift. Not only were the THz digital bit observed for potential wireless communications, but the refractivity of water vapor was also obtained by line-by-line summation.A series of accurate experiments of water vapor continuum absorption have been carried out by using the 170 m Long-Path THz-TDS system. With precise experimental results and MRT simulation, the parameters for general model of water vapor continuum absorption have been successfully obtained within several THz transparent windows. As another kind of atmospheric condition, artificial fog fully filled the 137 m long sample chamber, in order to investigate absorption and phase shift of the atmospheric diffusive scattering mediums.With all the quantitative understanding of the atmospheric effects, a comprehensive atmospheric model for the free-space THz signals has been established, including the humidity, temperature and distance. This model can be used to provide a theoretical verification for all of the free-space THz applications.To demonstrate another free-space THz application, a series of experiments of THz-TDS remote detection of small molecules vapor have been performed. Samples of CH3CN, D2O and HDO have been successfully detected using 170 m Long-Path THz-TDS system. Moreover, the reaction ratio of the transition from H2O and D2O to HDO has been monitored in time

SCATTERING EFFECTS BY SiO2 NANO-MICROPARTICLE AND FREE SPACE ATTENUATION MODELLING FOR DIVERSE WEATHER CONDITIONS

This article presents few empirical models to be used prediction of atmospheric attenuation due to airborne suspended particles such as sand, dust, fog and mist. The attenuation can be determined using the proposed models as standard methods considering various types of weather conditions including silicadominant sandstorm, dust storm, fog or mist. As atmospheric attenuation heavily affects the transmission of data using visible light communication, we study the effect of air suspended particles to scattering amplitude and turbulence phase of the light beam. The proposed models are compared with existing attenuation models in the case of attenuation under foggy weather condition. The proposed models especially proposed Model 2 is seen to be best fit for prediction of atmospheric attenuation under dense to very light fog weather

Exponentiated weibull fading channel model in free-space optical communications under atmospheric turbulence

Free-space optical (FSO) communications is drawing increasing attention as a promising technology to overcome bandwidth shortage, of an evermore crowded wireless marketplace. Currently radio-frequency (RF) technology struggles to cope with the ever increasing demand for high-bandwidth data. Moreover, as the number of users increases, the RF spectrum is getting so crowded that there is virtually no room for new wireless services, with the additional inconvenient of limited bandwidth restriction for using a RF band and the license fees that have to be paid for such bands. FSO communications offer clear advantages over other alternatives such as narrower and more secure beams, virtually limitless bandwidth and no regulatory policies for using optical frequencies and bandwidth. Moreover, in the space sector FSO technology is becoming more attractive for satellite communication systems due to the less mass and power requirements --compared to RF. The major drawback for deploying wireless links based on FSO technology is the perturbation of the optical wave as it propagates through the turbulent atmosphere. Many effects are produced, of which the most noticeable is the random fluctuations of the signal-carrying laser beam irradiance (intensity), phenomenon known as scintillation and quantified by the scintillation index (SI). The statistical analysis of the random irradiance fluctuations in FSO links is conducted through the probability density function (PDF), from which one can obtain other statistical tools to measure link performance such as the probability of fade and the bit error-rate (BER). Nowadays, the most widespread models for the irradiance data are, by far, the Lognormal (LN) and Gamma-Gamma (GG) distributions. Although both models comply with actual data in most scenarios neither of them is capable of fitting the irradiance data under all conditions of atmospheric turbulence for finite receiving aperture sizes, i.e. in the presence of aperture averaging. Furthermore, there are several cases where neither the LN or the GG model seem to accurately fit the irradiance data, specially in the left tail of the PDF. The work presented in this thesis is devoted to propose a new model for the irradiance fluctuations in FSO links under atmospheric turbulence, in the presence of aperture averaging; resulting in the exponentiated Weibull (EW) distribution. A physical justification for the appearance of the new model is provided along with numerous test scenarios in the weak-to-strong turbulence regime --including numerical simulations and experimental data-- to assess its suitability to model the irradiance data in terms of the PDF and probability of fade. Here, a semi-heuristic approach is used to find a set of equations relating the EW parameters directly to the SI. Such expressions were tested offering a fairly good fitting the actual PDF of irradiance data. Furthermore, for all the scenarios tested a best fit version of the EW PDF is obtained and always presents itself as an excellent fit to the PDF data. The new model has been compared to the LN and GG distributions proving to cope to the predictions made by those and, in some cases, even outperforming their predictions. Additionally, a new closed-form expression has been derived for estimating the BER performance under EW turbulence, for intensity-modulation/direct-detection (IM/DD) systems using on-off keying (OOK) modulation. Moreover, this expression has been extended to include pointing errors. Finally, the exponentiated Weibull PDF has been proved to be valid with fully and partially coherent beams. The results presented here suggest that the EW distribution presents the better fit for data under different scenarios, thus, the exponentiated Weibull distribution becomes an excellent alternative to model the PDF of irradiance data under all conditions of atmospheric turbulence in the presence of aperture averaging

Wireless optical communications through the turbulent atmosphere: a review

Peer ReviewedPostprint (published version